Acicular hematite particles and magnetic recording medium

Abstract

Acicular hematite particles of the present invention comprise an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35 and a pH value of not less than 8, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO4.Such acicular hematite particles are suitable as non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium which exhibits a low light transmittance, an excellent smooth surface and a high strength, and can be prevented from being deteriorated in magnetic properties due to the corrosion of magnetic acicular metal particles containing iron as a main component which are dispersed in a magnetic recording layer thereof.

Description

BACKGROUND OF THE INVENTION:

The present invention relates to acicular hematite particles and a magnetic recording medium, and more particularly, to acicular hematite particles suitable as non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium which exhibits a low light transmittance, an excellent smooth surface and a high strength, and can be prevented from being deteriorated in magnetic properties due to the corrosion of magnetic acicular metal particles containing iron as a main component which are dispersed in a magnetic recording layer thereof; a non-magnetic substrate for a magnetic recording medium provided with a non-magnetic undercoat layer containing the acicular hematite particles; and a magnetic recording medium comprising the non-magnetic substrate and a magnetic recording layer containing magnetic acicular metal particles containing iron as a main component.

With a development of miniaturized, lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been increasingly and strongly required to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, an improved frequency characteristic and a lower noise level.

Various attempts have been made at both enhancing the properties of magnetic particles and reducing the thickness of a magnetic recording layer in order to improve these properties of a magnetic recording medium.

The enhancement of the properties of magnetic particles is firstly described.

The required properties of magnetic particles in order to satisfy the above-described demands on a magnetic recording medium are a high coercive force and a large saturation magnetization.

As magnetic particles suitable for high-output and high-density recording, magnetic acicular metal particles containing iron as a main component, which are obtained by heat-treating acicular goethite particles or acicular hematite particles in a reducing gas, are widely known.

Although magnetic acicular metal particles containing iron as a main component have a high coercive force and a large saturation magnetization, since the magnetic acicular metal particles containing iron as a main component used for a magnetic recording medium are very fine particles having a particle size of not more than 1 μm, particularly, 0.01 to 0.3 μm, they easily corrode and the magnetic characteristics thereof are deteriorated, especially, the saturation magnetization and the coercive force are decreased.

Therefore, in order to maintain the characteristics of a magnetic recording medium which uses magnetic acicular metal particles containing iron as a main component as magnetic particles, over a long period, it is strongly demanded to suppress the corrosion of magnetic acicular metal particles containing iron as a main component as much as possible.

A reduction in the thickness of a magnetic recording layer is described. Video tapes have recently been required more and more to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the short-wave region have come to be used, and as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small.

With respect to short wavelength signals, a reduction in the thickness of a magnetic recording layer is also strongly demanded in order to improve the high output characteristics, especially, the S/N ratio of a magnetic recording medium. This fact is described, for example, on page 312 of Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, published by Sogo Gijutsu Center Co., Ltd. (1982), “. . . the conditions for high-density recording in a coated-layer type tape are that the noise level is low with respect to signals having a short wavelength and that the high output characteristics are maintained. To satisfy these conditions, it is necessary that the tape has large coercive force Hc and residual magnetization Br, . . . and the coating film has a smaller thickness. . . . ”.

Development of a thinner film for a magnetic recording layer has caused some problems.

Firstly, it is necessary to make a magnetic recording layer smooth and to eliminate the non-uniformity of thickness. As well known, in order to obtain a smooth magnetic recording layer having a uniform thickness, the surface of the base film must also be smooth. This fact is described on pages 180 and 181 of Materials for Synthetic Technology - Causes of Friction and Abrasion of Magnetic Tape and Head Running System and Measures for Solving the Problem (hereinunder referred to as “ Materials for Synthetic Technology” (1987), published by the Publishing Department of Technology Information Center,“. . . the surface roughness of a hardened magnetic coating film depends on the surface roughness of the base film (back surface roughness) so largely as to be approximately proportional, . . . , since the magnetic coating film is formed on the base film, the more smooth the surface of the base film is, the more uniform and larger head output is obtained and the more the S/N ratio is improved.”

Secondly, there has been caused a problem in the strength of a base film with a tendency of the reduction in the thickness of the base film in response to the demand for a thinner magnetic coating film. This fact is described, for example, on page 77 of the above-described Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, “. . . Higher recording density is a large problem assigned t the present magnetic tape. This is important in order to shorten the length of the tape so as to miniaturize the size of a cassette and to enable long-time recording. For this purpose, it is necessary to reduce the thickness of a base film . . . With the tendency of reduction in the film thickness, the stiffness of the tape also reduces to such an extent as to make smooth travel in a recorder difficult. Therefore, improvement of the stiffness of a video tape both in the machine direction and in the transverse direction is now strongly demanded. . . . ”

There is no end to a demand for a higher performance in recent magnetic recording media. Since the above-described reduction in the thickness of a magnetic recording layer and a base film lowers the durability of the magnetic recording medium, an improvement of the durability of the magnetic recording medium is in strong demand.

This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 5-298679, “. . . With the recent development in magnetic recording, a high picture quality and a high sound quality have been required more and more in recording. The signal recording property is, therefore, improved. Especially, finer and higher-density ferromagnetic particles have come to be used. It is further required to make the surface of a magnetic tape smooth so as to reduce noise and raise the C/N . . . However, the coefficient of friction between the magnetic recording layer and an apparatus during the travel of the magnetic recording tape increases, so that there is a tendency of the magnetic recording layer of the magnetic recording medium being damaged or exfoliated even in a short time. Especially, in a videotape, since the magnetic recording medium travels at a high speed in contact with the video head, the ferromagnetic particles are apt to be dropped from the magnetic recording layer, thereby causing clogging on the magnetic head. Therefore, an improvement in the running durability of the magnetic recording layer of a magnetic recording medium is expected . . .”

The end portion of a magnetic recording medium such as a magnetic tape, especially, a video tape is judged by detecting a portion of the magnetic recording medium at which the light transmittance is large by a video deck. If the light transmittance of the whole part of a magnetic recording layer is made large by the production of a thinner magnetic recording medium or the ultrafine magnetic particles dispersed in the magnetic recording layer, it is difficult to detect the portion having a large light transmittance by a video deck. For reducing the light transmittance of the whole part of a magnetic recording layer, carbon black or the like is added to the magnetic recording layer. It is, therefore, essential to add carbon black or the like to a magnetic recording layer in the present video tapes.

However, addition of a large amount of non-magnetic particles such as carbon black impairs not only the enhancement of the magnetic recording density but also the development of a thinner magnetic recording layer. In order to reduce the magnetization depth from the surface of the magnetic tape and to produce a thinner magnetic recording layer, it is strongly demanded to reduce, as much as possible, the quantity of non-magnetic particles such as carbon black which are added to a magnetic recording layer.

It is, therefore, strongly demanded that the light transmittance of a magnetic recording layer should be small even if the carbon black or the like which is added to the magnetic recording layer is reduced to a small amount. From this point of view, improvements in the magnetic recording medium are now in strong demand.

There has, also, been pointed out such a problem that the magnetic acicular metal particles containing iron as a main component dispersed in the magnetic recording layer are corroded in a passage of time after the production thereof, so that magnetic properties of the magnetic recording medium are considerably deteriorated.

Various efforts have been made to improve the substrate for a magnetic recording layer with a demand for a thinner magnetic recording layer and a thinner base film. A magnetic recording medium having at least one undercoat layer (hereinunder referred to “non-magnetic undercoat layer”) comprising a binder resin and non-magnetic particles such as hematite particles which are dispersed therein, on a base film has been proposed and put to practical use (Japanese Patent Publication (KOKOKU) No. 6-93297 (1994), Japanese Patent Application Laid-Open (KOKAI) Nos. 62-159338 (1987), 63-187418 (1988), 4-167225 (1992), 4-325915 (1992), 5-73882 (1993), 5-182177 (1993), 5-347017 (1993), 6-60362 (1994), etc.)

The above-described magnetic recording media composed of a base film and a non-magnetic undercoat layer composed of a binder resin and non-magnetic particles dispersed therein and formed on the base film, have a small light transmittance and a high strength, but the durability and the surface smoothness thereof are inconveniently poor.

This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 5-182177 (1993), “. . . Although the problem of surface roughness is solved by providing a magnetic recording layer as an upper layer after forming a thick non-magnetic undercoat layer on the base film, the problem of the abrasion of a head and the problem of durability are not solved and still remain. This is considered to be caused because a thermoset resin is usually used as a binder of the non-magnetic undercoat layer, so that the magnetic recording layer is brought into contact with a head or other members without any cushioning owing to the hardened non-magnetic undercoat layer, and a magnetic recording medium having such a non-magnetic undercoat layer has a considerably poor flexibility.”

Therefore, it has been strongly required to improve surface smoothness of the non-magnetic undercoat layer. Hitherto, it has been attempted to enhance a dispersibility of acicular hematite particles as non-magnetic particles by directing attention to the particle size distribution of major axis diameters of the particles (Japanese Patent Application Laid-open (KOKAI) No. 9-170003(1997), and the like).

That is, in the above Japanese Patent Application Laid-open (KOKAI) No. 9-170003(1997), there has been described a method of heat-treating acicular goethite particles or acicular hematite particles produced by heat-dehydrating the acicular goethite particles, at a temperature of not less than 550° C., thereby obtaining high-density acicular hematite particles having a good geometrical standard deviation of a major axis diameter. However, as shown in Comparative Examples hereinafter, the obtained acicular hematite particles are deteriorated in a geometrical standard deviation of the minor axis diameter, so that the dispersibility of the particles is still unsatisfactory.

Accordingly, with a recent tendency of reducing the thickness of the magnetic recording layer and non-magnetic base film, there have been most demanded acicular hematite particles having a uniform particle size, which are suitable as non-magnetic particles for a non-magnetic undercoat layer having a smooth surface and a high mechanical; and a magnetic recording medium provided with a non-magnetic undercoat layer containing the acicular hematite particles, which exhibits a low light transmittance, an excellent smooth surface, a high mechanical strength and an excellent durability, and which can be prevented from being deteriorated in magnetic properties due to the corrosion of magnetic acicular metal particles containing iron as a main component contained in a magnetic recording layer. However, such acicular hematite particles and magnetic recording medium capable of satisfying these requirements have not been obtained yet.

Further, in recent years, for purposes of short-wavelength recording and high-density recording, it has been inevitably necessary to use magnetic acicular metal particles containing iron as a main component, having a high coercive force and a large saturation magnetization. However, since the magnetic acicular metal particles containing iron as a main component are fine particles, the magnetic acicular metal particles containing iron as a main component tend to be corroded, resulting in deterioration in magnetic properties thereof. Therefore, it has been demanded to provide a magnetic recording medium in which magnetic properties thereof are not deteriorated and can be maintained for a long period of time.

As a result of the present inventors' earnest studies, it has been found that (a) by heat-treating specific acicular goethite particles at a specific temperature and then heat-dehydrating the thus-treated acicular goethite particles to obtain specific acicular hematite particles, or (b) by subjecting specific acicular hematite particles to a acid-dissolving treatment under specific conditions to obtain specific acicular hematite particles, and then heating the acicular hematite particles obtained by the method (a) or (b) in an alkaline suspension, there can be obtained high-purity acicular hematite particles which exhibit a uniform particle size, especially wherein the geometrical standard deviation nears 1.0, and which are useful as non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium which can be prevented from being deteriorated in magnetic properties due to the corrosion of magnetic acicular metal particles containing iron as a main component contained in a magnetic recording layer. The present invention has been attained on the basis of the finding.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide acicular hematite particles which are useful as non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium exhibiting an excellent surface smoothness, and have a uniform particle size, especially are excellent geometrical standard deviation of a minor axis diameter thereof.

It is another object of the present invention to provide acicular hematite particles suitable as non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium which can exhibit a low light transmittance, a smooth surface and a high strength, and can be prevented from being deteriorated in magnetic properties due to the corrosion of magnetic acicular metal particles containing iron as a main component contained in a magnetic recording layer thereof, and a magnetic recording medium provided with a non-magnetic undercoat layer containing the acicular hematite particles.

It is an other object of the present invention to provide acicular hematite particles suitable as non-magnetic particles for a non-magnetic undercoat layer having a low light transmittance, a smooth surface and a high, and a magnetic recording medium which can exhibit a low light transmittance, a smooth surface, a high strength and an excellent durability, and can be prevented from being deteriorated in magnetic properties due to the corrosion of magnetic acicular metal particles containing iron as a main component contained in a magnetic recording layer thereof.

To accomplish the aims, in a first aspect of the present invention, there are provided acicular hematite particles comprising an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35 and a pH value of not less than 8, and

containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 .

In a second aspect of the present invention, there are provided acicular hematite particles comprising an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35 and a pH value of not less than 8, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and

having a coat formed on at least a part of the surface of said acicular hematite particle, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in amount of 0.05 to 50% by weight, calculated as Al or SiO 2 , based on the weight of said acicular hematite particles.

In a third aspect of the present invention, there are provided acicular hematite particles comprising an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35 and a pH value of not less than 8,

containing aluminum existing within the particle in an amount of 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and

containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 .

In a fourth aspect of the present invention, there are provided acicular hematite particles comprising an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35 and a pH value of not less than 8, containing aluminum existing within the particle in an amount of 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and

having a coat formed on at least a part of the surface of said acicular hematite particle, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in amount of 0.05 to 50% by weight, calculated as Al or SiO 2 , based on the weight of said acicular hematite particles.

In a fifth aspect of the present invention, there are provided acicular hematite particles comprising an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35, a geometrical standard deviation of major axis diameter of not more than 1.50, a BET specific surface area of 35.9 to 180 m 2 /g and a pH value of not less than 8, and

containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 .

In a sixth aspect of the present invention, there are provided acicular hematite particles comprising an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35, a geometrical standard deviation of major axis diameter of not more than 1.50, a BET specific surface area of 35.9 to 180 m 2 /g and a pH value of not less than 8, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and

having a coat formed on at least a part of the surface of said acicular hematite particle, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in amount of 0.05 to 50% by weight, calculated as Al or SiO 2 , based on the weight of said acicular hematite particles.

In a seventh aspect of the present invention, there are provided acicular hematite particles comprising an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35, a geometrical standard deviation of major axis diameter of not more than 1.50, a BET specific surface area of 35.9 to 180 m 2 /g and a pH value of not less than 8,

containing aluminum existing within the particle in an amount of 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and

containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 .

In an eighth aspect of the present invention, there are provided acicular hematite particles comprising an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35, a geometrical standard deviation of major axis diameter of not more than 1.50, a BET specific surface area of 35.9 to 180 m 2 /g and a pH value of not less than 8, containing aluminum existing within the particle in an amount of 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and

having a coat formed on at least a part of the surface of said acicular hematite particle, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in amount of 0.05 to 50% by weight, calculated as Al or SiO 2 , based on the weight of said acicular hematite particles.

In a ninth aspect of the present invention, there is provided acicular hematite particles comprising a geometrical standard deviation of major axis diameter of not more than 1.50, a geometrical standard deviation of minor axis diameter of not more than 1.30, a BET specific surface area of 40 to 150 m 2 /g, an average major axis diameter of 0.01 to 0.2 μm and a pH value of not less than 8, and

containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 .

In a tenth aspect of the present invention, there are provided acicular hematite particles comprising a geometrical standard deviation of major axis diameter of not more than 1.50, a geometrical standard deviation of minor axis diameter of not more than 1.30, a BET specific surface area of 40 to 150 m 2 /g, an average major axis diameter of 0.01 to 0.2 μm and a pH value of not less than 8, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and

having a coat formed on at least a part of the surface of said acicular hematite particle, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in amount of 0.05 to 50% by weight, calculated as Al or SiO 2 , based on the weight of said acicular hematite particles.

In an eleventh aspect of the present invention, there are provided acicular hematite particles comprising a geometrical standard deviation of major axis diameter of not more than 1.50, a geometrical standard deviation of minor axis diameter of not more than 1.30, a BET specific surface area of 40 to 150 m 2 /g, an average major axis diameter of 0.01 to 0.2 μm and a pH value of not less than 8,

containing aluminum existing within the particle in an amount of 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and

containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 .

In a twelfth aspect of the present invention, there are provided acicular hematite particles comprising a geometrical standard deviation of major axis diameter of not more than 1.50, a geometrical standard deviation of minor axis diameter of not more than 1.30, a BET specific surface area of 40 to 150 m 2 /g, an average major axis diameter of 0.01 to 0.2 μm and a pH value of not less than 8, containing aluminum existing within the particle in an amount of 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and

having a coat formed on at least a part of the surface of said acicular hematite particle, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in amount of 0.05 to 50% by weight, calculated as Al or SiO 2 , based on the weight of said acicular hematite particles.

In a thirteenth aspect of the present invention, there is provided a magnetic recording medium comprising:

a non-magnetic base film;

a non-magnetic undercoat layer formed on said non-magnetic base film, comprising a binder resin and acicular hematite particles having an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35 and a pH value of not less than 8, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 ; and

a magnetic coating film comprising a binder resin and magnetic acicular metal particles containing iron as a main component.

In a fourteenth aspect of the present invention, there is provided a magnetic recording medium comprising:

a non-magnetic base film;

a non-magnetic undercoat layer formed on said non-magnetic base film, comprising a binder resin and acicular hematite particles comprising an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35 and a pH value of not less than 8, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and

having a coat formed on at least a part of the surface of said acicular hematite particle, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in amount of 0.05 to 50% by weight, calculated as Al or SiO 2 , based on the weight of said acicular hematite particles; and

a magnetic coating film comprising a binder resin and magnetic acicular metal particles containing iron as a main component.

In a fifteenth aspect of the present invention, there is provided a magnetic recording medium comprising:

a non-magnetic base film;

a non-magnetic undercoat layer formed on said non-magnetic base film, comprising a binder resin and acicular hematite particles comprising an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35 and a pH value of not less than 8,

containing aluminum existing within the particle in an amount of 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 ; and

a magnetic coating film comprising a binder resin and magnetic acicular metal particles containing iron as a main component.

In a sixteenth aspect of the present invention, there is provided a magnetic recording medium comprising:

a non-magnetic base film;

a non-magnetic undercoat layer formed on said non-magnetic base film, comprising a binder resin and acicular hematite particles comprising an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35 and a pH value of not less than 8, containing aluminum existing within the particle in an amount of 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and

having a coat formed on at least a part of the surface of said acicular hematite particle, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in amount of 0.05 to 50% by weight, calculated as Al or SiO 2 , based on the weight of said acicular hematite particles; and

a magnetic coating film comprising a binder resin and magnetic acicular metal particles containing iron as a main component.

In a seventeenth aspect of the present invention, there is provided a non-magnetic substrate comprising:

a non-magnetic base film; and

a non-magnetic undercoat layer formed on said non-magnetic base film, comprising a binder resin and acicular hematite particles having an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35 and a pH value of not less than 8, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 .

In an eighteenth aspect of the present invention, there is provided a non-magnetic substrate comprising:

a non-magnetic base film; and

a non-magnetic undercoat layer formed on said non-magnetic base film, comprising a binder resin and acicular hematite particles comprising an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35 and a pH value of not less than 8, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and

having a coat formed on at least a part of the surface of said acicular hematite particle, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in amount of 0.05 to 50% by weight, calculated as Al or SiO 2 , based on the weight of said acicular hematite particles.

In a nineteenth aspect of the present invention, there is provided a non-magnetic substrate comprising:

a non-magnetic base film; and

a non-magnetic undercoat layer formed on said non-magnetic base film, comprising a binder resin and acicular hematite particles comprising an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35 and a pH value of not less than 8,

containing aluminum existing within the particle in an amount of 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and

containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 .

In a twentieth aspect of the present invention, there is provided a non-magnetic substrate comprising:

a non-magnetic base film; and

a non-magnetic undercoat layer formed on said non-magnetic base film, comprising a binder resin and acicular hematite particles comprising an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35 and a pH value of not less than 8, containing aluminum existing within the particle in an amount of 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and

having a coat formed on at least a part of the surface of said acicular hematite particle, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in amount of 0.05 to 50% by weight, calculated as Al or SiO 2 , based on the weight of said acicular hematite particles.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is explained in more detail below.

First, the acicular hematite particles for a non-magnetic undercoat layer of a magnetic recording medium according to the present invention are described.

The acicular hematite particles according to the present invention have substantially the same acicular shape as that of particles to be treated. The “acicular” shape may include not only a needle shape but also a spindle shape or a rice-ball shape. In addition, the lower limit of an aspect ratio of the particles (=average major axis diameter:average minor axis diameter, hereinafter referred to merely as “aspect ratio”) is usually 2:1, preferably 3:1. With the consideration of dispersibility of the particles in vehicle, the upper limit of the aspect ratio is preferably 20:1, more preferably 10:1. When the aspect ratio is less than 2:1, it may be difficult to obtain a coating film having a sufficient strength. On the other hand, when the aspect ratio is more than 20:1, the particles may be entangled with each other in vehicle, thereby causing a tendency that the dispersibility thereof may be deteriorated or the viscosity thereof may be increased.

The upper limit of the average major axis diameter of the acicular hematite particles according to the present invention is usually 0.295 μm. The lower limit of the average major axis diameter of the acicular hematite particles according to the present invention is usually 0.004 μm. If the upper limit of the average major axis diameter exceeds 0.295 μm, the surface smoothness of the coating film formed using such particles may be impaired because the particle size is large. On the other hand, if the lower limit of the average major axis diameter is less than 0.004 μm, the dispersion in the vehicle may be difficult because of the increase of the intermolecular force due to the fine particles. With the consideration of the dispersibility in the vehicle and the surface smoothness of the coating film, the upper limit thereof is preferably 0.275 μm, more preferably 0.200 μm, still more preferably 0.100 μm, and the lower limit thereof is preferably 0.008 μm, more preferably 0.010 μm, more preferably 0.020 μm.

The upper limit of the average minor axis diameter of the acicular hematite particles according to the present invention is usually 0.147 μm. The lower limit of the average minor axis diameter of the acicular hematite particles according to the present invention is usually 0.002 μm. If the upper limit of the average minor axis diameter exceeds 0.147 μm, the surface smoothness of the coating film formed using such particles may be impaired because the particle size is large. On the other hand, if the lower limit of the average minor axis diameter is less than 0.002 μm, the dispersion in the vehicle may be difficult because of the increase of the intermolecular force due to the fine particles. With the consideration of the dispersibility in the vehicle and the surface smoothness of the coating film, the upper limit thereof is preferably 0.123 μm, more preferably 0.100 μm, still more preferably 0.050 μm, and the lower limit thereof is preferably 0.004 μm, more preferably 0.005 μm, more preferably 0.010 μm.

The upper limit of the BET specific surface area (S BET ) of the acicular hematite particles according to the present invention is usually 180 m 2 /g. The lower limit of the BET specific surface area (S BET ) of the acicular hematite particles according to the present invention is usually 35.9 m 2 /g. If the upper limit thereof is more than 180 m 2 /g, the dispersion in the vehicle may be difficult because of the increase of the intermolecular force due to the fine particles. On the other hand, if the lower limit thereof is less than 35.9 m 2 /g, the acicular hematite particles may be coarse particles or large particles produced by sintering a particle and between particles, which are apt to exert a deleterious influence on the surface smoothness of the coating film. With the consideration of the dispersibility in the vehicle and the surface smoothness of the coating film, the upper limit thereof (S BET ) is preferably 160 m 2 /g, more preferably 150 m 2 /g and the lower limit thereof (S BET ) is preferably 38 m 2 /g, more preferably 40 m 2 /g.

The upper limit of the geometrical standard deviation of the major axis diameter of the acicular hematite particles according to the present invention is usually not more than 1.50. If the upper limit of the geometrical standard deviation of the major axis diameter exceeds 1.50, the coarse particles existent sometimes exert a deleterious influence on the surface smoothness of the coating film. With the consideration of the surface smoothness of the coating film, the upper limit thereof is preferably 1.45, more preferably not more than 1.40. From the point of view of industrial productivity, the lower limit thereof is preferably 1.01.

The upper limit of the geometrical standard deviation of the minor axis diameter of the acicular hematite particles according to the present invention is usually not more than 1.35. If the upper limit of the geometrical standard deviation of the minor axis diameter exceeds 1.35, the coarse particles existent sometimes exert a deleterious influence on the surface smoothness of the coating film. With the consideration of the surface smoothness of the coating film, the upper limit thereof is preferably 1.33, more preferably not more than 1.30, still more preferably not more than 1.28. From the point of view of industrial productivity, the lower limit thereof is preferably 1.01.

The acicular hematite particles for a non-magnetic undercoat layer of a magnetic recording medium according to the present invention have a high degree of densification. When the degree of densification is represented by a ratio value of the specific surface area (S BET ) measured by a BET method to the surface area (S BET ) calculated from the major axis diameter and the minor axis diameter which were measured from the particles in an electron micrograph of the acicular hematite particles (hereinafter referred to merely as “S BET /S TEM value”), the S BET /S TEM value is 0.5 to 2.5.

When the S BET/S TEM value is less than 0.5, although the acicular hematite particles are highly densified, the particle diameter thereof may be increased due to sintering in each particle or between particles, so that a coating film formed using these particles, may not have a sufficient smooth surface. On the other hand, when the S BET /S TEM value is more than 2.5, the degree of densification of the particles is insufficient, so that many pores tend to be formed on the surface and inside of the particle, resulting in insufficient dispersibility of the particles in vehicle. With the consideration of the smooth surface of the coating film and the dispersibility in the vehicle, the S BET /S TEM value is preferably 0.7 to 2.0, more preferably 0.8 to 1.6.

The pH value of the acicular hematite particles when suspended in an aqueous solution (concentration: 50 g/liter), is not less than 8. If it is less than 8, the magnetic acicular metal particles containing iron as a main component contained in the magnetic recording layer formed on the non-magnetic undercoat layer are gradually corroded, which leads to a deterioration in the magnetic characteristics. In consideration of a corrosion preventive effect on the magnetic acicular metal particles containing iron as a main component, the lower limit of the pH value of the acicular hematite particles is preferably not less than 8.5, more preferably not less than 9.0. The upper limit of the pH value of the acicular hematite particles is preferably 11, more preferably 10.5.

The content of soluble sodium salt in the acicular hematite particles is not more than 300 ppm (calculated as Na). If it exceeds 300 ppm, the magnetic acicular metal particles containing iron as a main component contained in the magnetic recording layer formed on the non-magnetic undercoat layer are gradually corroded, thereby causing a deterioration in the magnetic characteristics. In addition, the dispersion property of the acicular hematite particles in the vehicle is easily impaired, and the preservation of the magnetic recording medium is deteriorated and chalking is sometimes caused in a highly humid environment. In consideration of a corrosion preventive effect on the magnetic acicular metal particles containing iron as a main component, the content of soluble sodium salt in the acicular hematite particles is preferably not more than 250 ppm, more preferably not more than 200 ppm, even more preferably not more than 150 ppm. From the point of view of industry such as productivity, the lower limit thereof is about 0.01 ppm.

The content of soluble sulfate in the acicular hematite particles is not more than 150 ppm (calculated as SO 4 ). If it exceeds 150 ppm, the magnetic acicular metal particles containing iron as a main component contained in the magnetic recording layer formed on the non-magnetic undercoat layer are gradually corroded, thereby causing a deterioration in the magnetic properties. In addition, the dispersion property of the acicular hematite particles in the vehicle is easily impaired, and the preservation of the magnetic recording medium is deteriorated and chalking is sometimes caused in a highly humid environment. In consideration of a corrosion preventive effect on the magnetic acicular metal particles containing iron as a main component, the content of soluble sodium salt in the acicular hematite particles is preferably not more than 70 ppm, more preferably not more than 50 ppm. From the point of view of industry such as productivity, the lower limit thereof is about 0.01 ppm.

With the consideration of the durability of the magnetic recording medium having a non-magnetic undercoat layer containing such acicular hematite particles, it is preferable acicular hematite particles contain aluminum present substantially uniformly within the particle in an amount of 0.05 to 50% by weight (calculated as Al) based on the total weight of the acicular hematite particles. When the aluminum content exceeds 50% by weight, although a magnetic recording medium having the non-magnetic undercoat layer containing such acicular hematite particles has a sufficient durability, the durability-improving effect becomes saturated, so that it is meaningless to add aluminum more than necessary. From the point of view of durability-improving effect of a magnetic recording medium and industrial productivity, the aluminum content therein is preferably 0.1 to 30% by weight, more preferably 0.2 to 20% by weight (calculated as Al) based on the total weight of the particles.

Various properties of the acicular hematite particles which contain aluminum within the particle, such as aspect ratio, average major axis diameter, average minor axis diameter, BET specific surface area, geometrical standard deviation of major axis diameter, geometrical standard deviation of minor axis diameter and degree of densification are approximately equivalent in values to those of the acicular hematite particles in which aluminum is not contained within the particle.

The resin adsorptivity of the acicular hematite particles which contain aluminum within the particle according to the present invention is usually not less than 65%, preferably not less than 68%, more preferably not less than 70%. The upper limit thereof is preferably 95%.

At least a part of the surface of the acicular hematite particle which may contain aluminum within the particle according to the present invention may be coated with at least one selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon. When the acicular hematite particles which are coated with the above-described coating material are dispersed in a vehicle, the treated particles have an affinity with the binder resin and it is more easy to obtain a desired dispersibility.

The amount of the hydroxide of aluminum, the oxide of aluminum, the hydroxide of silicon or the oxide of silicon used as the coating material is usually not less than 50% by weight, preferably 0.01 to 50% by weight (calculated as Al and/or SiO 2 ) based on the total weight of the particles. If it is less than 0.01% by weight (calculated as Al and/or SiO 2 ) based on the total weight of the particles, the dispersibility-improving effect by coating therewith may be insufficient. If the amount exceeds 50% by weight (calculated as Al and/or SiO 2 ) based on the total weight of the particles, the dispersibility-improving effect by coating therewith becomes saturated, so that it is meaningless to add a coating material more than necessary. From the point of view of dispersibility in the vehicle and industrial productivity, the more preferable amount of coating material is 0.05 to 20% by weight (calculated as Al and/or SiO 2 ) based on the total weight of the particles.

Various properties of the acicular hematite particles which are coated with above-mentioned coating material, such as aspect ratio, average major axis diameter, average minor axis diameter, BET specific surface area, geometrical standard deviation of major axis diameter, geometrical standard deviation of minor axis diameter and degree of densification are approximately equivalent in values to those of the acicular hematite particles in which the surfaces thereof is not coated with the above-mentioned coating material.

The process for producing the acicular hematite particles according to the present invention is exemplified as follows.

The acicular hematite particles for a non-magnetic undercoat layer of a magnetic recording medium according to the present invention are produced by the following processes (I) and (II).

(I) The acicular hematite particles according to the present invention can be obtained by subjecting acicular hematite particles to be treated to acid-dissolving treatment under specific conditions.

The acicular hematite particles to be treated can be produced by various methods. Examples of these methods may include a method of directly producing the hematite particles by a wet process; a method of producing akaganeite particles (β-FeOOH) and then heat-dehydrating the akaganeite particles; or the like. As an ordinary production method, there may be industrially preferably used a method of producing acicular goethite particles as a precursor of the acicular hematite particles by the following wet process and then heat-dehydrating the obtained acicular goethite particles.

An ordinary method of producing acicular goethite particles as one of precursors of the acicular hematite particles is described below.

As described hereinafter, the acicular goethite particles can be produced by passing an oxygen-containing gas through a suspension containing ferrous precipitates such as hydroxides of iron or iron carbonate which are obtained by reacting a ferrous salt with either alkali hydroxide, alkali carbonate or mixed alkali composed of alkali hydroxide and alkali carbonate.

Acicular goethite particles are produced by an ordinary method:

(A) a method of producing needle-shaped goethite particles comprising oxidizing a suspension having a pH value of not less than 11 and containing colloidal ferrous hydroxide particles which is obtained by adding not less than an equivalent of an alkali hydroxide solution to an aqueous ferrous salt solution, by passing an oxygen-containing gas thereinto at a temperature of not higher than 80° C.;

(B) a method of producing spindle-shaped goethite particles comprising oxidizing a suspension containing FeCO 3 which is obtained by reacting an aqueous ferrous salt solution with an aqueous alkali carbonate solution, by passing an oxygen-containing gas thereinto after aging the suspension, if necessary;

(C) a method of producing spindle-shaped goethite particles comprising oxidizing a suspension containing precipitates containing iron which is obtained by reacting an aqueous ferrous salt solution with an aqueous alkali carbonate solution and an alkali hydroxide solution, by passing an oxygen-containing gas thereinto after aging the suspension, if necessary;

(D) a method of growing needle-shaped seed goethite particles comprising oxidizing a ferrous hydroxide solution containing colloidal ferrous hydroxide particles which is obtained by adding less than an equivalent of an alkali hydroxide solution or an alkali carbonate solution to an aqueous ferrous salt solution, by passing an oxygen-containing gas thereinto, thereby producing needle-shaped seed goethite particles, adding not less than an equivalent of an alkali hydroxide solution to the Fe 2+ in the aqueous ferrous salt solution, to the aqueous ferrous salt solution containing the needle-shaped seed goethite particles, and passing an oxygen-containing gas into the aqueous ferrous salt solution;

(E) a method of growing needle-shaped seed goethite particles comprising oxidizing a ferrous hydroxide solution containing colloidal ferrous hydroxide particles which is obtained by adding less than an equivalent of an alkali hydroxide solution or an alkali carbonate solution to an aqueous ferrous salt solution, by passing an oxygen-containing gas thereinto, thereby producing needle-shaped seed goethite particles, adding not less than an equivalent of an aqueous alkali carbonate solution to the Fe 2+ in the aqueous ferrous salt solution, to the aqueous ferrous salt solution containing the needle-shaped seed goethite particles, and passing an oxygen-containing gas into the aqueous ferrous salt solution; and

(F) a method of growing needle-shaped seed goethite particles comprising oxidizing a ferrous hydroxide solution containing colloidal ferrous hydroxide particles which is obtained by adding less than an equivalent of an alkali hydroxide solution or an alkali carbonate solution to an aqueous ferrous salt solution, by passing an oxygen-containing gas thereinto, thereby producing needle-shaped seed goethite particles and growing the obtained needle-shaped seed goethite particles in an acidic or neutral region.

Elements other than Fe such as Ni, Zn, P and Si, which are generally added in order to enhance various properties of the particles such as the major axis diameter, the minor axis diameter and the aspect ratio, may be added during the reaction system for producing the goethite particles.

The acicular goethite particles obtained have an average major axial diameter of usually 0.005 to 0.4 μm, an average minor axial diameter of usually 0.0025 to 0.20 μm, a geometrical standard deviation of the major axis diameter of not more than 1.70, a geometrical standard deviation of the minor axis diameter of 1.37 to 1.50 and a BET specific surface area of about usually 50 to 250 m 2 /g, and contain soluble sodium salts of usually 300 to 1500 ppm (calculated as Na) and ordinarily soluble sulfates of 150 to 3000 ppm (calculated as SO 4 ).

Alternatively, the acicular hematite particles to be treated which contain aluminum therewithin in an amount of 0.05 to 50% by weight (calculated as Al) based on the total weight of the particles, can be produced by containing aluminum in the acicular goethite particles by preliminarily adding an aluminum compound upon the above-mentioned production reaction of the acicular goethite particles.

Further, in the production reaction of the acicular goethite particles which contain aluminum within the particle, an aluminum compound may be added to at least one solution selected from suspensions containing a ferrous salt, alkali hydroxide, alkali carbonate, mixed alkali composed of alkali hydroxide and alkali carbonate, or ferrous precipitates such as hydroxides of iron or iron carbonate. It is preferred that the aluminum compound is added to an aqueous ferrous salt solution.

As the above-mentioned aluminum compounds, there may be used aluminum salts such as aluminum acetate, aluminum sulfate, aluminum chloride or aluminum nitrate, alkali aluminates such as sodium aluminate, alumina sol, aluminum hydroxide or the like.

The amount of the aluminum compound added is 0.05 to 50% by weight (calculated as Al) based on the total weight of the particles.

The obtained acicular goethite particles usually have an average major axial diameter of ordinarily 0.005 to 0.4 μm, an average minor axial diameter of ordinarily 0.0025 to 0.20 μm, a geometrical standard deviation of the major axis diameter of not more than 1.70, a geometrical standard deviation of the minor axis diameter of 1.37 to 1.50 and a BET specific surface area of about ordinarily 50 to 250 m 2 /g, contain aluminum in an amount of usually 0.05 to 50% by weight (calculated as Al) based on the total weight of the particles, and contain soluble sodium salts of usually 300 to 1500 ppm (calculated as Na) and soluble sulfates of usually 100 to 3000 ppm (calculated as SO 4 ).

