Mixed Cross-Link Systems in Elastomers

Rubbers are long-chain molecules which are plastic in nature in the raw or unvulcanized state. Vulcanization is an irreversible process by which the predominantly plastic rubber is converted to predominantly elastic and a three-dimensional network structure through the anchoring between two polymer chains. Chemically, this is done by an intermolecular cross-linking reaction. Cross-linking increases the retractive force and reduces the amount of permanent deformation remaining after removal of the deforming force. These links between polymer chains may be chains of sulfur atom or atoms, carbon-carbon bonds, polyvalent metal ions, etc., depending on the nature of the vulcanizing system. Properties of elastomers depend on how efficiently this cross-linking has been achieved and also which types of cross-linking agents are used. For example, the retractive force resisting a deformation is proportional to the number of network supporting polymer chains per unit volume of elastomers 1]. Again the strength of elastomers depends on the stress relaxation mechanism in the cross-linked material 2]. The modulus of a vulcanizate is proportional to the number of cross-links formed, while the tensile strength normally passes through a maximum with an increase in the number of cross-links. The resilience, heat build-up, and fatigue properties depend on the chemical nature of the cross-links and on the chemical structure of the base polymer 3]. Different types of cross-links have both advantages and disadvantages with respect to technical properties. To cite one example, a sulfur cross-linking system produces good tensile strength but poor aging properties. On the other hand, carbon-carbon cross-links produce good aging properties but poor tensile properties. Thus there are reasons for using a mixed cross-link system in order to obtain the right compromise.