Nitrile rubber is a copolymer of butadiene and acrylonitrile in which the unsaturated diene segments form the elastomeric network and the nitrile groups supply polarity ^[1]. The acrylonitrile fraction, usually between 18 and 50 percent, sets the balance: higher content raises oil resistance and the glass transition temperature, while lower content preserves flexibility in the cold ^[1]. Vulcanisation proceeds through the residual double bonds, and the resulting crosslink density fixes compression set and swelling ^[2]. Hydrogenated grades convert backbone double bonds into methylene sequences, which removes the sites at which oxidation and ozonolysis begin ^[3]. The nitrile dipole also makes the polymer compatible with polar plasticisers and incompatible with non-polar oils ^[1].