Structural characteristics and swelling mechanism of two commercial nitrile-butadiene elastomers in various fluids

For initial times, the swelling is interpreted in terms of diffusion of the fluid into the elastomer. However, diffusion is not strictly Fickian since the exponent, varying between 0.4 and 0.6 for different fluids, differs from 0.5 expected for Fickian diffusion. It is shown that the results of maximum swelling can be interpreted in terms of compatibility of the Hansen solubility parameters δT of the fluid and the elastomer, swelling decreasing on a Gaussian curve as δT of the fluid deviates from that of the elastomer. Also, swelling is determined by the polybutadiene component of the elastomer. In HISOL-15, a commercial aromatic solvent, nitriles swell by about 55%, and mixing of HISOL-15 with S-8 produces swelling linearly proportional to the % of HISOL-15. The observed few percent decrease in swelling on long-term exposure may be related to loss of material from the elastomers.