The evolution of morphology, crystallization and static and dynamic mechanical properties of long glass-fibre-reinforced polypropylene composites under thermo-oxidative ageing

In this work, the static and dynamic mechanical properties, crystallization behaviours, and morphology of long glass-fibre-reinforced polypropylene (PP) composites with thermo-oxidative ageing time from 0 day to 50 days at 120 degrees C were investigated and discussed. The static mechanical properties showed a global decrease in tensile, bending and impact strengths with increasing ageing time. From the results obtained by scanning electronic microscopic observations, interface debonding clearly occurred between the glass fibre and PP matrix in the aged samples. The crystallinity (X-c) of the composites was analyzed by differential scanning calorimetry; annealing process played the leading role in the early period of ageing, while as ageing progressed, the degradation of PP matrix dominated the ageing process and X-c decreased. The dynamic mechanical analysis results indicated that the storage modulus and glass transition temperature of the composites also decreased with prolonging ageing time. Then, the apparent activation energy (E) of glass transition was calculated by the Arrhenius equation with different scanning frequencies. A higher value of E was obtained for the samples in the later ageing period, which means a higher energy barrier for glass transition.