Patent Publication Number: US-8524144-B2

Title: Method for enhanced fiber bundle dispersion with a divergent fiber draw unit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of application Ser. No. 11/725,593 filed Mar. 10, 2007. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a method for forming nonwoven webs, and to an apparatus for forming such webs. 
     BACKGROUND OF THE INVENTION 
     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. 
     SUMMARY OF THE INVENTION 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an exemplary prior art process for producing nonwoven webs. 
         FIGS. 2A through 2G  are schematic illustrations of various embodiments of fiber drawing units in accordance with aspects of the invention. 
         FIG. 3  is a schematic illustration of an exemplary prior art diffusion chamber configuration. 
     
    
    
     DEFINITIONS 
     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. 
     DETAILED DESCRIPTION 
     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. 
       FIG. 1  corresponds to  FIG. 1  of the prior art PCT Publication No. WO 2005/045116 and is used herein to describe various conventional features of the present method and apparatus for forming a nonwoven web in a melt extrusion process. With reference to  FIG. 1 , a process line  10  is presented for production of monocomponent or multicomponent continuous fibers. The process line  10  is an open system and includes an extrusion device, such as the conventional extruder  30 , for melting and extruding polymer fed from polymer hopper  20 . The polymer is fed from extruder  30  through polymer conduit  40  to a spinneret  50  that forms fibers  60 , which may be monocomponent or multicomponent fibers. Where multicomponent fibers are desired, a second extruder fed from a second polymer hopper would be used. The spinneret  50  has openings or capillaries arranged in one or more rows. The spinneret openings form a downwardly extending “curtain” or “bundle” of fibers  60  when polymer is extruded through the spinneret  50 . Spinnerets  50  for extruding multicomponent continuous fibers are well known in the art and need not be described in detail herein detail. An exemplary spin pack for producing multicomponent fibers is described in U.S. Pat. No. 5,989,004 to Cook, which is incorporated herein in its entirety for all purposes. 
     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  FIG. 1  also includes a quench blower  64  positioned adjacent the curtain of fibers  60  extending from the spinneret  50 . Air from the quench air blower  64  quenches the fibers  60  extending from the spinneret  50 . The quench air can be directed from one side of the fiber curtain as shown in  FIG. 1 , or both sides of the fiber curtain. As used herein, the term “quench” simply means reducing the temperature of the fibers using a medium that is cooler than the fibers, such as, for example, chilled air streams, ambient temperature air streams, or slightly to moderately heated air streams. 
     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  FIG. 1 . In various prior art configurations, including that of  FIG. 1 , the opposed sidewalls  72  and  74  are substantially parallel to each other and generally perpendicular to a horizontal plane. The fiber drawing unit  70  utilizes a moving pneumatic stream, such as aspirating air supplied by a blower (not shown), to draw the fibers through the slot. The aspirating air may be heated or unheated. The aspirating air accelerates the fibers and applies an attenuating or drawing force on the fibers to reduce the diameter of the fibers. The aspirating air also acts to guide and pull the curtain or bundle of fibers through the drawing slot of the fiber drawing unit  70 . The aspirating air may be heated, for example to activate latent helical crimp in multicomponent fibers prior to fiber laydown. 
     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  FIG. 1 , diffusion chamber  80  is formed between the opposed diverging sidewalls  82  and  84 . The opposed sidewalls  82  and  84  diverge outwardly toward the outlet of the chamber  80  in such a way that the volume of the chamber defined between the sidewalls expands towards the bottom end of the diffuser. Desirably, the opposed sidewalls  82  and  84  are substantially continuous and unvented, so that air from the jet of attenuation air does not escape from the walls of the diffusion chamber but rather exits the bottom of the diffusion chamber  80  after traveling therethrough. The gradually expanding or increasing volume of diffusion chamber  80  allows for the jet of fast-moving attenuation air to gradually expand into the increasing volume as it exits the fiber drawing unit  70  and passes through the diffusion chamber  80 . The diverging sidewalls  82  and  84  may be substantially parallel to one another in the upper portion of the diffusion chamber  80  and are inclined or diverge at about a 5 degree angle from the vertical centerline of the chamber at the point where they begin to diverge from one another. The sidewalls of the diffusion chamber  80 , and thus the angle of divergence, may be adjustable, and the angle of divergence may be much less than 5 degrees or may be greater than 5 degrees. 
