EP0893530A1

EP0893530A1 - Breathable nonwoven liquid barrier fabric and method

Info

Publication number: EP0893530A1
Application number: EP98108294A
Authority: EP
Inventors: Jerry Zucker; Arthur J. Sampson; Richard L. Ferencz
Original assignee: Polymer Group Inc
Current assignee: Avintiv Specialty Materials Inc
Priority date: 1997-07-24
Filing date: 1998-05-07
Publication date: 1999-01-27
Also published as: CA2236402A1; JPH11158777A

Abstract

A porous, nonwoven fabric made from bonded fibers and/or filaments is provided, and a continuous film or layer of a liquid radiation curable resin is applied to the fabric surface. The liquid film is subjected to radiation to cure the liquid into a solid film bonded to the surface of the fabric. The film renders the fabric impervious to liquids but allows passage of liquid vapor, and the technique provides an alternative to more expensive ways to achieve the same barrier properties.

Description

Background of the Invention

This invention relates to nonwoven fabrics having a vapor breathable film layer incorporated therein, with said fabrics being substantially impermeable to liquids.

Nonwoven fabrics are produced by a variety of processes distinguished from weaving of yarns on a loom. In general, nonwoven fabrics are made from fibers or continuous filaments, and the fibers or filaments are consolidated at points of intersection by thermal or adhesive bonding, or by fiber or filament entanglement.

Most nonwovens, as commercially produced, are porous and easily transmit liquids and any agents carried by or in the liquids. Many existing and proposed uses for nonwovens require a fabric which is essentially liquid proof, but which can breathe by allowing transmission of water vapor. End uses for breathable nonwovens include clothing, such as medical apparel, components of diapers or other disposable sanitary articles, building covering materials and the like.

Many attempts have been made to provide a nonwoven with the above properties. As one example, a nonwoven fabric may be laminated or extrusion coated with a film, and the film may be of a non-porous or microporous type, but the lamination or extrusion coating process is expensive and requires substantial space and equipment, especially if the laminate is produced on the same line with the nonwoven. It is also known to treat a nonwoven with a waterproofing agent, such as a fluoro compound, but such compounds are relatively expensive, do not fill the pores of the fabric, and require separate treatment and drying steps. The average pore size of nonwoven fabrics can be decreased by providing composite fabrics, such as spunbond-meltblown-spunbond (SMS) fabrics, wherein the meltblown layer comprises very fine fibers. Even composite fabrics, however, do not provide a high barrier to liquids and normally require topical treatments or an extruded film layer to achieve high liquid barrier properties.

Many of the present solutions to the barrier problem are inadequate in that the treated or film laminated nonwoven is not very durable, and the initial properties are degraded upon handling, fabrication, washing and the like. Also, treated and film laminated fabrics tend to be stiff and unsuitable for many applications.

Summary of the Invention

In accordance with the present invention, a nonwoven fabric is coated with a liquid, radiation curable, film forming polymer or resin to provide a continuous and unbroken film on the fabric, and the liquid polymer film is subjected to radiation to cure the polymer and to cause a durable unbroken, flexible film to form and to adhere and bond to the fabric. Surprisingly, a continuous, flexible and liquid impervious film can be formed on substrates which are porous. Within ordinary levels of resin application, the resulting coated fabric is impervious to the transfer or passage of liquids such as water, blood and alcohol but is capable of transmitting water vapor.

The coating and curing steps require a minimal amount of space, and the operation may be carried out on line during production of the nonwoven fabric using a small production space at normal nonwoven production line speeds. Advantageously, the coating can be selectively applied to only portions of a moving nonwoven fabric web, for example, to render only certain portions impervious to liquids, as may be desired in a component of a diaper, or in medical products.

In another aspect of the present invention, liquid radiation curable resins are applied to a surface of a nonwoven, followed by curing of the resin, in order to increase the hydrostatic head of the fabric by a minimum of two to three times the original hydrostatic head.

One preferred type of radiation curable resins are ultraviolet curable resins containing a monomer having one or more functional groups, a polymer or oligomer having unsaturation sites, and a photoinitiator sensitive to UV radiation. The photoinitiator supplies free radicals or cations to cause polymerization of the monomer with the polymer and provide a solid film.

The use of a radiation curable, film forming polymer, greatly reduces the costs compared to current alternative practices, since only a liquid applicator and radiation source are required. The cost of relatively expensive treating agents, and the use of extra heating, film extrusion, and lamination steps, are eliminated.

