Patent Application
20050028951
The present invention relates to smooth base stock for high quality imaging, and, more particularly, to smooth base stock composed of nonstandard fibers.
Obtaining photo quality prints by conventional silver halide photography, ink jet printing, dye sublimation, thermal dye transfer, or other like methods is dependent upon the smoothness of the base stock forming the base material for such printing papers. The base stock is typically comprised of cellulose fibers, upon the top and/or bottom side of which polyolefin or polyester films may be extruded or laminated. An imaging coating is applied on top of the poly film on one or both sides. Examples of imaging coatings include photosensitive emulsions, water or pigment-based ink receptive layers, thermal dye-recording layers, or other specialty coatings appropriate for a given imaging method. The smoothness of the base stock determines the appearance and surface uniformity of the final image.
It is known that the addition of mineral pigments to a raw stock in place of fiber can improve surface smoothness. Doing so however, can create problems when the basis weight, caliper, and stiffness properties of the base stock are specified. Too much filler pigment can result in a decrease in caliper and stiffness for a desired basis weight and smoothness.
Being able to achieve high levels of smoothness in a paper base stock is primarily influenced by the cellulose fiber properties, the uniformity of fiber distribution comprising the base stock (formation), and the calendering or densification processes of the base stock. Fibers can be thought of as hollow tubes with a characteristic length and cell wall thickness.
It is well known that formation (uniformity of fiber distribution within a paper base stock), fiber length and fiber wall thickness contribute to smoothness. The more uniform the fiber distribution, the better formation, and the less chance of high and low areas occurring on the surface of paper causing a rough surface. Softwood fibers are significantly longer than hardwood fibers; therefore, hardwood fibers are preferred when smoothness is a desired property. Shorter fibers also contribute to better formation due to a lower tendency to flocculate and by their ability to pack together more tightly. Fiber cell wall thickness will influence the compressibility or ability of fibers to flatten during drying and when compressed during densification. Fibers with thick cell walls are stiffer and resist collapse whereas fibers with thin cell walls are easily compressed and produce a dense, well-formed base stock. Hardwood fibers with a thin cell wall also provide more fibers per gram, further contributing to formation and smoothness. To optimize the benefits that fiber characteristics contribute to a smooth base stock, base stocks used for photographic quality papers typically are comprised of fibers that are short and/or have thin fiber cell walls.
Accordingly, conventional techniques for providing a uniform base sheet having the desired smoothness utilize short fibers. In accordance with these conventional methods, the desired fiber length-weighted average, as measured by a Kajaani fiber analyzer, has been described to be between 0.4 to 0.58 mm (U.S. Pat. Nos. 6,391,532, 6,364,997, and 6,107,014), equal to or less than 0.5 mm (U.S. Pat. Nos. 5,250,496 and 5,288,690) or a length between 0.4 to 0.9 mm (JP-A-61-69649).
The present invention provides a smooth base stock for high quality imaging. The base stock is characterized in that it utilizes nonstandard fibers yet provides the smoothness and other physical properties required for high quality imaging. In accordance with certain embodiments, the base stock of the present invention comprises fibers having a length weighted average of greater than 0.9 mm and the base stock has a roughness of from about 1.2 micron Ra to about 1.5 micron Ra.
In accordance with another embodiment of the present invention, the base stock is composed of fibers having a population below about 10×106 fibers per gram. According to certain aspects of the invention, the base stock in accordance with the present invention exhibits the desired smoothness and yet is substantially free of filler pigments.
In accordance with another aspect of the invention, a method for producing a base stock for use in forming a recording material is disclosed. The method includes the steps of providing a fiber slurry composed of fibers having a length weighted average fiber length above about 1 mm, forming a web of the fibers and subjecting the web to a smoothing operation to provide a base stock having a roughness of from about 1.2 micron Ra to about 1.5 micron Ra. The method may also include a refining step wherein the fibers, after the refinement treatment, have a length-weighted average fiber length of above about 0.9 mm.
In accordance with another aspect of the invention, a support material for an image forming system is provided. The support material includes a base stock composed of fibers having a length weighted average of greater than about 0.9 mm and a roughness of from about 1.2 micron Ra to about 1.5 micron Ra and an image forming or image receptive coating layer on at least one side of the base stock wherein the image forming or image receptive coating is selected from the group consisting of a photosensitive emulsion, an ink jet receptive coating, a thermal dye recording layer and a pigment based ink receptive layer.
