EFFECT OF ENVIRONMENTAL EXPOSURES ON THE ARAMID FIBRE KEVLAR

摘要

The effects of exposure of Kevlar 49 fibres to thermal, ultrasonic and chemical environments have been analysed. Both the tensile strength and the tensile modulus deteriorate with thermal exposures. However, the former is more sensitive than the latter. The prominent crystal structural changes induced by thermal exposures are reduction in crystallinity, molecular misalignment about the fibre axis, increase in the inter layer distance, changes in crystallite size and/or microstrain. Thermally induced macro changes include introduction of surface holes, partial hollowness near the core, localised thinning, material loss in the vicinity of surface impurities etc. The structural changes could be satisfactorily correlated with the corresponding changes in tensile properties. All the thermally induced effects are controlled by two parameters, viz., the temperature (T) and the duration of the cumulative exposure to T, t_(cum) (T). The T-t_(cum)(T) effect leads to the observation of isothermal decomposition of Kevlar over a range of temperatures which include T's, far below the reported decomposition temperature of 500/550 deg C. A hitherto unknown direct correlation between X-ray diffraction peak positions and the tensile strength has also been identified. Exposure to ultrasonic waves introduces axial compression, evinced by the formation of kink bands, fibrillation and macro buckling. The unit cell of axially compressed fibres manifests an anisotropic deformation. The process of moisture uptake by Kevlar 49 fibres is controlled by the relative humidity (RH) of ambient atmosphere. The saturation moisture content is, however, determined by structural characteristics such as the degree of crystallinity, molecular alignment etc. The uptake of common organic solvents by Kevlar is dependent on the molar volume of the former.