Copper Naphthenate: An Update and Status Report on an Effective Wood Pole Preservative

by

M. H. Freeman¹,
J. A. Brient²

Abstract

This paper reviews the efficacy and field trials of copper naphthenate in various wood species. Included in this discussion are chemical and physical characteristics of copper naphthenate preservative systems, the physical effects on wood treated with copper naphthenate, a review of long-term efficacy trials, and a discussion of causes of premature pole failures. Also described are typical plant handling characteristics of copper naphthenate and its diluted solutions, and information relating to conversion of oilborne pressure treating plants to copper naphthenate.

Background and History

The use of copper naphthenate as an industrial biocide was established in the late 1800's (29). Copper naphthenate is the reaction product of a copper compound with naphthenic acids. Naphthenic acids are naturally occurring byproducts of petroleum, recovered from petroleum distillates by caustic extraction followed by acidification. Typical crude petroleum contains 0.5-2% naphthenic acid by weight, with the highest concentrations found in petroleum from South America/Caribbean areas, western North America, Romania, China and Russia. Naphthenic acids are predominantly alicyclic (saturated, non-aromatic) acids characterized by the formula C_nH_2n-zO₂ (z=2,4,6,8) with typical structures shown in Figure 1. One-, two-, and three-ringed acids predominate in commercial naphthenic acids.

The physical and chemical characteristics of copper naphthenate and naphthenic acids have been described in detail (33) and their use in wood preservation discussed by Hartford et al. (23). Many grades of naphthenic acids can be used in wood preservation since the specifications written for copper naphthenate allow a range of acid values, all of which are known to perform extremely well in ground contact. Copper soaps of synthetic acids such as 2-ethylhexanoic acid (octoates) and neodecanoic acid are slightly less effective as wood preservatives, however (46).

Trade names for copper naphthenate in commercial use include CuNap-8, Perm-E8, Cop-R-Nap, Cunapsol, and Cuprinol. Of these, the oldest name is Cuprinol, dating back to the Danish of over a century ago, meaning "copper in oil" (6). A review of the literature cites many applications for use, including field boxes, beehives, benches, flats, fence posts, water tanks, canvas, tents, burlap, ropes, nets, greenhouses, utility poles, cross arms, and wooden structures in ground contact and above ground contact (35). Copper naphthenate is known to control many decay fungi, molds, mildew, dry rot, certain marine growths, termites, wood parasites, and bacteria. Recent studies by Grace et al. (40) have proven copper naphthenate to be effective in preventing the consumption of wood by the aggressive Formosan termite in Hawaiian field and lab tests.

Copper naphthenate began its strong leap into the wood preservation business in the mid 1940's with the need to extend the useful volume of creosote available in the postwar effort. Due to a modification of operating practices of the steel mills, creosote, whose main source is the coking of coal and of petroleum products, was in short supply. The American Wood Preservers' Association (AWPA) began a search for combination biocides that could be added to creosote to effectively extend its service life. Colley et al. (47) determined that copper naphthenate was a likely extender for creosote that did not offer some of the proposed corrosion problems that addition of pentachlorophenol as a phenolic acid would pose.

Minich and Goll (29) gave a broad background of the technical aspects of copper naphthenate as a wood preserving chemical, including its solubility in organic solvents, relative vapor pressure, electrical conductivity, compatibility with commercially available oils, and effectiveness against wood decay fungi. A specification was proposed to add copper naphthenate to the AWPA Book of Standards. The key issues introduced by the proposal by Minich and Goll included copper naphthenate's uniform and highly effective performance as a permanent wood preservative, easy application, and safety in handling to workers. Copper naphthenate is listed in the AWPA Wood Preservative Standards, P-8, with the following specifications:

• The acid used in the manufacture of copper naphthenate shall be naphthenic acid of the group of alicyclic carboxylic acids occurring in petroleum and shall have an acid number of not less than 180 and not more than 250, on an oil-free basis.
• The copper naphthenate concentrate used to prepare wood preserving solutions shall contain not less than 6%, nor more than 8%, copper in the form of copper naphthenate.
• All of the copper present in the concentrate shall be combined as copper naphthenate.
• The copper naphthenate concentrate shall not contain more than 0.5% water.
• The foregoing tests shall be made in accordance with the standard methods of the AWPA Standards A-5.
• Solvents used to prepare solutions of copper naphthenate shall comply with the standards of the AWPA Standard P-9.
• The copper naphthenate concentrate shall not contain more than 2% (relative) of the total copper in the concentrate as being water extractable as determined by the analytical method A-14 on Page 248 of the 1987 AWPA Proceedings.

These values in the current AWPA Standards vary slightly from the original proposal prepared by Minich and Goll, including an upper limitation of the naphthenic acid oil-free acid value in an attempt to preclude the use of synthetic carboxylic acids in preparing copper carboxylate solutions. Data has been presented to the Association as well as to the International Research Group on Wood Preservation that certain carboxylic acids do not provide adequate protection. Of these, the synthetic carboxylic acids having acid numbers in excess of 250 and less than 350 have proven to be highly leachable and less effective wood preservatives for ground contact (46). An analytical method was developed to determine the presence of synthetic acids and other non-naphthenic adulterants in copper naphthenate (50).

Additionally, the use of low molecular weight acids can cause increased water solubility of both the copper and act as a coupling agent for water/hydrophobic sections of the naphthenate molecule and increase the solution's propensity for the formation of stable emulsions. Copper naphthenate free of any solvent exists as an amorphous, glassy solid with the copper content ranging from 9.3-12.5% by weight. When using an acid value number (TAN) for copper naphthenate of 200, the ratio of copper to total copper naphthenate fraction is approximately 1 to 10.

Copper naphthenate is typically supplied as an 8% concentrate for dilution in solvents such as P9-A oil, diesel fuel or mineral spirits. Typical properties for the 8% concentrate and for a 1 % (copper as metal) solution (RTU - ready to use) when the 8% concentrate is diluted with 8.3 parts fuel oil are shown in Table I.

One of the principal reasons that copper naphthenate is gaining market acceptance and is being compared to other oilborne wood preservatives is its low mammalian toxicity. The acute toxicity profile of copper naphthenate (8%) has been documented, including studies conducted by the U.S. Army Industrial Hygiene Group. The Naphthenate Salts Task Force answering the questions on the 1985 U.S. EPA Data Call-In reviewed the acute and chronic toxicity package of copper naphthenate and copper naphthenate treating solutions. A typical acute toxicity profile of copper naphthenate concentrate is published in Table II (22,23,26,36). A complete review of the regulatory status by Talerek (41) is summarized below in the section entitled Regulatory Status.

1. Wood Scientist, Memphis, TN
2. Manager, Naphthenic Acid Technology, Merichem, Houston, TX

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