Kevlar 49 fibres are aromatic polyamide fibres spun from the polymer poly(p-phenylene terephalamide). They have linear chain molecules. They are classified as high strength / high modulus fibres, and are useful for a wide range of applications such as cables, tire cord, sporting goods, such as (skiis, golf clubs, surfacing board etc.), circuit boards, fan blades , low weight panels for the air craft interiors, clothes of the firemen, in rigid composites where glass fibres have limited utility and for forming new types of fibre reinforced composites.                             Composites are often subjected to thermal-atmoshere applications. The present study has dealt with the effect of annealing in the temperature range (150-500°C), and long –term ageing (1-150days) at (150°C) on the structure, and mechanical properties of the Kevlar 49 fibres.                                    

     Wide-angle X-ray diffraction techniques were used to characterize quantitatively the structure in terms of crystallinity , crystallite size and orientation. The mechanical properties were characterized in terms of initial Young’s modulus, tensile strength, and elongation at break.                    Thermogravimetric analysis was performed to determine the loss in weight after annealing. Differential thermal analysis scan was performed to determine the “Peak melting point” of the untreated Kevalr 49 fibres. The surface topography, fracture behaviour and microfibrillar character of Kevlar 49 fibres were studied in the scanning electron microscope.               Kevlar 49 fibres have a high crystallinity (75.5%) as determined by X-ray techniques, good orientation (width at half – amplitude equal (12.3°) and the orientation parameter <sin2 Ψ> = 0.020), medium crystallite sizes (D110 =4.23nm and D200= 3.85 nm) . They have ahigh initial Young’s modulus (112.7Gnm-2 ), high tensile strength (1.75 Gnm-2 ) and small elongation at break (2.7%). The Humidity absorption is low (2.8%), and the “Peak melting point”  is high (536.7°C) as determined by DTA.

     The constant-length annealing as well as the free length annealing of Kevlar 49 fibres in the temperature  range (150-350°C) did not cause any change in the crystallinity. At (400°C) the maximum crystallinity (81.0%) was obtained for constant-length annealing and (77.0%) for free length annealing.                                                The microparacrystallite size D110 increased from (4.23nm) to (4.89nm) at (400 °C)  and D200 increased from (3.85nm) to (4.27nm) for constant-length annealing, while D110 increased from (4,23nm) to (5.30nm) at (400 °C)  and D200 increased from (3.85nm) to (5.03nm) for free-length annealing.                          

       There is a linear relation between the crystallinity and the initial Young’s modulus. The tensile strength and elongation at break are inversely proportional to the annealing temperature due to degradation affects at high temperature.                           

       The crystallinity  of Kevlar 49 fibres was practically unaffected by thermal ageing up to (7days) at (150 °C) , which make Kevlar 49 suitable for use in composites at this temperature level for several days.                                                      The surface of Kevlar 49 as seen as in the stereoscan Electron Microscope is smooth, but become rough at high temperatures. “ Elastica loop” test indicated that kinks and cracks occur on the surface in compression. The fibrillar structure at Kevlar 49 was observed after cracking the surface.

     Thermogravimetric analysis has shown that until  (300 °C), there was no appreciable loss in weight for Kevlar 49 fibres.