Bound antioxidants in elastomers

Abstract

Amine and phenolic antioxidants are found to react with chloroprene (CR) and epichlorohydrin (ECH) rubbers with their reactive chlorine atoms. These reactions take place during high shear mixing in a torque rheometer or in a Winkworth mixer at high temperature or during the vulcanization of the elastomers at 160°C. Mixed alkyl, aryl secondary amines are shown to react only at the alkyl substituted amino group. Mono substituted sodium salt of dihydric phenols were initially used to facilitate reaction with the rubbers but this was found unnecessary as the free phenols will react to give high yields of bound antioxidant. The concentration of the bound antioxidants are estimated by I.R. spectroscopy after complete extraction. Monsanto rheometer studies show that in many cases the crosslinking reaction of CR and ECH rubbers which take place through the chlorine atom are reduced by the presence of the antioxidants. The effectiveness of the rubber-bound antioxidant is assessed by accelerated oven ageing, U.V. irradiation and stress-relaxation techniques. It is shown that immobilization of amine antioxidants by combining them with the network does not impair their antioxidant activity. Phenolic antioxidants whether free or bound give very good effective protection to chloroprene rubbers but act as pro-oxidant with epichlorohydrin rubber. The effect is increased in the presence of nickel dibutyldithiocarbamate which is commonly employed as an antioxidant for ECH rubber. Epichlorohydrin rubber bound antioxidant can be used as a very effective polymeric antioxidant in chloroprene rubbers.