Development and mode of action of thermal modification processes
Due to an increasingly strict regimentation of biocidal wood preservation, in several European countries the development and establishment of environmental friendly protection measures was forced since the 1980s. Especially the thermal modification of timber at temperatures between 180 and 220°C became important in research. Nowadays this technique is implemented in different processes and reached industrial scale.
All thermal modification processes have in common, that wood is exposed to high temperatures in a low-oxygen atmosphere, which leads to a heat-induced modification of the cell walls. Degradation of wood cell wall components (mainly hemicelluloses) as well as a reorganization of molecules (mainly lignin) occurs, which causes a reduction in moisture uptake and swelling of up to 50%. Furthermore the resistance against wood-destroying fungi is increased. However, the mechanical properties of thermally modified timber (TMT) are reduced due to the gasification of wood substance.
Thermal modification processes differ in the way to avoid burning of the timber during the high temperature treatment. Therefore the oxygen content needs to be minimized through a water steam atmosphere, nitrogen gas, or a liquid medium, e.g. vegetable oil.
Properties of thermally modified timber (TMT)
Properties of differently heat treated timber were examined in the frame of research projects at the University Hamburg and the Federal Research Centre for Forestry and Forest Products (BFH) since 2000.
The impact of high temperatures on wood during heat treatments is connected to a reduction of static but even more on dynamic strength properties. However, the modulus of elasticity (MOE) is less affected and thus the flexural stiffness of TMT is not to be seen critically.
In comparative studies the mechanical properties of differently manufactured TMT (TMT A, B, and C = processes in gaseous atmosphere; OHT = oil heat treatment) were determined in standard laboratory tests. The MOR was reduced by up to 38 %, while the maximum los in MOE was 15 % only. Even more drastic strength loss was observed for dynamic load tests: the impact bending strength decreased by 42 – 64 %.
Besides mechanical strength, some other properties, which are important for many typical applications of TMT, e. g. the resistance to wear and abrasion and the surface hardness. Both properties decrease with increasing treatment intensity, which comes along with increasing brittleness of the material. For flooring and decking applications a reduced wear resistance can be seen as critical, while the slight reduction in Brinell hardness seems to be negligible.
Resistance against wood-destroying fungi
Wooden components exposed outdoors above ground, e.g. terrace decking boards, are falling into European use class 3 (UC 3), while poles, posts, and other components, which are in direct contact with the ground or fresh water, belong to UC 4 (EN 335, 2006). Therefore field studies were carried out with horizontal double layers and grave yard tests representing both exposures, UC 3 and UC4, since 2001. Decay was assessed on all specimens once a year using the 5-step rating scheme according to EN 252 (1990): 0 =sound; 1= slight attack; 2 = moderate attack; 3 = severe attack; 4 = failure). Durability classes were determined according to EN 350-1 (1994). In addition, the resistance of all materials tests against Poria placenta, which is regarded as one of the most aggressive fungi to TMT, was tested.
The results from lab and field tests have shown that TMT is not suitable for use in ground contact, independently from the modification process. Nevertheless its durability is increased and predestines it for above ground exposures, e.g. decking, cladding, or garden furniture.
The optical appearance of wood products play an important role in practice: Without pigmented coatings exposed wood will turn grey rapidly. TMT also grays, sometimes even faster than untreated wood. Furthermore thermally modified sapwood is also susceptible to blue stain fungi.
Although graying and blue stain do not affect the functional properties of TMT, the end user usually expects that the appearance of a wooden products does not change with time. Thus, the natural process of graying is often seen as quality defect.
Current research projects at IBW are therefore focusing on the mechanisms responsible for graying of TMT and the development of suitable protection measures. Presently various UV stabilization systems undergo laboratory and field exposure tests.
Resistance against marine borers
Since 2002 resistance tests against Teredo navalis (Ship worm, Fig. 2A) - one of the most aggressive marine borers – are running in South Sweden. The very high inoculums potential can be seen from untreated control specimens, which were completely destroyed after only one year of exposure (Fig. 2B+C) and need to be replaced every year.
The assessment of TMT specimens after 5 years of exposure the oil-heat treated material showed only slight attack (Fig. 3). The other TMT materials were already destroyed completely after one year. In this case probably the combination of thermal modification and a high oil uptake led to the increased resistance against Teredo. Heat treatment or oil impregnation as single measures did not achieve a durability increase.
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