The cure of elastomers by dicumyl peroxide as observed in differential scanning calorimetry

Abstract

Differential scanning calorimetry (DSC) has been used to determine the enthalpy changes accompanying the thermal homolysis and subsequent radical reactions occuring in the dicumyl peroxide (DCP) cure of elastomers (NR, EPDM, SBR, BR, NBR, and EPM). The thermal degradation of DCP alone, dispersed on kaolin clay in a hydrocarbon solvent, was also studied. The degradation of DCP alone results in an exothermic enthalpy change of ∼ 215 kJ mole −1. In elastomer systems, the observed enthalpy exchange can be ordered BR >; SBR ; NBR (34% ACN) > NBR (27% ACN) > NR > EPDM ∼ EPM. Low enthalpy values are associated with systems containing predominantly secondary hydrogen atoms (EPDM, EPM). The high enthalpy of cure for BR appears associated with the known ability of the polybutadienyl radicals to propagate a limited chain reaction. The relative extent of cumyloxy radical disproportionation to hydrogen abstraction was determined in each system and this has no apparent effect on the observed enthalpy change. The method of Borchardt and Daniels was used for data reduction and calculation of apparent activation energies. For DCP degradation alone the calculated activation energy is in good agreement with literature values. In elastomer systems, the calculated activation energies must be treated with caution because, as pointed out by Borchardt and Daniels, their method does not apply to solid state reactions.