THE EFFICIENCY OF PEROXIDES FOR CURING SILICONE ELASTOMERS

Peroxides are preferred for heat curing vinylmethylsilicone (VMQ) elastomers because the free-radical-initiated crosslink does not reduce the inherent stability of the polymer. The objectives of this work were: (1) to obtain information on the relationship between peroxide concentration and the physical properties of heat-cured silicone elastomers; and (2) to explain the suspected difference in efficiency of two peroxides, alpha,alpha-di(tert-butylperoxy)-m/p-diisopropylbenzene (DBPIB) and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (DBPH). VMQ bases and gums were cured with DBPIB and DBPH over a range of peroxide concentrations. Higher tensile modulus and higher delta torque (oscillating disk rheometer data) were observed when curing with DBPIB over the entire concentration range. This indicates that DBPIB produces a higher crosslink density than DBPH at equivalent molar concentrations. A series of calculations were performed, using computational chemistry techniques, to gain insight into the reason for the observed difference in crosslinking efficiency between DBPIB and DBPH. These calculations show that DBPIB is not more efficient than DBPH in abstracting hydrogen from the methyl groups of silicone elastomers. The predominant cause for the difference in efficiency is related to the stability of the DBPH free radical. This radical is formed from hydrogen abstraction at the central ethylene moiety of DBPH by neighboring peroxy fragments. Therefore, DBPH is more susceptible to hydrogen abstraction, which consumes radicals in nonproductive (noncrosslinking) pathways. This difference in peroxide efficiency may apply to other polymers.