Wood Utility Poles and Preservative Choices

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By Elaina Jackson

There are some 160 to 180 million wood utility poles in service in the US. They are the backbone of overhead line construction, and most of these poles are pressure treated with some type of preservative. Choosing a pole treated with an appropriate wood preservative for the environment can save a utility time, frustration and money. And, there are a number of chemical choices available.

Species and Options

According to "Wood Pole Purchasing, Inspection, and Maintenance: A survey of Utility Practices," an article in the Forest Products Journal, 69 percent of poles in service are southern pine, followed by Douglas-fir (15 percent) and western red cedar (13 percent).

The most prevalent wood preservative for poles in service is pentachlorophenol (Penta). Approximately 63 percent of poles are treated with this preservative. This is followed by chromated copper arsenate (CCA) (16 percent), creosote (16 percent), copper naphthenate (3 percent) and ammoniacal copper arsenate or ammoniacal copper zinc arsenate (1 percent).

Pacific Wood Preserving's treated CuNap poles await shipment to utility customers.
Pacific Wood Preserving's treated CuNap poles await shipment to utility customers.

According to industry expert Mike Freeman, who was quoted in "AWPA Penta Re-Affirmation Data Package for AWPA Standards P8.1 and P35-08" in April 2010, some 16.5 million pounds of technical Penta are used annually, each year resulting in the Penta treatment of an estimated 2 million wood utility poles. Freeman estimates a total of 4.2 million poles treated annually with all preservatives.

Southern pine species can be treated with a variety of wood preservatives, including waterborne (treatments where the carrier for the preservative is water) and oilborne preservatives (where the carrier is oil or the treatment is creosote). Southern pine poles readily accept chemical treatment, although the trees do not generally grow as large as Douglas-fir, which is a refractory species and more difficult to treat. In general, East Coast distribution poles are southern pine, while wood transmission poles are made from Douglas-fir or cedar. In the West, both distribution and transmission poles are usually Douglas-fir or cedar.

All wood preservatives typically used for utility poles are robust, with many decades of data supporting their effectiveness. The cost to install poles and the need for reliable performance make utilities reluctant to change preservatives or preservative systems.

Chemical Treatment Option 1: Pentachlorophenol

Penta has been used for utility poles since the early 1940s. While Penta has a "bad rap" from some environmental organizations, the Penta manufactured today in the US and Mexico has reduced toxic characteristics because of regulatory mandates. According to Freeman, there is some confusion with toxicity between Penta pressure-treated wood and more toxic options, though the latter are no longer used. Some countries have banned the use of Penta for pressure treatment.

Wood treaters purchase Penta in block form and dissolve it in cosolvent or whole P9 oil-as defined by the American Wood Protection Association (AWPA)-or purchase it as a 40 percent concentrate and mix it with blending oil (typically #2 fuel oil-petroleum diesel). To ensure AWPA compliance, each treater is required, at least quarterly, to obtain third-party certification that its P9 oil (carrier) meets AWPA standards for solvency, distillation range, flash point, specific gravity and other physical characteristics. The actual work solution applied by pressure treatment into the wood pole is typically a 5-8 percent solution by weight, with the remainder consisting of the AWPA P9 oil as a carrier.

One of five 160-feet long treating cylinders at Pacific Wood Preserving's plant is ready to be pushed and unloaded.
One of five 160-feet long treating cylinders at Pacific Wood Preserving's plant is ready to be pushed and unloaded.

Penta recently underwent an extensive data review process, resulting in its re-registration by the US Environmental Protection Agency.

AWPA standards have also been modified to allow the use of biodiesel-hydrocarbon blends as a carrier solvent, provided it meets the physical characteristics requirements in AWPA P9-though long-term data regarding its combined efficacy with Penta has not been established. Recent studies by Mississippi State University have shown that Penta dissolved in biodiesel-based systems might not be as effective as petroleum-based carrier systems. Reports from laboratory tests and limited field testing on a proprietary biodiesel containing P9 oil indicate the biodiesel may not effect efficacy, but long-term data is not yet available.

