The role of micrometric wollastonite particles on stress whitening behavior of polypropylene composites

The micromechanism and susceptibility to stress whitening during tensile straining of micrometric wollastonite mineral-reinforced polypropylenes is studied by electron microscopy and compared with unreinforced neat polypropylenes. Mineral-reinforced polypropylene composite exhibit significantly reduced susceptibility to stress whitening, and are characterized by lower gray level in the plastically deformed stress whitened zone. This behavior is attributed to the effective reinforcement of polypropylene by wollastonite that acts in concert increasing the tensile modulus of the composite and restricts plastic deformation of the matrix. The increase in tensile modulus is explained in terms of a three-phase model involving matrix, particle, and interface zone. Furthermore, isothermal crystallization indicated that the reinforcement mineral increases the rate of nucleation with consequent increase in % bulk crystallinity. The reinforcement of polypropylene alters the primary micromechanism of stress whitening from deformation bands/crazing in neat polypropylenes to wedge/ridge-tearing in mineral-reinforced polypropylene composites. The final fracture in reinforced polypropylene occurs by a mixed mode consisting of fibrillation and brittle mode, while crazing-tearing and brittle deformation are fracture modes for neat polypropylene.