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Thermal modification does not always reduce water absorption of wood

30.11.2011

Currently, wood is the most ecological alternative in building construction. The trend for its use in small houses and apartments is increasing. The ThermoWood thermal modification treatment is commonly used in Finland's thermal modification plants to prevent wood decay.

It is a popular belief that thermal modification reduces water absorption of wood. However, in her doctoral dissertation M.Sc. Sini Metsä-Kortelainen shows that this is not always the case. The wood species used, its sapwood and heartwood portions, and thermal modification temperature all have an important influence on the wood's biological and physical properties.

Even after thermal modification the water resistance of sapwood and heartwood differ

– During experiments it was noted that the water absorption of sapwood and heartwood of pine differ a lot.   When sapwood of pine, for example, was modified in different temperatures, it might, in some cases, absorb even more water after the treatment, Metsä-Kortelainen explains.

It is thought also that thermal modification evens the humidity resistance of wood's different portions. Research indicates that even after thermal modification the influence of the internal structure of the wood material is apparent: even then the water resistance of sapwood and heartwood differ.

In the opinion of Metsä-Kortelainen, those involved in the industry's production processes, in applications of wood as well as in testing should pay more attention to the differences among tree species, between sapwood and heartwood and between processing temperatures.

– Sawmills do not pay sufficient attention to this matter, and the separation of sapwood and heartwood from the raw material is not very efficient. Their separation would help in finding a suitable application for the wood material; a wrong kind of application can lead to problems, such as decay.

Enormous amount of research data and material

According to Metsä-Kortelainen, the most challenging factor in her work was the management of huge amount of test material, as the research included two tree species, pine and spruce, and their sapwood and heartwood. The reference materials were larch, bangkirai, Western red cedar, merbau, as well as pressure-treated wood materials, depending on the test.

There were several decay and water absorption tests in the research. The total number of specimens in one of the decay tests was 640. There was just about enough room to fit it all in inside the laboratory. Soil box and brown-rot tests were quick methods to investigate decay, but their practical connection was further examined with outdoor field experiments, the longest of which took nine years.

Sini Metsä-Kortelainen initiated her research at the Aalto University Department of Forest Products Technology with the help of a grant from the Finnish Foundation for Technology Promotion and in cooperation with the VTT Technical Research Centre of Finland. Later on she moved to VTT to continue her work there.

Dissertation on 9th of December 2011 at 12 noon

M.Sc. Sini Metsä-Kortelainen will publicly defend her doctoral dissertation on 9th of December 2011 at Aalto University Department of Forest Products Technology (School of Chemical Technology), Auditorium, Tekniikantie 3, Espoo.  The topic of her dissertation is "Differences between sapwood and heartwood of thermally modified Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) under water and decay exposure". The opponent is Professor Holger Militz from Georg-August-Universität, Göttingen, Germany, and the custos is Professor Mark Hughes from the Aalto University Department of Forest Products Technology.

Additional information:

M.Sc. Sini Metsä-Kortelainen, sini.metsa-kortelainen [at] vtt [dot] fi

 

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