Polyurethane elastic yarn and preparation thereof and covering yarn of using said elastic yarn as core

This invention relates to polyurethane elastic yarn obtained by melt-spinning characterized in that the brightness is less than 70 when the brightness is defined as (I/Io) 100, wherein I represents the light reflectance of said yarn's surface, and I o represents the light reflectance of a standard white board. Preferably, there are more than 10 of angular protrudes with height of 0.2-5.0 um on the fiber's surface, per 10 um of the length of the fiber in axial direction. Process for preparing said elastic yarn by melt-spinning (A) crystalline polyester comprising butylene terephthalate as main component and (B) thermoplastic polyurethane, characterized by melt-mixing the above-mentioned (A) with the thermoplastic polyurethane having 150-500 umol/g of isocyanate group before spinning. Further, the present invention provides a covering yarn comprising said polyurethane elastic yarn as a core. When stocking, tight, socks etc. are produced by using the covering yarn of the present invention, there is no defects in brightness phenomenon caused by melt spinning urethane, and the product having an excellent appearance can be obtained.