Wood preservation facilities, creosote: chapter D-2

2. Physical and Chemical Properties

Creosote is a complex and variable mixture produced from coal that is made up of more than 250 compounds.

There are five major classes of compounds in creosote (3):

• Aromatic Hydrocarbons including PAHs, alkylated PAHs, benzene, toluene, and xylene (PAHs can constitute up to 90% of creosote);
• Phenolics including phenols, cresols, xylenols, and naphthols (1 to 3% of creosote);
• Nitrogen-containing Heterocycles including pyridines, quinolines, acridines, indolines, carbazoles (1 to 3% of creosote);
• Sulphur-containing Heterocycles including benzothiophenes (1 to 3% of creosote); and
• Oxygen-containing Heterocycles including dibenzofurans (5 to 7.5% of creosote) (United States Environmental Protection Agency, 1987).

Polycyclic aromatic hydrocarbons (PAHs) are the main constituents of creosote, with the most toxic being anthracene, fluoranthene, acenaphthene, fluorene, naphthalene, chrysene, pyrene and phenanthrene) (6). Other components can include phenols, cresols, cresylic acid, pyridines, quinolines and acridines. All components may vary in proportion (3).

Creosote oil is the common name used to describe different fractions of creosote.

• Creosote
  • P1/P13: Creosote preservative - 50% coal tar creosote, 50% petroleum oil
  • P2: Creosote solution
  • P3: Creosote & petroleum oil solution
  • P 13: Petroleum oil for blending with creosote
• Creosote Additives and/or Extenders
  • Naphthalene oil - naphthalene is a coal tar derived product
  • Coal tar distillate blend

There are two major types of creosote: P1/P13 fraction (creosote preservative being 50% coal tar creosote and 50% petroleum oil), which is used in the treatment of poles and pilings; and the P2 fraction (creosote solution) which is used in the treatment of railroad ties.

These two fractions of creosote are derived by carbonizing coal through high-temperature distillation and collecting the coal tar fractions that are composed of light oil, middle oil and heavy (oil) anthracene. The middle oil fraction is further distilled, creating additional fractions. P1/P13 and P2 fractions are collected when the middle oil temperature is between 210°C and 355°C (6).

The many components in creosote complement each other with respect to wood preservation. The lighter molecular weight PAHs in creosote are generally more toxic to decay organisms. The heavier molecular weight components of creosote help “retain” the more toxic lighter components within the wood by minimizing leaching or volatilization. The heavier residues of creosote, when impregnated into wood, also act as water repellents, limiting moisture changes (2). This improves the wood’s dimensional stability and reduces checking and splitting (1).

Because of the many components of creosote and their varying concentrations, the physical and chemical properties of creosote per se can only be generalized. Table 2 summarizes properties that have been compiled in various documents.

Properties that warrant special consideration for the safe handling of creosote include the following:

• combustibility
• moderate vapour pressure
• solubility of minimal fractions in water
• density, which is greater than water, implying that creosote will sink to the bottom of fresh and marine waters

The physical and chemical properties of creosote are outlined in Table 2. General physical and chemical properties can be obtained from manufacturers’ material safety data sheets (MSDS) and the labels for pesticide products (pesticide label). Electronic copies of the pesticide label can be obtained from the Health Canada website (4).