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Abdul Wahab, Mohammad Azizol (2007) Set behaviour of epichlorohydrin elastomers. Doctoral thesis, London Metropolitan University.
Traditionally Epichlorohydrin (ECO) elastomers are cured by thiourea (ETU)-Lead (PbO) curing systems. However, owing to the current concerns regarding health, safety and the environment such systems are now classified as relatively high risk systems. ECO elastomers are widely used as seals and gaskets; in these applications the minimisation of compression-set and compression stress-relaxation is highly desirable. It has been suggested that a curing system designated ZisnetFPT is not only relatively innocuous but it also has the potential to satisfy property requirements needed for providing adequate sealing efficiency. It is common practice to incorporate acid acceptors, which are merely metal oxides, in ECO rubber mixes in order to neutralise HC1 that might be liberated during vulcanisation. However, the addition of acid acceptors enhances the hydrophilic nature of ECO vulcanisates and leads to excessive moisture absorption. Water can plasticise most rubbers leading to high rates of stress-relaxation. In view of the deleterious nature of moisture, several acid acceptors commonly used in ECO elastomers were evaluated. The stiffening of vulcanisates conferred by carbon black was assessed using a relative stiffening parameter, af. was found to be greater in ETU-Lead based systems as compared with systems based on Zisnet-FPT. The crosslink densities of both rubber vulcanisates is generally determined by swelling measurements. The effect of various additives incorporated in ECO vulcanisates upon properties either singularly or interactively was assessed using an advanced experimental statistical method known as the Central Composite Rotatable Design (CCRD). The CCRD evaluation revealed that the compression-set behaviour exhibited by the Zisnet-FPT cure system is strongly carbon black-dependent. It was also found that for vulcanisates cured by the traditional curing system the effect of carbon black upon compression-set was negligible. The longevity as assessed by compression-set performance of ECO vulcanisates cured by both systems was evaluated using the classical Arrhenius approach. The relationship between compression-set and stress-relaxation was studied by means of a mathematical model proposed by Ellul and Southern. It was found that there is a broad correlation between compression-set and stress-relaxation provided certain parameters in the model do not vary excessively.
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