In the reaction of wood with acetic anhydride, when no catalysis or decrystalizing solvent are used, there is always hydroxyl functionality remaining after the reaction has gone to “completion” as indicated by IR spectra. This indicates that not all hydroxyl groups have been involved in the reaction. Acetylation of wood is a good reaction to study in the accessibility and reactivity of wood as one acetyl group reacts with one hydroxyl group with no polymerization. The rate controlling parameter in this reaction is the accessibility of the reagent to the hydroxyl site. In the early stages of the reaction, lignin reacts the fastest due to the acetic phenolic hydroxyl. This is followed by the reaction with the hemicellulose polymers. No reaction seems to occur in the cellulose. Softwoods react with acetic anhydride faster and to a higher extent as compared to hardwoods. Primary hydroxyl groups react faster than secondary hydroxyl groups so woods high in xylans (hardwoods) show a slower rate of reaction and a lower level of hemicelluloses substitution as compared to softwoods. The primary intent of this research was to study the effects of acetylation on the change in equilibrium moisture content.
Keywords: Accessibility, hydroxyl groups, acetylation, reaction rate, hardwoods, softwoods
Authors
Roger Rowell
Biological Systems Engineering, University of Wisconsin, Madison, WI
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