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.