This project represents a collaboration between industrial researchers at PPG Industries Inc. and three professors at the University of Oklahoma. This work is funded by the National Science Foundation through the GOALI program. We are exploring a new method for compatibilizing silica filler for tires. Silica filled tires can have notable performance advantages over normal carbon black filled tires including lower heat buildup and improved resistance to chipping/chunking.(1) Silica filled tires can also have a lower rolling resistance without losing wet pavement adhesion, leading to better fuel economy without any change in braking distance.(2) In spite of these performance advantages, carbon black is still the particulate filler of choice due to cost. Further, the reinforcing effect of unmodified silica in hydrocarbon elastomers is not comparable primarily due to the poor adhesion of the polymer to the silica. To make the adhesion of silica to rubber comparable to the adhesion of carbon black to rubber, an expensive organosilane coupling agent is normally necessary.
In this project an alternative technology, admicellar polymerization, is used to improve silica/rubber adhesion. In admicellar polymerization,(3)(4)(5) a monomer is polymerized in the hydrophobic region of a surfactant bilayer adsorbed on a surface. We have discovered that a styrene-diene copolymer polymerized on a precipitated silica improves the properties of a rubber filled with the modified silica relative to the rubber filled with the unmodified silica. If this process can be successfully commercialized, the new silica product would be significantly cheaper than any silane-coupled product currently available on the market. This product would represent a significant competitive advantage for PPG Industries Inc., an American corporation, in the very competitive worldwide automobile tire market. It would also represent a significant benefit for the American consumer; we could buy tires that last longer and improve gas mileage thereby reducing the number of waste tires and the emission of greenhouse gases.
Currently, our efforts are directed towards understanding the kinetics of admicellar polymerization and the effect of copolymer composition on reinforcement efficiency. A screening study of different reaction variables has been completed. The purpose of this study was to evaluate effects on pore structure and also evaluate the resulting silica as a rubber reinforcer. We tested four independent variables: initiator type, monomer level, surfactant level and diene copolymer identity. In addition, three different silicas were tested. Due to space limitations, all of the major conclusions of this study are not discussed here. The most significant result is that there is an optimal amount of polymer that should be reacted on the surface for maximum property improvement, i.e. more admicellar polymer can actually degrade reinforcing behavior. Our efforts are currently concentrated on understanding the characteristics of the polymer that is formed via admicellar polymerization i.e. what are the molecular weight and branching level.
For a related project that uses admicellar polymerization to improve interfacial adhesion, see the description of structural composites.
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