Detailed Description

As used herein, the term "nonwoven fabric" means a fabric comprising man-made or polymer fibers or filaments which are bonded together at points or are entangled to provide a coherent, planar fabric. Various types of bonding include thermal bonding and chemical and adhesive bonding. The types of entanglement include needling and hydroentanglement, the latter involving impingement of jets of water on an unconsolidated web supported on a porous support. Fibers may also be consolidated by wet laying techniques.

The various methods for making nonwovens are well known and will not be described herein in detail. The term spunbonding refers to a method by which a large number of continuous filaments are spun or extruded, drawn and deposited on a moving conveyor as a moving web. The filaments are then thermally bonded, either by virtue of the filaments having a low softening temperature, or by incorporating some fraction of binder or lower melting point filaments into the web. A related process is melt blowing, in which molten filaments are extruded from small apertures, and are blown and drawn by impingement of high velocity hot air toward a conveyor to form a continuous web.

Fabrics made from fibers usually involve the formation of a fiber web by carding or air laying, and then bonding or consolidating the web. For example, the web may be pattern bonded using an engraved heated calender or an ultrasonic bonder, hot air bonded, or it may be resin bonded and heated. Entanglement procedures have been described above.

The polymers presently used to make nonwovens include polyolefins, such as polyethylene and polypropylene, and polyester, although many others are known and can be employed, such as nylon, rayon, polyurethane, and others. It is not believed that the present invention is limited to the type of polymer or fiber employed in the nonwoven, since the film as applied mechanically interacts with the fabric and surrounds at least portions of the fibers or filaments to provide a strong mechanical bond. Continuous films are more easily applied to fabrics having a small pore size. The weight of the nonwoven fabric is not critical and may range in the order of 3 to 100 grams per square meter.

The unconsolidated web of fibers or filaments is preferably bonded prior to the application of the film coating, such that the fibers or filaments are fixed in position and the web is cohesive and shape retaining. The degree of bonding is not critical, as long as the web is sufficiently shape retaining to allow application of the liquid film. As used herein, the term "nonwoven fabric" means a web of fibers or filaments which has been bonded.

The nonwoven substrate may comprise a single layer or multiple layers, and these layers may be produced by identical or different processes. For example, a composite nonwoven may include spunbond and meltblown layers. The film may reside as an outer bonded layer or may reside as an inner layer of a composite. One preferred nonwoven substrate is a fabric having at least one meltblown layer and at least one spunbond layer.

Radiation curable polymers are available from a variety of commercial sources and are furnished in liquid form, generally free of any solvents. The resin is normally a mixture comprising a monomer having one or more functional groups, and a polymer or oligomer having a degree of unsaturation. An oligomer is selected from those capable of forming a flexible film upon curing. The monomer is referred to as a dilutent for the polymer, with the ratio of the two being adjustable to control viscosity. For the purposes of the present invention, the uncured liquid resin must be sufficiently viscous to be applied and retained as a continuous liquid film on a nonwoven fabric for a period of several seconds up to several minutes. This parameter is usually not significant, since it is normally possible to apply and cure the liquid film within a fraction of a second and up to several seconds.

The liquid resin may be cured directly by electron beam radiation or may be cured by ultraviolet light if a photoinitiator is incorporated into the resin. There are two principal types of photoinitiators, free radical and cationic, which are selected depending on the species of resins used. Free radical initiators are employed with acrylates, while cationic photoinitiators are employed with vinyl ethers and epoxy compounds. Formulations of a large variety of radiation curable resins are available from RadTech, Northbrook, Illinois. Radiation curable resins are described in the following United States patents, incorporated herein by references: 4,125,503; 4,649,082; 4,937,173; 5,098,982; 5,281,662; and 5,352,713.

For the purpose of the present invention, a radiation curable resin is selected, which when polymerized into a film, will be flexible, will be impervious to the transfer or liquids, but will have a high rate of vapor transmission, allowing the film coated fabric to breathe. In general, film flexibility and breathability is determined by film thickness and film chemistry, such as, for example, the percentage of monomers, the crossline density and chemical nature of the polymer backbone. If the fabric is to be used in articles such as clothing or disposable sanitary articles, the cured resin should be non-toxic and non-reactive. For many of these articles, it is important to prevent penetration and transfer of liquids such as blood or liquid waste, while allowing transmission of water vapor for increased comfort. Preferably, the composite film and fabric will have a moisture or vapor transmission rate which exceeds 500 grams per square meter per day, and preferably exceeds 3,000 g/m² per day, with a hydrohead in excess of 65 cm.

