Rheology of polyflavonoid tannin–Formaldehyde reactions before and after gelling. I. Methods

Abstract

ABSTRACT: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 gel 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 ϱ of the zero-frequency viscosity, and by determining the point at which the value of tan ␦ 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 conversion 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 diffusioncontrolled 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.

Study of miscibility of melamine-formaldehyde resin and poly(vinyl acetate) blends for use as adhesives in engineered flooring

“…The small peaks in the region 2700-2800 cm −1 are associated with the CH stretching of the MF resin and PVAc, the methylene ( CH 2 ) and dimethylene ether ( CH 2 OCH 2 ) bridges of the MF resin, and the methylene unit ( CH 2 ) of the reaction with formaldehyde. It has been demonstrated that a large number of methylene ether bridges ( CH 2 OCH 2 ) occur, which rearrange themselves with relative ease to form methylene ( CH 2 ) bridges with liberation of formaldehyde [17]. However, the N H wagging peak in the 700-750 cm −1 range could not be recognized and the N H stretch peak in the 3280-3320 cm −1 range overlapped with the OH peak in the range 3000-3700 cm −1 .…”

Tannins: Major Sources, Properties and Applications

“…(3) In the case of some tannins ( e.g . quebracho tannin) a third important reaction occurs, namely the simultaneous hydrolysis of some interfl avonoid bonds, hence a depolymerization reaction, partly counteracting and thus slowing down the hardening process [12,42,43] .…”

“…In recent years, the importance of the marked colloidal nature of tannin extract solutions has come to the fore [42,43,[52][53][54][55][56][57][58][59][60] . It is the presence of both polymeric carbohydrates in the extract, as well as of the higher molecular fraction of the polyphenolic tannins, which determines the colloidal state of tannin extract solutions in water [42,52] .…”

Aningre ( Aningeria spp.) tannin extract characterization and performance as an adhesive resin

“…Thermomechanical analysis (TMA) has aready been used in resin characterization. In 2002, Garnier and Pizzi (2002) studied the rheology of polyflavonoid tannin using TMA, concluding that TMA takes into account the polymers interactions with the lignocellulose substrate and links experimental data with panels manufacturing. The gelling and hardening reactions of different commercial polyflavonoid tannins (quebracho, mimosa, pine, pecan) extract of prorobitenedin/profisetinidin, procyanidin and prodelphinidin types with formaldehyde were also studied using parallel plates rheomety.…”