Structural Characteristics and Swelling Mechanism of Two Commercial Nitrile-Butadiene Elastomers in Various Fluids

Abstract Results on the structural characteristics and swelling mechanisms of two commercial elastomers, viz. N0674-70 and peroxide cured N0741-75, in a variety of fluids (toluene, o-xylene, p-xylene, mesitylene, HISOL-15, n-hexane, ethanol, tert-butyl alcohol, acetone, JP-8, S-8 and blends of S-8 plus HISOL-15 and S-8 plus toluene) are reported. Both changes in mass and volume of rectangular test pieces were monitored as a function of time. Structural characterization of the elastomers was done using x-ray diffraction and FTIR spectroscopy. Time dependence of the recovery of swelling after the elastomers were taken out of the fluids was monitored by x-ray diffraction and changes in mass. In general, swelling of the peroxide cured N0741-75 was less than that of N0674-70, percentage changes in mass were less than those in volume, and the swelling is strongly fluid dependent. 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 δ T of the fluid and the elastomer, swelling decreasing on a Gaussian curve as δ T 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.