Silanization Efficiency of Silica/Silane in Dependence of Amines in Natural Rubber-based Tire Compounds

Silica-silane technology for low rolling resistance tire compounds requires efficient bridging between the silica surface and rubber molecules through silanization and coupling reactions. The presence of diphenylguanidine (DPG) as secondary vulcanization accelerator is also needed to catalyze the silanization reaction between the alkoxy groups of silane coupling agents and the silanol groups on the silica surface. However, DPG can liberate toxic aniline under high mixing temperatures and therefore safer alternatives are required. This study investigates the influence of amines with different structures, i.e. hexylamine (HEX), octadecylamine (OCT), cyclohexylamine (CYC) and dicyclohexylamine (DIC) on the primary silanization reaction rate constant in a model system, and on interfacial compatibility of practical silica-reinforced NR compounds. Compared to the system without, the amines clearly increase the reaction rate constant for which linear aliphatic amines work better than cyclic ones. This is due to better accessibility of the amines towards the silica surface, in agreement with the values of Payne effect as observed in the rubber compounds, except for the OCT case. The lowest Payne effect of the OCT-containing rubber compound is attributed to the additional shielding effect obtained from the long alkyl-chain that leads to more hydrophobicity, resulting in good physical interaction between silica and rubber. The presence of all amines improves the cure properties in which the linear aliphatic amines give shorter cure times than the cyclic aliphatic ones. As a result of good interfacial compatibility, the OCT-containing compound which shows lowest filler-filler interaction gives good mechanical properties that are closest to the reference compound with DPG.