Effects of Acid Pre-Treatments on the Swelling and Vapor Sorption of Thermally Modified Scots Pine (Pinus sylvestris L.) Wood

Abstract

Scots pine sapwood samples were pre-treated with a Lewis acid (AlCl3) and a combination of Lewis and protonic acids (AlCl3 and H2SO4), and were subsequently exposed to respective temperatures of 180 °C and 120 °C for establishing a comparable mass loss with those impregnated with demineralized water and solely thermally modified at 220 °C. Water impregnated samples dried at 120 °C also served as controls. The swelling behavior of all wood samples was examined with respect to maximum swelling in water, anti-swelling efficiency (ASE), shrinkage, and dynamic water vapor sorption at relative humidity ranges of 0% to 95%. The thermal modification at 220 °C diminished swelling and moisture adsorption, and also reduced moisture increment and decrement compared with the unmodified control. However, it was less obvious than both acid pre-treated samples. Excess surface work and Hailwood-Horrobin results calculated from water vapor sorption studies demonstrated that, at comparable mass loss, the available sorption sites were reduced to a greater extent by Lewis acid and combination of Lewis and protonic acids pre-treatment than the sole thermal treatment. This was attributed to more pronounced degradation of polysaccharides, mainly hemicelluloses and amorphous parts of cellulose, and to cross-linking of cell wall polymers due to the acid pre-treatments.