Reinforcement of EPDM-based ionic thermoplastic elastomer by precipitated silica filler

Incorporation of up to 20 phr of precipitated silica filler into a thermoplastic elastomer of composition 100 phr maleated EPDM rubber, 10 phr zinc oxide, 1 phr stearic acid, 30 phr zinc stearate, results in an improvement of physical properties such as modulus, tensile strength and tear resistance. While the glass-rubber transition temperature (T-g) of the thermoplastic elastomer occurring around -37 degrees C remains unaltered in the presence of the filler, the ionic transition (T-i) occurring around +50 degrees C is shifted to higher temperature on filler incorporation. Furthermore, the filler causes a decrease in tan delta at T-g, but an increase in tan delta at T-i. It is believed that, besides the dispersive-type forces operative in the interaction between the backbone chains and the filler particles, the ionic domains in the thermoplastic elastomer interact strongly with the polar sites on the filler surface. Reprocessability studies in the Monsanto Processability Tester (MPT) show that the silica-filled polymer can be reprocessed like a thermoplastic, there being no reduction in properties even after four cycles of extrusion through the MPT. (C) 1996 Elsevier Science Ltd.