FIELD OF THE INVENTION
- BACKGROUND OF THE INVENTION
This invention relates to a bicomponent sheath core fiber and textiles made from this fiber.
Natural leather has properties such as good water vapor permeability, high tear strength, flexibility and durability. Leather has these properties due to its cellular matrix and the entangling of collagen fibers within this matrix.
High quality synthetic leathers can be difficult to produce, though many efforts in this regard have been made. Prior art synthetic leathers are produced from dispersions of polyurethanes in organic solvents. For example, in one such process, textile material is impregnated with an organic solution of a polyurethane, optionally containing a polyurethane dispersion, and then treated with mixtures of dimethylformamide. Polyester webs have also been impregnated with polyurethane solutions in DMF/toluene and then treated with aqueous sodium hydroxide solution. Though this synthetic leather is reported to have good characteristics, the process uses undesirable organic solvents. In addition, solvent disposal becomes problematic.
U.S. Pat. No. 6,231,926 (Ronzani et al.) reports needled non-woven textiles having good synthetic leather-like qualities. The textiles comprise fiber forming polymers such as polyamides, polyurethanes, polypropylene, polyethylene, polyacrylonitrile and polyesters. These may be used in combination with natural fibers such as, for example, wool, cotton, viscose or silk. The polyesters are preferred and include polyethylene terephthalate, polytetramethylene terephthalate or poly-(1,4-cyclohexane-dimethylene terephthalate).
The technology to produce polymer fibers (or filaments) by melt spinning is well known. Typically, a thermoplastic composition is extruded through a spinneret in which the polymer molecules are unoriented. A second step, either in line or as a subsequent operation, is conducted to stretch or draw and orient the filament. Fibers may also be made by extruding a thermoplastic composition through a spinneret and drawing the molten extrudate into filaments by means of a stream of high velocity air. The filaments can be produced to a desired diameter and length.
Bi-component and multi-component fibers as well as fabrics also have been produced. Bi-component fabrics include first and second distinct fibers. Bi-component fibers comprise two components of different compositions. Bi-component fibers may be formed by extruding two components side by side or in a sheath core arrangement. Sheath core fibers may be used to produce non-woven webs, and the sheath may comprise an adhesive, or meltable, component that is used to bond fibers in a web together upon application of heat.
Various methods are used to make fibers into non-woven webs. Extruded filaments can be laid into a random web by means of air laying or by carding. These dry methods contrast to “wet laying” methods in which fibers, typically staple fibers, are placed into a liquid medium and then cast onto a scrim, similar to that of paper manufacture.
Fibers may be continuous or they may be cut or chopped into staple fibers. Fibers may also be entangled to a desired degree, thus producing desired textile properties. In needle-punching, hooked needles punch into and out of a non-woven fiber mat to entangle the fibers. In hydroentangling, jets of water, which act much the same as needles, produce an entangled non-woven fabric. Fibers can be bonded together into a web or mat by the addition of adhesive particles or binder fibers, both of which melt upon application of heat. Alternatively, the fibers can be bonded together by reaction between fibers or by melting of the surface (e.g., the sheath) of a conventional sheath core fiber which enables it to bond with other fibers which may be bicomponent or may be made from a single component.
One problem with the use of adhesive particles and thermo-bonding methods is that a substantial amount of adhesive must be used to provide integrity to the fabric so that it can be handled. Side-by-side or sheath core bicomponent fibers typically have a substantial adhesive component. In any case, the use of excessive adhesive material results in a stiff and non-drapeable fabric.
- SUMMARY OF THE INVENTION
Needs in the art remain for a synthetic material having good textile properties and tear resistance, such as the properties of a leather-like material, which would be easy to produce and avoid the use of undesirable chemical solvents during processing.
A polymeric bicomponent sheath core fiber is used to make non-woven material having textile qualities.
In one aspect, this invention is a polymeric sheath core fiber, comprising a sheath material disposed about the periphery of a core material, the sheath material having a first melting point, and the core material a material having a second melting point or a glass transition temperature at least about 10° C. less than the first melting point. The sheath may comprise an oriented semicrystalline polymer. The melting point or glass transition temperature of the core material is preferably is greater than about 35° C. The diameter of the fiber preferably is at least 20 micrometers. The sheath material or the core material may be selected from the group of homopolymers and copolymers of polyethylene, polypropylene, polymethylpentene, polytetrafluorethylene, polyamide, polyester, polyethylene oxide, polyoxymethylene, and cellulose acetate. In one preferred embodiment, the sheath comprises oriented polypropylene and the core is selected from polyethylene and a copolymer of ethylene and vinyl acetate. In another preferred embodiment, the sheath comprises oriented polyethyleneterephthalate and the core comprises a copolymer of an acid or a glycol with ethylene terephthalate.
