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New silane coupling agents for NRtruck tire applications

Article Excerpt
The use of silica to reinforce elastomers is well known in the passenger car tire industry. The silica is chemically bonded to the rubber matrix using silane coupling agents. Sulfur-containing alkoxysilanes, such as 1-(trietboxy-silanyl)-3-[3-(triethoxy-silanyl)-propyltetrasulfanyl]-propane (5), often referred to as TESPT, and octanethioic acid S-[3-(triethoxy-silanyl)-propyl] ester (6), are used to covalently bond the rubber to the silica. The silanes (5) and (6) contain functional groups that react well with the rubbers typically used in passenger tire tread compound, such as styrene-butadiene rubber and butadiene rubber. Because these rubbers are chemically bonded or linked to the silica, the silica-filled elastomer is better able to transfer stress and has less hysteresis than carbon black-filled elastomers. In carbon black-filled elastomers, the rubber is physi-adsorbed onto the surface of the filler. These characteristics of the silica-filled elastomers reduce the rolling resistance and improve the wet traction of passenger car tires.

Reducing rolling resistance and extending wear life of tread compound are also of strong interest for truck tires. The tread compounds used in fabricating truck tires are based upon natural rubber. When silica replaces carbon black in natural rubber-based formulations, a reduction of rolling resistance is observed. However, these compounds have poor wear properties. Tread compounds containing silica filler and silanes (5) or (6) are unable to match the wear performance of tread compounds containing highly reinforcing carbon black fillers. The replacement of carbon black by silica in truck tread compounds has been stymied by ineffective coupling of the silica to natural rubber.

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The reasons for the ineffective coupling of the silica to natural rubber are not well understood. The interlace model for carbon black-filled elastomers (ref. 1) provides insights into possible sources of the poor coupling. The model proposes a double layer structure of bound rubber composed of an inner polymer layer, referred to as glassy hard layer, and an outer polymer layer, referred to as sticky hard layer. The double layer of bound rubber is changeable and deformable, depending on the extension magnitude, and is therefore capable of transferring stress from the matrix to the filler. Improving the coupling of the bound rubber to the silica may enhance this ability of the interphase to transfer stress to the filler without failure of the interphase, and thereby extend the wear life of the tread compound.

The source of the poor coupling may arise from the inability of silanes (5) and (6) to form covalent bonds with the natural rubber. The polysulfide silane (5) is bonded to the allylic position of the rubber compound (refs. 2 and 3) during cure. Side reactions may decrease the yield of this chemical reaction between the natural rubber and silane (5). These side reactions may be more pronounced in natural rubber-based formulations. Natural rubber has a different structure and reactivity than styrene-butadiene rubber. It contains impurities that originate from its natural biosynthesis. Different chemical reactions may therefore be required to enhance the coupling efficiency between the natural rubber and silane coupling agent.

Natural rubber is naturally produced. Its biosynthesis involves the reaction of 3-methyl-2-buten-1-ol pyrophosphate (7) to generate a transient allylic cation intermediate (8), which rapidly reacts with the electron-rich 3-methyl-3-buene-1-ol pyrophosphate (9) to form a carbon-carbon chemical bond and a second carbocation intermediate (10). The transient intermediate (10) undergoes an El elimination to generate a new carbon-carbon double bond and form geranyl pyrophosphate (11) (ref. 4). The formation of this intermediate is helped not only by a good leaving group...

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