Rheology of Semi-Cured Silicone Rubber Compounds

The rheology and swelling behavior of a silicone rubber was studied at various cure states using bis (2،4-dichlorobenzoyl) peroxide as the curing agent. To this end، first an optimization was carried out on the percentage of crosslinking agent، the time and temperature of curing process. Then، samples، at various cured states، were prepared at zero (uncured sample)، gel point، 5%، 15%، 30%، 60% and 100% gel content using a moving die rheometer RPA2000. The moving die rheometer RPA2000 offered obvious priorities over the old oscillating disc rheometers and it was able to measure the behavior of rubber compounds prior، during and after cure in a single test. The results of frequency sweep tests indicated that the modulus of semi-cured samples increased with increasing the frequency and cure state. The increase in frequency was ineffective on the storage modulus of samples of higher cure states، exhibiting a plateau throughout the whole frequency range. The samples with lower percentage of the crosslinking exhibited a shoulder at low frequencies resembling the behavior commonly observed for immiscible polymer blends at frequency sweep tests. The viscosity behavior versus frequency for all semi-cured compounds obeyed the power law model with a power law index being quite far from Newtonian behavior. The crosslink density was determined using the Flory-Rhener equation. The swelling magnitude، the weight decreasing rate as a result of the extraction of the sol part and the swelling rate decreased with increasing the cure state.