WO1999025551A1

WO1999025551A1 - Microporous film/staple fiber composites

Info

Publication number: WO1999025551A1
Application number: PCT/US1998/024330
Authority: WO
Inventors: Larry C. Wadsworth; Nataraj Gosavi; Ahamad Yacub A. Khan
Original assignee: The University Of Tennessee Research Corporation
Priority date: 1997-11-13
Filing date: 1998-11-13
Publication date: 1999-05-27

Abstract

A composite comprising three layers adhesively bonded together exhibits a balance of breathability, barrier, strength, comfort, and appearance properties, making the composite ideally suited for a variety of medical, hygiene and sportswear uses. The composite comprises a core web of synthetic staple fibers (10) flanking webs of microporous films (14, 16).

Description

Title: MICROPOROUS FILM/STAPLE FIBER COMPOSITES

Inventors: LARRY C. WADSWORTH, NATARAJ GOSAVI, AHAMAD YACUB A. KHAN

BACKGROUND OF THE INVENTION

This invention relates generally to microporous/staple fiber web composites which exhibit breathability and good barrier properties. In one aspect it relates to a composite comprising a staple fiber layer flanked by two microporous film layers.

Microporous films (or membranes as they are frequently referred to) have long been used in applications requiring both breathability (or water vapor transmissibility) and barrier to liquids. Commercially available microporous films include Celgard 2400 polypropylene film, EXXAIRE polyethylene film produced by Exxon Chemical Company, and TetraTex, a microporous polytetrafluoroethylene film produced by TetraTek Corporation, and Gor-Tex produced by W.L. Gore & Associates. Other microporous films include those made from polyamides, polyesters, polyurethane, and polypropylene. A paper entitled "EXXAIRE PLUS NON-WOVENS - MADE FOR

EACH OTHER" was presented at the First Annual TANDEC Conference during October 22-25, 1991 in Knoxville, Tennessee. This paper disclosed a two layer composite comprising a microporous film and a nonwoven HDPE.

An article appearing in Nonwovens Industry dated June 1991 , page 38, and entitled "New Light-weight Film Creating Markets for Nonwoven

Composites" discloses a no-heat process for laminating a microporous film to nonwovens using discrete bonding patterns of an adhesive.

Patents which disclose microporous films and microporous film composites include the following: (a) U.S. Patent 4,777,073 discloses a breathable polyolefin film prepared by melt embossing a highly filled polyolefin film which is stretched to impart greater permeability to the film;

(b) U.S. Patent 4,929,303 discloses a breathable polyolefin film heat laminated to a nonwoven HDPE fabric.

SUMMARY OF THE INVENTION The present invention relates to composite web structures which are breathable and possess gpod strength and barrier properties. In addition, the composite constructed according to the present invention exhibits desirable aesthetic and comfort properties.

Although the composite of the present invention has a wide range of uses where breathability and barrier properties are necessary, it is particularly adapted for use as protective apparel.

The composite of the present invention comprises, in its broadest embodiment, a microporous (MP) film layer and a staple fiber layer of at least

80 wt% synthetic staple fibers. In a preferred embodiment the composite comprises a three layered structure having a core layer of the staple fiber web, bonded to flanking microporous films, as shown in Figure 1.

Optionally the composite may include one or two outer layers of nonwovens for aesthetics and comfort, as shown in Figures 2 and 3.

The structure combines the barrier properties of the microporous film and the comfort of the staple fibers. The nonwoven web (if used) also improve the aesthetics (appearance and soft hand) and add to the comfort of the user. The synthetic staple fiber web exhibits containment properties for aqueous liquids. Tests have shown that the dry synthetic staple fiber located downstream of the MP film layer acts as a barrier (containment) for any small amount of liquid which passes the MP film. It is believed that the hydrophobic property of the synthetic fibers contributes to the liquid containment. The preferred method of laminating the webs to form the composite involves the steps of (a) applying a thin coat of an adhesive onto the webs so that the adhesive is at the interface of each web to form a composite and (b) feeding the composite into the nip of calender rolls maintained at a low temperature (e.g. less than 100° C) to pressure bond the webs together. The method may be carried out in one pass through the calender wherein all four layers are pressure bonded together, or in two or more passes through the calender wherein two or three layers are bonded together in one pass followed by the addition of one or more layers in subsequent passes.

