Patent Publication Number: US-2005130540-A1

Title: Multicomponent spunbond filaments having a melt-processable superabsorbent polymer core

Description:
FIELD OF THE INVENTION  
      The invention relates generally to methods for manufacturing spunbond nonwoven webs and, more particularly, to methods for manufacturing spunbond nonwoven webs from multicomponent filaments incorporating a melt-processable superabsorbent polymer core.  
     BACKGROUND OF THE INVENTION  
      Nonwoven webs and their manufacture from melt-processable thermoplastic polymers has been the subject of extensive development resulting in a wide variety of materials converted for numerous commercial and consumer applications, such as disposable hygienic articles. Nonwoven webs consist of a sheet of overlapped and intermingled filaments or fibers of melt-processable thermoplastic polymers manufactured using, for example, spunbond processes. A spunbond process generally involves distributing one or more thermoplastic polymers in a spin pack for extrusion as a dense curtain of semi-solid filaments from a spinneret of the spin pack. The descending curtain of filaments is cooled by a cross-flow of cooling air and the individual filaments are attenuated by a drawing device or aspirator. Spunbond filaments are generally lengthwise continuous and have average diameters in the range of about 10 to 20 microns. The filaments discharged from the drawing device are deposited on a collector, such as a forming belt or a forming drum, as a continuous length nonwoven web, which is slit and shaped for use.  
      Disposable hygienic articles commonly incorporate an absorbent core containing a superabsorbent polymer (SAP) capable of absorbing several times its weight of aqueous body fluids while retaining the absorbed fluids under moderate pressure. SAP&#39;s contain water-insoluble, cross-linked chain molecules that are capable of forming a hydrogel when hydrated by aqueous body fluids. The degree of polymer crosslinking of the SAP affects the absorbent capacity and gel strength of the hydrogel. In a physical property tradeoff, an SAP having high gel strength generally possesses a low absorption capacity, and an SAP having a high absorption capacity typically possesses low gel strength. Generally, SAP&#39;s characterized by a low absorbent capacity are incapable of absorbing a sufficient amount of fluid for practical use in a disposable hygienic article. To be useful in a disposable hygienic article, the SAP must have adequately high absorption capacity and the hydrogels formed therefrom must have adequately high gel strength.  
      In a conventional arrangement, the absorbent core includes granules of SAP that are dispersed in a porous matrix of cellulose fibers. The swelling of the SAP granules occurs in such a way that a very high absorption rate is observed shortly after an aqueous body fluid is introduced. However, the swollen SAP granules tend to close the open spaces in the SAP-fiber matrix, which slows or prevents the entry of aqueous body fluids by reducing porosity and permeability. Subsequent amounts of aqueous body fluids that can no longer penetrate into the interior of the absorbent core may leak from the hygienic article. As SAP granules embedded deeper in the absorbent core are shielded, the total storage capacity of the absorbent core is effectively reduced. This surface-limiting blocking phenomenon is particularly acute for low gel strength SAP&#39;s of high absorbency. Another disadvantage of conventional absorbent cores arises because the matrix of cellulose fibers is bulky, which conflicts with consumer demand for thin diapers. Yet another disadvantage of conventional absorbent cores is that the SAP granules tend to leak when a weight-bearing load is applied to the absorbent core.  
      It would be desirable, therefore, to arrange a superabsorbent polymer in an absorbent core of a hygienic article with a configuration that enhances the total storage capacity.  
     SUMMARY  
      The invention provides a nonwoven web and a product formed using the nonwoven web each of which includes a plurality of multicomponent filament. Each filament features a liquid-permeable or liquid-pervious sheath region and a core region encased within the sheath region. The sheath region comprises a melt-processable thermoplastic polymer and the core region comprises a melt-processable superabsorbent polymer capable of absorbing liquid that penetrates through said sheath region to said core region. The nonwoven web may be used to fabricate products or articles, such as an absorbent core for a hygienic article.  
      In another aspect, the invention is directed to a method of manufacturing a nonwoven web that includes heating a thermoplastic polymer to a flowable state, heating a superabsorbent polymer to a flowable state, and combining the thermoplastic polymer and the superabsorbent polymer to form multicomponent filaments. Each multicomponent filament has a liquid-pervious sheath region including the thermoplastic polymer and a core region including the superabsorbent polymer. The filaments are collected to form a nonwoven web.  
