Microstructure-properties correlation in silica-reinforced dynamically vulcanized EPDM/PP thermoplastic elastomers

Silica filled dynamically crosslinked thermoplastic elastomers (TPEs) based on ethylene-propylene diene monomer (EPDM) and polypropylene (PP) with 60/40 (w/w) ratio have been prepared to investigate the effects of silica upon the interfacial interaction between the two phases as well as microstructure/properties relationship. Blends loaded with 10 to 30 phr silica (premixed with EPDM) showed more reduction in the mixing torque after the maximum torque peak with lower and constant viscosity and hence better melt processability compared with similar but unfilled sample. This is showed to be due to the formation of tight agglomerate structure as a result of lower EPDM/PP viscosity ratio for the unfilled sample. Increasing the concentration of silica to 50 phr leads to the sharp increase in the mixing torque during dynamic vulcanization with high final melt viscosity. SEM examination showed that silica remains shelled by EPDM, but at high concentration, migration of silica occurs from EPDM onto PP during melt mixing leading to the enhancement of the adhesion between the PP shelled rubber particles which gives rise to the formation of strong networks. Rheometric mechanical spectrometer (RMS) showed less fractionation of the relaxation times for the PP segments in the silica filled samples with lower damping behavior (Tan. delta(Max)). Addition of functionalized PP promotes the interfacial interaction between the matrix and the silica filled crosslinked rubber networks. More rubbery behavior and increased extensibility observed for the blends incorporated by 10 to 30 phr silica, while the 50 phr filled sample is characterized by higher elastic modulus. Crystallization of the highly silica filled blends is promoted as the migrated silica acts as nucleating agent for PP.