Patent Publication Number: US-2005130539-A1

Title: Nonwoven webs manufactured from additive-loaded multicomponent filaments

Description:
FIELD OF THE INVENTION  
      The invention relates generally to nonwoven webs and, more particularly, to nonwoven webs of multicomponent filaments loaded with surfactant and methods of manufacturing such nonwoven webs.  
     BACKGROUND OF THE INVENTION  
      Nonwoven webs and their manufacture in meltspinning devices have been the subject of extensive development resulting in a wide variety of materials for numerous commercial applications. Nonwoven webs consist of a sheet of overlapped or entangled filaments or fibers of melt-processable thermoplastic polymers manufactured using spunbond and meltblown processes. A spunbond process generally involves extruding a curtain of semi-solid filaments of one or more thermoplastic polymers from multiple rows of fine orifices in a spinneret and attenuating or drawing the extruded filaments with drag forces created by a high-velocity flow of process air. Spunbond filaments are generally continuous and may have average diameters in the range of about 10 to 20 microns. A meltblown process generally involves pumping a thermoplastic polymer from an extruder through a die to form a curtain of fibers and directing a high pressure gas stream at the exit of a die to attenuate the fibers while they are in their extensible molten state. Meltblown fibers may be continuous or discontinuous and are usually smaller than 10 microns in average diameter. The filaments or fibers are deposited on a moving collector in a substantially random manner thereby forming a continuous-length nonwoven web.  
      As many thermoplastic polymers are normally hydrophobic, the thermoplastic filaments or fibers must be modified to produce a nonwoven web that is water permeable or hydrophilic. One modification technique involves mixing a surfactant with the thermoplastic polymer before extrusion so that the fibers or filaments are impregnated with surfactant upon extrusion. The surfactant migrates or diffuses to the external surface of the impregnated fibers or filaments in a process called blooming. Surfactant migration may occur during and/or after filament formation and may be coerced by heating. Alternatively, the filaments or fibers may be surface-treated with a chemical agent by a conventional post-deposition topical treatment. One type of post-deposition surface treatment involves dipping the nonwoven web in a treatment bath containing a surfactant. Another type of post-deposition surface treatment involves coating or spraying the nonwoven web with a treatment solution containing a surfactant.  
      Surfactant-treated nonwoven webs in an article are susceptible to surfactant loss when exposed to stresses capable of removing the surfactant. Surfactant may be transferred from the external surface of a surfactant-treated nonwoven web to a contacting hydrophobic material, such as when an article containing the surfactant-treated nonwoven web is packaged and stored. As a specific example, surfactant is transferred from top sheets in hygienic articles to contacting hydrophobic surfaces of the hygienic article and/or to the product packaging when compression packaged and stored before use. As a result, the hygienic article will not perform as expected when used or has a shortened shelf life the top sheet gradually loses its ability to transfer liquids, and. Elevated temperatures experienced during shipping and storage may accelerate surfactant migration to contacting hydrophobic surfaces rendering them hydrophilic with a concomitant loss of barrier properties. The incremental conversion of hydrophobic surfaces further reduces product shelf life.  
      Surfactant also tends to be removed from the surfactant-treated nonwoven web by recurring exposure to an aqueous medium. For example, washing cycles gradually diminish the hydrophilicity of a surfactant-treated nonwoven web. As another example, when used as a top sheet in a hygienic absorbent article, a surfactant-treated nonwoven web loses its aqueous permeability with accumulating exposures to soilings by aqueous body fluids.  
      It would be desirable, therefore, to provide a nonwoven web treated with a chemical agent or additive, such as a surfactant, that can better withstand stresses and adverse conditions such as those discussed above.  
     SUMMARY  
      The invention provides a multicomponent filament having a sheath region of a first melt-processable polymer and a core region of a second melt-processable polymer encased within the sheath region. The core region includes an additive distributed with a first concentration that, over time and with the occurrence of stresses, migrates outwardly from the core region to the sheath region due to the presence of a concentration gradient decreasing in a radially outward direction. The first melt-processible polymer of the sheath region operates to impede the radial migration of additive from the core region to the filament&#39;s external surface.  
      In certain embodiments of the invention, the sheath region may initially contain a concentration of the additive that is smaller than the first concentration of the additive in the core region. Alternatively, the additive may be absent from the sheath region when the filaments are formed. In other embodiments of the invention, the sheath region may contain a second concentration of an additive that differs in chemical composition from the additive in the core region, where the additives produce the same web characteristic. The additive(s) may be a concentration gradient of surfactant that produces a hydrophilic web or other non-surfactant additives, such as colorants, anti-static agents, lubricants, flame retardants, antibacterial agents, softeners, ultraviolet absorbers, and polymer stabilizers in which the non-surfactant additive migrates from the core region to the sheath region and external filament surface.  