In accordance with specific embodiments of the invention, the support material further comprises a polyolefin or polyester coating layer disposed between the base stock and the image forming or image receptive coating layer on one or both sides of the support material.
In describing the preferred embodiment, certain terminology will be utilized for the sake of clarity. It is intended that such terminology include not only the recited embodiments but all technical equivalents which operate in a similar manner, for a similar purpose, to achieve a similar result. All documents cited are, in relevant part, incorporated herein by reference; the citation of any document is not to be construed as an admission that it is prior art with respect to the present invention.
The term “nonstandard fibers” refers to fibers that are not typically used for producing smooth, photo-quality papers. More specifically, nonstandard fibers are those having an unrefined average fiber length of greater than about 1.0 mm.
The term “average fiber length,” refers to the length weighted average fiber length as determined with a suitable fiber length analysis instrument such as a Kajaani Model FS-200 fiber analyzer.
The term “population” refers to the number of fibers per gram.
The base stock of the present invention may be used in any image forming system in which a smooth base sheet is required to provide a high quality image. By way of example, it may be used in conjunction with photosensitive emulsions, water or pigment-based ink receptive layers, thermal dye-recording layers, or other specialty coatings appropriate for a given imaging method.
Base stock for papers that produce high quality images require exceptionally smooth surfaces. Fiber used in the manufacture of these papers is generally natural cellulose fiber, but synthetic fiber may also be used. The base stock can be produced on a Fourdrinier or cylinder paper machine. Achieving the desired level of smoothness is dependent on fiber characteristics, fiber treatment, and method of base stock densification.
The base stock of the present invention can include various additives as typically used in producing base stocks. During manufacture of the base stock, chemicals may be added to impart water resistance, wet or dry strength, and to achieve color and brightness targets, depending upon the final product application. For example, sizing agents, dry and wet strengthening agents, fillers, pH adjusters, pigments, dyes and fluorescent brighteners may be added. The additives and amounts needed to obtain a base stock having the desired properties are known to those skilled in the art.
Fiber content analysis of a conventional photographic base stock gave the following wood species results: 34% eucalyptus, 24% maple, 23% aspen, 12% birch, 5% beech, and 2% ash. Unrefined fiber lengths of these species are presented. The Eucalyptus family provides the shortest hardwood fiber available with an average fiber length as low as 0.65 mm. Maple is about 0.9 mm and aspen is about 1.0 mm. Other hardwood fiber species are greater than 1.0 mm in length. Cell wall thickness of eucalyptus, maple, aspen, and birch is in the range of 3 to 4 micrometers.
Fiber content analysis for a base stock sample in accordance with a specific embodiment of the present invention gave the following wood species: 72% oak, 10% yellow poplar, 9% gum, 4% maple, the remainder being small amounts of other various hardwood species. Analysis of other samples of base stock in accordance with this aspect of the invention gave results that varied somewhat for percentages of species fibers present, but the predominant specie in all samples was oak, and the four species listed above comprised no less than 83% and generally more than 90% of the fiber in accordance with these embodiments of the invention. Unrefined fiber lengths are: oak is 1.25 mm, yellow poplar is 1.9 mm, and gum is 1.85 mm. Averaged fiber cell wall thickness for oak, yellow poplar, and gum is approximately 6.0 microns. Accordingly, the fiber composition of the invention base stock is significantly different from the fiber composition described in the prior art as being essential for producing a smooth base stock necessary for photo quality images. Based on the teachings in the prior art, one of skill in the art looking to produce a base stock for photo-quality images would use short fibers and expect better results with shorter fibers. By contrast, the present invention is directed to a base stock exhibiting the desired smoothness even though the fibers used are relatively long and typically considered as unsuitable for photo-quality paper. The ability to use longer fibers is advantageous from a cost perspective as the longer fibers typically are less expensive than the short fibers used with conventional photo-quality papers.
Base stock in accordance with certain aspects of the present invention includes at least 50% nonstandard fibers, more particularly at least 80% nonstandard fibers and in certain embodiments at least 90% nonstandard fibers based on the total fiber content.
