Influence of the thermo-hydro-mechanical treatments of wood on the performance against wood-degrading fungi

Hybrid poplar (Populus deltoides × Populus trichocarpa) and Douglas-fir (Pseudotsuga menziesii) wood specimens were densified with three variations of thermo-hydro-mechanical (THM) treatment. The THM treatments differed in the steam environment, including transient steam (TS), saturated steam (SS), and saturated steam with 1-min post–heat treatment at 200 °C (SS+PHT). The bending properties, FTIR spectra, and colour of the THM wood specimens were studied before and after exposure to two different wood decay fungi, brown rot Gloeophyllum trabeum, and white rot Trametes versicolor. The results showed that the performance of densified hybrid poplar wood was considerably poorer than the performance of Douglas-fir heartwood. The FTIR spectra measurements did not show changes in the densified hybrid poplar wood, while some changes were evident in densified Douglas-fir specimens. After fungal degradation, the most prominent changes were observed on the SS+PHT specimens. Colour is one of the most important parameter predominantly influenced by the wood species and the intensity of the densification process for both wood species, while after fungal exposure, the colour of all densified Douglas-fir specimens obtained more or less the same appearance, and densified hybrid poplar specimens resulted in lighter colour tones, indicating that the pattern of degradation of the densified and non-densified specimens are similar. The 3-point bending test results determined that the THM treatment significantly increased the modulus of rupture (MOR) and modulus of elasticity (MOE) of the densified wood specimens, while fungal exposure decreased the MOE and MOR in hybrid poplar and Douglas-fir specimens.