Dynamic vapour sorption and water-related properties of thermally modified Scots pine (Pinus sylvestris L.) wood pre-treated with proton acid

This study investigates the effect of proton acid pre-treatment and subsequent thermal modification at relatively low temperatures (up to 180 °C) on wood with respect to dimensional stability and water vapour sorption properties. The effects are compared to those of solely thermally-modified wood at higher temperatures (up to 250 °C). Scots pine sapwood (P. sylvestris L.) was impregnated with a proton acid or demineralised water, and subsequently, thermally modified to various mass losses (ML). Acid pre-treatment and thermal modification improved the dimensional stability and reduced the equilibrium moisture content (EMC) until certain ML. Excess surface work (ESW) obtained from vapour sorption studies indicated that, at comparable ML, the acid pre-treatment reduced the available sorption sites as compared to sole thermal treatment. Samples pre-treated with acid also showed stronger decreasing courses of EMC- and ESW-ratios than sole thermally modified ones. This was attributed to degradation of amorphous wood polymers and a stiffer matrix due to cross-linking of the cell wall polymers as a consequence of acid pre-treatment. Electron spin resonance (ESR) analysis indicated that acid pre-treatment did not enhance the concentration of phenoxy radicals, whereas thermally modified wood showed a considerably higher concentration of phenoxy radicals, suggesting that high radical density cannot be used as an indicator for high matrix stiffness.