      In accordance with the principles of the invention, the superabsorbent polymer (SAP) in the core of the multicomponent filaments is mechanically strengthened by the presence of the sheath of liquid-pervious or liquid-permeable thermoplastic polymer. This strengthening permits the SAP to be characterized by lower gel strength and, therefore, a higher absorbency than in conventional absorbent cores for hygienic articles and other fluid-absorbing items featuring SAP granules dispersed in a cellulose fiber matrix. Because the SAP is confined in the core of the multicomponent filaments, the effect of surface-limited blocking on liquid absorption is significantly reduced and the SAP is more efficiently utilized for absorbing liquids. As compared with absorbent cores of conventional SAP-fiber matrices, an absorbent core including the multicomponent filaments of the invention requires a reduced amount of SAP to achieve an equivalent total storage capacity. Therefore, the absorbent core may be manufactured at a reduced cost. By eliminating the matrix of cellulose fibers found in conventional absorbent cores, hygienic articles and other fluid-absorbing items may be made thinner. The polymer sheath surrounding the SAP core also assists for preventing leakage under a weight-bearing load.  
      These and other objects and advantages of the present invention shall become more apparent from the accompanying drawings and description thereof. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
      The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the principles of the invention.  
       FIG. 1  is a side view of a spunbonding apparatus for forming a plurality of filaments in accordance with the principles of the invention;  
       FIG. 2  is a perspective view of a portion of a multicomponent spunbond filament formed by the spunbonding apparatus of  FIG. 1 ;  
       FIG. 3  is a cross-sectional view of the multicomponent spunbond filament of  FIG. 2 ;  
       FIG. 4  is a cross-sectional view of a portion of a multicomponent spunbond filament in accordance with an alternative embodiment of the invention;  
       FIGS. 5A and 5B  are cross-sectional views of a portion of an alternative embodiment of a multicomponent spunbond filament in accordance with the principles of the invention;  
       FIG. 6  is a cross-sectional view similar to  FIG. 4  of a portion of a multicomponent spunbond filament in accordance with an alternative embodiment of the invention; and  
       FIG. 7  is a perspective view of a hygienic article having an absorbent core including multicomponent spunbond filaments in accordance with the principles of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The invention is directed to methods for boosting the effectiveness of superabsorbent polymers (SAP&#39;s) used for absorbing large quantities of aqueous body fluids. To that end, the SAP is incorporated as a core region of multicomponent spunbond filaments encased in a porous thermoplastic polymer sheath. Although the filaments will be described herein as being formed using an exemplary meltspinning apparatus, it should be understood that modifications to the exemplary meltspinning apparatus described herein could be made without departing from the intended spirit and scope of the invention.  
      With reference to  FIG. 1 , a spunbonding apparatus  10  is equipped with a screw extruder  12  that converts a solid melt-processable polymer into a molten state and transfers the molten polymer under pressure to a set of metering pumps  16 . Another screw extruder  14  converts another solid melt-processable polymer into a molten state and transfers the molten polymer under pressure to a set of metering pumps  18 . Pellets of the solid polymers are placed in hoppers  11 ,  13  and fed to the corresponding one of screw extruders  12 ,  14  for melting and transfer.  
      Each set of metering pumps  16 ,  18  pumps metered amounts of the corresponding polymers at corresponding volumetric flow rates to a spin pack  20 , which contains flow passageways that cooperate for combining the polymers. Spin packs are familiar to persons of ordinary skill in the art and, therefore, are not described here in detail. Generally, spin pack  20  includes flow passageways arranged to separately direct the polymers to a spinneret  22 , in which the polymers are combined. The spinneret  22  includes spinning orifices (not shown) arranged in rows from which a dense curtain of filaments  24  each constituted collectively by the two polymers is discharged. An exemplary spin pack  20  is disclosed in U.S. Pat. No. 5,162,074, the disclosure of which is hereby incorporated by reference herein in its entirety. The shape of the spinning orifices in spinneret  22  can be selected to accommodate the cross-section desired for the extruded filaments  24 , such as round, oval, trilobal, triangular, dog-boned, or flat.  
      The descending curtain of filaments  24  is quenched to accelerate solidification by a cross-flow of cooling air from a quench blower  25 . The filaments  24  are drawn into a filament-drawing device  26  that directs high velocity sheets of process air in a downwardly direction generally parallel to the length of the filaments  24 . Because the filaments  24  are extensible, the converging, downwardly-directed sheets of high-velocity process air apply a downward drag that attenuates the filaments  24 . Other exemplary filament-drawing devices  26  are disclosed in U.S. patent application Ser. No. 10/072,550, U.S. Pat. No. 4,340,563, and U.S. Pat. No. 6,182,732, the disclosures of which are hereby incorporated herein by reference in their entirety.  
