Water-Related Properties of Wood after Thermal Modification in Closed Process under Pressure in Nitrogen

Silver birch (Betula pendula) and Scots pine (Pinus sylvestris) wood boards (1000 × 100 × 25 mm) were thermally modified in a 340 L pilot-scale device in nitrogen with a special focus on increasing dimensional stability and reducing hydrophilicity. The research expands our understanding of the TM process in a closed system under pressure of nitrogen and its impact on the water absorption capabilities of wood. Several thermal modification (TM) parameters were tested, including temperature (160–180 °C), maximum temperature duration (30–180 min), and TM chamber initial pressure (3–6 bar). TM wood dimensional changes, mass loss (ML), equilibrium moisture content (EMC), and anti-swelling efficiency (ASE) were determined to characterize the TM process intensity and evaluate the hydrophilicity. Birch wood exhibited a higher ML (5.9–12%) than pine wood (2.6–9%) after TM. TM caused a shrinkage in the tangential, radial, and total volume of both wood species. The TM birch wood ASE values varied from 22 to 69%, while the pine wood ASE was 27 to 58%. The cell wall total water capacity (CWTWC) of TM wood was greatly reduced. The EMC and volumetric swelling (VS) of TM birch and pine wood were 29 to 67% lower, respectively, at all relative humidities (65, 75, and 95%).