Chemical degradation of crosslinked ethylene-propylene-diene rubber in an acidic environment. Part II. Effect of peroxide crosslinking in the presence of a coagent

An investigation on the time-dependent chemical degradation of ethylene-propylene diene rubber containing 5-ethylidene-2-norbornene as diene cured by peroxide crosslinking in the presence of a coagent in an acidic environment (20% Cr/H₂SO₄) has been made. Two types of rubber, with comparable monomer composition, but having significant differences in molar mass and levels of long chain branching were tested. Dicumyl peroxide and triallylcyanurate under similar conditions were used for curing the rubbers. The molecular mechanisms of chemical degradation at the surface were studied using X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy, which demonstrate that several oxygenated species evolve during exposure. The primary process of degradation is hydrolytic attack on the crosslink sites, which is manifested by a decrease in crosslink density. The surface degradation is found to be strong enough to alter the bulk mechanical properties as observed by the change in retention in tensile strength, elongation at break, modulus at 50% elongation and, the change in micro-hardness. Retention in modulus at 50% elongation is found to follow a negative linear correlation with decrease in crosslink density. With higher molar mass and level of long chain branching more crosslinking occurs and thus comparatively more hydrolytic attack ensues. Scanning electron microscopy shows that the surface topography is significantly altered upon exposure and supports the notion of the dependence of degradation on the crosslinking density of the samples. Importantly, the coagent used in this study is shown to enhance the chemical degradation through formation of weaker sites for hydrolysis. The results also show that upon prolonged exposure the resulting oxygenated species tend to combine with each other.     

Chemical degradation of crosslinked ethylene-propylene-diene rubber in an acidic environment. Part I. Effect on accelerated sulphur crosslinks

The time-dependent chemical degradation of accelerated sulphur cured ethylene propylene diene rubber containing 5-ethylidene-2-norbornene as diene in an acidic environment (20% Cr/H₂SO₄) was investigated. Two different rubbers with a similar ethylene to propylene ratio and diene content but with a significant difference in molar mass and level of long chain branching were used in the study. The molecular mechanisms of the chemical degradation occurring at the surface were determined using surface analysis (X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy). The results reveal formation of several oxygenated species on the surface as a consequence of the acid attack. Furthermore, the crosslink sites of the exposed rubber samples are also found vulnerable to hydrolytic attack as evidenced by the decrease in crosslink density. The extent of surface degradation was strong enough to affect the bulk mechanical properties. Changes in mechanical properties were also monitored through determining retention in tensile strength, (%) elongation at break, modulus at 50% elongation, and change in micro-hardness. A negative correlation is also established between retention in modulus at 50% elongation and decrease in crosslink density. Scanning electron microscopy reveals the topographical damage at the surface due to the aqueous acid induced chemical degradation. The results indicate that the chemical degradation proceeds mainly via hydrolysis of crosslinks but upon prolonged exposure, the oxygenated species tend to combine with each other. The effect of molar mass and level of long chain branching also influences the chemical degradation.