One-stage thermo-hydro treatment (THT) of hardwoods: an analysis of form stability after five soaking-drying cycles

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 degrees 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 degrees C for 1 h), and the original ASE of 33% decreased to 5% (140 degrees C for 1 h). Wood modified at 170 degrees 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 degrees C for 1 h, 160 degrees C for 3 h, and 180 degrees 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.