Abstract

A comparison is made of the vulcanization kinetics as studied with rheometry and of the mechanical and elastic properties for 65 ethylene—propylene—diene rubbers (EPDMs) vulcanized with an activated sulfur system according to the ISO 4097 recipe. The selected 50 commercial EPDMs and 15 development grades provide a broad range of chemical and physical structures, including different third monomer (TM) structures and contents ([TM]). At low [TM] (up to 40 mmol TM/100 g EPDM: for example ∼4.5 wt % 5-ethylidene-2-norbornene, ENB) the mechanical and elastic properties of EPDM are strongly affected by [TM], but at higher [TM] they are more or less unaffected. For fully-cured EPDMs this transition at ∼40 mmol TM/100 g is independent of the TM type (ENB, dicyclopentadiene, DCPD, 1,4-hexadiene, HD, etc.). In previous Fourier transform Raman (FT-Raman) spectroscopy and ¹H NMR relaxation time studies it was established that for the ISO 4097 vulcanization system the average length of the sulfur crosslinks is ∼3 S-atoms/crosslink. From this it can be calculated that at ∼35 mmol TM/100 g EPDM a stoichiometric point is reached, where all sulfur is used for crosslinking. As a result, the properties of ISO 4097 EPDM vulcanizates level off when [TM] is increased beyond the stoichiometric point. The scatter in data at high [TM] is due to both experimental errors and differences in ethene/propene ratio, molecular weight (distribution) and/or degree of branching. It should be noted that the maximum cure rate (RH), being a kinetic parameter, remains dependent on [TM] over the whole chemical composition range although it also levels off at very high [TM]. In contrast to other studies it was found that RH is first order in both [TM] and [sulfur] at low [TM].