Ultrasound devulcanization of silica filled silicone rubber: Processing, properties and molecular aspects

Continuous devulcanization of precipitated and fumed silica filled silicone rubber (PDMS) is carried out using a single screw extruder based ultrasound co-axial reactor. An increase of the reactor throughput is achieved during the devulcanization in the presence of water. Process characteristics including ultrasound power consumption and die entrance pressure are measured. Structural characteristics and mechanical properties of virgin vulcanizates, devulcanized and revulcanized PDMS are investigated indicating the process is suitable for devulcanization of peroxide cured industrial rubbers. Unique correlation between gel fraction and crosslink density is obtained for PDMS devulcanized at various processing conditions. The existence of strong interaction between precipitated silica and PDMS is detected via rheological measurements. These investigations are supplemented by proton NMR studies with measurements of T₂ relaxation time and diffusivity. It is found that the devulcanization process leads to formation of sol with high molecular weight and branching. In addition, ultrasound treatment of unfilled cured PDMS disks under static conditions is carried out to show the phenomenon of bubble formation (cavitation) in solid state.