The influence of curing systems on the properties of natural rubber

Abstract

AbstractAfter a brief historical introduction the influence of curatives on the chemical structure of vulcanizates is discussed. Emphasis is given to physically important features of network structure such as crosslink density, crosslink type, and various main‐chain modifications. The significance of extra‐network materials such as curative residues and zinc soaps is introduced. Nonsulfur systems are briefly considered.The effects of differences in network structure and in extra‐network materials on aging properties is outlined with special emphasis on the importance of total combined sulfur, conjugated triene groups, and curative residues. The special problem of finding suitable antioxidants and antiozonants for peroxide cured rubber is mentioned. The influence of different curing systems on the crystallization of natural rubber both at low temperatures and at large extensions is described and shown to be a similar function of vulcanizate structure in both cases when compared at constant degrees of crosslinking. Problems in the interpretation of stress‐strain behavior are reviewed.Changes in the C1 term of the Mooney‐Rivlin relation can only be interpreted as changes in crosslink density or as a filler stiffening effect of insoluble extra‐network material. Changes in the C2 term of this relation also take place as the curing system is changed and are correlated with inelastic behavior. Analysis of stress‐strain relations at high strain using the Martin‐Roth‐Stiehler equation indicates significant differences in the strain‐induced crystallization behavior of vulcanizates based on various curing systems, even when compared at similar crosslink densities.The inelastic stress‐strain properties of natural rubber vulcanizates are divided into three main regions: (a) the glass transition region; (b) the chemical transition region, and (c) the intermediate region. The effects of network structure and extra‐network materials in each region are considered separately. In particular, the effects of zinc soap structurization on stress relaxation behavior in the intermediate region is described, including the likely mechanisms by which rubber‐soluble zinc soaps reduce the stress relaxation rate.The influence of curing systems on the strength properties of homogeneous and isotropic natural rubber vulcanizates is described briefly, with particular emphasis on the role of polysulfidic crosslinks and main‐chain modifications.