Tensile creep behaviour of polypropylene fibre reinforced polypropylene composites

Creep, the deformation over time of a material under stress, is one characteristic of composites that has resulted in poor performance in certain applications. This work was undertaken to investigate the advantages of using PP fibres in random poly(propylene-co-ethylene) (PPE) with different fibre concentrations. Addition of long polypropylene fibres into PPE greatly improved the creep resistance and modulus of elasticity of the composite compared with the matrix polymer. However, high concentrations of long fibres (>50%, w/w) resulted in fibre packing problems and an increase in void content, which lead to a reduction in creep resistance. The curves of creep modulus and relative creep versus time were shifted along the logarithmic time scale to develop a creep master curve, which allows for the prediction of very long and short time behaviour of the composite. To verify the experimental results, a four-parameter viscoelastic model was applied in order to quantify the viscoelastic behaviour of the composites. Morphology of the composites was investigated using optical and scanning electron microscopy (SEM). Optical microscopy pictures showed that trans-crystallization of the matrix polypropylene was observed in the PPE. SEM photographs displayed a thin layer of matrix on the reinforcement, which was attributed to good impregnation and wetting of the fibres.