6 Wood Discoloration

7.4 Protection 155
Table 7.9 shows the applicability of wood preservatives and treating processes
depending upon the wood moisture content.
For pressure treatments [full-cell process, empty-cell process (Lowryprocess,
Rüping-process), vacuum-process] and principally for creosote, the
moisture content should be below fiber saturation. For short-term procedures
(superficial treatments) and also for water-based preservatives, at least the
wood surface must begin to dry. To bring the active substances into the wood,
the procedures may be arranged into four major groups (Table 7.10).
In Germany, there are 234 pressure plants and 2,115 plants that use soaking
(Quitt 2005). The necessary retention of a preservative depends on the endangerment
of the wood, on the efficacy of the active ingredient, and the treatment
procedure. The minimum quantities are shown in the respective DIBt approval.
The preventing chemical preservation of wood-based composites is regulated
in DIN 68800 part 5 (1978).
Wood plastic composites (WPCs) are a new material with plastic as a matrix
and embedded wood particles and fibers as well as distinctive additives
(Teischinger et al. 2005). The material is produced. e.g., by an extrusion process
or injection molding process. Wood is added for better technical properties
and for cost reduction. WPCs are increasingly used as a substitute for wooden
decks especially in North America. Marketing of WPC products as “maintenance
free” has been a key factor contributing to their success with the
consumer. WPCs are nevertheless susceptible to fungal degradation despite
the close association of wood with the plastic. Wood particles close to the surface
of WPC products can attain moisture levels high enough to facilitate the
onset of decay. Borates markedly reduced mass loss of WPC by Gloeophyllum
trabeum in a soil block test (Simonsen et al. 2004). Mankowski et al. (2005)
showed almost no mass loss by G. trabeum and Trametes versicolor in samples
that had been treated with zinc borate.
The non-chemical protection and chemical preservation of bamboo are
described by Liese (2002) and Liese and Kumar (2003).
Methods to determine the amount of active substances in the wood and
to measure penetration depth are described by Petrowitz and Kottlors (1992),
Schoknecht et al. (1998) and Schoknecht and Bergmann (2000). An overview
is in the Internet (www.holzfragen.de/seiten/hsm reagenzien.html). N-cyclohexyl-diazeniumdioxide
in impregnated pine wood was measured by direct
thermal desorption-gas chromatography-mass spectrometry (Jüngel et al.
2002).
Since about 1975 critical reports increase with regard of possible environmental
impacts by chemical wood preservation, like by pentachlorophenol and
chromate-containing preparations, the pollution of the soil by leached chemicals
(Willeitner 1973; Willeitner et al. 1991; Leiße 1992; Hartford 1993) and
due to problems arising from the disposal of treated timber (Marutzky 1990;
Voß and Willeitner 1992). Pentachlorophenol (PCP) has protected wood since
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