Thermal modification of wood significantly improves the natural decay resistance of coniferous wood. Improved decay resistance is based on changes in the chemical structure of the wood cells.

Thermal modification of wood results in acetic acid being formed as a consequence of the hydrolysis of hemicellulose and the amount of hemicellulose in wood is reduced following heat treatment. This means that there is distinctly less nourishment available for decay-causing fungi and partly for this reason Thermowood®  is far better able to withstand the impacts of decay-causing fungi than untreated wood. Decay is also slowed down by the lower pH of Thermowood®.

The results of tests conducted to study decay resistance of thermally modified wood have been very encouraging. These decay tests have been conducted in accordance with the standard EN 113. Decay tests conducted in laboratory conditions involved measuring weight loss of wood, e.g. following 8, 16 and 32 weeks. Coniophora puteana, Gleophyllum trapeum, Coriolus versicolor, and Poria placenta are examples of the fungi studied.

The biological durability of Thermowood® has also been studied in 5-year field experiments. On the basis of the field tests conducted, the decay resistance of thermally modified pine wood has proved to be better than that of the present-day AB class of pressure-impregnated pine wood.

On the basis of the test results, thermally modified wood is suitable for use in external applications, but it is not recommended to be used in applications involving direct ground contact. When constructing outdoor decking or patios, for example on gravel surface, these can be made entirely of Thermowood®. Alternative products for these uses are products treated as per Thermo-D and Stellac®Wood D2 or D3.

Improved decay resistance can only be reached when the treatment temperature clearly exceeds 200°C. SWM-WOOD is the first manufacturer to have been awarded the international KOMO® product approval in the Netherlands. According to this award, thermally modified spruce wood achieves the decay resistance class 2 (EN 350-1) when the treatment temperature is 220°C. In 2004, the KOMO product approval was expanded to also cover pine and radiata pine. The same decay-resistance class currently includes impregnated wood products as per AB class.

Research results

EN-113 decay test, pine »
Weight-loss ratio/Treatment intensity, pine »
Swedish 5 years in-ground field test »