Patent Application
20080230943
The present invention relates to a method for forming nonwoven webs, and to an apparatus for forming such webs.
Melt extruded nonwoven webs have many uses, including medical care garments and products, protective wear garments, mortuary and veterinary products, and personal care products. For these applications, nonwoven fibrous webs provide tactile, comfort and aesthetic properties that approach those of traditional woven or knitted cloth materials. Nonwoven web materials are also widely utilized as filtration media for both liquid and gas or air filtration applications since they can be formed into a filter mesh of fine fibers having a low average pore size suitable for trapping particulate matter while still having a low pressure drop across the mesh.
Melt extrusion processes for spinning continuous filament yarns, filaments or fibers such as spunbond fibers, and for spinning microfibers such as meltblown fibers, are well known in the art, as are the associated processes for forming nonwoven webs or fabrics therefrom. Typically, fibrous nonwoven webs such as spunbond nonwoven webs are formed with a fiber extrusion apparatus, such as a spinneret, and fiber attenuating apparatus, such as a fiber drawing unit (FDU), oriented in the cross-machine direction (“CD”). That is, the apparatus is oriented at a 90-degree angle to the direction of web production (the “machine direction” or “MD”). Although the fibers are laid on the forming surface in a generally random manner, still, because the fibers exit the CD oriented spinneret and FDU and are deposited on the MD-moving forming surface, the resulting nonwoven webs have an overall average fiber directionality wherein more of the fibers are oriented in the MD than in the CD. A fiber diffuser may be positioned below the FDU to reduce the fiber velocity prior to laying the fibers onto the forming surface. It is widely recognized that such properties as material tensile strength, porosity, permeability, extensibility and material barrier, for example, are a function of the material uniformity and the directionality of the fibers or filaments in the web.
Various attempts have been made to distribute the fibers or filaments within the web in a controlled manner, attempts including the use of electrostatics to impart a charge to the fibers or filaments, the use of spreader devices to direct the fibers or filaments in a desired orientation, the use of mechanical deflection means for the same purpose, and reorienting the fiber forming means. For example, WO 2005/045116 describes a method an apparatus for the production of nonwoven web materials wherein the fibers are attenuated with a fiber drawing unit and the velocity of the fibers is reduced in a downstream diffusion chamber defined between opposed diverging sidewalls. An electrostatic charge is applied to the fibers either before they enter the diffusion chamber or within the diffusion chamber by two or more oppositely directed electrostatic charging units.
WO 02/052071 describes a method and apparatus for the production of nonwoven web materials wherein the fibers are subjected to an electrostatic charge and then directed to a deflector device while under the influence of the charge. The fibers are then collected on a forming surface to form the nonwoven web. The deflector device may include a series of teeth separated by a distance determined by the desired orientation of the fibers in the nonwoven web.
The art is continuously seeking improved methods and devices to still further improve the process of distributing the fibers in melt extrusion processes to achieve superior nonwoven materials. The present invention relates to such an improved method and apparatus.
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.
The present invention provides a method and related apparatus for making a nonwoven web, the method including the step of providing a plurality of fibers from an open melt extrusion system. After leaving an extruder device, such as a conventional spinneret, the fibers may be quenched and then subjected to a pneumatic attenuation force by a drawing slot of a separate fiber draw unit (FDU) having an inlet and an outlet, the attenuation force imparting a velocity to the fibers and causing the fibers to be attenuated (reduced in diameter) in the quench zone. In an open type of system, the quench air is provided by one or more blowers, and the pneumatic attenuation force may be generated within the separate FDU by any combination of air nozzles or plenums (referred to collectively as air nozzles) that direct relatively high velocity aspirating air through the drawing slot. In a closed system, the FDU is generally combined with the quench air housing such that the quench air also serves as the attenuating air. In certain configurations of the FDU, some degree of attenuation of the fibers may occur in the FDU as well.
It may be desired to pertubate the attenuating airstreams within the drawing slot of the FDU to further improve the machine direction bundle spread of the fiber bundle. This may be done, for example, by alternately pulsing the air from the air nozzles in the opposite walls of the FDU. This feature may be accomplished with single or multiple air nozzles in the respective FDU walls.
