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  Elastomeric spun bonded fabric of polypropylene and process for makingUSPTO Application #: 20060073755Title: Elastomeric spun bonded fabric of polypropylene and process for making Abstract: The present invention provides for a non-woven fabric composite consisting essentially of a mutually interbonded mixture of crimped isotactic polypropylene fiber segments and uncrimped hard elastic isotactic polypropylene fiber segments. A process to manufacture the non-woven fabric composite and textile goods fabricated from the disclosed non-woven fabric composite are also provided. (end of abstract) Agent: E I Du Pont De Nemours And Company Legal Patent Records Center - Wilmington, DE, US Inventors: Matthew W. George, William W. Hyde, James J. Lang, Debora Flanagan Massouda, Carl Arnold USPTO Applicaton #: 20060073755 - Class: 442352000 (USPTO) Related Patent Categories: Fabric (woven, Knitted, Or Nonwoven Textile Or Cloth, Etc.), Nonwoven Fabric (i.e., Nonwoven Strand Or Fiber Material), Including Strand Or Fiber Material Which Is Of Specific Structural Definition, Strand Or Fiber Material Is Specified As Non-linear (e.g., Crimped, Coiled, Etc.) The Patent Description & Claims data below is from USPTO Patent Application 20060073755. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] The present invention is directed to the field of so-called stretch non-woven fabrics, and specifically to spun bonded fabrics of polypropylene characterized by a high degree of crimping and high elastic recovery. The invention is also directed to a process for preparing the fabric. BACKGROUND OF THE INVENTION [0002] It is known in the art to subject oriented isotactic polypropylene (PP) fibers to elevated temperatures in order to achieve what is variously known as a "hard elastic," "high stretch-high recovery" or "springy" fiber. As discussed in Polypropylene Fibers: Science and Technology, M. Ahmed, pp. 412 ff, Elsevier Scientific Publishing Company (1982), these fibers have distinctive properties. In one aspect they are characterized by stacked crystalline lamellae wherein the lamellae surfaces are normal to the fiber axis --said to be "row-nucleated morphology." The "hard elastic fibers" therein described are characterized by recovery of ca. 80% from a 25% extension. They are also characterized by initial tensile modulus on the order of 2,800 MPa. This initial tensile modulus is orders of magnitude higher than that of known elastic fibers in wide commercial use in textiles such as Lycra .RTM.. [0003] The preparation of "hard elastic" polypropylene is described in detail in Herrman (U.S. Pat. No. 3,256,258). Disclosed therein are fibers having gamma orientation of 10-30.degree. (as determined by X-ray diffraction) which fibers are produced in a melt spinning operation, subjected to spin-stretching, preferably without a post-draw step to produce a fiber having orientation of 10-50.degree.. This step is followed in turn by heating to a temperature of 105 to 160.degree. C., preferably 130-140.degree. C., for a duration of 0.6 seconds to 24 hours, resulting in exposing individual fibers or fiber bundles to elevated temperatures. [0004] The "hard elastic" fibers of Hermann are highly elastic, with low set and high and rapid elastic recovery. However, they are characterized by quite high elastic or Young's modulus. This means that considerable force needs to be applied to effect any degree of stretching. The hard elastic fibers of Hermann have had limited use in textiles because the fabric basis weight required for good coverage will not appear to the user to be elastic because of its high modulus, while low basis weight fabrics (which would appear to the user to be more elastic) provide insufficient cover and insufficient strength. [0005] Preparation of spun bonded PP fabrics is a known art, and is described in detail in Ahmed, op.cit., pp. 443ff. [0006] Hassenboehler et al., U.S. Pat. No. 5,244,482, discloses inelastic nonwovens of PP fibers heated to below their melting point and stretched in the machine direction to create nonwovens with a high degree of cross-direction (CD) recoverable stretch. Heating and stretching are accomplished by passing the nonwoven into an oven and withdrawing it at a higher velocity. Velocity ratios range from 1.1:1 to 2:1. Resulting webs recover at least 70% from a 50% elongation in the cross direction. [0007] Mormon, U.S. Pat. No. 4,965,122, discloses a "reversibly necked" elastic material attained by, among other materials, a multilayer structure comprising a spun bonded PP fabric, the disclosed structure being subject to stretching, heating, and cooling in order to make the desired stretch product of the invention. [0008] Both Hermann, op.cit., and Bersted et al. (U.S. Pat. No. 5,945,215) mention