Aggregation, fracture initiation, and strength of PP/CaCO3 composites

Polypropylene/CaCO3 composites were homogenized in a twin-screw compounder and then injection molded into tensile bars. Six different fillers were use d in a wide range of average particle sizes between 0.08 and 12 mu m Tensil e and flexural properties were measured by standard techniques, while impac t resistance was determined by instrumented impact testing. Structure was c haracterized by light and electron microscopy, while failure initiation and propagation was studied with in situ high-voltage electron microscopy. The results showed that aggregation of particulate fillers occurs when their p article size is smaller than a critical value. This critical size depends o n component properties and processing conditions. Strength and impact resis tance usually decrease with increasing number of aggregates. The presence o f aggregation can be detected by the use of a simple semiempirical model. C omparison of samples prepared by two different technologies showed that twi n-screw extrusion and injection molding leads to relatively homogeneous com posites, which was indicated by smaller deviations of the properties from t heoretical predictions. In spite of the acceptable dispersion, impact resis tance showed a large standard deviation, probably determined by the local v ariation of structure. In composites containing relatively large particles, the dominating micromechanical deformation process is debonding, while in the presence of extensive aggregation of small particles, cracks are initia ted inside and propagate through aggregates. Mixed-mode failure may also oc cur at certain intermediate particle sizes.