Stable and durable wood building materials based on molecular level chemical modification

Abstract

Wood is referred to as a material but in the true material sciences definition, a material is uniform, predictable, continuous, and reproducible. No two pieces of wood are the same even if they came from the same tree and the same board. Wood is better described as a composite and, more accurately, as a porous three-dimensional, hydroscopic, viscoelastic, anisotropic bio-polymer composite composed of an interconnecting matrix of cellulose, hemicelluloses, and lignin with minor amounts of inorganic elements and organic extractives. So, even solid wood is a composite. The characteristics we deal with at the solid wood level (swelling/shrinking, biological attack, and strength) are derived from the properties at the cell wall matrix and polymer level. Moisture sorption and desorption in the cell wall polymers results in dimensional instability and changing mechanical properties. Many different types of microorganisms recognize wood as a food source and are able to break it down resulting in both weight and strength losses. One technology that has now been commercialized to achieve high levels of stability, durability, and improved wet mechanical properties is acetylation: a reaction between the hydroxyl groups in the wood cell wall polymers and acetic anhydride. While all woods contain a low level of acetyl groups, increasing this acetyl content changes the properties and, thereby, the performance of the reacted wood. When a substantial number of the accessible hydroxyl groups are acetylated consistently across the entire cell wall, the wood reaches its highest level of stability and durability.