A particularly surprising aspect of the present invention is that it produces composite structures that are capable of passing the Viral

Penetration Tests under ASTM F1671. As noted above, it is important that the synthetic staple fiber layer be located downstream of the MP film. Note that a staple fiber layer flanked by MP films offers protection in both directions of flow. DESCRIPTION OF THE FIGURES

Figure 1 is a side crosssectional view of the first embodiment of the present invention.

Figure 2 is a side crosssectional view of the second embodiment of the present invention. Figure 3 is a side crosssectional view of the third embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS

The composites of the present invention exhibit both breathability and resistance to liquids, and are useful in a variety of medical and/or hygiene applications such as protective apparel, wound dressings, sterile dressings, absorbents (e.g. diapers), face masks and the like. Composites used in many of these applications should have the following properties:

(a) breathable,

(b) resistance to liquid,

(c) relatively high strength, (d) wear resistant,

(e) comfort,

(f) appearance, and

(g) relatively low cost.

In some of the applications, the composite should in addition have blocking properties for aqueous liquids.

The composite constructed according to the present invention combines the barrier properties of microporous film, the breathability, and comfort of staple fibers.

In its broadest embodiment, the composite comprises a core web of synthetic staple fibers 10 which exhibits predominantly hydrophobic wetting characteristics, and flanking layers of microporous films. As shown in Figure 1 , the staple fiber layer has a first side 11 and a second side 12. Each of the layers of the composite structure may be adhesively bonded together by a low temperature, low pressing bonding process. In the preferred embodiment, the composite further includes a web of a nonwoven 24 bonded to the outer or inner surface of the composite. The description of each of the webs used in the composite, the method of lamination, and properties of the composite, are described below. Microporous Film

The term "microporous film" means microporous membrane. The terms film and membrane are used interchangeably herein.

Microporous films are defined as having a narrow pore sized distribution int he submicron range, from 1.0 to 10 microns. The microporous films can be made by a number of processes, which include (a) dissolving polymers in solution followed by extraction of the solvent by water vapor, (b) stretching of crystallizable polymers which results in microsized tears, and (c) stretching of a mineral filled polyolefin film. The polymers used in the microporous films include PTFE, polyolefins, polyurethanes, polyamides, and polyesters.

The preferred microporous film 14, 16 used in the present invention is a polyolefin prepared by stretching a highly filled polyolefin film to impart permeability therein, in accordance with U.S. Patent 4,777,073, the disclosure of which is incorporated herein by reference. The microporous film prepared by this process exhibits excellent breathability, at least 3,000, and generally from 4,000 to 10,000 grams per square meter per day, and in comparison to other microporous film is inexpensive. As shown in Figure 2, the microporous film layers have an inside 20 and an outside 22. The inside is adjacent the staple fiber layer.

Polyolefins used to make the film include polypropylene, copolymers of propylene, homopolymers, and copolymers of ethylene and blends thereof. A preferred polyolefin is a copolymer of polypropylene and low density polyethylene, particularly linear low density polyethylene (LLDPE). The preferred filler at concentrations of from 30 to 70 wt% include inorganic fillers such as calcium carbonate, Ti0₂, barium carbonate, magnesium sulfate, and the other inorganic fillers listed in the above reference, U.S. Patent 4,777,073. Calcium carbonate is the preferred filler. The pore size of this film ranges from 0.1 to 0.5 microns. Staple Fiber Web

The term "Staple Fibers" as used herein includes natural or synthetic discrete fibers having a length from less than 1 inch to about 8 inches, preferably from about 0.5 inch to about 5 inches and most preferably from about 1 inch to about 3 inches. The preferred fibers are synthetic fibers, although a small amount (up to 20%) of natural fibers may be used in the blend. The synthetic staple fibers may include only one type of fibers or may include blends. For purposes of the present invention, the fibers preferably exhibit at least some hydrophobic properties. The preferred concentration of the synthetic fibers in a blend with natural fibers is more than 80 wt%, preferably more than 90 wt%. The most preferred staple fiber web is 100% synthetic fibers (e.g. polypropylene).

The synthetic man-made fibers may be made from thermoplastics such as polyolefins (including polypropylene and polyethylene), polyesters, polyamides and carbon fibers (activated) which are extruded to the proper diameter (usually from 10 to 50 microns) and cut in the desired length, usually from 0.5 to 5.0 inches. The natural staple fibers may be any cellulosic base fibers, such as cotton, ramie, hemp, flax, jute, kenaf, bagasse, eucalyptus, rayon, and combinations thereof but do not include wood fibers. The preferred synthetic staple fibers are polypropylene (PP) staple fibers. The preferred natural fibers (in a blend) are cotton fibers. The staple fibers may be formed into a web by any of the presently known processes, including, but not limited to, thermal bonding, latex bonding, or carding, or needle-punching, or hydroentangling.

