Thermally modified wood

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Beech wood from different intensive treatment processes. 200 ° C, 190 ° C, untreated (red heart)
Left untreated ash wood, right thermally modified ash wood
Wood surface, ash thermowood

Thermally modified wood (English: Thermally Modified Timber , short TMT) is the final product to a thermal treatment (heating) of wood to at least 160 ° C with lack of oxygen. The term thermowood is often used synonymously . The aim of thermal wood modification is to improve the technical properties of wood as a building material over the entire wood cross-section for specific purposes. So z. For example, the high resistance to rot achieved through heat treatment means that domestic woods are also suitable for use outdoors and in wet areas, without damage from fungal attack after a short period of time . The reduced water absorption capacity of thermowood reduces the tendency for wood to swell and shrink , bowl and crack. After several decades of research, the first industrial plants began to produce in Finland in the late 1990s.

Physical and chemical processes

The first technical approach to the thermal modification of wood is the publication by Burmester. Chemically, TMT is the result of partial pyrolysis in a low -oxygen atmosphere . Temperatures of 170 ° C to 250 ° C are used for about 24 to 48 hours.

The heating affects the OH groups ( hydroxyl groups ) that are linked between hemicellulose and lignin . Hemicellulose is partially degraded from around 140 ° C and crystallizes out again in a different form. When the wood is heated, acetyl groups are split off on the hemicelluloses and acetic acid is formed . The acetic acid acts as a catalyst in the breakdown of the hemicellulose and causes a decrease in the degree of polymerization of the hemicelluloses. Alpha- cellulose is also broken down from around 150 ° C. Lignin condensation increases the relative proportion of lignin in the wood. The wood is caramelized . Volatile substances such as resins and degradation products of hemicellulose and lignin such as B. furfural and 5-hydroxymethylfurfural are at least partially expelled.

Thermal modification must be distinguished from other methods of increasing resistance such as acetylation and furfurylation, as well as softening by steaming (e.g. of beech wood ) and darkening oak by smoking .

properties

The conversion or occupation of free OH groups reduces the shrinkage and swelling in tangential, axial and radial directions by up to 70%. An increase in the natural durability against animal wood pests and fungi was also found. When using red beech wood, thermally modified wood achieves durability class 1, with spruce wood up to class 2 and with ash wood class 1–2, depending on the intensity of the thermal process . The wood color becomes darker (through the entire cross-section), but is not UV-resistant (lightening). The temperature treatment leads to a significant reduction in the pH value to 1.5. This deprives the microorganisms of their nutrient medium and the absorption of water is limited.

There is a big difference between thermo softwood and thermo hardwood . The heat-treated softwoods have a reduced density due to the degradation of the substance and the escape of resin and are very soft, which is not the case with hardwoods to the same extent. Depending on the intensity or method of treatment, the strength values ​​of the wood are reduced by the treatment. In particular, the decrease in splitting strength can be critical here.

Due to the changes at the molecular level, not all glues or coatings that are used for the starting material can also be used for the thermally modified wood. An important disadvantage of thermal treatment is the decrease in flexural strength and thus a reduction in the load-bearing capacity of the wood, which limits the possible uses. The wood also has a smoky smell that subsides over time.

production method

There are various processes for the production of thermowood that are being further developed by companies and research institutions. The Stellac process, based on steam and heat, and the Russian BICOS process are leading in large-scale industrial use. In the oil-heat process / OHT (Menz wood), pure vegetable oil is used as a heat carrier, the wood is heated in this at temperatures of up to 220 ° C. In the vacuum press drying process (Timura wood), heating plates transfer the heat to the wood.

The company Stellac Oy is considered a pioneer in the research and further development of the steam-heat process, but on February 22, 2011 Stellac applied to the competent court in Mikkeli to initiate bankruptcy proceedings. Stellac has been working on the development of an industrial production process to improve wood properties on the basis of purely thermal treatment of the wood cell structures since 1990. This has been used in a fully developed form in large-scale thermal chambers since 1996.

