Biological Properties of Thermally Modified Wood

5 Biological Properties of Thermally Modified Wood 5.5.1 Microbiological Attack 

Loss of polysaccharides during heat treatment might simplistically be expected to lead to enhanced protection against brown rot fungi compared to white rotters. This has been found in some studies of the effect of thermal treatment upon decay resistance. Tjeerdsma etal. (1998a, 2000,2002) found that Plato heat treatment resulted in significant improvements 

Kim etal. (1998) investigated the effect of different treatment times and temperatures upon the decay resistance of wood thermally modified in air against T. palustris. The relationship between heating period and weight loss due to decay was modelled using a regression equation. Improvements in decay resistance were found as the heating period and temperature increased. Erom the numerical analysis employed, it was predicted that heat treatment at 150 °C for about 150 hours would achieve decay resistance comparable to that of a CCA treatment at 1 % retention. 

In a study of the effect of heat treatment upon the durability of spruce and larch heated at 200 °C in a vacuum, both spruce and larch exhibited good resistance to decay by G. trabeum, but treated spruce exhibited a decay mass loss of 9 % compared to 1 % with larch against C. puteana in an 8-week mini soil block test (Rep etal, 2004). 

Tjeerdsma etal. (1998a) subjected thermally modified wood to soft rot decay in unsterile soil tests. Although decay resistance was enhanced by thermal treatment, it was not possible to prevent attack entirely. Treatments giving the best performance in terms of decay resistance also exhibited the highest strength losses. Viitanen etal. (1994) have also reported on some preliminary experiments on heat-treated wood in unsterile soil. 

---

The properties of thermally modified wood are highly dependent upon the thermal treatment employed, and it is very important to take these into account when comparing the various treatment methods employed. This chapter examines the effect of the process variables upon the properties of thermally modified wood, and then considers the chemistry of thermal modification. Studies of physical changes are discussed, followed by an overview of the biological properties of thermally modified wood. A short description of some recent literature on the use of thermal treatment combined with compression and on hot oil treatments is also included. 

---