Patent Application
20060141086
The present invention relates generally to the formation of fibers and nonwoven webs by a meltblowing process. More particularly, the present invention relates to an improved die assembly for use in a melt blowing process.
The formation of fibers and nonwoven webs by meltblowing is well known in the art. See, by way of example, U.S. Pat. No. 3,016,599 to R. W. Perry, Jr.; U.S. Pat. No. 3,704,198 to J. S. Prentice; U.S. Pat. No. 3,755,527 to J. P. Keller et al.; U.S. Pat. No. 3,849,241 to R. R. Butin et al.; U.S. Pat. No. 3,978,185 to R. R. Butin et al.; U.S. Pat. No. 4,100,324 to R. A. Anderson et al.; U.S. Pat. No. 4,118,531 to E. R. Hauser; and U.S. Pat. No. 4,663,220 to T. J. Wisneski et al.
Briefly, meltblowing is a process type developed for the formation of fibers and nonwoven webs; the fibers are formed by extruding a molten thermoplastic polymeric material, or polymer, through a plurality of small holes. The resulting molten threads or filaments pass into converging high velocity gas streams that attenuate or draw the filaments of molten polymer to reduce their diameters. Thereafter, the meltblown fibers are carried by the high velocity gas stream and deposited on a collecting surface, or forming wire, to form a nonwoven web of randomly dispersed meltblown fibers.
Generally, meltblowing utilizes a specialized apparatus to form the meltblown webs from a polymer. Often, the polymer flows from a die through narrow cylindrical outlets and forms meltblown fibers. The narrow cylindrical outlets may be arrayed in a substantially straight line and lie in a plane which is the bisector of a V-shaped die tip. Typically the included angle formed by the exterior walls or faces of the V-shaped die tip is 60 degrees and is positioned proximate to a pair of air plates, thereby forming two slotted channels therebetween along each face of the die tip. Thus, air may flow through these channels to impinge on the fibers exiting from the die tip, thereby attenuating them. As a result of various fluid dynamic actions, the air flow is capable of attenuating the fibers to diameters of from about 0.1 to 10 micrometers; such fibers generally are referred to as microfibers. Larger diameter fibers, of course, also are possible depending on polymer viscosity and processing conditions, with the diameters ranging from around 10 micrometers to about 100 micrometers.
Investigation has been done in the art with respect to the effect of varying certain parameters of the attenuating air flows. For example, U.S. Pat. Nos. 6,074,597 and 5,902,540 disclose a meltblowing method and apparatus utilizing a die assembly formed from a stack of laminated plates having aligned orifices that define an adhesive flow path flanked on each side by air flows. The adhesive flow is drawn and attenuated by the air flows. These patents allege that convergent air flows in the conventional V-shaped die assemblies are inefficient, and that the air flows should be non-convergent with respect to the adhesive flow to maximize the shear component of the compressed air flows.
U.S. Pat. No. 6,336,801 discusses the advantages of using as a primary drawing medium attenuating air that is cooler than the temperature of the polymer within the die tip and exiting from the nozzle outlets. One advantage is that the fibers quench more rapidly and efficiently, resulting in a softer web and less likelihood of formation of undesirable shot. (“Shot” is the accumulation of molten polymer at the die tip apex that eventually reaches a relatively large size and is blown from the die nose, not as a fiber, but as a blob or “shot.”) Another advantage is that faster quenching may reduce the required forming distance between the die tip and the forming wire, thereby permitting the formation of webs with better properties, such as appearance, coverage, opacity, and strength. The '801 patent describes a novel die assembly that focuses heat at the die tip to maintain a desired polymer viscosity and thereby permitting use of significantly cooler attenuating air.
The art is continuously seeking ways to improve the meltblowing process to maximize efficiency and provide an improved meltblown web. The present invention relates to an improved die tip assembly for this purpose.
Objects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.
An embodiment of the present invention is an apparatus for forming meltblown material. The apparatus includes a generally V-shaped die head body having a die tip forming a die tip apex. A channel is defined through the die tip and apex through which a molten polymer is expelled. Air plates are positioned at opposite sides of the die tip and define (with the die tip) air channels through which pressurized attenuating air is directed towards the die tip apex.
Applicants have found that a particularly beneficial meltblowing process is established by reducing the degree of convergence of the air channels in the known wedge or V-shaped type of die assemblies. Through careful observation and experimentation, the present inventors have determined that shot formation is also largely a result of a relatively high degree of turbulence generated by the diverging air streams in conventional die assemblies. It has generally been believed in the art that an included convergence angle for the attenuating air channels of about 60 degrees was necessary for proper drawing of the molten polymer extruded from the die tip apex, and this belief has gone generally unchallenged. Applicants have found that shot formation can be significantly reduced without adversely affecting the quality of meltblown fibers produced by decreasing the convergence angles of at least one, and preferably both of the air channels while maintaining a relatively high velocity profile of the attenuating air exiting the air channels. The velocity of the air is a function of a number of variables, including air pressure, channel dimensions and shape, and so forth, and for a given channel configuration, can be controlled by varying the pressure of the attenuating air supplied to the channels. The decreased angle of impact of the air streams with respect to the axis of the die tip results in significantly reduced air turbulence at the die tip apex, yet the velocity of the air streams is sufficient to draw the molten polymer into fine fibers.
