The Effect of Acrylonitrile Content on the Thermo-Oxidative Aging of Nitrile Rubber

The level of thermo-oxidative degradation in a series of unstabilized and unfilled nitrile rubbers (NBR) varying in acrylonitrile (ACN) content (18–43.5 wt%) was investigated on heat-aged samples (40–120 °C) by Attenuated Total Reflectance–Fourier Transform Infrared (ATR-FTIR) spectroscopy. A similar degradation profile evolution was observed regardless of ACN content with the generation of hydroxyl-, carbonyl-, and ester-based products with a concomitant loss of the 1,4- trans , 1,4- cis , and 1,2-vinyl butadienes. The magnitude of IR active group absorption loss is greatest in the lowest ACN NBR concentration and steadily lessens toward higher ACN levels (1,4- cis > 1,2-vinyl > 1,4- trans >> butadiene methylenes). The 18% ACN NBR possesses two distinct kinetically different degradation regimes (80–120 and 40–80 °C). Activation energies by carbonyl growth and 1,4- trans loss increase from 71 to 87 kJ mol −1 and from 71 to 79 kJ mol −1 respectively, for decreasing ACN (43.5–18%) content. The rate of consumption of the 1,4- trans butadiene group is mainly affected by thermo-oxidative carbonyl-based and addition-cross-linking reactions, the latter being lower in activation energy for low to mid ACN NBRs. The high oxidation rate behavior of the lowest acrylonitrile rubber is attributed to its higher oxygen permeability rates. Cross-linking due to addition-type reactions is favored for higher 1,4 unsaturation levels.