Fiber population, or fibers per gram, is another measure of fiber properties indicative of the differences between fibers used in conventional base stocks for high image quality imaging and the fibers used in the present invention. The population of conventional fibers typically is greater than about 10×106 fibers per gram. By contrast, the population for the majority of fibers used in the base stock of the present invention is less than about 10×106 fibers per gram, typically less than about 7×106 fibers per gram. Base stock in accordance with certain aspects of the present invention includes at least 50% fibers having a population less than about 10×106 fibers per gram and more particularly at least 80% fibers having a population less than about 10×106 fibers per gram based on the total fiber content.
In a conventional paper making process, a number of steps are required to make a base stock. For the purposes of certain embodiments of this invention, one important step relates to the refining process. The fibers are refined prior to being pumped to the paper machine head box and formed into a paper web. During the refining step, fibers that have been previously treated in the pulping and bleaching stages are passed through refiners where the fibers experience a high degree of shear to further change the characteristics of the fiber. Refining can alter the characteristics of the fiber by increasing fiber surface area by fillibrating and by reducing fiber length. In some cases, specially designed refiners are used to reduce fiber length by employing a cutting action. In accordance with conventional paper making processes for producing base stock for high quality images, the refining treatment typically includes a combination of two refining methods. The first step is disc refining which will fillibrate the fiber and cause some reduction in fiber length. The second refining step utilizes a conical refiner or mixer to shorten or cut fibers as described in U.S. Pat. No. 6,107,014. Using a fiber mixture described previously, the resulting conventional fiber mix has a preferred fiber length-weighted average between 0.4 and 0.58 mm. References in the prior art for the manufacture of base stock for high quality images refer to a fiber length-weighted average equal to or less than 0.5 mm (U.S. Pat. Nos. 5,250,496 and 5,288,690). With the base stock of the present invention, the cutting refining step may be eliminated. The refining treatment in accordance with certain aspects of the present invention results in base stock composed of a fiber mixture having a length-weighted average greater than about 0.9 mm. The ability to provide a base stock having the desired smoothness while eliminating a refining step typically utilized in the prior art is another advantage obtained in accordance with certain aspects of the present invention.
The basis weight of the raw base paper will typically be from about 50 to about 250 g/m2, more particularly from about 100 to about 200 g/m2. The present invention is not limited to these basis weights and could be applicable to lighter or heavier basis weight papers.
The base stock of the present invention can also be provided with a pigment coating to improve smoothness of the base stock, particularly for base stock used for imaging. The pigment coating may include a binder present in the pigment coating composition at from about 8 to about 30% by weight of the dry coating. The pigment to binder ratio may range from about 100:15 to about 100:40, more particularly from about 100:20 to about 100:30.
The pigment coating composition of the present invention may include binders and pigments typically used in pigment coatings as would be known to those skilled in the art. Examples of pigments that may be included in the pigment composition include, but are not limited to, calcium carbonate pigments, clay, titanium dioxide, aluminum silicate, magnesium silicate, magnesium carbonate, zinc oxide, talc, satin white, barium sulfate, calcium silicate, zinc hydroxide, etc.
Examples of binders that may be included in the pigment composition include, but are not limited to, styrene-butadiene polymers, acrylic polymers, styrene-acrylic polymers, vinyl acetate and ethylenevinyl acetate polymers.
In accordance with one aspect of the present invention, the binder used in the coating is an acrylic latex. Examples of acrylic latexes, include but are not limited to, acrylic esters, modified acrylic esters, acrylic ester co-polymers, and modified acrylic ester co-polymers. Examples of useful binders include Rhoplex B-15P, Rhoplex P-554, and Rhoplex 60-A. A particularly preferred acrylic latex is Rhoplex B15-P available from the Rohm and Haas Company. The binder is usually used in an amount of about 8% to 30% by weight, more particularly from about 15% to about 25% by weight, based on the total solids content of the coating. For some applications, the binder may be used in amounts ranging from about 20% to about 30% binder by weight. In accordance with particular embodiments of the invention, the coating composition includes binders described in copending application Ser. No. ______ entitled “BIDER SELECTION FOR COATED PHOTOGRAPHIC BASE STOCK.”
