What determines the quality of thermal modified timber ? An number of properties of timber are enhanced / increased or reduced during the thermal modification process. Such as:
The density is measured by measuring the weight and the dimensions of the sample. The unit of density is kg/m3. The ThermoWood process reduces the density by about 10 % on average.
Generally the strength of wood has direct correlation with density. The ThermoWood process slightly lowers the density and therefore some effects on the strength values occur, but weight-strength-value can be practically unchanged.
Bending strength and modulus of elasticity, Material treated at temperatures below 200 °C does not experience a significant loss in bending strength. A clear reduction in horizontal bending strength can be found in material treated at temperatures above 200 °C. The ThermoWood process has been found to maintain or even slightly improve the modulus of elasticity. At this stage it is recommended that ThermoWood is NOT used for horizontal load bearing structural usage.
Compression strength, The Compression strength is mainly dependent on the actual density of wood. According to tests it is has been found that the ThermoWood process has no significant effect on the compression strength values.
Splitting strength,The ThermoWood process can cause some reduction in the splitting strength depending on treatment temperature, the reduction increases as the temperature goes over 200 °C.
Screw holding strength, The screw holding strength has a strong correlation with density. The main effect on screw holding strength comes from the general variance in wood density rather than from the ThermoWood process. It was found that material with lower density has better results when narrower pre-drilled holes are used.
Surface Hardness The ThermoWood process has limited effects on the Brinell hardness properties, variance is more related to the density and the wood species being used.
4. Equilibrium moisture content
The ThermoWood process leads to a reduction in equilibrium moisture content. When treated at the highest temperatures the equilibrium moisture content can be 40-50 percent lower compared to untreated wood.
Because of lower equilibrium moisture content and the changes in the chemical composition of the wood the tangential and radial swelling decreases significantly compared with the original material. In some cases the reduction in dimensional movement can be as much as 40-50%.
The ThermoWood process reduces the water uptake of wood, the levels may differ depending on the original wood species.
7. Thermal properties
The tests have shown that the thermal conductivity of ThermoWood is 20 - 25 percent lower compared to untreated wood, thus giving improved insulation performance.
8. Biological durability Standard tests (EN 113, ENV 807)
made in laboratory conditions have proven a significant improvement in biological durability. Improvements in biological durability are a result of the removal of natural food sources in the wood and also changes in the chemical and structural composition. Levels of resistance to fungal decay increase as higher temperatures are used. ThermoWood is recommended to be used in hazard classes 1 to 3 in accordance with EN-335-1 without the need for any further chemical protection. The treatment is throughout the wood piece and is not subject to leaching problems.
9. Weather resistance
As with most materials, ThermoWood is unable to resist the effects of ultra violet radiation. As a result, over a fairly short period of time when exposed to direct sunlight, the colour changes from the original brown appearance to a grey weathered colour. In addition the ultra violet radiation can cause small surface shakes to occur. Natural effects of rain and sun will cause some early wood to be removed from the surface, especially on un-coated boards, this occurs with all wood material over time. It is highly recommended to apply a pigment based surface protection to prevent colour changes and other natural effects of the weather, more information can be found from the Finnish ThermoWood association surface coating handbook.
Summary of the effects of the ThermoWood process on wood properties, by treatment class
Softwoods (pine and spruce) Thermo-S Thermo- D Treatment temperature 190 °C 212 °C Weather resistance + ++ Dimensional stability + ++ Bending strength no change - Colour darkness + + +
Hardwoods (birch and aspen) Thermo-S Thermo- D Treatment temperature 185 °C 200 °C Weather resistance no change + Dimensional stability + + Bending strength no change - Colour darkness + + +
Thermowood Treatment Classes
Two standard treatment classes for softwoods and for hardwoods have been introduced. These classes have been developed to take into account the key characteristics of ThermoWood, standard temperatures have been selected to ensure best overall technical performance of the final product. The standard treatment classes are named Thermo-S (190°C +/-3°C ) and Thermo-D (212°C +/-3°C ) for softwoods, and hardwoods Thermo-S (185°C +/-3°C ) and Thermo-D (200°C +/-3°C). End use recommendations:
Softwood (pine, spruce)
- building components - wall and ceiling panels in dry conditions - furniture - garden furniture - door and window components - sauna products - flooring - fascia boards - cladding - shutters
- cladding - fascia boards - exterior joinery - shutters - sound barriers - sauna and bathroom furnishing - decking - garden furniture - other exterior structures
Hardwood (birch, aspen)
- wall and ceiling panels - furniture - garden furniture - flooring - sauna products
End uses of hardwood Thermo-D products are same as Thermo-S products. The colour is darker because of higher treatment temperature
In addition to standard classes it is possible to produce ThermoWood in higher or lower temperatures for special purposes. Specific treatment levels can be agreed between the ThermoWood producers and industrial customers so as to optimise the needed characteristics in relation to the end use application.