making nonwovens from hard elastic PP fibers but provide no teaching therefore. There is no teaching in the art for subjecting spun bonded isotactic polypropylene fabrics to elevated temperatures without constraint nor is there any suggestion of a process to produce a composite fabric having desirable stretch characteristics. SUMMARY OF THE INVENTION [0009] The present invention provides for a non-woven fabric composite consisting essentially of a mutually interbonded mixture of crimped isotactic polypropylene fiber segments and uncrimped hard elastic isotactic polypropylene fiber segments. [0010] The present invention further provides for a process comprising exposing a spun bonded isotactic polypropylene fabric to a temperature of 150.degree. C. to 170.degree. C. over a period of 10 seconds to 5 minutes followed by cooling, said fabric being disposed in a manner to permit free-shrinkage in at least one dimension to occur during exposure, so that its thickness at least doubles. [0011] The present invention further provides for textile goods fabricated from a fabric composite consisting essentially of a mutually interbonded mixture of crimped isotactic polypropylene fiber segments and uncrimped hard elastic isotactic polypropylene fiber segments. BRIEF DESCRIPTION OF THE FIGURES [0012] FIG. 1A is a photomicrograph depicting the as received spun bonded polypropylene fabric employed in the Examples. [0013] FIG. 1B is a photomicrograph depicting the non-woven fabric composite of the invention showing crimped and uncrimped fiber segments extending between point-bonds, said point-bonds occupying less than 50% of the fabric area. DETAILED DESCRIPTION [0014] While it is known that heat-treatment of polypropylene fibers can make them "hard elastic," the instant invention provides a way to take advantage of this effect to make a nonwoven with a "soft-stretch feel". By heating in a way that allows unconstrained shrinkage, some of the fibers shrink first and pull in the other fibers. The fibers that are pulled in buckle out of plane. The fibers that shrink first carry the load and since they are elastic and there are relatively few of them, the initial tensile properties of the fabric show a low modulus and elasticity. The fibers that buckle provide coverage and ultimate strength under high extension. [0015] The present invention provides for a non-woven fabric composite consisting essentially of a mutually interbonded mixture of crimped polypropylene fiber segments and uncrimped hard elastic polypropylene fiber segments. The ends of the fiber segments are point-bonded to other fiber segments, the length of the fiber segments being defined by the distance between the point-bonded ends thereof, and the fiber segments being of a length determined by the areal density and size of the point-bonds. The area of the point-bonds is in the range of 10-60% of the total fabric area. [0016] The non-woven composite fabric of the present invention is characterized by fibers that are arranged in an array of randomly oriented fibers in a sheet structure. These fibers form an integrated sheet by virtue of being bonded to one another at some, but not all, crossover points. This is referred to in the art of non-wovens as point bonding. The distance between adjacent point-bonds along the length of any given fiber thereof defines a fiber segment that is the fundamental unit of the structure hereof. [0017] The present invention is directed to the use of isotactic polypropylene fibers. The practitioner hereof shall understand that the recitation herein of "polypropylene" shall mean in every instance isotactic polypropylene unless specifically stated to be otherwise. Contemplated to fall within the scope of the present invention are the numerous commercial isotactic polypropylene grades sold as homopolymers when they actually contain a small fraction of ethylene comonomer. [0018] The term "mutually interbonded" as employed herein shall be understood to mean that at least a portion of the crimped fiber segments are point-bonded to at least a portion of the uncrimped fiber segments so as to form a unitary structure made up of fibers having two different morphologies, namely crimped and uncrimped. [0019] The average length of a fiber segment in the non-woven composite fabric hereof will be determined by the average area of the individual point-bonds, and by the density thereof. The point-bonds are characterized by the average area of individual point-bonds, and by the percentage of the total area of the fabric occupied by point-bonds. Thus, it is possible to have a given percentage of the total area of the fabric made up of relatively many point-bonds of small area or relatively few point-bonds of larger area. Continue reading... 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