Process for minimizing heat-promoted interaction between brominated butyl and carbon black during compounding

An improved process for reducing the heat promoted interaction of brominated butyl and carbon blacks other than channel carbon black in which the brominated butyl containing from 0.5 to 5 weight per cent of bromine and carbon black are mixed at temperatures from 135.degree.C to about 180.degree.C, characterized in that said mixing is in the presence of from 0.1 to 2 parts by weight per 100 parts by weight of brominated butyl of an interaction retarding agent selected from boron compounds having at least one oxygen atom chemically bonded to boron and selected from boron oxide, boric acid and borate esters of formula R.sub.3 O--B(OR.sub.2)--OR.sub.1, wherein R.sub.1, R.sub.2 and R.sub.3 may be the same or different and are C.sub.1 -C.sub.12 hydrocarbyl radicals.