Acetylated wood shows improved properties largely due to the reduced amount of water in the acetylated cell wall. However, the exact mechanism by which water is excluded in acetylated wood remains unclear. Acetylation reduces hydroxyl content by substitution of hydroxyl groups in wood polymers but may also hinder access to unmodified hydroxyls by physical bulking. This work assesses hydroxyl accessibility in Radiata pine (Pinus radiata) samples that were acetylated and propionylated to different levels by means of deuterium exchange. The deuterium exchange experiments were performed in a novel way using an instrument designed for thermogravimetric analysis. Both acetylated and propionylated samples tend to reduce hydroxyl accessibility, but the relationship between accessibility and weight percentage gain for both modifications deviates from the expected theoretical relationship. The acetylated results suggest that, in addition to hydroxyl substitution, blocking of unmodified hydroxyl groups may play a role in hydroxyl accessibility reduction. The propionylated samples seemed to be damaged by the propionylation reaction, showing higher than expected accessibility. This may be a result of molecular restructuring within the cell wall which exposes new hydroxyl groups after propionylation.

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