      In accordance with the principles of the invention, a nonwoven web is manufactured by heating two thermoplastic polymers to a flowable state and adding a concentration of an additive, such as a surfactant, to at least one of the two thermoplastic polymers. The thermoplastic polymers are combined to form multicomponent filaments each having a core region of the additive-containing thermoplastic polymer and a sheath region of the other thermoplastic polymer, which may also include a concentration of an additive. The multicomponent filaments are collected to form the nonwoven web. If the additive is a surfactant, the period over which the non-woven web exhibits effective hydrophilicity may be extended as the sheath region impedes the radial migration of additive from the core region to the filament&#39;s external surface.  
      The nonwoven web of the invention may be used in diverse commercial product applications including, but not limited to, hygienic articles such as diapers, adult incontinence products, and feminine hygiene products. Nonwoven webs loaded with surfactant in accordance with the principles of the invention may be used as a top sheet for an absorbent medium in a hygienic article. Articles formed from nonwoven webs of such surfactant-loaded filaments will demonstrate a lengthened shelf life and an improved performance when subject to successive wettings by liquids. Additionally, high fiber basis weight nonwoven webs require an internal surfactant or bloom additive to insure hydrophilicity without the need to fully wet the material with a conventional topical treatment. Such topical treatments are disadvantageous as these relatively-thick nonwoven webs must be dried throughout their full thickness, which is time consuming and adds needless cost to the production of a hydrophilic nonwoven web. In accordance with the invention, the surfactant is not applied topically as a surface treatment and, as a result, the nonwoven web does not have to be dried.  
      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 diagrammatic view of an apparatus for forming a nonwoven web in accordance with the principles of the invention;  
       FIG. 2  is a multicomponent filament in accordance with the principles of the invention;  
       FIG. 3  is an axial cross-sectional view of the multicomponent filament of  FIG. 2 ;  
       FIGS. 4-6  are end views of multicomponent filaments in accordance with alternative embodiments of the invention; and  
       FIG. 7  is a perspective view of a hygienic article in accordance with the principles of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The invention is directed to nonwoven webs having a lengthened property, such as hydrophilicity, formed from multicomponent filaments including a sheath and a core embedded in the sheath, in which the core has a concentration of an additive, such as surfactant, that serves as a reservoir for replenishing additive depleted from the sheath. Although the invention will be described herein as being manufactured by an exemplary meltspinning system, it should be understood that modifications to the exemplary system described herein could be made so as to conform any portion or the entire system to produce any type of airlaid nonwoven web or a collection of unbonded filaments or fibers without departing from the intended spirit and scope of the invention.  
      With reference to  FIG. 1 , a spunbonding apparatus  10  is equipped with a pair of extruders  12 ,  14  that each convert a solid thermoplastic polymer into a molten state and provide the molten thermoplastic polymers under pressure to a corresponding set of gear pumps  16 , 18 . Extruder  12  is provided initially with a solid mass of Polymer A from a bulk source  20 , and extruder  14  is provided initially with a solid mass of Polymer B from a bulk source  22 , which are replenished from bulk sources  20 ,  22  as consumed during the melt spinning process. The gear pumps  16 ,  18  pump metered amounts of each thermoplastic polymer to an extrusion die or spinneret  24 , which has a spin pack that combines the thermoplastic polymers and discharges a curtain of multicomponent filaments  26  constituted collectively by the two thermoplastic polymers. For ease of reference, one thermoplastic polymer provided to spinneret  24  will be referred to as Polymer A, while the second thermoplastic polymer provided to spinneret  24  will be referred to as Polymer B. As will be understood in accordance with the principles of the invention, other embodiments may utilize more than two thermoplastic polymers. An exemplary multiple-component spin pack for a spinneret  24  is disclosed in U.S. Pat. No. 5,162,074, which is hereby incorporated by reference herein in its entirety.  
