Influence of Silane Coupling Agent Content on Crosslink Type and Density of Silica-Filled Natural Rubber Vulcanizates

Crosslink type and degree of crosslink density of a rubber vulcanizate determine the physical properties such as modulus, hardness, resilience, elongation at break, heat build-up, and so forth. By increasing the crosslink density, the modulus, hardness, resilience, and abrasion resistance increase, whereas the elongation at break, heat build-up, and stress relaxation decrease. The stress relaxation, tensile strength, and resilience increase in proportion to the content of polysulfides, whereas the fatigue and thermal aging resistance decrease. Silica and carbon black have been used as the main reinforcing agents in rubber compounds but their surface chemistries are very different. Silica has a number of hydroxyl groups on the surface, which results in strong filler-filler interactions and adsorption of polar materials by hydrogen bonds. Since intermolecular hydrogen bonds between hydroxyl groups on the silica surface are very strong, it can aggregate tightly. Its property can cause a poor dispersion of silica in a rubber compound. The adsorption of curatives by silica results in delay of the scorch time and reduction of the delta torque of a silica-filled rubber compound. The polar surface of silica makes hydrogen bonds with polar materials in rubber compounds. Since the silica surface is acidic, especially it forms a strong hydrogen bond with basic materials. N-tert-Butyl-2-benzothiazole sulfenamide (TBBS) and N-cyclohexyl benzothiazole sulfenamide (CBS) are generally used as cure accelerators for rubbers. Since they have basic functional groups such as amide, they are adsorbed well on the acidic silica surface. The adsorption of curatives to silica results in delay of the scorch time and reduction of the crosslink density in silicafilled rubber compounds. In general, silane coupling agent such as bis-(3-(triethoxysilyl)-propyl)-tetrasulfide (TESPT) is used to improve the silica dispersion as well as to prevent adsorption of curatives on the silica surface. The silane coupling agent reacts with silanol on the silica surface and then a siloxane bond is formed. The silane molecule is bound to the silica surface. In the present work, we studied the influence of silane coupling agent content on the crosslink type and level of crosslink density using silica-filled natural rubber (NR) compounds with different TESP contents. Variation of the crosslink density and type after the thermal aging was also investigated. Sulfur linkages are composed of monosulfide, disulfide, and polysulfides. Polysulfides of the total sulfur crosslinks are enhanced by increasing the elemental sulfur content in a rubber compound. Cure accelerators such as TBBS, CBS, 2-mercaptobenzothiazole (MBT), and tetramethyl thiuramdisulfide (TMTD) make the cure rate of a rubber compound faster. Sulfur linkages, especially polysulfides, can be dissociated by heating and this brings about decrease of the crosslink density. Curatives, especially sulfur, remained in a rubber vulcanizate make new crosslinks and this results in increase of the crosslink density. Figure 1 shows the variations of the crosslink densities with the silane coupling agent content. The crosslink densities of the monoand disulfides, polysulfides, and total sulfur linkages were investigated. All the crosslink densities are enhanced by increasing the silane coupling agent