. As the CLD decreased, the compression set went up except for Compounds 12–14 and 17–19. Note however that the silica-filled rubber vulcanizates had much higher crosslink densities (Compounds 31–40; Table III). This was because TESPT is a sulfur-bearing silane and was used as a coupling agent. The silica particles reacted with the rubber chains via the tetrasulfane groups of TESPT and produced additional crosslinks to those monosulfidic ones formed by elemental sulfur