Resistance of thermally modified and pressurized hot water extracted Scots pine sapwood against decay by the brown-rot fungus Rhodonia placenta

Abstract

The thermal degradation of wood is affected by a number of process parameters, which may also cause variations in the resistance against decay fungi. This study compares changes in the chemical composition, water-related properties and decay resistance of Scots pine sapwood that was either thermally modified (TM) in dry state at elevated temperatures (≥ 185 °C) or treated in pressurized hot water at mild temperatures (≤ 170 °C). The thermal decomposition of easily degradable hemicelluloses reduced the mass loss caused by Rhodonia placenta, and it was suggested that the cumulative mass loss is a better indicator of an actual decay inhibition. Pressurized hot water extraction (HWE) did not improve the decay resistance to the same extent as TM, which was assigned to differences in the wood-water interactions. Cross-linking reactions during TM caused a swelling restraint and an effective reduction in moisture content. This decreased the water-swollen cell wall porosity, which presumably hindered the transport of degradation agents through the cell wall and/or reduced the accessibility of wood constituents for degradation agents. This effect was absent in hot water-extracted wood and strong decay occurred even when most hemicelluloses were already removed during HWE.

Micro-tensile behavior of Scots pine sapwood after heat treatments in superheated steam or pressurized hot water

“…The chemical analyses confirmed earlier studies on the difference between HT of wood in dry and watersaturated state [6,26,30,31]. The presence of water catalyzed the hydrolytic cleavage of covalent bonds during the hydrothermal treatments wood [32].…”

“…Some studies explained that cross-linking reactions during Dry-HT enhance the cell wall matrix stiffness and restrict the expansion of the polymers to accommodate water molecules [30,31,43]. Other studies speculated that changes in the conformation of the matrix polymers hinder the relaxation of the cell wall polymers toward their thermodynamically most favorable arrangement [31,37,44]. Similar to the reduction in wood MC, the present study also showed differences in the change in tensile properties by the two HT techniques.…”

“…Although there is uncertainty about the exact nature of this additional mechanism, previous studies suggested that the various modifications of the residual cell wall matrix play a major role. Some studies explained that cross-linking reactions during Dry-HT enhance the cell wall matrix stiffness and restrict the expansion of the polymers to accommodate water molecules [30,31,43]. Other studies speculated that changes in the conformation of the matrix polymers hinder the relaxation of the cell wall polymers toward their thermodynamically most favorable arrangement [31,37,44].…”

Pressurized hot water extraction of Scots pine sapwood: effect of wood size on obtained treatment products

“…Thermal modification utilizes the partial degradation of wood at elevated temperatures to enhance the hygroscopic properties and decay resistance of solid wood products [10]. While HWE is less efficient in decreasing the hygroscopicity [11,12] or decay resistance [13] of wood compared with thermal modification, it enables the utilization of removed hemicelluloses and limits the loss in strength and toughness of the solid residue [14]. However, increasing the wood size may have a significant effect on the effectiveness of HWE and the obtained treatment products.…”

Thermal modification kinetics and chemistry of poplar wood in dry and saturated steam media

“…Studies by Altgen et al (2020) have shown that a leaching process can remove the residues produced during the thermal treatment under dry or pressurized hot water conditions. One main consequence of no leaching of the samples was the eventual underestimation of the real mass loss if ones aim to compare results under such peculiar conditions.…”