FUNCTIONALIZATION OF ETHYLENE-PROPYLENE RUBBER VIA MELT MIXING

Grafting of maleic anhydride (MAH) onto ethylene-propylene rubber (EPR) of approximately unitary ethylene/propylene molar ratio was performed by melt mixing in a laboratory-scale internal mixer with the addition of dicumyl peroxide (DCP) as an initiator. Concurrent with the graft reaction, the simultaneous presence of DCP and MAH enhanced the cross-linking of EPR chains as suggested by the emergence of a cure peak in the mixing torque curve and supported by dynamic rheological evidence; the DCP-induced chain scission, however, remained important. The height of the cure peak increased consistently with the amounts of DCP and MAH in the reaction mixture. At the end of ca. 30 min of melt mixing, the gel content (determined via xylene extraction) of the functionalized EPR increased with the height of cure peak but then levelled off in the vicinity of ca. 50 wt%. The limited gel content was attributed to the competition from DCP-initiated scission reaction. This competition resulted in a wide distribution of chain connectivity, ranging from highly degraded short chains to partially cross-linked chains. At low DCP levels, the extent of grafting (estimated by means of Fourier-transform infrared spectroscopy) increased with the MAH content and then remained at a plateau value; at higher DCP levels, the plateau appeared to have already been reached even at the lowest MAH content (i.e., 0.5 phr) here. The limited extent of grafting was attributed to the immiscible nature and the low diffusion rate of MAH in EPR.