The crosslinking efficiencies of several curing agents have been determined from swelling measurements. The results indicate that dicumyl peroxide is a quantitative crosslinking agent for natural rubber, as was previously found to be true of dimethyloctadiene. As one molecule of dicumyl peroxide is able to form one crosslink, 3.2 to 4.0 molecules of tetramethylthiuram disulfide or 9–10 sulfur atoms in a sulfur‐sulfeneamide system are needed for the same purpose. A comparison is made between the number of chemically introduced crosslinks and a corresponding value obtained from swelling measurements with the help of the Flory‐Rehner equation. It was found that a filler can effect the properties of a vulcanizate through a filler‐curing agent interaction as well as through a filler‐rubber interaction. When a filler‐curing agent interaction takes place, a certain amount of curing agent or accelerator becomes unavailable for cure. This lowers the number of crosslinks introduced by the curing agent. If the filler‐curing agent interaction is insignificant and can be neglected, the swelling value Q for a filled stock can be related to that of an unfilled, analogous gum vulcanizate, Qgum, by the simple equation Q = kQgum; k is a function of the nature and amount of the filler and the elastomer, but is independent of the curing agent.

SynopsisThe crosslink density of small microtomed samples of rubber vulcanizates has been determined using a gas-chromatographic method for estimating the amount of solvent in the swollen rubber. The method may be used for determining the variation in crosslink density through thick rubber articles, for determining the change in crosslink density after aging service, or, for the investigation of rubber bonds and interfaces.

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.

A rapid build-up of pendent accelerator groups in the initial stages of vulcanization of natural rubber has shown that they are the precursors to the formation of crosslinks. Larger amounts of attached accelerator fragments were found for efficient and so-called sulfurless systems than for conventional accelerator-sulfur systems especially at short cure times. Cleavage of disulfides and polysulfides in the pendent groups by hexanethiol in piperidine showed a build-up of di- and polysulfides in the initial stages corresponding to that of accelerator fragments. Analyses were made by liquid scintillation counting using 14C-labeled accelerators. Due to the necessity of using liquid samples, vulcanizates were degraded in xylene solution with tert butyl hydroperoxide using osmium tetroxide as a catalyst. The use of a dual vulcanization system of dicumyl peroxide followed by the addition of sulfur and labeled accelerator by swelling techniques made possible the determination of accelerator fragments at short cure times when their concentration was the greatest. Polysulfides were found to have a profound and detrimental effect on the hardening of SBR during accelerated aging in air. An increase in attached accelerator fragments was found to take place during aging which may contribute to additional crosslinking and hardening.

As pendent accelerator groups are believed to be precursors to the formation of crosslinks in accelerated sulfur vulcanization, their original composition and concentration are largely responsible for the characteristics and properties of the final vulcanizate, including aging. This means that aging of rubber vulcanizates is inherently tied into the curing system. Dual labeled experiments using tritium-labeled accelerator and sulfur-35 were carried out to determine the number of sulfur atoms in the original pendent groups. As attached pendent accelerator groups are built up in the initial part of the vulcanization reaction before a network is formed, a dual vulcanization system was used, first curing with dicumyl peroxide followed by the addition of labeled sulfur and accelerator by infusion from benzene solution. Analyses were made by liquid scintillation counting after degrading the vulcanizates in p-xylene solution with t-butyl hydroperoxide using osmium tetroxide as a catalyst. The initial number of sulfur atoms in the pendent groups was found to be 3 for an efficient low sulfur system and 16–17 for a conventional high sulfur system. Oxygen absorption measurements have shown that vulcanizates having a low amount of sulfur in the original pendent groups resulted in superior aging. The presently proposed scheme of vulcanization requires the formation of a zinc perthiomercaptide as the actual sulfurating agent. Reaction with rubber gives rise to attached pendent groups which are the precursors to the formation of crosslinks. Results have shown, however, that pendent accelerator groups were formed not only in the presence of zinc but also in its absence where no zinc complex is possible. This would mean that the mechanism for the formation of pendent groups, at least in the absence of zinc, may be different from that presently accepted.