silica-reinforced natural rubber tire compounds with safe compounding ingredients
Chesidi Hayichelaeh is a PhD student in the research group Elastomer Technology and Engineering (ETE). His supervisor is prof.dr.ir. J.W.M. Noordermeer from the Faculty of Engineering Technology (ET).
This doctoral degree program was undertaken in cooperation with the Prince of Songkla University, Thailand.
For heavy duty truck tire tread compounds, Natural Rubber (NR) is necessarily needed due to the fact that NR provides low heat build-up and excellent mechanical properties, which derive from crystal structures upon being stretched, i.e. strain-induced crystallization. Based on the innovation of the “Green Tire” by Michelin as introduced in the 1990s, tire tread compounds reinforced with a silica/silane system show improved key tire performances, i.e. lower rolling resistance and better wet traction, while maintaining the abrasion resistance when compared to conventional carbon black-reinforced tire tread compounds. The reduction in tire rolling resistance of silica-reinforced tire tread compounds leads to low fuel consumption and less CO2 emission. In addition to reinforcing fillers, several other ingredients are used in rubber compounds depending on the role of those substances such as process oils, antioxidants, activators, accelerators and curing agents. For rubber compounds reinforced with a silica/silane system, the reinforcement efficiency strongly depends on the reaction between the silica surface and silane molecules, the so-called silanization reaction. DiPhenyl Guanidine (DPG) as secondary accelerator for rubber compounds vulcanized with a sulfur system gives additional positive effects in silica-reinforced rubber compounds, while DPG can act as a silanization catalyst and de-activate free silanol groups that are left over after the silanization reaction. However, DPG liberates toxic aniline which has been classified as a carcinogen, during compounding under high mixing temperatures. Concern over the aniline toxicity leads to a search for safe alternatives. In addition, Distillated Aromatic Extract (DAE) which is a petroleum-based process oil conventionally used to improve the processibility and filler dispersion of rubber compounds with high filler loadings, has been banned due to its composition that contains Polycyclic Aromatic Hydrocarbons (PAHs) of which some have been classified as carcinogenic materials. The replacement of DAE with safe process oils such as Treated Distillate Aromatic Extract (TDAE) for tire compounds has come into practice.
In order to enhance safe working conditions and favor the ecological environment, to comply with tightened regulations and/or new legislations for the tire industry towards more “green” products, less or non-toxic compounding ingredients and bio-based raw materials such as bio process oils shall be used. This present thesis provides an in-depth study into the application of amines as alternatives for DPG and modified palm oils as sustainable alternatives for petroleum-based TDAE oil in silica-reinforced NR compounds, for low rolling resistance tires. Both model compound systems and practical rubber compounds were applied in order to gain information on the kinetics and performance properties of the compounds. Based on the results in this research, replacement of DPG by OCT is feasible, and EPO or mEPO are potential candidates to substitute TDAE in silica-reinforced NR compounds without scarifying the properties. With optimum loading, vulcanized rubber with OCT even shows better mechanical properties and tan δ at 60°C when compared to the DPG counterpart. The utilization of mEPOs results in compounds with enhanced reinforcement index and tensile strength, and reduced loss tangent at 60°C, compared to the mix with TDAE. Therefore, from the perspective of “safe and green” tires, the application of OCT and modified bio oils as alternatives for DPG and TDAE oil, respectively, are fulfilled and bring even lower tire rolling resistance, i.e. less fuel consumption and less CO2 emission.