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 (EMC R ), 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.

The water vapour sorption data of untreated (W untr ), acetylated (W ac ) and formaldehyde-treated (W FA ) Scots pine ( Pinus sylvestris L.) sapwood were analysed in terms of their sorption kinetics and were transformed into excess surface work (ESW) isotherms. The sorption kinetics were studied by fitting the non-linear parallel exponential kinetics (PEK) model to the experimental data in which the sorption kinetics curve is composed of two processes (fast and slow components). W ac and W FA showed evident differences in their sorption kinetics and their thermodynamic sorption behaviour. In contrast to acetylation, formalisation influenced both the extent of the slow sorption process and the shape of its pseudoisotherm. For W untr and W ac , it appears that some water associated with the slow process is adsorbed at sites for fast sorption newly generated upon swelling (previously postulated as extra water) and subsequently desorbed by the fast process. For W FA , the formation of extra water hardly occurs. ESW was reduced through acetylation with a constant factor over the whole hydroscopic range, whereas the ESW of W FA was reduced only after reaching the monolayer capacity compared to its control. The sorption behaviour of W ac was solely determined by cell wall bulking, whereas that of W FA was governed by the increased matrix stiffness due to cross-linking of the cell wall polymers.