      The filaments  24  discharged from filament-drawing device  26  are propelled toward a formaminous or porous collector  28 , such as a moving screen belt. The filaments  24  deposit in a substantially random manner as substantially flat loops on the collector  28  to collectively form a nonwoven web  30 . The collection device  28  moves in a machine direction, represented by the arrow labeled MD, parallel to the length of the nonwoven web  30 . The width of the nonwoven web  30  deposited on collector  28  is substantially equal to the width of the curtain of filaments  24 .  
      An air management system  32  positioned below the collector  28  supplies a vacuum that is transferred through the collector  28  to attract the filaments  24  onto the collector  28 . The air management system  32  disposes of the high-velocity process air discharged from the filament drawing device  26  so that filament laydown is relatively undisturbed. Exemplary air management systems  32  are disclosed in U.S. Pat. No. 6,499,982, the disclosure of which is hereby incorporated by reference herein in its entirety.  
      Additional spunbonding apparatus, not shown but similar to spunbonding apparatus  10 , and meltblowing apparatus (not shown) may be provided downstream or upstream of spunbonding apparatus  10  for depositing one or more additional spunbond and/or meltblown nonwoven webs of either monocomponent or multicomponent filaments either as a substrate for receiving nonwoven web  30  or onto nonwoven web  30 . An example of such a multilayer laminate in which some of the individual layers are spunbond and some meltblown is a spunbond/meltblown/spunbond (SMS) laminate made by sequentially depositing onto a moving forming belt first a spunbond fabric layer, then a meltblown fabric layer and last another spunbond layer containing filaments  24 .  
      With reference to  FIGS. 2 and 3  and in accordance with the principles of the invention, filaments  24  are multicomponent filaments including a sheath  34  of a melt-processable thermoplastic polymer concentrically surrounding a core  36  formed from a melt-processable SAP, including but not limited to acrylate-based materials such as polyacrylate or, more specifically, sodium polyacrylate which is a sodium salt of polyacrylic acid. The thermoplastic polymer forming sheath  34  may be selected from among any commercially available spunbond grade of a wide range of polymer resins, copolymers, and blends of polymer resins, including, without limitation, polyolefins, such as polyethylene, polypropylene, nylons, polyamides, polyesters, polyvinyl acetate, polyvinyl chloride, polyvinyl alcohol, and cellulose acetate. The core  36  may be concentrically arranged with the sheath  34  as depicted in  FIGS. 2 and 3  or, alternatively, may be eccentric or offset from the sheath axial centerline. The core  36  may also be distributed in multiple isolated cores within sheath  34  in an islands-in-the-sea configuration.  
      The sheath  34  is pervious or permeable to liquids or hydrophilic so that liquids can penetrate to the core  36  for absorption by the constituent SAP. The sheath  34  provides a mechanical support that strengthens the core  36  after an amount of a liquid is absorbed to form a hydrogel. As a result, a low gel strength and high absorbency SAP may be used in the core  36  of filament  24  for boosting the total storage capacity for liquids. Specifically, the SAP in the core  36  may exhibit an absorbency exceeding about  50  grams of saline per gram of SAP. Absorbency is a measure of the mass or volume of fluid that a given amount of SAP will absorb before saturation. Gel strength indicates the tendency of the hydrogel, once formed from the SAP, to deform or flow under an applied pressure or stress. The recipe or chemistry of the SAP may be altered to increase or decrease the rate of absorption while maintaining a high absorbency. Increases or decreases in the thickness of the sheath  34  may be used to regulate the absorption rate of the SAP core  36 . Typically, the filaments  24  contain at least about 50% by weight of SAP. The SAP in core  36  forms a hydrogel that expands volumetrically after absorbing liquid. As a result, filament  24  will expand in a radial dimension after liquid absorption. The SAP in core  36  retains the absorbed liquids under moderate applied pressures.  
      With reference to  FIG. 4  in which like reference numbers refer to like features in  FIG. 3 , a sheath  38  of filament  24  may be formed under melt-spining conditions such that an outer surface includes multiple pathways  40  each extending radially through the sheath thickness. The pathways  40  are direct passageways penetrating the sheath  38  and may be present as fractures in the sheath  38 . Alternatively, the pathways  40  may include a plurality of interconnecting interstices that communicate with the outside and inside of the sheath  38 . Because of the presence of pathways  40 , liquids are able to more readily permeate or pervade the sheath  38  with a higher permeation rate and reach the SAP core  36  more efficiently for absorption.  