Next, there is described a process for producing acicular hematite particles to be treated, which may substantially uniformly containing aluminum within the particle.

The acicular hematite particles which may substantially uniformly contain aluminum within the particle may be produced by heat-dehydrating the above-mentioned acicular goethite particles which may substantially uniformly contain aluminum within the particle.

The temperature of the heat-dehydration is preferably 550 to 850° C. to obtain high-density acicular hematite particles which may substantially uniformly contain aluminum within the particle.

Especially, in the case where the heat-dehydration is conducted at an elevated temperature as high as not less than 550° C., it is preferred that the surface of the acicular goethite particle which may substantially uniformly contain aluminum within the particle, is coated with an anti-sintering agent prior to the heat-dehydration, as is well known in the art.

As the sintering preventive, sintering preventives generally used are usable. For example, phosphorus compounds such as sodium hexametaphosphate, polyphospholic acid and orthophosphoric acid, silicon compounds such as #3 water glass, sodium orthosilicate, sodium metasilicate and colloidal silica, boron compounds such as boric acid, aluminum compounds including aluminum salts such as aluminum acetate, aluminum sulfate, aluminum chloride and aluminum nitrate, alkali aluminate such as sodium aluminate, alumina sol and aluminum hydroxide, and titanium compounds such as titanyl sulfate may be exemplified.

The amount of the sintering preventive applied onto the surface of the acicular goethite particle is about 0.05 to 10% by weight based on the total weight of the particles, though the amount is varied depending upon kinds of sintering preventives used, pH value of the alkali aqueous solution or various conditions such as heat-treating temperature or the like.

The acicular goethite particles coated with a sintering preventive have the BET specific surface area (S BET ) of usually about 50 to 250 m 2 /g. The coating treatment using a sintering preventive is composed of the steps of: adding a sintering preventive to an aqueous suspension containing the acicular goethite particles, mixing and stirring the resultant suspension, filtering out the particles, washing the particles with water, and drying the particles.

Meanwhile, as the acicular hematite particles to be treated, there may be preferably used high-density acicular hematite particles which may substantially uniformly contain aluminum within the particle. In the case of low-density acicular hematite particles, many dehydration pores are present within particles or on surfaces thereof. Therefore, upon subjecting the particles to the acid-dissolving treatment, the dissolution is initiated from the dehydration pores, so that the particle shape can be no longer maintained, resulting in deteriorated dispersibility of the obtained particles.

In order to obtain high-density acicular hematite particles which can maintain a particle shape of the acicular goethite particles, it is preferred that the acicular goethite particles which may substantially uniformly contain aluminum within the particle, are first heat-treated at a temperature as low as not less than 250° C. and less than 550° C. to form low-density acicular hematite particles which may contain aluminum which is present within the particle, and then the low-density hematite particles are heat-treated at an elevated temperature as high as 550 to 850° C.

If the temperature for heat-treating the goethite particles is less than 250° C., the dehydration reaction takes a long time. On the other hand, if the temperature is not less than 550° C., the dehydration reaction is abruptly brought out, so that it is difficult to retain the shapes because the sintering between particles is caused. The low-density acicular hematite particles obtained by heat-treating the acicular goethite particles at a low temperature are low-density particles having a large number of dehydration pores through which H 2 O is removed from the acicular goethite particles and the BET specific surface area thereof is about 1.2 to 2 times larger than that of the acicular goethite particles as the precursor.

The low-density acicular hematite particles obtained by heat-treating the acicular goethite particles coated with a sintering preventive at a temperature of not less than 250° C. and less than 550° C., have an average major axis diameter of usually 0.005 to 0.30 μm, an average minor axis diameter of usually 0.0025 to 0.15 μm, a geometrical standard deviation of a major axis diameter of usually not more than 1.70, a geometrical standard deviation of a minor axis diameter of usually 1.37 to 1.50 and a BET specific surface area (S BET ) of usually about 70 to 350 m 2 /g, and contain ordinarily soluble sodium salts of usually 300 to 1500 ppm (calculated as Na) and ordinarily soluble sulfates of usually 100 to 3000 ppm (calculated as SO 4 ).

The low-density acicular hematite particles containing aluminum which is present within the particle, obtained by heat-treating the acicular goethite particles coated with a sintering preventive at a temperature of not less than 250° C. and less than 550° C., have an average major axis diameter of usually 0.005 to 0.30 μm, an average minor axis diameter of usually 0.0025 to 0.15 μm, a geometrical standard deviation of a major axis diameter of usually not more than 1.70, a geometrical standard deviation of a minor axis diameter of usually 1.37 to 1.50 and a BET specific surface area (S BET ) of usually about 70 to 350 m 2 /g, and containing aluminum in an amount of 0.05 to 50% by weight (calculated as Al) based on the total weight of the particles, and contain soluble sodium salts of usually 500 to 3000 ppm (calculated as Na) and soluble sulfates of usually 300 to 4000 ppm (calculated as SO 4 ).

The low-density acicular hematite particles which may substantially uniformly contain aluminum within the particle, are then heat-treated at a temperature of usually 550 to 850° C., preferably 550 to 800° C. to obtain a high-density acicular hematite particles which may substantially uniformly contain aluminum within the particle. If the heat-treating temperature is less than 550° C., since the densification may be insufficient, a large number of dehydration pores may exist within and on the surface of the acicular hematite particles, so that the dispersion in the vehicle may become insufficient. Further, when the non-magnetic undercoat layer is formed from these particles, it may be difficult to obtain a coating film having a smooth surface. On the other hand, if the temperature exceeds 850° C., although the densification of the acicular hematite particles may be sufficient, since sintering is caused on and between particles, the particle size may increase, so that it may be difficult to obtain a coating film having a smooth surface.

The BET specific surface area of the high-density acicular hematite particles having the average major axis diameter of usually 0.005 to 0.30 μm, the average minor axis diameter of usually 0.0025 to 0.15 μm, the geometrical standard deviation of a major axis diameter of usually not more than 1.70 and the geometrical standard deviation of a minor axis diameter of usually 1.37 to 1.50, is usually about 35 to 180 m 2 /g. The high-density acicular hematite particles contain soluble sodium salts of usually 500 to 4000 ppm (calculated as Na) and soluble sulfates of usually 300 to 5000 ppm (calculated as SO 4 ).

The BET specific surface area of the high-density acicular hematite particles having the average major axis diameter of usually 0.005 to 0.30 μm, the average minor axis diameter of usually 0.0025 to 0.15 μm, the geometrical standard deviation of a major axis diameter of usually not more than 1.70 and the geometrical standard deviation of a minor axis diameter of usually 1.37 to 1.50, and containing aluminum within the particle, is usually about 35 to 180 m 2 /g. The high-density acicular hematite particles contain soluble sodium salts of usually 500 to 4000 ppm (calculated as Na) and soluble sulfates of usually 300 to 5000 ppm (calculated as SO 4 ), and contain aluminum in amount of 0.05 to 50% by weight (calculated as Al) based on the total weight of the particles.

The high-density acicular hematite particles which may substantially uniformly contain aluminum within the particle, are diaggregated by a dry-process, and formed into a slurry. The coarse particles thereof contained in the slurry are then deagglomerated by a wet-process. In the wet-deagglomeration, ball mill, sand grinder, colloid mill or the like is used until the coarse particles having a particle size of at least 44 μm are substantially removed. That is, the wet-pulverization is carried out until the amount of the coarse particles having a particle size of not less than 44 μm becomes to usually not more than 10% by weight, preferably not more than 5% by weight, more preferably 0% by weight based on the total weight of the particles. If the amount of the coarse particles having a particle size of not less than 44 μm is more than 10% by weight, the effect of treating the particles in an acid-dissolving treatment at the next step is not attained.

Next, there is described a process for producing acicular hematite particles having a specific geometrical standard deviation of the minor axis diameter.

The acicular hematite particles may be produced by subjecting a water suspension of acicular hematite particles as particles to be treated to an acid-dissolving treatment at an acid concentration of not less than 1.0 N, a pH value of not more than 3.0 and a temperature of 20 to 100° C. so as to dissolve 5 to 50% by weight of the acicular hematite particles based on the total weight of acicular hematite particles present in the water suspension, followed by washing with water; adding an aqueous alkali solution to the water suspension containing residual acicular hematite particles so as to adjust the pH value of the water suspension to not less than 13; and then heat-treating the water suspension at a temperature of 80 to 103° C., followed by filtering, washing with water and drying.

First, the acicular hematite particles to be treated with acid, are described below.

The particle shape of the hematite particles to be treated, is an acicular shape. The “acicular” shape may include not only a needle shape but also a spindle shape or a rice ball shape. In addition, the aspect ratio of the particles is usually 2:1 to 20:1.

The average major axis diameter of the acicular hematite particles to be treated, is usually 0.005 to 0.3 μm, preferably 0.02 to 0.28 μm.

The average minor axis diameter of the acicular hematite particles to be treated, is usually 0.0025 to 0.15 μm, preferably 0.01 to 0.14 μm.

The geometrical standard deviation of the major axis diameter of the acicular hematite particles to be treated, is usually not more than 1.70, and the geometrical standard deviation of the minor axis diameter of the acicular hematite particles to be treated, is usually 1.37 to 1.50.

The BET specific surface area (S BET ) of the acicular hematite particles to be treated, is usually 35 to 180 m 2 /g.

The acicular hematite particles to be treated contain soluble sodium salts of usually 500 to 4000 ppm (calculated as Na) and soluble sulfates of usually 300 to 5000 ppm (calculated as SO 4 ).

The acicular hematite particles to be treated, may contain aluminum in an amount of usually 0.05 to 50% by weight (calculated as Al) which is present within the particle.

Next, the acid-dissolving treatment of the acicular hematite particles to be treated is described in detail.

The concentration of the acicular hematite particles in the water suspension is usually 1 to 500 g/liter, preferably 10 to 250 g/liter. When the concentration of the acicular hematite particles in the water suspension is less than 1 g/liter, the amount of the particles treated is too small, which is industrially disadvantageous. On the other hand, when the concentration of the acicular hematite particles in the water suspension is more than 500 g/liter, it becomes difficult to evenly subject the particles to the acid-dissolving treatment.

As the acids, there may be used any of sulfuric acid, hydrochloric acid, nitric acid, sulfurous acid, chloric acid, perchloric acid and oxalic acid. With the consideration of corrosion or deterioration of a container in which a high-temperature treatment or a dissolving treatment is conducted, economy or the like, sulfuric acid is preferred.

The concentration of the acid is usually not less than 1.0 N, preferably not less than 1.2 N, more preferably not less than 1.5 N. The upper limit of concentration of the acid is preferably 5 N. When the acid concentration is less than 1.0 N, the dissolution of the acicular hematite particles requires an extremely long period of time and is, therefore, industrially disadvantageous.

Upon the acid-dissolving treatment, the initial pH value is usually not more than 3.0, preferably not more than 2.0, more preferably not more than 1.0. With the consideration of the dissolving time or the like, it is industrially preferred that the pH value is not more than 1.0. When the pH value is more than 3.0, the dissolution of the acicular hematite particles requires an extremely long period of time and is, therefore, industrially disadvantageous.

The temperature of the water suspension is usually 20 to 100° C., preferably 50 to 100° C., more preferably 70 to 100° C. When the temperature of the water suspension is less than 20° C., the dissolution of the acicular hematite particles requires an extremely long period of time and is, therefore, industrially disadvantageous. On the other hand, when the temperature of the water suspension is more than 100° C., the dissolution of the particles proceeds too rapidly, so that it becomes difficult to control the dissolving treatment. Further, in that case, the dissolving treatment requires a special apparatus such as autoclave, resulting in industrially disadvantageous process.

Incidentally, in the case where the acicular hematite particles to be treated have a relatively large average major axis diameter, i.e., in the range of 0.05 to 0.30 μm, it is preferred that the dissolving treatment is conducted under the hard conditions, e.g., at a pH value of not more than 1.0 and a temperature of 70 to 100° C. On the other hand, in the case where the acicular hematite particles to be treated have a relatively small average major axis diameter, i.e., in the range of 0.005 to 0.05 μm, it is preferred that the dissolving treatment is conducted under the soft conditions, e.g., at a pH value of 1.0 to 3.0 and a temperature of 20 to 70° C.

The dissolving treatment with acid may be conducted until the amount of the acicular hematite particles dissolved reaches 5 to 50% by weight, preferably 10 to 45% by weight, more preferably 15 to 40% by weight based on the total weight of the acicular hematite particles to be treated. When the amount of the acicular hematite particles dissolved is less than 5% by weight, the fine particle components may not be sufficiently removed by the dissolving treatment. On the other hand, when the amount of the acicular hematite particles dissolved is more than 50% by weight, the particles as a whole may be finely divided and the dissolving loss is increased, resulting in industrially disadvantageous process.

Incidentally, the aqueous solution in which iron salts are dissolved in the above dissolving treatment, is separated from the slurry by filtration. From the standpoint of the reuse of resources, the thus-separated iron salts may be used as a raw material of the ferrous salt used for the production of acicular goethite particles.

After completion of the above acid-dissolving treatment with acid, the acicular hematite particles remaining in the water suspension is filtered, washed with water and dried.

As the water-washing methods, there may be used those ordinarily used in industrial fields, such as a washing method by decantation, a washing method conducted according to a diluting process by using a filter thickener, a washing method of feeding water through a filter press, or the like.

(II) The acicular hematite particles according to the present invention are produced by heat-treating the specific acicular goethite particles which are produced by the afore-mentioned method, at a temperature of 100 to 200° C., whereby superfine goethite particles are adhered to the surface of the acicular goethite particles, and heat-treating at a temperature of 550 to 850° C.

As the acicular goethite particles as precursor, acicular goethite particles having an average major axis diameter of usually 0.01 to 0.25 μm and a geometrical standard deviation of a minor axis diameter of usually 1.37 to 1.50, are used. Especially, acicular goethite particles having an average major axis diameter of usually 0.01 to 0.25 μm, an average minor axis diameter of usually 0.05 to 0.17 μm, a geometrical standard deviation of a major axis diameter of usually not more than 1.70, a geometrical standard deviation of a minor axis diameter of usually 1.37 to 1.50, and a BET specific surface area (S BET ) of usually 50 to 250 m 2 /g, containing soluble sodium salts of usually 500 to 1500 ppm (calculated as Na) and soluble sulfates of usually 150 to 3000 ppm (calculated as SO 4 ), are preferable.

Also, the acicular goethite particles which substantially uniformly contain aluminum within the particle as starting particles, have an average major axis diameter of usually 0.01 to 0.25 μm and a geometrical standard deviation of a minor axis diameter of usually 1.37 to 1.50, are used. Especially, acicular goethite particles having an average major axis diameter of usually 0.01 to 0.25 μm, an average minor axis diameter of usually 0.05 to 0.17 μm, a geometrical standard deviation of a major axis diameter of usually not more than 1.70, a geometrical standard deviation of a minor axis diameter of usually 1.37 to 1.50, and a BET specific surface area (S BET ) of usually 50 to 250 m 2 /g, and containing aluminum in amount of 0.05 to 50% by weight (calculated as Al) based on the total weight of the particles and containing soluble sodium salts of usually 500 to 1500 ppm (calculated as Na) and soluble sulfates of usually 150 to 3000 ppm (calculated as SO 4 ), are preferable.

When the heating temperature is less than 100° C., it may be difficult to absorb a sufficient amount of the superfine goethite particles into the acicular goethite particles, thereby failing to obtain particles having a uniform particle size. On the other hand, when the heating temperature is more than 200° C., the acicular goethite particles is heat-dehydrated under such a condition that the superfine goethite particles still remain therein. As a result, the sintering between the particles is disadvantageously caused, thereby also failing to obtain particles having a uniform particle size. With the consideration of industrial productivity or the like, the heat-treating temperature is preferably 120 to 200° C.

The heat-treating time is preferably 5 to 60 minutes.

The acicular goethite particles obtained by heat-treating at a temperature of 100 to 200° C., have an average major axis diameter of usually 0.011 to 0.26 μm, and a geometrical standard deviation of minor axis diameter of usually not more than 1.30.

Especially, it is preferred that the acicular goethite particles obtained by heat-treating at a temperature of 100 to 200° C., have an average major axis diameter of usually 0.011 to 0.26 μm, an average minor axis diameter of usually 0.0055 to 0.13 μm, a geometrical standard deviation of major axis diameter of usually not more than 1.50, a geometrical standard deviation of minor axis diameter of usually not more than 1.30, and a BET specific surface area (S BET ) of usually 50 to 250 m 2 /g, and contain soluble sodium salts of usually 300 to 1500 ppm (calculated as Na) and soluble sulfates of usually 150 to 3000 ppm (calculated as SO 4 ).

Also, it is preferred that the acicular goethite particles which substantially uniformly contain aluminum within the particle as starting particles and are obtained by heat-treating at a temperature of 100 to 200° C., have an average major axis diameter of usually 0.011 to 0.26 and a geometrical standard deviation of a minor axis diameter of usually not more than 1.30, and containing aluminum in amount of 0.05 to 50% by weight (calculated as Al) based on the total weight of the particles.

Especially, the acicular goethite particles which substantially uniformly contain aluminum within the particle as starting particles and are obtained by heat-treating at a temperature of 100 to 200° C., have an average major axis diameter of usually 0.011 to 0.26, an average minor axis diameter of usually 0.0055 to 0.13 μm, a geometrical standard deviation of a major axis diameter of usually not more than 1.50, a geometrical standard deviation of a minor axis diameter of usually not more than 1.30, and a BET specific surface area (S BET ) of usually 50 to 250 m 2 /g, and containing aluminum in amount of 0.05 to 50% by weight (calculated as Al) based on the total weight of the particles, and contain soluble sodium salts of usually 300 to 1500 ppm (calculated as Na) and soluble sulfates of usually 150 to 3000 ppm (calculated as SO 4 ).

The obtained acicular goethite particles are heat-treated at 550 to 850° C., thereby producing acicular hematite particles.

Incidentally, as the acicular hematite particles according to the present invention, high-density acicular hematite particles produced by heat-treating acicular goethite particles at 100 to 200° C., heat-dehydrating the acicular goethite particles at 250 to 500° C., thereby obtaining low-density acicular hematite particles, and then heat-treating the obtained low-density acicular hematite particles at a temperature as high as 550 to 850° C., are preferable.

Next, the high-purification treatment of the acicular hematite particles is explained.

The high-density acicular hematite particles having a specific geometrical standard deviation of a minor axis diameter, which may substantially uniformly contain aluminum within the particle, are diaggregated by a dry-process, and formed into a slurry. The coarse particles thereof contained in the slurry are then deagglomerated by a wet-process. In the wet-deagglomeration, ball mill, sand grinder, colloid mill or the like is used until the coarse particles having a particle size of at least 44 μm are substantially removed. That is, the wet-pulverization is carried out until the amount of the coarse particles having a particle size of not less than 44 μm becomes to usually not more than 10% by weight, preferably not more than 5% by weight, more preferably 0% by weight based on the total weight of the particles. If the amount of the coarse particles having a particle size of not less than 44 μm is more than 10% by weight, the effect of treating the particles in a high-purification treatment at the next step is not attained.

The acicular hematite particles for a non-magnetic undercoat layer of a magnetic recording medium according to the present invention can be produced by adding an aqueous alkali solution to a water suspension containing acicular hematite particles so as to adjust the pH value of the water suspension to not less than 13, and then heat-treating the water suspension at 80 to 103° C., followed by filtering, washing with water and drying.

Upon the addition of the aqueous alkali solution, it is preferred that a filter cake obtained by filtering and water-washing the water suspension containing the acicular hematite particles, is preliminarily dispersed in water, thereby forming a water suspension again, or that the water suspension containing the acicular hematite particles is preliminarily washed with water by a decantation method.

The concentration of the alkaline suspension containing the acicular hematite particles is preferably 50 to 250 g/liter.

As the above-mentioned aqueous alkali solution, there may be used aqueous solutions containing alkali hydroxides such as sodium hydroxide, potassium hydroxide, calcium hydroxide or the like.

If the pH value of the aqueous alkali solution containing the acicular hematite particles is less than 13, it is difficult to effectively remove the solid crosslinking caused by the sintering preventive which exists on the surfaces of the acicular hematite particles, so that it is difficult to wash out the soluble sodium salt, soluble sulfate, etc. existing within and on the surfaces of the particles. The upper limit of the pH value is about 14. In consideration of the effect of removing the solid crosslinking caused by the sintering preventive which exists on the surfaces of the acicular hematite particles, the effect of washing out the soluble sodium slat, soluble sulfate, etc., and the effect of removing the alkali which adheres to the surfaces of acicular hematite particles in the process of treatment with the aqueous alkali solution, the preferable pH value is in the range of 13.1 to 13.8.

The heating temperature in the aqueous alkali suspension is preferably 80 to 103° C., more preferably 90 to 100° C. If the temperature is lower than 80° C., it is difficult to effectively remove the solid crosslinking caused by the sintering preventive which exists on the surfaces of the acicular hematite particles. If the heating temperature exceeds 103° C., although it is possible to effectively remove the solid crosslinking, since an autoclave is necessary or the treated solution boils under a normal pressure, it is not advantageous from the point of view of industry.

The acicular hematite particles heat-treated in the aqueous alkali suspension are thereafter filtered out and washed with water by an ordinary method so as to remove the soluble sodium salt and soluble sulfate which are washed out of the interiors and the surfaces of the particles and to remove the alkali adhered to the surfaces of the acicular hematite particles in the process of treatment with the aqueous alkali solution, and then dried.

As the method of washing the acicular hematite particles with water, a method generally industrially used such as a decantation method, a dilution method using a filter thickener and a method of passing water into a filter press is adopted.

If the soluble sodium salt and soluble sulfate which are contained within the acicular hematite particles are washed out with water, even if soluble sodium salt and soluble sulfate adhere to the surfaces when the surfaces of the acicular hematite particles are coated with a coating material, the soluble sodium salt and soluble sulfate can be easily removed by water washing.

The acicular hematite particles for a non-magnetic undercoat layer of a magnetic recording medium according to the present invention, may be coated with at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon, if required.

In order to coat the acicular hematite particles, an aluminum compound and/or a silicon compound is added to an aqueous suspension containing the acicular hematite particles, and mixed under stirring. After mixing and stirring, the pH value of the mixed solution is adjusted by using an alkali or acid, if necessary. The acicular hematite particles thus coated with at least one selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon are then filtered out, washed with water, dried and pulverized. They may be further deaerated and compacted, if necessary.

As the aluminum compound for the coating, the same aluminum compounds as those described above as the sintering preventive are usable.

The amount of aluminum compound added is usually 0.01 to 50% by weight (calculated as Al) based on the total weight of the particles. If the amount is less than 0.01% by weight, the dispersibility-improving effect in the vehicle may be insufficient. On the other hand, if the amount exceeds 50% by weight, the coating dispersibility-improving effect becomes saturated, so that it is meaningless to add an aluminum compound more than necessary.

As the silicon compound, the same silicon compounds as those described above as the sintering preventive are usable.

The amount of silicon compound added is 0.01 to 50% by weight (calculated as SiO 2 ) based on the total weight of the particles. If the amount is less than 0.01% by weight, the improvement of the dispersibility in the vehicle may be insufficient. On the other hand, if the amount exceeds 50.00% by weight, the coating dispersibility-improving effect becomes saturated, so that it is meaningless to add an silicon compound more than necessary.

When both an aluminum compound and a silicon compound are used, the amount thereof used is preferably 0.01 to 50% by weight (calculated as Al and SiO 2 ) based on the total weight of the particles.

Next, the magnetic recording medium according to the present invention is described.

The magnetic recording medium according to the present invention comprises:

a non-magnetic base film;

a non-magnetic undercoat layer formed on the non-magnetic base film, comprising a binder resin and the acicular hematite particles; and

a magnetic coating film formed on the non-magnetic undercoat layer, comprising a binder resin and magnetic particles.

Firstly, the non-magnetic substrate having the non-magnetic undercoat layer according to the present invention is described.

The non-magnetic substrate according to the present invention comprises:

a non-magnetic base film; and

a non-magnetic undercoat layer formed on said non-magnetic base film, comprising a binder resin and the acicular hematite particles.

The non-magnetic substrate of the present invention is produced by forming a coating film on the base film and drying the coating film. The coating film is formed by applying a non-magnetic coating composition which contains the acicular hematite particles, a binder resin and a solvent, to the surface of the base film.

As the base film, the following materials which are at present generally used for the production of a magnetic recording medium are usable as a raw material: a synthetic resin such as polyethylene terephthalate, polyethylene, polypropylene, polycarbonate, polyethylene naphthalate, polyamide, polyamideimide and polyimide; foil and plate of a metal such as aluminum and stainless steel; and various kinds of paper. The thickness of the base film varies depending upon the material, but it is usually about 1.0 to 300 μm, preferably 2.0 to 200 μm.

In the case of a magnetic disc, polyethylene terephthalate is usually used as the base film, and the thickness thereof is usually 50 to 300 μm, preferably 60 to 200 μm.

In a magnetic tape, when polyethylene terephthalate is used as the base film, the thickness thereof is usually 3 to 100 μm, preferably 4 to 20 μm; when polyethylene naphthalate is used, the thickness thereof is usually 3 to 50 μm, preferably 4 to 20 μm; and when polyamide is used, the thickness thereof is usually 2 to 10 μm, preferably 3 to 7 μm.

As the binder resin used in the present invention, the following resins which are at present generally used for the production of a magnetic recording medium are usable: vinyl chloride-vinyl acetate copolymer, urethane resin, vinyl chloride-vinyl acetate-maleic acid copolymer, urethane elastomer, butadiene-acrylonitrile copolymer, polyvinyl butyral, cellulose derivative such as nitrocellulose, polyester resin, synthetic rubber resin such as polybutadiene, epoxy resin, polyamide resin, polyisocyanate, electron radiation curing acryl urethane resin and mixtures thereof. Each of these resin binders may contain a functional group such as —OH, —COOH, —SO 3 M, —OPO 2 M 2 and —NH 2 , wherein M represents H, Na or K. With the consideration of the dispersibility of the particles, a binder resin containing a functional group —COOH or —SO 3 M is preferable.

The mixing ratio of the acicular hematite particles with the binder resin is usually 5 to 2000 parts by weight, preferably 100 to 1000 parts by weight based on 100 parts by weight of the binder resin.

As the solvents, there may be used methyl ethyl ketone, toluene, cyclohexanone, methyl isobutyl ketone, tetrahydrofuran, a mixture of these solvents or the like.

The total amount of the solvent used is 50 to 1,000 parts by weight based on 100 parts by weight of the acicular hematite particles. When the amount of the solvent used is less than 50 parts by weight, the viscosity of the non-magnetic coating composition prepared therefrom becomes too high, thereby making it difficult to apply the non-magnetic coating composition. On the other hand, when the amount of the solvent used is more than 1,000 parts by weight, the amount of the solvent volatilized during the formation of the coating film becomes too large, thereby rendering the coating process industrially disadvantageous.

It is possible to add a lubricant, a polishing agent, an antistatic agent, etc. which are generally used for the production of a magnetic recording medium to the non-magnetic undercoat layer.

The thickness of the non-magnetic undercoat layer obtained by applying a non-magnetic coating composition on the surface of the base film and drying, is usually 0.2 to 10.0 μm, preferably 0.5 to 5.0 μm. If the thickness is less than 0.2 μm, not only it is impossible to ameliorate the surface roughness of the non-magnetic substrate but also the strength is insufficient. If the thickness is more than 10 μm, it is difficult to reduce the thickness of the magnetic recording medium.

In case of using the acicular hematite particles as defined in first aspect as non-magnetic particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 189 to 300%, preferably 194 to 300%, more preferably 198 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 9.6 nm, preferably 0.5 to 9.1 nm, more preferably 0.5 to 8.6 nm; a Young's modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 120 to 160, preferably 121 to 160.

In case of using the acicular hematite particles as defined in second aspect as non-magnetic particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 191 to 300%, preferably 196 to 300%, more preferably 201 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 9.4 nm, preferably 0.5 to 8.9 nm, more preferably 0.5 to 8.4 nm; a Young's modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 122 to 160, preferably 124 to 160.

In case of using the acicular hematite particles as defined in third aspect as non-magnetic particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 191 to 300%, preferably 196 to 300%, more preferably 201 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 9.3 nm, preferably 0.5 to 8.8 nm, more preferably 0.5 to 8.3 nm; a Young's modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 122 to 160, preferably 124 to 160.

In case of using the acicular hematite particles as defined in fourth aspect as non-magnetic particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 193 to 300%, preferably 198 to 300%, more preferably 203 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 9.1 nm, preferably 0.5 to 8.6 nm, more preferably 0.5 to 8.1 nm; a Young's modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160, preferably 126 to 160.

In case of using the acicular hematite particles as defined in ninth aspect as non-magnetic particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 193 to 300%, preferably 198 to 300%, more preferably 203 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 9.0 nm, preferably 0.5 to 8.5 nm, more preferably 0.5 to 8.0 nm; a Young's modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160, preferably 126 to 160.

In case of using the acicular hematite particles as defined in tenth aspect as non-magnetic particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 195 to 300%, preferably 200 to 300%, more preferably 205 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 8.8 nm, preferably 0.5 to 8.3 nm, more preferably 0.5 to 7.8 nm; a Young's modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 126 to 160, preferably 128 to 160.

In case of using the acicular hematite particles as defined in eleventh aspect as non-magnetic particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 196 to 300%, preferably 201 to 300%, more preferably 206 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 8.7 nm, preferably 0.5 to 8.2 nm, more preferably 0.5 to 7.7 nm; a Young's modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 126 to 160, preferably 128 to 160.

In case of using the acicular hematite particles as defined in twelfth aspect as non-magnetic particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 198 to 300%, preferably 203 to 300%, more preferably 208 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 8.5 nm, preferably 0.5 to 8.0 nm, more preferably 0.5 to 7.5 nm; a Young's modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 128 to 160, preferably 130 to 160.

The magnetic recording medium according to the present invention can be produced by applying a magnetic coating composition containing the magnetic particles, a binder resin and a solvent, on the non-magnetic undercoat layer, followed by drying, to form a magnetic recording layer thereon.

As the magnetic particles used in the present invention, magnetic particles containing iron as a main component are usable.

The magnetic acicular metal particles containing iron as a main ingredient used in the present invention comprises iron and at least one selected from the group consisting of Co, Al, Ni, P, Si, Zn, Ti, B, Nd, La and Y. Further, the following magnetic acicular metal particles containing iron as a main component may be exemplified.

1) Magnetic acicular metal particles containing iron as a main component comprises iron; and cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles containing iron as a main component.

2) Magnetic acicular metal particles containing iron as a main component comprises iron; and aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component.

3) Magnetic acicular metal particles containing iron as a main component comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles containing iron as a main component; and aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component.

4) Magnetic acicular metal particles containing iron as a main component comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component.

5) Magnetic acicular metal particles containing iron as a main component comprises iron; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component.

6) Magnetic acicular metal particles containing iron as a main component comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles containing iron as a main component; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component.

7) Magnetic acicular metal particles containing iron as a main component comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component.

8) Magnetic acicular metal particles containing iron as a main component comprises iron; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component.

9) Magnetic acicular metal particles containing iron as a main component comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles containing iron as a main component; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component.

10) Magnetic acicular metal particles containing iron as a main component comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles containing iron as a main component; at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component.

11) Magnetic acicular metal particles containing iron as a main component comprises iron; aluminum of usually y 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component; at least one selected from the group consisting of Nd, La and Y of ordinarily 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component.

12) Magnetic acicular metal particles containing iron as a main component comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles containing iron as a main component; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component; at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component.

The iron content in the particles is the balance, and is preferably 50 to 99% by weight, more preferably 60 to 95% by weight (calculated as Fe) based on the weight of the magnetic acicular metal particles containing iron as a main component.

The magnetic acicular metal particles containing iron as a main component comprising (i) iron and Al; (ii) iron, cobalt and aluminum, (iii) iron, aluminum and at least one rare-earth metal such as Nd, La and Y, or (iv) iron, cobalt, aluminum and at least one rare-earth metal such as Nd, La and Y is preferable from the point of the durability of the magnetic recording medium.

Further, the magnetic acicular metal particles containing iron as a main component comprising iron, containing aluminum of 0.05 to 10% by weight (calculated as Al) which are present within the particle, and optionally containing at least one rare-earth metal such as Nd, La and Y, are more preferable.

With respect to the existing position of aluminum of usually 0.05 to 10% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component, it may be contained only in the core and inside portions, or in the surface portion of the magnetic acicular metal particles containing iron as a main component. Alternatively, aluminum may be approximately uniformly contained in the magnetic acicular metal particles containing iron as a main component from the core portion to the surface. An aluminum-coating layer may be formed on the surfaces of the particles. In addition, any of these positions may be combined. In the consideration of the effect of improving the surface property of the magnetic recording layer or the durability of the magnetic recording medium, magnetic acicular metal particles containing iron as a main component uniformly containing aluminum from the core portion to the surface and coated with an aluminum-coating layer are more preferable.

When the content of aluminum is less than 0.05% by weight (calculated as Al), the adsorption of the resin to the magnetic acicular metal particles containing iron as a main component in the vehicle may not be said sufficient, so that it may be difficult to produce a magnetic recording layer or a magnetic recording medium having a high durability. When the content of aluminum exceeds 10% by weight, the effect of improving the durability of the magnetic recording layer or the magnetic recording medium is observed, but the effect is saturated and it is meaningless to add aluminum more than necessary. Furthermore, the magnetic characteristics of the magnetic acicular metal particles containing iron as a main component may be sometimes deteriorated due to an increase in the aluminum as a non-magnetic component. The existing amount of aluminum of the magnetic acicular metal particles containing iron as a main component is preferably 0.1 to 7% by weight.

It is still more preferable to produce a magnetic recording medium of the present invention using the magnetic acicular metal particles containing iron as a main component containing Al and a rare-earth metal such as Nd, La and Y therein, because the magnetic recording layer or magnetic recording medium produced is apt to have a more excellent durability. Especially, magnetic acicular metal particles containing iron as a main component containing Al and Nd therein are the even more preferable.

The magnetic acicular metal particles containing iron as a main component comprising iron and aluminum within the particles are produced, as is well known, by adding an aluminum compound at an appropriate stage during the above-described process for producing acicular goethite particles to produce acicular goethite particles containing aluminum at desired positions of the particles, and heat-treating, at a temperature of 300 to 500° C., the acicular goethite particles or the acicular hematite particles containing aluminum at desired positions within the particles which are obtained by dehydrating the acicular goethite particles.

The magnetic acicular metal particles containing iron as a main component coated with aluminum are produced by heat-treating, at a temperature of 300 to 500° C., the acicular goethite particles coated with an oxide or hydroxide of aluminum, or the acicular hematite particles coated with the oxide or hydroxide of aluminum which are obtained by dehydrating the acicular goethite particles.

The magnetic particles containing iron as a main component used in the present invention have an average major axial diameter of usually 0.01 to 0.50 μm, preferably 0.03 to 0.30 μm, an average minor axial diameter of usually 0.0007 to 0.17 μm, preferably 0.003 to 0.10 μm, and an aspect ratio of usually not less than 3:1, preferably not less than 5:1. The upper limit of the aspect ratio is usually 15:1, preferably 10:1 with the consideration of the dispersibility in the vehicle. The shape of the a magnetic particles containing iron as a main component may have not only acicular but also spindle-shaped, rice ball-shaped, cubic-shaped, plate-like shaped or the like.

The geometrical standard deviation of the major axis diameter of the magnetic particles used in the present invention is preferably not more than 2.50. If it exceeds 2.50, the coarse particles existent sometimes exert a deleterious influence on the surface smoothness of the magnetic recording layer. From the point of view of industrial productivity, the lower limit of the geometrical standard deviation of the major axis diameter is preferably 1.01.

As to the magnetic properties of the magnetic acicular metal particles containing iron as a main component used in the present invention, the coercive force is usually 800 to 3500 Oe, preferably 900 to 3500 Oe, more preferably 1000 to 3500 Oe, and the saturation magnetization is usually preferably 90 to 170 emu/g, preferably 100 to 170 emu/g, more preferably 110 to 170 emu/g with the consideration of the properties such as high-density recording.

