シリカ/シランガップリング剤の反応機構解析

抄録

Silica in combination with bifunctional organosilanes (e.g. [Bis(3-triethoxysilylpropyl) tetrasulfane] =TESPT) have recently become more important in the tire application. Their use in tire treads as a replacement of carbon black leads to an improvement in rolling resistance and wet traction without loss in abrasion resistance. The requirements for the attainment of this combination of 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 (silane concentration, pH, moisture content) on the reaction type, degree of reaction and the reaction kinetics were considered. The results of the investigation obtained using ²⁹Si-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), either directly or after (partial) hydrolysis of the silane. After having given off further ethanol, it is followed by condensation reactions between silanol groups of silane molecules which are already bound to the silica surface (secondary reaction).
The data of the kinetic studies show a clear difference between the quick primary reaction and the slow secondary reaction. The energy of activation according to Arrhenius was determined for both reactions and was found to be much lower for the secondary reaction. Both reactions become quicker in the acidic and alkaline ranges respectively.
The primary reaction accelerates up to a particular H₂O content after which the rate remains constant. The secondary reaction on the other hand keeps on accelerating with rising H₂O content.
Modification with different silane concentrations showed a noticeably higher rate constant at low concentrations and a greater amount of ethanol formed per mol of silane.