Abstract: The present invention relates to melt-spun polyurethane elastic fiber having a degree of luster of 70 or less, the degree of luster being defined as (I/Io).times.100 where the amount of light reflecting off the surface of the fiber is I and the amount of light reflecting off a standard white plate is Io. On the surface of preferable polyurethane elastic fiber, 10 or more mountain-like protrusions of 0.2 to 5.05 to 110 parts by weight of 5 to 110 parts by weight of .mu.m in height are present every 10 .mu.m fiber in the axial direction.Also, the present invention relates to a process for producing polyurethane elastic fiber, comprising melt-spinning butylene terephthalate-based crystalline polyester (A) and thermoplastic polyurethane (B) wherein before spinning, the compound (A) is melt-mixed with thermoplastic polyurethane (B-1) having isocyanate groups in an amount of 150 to 500 .mu.mol/g.Further, the present invention relates to covered fiber comprising the polyurethane elastic fiber as a core.

Abstract: A composite elastic filament with a rough surface, consisting of a sheath component composed of a fiber-forming thermoplastic polymer, such as polyamide, polyester or polyolefin, and a core component composed of a fiber-forming elastomer, such as polyurethane or polyester elastomer, wherein the core/sheath conjugate ratio ranges from 1/1 to 100/1 by cross-sectional area and the core portion has a smooth peripheral surface uniformly extending in the direction of the filament axis while the sheath portion covering the core portion has numerous ridges rising along the circumference of the filament and closely spaced along the length of the filament. This filament can be readily produced by the melt conjugate spinning of the core and sheath components at the above-specified conjugate ratio, followed by drawing 1.1- to 10.0-fold and relaxation.

Abstract: A potentially elastic composite filament, preferably a core-sheath composite filament composed of a polyurethane as the core and a polyester as the sheath, wherein the ratio of a polyurethane component having allophanate cross-linkages to a linear polyester component readily soluble in water or an aqueous alkali solution ranges from 1/1 to 90/1 and the polyester component is exposed on the surface of the filament in its cross-section. Although the core component has a high tensile strength and a large elongation at break, these properties are controlled by the sheath component. Therefore, this filament can be formed into a textile structure with good workability similar to that of ordinary synthetic fibers, and the obtained structure develops characteristics as an elastic polyurethane filament when treated with water or alkali.

Abstract: A composite elastic filament yarn having excellent heat resistance is produced by conducting conjugate-spinning, in a core-sheath arrangement, preferably a concentric core-sheath arrangement, of a polyurethane, as a core component, which is crosslinked by a polyisocyanate at a crosslink density (Y) of 15 .eta.mol/g or more to thereby have improved heat resistance, and a non-polyurethane thermoplastic elastomer, as a sheath component, particularly a polyester elastomer, a polyamide elastomer or polystyrene elastomer, under such conditions that the core to sheath conjugate ratio (X) is in the range of from 3/1 to 100/1, and that the relationship of Y.gtoreq.-X+35 is satisfied. This elastic yarn is free from tackiness and can be wound at a high speed. Further, the yarn is easy to unwind and its workability is excellent. This elastic yarn is suited for use in various fields, such as socks, tricots, panty hose, swimsuits and foundations.