The pigments useful in accordance with the present invention are not particularly limited and any pigments can be used which are suitable for the end use application of the coated paper. In accordance with certain embodiments of the invention, the pigment coating contains pigments which are particularly useful in improving smoothness of the base stock as described in commonly assigned and contemporaneously filed U.S. application Ser. No. ______, entitled “PIGMENT SELECTION FOR COATED PHOTOGRAPHIC BASE STOCK.” Examples of the pigments useful in accordance with this aspect of the present invention include, but are not limited to, anisotropic particles in the form of needle-shaped aragonite precipitated calcium carbonate, high-aspect-ratio clay, low bulk density pigment in the form of hollow sphere polystyrene pigment and combinations thereof. In accordance with a more specific aspect of the present invention, a specific pigment formulation comprising a combination of these pigments is provided. By providing a particular range of concentrations of needle-shaped precipitated calcium carbonate, hollow sphere polystyrene pigment and high-aspect-ratio clay, the roughness of a coated photographic base paper can be minimized. In accordance with a particular embodiment of the invention a pigment coating containing from about 10 to about 14% high aspect ratio clay, from about 18 to about 22% hollow sphere polystyrene pigment and from about 65 to about 75% aragonite precipitated calcium carbonate based on total pigment weight can be used to minimize surface roughness. The present invention provides levels of smoothness that are not typically attained using prior art techniques.
In accordance with another particularly useful embodiment of the invention, a coated paper is provided having a pigment coating containing from about 20 to about 30% of an acrylic binder, from about 40 to about 80% of aragonite precipitated calcium carbonate and from about 15 to about 25% hollow sphere polystyrene pigment by weight based on the dry pigment coating.
The base stock of the present invention may be used in any image forming system in which a smooth base sheet is required to provide a high quality image. By way of example, it may be used in conjunction with photosensitive emulsions, water or pigment-based ink receptive layers, thermal dye-recording layers, or other specialty coatings appropriate for a given imaging method. More particularly, a base stock produced in accordance with the present invention can be converted into photographic products or used to produce photographic ink jet products.
The pigment coating as described herein may be applied to the uncoated base stock using any conventional coating devices, such as a gate roll coater, a bill blade coater, an air knife coater, and the like. The pigment coating will typically be applied to provide a coat weight of from about 4 to about 15 lb/3300 ft2, more particularly from about 8 to 10 lb/3300 ft2.
After being formed and dried on the paper machine to form a web, final smoothness of the base paper (or coated paper) is generally achieved by subjecting the web (or coated paper) to various smoothing operations. One particularly useful method involves a densification process known as calendering, during which a paper web is passed between nips formed by multiple rolls stacked upon one another, creating pressure to compress the paper and make it smoother. Generally, the compression step is accomplished with a stack of four or more metallic rolls (U.S. Pat. No. 5,060,565). In such a stack, the nip load and compression force increase in each successive nip from the top down due to the weight of the rolls and whatever additional load force is applied. In U.S. Pat. No. 5,200,258, a process is described using a nip formed by two rolls of dissimilar material (i.e. metallic and a polymeric resin covered roll) followed by a nip formed by two metallic rolls. This is a process used for production of standard base stocks as well. In accordance with certain aspects of the present invention, the paper may be compressed by a succession of nips formed by either a polymeric resin covered roll and a metallic roll or by two metallic rolls. It is known in the art of calendering that a nip formed by a polymeric covered roll and a metallic roll will give improved fine scale smoothness to the web contacting the polymeric covered roll. A nip formed by two metallic rolls will improve large to medium scale roughness resulting from paper formation-related roughness. In accordance with a particular embodiment of the present invention, the smoothing operation involves passing the paper web through a plurality of nips in a calender stack wherein the first nips are formed by polymeric covered rolls adjacent to metallic rolls and the last two nips are formed by pairs of adjacent metallic rolls. Therefore, the fine scale smoothness is improved initially with the large and medium scale smoothness improved in the last two nips. A means is employed to control nip pressures so calender roll weight and loading pressure are not the only factors in determining individual nip loads. The described calendering sequence allows a high level of smoothness without a blackening effect that can occur when paper is calendered through multiple metallic nips.
