A Ripening Effect on Processability and Physical Properties for Silica Filled Unvulcanized EPDM

Silica filled ethylene propylene diene monomer rubber (EPDM) with bis (3-triethoxysilylpropyl) tetrasulfide (TESPT) were mixed in an internal mixer ripened at 23 ± 2℃ and 50 ± 5% RH for several hours. The reaction of TESPT with silanol groups on the silica surface during ripening was pursued on the basis of 29Si-CP/MAS solid state NMR spectroscopy in conjunction with the amount of ethanol released from unvulcanized rubber. The dispersion of silica was evaluated by a nonlinear effect at a small strain, called Payne effect, through dynamic mechanical analysis. The silica dispersion in the compound mixed at a lower temperature was improved during ripening, in which the reaction proceeded. On the other hand, for the mixture prepared at a higher temperature, unreacted silica particles were aggregated upon ripening. Mooney viscosity (ML1+4) of the unvulcanized compound containing TESPT increased during ripening, regardless of the mixing temperature. The vulcanized rubber prepared after ripening exhibited a better dispersion of silica, leading to a decrease in hardness and an increase in reinforcement. The physical properties would be better with a higher mixing temperature and a longer ripening time.