Investigations into the silica/silane reaction system

Silica in combination with organosilanes (e.g. [bis(3-triethoxysilylpropyl)tetrasulfane] = TESPT) has recently become more important in tire applications. Their use in tire treads leads to an improvement in rolling resistance and wet traction. The requirements for the attainment of these properties are, that the triethoxysilyl groups of TESPT react with the silanol groups on the silica surface during compounding, and the polymer active groups react with the polymer during cure. The reaction of precipitated silica with this silane was investigated. The influence of various parameters on the reaction type and the reaction kinetics was considered. The results of the investigation obtained using Si-29-CP/MAS solid state NMR spectroscopy agree well with a horizontal reaction model in which a single siloxane bond is first formed with the silica surface (primary reaction). It is followed by condensation reactions between silanol groups of silane molecules which are already bound to the silica surface (secondary reaction). The kinetic studies data show a clear difference between the fast primary reaction and the slow secondary reaction. Both reactions become more rapid in acidic and alkaline pH ranges. The primary reaction accelerates up to a particular H2O content after which the rate remains constant. The secondary reaction keeps on accelerating with rising H2O content. Modification with different silane concentrations showed a higher rate constant at low concentrations.