Polymerization of silicic acid .3. Determination of the rate of polycondensation from rheological analysis

The kinetics of the polymerization-nucleation step was studied for time values higher than the induction time from the apparent viscosity at the evolution curves of high shear rates of polysilicic acid sols. For the hypothesis that the polymerization rate obeys an approximate second order dependence on silicic acid concentration, the half-life value was used to outline the kinetic curves N = f(t) at different experimental conditions and to prove the effect of concentration, pH, temperature upon the polymerization rate constant and activation free energy of the initial polymerization process. The predominant parameter to this process was the silicic acid concentration. The values of activation free energy of polymerization have been found to be of the same order of magnitude, whatever the experimental conditions may be; these values are always lower than the ones obtained for the nucleation phase.