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

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 radiataD. Don) wood (WAcand 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 WPrsamples seems to be modified by the propionylation reaction, which leads to increased OHAat 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 WPrexhibited less OHAthan expected indicating steric hindrance from the propionyl groups.

The Importance of Moisture for Brown Rot Degradation of Modified Wood: A Critical Discussion

“…Different types of medium of transport for the percolating network have been proposed [103][104][105]. Inhibition of percolation in modified wood may occur through the filling of nanopores and physical blocking of the solute molecules [62][63][64][65][66][67][68][69][70]. Solute exclusion was identified as an appropriate technique to test this theory.…”

“…Several studies have been conducted to investigate whether steric hindrance of water molecules by acetyl groups plays a role in MC reduction in acetylated wood [60,[62][63][64][65]. In a recent study, Beck et al [66] showed that acetylation reduced OH accessibility to a greater extent than would be predicted if OH substitution was solely responsible for accessibility reduction. This suggests that steric hindrance further reduces the accessibility of water molecules to unmodified OH groups in acetylated wood.…”

“…With fewer sorption sites, water may not affect the plasticity of acetylated hemicelluloses as strongly as it does in untreated wood, resulting in an increased glass transition temperature [97]. Furthermore, Beck et al [66] showed that the acetyl groups block more OH-groups than they substitute due to steric hindrance, which further reduces the number of accessible sorption sites. A dynamic vapor sorption (DVS) based study showed that the slow sorption process is more heavily affected by acetylation than the fast sorption process as described by the parallel exponential kinetics (PEK) model [56].…”

On Wood–Water Interactions in the Over-Hygroscopic Moisture Range—Mechanisms, Methods, and Influence of Wood Modification

“…In acetylation, wood reacts with acetic anhydride, producing acetic acid as a by-product [90]. During the reaction, accessible hydroxyl groups are replaced with acetyl groups, thus reducing the number of hydroxyl groups accessible for water molecules [91,92]. In addition, acetylation swells the cell wall, leaving less available space for water.…”

Quantification of accessible hydroxyl groups in cellulosic pulps by dynamic vapor sorption with deuterium exchange

“…So far, DVS with deuterium exchange has been applied to determine the accessible OH group content mostly in wood (Beck et al 2018;Kymäläinen et al 2015Kymäläinen et al , 2018Popescu et al 2014;Rautkari et al 2013;Thybring et al 2017), although some accessibility studies have been done to other cellulosic materials as well, e.g. bacterial cellulose (Lee et al 2011).…”