TITANIUM-DIOXIDE INDUCED FAILURE IN POLYCARBONATE

Incorporation of low levels of fillers into thermoplastics can often l ead to an increase in the modulus and a general improvement in the mec hanical properties. However, increasing the level of filler loading ca n lead to a dramatic loss of impact strength and related mechanical pr operties. In this study the pigmentation of polycarbonate (PC) with ti tanium dioxide (TiO2) is shown to lead to a reduction in the fracture energy and changes in the failure mechanism of injection moulded test samples. Electron microscopic examination of the fracture surfaces ind icates that changes in the failure mechanism are a result of the occur rence of large areas of densified polymer around pigment particles. De nsification reduces the extent of segmental motion within the polymer matrix, as indicated by dielectric and thermally stimulated discharge current measurements. The influence on the molecular motion and mechan ical properties of the degree of drying, molecular weight of the polym er and concentration of TiO2 are reported. Complementary positron anni hilation measurements allow a tentative mechanism to be proposed to ex plain the loss of impact strength in these materials. Data is also pre sented on polybutylene terephthalate (PET) pigmented materials and a P C system in which the TiO2 has been mixed with polyethylene prior to d ispersion in the PC matrix. These observations confirm the importance of polymer-pigment interactions in determining the fracture properties of samples.