With the consideration of the durability of the magnetic recording medium, it is preferred to use magnetic acicular metal particles containing iron as a main component, which contain aluminum, as magnetic particles. The resin adsorptivity of such magnetic acicular metal particles containing iron as a main component used in the present invention is usually not less than 65%, preferably not less than 68%, more preferably not less than 70%.

As the binder resin for the magnetic recording layer, the same binder resin as that used for the production of the non-magnetic undercoat layer is usable.

The mixing ratio of the magnetic acicular metal particles containing iron as a main component with the binder resin in the magnetic recording layer is usually 200 to 2000 parts by weight, preferably 300 to 1500 parts by weight based on 100 parts by weight of the binder resin.

As the solvents, the same solvent as that used for the production of the non-magnetic undercoat layer is usable.

The total amount of the solvent used is 65 to 1,000 parts by weight based on 100 parts by weight of the magnetic particles. When the amount of the solvent used is less than 65 parts by weight, the viscosity of the magnetic coating composition prepared therefrom becomes too high, thereby making it difficult to apply the magnetic coating composition. On the other hand, when the amount of the solvent used is more than 1,000 parts by weight, the amount of the solvent volatilized during the formation of the coating film becomes too large, thereby rendering the coating process industrially disadvantageous.

It is possible to add a lubricant, a polishing agent, an antistatic agent, etc., which are generally used for the production of a magnetic recording medium to the magnetic recording layer.

The thickness of the magnetic recording layer obtained by applying the magnetic coating composition on the surface of the non-magnetic undercoat layer and dried, is usually in the range of 0.01 to 5.0 μm. If the thickness is less than 0.01 μm, uniform coating may be difficult, so that unfavorable phenomenon such as unevenness on the coating surface is observed. On the other hand, when the thickness exceeds 5.0 μm, it may be difficult to obtain desired signal recording property due to an influence of diamagnetism. The preferable thickness is in the range of 0.05 to 1.0 μm.

The magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95; a gloss (of the coating film) of usually 192 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.8 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 120 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm −1 .

In case of using the acicular hematite particles as defined in the first aspect as non-magnetic particles, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95; a gloss (of the coating film) of usually 192 to 300%, preferably 197 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.8 nm, preferably 0.5 to 9.3 nm,; more preferably 0.5 to 8.8 nm, a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 120 to 160, preferably 122 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm −1 , preferably 1.20 to 2.00 μm −1 .

In case of using the acicular hematite particles as defined in the second aspect as non-magnetic particles, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95; a gloss (of the coating film) of usually 194 to 300%, preferably 199 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.6 nm, preferably 0.5 to 9.1 nm, more preferably 0.5 to 8.6 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 122 to 160, preferably 124 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm − , preferably 1.20 to 2.00 μm −1 .

In case of using the acicular hematite particles as defined in the first aspect as non-magnetic particles and magnetic acicular metal particles containing iron as a main component and aluminum which are present in and/or on the surface of the particle, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 194 to 300%, preferably 199 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.6 nm, preferably 0.5 to 9.1 nm, more preferably 0.5 to 8.6 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 122 to 160, preferably 124 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm −1 , preferably 1.20 to 2.00 μm − . As to the durability, the running durability thereof is usually not less than 22 minutes, preferably not less than 24 minutes. Also, the scratch resistance thereof is usually A or B, preferably A.

In case of using the acicular hematite particles as defined in the second aspect as non-magnetic particles and magnetic acicular metal particles containing iron as a main component and aluminum which are present in and/or on the surface of the particle, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 196 to 300%, preferably 201 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.4 nm, preferably 0.5 to 8.9 nm, more preferably 0.5 to 8.4 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160, preferably 126 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm −1 , preferably 1.20 to 2.00 μm −1 . As to the durability, the running durability thereof is usually not less than 23 minutes, preferably not less than 25 minutes. Also, the scratch resistance thereof is usually A or B, preferably A.

In case of using the acicular hematite particles as defined in the third aspect as non-magnetic particles, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 194 to 300%, preferably 199 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.6 nm, preferably 0.5 to 9.1 nm, more preferably 0.5 to 8.6 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 122 to 160, preferably 124 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm −1 , preferably 1.20 to 2.00 μm −1 . As to the durability, the running durability thereof is usually not less than 22 minutes, preferably not less than 23 minutes. Also, the scratch resistance thereof is usually A or B, preferably A.

In case of using the acicular hematite particles as defined in the fourth aspect as non-magnetic particles, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 196 to 300%, preferably 201 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.4 nm, preferably 0.5 to 8.9 nm, more preferably 0.5 to 8.4 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160, preferably 126 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm −1 , preferably 1.20 to 2.00 μm −1 . As to the durability, the running durability thereof is usually not less than 23 minutes, preferably not less than 24 minutes. Also, the scratch resistance thereof is usually A or B, preferably A.

In case of using the acicular hematite particles as defined in the third aspect as non-magnetic particles and magnetic acicular metal particles containing iron as a main component and aluminum which are present in and/or on the surface of the particle, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 196 to 300%, preferably 201 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.4 nm, preferably 0.5 to 8.9 nm, more preferably 0.5 to 8.4 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160, preferably 126 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm −1 , preferably 1.20 to 2.00 μm −1 . As to the durability, the running durability thereof is usually not less than 23 minutes, preferably not less than 24 minutes. Also, the scratch resistance thereof is usually A or B, preferably A.

In case of using the acicular hematite particles as defined in the fourth aspect as non-magnetic particles and magnetic acicular metal particles containing iron as a main component and aluminum which are present in and/or on the surface of the particle, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 198 to 300%, preferably 203 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.2 nm, preferably 0.5 to 8.7 nm, more preferably 0.5 to 8.2 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 126 to 160, preferably 127 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm −1 , preferably 1.20 to 2.00 μm −1 . As to the durability, the running durability thereof is usually not less than 24 minutes, preferably not less than 26 minutes. Also, the scratch resistance thereof is usually A or B, preferably A.

In case of using the acicular hematite particles as defined in the ninth aspect as non-magnetic particles, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 196 to 300%, preferably 201 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.4 nm, preferably 0.5 to 8.9 nm, more preferably 0.5 to 8.4 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160, preferably 126 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm −1 , preferably 1.20 to 2.00 μm −1 .

In case of using the acicular hematite particles as defined in the tenth aspect as non-magnetic, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 198 to 300%, preferably 203 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.2 nm, preferably 0.5 to 8.7 nm, more preferably 0.5 to 8.2 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 126 to 160, preferably 128 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm −1 , preferably 1.20 to 2.00 μm −1 .

In case of using the acicular hematite particles as defined in the ninth aspect as non-magnetic particles and magnetic acicular metal particles containing iron as a main component and aluminum which are present in and/or on the surface of the particle, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 198 to 300%, preferably 203 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.2 nm, preferably 0.5 to 8.7 nm, more preferably 0.5 to 8.2 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 126 to 160, preferably 128 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm −1 , preferably 1.20 to 2.00 μm −1 . As to the durability, the running durability thereof is usually not less than 24 minutes, preferably not less than 26 minutes. Also, the scratch resistance thereof is usually A or B, preferably A.

In case of using the acicular hematite particles as defined in the tenth aspect as non-magnetic particles and magnetic acicular metal particles containing iron as a main component and aluminum which are present in and/or on the surface of the particle, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 200 to 300%, preferably 205 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.0 nm, preferably 0.5 to 8.5 nm, more preferably 0.5 to 8.0 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 128 to 160, preferably 130 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm −1 , preferably 1.20 to 2.00 μm −1 . As to the durability, the running durability thereof is usually not less than 25 minutes, preferably not less than 27 minutes. Also, the scratch resistance thereof is usually A or B, preferably A.

In case of using the acicular hematite particles as defined in the eleventh aspect as non-magnetic particles, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 198 to 300%, preferably 203 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.2 nm, preferably 0.5 to 8.7 nm, more preferably 0.5 to 8.2 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 126 to 160, preferably 128 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm −1 , preferably 1.20 to 2.00 μm −1 . As to the durability, the running durability thereof is usually not less than 24 minutes, preferably not less than 26 minutes. Also, the scratch resistance thereof is usually A or B, preferably A.

In case of using the acicular hematite particles as defined in the twelfth aspect as non-magnetic, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 200 to 300%, preferably 205 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.0 nm, preferably 0.5 to 8.5 nm, more preferably 0.5 to 8.0 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 128 to 160, preferably 130 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm −1 , preferably 1.20 to 2.00 μm −1 . As to the durability, the running durability thereof is usually not less than 25 minutes, preferably not less than 27 minutes. Also, the scratch resistance thereof is usually A or B, preferably A.

In case of using the acicular hematite particles as defined in the eleventh aspect as non-magnetic particles and magnetic acicular metal particles containing iron as a main component and aluminum which are present in and/or on the surface of the particle, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 200 to 300%, preferably 205 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.0 nm, preferably 0.5 to 8.5 nm, more preferably 0.5 to 8.0 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 128 to 160, preferably 130 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm −1 , preferably 1.20 to 2.00 μm −1 . As to the durability, the running durability thereof is usually not less than 25 minutes, preferably not less than 27 minutes. Also, the scratch resistance thereof is usually A or B, preferably A.

In case of using the acicular hematite particles as defined in the twelfth aspect as non-magnetic particles and magnetic acicular metal particles containing iron as a main component and aluminum which are present in and/or on the surface of the particle, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 202 to 300%, preferably 207 to 300%; a surface roughness Ra (of the coating film) of usually not more than 8.8 nm, preferably 0.5 to 8.3 nm, more preferably 0.5 to 7.8 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 130 to 160, preferably 132 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 μm −1 , preferably 1.20 to 2.00 μm −1 . As to the durability, the running durability thereof is usually not less than 26 minutes, preferably not less than 28 minutes. Also, the scratch resistance thereof is usually A or B, preferably A.

The magnetic recording medium according to the present invention shows an anti-corrosion property of not more than 10.0%, preferably not more than 9.5% when expressed by he percentage (%) of change in coercive force thereof, and also shows an anti-corrosion property of not more than 10.0%, preferably not more than 9.5% when expressed by the percentage (%) of change in saturation magnetic flux density thereof.

The important feature of the present invention lies in such a fact that when the acicular hematite particles are acid-dissolving treatment, there can be obtained acicular hematite particles which have such a uniform particle size as being expressed by the geometrical standard deviation of major axis diameter of not more than 1.50 and the geometrical standard deviation of minor axis diameter of preferably not more than 1.35, and are, in particular, excellent in particle size distribution of minor axis diameter thereof.

The reason why the acicular hematite particles according to the present invention can exhibit a uniform particle size, is considered, such that by acid-treating acicular hematite particles in the strongly acidic solution, superfine hematite particles can be removed therefrom, whereby it is possible to obtain acicular goethite particles having a uniform particle size with respect to both major axis diameter and minor axis diameter thereof, because the amount of the superfine hematite particles be reduced.

Further, another important feature of the present invention lies in such a fact that when the acicular goethite particles are heat-treated at a temperature of 100 to 200° C. before subjecting to heat-dehydration treatment, there can be obtained acicular hematite particles which have such a uniform particle size as being expressed by the geometrical standard deviation of major axis diameter of not more than 1.50 and the geometrical standard deviation of minor axis diameter of preferably not more than 1.30, and are, in particular, excellent in particle size distribution of minor axis diameter thereof.

The reason why the acicular hematite particles according to the present invention can exhibit a uniform particle size, is considered, such that by heat-treating acicular goethite particles at a temperature of 100 to 200° C., superfine goethite particles can be absorbed into the acicular goethite particles, whereby it is possible to obtain acicular goethite particles having a uniform particle size with respect to both major axis diameter and minor axis diameter thereof, and further since the amount of the superfine goethite particles be reduced, the sintering between the particles due to the presence of the superfine goethite particles is unlikely to occur upon the subsequent heat-dehydration treatment, whereby it is possible to obtain acicular hematite particles which still maintain the uniform particle size of the acicular goethite particles.

In the magnetic recording medium according to the present invention, when the acicular hematite particles of the present invention are used as non-magnetic particles for non-magnetic undercoat layer, there can be obtained such a magnetic recording medium having a small light transmittance, a more excellent smooth surface and a high strength.

The reason why the magnetic recording medium according to the present invention can exhibit a more excellent smooth surface, is considered by the present inventors, such that by the synergistic effect of such a uniform particle size as being expressed by the geometrical standard deviation of major axis diameter of not more than 1.50 and the geometrical standard deviation of minor axis diameter of not more than 1.35 which results in less amount of coarse or fine particles, and the BET specific surface area of 35.9 to 180 m 2 /g which results in less amount of dehydration pores in the particles or on the surfaces of the particles, the acicular hematite particles according to the present invention can exhibit a more excellent dispersibility in vehicle, so that the obtained non-magnetic undercoat layer can have a more excellent smooth surface.

The another reason why the surface smoothness of the magnetic recording medium is excellent is considered as follows. Since it is possible to sufficiently remove the soluble sodium and the soluble sulfate, which agglomerate acicular hematite particles by firmly crosslinking, the agglomerates are separated into substantially discrete particles by washing the particles with water, so that acicular hematite particles having an excellent dispersion in the vehicle are obtained.

It has been found that the deterioration in magnetic properties of the magnetic acicular metal particles containing iron as a main component and dispersed in the magnetic layer, is considerably influenced by the pH value of non-magnetic particles in the non-magnetic undercoat layer and by the content of soluble sodium salts and soluble sulfates contained in the non-magnetic particles.

That is, as described above, the acicular goethite particles used as a precursor have been produced by various methods. However, in any method, in the case where ferrous sulfate is used as a main raw material for the production of the acicular goethite particles, a large amount of sulfate [SO 4 2− ] is necessarily present in a reacted slurry.

In particular, when the goethite particles are produced in an acid solution, water-soluble sulfates such as Na 2 SO 4 are produced simultaneously, and the reaction mother liquor contains alkali metals such as K + , NH 4 + , Na + or the like, so that precipitates containing alkali metals or sulfates tend to be formed. The thus-formed precipitates are represented by the formula of RFe 3 (SO 4 ) (OH) 6 , where R is K + , NH 4 + or Na + . Such precipitates are composed of insoluble sulfur-containing iron salts and, therefore, cannot be removed merely by an ordinary water-washing method. The insoluble salts are converted into soluble sodium salts or soluble sulfates by the subsequent heat-treatment. However, the soluble sodium salts or the soluble sulfates cause the acicular hematite particles to be cross-linked with each other by the action of a sintering preventive which is added as an essential component for preventing the deformation of the acicular hematite particles and the sintering therebetween in the high-temperature treatment for high densification, and are strongly bonded to the inside or the surface of the acicular hematite particles. Thus, the agglomeration of the acicular hematite particles are further accelerated by the soluble sodium salts or the soluble sulfates. As a result, it becomes extremely difficult to remove especially such soluble sodium salts or soluble sulfates enclosed within the particles or the agglomerated particles, by an ordinary water-washing method.

In the case where the acicular goethite particles are produced in an aqueous alkali solution using ferrous sulfate and sodium hydroxide, a sulfate (Na 2 SO 4 ) is simultaneously produced, and NaOH is contained in the reacted slurry. Since Na 2 SO 4 and NaOH both are water-soluble, it is considered that these compounds can be essentially removed by sufficiently washing the acicular goethite particles with water. However, in general, the acicular goethite particles are deteriorated in water-washing efficiency due to a low crystallizability thereof, so that even when the acicular goethite particles are washed with water by an ordinary method, water-soluble components such as soluble sulfates [SO 4 2− ] or soluble sodium salts [Na + ] still remain in the acicular goethite particles. Further, as described above, these water-soluble components cause the acicular hematite particles to be cross-linked with each other by the action of the sintering preventive, and are strongly bonded to the inside or the surface of the acicular hematite particles, thereby further accelerating the agglomeration of the acicular hematite particles. As a result, it also becomes extremely difficult to remove, especially, such soluble sodium salts or soluble sulfates enclosed within the particles or the agglomerated particles by an ordinary water-washing method.

As described above, it is considered that when the high-density hematite particles containing the soluble sodium salts or the soluble sulfates which are strongly bonded to the inside or the surface of the particles and to the inside of the agglomerated particles through the sintering preventive, are wet-pulverized to deagglomerate coarse particles, then the pH value of the slurry containing the high-density hematite particles is adjusted to not less than 13 and the slurry is heat-treated at not less than 80° C., the aqueous alkali solution can be sufficiently immersed into the high-density hematite particles, so that the bonding force of the sintering preventive strongly bonded to the inside or the surface of the particles and to the inside of the agglomerated particles is gradually weakened, whereby the sintering preventive is released from the inside or the surface of the particles and the inside of the agglomerated particles and simultaneously the water-soluble sodium salts or the water-soluble sulfates tend to be readily removed by washing with water.

In addition, the reason why the deterioration in magnetic properties due to the corrosion of the magnetic acicular metal particles containing iron as a main component and dispersed in the magnetic recording layer, can be suppressed, is considered, such that the amount of water-soluble components such as the soluble sodium salts or the soluble sulfates which accelerate the corrosion of metals contained in the acicular hematite particles for non-magnetic undercoat layer, is reduced and the particles have a pH value as high as not less than 8, so that the corrosion of the acicular hematite particles containing iron as a main component can be prevented from proceeding.

In the magnetic recording medium according to the present invention, when the acicular hematite particles containing aluminum in specific amount within the particle of the present invention are used as non-magnetic particles for non-magnetic undercoat layer, there can be obtained such a magnetic recording medium having a small light transmittance, a more excellent smooth surface, a high strength and a high durability.

The reason why the strength and the durability of the magnetic recording medium is more enhanced is considered that by using the acicular hematite particles containing aluminum uniformly within the particle, the resin adsorptivity of the acicular hematite particles to the binder resin in the vehicles are enhanced due to the use of the above-described particles, as will be shown in later-described examples, the degree of adhesion of the acicular hematite particles in the non-magnetic undercoat layer or the non-magnetic undercoat layer itself to the base film is enhanced; and the increase in adsorptivity of the binder resin for the magnetic particles in the coating composition by using as magnetic particles, the magnetic acicular metal particles containing iron as a main component and further containing aluminum, which results in increase in adhesion between the magnetic particles in the magnetic recording layer and the non-magnetic undercoat layer or between the magnetic recording layer itself and the non-magnetic undercoat layer.

As described in Examples hereinafter, in the case where the acicular hematite particles according to the present invention is used as non-magnetic particles for the non-magnetic undercoat layer, since these particles contain less amount of fine particle components and, therefore, can show an excellent dispersibility in vehicle, there can be obtained such a non-magnetic undercoat layer having an excellent strength and an excellent smooth surface. Further, in the case where a magnetic recording medium is produced by using the non-magnetic undercoat layer, there can be obtained such a magnetic recording medium which can exhibit a low light transmittance, an excellent smooth surface and a high strength, and whose magnetic properties can be prevented from being deteriorated due to the corrosion of the magnetic acicular metal particles containing iron as a main component and dispersed in the magnetic recording layer, since the pH value of the acicular hematite particles used as non-magnetic particles is low and the amount of the soluble sodium salts or the soluble sulfates contained is reduced. Therefore, the acicular hematite particles according to the present invention are suitable as non-magnetic particles for a non-magnetic undercoat layer of a high-density magnetic recording medium.

As described above, the magnetic recording medium according to the present invention can exhibit a low light transmittance, a smooth surface and a high strength, and can be prevented from being deteriorated in magnetic properties due to the corrosion of the magnetic acicular particles containing iron as a main component and dispersed in the magnetic recording layer. Therefore, the magnetic recording medium according to the present invention is suitable as a high-density magnetic recording medium.

Further, in the magnetic recording medium according to the present invention, in the case where specific acicular hematite particles are used as non-magnetic particles for the non-magnetic undercoat layer, there can be obtained such a non-magnetic undercoat layer having a high strength and an excellent smooth surface, because the acicular hematite particles used as non-magnetic particles contain less amount of fine particle components, and have a pH value as high as not less than 8 and an excellent dispersibility in vehicle due to less content of the soluble salts. Consequently, when the magnetic recording medium is produced by using the above-mentioned non-magnetic undercoat layer and using as magnetic particles for the magnetic recording layer, the magnetic acicular metal particles containing iron as a main component and further containing a predetermined amount of Al, there can be obtained such a magnetic recording medium which can exhibit a low light transmittance, an excellent smooth surface, a high strength and an excellent durability, and can be prevented from being deteriorated in magnetic properties due to the corrosion of the magnetic acicular metal particles containing iron as a main component and dispersed in the magnetic recording layer. Therefore, the magnetic recording medium according to the present invention is suitable as a high-density magnetic recording medium.

EXAMPLES

The present invention is described in more detail by Examples and Comparative Examples, but the Examples are only illustrative and, therefore, not intended to limit the scope of the present invention.

Various properties were evaluated by the following methods.

(1) The residue on sieve after the wet-pulverization was obtained by measuring the concentration of the slurry after pulverization by a wet-process in advance, and determining the quantity of the solid content on the sieve remaining after the slurry equivalent to 100 g of the particles content was passed through the sieve of 325 meshes (mesh size: 44 μm).

(2) The average major axis diameter and the average minor axis diameter of the acicular particles are expressed by the average values of 350 particles measured in the photograph obtained by magnifying an electron micrograph (×30000) by 4 times in the vertical and horizontal directions, respectively.

(3) The aspect ratio is the ratio of the average major axis diameter and the average minor axis diameter.

(4) The geometrical standard deviation for particle size distribution of the major axis diameter and minor axis diameter was obtained by the following method.

The major axis diameters and minor axis diameters of the particles were measured from the magnified electron microphotograph in the above-mentioned (2). The actual major axis diameters and minor axis diameters of the particles and the number of particles were obtained from the calculation on the basis of the measured values. On logarithmico-normal probability paper, the major axis diameters or minor axis diameters were plotted at regular intervals on the abscissa-axis and the accumulative number of particles belonging to each interval of the major axis diameters or minor axis diameters was plotted by percentage on the ordinate-axis by a statistical technique. The major axis diameters or minor axis diameters corresponding to the number of particles of 50% and 84.13%, respectively, were read from the graph, and each geometrical standard deviation was measured from the following formulae:

Geometrical standard deviation of the major axis diameter or minor axis diameter={major axis diameter (μm) or

minor axis diameter (μm) corresponding to 84.13% under integration sieve}/{major axis diameter or minor axis diameter (geometrical average diameter) corresponding to 50% under integration sieve}.

The more the geometrical standard deviation nears 1.0, the more excellent the particle size distribution of the major axis diameters and minor axis diameters of the particles.

(5) The specific surface area is expressed by the value measured by a BET method.

(6) The degree of densification of the particles is represented by S BET /S TEM value as described above.

S BET is a specific surface area measured by the above-described BET method.

S TEM is a value calculated from the average major axis diameter d cm and the average minor axis diameter w cm measured from the electron microphotograph described in (2) on the assumption that a particle is a rectangular parallellopiped in accordance with the following formula:

S TEM ( m 2 /g )={(4 ·d·w+ 2 w 2 )/( d·w 2 ·ρ p )}×10 −4

wherein ρ p is the true specific gravity of the hematite particles, and 5.2 g/cm 3 was used.

(7) The content of each of Al, Si, P, and Nd in and/or on the particle was measured according to JIS K0119 using “fluorescent X-ray spectroscopy device 3063 M” (manufactured by Rigaku Denki Kogyo Co., Ltd.).

(8) The pH value of the particles was measured as follows.

5 g of a sample was weighed and placed in a 300-ml conical flask. 100 ml of boiling pure water was added into the flask and the contents thereof were heated and maintained in a boiling condition for about 5 minutes. Thereafter, the flask was plugged and allowed to stand for cooling up to an ordinary temperature. After boiling pure water was added in such an amount corresponding to the weight loss and the flask was plugged again, the contents of the flask were shaken and mixed for one minute and then allowed to stand for 5 minutes, thereby obtaining a supernatant. The pH value of the thus obtained supernatant was measured according to JIS Z 8802-7, and the pH value of the particles was expressed by the measured value.

(9) The content of soluble sodium salts and the content of soluble sulfates were determined by filtering the supernatant prepared for the above measurement of the pH value of the particles using a filter paper No. 5, and measuring amounts of Na + and SO 4 2− in the filtrate by an inductively coupled plasma atomic emission spectrometry device (manufactured by Seiko Denshi Kogyo Co., Ltd.).

(10) The resin adsorptivity of the particles represents the degree at which a resin is adsorbed to the particles. The closer to 100% the value obtained in the following manner, the firmer the resin adsorptivity to the particles surfaces in the vehicle and the more favorable.

The resin adsorptivity Wa was first obtained. 20 g of particles and 56 g of a mixed solvent (27.0 g of methyl ethyl ketone, 16.2 g of toluene, and 10.8 g of cyclohexanone) with 2 g of a vinyl chloride-vinyl acetate copolymer having a sodium sulfonate group dissolved therein were charged into a 100-ml polyethylene bottle together with 120 g of 3 mmφ steel beads. The particles and the solvent were mixed and dispersed by a paint shaker for 60 minutes.

Thereafter, 50 g of the coating composition was taken out, and charged into a 50-ml settling cylinder. The solid content was separated from the solvent portion by the centrifugalization at a rate of 10000 rpm for 15 minutes. The concentration of the solid resin content contained in the solvent portion was determined by a gravimetric method and the resin content existing in the solid portion was determined by deducting the obtained resin content from the amount of the resin charged as the resin adsorptivity Wa (mg/g) to the particles.

The total quantity of separated solid content was taken into a 100 ml-tall beaker, and 50 g of a mixed solvent (25.0 g of methyl ethyl ketone, 15.0 g of toluene, and 10.0 g of cyclohexanone) was added thereto. The obtained mixture was to ultrasonic dispersion for 15 minutes, and the thus-obtained suspension was charged into a 50-ml settling cylinder. The solid content was separated from the solvent portion by centrifuging them at a rate of 10000 rpm for 15 minutes. The concentration of the solid resin content contained in the solvent portion was measured so as to determine the resin content dissolved from the resin which had been adsorbed to the particle surfaces into the solvent phase.

The process from the step of taking the solid content into the 100 ml-tall beaker to the determination of the resin content dissolved into the solvent phase was repeated twice. The total quantity We (mg/g) of resin content dissolved into the solvent phase in the three cycles was obtained, and the value calculated in accordance with the following formula is expressed as the resin adsorptivity T(%):

T (%)=[( Wa−We )/ Wa]× 100.

(11) The viscosity of the coating composition was obtained by measuring the viscosity of the coating composition at 25° C. at a shear rate D of 1.92 sec −1 by using “E type viscometer EMD-R” (manufactured by Tokyo Keiki, Co., Ltd.).

(12) The gloss of the surface of the coating film of each of the non-magnetic undercoat layer and the magnetic coating layer was measured at an angle of incidence of 45° by “glossmeter UGV-5D” (manufactured by Suga Shikenki, Co., Ltd.).

(13) The surface roughness Ra is expressed by the average value of the center-line average roughness of the profile curve of the surface of the coating film by using “Surfcom-575A” (manufactured by Tokyo Seimitsu Co., Ltd.).

(14) The strength of the coating film was expressed the Young's modulus obtained by “Autograph” (produced by Shimazu Seisakusho Co., Ltd.). The Young's modulus was expressed by the ratio of the Young's modulus of the coating film to that of a commercially available video tape “AV T-120” (produce by Victor Company of Japan, Limited). The higher the relative value, the more favorable.

(15) The magnetic properties of the magnetic particles and magnetic recording medium were measured under an external magnetic field of 10 kOe by “Vibration Sample Magnetometer VSM-3S-15 (manufactured by Toei Kogyo, Co., Ltd.)”.

(16) The change with the passage of time in magnetic properties of the magnetic recording medium due to the corrosion of the magnetic acicular metal particles containing iron as a main component, which are dispersed in the magnetic coating film, was determined as follows.

The magnetic recording medium was allowed to stand at a temperature of 60° C. and a relative humidity of 90% for 14 days. The coercive force values and the saturation magnetization values of the magnetic recording medium before and after the keeping test were measured, and the difference between the measured values before and after the keeping test was divided by the value before the keeping test, thereby obtaining an amount of change in each magnetic property which was expressed by a percentage.

(17) The light transmittance is expressed by the linear adsorption coefficient calculated by substituting the light transmittance measured by using “UV-Vis Recording Spectrophotometer UV-2100” (manufactured by Shimazu Seisakusho, Ltd.) for the following formula. The larger the value, the more difficult it is for the magnetic recording medium to transmit light:

Linear adsorption coefficient (μm − )={1n (1/t)}/FT wherein t represents a light transmittance (−) at λ=900 nm, and FT represents thickness (μm) of the coating film used for the measurement.

(18) The durability of the magnetic medium was evaluated by the following running durability and the scratch resistance.

The running durability was evaluated by the actual operating time under the conditions that the load was 200 gw and the relative speed of the head and the tape was 16 m/s by using “Media Durability Tester MDT-3000” (manufactured by Steinberg Associates). The longer the actual operating time, the higher the running durability.

The scratch resistance was evaluated by observing through the microscope the surface of the magnetic tape after running and visually judging the degree of scratching. Evaluation was divided into the following four ranks.

A: No scratch

B: A few scratches

C: Many scratches

D: Great many scratches

(19) The thickness of each of the base film, the non-magnetic undercoat layer and the magnetic coating film constituting the magnetic recording medium was measured in the following manner by using “Digital Electronic Micrometer R351C” (manufactured by Anritsu Corp.)

The thickness (A) of a base film was first measured. Similarly, the thickness (B) (B=the sum of the thicknesses of the base film and the non-magnetic undercoat layer) of a non-magnetic substrate obtained by forming a non-magnetic undercoat layer on the base film was measured. Furthermore, the thickness (C) (C=the sum of the thicknesses of the base film, the non-magnetic undercoat layer and the magnetic recording layer) of a magnetic recording medium obtained by forming a magnetic recording layer on the non-magnetic substrata was measured. The thickness of the non-magnetic undercoat layer is expressed by (B)−(A), and the thickness of the magnetic recording layer is expressed by (C)−(B).

Example 1

<Production of spindle-shaped hematite Particles>

1200 g of spindle-shaped goethite particles obtained by the above production method (B) of goethite particles using a ferrous sulfate aqueous solution and a sodium carbonate aqueous solution (average major axis diameter: 0.171 μm, average minor axis diameter: 0.0213 μm, aspect ratio: 8.0:1, geometrical standard deviation of major axis diameter: 1.34, geometrical standard deviation of minor axis diameter: 1.39, BET specific surface area: 151.6 m 2 /g, content of soluble sodium salts: 701 ppm (calculated as Na), content of soluble sulfate: 422 ppm (calculated as SO 4 ), pH value: 6.7) were suspended in water so as to obtain a slurry, and the concentration of the solid content was adjusted to 8 g/liter. 150 liter of the slurry was heated to 60° C. and the pH value of the slurry was adjusted to 10.0 by adding a 0.1-N aqueous NaOH solution.

To the alkali slurry was gradually added 36 g of #3 water glass as a sintering preventive, and after the end of addition, the resultant mixture was aged for 60 minutes. The pH value of the slurry was then adjusted to 6.0 by adding a 0.1-N acetic acid solution. Thereafter, the particles were filtered out, washed with water, dried and pulverized by an ordinary method, thereby producing spindle-shaped goethite particles coated with an oxide of silicon. The silicon content was 0.78% by weight (calculated as SiO 2 ).

1000 g of the spindle-shaped goethite particles obtained were charged into a stainless steel rotary furnace, and heat-dehydrated in the air at 350° C. for 40 minutes while rotating the furnace, to obtain low-density spindle-shaped hematite particles. The thus obtained low-density spindle-shaped hematite particles had an average major axis diameter of 0.145 μm, an average minor axis diameter of 0.0193 μm, an aspect ratio of 7.5:1, a geometrical standard deviation of major axis diameter of 1.34, a geometrical standard deviation of minor axis diameter of 1.39, a BET specific surface area of 176.5 m 2 /g and a S BET /S TEM value of 4.15. The content of soluble sodium salt of the low-density spindle-shaped hematite particles was 1717 ppm (calculated as Na) and the content of soluble sulfate was 1011 ppm (calculated as SO 4 ). The pH value of the low-density spindle-shaped hematite particles was 6.3.

850 g of the low-density spindle-shaped hematite particles were then charged into a ceramic rotary furnace, and heat-treated in the air at 610° C. for 30 minutes while rotating the furnace so as to fill in dehydration pores. The resultant high-density spindle-shaped hematite particles had an average major axis diameter of 0.137 μm, an average minor axis diameter of 0.0190 μm, an aspect ratio of 7.2:1, a geometrical standard deviation of major axis diameter of 1.35, a geometrical standard deviation of minor axis diameter of 1.40, a BET specific surface area (S BET ) of 55.3 m 2 /g and a S BET /S TEM value of 1.28. The silicon content was 0.85% by weight (calculated as SiO 2 ). The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 2626 ppm (calculated as Na) and the content of soluble sulfate was 3104 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 5.3.

After 800 g of the high-density spindle-shaped hematite particles obtained were roughly pulverized by a Nara mill in advance, the obtained high-density spindle-shaped hematite particles were charged into 4.7 liter of pure water and deagglomerated by a homomixer (manufactured by Tokushu-kika Kogyo, CO., Ltd.) for 60 minutes.

The slurry of the high-density spindle-shaped hematite particles obtained was then dispersed for 3 hours at an axial rotation frequency of 2000 rpm while being circulated by a horizontal SGM (Dispermat SL, manufactured by S. C. Adichem, CO., Ltd.). The high-density spindle-shaped hematite particles in the slurry remaining on a sieve of 325 meshes (mesh size: 44 μm) was 0% by weight.

<Dissolving treatment with acid>

The slurry of the high-density spindle-shaped hematite particles obtained was mixed with water, thereby adjusting the concentration of the slurry to 100 g/liter. A 70% aqueous sulfuric acid solution was added to 7 liter of the slurry under stirring so as to adjust the sulfuric acid concentration to 1.3 N and the pH value to 0.65. The slurry was then heated to 80° C. under stirring, and was held for 5 hours at 80° C., thereby dissolving 29.7% by weight of the spindle-shaped hematite particles based on the total weight of the spindle-shaped hematite in the slurry.

The slurry was filtered to separate a filtrate (aqueous acid solution of iron sulfate) therefrom. The slurry from which the filtrate had been separated, was then washed with water by a decantation method and the pH value of the slurry was adjusted to 5.0. When the concentration of the slurry at this point was checked so as to ensure the accuracy, it was 68 g/liter.

2 liter of the obtained slurry was filtered through a Buchner filter, and pure water was passed until the electric conductivity of the filtrate became not more than 30 μs. The high-density spindle-shaped hematite particles were then dried by an ordinary method and pulverized so as to obtain the high-density spindle-shaped hematite particles. The high-density spindle-shaped hematite particles obtained had an average major axis diameter of 0.131 μm, an average minor axis diameter of 0.0181 μm, an aspect ratio of 7.2:1, a geometric standard deviation of major axis diameter of 1.35, a geometric standard deviation of major axis diameter of 1.33, a BET specific surface area (S BET ) of 58.1 m 2 /g and a S BET /S TEM value of 1.28. The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 138 ppm (calculated as Na) and the content of soluble sulfate was 436 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 4.8.

S<Treatment of spindle-shaped hematite particles in an aqueous alkali solution>

The concentration of the high-density spindle-shaped hematite particles in the slurry was adjusted to 50 g/liter, and 5 liter of the slurry was adjusted to pH value 13.6 by adding 6N-aqueous NaOH solution. The resulting slurry was then heated to 95° C. under stirring, and was held for 3 hours at 95° C.

The resultant slurry was then washed with water by a decantation method and the pH value of the slurry was adjusted to 10.5. The concentration of the slurry at this point was 98 g/liter.

The high-density spindle-shaped hematite particles were filtered out from 1 liter of the obtained slurry through a Buchner filter, and the purified water was passed into the filtrate until the electric conductivity of the filtrate became not more than 30 μs. The high-density spindle-shaped hematite particles were then dried by an ordinary method and pulverized to obtain the target high-density spindle-shaped hematite particles. The high-density spindle-shaped hematite particles obtained had an average major axial diameter of not more than 0.131 μm, a minor axial diameter of 0.0181 μm, and a specific ratio of 7.2:1. The geometric standard deviation of major axial diameter was 1.35, the geometric standard deviation of minor axial diameter was 1.33, the BET specific surface (S BET ) was 57.8 m 2 /g, the S BET /S TEM value was 1.27. The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 99 ppm (calculated as Na) and the content of soluble sulfate was 21 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 9.3.