      The descending airborne curtain of multicomponent filaments  26  are quenched with cross-flow cooling air, as represented by arrows  28 , from a quench blower (not shown) to accelerate solidification and drawn through a filament-drawing device  30 . The filament-drawing device  30  applies a tangential high velocity flow of process air, as represented by arrows  31 , in a direction substantially parallel to the length of the multicomponent filaments  26 . Because the multicomponent filaments  26  are extensible, the drag force of the spunbonding process pneumatically attenuates and molecularly orients the multicomponent filaments  26 . The multicomponent filaments  26  discharged from the filament-drawing device  30  are deposited in a substantially random manner as a nonwoven web  32  on a horizontally and linearly moving perforated collector  34 . The collector  34  moves in a machine direction, represented by the arrow labeled MD, that represents the length of the nonwoven web  32  in the direction in which it is produced. The collector  34  also spans the width of the curtain of multicomponent filaments  26  in a cross-machine direction perpendicular to the machine direction and into and out of the plane of the page of  FIG. 1 .  
      Additional spunbonding apparatus, not shown but similar to spunbonding apparatus  10 , and meltblowing apparatus (not shown) may be provided downstream of spunbonding apparatus  10  for depositing one or more spunbond and/or meltblown nonwoven webs of either monocomponent or multicomponent filaments on nonwoven web  32 . 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.  
      With continued reference to  FIG. 1 , a surfactant from a surfactant source  36  is added along with the mass of Polymer B from bulk source  22  to the hopper of extruder  14 . A mass or volume of the surfactant is blended, preferably homogenously, with Polymer B to create a mixture that is pumped by gear pump  18  from the extruder  14  to the spinneret  24 . The surfactant may be any suitable chemical agent that increases the hydrophilicity of the multicomponent filaments  26  so that nonwoven web  32  is wettable by and has the strong ability to absorb an aqueous medium containing water or another liquid. A mass or volume of a surfactant from a different surfactant source  38  may be mixed with the mass of Polymer A from bulk source  20  added to the hopper of extruder  12  and pumped by gear pump  18  from extruder  12  to spinneret  24 . In this manner and consistent with the principles of the invention, each of the thermoplastic polymers provided to the spinneret  24  may include a surfactant concentration.  
      The surfactant may be blended with the thermoplastic polymer in a dry form, such as powder or pellets, using conventional mechanical mixing techniques before being placed into the hopper of either extruder  12 ,  14 . Mechanical mixing techniques using devices for homogenizing an admixture of powders and pellets, such as V-blenders or double cone blenders, are familiar to persons of ordinary skill.  
      The melt-processable thermoplastic polymer or polymers used to fabricate the multicomponent filaments  26  may be any of the commercially available spunbond grades of a wide range of thermoplastic polymer resins, copolymers, and blends of thermoplastic polymer resins, including without limitation polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate, polybutylene, polyamides, nylons, polyvinyl acetate, polyvinyl chloride, polyvinyl alcohol, cellulose acetate, and blends and copolymers thereof. The invention contemplates that each of the thermoplastic polymers (Polymers A and B) constituting the multicomponent filaments  26  may be identical in base composition and differ only in the concentration of the added surfactant. For example, the core may be formed from polypropylene containing a concentration of surfactant and the sheath may be formed from polypropylene having identical material characteristics and a lower concentration of surfactant. Most of these thermoplastic polymer resins are hydrophobic and, therefore, are rendered hydrophilic (wettable) by the presence of the surfactant at the external surface. The surfactant may also be added to normally hydrophilic thermoplastic resins for enhancing their wettability.  
      Typically, the surfactants supplied by the surfactant sources  36 ,  38  may be identical, although the invention is not so limited. Each surfactant may be classified as a fast wetting surfactant that causes liquids to permeate the nonwoven web  32  at a fast rate or, alternatively, as a low wetting surfactant which causes liquids to permeate the nonwoven web  32  at a relatively slow rate. Each surfactant must be miscible with the associated thermoplastic polymer so as to be capable of forming homogeneous mixtures. Each surfactant may be anionic, cationic, amphoteric or non-ionic, in which non-ionic surfactants are believed to be less irritating to human skin tissue. Preferred non-ionic surfactants include, but are not limited to, sorbitan esters, ethoxylated sorbitan esters, silicone copolymers, fluorochemical-based surfactants, alcohol ethoxylates, alkylphenol ethoxylates, carboxylic acid esters, glycerol esters, polyoxyethylene esters of fatty acids, polyoxyethylene esters of aliphatic carboxylic acids related to abietic acid, anhydrosorbitol esters, ethoxylated anhydrosorbitol esters, ethoxylated natural fats, oils, and waxes, glycol esters of fatty acids, carboxylic amides, diethanolamine condensates, monoalkanolamine condensates, polyoxyethylene fatty acid amides, and polyalkyleneoxide block copolymers.  
