Poly(P-phenyleneterephthalamide) fibers

Claim 1

Poly(p-phenyleneterephthalamide) fibers consisting essentially of poly(p-phenyleneterephthal- amide) having an inherent viscosity of at least 5 dl/g as determined at 25°C in sulfuric acid having a concentration of 98% by weight at a polymer concentration of 0.5 g/dl, said fibers being composed of single filaments of 0.5 to 3 denier, wherein the single filament tensile strength is at least 35 g/denier, the macrovoid number, defined below, is not more than 10 per 100 mm, and the asymmetry, defined below, is not more than 0.2, the macrovoid number being the number of voids having a size of at least about 1 µm, that can be counted when optional 20 single filaments are selected from the fibers and each single filament is observed along a length of 5 mm from an optional point by an optical microscope at 400 magnifications, with the proviso that in case of a void elongated in the direction of the fiber axis, the number of portions having an increased width is regarded as the number of macrovoids, and the asymmetry being the value obtained by determining a W-shaped interference fringe of the single filament by using a quantitative transmittance type interference microscope using polarized light vibrating in a direction perpendicular to the fiber axis, measuring the absolute value of the difference between the angle ABC and the angle ACB in the interference fringe having apexes A, B and C (A being the central apex) with respect to at least 20 points in optional 5 single filaments selected from the fibers and dividing the number of the measuring points where the absolute value of the angle difference is at least 20° by the number of all the measuring points.