Wood preservation facilities, creosote: chapter D-1

Wood Preservation Canada ( WPC ) describes creosote as the oldest and one of the most effective industrial preservatives for protecting wood from deterioration and decay caused by fungi, insects and marine organisms.

Creosote has been known for its preservative properties since 1706. In 1838, the Bethell (full cell) process using creosote was patented. Creosote has been applied to a large variety of wood products for more than 150 years. It is used primarily for railway ties (where it is often blended with heavy petroleum oil), utility poles, marine piling and timbers, and highway construction (1). Creosote has been described as one of the most effective substances known for the protection of wood against all forms of wood-destroying organisms. It has a marked toxicity to a wide spectrum of wood-destroying fungi, marine borers and insects.

Creosote’s attributes, aside from its broad-spectrum efficacy, are that it gives the treated wood water repellency, improved dimensional stability and mechanical wear, corrosion resistance, reduced electrical conductivity, and increased resistance to corrosive chemicals.

Creosote, as defined by the American Wood Protection Association (AWPA), is a distillate derived from coal tar, derived by the high-temperature carbonization of bituminous coal. Creosote consists primarily of liquid, solid polycyclic aromatic hydrocarbons (PAHs), other heteronuclear aromatic substances, and some tar acids and bases (2).

 

This carbonization process takes place during the making of coke by the steel industry. The coal tar is then distilled to produce creosote and other products.

Many factors affect the character and composition of creosote, including the characteristics of the coal that is used, the method of coal tar distillation and the temperature range in which the creosote fractions are collected. Creosote has approximately 200 to 250 chemical compounds. Therefore, relative concentrations of creosote components can vary from batch to batch.

During the distillation of coal tar, the first fractions contain the light oils (or low molecular weight oils), with pitch being the main product. The higher boiling point liquid fraction recovered between the light oils and pitch is designated creosote. It is heavier than water, and has a continuous boiling range beginning at about 200°C (3)

Because creosote is oily, the treated wood is somewhat water repellant. This improves the wood’s dimensional stability and reduces checking and splitting. Creosote-treated wood is also more resistant to mechanical wear, which is of vital importance for such applications as railway ties and bridge decking.

Creosote for wood preservation uses is currently produced in Canada and imported from the United States. Table 1 provides an overview of creosote use in Canadian pressure treatment facilities (4).

The CSA O80 Series of Standards specifies requirements related to the preservation through chemical treatment (pressure), which includes creosote treated products. (5)
Treatment conditions must be calibrated to yield the target retention levels described on the pesticide label.