A low viscosity, fully hydrogenated butadiene acrylonitrile copolymer (less than 1% residual doublebonds), which provides vulcanizates with superior resistance to heat, ozone and excellent resistance to non - polar hydrocarbon fluids including those containing aggressive additives. With this level of unsaturation, Therban^® AT 3904 VP-based compounds can only be vulcanized with peroxides or high-energy radiation. The incorporation of a crosslinking co-agent is recommended for optimized physical properties of both unaged and aged vulcanizates. Low viscosity AT - technology provides a broad processing window, low mould fouling and the potential for higher filler loads compared to standard viscosity HNBR types.