From the FDU, the fibers are conveyed through a diverging diffusion chamber spaced from the outlet of the FDU wherein the velocity of the fibers is reduced. The fibers may also be subjected to an applied electrostatic charge in either the diffusion chamber or the FDU. The fibers exit the diffusion chamber and are collected as a web on a moving forming surface.
Linear drawing devices, slot drawing and fiber drawing units that utilize high velocity jets to impart the draw forces on the fibers are known to compress or densify the fiber bundle in the fiber/air stream. This densified or compressed fiber bundle then needs to be expanded in order to form the desired web. Diffusion devices and other types of fiber deflectors or spreading devices and electrostatics are used to expand the fibers to ensure a high level of dispersion prior to the web forming process.
A unique feature of the method and apparatus of the invention includes conveying the fibers through a diverging profile portion of the FDU drawing slot to expand and spread the fibers in the machine direction within the FDU. The diverging profile causes the fiber bundle to expand and spread in the machine direction within the drawing slot prior to the inlet of the diffuser. This machine direction spreading of the fiber bundle within the FDU in combination with a diverging diffuser results in improved web formation as compared to straight drawing slots (parallel sidewalls) or converging drawing slots (converging sidewalls) under comparable processing parameters.
The diverging profile portion of the FDU drawing slot may take on various shapes. In one embodiment, the diverging portion is defined by symmetrically diverging sidewalls (curved, straight, or a combination thereof) of the FDU such that a symmetric divergence angle is defined with respect to the longitudinal centerline of the drawing slot. In an alternate embodiment, the diverging portion is defined by asymmetrically diverging sidewalls, or one diverging sidewall. The diverging portion of the drawing slot may diverge substantially continuously (at a constant or varying rate) from a minimum width to a maximum width. Alternatively, the diverging portion may diverge in a discontinuous manner (e.g., stepwise) between the minimum and maximum width The diverging profile portion of the FDU drawing slot may encompass the total longitudinal length of the drawing slot. For example, the drawing slot may diverge from a minimum width at the inlet of the drawing slot to a maximum width at the outlet of the drawing slot. In different embodiments, the diverging profile portion may be defined only in a portion of the overall length of the drawing slot. For example, the FDU drawing slot may include an upstream (with reference to the direction of fiber travel) non-diverging portion adjacent to the diverging profile portion. This non-diverging portion may have essentially parallel sidewalls, or converging sidewalls.
The diverging profile portion of the FDU may be defined by curved wall sections, straight wall sections, or a combination of curved and straight wall sections.
Similar to the diverging profile portion of the FDU drawing slot, the diverging diffusion chamber may be defined by symmetrically or asymmetrically diverging sidewalls.
In a particular embodiment, the electrostatic charge is applied to the fibers as the fibers are conveyed through the FDU drawing slot by one or more electrostatic charging units. For example, the charge may be applied with opposed electrostatic charging units within the FDU, with one of the electrostatic charging units located substantially closer to the diffusion chamber than the other electrostatic charging unit. In alternate embodiments, the electrostatic charge is applied to the fibers as the fibers are conveyed through the diffusion chamber, for example by opposed electrostatic charging units within the diffusion chamber.
As used herein the term “polymer” generally includes but is not limited to, homopolymers, copolymers, such as for example, block, graft, random and alternating copolymers, terpolymers, etc. and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term “polymer” shall include all possible geometrical configurations of the chemical formula structure. These configurations include, but are not limited to isotactic, syndiotactic and random symmetries.
As used herein the term “fibers” refers to both staple length fibers and continuous fibers, unless otherwise indicated. The term “fiber bundle” refers to a grouping of individual fibers.
As used herein the term “nonwoven web” or “nonwoven material” means a web having a structure of individual fibers or filaments which are interlaid, but not in an identifiable manner as in a knitted or woven fabric. Nonwoven webs may be formed from many processes, such as, for example, meltblowing processes, spunbonding processes, air-laying processes and carded web processes. The basis weight of nonwoven fabrics is usually expressed in grams per square meter (gsm) or ounces of material per square yard (osy), and the fiber diameters useful are usually expressed in microns.
