The influence of curing systems on the properties of natural rubber

Abstract

SYNOPSISAfter 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 C, 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 C, 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. 61

Oil palm wood flour filled natural rubber composites: fatigue and hysteresis behaviour

“…The slow deterioration of vulcanizates by moderate heat in the absence of oxygen is really a continuation of the vulcanization chemistry. More conventionally, the term ‘ageing’ is used to describe the deterioration of rubber by reaction with atmospheric oxygen, although clearly both processes may take place simultaneously 13…”

“…Irrespective of the OPWF loading, the fatigue life of the thermo‐oxidatively aged composites was reduced with increasing extension ratio. In this work, the composites were prepared using a conventional vulcanization (CV) system where the composites consist largely of polysulfidic crosslinks with some mono‐ and disulfidic crosslinks 13. At ageing temperature, polysulfidic crosslinks were undergoing desulfuration to disulfidic and to a lesser extent, monosulfidic linkages but they were not decomposing.…”

Effect of Polymer Chain Modifications on Elastomer Properties

“…It is believed that the type of crosslinks is another structural parameter, which strongly influences the behavior of the polymer chains and resulting properties of the material. 1,3,4,7,15,16,18,[23][24][25][26][27][28] Due to this, much attention is devoted to the crosslink density and structures analyses, and interpretation of the macroscopic properties in the context of these parameters.…”

“…As a result, beside formation of intermolecular crosslinks, various side reactions can occur during the curing proces,s. 1,3,7,11,12,15,18,23,24,25 As a result of them, the curatives bond chemically to the polymer chains without formation of the elastically effective crosslinks. Part of the polysulfidic accelerator-terminated pendant groups, being crosslink precursors, can remain in this form.…”

“…During the curing process, one of the critical steps is the formation of polysulfidic acceleratorterminated pendant groups (RS y Acc). 1,[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] These crosslinks precursors are bound to the polymer chains. Further, they react with other pendant groups or directly with the polymer chains.…”