     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  FIG. 1 , exemplary electrostatic charging units  76  and  78  are shown in opposed relationship located on opposed sidewalls  72  and  74  of the fiber drawing unit  70 . Where opposed electrostatic charging units are utilized, they may be configured in an offset or staggered relationship such that one electrostatic charging unit is higher or lower than the other. For example, referring to  FIG. 1 , electrostatic charging unit  78  is mounted lower on its respective sidewall, i.e., closer to the diffusion chamber, than is electrostatic charging unit  76 . Generally described, an electrostatic charging device, such as charging units  76 ,  78 , may consist of one or more rows of electric emitter pins which produce a corona discharge, thereby imparting an electrostatic charge to the fibers, and the fibers, once charged, will tend to repel one another and help prevent groups of individual fibers from clumping or “roping” together. An exemplary process for charging fibers to produce nonwovens with improved fiber distribution is disclosed in PCT Pub. No. WO 02/52071 to Haynes et al. published Jul. 4, 2002, the disclosure of which is incorporated herein by reference in its entirety for all purposes. The function and operation of such electrostatic charging devices is well known in the art and need not be described in detail herein. 
     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  FIG. 1 . 
     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  FIG. 1  that the fibers are drawn through the drawing slot of the fiber drawing unit by aspirating air entering generally from both sides of the passage, where an electrostatic charging unit is located only on one of the walls forming the drawing slot of the fiber drawing unit, the fiber bundle spread in the machine direction may be enhanced by utilizing attenuation air entering the fiber drawing unit only from the opposing sidewall of the fiber drawing slot. 
       FIGS. 2A through 2G  illustrate aspects of a various fiber drawing units  270  in accordance with the method and apparatus of the present invention that may be utilized in the process line illustrated in  FIG. 1 , or other suitable process lines. It should be understood that the illustrations are schematic and grossly exaggerated in order to more clearly illustrate aspects of the invention. 
     Referring to  FIG. 2A , a process line  200  is illustrated with an extrusion device in the form of a spinneret  250  for forming individual fibers from a molten polymer, as described above. Dual quench air blowers  264  are provided at the outlet of the spinneret  250 . A fiber draw unit (FDU)  270  having an inlet  271  and an outlet  275  receives the quenched fibers. Attenuating air (heated or unheated) is directed into the FDU  270  by any combination of nozzles, plenums, or jets  210  (referred to collectively as nozzles). In the illustrated embodiment, two nozzles  210  are provided at each wall  272 ,  274 . These dual air nozzles are located at or very near to a point of divergence of the sidewalls  272 ,  274 , as explained in greater detail below. It should be appreciated that any number, configuration, and location of nozzles  210  for supplying attenuating air within the FDU are within the scope and spirit of the invention. 
     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  FIG. 1 ) as a nonwoven web. 
     Still referring to  FIG. 2A , the FDU  270  defines a drawing slot  273  that includes a diverging profile portion  277 . In general the diverging profile portion is the longitudinal portion of the drawing slot  273  wherein the cross-sectional width of the slot  273  increases from a minimum to a maximum. In the particular embodiment of  FIG. 2A , the diverging profile portion  277  corresponds generally to the entire length of the drawing slot  273 , with the inlet  271  of the drawing slot defining the minimum width and the outlet  275  defining the maximum width. The diverging profile portion  277  in this embodiment has a generally constant angle of divergence defined by symmetrically diverging sidewalls  272  and  274 . With respect to a longitudinal centerline of the drawing slot  273 , the walls  272 ,  274  diverge equally over the length of the slot  273 . As described above, the diverging profile portion  277  of the FDU causes the fibers conveyed through the drawing slot to open or expand in the machine direction prior to exiting the FDU outlet  275  and before entering the diffusion chamber  280 . This initial machine direction spreading is believed to significantly enhance the function of the diffuser without detrimentally affecting the degree of attenuation within the FDU  270 , thus having an insignificant impact on fiber size. The diverging profile portion  277  provides an improved nonwoven web and does not require increased energy (i.e., increased attenuating air pressure) to provide this benefit. 
