Preservation of plywood against biological attack with low environmental impact using tannin-boron preservative

An experimental investigation was conducted to protect plywood panels made of perishable wood species (beech and poplar) against biological attacks with low environmental impact. The newly developed tannin-boron preservative which was proposed to reduce the leaching of borates was selected for this purpose. This system was used to protect plywood with two approaches: (1) in the glue line to replace formaldehyde based adhesives and gluing wood veneers, (2) pretreatment of wood veneers with the diluted formulations and bonded with melamine-urea-formaldehyde resin The research data in this thesis were obtained from three main groups of experiments: chemicals and thermomechanical testing on the tannin-boron resin, measuring physical and mechanical properties, and different biological tests before and after leaching processes. The chemicals testing on the tannin/hexamine adhesive showed that the addition of boron, in the form of boric acid, can contribute more inter-flavonoid linkages and the opening of pyran rings which leads to the acceleration of polymerization reactions. Thermomechanical analysis on the adhesives of mimosa and quebracheo tannins confirmed that the addition of boric acid (1) lowered time and temperature of hardening, (2) and increased modulus of elasticity values of the adhesive. The addition of optimal boric acid into the tannin adhesive upgraded physical properties and tensile shear strength. An excess loading of boric acid (10%) in the glue line caused imperfect physical and mechanical features. Despite the low uptake of boric acid but its addition into the tannin glue provided effective resistance against fungal attack even after mild leaching test according to the EN 1250-2. The results of termites test showed that increase in the boric acid content of the adhesive caused the lower survival rate and the weight loss but samples underwent the high level of attack. No improvement against termite attack was obtained after leaching by a choice feeding test. The plywoods made of pretreated veneers by tannin-boron solutions showed partially lower tensile shear strength but still in the range of standards requirements for the humid applications. The satisfactory results were obtained even after severe leaching test against fungal attack with a strong polymeric network of tannin. The control plywoods made of pretreated veneers with boric acid alone solutions presented significant sensitivity against biological attack after leaching test. The results of termite tests showed the lower survival rate and the weight loss even after severe leaching test for the plywoods made of pretreated veneers with tannin-boron systems. The results of choice feeding test showed that termites preferred feeding from the controls when alternative samples were treated with the tannin-boron system. In general the evidence from this study suggests that tannin-boron system can significantly slow down the leaching of boron and it rendered high potential to preserve plywood