Chlorobutyl Rubber—Optimum Processing Procedures

Abstract

Chlorinated butyl rubber cures faster, forms more stable crosslinks, and provides wider compounding versatility than conventional butyl rubbers. Because of chlorobutyl's increased reactivity MgO is added to control scorch. Unfortunately, MgO also retards development of stress-strain properties in cured vulcanizates. While these effects can be explained on the basis of an interference with the vulcanization process, data indicate that MgO retards polymer-pigment interaction as well. Heat treating has been shown to reduce the retarding influence of MgO without sacrificing processing safety. Cure systems respond differently to heat treatment, some showing little or unfavorable effects. Heat treating is effective only with vulcanization systems that crosslink through unsaturation rather than through bis-alkylation reactions involving chlorine. A mechanism is proposed which explains these phenomena in terms of a competition for available chlorine between polymer-polymer crosslinking and polymer-carbon black interaction which together contribute to the final crosslinked network. Processing safety is maintained because of a reaction between MgO and a small fraction of tertiary-allylic chlorine which is believed to contribute to scorch. In processing chlorobutyl compounds, therefore, whenever MgO is used to control scorch safety, heat treatment is desirable in order to reduce the retarding influence on vulcanization and reinforcement. Heat treating is effective, however, only in compounds cured with vulcanization systems capable of crosslinking through unsaturation. Since HCl may be a byproduct of thermal reactions, heat treatment should not be carried at temperatures above 330° F unless a suitable acid scavenger such as MgO, PbO, or CaSiO₃ is employed. When such materials are present in the compound, temperatures up to 400° F can be tolerated. Optimum processing procedures for chlorobutyl compounds will depend on the requirements of a given application, the cure system required, and the limitations of available processing equipment.