The term “spunbond” or “spunbond nonwoven web” refers to a nonwoven fiber or filament material of small diameter fibers that are formed by extruding molten thermoplastic polymer as fibers from a plurality of capillaries of a spinneret. The extruded fibers are cooled while being drawn by an eductive or other well-known drawing mechanism. The drawn fibers are deposited or laid onto a forming surface in a generally random manner to form a loosely entangled fiber web, and then the laid fiber web is subjected to a bonding process to impart physical integrity and dimensional stability. The production of spunbond fabrics is disclosed, for example, in U.S. Pat. No. 4,340,563 to Appel et al., U.S. Pat. No. 3,692,618 to Dorschner et al., and U.S. Pat. No. 3,802,817 to Matsuki et al. Typically, spunbond fibers or filaments have a weight-per-unit-length in excess of about 1 denier and up to about 6 denier or higher, although both finer and heavier spunbond fibers can be produced. In terms of fiber diameter, spunbond fibers often have an average diameter of larger than 7 microns, and more particularly between about 10 and about 25 microns, and up to about 30 microns or more.
Reference is now made to particular embodiments of the inventive method and apparatus according to the invention, one or more examples of which are illustrated in the figures. It should be appreciated that the embodiments are provided by way of explanation of the invention, and are not meant as a limitation of the invention. For example, features illustrated or described with respect to one embodiment may be used in another embodiment to yield still a further embodiment. The present invention encompasses these and other modifications and variations made to the embodiments described and illustrated herein.
Polymers suitable for the present invention include the known polymers suitable for production of nonwoven webs and materials such as for example polyolefins, polyesters, polyamides, polycarbonates and copolymers and blends thereof. It should be appreciated that the particular type of polymer is not a limiting feature.
The exemplary open process line 10 in
An aspirator or “fiber drawing unit” (FDU) 70 is positioned spaced from and below the spinneret 50 to receive the quenched curtain or bundle of fibers. The function and operation of fiber drawing units for use in melt spinning polymers are well known in the art. Generally, the fiber drawing unit 70 includes an elongated vertical passage or drawing slot defined by parallel side walls of the FDU 70 through which the fibers are drawn by aspirating air entering generally from both of the sides of the drawing slot and flowing downwardly through the passage. The attenuation chamber or fiber drawing slot is formed by opposed plates or sidewalls, designated 72 and 74 in
As the fibers exit the fiber drawing unit 70 they are passed through a diffusion chamber 80 to reduce the fiber velocity prior to laying the fibers down into a nonwoven web. Diffusion chambers or diffusers in general are disclosed in U.S. Pat. No. 5,814,349 to Geus et al., incorporated herein by reference in its entirety for all purposes. Other diffusion chamber configurations are described in U.S. Pat. Nos. 6,918,750 and 6,932,590 also to Geus et al. As described in U.S. Pat. No. 5,814,349, it is desirable for the diffuser to be mounted slightly below the exit of the fiber drawing unit to allow for ambient air to be drawn into the diffusion chamber from the sides.
Desirably, as shown in
As the pneumatic jet expands in the diffusion chamber 80, it decreases in velocity, and the fiber velocity also decreases, which allows for the fiber bundle to spread out somewhat in the machine direction. That is, as the fiber bundle travels downward through the diffusion chamber, it begins to take on a machine direction dimension which is somewhat larger than it had at the outlet of the fiber drawing unit 70.
