Polyethylene/crystalline ethylene-propylene copolymer/amorphous ethylene-propylene copolymer in-reactor alloys synthesized by periodic switching polymerization process: An excellent toughener for polypropylene

Abstract

In order to obtain softer polyolefins applicable as toughener for polypropylene (PP), a series of in-reactor alloys composed of polyethylene, crystalline ethylene-propylene copolymer (cEP) and amorphous ethylene-propylene copolymer (aEP) were synthesized by sequential ethylene polymerization followed by ethylene-propylene copolymerization in periodic switching polymerization process (PSPP) using a spherical MgCl2-supported Ziegler-Natta catalyst. In the PSPP step, ethylene-propylene copolymerization was repeatedly switched between two different monomer feed ratios: one with high propylene/ethylene ratio to form aEP component, and the other with low propylene/ethylene ratio to form cEP component. In the synthesized PE/cEP/aEP in-reactor alloys, aEP accounted for 40~56 wt%, and cEP accounted for 18~30 wt% of the alloy. With aEP content rising above 48 wt%, the alloys showed rather low modulus, and no yielding appeared in their stress–strain curve, implying that they were a sort of elastomeric material. By blending the PE/cEP/aEP alloys with isotactic PP, their PP-toughening performances were evaluated and compared with a commercial EPDM. Impact strength of PP toughened by the alloy was significantly higher than that toughened by EPDM at the same blend ratio. Cryo-fractured surface of PP/alloy blends showed that the alloy particles were uniformly dispersed in PP matrix, and showed a core–shell type morphology, in which the core domain was composed of polyethylene, and aEP constituted the shell domain in between the core and the PP matrix. Segmented EP copolymer chains (s-EP) located inside the aEP domains formed thin filaments bridging the core and the PP matrix, which are believed to act as compatibilizer that significantly enhanced interfacial adhesion between the PP matrix and the dispersed phase.