Optimisation of a two-stage heat treatment process: durability aspects |
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| Authors: | M J Boonstra J van Acker E Kegel M Stevens |
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| Institution: | (1) Plato International BV, P.O. Box 2159, 6802 CD Arnhem, The Netherlands;(2) Laboratory of Wood Technology, Ghent University, Coupure links 653, 9000 Ghent, Belgium |
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| Abstract: | Heat treatment of wood at relatively high temperatures (in the range of 150–280°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°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. |
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