CHEMICAL MODIFICATION OF WOOD TO PRODUCE STABLE AND DURABLE COMPOSITES

Dimensionally stable solid wood and wood composites can be achieved by several methods, including cell wall bulking, cell wall polymer crosslinking and removal of hygroscopic components in the cell wall. Bulking the cell wall with bonded chemicals produces dimensionally stable products that can withstand wet and dry cycles. Crosslinking with a short difunctional chemical, such as formaldehyde, can produce dimensionally stable products that can also be used in wet-dry cycles and the amount of chemical add-on is very small (4 to 6%), but the wood is brittle. Wood heated to temperatures where the hygroscopic hemicellulose polymers are degraded produces a dimensionally stable product, but with reduced strength. Controlling the moisture content in the cell wall is one of the keys to biological resistance. If the equilibrium moisture content in the cell wall is below that required for a microorganism to attack wood, the chemically modified wood is then resistant to attack. The modification of the configuration and conformation of the substrate polymers, so that enzyme recognition cannot take place, may also be part of the protection mechanism.