State of Cure Measurements in Peroxide and Sulfur cured EPDM

This paper will serve as a review as the current methods used for measuring the state of cure in finished rubber products. It is well known that the level of state of cure has a profound impact on rubber properties measured on finished parts: mechanical (hardness, modulus, tensile strength, elongation to break, tear strength), dynamic (tan delta, hysteresis, fatigue behavior) and performance properties such as resistance to compression set and to abrasion. Flory-Rhener equilibrium volume swell, stress-strain modulus equation of state and nuclear magnetic resonance (NMR) spin echo measurements (T2 relaxation) will be employed for the measurement of state of cure. Peroxide and sulfur cured unfilled EPDM compounds will be used as model systems for this study. Crosslink densities and/or average molecular weights between crosslinks (Mc) will be compared to rheometer torque, hardness, physical property and compression set data. A biexponential analysis of the NMR T2 relaxation data provided satisfactory curve fitting results and an average molecular weight between crosslinks was calculated using the short decay time constant. The NMR measurement of crosslink density gave higher results compared to numbers predicted by stress strain analysis. Combining NMR Mc and volume swell Mc data (in particular, from the phantom model analysis) resulted in a successful estimation of the molecular weight between chain entanglements. It was also possible by NMR T2 relaxation curve analysis on unvulcanized EPDM to estimate the molecular weight between physical entanglements. Critical properties such as hardness, modulus and delta torque rheometry are in good correlation with crosslink density data from both NMR and stress strain techniques while the average molecular weight between crosslinks by equilibrium volume swell follows more closely properties such as elongation to break and compression set. Finally, it was concluded that both NMR T2 data and equilibrium volume swell measurements can follow the effects of sulfur reversion, with the latter being most sensitive to the crosslink density changes taking place during reversion.