|Wooden Pole Treatment|
Wood preservatives fall into the following two general classes:
1) Oilborne preservatives such as creosote and petroleum solutions of pentachlorophenol and copper naphthenate (CuN)
2) Waterborne preservatives such as inorganic arsenicals that are applied as water solutions
Wood may swell slightly from preservative oils, but will shrink as it loses moisture during the treating process. Creosote and solutions with heavier, less volatile petroleum oils often help protect wood from weathering, but may adversely influence its cleanliness, odor, color, and paintability.
Coal-tar creosote is a black or brownish oil made by distilling coal tar. Its advantages are as follows:
-High toxicity to wood-destroying organisms
-Relative insolubility in water and low volatility, which impart to it a degree of permanence under the most varied-use conditions
-Ease of application
-Ease with which its depth of penetration can be determined
-Long record of satisfactory use
-Recommended for marine applications
Although coal-tar creosote or creosote-coal-tar solutions are well suited for general outdoor service in structural timbers, they have properties that are disadvantageous for some purposes. The color of creosote and the fact that creosote-treated wood cannot be painted satisfactorily make this preservative unsuitable where appearance and paintability are important.
Water-repellent solutions containing chlorinated phenols, principally pentachlorophenol, in solvents of the mineral spirits type, were first used in commercial treatment of wood by the millwork industry in about 1931.
Pentachlorophenol solutions for wood preservation generally contain 57% (by weight) of this chemical, although solutions with volatile solvents may contain lower or higher concentrations. The performance of pentachlorophenol and the properties of the treated wood are influenced by the properties of the solvent used.
The heavy oils remain in the wood for a long time and do not usually provide a clean or paintable surface. Because of the toxicity of pentachlorophenol, care is necessary to avoid excessive personal contact with the solution or vapor in handling and using it. This treatment is not recommended for use in the marine environment.
Standard wood preservatives used in waterborne solutions include ammoniacal copper zinc arsenate (ACZA) and chromated copper arsenate (CCA) (types A, B, and C). These preservatives are often employed when cleanliness and paintability of the treated wood are required.
Several formulations involving combinations of copper, chromium, and arsenic have shown high resistance to leaching and very good service performance. Both ACZA and CCA are included in many product specifications for materials such as building foundations, building poles, utility poles, marine piles, and piles for land and fresh-water use.
Waterborne preservatives leave the wood surface comparatively clean, paintable, and free from objectionable odor. In the retentions normally specified for wood preservation, waterborne preservatives decrease the danger of ignition and rapid spread of flames. However, once ignited, poles treated with copper and chromium tend to smolder for a prolonged period. This is known as the afterglow effect.
Ammoniacal Copper Zinc Arsenate
ACZA (chemonite) should contain approximately 50.0% copper oxide, 25.0% arsenic pentoxic, and 25.0% zinc oxide. The net retention of the preservative is calculated as weight of copper oxide plus arsenic pentoxide and zinc oxide per volume of wood treated within the proportions in the specification.
Service records on structures treated with ACZA show that this preservative provides very good protection against decay and termites. High retentions of preservative will provide extended service life to wood exposed to the marine environment.
Chromated Copper Arsenate
CCA Type A. Service data on treated poles, posts, and stakes installed in the U.S. since 1938 has shown excellent protection by CCA Type A against decay fungi and termites.
CCA Type B. This type has been used commercially in Sweden since 1950 and now throughout the world. It was included in stake tests in the U.S. in 1949 and gives excellent protection.
CCA Type C. Composition of CCA Type C was arrived at by AWPA technical committees in encouraging a single standard for CCA preservatives. Type C is the most commonly used formulation of CCA for utility poles because of low conductivity and because it is less corrosive than the other formulations.
CuN is a commercially available preservative treatment for utility structures that is classified as a nonrestricted-use pesticide by the Environmental Protection Agency (EPA). Concentrate used to prepare wood-preserving solutions should contain not less than 6% nor more than 8% copper in the form of CuN.
The physical characteristics of CuN are similar to those of pentachlorophenol in heavy oil because they use the same carrier. CuN was first used to treat poles during the mid-1940s when, because of the limited availability of creosote, CuN was mixed 50/50 with creosote. It was used for only a limited time because of economic reasons.