Abstract
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.
Highlights
Natural rubber (NR) is a renewable resource that is used for several applications in which tires take up a large portion of NR consumption
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
The lowest Payne effect of the OCTcontaining 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
Summary
Natural rubber (NR) is a renewable resource that is used for several applications in which tires take up a large portion of NR consumption. NR is used in many parts of the tires, especially for truck tire tread compounds while it provides low heat build-up and excellent mechanical properties derived from its ability to form crystals upon being stretched, i.e. strain-induced crystallization. Reinforcing fillers are a crucial ingredient in tire compounds, added to improve tire performances. After the emergence of the “Green Tire” introduced by Michelin for low rolling resistance tire treads, i.e. energy saving tires based on a silica/silane reinforcement system that can reduce about 25% rolling resistance compared to conventional carbon black-filled tire tread compounds [1], the silica technology was widely implemented. The mixing of silica into rubber compounds encounters the incompatibility problem, that can be overcome by using a silane coupling agent.
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