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| 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
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
-General availability
-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.
Pentachlorophenol Solutions
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 5–7% (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.
Waterborne Preservatives
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.
Copper Naphthenate
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.
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