KEVLAR FIBRE S IN THERMAL ENVIRONMENTS. Kalyani Vijayan, R. V. Iyer, Materials Science Division, National Aerospace Laboratories, Bangalore 560 017, India

Kevlar fibres are found to undergo isothermal decomposition. Features which are typical of decomposition at 500deg.C are found to be introduced at much lower temperatures. The parameter which critically controls such behavior is the duration of the cumulative exposure to any temperature. On the basis of X-ray diffraction data, optical and scanning electron micrographs, weight loss and tensile data, detailed characterization of isothermal decomposition occurring in the temperature range of 250 to 500deg.C has been carried out. It is found that decomposition is preceded by damage to the surface of individual filaments. Presence of surface impurities enhance the damage. There is also evidence for evolution of volatile components via the surface. Progressive changes in crystal structural and tensile characteristics and the weight of the fibre have also been observed. Based on data collected at various stages of isothermal ageing at different temperatures, a structural model has been proposed to explain the decomposition behavior of Kevlar fibres.

The effect of thermal spikes, each of 10s duration, on Kevlar fibres has also been examined. Fibres were exposed to cumulative spikes, upto 60s duration, in the temperature range 100 to 700deg.C. Above a threshold temperature, the spikes are found to leave a detectable impression on the tensile as well as the structural characteristics of the fibre.