Thermal modification of wood involves heating wood material to a temperature of at least 180°C while at the same time protecting it with steam. The steam protects the wood, but it also influences the chemical changes taking place in wood. Thermal modification involves using heat and water vapour to produce environmentally-friendly modified (wood) products, which are free of any impregnation substances.
Thermal modification impacts on several wood properties changing them permanently. The dimensional stability of wood significantly improves in conditions where the moisture level varies. Also, the thermal insulation properties of wood thus treated are superior to those of non-treated wood. The effect of treatment applied at a sufficiently high temperature is such that wood becomes decay-resistant. Thermal modification does cause a slight reduction in the strength properties of wood. Thermal modification can be successfully applied to the wood of nearly all tree species. The lovely shades of colours produced by heat treatment can be preserved by applying oil or pigments to the wood surface.
Thermal modification has an effect on the strength properties of wood. The strength and density of wood usually correlate with one another quite well. The density of thermally modified wood diminished a little lower following treatment.
The bending strength of thermally modified wood improves following treatments at low temperatures (wood thus treated is suitable for interior applications) and correspondingly diminished following treatments at high temperatures (wood thus treated is suitable for exterior applications). Two methods of testing bending strength have been used in studies: in one, flawless material was used applying a short span, while in the other pieces containing natural defects were tested applying longer spans. The results demonstrated that significant reduction in strength pine wood begins when treatment temperatures exceed 220°C.
In addition to the treatment temperature, the bending strength of thermally modified wood depends the moisture content of the wood. Because the equilibrium moisture content of thermally modified wood is lower, its moisture content is lower in specific conditions, and thus its strength values can be higher than those of untreated wood.
Thermally modified wood is not recommended to be used in supporting structures in applications where structural plans require strength grading for structural material. thermally modified wood is well suited for external applications, e.g. as cladding for decks and patios.
The hardness of the thermally modified wood improves following different intensities of treatment when compared to untreated wood. Especially in the case of coniferous timbers, improved hardness provides many opportunities for benefiting from this as improved hardness means increases wear resistance.
Thermal modification of wood improves the weatherproofing properties of wood. Due to diminished equilibrium moisture content, the moisture content of thermally modified wood when subjected to various weather conditions also diminishes. Other significant advantages imparted by heat treatment on wood regarding its use in external applications are in improving dimensional stability and in the consequent reduced stress caused by air temperature, moisture in wood, sunlight, and other physical properties.
Surface mould can occur also on the surface of thermally modified wood. Due to the impurities spread by bacteria contained in air and by rain, mould colonies can form on untreated surfaces. However, these mould colonies are to be found only on the surface of wood and they do not weaken wood structurally. Moulds can be removed by washing, wiping or scraping.
In a similar manner, and as is the case with most natural materials, the UV radiation of sunlight makes thermally modified wood susceptible to colour changes. Direct exposure to sunlight causes colour changes with time in the original brown colour and at first leads to lightening of the colour and then turning grey within 2-3 years.
Even though the moisture content and moisture-related dimensional changes of thermally modified wood are reduced, UV radiation does cause minute surface cracks in exposed, non-surface-coated boards. The number of these surface cracks is equivalent to those found in untreated wood. Five-year weather-stress trials have revealed that the moisture content of thermally modified wood is about 50% lower than that of untreated wood. Surface-dressing substances not containing pigment or those composed of thin coats of stain wear off thermally modified wood in the same way as they wear off untreated wood.
Thermally modified wood behaves better than unmodified wood as a surface to be painted. Exposure trials lasting 5 years have revealed that the functionality of vulcanised and water-dilutable acrylic paints was better on thermally modified boards than on untreated boards. Boards treated with these paints demonstrated less peeling off of the paint.
Exterior wall paints work well with thermally modified wood. The best surface-dressing systems for thermally modified wood consist of priming oil and solvent-dilutable alkyd paint or water-dilutable acryl finishing paint
Improved decay resistance
Thermal modification of wood significantly improves the decay resistance of wood. Improved decay resistance is based on changes in the chemical structure of the wood cells.
Thermal modification of wood results in acetic acid being formed as a consequence of the hydrolysis of hemicellulose and the amount of hemicellulose in wood is reduced following heat treatment. This means that there is distinctly less nourishment available for decay-causing fungi and partly for this reason thermally modified wood is far better able to withstand the impacts of decay-causing fungi than untreated wood. Decay is also slowed down by the lower pH of thermally modified wood.
The results of tests conducted to study decay resistance of thermally modified wood have been very encouraging. These decay tests have been conducted in accordance with the standard EN 113. Decay tests conducted in laboratory conditions involved measuring weight loss of wood, e.g. following 8, 16 and 32 weeks. Coniophora puteana, Gleophyllum trapeum, Coriolus versicolor, and Poria placenta are examples of the fungi studied.
The biological durability of thermally modified wood has also been studied in 5-year field experiments. On the basis of the field tests conducted, the decay resistance of thermally modified pine wood has proved to be better than that of the present-day AB class of pressure-impregnated pine wood. On the basis of the field tests, the decay resistance of thermally modified wood improved by an average of 70% when compared to that of untreated wood .
