6 Wood Discoloration

7.4 Protection 157
unpleasant smell. Stake tests according to EN 252 with acetylated pine wood
samples showed that the resistance of samples with an acetyl content of about
20% equals that of CCA treated wood with 10 kg/m 3 retention (Larsson Brelid
et al. 2000). Brown-rot decay became zero at a weight percent gain (WPG) of
about 20% due to acetylation, and white-rot was prevented at a WPG of about
12% (Ohkoshi et al. 1999). As other anhydrides, propionic, butyric and hexanoic
anhydrides were tested against brown, white and soft-rot fungi (Suttie
et al. 1999; Papadopoulus 2004). Several carboxylic acid anhydrides were used
for pine sapwood (Dawson et al. 1999).
Impregnation with melamine resins leads to a deposition of the resin in the
cell wall (Rapp et al. 1999) and there to the “blockade of hydroxyl groups”
without chemical linkage, which also improves the mechanical properties and
durability of wood (Rapp and Peek 1996; Lukowsky et al. 1999).
Impregnation with 1,3-dimethylol-4,5-dihydroxyethylen urea (DMDHEU)
effects the “linking-up of neighbored hydroxyl groups” by etherification with
the N-methylol groups (Rapp and Müller 2005). There was no significant
weight loss by Trametes versicolor of beech wood samples with 25% WPG of
DMDHEU (Verma et al. 2005).
There are various methods to produce thermally modified timber (“thermal
modification of wood”) which leads to improved dimensional stability
(Tjeerdsma et al. 1998) and biological resistance, but also partial wood degradation
and discoloration. The processes have in common that the wood is
subjected to temperatures between 160 and 260 ◦ Cinanatmospherewithlow
oxygen content (Leithoff and Peek 1998; Rapp 2001; Ewert and Scheiding 2005).
Potentially toxic byproducts have been considered by Kamdem et al. (2000). In
Europe, about 45,000 m 3 of thermally modified timber were produced in 2004.
Four basic technologies have been established: the Finnish “Thermo wood”,
the Dutch “Plato wood”, the French “Retification”. Heat is transferred to the
wood in the gas phase of air, exhaust fumes of combustion gases or nitrogen.
The German “oil heat treatment” uses a vegetable oil (rape) for heat transfer,
which additionally affects hydrophobization (Sailer et al. 2000; Bächle et al.
2004). The wood is used outdoors, e.g., for façade covering, noise barriers,
and in gardens for decks, and indoors, e.g., for floorings. Four years lasting
field tests of wood samples from the four European industrial heat treatment
processes indicated that heat treated wood appears to be not suitable for in
ground application, since only durability classes in the range from 2 (durable)
to 4 (slightly durable) were achieved (Welzbacher and Rapp 2005). Thermalhygro-mechanically
densified wood showed reduced hygroscopy and improved
mechanical performance, and resistance to fungal degradation (Schwarze and
Spycher 2005).
Wood hydrophobization occurs by oils, waxes, paraffins, and silicons. Sailer
(2001) and Rapp et al. (2005) used vegetable oils. The oil, which is deposited in
the cell lumina, reduces water uptake without inhibiting vapor release. A wax-
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