Organofunctional silanes hydrolysis

Figure 9.7 Organofunctional silane hydrolysis, condensation and covalent bonding with inorganic substrate e.g. glass fibre).

Abstract—A review of the literature is presented for the hydrolysis of alkoxysilane esters and for the condensation of silanols in solution or with surfaces. Studies using mono-, di-, and trifunctional silane esters and silanols with different alkyl substituents are used to discuss the steric and electronic effects of alkyl substitution on the reaction rates and kinetics. The influences of acids, bases, pH, solvent, and temperature on the reaction kinetics are examined. Using these rate data. Taft equations and Brensied plots are constructed and then used to discuss the mechanisms for acid and base-catalyzed hydrolysis of silane esters and condensation of silanols. Practical implications for using organofunctional silane esters and silanols in industrial applications are presented. 

In this review, we will briefly discuss selected literature on hydrolysis and condensation reactions at silicon which are relevant to organofunctional silanes. Some studies will involve simpler (lower functionality) models some may involve related chemical reaction classes illustrating a mechanistic point. We will then discuss selected references focused on silanes of the types used as coupling agents and crosslinkers. Finally, some reactions at the interface will be mentioned. 

One approach to this problem has been to characterize the practical consequences of silane hydrolysis. Visual observation of the hydrolysis behavior of typical organofunctional silanes, supplemented by some spectroscopic data, and trapping of silanols with trimethylsilanol were reported by Plueddemann [ 1, 14], Comparative data give some measures of the ease of hydrolysis and the solution stability. The data are quite helpful in the practical use of hydrolyzed silane solutions. They are not presented in a way that allows quantitative kinetic conclusions. 

A critical concern in the use of organofunctional silanes is the effects that the organofunctional groups and the silane ester groups have on the hydrolysis reactions occurring at silicon. 

Keywords organofunctional silanes, methacryloyloxyalkyl silanes, hydrolysis, kinetics, NMR spectroscopy, copolymerization, coatings, scratch resistance... 

Silane coupling agents have the structure R—Si—X3, where R represents an organofunctional group and X is generally alkoxy. They are generally applied from water solutions or from organic solvent-water mixtures. In the presence of water, hydrolysis occurs step-wise to form alkoxysilanols, and eventually silane triols. Condensation of the silanols to siloxanes also occurs. Reactions shown schematically below ... 

Osterholtz ED, Pohl ER. Kinetics of the hydrolysis and condensation of organofunctional alkoxysilanes a review. In Mittal KL, editor. Silanes and other coupling agents. The Netherlands VSP Utrecht 1992. p. 119-41. 

---