In another aspect, this invention is a method of forming a polymeric non-woven textile, comprising providing a sheath core fiber having a sheath disposed about the periphery of a core, the fiber having a cross sectional shape, the sheath comprising an oriented semicrystalline polymer having a first melting point and the core comprising a material having a second melting point or a glass transition temperature at least 10° C. lower than the first melting point; cutting the sheath core fiber to lengths of at least about 5 millimeters; entangling the fibers to form a fiber mat; and heating the fiber mat to melt the material in the core. In a preferred embodiment, the cutting step produces a fiber end that has a cross sectional shape that is the same as the cross sectional shape of the fiber. The entangling step may be done by hydroentangling or by needle punching.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In a third aspect, this invention is a polymeric non-woven mat, comprising a sheath core fiber having a sheath material disposed about the periphery of a core material, the sheath material having a first melting point, and the core material comprising a material having a second melting point or a glass transition temperature at least about 10° C. less than the first melting point. In a preferred embodiment, the diameter of the fiber is about 20 micrometers, and the length of the fiber is at least about 5 millimeters. The non-woven mat may also comprise one or more filler materials selected from the group of polyester, nylon, acrylic, wool, cotton, viscose, and silk.
In this invention, a bicomponent sheath core fiber is used to produce a non-woven material, such as a mat, web, or fabric. The sheath comprises an oriented polymer. The core comprises a material that has a melting point or glass transition temperature lower than the melting point of the material comprising the sheath. Chopped, or cut, fibers are used to make non-woven mats having textile properties. The mats may be entangled by methods known in the art. Then the mat is treated so that the core material melts or flows, and bonds to adjacent fibers in the non-woven mat. Point bonds thus are formed at the ends of the cut fibers where the core material is exposed. The relatively small amount of adhesive in the mat produces a textile material of desirable strength, hand, and drape. Depending upon the thickness of the mat and the materials comprising it, the non-woven material may have the qualities of a synthetic leather.
As used herein, “filaments” or “fibers” refer to continuous strands of material. Desirable fibers have a diameter of at least about 5 micrometers. “Continuous” is a term that refers to a fiber at least one meter long. “Cut” or “chopped” fibers refer to filaments or fibers ranging in length from about one millimeter (mm) to less than about one meter. Preferably, the fibers of this invention have a length of at least about 5 millimeters (mm).
Fibers should be distinguished from particles, which may have various shapes, and may have a width (or diameter) and a length approximately the same. The fibers of this invention are core-sheath composite fibers comprising a core component and a sheath component. The sheath comprises an oriented semicrystalline polymer and it covers the periphery of the core. Upon solidification of some polymers, a portion of the polymer forms randomly oriented crystals. Such polymers are referred to as semi-crystalline. Aligning these crystalline regions by orienting the polymer is known to result in improved mechanical strength. A common technique for orienting some polymers, such as polyethylene, is by plastic flow at temperatures below the melting point. That is, the polymer is drawn, or pulled, to produce orientation of the crystals in the polymer. Suitable semicrystalline polymers for the sheath component of the fiber of this invention include polyethylene, polypropylene, and polymethylpentene. Also suitable are fluorinated and oxygen-substituted polymers such as polytetrafluorethylene (i.e., commercially available under the trade designation TEFLON), polyamides (i.e., commercially available under the trade designation NYLON), polyester such as polyethylene terephthalates (commercially available under the trade designation DACRON), polyethylene oxide, polyoxymethylene, and cellulose acetate.
It is also contemplated that a hollow core fiber is useful for this invention. That is, a fiber comprising oriented polymer has a lumen, or hollow core. This fiber is used by “potting” the ends of the fibers, such as is done for fibers used in a blood oxygenator, and then filling the lumen, or just the lumen near the fiber ends, with a desired composition. This composition is an adhesive or one component of a two component adhesive system (e.g., a two part epoxy). The fiber is then cut away from the potting and used to form a mat.
