Effect of Vulcanization and CO Plasticization on Cell Morphology of Silicone Rubber in Temperature Rise Foaming Process.

Both vulcanization reaction and CO plasticization play key roles in the temperature rise foaming process of silicone rubber. The chosen methyl-vinyl silicone rubber system with a pre-vulcanization degree of 36% had proper crosslinked networks, which not only could ensure enough polymer matrix strength to avoid bubble rupture but also had enough dissolved CO content in silicone rubber for induced bubble nucleation. The CO diffusion and further vulcanization reaction occur simultaneously in the CO plasticized polymer during bubble nucleation and growth. The dissolved CO in the pre-vulcanized silicone rubber caused a temperature delay to start while accelerating further vulcanization reactions, but the lower viscoelasticity caused by either CO plasticization or fewer crosslinking networks was still the dominating factor for larger cell formation. There was a sudden increase in elastic modulus and complex viscosity for pre-vulcanized silicone rubbers at higher temperature because of the occurrence of further vulcanization, but CO plasticization reduced the scope of change of rheological properties, and the loss factor was close to 1 around 170 °C, which is corresponding to the optimum foaming temperature. The foamed silicone rubber had a higher cell density and smaller cell size at a higher temperature rising rate, which is due to higher CO supersaturation and faster vulcanization reaction. These results provide some insight into the coupling mode and effect of CO plasticization and vulcanization for regulating cell structure in foaming silicone rubber process.