The reduction in the fibre saturation point of wood due to chemical modification using anhydride reagents: A reappraisal

The data from a previous study of the moisture adsorption of Corsican pine modified with linear chain carboxylic acid anhydrides (acetic, propionic, butyric, valeric, hexanoic) has been reanalysed so that moisture content is reported as a percentage of cell wall mass only. As part of this work, the change in fibre saturation point (FSP, determined from fits of the Hailwood-Horrobin model) of the modified wood has been determined as a function of weight percentage gain (WPG). The change in FSP is not determined by the extent of hydroxyl substitution of the cell wall, but is related to WPG only (as reported previously). This is also true for the relationship between monolayer water, polylayer water, molecular weight per sorption site at saturation and WPG. However, there is no simple linear relationship between calculated FSP and WPG. When the FSP was determined by solute exclusion methods, the reduction thereof could be simply explained as due to bulking of the cell wall by the bonded acyl substituents. This is not the case when the FSP is determined from adsorption isotherms. This result is interpreted as an ambiguity of determination of FSP from projections of sorption isotherms. Differences in FSP determined by the two methods (sorption versus solute exclusion) are considered to be due to incomplete wetting of the lignocellulosic material when absorbing moisture from the initial dry state.