     It should also be appreciated that, although the walls of the FDU  270  in  FIGS. 2A through 2G  are illustrated as straight (non-curved), the walls may be curved, or include a combination of curved and straight walls, to achieve the purposes of a diverging profile portion  277  of the drawing slot  273 . 
       FIG. 2B  illustrates an embodiment of an FDU  270  wherein the diverging profile portion  277  is less than the entire length of the drawing slot  273 . In this embodiment, the sidewalls  272 ,  274  are essentially parallel from the inlet  271  to a downstream location within the FDU, at which point the walls  272 ,  274  diverge symmetrically to the outlet  275 . Thus, the diverging profile portion  277  is preceded by an initial non-diverging portion of the drawing slot  273  defined by parallel sidewalls. 
     As with  FIG. 2A , the diverging profile portion  277  of  FIG. 2B  is defined by generally continuously diverging wall portions (straight or curved). It should be appreciated that the diverging profile portion  277  may be defined by a non-continuous wall profile, such as a step profile. Various diverging profiles may be used to achieve the objects and function of the diverging profile portion  277  within the scope and spirit of the invention. 
     The processing line  200  of  FIG. 2B  also incorporates one or more electrostatic charging units  276 ,  278  within the diverging diffusion chamber  280 , as discussed above. These units may be directly opposed as illustrated in the figure, or provided in a staggered configuration. Also, it should be understood that any one of the embodiments of  FIGS. 2A through 2G  may include any combination of charging units within the FDU  20 , such as the units  276 ,  278  in  FIG. 2C . 
       FIG. 2C  illustrates an embodiment of an FDU  270  wherein the diverging profile portion  277  is preceded and followed by a non-diverging portion of the drawing slot  273 . In this embodiment, the initial longitudinal portion of the drawing slot  273  that precedes the diverging portion has a converging profile such that the minimum width of the diverging profile portion  277  is defined at the point of maximum convergence of the initial portion. In essence, a nozzle is created by this unique profile that may function to accelerate the fibers before the fibers enter the diverging profile portion  277 . The downstream non-diverging portion of the drawing slot  273  is defined by a parallel portion of the sidewalls  272 ,  274 . 
       FIG. 2D  illustrates an embodiment of an FDU  270  wherein the diverging profile portion  277  is defined by asymmetrically diverging sidewalls  272 ,  274 . In this embodiment, the sidewall  274  is straight and essentially parallel to a longitudinal centerline of the drawing slot  273 . The opposite sidewall  272  is parallel to the sidewall  274  in an upper portion of the slot  273 , and then diverges to the outlet  275 . The diverging profile of the diffusion chamber  280  is also defined by asymmetrically diverging sidewalls  282  and  284 . In an alternate embodiment, the FDU  270  may have a asymmetrically diverging profile, with the diffusion chamber  280  having a symmetrically diverging profile. 
     The embodiment of  FIG. 2D  is similar to that of  FIG. 2E  in that the sidewall  274  of the FDU  270  and sidewall  284  of the diffusion chamber  280  are straight. In this particular configuration, the sidewalls  274 ,  284  are disposed in the same plane, and may constitute a continuous wall. The inlet  286  of the diffusion chamber  280  is still spaced from the outlet  275  of the FDU  270  by the space between the sidewalls  272  and  282 . 
       FIG. 2F  illustrates an embodiment of a compact or “short” process line  200  wherein the FDU  270  shorter in the longitudinal dimension as compared to the other embodiments, yet still incorporates the diverging profile portion  277 . In fact, the longitudinal length of the drawing slot  273  may be less than the longitudinal length of the diffusion chamber  280 . It should be appreciated that the benefits of the present invention may be realized on various sizes and configurations of diffusion chambers and fiber drawing units, including symmetric and asymmetric diverging walls of the fiber drawing units and/or diffusion units. 
       FIG. 2G  illustrates an embodiment of an FDU  270  wherein the drawing slot  273  is defined by an initial converging section of the walls  272 ,  274 , followed by a parallel section of the walls  272 ,  274 . The parallel section merges into the diverging profile portion  277  defined by asymmetrically diverging sidewalls  272 ,  274 . In the illustrated embodiment, the converging/parallel/diverging profile of the drawing slot  273  is symmetric with respect to a longitudinal centerline of the slot  273 . It should be appreciated that any one or all of the different profile sections may be asymmetric as well. 
     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. 
