Influence of Thermal Modification in Nitrogen Atmosphere on Physical and Technological Properties of European Wood Species with Different Structural Features

Abstract

The wood of five European species: black poplar (Populus nigra L.), European beech (Fagus sylvatica L.), European ash (Fraxinus excelsior L.), European oak (Quercus robur L.), and Scots pine (Pinus sylvestris L.) was subjected to thermal modification in nitrogen atmosphere at 190 °C during 6 h. Native and modified wood was varnished and oiled in industrial conditions. Thermally modified (TM) wood was characterized by a greater absorption of varnish and oil when applying the first layer to the surface, which finally resulted in higher application values compared to native wood. In particular, after varnishing, there was a significant increase in gloss and radical change of colour. Regardless of the wood species, finishing process (varnishing, oiling), the ΔE values were close to or higher than 6, which proves high colour changes. Modified poplar, ash, and oak after varnishing had a different colour (ΔE higher than 12). The surface colour changes as a result of UV photoaging was individual, depending on the wood species and the method of finishing. In the case of the thickness of varnish coatings, the wood structure was important, i.e., on ring-porous hardwood and softwood they were thicker. In the case of wood species with a lower density, i.e., black poplar and pine, the thermal modification in nitrogen atmosphere process did not reduce the resistance of the varnish coat, and in the case of species with a higher density (oak, ash, beech) it decreased by one level. Thermal modification reduced the Brinell hardness of wood with wide rays (oak and beech) by 11%. The applied process of surface finishing by double varnishing or oiling did not significantly change the hardness of tested wood.

Chemical Composition and Mechanical Properties of Wood after Thermal Modification in Closed Process under Pressure in Nitrogen

“…As a result, MOR and Brinell hardness were lowered significantly. The Brinell hardness of European beech (Fagus sylvatica L.) wood was lowered from 44.8 to 39.9 N×mm −2 after 6 h of TM in a nitrogen environment at 190 • C. However, the specific anatomical surface evaluated for indentation was not mentioned [21]. Beech wood density (681 ± 20 kg×m −3 ) utilized in that investigation was equivalent to that of silver birch wood; however, it recorded more than 2 fold higher Brinell hardness than birch wood.…”

“…The Brinell hardness of Scots pine (Pinus sylvestris L.) decreased from 40.7 to 37 N×mm −2 after 6 h of TM in nitrogen atmosphere at 190 • C, but the changes were not statistically significant. The anatomical surface that was evaluated for indentation was not mentioned; however, the density (595 ± 7 kg×m −3 ) was quite similar to that of the pine wood we utilized for our research [21]. Since the approach employed by the authors of that study was the same, it is impossible to explain how they arrived at such values.…”

“…TM at 160 and 180 • C increased the biological resistance of C. citriodora wood, but showed little benefit for Pinus taeda [12]. The European species black poplar (Populus nigra L.), European beech (Fagus sylvatica L.), European ash (Fraxinus excelsior L.), European oak (Quercus robur L.) and Scots pine (Pinus sylvestris L.) were thermally modified in a nitrogen environment at 190 • C for 6 h. Beech and oak woods' Brinell hardness decreased following TM, although the variations in poplar, ash, and pine wood were not statistically significant [21].…”

Fire Properties of Paper Sheets Made of Cellulose Fibers Treated with Various Retardants

“…These compounds can act by lowering the ignition temperature, releasing non-flammable gases, or forming protective barriers in the form of char. Mechanical methods involve structural changes in the cellulose, such as surface modifications, which make it more difficult for the fire to spread [5].…”

Effect of Low-Thermal Treatment on the Particle Size Distribution in Wood Dust after Milling

“…Thermal treatment (thermal modification, hydrothermal treatment) processes are applied to a well-established commercial technology for improving wood’s appearance, dimensional stability, and durability. There are reports about hygrothermal treated hardwood [ 29 , 30 , 31 ], softwood [ 32 , 33 ], and bamboo [ 34 ]. Heat-treated wood, due to its properties, is suitable for flooring, various building facade elements, and especially for humid areas such as saunas and bathrooms.…”