However, in order to provide for high uniformity of material formation on fiber laydown, it is highly desirable for the machine direction fiber bundle spread to be larger than the bundle spread generated by the diffusion chamber alone. In this regard, one or more electrostatic charging devices may be used to impart an electrostatic charge to the fibers of the fiber bundle either as they travel through the drawing slot of fiber drawing unit 70 or as they travel through diffusion chamber 80, or both. In
In still another embodiment to assist machine direction bundle spreading, it may be desirable to utilize one or more electrostatic charging units inside the diffusion chamber 80. For example, one or more electrostatic charging units may be located on the same diffusion chamber sidewall. It may also be desirable to have at least one electrostatic charging unit located on each sidewall of the diffusion chamber. Where electrostatic charging units are located on both sidewalls, they may be located substantially directly across from one another, that is, the electrostatic charging units may be located at substantially the same vertical height within diffusion chamber 80. It may also be advantageous to have the electrostatic charging units in the diffusion chamber located in a staggered configuration, similar to the staggered configuration described with respect to electrostatic charging units 76 and 78 in fiber drawing unit 70 in
In still another embodiment, a single electrostatic charging unit may be used, in either the diffusion chamber or in the fiber drawing slot, in conjunction with specific application of aerodynamic forces to balance the repulsion forces created by the electrostatic charging unit. As an example, although it was stated above with reference to
Referring to
To further enhance machine direction spreading of the fiber bundle, it may be desired to pertubate the air supplied by the nozzles 210, for example by pulsing or otherwise disturbing or disrupting the airstreams. This may be accomplished by the use of one or more mechanical valves that alternately pulse or modify the air flow fed to the nozzles 210. Such pertubation can be accomplished with single, dual, or other multiple arrangements of nozzles 210 within the respective walls 272, 274 of the FDU. Pertubation of drawing air is described in U.S. Pat. No. 5,807,795 to Lau et al., which is incorporated herein in its entirety for all purposes.
A diverging diffusion chamber 280 having an inlet 286 and an outlet 288, and symmetrically diverging walls 282 and 284 is disposed below the outlet 275 and functions as described above. The outlet 275 of the FDU 270 has a width generally equal to or less than the width of the inlet 286 to the diffusion chamber 280. The fibers exit the diffusion chamber 280 and are laid onto a traveling forming belt 212 (110 in
Still referring to
It should also be appreciated that, although the walls of the FDU 270 in
As with
The processing line 200 of
The embodiment of
In general, fiber drawing units may have an effective longitudinal length of the drawing slot of between about 10 inches to about 100 inches. A portion or the entire length of the drawing slot may diverge within the scope and spirit of the invention. The magnitude of divergence will thus depend on the length and divergence angle of the sidewalls, and can be readily empirically determined by those skilled in the art as a function of processing parameters. Although not meant as a limitation of the invention, it is believed that, with certain embodiments, the diverging profile portion should have an inlet width of from about 0.125 to about 0.60 inches, and that the outlet width of the divergence portion should be less than about 1.0 inches. In alternate embodiments, the total inclusive divergence angle (from one sidewall to the opposite sidewall) may vary within a range up to about 5 degrees, or greater.
Still referring to
Returning to
The process line 10 further includes a bonding device such as the calender rolls 150 and 160 shown in
Lastly, the process line 10 further includes a winding roll 180 for taking up the bonded web 170. While not shown here, various additional potential processing and/or finishing steps known in the art such as web slitting, stretching, treating, or lamination of the nonwoven fabric into a composite with other materials, such as films or other nonwoven layers, may be performed without departing from the spirit and scope of the invention.
In still another embodiment, the uniformity of the nonwoven web formation may be further improved or enhanced by utilizing vortex generators on or near the inner surface of the diverging sidewalls of the diffusion chamber. Vortex generators may be placed along one or more walls at spaced apart locations across the cross machine direction of the sidewall, to induce vortices into the airstream. The vortices induced will act to increase turbulence in the inner layer of the airstream close to the sidewall, adding energy to the flow in that area, and reduce flow separation, allowing for the airstream to more effectively conform to the sidewalls as the sidewalls diverge, and thus providing for a more complete machine direction dispersion of the airstream and consequently a larger machine direction fiber bundle spread. Vortices may be generated by having tabs or protrusions on one or more sidewalls at spaced apart locations, such as are described in U.S. Pat. No. 5,695,377 to Triebes et al., incorporated herein by reference in its entirety. Depending on placement of the vortex generators and amount of machine direction fiber bundle spread inside the diffusion chamber, catching or dragging of the fibers upon the vortex generators may be an issue. In that instance, it may be desirable to utilize as vortex generators dimples or inverted tabs which extend into the surface of the material forming the sidewall, rather than vortex generators which extend outwardly from the inner surface of the sidewall into the diffusion chamber.