Example 2

<Production of non-magnetic substrate: Formation of non-magnetic undercoat layer on base film>

12 g of the high-density spindle-shaped hematite particles obtained in Example 1 were mixed with a binder resin solution (30% by weight of vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group and 70% by weight of cyclohexanone) and cyclohexanone, and each of the obtained mixtures (solid content: 72% by weight) was kneaded by a plast-mill for 30 minutes.

Each of the thus-obtained kneaded material was charged into a 140 ml-glass bottle together with 95 g of 1.5 mm glass beads, a binder resin solution (30% by weight of polyurethane resin having a sodium sulfonate group and 70% by weight of a solvent (methyl ethyl ketone : toluene=1:1)), cyclohexanone, methyl ethyl ketone and toluene, and the obtained mixture was mixed and dispersed by a paint shaker for 6 hours to obtain a non-magnetic coating composition. The viscosity of the obtained coating film composition was 367 cP.

The thus-obtained non-magnetic coating composition containing the high-density spindle-shaped hematite particles was as follows:

	High-density spindle-shaped	100	parts by weight
	hematite particles
	Vinyl chloride-vinyl acetate	10	parts by weight
	copolymer resin having a sodium
	sulfonate group
	Polyurethane resin having a	10	parts by weight
	sodium sulfonate group
	Cyclohexanone	44.6	parts by weight
	Methylethyl ketone	111.4	parts by weight
	Toluene	66.9	parts by weight

The non-magnetic coating composition obtained was applied to a polyethylene terephthalate film of 12 μm thick to a thickness of 55 μm by an applicator, and the coating film was then dried, thereby forming a non-magnetic undercoat layer. The thickness of the non-magnetic undercoat layer was 3.5 μm.

The non-magnetic undercoat layer produced from the high-density spindle-shaped hematite particles as the non-magnetic particles had a gloss of 204%, and a surface roughness Ra of 6.6 nm. The Young's modulus (relative value) thereof was 124.

Example 3

<Production of magnetic recording medium: Formation of magnetic recording layer>

12 g of magnetic acicular metal particles containing iron as a main component (average major axis diameter: 0.120 μm, average minor axis diameter: 0.0154 μm, a geometrical standard deviation of major axis diameter of 1.37, aspect ratio: 7.8:1, coercive force value: 1896 Oe, saturation magnetization value: 133.8 emu/g, pH value: 9.7), 1.2 g of a polishing agent (AKP-30: trade name, produced by Sumitomo Chemical Co., Ltd.), 0.12 g of carbon black (#3250B, trade name, produced by Mitsubishi Chemical Corp.), a binder resin solution (30% by weight of vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group and 70% by weight of cyclohexanone) and cyclohexanone were mixed to obtain a mixture (solid content: 78% by weight). The mixture was further kneaded by a plast-mill for 30 minutes to obtain a kneaded material.

The thus-obtained kneaded material was charged into a 140 ml-glass bottle together with 95 g of 1.5 mm glass beads, a binder resin solution (30% by weight of polyurethane resin having a sodium sulfonate group and 70% by weight of a solvent (methyl ethyl ketone : toluene=1:1)), cyclohexanone, methyl ethyl ketone and toluene, and the mixture was mixed and dispersed by a paint shaker for 6 hours. Then, the lubricant and hardening agent were added to the mixture, and the resultant mixture was mixed and dispersed by a paint shaker for 15 minutes.

The thus-obtained magnetic coating composition was as follows:

	Magnetic acicular metal	100	parts by weight
	particles containing iron as a
	main component
	Vinyl chloride-vinyl acetate	10	parts by weight
	copolymer resin having a sodium
	sulfonate group
	Polyurethane resin having a	10	parts by weight
	sodium sulfonate group
	Polishing agent (AKP-30)	10	parts by weight
	Carbon black (#3250B)	3.0	parts by weight
	Lubricant (myristic acid:butyl	3.0	parts by weight
	stearate = 1:2)
	Hardening agent (polyisocyanate)	5.0	parts by weight
	Cyclohexanone	65.8	parts by weight
	Methyl ethyl ketone	164.5	parts by weight
	Toluene	98.7	parts by weight

The magnetic coating composition obtained was applied to the non-magnetic undercoat layer to a thickness of 15 μm by an applicator, and the magnetic recording medium obtained was oriented and dried in a magnetic field, and then calendered. The magnetic recording medium was then subjected to a curing reaction at 60° C. for 24 hours, and thereafter slit into a width of 0.5 inch, thereby obtaining a magnetic tape. The thickness of the respective magnetic recording layer was 1.1

The coercive force Hc of the magnetic tape produced by forming a magnetic recording layer on the non-magnetic undercoat layer was 1986 Oe, the squareness (Br/Bm) thereof was 0.87, the gloss thereof was 220%, the surface roughness Ra thereof was 6.2 nm, the Young's modulus (relative value) thereof was 128, the linear absorption coefficient thereof was 1.24 μm −1 .

The percentages of change in coercive force and saturation magnetization which represent an anti-corrosion is property of the magnetic tape, were 4.3% and 4.6%, respectively.

Example 4

<Production of magnetic recording medium: Formation of magnetic recording layer>

The same procedure as defined in Example 3 was conducted except that the magnetic particles were changed to magnetic acicular metal particles (average major axis diameter: 0.115 μm, average minor axis diameter: 0.0145 μm, a geometric standard deviation of major axis diameter of 1.36, aspect ratio: 7.9:1, coercive force: 1909 Oe, saturation magnetization: 133.8 emu/g, aluminum content of 2.85% by weight, calculated as Al, (1.26% by weight, calculated as Al, of aluminum in the central portion, 0.84% by weight, calculated as Al, of aluminum in the surface layer portion, and 0.75% by weight, calculated as Al, of aluminum on the surface coating), Nd content of 0.12% by weight, resin adsorptivity: 81.6%), thereby producing a magnetic tape.

The thickness of the magnetic coating film was 1.0 μm.

The thus obtained magnetic tape had a coercive force Hc of 1980 Oe, a squareness (Br/Bm) of 0.88, a gloss of 223%, a surface roughness Ra of 6.4 nm, a Young's modulus (relative value) of coating film of 128, a linear absorption coefficient of 1.24 μm −1 , a running durability of 28.9 minutes and a scratch resistance of A.

The percentages of change in coercive force and saturation magnetization which represent an anti-corrosion property of the magnetic tape, were 4.6% and 2.7%, respectively.

Example 5

<Production of single-shaped hematite particles>

1200 g of spindle-shaped goethite particles obtained by the above production method (B) of goethite particles using a ferrous sulfate aqueous solution, a sodium carbonate aqueous solution and aluminum sulfate aqueous solution, and containing aluminum in an amount of 1.12% by weight (calculated as Al) based on the total weight of the particles, uniformly within the particles (average major axis diameter: 0.167 μm, average minor axis diameter: 0.0196 μm, aspect ratio: 8.5:1, geometrical standard deviation of major axis diameter: 1.32, geometrical standard deviation of minor axis diameter: 1.40, BET specific surface area: 165.3 m 2 /g, content of soluble sodium salts: 1821 ppm (calculated as Na), content of soluble sulfate: 2162 ppm (calculated as SO 4 ), pH value: 6.8) were suspended in water so as to obtain a slurry, and the concentration of the solid content was adjusted to 8 g/liter. 150 liter of the slurry was heated to 60° C. and the pH value of the slurry was adjusted to 10.0 by adding a 0.1-N aqueous NaOH solution.

To the alkali slurry was gradually added 24 g of #3 water glass as a sintering preventive, and after the end of addition, the resultant mixture was aged for 60 minutes. The pH value of the slurry was then adjusted to 6.0 by adding a 0.1-N acetic acid solution. Thereafter, the particles were filtered out, washed with water, dried and pulverized by an ordinary method, thereby producing spindle-shaped goethite particles which substantially uniformly contain aluminum within the particle, coated with an oxide of silicon. The silicon content was 0.52% by weight (calculated as SiO 2 ) based on the total weight of the particles.

1000 g of the spindle-shaped goethite particles which substantially uniformly contain aluminum within the particle, obtained were charged into a stainless steel rotary furnace, and heat-dehydrated in the air at 350° C. for 40 minutes while rotating the furnace, to obtain low-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle. The thus obtained low-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, had an average major axial diameter of 0.143 μm, an average minor axial diameter of 0.0191 μm, a geometric standard deviation of major axis diameter of 1.32, a geometric standard deviation of minor axis diameter of 1.40, an aspect ratio of 7.5:1, a BET specific surface area (S BET ) of 188.9 m 2 /g and a S BET /S TEM value of 4.40. The low-density spindle-shaped hematite particles contained soluble sodium salts of 1682 ppm (calculated as Na) and soluble sulfates of 976 ppm (calculated as SO 4 ). The aluminum content was 1.23% by weight (calculated as Al), the pH value of the low-density spindle-shaped hematite particles was 6.1. The SiO 2 content thereof was 0.57% by weight (calculated as SiO 2 ) based on the total weight of the particles.

850 g of the low-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, were then charged into a ceramic rotary furnace, and heat-treated in the air at 650° C. for 30 minutes while rotating the furnace so as to fill in dehydration pores. The resultant high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, had an average major axial diameter of 0.141 μm, an average minor axial diameter of 0.0192 μm, a geometric standard deviation of major axis diameter of 1.33, a geometric standard deviation of minor axis diameter of 1.41, an aspect ratio of 7.3:1, a BET specific surface area (S BET ) of 56.1 m 2 /g and a S BET /S TEM value of 1.31. The silicon content was 0.57% by weight (calculated as SiO 2 ) and the aluminum content was 1.23% by weight (calculated as Al). The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 2538 ppm (calculated as Na) and the content of soluble sulfate was 2859 ppm (calculated as SO 4 ). The pH thereof was 5.6.

After 800 g of the high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, obtained were roughly pulverized by a Nara mill in advance, the obtained high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, were charged into 4.7 liter of pure water and deagglomerated by a homomixer (manufactured by Tokushu-kika Kogyo, CO., Ltd.) for 60 minutes.

The slurry of the high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, obtained was then dispersed for 3 hours at an axial rotation frequency of 2000 rpm while being circulated by a horizontal SGM (Dispermat SL, manufactured by S. C. Adichem, CO., Ltd.). The high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, in the slurry remaining on a sieve of 325 meshes (mesh size: 44 μm) was 0% by weight.

<Dissolving treatment with acid>

The slurry of the high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, obtained was mixed with water, thereby adjusting the concentration of the slurry to 100 g/liter. A 70% aqueous sulfuric acid solution was added to 7 liter of the slurry under stirring so as to adjust the sulfuric acid concentration to 1.3N and the pH value to 0.58. The slurry was then heated to 80° C. under stirring, and was held for 5 hours at 80° C., thereby dissolving 29.5% by weight of the high density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, based on the total weight of the high-density spindle-shaped hematite particles in the slurry.

The slurry was filtered to separate a filtrate (aqueous acid solution of iron sulfate) therefrom. The slurry from which the filtrate had been separated, was then washed with water by a decantation method and the pH value of the slurry was adjusted to 5.0. When the concentration of the slurry at this point was checked so as to ensure the accuracy, it was 68 g/liter.

A part of the water-washed slurry obtained was separated and filtered through a Buchner filter, and pure water was passed until the electric conductivity of the filtrate became not more than 30 μs. The resultant particles were then dried by an ordinary method and pulverized so as to obtain the high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle. The high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, obtained had an average major axis diameter of 0.133 μm, an average minor axis diameter of 0.0182 μm, an aspect ratio of 7.3:1, a geometric standard deviation of major axis diameter of 1.34, a geometric standard deviation of minor axis diameter of 1.33, a BET specific surface area (S BET ) of 60.3 m 2 /g, a S BET /S TEM value of 1.34, an aluminum content of 1.23% by weight (calculated as Al). The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 131 ppm (calculated as Na) and the content of soluble sulfate was 424 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 4.9.

<Treatment of high-density spindle-shaped hematite particles in an aqueous alkali solution>

The concentration of the high-density spindle-shaped hematite particles in the slurry was adjusted to 50 g/liter, and 5 liter of the slurry was adjusted to pH value 13.6 by adding a 6N-aqueous NaOH solution. The resulting slurry was then heated to 95° C. under stirring, and was held for 3 hours at 95° C.

The resultant slurry was then washed with water by a decantation method and the pH value of the slurry was adjusted to 10.5. The concentration of the slurry at this point was 98 g/liter.

The particles were filtered out from the obtained slurry of 1 liter through a Buchner filter, and the purified water was passed into the filtrate until the electric conductivity of the filtrate became not more than 30 μs. The particles were then dried by an ordinary method and pulverized to obtain the target high-density spindle-shaped hematite particles. The high-density spindle-shaped hematite particles obtained had an average major axial diameter of not more than 0.133 μm, a minor axial diameter of 0.0182 μm, and a specific ratio of 7.3:1. The geometric standard deviation of major axial diameter was 1.33, the geometric standard deviation of minor axial diameter was 1.34, the BET specific surface (S BET ) was 60.1 m 2 /g, the S BET /S TEM value was 1.33. The aluminum content was 1.23% by weight (calculated as Al). The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 97 ppm (calculated as Na) and the content of soluble sulfate was 20 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 9.4, and the resin adsorptivity thereof was 72.5%.

Example 6

<Production of non-magnetic substrate: Formation of non-magnetic undercoat layer on base film>

The same procedure as defined in Example 2 was conducted except that the spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, obtained in Example 5 were used instead of the spindle-shaped hematite particles, thereby obtaining a non-magnetic undercoat layer.

The thickness of the non-magnetic undercoat layer was 3.5 μm.

The thus obtained non-magnetic undercoat layer had a gloss of 212%, a surface roughness Ra of 6.1 nm, and a Young's modulus (relative value) of coating film of 127.

Example 7

<Production of magnetic recording medium: Formation of magnetic recording layer>

The same procedure as defined in Example 3 was conducted except that magnetic acicular metal particles containing iron as a main component and further containing aluminum in amounts of 1.24% by weight (calculated as Al) at a central portion of particle; 0.83% by weight (calculated as Al) at a surface portion thereof; and 0.94% by weight (calculated as Al) in a coating layer formed on the surface of particle (average major axis diameter: 0.112 μm, average minor axis diameter: 0.0147 μm, geometrical standard deviation of major axis diameter: 1.36, aspect ratio: 7.6:1, coercive force: 1908 Oe, saturation magnetization: 136.3 emu/g, resin adsorptivity: 82.0%) were used, thereby obtaining a magnetic coating composition.

The thus obtained magnetic coating composition was applied onto the non-magnetic undercoat layer obtained in Example 6 in the same manner as in Example 3, thereby forming a magnetic recording layer and producing a magnetic tape.

The thickness of the magnetic coating film was 1.1 μm.

The thus obtained magnetic tape had a coercive force Hc of 1983 Oe, a squareness (Br/Bm) of 0.87, a gloss of 234%, a surface roughness Ra of 6.1 nm, a Young's modulus (relative value) of coating film of 131, a linear absorption coefficient of 1.24 μm −1 , a running durability of 29.2 minutes, and a scratch resistance of A.

Changes in the coercive force and the saturation magnetic flux density Bit with passage time were 4.5% and 2.6%, respectively.

Example 8

<Production of spindle-shaped hematite particles>

1200 g of spindle-shaped goethite particles obtained by the above production method (B) of goethite particles using a ferrous sulfate aqueous solution and a sodium carbonate aqueous solution (average major axis diameter: 0.0812 μm, geometrical standard deviation of major axis diameter: 1.53, average minor axis diameter: 0.0110 μm, geometrical standard deviation of minor axis diameter: 1.37, aspect ratio: 7.4:1, BET specific surface area: 168.9 m 2 /g, soluble sodium salt content: 1212 ppm (calculated as Na), soluble sulfate content: 1816 ppm (calculated as SO 4 ), pH value: 6.8) were suspended in water so as to obtain a slurry, and the concentration of the solid content was adjusted to 8 g/liter. 150 liter of the slurry was heated to 60° C. and the pH value of the slurry was adjusted to 10.0 by adding a 0.1-N aqueous NaOH solution.

To the alkali slurry was gradually added 36.0 g of #3 water glass as a sintering preventive, and after the end of addition, the resultant mixture was aged for 60 minutes. The pH value of the slurry was then adjusted to 6.0 by adding a 0.1-N acetic acid solution. Thereafter, the particles were filtered out, washed with water, dried and pulverized by an ordinary method, thereby producing spindle-shaped goethite particles coated with an oxide of silicon. The silicon content was 0.78% by weight (calculated as SiO 2 ).

The spindle-shaped goethite particles obtained were charged into a heat-treatment metal furnace, and heat-treated therein at 150° C. for 30 minutes, thereby absorbing superfine goethite particles present within the spindle-shaped goethite particles, into the spindle-shaped goethite particles.

The thus obtained spindle-shaped goethite particles were charged again into the heat treatment metal furnace, and heat-dehydrated therein at 320° C. for 30 minutes, thereby obtaining low-density spindle-shaped hematite particles. The thus obtained low-density spindle-shaped hematite particles had an average major axis diameter of 0.0736 μm, a geometrical standard deviation of major axis diameter of 1.38, an average minor axis diameter of 0.0118 μm, a geometrical standard deviation of minor axis diameter of 1.16, an aspect ratio of 6.1:1, a BET specific surface area of 190.3 m 2 /g, a S BET /S TEM value of 2.70. The silicon content was 0.78% by weight (calculated as SiO 2 ). The content of soluble sodium salt of the low-density spindle-shaped hematite particles was 1826 ppm (calculated as Na) and the content of soluble sulfate was 2512 ppm (calculated as SO 4 ). The pH value of the low-density spindle-shaped hematite particles was 6.1.

850 g of the low-density spindle-shaped hematite particles were then charged into a ceramic rotary furnace, and heat-treated in the air at 650° C. for 30 minutes while rotating the furnace so as to fill in dehydration pores. The resultant high-density spindle-shaped hematite particles had an average major axis diameter of 0.0727 μm, a geometrical standard deviation of major axis diameter of 1.38, an average minor axis diameter of 0.0120 μm, a geometrical standard deviation of minor axis diameter of 1.17, an aspect ratio of 6.1:1, a BET specific surface area (S BET ) of 86.8 m 2 /g, a S BET /S TEM value of 1.25. The silicon content was 0.87% by weight (calculated as SiO 2 ). The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 2121 ppm (calculated as Na) and the content of soluble sulfate was 2832 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 5.8.

<Treatment of high-density spindle-shaped hematite particles in an aqueous alkali solution>

The concentration of the high-density spindle-shaped hematite particles in the slurry was adjusted to 50 g/liter, and 5 liter of the slurry was adjusted to pH value 13.4 by adding a 6N-aqueous NaOH solution. The resulting slurry was then heated to 95° C. under stirring, and was held for 3 hours at 95° C.

The resultant slurry was then washed with water by a decantation method and the pH value of the slurry was adjusted to 10.5. The concentration of the slurry at this point was 98 g/liter.

The particles were filtered out through a Buchner filter, and the purified water was passed into the filtrate until the electric conductivity of the filtrate became not more than 30 μs. The particles were then dried by an ordinary method and pulverized to obtain the target high-density spindle-shaped hematite particles. The high-density spindle-shaped hematite particles obtained had an average major axial diameter of 0.0726 μm, a minor axial diameter of 0.0120 μm, and a specific ratio of 6.1. The geometric standard deviation of major axial diameter was 1.38, the geometric standard deviation of minor axial diameter was 1.17, the BET specific surface (S BET ) was 86.2 m 2 /g, the S BET /S TEM value was 1.24. The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 86 ppm (calculated as Na) and the content of soluble sulfate was 32 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 9.0, and the resin adsorptivity thereof was 75.1%.

Example 9

<Production of non-magnetic substrate: Formation of nonmagnetic undercoat layer on base film>

The same procedure as defined in Example 2 was conducted except that the high-density spindle-shaped hematite particles obtained in Example 8 were used instead of the spindle-shaped hematite particles, thereby obtaining a non-magnetic undercoat layer. The thickness of the non-magnetic undercoat layer was 3.4 μm.

The thus obtained non-magnetic undercoat layer had a gloss of 208%, a surface roughness Ra of 6.4 nm, and a Young's modulus (relative value) of coating film of 134.

Example 10

<Production of magnetic recording medium: Formation of magnetic recording layer>

The same procedure as defined in Example 3 was conducted except that the magnetic particles were changed to magnetic acicular metal particles containing iron as a main component (average major axis diameter: 0.103 μm, average minor axis diameter: 0.0152 μm, a geometric standard deviation of major axis diameter of 1.38, aspect ratio: 6.8:1, coercive force: 1910 Oe, saturation magnetization: 136 emu/g), and that the non-magnetic undercoat layer obtained in Example 9 was used, thereby producing a magnetic tape.

The thickness of the magnetic coating film was 1.0 μm.

The thus obtained magnetic tape had a coercive force Hc of 1991 Oe, a squareness (Br/Bm) of 0.88, a gloss of 235%, a surface roughness Ra of 5.8 nm, a Young's modulus (relative value) of coating film of 137, a linear absorption coefficient of 1.26 μm −1 , a running durability of 298 minutes and a scratch resistance of A.

Changes in the coercive force and the saturation magnetic flux density Bm with passage time were 4.6% and 3.8%, respectively.

Example 11

<Production of magnetic recording medium: Formation of magnetic recording layer>

The same procedure as defined in Example 3 was conducted except that the magnetic particles were changed to magnetic acicular metal particles (average major axis diameter: 0.115 μm, average minor axis diameter: 0.0145 μm, a geometric standard deviation of major axis diameter of 1.36, aspect ratio: 7.9:1, coercive force: 1909 Oe, saturation magnetization: 133.8 emu/g,,aluminum content of 2.85% by weight, calculated as Al, (1.26% by weight, calculated as Al, of aluminum in the central portion, 0.84% by weight, calculated as Al, of aluminum in the surface layer portion, and 0.75% by weight, calculated as Al, of aluminum on the surface coating), Nd content of 0.12% by weight, resin adsorptivity: 81.6%), and that the non-magnetic undercoat layer obtained in Example 9 was used, thereby producing a magnetic tape.

The thickness of the magnetic coating film was 1.0 μm.

The thus obtained magnetic tape had a coercive force Hc of 1985 Oe, a squareness (Br/Bm) of 0.89, a gloss of 238%, a surface roughness Ra of 5.6 nm, a Young's modulus (relative value) of coating film of 138, a linear absorption coefficient of 1.27 μm −1 , a running durability of not less than 30 minutes and a scratch resistance of A.

Changes in the coercive force and the saturation magnetic flux density Bm with passage time were 2.8% and 2.6%, respectively.

Example 12

<Production of spindle-shaped hematite Particles>

1200 g of spindle-shaped goethite particles obtained by the above production method (B) of goethite particles using a ferrous sulfate aqueous solution, a sodium carbonate aqueous solution and aluminum sulfate aqueous solution (average major axis diameter: 0.0846 μm, geometrical standard deviation of major axis diameter: 1.49, average minor axis diameter: 0.0115 μm, geometrical standard deviation of minor axis diameter: 1.38, aspect ratio: 7.4:1, BET specific surface area: 161.6 m 2 /g, aluminum content: 2.12% by weight (calculated as Al), soluble sodium salt content: 1168 ppm (calculated as Na), soluble sulfate content: 1721 ppm (calculated as SO 4 ), pH value: 6.0) were suspended in water so as to obtain a slurry, and the concentration of the solid content was adjusted to 8 g/liter. 150 liter of the slurry was heated to 60° C. and the pH value of the slurry was adjusted to 10.0 by adding a 0.1-N aqueous NaOH solution.

To the alkali slurry was gradually added 42.0 g of #3 water glass as a sintering preventive, and after the end of addition, the resultant mixture was aged for 60 minutes. The pH value of the slurry was then adjusted to 6.0 by adding a 0.1-N acetic acid solution. Thereafter, the particles were filtered out, washed with water, dried and pulverized by an ordinary method, thereby producing spindle-shaped goethite particles which substantially uniformly contain aluminum within the particle, coated with an oxide of silicon. The silicon content was 0.90% by weight (calculated as SiO 2 ).

The spindle-shaped goethite particles which substantially uniformly contain aluminum within the particle, obtained were charged into a heat treatment metal furnace, and heat-treated therein at 140° C. for 30 minutes, thereby absorbing superfine goethite particles present within the spindle-shaped goethite particles, into the spindle-shaped goethite particles.

The thus obtained spindle-shaped goethite particles which substantially uniformly contain aluminum within the particle, were charged again into the heat treatment metal furnace, and heat-dehydrated therein at 340° C. for 30 minutes, thereby obtaining low-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle. The thus obtained low-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, had an average major axis diameter of 0.0793 μm, a geometrical standard deviation of major axis diameter of 1.37, an average minor axis diameter of 0.0119 μm, a geometrical standard deviation of minor axis diameter of 1.23, an aspect ratio of 6.7:1, a BET specific surface area (S BET ) of 181.0 m 2 /g, a S BET /S TEM value of 2.60. The silicon content was 0.99% by weight (calculated as SiO 2 ) and the aluminum content was 2.35% by weight (calculated as Al). The content of soluble sodium salt of the low-density spindle-shaped hematite particles was 1465 ppm (calculated as Na) and the content of soluble sulfate was 1965 ppm (calculated as SO 4 ). The pH value of the low-density spindle-shaped hematite particles was 5.9.

850 g of the low-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, were then charged into a ceramic rotary furnace, and heat-treated in the air at 650° C. for 30 minutes while rotating the furnace so as to fill in dehydration pores. The resultant high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, had an average major axis diameter of 0.0753 μm, a geometrical standard deviation of major axis diameter of 1.37, an average minor axis diameter of 0.0122 μm, a geometrical standard deviation of minor axis diameter of 1.24, an aspect ratio of 6.2:1, a BET specific surface area (S BET ) of 83.8 m 2 /g, a S BET /S TEM value of 1.23. The silicon content was 1.00% by weight (calculated as SiO 2 ) and the aluminum content was 2.35% by weight (calculated as Al). The silicon content was 1.00% by weight (calculated as SiO 2 ). The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 1612 ppm (calculated as Na) and the content of soluble sulfate was 2101 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 5.7.

<Treatment of high-density spindle-shaped hematite particles in an aqueous alkali solution>

The concentration of the high-density spindle-shaped hematite particles in the slurry was adjusted to 50 g/liter, and 5 liter of the slurry was adjusted to pH value 13.8 by adding a 6N-aqueous NaOH solution. The resulting slurry was then heated to 95° C. under stirring, and was held for 3 hours at 95° C.

The resultant slurry was then washed with water by a decantation method and the pH value of the slurry was adjusted to 10.5. The concentration of the slurry at this point was 98 g/liter.

The particles were filtered out through a Buchner filter, and the purified water was passed into the filtrate until the electric conductivity of the filtrate became not more than 30 μs. The particles were then dried by an ordinary method and pulverized to obtain the target high-density spindle-shaped hematite particles. The high-density spindle-shaped hematite particles obtained had an average major axial diameter of 0.0752 μm, a minor axial diameter of 0.0122 μm, and a specific ratio of 6.2. The geometric standard deviation of major axial diameter was 1.37, the geometric standard deviation of minor axial diameter was 1.24, the BET specific surface (S BET ) was 83.4 m 2 /g, the S BET /S TEM value was 1.22. The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 63 ppm (calculated as Na) and the content of soluble sulfate was 21 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 9.1, and the resin adsorptivity thereof was 79.6%.

Example 13

<Production of non-magnetic substrate: Formation of non-magnetic undercoat layer on base film>

The same procedure as defined in Example 2 was conducted except that the spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, obtained in Example 12 were used instead of the spindle-shaped hematite particles, thereby obtaining a non-magnetic undercoat layer.

The thickness of the magnetic coating film was 3.5 μm.

The thus obtained non-magnetic undercoat layer had a gloss of 218%, a surface roughness Ra of 6.2 nm, and a Young's modulus (relative value) of coating film of 135.

Example 14

<Production of magnetic recording medium: Formation of magnetic recording layer>

The same procedure as defined in Example 3 was conducted except that the magnetic particles were changed to acicular magnetic metal particles (average major axis diameter: 0.110 μm, average minor axis diameter: 0.0146 μm, a geometric standard deviation of major axis diameter of 1.38, aspect ratio: 7.5:1, coercive force: 1943 Oe, saturation magnetization: 132 emu/g), and that the non-magnetic undercoat layer obtained in Example 13 was used, thereby producing a magnetic tape.

The thickness of the magnetic coating film was 1.0 μm.

The thus obtained magnetic tape had a coercive force Hc of 1989 Oe, a squareness (Br/Bm) of 0.88, a gloss of 238%, a surface roughness Ra of 5.8 nm, a Young's modulus (relative value) of coating film of 136, a linear absorption coefficient of 1.27 μm −1 , a running durability of not less than 30 minutes and a scratch resistance of A.

Changes in the coercive force and the saturation magnetic flux density Bm with passage time were 2.8% and 2.1%, respectively.

Example 15

<Production of magnetic recording medium: Formation of magnetic recording layer>

The same procedure as defined in Example 3 was conducted except that the magnetic particles were changed to magnetic acicular metal particles (average major axis diameter: 0.115 μm, average minor axis diameter: 0.0145 pm, a geometric standard deviation of major axis diameter of 1.36, aspect ratio: 7.9:1, coercive force: 1909 Oe, saturation magnetization: 133.8 emu/g, aluminum content of 2.85% by weight, calculated as Al, (1.26% by weight, calculated as Al, of aluminum in the central portion, 0.84% by weight, calculated as Al, of aluminum in the surface layer portion, and 0.75% by weight, calculated as Al, of aluminum on the surface coating), Nd content of 0.12% by weight, resin adsorptivity: 81.6%), and that the non-magnetic undercoat layer obtained in Example 13 was used, thereby producing a magnetic tape.

The thickness of the magnetic coating film was 1.0 μm.

The thus obtained magnetic tape had a coercive force Hc of 1990 Oe, a squareness (Br/Bm) of 0.89, a gloss of 243%, a surface roughness Ra of 5.4 nm, a Young's modulus (relative value) of coating film of 138, a linear absorption coefficient of 1.26 μm −1 , a running durability of not less than 30 minutes and a scratch resistance of A.

Changes in the coercive force and the saturation magnetic flux density Bm with passage time were 1.8% and 1.6%, respectively.

<Kinds of acicular goethite particles>

Precursor 1 to 6:

Various properties of acicular goethite particles as a precursor of acicular hematite particles are shown in Table 1.

Examples 16 to 22 and Comparative Examples 1 to 6

<Production of low-density acicular hematite particles>

Low-density acicular hematite particles were obtained in the same way as in Example 1 except for varying the kind of acicular goethite particles as a precursor, the kind and amount added of sintering preventive, and heat-dehydration temperature and time. Incidentally, the particles obtained in Comparative Example 4 were acicular goethite particles.

The main producing conditions and various properties are shown in Tables 2 to 3.

Examples 23 to 29 and Comparative Examples 7 to 11

<Production of high-density acicular hematite particles>

High-density acicular hematite particles were obtained in the same way as in Example 1 except for varying the kind of low-density acicular hematite particles, and heat-treating temperature and time for high densification.

The main producing conditions and various properties are shown in Tables 4 to 5.

Examples 30 to 36 and Comparative Examples 12 to 13

<Acid-dissolving treatment of acicular hematite particles>

Acicular hematite particles were obtained in the same way as in Example 1 except for varying the kind of high-density acicular hematite particles, use or non-use of the wet pulverization, the acid concentration, the pH value of slurry, and the heating temperature and time.

The main producing conditions and various properties are shown in Tables 6 to 7.

Examples 37 to 43 and Reference Examples 1 to 2

<Heat-treatment of acicular hematite particles in alkaline suspension>

The same procedure as defined in Example 1 was conducted except that kind of acicular hematite particles, pH value of slurry, heating temperature and heating time were varied, thereby obtaining acicular hematite particles.

Main production conditions and various properties are shown in Tables 8 and 9.

Example 44

<Surface-coating treatment of acicular hematite particles>

The concentration of the slurry having a pH value of 10.5 which was obtained in Example 37 by washing the slurry heat-treated in the alkaline suspension, with water by a decantation method, was 50 g/liter. 4 liters of the water-washed slurry was heated again to 60° C., and then mixed with 74.1 ml of 1.0N sodium aluminate aqueous solution (corresponding to 1.0% by weight calculated as Al based on the weight of the acicular hematite particles). After maintaining the slurry at that temperature for 30 minutes, the pH value of the slurry was adjusted to 8.0 using an aqueous acetic acid solution. Next, the slurry was sequentially subjected to filtration, washing with water, drying and pulverization in the same manner as in Example 1, thereby obtaining acicular hematite particles whose surfaces were coated with a hydroxide of aluminum.

Main production conditions and various properties are shown in Tables 10 and 11.

Examples 45 to 50

Acicular hematite particles coated with a coating material by an ordinary method were obtained in the same way as in Example 44 except for varying the kind of acicular hematite particles and the kind and amount of the coating material.

The main producing conditions and various properties are shown in Tables 10 and 11, respectively.

Examples 51 to 64. Comparative Examples 14 to 22 and Reference Examples 3 to 11

<Production of non-magnetic substrate: Formation of non-magnetic undercoat layer on non-magnetic base film>

By using the acicular hematite particles obtained in Example 30 to 50, Comparative Examples 1, 3 and 7 to 13 and Reference Examples 1 to 2, non-magnetic undercoat layers were formed in the same way as in Example 2.

The main producing conditions and various properties are shown in Tables 12 and 13, respectively.

<Production of magnetic recording medium: Formation of magnetic coating film>

Magnetic particles used for forming the magnetic recording layers and various properties thereof are shown in Table 14.

Examples 65 to 78, Comparative Examples 23 to 31 and Reference Examples 12 to 20

Magnetic recording media were produced in the same way as in Example 3 except for varying the kind of non-magnetic substrate and the kind of magnetic particles.

The main producing conditions and various properties are shown in Tables 15 and 16.

Examples 79 to 92, Comparative Examples 32 to 40 and Reference Examples 21 to 29

Magnetic recording media were produced in the same way as in Example 4 except for varying the kind of non-magnetic substrate and the kind of magnetic particles.

The main producing conditions and various properties are shown in Tables 17 and 18.

<Kinds of acicular goethite particles which substantially uniformly contain aluminum within the particle >

Precursor 7 to 13:

Various properties of acicular goethite particles as a precursor of acicular hematite particles are shown in Table 19.

Examples 93 to 99 and Comparative Examples 41 to 46

<Production of low-density acicular hematite particles which substantially uniformly contain aluminum within the particle>

Low-density acicular hematite particles were obtained in the same way as in Example 5 except for varying the kind of acicular goethite particles as a precursor, the kind and amount added of sintering preventive, and heat-dehydration temperature and time. Incidentally, the particles obtained in Comparative Example 44 were acicular goethite particles.

The main producing conditions and various properties are shown in Tables 20 to 21.

Examples 100 to 106 and Comparative Examples 47 to 51

<Production of high-density acicular hematite Particles which substantially uniformly contain aluminum within the Particle>

High-density acicular hematite particles were obtained in the same way as in Example 5 except for varying the kind of low-density acicular hematite particles, and heat-treating temperature and time for high densification.

The main producing conditions and various properties are shown in Tables 22 to 23.

Examples 107 to 113 and Comparative Examples 52 to 53

<Acid-dissolving treatment of acicular hematite particles which substantially uniformly contain aluminum within the particle>

Acicular hematite particles were obtained in the same way as in Example 5 except for varying the kind of high-density acicular hematite particles, use or non-use of the wet pulverization, the acid concentration, the pH value of slurry, and the heating temperature and time.

The main producing conditions and various properties are shown in Tables 24 to 25.

Examples 114 to 120 and Reference Examples 30 to 31

<Heat-treatment of acicular hematite Particles in alkaline suspension>

The same procedure as defined in Example 5 was conducted except that kind of acicular hematite particles, pH value of slurry, heating temperature and heating time were varied, thereby obtaining acicular hematite particles.

Main production conditions and various properties are shown in Tables 26 and 27.

Example 121

<Surface coating of acicular hematite particles which substantially uniformly contain aluminum within the particle>

The concentration of the slurry having a pH value of 10.5 which was obtained in Example 114 by washing the slurry heat-treated in the alkaline suspension, with water by a decantation method, was 50 g/liter. 4 liters of the water-washed slurry was heated again to 60° C., and then mixed with 74.1 ml of 1.ON sodium aluminate aqueous solution (corresponding to 1.0% by weight calculated as Al based on the weight of the acicular hematite particles). After maintaining the slurry at that temperature for 30 minutes, the pH value of the slurry was adjusted to 8.0 using an aqueous acetic acid solution. Next, the slurry was sequentially subjected to filtration, washing with water, drying and pulverization in the same manner as in Example 5, thereby obtaining acicular hematite particles whose surfaces were coated with a hydroxide of aluminum.