On the basis of the test results, thermally modified wood is suitable for use in external applications and also in applications involving ground contact as long as the structural properties are not critical and drainage of the underlying soil has been arranged. When using wood in constructing decking or patios exposed to the weather, for example, these can be made entirely of thermally modified wood. Alternative products for these uses are products treated as per Stellac®Wood D2 or D3 and Thermo-D.
Improved decay resistance can be reached only when the treatment temperature clearly exceeds 200°C. SWM WOOD is the first manufacturer to have been awarded the international KOMO® product approval in the Netherlands. According to this award, thermally modified spruce wood achieves the decay resistance class 2 (EN 350-1) when the treatment temperature is 220°C. The same decay-resistance class currently includes impregnated wood products as per AB class.
Equilibrium moisture content
Reduced equilibrium moisture content produces new application possibilities for thermally modified wood. In applications where changes in wood moisture and the associated swelling and shrinking cause problems, one can solve these problems by using thermally modified wood. For example, the summer cottages floor made of thermally modified floorboards has no gaps between the boards.
At high temperatures (220°C), equilibrium moisture content is no more than about half of that of untreated wood. The difference in the moisture content values of wood increases with increasing relative humidity of the air.
The reduced equilibrium moisture content of thermally modified wood in turn means reduced susceptibility to decay. In practice, the moisture content of air is not able to increase the equilibrium moisture content of thermally modified wood to a level congenial for decay-causing fungi to damage wood. When the relative humidity of the air is more than 90%, the equilibrium moisture content of thermally modified wood will still remain below 15%, whereas decay-causing fungi require moisture contents in excess of 20%
Reduced moisture-related dimensional instability
The dimensional instability of wood caused by variation in the humidity of the surrounding air decreases by 20%...90% following heat treatment when compared to that of untreated wood. This is caused by reduced shrinking of wood, reduced equilibrium moisture content of wood, and by the slowing down of absorption of moisture into wood. The seasonal dimensional stability of thermally modified wood is far better than that of untreated wood.
These very slight moisture-related dimensional changes of thermally modified wood are based on the decomposition of hemicellulose in the wood. The decomposition of hemicellulose is accompanied by a reduction in the amount of hydroxyl groups capable of binding water, and the wood becomes dimensionally more stable than untreated wood
These diminished moisture-related dimensional changes are also evident in that even though the moisture content of thermally modified wood varies, the dimensions of treated pieces of wood do not change despite changes in the moisture content of the wood.
The moisture-related dimensional changes of thermally modified wood diminish with increasing intensity of heat treatment. Already at low temperatures (approx. 190°C) these moisture-related dimensional changes are reduced by about 20% when compared to untreated wood. Following heat treatments making wood suitable for outdoor use, the moisture-related dimensional changes are further reduced and following treatments involving temperatures of about 210-220°C, these changes are already about 40-50% lower than in untreated wood.
Reduced moisture-related dimensional changes represent significant advantages in those applications and in the wood-product industry where dimensional stability of wood is of great significance.
Thermally modified wood provides excellent application opportunities in situations where changes in the temperature and humidity of the air presuppose dimensional stability on the part of wood.
The wood-product industry, building products, wooden houses, door and window frames are examples of applications where thermally modified wood represents a new option as a raw material whose diminished swelling and shrinking are of significant benefit to both its processing and the end user.
The effect of thermal modification in reducing dimensional changes in wood has been clearly observed also in regard to warping of the end product. According to research results, coated and uncoated thermally modified wood retains its dimensions better than impregnated and untreated wood.
An added bonus related to reduced moisture-related dimensional changes is in the diminished drying tensions of thermally modified wood when compared to untreated wood. This is an advantage when splitting pieces of wood and when manufacturing joinery products.
The colour changes manifested in thermally modified wood provide a basis for new solutions both in planning and in decisions related to interior decoration.
The colour changes of thermally modified wood are influenced by the treatment temperature and duration. The higher temperature, the darker the appearance of the modified wood. The colour change is similar for nearly all timber species and it is tied up with the treatment temperature. Colour change occurs primarily as a consequence of the decomposition of the lignin contained in wood.
Thermal modification changes the colour of wood throughout to the same shade. Raw material and surface-treatment solutions in the wood-products industry can be closer to one another and the problematic colour differences in wood products caused by blows and scratches no longer occur. The final hue is formed as the interaction of the intensity of the treatment and of the surface-coating substance.
Wood needs to be protected from UV radiation of sunlight in order to retain its colour. Unless wood is given a surface coating, UV radiation will cause it to become paler and grey in the same way as all other wood material.
In the case of conifer wood, the normal changes caused by density and the earlywood and latewood affect the consistency of its colour.
The practice at SWM-WOOD in its production is for the precisely the same shade for various end-use purposes to be replicated per production batch. The colour of the wood can be measured using the component value L, for example.