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Diphenyl guanidine (DPG) is an essential ingredient in silica-reinforced rubber compounds for low rolling resistance tires, as it not only acts as a secondary accelerator, but also as a catalyst for the silanization reaction. However, because of concern over the toxicity of DPG that liberates aniline during high-temperature processing, safe alternatives are required. The present work studies several amines as potential alternatives for DPG. Different amines (i.e., hexylamine, decylamine, octadecylamine, cyclohexylamine, dicyclohexylamine, and quinuclidine) are investigated in a model system, as well as in a practical rubber compound by taking the ones with DPG and without amine as references. The kinetics of the silanization reaction of the silica/silane mixtures are evaluated using model compounds. The mixtures with amines show up to 3.7 times higher rate constants of the primary silanization reaction compared to the compound without amine. Linear aliphatic amines promote the rate constant of the primary silanization reaction to a greater extent compared to amines with a cyclic structure. The amines with short-alkyl chains that provide better accessibility towards the silica surface, enhance the primary silanization reaction more than the ones with long-alkyl chains. The different amines have no significant influence on the rate constant of the secondary silanization reaction. The amine types that give a higher primary silanization reaction rate constant show a lower flocculation rate in the practical compounds. For the systems with a bit lower primary silanization reaction rate, but higher extent of shielding or physical adsorption that still promotes higher interfacial compatibility between the elastomer and the filler surface, the rubber compounds show a lower Payne effect which would indicate lower filler-filler interaction. However, the flocculation rate constant remained high.

Enhancing the Silanization Reaction of the Silica-Silane System by Different Amines in Model and Practical Silica-Filled Natural Rubber Compounds