Methods Of Active Impregnation

Experience has shown that timber with a high content of tar and resin lasts longer than timber with a low content of either. This is partly because the timber is harder and partly because these substances have ingredients that are poisonous to fungus and certain insects. Traditional types of timber protection aim to increase the quantity of such materials by covering the timber with tar. Extract from bark has also been used to impregnate oak, birch and spruce, with good results. This method was once so popular that bark extract became a major Norwegian export. Over 2000 years ago the Chinese used salt water as an impregnating agent. Wood containing more than 5% of table salt (sodium chloride) is not susceptible to fungus. The modern version of this is the use of metal salts. Wood tar has mostly been replaced by derivatives of fossil oil (Table 19.3).

Since forestry was industrialized, the general quality of timber has deteriorated considerably, and the need for biocides has rocketed over the last decades. New fashions in architecture, which include highly exposed exterior timber structures, have accelerated this trend.

For active impregnation the following functional qualities are desirable:

• Enough poison to prevent attack from fungus and insects.

• Not be poisonous to people or animals.

• The ability to penetrate into the material.

• Resistant to being washed out or vaporized from the material.

• Free from damaging technical side effects such as miscolouring, corrosion of nails, etc.

Unfortunately, an impregnating substance with all these qualities does not exist. There is generally a clear relationship between toxicity and effectiveness. Effective poisons such as metal salts have particularly damaging effects on the environment, including humans. Less damaging substances such as bark extract and ferrous sulphate are at the same time less effective and susceptible to rapid leaching.

Preventive impregnating agents must be differentiated from treatments that are used after the material has been attacked. The same substance can, however, often be used in both cases. To make the mixtures fully effective, both fungicide and insecticide may be needed in the same mix. They are dissolved in water or organic solvents.

|Table 19.3 Active substances in fungicides and insecticides

I

Type

Fungicide

Insecticide

Toxicity

DC

Mineral based

< Û.

Arsenic salts

x

x

Very high

Aluminium sulphate

x

Low

Boric acid, oxides and salts

x

x

Medium

Copper and copper salts

x

Medium

Ferrous sulphate

x

Low

Fluorine salts

x

Medium

Sodium chloride (table salt)

x

Low

Zinc and zinc salts

x

x

Medium

Oil and coal based

Creosote

x

Very high

Endosulphane

x

Very high

Hexachlorobenzene (Lindane)

x

Very high

Parathion

x

Very high

Pentachlorophenol

x

Very high

2-Phenylphenol (Preventol)

x

Medium

Pyrethrin (Permethrin)

x

High

Triazole (Propiconazole)

x

Medium

Tributyl tin

x

Very high

Wood based

Bark extract

x

Low

Tar from softwood

x

High

Tar from beech

x

Low

Wood vinegar

(x)

(x)

Low

The substances are applied to the timber by pressure/vacuum treatment or by dipping/brushing on.

Apart from creosote (composed of polycyclic aromatic hydrocarbons), pyrethrine (Permethrine) is the most common oil derivative and has superseded such derivatives as pentachlorophenol, which were phased out during the 1980s and 1990s because of high environmental and health risks. The most important metals used for impregnation are arsenic, chrome and copper where copper is regarded as least toxic.

There are different classes of impregnating substances; when in contact with the ground, timber requires strong substances in large doses, but in well-ventilated, outdoor cladding, a much weaker mix will be effective enough. A strong salt impregnation agent usually contains a mixture of copper, chrome and arsenic salts (CCA) where arsenic and copper are the biocides and chrome acts as fixative, however still highly toxic. For timber above ground level it is quite adequate just to use copper compounds.

Both metal and fossil oil products are based on very restricted resources.

Production of impregnating substances and the work at manufacturing workshops can result in emissions of strong biological toxins to earth, air and water. Heavy metals are highly toxic and have large biological amplification capacities. Also frequently used fluorine salts, zinc salts and borates have serious toxic effects. From the impregnation industry based on fossil oils, vaporized solvents can be released as well as a range of chlorinated hydrocarbons. Many of these will, in the same way as heavy metals, have a capacity for biological amplification.

In the house, solvent-based tributyltin, zinc and copper-naphthe-nates volatize, potentially exposing the occupants to their toxic fumes. Water-soluble metal salts are usually stable in buildings. They are, however, released from exterior surfaces exposed to rain and may contaminate ground water and soil. Normally about 30% of the metal salts will leach out in the course of a 30-year period (Hansen, 1997). Acid rain increases the rate of leaching. These substances will quite easily combine with earth particles which delays the drainage and spread of the substances to some extent. Pure sand will not have this effect. Leaching will also occur with the oil-based products. These also have gaseous emissions to air. In creosote impregnated buildings, considerable concentrations of naphthalene have been registered inside buildings even when the application has been outdoors (Gustafsson, 1990). Creosote combined with solar radiation can also cause rapid and serious burning of the skin.

If creosote-impregnated timber is combusted at temperatures under 350 °C, the entire contents of polycyclic aromatic hydrocarbons (PAH) will be emitted with the chimney gases. Much the same is the case with products impregnated with CCA; for example, about 80% of the arsenic compounds are released. Alternatives are either disposal at strictly controlled tips, where the substances will eventually return to nature, or combustion at very high temperatures, as in cement factories.

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