THERMALLY MODIFIED WOOD

Thermally modified wood is wood that has been modified by a controlled pyrolysis process of wood being heated (> 180 °C) in absence of oxygen inducing some chemical changes to the chemical structures of cell wall components (lignin, cellulose and hemicellulose) in the wood in order to increase its durability. Low oxygen content prevents the wood from burning at these high temperatures. Several different technologies are introduced using different media including nitrogen gas, steam and hot oil.

Beechwood processed at different temperatures, 200 °C, 190 °C, not treated

Modifications of The Proceses

The first technological approach of thermal modification processes of wood is the paper of Burmester. There are 5 different thermal modification processes : Thermowood (or Premium wood) in Finland, Retification process (Retiwood, New Option Wood) and Le -Bois Perdure in France, Plato process in Netherlands, Oil-heat treatment in Germany (OHT-Process)
Description of the Process

Three of the processes are performed in one step, using oil (oil-heat treatment), nitrogen (Reti wood) and steam (Le-Bois Perdure). The Thermo wood process consists of drying, heat treatment and finally cooling/conditioning, and takes up to 72 hours. The Plato process consists of hydrothermolysis, dry curing and conditioning, and can take up to 7 days. The required time depends on wood species, width and initial moisture content.

comparison European Ash,  left= untreated, right= thermally modified

Characteristics of Thermally Modified Wood

The main advantage is that softwood can be used for applications that require high durability, which is environmentally friendly. Durability class 1–3, according European Standard EN 350-2, can be obtained out of non durable (class 5) softwood species. 

The main disadvantage is that the strength is decreased as a result of the high temperatures. In general the bending strength is reduced up to 30% with more reduction at higher temperatures. 

The biological resistance against some (not all) micro-organisms and insects is improved. Shrinking and swelling is reduced up to 50–90%. The treated wood is somewhat darkened in colour. As a principle the heat treatment process can be done on all wood species.

wood surface, thermally modified European Ash wood

Matureness of the Technology

Thermally modified wood is not just an academic topic these days. Thermal wood treatment has already been penetrating the market for a number of years. There is also some industrial input in the research and development. The Natural Resources Research Institute at the University of Minnesota Duluth has a pilot-scale kiln to research the effectiveness of thermal modification on underutilized Minnesota tree species to expand the wood markets.

In the guitar-making industry, this process is called "Thermo Curing", "Baked" and "Roasted", among other names. Some guitar manufacturers have begun using acoustic sound boards and electric guitar fretboards that are thermally cured in order to help prevent the typical warping and cracking that often occurs from seasonal humidity swings. As a secondary benefit, acoustic guitars tend to sound like well-broken-in aged instruments much sooner than do non-thermally modified guitars.

Ongoing Research

There is ongoing research to optimize the industrial processes and the various parameters to produce it and to develop the applications for this wood. There is also research in order to find new wood heat treatment processes, even trying to combine different wood modification processes. Last but not least there is ongoing research trying to clarify the chemical changes of wood during the thermal treatment process.

References

1. ^ Burmester, A. (1973): Einfluß einer Wärme-Druck Behandlung halbtrockenen Holzes auf seine Formbeständigkeit. Holz als Roh- und Werkstoff 31, pp. 237–243
2. ^ European Standard EN 350-2 (1994); Durability of Wood and Wood-based Products – Natural Durability of Solid Wood: Guide to natural durability and treatability of selected wood species of importance in Europe
3. ^ "Recent Development of Thermal Wood Treatments". (PDF). International Association of Wood Products Societies (IAWPS). Retrieved January 24, 2016.
4. ^ "Wood modification developments.  (PDF).HERON. Retrieved January 24, 2016.
5. ^ "8th European Conference on Wood Modification October 2015 Helsinki". ECWM 2015. Retrieved January 24, 2016.

- Wikipedia