Types of Preservatives
Preservative may be classified broadly into three groups:
- Waterborne inorganic compounds are water soluble chemicals used to protect against insects and decay. The water evaporates leaving the chemical affixed to the wood cell walls.
- Creosote or creosote solutions are oil-based distillates of coal tar used to protect against insects and decay. Creosote occupies the cell cavities of wood fibre.
There are two types of waterborne preservatives in current use. The most common one is chromated copper arsenate (CCA) which is easily identified by the green coloration it imparts to wood (brown coloured formulations are also available). The other is ammoniacal copper arsenate (ACA).
Both these chemicals are mixtures of stable metal oxides dissolved in water. CCA is an aqueous solution of copper, chrome, and arsenic while AVA is an aqueous solution of copper arsenate and ammonium hydroxide.
Copper acts as a fungicide to give the chemicals resistance to decay. The arsenate is toxic to wood destroying insects such as termites.
CCA and ACA affix chemically to the wood fibre by uniting the wood sugars to render wood unsuitable as food to insects and fungi. The copper in the preservative also deters the formation of moss on the treated wood surface in damp climates.
CCA and ACA treated wood do not release excess chemical when properly treated and produce a clean, dry surface, which is odourless.
The treating process does not obscure the natural grain of the wood. Treated surfaces may be stained or painted. Treated wood can also be left uncoated, in which case it will acquire a driftwood grey colour.
Creosote is a distillate of coal tar produced bu the high temperature carbonization of bituminous coal. Creosote is primarily composed of liquid and solid aromatic hydrocarbons as well as some tar acids and tar bases, which provide protection against destructive insects and organisms. It is not often used in building construction.
Pentaclorophenol, or penta, is the normally used oilborne preservative. Penta pressure-treated wood is highly resistant to fungi and insect attack, but because of it petroleum base, is not usually associated with building construction. It is used as a preservative for applications such as utility poles.
The waterborne preservatives are the chemicals most used for building construction. These chemicals are used to provide protection for plywood, parallel strand lumber (PSL), glulam, dimensional lumber and timbers used for wood foundations, roofing accessories, facias and soffits, structural members in exposed locations, and for any other building application where wood is exposed to damaging conditions, including landscape features.
Creosote and oilborne preservative-treated wood products are not usually associated with building construction because of odour and appearance but rather find use in heavy industrial applications such as in dock and railroad construction.