Radiation-Induced Crosslinking of Chlorobutyl and Polydiene Elastomers. Promotion by Polythiols

Abstract

Abstract The response of Chlorobutyl (CIIR), an elastomer with 1,1-disubstituted carbon atoms along the polymer chain, to ionizing radiation has been directed toward a predominantly crosslinking reaction by addition of 1–4 phr of thioetherpolythiols (TEPT) prepared by reaction of trivinylcyclohexane or cyclododecatriene with H2S or low-molecular-weight dithiols. From a fundamental viewpoint, the unique feature of the CIIR-TEPT system is the reversal of expected behavior for a polymer molecule containing 1,1-disubstituted carbon atoms when exposed to high energy radiation: instead of chain scission, cross-linking is the predominant physical change to an extent comparable to that obtained from thermal curing processes. The TEPT becomes part of the radiation-cured network, for, as the radiation dose is increased, there is progressive addition of thiol sulfur to the crosslinked network. Concomitantly, there is a progressive reduction in the chlorine content as radiation-induced vulcanization proceeds. These polythiols are also powerful accelerators of radiation-induced cross-linking of polydiene elastomers, especially those with appreciable amounts of pendent vinyl unsaturation, such as conventional SBR. Evidence exists that a blend of CIIR and SBR can be covulcanized by radiation-induced curing in the presence of TEPT. Experiments were conducted with gamma radiation from a 60Co source and with a 1.4 MeV electron beam at dose rates of 20–120 kJ/kg (2–12 megarads). Physical properties of fully compounded polythiol-promoted CIIR systems crosslinked by radiation are lower than comparable thermally cured compounds, but the strengths would be adequate for many applications. Compounding ingredients, especially common accelerators of sulfur vulcanization, provide a wide range of inhibitory action toward radiation precure. However, with selected types, especially organometallic derivatives which are less soluble in the compound, radiation cure can proceed with a minimum of inhibition.