Tannin-boron complex as a preservative for 3-ply beech plywoods designed for humid conditions

Abstract Boric acid (BA) effectively controls wood-rotting fungi and termite attack but is readily leached out in outdoor exposure. A newly developed tannin-boron (TB) complex has been investigated to reduce BA leaching and to preserve beech plywood from biological attack. Wood veneers were impregnated with different TB solutions and then bonded with a melamine urea formaldehyde (MUF) adhesive for bond class 2 applications. The formation of a solid hydrophobic TB network on the veneer surfaces partially decreased the tensile shear strength of plywood but the prepared panels met the requirements for bond class 2. The plywood samples made from veneers treated with only BA solutions showed fair resistance against white rot fungus and termite attack, but the durability decreased even after mild leaching tests. The panels made of treated veneers with TB formulations showed instead a high resistance against biological attack after mild leaching and to some extent after severe leaching tests. This depended on the concentration of tannin and BA in the solutions. Favorable results were obtained with 20% tannin in the formulation. The results of the termites feeding choice test were more positive than those obtained by a forced feeding test. The control samples were attacked faster and showed higher weight loss when alternative samples included TB formulations.