The hydrolysis and polycondensation reactions involving alkoxysilane groups have been studied in bulk using model compounds prepared from an isocyanate-functionalized prepolymer and (7-aminopropyl)triethoxysilane. The molecular weight evolution and the appearance of the different condensed species are followed by ^Si NMR and size exclusion chromatography (SEC). Peak assignments of the MSi NMR experiments were determined by comparing two acid-catalyzed reactions. If hydrolysis is fast, the condensation reaction occurs at a upper rate at the first time and slower rate after. During the same reaction, the gel time is determined by dynamic mechanical measurements and by the appearance of insoluble fractions in different solvents. The scaling exponent for the variation of G' and G" with frequency at the gel point is = 0.63 ± 0.02, in agreement with the percolation theory. The extent of the reaction may be calculated from NMR and SEC results. The experimental conversion at gelation is found equal to 0.50, whereas the predicted value is 0.33. This difference can be explained by substitution effects and by intramolecular reactions.

SYNOPSISThe reactions of an epoxy prepolymer based on bisphenol A diglycidylether (DGEBA) with y-aminopropyltriethoxysilane ( y-APS) are studied. The results of different techniques are compared: size exclusion chromatography, differential scanning calorimetry, chemical titration, and Fourier Transform Infrared absorption. Epoxy amine reactions are shown to be faster than the crosslinking reactions between alkoxysilane and hydroxy groups, and thus, can be studied separately. The reactivity of the epoxy group in DGEBA is compared with that of phenylglycidylether ( P G E ) . And the reactivity of the amine group of 7-APS is compared with that of hexylamine. The kinetic constants are calculated with a mechanism which takes into account both the catalytic and noncatalytic reactions. The ratio r = k,/ kl of the reactivity of the secondary to the primary amino-hydrogens was also determined. The values of r are 1.4 for hexylamine and 1.2 for y-APS. The reactivities of the epoxy groups are the same for both PGE and DGEBA.

The system studied here principally is y-APS + BADGE ( y-aminopropyltriethoxysilane + diglycidyl ether of bisphenol A).The reaction takes place even at -20°C. It is first the epoxy-mine autocatalytic second-order reaction, but we also have a cross-linking reaction which needs more time and higher temperature. The behaviour of this system is the same as Y-APS -PGE (phenylglycidyl ether) system: we can obtain the complete disappearance of OH groups. The thermomechanical properties of the reaction product considerably change with the temperature, moisture and curing time.

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