The synthetic staple fibers, which exhibit hydrophobic properties function to contain aqueous liquid or blood. Moreover, the staple fiber web improves the comfort property of the composite.

Nonwoven Webs (Optional)

Nonwoven webs 24 are webs made of randomly oriented fibers or filaments of thermoplastic polymer by entangling the fibers or filaments through mechanical, thermal, or chemical means. Nonwovens exclude paper and products which are rewoven, knitted, tufted, or felted by wet milling. The preferred nonwoven webs for use in the present invention are the spunbonded and meltblown webs.

The spunbonded webs are formed by filaments that have been extruded, drawn, laid on a continuous belt, and then immediately thermally bonded by passing through a heated calender. These webs are continuous filament fiber structures having an average fiber diameter between 12 and 50 microns.

Meltblown webs are made by extruding a molten plastic through a row of die openings to form filaments and contacting the extruded filaments with high velocity sheets of converging hot air. The converging air contacts the attenuates or draws the filaments down, depositing them as fibers onto a collector in a random pattern, forming a meltblown web. The meltblown webs have average fiber size between 0.5 to 15 microns which is substantially smaller than the average fiber size of the spunbond web. Another difference between the meltblown and spunbond webs is that the meltblown webs are generally held together by fiber entanglement with some thermal bonding whereas the spunbond webs are generally thermal bonded by the calendar, although spunbond webs are also bonded by chemical, adhesive, and needling processes.

Both the spunbonding and meltblowing techniques are well known in the art. For example, U.S. Patent 4,405,297 discloses a spunbond process and U.S. Patent 3,978,185 discloses a meltblowing process, both of which are incorporated herein for reference.

For purposes of the present invention, the nonwoven webs can be made of any synthetic thermoplastic polymer used in meltblowing or spunbond processes. By way of example, these include the following: polyolefins, particularly ethylene and propylene homopolymers and copolymers (including EVA and EMA copolymers), nylon, polyamines, polyester, polystyrene, poly-4-methylpentene, polymethylmethacrylates, polyt fluorochloroethylene, polyurethanes, polycarbonates, silicones, polyphenylene sulfide, and polypropylene or polyethylene therephathalate.

The most common polymers used in spunbonded fabrics include polypropylene having a melt flow rate of 12-40. These polymers generally are extruded at temperatures ranging from 180° to 350°C.

The most common polymers used in meltblown fabrics include polypropylene having melt flow rates of 10-2500. These polymers are generally extruded at temperatures of between 180° to 350°C and contacted with high velocity air from 180° to 375°C.

The preferred weight of the nonwoven web is from about 0.1 to 2 oz. per square yard. The preferred weights are between about 0.25 to about 1.5 oz. per square yard for the spunbond webs. The nonwovens may include additives to impart desired properties to the webs. Examples of these additives are wetting agents, fluorochemicals, antistatics, and antimicrobiotic agents.

As indicated above, the nonwoven webs in a preferred embodiment comprise the outer two layers of the four layer structure. These webs impart strength to the structure, improve the hand (softness to the feel) and wearability, and improve the comfort and appearance giving the composite fabric a clothlike appearance and feel. Adhesive Any of the adhesives compatible with polyolefins and the synthetic staple fibers may be used. The preferred adhesives are the hot melt adhesives such as the polypropylene based adhesives, and EVA adhesive, (e.g. 20-40 wt% VA). Adhesive Bonding It is important in laminating the composites of the present invention to use nonthermal bonding techniques. Thermal bonding has a tendency to damage the microporous film by introducing pinholes. The preferred technique for applying the adhesive is by melt-spraying or meltblowing of adhesives wherein an air/adhesive spray is deposited on one of the surfaces to be bonded. Both the meltblowing or meltspraying involve extruding a filament or filaments of a hot melt adhesive from a die and contacting the filament or filaments with air to either stretch or attenuate the filament or break it up into droplets which are deposited on the surface of the web. The amount of adhesive deposited on the web may vary within a wide range, but it should be sufficient to ensure good adhesion but not so much that substantial amounts of the pores of the web are plugged. Application from about 1 to 10 grams per square meter of the adhesive should be sufficient, with 1 to 5 grams per square meter being preferred.

