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.
The feasibility of the use of epoxidized palm oil (EPO) and amine-modified epoxidized palm oil (mEPO) as process oils in silica-reinforced natural rubber compounds is studied. The chemical structures of EPO and mEPO are characterized by Fourier transform infrared spectroscopy and proton nuclear magnetic resonance spectroscopy (1H-NMR). Amine modification for 3 and 5 h leads to mEPOs with 0.03 and 0.04 mmol of amine in 1 g of oil, referred to as 0.03 mEPO and 0.04 mEPO, respectively. The properties of rubber compounds containing modified palm oils are investigated by taking those with TDAE oil and those without oil as references. The use of process oils clearly enhances the processibility (i.e., lower mixing torque and complex viscosity) and mechanical and dynamic mechanical properties of the rubber compounds as compared with compounds without oil. The rubber compounds with EPO and 0.03 mEPO show a lower Payne effect (i.e., less filler–filler interaction) than the rubber compound with TDAE because of the shielding effect of the oils on the silica surface. The use of mEPO boosts the vulcanization reaction, resulting in much better cure torque difference, which indicates a higher crosslink density due to the amino groups present in mEPO as compared with TDAE. Therefore, rubber compounds with mEPOs have better mechanical properties (i.e., reinforcement index, tensile strength, and elongation at break) and better elastic response under dynamic deformation, as indicated by a lower loss tangent at 60 °C as compared with the mix with TDAE.
Diphenyl guanidine (DPG) is the most commonly used secondary accelerator in silica-reinforced rubber compounds because of its additional positive effect on the silanization reaction and deactivation of free silanol groups that are left over after the silanization. However, because of health and safety concerns about the use of DPG, which decomposes to give highly toxic aniline during high processing temperature, safe alternatives are required. This work investigates the effect of various types of aliphatic amines having alkyl or cyclic structures and similar pKa (i.e., hexylamine [HEX], decylamine [DEC], octadecylamine [OCT], cyclohexylamine [CYC], dicyclohexylamine [DIC], and quinuclidine [QUI]) on the properties of silica-reinforced natural rubber (NR) compounds by taking the ones with DPG and without amine as references. When compared with the compound without amine, the use of all amine types reduces filler–filler interaction (i.e., the Payne effect) and enhances filler–rubber interaction, as indicated by bound rubber content and decreased heat capacity increment. The amines with alkyl chains can reduce the Payne effect and enhance cure rate to a greater extent compared with the amines with cyclic rings as a result of better accessibility toward the silica surface and a shielding effect because of less steric hindrance. The longer carbon tails on linear aliphatic amines ranging from HEX, DEC, to OCT lead to a lower Payne effect, lower heat capacity increment, higher bound rubber content, and higher modulus as well as tensile strength. Overall, the use of OCT provides silica-reinforced NR compounds with properties closest to the reference one with DPG and can act as a potential alternative for DPG.
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