Preservative Choices for Wood Utility Poles

A Penta-treated wood transmission pole in an Arizona sunset.

By Elaina Jackson, Pacific Wood Preserving Co.

There are approximately 160 million to 180 million wood utility poles in service in the U. S. They are the backbone of overhead line construction, and most of these poles are pressure-treated with a wood preservative. Choosing a pole treated with an appropriate and proper wood preservative for the environment can save a utility time, frustration and money. 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 utilized for poles in service is pentachlorophenol (penta). Approximately 63 percent of poles are treated with this preservative, followed by CCA (16 percent), creosote (16 percent), copper naphthenate (3 percent) and ammoniacal copper arsenate or ammoniacal copper zinc arsenate (1 percent).

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, approximately 16.5 million pounds of technical penta are used annually, resulting each year in the penta treatment of an estimated 2 million wood utility poles. He estimates that 4.2 million poles are 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, though the trees do not generally grow as large as Douglas-fir, which is a refractory species and more difficult to treat. Generally, 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 typically Douglas-fir or cedar.

All wood preservatives typically utilized for utility poles are robust, with many decades of data supporting 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: Penta

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 U.S. and Mexico has reduced toxic characteristics due to 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 today. Some countries have even banned the use of penta for pressure treatment.

Wood treaters purchase penta in block form and dissolve it in cosolvent or whole P9 oil, which is defined by the American Wood Protection Association (AWPA). In addition, treaters can purchase penta as a 40 percent concentrate and mix it with blending oil, which typically is No. 2 fuel oil-petroleum diesel. To ensure AWPA compliance, each treater is required to obtain third-party certification quarterly that its P9 oil (carrier) meets AWPA standards for solvency, distillation range, flash point, specific gravity and other physical characteristics. The actual work solution that is applied by pressure treatment into the wood pole is typically a 5 percent to 8 percent solution by weight, with the remainder consisting of the AWPA P9 oil as a carrier.

Penta recently underwent an extensive data review process resulting in its re-registration by the U.S. Environmental Protection Agency (EPA).

AWPA standards have also been modified to allow the use of biodiesel-hydrocarbon blends as a carrier solvent provided the solvent meets the requirements of the physical characteristics in AWPA P9. Recent studies by Mississippi State University have shown that penta dissolved in biodiesel-based systems may not be as effective as petroleum-based carrier systems. There are reports from laboratory tests and limited field testing on a proprietary biodiesel containing P9 oil that indicate that the biodiesel may not impact efficacy, but long-term data is not yet available.

Penta is a restricted-use pesticide, meaning the product or its uses are restricted “to use [and purchase] by a certified pesticide applicator or under the direct supervision of a certified applicator” (EPA 40 CFR Subpart I, 152.160). The only major company currently manufacturing and selling penta as a wood preservative in the U.S. is KMG-Bernuth.

Chemical Treatment Option 2: CCA

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

CuNap-treated poles at PWP’s yard in Sheridan, Ore., await shipment to utility customers.

CCA was effectively banned for residential lumber uses in 2003 when producers and the 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/efficacy when treated with CCA and are treated with penta, creosote or copper naphthenate. Cedar poles can be effectively treated with CCA, although it is more common to see penta-treated cedar poles, either thermal butt or full-length treated. Several CCA suppliers exist, but there is only one supplier of the oil emulsion that makes the poles more climbable.

Chemical Treatment Option 3: CuNap

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. Most wood preservatives are classified as restricted use due to different toxicity profiles. A fair amount of CuNap was used in combination with creosote during World War II due to a creosote shortage.

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

A PWP employee in Sheridan, Ore., stands by as CuNap-treated poles are pushed out of the treatment cylinder.

Prior to this, Merichem had 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/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 utilize biodiesel as a carrier anyway. As a result of this negative publicity and the supply issues, the use of this wood preservative has plummeted this year for Douglas-fir. Pacific Wood Preserving Co., which never used biodiesel, recently added penta to its Oregon plant for these reasons.

One of five 160′-long treating cylinders is ready to be pushed and unloaded.

In August, Nisus Corp. announced that it had received EPA registration for CuNap. Nisus is currently working on manufacturing product and anticipates that CuNap will be available soon, although no specific date was available at press time. Concerns about this product’s ability to be economically competitive 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.

Chemical Treatment Options NO. 4 and No. 5: Creosote and ACZA

Once widely used for poles, creosote is now primarily used for railroad ties, except in Texas and Louisiana where creosote is still commonly used for utility poles. Creosote is a robust preservative, but utilities generally prefer penta or CCA poles due to cost and environmental considerations. Creosote also recently underwent EPA’s re-registration process.

Ammoniacal copper zinc arsenate (ACZA) is not a widely used preservative for poles because of its tendency to be brittle and experience after-glow. It is an excellent preservative with a long history of performance, however. With increased costs on the oilborne side of the business, ACZA may be more attractive as a wood preservative since it is waterborne.

Elaina Jackson is the Pacific Wood Preserving Co.’s (PWP) ( chief operating officer. After more than a decade in commercial banking, Jackson entered the business world running and selling a manufacturing company in the 1990s and joining PWP in 2000.

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