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

The primary objective of this study is to investigate the level of thermo-oxidative degradation in a series of unstabilized and unfilled nitrile rubbers varying in acrylonitrile content (18 to 43.5%).  Specimen aging was carried out from 40 up to 120ºC in a hot air oven and Attenuated Total Reflectance - Fourier Transform - Infrared spectroscopy (ATR-FT-IR) was the preferred method to follow the degradation process.    A similar degradation profile evolution is observed regardless of acrylonitrile 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 as a function of oxidative heat aging. The magnitude of IR active group absorption loss is highest in the lowest ACN NBR and steadily lowers in going to higher ACN levels (1,4 cis > 1,2 vinyl > 1,4 trans >> butadiene methylenes).  In the context of the experimental conditions of temperature and time, no significant change in the nitrile absorption was noted.  Time/temperature superposition was successful for each acrylonitrile level and shift factors were calculated using a reference temperature of 80°C.  The heterogeneous behaviour of the degradation process on the sample surface, likely due to impurities, brought about increased scatter in the absorption results. Shift factor values were similar for each functional group followed at each acrylonitrile level and activation energies were calculated assuming Arrhenius kinetic behavior.  The 18% ACN nitrile possesses two distinct kinetically different degradation regimes, one from 80 to 120°C and the second from 40 to 80°C.    A second shift factor was applied to the data to account for the acrylonitrile level resulting in an excellent superposition of all data points for the carbonyl and 1,4 trans groups. The high oxidation rate behaviour of the lowest acrylonitrile rubber is attributed to the higher oxygen permeability rates compared to the higher acrylonitrile containing nitriles.  General oxidation degradation mechanisms are proposed and summarized to support the functional group information gathered from ATR-FT-IR.