Enhanced auto adhesion of wood fibers using phenol oxidases

Enzymatic oxidation of lignin phenolic hydroxyl groups can enhance the level of auto adhesion between wood fibers. Previous investigations within the area have not taken the adhesive effect of proteins and carbohydrates in the enzyme solution into consideration, and part of the claimed enzyme generated adhesion may merely be a result of the adhesive effect of the proteins and carbohydrates in the enzyme solution. In this paper the enzyme laccase is used to oxidize beech (Fagus sylvatica) wood fibers. Laccase catalyzes a one-electron oxidation of phenolic hydroxyl groups while reducing oxygen, yielding phenoxy radicals and water. Wet and dry process fiberboards are made from enzyme treated and untreated beech wood fibers. The enzyme treated fiberboards have significantly higher modulus of rupture and modulus of elasticity as well as better dimensional stability. No effect of the protein and carbohydrate content in the enzyme solution upon the mechanical properties and dimensional stability can be found. The bonding effect is caused by the catalytic effect of the enzyme only. Electron spin resonance spectroscopy of beech wood fibers shows that a considerable amount of the laccase generated radicals is stabilized in the lignin polymer, and the bonding mechanism appears to be associated with reactions of these free radicals. The physico-mechanical parameters of the enzyme treated fiberboards have not been optimized in the present work. Hence, strength improvements may be obtained simply by optimizing the process parameters. The possibility of producing a medium density fiberboard without the use of synthetic adhesives appears to be promising. The use of oxidative enzymes in processing and manufacturing of wood-based materials may yield new and more environmentally safe products.