Study on the phase structures and toughening mechanism in PP/EPDM/SiO2 ternary composites

In this paper, EPDM rubber and nano-SiO₂ particles were employed to modify PP simultaneously. Our goal was to control the distribution and dispersion of EPDM and nano-SiO₂ particles in PP matrix by using an appropriate processing method and adjusting the wettability of nano-SiO₂ particles toward PP and EPDM, so as to achieve a simultaneous enhancement of toughness and modulus of PP. With regard to this, two kinds of nano-SiO₂ particles (with hydrophilic or hydrophobic) as well as two processing methods (one-step or two-step) were employed to prepare PP/EPDM/SiO₂ ternary composites. A unique structure with the majority of EPDM particles surrounded by SiO₂ particles was first observed by using hydrophilic SiO₂ and two-step processing method, resulting in a dramatically increase of Izod impact strength as the rubber content in the range of brittle-ductile transition (15-20 wt%). The observation that poor adhesion and poor compatibility between particles and PP matrix could result in a significant increase in Izod impact strength was unusual and needed further investigation. This could be tentatively understood as a consequence of the overlap of the ‘stress volume’ between EPDM and SiO₂ particles due to the formation of the unique structure. Our work provided a deep understanding of the toughening mechanism and a new way for the preparation of high performance polymer composites.