During the lengthy five-step process, the wood is carefully modified so that cracks caused by strong and rapid temperature fluctuations are prevented. The wood goes through the following phases:

  1. Initial warm-up phase (to 100 ° C)
  2. Preconditioning and drying phase (controlled reduction of wood moisture to 0%)
  3. High temperature phase (heating up to 230 ° C depending on the type of wood and finishing class)
  4. Conditioning phase (restoring the optimal moisture level)
  5. Cooling phase. The Stellac process is fully automated, so a high level of quality is guaranteed.

Work is currently underway on a new process that works without pressure and thus significantly reduces the outlay on equipment. A combination of steam and nitrogen is used. The Eberswalde University for Sustainable Development is in charge here .

use

In principle, all types of wood are suitable for thermal modification. The previous production concentrated on the hardwoods alder , beech , oak , maple , ash , birch and robinia as well as the conifers spruce and pine . In thermally modified wood from local coniferous and hardwoods, an ecological alternative to tropical woods such as Bangkirai or teak is seen. This is due on the one hand to the possibility of using thermally modified wood in wet or outdoor areas, and on the other to the dark color that the wood receives depending on the intensity of the thermal treatment. Thermowood also offers an alternative to the use of chemical wood preservatives that are potentially harmful to health . Typical applications include decking and furniture and wooden floors in the sanitary and sauna area.

Since the extent of the reduction in load-bearing capacity has not yet been adequately researched, TMT (in contrast to conventional construction timber ) is currently not a regulated construction product. It must therefore not be used for load-bearing and bracing purposes without a special proof of usability.

market

In terms of the total wood consumption, thermowood occupies a marginal position, but shows high annual growth rates. The growth in European production between 2000 and 2004 is estimated to be more than 300%. With (2004) around 40,000 m³ and thus around 75% of European production, Finland is by far the most important producer country, followed by the Netherlands , Austria and France .

Manufacturers using the Finnish ThermoWood process state production figures of 145,000 m³ for 2014, 7% of which were marketed in Finland, 74% in the rest of the EU and 19% in other countries. Finnish manufacturers mainly modify pine and spruce wood.

In the first half of 2010, the thermal wood production of the companies in the thermal wood specialist group of the "Bundesverband Säge-Holzindustrie Deutschland" (BSHD) increased by 43% compared to the first half of 2009 to 14,000 m³. The increase in production can partly be explained by new production facilities. 80% of the wood is used as terrace covering. Indoor use has declined compared to 2009. Use in window production increased from 5 to 12%. Beech and ash are used most frequently with around 40% each, the conifers spruce and pine are less important, and oak, fir and poplar continue to be thermally treated. The market development for thermo wood in production and in import trade is assessed positively, 75% of the respondents assume an increase, 25% expect a stagnating market, no one is expecting a decline. The Thermo Wood specialist group with eight members represents 85% of the capacities in Germany, plus two Austrian manufacturers.

Manufacturer

Finnforest, StellacWood and StoraEnso produce in Finland (ThermoWood process). In Austria there are the companies Mitteramskogler (remuneration according to the ThermoWood process) and STIA or Aberger (both according to the Stellac process). Firstwood GmbH (Stellac process), Menz-Holz (oil-heat remuneration, OHT), timura-Holzmanufaktur (vacuum press-drying process), Hagensieker / proGOODWOOD (remuneration according to ThermoWood process) and ThermoWood Spreewald work in Germany (Steam heat, BICOS process). In the Netherlands, Plato wood (Plato process) produces thermowood and in France NOW (Rectifications process).

Individual evidence

  1. ↑ European Committee for Standardization, 2007: Technical Specification prCEN / TS 15679 Thermal modified Timber - Definitions and characteristics (pdf).
  2. ↑ a b TMT leaflet of the Institute for Wood Technology Dresden gGmbH (PDF; 667 kB).
  3. ↑ A. Burmester: Influence of a heat-pressure treatment of semi-dry wood on its dimensional stability. In: Wood as a raw material. 31, 1973, pp. 237-243.
  4. ↑ Stellac Oy bankruptcy notification .
  5. ↑ Leaflet from the Institute for Wood Technology Dresden on the usability of TMT for load-bearing and stiffening components (PDF file; 230 kB).
  6. ↑ Thermowood market - demand is still low. In: ParkettMagazin. 05/04.
  7. ↑ Production statistics 2014. International ThermoWood Association
  8. ↑ Thermowood production increased by 43%. BSHD Speedletter, October 1st, 2010.

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