Brittle-Ductile Transition and Toughening Mechanism in POM/TPU/CaCO3 Ternary Composites

Polyoxymethylene (POM)/elastomer/filler ternary composites were prepared, in which thermoplastic polyurethane (TPU) and an inorganic filler, CaCO 3 were used to achieve balanced machanical properties of POM. A two -step processing method, in which the elastomer and the filler were mixed to a masterbatch first and then the masterbatch was melt-blendend with pure POM, was used to obtain a coreshell microstructure with CaCO 3 covered by TPU. A brittleductile transition phenomenon was observed with increasing TPU content for this ternary system. To better understand the toughening mechanism, we investigated the fractured surface, interparticle distance, and the spherulite size of POM as function of the TPU and Ca CO 3 content. The critical TPU content depended on not only the content of CaCO 3 , but also the size of CaCO 3 particles. The observed brittle-ductile transition was discussed based on the crystallinity and spherulite size of POM as well as Wu's critical interparticle distance theory. The results showed that the impact strengh of POM/TPU/CaCO 3 ternary system depends on a critical, interparticle distance, which varies from one system to another. The dependence of the impact strengh on the spherulite size was considered for the first time, and a single curve was constructed. A critical spherulite size of 40 micron was found, at which brittle-ductile transition occurs, regardless of the TPU and CaCO 3 content or the size of CaCO 3 particles. Our results indicate that the spherulite size of POM indeed plays a role in determining the toughness, and must be considered when discussing the toughening mechanism.