Prevulcanization Inhibitors

Abstract

Abstract Conversion of raw rubber to a usable product is basically a three-step process. Crosslinking ingredients, fillers, and other components are added, the compounded green stock is shaped into the desired form, and the form is heated (cured) to introduce crosslinks (covalent bonds) between the rubber polymer chains and produce a three-dimensional network with vastly improved properties. The crosslinked product no longer flows and thus cannot be reprocessed. The curing system is activated by heat, heat is required for and generated by processing, and faster processing is desirable for economic reasons. Thus, a basic conflict exists between the processing and curing stages. The balance is delicate because the compounder, again for best economics, wants the product to cure rapidly after it is processed. Modern curing systems provide a scorch delay time, during which heating produces no crosslinks, followed by a period of rapid crosslink formation. The compounder then adjusts process conditions to provide enough heat history to eliminate most of the scorch delay so that the cure time will be minimized. Since the compounder is balancing processing against cure, subtle and sometimes uncontrollable changes in factory equipment or operating parameters can produce unprocessable (scorched) rubber stocks. Such changes can be compensated for by making adjustments in the curing system or in the processing conditions. However, relationships between curing system and properties are complex, and considerable effort is generally required to develop a new formulation. Thus, the need exists for an additive which increases only the scorch delay time with minimal effects on other cure parameters and vulcanizate properties. Such compounds, called prevulcanization inhibitors, scorch inhibitors, or retarders, are in commercial use. A number of papers and patents describing the chemistry and use of these compounds have been published and are reviewed here.