Modeling and Optimization of Polyphenol Extraction for Tannin-Formaldehyde Resin Synthesis

The properties of plywood depend on many factors, including the quality of resin applied, and the curing or pressing conditions. The bark extracts of various commercially important trees containing polyphenolics in the form of tannins can be condensed with formaldehyde to produce wood resins. In the present work, aqueous ethanol was used as solvent to extract formaldehyde-condensable polyphenolics from Pinus caribaea Morelet bark. The contents of the co-extracted tannin and sugars were determined as well as the Stiasny number. Applying response surface methodology, a four factor, five level central composite rotatable design (CCRD) was employed to examine the optimum conditions of extraction variables for the pine bark polyphenolics. To study the effect of extraction optimization conditions on the resin quality, resins were processed from tannins extracted at the following response parameters; i) only tannin content was optimized, ii) only sugar content was optimized, iii) only Stiasny number was optimized and iv) tannin content, sugar content and Stiasny number were optimized simultaneously. The resins were characterized by viscosity, pot life and plywood bond strength. Each experiment was done in triplicate. Industrial grade phenol-formaldehyde adhesive was used as the control. The optimization models developed were found to adequately represent the extraction of polyphenolic compounds from P. caribaea bark. The quality of resin prepared from the extracted polyphenolics was found to be dependent on the extraction process parameters employed. The extraction process that gave a high tannin yield (18.85%) with a corresponding good quality resin (shear strength = 2.26 MPa, 11.4% delamination) similar to that of the control (industrial phenol-formaldehyde resin) was found for the optimization model when the objective function was to maximize both the tannin yield and Stiasny number and minimize the sugar content simultaneously. This corresponded to optimum extraction conditions of 52.2°C extraction temperature, 150 min extraction time, 43.6% solvent concentration and 10:1 liquid-solid ratio.