Thermally Modified Wood

Thermally modified wood is produced by subjecting solid wood to high temperatures to improve their dimensional stability and biological durability. Solid wood is exposed t 160-240 ^oC in an atmosphere of low oxygen content or oil is used to transfer heat to the wood.

The wood’s properties can be altered by modifying its cell wall components using heat. Portions of hemicelluloses are hydrolyzed into to their monosaccharide components such as glucose, galactose, mannose, arabinose and xylose The amorphous regions of cellulose are also hydrolyzed, breaking cellulose into shorter chains. The degradation of the cell wall’s two major components leads to reduced free hydroxyl groups in the chemical structures and increased cross-linking of lignin.

The modifications result in several changes in the wood’s properties. The following are basic properties of thermally modified wood, regardless of the kind of thermal modification ™ process used: (1) decreased heat conductivity by 10 – 30%; (2) decreased mechanical strength by up to 30%; (3) reduced weight by 5 – 15%; (4) decreased shrinking and swelling by 50-90% due to reduced equilibrium moisture content of the wood; (5) improved biological durability; (6) extractives migration to the wood surface, and (7) color change from whit or yellow to dark brown.

Commercial production of thermally modified wood started in the late 1990’s in Europe. ThermoWood. PlatoWood, Retification and Perdure, and Oil-heated wood of Finland, Netherland, France and Germany, respectively, are some of the popular TM processes.

Green, air-dried and kiln dried wood can be thermally modified depending on the TM process used. For example, in the Finnish Thermowood process, green wood is first dried above 100 0c. Thermal modification follows after the desired moisture content of 10% is reached. The treatment temperature and duration will vary depending on the wood’s end-uses.

In 1974, FPRDI started conducting studies on thermal modification. Mailum and Arenas subjected five local wood species to different temperatures ( 130, 150 and 175 ⁰C). Results showed improvement in the natural durability of acacia [Samanea saman (Jacq.) Merr.], guijo [Shorea guiso (Blanco) Blume], mayapis [Shorea palosapis (Blanco) Merr.] and palosapis {Anisoptera thurifera (Blanco) Blume spp. Thurifera] against two decay fungi species, i. e. Fomes liviudus, a white-rot fungus and Lenzies striata, a brown-rot fungus.

However, no similar studies followed thereafter until a 2009 research on plantation species, malapapaya [Polyscias nodosa (Blume) Seem.]. Malapapaya was thermally modified to improve its physico-mechanical properties and decay resistance using different combinations of temperature and treatment durations. Results showed significant improvement on the biological durability and dimensional stability, as well as color change in malapapaya wood.