Rv Iyer

Decomposition behaviour of Kevlar 49 fibres: Part I. AtT≈T d

The structural changes which accompany decomposition of Kevlar 49 fibres atT=500°C and 550°C, respectively, have been elucidated. At both these temperatures, cumulative thermal exposures of specific durations,tcum(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 properties.

Decomposition Behavior of Kevlar 49 Fibers: Part II. At T values < Td

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, t cum(T), the cumulative exposure to any temperature T, on thermally induced effects has been unambiguously established. In particular, the influence of the T- t cum(T) effect on crystallographic parameters has been observed for the first time.

Low temperature crystallographic data on Kevlar 49 fibres

Using X-ray diffraction data, the behaviour of Kevlar 49 fibres at low temperatures, up to −100°C, has been analysed. During cooling, the basal plane of the monoclinic unit cell shrinks whereas the c- (unique, chain axis) length is not significantly affected. In contrast, in the return heating cycle to ambient temperature, the basal plane expands and contraction occurs along the chain direction. The unit cell registers a reduction in volume in both the cooling and heating cycles. Conspicuously, after a cycle of cooling and heating, the unit cell does not return to its initial volume.

Effect of thermal spikes on the structural characteristics of Kevlar fibres

The effect of upto six cumulative exposures to thermal spikes, each of 10 s duration, on Kevlar 49 fibres has been analysed. X-ray data show that exposures to spikes corresponding to Ts ≥ 400°C cause changes at the level of the crystal lattice. At and above 500°C, severe surface damages such as introduction of longitudinal openings, peel-offs and extraneous material are found to occur. The tensile properties of the spike-exposed fibres manifest changes which conform well with the structural changes. As in the case of prolonged thermal exposures, the spike induced effects are also controlled by two parameters, viz., the temperature and the duration of the cumulative exposure. The data from spike exposed fibres indicate that the thermally induced changes in the structural and tensile characteristics get initiated at the very early stages of thermal exposure viz., of the order of 10 s.

Crystallographic data on axially compressed Kevlar 49 fibres

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 thec-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 azimuthal spread of reflections also exhibit changes. The compression induced structural changes provide qualitative support to the experimentally observed reduction in tensile strength and modulus.