Handling and storage of ThermoWood
ThermoWood must be stored in a dry place. Since no special storage temperature is required, cold warehouses are among the suitable locations. The product should be carefully covered or stored in a covered warehouse.
The packages should be stored horizontally with a sufficient number of supports under the packages to prevent the lowest boards from distortion. The packages must be stored so they do not touch the ground
Before use or further working where gluing and/or surface treatment is taking place, the material must be allowed sufficient time for conditioning to the suitable MC and temperature as per the manufacturer’s recommendations. When ThermoWood packages are lifted with a crane, forklift, or similar device, the forks should be adjusted to their maximum distance apart because of the slightly decreased bending strength of the material.
The bundles are not to be opened before use.
Handling of residual and discarded products
ThermoWood is a natural wood product without any chemicals added to it. When not glued or painted, ThermoWood waste can be handled like any other untreated wood waste.
ThermoWood can be burned. It produces about 30% less energy than untreated wood because the majority of the energy containing extractives has already been removed in the heat-treatment process. ThermoWood burns with a smaller flame and produces less smoke and harmful gases because of the factors mentioned above. Flammability is normally better due to the lower equilibrium moisture content of the wood; i.e., the wood is drier. There is no significant difference between the compounds in smoke from ThermoWood and those in the smoke from normal wood.
Different surface treatments have different maintenance intervals. The more pigment used, the longer the maintenance interval. However, if opaque paint is used, the original ThermoWood colour and appearance are obscured. As a rule of thumb, pigment-containing transparent surface treatment has a doubled or tripled maintenance interval in comparison to a treatment product without pigment. Furthermore, opaque paints have maintenance intervals twice as long as pigment-containing transparent paints.
The environment and climate have a crucial effect on the life span of a surface treatment. UV radiation from sunlight and moisture are major obstacles that must be surmounted by surface treatment. These factors mean that, for example, the south side of a building needs maintenance more often than the north side. Moreover, buildings in a continental climate have a longer life span for their surface treatment than buildings by the sea.
To ensure maximum performance of coating and avoid damages, the surfaces should be cleaned and checked annually, with any defects repaired immediately. Always refer to the paint manufacturer’s specific maintenance instructions, if available.
Health and safety
There is no significant difference between the health and safety considerations for ThermoWood and those for normal softwood or hardwood species. There are still two detectable differences: the smell of the material and the dust resulting from the processing of ThermoWood.
ThermoWood has a smoke-like smell, which likely comes from chemical compounds called furfurals. Although the human senses can easily recognise
the smell and it appears stronger than that of untreated wood, the volatile organic compound (VOC) emissions from ThermoWood are only a fraction of those of normal pine.
There have been no toxic or harmful components found in ThermoWood. It has even been tested as a bone substitute material. However, if a wood splinter penetrates the skin, it should be removed as soon as possible, just as with normal wood.
ThermoWood dust has a smaller particle size than dust from normal softwoods. It is comparable to MDF (although the density is lower) or hardwood dust. The dust can cause problems for people suffering from asthma. For the reasons mentioned above, one has to pay special attention to the utilisation of an appropriate dust extraction system.
If the dust extraction system is not sufficient, a dust mask must be used.
When gluing or painting ThermoWood, always follow the paint or adhesive manufacturers' specific health and safety instructions.
1. Sawing Sawing of ThermoWood does not significantly differ from sawing of untreated wood. Due to the stabilisation of the wood after ThermoWood process the effect of further distortion after sawing is reduced. As the resinous substances have been removed during the ThermoWood process, the machines work well and are cleaner after processing. Good dust extraction systems will be needed when sawing in factories.
2. Planing Standard planing equipment can be used when further processing ThermoWood. Some care should be taken in the set up of the infeed rollers to reduce the risk of cracking the material. Excellent planed surface quality can be achieved. Best results are achieved when hard metal blade cutters are used. Similar processing parameters to planing hardwoods should be followed. Good dust extractions systems will be needed. More information is available from the Finnish ThermoWood Association planing handbook.
3. Milling In order to get a good surface quality, blades must be sharp, otherwise tearing may occur. Greater tearing is observed when the wood is milled across the grain. The highest risk of tearing occurs at the start and end of the milling. The best results are obtained when there is sufficient solid wood material behind the blade. Processing must be pre-planned carefully.
4. Sanding There is very often no need for sanding, because after planing or milling ThermoWood has an excellent surface quality. Sanding is easy and the sand paper does not become clogged up by resin. When machine sanding, good extraction systems are needed.
