Effects of acetylation and formalization on the dynamic water vapor sorption behavior of wood

The dynamic water vapor sorption of untreated, acetylated (W-ac), and formaldehyde-treated (W-FA) Scots pine (Pinus sylvestris L.) sapwood was studied in a dynamic vapor sorption apparatus to assess the effects of cell wall bulking and cross-linking. Both modifications resulted in a considerable reduction of reduced equilibrium moisture content (EMCR), the corresponding equilibrium times, and hysteresis in the hydroscopic range of wood. Acetylation reduced the adsorption and desorption of water at each given relative humidity (RH) step from 0% to 95% RH, whereas formalization affected the sorption behavior of wood solely above 20% RH. From 20% to 95% RH, the EMC ratio of W-FA to its control steadily decreased, whereas the EMC ratio of W-ac was still constant in this RH range. Below 20% RH, the sorption behavior of W-ac was governed by hydroxyl blocking, whereas that of W-FA was hardly influenced compared with the control. Above 20% RH, the sorption behavior of W-ac was solely determined by cell wall bulking, whereas that of W-FA was governed by the increased matrix stiffness due to the cross-linking of cell wall polymers.