Background
Heat treatment of wood was scientifically studied by Stamm and Hansen in the 1930s in Germany and by White in the 1940s in the United States. In the 1950s, Germans Bavendam, Runkel and Buro continued research into the subject. Kollman and Schneider published their findings in the 1960s, and Rusche and Burmester in the 1970s. More recently, research work was carried out in Finland, France and the Netherlands in the 1990s.The most comprehensive research work was conducted by
VTT (Finnish State Research Center) in Finland.
ThermoWood® is manufactured using a method developed by VTT.The wood material is heated to a temperature of at least 180 degrees Celsius while it is protected with steam. Besides providing protection, the steam also affects the chemical changes taking place in the wood. As a result of the treatment, environmentally friendly ThermoWood® is created. Its colour darkens, it
is more stable than normal wood in conditions of changing humidity and its thermal insulation properties are improved.
If carried out at a sufficiently high temperature, treatment also makes the wood resistant to decay.
The ThermoWood® process
An industrial scale heat-treatment process for wood has been developed at VTT in co-operation with the Finnish wood product industry.The ThermoWood® process is licensed to the members of the Finnish ThermoWood® Association. Finnforest ThermoWood® is available in two treatment classes Thermo-S (Scandinavian pine or spruce, heat treatment 190ºC, internal use) and Thermo-D (Scandinavian pine, heat treatment 212ºC, internal and external use).
The ThermoWood® process can be divided into three main phases:
Phase 1.Temperature increase and high-temperature drying
Using heat and steam, the kiln temperature is raised rapidly to a level of around 100ºC.Thereafter, the temperature is increased steadily to 130ºC, during which time the high-temperature drying takes place and the moisture content in the wood decreases to nearly zero.
Phase 2. Heat treatment
Once high-temperature drying has taken place, the temperature inside the kiln is increased to between 185ºC and 215ºC.When the target level has been reached, the temperature remains constant for 2–3 hours depending on the end-use application.
Phase 3. Cooling and moisture conditioning
The final stage is to lower the temperature by using water spray systems; when the temperature has reached 80–90 ºC, re-moisturising takes place to bring the wood moisture content to a useable level, 4–7%.
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