5. Surface treatment To prevent colour changes and other natural effects of weathering it is recommended that surface treatment is used. Oil-based substances work well and in a similar way as with untreated wood. When working with water-solvent substances it has to be taken into account that ThermoWood has a lower water absorption rate than normal wood, this can have an effect on drying time and penetration. Results are also dependent on the paint application and drying process. The paint manufacturer’s instructions should be followed. More information can be found from the Finnish ThermoWood Association surface coating handbook.
6. Gluing ThermoWood has a slower water absorption rate, high moisture content glues, such as PVCa can take longer to dry and longer pressing times may be required. When working with PVCa glue the moisture content of the glue should be as low as possible. Two pack PVCa glues which include a chemical hardener give good results and speed up the drying time significantly. PU (polyurethane) glues work well with ThermoWood. When using PU-glues, it has to be taken into account that the hardening reaction of PU needs water. The water can be absorbed either from the wood or surrounding air. If both wood and air are very dry, gluing may fail. When gluing ThermoWood, the glue manufacturer’s specific instructions must always be referred to.
7. Mechanical joints • Screwing The ThermoWood process can reduce splitting strength of wood. The use of self-tapping screws or pre-drilling of holes must be made to avoid cracking of the material. It is recommended to use low threaded screws. It is very important to use stainless steel screws with countersunk heads for external usage or in other humid environments. • Nailing Best results are gained when using a compressed air nail gun with adjustable nailing depth on the gun. Using a normal hammer increases risk of splitting due to accidental hammer contact with the wood. It is very important to use stainless steel or other rust free nails when fixing ThermoWood outside or in humid conditions. If using a compressed air nail gun, galvanised nails can be used as no metal on metal contact occurs to break the galvanised seal. Galvanised nails are also working, if ThermoWood will be treated with covering paint after nailing. It is also recommended to use small oval head nails as this also helps to reduce the risk of splitting. Additional points when working with ThermoWood: • Sharp tools should be used to achieve best results • The dust has smaller particle size than normal wood. Special attention has to be paid to the dust extraction system and when working in confined spaces dust masks should be used.
List of standards applicable to wood modification
List of standards
– EN 20 – 1 Wood preservatives. Determination of the protective effectiveness against Lyctus Brunneus (Stephens). Part 1: Application by surface treatment (laboratory method)
– EN 21 Wood preservatives. Determination of the toxic values against Anobium punctatum (De Geer) by larval transfer (Laboratory method) – EN 46 Wood preservatives. Determination of the preventive action against recently hatched larvae of Hylotrupes bajulus (Linnaeus) (Laboratory method)
– EN 47 Wood preservatives. Determination of the toxic values against Hylotrupes bajulus (Linnaeus) larvae (Laboratory method)
– EN 84 Wood preservatives. Accelerated ageing of treated wood prior to biological testing. Leaching procedure
– EN 113 Wood preservatives. Test method for determining the protective effectiveness against wood destroying basidiomycetes. Determination of the toxic values
– EN 117 Wood preservatives. Determination of toxic values against Reticulitermes santonensis de Feytaud (Laboratory method)
– EN 252 Field test method for determining the relative protective effectiveness of a wood preservative in ground contact
– EN 302-2 Adhesives for load-bearing timber structures; test methods; part 2: determination of resistance to delamination (laboratory method)
– EN 335 – 1 Durability of wood and wood-based products - Definition of hazard classes of biological attack - Part 1: General
– EN 335 – 2 Durability of wood and wood-based products - Definition of hazard classes of biological attack - Part 2: Application to solid wood
– EN 350 – 1 Durability of wood and wood-based products. Natural durability of solid wood. Part 1: Guide to the principles of testing and classification of the natural durability of wood
– EN 350 – 2 Durability of wood and wood-based products. Natural durability of solid wood. Part 2: Guide to natural durability and treatability of selected wood species of importance in Europe – EN 392 Glued laminated timber - Shear test glue lines – EN 408 Timber structures. Structural timber and glued laminated timber
EN 927 – 3 Paints and varnishes. Coating materials and coating systems for exterior wood. Part 3: Natural weathering test
– EN 927 – 4 Paints and varnishes. Coating materials and coating systems for exterior wood. Part 4: Assessment of the water-vapour permeability
– EN 927 – 5 Paints and varnishes. Coating materials and coating systems for exterior wood. Part 5: Assessment of the liquid water permeability
– EN 12037 Wood preservatives - Field test method for determining the relative protective effectiveness of a wood preservative exposed out of ground contact - Horizontal lap-joint method – ISO 5660 – 1 Fire tests; reaction to fire; part 1: rate of heat release from building products (cone calorimeter method) – ISO 6341 Water quality -- Determination of the inhibition of the mobility of Daphnia magna Straus (Cladocera, Crustacea) -- Acute toxicity test
– ASTM D 3273 Test Method for Resistance to Growth of Mold on the Surface of Interior Coatings In an Environmental Chamber