      With reference to  FIGS. 2 and 3 , the thermoplastic polymers constituting each multicomponent filament  26  are arranged a sheath/core configuration in which one thermoplastic polymer (Polymer B) is disposed in a core  40  surrounded by a sheath  42  of the other thermoplastic polymer (Polymer A). The core  40  and sheath  42  each extend continuously along the length of each multicomponent filament  26  and are coextensive along an annular or cylindrical interface  43 . The core  40  and sheath  42  may be arranged coaxially in a concentric configuration with the sheath  42  radially outward of the core  40 .  
      In accordance with the principles of the invention and with reference to  FIGS. 2 and 3 , a surfactant  44   a  is present in the core  40  in a greater concentration than a surfactant  44   b  in sheath  42 . Preferably, the concentration of surfactant  44   a  in Polymer B constituting the core  40  is about 5% by weight to about 10% by weight, and the concentration of surfactant  44   b  in Polymer A constituting the sheath  42  ranges up to about 3% by weight, typically about 1% by weight to about 3% by weight. In certain specific embodiments of the invention, Polymer A may be the same thermoplastic polymer as Polymer B and the chemical composition of the surfactant in each of the polymers may be identical.  
      In accordance with the principles of the invention, the surfactant  44   a  initially present in the core  40  will tend to diffuse or migrate from the region of high concentration across the interface  43  to the region of low concentration in the sheath  42 , as represented by the arrows labeled with reference numeral  41 . Molecules of surfactant  44   b  initially present at an external surface  48  of the sheath  42  produce hydrophilicity or wettability. Surfactant molecules at the external surface  48  of the sheath  42  are lost, as represented by the arrows labeled with reference numeral  47 , by contact with another hydrophobic surface or by repeated wetting with liquid. Amounts of surfactant  44   b  and surfactant  44   a  present in sheath  42  migrate to the external surface  48  as represented by the arrows labeled with reference numeral  49 . The radially outward migration reduces the surfactant concentration in the sheath  42 . As the surfactant concentration drops in the sheath  42 , surfactant  44   a  migrating from the core  40  into the sheath  42  replenishes the depleted concentration of surfactant  44   a  and surfactant  44   b . As a result, the nonwoven web  32  ( FIG. 1 ) will remain hydrophilic for a, longer period after manufacture when packaged and with repeated exposure to liquids. The surfactant  44   a  in the core  40  serves as a reservoir for surfactant transfer, as required or otherwise on a time-delayed basis due to the difference in concentration, across the annular interface  43  to the sheath  42  and subsequently to the external surface  48 .  
      With reference to  FIG. 4  and in accordance with an alternative embodiment of the invention, sheath  42  may contain no surfactant  44   b  at least at the moment of discharge from the spinneret  24  ( FIG. 1 ). Surfactant  44   a  migrates from the core  40  into the sheath  42  and subsequently to the external surface  48  makes the multicomponent filament  26  hydrophilic. The initial migration occurs subsequent to discharge of the multicomponent filament  26  from the spinneret  24 , such as during web processing to complete web formation, during web processing to form an article, and while the web and/or article are stored before the time of use.  
      The invention contemplates that the chemical composition of the surfactant  44   a  may differ from the chemical composition of the surfactant  44   b . In this alternative embodiment, surfactant  44   a  is not present in the sheath  42  (i.e., has a zero concentration) at least at the moment of discharge from the spinneret  24  ( FIG. 1 ). The hydrophilicity of the multicomponent filament  26  is supplied initially by molecules of surfactant  44   b  present at the external surface  48 . Due to the concentration disparity, surfactant  44   a  migrates radially outwardly from the core  40  into the sheath  42  and amounts of surfactant  44   a  eventually reach the external surface  48 . Eventually, surfactant  44   a  is lost from the external surface  48  along with with surfactant  44   b . Amounts of surfactant  44   b  lost from the external surface  48  are replenished by the outward migration of stored amounts of surfactant  44   b  from the sheath  42 . Surfactant  44   a  lost from the external surface  48  is replenished by amounts of surfactant  44   a  migrating from the underlying sheath  42 . The store of surfactant  44   a  in the sheath  42  is replenished by migration of amounts of surfactant  44   a  from core  40 . The migration and loss of surfactant  44   a  may be independent of the migration and loss of surfactant  44   b  or the migration and loss of surfactants  44   a,b  may be interrelated.  
