The service life of nitrile O-rings exposed to hydraulic fluid was determined by accelerated aging at nine temperatures and four immersion times. Tensile mechanical properties (elongation and low strain modulus), volume swell, compression set, and chemical crosslink density by solvent swell were measured. Calculated activation energies based on Arrhenius rate behavior ranged from 52 to 65 kJmol−1, approximately 20–30 kJmol−1 lower than nitrile rubber heat aged in air environments. Using a 50% loss of elongation as a failure criterion, an estimate of 15 yr of service life at 23 °C was calculated. This corresponds to a compression set of 50% and an increase of approximately 30% of the chemical crosslink density. Replacement of the plasticizer with the mineral oil and its additives increased total inorganic levels in degraded O-rings as measured by thermogravimetric analysis. Besides additional sulfur and sodium, energy dispersive spectroscopy identified the presence of phosphorous, chlorine, and potassium. Hydraulic oil additives are likely responsible in facilitating the O-ring degradation through lower energy pathways that accelerate nitrile rubber hardening.
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Pazur et al. (2013) studied this question.