An especially suitable resin system is described in U.S. patent no. 5,536,760, incorporated herein by reference. The liquid resin composition comprises (A) an unsaturated polyester component containing an unsaturated polyester polymer, an unsaturated polyester oligomer or a mixture thereof; and (B) a non-polymerized, cocurable vinyl ether component which may be separate from or structurally incorporated in the unsaturated polyester component, provided that the vinyl ether component contains an average of at least two vinyl ether groups per molecule of the vinyl either component. A photoinitiator is added if UV curing is desired.

Various additives may be added to the liquid resin prior to application. Possible additions include antistatic agents, antimicrobial agents, fillers to control weight and viscosity, pigments and the like. The limitations on additives are that they should not interfere with the radiation curing process and should be compatible with the resin system employed. Other typical additives include defoamer, adhesion promoters, flatting agents and stabilizers.

The liquid resin coating is applied to the nonwoven, preferably a nonwoven web carried by a conveyor, by known conventional coating processes. Examples of coating processes include a gravure or flexigraphic printing, roll coating, blade coating, and spray coating. A preferred method involves transfer coating a rubber roll with a thin layer using a gravure with a doctor blade assembly and allowing the nonwoven moving on a conveyor belt to contact the web on one side. Depending on end use requirements, the coatings will be applied zonally or uniformly to the web at a coating thickness of 3 to 500 µm, with 3 to 100 µm being preferred. Coatings may be applied to one or both sides of the fabric.

A minimum amount of liquid resin is applied to the fabric substrate to achieve the desired properties, which also allows the coating to be cured more rapidly. To provide a cured film free of pinholes, it has been found that the rate of application of the liquid as a uniform coating should exceed about three grams per square meter (gsm). The upper limit of coating application is based on cost considerations, and generally an application rate of twenty-five gsm will not be exceeded, providing a coating thickness of 25 microns. In general, if the liquid resin does not contain solvents, the liquid application rate will be equal to the cured film rate.

Although less preferred, a low basis weight of the liquid resin may be applied, such that when the resin is cured, the fabric will exhibit a substantial increase in hydrostatic head without any substantial impact on the softness, drape and flexibility of the fabric. At low application rates, for example three to seven gsm, it is normally possible to increase the hydrostatic head by a factor of two to three times, with the fabric remaining permeable to the transfer of liquid vapors.

The liquid film resident on the surface is then rapidly cured, or is cured while the liquid resin still resides as a continuous and unbroken film on the nonwoven fabric. This is accomplished by exposing the resin coated nonwoven fabric to a source of suitable radiation. For example, if the photoinitiator in the resin is activated by ultraviolet light, the coated substrate may be passed through an enclosure having a series of UV lamps, such as mercury lamps.

Suitable curing procedures are described in the aforesaid U.S. patent no. 5,536,760. If ionizing radiation is employed by use of an electron beam, a one mil thick liquid film can be cured in air through its thickness upon exposure to a 0.5 to 5 megarads of ionizing radiation.

If a photoinitiator is incorporated into the resin, ultraviolet radiation having a wavelength of 180-400 nanometers may be used to effect the cure. Typical medium pressure tubular mercury lamps have an output of 200 watts per inch along the length of the tube. The tubes can be in series and in parallel and are typically spaced a few inches from the uncured film.

Both photosensitizers and photoinitiators are used in UV curing. Examples of photosensitizers include benzophenone, anthraquinone, and thioxamthone. Examples of photoinitiators include isobutyl benzoin ether, alpha, alpha-diethyozyacetophenone, and alpha, alpha-dimethoxy-alpha-phenylacetophenone.

If cured coatings are to be applied on a continuous nonwoven production line, the apparatus for applying and curing the coating will be applied at the exit end of the line, that is, between the exit of the finished nonwoven web and a winding apparatus for winding the web into a roll. During application of the liquid resin, the web may be supported on a flat conveyor or a rotating roll, or may be passed unsupported through the coating apparatus. The web, coated with liquid resin, is then passed through an enclosure containing the source of radiation. Line speeds are not critical, since continuous curing is possible at line speeds up to at least 300 meters per minute. The radiation capacity and amount of exposure is determined by liquid resin chemistry and thickness, fillers, if any, and line speed.

As the coating is being applied to the web and cured, it is desirable to prevent excessive penetration of the coating into the fabric. This can be accomplished, for example, by using a low pore size web or by initiating the radiation cure immediately after application. Also, a viscous coating may be used, which is heated to reduce viscosity during application, or a high shear coating method may be used.

