Rheology of polyflavonoid tannin-formaldehyde reactions before and after gelling. I. Methods

The gelling and hardening reactions of different, commercial polyflavonoid tannin extracts of the prorobitenedin/profisetinidin, procyanidin, and prodelphinidin types with formaldehyde were studied by parallel-plate rheometry. To do this, methods to determine the rheological characteristics of tannin-formaldehyde polycondensation reactions both before and after the get point were developed. The validity of some modifications of known methods was checked on the best-known commercial tannin extract, namely, the reaction of the mimosa tannin extract with 5% paraformaldehyde, over a range of different temperatures. The gel point was determined by three different methods, namely, by the crossover point of viscous and elastic moduli, by the extrapolation to infinity of the zero-frequency viscosity, and by determining the point at which the value of tan delta is constant whatever frequency is used in the measurement. The results obtained were critically compared. The comparisons obtained were good and were reproducible when the first two methods were used, but not when the third one was used. This allowed the determination of the energy of activation at different stages in the tannin-aldehyde polycondensation reaction and the calculation of the degree of con-version of the tannin-aldehyde polycondensation before, at, and after the gel point. Mathematical expressions defining the degree of conversion for tannin-aldehyde reactions before, at, and after the gel point as a function of different moduli were developed and checked with the experimental data obtained. Methods and mathematical expressions for the determination of the rate constants of the different stages of the reaction were also developed and checked. After the gel point, the system was modeled using both a diffusion controlled model and a second-order kinetic law. The applied results obtained indicated that, although a certain element of diffusion control is present, a second order, diffusion-independent kinetic is more valid under the experimental conditions used. This indicates that the rheometry approach to a polycondensation network is limited mostly by the capability of the equipment to the region after the gel point, where diffusion control does not as yet play a predominant role. (C) 2002 Wiley Periodicals, Inc.