EFFECT OF MINERAL FILLERS IN LOW CONCENTRATION ON THE MECHANICAL-PROPERTIES OF POLYMERIC MATERIALS .1. STATIC AND FATIGUE FRACTURE OF POLYPROPYLENE, QUALITATIVE ASPECTS

Injection moulded polypropylene samples containing low concentrations of talc, mica or wollastonite fillers were studied by differential scanning calorimetry, scanning electron microscopy (SEM), and static and dynamic (fatigue) flexural testing. All the fillers under study displayed a nucleating effect on crystallization. This effect is probably responsible for a decrease of the macromolecular orientation, especially in the 'shear zone'. The most noticeable change in static flexural properties was an increase (50%) of the ultimate stress, presumably due to a change of fracture mechanism induced by the presence of mineral particles. The mineral particles also induced a decrease of the flexural fatigue lifetime. This effect can be attributed directly to the initiating effect of mineral particles on cracking, but also indirectly to a change of skin-core structure as a result of filler nucleating effects during processing. Three crack propagation modes were observed by SEM: matrix/filler debonding, partial debonding with crack reinitiation at the particle edge, and linear propagation with particle rupture. However, the mode of crack propagation seemed to have little influence on the fatigue lifetime, which was essentially controlled by the number of particles per unit volume.