Water vapour sorption of wood modified by acetylation and formalisation – analysed by a sorption kinetics model and thermodynamic considerations

Abstract

Abstract The water vapour sorption data of untreated (Wuntr), acetylated (Wac) and formaldehyde-treated (WFA) 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). Wac and WFA 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 Wuntr and Wac, 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 WFA, 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 WFA was reduced only after reaching the monolayer capacity compared to its control. The sorption behaviour of Wac was solely determined by cell wall bulking, whereas that of WFA was governed by the increased matrix stiffness due to cross-linking of the cell wall polymers.