Alkoxysilane hydrolysis

Alkoxysilanes undergo hydrolysis, condensation (catalysts for alkoxysilane hydrolysis are usually catalysts for condensation), and a bond formation stage under base as well as under acid catalyzed mechanisms. In addition to this reaction of silanols with hydroxyls of the fiber surface, the formation of polysiloxane structures also can take place. 

It is important to note that catalysts for alkoxysilane hydrolysis are usually catalysts for condensation. In typical silane surface treatment applications, alkoxysilane reaction products are removed from equilibrium by phase separation and deposition of condensation products. The overall complexity of hydrolysis and condensation has not allowed simultaneous determination of the kinetics of silanol formation and reaction. Equilibrium data for silanol formation and condensation, until now, have not been reported. 

Alkoxysilane hydrolysis—effects of substituents. A series of hydrolysis studies [4-9] have elucidated general trends. Under basic conditions, the hydrolysis of alkoxy groups usually takes place in a stepwise manner. Carbon-bonded substituents can have profound effects on the rate of hydrolysis. With the... 

Figure 2. Numeric equivalents for the alkoxysilane hydrolysis cascade reaction.

Figure 3. Effect of pH on alkoxysilane hydrolysis, adapted from Prassas f 20].

The equilibrium reactions for alkoxysilane hydrolysis mixtures are given in equations (l)-(3). Data on equilibrium constants for these reactions have not been reported. 

A series of studies were conducted to define parameters related to reactivity, solubility, and stability of alkoxysilane hydrolysis mixtures for the purpose of generating model compounds stable in water solution and maintaining coupling agent activity. 

Equilibrium constant determination for alkoxysilane hydrolysis. Triethyl-silanol was selected as a model compound for determination of the equilibrium constant for equation (1), since under neutral conditions the condensation to disiloxane was observed to take place only over an extended period of time (i.e. years), eliminating equilibria (2) and (3) as interfering factors. 

New data have been presented in the context of a review of the aqueous behavior of silanes which elucidate their behavior, including mixed alkoxysilane hydrolysis kinetics, silane solubility, and the determination of the equilibrium constant for the alkoxy hydrolysis reaction. 

A number of schemes were developed which incorporated the conclusions of the alkoxysilane hydrolysis studies in the first part of this paper. A series of waterborne silanes were developed having high active silanol contents which are stable in water for periods of more than 6 months. Low molecular weight alcohols were not incorporated in the solutions since even at concentrations as low as 1% they contribute to flammability. 

Keywords Alkoxysilanes hydrolysis Fourier transform infrared spectroscopy. 

Keywords Alkoxysilanes hydrolysis alcoholysis siliconium coupling agents. 

The kinetics of alkoxysilane hydrolysis have not yet been resolved successfully. Reuther (56) published a preliminary report a few years ago on conductometric measurements on some alkoxysilanes, but kinetic expressions were not devised. Shaffer and Flanigen (66) also found that conductometric data obtained on hydrolysis of alkoxysilanes were difficult to interpret. 

Summary Compared with the fiequently investigated alkoxysilane hydrolysis the reverse silanol aleoholysis has been investigated at model silanols of type XMe2SiOH. The equilibrium constants are within the order of 0.1. The reaction rate of the acid-catalyzed silanol alcoholysis decreases with increasing electronegativity of the substituents X at Si, whereas the base-catalyzed reaction is accelerated. Various alcohols ROH affect the reaction rates preferably by the sterical influence of R. The acid-catalyzed silanol alcoholysis is slighly more affected by substituents X as the reverse alkoxysilane hydrolysis. 

Whereas the alkoxysilane hydrolysis is already frequently investigated, there is only little information on the reverse silanol alcoholysis. Thus, we investigated the reaction of silanols XMe2SiOH (for X see Table 1) with methanol and other alcohols with respect to equilibrium constants K and rate constants k and of the acid and base-catalyzed reaction and their substituent dependence. 

In this study we describe a meftiod used to measure the reactions that silylated latexes can undergo in coatings formulations. Si NMR is shown to reveal crosslinking by silanol condensation occurring in silylated latex synthesis and cured films. In addition, a complementary method for quantitative determination of degree of alkoxysilane hydrolysis is described. Low-temperature separation of the latex solids from the volatile components followed by gas chromatographic analysis of the distillate can provide accurate and reproducible measurement of the alcohol generated by the hydrolysis of the alkoxysilanes used in the formulation. 

Silane- and siloxanediols are important intermediates in silicone synthesis. During the technical chloro- or alkoxysilane hydrolysis the primary-formed silanols and siloxanols react preferably in the aqueous phase due to their considerable water solubility. To get more information on these reactions in water we investigated some silane- and siloxanediols using a method already described [1]. 

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