Accessibility of hydroxyl groups in anhydride modified wood as measured by deuterium exchange and saponification

Abstract

Abstract Acetylated wood (WAc) shows improved properties largely due to the reduced amount of water in its cell wall, but the exact mechanism of water reduction remains unclear. Acetylation reduces hydroxyl (OH) content by acetyl (Ac) substitution but may also limit water access to unmodified OH groups by steric hindrance. In the present work, the accessibility of OH groups in acetylated or propionylated Radiata pine (Pinus radiata D. Don) wood (WAc and WPr) was investigated by deuterium exchange, saponification in sodium hydroxide followed by high-performance liquid chromatography (HPLC) analysis and weight percentage gain determination of the modified samples. Acetylation reduced OH accessibility (OHA) to a greater extent than would be predicted, if OH substitution were the only responsible mechanism for accessibility reduction. The combination of deuterium exchange and saponification results provides strong evidence that steric hindrance plays a key role in reduction of water accessibility to unmodified OH groups in WAc. The supramolecular architecture of WPr samples seems to be modified by the propionylation reaction, which leads to increased OHA at low levels of substitution. This suggests that molecular restructuring within the cell wall exposes new OH groups after propionylation. At higher levels of substitution, however, the WPr exhibited less OHA than expected indicating steric hindrance from the propionyl groups.