Antioxidants and Antiozonants for General Purpose Elastomers

Abstract

Abstract The purpose of this article is to review the developments in antioxidants for rubber over the past 25 years since the publication of Davis and Blake's “Chemistry and Technology of Rubber”. The intent is to provide a guide for rational use and further development. Accordingly, the organization of the material is as follows: a) Outline of the mechanism of oxidative attack and antioxidant action. b) Critical review of methods of evaluation. c) Survey of antioxidants in the literature. Numerous reviews of antioxidants in one or more aspects have been published. These have been duplicated only so far as necessary to form a complete outline. Patent references are included only when the specification cites experimental data in support of the claims of antioxidant action. Usage divides rubber antioxidants into four categories: 1) Polymer stabilizers, which are added in the polymerization plant to repress the action of oxygen during drying and storage of the uncured rubber. 2) Antioxidants, which are added in compounding to prolong the useful life of the vulcanizate by retarding the oxidative changes that occur over a long period of service. 3) Flex-crack inhibitors, antioxidants which retard crack initiation or crack growth under conditions of cyclic deformation. 4) Antiozonants, substances which by chemical action (or by a combination of physical and chemical processes) delay the onset or growth of cracks resulting from ozone attack. (This definition is written to exclude waxes, which protect from ozone by forming a chemically nonreactive surface film.) There is a closer fundamental connection between polymer stabilizers and vulcanizate antioxidants than among other classes. The discussion of mechanisms of oxidation and antioxidant action will introduce the first two. Evaluation of the two classes of materials are operationally different and will be discussed separately. The present discussion is limited to “general purpose” elastomers of the present time; namely, NR, IR, SBR and BR.