In particular embodiments of die assemblies according to the invention, the included angle of convergence between the air channels is between about 10 degrees to about 20 degrees such that each air channel defines a convergence angle with respect to a longitudinal axis of the diet tip of between about 5 degrees to about 10 degrees. It is not necessary that each of the air channels have the same convergence angle with respect to the axis of the die tip. For example, one channel may have a convergence angle of 5 degrees and the other channel may have a convergence angle of 7 degrees. It may also be desired that only one of the air channels have a convergence angle that is less than 20 degrees.
In yet another embodiment, the air channels define a first zone of convergence at a first included angle, and a second zone of convergence adjacent to the die tip apex at a second included angle that is less than the first included angle. The second included angle may be within the range of between about 10 degrees to about 20 degrees. The first included angle may be greater than about 30 degrees, and more particularly about 60 degrees.
The air channels may have various configurations and cross-sectional shapes. In a particular embodiment, the air channels have a substantially constant cross-sectional area along the zone of convergence that is adjacent to the die tip apex, for example along the second zone of convergence in the embodiment having first and second zones of convergence. The air channels may have a varying cross-sectional area along the first zone of convergence.
The air channels may be defined with a step angular change between the first and second zones of convergence. Alternately, the channels may include a gradual angular change between the first and second zones of convergence.
The air channels may be defined by a space between the air plates and the sides of the die tip. In this embodiment, the die tip comprises side walls at a first angle along the first zone of convergence, and at a second angle along the second zone of convergence. Alternately, the side walls of the die tip may have a gradual or radial component defining the change in convergence of the air channels.
In the embodiment wherein a first convergence zone precedes the second convergence zone having a decreased convergence angle between the air channels, attenuating air may be supplied at a pressure greater than in conventional systems. For example, the air may be supplied at a pressure up to about 30 psig, as compared to 10 psig for many conventional systems. The air may be delivered at a relatively constant velocity, or at an increasing velocity profile as a result of convergence (i.e., reduction) of the cross-sectional profiles of the air channels in a direction towards the die tip apex.
The invention will be described in greater detail below with reference to particular embodiments illustrated in the figures.
Reference will now be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, and not meant as a limitation of the invention. For example, features illustrated or described as part of one embodiment, may be used with another embodiment, to yield still a further embodiment. It is intended that the present invention include modifications and variations to the embodiments described herein.
A conventional apparatus and process for forming a meltblown fabric is shown in
The melted polymer is forced out of slot 24 through a plurality of small diameter capillaries 36 extending across tip 38 of die 14. The capillaries 36 generally have a diameter on the order of 0.0065 to 0.0180 in., and are spaced from 9-100 capillaries per inch. The gas passes from chambers 26 and 28 through passageways 40 and 42. The two streams of gas from passageways 40 and 42 converge to entrain and attenuate molten polymer threads 44 (see
The extruded threads 44 form a coherent, i.e. cohesive, fibrous nonwoven web 18 that may be removed by rollers 47, which may be designed to press web 18 together to improve the integrity of web 18. Thereafter, web 18 may be transported by conventional arrangement to a wind-up roll, pattern-embossed, etc. U.S. Pat. No. 4,663,220 discloses in greater detail an apparatus and process using the above-described elements, and is incorporated by reference herein.
Embodiments of an apparatus 100 according to the invention are shown in
It should be understood that
It should also be understood that a die tip configuration according to the invention may contain additional or fewer components than are illustrated in the figures. For example,
Air plates 120a and 120b are disposed along opposite sides 116 of the die tip 112. The plates 120a and 120b cooperate with the die tip sides 116 to define air channels 122a and 122b. The air channels 122a and 122b direct pressurized attenuating air 136 at the die tip apex 114 to draw and attenuate the molten polymer extruded from the exit orifice of the polymer channel 118 into a relatively fine continuous fiber, as is well known to those skilled in the art.
Referring to
As illustrated in
The air channels 122a and 122b may have various configurations and cross-sectional shapes. For example, in the embodiment of
As in the embodiment of
The air channels 122a and 122b may be defined by a space between the air plates 120a and 120b and the sides 116 of the die tip 112, as illustrated in
It should be appreciated that the pressure of the attenuating air supplied to the air channels 122a and 122b to achieve a desired velocity profile at the exit may vary as a function of a number of variables, including the shape and configuration of the air channels, angle of convergence of the air channels, viscosity of the molten polymer, and so forth. In the embodiment wherein a first convergence zone 124 precedes a second convergence zone 128 having a decreased convergence angle between the air channels 122a and 122b, attenuating air may be supplied within a pressure range of between about 2 psig and about 30 psig. In an embodiment, wherein the included angle of convergence 130 of the air channels along the second zone of convergence 128 is about 16 degrees, the pressure of the attenuating air supplied to the air channels may be about 20 psig.
A small scale prototype system of the embodiment depicted in
The design demonstrated the ability to process at high pressures and obtain fine fibers even at high polymer throughputs. A photograph of the system running is shown in
It should be appreciated by those skilled in the art that various modifications and variations can be made to the embodiments of the invention described or illustrated herein without departing from the scope and spirit of the invention as set forth in the appended claims.
