Photochemical Ageing of Kevlar® 49

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Abstract
Photochemical ageing of yarns from two similar Kevlar 49 fabrics, one woven in the U.K. and the other woven in the U.S., and unwoven Kevlar 49 yarns and fila ments, was studied by changes in mechanical and molecular properties. The warp and weft yarns from each fabric showed different susceptibilities to photo-induced deterioration, as indicated by their respective tensile properties. The warp yarn of the U.K. fabric photodegraded faster than the weft yam. The reverse occurred in the U.S. material. Unwoven yarns were less susceptible to photodegradation than the fabric yams. The degradation rates remained unaltered on solvent extraction of yarn surface finish and after abrasion damage (which may occur during the weaving pro cess). Although radiation self-screening occurs within the yams, it is not sufficient to account for the differences in their photodegradation rates. The reduction in me chanical properties was not accompanied by any appreciable molecular weight changes and is therefore unlikely to be due to molecular chain scission reactions; however, the lower the initial strength of the yarn, the more rapid is the reduction in tensile strength. Photochemical ageing behavior may thus be determined by small variations in the degree of crystallinity in the fibers.

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