Storage hardening of natural rubber

Natural rubber exhibits different storage properties when compared with synthetic polyisoprenes. Stored over an extended period of a few months, natural rubber is found to become progressively harder while the synthetic rubber is known to soften. This phenomenon of spontaneous crosslinking of natural rubber in storage is of considerable importance since it affects the processing properties. The process of storage hardening has been actively investigated and is believed to involve abnormal groups in the rubber molecules. This paper reviews the various proposals by different workers involving aldehyde, epoxide, and amino acids. Our recent results show that amino acids react with certain abnormal groups in rubber at the latex stage. The net effect of the reaction is to functionalize the rubber with carboxylic, hydroxylic, and amine groups in addition to the original abnormal groups. Storage of the dry rubber could lead to slow crosslinking reactions which are accelerated by anhydrous conditions.