Penta is a restricted use pesticide. The only major company manufacturing and selling Penta as a wood preservative in the US is KMG-Bernuth Inc.

Chemical Treatment Option 2: Chromated Copper Arsenate

Chromated copper arsenate- (CCA-) treated southern pine poles are commonplace, especially since the rising costs of oil-borne systems have made a waterborne treatment more economically attractive. Some utilities prefer the softer oil-borne treatments for lineman climbing; some CCA treaters, however, apply a refined hydrocarbon oil emulsion to the pole's outer layer to combat this issue. The viscous oil additive serves as a lubricant, making the pole easier to climb.

CCA was banned for residential lumber uses in 2003 when producers and the Environmental Protection Agency (EPA) reached a voluntary agreement to limit uses to industrial applications. Like Penta, CCA is also a restricted use pesticide.

Douglas-fir poles do not obtain sufficient penetration, retention or efficacy when treated with CCA and generally are treated with Penta, creosote or copper naphthenate. Cedar poles can effectively be treated with CCA, although it is more common to see Penta-treated cedar poles-either thermal butt treated or full length treated. There are several CCA suppliers, but only one supplier of the oil emulsion that makes the poles more climbable.

 A Penta treated wood transmission line.
A Penta treated wood transmission line.

Chemical Teatment Option 3: Copper Naphthenate

Copper Naphthenate (CuNap) was first used in Germany and has been in commercial use since 1911. It was recognized in the AWPA standards in 1949 but was not widely used for pressure treatments until the late 1980s when regulatory activities stimulated interest in the product because of its general use classification by the EPA-as opposed to most wood preservatives, which are classified as restricted use because of different toxicity profiles.

CuNap recently had several supply and availability issues. CuNap is up for re-registration, which means the EPA will require an extensive and expensive review of toxicology data, which occurs about every 10 years. Merichem, the company that until recently manufactured this product, decided not to expend the resources required to re-register it. In March 2011, Merichem announced it was exiting the CuNap business.

Prior to this, Merichem experienced issues causing severe product availability problems, and many users were frustrated with unreliable product supply and escalating costs. Exacerbating the supply issues was a recent study by Oregon State University finding that poles treated with a biodiesel and CuNap solution were not efficacious and could experience premature failures. While most treaters used petroleum oils for blending-and it was widely publicized at industry AWPA meetings that biodiesel was an unproven carrier for this preservative-a few treaters made the decision to use biodiesel anyway as a carrier. As a result of the negative publicity and supply issues, the use of this wood preservative has plummeted for Douglas-fir.

In August 2011, Nisus Corp. announced it had received EPA registration for CuNap. Nisus is currently working on manufacturing product and anticipates CuNap will be available soon. Availability concerns about this product's economic competitiveness with competing preservatives still exist. When properly blended with appropriate P9 oils, however, this is a robust, relatively safe and proven wood preservative with decades of data supporting its use.

A Pacific Wood Preserving employee stands by as CuNap treated poles are finished treating and being pushed out of the treatment cylinder.
A Pacific Wood Preserving employee stands by as CuNap treated poles are finished treating and being pushed out of the treatment cylinder.

Chemical Treatment Options 4 and 5: Creosote and Ammoniacal Copper Zinc Arsenate

Once widely used for poles, creosote is now primarily used for railroad ties-except in Texas and Louisiana where creosote use is more common. Creosote is a robust preservative, but utilities generally prefer Penta or CCA poles because of cost and environmental considerations. Creosote also recently underwent the EPA's re-registration process.

Ammoniacal Copper Zinc Arsenate (ACZA) is not a widely used pole preservative because of its tendency to be brittle and experience after-glow. It is an excellent preservative, however, with a long history of performance. With increased costs on the oil-borne side of the business, ACZA may be more attractive as a wood preservative since it is waterborne.


About the author: Elaina Jackson is the chief operating officer of the Pacific Wood Preserving Companies (www.pacificwood.com). She has a BA in journalism from Humboldt State University and attended Golden Gate University, majoring in finance. After more than a decade in commercial banking, Jackson entered the business world running and selling a manufacturing company in the 1990s and joining Pacific Wood Preserving in 2000.

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