Main production conditions and various properties are shown in Tables 28 and 29.

Examples 122 to 127

Acicular hematite particles coated with a coating material by an ordinary method were obtained in the same way as in Example 121 except for varying the kind of acicular hematite particles and the kind and amount of the coating material.

The main producing conditions and various properties are shown in Tables 28 and 29.

Examples 128 to 141. Comparative Examples 54 to 62 and Reference Examples 32 to 40

<Production of non-magnetic substrate: Formation of non-magnetic undercoat layer on non-magnetic base film>

By using the acicular hematite particles obtained in Example 107 to 127 and Comparative Examples 41, 43 and 47 to 53 and Reference Examples 30 to 31, non-magnetic undercoat layers were formed in the same way as in Example 2.

The main producing conditions and various properties are shown in Tables 30 and 31.

Examples 142 to 155. Comparative Examples 63 to 71 and Reference Examples 41 to 49

<Production of magnetic recording medium: Formation of magnetic coating film>

Magnetic recording media were produced in the same way as in Example 3 except for varying the kind of non-magnetic substrate and the kind of magnetic particles.

The main producing conditions and various properties are shown in Tables 32 and 33.

<Kind of acicular goethite particles>

Goethite Particles 1 to 2:

Acicular goethite particles 1 and 2 as precursors having properties shown in Table 34 were prepared.

Goethite particles 3 to 5:

Acicular goethite particles 3 to 5 which were subjected to a sintering preventing treatmnent, were obtained in the same way as in Example 8 except for varying the kind of starting particles and the element and amount of sintering preventive.

Various properties of the obtained acicular goethite particles are shown in Table 35.

<Heat treatment>

Goethite particles 6 to 9:

Acicular goethite particles 6 to 9 were obtained in the same way as in Example 8 except for varying the kind of acicular goethite particles as precursors and the heat-treating temperature and time.

The main producing conditions are shown in Table 36 and various properties of the obtained acicular goethite particles are shown in Table 37.

<Production of low-density acicular hematite particles>

Hematite particles 1 to 4:

Low-density acicular hematite particles 1 to 4 were obtained in the same way as in Example 8 except for varying the kind of acicular goethite particles as precursors and the heat-dehydrating temperature and time.

The main producing conditions are shown in Table 38 and various properties of the obtained low-density acicular hematite particles are shown in Table 39.

<Production of high-density acicular hematite particles>

Hematite particles 5 to 13:

High-density acicular hematite particles were obtained in the same way as in Example 8 except for varying the kind of particles to be treated and the heat-treating temperature and time.

The main producing conditions are shown in Table 40 and various properties of the obtained high-density hematite particles are shown in Table 41.

Examples 156 to 159 and Reference Examples 50 to 51

<Heat-treatment of acicular hematite particles in alkaline suspension>

The same procedure as defined in Example 1 was conducted except that kind of acicular hematite particles, pH value of slurry, heating temperature and heating time were varied, thereby obtaining acicular hematite particles.

Main production conditions and various properties are shown in Tables 42 and 43.

Examples 160

<Surface coating treatment of acicular hematite particles>

After 700 g of the high-density acicular hematite particles obtained in the Example 156 were roughly pulverized by a Nara mill in advance, the obtained particles were charged into 7 liters of pure water and diagglomerated by a homomixer (manufactured by Tokushu-kika Kogyo, CO., Ltd.) for 60 minutes.

The slurry of the acicular hematite particles obtained was then dispersed for 6 hours at an axial rotation frequency of 2000 rpm while being circulated by a horizontal sand grinder (Dispermat SL, manufactured by S. C. Adichem, Co., Ltd.).

The pH value of the obtained slurry containing the acicular hematite particles was adjusted to 4.0 by using a 0.1 N acetic acid aqueous solution. By adding water to the slurry, the concentration of the resultant slurry was adjusted to 45 g/liter. 10 liter of the slurry was re-heated to 60° C., and 500 ml (equivalent to 3.0% by weight (calculated as Al) based on the acicular hematite particles) of a 1.0 mol/liter aqueous aluminum acetate solution was added to the slurry, and the mixture was held for 30 minutes. Thereafter, the pH value of the mixture was adjusted to 7.1 by using aqueous sodium hydroxide solution. After holding the mixture for 30 minutes, the particles were then filtered out, washed with water, dried and, thereby obtaining acicular hematite particles coated with a hydroxide of aluminum.

The main producing conditions and various properties are shown in Tables 44 and 45.

Examples 161 to 163

Acicular hematite particles coated with a coating material were obtained in the same way as in Example 160 except for varying the kind of acicular hematite particles to be treated, the pH value before coating, the kind and amount of additives, and the final pH value.

The main producing conditions are shown in Table 44 and various properties of the obtained acicular hematite particles are shown in Table 45.

Examples 164 to 171 Comparative Examples 72 to 76 and Reference Examples 52 to 53

<Production of non-magnetic substrate: Formation of non-magnetic undercoat layer on non-magnetic base film>

By using the acicular hematite particles 9 to 13 and the non-magnetic particles obtained in Example 156 to 163 and Reference Examples 50 to 51, non-magnetic undercoat layers were formed in the same way as in Example 2.

The main producing conditions and various properties of the obtained non-magnetic undercoat layers are shown in Table 46.

Examples 172 to 183. Comparative Examples 77 and 81 and Reference examples 54 to 55

<Production of magnetic recording medium: Formation of magnetic coating film>

Magnetic recording media were produced in the same way as in Example 3 except for varying the kind of non-magnetic substrate and the kind of magnetic particles.

The main producing conditions and various properties of the obtained magnetic recording media are shown in Table 47.

<Kind of acicular goethite particles which substantially uniformly contain aluminum within the particle>

Goethite particles 10 to 11:

Acicular goethite particles 10 and 11 as precursors having properties shown in Table 48 were prepared.

Goethite particles 12 to 14:

Acicular goethite particles 12 to 14 which were subjected to a sintering preventing treatment, were obtained in the same way as in Example 12 except for varying the kind of acicular goethite particles as precursors and the element and amount of sintering preventive.

Various properties of the obtained acicular goethite particles are shown in Table 49.

<Heat treatment>

Goethite particles 15 to 18:

Acicular goethite particles 15 to 18 were obtained in the same way as in Example 12 except for varying the kind of starting particles and the heat-treating conditions.

The main producing conditions are shown in Table 50 and various properties of the obtained acicular goethite particles are shown in Table 51.

<Production of low-density acicular hematite particles which substantially uniformly contain aluminum within the particle>

Hematite Particles 14 to 17:

Low-density acicular hematite particles 14 to 17 were obtained in the same way as in Example 12 except for varying the kind of acicular goethite particles as precursors and the heat-dehydrating temperature and time.

The main producing conditions are shown in Table 52 and various properties of the obtained low-density acicular hematite particles are shown in Table 53.

<Production of high-density acicular hematite particles which substantially uniformly contain aluminum within the particle >Hematite particles 18 to 26:

High-density acicular hematite particles were obtained in the same way as in Example 12 except for varying the kind of particles to be treated and the heat-treating temperature and time.

The main producing conditions are shown in Table 54 and various properties of the obtained high-density acicular hematite particles are shown in Table 55.

Examples 184 to 187 and Reference Examples 56 to 457

<Heat-treatment of acicular hematite particles in alkaline suspension>

The same procedure as defined in Example 12 was conducted except that kind of acicular hematite particles, use or non-use of heat-treatment in alkaline suspension, pH value of slurry, heating temperature and heating time were varied, thereby obtaining acicular hematite particles.

Main production conditions and various properties are shown in Tables 56 and 57.

Example 188

<Surface coating treatment of acicular hematite particles>

After 700 g of the high-density acicular hematite particles obtained in the Example 184 were roughly pulverized by a Nara mill in advance, the obtained particles were charged into 7 liters of pure water and diagglomerated by a homomixer (manufactured by Tokushu-kika Kogyo, CO., Ltd.) for 60 minutes.

The slurry of the acicular hematite particles obtained was then dispersed for 6 hours at an axial rotation frequency of 2000 rpm while being circulated by a horizontal sand grinder (Dispermat SL, manufactured by S.C. Adichem, CO., Ltd.).

The pH value of the obtained slurry containing the acicular hematite particles was adjusted to 4.0 by using a 0.1 N acetic acid aqueous solution. By adding water to the slurry, the concentration of the resultant slurry was adjusted to 45 g/liter. 10 liter of the slurry was re-heated to 60° C., and 500 ml (equivalent to 3.0% by weight (calculated as Al) based on the acicular hematite particles) of a 1.0 mol/liter aqueous aluminum acetate solution was added to the slurry, and the mixture was held for 30 minutes. Thereafter, the pH value of the mixture was adjusted to 7.0 by using aqueous sodium hydroxide solution. After holding the mixture for 30 minutes, the particles were then filtered out, washed with water, dried and, thereby obtaining acicular hematite particles coated with a hydroxide of aluminum.

The main producing conditions and various properties are shown in Tables 58 and 59.

Examples 189 to 191

Acicular hematite particles coated with a coating material were obtained in the same way as in Example 188 except for varying the kind of particles to be treated, the pH value before coating, the kind and amount of additives, and the final pH value.

The main producing conditions are shown in Table 58 and various properties of the obtained acicular hematite particles are shown in Table 59.

Example 192 to 199. Comparative Examples 82 to 986 and Reference examples 58 to 59

<Production of non-magnetic substrate: Formation of non-magnetic undercoat layer on non-magnetic base film>

By using the acicular hematite particles 22 to 26 and the non-magnetic particles obtained in Example 184 to 191, and Reference Examples 56 to 57, non-magnetic undercoat layers were formed in the same way as in Example 2.

The main producing conditions and various properties of the obtained non-magnetic undercoat layers are shown in Table 60.

Examples 200 to 211. Comparative Examples 87 and 91 and Reference examples 60 to 61

<Production of magnetic recording medium: Formation of magnetic coating film>

Magnetic recording media were produced in the same way as in Example 3 except for varying the kind of non-magnetic substrate and the kind of magnetic particles.

The main producing conditions and various properties of the obtained magnetic recording media are shown in Table 61.

	TABLE 1
	Properties of acicular goethite particles
	Production		Geometrical
	method of		standard
	acicular	Average major	deviation of
Kind of	goethite	axial diameter	major axial
Precursor	particles	(μm)	diameter (−)
Precursor	(B)	0.181	1.32
1
Precursor	(C)	0.230	1.31
2
Precursor	(D)	0.251	1.28
3
Precursor	(A)	0.200	1.36
4
Precursor	(E)	0.160	1.35
5
Precursor	(F)	0.271	1.32
6
	Properties of acicular goethite particles
			Geometrical
			standard
		Average minor	deviation of
	Kind of	axial diameter	minor axial	Aspect ratio
	Precursor	(μm)	diameter (−)	(−)
	Precursor	0.0229	1.38	7.9:1
	1
	Precursor	0.0277	1.37	8.3:1
	2
	Precursor	0.0285	1.38	8.8:1
	3
	Precursor	0.0225	1.40	8.9:1
	4
	Precursor	0.0210	1.38	7.6:1
	5
	Precursor	0.0319	1.38	8.5:1
	6
	Properties of acicular goethite particles
		Soluble	Soluble
		sodium salt	sulfate
	BET specific	(calculated	(calculated	pH
Kind of	surface area	as Na)	as SO 4 )	value
Precursor	(m 2 /g)	(ppm)	(ppm)	(−)
Precursor	138.2	678	387	6.8
1
Precursor	98.6	412	1,234	5.3
2
Precursor	86.1	612	823	6.3
3
Precursor	81.8	986	343	8.2
4
Precursor	163.8	789	546	6.6
5
Precursor	65.1	423	751	6.0
6

	TABLE 2
	Kind of	Anti-sintering treatment
Examples and	acicular			Amount
Comparative	goethite		Calcu-	added
Examples	particles	Kind	lated as	(wt %)
Example 16	Particles	Water glass #3	SiO 2	1.0
	obtained in
	Example 1
Example 17	Precursor 1	Water glass #3	SiO 2	2.0
		Phosphoric acid	P	1.0
Example 18	Precursor 2	Phosphoric acid	P	2.5
Example 19	Precursor 3	Sodium hexa-	P	1.8
		metaphosphate
Example 20	Precursor 4	Water glass #3	SiO 2	3.0
Example 21	Precursor 5	Sodium hexa-	P	1.0
		metaphosphate
Example 22	Precursor 6	Water glass #3	SiO 2	3.5
		Phosphoric acid	P	1.5
Comparative	Particles	—	—	—
Example 1	obtained in
	Example 1
Comparative	Particles	—	—	—
Example 2	obtained in
	Example 1
Comparative	Particles	Water glass #3	SiO 2	1.0
Example 3	obtained in
	Example 1
Comparative	Particles	Phosphoric acid	P	1.0
Example 4	obtained in
	Example 1
Comparative	Particles	Phosphoric acid	P	1.5
Example 5	obtained in
	Example 1
Comparative	Precursor 6	Water glass #3	SiO 2	1.5
Example 6
	Examples and
	Comparative	Heat-dehydration
	Examples	Temperature (° C.)	Time (min)
	Example 16	330	60
	Example 17	360	30
	Example 18	340	60
	Example 19	310	120
	Example 20	360	75
	Example 21	330	90
	Example 22	380	60
	Comparative	310	60
	Example 1
	Comparative	340	30
	Example 2
	Comparative	320	60
	Example 3
	Comparative	—	—
	Example 4
	Comparative	350	60
	Example 5
	Comparative	330	60
	Example 6

	TABLE 3
	Properties of low-density acicular hematite particles
	Average	Geometrical	Average	Geometrical
Examples	major	standard	minor	standard
and	axial	deviation of	axial	deviation of
Comparative	diameter	major axial	diameter	minor axial
Examples	(μm)	diameter (−)	(μm)	diameter (−)
Example 16	0.142	1.37	0.0192	1.38
Example 17	0.153	1.35	0.0196	1.38
Example 18	0.199	1.34	0.0246	1.38
Example 19	0.213	1.31	0.0250	1.38
Example 20	0.168	1.39	0.0195	1.41
Example 21	0.126	1.36	0.0172	1.37
Example 22	0.230	1.35	0.0291	1.39
Comparative	0.139	1.37	0.0194	1.38
Example 1
Comparative	0.138	1.37	0.0197	1.38
Example 2
Comparative	0.141	1.36	0.0191	1.38
Example 3
Comparative	—	—	—	—
Example 4
Comparative	0.142	1.37	0.0191	1.38
Example 5
Comparative	0.230	1.35	0.0291	1.38
Example 6
Examples	Properties of low-density acicular hematite particles
and	Aspect			S BET /S TEM
Comparative	ratio	S BET	S TEM	value
Examples	(−)	(m 2 /g)	(m 2 /g)	(−)
Example 16	7.4:1	151.6	42.8	3.54
Example 17	7.8:1	179.4	41.8	4.30
Example 18	8.1:1	143.0	33.2	4.31
Example 19	8.5:1	123.7	32.6	3.80
Example 20	8.6:1	143.7	41.7	3.44
Example 21	7.3:1	196.6	47.8	4.12
Example 22	7.9:1	110.5	28.1	3.93
Comparative	7.2:1	146.8	42.4	3.46
Example 1
Comparative	7.0:1	130.6	41.8	3.12
Example 2
Comparative	7.4:1	168.6	43.0	3.92
Example 3
Comparative	—	—	—	—
Example 4
Comparative	7.4:1	163.2	43.0	3.80
Example 5
Comparative	7.9:1	91.2	28.1	3.24
Example 6
	Properties of low-density acicular hematite particles
		Soluble
		sodium	Soluble
		salt	sulfate
Examples	Amount of sintering	(calcu-	(calcu-
and	preventive	lated as	lated as	pH
Comparative	Calcu-	Content	Na)	SO 4 )	value
Examples	lated as	(wt %)	(ppm)	(ppm)	(−)
Example 16	SiO 2	1.09	1,653	925	6.0
Example 17	SiO 2	2.77	1,561	1,110	5.9
	P	1.07
Example 18	P	2.73	1,356	1,610	6.1
Example 19	P	1.95	1,282	1,583	6.6
Example 20	SiO 2	3.21	2,582	632	7.8
Example 21	P	1.10	1,376	652	5.6
Example 22	SiO 2	3.81	1,168	3,265	4.8
	P	1.64
Comparative	—	—	726	586	5.7
Example 1
Comparative	—	—	832	612	5.6
Example 2
Comparative	SiO 2	1.09	1,231	1,010	5.8
Example 3
Comparative	—	—	2,663	713	7.5
Example 4
Comparative	P	1.64	1,183	1,121	6.1
Example 5
Comparative	SiO 2	1.63	1,216	892	6.4
Example 6

	TABLE 4
		Kind of
		low-density	Heat treatment for high
	Examples and	acicular	densification
	Comparative	hematite	Temperature	Time
	Examples	particles	(° C.)	(min)
	Example 23	Example 16	680	60
	Example 24	Example 17	690	30
	Example 25	Example 18	720	20
	Example 26	Example 19	660	60
	Example 27	Example 20	640	30
	Example 28	Example 21	680	60
	Example 29	Example 22	730	30
	Comparative	Comparative	680	20
	Example 7	Example 2
	Comparative	Comparative	670	30
	Example 8	Example 4
	Comparative	Comparative	450	60
	Example 9	Example 5
	Comparative	Comparative	850	20
	Example 10	Example 6
	Comparative	Comparative	680	30
	Example 11	Example 6

	TABLE 5
	Properties of high-density acicular hematite particles
	Average	Geometrical	Average	Geometrical
	major	standard	minor	standard
Examples and	axial	deviation of	axial	deviation of
Comparative	diameter	major axial	diameter	minor axial
Examples	(μm)	diameter (−)	(μm)	diameter (−)
Example 23	0.136	1.37	0.0195	1.39
Example 24	0.152	1.36	0.0200	1.38
Example 25	0.197	1.35	0.0249	1.39
Example 26	0.211	1.33	0.0252	1.38
Example 27	0.166	1.41	0.0195	1.42
Example 28	0.126	1.36	0.0173	1.38
Example 29	0.231	1.36	0.0296	1.40
Comparative	0.070	1.83	0.0330	1.68
Example 7
Comparative	0.126	1.64	0.0228	1.61
Example 8
Comparative	0.142	1.37	0.0191	1.39
Example 9
Comparative	0.100	1.71	0.0263	1.38
Example 10
Comparative	0.229	1.36	0.0292	1.38
Example 11
Examples	Properties of high-density acicular hematite particles
and	Aspect			S BET /S TEM
Comparative	ratio	S BET	S TEM	value
Examples	(−)	(m 2 /g)	(m 2 /g)	(−)
Example 23	7.0:1	50.4	42.3	1.19
Example 24	7.6:1	52.5	41.0	1.28
Example 25	7.9:1	41.4	32.8	1.26
Example 26	8.4:1	41.4	32.3	1.28
Example 27	8.5:1	53.6	41.8	1.28
Example 28	7.3:1	55.1	47.5	1.16
Example 29	7.8:1	42.9	27.7	1.55
Comparative	2.1:1	14.6	28.8	0.51
Example 7
Comparative	5.5:1	26.7	36.8	0.73
Example 8
Comparative	7.4:1	111.6	43.0	2.60
Example 9
Comparative	3.8:1	25.6	33.1	0.77
Example 10
Comparative	7.8:1	41.3	28.0	1.47
Example 11
	Properties of high-density acicular hematite particles
		Soluble
		sodium	Soluble
		salt	sulfate
	Amount of sintering	(calcu-	(calcu-
Examples and	preventive	lated as	lated as	pH
Comparative	Calcu-	Content	Na)	SO 4 )	value
Examples	lated as	(wt %)	(ppm)	(ppm)	(−)
Example 23	SiO 2	1.08	2,382	2,652	5.5
Example 24	SiO 2	2.76	2,265	2,783	4.9
	P	1.07
Example 25	P	2.73	1,982	3,326	5.6
Example 26	P	1.95	1,765	3,273	5.1
Example 27	SiO 2	3.22	3,039	1,076	7.5
Example 28	P	1.11	2,162	2,765	4.6
Example 29	SiO 2	3.82	1,963	2,838	4.3
	P	1.65
Comparative	—	—	1,365	1,863	5.8
Example 7
Comparative	—	—	1,462	1,921	6.1
Example 8
Comparative	P	1.64	1,763	2,563	5.1
Example 9
Comparative	SiO 2	1.63	2,762	3,362	4.8
Example 10
Comparative	SiO 2	1.64	2,562	2,863	5.5
Example 11

	TABLE 6
		Kind of high-
	Examples and	density acicular	Wet-pulverization
	Comparative	hematite	Use or	Residue on
	Examples	particles	non-use	sieve (wt %)
	Example 30	Example 23	used	0
	Example 31	Example 24	used	0
	Example 32	Example 25	used	0
	Example 33	Example 26	used	0
	Example 34	Example 27	used	0
	Example 35	Example 28	used	0
	Example 36	Example 29	used	0
	Comparative	Particles	used	0
	Example 12	obtained in
		Example 1
	Comparative	Comparative	used	0
	Example 13	Example 11
Examples	Acid-dissolving treatment
and		Concent-	Tempera-		pH
Comparative	Kind of	ration	ture	Time	value
Examples	acid	(N)	(° C.)	(Hr)	(−)
Example 30	Sulfuric	1.4	90	3.0	0.68
	acid
Example 31	Sulfuric	1.5	85	5.5	0.56
	acid
Example 32	Sulfuric	2.0	90	5.0	0.32
	acid
Example 33	Sulfuric	1.5	75	7.0	0.74
	acid
Example 34	Sulfuric	1.2	70	2.0	0.81
	acid
Example 35	Sulfuric	1.3	90	1.0	0.91
	acid
Example 36	Sulfuric	1.5	85	8.0	0.70
	acid
Comparative	Sulfuric	3.2E−05	80	5.0	4.5
Example 12	acid
Comparative	Sulfuric	3.8E−04	80	7.0	3.8
Example 13	acid

	TABLE 7
	Properties of acicular hematite particles washed
	with water after acid-dissolving treatment
		Geometrical		Geometrical
	Average	standard	Average	standard
Examples	major	deviation of	minor	deviation of
and	axial	major axial	axial	minor axial
Comparative	diameter	diameter	diameter	diameter
Examples	(μm)	(−)	(μm)	(−)
Example 30	0.130	1.34	0.0188	1.33
Example 31	0.143	1.32	0.0181	1.32
Example 32	0.175	1.31	0.0216	1.33
Example 33	0.200	1.31	0.0235	1.31
Example 34	0.164	1.39	0.0193	1.34
Example 35	0.126	1.35	0.0173	1.32
Example 36	0.193	1.32	0.0238	1.34
Comparative	0.136	1.37	0.0195	1.38
Example 12
Comparative	0.230	1.36	0.0296	1.38
Example 13
	Properties of acicular hematite particles washed
Examples	with water after acid-dissolving treatment
and	Aspect	Amount			S BET /S TEM
Comparative	ratio	dissolved	S BET	S TEM	value
Examples	(−)	(wt %)	(m 2 /g)	(m 2 /g)	(−)
Example 30	6.9:1	26.5	55.4	43.9	1.26
Example 31	7.9:1	35.6	58.1	45.2	1.29
Example 32	8.1:1	41.2	46.6	37.8	1.23
Example 33	8.5:1	21.6	43.2	34.7	1.25
Example 34	8.5:1	10.2	57.8	42.2	1.37
Example 35	7.3:1	6.8	56.2	47.5	1.18
Example 36	8.1:1	49.6	46.6	34.3	1.36
Comparative	7.0:1	0.3	51.0	42.3	1.21
Example 12
Comparative	7.8:1	1.2	41.8	27.7	1.51
Example 13
	Properties of acicular hematite particles washed
	with water after acid-dissolving treatment
		Soluble sodium
	Examples	salt	Soluble sulfate
	and	(calculated as	(calculated as
	Comparative	Na)	SO 4 )	pH value
	Examples	(ppm)	(ppm)	(−)
	Example 30	126	326	4.6
	Example 31	141	412	5.3
	Example 32	98	512	5.1
	Example 33	121	286	5.3
	Example 34	68	211	5.1
	Example 35	111	268	4.6
	Example 36	123	312	4.5
	Comparative	368	512	5.0
	Example 12
	Comparative	346	536	4.8
	Example 13

TABLE 8
	Kind of
	acicular hematite	Heat treatment in aqueous
Examples	particles	alkali solution
and	subjected to acid-	pH
Reference	dissolving	value	Temperature	Time
Examples	treatment	(−)	(° C.)	(min)
Example 37	Example 30	13.5	90	180
Example 38	Example 31	13.8	85	120
Example 39	Example 32	13.1	95	180
Example 40	Example 33	13.5	98	240
Example 41	Example 34	13.3	80	120
Example 42	Example 35	13.5	90	300
Example 43	Example 36	13.6	85	90
Reference	Example 30	9.5	90	180
Example 1
Reference	Example 30	13.1	45	180
Example 2

	TABLE 9
	Properties of acicular hematite particles washed
	with water after heat-treatment in aqueous alkali solution
	Average	Geometrical	Average	Geometrical
Examples	major	standard	minor	standard
and	axial	deviation of	axial	deviation of
Reference	diameter	major axial	diameter	minor axial
Examples	(μm)	diameter (−)	(μm)	diameter (−)
Example 37	0.130	1.34	0.0188	1.33
Example 38	0.143	1.33	0.0180	1.32
Example 39	0.174	1.31	0.0216	1.33
Example 40	0.201	1.31	0.0235	1.31
Example 41	0.163	1.38	0.0192	1.34
Example 42	0.126	1.35	0.0173	1.33
Example 43	0.194	1.32	0.0238	1.33
Reference	0.130	1.34	0.0188	1.38
Example 1
Reference	0.130	1.34	0.0188	1.39
Example 2
	Properties of acicular hematite particles washed
Examples	with water after heat-treatment in aqueous alkali solution
and				S BET /S TEM
Reference	Aspect	S BET	S TEM	value
Examples	ratio (−)	(m 2 /g)	(m 2 /g)	(−)
Example 37	6.9:1	55.2	43.9	1.26
Example 38	7.9:1	58.3	45.4	1.28
Example 39	8.1:1	46.7	37.8	1.23
Example 40	8.6:1	43.4	34.6	1.25
Example 41	8.5:1	58.1	42.4	1.37
Example 42	7.3:1	56.6	47.5	1.19
Example 43	8.2:1	47.0	34.3	1.37
Reference	6.9:1	55.5	43.9	1.26
Example 1
Reference	6.9:1	55.6	43.9	1.27
Example 2
	Properties of acicular hematite particles washed
	with water after heat-treatment in aqueous alkali solution
Examples	Soluble sodium	Soluble sulfate
and	salt	(calculated as
Reference	(calculated as Na)	SO 4 )	pH value
Examples	(ppm)	(ppm)	(−)
Example 37	116	13	9.1
Example 38	98	6	9.3
Example 39	86	12	8.9
Example 40	68	31	8.7
Example 41	72	1	9.3
Example 42	96	3	9.0
Example 43	121	10	9.5
Reference	116	286	7.6
Example 1
Reference	106	182	7.8
Example 2

	TABLE 10
	Surface treatment
	Kind of acicular		Amount
	hematite		added
	particles heat-	Kind of	(calculated
	treated in	surface	as Al or
	aqueous alkali	treatment	SiO 2 )
Examples	solution	material	(wt %)
Example 44	Example 37	Sodium aluminate	1.0
Example 45	Example 38	Water glass #3	0.5
Example 46	Example 39	Aluminum sulfate	2.0
		Water glass #3	1.0
Example 47	Example 40	Aluminum acetate	5.0
Example 48	Example 41	Sodium aluminate	1.0
		Colloidal silica	1.5
Example 49	Example 42	Colloidal silica	3.0
Example 50	Example 43	Sodium aluminate	12.0
	Coating material
	Coating amount
	(calculated as Al or
	SiO 2 )
	Examples	Kind*	(wt %)
	Example 44	A	0.99
	Example 45	S	0.49
	Example 46	A	1.95
		S	0.98
	Example 47	A	4.78
	Example 48	A	0.98
		S	1.47
	Example 49	S	2.90
	Example 50	A	10.75
	Note
	*“A” represents a hydroxide of aluminum.
	“S” represents an oxide of silicon.

	TABLE 11
	Properties of acicular hematite particles washed
	with water after surface treatment
		Geometrical		Geometrical
	Average	standard	Average	standard
	major	deviation of	minor	deviation of
	axial	major axial	axial	minor axial
	diameter	diameter	diameter	diameter
Examples	(μm)	(−)	(μm)	(−)
Example 44	0.130	1.34	0.0189	1.33
Example 45	0.143	1.33	0.0180	1.32
Example 46	0.174	1.31	0.0216	1.33
Example 47	0.200	1.31	0.0235	1.31
Example 48	0.164	1.38	0.0192	1.34
Example 49	0.126	1.35	0.0173	1.33
Example 50	0.194	1.32	0.0239	1.32
	Properties of acicular hematite particles washed
	with water after surface treatment
					S BET /S TEM
		Aspect	S BET	S TEM	value
	Examples	ratio (−)	(m 2 /g)	(m 2 /g)	(−)
	Example 44	6.9:1	55.8	43.7	1.28
	Example 45	7.9:1	58.0	45.4	1.28
	Example 46	8.1:1	47.0	37.8	1.24
	Example 47	8.5:1	43.5	34.7	1.26
	Example 48	8.5:1	57.9	42.4	1.37
	Example 49	7.3:1	56.0	47.5	1.18
	Example 50	8.1:1	45.5	34.2	1.33
	Properties of acicular hematite particles washed
	with water after surface treatment
		Soluble sodium
		salt	Soluble sulfate
		(calculated as	(calculated as
		Na)	SO 4 )	pH value
	Examples	(ppm)	(ppm)	(−)
	Example 44	86	2	9.3
	Example 45	63	3	9.1
	Example 46	52	5	8.9
	Example 47	32	8	9.0
	Example 48	16	10	8.8
	Example 49	72	12	9.1
	Example 50	68	6	9.4

	TABLE 12
	Production of non-magnetic
	coating composition	Non-magnetic
		Kind of	Weight ratio	coating
		acicular	of particles	composition
		hematite	to resin	Viscosity
	Examples	particles	(−)	(cP)
	Example 51	Example 37	5.0	358
	Example 52	Example 38	5.0	384
	Example 53	Example 39	5.0	384
	Example 54	Example 40	5.0	205
	Example 55	Example 41	5.0	358
	Example 56	Example 42	5.0	435
	Example 57	Example 43	5.0	333
	Example 58	Example 44	5.0	333
	Example 59	Example 45	5.0	310
	Example 60	Example 46	5.0	384
	Example 61	Example 47	5.0	230
	Example 62	Example 48	5.0	218
	Example 63	Example 49	5.0	192
	Example 64	Example 50	5.0	179
	Properties of non-magnetic undercoat layer
				Surface	Young's
				roughness	modulus
		Thickness	Gloss	Ra	(relative
	Examples	(μm)	(%)	(nm)	value)
	Example 51	3.5	210	6.4	122
	Example 52	3.5	198	6.8	121
	Example 53	3.5	206	6.4	126
	Example 54	3.3	201	6.8	131
	Example 55	3.4	215	6.0	124
	Example 56	3.5	218	5.6	122
	Example 57	3.5	215	5.7	133
	Example 58	3.4	211	5.6	125
	Example 59	3.3	206	6.0	126
	Example 60	3.4	210	6.2	129
	Example 61	3.3	209	6.8	139
	Example 62	3.5	219	6.3	128
	Example 63	3.5	223	5.0	124
	Example 64	3.5	218	6.8	136

	TABLE 13
	Production of non-magnetic
	Comparative	coating composition	Non-magnetic
	Examples	Kind of	Weight ratio	coating
	and	acicular	of particles	composition
	Reference	hematite	to resin	Viscosity
	Examples	particles	(−)	(cP)
	Comparative	Comparative	5.0	8,320
	Example 14	Example 1
	Comparative	Comparative	5.0	10,880
	Example 15	Example 3
	Comparative	Comparative	5.0	435
	Example 16	Example 7
	Comparative	Comparative	5.0	563
	Example 17	Example 8
	Comparative	Comparative	5.0	5,760
	Example 18	Example 9
	Comparative	Comparative	5.0	230
	Example 19	Example 10
	Comparative	Comparative	5.0	384
	Example 20	Example 11
	Comparative	Comparative	5.0	435
	Example 21	Example 12
	Comparative	Comparative	5.0	410
	Example 22	Example 13
	Reference	Example 30	5.0	384
	Example 3
	Reference	Example 31	5.0	410
	Example 4
	Reference	Example 32	5.0	435
	Example 5
	Reference	Example 33	5.0	205
	Example 6
	Reference	Example 34	5.0	384
	Example 7
	Reference	Example 35	5.0	358
	Example 8
	Reference	Example 36	5.0	410
	Example 9
	Reference	Reference	5.0	384
	Example 10	Example 1
	Reference	Reference	5.0	358
	Example 11	Example 2
	Comparative	Properties of non-magnetic undercoat layer
	Examples			Surface	Young's
	and			roughness	modulus
	Reference	Thickness	Gloss	Ra	(relative
	Examples	(μm)	(%)	(nm)	value)
	Comparative	3.6	128	21.6	108
	Example 14
	Comparative	3.8	86	42.6	96
	Example 15
	Comparative	3.5	141	19.8	109
	Example 16
	Comparative	3.5	176	12.4	113
	Example 17
	Comparative	3.6	121	27.6	105
	Example 18
	Comparative	3.5	159	16.6	109
	Example 19
	Comparative	3.4	186	9.6	116
	Example 20
	Comparative	3.5	188	9.3	118
	Example 21
	Comparative	3.5	176	13.1	115
	Example 22
	Reference	3.5	203	6.8	121
	Example 3
	Reference	3.4	193	7.0	122
	Example 4
	Reference	3.3	201	6.8	125
	Example 5
	Reference	3.4	196	7.6	135
	Example 6
	Reference	3.3	211	6.4	123
	Example 7
	Reference	3.5	216	5.8	121
	Example 8
	Reference	3.4	209	6.1	131
	Example 9
	Reference	3.5	204	6.8	121
	Example 10
	Reference	3.5	206	6.6	121
	Example 11

	TABLE 14
	Properties of acicular magnetic metal particles
	containing iron as main component
			Geometrical
	Average	Average	standard
	major	minor	deviation of
Kind of	axial	axial	major axial	Aspect
magnetic	diameter	diameter	diameter	ratio
particles	(μm)	(μm)	(−)	(−)
Magnetic metal	0.110	0.0150	1.36	7.3:1
particles (a)
Magnetic metal	0.098	0.0134	1.35	7.3:1
particles (b)
Magnetic metal	0.101	0.0144	1.38	7.0:1
particles (c)
Magnetic metal	0.125	0.0184	1.35	6.8:1
particles (d)
Magnetic metal	0.127	0.0177	1.39	7.2:1
particles (e)
Magnetic metal	0.105	0.0148	1.36	7.1:1
particles (f)
	Properties of acicular magnetic metal particles
	containing iron as main component
	Kind of	Coercive	Saturation
	magnetic	force (Hc)	magnetization	pH value
	particles	(Oe)	(emu/g)	(−)
	Magnetic metal	1,915	131.6	9.5
	particles (a)
	Magnetic metal	1,938	130.5	10.1
	particles (b)
	Magnetic metal	2,065	128.9	10.0
	particles (c)
	Magnetic metal	1,896	130.8	9.8
	particles (d)
	Magnetic metal	1,915	135.6	9.5
	particles (e)
	Magnetic metal	1,680	128.3	9.9
	particles (f)
	Properties of acicular magnetic metal particles
	containing iron as main component
	Content of Al
	Kind of	Central	Surface	Surface
	magnetic	Portion	portion	coating layer
	particles	(wt %)	(wt %)	(wt %)
	Magnetic metal	2.61	1.36	0.01
	particles (a)
	Magnetic metal	1.32	2.84	0.01
	particles (b)
	Magnetic metal	1.38	2.65	0.78
	particles (c)
	Magnetic metal	0.01	0.01	0.01
	particles (d)
	Magnetic metal	0.01	0.01	0.01
	particles (e)
	Magnetic metal	0.01	0.01	0.01
	particles (f)
	Properties of acicular magnetic metal particles
Kind of	containing iron as main component
magnetic	Content of Nd	Resin adsorption
particles	(wt %)	(%)
Magnetic metal	0.01	72.5
particles (a)
Magnetic metal	0.36	80.1
particles (b)
Magnetic metal	2.78	83.6
particles (c)
Magnetic metal	0.01	57.6
particles (d)
Magnetic metal	0.01	56.5
particles (e)
Magnetic metal	0.01	58.1
particles (f)