      By adjusting the relative concentrations of the surfactants  44   a,b  in the two thermoplastic polymers (Polymers A and B), the shelf life of the surfactant-treated nonwoven web  32  may be significantly extended as the hydrophilic nature of the external surface  48  is lengthened by the net transport or migration of surfactant  44   a  from the relatively-high surfactant concentration present in the core  40  into the sheath  42  and to the external surface  48 . In addition to the gradient in surfactant concentration, the migration rate from the core  40  to the sheath  42  and from the sheath  42  to the external surface  48  is also influenced by other factors, including the sheath thickness, the chemical properties of the surfactant(s), the environmental temperature, extrusion conditions, and the characteristics of the thermoplastic polymer(s). In particular, the diffusion of a surfactant in a thermoplastic polymer depends upon the affinity between the surfactant and the thermoplastic polymer. The affinity varies among the different possible combinations of thermoplastic polymer and surfactant, which will influence transfer of surfactant  44   a  from the core  40  to the sheath  42  and surfactant  44   a  and surfactant  44   b  from the sheath  42  to the external surface. Accordingly, the affinity between the selected surfactant and the thermoplastic material forming the core  40  should permit surfactant transfer from the core  40  to the sheath  42 . Likewise, the affinity between the thermoplastic polymer forming sheath  42  and surfactant  44   a  will influence migration of surfactant  44   a  from the sheath  42  to the external surface  48 . Similarly, the affinity between the thermoplastic polymer forming sheath  42  and surfactant  44   b  will influence migration of surfactant  44   b  from the sheath  42  to the external surface  48 .  
      The invention contemplates that one or more non-surfactant additives, such as compatibilizing agents, colorants or pigments, optical brighteners, ultraviolet light stabilizers, antistatic agents, abrasion resistance enhancing agents, nucleating agents, fillers and/or other additives and processing aids, may be added in a concentration gradient to one or more of the polymers constituting multicomponent filaments  26 . In these various alternative embodiments, the non-surfactant additive is added to filaments  26  with a greater concentration in the core  40  than in the sheath  42 , as described herein with specific regard to surfactant. The subsequent additive migration of the non-surfactant additive(s) radially outward from the core  40  lengthens or sustains the manifestation of a corresponding property(ies) or a characteristic(s) of the filaments  26 . The invention contemplates that one or more of the non-surfactant additives may be distributed in multicomponent filaments  26  in a manner similar to the concentration gradient of surfactant, as described herein, either jointly with the surfactant or in the absence of surfactant.  
      With reference to  FIG. 5  and in accordance with an alternative embodiment of the invention, multicomponent filament  26  may have a core  50  that is offset or non-concentric within a sheath  52  so as to have an eccentric or asymmetrical configuration. With reference to  FIG. 6  and in accordance with another alterative embodiment of the invention, the thermoplastic polymers of multicomponent filament  26  may have an “islands-in-the-sea” configuration in which multiple core regions  60  of Polymer B reside inside a sheath  62  of Polymer A. Although the components of filament  26  are depicted in  FIGS. 2-6  as having round cross-sections, the invention contemplates that the nonwoven web  32  might be formed from filaments (not shown) of different cross-sectional shapes.  
      With reference to  FIG. 7 , a disposable hygienic article  70  generally includes a top sheet  72 , a back sheet  74 , a fluid storage layer  76  separating the top sheet  72  from the back sheet  74 , and a fluid acquisition and transfer layer  78  separating the fluid storage layer  76  from the top sheet  72 . The top sheet  72 , which faces and contacts the wearer, is fluid previous so that aqueous body fluids may readily penetrate through its thickness to the fluid storage layer  76 . Fluid acquisition and transfer layer  78  distributes aqueous body fluids transferred from top sheet  72  to the underlying fluid storage layer  76 , which includes an absorbent material capable of absorbing large quantities of aqueous body fluids and retaining the absorbed body fluids under moderate applied pressures. The back sheet  74  prevents aqueous body fluids absorbed in the fluid storage layer  76  from wetting items in the surrounding environment, such as pants, pajamas and undergarments. Loop-type fasteners  80  on the back sheet  74  cooperate with hook-type fasteners  82  on corresponding attachment tabs  84  extending laterally of the back sheet  74  cooperate for attaching the hygienic article  70  to a wearer.  
      In accordance with the principles of the invention, all or part of the components of hygienic article  70 , including but not limited to the top sheet  72  and the back sheet  74 , may incorporate portions of a nonwoven web formed from the multicomponent filaments of the invention. The invention contemplates that various other consumer and commercial articles may incorporate a portion of a nonwoven web formed from the multicomponent filaments of the invention.  
      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 applicants 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. For example, the multicomponent filaments from which the nonwoven web of this invention is made may be produced by meltblown processes as well known to persons of ordinary skill 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 applicants&#39; general inventive concept. The scope of the invention itself should only be defined by the appended claims, wherein we claim: