Silane coupling agents for silica-filled tire-tread compounds: the link between chemistry and performance

Silanes are commonly used as coupling agents for silica-filled compounds in the rubber industry, mainly for tire-tread compounds. The replacement of carbon black by silica results in an improvement of tire performance in terms of wet grip, abrasion resistance and rolling resistance. The most widely used silanes are bis-(triethoxysilylpropyl)tetrasulfide (TESPT) and the corresponding disulfide (TESPD). The coupling agent reacts first with the silanol groups of the silica filler, forming a hydrophobic shell around the filler particle and improving the compatibility between the filler and the rubber. Secondly, the sulfur moiety reacts with the rubber with formation of a stable network between the filler and the rubber polymer. The different steps of the formation of the filler-rubber network and their influence on the in-rubber properties have been investigated separately. The silanisation efficiency of different types and numbers of alkoxygroups, as well as the effect of the coupling of the polysulfide groups to the rubber polymer were analyzed. It was found that all functionalities have to be combined in a single molecule to provide the well-balanced property profile that is characteristic for silica compounds.