IRSA Lackfabrik Irmgard Sallinger GmbH

The special wood for interior fittings: Thermowood

The special wood for interior fittings: Thermowood

Natural and environmentally-friendly building materials are the trend of our time. Not lastly due to reasons of cost native wood then often comes into play. In order, nevertheless, to achieve the visual effect of an exotic wood, the native wood can be altered in such a way until it gets a darker look and feel.

All types of wood are basically suitable for thermo modification; to all intents and purposes it is practised on the following woods: alder, beech, oak, maple, ash, birch as well as robinia amongst the deciduous trees as well as spruce and pine among the coniferous. Thermally modified timber from native spruce and deciduous trees is considered as an ecological alternative to tropical modified wood like Bangkirau or Teak. On the one hand this is because of the possibility to be able to use thermally modified timber in wet or outside areas, on the other hand also because of the dark colour which the wood gets depending on the intensity of the thermal treatment. Typical areas of application are terrace wood block paving and terrace furniture as well as wooden flooring in washrooms or saunas.

Thermally modified timber (TMT) is the end product of a thermal treatment ((heating) of wood up to at least 160°C under oxygen deficiency. The term Thermowood is frequently used synonymously.

The aim of thermal timber modification is to improve the technical properties of the building material wood across the entire cross-section for particular areas of application. After several decades of research the industrial plants in Finland began production towards the end of the 1990s.

Chemically TMT is the result of a partial pyrolysis in an atmosphere low in oxygen. Temperatures of 170 °C to 250 °C are applied for approximately 24-48 hours.

The thermal modification must be differentiated from other processes of timber modification such as steaming (for instance of beech wood) or smoking.

Due to the conversion and/or occupancy of free OH Groups the degree of shrinkage and swelling is decreased in tangential, axial and radial direction by up to 70%. Additionally an increase in the natural durability against animal and fungal wood pests was ascertained. The colour of the wood darkens (throughout the entire cross section), however it is not UV resistant (lightening).

There is a great difference between thermo pinewood and thermo hardwood. The density of the thermally treated pinewood is reduced due to matter decomposition and resin discharge and it becomes very soft, which is not the case to such a degree with hardwood. Depending on the degree of intensity and/or method of treatment the strength properties of the wood are decreased by the treatment. In particular the reduction in interlaminar strength can prove critical at this point.

Due to changes at the molecular level not all glues or coatings which are used for the base material are suitable for the thermally modified wood. An important disadvantage of the thermo treatment is the decrease in the bending strength and thus a reduction in the bearing strength of the wood, which then limits its possibilities of application. Furthermore the wood gets a smoky smell, which, however, diminishes over time.

Compared to the overall wood consumption, thermo timber only takes up a marginal proportion; however, it shows a high yearly rate of growth. Thus the growth in the European production between 2000 and 2004 is estimated at more than 300%. With (2004) around 40.000 m3, and thus around 75% of the European production, Finland is by far the most important country of production, followed by the Netherlands, Austria and France.

For the year 2007 producers using the Finish ThermoWood-Process quote production figures of

72.000 m3, of which 19% is sold in Finland, 73% in the rest of the European Union, and 8% in other countries. Far and away the most widely used timber by these producers is pine and alder.

Literature sources:

  • European Committee for Standardisation, 2007: Technical Specification prCEN/TS 15679 Thermal modified Timber - Definitions and characteristics
  • TMT Merkblatt des Instituts für Holztechnologie Dresden GmbH
  • Merkblatt des Instituts für Holztechnologie Dresden zur Verwendbarkeit von TMT für tragende und aussteifende Bauteile
  • Thermoholz-Markt – Nachfrage noch klein. In: ParkettMagazin 05/04
  • Finnish Thermowood Association: Thermowood Products