Kalyani Vijayan

Thermally induced structural changes in Nomex fibres

Thermally aged Nomex fibres manifest several residual effects viz. reduction in X-ray crystallinity, weight loss and deterioration in tensile characteristics. Surface damages in the form of longitudinal openings, holes, material deposits etc have also been observed. Based on the data from thermally exposed fibres, the time needed for states of zero tensile strength and modulus have been predicted.

Effect of thermal ageing on Nylon 6,6 fibres

The residual effects of thermal ageing at various temperatures on fibres of the aliphatic polyamide Nylon 6,6 have been studied. Both crystal and macro structural characteristics manifest the residual effects. The former category includes changes in intensity, 2θ values and half width. The macro changes include introduction of surface damages in the form of holes, material deposits etc. The fibre also undergoes reduction in weight. The structural changes suggest deterioration in the initial tensile characteristics which has been verified experimentally. A direct correlation between the tensile strength and the angular separation of the equatorial reflections has also been observed.

Thermal Aging of Twaron Fibers

The residual effects of thermal aging on Twaron fibers have been studied using X-ray diffraction, weight analysis, scanning electron microscopy and tensile testing. X-ray data from isothermally aged fibers indicate the introduction of changes at the level of the crystal lattice, which include reduction in crystallinity, variations in 20 values, half width and azimuthal spread of reflections. Isothermal aging also introduces weight loss. Examination of the surface of fibers has shown that damages in the form of holes, materials deposits, etc., are introduced by isothermal aging. These data on Twaron further show that the thermally induced effects are controlled by two parameters: temperature T and cumulative exposure time to T, i.e. tcum(T). Among the tensile properties, the tensile strength is found to be more sensitive to thermal aging than modulus. In addition to the structural studies on the thermal aging behavior of Twaron, a one-to-one comparison has also been attempted between the tensile behavior of Twaron and Kevlar 49 fibers, which are also made up of the same polymer.

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.

Forbidden reflections from the aramid PPTA - A novel correlation with stacking faults

The occurrence of space group forbidden reflections in the X-ray diffraction patterns from the aramid PPTA has been correlated with the presence of stacking faults. The fraction of sample affected by the presence of such faults has also been estimated.

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 penetrants on the aramid Nomex

The response of Nomex sheets to some penetrants has been analyzed. The process of moisture uptake depends on the relative humidity (RH) of the ambient atmosphere and the initial characteristics of the polymer. In the case of common laboratory solvents,the uptake by the polymer shows an inverse dependence on the molar volume of the former. Although the calendered and the uncalendered Nomex sheets exhibit an overall similarity in their response to various penetrants,quantitative comparison reveals distinct differences.

X-ray analysis of polycrystalline aluminium subjected to fatigue cycling

Variations in the halfwidth values of X-ray reflections from fatigue-cycled, polycrystalline aluminium samples have been analysed. An oscillatory variation of the halfwidths with fatigue cycling has been observed. Analysis of the diffraction line profiles indicates that broadening arises mainly because of the build-up of microstrains during fatigue cycling. The present data indicate that (i) broadening due to fatigue cycling increases with glancing angle; (ii) changes in halfwidth and integral widths, due to fatigue cycling, are comparable and (iii) (b/b0) versusN curves for fatigue cycling under constant stress amplitude and flight loading conditions are comparable.

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.