An important aspect of the present invention is the ability to significantly increase the hydrohead of light weight nonwoven fabrics while maintaining high vapor transmission. Treated fabrics having a basis weight of less than 50 gsm can easily obtain a hydrohead of greater than 90 cm while maintaining a moisture vapor transmission rate of greater than 3,000 g/m² per day. For example, the hydrostatic head of a 20 gsm (grams per square meter) SMS nonwoven fabric can be increased from about 30 cm before coating to greater than 100 cm after coating. In spite of the significant increase in repellant characteristics, the moisture transmission rate of the fabric exceeds 5,000 g/m² per day. Since there are substantially no pores in the film layer, vapor transmission occurs through absorption at one surface and transmission to the other surface of the film.

The above figures can be compared to SMS fabrics treated with fluorocarbon repellents. For example, U.S. patent 5,482,765 reports a 54 gsm SMS fabric having a hydrohead of 92-96 cm and a moisture vapor transmission rate of 4720-4830 g/m² per day.

The term "hydrohead" as used herein refers to a standard test to measure the liquid barrier properties of a fabric. The hydrohead test determines the height of water (in centimeters) which the fabric will support before a predetermined amount of liquid will pass through, and is defined in Federal Test Standard No. 191A, Method 5514. The water vapor transmission rate (WVTR) of fabrics is specified in ASTM Standard Test Method E-96-80.

Nonwoven fabrics are produced in bulk in the form of rolls and are later cut and converted into a wide variety of useful articles. Presently, there is a need for breathable nonwoven fabrics which are fluid repellent, and which can be supplied at a low cost. Possible end uses include single or multiple use protective apparel, such as medical gowns and laboratory coats, industrial protective clothing and rain wear. In some cases, the resin coating can be applied only to areas of the fabric requiring high repellency, such as the front panels and sleeves of an operating gown.

The radiation curable resin can also be applied when the fabric is being converted to a final product. For example, during the production of disposable sanitary articles, such as diapers, preselected areas of fabric may be coated, such as the inner surface of the outermost layer of fabric, in order to prevent leaking of liquids, while retaining a soft, cloth-like feel.

For most end use applications, a resin coating is applied to only one side of the fabric, and as thinly as possible, while still providing a good liquid barrier. This will assure that the composite will continue to have the soft feel of a fabric, and the uncoated side will have the appearance and tactile qualities of a porous fabric.

Claims

A method for making a breathable nonwoven liquid barrier fabric, said method comprising the steps of preparing a bonded porous nonwoven fabric, applying a coating of a liquid, radiation curable resin onto at least one surface of the fabric to form a continuous film thereon, and then radiating said film to provide a cured, continuous solid film on said fabric, said cured film being substantially impervious to liquids and pervious to vapors.
The method of claim 1 wherein said nonwoven fabric is produced continuously on a production line, and wherein said radiation curable resin is applied and cured in said production line.
The method of claim 1 wherein said bonded nonwoven fabric comprises continuous filaments.
The method of claim 1 wherein said bonded nonwoven fabric comprises fibers.
The method of claim 1 wherein said coating is applied at a thickness of from about five to five hundred microns.
The method of claim 5 wherein said coating is applied at a thickness of from about three to about one hundred microns.
The method of claim 1 wherein said liquid, radiation curable resin comprises a monomer, a polymer and a photoinitiator.
The method of claim 1 wherein said radiation curable resin comprises a vinyl ether ester.
The method of claim 1 wherein said liquid radiation curable resin is coated only on portions of said nonwoven fabric.
A breathable nonwoven liquid barrier fabric comprising a nonwoven fabric having opposed outer surfaces, and a continuous film of radiation cured resin adhered to one of said surfaces, said film being substantially impervious to liquids and pervious to vapor.
A vapor permeable, liquid impermeable composite, said composite comprising a porous nonwoven fabric having first and second surfaces, and a radiation cured film having a thickness of 3 to 100 microns bonded to said first surface, said second surface being free of radiation cured film.
The composite of claim 11 wherein said radiation cured film comprises polyester polymers and a cocured vinyl ether.
The composite of claim 11 wherein said composite has a vapor transmission rate in excess of 500 grams per square meter per day, and a hydrohead in excess of 65 cm.
The composite of claim 11 wherein said radiation cured film extends only over a portion of said nonwoven fabric.