Abstract
Hot water extraction (HWE) treatment changes the physicochemical properties of the wood, including hygroscopic properties. HWE treatment decreases the hydroxyl accessibility of the wood, but the relevance of other mechanisms that change hygroscopic properties are not fully understood. This study investigates the effect of drying on the hydroxyl accessibility and sorption properties of wood. Pressurized hot water extraction (HWE) treatment was applied at 140 °C for 1–5 h to Scots pine (Pinus sylvestris L.) sapwood samples in order to remove increasingly more hemicellulose from the cell wall matrix. Following HWE treatment, half of the wood samples were oven-dried and then re-soaked, while the other half was kept in a fully saturated state. The samples were investigated by applying a new approach that was based on the deuteration of accessible hydroxyl groups, which was followed by the measurement of mass loss due to re-protonation. Sorption properties of the wood samples were studied by measuring moisture content, sorption isotherms and dimensional changes. The present results showed that accessible hydroxyl group content decreased only due to hemicellulose removal during the HWE treatment and was unaffected by oven-drying. However, oven-drying enhanced the effect of HWE treatments in reducing the water-saturated dimensions and the moisture content of wood. Therefore, the additional reductions in hygroscopicity and water-saturated dimensions were not related to changes in sorption site density.
Highlights
Wood is a hygroscopic material that takes up moisture depending on its surrounding conditions
1.0 0.7 0.6 0.7 for the dried and re-soaked sample group. These differences were evident in the reference samples and were small compared to the effects of hot water extraction (HWE) treatment
Oven-drying enhanced the effect of HWE treatments in reducing the water-saturated dimensions and the moisture content of wood. This drying effect became more noticeable at longer HWE treatment times and could not be fully reversed by resoaking in water
Summary
Wood is a hygroscopic material that takes up moisture depending on its surrounding conditions. Several studies have explored the mechanisms behind the reduced hygroscopicity of hydrothermally modified wood (Borrega and Kärenlampi 2010; Altgen et al 2016, 2018a; Wentzel et al 2018). Previous studies conclude that the hygroscopicity of wood is linked to the presence of accessible polar groups, i.e. hydroxyl (OH) groups, which act as sorption sites for water (Venkateswaran 1970; Berthold et al 1996). Besides the sorption site density, further mechanisms influence the sorption behaviour of thermally modified wood (Repellin and Guyonnet 2005; Borrega and Kärenlampi 2010; Rautkari et al 2013; Altgen et al 2016, 2018a; Wentzel et al 2018). Hydrothermal treatments of wood affect the sorption of wood by reduction of accessible OH groups due to changes in chemical composition, and by other mechanisms that are not fully understood
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