Other methods of vortex generation may be employed with or in place of those described above. For example, one or more backward facing steps running substantially in the cross-machine direction width of the diffusion chamber may be used on the inner sidewall surface to generate vortices. As another example, air jets may be used on one or both sidewalls of the diffusion chamber at or near the point of divergence to generate vortices by blowing fine jets of a fluid such as air through pores or holes drilled or otherwise formed in the sidewall surface material. As an alternative to actual air jets, synthetic jets such as are generally described in U.S. Pat. No. 5,988,522 to Glezer et al., incorporated herein by reference in its entirety, may be used on one or both sidewalls to generate vortices. Generally described, a synthetic jet may be produced from a fluid-filled chamber having a flexible actuatable membrane at one end and a more rigid wall at the other end, the rigid wall having a small hole. The flexible membrane may then be repeatedly actuated by acoustical wave energy, mechanical energy or piezoelectric energy, thereby causing a jet of fluid (such as air) to emanate from the hole in the more rigid wall at the other end of the chamber.
The following example is provided for illustration purposes and the invention is not limited thereto.
Example spunbond nonwoven materials were produced using commercially available isotactic polypropylene of approximately 35 melt flow rate, available from ExxonMobil Chemical Co. (Houston, Tex.) and designated as Exxon 3155. All materials were produced using a spunbond type slot-draw nonwoven spinning system such as described in the above-mentioned U.S. Pat. No. 3,802,817 to Matsuki et al. and, after being collected on a forming surface, all materials were thermally bonded using a heated calendar roll. For all the materials, an electrostatic charging system was located near the drawing slot exit of the fiber drawing unit to charge the filament curtain, as generally described in the PCT Pub. No. WO 2005/045116 cited above, wherein the fibers were subjected to an applied electrostatic charge before the fibers entered the diffusion chamber.
Also for the production of the Example materials, a diffusion chamber substantially as described in U.S. Pat. No. 5,814,349 to Geus et al. and as generally described above (except that no electrostatic charging units were located within the diffuser) was located below the fiber drawing unit drawing slot. The diffusion chamber was mounted slightly lower than the exit of the fiber drawing unit to allow for air to be drawn into the diffusion chamber. The diffusion chamber was set up using control rods to produce a venturi shape, with the sidewalls initially converging before diverging out at the bottom or exit of the diffusion chamber.
The control samples (STRAIGHT FDU) were produced using a Fiber Drawing Unit (FDU) with parallel sidewalls that established an entrance opening and an exit opening on the FDU of the same dimension. The example materials (DIVERGING FDU) were produced using a FDU with diverging sidewalls that established an exit opening greater than the entrance opening dimension. One set of example materials was produced using an electrostatic charging system to impart a charge on the fibers. For all materials, the spinning and drawing conditions were held constant. The polymer throughput rate, the fiber drawing rates were held constant thereby resulting in the same fiber size. For all materials, the fibers had an average diameter of about 18 microns (about 2.0 denier).
The formed nonwoven webs were tested for Air Permeability according to the ASTM D737 test method, and using a TEXTEST FX 3300 Air Permeability tester available from the Schmid Corp. (Spartanburg, S.C.). The materials were tested for air permeability and the results of fifteen repetitions for each sample were averaged for each material. The permeability results measured in CFM (cubic feet per minute) are shown in TABLE I below.
In the present Example, air permeability is a measure of airflow through the Spunbond Web. Higher numbers indicate a lower pressure drop. Pressure drop is a direct indicator of web formation. Better formation materials have smaller pore structure, which causes the pressure drop to increase. Therefore better formation is indicated by lower Permeability values. The data in Table 1 shows that the permeability values for the diverging FDU samples are ˜11% to 13% lower than the comparative materials. All materials listed in TABLE were the same basis weight, about 0.50 osy (about 17 gsm), and were produced at the same polymer throughput rate, of about 10.6 PIH (about 190 kg/meter/hour). The results indicate that, with all other parameters being essentially constant, the diverging FDU produces a better-formed web.
