Storage Stability of Solvent-Free Condensates of Functionalized Trialkoxysilanes
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ABSTRACT The lack of storage stability of organic-inorganic solvent-free sols is a well-known problem. We investigated the influence of different solvents in the acid catalyzed hydrolysis of various sols on their storage stability. Our experiments were based on two different silanes, 3-methacryloxypropyltrimethoxysilane (MPTS) and 1,1,1-tris(allyloxymethyl)-l-[[[3-(triethoxysilyl)propyl]aminocarbonyl]oxymethyl]methane (silane 31), synthesized by reaction of pentaerythritol triallyl ether and 3-isocyanatopropyltriethoxysilane. The employed solvents were alcohol, tetrahydofuran and tert-butyl methyl ether in a range of 95 wt% to 30 wt% in solution. In order to get comparable chemical conditions the degree of condensation of the various sols were adjusted between 75% and 85%. Furthermore, the sols were modified by silylation with trimethylchlorosilane. A comparison of silylated and not silylated silane 31 clearly showed that a decrease in residual silanol groups increase the storage stability. The results of the investigations demonstrated that both the kind and the amount of solvent used in hydrolysis affects the initial viscosity of solvent-free condensates of silane 31 and MPTS.
INTRODUCTION During the last decades new hybrid inorganic-organic materials offering a wide range of applications in mechanical and chemical protection [1,2] and in dental fields [3-5] were prepared by the sol-gel process. The incorporation of organic groups into the inorganic network leads to an increase in structural flexibility by reduction of the degree of crosslinking and provides new functional properties. The precursors usually used are
organically modified silicon alkoxides R'nSi(OR) 4 -, (n=1 or 2) where R' is an organic group which behaves as a network modifier like alkyl or aryl groups [6] or as a network former like vinyl or methacryl groups [7]. Among the various compounds, trifunctional alkoxysilanes R'Si(OR) 3 are the most common precursors used to introduce organic groups within an inorganic network [8,9]. Although organically modified trialkoxysilanes are often used precursors in sol-gel process there are hardly any literature concerning the storage stability of these silanes. Especially the lack of storage stability of organic-inorganic solvent-free sols is a wellknown problem. Nevertheless only a few studies have been reported on the influence of the nature of solvent, the amount of solvent and the amount of water used in hydrolysis and condensation of tetraalkoxysilanes [10-12]. Silylation of residual silanol groups on silica surfaces by reacting the surface with trimethyl chlorosilanes is a well-known method to obtain organophilic and hydrophobic surfaces, respectively [13]. In the present paper the influence of silylation of residual silanol groups and the influence of the amount and kind of solvent used for hydrolysis on the storage stability of organically modified trialkoxysilanes is described. For our investigations we used two different silanes. On the one hand 1,1,1-tris(allyloxymethyl)-l-[[[3-(triethoxysilyl)p
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