Optimisation of a two-stage heat treatment process: durability aspects

Abstract

Heat treatment of wood at relatively high temperatures (in the range of 150-280 degrees C) is an effective method to improve biological durability of wood. This study was performed to investigate the effect of heat treatment process optimisation on the resistance against fungal attack, including basidiomycetes, molds and blue stain fungi. An industrially used two-stage heat treatment method under relatively mild conditions (< 200 degrees C) was used to treat the boards. Heat treatment of radiata pine sapwood revealed a clear improvement of the resistance against the brown rot fungi Coniophora puteana and Poria placenta. Increasing process temperature and/or effective process time during the first process stage, the hydro thermolysis, appeared to affect the resistance against C. puteana attack, but the effect on the resistance against P. placenta was rather limited. Heat treated radiata pine showed a limited resistance against the white rot fungus Coriolus versicolor and process variations during the hydro thermolysis stage appeared not to affect this resistance. A clear difference between the resistance of heat treated Scots pine sapwood and heartwood against fungal attack is observed. Scots pine heartwood showed a higher resistance against C. puteana and P. placenta but also against the white rot fungus C. versicolor. Similar results were obtained when heat treated birch was exposed to brown and white rot fungi. Heat treatment showed an improved resistance against C. puteana attack, especially at higher temperatures during the hydro thermolysis stage. A clear improvement of the durability was also observed after exposure to the white rot fungus C. versicolor and especially Stereum hirsutum. Increasing the process temperature or process time during the hydro thermolysis stage appeared to have a limited effect on the resistance against C. versicolor attack. Heat treated radiata pine and Norway spruce were still susceptible to mold growth on the wood surface, probably due to the formation of hemicelluloses degradation products (e.g. sugars) during heat treatment. Remarkable is the absence of blue stain fungi on heat treated wood specimen, also because the abandant blue stain fungi were observed on untreated specimen. Molecular reasons for the resistance of heat treated wood against fungal attack are discussed in detail contributing to a better understanding of heat treatment methods

Effect of oil impregnation on water repellency, dimensional stability and mold susceptibility of thermally modified European aspen and downy birch wood

“…Figure 4 shows the comparison between the different treatments and highlighted the significant differences. The TM wood studied here indicated that heat treatment did not obviously reduce, but rather accelerated the mold growth on the outer surface of the boards and the result is in agreement with other findings [1]. This effect might be due to the hemicellulose that hydrolyzed into oligomeric and monomeric structures even though it changes the cellulose, hemicelluloses and lignin content [1,31].…”

“…To determine the water uptake capacity and swelling properties, the samples were oven-dried at 50°C for 72 h and submerged in distilled water at 21°C for periods of 1,3,6,12,24,48,96,192, 384 and 768 h. The distilled water was replaced after every soaking interval. After each saturation period, the masses and volumes were recorded to measure water absorption (WA; defined as the absorbed water divided by the dried mass) and volumetric swelling coefficient (S).…”

“…In Europe, five processes are currently available at the industrial scale, including ThermoWood Ò (or Premium wood) using steam in Finland; the Perdure process (BOIS PERDURE), which is conducted in a steam atmosphere in France; the Retification process using nitrogen (Retiwood) in France; the Plato process (Plato Ò WOOD) in an aqueous environment at super-atmospheric pressures in the Netherlands; and an oilheat treatment (Menz Holz Ò ) in Germany. All these processes are used to improve wood properties by reducing hygroscopicity, improving dimensional stability and enhancing resistance against biological attack by modifying the chemical structure of the wood at temperatures ranging from 160 to 260°C [1][2][3][4]. The extent to which the wood properties are modified after the thermal treatment depends on the method, the wood species, inherent wood properties, the type and condition of the heating gas and the treatment schedules.…”

Mode of action of brown rot decay resistance in modified wood: a review

“…As pointed out above, brown rot fungi degrade hemicelluloses before cellulose (Militz 2002;Weiland and Guyonnet 2003;Rowell et al 2009) and this order of degradation is considered by many investigators to be a key step (Boonstra et al 2007;Rowell et al 2009). Rowell et al (2009) proposed that arabinose, which is the only L-pentose sugar found in wood, is a key compound that triggers the chain reactions of degradation and that its absence would prevent the initiation of degradation.…”

“…Rowell et al (2009) proposed that arabinose, which is the only L-pentose sugar found in wood, is a key compound that triggers the chain reactions of degradation and that its absence would prevent the initiation of degradation. Indeed, most hemicelluloses are degraded in thermally modified wood (Boonstra et al 2007) and hemicelluloses are considerably modified by acetyl groups in acetylated wood (Rowell et al 2009). …”