As indicated above, the bonding should be by nonthermal pressure techniques. "Nonthermal" means the composite is formed by applying a bonding pressure at temperatures below melting point or softening temperature of the polymers used in the laminate. With polyolefins, this means that the temperatures are carried out below 100°C and preferably below 50° C, most preferably below 30°C. The lower limit of the geographic location, may vary widely from 0°C to 50°C.

The following table presents some of the preferred properties or specifications for each layer.

*one or both of the MP films may be two or more layers or plies of MP films

The thickness of the staple fiber layer is preferably 4 mils or larger, most preferably 10 to 50 mils to provide containment for any liquid passing the MP film layer. Uses of Composites

As amply demonstrated by the test results presented below, the composite of the present invention exhibits a combination of properties making it ideally suited for a number of applications: (a) The composite is breathable.

(b) The composite exhibits good barrier to liquids.

(c) The composite has a soft, clothlike hand and appearance.

(d) The composite with the preferred microporous film (EXXAIRE⁸) costs less than many composites having other microporous films. A particularly useful application of the composite of the present invention is in medical protective apparel designed to protect the wearer from contact with external blood or toxic liquids, or to protect the environment from blood or liquid contamination emanating from the wearer.

Other medical and hygiene uses include feminine hygiene absorbents, baby diapers, adult incontients, industrial protective apparel, wound dressing, transdermal patches, and the like. Other uses include sportswear, rain gear, footwear, and the like.

Although the reasons for the improved performance (Virus Resistance Tests) of the composite of the present invention are not fully understood, it is believed that the combination of the microporous film and the synthetic staple fiber layer both contribute. The MP film provides in initial barrier for the blood and the staple fiber layer contains any minute quantitives of blood passing the MP film. Tests have shown that a dry staple fiber layer effectively contains liquid penetrating the MP film. EXPERIMENTS

Tests were carried out to demonstrate the effectiveness of a dry synthetic staple fibers downstream of the microporous film (MF) to kill any of the viruses that could travel with the blood through the MF layer because virus cannot normally live in dry environments.

To study the "dry layer" layer effect, laminates containing thermally bonded 100% polypropylene webs (TPP), TPP as is (hydrophilic), scoured TPP (hydrophobic), scoured and plasma-treated TPP (hydrophilic), and scoured and surfacant-treated TPP (hydrophilic due to a wetting agent) were prepared. Prior to performing the viral penetration test, it was determined that the virus was not adversely affected. The laminates were subjected to the viral penetration test results by the ASTM F1671 method with the TPP center layers dry and wetted with sterile, deionized, distilled water. The webs were about 15 to 25 mils thick.

The viral penetration results of the laminates having different TPP's providing different environment in terms of hydrophilicity are given in Table 1. The test results of Samples XV and XVIII, MF TPP (as is)/MF and MF TPP (scoured) with plasma/MF, indicated the importance of the dry center layer in passing the vigorous viral penetration test. By wetting the center layer, the hydrophilic TPP's, both samples failed the test. However, Sample XVI, MF/TPP (scoured)/MF, passed the test whether the center layer, the hydrophopbic TPP, was dry or wet. The laminates, Sample XVII, MF/TPP (scoured) with surfacant MF also passed the test regardless of the dry or wet environment.

To confirm the importance of two MF layers as both front and back layers in the fabrication of laminates, two layers of MF's, and two laminates samples containing SB as a center layer and TCPP as front and back layers (Samples XVIII, #6 and #7) were tested. In dry environment, the MF staple passed the test but the samples #6 and #7, TCPP-SB-TCPP's, failed. The hydrophobic center layer, SB, did not prevent the virus penetration when the hydrophilic TCPP were used as front and back layers. The test result (Table 2) indicated that using two MF layers seems to be the most effective fabric construction to pass the viral penetration test.

The data clearly demonstrate that of the dry staple fibers (e.g. PP) plays an important role passing in the Resistance to Blood and Virus tests.

Although the composites of the present invention have been described mainly as comprising two or three layers adhesively bonded together, it is to be emphasized that this represents the preferred structure. Variations include adhesively bonding intermediate layers between two or more of the recited layers or adding a cover layer (e.g. nonwoven) to the outside surface or inside surface of the composite, as shown in Figures 2 and 3.

Table 1. Viral Penetration Test Results of the Laminates Containing a TPP Inner Layer by the ASTM F1671 Method.