      The pathways  40  are introduced into the sheath  38  by various different techniques including, but not limited to, adding a pathway-promoting agent to the polymer of the sheath  38 . In certain embodiments of the invention, the pathways  40  may be introduced in sheath  38  by conventional phase separation methods. For example, the pathways  40  may be formed by mixing the thermoplastic polymer with a diluent or plasticizer, quenching in a liquid medium to induce phase separation, and washing away the diluent to leave behind an interconnected porous structure. Pathways  40  may also be formed in sheath  38  by introducing a blowing agent or a swelling agent into the thermoplastic polymer before the filaments  24  are formed. Another technique for forming pathways  40  in sheath  38  is to add a filler material, such as a concentration of particulate filler like calcium carbonate, capable of initiating pathway formation to the sheath polymer. Alternatively, pathways  40  may be introduced into the polymer forming sheath  38  by introduction of an additive, such as polyethylene glycol as disclosed in U.S. Pat. No. 6,623,853, the disclosure of which is hereby incorporated by reference herein in its entirety.  
      With reference to  FIGS. 5A and 5B , filament  24  may be meltspun under conditions that form a central, axially-extending lumen in which the SAP is distributed in discrete, spaced-apart portions  42  surrounded by a sheath  44 . Adjacent portions  42  are separated by one of multiple axially-extending voids  46 . When liquid is absorbed, the portions  42  expand volumetrically or swell to at least partially fill the axially-extending voids  46 , as shown in  FIG. 5B , which reduces the volume of the voids  46 . The portions  42  may be arranged with regular axial spacing or may have a less periodic arrangement. The arrangement consisting of portions  42  of SAP separated by voids  46  may be formed by controlling the relative volumetric flow rates of sheath polymer and SAP to the spin pack, such as spin pack  20  ( FIG. 1 ) so that the flow rate of the sheath polymer is less than a flow rate required to has a structure in which the SAP core is uninterrupted by voids.  
      With reference to  FIG. 6 , filament  24  may include a core  48  constituted by an SAP matrix  50  containing SAP granules  52  and/or SAP agglomerates  54  and a sheath  56  surrounding the core  48 . The effect of the dispersion of SAP granules  52  or SAP agglomerates  54  in the SAP matrix  50  is to increase the gel strength at the expense of a slight reduction in absorbency. The SAP granules  52  or SAP agglomerates  54  are added along with the solid-phase SAP to the appropriate one of the hoppers  11 ,  13  ( FIG. 1 ).  
      With reference to  FIG. 7 , a disposable hygienic article  58  generally includes a top sheet  60 , a back sheet  62 , an absorbent core  64  separating the top sheet  60  from the back sheet  62 , and a fluid acquisition and transfer layer  66  separating the absorbent core  64  from the top sheet  60 . The top sheet  60 , which is intended to be placed adjacent to the wearer&#39;s skin when the hygienic article  58  is worn, is fluid pervious so that aqueous body fluids may readily penetrate through its thickness to the absorbent core  64 . Fluid acquisition and transfer layer  66  distributes aqueous body fluids transferred from top sheet  60  to the underlying absorbent core  64 . The nonporous, hydrophobic back sheet  62  prevents aqueous body fluids absorbed in the absorbent core  64  from wetting the wearer&#39;s clothing, such as pants, pajamas and undergarments. Fastener elements  68  on the back sheet  62  cooperate with complementary fastener elements  70  on corresponding attachment tabs  72  extending laterally from the back sheet  62  for attaching the hygienic article  58  to a wearer.  
      The absorbent core  64  includes filaments  24  in accordance with the principles of the invention capable of absorbing large quantities of aqueous body fluids and retaining the absorbed body fluids under moderate applied pressures. The absorbent core  64  may be formed in its entirety from filaments  24 , from a commingled mixture of filaments  24  and other non-absorbent spunbond filaments, or from a multi-ply laminate structure in which nonwoven web  30  forms one layer. The absorbent capacity of the absorbent core  64  may be optimized for the intended use of the hygienic article  58 .  
      After a limited number of soilings by aqueous body fluids is absorbed by the absorbent core  64 , the hygienic article  58  is intended to be discarded. The invention contemplates that hygienic article  58  may be any item used to absorb and contain aqueous body fluids, and more specifically refers to devices that are placed against or in proximity to the body of the wearer to absorb and contain the various aqueous body fluids discharged from the wearer. For example, hygienic article  58  may be a diaper, a catamenial pad, a feminine hygiene product such as tampons and sanitary napkins, absorbent underpants, an incontinence pad, a training pant and the like, as well as a wipe, a bandage, a wound dressing, and other articles. The invention contemplates that various other consumer and commercial articles or applications that require a heightened level of liquid absorption may incorporate the multicomponent filaments  24  of the invention. As examples, the multicomponent filaments  24  of the invention may be used to for protecting power and communication cables from moisture, in agriculture to increase the capability of soil to retain moisture and nutrients, for treating and containing wastewater, and in the hygienic packaging of food products.  
      While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant&#39;s general inventive concept. The scope of the invention itself should only be defined by the appended claims,