The core comprises any suitable amorphous, semicrystalline, or crystalline polymer. Suitability is determined by the flow characteristics of the polymer at the desired temperature range. That is, useful core polymers flow sufficiently at a desired temperature to exude from the fiber ends and to bond to adjacent fibers under processing conditions described further below. One way to characterize amorphous polymers is their glass transition temperature, Tg, which is the temperature at which an amorphous polymer changes from a ‘vitreous’ state to a ‘plastic’ state. It should be noted that this is not the same as melting point, which is the temperature at which a crystalline material is at equilibrium with its liquid state. However, for this invention, the effect of reaching or exceeding either the Tg or the melting point may be the same.
Preferably, the core comprises a non-oriented polymer. The polymer in the core has a melting point or a glass transition temperature at least about 10 degrees C. less than the melting point of the sheath material. Preferably, the core material does not soften or melt at room temperature or at common processing and handling temperatures, e.g., below about 35° C.
Although bicomponent fibers are herein described and typically preferred, it is to be understood that multicomponent fibers are suitable for practice in this invention. That is, the fiber may comprise two or more components in the sheath, or there may be two concentric sheaths of different compositions, for example, around one core of another composition. The core may comprise more than one composition.
Any non-oriented polymeric material having a melting point or glass transition temperature greater than about 35° C. and at least about 10° C. less than the melting point of the sheath component is suitable for the practice of this invention. Preferred core materials include thermoplastic polymers and copolymers of polyethylene, polypropylene, polyester and polyamide. The core polymer may be amorphous polyester, such as that commercially available from Eastman Chemical Co. as Eastman 20110. This polyester is the condensation product of terephthallic acid with a mixture of ethylene glycol and cyclohexane dimethanol.
For example, suitable core sheath fibers comprise oriented polypropylene as the sheath and polyethylene or a copolymer of ethylene and vinyl acetate for the core. An alternative core sheath fiber has a sheath of oriented polyethyleneterephthalate and a core of a copolymer of ethylene terephthalate with another glycol or another diacid or both. The copolymer may be oriented or may be amorphous.
Once the bicomponent fiber has been produced and the desired qualities of the fiber selected, the non-woven mats of this invention are prepared by chopping the desired fibers, laying them into a mat, entangling the fibers of the mat, and then exposing the mat to elevated temperatures and optionally, elevated pressures, to cause the core material to bond to adjacent fibers in the mat.
Fibers are cut or chopped to a desired length, typically at least 5 mm long, by conventional methods. The fibers are laid into a mat, typically by carding and crosslapping, and then entangled by any method known in the art, including, but not limited to, needle-punching and hydroentangling. The entangled mat is then exposed to elevated temperatures sufficient to cause the polymer in the core to soften and melt and flow out of the end of the fiber, and bond to an adjacent fiber.
The non-woven material further may include additional filler materials (e.g., fibers or particles), such as those formed from polyester, nylon, and acrylic, and natural fibers such as wool, cotton, viscose or silk.
In the process of this invention, bicomponent fibers are prepared by extruding two different polymer compositions through a spinneret, as is known in the art, into continuous fibers having the desired diameter. The sheath component is oriented by stretching the fiber either as it is extruded or subsequently in a separate operation down line from the extrusion. The fibers are then cut or chopped to lengths of at least about 5 mm. Preferably, the shape in cross-section at the cut end of the fiber is substantially the same as the shape in cross-section of the remainder of the fiber, which preferably may be oval or round. Alternatively, other shapes can be made, for example, tri-lobed fibers.
The fibers are formed into a mat be means of conventional carding and crosslapping equipment, in which the chopped fibers are first mixed with any desired additives (such as particles, filler fibers, adhesives, anti-microbial compounds, etc.) and placed into a mixer that combines the fibers with any additives before forming the web. Alternatively, the fibers may be mixed with desired additives and air laid onto a scrim or platen. Vacuum may be used to urge the fibers against the platen.
At this point the fibers are entangled by needle punching or hydroentangling. Then the fiber mat is heated to at least about the melting point of the composition comprising the core to cause bonding of the core material exposed at the fiber ends to adjacent intersecting fibers in the mat. Pressure can also be used to assist in the fiber bonding step.
The bonding that occurs at the fiber ends with adjacent fibers is sufficient to get excellent drape and hand properties of the mat, suitable for use as disposable medical products such as surgical gowns and operating room drapes, for soft furnishing fabrics such as window drapes and upholstery covers and for implanted devices such as vascular grafts.
Although particular embodiments of the invention have been disclosed herein in detail, this has been done for the purposes of illustration only, and is not intended to be limiting with respect to the scope of the appended claims. It is contemplated that various substitutions, alterations, and modifications may be made to the embodiments of the invention described herein without departing from the spirit and scope of the invention as defined by the claims.