A newly developed thermo-hydro treatment (THT) for use in a one-stage heat treatment process was examined by focusing on the form stability-related properties of European aspen (Populus tremula), birch silver (Betula pendula), and gray alder (Alnus incana). In particular, wood specimens were subjected to THT in a saturated steam atmosphere in a pilot-scale autoclave heated between 140 and 180°C for 1–3 h. Several parameters of untreated and treated samples after several soaking and drying cycles were compared, namely, the changes in the volumetric swelling, swelling in the radial and tangential directions, cell wall total water capacity, and anti-swelling efficiency (ASE). Due to repeated wetting in the cyclic water submersion-drying test, the original ASE of 73% decreased to 65% (180°C for 1 h), and the original ASE of 33% decreased to 5% (140°C for 1 h). Wood modified at 170°C presented good results that were not significantly lower than wood treated at higher temperatures while consuming less energy to deliver ASE improvement and was selected as optimum. To increase the ASE by 1%, the amount of energy consumed was decreased by 41%, 39%, and 17% compared with the treatment regimes of 160°C for 1 h, 160°C for 3 h, and 180°C for 1 h, respectively. The new THT regime led to improved long-term dimensional stability due to the cross-linking of cell wall polymers, which resulted in increased cell wall rigidity.
The effect of thermo-hydro treatment (THT) on the properties of birch (Betula spp.) wood veneers has been studied. THT was carried out in a multi-functional pilot scale wood modification device of wood treatment technology (WTT, Latvia) under elevated water vapor pressure conditions at four combinations of temperature and treatment time (°C/min): 150/10; 150/50; 160/10 and 160/50. After THT, the following veneer properties were examined: mass loss (ML), chemical composition, bending strength (BS), tensile strength (TS), equilibrium moisture content (EMC), resistance to decay by mould and blue stain fungi, and surface contact angle (CA). The chemical components were changed by THT. Increased THT temperature and time resulted in hydrophobization of veneers as indicated by decreasing EMC and increasing CA data. All THT were effective against wood discoloring fungi, although insufficient decay resistance was observed. The mechanical strength properties of THT veneers were also deteriorated.