	TABLE 15
	Production of magnetic recording medium
				Weight
		Kind of		ratio of
		non-		particles
		magnetic	Kind of magnetic	to resin
	Examples	substrate	particles	(−)
	Example 65	Example 51	Particles used in	5.0
			Example 3
	Example 66	Example 52	Magnetic metal	5.0
			particles (e)
	Example 67	Example 53	Magnetic metal	5.0
			particles (e)
	Example 68	Example 54	Magnetic metal	5.0
			particles (e)
	Example 69	Example 55	Magnetic metal	5.0
			particles (f)
	Example 70	Example 56	Magnetic metal	5.0
			particles (f)
	Example 71	Example 57	Magnetic metal	5.0
			particles (f)
	Example 72	Example 58	Particles used in	5.0
			Example 3
	Example 73	Example 59	Magnetic metal	5.0
			particles (e)
	Example 74	Example 60	Magnetic metal	5.0
			particles (e)
	Example 75	Example 61	Magnetic metal	5.0
			particles (e)
	Example 76	Example 62	Magnetic metal	5.0
			particles (f)
	Example 77	Example 63	Magnetic metal	5.0
			particles (f)
	Example 78	Example 64	Magnetic metal	5.0
			particles (f)
	Properties of magnetic recording medium
		Thickness
		of magnetic	Coercive	Br/Bm
		layer	force (Hc)	value	Gloss
	Examples	(μm)	(Oe)	(−)	(%)
	Example 65	1.1	1,975	0.87	225
	Example 66	1.1	1,986	0.87	219
	Example 67	1.1	1,990	0.88	221
	Example 68	1.0	1,982	0.88	215
	Example 69	1.1	1,781	0.88	238
	Example 70	1.0	1,785	0.88	241
	Example 71	1.1	1,780	0.89	231
	Example 72	1.0	1,978	0.88	231
	Example 73	1.1	1,980	0.87	230
	Example 74	1.0	1,983	0.87	228
	Example 75	1.1	1,991	0.88	233
	Example 76	1.0	1,780	0.89	240
	Example 77	1.1	1,778	0.89	243
	Example 78	1.0	1,785	0.89	246
	Properties of magnetic recording medium
	Corrosion resistance
			Linear	Percen-
			absorp-	tage of	Percen-
	Surface	Young's	tion	change in	tage of
	rough-	modulus	coeffi-	coercive	change
	ness Ra	(relative	cient	force	in Bm
Examples	(nm)	value)	(μm −1 )	(%)	(%)
Example 65	6.2	124	1.22	4.6	4.2
Example 66	7.0	126	1.24	3.8	4.2
Example 67	6.6	125	1.25	3.2	3.9
Example 68	6.8	126	1.27	2.8	3.1
Example 69	6.2	127	1.24	3.1	2.8
Example 70	6.0	126	1.24	4.1	4.9
Example 71	6.2	128	1.23	3.9	3.5
Example 72	5.8	123	1.26	2.6	2.8
Example 73	6.0	126	1.24	1.6	1.3
Example 74	6.5	130	1.23	2.1	1.5
Example 75	5.6	143	1.28	1.6	3.2
Example 76	6.2	130	1.23	2.6	1.6
Example 77	5.8	126	1.22	1.1	2.8
Example 78	5.6	136	1.26	1.3	2.6

	TABLE 16
	Production of magnetic recording medium
	Comparative			Weight
	Examples			ratio of
	and	Kind of non-		particles
	Reference	magnetic	Kind of magnetic	to resin
	Examples	substrate	particles	(−)
	Comparative	Comparative	Magnetic metal	5.0
	Example 23	Example 14	particles (e)
	Comparative	Comparative	Magnetic metal	5.0
	Example 24	Example 15	particles (e)
	Comparative	Comparative	Magnetic metal	5.0
	Example 25	Example 16	particles (e)
	Comparative	Comparative	Magnetic metal	5.0
	Example 26	Example 17	particles (e)
	Comparative	Comparative	Magnetic metal	5.0
	Example 27	Example 18	particles (e)
	Comparative	Comparative	Magnetic metal	5.0
	Example 28	Example 19	particles (f)
	Comparative	Comparative	Magnetic metal	5.0
	Example 29	Example 20	particles (f)
	Comparative	Comparative	Magnetic metal	5.0
	Example 30	Example 21	particles (f)
	Comparative	Comparative	Magnetic metal	5.0
	Example 31	Example 22	particles (f)
	Reference	Reference	Magnetic metal	5.0
	Example 12	Example 3	particles (e)
	Reference	Reference	Magnetic metal	5.0
	Example 13	Example 4	particles (e)
	Reference	Reference	Magnetic metal	5.0
	Example 14	Example 5	particles (e)
	Reference	Reference	Magnetic metal	5.0
	Example 15	Example 6	particles (e)
	Reference	Reference	Magnetic metal	5.0
	Example 16	Example 7	particles (e)
	Reference	Reference	Magnetic metal	5.0
	Example 17	Example 8	particles (f)
	Reference	Reference	Magnetic metal	5.0
	Example 18	Example 9	particles (f)
	Reference	Reference	Magnetic metal	5.0
	Example 19	Example 10	particles (f)
	Reference	Reference	Magnetic metal	5.0
	Example 20	Example 11	particles (f)
	Properties of magnetic recording medium
	Comparative	Thickness
	Examples	of
	and	magnetic	Coercive	Br/Bm
	Reference	layer	force (Hc)	value	Gloss
	Examples	(μm)	(Oe)	(−)	(%)
	Comparative	1.3	1,958	0.74	136
	Example 23
	Comparative	1.2	1,941	0.78	121
	Example 24
	Comparative	1.1	1,975	0.83	171
	Example 25
	Comparative	1.0	1,969	0.84	186
	Example 26
	Comparative	1.3	1,953	0.76	158
	Example 27
	Comparative	1.1	1,775	0.82	183
	Example 28
	Comparative	1.1	1,768	0.85	194
	Example 29
	Comparative	1.0	1,732	0.85	193
	Example 30
	Comparative	1.1	1,763	0.84	180
	Example 31
	Reference	1.1	1,980	0.87	217
	Example 12
	Reference	1.1	1,983	0.87	212
	Example 13
	Reference	1.0	1,985	0.87	213
	Example 14
	Reference	1.1	1,982	0.87	210
	Example 15
	Reference	1.0	1,988	0.88	226
	Example 16
	Reference	1.1	1,780	0.88	226
	Example 17
	Reference	1.1	1,782	0.88	214
	Example 18
	Reference	1.0	1,775	0.88	211
	Example 19
	Reference	1.0	1,781	0.88	216
	Example 20
Comparative	Properties of magnetic recording medium
Examples			Linear
and	Surface		absorption
Reference	roughness Ra	Young's modulus	coefficient
Examples	(nm)	(relative value)	(μm −1 )
Comparative	18.8	116	1.03
Example 23
Comparative	26.5	100	1.10
Example 24
Comparative	14.4	113	1.18
Example 25
Comparative	10.2	116	1.16
Example 26
Comparative	16.6	109	1.06
Example 27
Comparative	11.2	114	1.12
Example 28
Comparative	8.2	119	1.19
Example 29
Comparative	8.4	119	1.16
Example 30
Comparative	10.2	117	1.13
Example 31
Reference	6.8	123	1.23
Example 12
Reference	6.8	125	1.24
Example 13
Reference	6.4	128	1.22
Example 14
Reference	7.2	136	1.28
Example 15
Reference	6.8	125	1.26
Example 16
Reference	6.0	124	1.24
Example 17
Reference	5.9	135	1.22
Example 18
Reference	6.3	124	1.23
Example 19
Reference	6.2	124	1.23
Example 20
	Comparative	Properties of magnetic recording medium
	Examples	Corrosion resistance
	and	Percentage of change	Percentage of change
	Reference	in coercive force	in Bm
	Examples	(%)	(%)
	Comparative	46.3	36.5
	Example 23
	Comparative	53.6	32.1
	Example 24
	Comparative	46.0	36.8
	Example 25
	Comparative	39.3	31.5
	Example 26
	Comparative	39.8	29.6
	Example 27
	Comparative	41.6	32.0
	Example 28
	Comparative	23.8	21.8
	Example 29
	Comparative	19.6	17.8
	Example 30
	Comparative	23.9	25.6
	Example 31
	Reference	15.6	18.2
	Example 12
	Reference	18.2	17.6
	Example 13
	Reference	18.6	18.8
	Example 14
	Reference	14.8	16.1
	Example 15
	Reference	11.8	13.6
	Example 16
	Reference	15.1	16.8
	Example 17
	Reference	12.9	18.1
	Example 18
	Reference	12.1	14.6
	Example 19
	Reference	11.6	10.9
	Example 20

	TABLE 17
	Production of magnetic recording medium
				Weight
				ratio of
		Kind of non-		particles
		magnetic	Kind of magnetic	to resin
	Examples	substrate	particles	(−)
	Example 79	Example 51	Particles used in	5.0
			Example 4
	Example 80	Example 52	Magnetic metal	5.0
			particles (a)
	Example 81	Example 53	Magnetic metal	5.0
			particles (a)
	Example 82	Example 54	Magnetic metal	5.0
			particles (b)
	Example 83	Example 55	Magnetic metal	5.0
			particles (b)
	Example 84	Example 56	Magnetic metal	5.0
			particles (c)
	Example 85	Example 57	Magnetic metal	5.0
			particles (c)
	Example 86	Example 58	Particles used in	5.0
			Example 4
	Example 87	Example 59	Magnetic metal	5.0
			particles (a)
	Example 88	Example 60	Magnetic metal	5.0
			particles (a)
	Example 89	Example 61	Magnetic metal	5.0
			particles (b)
	Example 90	Example 62	Magnetic metal	5.0
			particles (b)
	Example 91	Example 63	Magnetic metal	5.0
			particles (c)
	Example 92	Example 64	Magnetic metal	5.0
			particles (c)
	Properties of magnetic recording medium
		Thickness
		of
		magnetic	Coercive	Br/Bm
		layer	force (Hc)	value	Gloss
	Examples	(μm)	(Oe)	(−)	(%)
	Example 79	1.1	1,976	0.87	226
	Example 80	1.0	1,981	0.87	221
	Example 81	1.1	1,986	0.87	223
	Example 82	1.0	2,000	0.87	214
	Example 83	1.0	1,997	0.88	235
	Example 84	1.1	2,138	0.88	240
	Example 85	1.0	2,145	0.88	233
	Example 86	1.1	1,978	0.88	230
	Example 87	1.1	1,990	0.88	235
	Example 88	1.0	1,984	0.88	226
	Example 89	1.0	1,998	0.89	234
	Example 90	1.0	2,006	0.89	241
	Example 91	1.1	2,135	0.90	245
	Example 92	1.0	2,141	0.89	245
	Properties of magnetic recording medium
			Linear
			absorp-
	Surface	Young's	tion	Durability
	rough-	modulus	coeffi-	Running
	ness Ra	(relative	cient	time	Scratch
Examples	(nm)	value)	(μm −1 )	(min)	resistance
Example 79	6.0	126	1.23	29.8	A
Example 80	6.8	128	1.24	22.6	B
Example 81	6.4	128	1.25	23.2	B
Example 82	6.8	130	1.27	26.5	A
Example 83	6.4	131	1.26	23.8	B
Example 84	6.0	131	1.25	≧30	A
Example 85	6.3	130	1.24	≧30	A
Example 86	6.1	127	1.24	≧30	A
Example 87	6.0	130	1.25	27.2	B
Example 88	6.8	132	1.25	28.8	A
Example 89	6.2	135	1.27	29.6	A
Example 90	5.8	136	1.27	≧30	A
Example 91	5.7	133	1.25	≧30	A
Example 92	5.7	133	1.24	≧30	A
	Properties of magnetic recording medium
	Corrosion resistance
		Percentage of change	Percentage of change
	Examples	in coercive force (%)	in B m (%)
	Example 79	3.6	3.2
	Example 80	4.9	4.8
	Example 81	4.2	4.3
	Example 82	2.1	2.5
	Example 83	3.1	2.6
	Example 84	3.6	3.8
	Example 85	3.5	3.7
	Example 86	3.3	3.0
	Example 87	3.9	2.8
	Example 88	3.7	3.0
	Example 89	1.6	2.8
	Example 90	1.3	2.6
	Example 91	3.1	4.0
	Example 92	2.8	3.8

	TABLE 18
	Production of magnetic recording medium
	Comparative			Weight
	Examples			ratio of
	and	Kind of non-		particles
	Reference	magnetic	Kind of magnetic	to resin
	Examples	substrate	particles	(−)
	Comparative	Comparative	Magnetic metal	5.0
	Example 32	Example 14	particles (a)
	Comparative	Comparative	Magnetic metal	5.0
	Example 33	Example 15	particles (a)
	Comparative	Comparative	Magnetic metal	5.0
	Example 34	Example 16	particles (a)
	Comparative	Comparative	Magnetic metal	5.0
	Example 35	Example 17	particles (a)
	Comparative	Comparative	Magnetic metal	5.0
	Example 36	Example 18	particles (a)
	Comparative	Comparative	Magnetic metal	5.0
	Example 37	Example 19	particles (d)
	Comparative	Comparative	Magnetic metal	5.0
	Example 38	Example 20	particles (d)
	Comparative	Comparative	Magnetic metal	5.0
	Example 39	Example 21	particles (d)
	Comparative	Comparative	Magnetic metal	5.0
	Example 40	Example 22	particles (d)
	Reference	Reference	Magnetic metal	5.0
	Example 21	Example 3	particles (a)
	Reference	Reference	Magnetic metal	5.0
	Example 22	Example 4	particles (a)
	Reference	Reference	Magnetic metal	5.0
	Example 23	Example 5	particles (a)
	Reference	Reference	Magnetic metal	5.0
	Example 24	Example 6	particles (a)
	Reference	Reference	Magnetic metal	5.0
	Example 25	Example 7	particles (a)
	Reference	Reference	Magnetic metal	5.0
	Example 26	Example 8	particles (d)
	Reference	Reference	Magnetic metal	5.0
	Example 27	Example 9	particles (d)
	Reference	Reference	Magnetic metal	5.0
	Example 28	Example 10	particles (d)
	Reference	Reference	Magnetic metal	5.0
	Example 29	Example 11	particles (d)
	Properties of magnetic recording medium
	Comparative	Thickness
	Examples	of
	and	magnetic	Coercive	Br/Bm
	Reference	layer	force (Hc)	value	Gloss
	Examples	(μm)	(Oe)	(−)	(%)
	Comparative	1.2	1,957	0.78	135
	Example 32
	Comparative	1.3	1,961	0.76	118
	Example 33
	Comparative	1.1	1,968	0.82	173
	Example 34
	Comparative	1.1	1,963	0.84	182
	Example 35
	Comparative	1.2	1,954	0.78	161
	Example 36
	Comparative	1.1	1,863	0.84	178
	Example 37
	Comparative	1.0	1,871	0.85	190
	Example 38
	Comparative	1.1	1,878	0.85	189
	Example 39
	Comparative	1.0	1,876	0.83	182
	Example 40
	Reference	1.1	1,981	0.87	218
	Example 21
	Reference	1.2	1,986	0.87	213
	Example 22
	Reference	1.1	1,983	0.87	210
	Example 23
	Reference	1.2	1,988	0.87	205
	Example 24
	Reference	1.1	1,983	0.87	218
	Example 25
	Reference	1.1	1,910	0.87	220
	Example 26
	Reference	1.1	1,906	0.88	218
	Example 27
	Reference	1.0	1,897	0.88	213
	Example 28
	Reference	1.1	1,907	0.88	217
	Example 29
	Comparative	Properties of magnetic recording medium
	Examples			Linear
	and	Surface		absorption
	Reference	roughness Ra	Young's modulus	coefficient
	Examples	(nm)	(relative value)	(μm −1 )
	Comparative	24.3	113	1.04
	Example 32
	Comparative	38.5	96	1.09
	Example 33
	Comparative	19.8	112	1.16
	Example 34
	Comparative	12.6	113	1.14
	Example 35
	Comparative	21.9	110	1.06
	Example 36
	Comparative	12.5	113	1.09
	Example 37
	Comparative	10.7	116	1.15
	Example 38
	Comparative	11.0	119	1.17
	Example 39
	Comparative	13.1	119	1.13
	Example 40
	Reference	6.6	122	1.22
	Example 21
	Reference	7.0	124	1.24
	Example 22
	Reference	7.3	126	1.23
	Example 23
	Reference	7.6	133	1.27
	Example 24
	Reference	6.4	124	1.27
	Example 25
	Reference	6.1	123	1.25
	Example 26
	Reference	6.0	126	1.23
	Example 27
	Reference	6.4	125	1.25
	Example 28
	Reference	6.1	124	1.24
	Example 29
	Comparative
	Examples	Properties of magnetic recording medium
	and	Durability
	Reference	Running time
	Examples	(min)	Scratch resistance
	Comparative	8.3	D
	Example 32
	Comparative	1.6	D
	Example 33
	Comparative	9.2	D
	Example 34
	Comparative	14.5	C
	Example 35
	Comparative	12.6	C
	Example 36
	Comparative	13.6	C
	Example 37
	Comparative	11.8	D
	Example 38
	Comparative	12.3	C
	Example 39
	Comparative	13.6	C
	Example 40
	Reference	22.9	B
	Example 21
	Reference	24.8	A
	Example 22
	Reference	24.2	B
	Example 23
	Reference	23.6	B
	Example 24
	Reference	25.2	A
	Example 25
	Reference	18.2	B
	Example 26
	Reference	20.1	B
	Example 27
	Reference	18.9	B
	Example 28
	Reference	20.6	B
	Example 29
	Comparative
	Examples	Properties of magnetic recording medium
	and	Corrosion resistance
	Reference	Percentage of change	Percentage of change
	Examples	in coercive force (%)	in B m (%)
	Comparative	38.2	32.6
	Example 32
	Comparative	41.2	28.1
	Example 33
	Comparative	36.5	29.2
	Example 34
	Comparative	31.3	26.5
	Example 35
	Comparative	32.1	21.3
	Example 36
	Comparative	36.5	16.8
	Example 37
	Comparative	17.9	17.8
	Example 38
	Comparative	16.5	21.6
	Example 39
	Comparative	19.8	17.3
	Example 40
	Reference	13.6	12.6
	Example 21
	Reference	17.8	16.5
	Example 22
	Reference	13.2	17.1
	Example 23
	Reference	12.6	15.6
	Example 24
	Reference	10.9	11.6
	Example 25
	Reference	13.1	12.8
	Example 26
	Reference	12.2	12.4
	Example 27
	Reference	13.6	13.1
	Example 28
	Reference	10.8	10.4
	Example 29

	TABLE 19
		Properties of acicular
	Production of	goethite particles
	acicular goethite		Geometrical
	particles	Average	standard
		Kind of	major	deviation of
		aluminum	axial	major axial
Kind of	Production	compound	diameter	diameter
Precursor	method	added	(μm)	(−)
Precursor	(B)	Aluminum	0.185	1.31
7		sulfate
Precursor	(C)	Aluminum	0.233	1.33
8		acetate
Precursor	(D)	Aluminum	0.246	1.26
9		sulfate
Precursor	(A)	Sodium	0.218	1.31
10		aluminate
Precursor	(E)	Aluminum	0.150	1.34
11		sulfate
Precursor	(F)	Aluminum	0.268	1.31
12		sulfate
Precursor	(A)	—	0.333	1.42
13
	Properties of acicular goethite particles
		Geometrical
	Average	standard		BET
	minor	deviation of		specific
	axial	minor axial	Aspect	surface	Content
Kind of	diameter	diameter	ratio	area	of Al
Precursor	(μm)	(−)	(−)	(m 2 /g)	(wt %)
Precursor	0.0235	1.38	7.9:1	141.1	1.68
7
Precursor	0.0281	1.37	8.3:1	96.1	0.82
8
Precursor	0.0289	1.41	8.5:1	85.8	3.58
9
Precursor	0.0228	1.38	9.6:1	73.8	1.65
10
Precursor	0.0205	1.38	7.3:1	171.6	2.13
11
Precursor	0.0331	1.37	8.1:1	60.6	0.46
12
Precursor	0.0370	1.39	9.0:1	55.8	—
13
	Properties of acicular goethite particles
		Soluble	Soluble
		sodium salt	sulfate
		(calculated	(calculated
	Kind of	as Na)	as SO 4 )	pH value
	Precursor	(ppm)	(ppm)	(−)
	Precursor	1,378	902	6.3
	7
	Precursor	889	774	6.5
	8
	Precursor	726	1,251	6.2
	9
	Precursor	583	645	7.9
	10
	Precursor	1,020	1,009	6.0
	11
	Precursor	1,406	503	5.3
	12
	Precursor	552	618	7.2
	13

TABLE 20
Examples	Kind of	Anti-sintering treatment
and	acicular	Kind of
Comparative	goethite	sintering	Calcu-	Amount
Examples	particles	preventive	lated as	(w %)
Example 93	Particles	Water glass #3	SiO 2	0.5
	used in
	Example 5
Example 94	Precursor	Water glass #3	SiO 2	1.0
	7	Phosphoric acid	P	1.0
Example 95	Precursor	Phosphoric acid	P	1.5
	8
Example 96	Precursor	Sodium hexa-	P	2.0
	9	metaphosphate
Example 97	Precursor	Water glass #3	SiO 2	3.0
	10
Example 98	Precursor	Sodium hexa-	P	1.5
	11	metaphosphate
Example 99	Precursor	Water glass #3	SiO 2	2.0
	12	Phosphoric acid	P	1.0
Comparative	Particles	—	—	—
Example 41	used in
	Example 5
Comparative	Particles	—	—	—
Example 42	used in
	Example 5
Comparative	Particles	Water glass #3	SiO 2	1.5
Example 43	used in
	Example 5
Comparative	Particles	Phosphoric acid	P	1.5
Example 44	used in
	Example 5
Comparative	Particles	Phosphoric acid	P	1.0
Example 45	used in
	Example 5
Comparative	Precursor	Water glass #3	SiO 2	1.0
Example 46	13
	Examples and
	Comparative	Heat-dehydration
	Examples	Temperature (° C.)	Time (min)
	Example 93	340	30
	Example 94	350	60
	Example 95	330	60
	Example 96	330	60
	Example 97	340	45
	Example 98	300	120
	Example 99	360	30
	Comparative	320	60
	Example 41
	Comparative	330	45
	Example 42
	Comparative	340	30
	Example 43
	Comparative	—	—
	Example 44
	Comparative	340	30
	Example 45
	Comparative	320	60
	Example 46

	TABLE 21
	Properties of low-density acicular hematite particles
		Geometrical		Geometrical
	Average	standard	Average	standard
Examples	major	deviation of	minor	deviation of
and	axial	major axial	axial	minor axial
Comparative	diameter	diameter	diameter	diameter
Examples	(μm)	(−)	(μm)	(−)
Example 93	0.144	1.35	0.0194	1.38
Example 94	0.150	1.36	0.0191	1.38
Example 95	0.195	1.29	0.0240	1.37
Example 96	0.215	1.33	0.0250	1.41
Example 97	0.171	1.33	0.0193	1.38
Example 98	0.120	1.36	0.0168	1.38
Example 99	0.234	1.33	0.0285	1.37
Comparative	0.142	1.36	0.0193	1.38
Example 41
Comparative	0.137	1.37	0.0198	1.38
Example 42
Comparative	0.140	1.36	0.0193	1.38
Example 43
Comparative	—	—	—	—
Example 44
Comparative	0.143	1.37	0.0190	1.38
Example 45
Comparative	0.256	1.42	0.0331	1.39
Example 46
Examples	Properties of low-density acicular hematite particles
and	Aspect	Content			S BET /S TEM
Comparative	ratio	of Al	S BET	S TEM	value
Examples	(−)	(wt %)	(m 2 /g)	(m 2 /g)	(−)
Example 93	7.4:1	1.23	153.8	42.3	3.63
Example 94	7.9:1	1.82	149.0	42.8	3.48
Example 95	8.1:1	0.90	115.3	34.0	3.39
Example 96	8.6:1	3.90	111.5	32.6	3.42
Example 97	8.9:1	1.83	95.9	42.1	2.28
Example 98	7.1:1	2.35	180.6	49.0	3.69
Example 99	8.2:1	0.51	81.2	28.6	2.84
Comparative	7.4:1	1.23	148.3	42.6	3.48
Example 41
Comparative	6.9:1	1.24	136.8	41.7	3.28
Example 42
Comparative	7.3:1	1.23	151.2	42.6	3.55
Example 43
Comparative	—	—	—	—	—
Example 44
Comparative	7.5:1	1.22	158.6	43.2	3.67
Example 45
Comparative	7.7:1	—	78.8	24.7	3.18
Example 46
	Properties of low-density acicular hematite particles
		Soluble
	Amount of	sodium	Soluble
	sintering	salt	sulfate
	preventive	(calcu-	(calcu-		Resin
	Calcu-		lated	lated	pH	adsorp-
Ex. and	lated	Content	as Na)	as SO 4 )	value	tion
Com. Ex.	as	(wt %)	(ppm)	(ppm)	(−)	(%)
Ex. 93	SiO 2	0.55	1,705	964	6.3	—
Ex. 94	SiO 2	1.07	2,108	1,332	6.2	—
	P	1.07
Ex. 95	P	1.68	1,811	1,290	6.4	—
Ex. 96	P	2.27	1,493	1,688	6.1	—
Ex. 97	SiO 2	3.27	1,276	919	7.3	—
Ex. 98	P	1.67	1,929	1,456	5.9	—
Ex. 99	SiO 2	2.21	2,352	827	5.8	—
	P	1.11
Com. Ex.	—	—	1,695	953	6.3	46.8
41
Com. Ex.	—	—	1,714	987	6.4	—
42
Com. Ex.	SiO 2	1.65	1,742	1,001	6.2	40.6
43
Com. Ex.	P	1.50	987	685	6.5	—
44
Com. Ex.	P	1.09	1,671	972	6.3	—
45
Com. Ex.	SiO 2	1.10	1,198	1,045	6.9	—
46

	TABLE 22
		Kind of low-
	Examples	density	Heat-treatment for high
	and	acicular	densification
	Comparative	hematite	Temperature	Time
	Examples	particles	(° C.)	(min)
	Example 100	Example 93	630	60
	Example 101	Example 94	650	60
	Example 102	Example 95	680	30
	Example 103	Example 96	640	75
	Example 104	Example 97	680	30
	Example 105	Example 98	660	30
	Example 106	Example 99	710	20
	Comparative	Comparative	680	30
	Example 47	Example 42
	Comparative	Comparative	680	30
	Example 48	Example 44
	Comparative	Comparative	450	30
	Example 49	Example 45
	Comparative	Comparative	850	30
	Example 50	Example 46
	Comparative	Comparative	670	30
	Example 51	Example 46

	TABLE 23
	Properties of high-density acicular hematite particles
		Geometrical		Geometrical
	Average	standard	Average	standard
Examples	major	deviation of	minor	deviation of
and	axial	major axial	axial	minor axial
Comparative	diameter	diameter	diameter	diameter
Examples	(μm)	(−)	(μm)	(−)
Example 100	0.138	1.35	0.0193	1.38
Example 101	0.151	1.36	0.0198	1.39
Example 102	0.195	1.31	0.0245	1.37
Example 103	0.213	1.34	0.0253	1.42
Example 104	0.168	1.34	0.0196	1.38
Example 105	0.120	1.35	0.0170	1.38
Example 106	0.230	1.34	0.0287	1.38
Comparative	0.068	1.86	0.0333	1.65
Example 47
Comparative	0.128	1.65	0.0226	1.38
Example 48
Comparative	0.142	1.37	0.0193	1.38
Example 49
Comparative	0.110	1.68	0.0260	1.39
Example 50
Comparative	0.234	1.44	0.0295	1.39
Example 51
Examples	Properties of high-density acicular hematite particles
and	Aspect	Content			S BET /S TEM
Comparative	ratio	of Al	S BET	S TEM	value
Examples	(−)	(wt %)	(m 2 /g)	(m 2 /g)	(−)
Example 100	7.2:1	1.23	51.4	42.6	1.21
Example 101	7.6:1	1.82	52.6	41.4	1.27
Example 102	8.0:1	0.90	43.1	33.4	1.29
Example 103	8.4:1	3.91	41.2	32.2	1.28
Example 104	8.6:1	1.83	55.2	41.5	1.33
Example 105	7.1:1	2.35	56.1	48.5	1.16
Example 106	8.0:1	0.51	41.0	28.5	1.44
Comparative	2.0:1	1.24	13.8	28.8	0.48
Example 47
Comparative	5.7:1	1.23	25.6	37.0	0.69
Example 48
Comparative	7.4:1	1.23	100.8	42.6	2.37
Example 49
Comparative	4.2:1	—	26.8	33.1	0.81
Example 50
Comparative	7.9:1	—	42.5	27.7	1.53
Example 51
	Properties of high-density acicular hematite particles
		Soluble
	Amount of	sodium	Soluble
	sintering	salt	sulfate
	preventive	(calcu-	(calcu-		Resin
Ex.	Calcu-		lated	lated	pH	adsorp-
and	lated	Content	as Na)	as SO 4 )	value	tion
Com. Ex.	as	(wt %)	(ppm)	(ppm)	(−)	(%)
Ex. 100	SiO 2	0.55	2,516	2,788	5.3	—
Ex. 101	SiO 2	1.07	3,009	3,072	5.2	—
	P	1.06
Ex. 102	P	1.66	2,641	2,904	5.3	—
Ex. 103	P	2.25	2,038	3,456	5.0	—
Ex. 104	SiO 2	3.29	1,515	2,622	6.2	—
Ex. 105	P	1.68	2,463	3,291	4.9	—
Ex. 106	SiO 2	2.23	3,652	2,543	4.8	—
	P	1.10
Com. Ex.	—	—	2,588	2,862	5.3	21.8
47
Com. Ex.	P	1.66	2,832	2,904	5.3	36.8
48
Com. Ex.	P	1.09	2,613	3,008	5.4	51.3
49
Com. Ex.	SiO 2	1.10	1,582	2,936	5.3	36.8
50
Com. Ex.	SiO 2	1.10	1,654	2,890	5.8	37.8
51

	TABLE 24
		Kind of high-
		density
	Examples and	acicular	Wet-pulverization
	Comparative	hematite	Use or	Residue on
	Examples	particles	non-use	sieve (wt %)
	Example 107	Example 100	used	0
	Example 108	Example 101	used	0
	Example 109	Example 102	used	0
	Example 110	Example 103	used	0
	Example 111	Example 104	used	0
	Example 112	Example 105	used	0
	Example 113	Example 106	used	0
	Comparative	Particles used	used	0
	Example 52	in Example 5
	Comparative	Comparative	used	0
	Example 53	Example 51
Examples	Acid-dissolving treatment
and		Concen-	Tempera-		pH
Comparative	Kind of	tration	ture	Time	value
Examples	acid	(N)	(° C.)	(Hr)	(−)
Example 107	Sulfuric	1.5	85	5.0	0.55
	acid
Example 108	Sulfuric	1.4	90	6.0	0.61
	acid
Example 109	Sulfuric	1.8	85	7.0	0.48
	acid
Example 110	Sulfuric	1.6	75	5.0	0.50
	acid
Example 111	Sulfuric	1.3	75	2.5	0.71
	acid
Example 112	Sulfuric	1.5	80	2.0	0.68
	acid
Example 113	Sulfuric	2.0	90	7.0	0.36
	acid
Comparative	Sulfuric	3.0E−05	80	5.5	4.7
Example 52	acid
Comparative	Sulfuric	4.0E−04	80	8.0	3.4
Example 53	acid

	TABLE 25
	Properties of acicular hematite particles
	washed with water after acid-dissolving treatment
		Geometrical		Geometrical
	Average	standard	Average	standard
Examples	major	deviation of	minor	deviation of
and	axial	major axial	axial	minor axial
Comparative	diameter	diameter	diameter	diameter
Examples	(μm)	(−)	(μm)	(−)
Example 107	0.132	1.34	0.0187	1.33
Example 108	0.142	1.35	0.0179	1.34
Example 109	0.173	1.31	0.0218	1.33
Example 110	0.195	1.33	0.0231	1.34
Example 111	0.163	1.33	0.0195	1.32
Example 112	0.121	1.35	0.0165	1.32
Example 113	0.195	1.33	0.0227	1.33
Comparative	0.138	1.35	0.0194	1.38
Example 52
Comparative	0.234	1.44	0.0296	1.39
Example 53
		Properties of acicular hematite particles
	Examples	washed with water after acid-dissolving treatment
	and		Amount
	Comparative	Aspect ratio	dissolved	Content of Al
	Examples	(−)	(wt %)	(wt %)
	Example 107	7.1:1	25.8	1.22
	Example 108	7.9:1	31.2	1.83
	Example 109	7.9:1	39.8	0.91
	Example 110	8.4:1	21.3	3.92
	Example 111	8.4:1	11.6	1.85
	Example 112	7.3:1	18.2	2.36
	Example 113	8.6:1	42.8	0.52
	Comparative	7.1:1	0.3	1.23
	Example 52
	Comparative	7.9:1	1.0	—
	Example 53
	Properties of acicular hematite particles
Examples	washed with water after acid-dissolving treatment
and			S BET /S TEM
Comparative	S BET	S TEM	value
Examples	(m 2 /g)	(m 2 /g)	(−)
Example 107	56.1	44.0	1.27
Example 108	57.6	45.7	1.26
Example 109	48.2	37.5	1.29
Example 110	45.6	35.3	1.29
Example 111	58.1	41.8	1.39
Example 112	61.2	49.8	1.23
Example 113	47.1	35.9	1.31
Comparative	50.8	42.4	1.20
Example 52
Comparative	40.6	27.6	1.47
Example 53
	Properties of acicular hematite particles washed
	with water after acid-dissolving treatment
		Soluble
	Amount of	sodium	Soluble
	sintering	salt	sulfate
	preventive	(calcu-	(calcu-		Resin
	Calcu-		lated	lated as	pH	adsorp-
Ex. and	lated	Content	as Na)	SO 4 )	value	tion
Com. Ex.	as	(wt %)	(ppm)	(ppm)	(−)	(%)
Ex. 107	SiO 2	0.54	118	343	5.0	68.6
Ex. 108	SiO 2	1.07	141	410	5.2	68.3
	P	1.05
Ex. 109	P	1.67	124	368	5.1	66.8
Ex. 110	P	2.26	89	532	4.9	67.1
Ex. 111	SiO 2	3.30	72	295	5.3	71.2
Ex. 112	P	1.68	103	491	5.1	70.6
Ex. 113	SiO 2	2.24	152	206	5.0	66.9
	P	1.11
Com. Ex.	SiO 2	0.58	107	314	5.3	61.2
52
Com. Ex.	SiO 2	1.11	96	357	5.2	54.3
53

TABLE 26
	Kind of high-
Examples	density	Heat-treatment in aqueous
and	acicular	alkali solution
Reference	hematite	pH value	Temperature	Time
Examples	particles	(−)	(° C.)	(min)
Example	Example 107	13.6	90	180
114
Example	Example 108	13.5	95	180
115
Example	Example 109	13.2	85	240
116
Example	Example 110	13.4	85	180
117
Example	Example 111	13.5	80	300
118
Example	Example 112	13.3	95	120
119
Example	Example 113	13.2	90	240
120
Reference	Example 107	9.3	90	180
Example 30
Reference	Example 107	13.1	43	180
Example 31

	TABLE 27
	Properties of acicular hematite particles
	washed with water after heat-treatment in
	aqueous alkali solution
		Geometrical		Geometrical
	Average	standard	Average	standard
Examples	major	deviation of	minor	deviation of
and	axial	major axial	axial	minor axial
Reference	diameter	diameter	diameter	diameter
Examples	(μm)	(−)	(μm)	(−)
Example 114	0.132	1.34	0.0188	1.33
Example 115	0.142	1.33	0.0179	1.34
Example 116	0.173	1.31	0.0217	1.32
Example 117	0.195	1.31	0.0231	1.34
Example 118	0.163	1.38	0.0195	1.31
Example 119	0.121	1.35	0.0166	1.32
Example 120	0.195	1.32	0.0227	1.33
Reference	0.132	1.34	0.0188	1.38
Example 30
Reference	0.132	1.34	0.0188	1.39
Example 31
	Properties of acicular hematite particles
	washed with water after heat-treatment in
	aqueous alkali solution
Examples
and	Aspect	Content			S BET /S TEM
Reference	ratio	of Al	S BET	S TEM	value
Examples	(−)	(wt %)	(m 2 /g)	(m 2 /g)	(−)
Example 114	7.0:1	1.22	57.0	43.8	1.30
Example 115	7.9:1	1.83	57.3	45.7	1.25
Example 116	8.0:1	0.91	49.6	37.7	1.32
Example 117	8.4:1	3.92	46.3	35.3	1.31
Example 118	8.4:1	1.85	59.6	41.8	1.43
Example 119	7.3:1	2.36	62.0	49.5	1.25
Example 120	8.6:1	0.52	47.1	35.9	1.31
Reference	7.0:1	1.22	57.1	43.8	1.30
Example 30
Reference	7.0:1	1.22	57.3	43.8	1.31
Example 31
	Properties of acicular hematite particles washed
	with water after heat-treatment in aqueous alkali
	solution
		Soluble
	Amount of	sodium	Soluble
	sintering	salt	sulfate
	preventive	(calcu-	(calcula		Resin
	Calcu-		lated	ted as	pH	adsorp-
Ex. and	lated	Content	as Na)	SO 4 )	value	tion
Com. Ex.	as	(wt %)	(ppm)	(ppm)	(−)	(%)
Ex. 114	SiO 2	0.44	96	7	9.2	70.8
Ex. 115	SiO 2	0.77	101	10	9.1	71.6
	P	0.32
Ex. 116	P	0.48	72	6	9.1	72.1
Ex. 117	P	0.61	56	21	8.9	73.2
Ex. 118	SiO 2	2.25	52	6	8.9	74.1
Ex. 119	P	0.50	92	16	9.3	74.2
Ex. 120	SiO 2	1.68	110	3	9.6	70.6
	P	0.34
Ref. Ex.	SiO 2	0.43	136	293	7.3	66.4
30
Ref. Ex.	SiO 2	0.44	156	216	7.7	65.8
31

	TABLE 28
	Kind of acicular	Surface treatment
	hematite		Amount
	particles	Kind of	added
	treated in	surface	(calculated
	aqueous alkaline	treatment	as Al or
Examples	solution	material	SiO 2 ) (wt %)
Example 121	Example 114	Sodium aluminate	1.0
Example 122	Example 115	Water glass #3	1.0
Example 123	Example 116	Aluminum sulfate	1.5
		Water glass #3	0.5
Example 124	Example 117	Aluminum acetate	4.0
Example 125	Example 118	Sodium aluminate	2.0
		Colloidal silica	0.5
Example 126	Example 119	Colloidal silica	2.0
Example 127	Example 120	Sodium aluminate	15.0
	Coating material
				Coating amount
				(calculated as Al
	Examples	Kind* 1	Calculated as	or SiO 2 ) (wt %)
	Example 121	A	Al	0.99
	Example 122	S	SiO 2	0.98
	Example 123	A	Al	1.47
		S	SiO 2	0.49
	Example 124	A	Al	3.85
	Example 125	A	Al	1.96
		S	SiO 2	0.49
	Example 126	S	SiO 2	1.96
	Example 127	A	Al	13.00
	Note * 1 : “A” represents a hydroxide of aluminum.
	“S” represents an oxide of silicon.

