Some features of the vulcanization of butadiene-nitrile rubbers☆

BUTADIENE-nitrile rubbers are fairly reactive polymers and consequently they can be vulcanized by various methods. I t has been shown previously that they readily undergo thermal vulcanization [1]. Vulcanization also occurs in the presence of hydroxides of alkaline earth metals [2] and zinc chloride [3]. The mechanism of vulcanization and the structure of the vulcanized network are naturally different when different vulcanizing agents arc used [4]. In contrast to SKB* thermal vulcanization of butadiene-nitrile rubbers involves elimination of a hydrogen atom from the carbon atom in the a-position with respect to the nitrile group [5]. The underlying process in vulcanization in the presence of bases is hydrolysis of the nitrile groups and formation of salt bonds [2]. Vulcanization in the presence of zinc chloride, at least in the initial stages, apparently involves complex formation at the nitrilc groups. The scheme of thermal vulcanization put forward in reference [5], which postulates elimination of a hydrogen atom in the a-position with respect to the CN group, reaction of a nitrile group with this carbon atom and formation of intraand intermolecular -CN--C-bonds, is supported by reports on studies of the properties and structure of polyacrylonitrile [6-9] and on the polymerizability of saturated alkyl nitriles [10, ll]. Two additional new experimental facts have provided definite confirmation of this mechanism of thermal vulcanization. SKN-40, which has a fairly high nitrile content, should after thermal vulcanization acquire to some extent semiconducting properties, as occurs with polyacrylonitrile [6-7]. The rubber was extracted with acetone for 80 hr and heated at 183 °, then the change in its electrical conductivity was determined. I t was found that heat t reatment brings about an increase in the specific conductivity from 0.8 X 107 to 0.5 x 106 ohm -1. cm -1,