Industrial & Engineering Chemistry Research, Vol.58, No.32, 14823-14838, 2019
Molecular Simulations and Experimental Characterization of Fluorinated Nitrile Butadiene Elastomers with Low H2S Permeability
The elastomer seals that are used in the processing of sour gas in the oil and gas industry must be able to withstand chemical degradation by corrosive materials such as hydrogen sulfide present in the sour gas. Nitrile rubbers, including hydrogenated nitrile butadiene (HNBR), are commonly used because they have low glass transition temperatures, good low-temperature sealing ability, and excellent resistance to swelling by hydrocarbons. They are, however, prone to chemical modifications, including chain scission, cross-linking, and sulfur incorporation, that result in changes in mechanical properties after contact with H2S. Fluoroelastomers are promising alternatives but have higher glass transition temperatures than nitrile rubbers, which affect their low-temperature performance. The approach of free-radical grafting of a semifluorinated alkyl methacrylate, 1H,1H,2H,2H-perfluorooctyl methacrylate (F6H2MA), to HNBR, to improve its resistance to H2S was investigated. The effects of grafting density of F6H2MA on the solubilities and diffusion coefficients of H2S, and carbon dioxide, which is also present in sour gas, were studied using molecular dynamics and Monte Carlo simulations. The influence of acrylonitrile concentration in the copolymer was characterized. Simulated dilatometry and differential scanning calorimetry measurements showed that the grafting of F6H2MA to HNBR did not result in a significant increase in the glass transition temperature of the copolymer. H2S permeability through the fluorinated HNBR showed a significant decrease with an increase in F6H2MA concentration. The results of molecular simulations were corroborated by measurement of mechanical properties of elastomer samples aged in a sour-gas environment at 250 degrees F and 1000 psig for 24 h. Gravimetric and volumetric swelling, as well as mechanical properties of the aged samples, was measured for comparison with the original unaged samples. The degrees of swelling and mechanical property deterioration after aging were significantly suppressed by grafting HNBR with even low concentrations of the semifluorinated methacrylate.