Coated paper in accordance with the certain aspects of the present invention is advantageous due to the improvement in smoothness obtained using the described fibers and pigment coating composition. Smoother papers provide images of higher quality in most image forming operations. Smoothness of photobase paper is particularly important for generating high quality images. The surface roughness or Ra of the base stock or coated paper is a measure of relatively finely spaced surface irregularities on the paper. Ra represents the center line roughness of the base stock or finished paper. The surface roughness measurement provides an indication of the maximum variations over the surface of the paper. Lower Ra values indicate smoother base stock or coated paper.
In accordance with one aspect of the present invention, the base stock is subjected to a smoothing operation to provide a base stock having a roughness of from about 1.2 micron Ra to about 1.5 micron Ra. Calender loads typically range from about 1000 pli to about 1500 pli to produce base stock having the desired smoothness. Ra represents the center line roughness of the base stock or finished paper. Ra is preferably 3.0 microns or less, more preferably 2.0 micron or less and most preferably 1.5 micron or less.
In accordance with certain embodiments of the present invention, the base stock (or coated paper) is further coated with a polymeric resin layer on one or both sides of the base stock (or coated paper). The polymer film is typically applied to the base stock by an extruding or laminating process although any method of coating the polymeric film to the base stock to provide a smooth surface can be used. One or more coating layers of polymer can be applied to the base stock (or coated paper). The polymers useful in accordance with this aspect of the invention are not particularly limited provided the polymer is capable of being extruded, laminated or coated onto the paper base stock.
Polyolefin resins typically are used in producing a photographic support to which a photosensitive emulsion is applied. Polyolefin resins useful in forming the polyolefin resin layer include homopolymers of olefins such as low density polyethylene, high density polyethylene, polypropylene, polybutene, polypentene, copolymers of two or more olefins and mixtures thereof. Polymers of various densities and melt indices can be used. Polyester resins or films may also be used in producing a photographic support. The polymer resin layer may also include other additives such as pigments, amides, metal salts of aliphatic acids, antioxidants, brighteners, ultraviolet absorbers, etc. Titanium dioxide is frequently added to the polymer resin layer to improve sharpness and image resolution. U.S. Pat. No. 4,994,357 to Uno et al. describes various polyolefin coating compositions and the use of the compositions in producing photographic supports.
The polymer layer may be applied to provide a dry coat weight of from about 5 to about 30 lb/3300 ft2, more particularly from about 15 to about 25 lb/3300 ft2. The polymer layer can be extruded as a single layer or co-extruded as a multi-layer.
The present invention is illustrated in more detail by the following non-limiting examples.
Base stock samples in accordance with particular embodiments of the invention were tested using a laser based UBM surface smoothness apparatus. Roughness of typical photographic base stock and the invention base stock samples both measured in a range of 1.3 to 1.4 micron Ra. Another surface measurement method using a stylus probe was also used. The stylus method separates roughness components by size into three categories or length scales: R1, R2, and R3. R1 represents the largest scale roughness and R3 the smallest “fine” scale roughness. For each roughness scale, a smaller R-value represents a smoother base paper. Average data for three reference base stocks were: R1=36, R2=37, and R3=48. By comparison, three invention base stocks averaged R1=29, R2=36, and R3=46.
Having described various aspects and embodiments of the invention and several advantages thereof, it will be recognized by those of ordinary skills that the invention is susceptible to various modifications, substitutions and revisions within the spirit and scope of the appended claims.
This application claims the benefit of U.S. Provisional Application No. 60/479,119, filed Jun. 17, 2003, which is related to U.S. Provisional Application No. 60/479,118, entitled “BINDER SELECTION FOR COATED PHOTOGRAPHIC BASE STOCK” and U.S. Provisional Application No. 60/478,991, entitled “PIGMENT SELECTION FOR COATED PHOTOGRAPHIC BASE STOCK.” The disclosures of these provisional applications are hereby incorporated by reference. This application is related to contemporaneously filed U.S. application Ser. No. ______, entitled “BINDER SELECTION FOR COATED PHOTOGRAPHIC BASE STOCK” and U.S. application Ser. No. ______, entitled “PIGMENT SELECTION FOR COATED PHOTOGRAPHIC BASE STOCK.” The disclosures of these applications are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60479119 | Jun 2003 | US | |
| 60479118 | Jun 2003 | US | |
| 60478991 | Jun 2003 | US |