	TABLE 29
	Properties of acicular hematite particles
	washed with water after surface-treatment
		Geometrical		Geometrical
	Average	standard	Average	standard
	major	deviation of	minor	deviation of
	axial	major axial	axial	minor axial
	diameter	diameter	diameter	diameter
Examples	(μm)	(−)	(μm)	(−)
Example 121	0.131	1.34	0.0188	1.33
Example 122	0.142	1.35	0.0179	1.34
Example 123	0.173	1.31	0.0217	1.32
Example 124	0.195	1.33	0.0232	1.34
Example 125	0.163	1.33	0.0195	1.31
Example 126	0.121	1.35	0.0166	1.32
Example 127	0.195	1.33	0.0227	1.33
	Properties of acicular hematite particles
	washed with water after surface-treatment
	Aspect	Content			S BET /S TEM
	ratio	of Al* 2	S BET	S TEM	value
Examples	(−)	(wt %)	(m 2 /g)	(m 2 /g)	(−)
Example 121	7.0:1	1.22	57.3	43.9	1.31
Example 122	7.9:1	1.83	58.1	45.7	1.27
Example 123	8.0:1	0.91	48.9	37.7	1.30
Example 124	8.4:1	3.92	46.9	35.1	1.34
Example 125	8.4:1	1.85	61.2	41.8	1.46
Example 126	7.3:1	2.36	63.6	49.5	1.28
Example 127	8.6:1	0.52	47.4	35.9	1.32
	Properties of acicular hematite particles washed
	with water after surface-treatment
		Soluble
	Amount of	sodium	Soluble
	sintering	salt	sulfate
	preventive	(calcu-	(Calcula		Resin
	Calcu-		lated	ted as	pH	adsorp
	lated	Content	as Na)	SO 4 )	value	-tion
Examples	as	(wt %)	(ppm)	(ppm)	(−)	(%)
Example	SiO 2	0.44	91	1	9.4	78.2
121
Example	SiO 2	0.77	78	5	9.2	81.2
122	P	0.32
Example	P	0.48	63	6	9.1	83.6
123
Example	P	0.61	71	2	9.0	84.1
124
Example	SiO 2	2.25	58	8	8.9	80.6
125
Example	P	0.50	62	3	9.0	79.8
126
Example	SiO 2	1.68	65	1	9.2	81.6
127	P	0.34
Note* 2 : Al content excluding amount of Al in surface-coating layer.

	TABLE 30
	Production of non-magnetic
	coating composition	Non-magnetic
	Kind of	Weight ratio	coating
	acicular	of particles	composition
	hematite	to resin	Viscosity
Examples	particles	(−)	(cP)
Example 128	Example 114	5.0	333
Example 129	Example 115	5.0	358
Example 130	Example 116	5.0	333
Example 131	Example 117	5.0	284
Example 132	Example 118	5.0	230
Example 133	Example 119	5.0	410
Example 134	Example 120	5.0	384
Example 135	Example 121	5.0	333
Example 136	Example 122	5.0	284
Example 137	Example 123	5.0	333
Example 138	Example 124	5.0	207
Example 139	Example 125	5.0	230
Example 140	Example 126	5.0	284
Example 141	Example 127	5.0	207
	Properties of non-magnetic undercoat layer
			Surface	Young's
			roughness	modulus
	Thickness	Gloss	Ra	(relative
Examples	(μm)	(%)	(nm)	value)
Example 128	3.5	212	6.2	123
Example 129	3.4	201	6.4	123
Example 130	3.4	205	6.4	128
Example 131	3.5	203	6.4	130
Example 132	3.4	218	5.4	128
Example 133	3.3	220	5.8	125
Example 134	3.4	216	5.7	134
Example 135	3.5	213	5.8	127
Example 136	3.5	210	6.0	127
Example 137	3.5	213	6.1	130
Example 138	3.5	216	5.8	136
Example 139	3.4	223	5.0	129
Example 140	3.3	221	5.6	125
Example 141	3.5	220	5.0	136

	TABLE 31
		Production of non-magnetic
	Comparative	coating composition	Non-magnetic
	Examples	Kind of	Weight ratio	coating
	and	acicular	of particles	composition
	Reference	hematite	to resin	Viscosity
	Examples	particles	(−)	(cP)
	Comparative	Comparative	5.0	7,680
	Example 54	Example 41
	Comparative	Comparative	5.0	10,240
	Example 55	Example 43
	Comparative	Comparative	5.0	384
	Example 56	Example 47
	Comparative	Comparative	5.0	640
	Example 57	Example 48
	Comparative	Comparative	5.0	5,632
	Example 58	Example 49
	Comparative	Comparative	5.0	230
	Example 59	Example 50
	Comparative	Comparative	5.0	410
	Example 60	Example 51
	Comparative	Comparative	5.0	384
	Example 61	Example 52
	Comparative	Comparative	5.0	435
	Example 62	Example 53
	Reference	Example 107	5.0	384
	Example 32
	Reference	Example 108	5.0	410
	Example 33
	Reference	Example 109	5.0	410
	Example 34
	Reference	Example 110	5.0	281
	Example 35
	Reference	Example 111	5.0	281
	Example 36
	Reference	Example 112	5.0	410
	Example 37
	Reference	Example 113	5.0	384
	Example 38
	Reference	Reference	5.0	384
	Example 39	Example 30
	Reference	Reference	5.0	320
	Example 40	Example 31
	Comparative	Properties of nonmagnetic undercoat layer
	Examples			Surface	Young's
	and			roughness	modulus
	Reference	Thickness	Gloss	Ra	(relative
	Examples	(μm)	(%)	(nm)	value)
	Comparative	3.5	126	20.6	106
	Example 54
	Comparative	3.7	88	38.9	99
	Example 55
	Comparative	3.5	142	20.2	110
	Example 56
	Comparative	3.5	173	12.3	111
	Example 57
	Comparative	3.6	126	26.5	106
	Example 58
	Comparative	3.5	160	18.2	106
	Example 59
	Comparative	3.5	183	9.9	114
	Example 60
	Comparative	3.5	189	9.2	116
	Example 61
	Comparative	3.5	175	13.6	116
	Example 62
	Reference	3.5	200	7.2	123
	Example 32
	Reference	3.4	195	7.6	121
	Example 33
	Reference	3.5	198	6.8	120
	Example 34
	Reference	3.4	196	7.4	132
	Example 35
	Reference	3.5	211	6.4	126
	Example 36
	Reference	3.5	211	6.0	124
	Example 37
	Reference	3.4	211	6.3	131
	Example 38
	Reference	3.5	210	6.8	121
	Example 39
	Reference	3.4	206	6.8	124
	Example 40

	TABLE 32
	Production of magnetic recording medium
				Weight
				ratio of
		Kind of non-		particles
		magnetic	Kind of magnetic	to resin
	Examples	substrate	particles	(−)
	Example 142	Example 128	Particles used in	5.0
			Example 7
	Example 143	Example 129	Magnetic metal	5.0
			particles (a)
	Example 144	Example 130	Magnetic metal	5.0
			particles (a)
	Example 145	Example 131	Magnetic metal	5.0
			particles (b)
	Example 146	Example 132	Magnetic metal	5.0
			particles (b)
	Example 147	Example 133	Magnetic metal	5.0
			particles (c)
	Example 148	Example 134	Magnetic metal	5.0
			particles (c)
	Example 149	Example 135	Particles used in	5.0
			Example 7
	Example 150	Example 136	Magnetic metal	5.0
			particles (a)
	Example 151	Example 137	Magnetic metal	5.0
			particles (a)
	Example 152	Example 138	Magnetic metal	5.0
			particles (b)
	Example 153	Example 139	Magnetic metal	5.0
			particles (b)
	Example 154	Example 140	Magnetic metal	5.0
			particles (c)
	Example 155	Example 141	Magnetic metal	5.0
			particles (c)
	Properties of magnetic recording medium
	Thickness
	of
	magnetic	Coercive	Br/Bm
	layer	force (Hc)	value	Gloss
Examples	(μm)	(Oe)	(−)	(%)
Example 142	1.0	1,979	0.87	230
Example 143	1.0	1,984	0.87	226
Example 144	1.0	1,990	0.88	226
Example 145	1.0	2,005	0.87	220
Example 146	1.0	1,994	0.87	243
Example 147	1.1	2,141	0.88	245
Example 148	1.1	2,146	0.88	238
Example 149	1.1	1,981	0.88	231
Example 150	1.0	1,994	0.88	236
Example 151	1.0	1,980	0.89	230
Example 152	1.1	2,001	0.90	237
Example 153	1.0	2,010	0.89	245
Example 154	1.0	2,141	0.90	241
Example 155	1.0	2,138	0.90	243
	Properties of magnetic recording medium
			Linear
			absorp-
	Surface	Young's	tion	Durability
	rough-	modulus	coeffi-	Running
	ness Ra	(relative	cient	time	Scratch
Examples	(nm)	value)	(μm −1 )	(min)	resistance
Example 142	5.8	127	1.22	29.6	A
Example 143	6.4	129	1.24	24.8	B
Example 144	6.2	128	1.26	24.8	B
Example 145	6.4	131	1.28	27.7	A
Example 146	6.0	132	1.27	28.9	A
Example 147	5.8	131	1.24	≧30	A
Example 148	6.1	132	1.25	≧30	A
Example 149	5.8	128	1.25	≧30	A
Example 150	5.5	132	1.25	≧30	A
Example 151	6.1	133	1.26	≧30	A
Example 152	6.0	136	1.28	≧30	A
Example 153	5.3	137	1.26	≧30	A
Example 154	5.8	135	1.25	≧30	A
Example 155	5.9	133	1.25	≧30	A
	Properties of magnetic recording medium
	Corrosion resistance
	Percentage of change	Percentage of change
Examples	in coercive force (%)	in B m (%)
Example 142	3.1	2.8
Example 143	4.2	4.6
Example 144	4.6	3.1
Example 145	2.6	2.1
Example 146	3.5	1.8
Example 147	3.2	3.4
Example 148	3.4	3.8
Example 149	3.4	2.6
Example 150	4.1	1.6
Example 151	4.6	4.1
Example 152	1.8	3.8
Example 153	0.8	0.6
Example 154	2.6	2.8
Example 155	2.6	1.6

	TABLE 33
	Production of magnetic recording medium
	Comparative			Weight
	Examples			ratio of
	and	Kind of non-		particles
	Reference	magnetic	Kind of magnetic	to resin
	Examples	substrate	particles	(−)
	Comparative	Comparative	Magnetic metal	5.0
	Example 63	Example 54	particles (a)
	Comparative	Comparative	Magnetic metal	5.0
	Example 64	Example 55	particles (a)
	Comparative	Comparative	Magnetic metal	5.0
	Example 65	Example 56	particles (a)
	Comparative	Comparative	Magnetic metal	5.0
	Example 66	Example 57	particles (a)
	Comparative	Comparative	Magnetic metal	5.0
	Example 67	Example 58	particles (a)
	Comparative	Comparative	Magnetic metal	5.0
	Example 68	Example 59	particles (d)
	Comparative	Comparative	Magnetic metal	5.0
	Example 69	Example 60	particles (d)
	Comparative	Comparative	Magnetic metal	5.0
	Example 70	Example 61	particles (d)
	Comparative	Comparative	Magnetic metal	5.0
	Example 71	Example 62	particles (d)
	Reference	Reference	Magnetic metal	5.0
	Example 41	Example 32	particles (a)
	Reference	Reference	Magnetic metal	5.0
	Example 42	Example 33	particles (a)
	Reference	Reference	Magnetic metal	5.0
	Example 43	Example 34	particles (a)
	Reference	Reference	Magnetic metal	5.0
	Example 44	Example 35	particles (a)
	Reference	Reference	Magnetic metal	5.0
	Example 45	Example 36	particles (a)
	Reference	Reference	Magnetic metal	5.0
	Example 46	Example 37	particles (d)
	Reference	Reference	Magnetic metal	5.0
	Example 47	Example 38	particles (d)
	Reference	Reference	Magnetic metal	5.0
	Example 48	Example 39	particles (d)
	Reference	Reference	Magnetic metal	5.0
	Example 49	Example 40	particles (d)
	Properties of magnetic recording medium
	Comparative	Thickness
	Examples	of
	and	magnetic	Coercive	Br/Bm
	Reference	layer	force (Hc)	value	Gloss
	Examples	(μm)	(Oe)	(−)	(%)
	Comparative	1.2	1,957	0.78	135
	Example 63
	Comparative	1.3	1,961	0.76	118
	Example 64
	Comparative	1.1	1,968	0.82	173
	Example 65
	Comparative	1.1	1,963	0.84	182
	Example 66
	Comparative	1.2	1,954	0.78	161
	Example 67
	Comparative	1.1	1,863	0.84	178
	Example 68
	Comparative	1.0	1,871	0.85	190
	Example 69
	Comparative	1.1	1,878	0.85	189
	Example 70
	Comparative	1.0	1,876	0.83	182
	Example 71
	Reference	1.1	1,986	0.87	217
	Example 41
	Reference	1.1	1,991	0.87	214
	Example 42
	Reference	1.1	1,984	0.87	212
	Example 43
	Reference	1.1	1,989	0.87	210
	Example 44
	Reference	1.1	1,986	0.87	213
	Example 45
	Reference	1.2	1,909	0.88	218
	Example 46
	Reference	1.1	1,910	0.88	220
	Example 47
	Reference	1.1	1,891	0.88	216
	Example 48
	Reference	1.1	1,900	0.88	220
	Example 49
	Properties of magnetic recording medium
			Linear
Comparative			absorp-
Examples	Surface	Young's	tion	Durability
and	rough-	modulus	coeffi-	Running
Reference	ness Ra	(relative	cient	time	Scratch
Examples	(nm)	value)	(μm −1 )	(min)	resistance
Comparative	24.3	113	1.04	8.3	D
Example 63
Comparative	38.5	96	1.09	1.6	D
Example 64
Comparative	19.8	112	1.16	9.2	D
Example 65
Comparative	12.6	113	1.14	14.5	C
Example 66
Comparative	21.9	110	1.06	12.6	C
Example 67
Comparative	12.5	113	1.09	13.6	C
Example 68
Comparative	10.7	116	1.15	11.8	D
Example 69
Comparative	11.0	119	1.17	12.3	C
Example 70
Comparative	13.1	119	1.13	13.6	C
Example 71
Reference	6.8	123	1.21	23.8	B
Example 41
Reference	6.8	123	1.22	24.2	B
Example 42
Reference	7.2	128	1.22	24.4	B
Example 43
Reference	7.2	130	1.24	23.4	B
Example 44
Reference	6.8	126	1.25	25.6	A
Example 45
Reference	6.2	125	1.24	22.3	B
Example 46
Reference	6.0	125	1.24	23.9	B
Example 47
Reference	6.2	125	1.25	24.2	B
Example 48
Reference	6.0	125	1.25	24.8	B
Example 49
	Comparative
	Examples	Properties of magnetic recording medium
	and	Corrosion resistance
	Reference	Percentage of change	Percentage of change
	Examples	in coercive force (%)	in B m (%)
	Comparative	38.2	32.6
	Example 63
	Comparative	41.2	28.1
	Example 64
	Comparative	36.5	29.2
	Example 65
	Comparative	31.3	26.5
	Example 66
	Comparative	32.1	21.3
	Example 67
	Comparative	36.5	16.8
	Example 68
	Comparative	17.9	17.8
	Example 69
	Comparative	16.5	21.6
	Example 70
	Comparative	19.8	17.3
	Example 71
	Reference	11.8	11.6
	Example 41
	Reference	15.8	16.8
	Example 42
	Reference	11.6	18.6
	Example 43
	Reference	10.8	13.2
	Example 44
	Reference	10.1	10.9
	Example 45
	Reference	13.2	14.1
	Example 46
	Reference	11.8	13.1
	Example 47
	Reference	14.1	12.8
	Example 48
	Reference	12.0	10.9
	Example 49

	TABLE 34
	Properties of acicular goethite particles
				Geometrical
				standard
			Average major	deviation of
	Kind of		axial diameter	major axial
	Precursor	Shape	(μm)	diameter (−)
	Goethite	Spindle-	0.0593	1.56
	particles 1	shaped
	Goethite	Acicular	0.0932	1.53
	particles 2
	Properties of acicular goethite particles
			Geometrical
			standard
		Average minor	deviation of
	Kind of	axial diameter	minor axial	Aspect ratio
	Precursor	(μm)	diameter (−)	(−)
	Goethite	0.0096	1.37	6.2:1
	particles 1
	Goethite	0.0126	1.39	7.4:1
	particles 2
	Properties of acicular goethite particles
		Soluble	Soluble
		sodium salt	sulfate
	BET specific	(calculated	(calculated	pH
Kind of	surface area	as Na)	as SO 4 )	value
Precursor	(m 2 /g)	(ppm)	(ppm)	(−)
Goethite	231.3	563	412	5.8
particles 1
Goethite	186.8	1,126	368	7.8
particles 2

	TABLE 35
	Properties of acicular goethite
	particles subjected to anti-
	sintering treatment
			Geometrical
		Average major	standard deviation
	Kind of	axial	of major axial
Precursor	Precursor	diameter (μm)	diameter (−)
Goethite	Particles	0.0813	1.53
particles 3	obtained in
	Example 8
Goethite	Goethite	0.0593	1.56
particles 4	particles 1
Goethite	Goethite	0.0931	1.53
particles 5	particles 2
	Properties of acicular goethite particles
	subjected to anti-sintering treatment
		Average	Geometrical
		minor axial	standard deviation	Aspect
		diameter	of minor axial	ratio
	Precursor	(μm)	diameter (−)	(−)
	Goethite	0.0110	1.37	7.4:1
	particles 3
	Goethite	0.0096	1.37	6.2:1
	particles 4
	Goethite	0.0128	1.39	7.3:1
	particles 5
	Properties of acicular goethite particles
	subjected to anti-sintering treatment
	BET specific
	surface area	Sintering preventive
	Precursor	(m 2 /g)	Calculated as	Amount (wt %)
	Goethite	190.8	P	1.08
	particles 3
	Goethite	228.6	P	1.64
	particles 4
	Goethite	185.2	SiO 2	1.09
	particles 5
	Properties of acicular goethite particles
	subjected to anti-sintering treatment
		Soluble	Soluble
		sodium salt	sulfate
		(calculated	(Calculated as
		as Na)	SO 4 )	pH value
	Precursor	(ppm)	(ppm)	(−)
	Goethite	813	412	7.6
	particles 3
	Goethite	612	515	6.1
	particles 4
	Goethite	1,326	412	8.0
	particles 5

	TABLE 36
	Conditions of heat-treatment
Goethite	Kind of		Temperature	Time
particles	Precursor	Atmosphere	(° C.)	(min)
Goethite	Goethite	Air	150	30
particles 6	particles 3
Goethite	Goethite	Air	180	30
particles 7	particles 4
Goethite	Goethite	Air	120	20
particles 8	particles 5
Goethite	Goethite	Air	80	30
particles 9	particles 4

	TABLE 37
	Properties of acicular goethite particles
	heat-treated
		Geometrical		Geometrical
	Average	standard	Average	standard
	major	deviation of	minor	deviation of
Kind of	axial	major axial	axial	minor axial
goethite	diameter	diameter	diameter	diameter
particles	(μm)	(−)	(μm)	(−)
Goethite	0.0812	1.38	0.0109	1.16
particles 6
Goethite	0.0591	1.43	0.0098	1.21
particles 7
Goethite	0.0930	1.36	0.0128	1.25
particles 8
Goethite	0.0590	1.53	0.0099	1.37
particles 9
	Properties of acicular goethite particles
	heat-treated
			BET
			specific	Sintering
	Kind of	Aspect	surface	preventive
	goethite	ratio	area	Calculated	Amount
	particles	(−)	(m 2 /g)	as	(wt %)
	Goethite	7.4:1	191.2	P	1.18
	particles 6
	Goethite	6.0:1	227.6	P	1.82
	particles 7
	Goethite	7.3:1	184.8	SiO 2	1.20
	particles 8
	Goethite	6.0:1	225.1	P	1.80
	particles 9
	Properties of acicular goethite particles
	heat-treated
		Soluble	Soluble
		sodium salt	sulfate
	Kind of	(calculated as	(calculated as
	goethite	Na)	SO 4 )	pH value
	particles	(ppm)	(ppm)	(−)
	Goethite	883	446	7.5
	particles 6
	Goethite	710	532	6.0
	particles 7
	Goethite	1,410	483	7.9
	particles 8
	Goethite	681	518	6.1
	particles 9

TABLE 38
Kind of		Conditions of heat-dehydration
particles	Kind of	treatment
to be	goethite		Temperature	Time
treated	particles	Atmosphere	(° C.)	(min)
Hematite	Goethite	Air	320	20
particles 1	particles 6
Hematite	Goethite	Air	340	30
particles 2	particles 7
Hematite	Goethite	Air	350	20
particles 3	particles 8
Hematite	Goethite	Air	340	20
particles 4	particles 4

	TABLE 39
	Properties of low-density acicular hematite
	particles
		Geometrical		Geometrical
	Average	standard	Average	standard
Kind of	major	deviation of	minor	deviation of
particles	axial	major axial	axial	minor axial
to be	diameter	diameter	diameter	diameter
treated	(μm)	(−)	(μm)	(−)
Hematite	0.0731	1.37	0.0113	1.16
particles 1
Hematite	0.0533	1.43	0.0103	1.20
particles 2
Hematite	0.0841	1.36	0.0131	1.25
particles 3
Hematite	0.0531	1.56	0.0104	1.37
particles 4
	Properties of low-density acicular hematite
	particles
	Kind of	Aspect			S BET /S TEM
	particles	ratio	S BET	S TEM	value
	to be treated	(−)	(m 2 /g)	(m 2 /g)	(−)
	Hematite	6.5:1	226.5	73.3	3.09
	particles 1
	Hematite	5.2:1	263.8	81.9	3.22
	particles 2
	Hematite	6.4:1	216.8	63.3	3.43
	particles 3
	Hematite	5.1:1	226.6	81.2	2.79
	particles 4
	Properties of low-density acicular hematite
	particles
		Soluble
		sodium	Soluble
		salt	sulfate
Kind of	Sintering	(calcu-	(calcu-
particles	preventive	lated as	lated as	pH
to be	Calcu-	Amount	Na)	SO 4 )	value
treated	lated as	(wt %)	(ppm)	(ppm)	(−)
Hematite	P	1.18	1,625	913	7.4
particles 1
Hematite	P	1.81	1,321	615	6.0
particles 2
Hematite	SiO 2	1.20	1,821	516	7.6
particles 3
Hematite	P	1.79	1,326	712	6.3
particles 4

TABLE 40
Kind of	Kind of	Conditions of high-temperature
particles	particles	heat-treatment
to be	to be		Temperature	Time
treated	treated	Atmosphere	(° C.)	(min)
Hematite	Hematite	Air	680	30
particles 5	particles 1
Hematite	Hematite	Air	630	40
particles 6	particles 2
Hematite	Hematite	Air	600	40
particles 7	particles 3
Hematite	Goethite	Air	650	20
particles 8	particles 7
Hematite	Goethite	Air	700	30
particles 9	particles 1
Hematite	Goethite	Air	630	30
particles 10	particles 9
Hematite	Hematite	Air	680	30
particles 11	particles 4
Hematite	Hematite	Air	450	30
particles 12	particles 3
Hematite	Hematite	Air	900	30
particles 13	particles 3

	TABLE 41
	Properties of high-density acicular hematite
	particles
		Geometrical		Geometrical
	Average	standard	Average	standard
Kind of	major	deviation of	minor	deviation of
particles	axial	major axial	axial	minor axial
to be	diameter	diameter	diameter	diameter
treated	(μm)	(−)	(μm)	(−)
Hematite	0.0730	1.37	0.0114	1.16
particles 5
Hematite	0.0531	1.43	0.0103	1.20
particles 6
Hematite	0.0840	1.36	0.0130	1.25
particles 7
Hematite	0.0583	1.40	0.0108	1.27
particles 8
Hematite	0.0432	1.61	0.0156	1.39
particles 9
Hematite	0.0456	1.59	0.0152	1.39
particles 10
Hematite	0.0528	1.56	0.0107	1.37
particles 11
Hematite	0.0841	1.35	0.0132	1.28
particles 12
Hematite	0.0613	1.59	0.0232	1.38
particles 13
		Properties of high-density acicular hematite
	Kind of	particles
	particles	Aspect			S BET /S TEM
	to be	ratio	S BET	S TEM	value
	treated	(−)	(m 2 /g)	(m 2 /g)	(−)
	Hematite	6.4:1	81.2	72.7	1.12
	particles 5
	Hematite	5.2:1	91.6	81.9	1.12
	particles 6
	Hematite	6.5:1	71.6	63.8	1.12
	particles 7
	Hematite	5.4:1	84.6	77.8	1.09
	particles 8
	Hematite	2.8:1	58.2	58.2	1.00
	particles 9
	Hematite	3.0:1	72.3	59.0	1.22
	particles 10
	Hematite	4.9:1	65.6	79.2	0.83
	particles 11
	Hematite	6.4:1	186.2	62.8	2.96
	particles 12
	Hematite	2.6:1	36.8	39.4	0.93
	particles 13
	Properties of high-density acicular hematite
	particles
		Soluble
		sodium	Soluble
		salt	sulfate
Kind of	Sintering	(calcu-	(calcu-
particles	preventive	lated as	lated	pH
to be	Calcu-	Amount	Na)	as SO 4 )	value
treated	lated as	(wt %)	(ppm)	(ppm)	(−)
Hematite	P	1.18	2,863	2,162	6.9
particles 5
Hematite	P	1.81	2,068	1,865	5.9
particles 6
Hematite	SiO 2	1.20	2,365	3,100	6.0
particles 7
Hematite	P	1.82	1,563	2,016	5.6
particles 8
Hematite	P	1.79	2,880	2,266	6.8
particles 9
Hematite	P	1.81	1,865	1,265	5.8
particles 10
Hematite	P	1.79	2,653	2,565	5.9
particles 11
Hematite	SiO 2	1.20	1,865	2,O11	5.6
particles 12
Hematite	SiO 2	1.21	1,780	2,025	5.6
particles 13

TABLE 42
Examples		Heat-treatment in aqueous
and	Kind of	alkali solution
Reference	particles to	pH value	Temperature	Time
Examples	be treated	(−)	(° C.)	(min)
Example	Hematite	13.7	85	210
156	particles 5
Example	Hematite	13.4	95	120
157	particles 6
Example	Hematite	13.8	90	240
158	particles 7
Example	Hematite	13.1	98	180
159	particles 8
Reference	Hematite	8.9	80	180
Example 50	particles 5
Reference	Hematite	13.5	40	180
Example 51	particles 5

	TABLE 43
	Properties of high-density acicular hematite
	particles washed with water after heat-treatment
	in aqueous alkali solution
	Average	Geometrical	Average	Geometrical
	major	standard	minor	standard
Examples and	axial	deviation of	axial	deviation of
Reference	diameter	major axial	diameter	minor axial
Examples	(μm)	diameter (−)	(μm)	diameter (−)
Example 156	0.0731	1.36	0.0114	1.16
Example 157	0.0532	1.42	0.0102	1.21
Example 158	0.0840	1.36	0.0129	1.26
Example 159	0.0583	1.39	0.0108	1.27
Reference	0.0730	1.37	0.0114	1.17
Example 50
Reference	0.0730	1.37	0.0114	1.17
Example 51
	Properties of high-density acicular hematite
	particles washed with water after heat-treatment
	in aqueous alkali solution
	Examples and	Aspect			S BET /S TEM
	Reference	ratio	S BET	S TEM	value
	Examples	(−)	(m 2 /g)	(m 2 /g)	(−)
	Example 156	6.4:1	81.2	72.7	1.13
	Example 157	5.2:1	92.6	82.6	1.12
	Example 158	6.5:1	72.1	64.2	1.12
	Example 159	5.4:1	84.8	77.8	1.09
	Reference	6.4:1	82.6	72.7	1.14
	Example 50
	Reference	6.4:1	81.9	72.7	1.13
	Example 51
	Properties of high-density acicular hematite
	particles washed with water after heat-
	treatment in aqueous alkali solution
		Soluble
		sodium	Soluble
		salt	sulfate
	Sintering	(calcu-	(calcula
Examples and	preventive	lated as	ted as	pH
Reference	Calcu-	Amount	Na)	SO 4 )	value
Examples	lated as	(wt %)	(ppm)	(ppm)	(−)
Example 156	P	0.56	86	12	9.1
Example 157	P	0.93	73	6	9.2
Example 158	SiO 2	1.16	125	38	9.3
Example 159	P	0.91	113	21	9.3
Reference	P	1.18	436	216	7.6
Example 50
Reference	P	1.18	365	182	7.7
Example 51

TABLE 44
	Kind of
	particles to be	Concentration of water
Examples	treated	suspension (g/liter)
Example	Example	45
160	156
Example	Example	45
161	157
Example	Example	45
162	158
Example	Example	45
163	159
	Coating treatment with aluminum compound or
	silicon compound
	Aluminum compound or silicon
	compound
			Amount added
	pH value		(calculated	Final
	before		as Al or	pH
	addition	Kind of	SiO 2 )	value
Examples	(−)	additives	(wt %)	(−)
Example	4.0	Aluminum acetate	3.0	7.1
160
Example	10.0	Water glass #3	1.0	6.9
161
Example	10.3	Sodium aluminate	5.0	7.5
162
Example	10.2	Sodium aluminate	2.0	7.0
163		Water glass #3	0.5

	TABLE 45
	Properties of acicular hematite particles after
	surface-coating treatment
		Geometrical		Geometrical
	Average	standard	Average	standard
	major	deviation of	minor	deviation of
	axial	major axial	axial	minor axial
	diameter	diameter	diameter	diameter
Examples	(μm)	(−)	(μm)	(−)
Example	0.0731	1.36	0.0114	1.17
160
Example	0.0531	1.42	0.0102	1.20
161
Example	0.0840	1.36	0.0129	1.26
162
Example	0.0582	1.39	0.0108	1.27
163
	Properties of acicular hematite particles
	after surface-coating treatment
	Aspect			S BET /S TEM
	ratio	S BET	S TEM	value
Examples	(−)	(m 2 /g)	(m 2 /g)	(−)
Example	6.4:1	83.1	72.7	1.14
160
Example	5.2:1	92.8	82.7	1.12
161
Example	6.5:1	74.1	64.2	1.15
162
Example	5.4:1	86.0	77.8	1.10
163
	Properties of acicular hematite particles after
	surface-coating treatment
			Surface-
			coating
		Surface-coating	amount of
		amount of	oxide of
	Sintering	hydroxide of	silicon
	preventive	aluminum	(calculated
	Calcu-	Amount	(calculated as	as
Examples	lated as	(wt %)	Al) (wt %)	SiO 2 ) (wt %)
Example	P	0.54	2.93	—
160
Example	P	0.92	—	0.99
161
Example	SiO 2	1.08	4.72	—
162
Example	P	0.89	1.95	0.50
163
	Properties of acicular hematite particles
	after surface-coating treatment
		Soluble sodium
		salt	Soluble sulfate
		(calculated as	(calculated as
		Na)	SO 4 )	pH value
	Examples	(ppm)	(ppm)	(−)
	Example	52	8	9.2
	160
	Example	61	16	9.1
	161
	Example	78	23	9.1
	162
	Example	96	18	9.3
	163

	TABLE 46
		Production of non-magnetic
	Examples,	coating
	Comparative	composition	Non-magnetic
	Examples		Weight ratio	coating
	and	Kind of non-	of particles	composition
	Reference	magnetic	to resin	Viscosity
	Examples	particles	(−)	(cP)
	Example 164	Example 155	5.0	512
	Example 165	Example 156	5.0	435
	Example 166	Example 157	5.0	410
	Example 167	Example 158	5.0	384
	Example 168	Example 159	5.0	307
	Example 169	Example 160	5.0	358
	Example 170	Example 161	5.0	461
	Example 171	Example 162	5.0	384
	Comparative	Hematite	5.0	717
	Example 72	particles 9
	Comparative	Hematite	5.0	614
	Example 73	particles 10
	Comparative	Hematite	5.0	563
	Example 74	particles 11
	Comparative	Hematite	5.0	14,410
	Example 75	particles 12
	Comparative	Hematite	5.0	307
	Example 76	particles 13
	Reference	Reference	5.0	563
	Example 52	Example 50
	Reference	Reference	5.0	589
	Example 53	Example 51
	Examples,
	Comparative	Properties of non-magnetic undercoat layer
	Examples			Surface	Young's
	and			roughness	modulus
	Reference	Thickness	Gloss	Ra	(relative
	Examples	(μm)	(%)	(nm)	value)
	Example 164	0.35	215	6.0	136
	Example 165	0.35	221	5.6	133
	Example 166	0.35	213	6.2	138
	Example 167	0.34	206	6.8	133
	Example 168	0.35	216	6.1	138
	Example 169	0.35	228	5.8	135
	Example 170	0.34	215	6.3	139
	Example 171	0.36	210	6.5	135
	Comparative	0.35	153	21.6	108
	Example 72
	Comparative	0.36	158	18.9	113
	Example 73
	Comparative	0.35	168	16.8	119
	Example 74
	Comparative	0.37	102	32.1	90
	Example 75
	Comparative	0.34	173	16.2	81
	Example 76
	Reference	0.35	194	8.6	128
	Example 52
	Reference	0.35	191	8.8	128
	Example 53

