Peroxide cured ethylene-propylene-diene/polypropylene (PP/EPDM) Thermoplastic vulcanizates: effects of methacrylate co-agents on properties
F.R. de Risi and J.W.M Noordermeer, Dutch Polymer Institute and University of Twente, The Netherlands
The development of thermoplastic vulcanizates (TPVs) as a type of thermoplastic elastomers, has gained large technical and economic importance. TPVs are typically characterized by a chemically crosslinked rubber phase, embedded in a thermoplastic matrix, produced by dynamic vulcanization during mixing the rubber with the thermoplastic in the molten state. In the present work, the effects of different methacrylate-type co-agents as co-curing agents for peroxide vulcanization of PP/EPDM TPVs are reported, whereby the weight fraction of the EPDM vs. the PP is varied from 30, 50 to 70%. The methacrylates tested were: Ethylene glycol dimethacrylate (EDMA), Trimethylol propane trimethacrylate (TMPTMA, often referred to as TRIM) and Zinc dimethacrylate (ZDMA). Crosslinking was carried out at a fixed content of dicumyl peroxide with the amounts of co-agents varied. The purpose of the co-agents was to boost EPDM-crosslinking and at the same time prevent degradation of the PP by β-scission. Significant differences are reported: although all three methacrylates have the same ethylenic moieties as the reactive groups, the observed differences in mechanical properties point to diverse structures of the crosslink networks of the PP/EPDM TPVs. Out of the three co-agents, TMPTMA gives the best overall balance of properties, in EPDM crosslinking and preventing PP-degradation. This is explained by the positioning of the solubility parameter of this co-agent just in between those of EPDM and PP. The solubility parameter for EDMA is far away from either one. ZDMA, although giving very good room temperature properties like Tensile Strength, shows signs of thermo-reversion of the crosslinks in the Compression Set measured at 70 C.