Degradation of hydrogenated nitrile rubber (HNBR) O-rings exposed to simulated servo system conditions

The degradation behaviors and mechanisms of hydrogenated nitrile rubber O-rings under the compression and free state were investigated in simulated hydraulic oil environments at different elevated temperatures. The chemical structures and mechanical properties of the uncompressed and compressed samples before and after exposure to oil for different days were evaluated by measuring attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, crosslinking density, compression set, weight loss, mechanical properties and fracture morphology. The ATR-FTIR results demonstrate that the hydroxyl groups were formed under the compression and free state, and the amide groups were only generated under the free state. The crosslinking density results reveal that the crosslinking dominated during the most of aging process and the chain scission might account for more at the elevated temperature in the middle and later stages. The compression set for the aged samples increased with increasing exposure time and temperature. The mechanical properties of the uncompressed and compressed samples significantly varied with the physical and chemical changes in the matrix structure under coupling actions of stress, temperature and oil. Moreover, the fracture morphology results indicate that the formation of the hardened brittle outer layer, voids and agglomerates might be attributed to the decrease in properties of the aged samples, especially for the free state.