       FIG. 3  (also from the PCT Pub. No. WO 2005/045116) illustrates an exemplary diverging diffusion chamber  300  bounded by generally opposed sidewalls  310  and  320 . Located within each sidewall  310  and  320 , respectively, is electrostatic charging unit  312  and  322 . The electrostatic charging units  312  and  322  are arranged in a staggered pattern or offset configuration such that the unit  322  would be located closer to the drawing slot of the fiber drawing unit  70  ( FIG. 1 ) than electrostatic charging unit  312 . In an alternate embodiment, the charging units  312 ,  322  may be located directly across from one another. Also, where three or more electrostatic charging units are used, they may continue the staggered pattern of  FIG. 2 , or may be configured such that certain of the electrostatic charging units are located directly across from one another while other electrostatic charging units are located in a staggered pattern. 
     Still referring to  FIG. 3 , the sidewalls of the diffusion chamber may be capable of adjustment, as is shown by adjusting rods  314 ,  316  and  318  attached to sidewall  310  and adjusting rods  324 ,  326  and  328  attached to sidewall  320 . By manipulation of the adjusting rods, it is possible to configure the diffusion chamber  300  such that the sidewalls  310  and  320  are substantially parallel to one another for a certain vertical portion of the diffuser (the region of the diffuser marked by bracket A) before beginning to slope outward or diverge from one another in the region of the diffuser marked by bracket B. Also, it is possible to cause the entire length of sidewalls  310  and  320  to diverge from one another along their entire lengths. Other configurations are possible and may be desirable depending on process variables such as rate of fiber production and amount of drawing air to be conducted through the diffusion chamber. For example, it may be desirable to have sidewalls  310  and  320  converge very slightly prior to divergence, producing the cross section of a venturi nozzle or throat. 
     Returning to  FIG. 1 , also shown is an endless foraminous forming surface such as belt  110  which is positioned below the fiber drawing unit  70  and the diffusion chamber  80  to receive the attenuated fibers  100  from the outlet opening of the diffusion chamber  80 . A vacuum source (not shown) positioned below the foraminous forming surface  110  may be beneficially employed to pull the attenuated fibers onto foraminous forming surface  110 . The fibers received onto foraminous forming surface  110  comprise a nonwoven web of loose continuous fibers, which may desirably be initially consolidated using consolidation means  130  to assist in transferring the web to a bonding device. Consolidation means  130  may be a mechanical compaction roll as is known in the art, or may be an air knife blowing heated air onto and through the web as is described in U.S. Pat. No. 5,707,468 to Arnold, et al., incorporated herein by reference in its entirety. 
     The process line  10  further includes a bonding device such as the calender rolls  150  and  160  shown in  FIG. 1 , which may be used to thermally point-bond or spot-bond the nonwoven web as described above. Alternatively, where the fibers are multicomponent fibers having component polymers with differing melting points, through-air bonders such as are well known to those skilled in the art may be advantageously utilized. Generally speaking, a through-air bonder directs a stream of heated air through the web of continuous multicomponent fibers thereby forming inter-fiber bonds by desirably utilizing heated air having a temperature at or above the polymer melting temperature of the lower melting polymer component and below the melting temperature of higher melting polymer component. The web may be bonded by utilizing other means known in the art, such as adhesive bonding means, ultrasonic bonding means or entanglement means such as hydroentangling or needling. 
     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 sidewail 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 wail 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 
     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 roil. 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 1 below. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Air Permeability (CFM) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Straight FDU 
                   
                 Diverging FDU 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Electrostatic 
                 No 
                 Yes 
                 No 
                 Yes 
               
               
                   
                 Charging 
               
               
                   
                 Permeability 
                 1060.8 
                 1011.7 
                 925.9 
                 905.4 
               
               
                   
                 Std Dev 
                 57.1 
                 61.7 
                 46.7 
                 65.7 
               
               
                   
                 Number of 
                 15 
                 15 
                 15 
                 15 
               
               
                   
                 samples 
               
               
                   
                   
               
            
           
         
       
     
     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.