Understanding decay resistance, dimensional stability and strength changes in heat treated and acetylated wood

Abstract

Reductions in hygroscopicity, increased dimensional stability and decay resistance of heat treated wood depends on decomposition of a large portion of the hemicelluloses in the wood cell wall. In theory, these hemicelluloses are converted to water and volatile furan-type intermediates that can polymerize in the cell wall. Reductions in hygroscopicity and improved dimensional stability of acetylated wood depends on esterification of the accessible hemicelluloses in the cell wall reducing hydrogen bonding with water and bulking the cell wall back to its green volume. Stability is not 100% since the water molecule is smaller than the acetyl group so water can access hydroxyl sites even when the wood is fully acetylated. The cell wall moisture content is too low in acetylated wood to support fungal attack so the initial enzymatic attack starting the colonization does not take place. The critical sugar in the hemicelluloses that may be the trigger for fungal attack is arabinose as it is the only sugar in a less stable five-member ring as compared to sugars in a stable six-membered ring. Strength properties are reduced in heat treated wood as a result of the degradation of the cell wall matrix resulting from the hemicellulose loss. Strength properties are not significantly changed in acetylated wood and acetylation results in greatly improved wet strength and wet stiffness properties.

Citation

Rowell, Roger M.; Ibach, Rebecca E.; McSweeny, James; Nilsson, Thomas. 2009. Understanding decay resistance, dimensional stability and strength changes in heat treated and acetylated wood. In: 4th European Conference on Wood Modification. 2009 April 27-29. Stockholm, Sweden. 489-502.