Toughening mechanism of polypropylene bends with polymer particles in core-shell structure: Equivalent rubber content effect related to core-shell interfacial strength

Abstract

Toughening mechanism of in-situ core-shell dispersed particle in polymer blends was re-examined by using different polymers as cores. Three kinds of polyethylene including high density polyethylene (HDPE), medium density polyethylene (MDPE) and low density polyethylene (LDPE) were adopted to prepare polypropylene/ethylene-propylene rubber/polyethylene (PP/EPR/PE) blends, and all of them presented simialr morphological characteristics of core-shell dispersed particles with similar size in which PE acts as the core. However, PP/EPR/HDPE and PP/EPR/MDPE presented excellent toughness while poor toughening effect appeared in PP/EPR/LDPE. The impact section morphology showed that shear yielding occurred not only in the matrix for PP/EPR/HDPE and PP/EPR/MDPE but also in the HDPE and MDPE cores. The results of crystallization enthalpy loss indicated that the interfacial strength between EPR and LDPE is much weaker than those of EPR with other PEs. By introducing LDPE into PP/EPR/HDPE blends, shear yielding in core disappeared and the impact strength significant declined, which was ascribed to weakening of core-shell interfacial strength caused by LDPE. Finally, a complete toughening mechanism of equivalent rubber content effect for polymer core-shell particles was proposed. Besides enhanced percolation of stress volumes by introduction of crystallizable core, an efficient shear yield of the core and the matrix based on good core-shell interfacial strength under impact is also critical for effective toughening.