Adhesive resins are traditionally used to supplement the properties of nonwoven fabrics. The adhesive resins can impart durability, dimensional stability, abrasion resistance, facial density and other similar properties to nonwoven fabrics. In one particular application, adhesive resins can be used in substrates for artificial or synthetic leathers. More specifically, the adhesive resin, usually in a soft, elastic and porous form, can serve as a filler, uniformizer and stabilizer. The adhesive resin is applied to one or both surfaces of the substrate or, alternatively, is used to impregnate the entire nonwoven fabric. After applying the adhesive resin, the nonwoven fabric is then usually finished by sanding, or brushing and shearing, to create a smooth surface. Uniform internal density is required for the leather-like product so that it does not develop an undesirable roughened surface, usually known as “orange peel,” when it is subsequently formed over curved surfaces such as, for example, on footwear, luggage and similar applications. Extra layers may also be laminated on the nonwoven fabric to improve appearance or physical properties.
The Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, John Wiley & Sons, Inc., Volume 15, “Leather-like Materials” p. 177-192, (1995) summarizes the development of artificial leather materials. The encyclopedia notes that in the 1930s, a plasticized poly(vinylchloride) film coated onto a woven or knitted fabric was developed as a leather-like material, but the material was stiff, exhibited plasticizer migration and had extremely low permeability, poor flex endurance and a cold touch. In the 1960s, synthetic leather materials having improved appearance, feel and resistance to grain breakage were made by laminating a cast polyurethane film to a brushed fabric. Further improvements in flexibility were achieved with the introduction of poromeric polyurethane-coated fabrics which were produced by applying a solution of a polyurethane resin in an organic solvent to a brushed fabric and then immersing the fabric in a non-solvent bath (e.g., water) to coagulate the polyurethane and form the poromeric structure. Typically, a polyurethane film was applied atop the poromeric structure. Polyurethane-impregnated and coated nonwoven fabrics made with ultra-fine fibers or microfibers (i.e., of less than 0.3 denier) also were developed to produce suede-like and other leather goods. Broadly, poromerics or poromeric imitation leathers are synthetic breathable leather substitutes. The term poromeric was coined by DuPont as derivatives of the terms microporous and polymeric.
Combinations of various woven or knitted fabrics with nonwoven materials also have been disclosed for use in making artificial leather. For example, U.S. Pat. No. 5,256,429, entitled “Composite Sheet for Artificial Leather,” to Honda et al., discloses making a composite sheet, with a high surface density without stiffness, by needle punching a surface layer of special staple fibers of very low denier into a tightly woven or tightly knit fabric of high twist multifilament yarn and then coating or impregnating the resulting structure with a binder. The special fibers are formed from “islands-in a-sea” composite fibers by dissolving the polymeric “sea” surrounding “islands” of a second polymer to leave fibers of very low titre (i.e., 0.001 to 0.1 denier). However, making artificial leather products from “islands-in-a-sea” fibers is costly.
U.S. Pat. No. 4,073,988, entitled “Suede-like Artificial Leathers and a Method for Manufacturing Same,” to Neshida et al., discloses an artificial suede leather made from pile-like fibers by subjecting the fibers to a heat treatment and a swelling treatment before impregnating or coating the fibers with a synthetic polymer solution or emulsion. The solution or emulsion is then coagulated, and dried. The sheet is then buffed to raise naps. The filaments making up the pile-like fibers have complex cross-sections, created by bonding different polymers having mutually low adhesive affinity with each other. An additional buffing step is required to produce raised naps.
U.S. Pat. No. 4,329,390, entitled “Method for Making Leather like Materials (B),” to Civardi et al. discloses a synthetic leather-like sheet made from laminating a thin micro-porous elastomeric layer, a fibrous backing and a very thin preformed skin. The thin preformed skin is embossed to create a grain appearance.
U.S. Pat. No. 5,922,445, entitled “Composite Material and Process for Production of Same,” to Yoshida et al. discloses another leather-like composite sheet. The composite sheet is obtained by coating or impregnating a fibrous base material with an elastic polymeric substance, in which the fibers constituting the fibrous base material and the elastic polymeric substance are prevented from bonding with each other. This sheet may be produced by applying a hydrophobicizing treatment to the fibrous base material, then impregnating or coating a solution of the elastic polymeric substance to which a hydrophilic silicone has been added, and wet solidifying.
Stitch-bonded composites suitable for use as artificial leather substrates are disclosed in U.S. Pat. No. 5,707,710 entitled “Composite Sheet for Artificial Leather” to Zafiroglu, U.S. Pat. No. 6,063,473, entitled “Abrasion-Resistant Composite Sheet” also to Zafiroglu, U.S. Pat. No. 6,407,018 B1 (col. 5, ln. 47 to col. 6, ln. 13), entitled “Stitchbonded Fabric and Process for Making Same” also to Zafiroglu, and U.S. Pat. No. 7,186,451, which is incorporated herein by reference in its entirety, entitled “Composite Sheet Suitable for Use as Artificial Leather” to Zafiroglu et al.
U.S. Pat. No. 3,245,854, entitled “Process of Manufacturing Nonwoven Fabrics,” to Etchison et al., discloses a low-cost process comprising the needlepunching of fibers into a thermoplastic film, followed by the application of heat through an oven, or through an oven and a calender, to melt the film. The surfaces of the nonwoven fabric do not become flat until the product is heated and calendered. Further, the molten film fuses and permeates throughout the needlepunched web. Thus, while this technique produces cohesive and durable or moldable products, which may be suitable as coating substrates, it is insufficient in other aspects. Particularly, the technique fails to produce the internal uniformity and the smooth surfaces required for many artificial leather applications, unless it is followed by the addition of resins, as well as mechanical surface finishing. Moreover, calendering densifies and exaggerates minor internal nonuniformities such as those resulting from web forming patterns or needling patterns, thereby exacerbating the “orange peel” effect.
Co-pending, commonly owned U.S. application Ser. No. 10/611,769 and U.S. application Ser. No. 11/284,377, by the same inventor, disclose needlepunched products which were also found unsuitable for artificial leather. The '769 application discloses in one embodiment the needlepunching of fibers into a thermoplastic adhesive layer followed by the application of highly concentrated heat and pressure exclusively from the needle-exit side; melting the thermoplastic adhesive layer; and bonding and anchoring the fibers in the adhesive layer which enters the fibrous layer in the back. The composite is attached with heat and pressure to a backing. The '377 application discloses similarly needlepunched structures, optionally shrunk after needling, and finished with heat and pressure applied from the needle-exit side, resulting in a thin, densified and resin-rich abrasion-resistant backface.
While the aforementioned leather substrate materials function well to various degrees, there remains a need in the art for lower-cost composite sheets with a smooth and uniform surface as well as a uniform internal structure, having a combination of uniform conformability, thermal stability, moldability, crease-resistance, tear-resistance, odor absorption and other desirable properties.