The structural changes which accompany decomposition of Kevlar 49 fibres at T = 500°C and 550°C, respectively, have been elucidated. At both these temperatures, cumulative thermal exposures of specific durations, teem(T), are required to result in decomposition. The conspicuous features which characterize isothermal decomposition of the fibres are: (i) progressive reduction and an eventual total loss in fibre crystallinity, (ii) progressive loss in weight, (iii) introduction of surface damages, (iv) introduction of hollowness, and (v) deterioration in tensile proptrties.
The residual effects of thermal aging of Kevlar 49 fibers in the temperature range 150-450′C have been analyzed. Thermal aging introduces crystallographic as well as macro-structural changes. Weight losses and deterioration in tensile properties were also observed. The order in which the deterioration in crystallinity, weight and tensile strength occur has been identified. Master curves for predicting the time needed for 50% deterioration at various temperatures and the corresponding activation energy have been estimated. The role of the parameter, tcum( T), the cumulative exposure to any temperature T, on thermally induced effects has been unambiguously established. In particular, the influence of the T- tcum( T) effect on crystallographic parameters has been observed for the first time.
Axially compressed Kevlar 49 fibres have been examined by X-ray diffraction methods. The most prominent effect of axial compression is the anisotropic deformation of the unit cell. Whereas the c-axial length, which corresponds to the chain axis, undergoes contraction, the basal plane dimensions manifest enlargement. The deformations increase with the extent of axial compression. The half-widths and the a~muthal spread of reflections also exhibit changes. The compression induced structural changes provide qualitative support to the experimentally observed reduction in tensile strength and modulus.
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