	TABLE 47
	Production of magnetic recording medium
	Examples,			Weight
	Comparative			ratio of
	Examples and	Kind of non-		particles
	Reference	magnetic	Kind of magnetic	to resin
	Examples	substrate	particles	(−)
	Example 172	Example 164	Magnetic metal	5.0
			particles (d)
	Example 173	Example 165	Magnetic metal	5.0
			particles (d)
	Example 174	Example 168	Magnetic metal	5.0
			particles (d)
	Example 175	Example 169	Magnetic metal	5.0
			particles (d)
	Example 176	Example 164	Magnetic metal	5.0
			particles (a)
	Example 177	Example 165	Magnetic metal	5.0
			particles (b)
	Example 178	Example 166	Magnetic metal	5.0
			particles (c)
	Example 179	Example 167	Magnetic metal	5.0
			particles (c)
	Example 180	Example 168	Magnetic metal	5.0
			particles (a)
	Example 181	Example 169	Magnetic metal	5.0
			particles (b)
	Example 182	Example 170	Magnetic metal	5.0
			particles (c)
	Example 183	Example 171	Magnetic metal	5.0
			particles (c)
	Comparative	Comparative	Magnetic metal	5.0
	Example 77	Example 72	particles (d)
	Comparative	Comparative	Magnetic metal	5.0
	Example 78	Example 73	particles (d)
	Comparative	Comparative	Magnetic metal	5.0
	Example 79	Example 74	particles (d)
	Comparative	Comparative	Magnetic metal	5.0
	Example 80	Example 75	particles (d)
	Comparative	Comparative	Magnetic metal	5.0
	Example 81	Example 76	particles (d)
	Reference	Reference	Magnetic metal	5.0
	Example 54	Example 52	particles (d)
	Reference	Reference	Magnetic metal	5.0
	Example 55	Example 53	particles (d)
	Examples,
	Comparative	Properties of magnetic recording medium
	Examples	Thickness
	and	of coating	Coercive	Br/Bm
	Reference	film	force (Hc)	value	Gloss
	Examples	(μm)	(Oe)	(−)	(%)
	Example 172	1.0	1,963	0.87	219
	Example 173	1.1	1,958	0.87	218
	Example 174	1.0	1,968	0.88	223
	Example 175	1.0	1,960	0.87	225
	Example 176	1.0	1,972	0.88	228
	Example 177	1.1	1,996	0.88	224
	Example 178	1.0	2,136	0.89	226
	Example 179	1.1	2,134	0.89	221
	Example 180	1.0	1,986	0.89	233
	Example 181	1.0	2,001	0.89	235
	Example 182	1.0	2,127	0.90	238
	Example 183	1.1	2,130	0.89	221
	Comparative	1.2	1,956	0.78	163
	Example 77
	Comparative	1.2	1,958	0.76	173
	Example 78
	Comparative	1.1	1,965	0.82	178
	Example 79
	Comparative	1.1	1,959	0.80	168
	Example 80
	Comparative	1.1	1,961	0.78	156
	Example 81
	Reference	1.1	1,976	0.86	213
	Example 54
	Reference	1.0	1,980	0.86	210
	Example 55
	Properties of magnetic recording medium
Examples,			Linear
Comparative			absorp-
Examples	Surface	Young's	tion	Durability
and	rough-	modulus	coeffi-	Running
Reference	ness Ra	(relative	cient	time	Scratch
Examples	(nm)	value)	(μm −1 )	(min)	resistance
Example 172	6.2	136	1.28	21.3	B
Example 173	6.2	136	1.27	21.6	B
Example 174	5.9	138	1.30	22.5	A
Example 175	5.8	137	1.31	22.4	A
Example 176	6.1	137	1.28	29.6	A
Example 177	5.7	135	1.27	28.9	A
Example 178	5.7	138	1.31	≧30	A
Example 179	6.2	135	1.29	27.1	B
Example 180	5.7	139	1.29	≧30	A
Example 181	5.6	136	1.32	≧30	A
Example 182	5.5	140	1.27	≧30	A
Example 183	6.1	138	1.26	≧30	A
Comparative	19.8	118	1.01	11.6	D
Example 77
Comparative	14.6	116	1.03	12.6	D
Example 78
Comparative	13.2	115	1.09	14.5	D
Example 79
Comparative	15.1	116	1.11	11.6	D
Example 80
Comparative	18.6	118	1.12	9.6	D
Example 81
Reference	7.4	129	1.18	18.6	C
Example 54
Reference	7.8	129	1.16	19.3	C
Example 55
	Examples,
	Comparative
	Examples	Properties of magnetic recording medium
	and	Corrosion resistance
	Reference	Percentage of change	Percentage of change
	Examples	in coercive force (%)	in B m (%)
	Example 172	4.5	4.3
	Example 173	3.8	3.6
	Example 174	4.0	2.8
	Example 175	4.2	3.8
	Example 176	4.1	1.6
	Example 177	3.6	0.8
	Example 178	2.1	0.6
	Example 179	1.8	0.9
	Example 180	2.6	2.1
	Example 181	4.6	1.8
	Example 182	1.6	2.1
	Example 183	3.1	4.2
	Comparative	38.6	26.8
	Example 77
	Comparative	43.0	26.5
	Example 78
	Comparative	28.3	32.5
	Example 79
	Comparative	31.6	28.2
	Example 80
	Comparative	21.3	19.6
	Example 81
	Reference	17.8	18.2
	Example 54
	Reference	16.5	16.8
	Example 55

	TABLE 48
	Properties of acicular goethite particles
				Geometrical
				standard
				deviation of
			Average major	major axial
	Kind of		axial diameter	diameter
	Precursor	Shape	(μm)	(−)
	Goethite	Spindle-	0.0586	1.53
	particles 10	shaped
	Goethite	Acicular	0.0893	1.52
	particles 11
	Properties of acicular goethite particles
			Geometrical
			standard
			deviation of
		Average minor	minor axial	Aspect
	Kind of	axial diameter	diameter	ratio
	Precursor	(μm)	(−)	(−)
	Goethite	0.0084	1.41	7.0:1
	particles 10
	Goethite	0.0118	1.42	7.6:1
	particles 11
	Properties of acicular goethite particles
			Soluble
			sodium	Soluble
			salt	sulfate
	BET		(calcu-	(calcu-
	specific	Content	lated	lated	pH
Kind of	surface	of Al	as Na)	as SO 4 )	value
Precursor	area (m 2 /g)	(wt %)	(ppm)	(ppm)	(−)
Goethite	242.6	2.86	412	381	5.9
particles 10
Goethite	190.6	1.32	832	265	7.6
particles 11

	TABLE 49
	Properties of acicular
	goethite particles subjected
	to anti-sintering treatment
				Geometrical
			Average	standard
			major axial	deviation of
	Kind of	Kind of	diameter	major axial
	Precursor	Precursor	(μm)	diameter (−)
	Goethite	Goethite	0.0808	1.52
	particles 12	particles
		used in
		Example 12
	Goethite	Goethite	0.0586	1.53
	particles 13	particles 10
	Goethite	Goethite	0.0893	1.52
	particles 14	particles 11
	Properties of acicular goethite particles
	subjected to anti-sintering treatment
			Geometrical
			standard
			deviation of
		Average minor	minor axial	Aspect
	Kind of	axial diameter	diameter	ratio
	Precursor	(μm)	(−)	(−)
	Goethite	0.0113	1.41	7.2:1
	particles 12
	Goethite	0.0084	1.41	7.0:1
	particles 13
	Goethite	0.0118	1.42	7.6:1
	particles 14
	Properties of acicular goethite particles
	subjected to anti-sintering treatment
		Sintering
	BET specific	preventive
	Kind of	surface area	Calculated	Amount
	Precursor	(m 2 /g)	as	(wt %)
	Goethite	200.6	P	1.16
	particles 12
	Goethite	240.6	P	1.59
	particles 13
	Goethite	189.8	SiO 2	1.11
	particles 14
	Properties of acicular goethite particles
	subjected to anti-sintering treatment
		Soluble	Soluble
		sodium salt	sulfate
	Content	(calculated	(calculated
Starting	of Al	as Na)	as SO 4 )	pH value
particles	(wt %)	(ppm)	(ppm)	(−)
Goethite	3.36	765	362	7.6
particles 12
Goethite	2.85	532	412	5.8
particles 13
Goethite	1.31	963	282	7.6
particles 14

TABLE 50
Kind of		Conditions of heat treatment
goethite	Kind of		Temperature	Time
particies	precursor	Atmosphere	(° C.)	(min)
Goethite	Goethite	Air	130	40
particles 15	particles 12
Goethite	Goethite	Air	150	30
particles 16	particles 13
Goethite	Goethite	Air	180	20
particles 17	particles 14
Goethite	Goethite	Air	80	30
particles 18	particles 13

	TABLE 51
	Properties of acicular goethite particles
	heat-treated
		Geometrical		Geometrical
	Average	standard	Average	standard
	major	deviation of	minor	deviation of
Kind of	axial	major axial	axial	minor axial
goethite	diameter	diameter	diameter	diameter
particles	(μm)	(−)	(μm)	(−)
Goethite	0.0808	0.37	0.0112	1.26
particles 15
Goethite	0.0586	1.38	0.0084	1.27
particles 16
Goethite	0.0893	1.36	0.0117	1.29
particles 17
Goethite	0.0586	1.52	0.0084	1.41
particles 18
	Properties of acicular goethite particles
	heat-treated
			BET
			specific	Sintering
	Kind of	Aspect	surface	preventive
	goethite	ratio	area	Calculated	Content
	particles	(−)	(m 2 /g)	as	(wt %)
	Goethite	7.2:1	201.8	P	1.17
	particles 15
	Goethite	7.0:1	239.4	P	1.59
	particles 16
	Goethite	7.6:1	190.1	SiO 2	1.12
	particles 17
	Goethite	7.0:1	239.8	P	1.59
	particles 18
	Properties of acicular goethite particles
	heat-treated
		Soluble	Soluble
		sodium salt	sulfate
Kind of	Content	(calculated	(calculated
goethite	of Al	as Na)	as SO 4 )	pH value
particles	(wt %)	(ppm)	(ppm)	(−)
Goethite	3.36	681	293	7.1
particles 15
Goethite	2.86	543	421	5.9
particles 16
Goethite	1.31	1,128	462	7.6
particles 17
Goethite	2.85	652	413	5.9
particles 18

TABLE 52
Kind of		Conditions of heat-dehydration
particles	Kind of	treatment
to be	goethite		Temperature	Time
treated	particles	Atmosphere	(° C.)	(min)
Hematite	Goethite	Air	330	30
particles 14	particles 15
Hematite	Goethite	Air	350	30
particles 15	particles 16
Hematite	Goethite	Air	370	25
particles 16	particles 17
Hematite	Goethite	Air	340	30
particles 17	particles 18

	TABLE 53
	Properties of low-density acicular hematite
	particles
		Geometrical		Geometrical
	Average	standard	Average	standard
Kind of	major	deviation of	minor	deviation of
particles	axial	major axial	axial	minor axial
to be	diameter	diameter	diameter	diameter
treated	(μm)	(−)	(μm)	(−)
Hematite	0.0727	1.37	0.0110	1.27
particles 14
Hematite	0.0533	1.38	0.0085	1.27
particles 15
Hematite	0.0822	1.36	0.0116	1.29
particles 16
Hematite	0.0539	1.52	0.0086	1.42
particles 17
	Properties of low-density acicular hematite
	particles
	Kind of	Aspect			S BET /S TEM
	particles	ratio	S BET	S TEM	value
	to be treated	(−)	(m 2 /g)	(m 2 /g)	(−)
	Hematite	6.6:1	218.6	75.2	2.91
	particles 14
	Hematite	6.3:1	251.2	97.7	2.57
	particles 15
	Hematite	7.1:1	209.6	71.0	2.95
	particles 16
	Hematite	6.3:1	246.8	96.6	2.56
	particles 17
	Properties of low-density acicular hematite
	particles
	Kind of	Sintering
	particles	preventive
	to be	Calculated	Amount	Content of Al
	treated	as	(wt %)	(wt %)
	Hematite	P	1.30	3.73
	particles 14
	Hematite	P	1.74	3.18
	particles 15
	Hematite	SiO 2	1.24	1.45
	particles 16
	Hematite	P	1.75	3.17
	particles 17
	Properties of low-density acicular hematite
	particles
		Soluble sodium
	Kind of	salt	Soluble sulfate
	particles	(calculated as	(calculated as
	to be	Na)	SO 4 )	pH value
	treated	(ppm)	(ppm)	(−)
	Hematite	1,321	721	7.2
	particles 14
	Hematite	1,165	583	5.5
	particles 15
	Hematite	1,811	412	7.8
	particles 16
	Hematite	1,011	562	6.1
	particles 17

TABLE 54
	Kind of
Kind of	particles	Conditions of heat-treatment
hematite	to be		Temperature	Time
particles	treated	Atmosphere	(° C.)	(min)
Hematite	Hematite	Air	670	20
particles	particles 14
18
Hematite	Hematite	Air	630	30
particles	particles 15
19
Hematite	Hematite	Air	610	40
particles	particles 16
20
Hematite	Goethite	Air	640	30
particles	particles 16
21
Hematite	Goethite	Air	650	30
particles	particles 17
22
Hematite	Goethite	Air	650	30
particles	particles 18
23
Hematite	Hematite	Air	650	30
particles	particles 17
24
Hematite	Hematite	Air	440	30
particles	particles 16
25
Hematite	Hematite	Air	910	30
particles	particles 16
26

	TABLE 55
	Properties of high-density acicular hematite particles
	Average	Geometrical	Average	Geometrical
	major	standard	minor	standard
Kind of	axial	deviation of	axial	deviation of
hematite	diameter	major axial	diameter	minor axial
particles	(μm)	diameter (−)	(μm)	diameter (−)
Hematite	0.0726	1.37	0.0111	1.28
particles 18
Hematite	0.0532	1.39	0.0086	1.27
particles 19
Hematite	0.0821	1.36	0.0117	1.29
particles 20
Hematite	0.0521	1.38	0.0087	1.29
particles 21
Hematite	0.0412	1.63	0.0150	1.44
particles 22
Hematite	0.0521	1.61	0.0091	1.41
particles 23
Hematite	0.0538	1.53	0.0087	1.42
particles 24
Hematite	0.0822	1.36	0.0116	1.29
particles 25
Hematite	0.0642	1.58	0.0213	1.43
particles 26
	Properties of high-density acicular hematite particles
Kind of	Aspect			S BET /S TEM
hematite	ratio	S BET	S TEM	value
particles	(−)	(m 2 /g)	(m 2 /g)	(−)
Hematite	6.5:1	84.6	74.6	1.13
particles 18
Hematite	6.2:1	95.6	96.7	0.99
particles 19
Hematite	7.0:1	73.2	70.4	1.04
particles 20
Hematite	6.0:1	91.8	95.8	0.96
particles 21
Hematite	2.7:1	38.3	60.6	0.63
particles 22
Hematite	5.7:1	63.6	91.9	0.69
particles 23
Hematite	6.2:1	73.8	95.6	0.77
particles 24
Hematite	7.1:1	189.9	71.0	2.67
particles 25
Hematite	3.0:1	39.6	42.1	0.94
particles 26
	Properties of high-density acicular hematite particles
Kind of	Sintering
hematite	preventive	Content of Al
particles	Calculated as	Amount (wt %)	(wt %)
Hematite	P	1.30	3.73
particles 18
Hematite	P	1.75	3.18
particles 19
Hematite	SiO 2	1.24	1.45
particles 20
Hematite	P	1.76	3.18
particles 21
Hematite	—	—	3.19
particles 22
Hematite	P	1.76	3.18
particles 23
Hematite	P	1.75	3.17
particles 24
Hematite	SiO 2	1.24	1.45
particles 25
Hematite	SiO 2	1.24	1.46
particles 26
	Properties of high-density acicular hematite particles
		Soluble	Soluble
		sodium salt	sulfate
Kind of	Resin	(calculated	(calculated
hematite	adsorp-	as Na)	as SO 4 )	pH value
particles	tion (%)	(ppm)	(ppm)	(−)
Hematite	74.6	2,221	1,652	6.9
particles 18
Hematite	73.2	2,102	1,632	6.3
particles 19
Hematite	71.3	1,821	2,236	5.6
particles 20
Hematite	72.6	1,321	1,821	5.3
particles 21
Hematite	61.6	2,016	1,621	6.3
particles 22
Hematite	67.1	1,211	821	5.9
particles 23
Hematite	68.3	2,182	2,265	5.5
particles 24
Hematite	67.2	1,683	1,821	5.3
particles 25
Hematite	62.1	1,321	1,865	5.7
particles 26

TABLE 56
Examples		Heat treatment in aqueous
and	Kind of	alkali solution
Reference	particles to	pH value	Temperature	Time
Examples	be treated	(−)	(° C.)	(min)
Example	Hematite	13.1	90	180
184	particles 18
Example	Hematite	13.5	95	120
185	particles 19
Example	Hematite	13.6	98	150
186	particles 20
Example	Hematite	13.8	85	240
187	particles 21
Reference	Hematite	8.2	85	180
Example 56	particles 18
Reference	Hematite	13.4	40	180
Example 57	particles 18

	TABLE 57
	Properties of high-density acicular hematite
	particles heat-treated in aqueous alkali solution
		Geometrical		Geometrical
	Average	standard	Average	standard
	major	deviation of	minor	deviation of
Examples and	axial	major axial	axial	minor axial
Reference	diameter	diameter	diameter	diameter
Examples	(μm)	(−)	(μm)	(−)
Example 184	0.0725	1.36	0.0110	1.28
Example 185	0.0531	1.38	0.0086	1.27
Example 186	0.0821	1.35	0.0117	1.29
Example 187	0.0520	1.37	0.0088	1.29
Reference	0.0726	1.37	0.0111	1.28
Example 56
Reference	0.0726	1.37	0.0111	1.28
Example 57
	Properties of high-density acicular hematite
	particles heat-treated in aqueous alkali
	solution
	Examples and	Aspect			S BET /S TEM
	Reference	ratio	S BET	S TEM	value
	Examples	(−)	(m 2 /g)	(m 2 /g)	(−)
	Example 184	6.6:1	85.1	75.2	1.13
	Example 185	6.2:1	96.1	96.7	0.99
	Example 186	7.0:1	74.1	70.4	1.05
	Example 187	5.9:1	90.6	94.8	0.96
	Reference	6.5:1	83.2	74.6	1.12
	Example 56
	Reference	6.5:1	84.6	74.6	1.13
	Example 57
	Properties of high-density acicular hematite
	particles heat-treated in aqueous alkali
	solution
		Sintering
	Examples and	preventive
	Reference	Calculated	Amount	Content of Al
	Examples	as	(wt %)	(wt %)
	Example 184	P	0.64	3.72
	Example 185	P	0.83	3.16
	Example 186	SiO 2	1.01	1.44
	Example 187	P	0.80	3.16
	Reference	P	1.30	3.73
	Example 56
	Reference	P	1.31	3.76
	Example 57
	Properties of high-density acicular hematite
	particles heat-treated in aqueous alkali
	solution
		Soluble	Soluble
		sodium salt	sulfate
Examples and	Resin	(calculated	(calculated
Reference	adsorp-	as Na)	as SO 4 )	pH value
Examples	tion (%)	(ppm)	(ppm)	(−)
Example 184	75.3	72	18	9.3
Example 185	76.8	112	10	9.1
Example 186	78.2	56	23	9.2
Example 187	77.8	81	29	8.9
Reference	61.8	396	192	7.3
Example 56
Reference	64.1	410	190	7.4
Example 57

TABLE 58
	Kind of
	particles to be	Concentration of water
Examples	treated	suspension (g/liter)
Example	Example	45
188	184
Example	Example	45
189	185
Example	Example	45
190	186
Example	Example	45
191	187
	Coating treatment with aluminum compound or
	silicon compound
	Aluminum compound or silicon
	compound
			Amount added
	pH value		(calculated	Final
	before		as Al or	pH
	addition	Kind of	SiO 2 )	value
Examples	(−)	additives	(wt %)	(−)
Example	4.0	Aluminum acetate	3.0	7.0
188
Example	10.0	Water glass #3	1.0	6.9
189
Example	10.3	Sodium aluminate	5.0	7.2
190
Example	10.1	Sodium aluminate	2.0	7.0
191		Water glass #3	0.5

	TABLE 59
	Properties of acicular hematite particles after
	surface-coating treatment
		Geometrical		Geometrical
	Average	standard	Average	standard
	major	deviation of	minor	deviation of
	axial	major axial	axial	minor axial
	diameter	diameter	diameter	diameter
Examples	(μm)	(−)	(μm)	(−)
Example	0.0725	1.36	0.0111	1.28
188
Example	0.0531	1.38	0.0086	1.26
189
Example	0.0820	1.36	0.0117	1.28
190
Example	0.0523	1.38	0.0087	1.29
191
	Properties of acicular hematite particles after
	surface-coating treatment
	Aspect			S BET /S TEM
	ratio	S BET	S TEM	value
Examples	(−)	(m 2 /g)	(m 2 /g)	(−)
Example	6.5:1	85.6	74.6	1.15
188
Example	6.2:1	97.1	96.7	1.00
189
Example	7.0:1	75.1	70.4	1.07
190
Example	6.0:1	92.9	95.8	0.97
191
	Properties of acicular hematite particles after
	surface-coating treatment
	Sintering
	preventive
		Calculated	Amount	Content of Al
	Examples	as	(wt %)	(wt %)
	Example	P	0.62	3.71
	188
	Example	P	0.80	3.15
	189
	Example	SiO 2	1.00	1.41
	190
	Example	P	0.78	3.14
	191
	Properties of acicular hematite particles after
	surface-coating treatment
	Surface-coating	Surface-coating
	amount of	amount of oxide of
	hydroxide of	silicon
	aluminum	(calculated as	Resin
	(calculated as Al)	SiO 2 )	adsorption
Examples	(wt %)	(wt %)	(%)
Example	2.93	—	81.6
188
Example	—	0.99	83.2
189
Example	4.71	—	84.1
190
Example	1.96	0.49	81.8
191
	Properties of acicular hematite particles after
	surface-coating treatment
		Soluble sodium	Soluble sulfate
		salt (calculated	(calculated as
		as Na)	SO 4 )	pH value
	Examples	(ppm)	(ppm)	(−)
	Example	78	13	9.1
	188
	Example	56	8	9.0
	189
	Example	82	12	9.0
	190
	Example	52	26	9.2
	191

	TABLE 60
	Examples,	Production of non-magnetic
	Comparative	coating composition	Non-magnetic
	Examples	Kind of	Weight ratio	coating
	and	acicular	of particles	composition
	Reference	hematite	to resin	Viscosity
	Examples	particles	(−)	(cP)
	Example 192	Example 184	5.0	384
	Example 193	Example 185	5.0	358
	Example 194	Example 186	5.0	364
	Example 195	Example 187	5.0	435
	Example 196	Example 188	5.0	512
	Example 197	Example 189	5.0	461
	Example 198	Example 190	5.0	307
	Example 199	Example 191	5.0	410
	Comparative	Hematite	5.0	870
	Example 82	particles 22
	Comparative	Hematite	5.0	794
	Example 83	particles 23
	Comparative	Hematite	5.0	717
	Example 84	particles 24
	Comparative	Hematite	5.0	11,240
	Example 85	particles 25
	Comparative	Hematite	5.0	307
	Example 86	particles 26
	Reference	Reference	5.0	461
	Example 58	Example 56
	Reference	Reference	5.0	502
	Example 59	Example 57
	Examples,	Properties of non-magnetic undercoat
	Comparative	layer
	Examples			Surface	Young's
	and			roughness	modulus
	Reference	Thickness	Gloss	Ra	(relative
	Examples	(μm)	(%)	(nm)	value)
	Example 192	0.35	218	6.1	136
	Example 193	0.35	225	5.7	133
	Example 194	0.35	218	6.0	138
	Example 195	0.35	211	6.4	134
	Example 196	0.35	218	6.0	137
	Example 197	0.35	225	5.6	136
	Example 198	0.35	216	6.1	137
	Example 199	0.35	214	6.3	135
	Comparative	0.35	154	20.1	110
	Example 82
	Comparative	0.35	161	17.8	111
	Example 83
	Comparative	0.35	164	17.2	116
	Example 84
	Comparative	0.37	113	28.3	98
	Example 85
	Comparative	0.35	176	15.4	79
	Example 86
	Reference	0.35	198	8.2	128
	Example 58
	Reference	0.36	196	8.5	128
	Example 59

	TABLE 61
	Examples,	Production of magnetic recording medium
	Weight
	Comparative			ratio of
	Examples and	Kind of non-		particles
	Reference	magnetic	Kind of magnetic	to resin
	Examples	substrate,	particles	(−)
	Example 200	Example 192	Magnetic metal	5.0
			particles (d)
	Example 201	Example 195	Magnetic metal	5.0
			particles (d)
	Example 202	Example 196	Magnetic metal	5.0
			particles (d)
	Example 203	Example 191	Magnetic metal	5.0
			particles (d)
	Example 204	Example 192	Magnetic metal	5.0
			particles (a)
	Example 205	Example 193	Magnetic metal	5.0
			particles (b)
	Example 206	Example 194	Magnetic metal	5.0
			particles (c)
	Example 207	Example 195	Magnetic metal	5.0
			particles (c)
	Example 208	Example 196	Magnetic metal	5.0
			particles (a)
	Example 209	Example 197	Magnetic metal	5.0
			particles (b)
	Example 210	Example 198	Magnetic metal	5.0
			particles (c)
	Example 211	Example 199	Magnetic metal	5.0
			particles (c)
	Comparative	Comparative	Magnetic metal	5.0
	Example 87	Example 82	particles (d)
	Comparative	Comparative	Magnetic metal	5.0
	Example 88	Example 83	particles (d)
	Comparative	Comparative	Magnetic metal	5.0
	Example 89	Example 84	particles (d)
	Comparative	Comparative	Magnetic metal	5.0
	Example 90	Example 85	particles (d)
	Comparative	Comparative	Magnetic metal	5.0
	Example 91	Example 86	particles (d)
	Reference	Reference	Magnetic metal	5.0
	Example 60	Example 58	particles (d)
	Reference	Reference	Magnetic metal	5.0
	Example 61	Example 59	particles (d)
	Examples,	Properties of magnetic recording medium
	Comparative	Thickness
	Examples	of
	and	magnetic	Coercive	Br/Bm
	Reference	layer	force (Hc)	value	Gloss
	Examples	(μm)	(Oe)	(−)	(%)
	Example 200	1.0	1,968	0.87	223
	Example 201	1.0	1,971	0.87	221
	Example 202	1.0	1,965	0.88	226
	Example 203	1.1	1,962	0.89	228
	Example 204	1.0	1,981	0.89	235
	Example 205	1.0	1,999	0.87	230
	Example 206	1.0	2,130	0.87	226
	Example 207	1.1	2,133	0.88	233
	Example 208	1.0	1,983	0.88	241
	Example 209	1.1	2,001	0.89	238
	Example 210	1.0	2,129	0.89	248
	Example 211	1.0	2,125	0.90	229
	Comparative	1.1	1,960	0.76	168
	Example 87
	Comparative	1.0	1,963	0.75	175
	Example 88
	Comparative	1.1	1,968	0.81	176
	Example 89
	Comparative	1.1	1,963	0.79	171
	Example 90
	Comparative	1.0	1,965	0.78	152
	Example 91
	Reference	1.1	1,978	0.86	214
	Example 60
	Reference	1.1	1,983	0.86	211
	Example 61
	Properties of magnetic recording medium
Examples,			Linear
Comparative			absorp-
Examples	Surface	Young's	tion	Durability
and	rough-	modulus	coeffi-	Running
Reference	ness Ra	(relative	cient	time	Scratch
Examples	(nm)	value)	(μm −1 )	(min)	resistance
Example 200	6.4	137	1.26	27.1	A
Example 201	6.3	136	1.27	27.3	B
Example 202	6.0	138	1.28	28.1	A
Example 203	5.8	137	1.28	28.5	A
Example 204	6.0	137	1.29	≧30	A
Example 205	6.2	135	1.29	29.6	B
Example 206	6.3	138	1.31	≧30	A
Example 207	6.0	135	1.28	28.6	B
Example 208	5.8	139	1.27	≧30	A
Example 209	5.6	136	1.31	≧30	A
Example 210	5.2	141	1.27	≧30	A
Example 211	5.9	136	1.28	≧30	A
Comparative	18.4	110	1.03	12.6	D
Example 87
Comparative	13.6	115	1.05	13.1	D
Example 88
Comparative	12.9	119	1.10	13.6	D
Example 89
Comparative	15.0	93	1.10	12.3	D
Example 90
Comparative	18.4	83	1.03	10.6	D
Example 91
Reference	7.8	129	1.17	19.2	C
Example 60
Reference	7.6	129	1.17	18.6	C
Example 61
	Examples,
	Comparative
	Examples	Properties of magnetic recording medium
	and	Corrosion resistance
	Reference	Percentage of change	Percentage of change
	Examples	in coercive force (%)	in B m (%)
	Example 200	4.1	3.9
	Example 201	3.8	4.4
	Example 202	4.0	2.6
	Example 203	3.6	3.1
	Example 204	3.1	2.1
	Example 205	2.6	1.6
	Example 206	1.6	1.8
	Example 207	2.1	1.8
	Example 208	2.6	2.3
	Example 209	1.8	3.6
	Example 210	0.6	0.4
	Example 211	3.2	0.9
	Comparative	31.2	28.1
	Example 87
	Comparative	26.5	26.5
	Example 88
	Comparative	31.2	23.1
	Example 89
	Comparative	41.3	32.6
	Example 90
	Comparative	26.5	31.2
	Example 91
	Reference	16.1	17.0
	Example 60
	Reference	15.6	16.0
	Example 61

Claims

1. Acicular hematite particles comprising an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35 and a pH value of not less than 8, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO4.

2. Acicular hematite particles according to claim 1, which further comprise a coat formed on at least a part of the surface of said acicular hematite particle, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in amount of 0.05 to 50% by weight, calculated as Al or SiO2, based on the weight of said acicular hematite particles.

3. Acicular hematite particles according to claim 1, which further comprise aluminum existing in the particle in an amount of 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles.

4. Acicular hematite particles according to claim 3, which further comprise a resin adsorptivity of not less than 65%.

5. Acicular hematite particles according to claim 3, which further comprise a coat formed on at least a part of the surface of said acicular hematite particle, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in amount of 0.05 to 50% by weight, calculated as Al or SiO2, based on the weight of said acicular hematite particles.

6. Acicular hematite particles according to claim 1, which further comprise a geometrical standard deviation of major axis diameter of not more than 1.50 and a BET specific surface area of 35.9 to 180 m2/g.

7. Acicular hematite particles according to claim 1, which further comprise an aspect ratio (average major axis diameter/average minor axis diameter) of 2:1 to 20:1, and a ratio value of a BET specific surface area SBET to a specific surface area STEM of 0.5 to 2.5 (the specific surface area STEM being calculated from the major axis diameter and the minor axis diameter which were measured from the particles in an electron micrograph of the acicular hematite particles).

8. Acicular hematite particles according to claim 1, which further comprise a geometrical standard deviation of major axis diameter of not more than 1.50, a geometrical standard deviation of minor axis diameter of not more than 1.30, a BET specific surface area of 40 to 150 m2/g and an average major axis diameter of 0.01 to 0.2 μm.

9. A magnetic recording medium comprising:a non-magnetic base film; a non-magnetic undercoat layer formed on said non-magnetic base film, comprising a binder resin and acicular hematite particles having an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35 and a pH value of not less than 8, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO4; and a magnetic coating film comprising a binder resin and magnetic acicular metal particles containing iron as a main component.

10. A magnetic recording medium according to claim 9, wherein said acicular hematite particles have a coat formed on at least a part of the surface of said acicular hematite particle and comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of 5 to 50% by weight based on the total weight of the acicular hematite particles.

11. A magnetic recording medium according to claim 9, wherein said acicular hematite particles contain aluminum existing in the particle in an amount of 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles.

12. Magnetic recording medium according to claim 11, wherein said acicular hematite particles have a resin adsorptivity of not less than 65%.

13. A magnetic recording medium according to claim 11, wherein said acicular hematite particles have a coat formed on at least a part of the surface of said acicular hematite particle and comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of 5 to 50% by weight based on the total weight of the acicular hematite particles.

14. A magnetic recording medium according to claim 11, which further comprises a gloss of coating film of 194 to 300%, a surface roughness Ra of coating film of not more than 9.6 nm, a linear absorption of coating film of 1.10 to 2.00 μm—1, and a running durability of not less than 22 minutes.

15. A magnetic recording medium according to claim 9, wherein said acicular hematite particles have a geometrical standard deviation of major axis diameter of not more than 1.50 and a BET specific surface area of 35.9 to 180 m2/g.

16. A magnetic recording medium according to claim 9, wherein said acicular hematite particles have a geometrical standard deviation of major axis diameter of not more than 1.50, a geometrical standard deviation of minor axis diameter of not more than 1.30, a BET specific surface area of 40 to 150 m2/g and an average major axis diameter of 0.01 to 0.2 μm.

17. A magnetic recording medium according to claim 16, which further comprises a gloss of coating film of 196 to 300%, a surface roughness Ra of coating film of not more than 9.4 nm, and a linear absorption of coating film of 1.10 to 2.00 μm−1.

18. A magnetic recording medium according to claim 9, wherein said magnetic acicular metal particles containing iron as a main component contain aluminum in an amount of 0.05 to 10% by weight, calculated as Al, based on the weight of said magnetic acicular metal particles containing iron as a main component.

19. A magnetic recording medium according to claim 18, which further comprises a gloss of coating film of 194 to 300%, a surface roughness Ra of coating film of not more than 9.6 nm, a linear absorption of coating film of 1.10 to 2.00 μm−1, and a running durability of not less than 24 minutes.

20. A magnetic recording medium according to claim 9, which further comprises a gloss of coating film of 192 to 300%, a surface roughness Ra of coating film of not more than 9.8 nm, and a linear absorption of coating film of 1.10 to 2.00 μm−1.

21. A magnetic recording medium according to claim 9, which further comprises the change in the coercive force of not more than 10.0% and the change in the saturation magnetic flux density of not more than 10.0%.

22. A non-magnetic substrate comprising:a non-magnetic base film; and a non-magnetic undercoat layer formed on said non-magnetic base film, comprising a binder resin and acicular hematite particles having an average major axis diameter of 0.004 to 0.295 μm, a geometrical standard deviation of minor axis diameter of not more than 1.35 and a pH value of not less than 8, and containing not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO4.

23. A non-magnetic substrate according to claim 22, wherein said acicular hematite particles have a coat formed on at least a part of the surface of said acicular hematite particle and comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of 5 to 50% by weight based on the total weight of the acicular hematite particles.

24. A non-magnetic substrate according to claim 22, wherein said acicular hematite particles contain aluminum existing within the particle in an amount of 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles.

25. A non-magnetic substrate according to claim 24, wherein said acicular hematite particles have a coat formed on at least a part of the surface of said acicular hematite particle and comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of 5 to 50% by weight based on the total weight of the acicular hematite particles.

26. A non-magnetic substrate according to claim 24, which further comprises a gloss of coating film of 191 to 300% and a surface roughness Ra of coating film of 0.5 to 9.3 nm.

27. A non-magnetic substrate according to claim 22, wherein said acicular hematite particles further have a geometrical standard deviation of major axis diameter of not more than 1.50 and a BET specific surface area of 35.9 to 180 m2/g.

28. A non-magnetic substrate according to claim 22, wherein said acicular hematite particles have a geometrical standard deviation of major axis diameter of not more than 1.50, a geometrical standard deviation of minor axis diameter of not more than 1.30, a BET specific surface area of 40 to 150 m2/g and an average major axis diameter of 0.01 to 0.2 μm.

29. A non-magnetic substrate according to claim 28, which further comprises a gloss of coating film of 193 to 300% and a surface roughness Ra of coating film of 0.5 to 9.0 nm.

30. A non-magnetic substrate according to claim 22, which further comprises a gloss of coating film of 189 to 300% and a surface roughness Ra of coating film of 0.5 to 9.6 nm.