Kan sangkhro tannin-formaldehyde chak pluak mai-kongkang bai lek.

Investigations were made to study the synthesis of tannin-formaldehyde resins from tannin extracts of Rhizophora apiculata Bl. bark for use as plywood adhesives. The first experiment was conducted to find the proper ways of tannin extraction from bark with three solvents i.e., water, ethanol, and acetone-water. The resultant tannin-formaldehyde precipitate in percent based on dry weight of bark were 27.45, 26.62, and 28.83, respectively, and tannin-formaldehyde precipitate in percent based on dry weight of bark extract were 83.33, 83.82, and 86.01, respectively. Water was chosen to be used as a solvent for all extraction in further experiments. The average pH and viscosity of bark extract with water in 45% solution at room temperature of about 28 deg Centigrade were 3.8 and 55 poise, respectively. Its minimum gelation rate of the reaction between tannin and formaldehyde occurred at about pH 3-4. The statistical analysis of the shear strength obtained from three-ply test panels of Shorea cutisii Dyer ex King. revealed that paraformaldehyde content, filler content, and pH level in the glu-mixture had significant influences on glu bond quality in descending order of importance, respectively. The more paraformaldehyde content was added in glue mix, the higher strength was improved. The shear strength values increased with an increasing filler content of glue mix from 2 to 8% and decreased at higher content. It was also observed that shear strength decreased with increasing the pH level from 4.5 of glue mix. The result of this investigation showed that tannin-formaldehyde resin as plywood adhesives did not pass the requirement of the standard of commercial adhesives for interior and exterior plywood (PHILSA, Revised, 1975). When fortified with the synthetic phenol-formaldehyde resin, the bond quality especially the water resistance of the adhesive and wood failure of plywood was greater than those of unfortified tannin-formaldehyde adhesive. They were found to pass to Interior Plywood Commercial Standard (PHILSA, Revised, 1975).