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, CaCO3, were used to achieve balanced mechanical 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-blended with pure POM, was used to obtain a core-shell microstructure with CaCO3 covered by TPU. A brittle-ductile 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 CaCO3 content. The critical TPU content depended on not only the content of CaCO3, but also the size of CaCO3 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 strength of POM/TPU/CaCO3 ternary system depends on a critical, interparticle distance, which varies from one system to another. The dependence of the impact strength 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 CaCO3 content or the size of CaCO3.