Inhibition of Hardening in Natural Rubber

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Abstract
It is well known that the polyisoprene of hevea latex undergoes considerable changes after its emergence from the tree. These changes take the form of chain scission or crosslinking depending on the conditions obtaining (see all References). In the solid state the predominant reaction that takes place on storage of rubber is one of crosslinking as indicated by the increase in Mooney viscosity, Williams plasticity and gel content. The plasticity of solid rubber changes continuously from the time it is prepared to the time it is used by the consuming industry. Considerable work has been carried out in the past to study this reaction with a view to controlling or inhibiting it. Wood suggested that peroxide or other free radical forming groups in the rubber may be responsible for the gelation of rubber on storage but gave no indication whether these groups were present in rubber as it leaves the tree or were formed subsequently. In a recent paper, it was shown by the writer that reactive groups are present in the rubber molecule as it leaves the tree. These reactive groups have the characteristic property of carbonyl groups or more specifically aldehyde groups. Condensation of these groups with other groups on the rubber hydrocarbon molecule, e.g., α-methyl or methylene groups, may be responsible for crosslinking of the polymer molecules. In support of this hypothesis, it was shown that reactive monofunctional amines or other carbonyl reagents are capable of inhibiting this crosslinking effectively, and that bifunctional amines can bring about crosslinking. The present investigation is an extension of this work and includes an examination of the effect of excess bifunctional amines, observations on the influence of formaldehyde in de-activating the naturally occurring amines in latex and a study of the reaction between hydroxylamine and a number of rubbers from different clones.

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