* MF: 1.0 mil polyethylene Exxaire™ microporous film TPP(as Is): thermally bonded PP nonwoven with a hydrophilic finish TPP(scoured): scoured and rinsed TPP

TPP(scoured) w surfactant: scoured TPP with 0.05% surfactant(Triton X-100) treatment TPP(scoured) w plasma: scoured TPP with plasma treatment

^** ND: Not determined by the test, and the test results were decided without counting these trials.

Table 2. Viral Penetration Test Results of the Laminates by the ASTM F1671 Method.

* MF: 1.0 mil polyethylene Exxaire™ microporous film SB1 - 1.0 oz/yd² polypropylene (PP) spunbond webs SB2 - 0.5 oz yd² polypropylene spunbond webs TCPP1 - Thermally bonded 60/40 cotton/PP with 0.7 oz/yd² basis weight, 18% bond area and 36" width

ND: Not determined by the test, and the test results were decided without counting these trials.

Claims

WHAT IS CLAIMED IS: 1. A composite web structure comprising: a. a staple fiber layer having a first side and a second side, said staple fiber layer comprising at least 80% synthetic staple fibers; and b. a first microporous film layer adjacent the first side of said staple fiber layer.

2. The web of claim 1 , wherein said staple fibers have a length in the range of 0.5 to 5 inches.

3. The web of claim 2, wherein said natural fibers are cotton, ramie, hemp, flax, jute kenaf, bagasse, eucalyptus, or combinations of the foregoing fibers.

4. The web of claim 3, wherein said synthetic fibers are made from thermoplastics and said natural fibers are cellulose base fibers.

5. The web of claim 1 , further comprising a second microporous film layer adjacent the second side of said staple fiber layer, such that said staple fiber layer is sandwiched between said first and second microporous film layers.

6. The web of claim 5, wherein said staple fiber layer exhibits predominantly hydrophobic wetting characteristics.

7. The web of claim 5, wherein said first and second microporous film layers are adhesively bonded to said staple fiber layer.

8. The web of claim 5, wherein said microporous film comprises polyolefin and has a breathability of at least 3000 grams per square meter per day.

9. The web of claim 8, wherein said polyolefin is polypropylene, copolymers of propylene, homopolymers, or copolymers of ethylene.

10. The web of claim 9, wherein said polyolefin is a copolymer of polypropylene and low density polyethylene.

11. The web of claim 5, wherein said microporous film comprises pores having a size in the range of 1.0 to 10 microns.

12. The web of claim 5, wherein said first and second microporous film layers have an inside and an outside, said insides being adjacent said staple fiber layer.

13. The web of claim 12, further comprising a nonwoven web layer adjacent the outside of said first microporous film layer.

14. The web of claim 1 , further comprising a nonwoven web layer adjacent the second side of said staple fiber layer.

15. A composite web structure comprising: a. a staple fiber layer having a first side and a second side, said staple fiber layer comprising at least 80% synthetic staple fibers and at most 20% natural fibers; b. a first microporous film layer adhesively bonded to the first side of said staple fiber layer, said first microporous film comprising polyolefin and having a breathability in the range of 4000-10000 grams per square meter per day; c. a second microporous film layer adhesively bonded to the second side of said staple fiber layer, said second microporous film comprising polyolefin and having a breathability in the range of 4000-10000 grams per square meter per day.

16. The web of claim 15, wherein said first and second microporous films comprise pores having sizes in the range of 1.0-10 microns.

17. The web of claim 15, wherein said synthetic staple fibers comprise polypropylene staple fibers.

18. A composite web structure comprising: a. a staple fiber layer having a first side and a second side, said staple fiber layer comprising at least 80% synthetic staple fibers and at most 20% natural fibers; b. a first microporous film layer having an inside and an outside, the inside of said microporous film layer being adhesively bonded to the first side of said staple fiber layer, said first microporous film comprising polyolefin; c. a second microporous film layer adhesively bonded to the second side of said staple fiber layer, said second microporous film comprising polyolefin; and d. a nonwoven web layer bonded to the outside of said first microporous film layer, said web layer comprising continuous filament fiber structures.

19. The composite web structure of claim 18, wherein said nonwoven web layer comprises wetting agents, fiuorochemicals, antistatics, or antimicrobiotic agents.

20. The composite web structure of claim 18, wherein said nonwoven web layer comprises a meltblown or spunbonded thermoplastic polymer.