Timber impregnation

The first distillation of gas-works tar from coal carbonization was carried out at Leith in Scotland in 1822. The tar oil was used in timber impregnation, while the distillation residue, namely pitch, was employed in coal briquetting. 

In the United States creosote specification AWPA PI/89 is intended for the treatment of timber for land and fresh-water use, and the heavier grade AWPA P13/89 for the preservation of marine piling and timber. In the United Kingdom a British Standard Specification, BS. 144/90, Part 1, specifies three grades of creosote two for pressure impregnation and one for bmshing appHcation. The standards of the West European Institute for Wood Preservation (WEI) are often used in Europe. 

Most wood species are subject to some form of biological attack, a hazard not encountered by most other constructional materials. Fungal decay and attack by termites and other insect borers and marine borers are the main problems. The heartwood of many timber species and the sapwood of most species can be impregnated with wood preservatives to prevent these problems. 

The sapwood of all timber species is susceptible to attack, so the use of a durable species usually requires the removal of the sapwood. However, since sapwood can usually be fully penetrated with preservative by an impregnation treatment, an overall durable product can be obtained from the timber of small trees of a durable species containing a high proportion of sapwood. 

Timber species also vary in the treatability of the heartwood - with preservatives in an impregnation plant, and are classified into four categories according to the depth of penetration which can be achieved permeable (complete penetration) moderately resistant (6-18 mm lateral penetration) resistant (3-6 mm lateral penetration) and extremely resistant (no appreciable lateral and very little end-grain penetration). 

There are, however, timber species of low natural durability which are impermeable and therefore cannot be effectively preserved. Such timbers can only be successfully used in dry situations. Likewise where preservative-treated timber is to be used in a situation of the highest hazard, long service life is best achieved by using easily treated species impregnated with a suitable preservative. Timber species that are difficult to treat can be incised to improve preservative penetration. 

Chlorinated phenols are used for the impregnation of timber and the production of phenoxyalkano-ate herbicides that are degraded by dioxygenation to chlorophenols. 

Bethell Also known as the Full-cell process. A method for impregnating timber with a creosote preservative. The wood is first degassed under partial vacuum and then impregnated under a pressure of up to 10 atm. See also Rueping. 

The metal-organic preservatives copper HDO and aluminum HDO are commonly used for timber used in landscape improvement. Therefore, sanq)les impregnated with these preservatives were also pyrolyzed at 500 °C. Aluminum could not be measured in the oil because the chamber had to be evacuated which would have contaminated the inner walls with pyrolysis liquids. Therefore, only the char was analyzed for A1 resulting in a recovery of 82.7 %. Hence, 17.3 % of the input aluminum is expected to remain in the oil. This is much more compared to Cu and Cr due to the lower melting and boiling point of Al. 

As part of its operations between 1955 and 1977, a Finnish sawmill had been impregnating timber with a preservative to inhibit microbial degradation. This product, called Ky-5, contained a mixture of chlorophenols, namely, 2,4,6-trichlorophenol (7-15%), 2,3,4,6-tetrachlorophenol ( 80%) and pentachlorophenol (6-10%). Ky-5 also contained traces of polychlorinated phenoxyphenols and dibenzo-p-dioxins as impurities. Over the years this product had contaminated the soils around the sawmill. A cost-effective bioremediation strategy was needed that could be used at this site but also throughout Finland where 800 other sites of this type existed. 

In one sense full impregnation of timber is a false goal. If the surface layer of the timber can be impregnated with a water resistant resin, the centre can remain... 

Bulking is not neeessary to ensure dimensional stability an alternative is to reduee the permeability of the wood by filling the lumens with resin. The solution used to impregnate the timber need not penetrate the eell wall or even be water-soluble. It may be suffieient to physieally fill the lumen spaees. The timber needs to be dried to about 10% moisture eontent before pulling a vaeuum to remove the air and then foreing the monomer into the lumens under pressure. 

Heat treatment by immersion of the timber in hot oil, thus excluding air during treatment, has also been developed for the market. Rapeseed oil, or in some cases linseed oil and oil-based resins, have been used to impregnate the timber and provide a rapid thermal transfer to the wood, while excluding oxygen. The thermally treated timber market has benefited from interest not only in improved dimensional stability, but also the improved resistance to biodegradation seen in these timbers (Welzbacher and Rapp, 2002). This area is set to expand as traditional CCA preservatives are phased out due to recent legislation. 

Pines such as P. radiata are permeable and ean be dried quiekly (Table 8.4a). However their pits aspirate and if subsequently pressure impregnated with an aqueous preservative such as copper-chrome-arsenate using the full eell proeess they eannot be redried nearly as rapidly. If sueh resaturated, preservative treated timber were to be dried using the schedule in Table 8.4a steep moisture gradients would develop leading to severe checking. A milder schedule is required. Treated pine takes approximately twiee as long to dry. 

Increasing resistance of timber to impregnation requires a severer, more I prolonged treatment... 

Huang L-H, Qin T-F, and Ohira Tatsuro (2004) Studies on preparations and analysis of essential oil from Chinese fir. Journal of Forestry Research, 75(1) 80-2 Hudson MS and Henrrksson ST (1956) The oscillation pressure method of wood impregnation. Forest Products Journal, 6(10) 381-6 Hughes C (1982) The natural durability of untreated timbers. What s New in Forest Research No. 112. New Zealand Ministry of Forestry, Forest Research Institute, Rotorua Humphrey PE and Bolton AJ (1989) The hot pressing of dry-formed wood-hased composites. Part 2 A simulation model for heat and moisture transfer, and typical results. Holzforschung, 43 T) 199-206... 

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