Copper Naphthenate Performance:
A New Way to Look at Old Data

By

Craig R.McIntyre

Abstract

Although copper naphthenate has over a 50-year test history, it is still considered as a "new" preservative in the United States when it is used for utility poles. It has also been extensively used in remedial treatments for poles and has considerable retail or over-the-counter sales. The test history includes a number of different tests and a rationale for evaluating this data and comparing the performance of copper naphthenate to other common pole preservatives is presented. Thus, the efficacy of copper naphthenate can be easily summarized.

Introduction

In 1946, Oscar Blew¹ of the USDA Forest Products Laboratory stated,

The effectiveness of the naphthenates as wood preservatives has not received adequate study and the time and extent of their use is insufficient to furnish conclusive information as to the absorptions that should be injected for best results.

He noted that FPL started a field test in 1941 and that although copper naphthenate was showing promise

It seems doubtful that copper naphthenate will be sufficiently plentiful or cheap in the near future to be of much use as a substitute for creosote in pressure treatments.

Obviously since then a number of exposure tests have been published to adequately develop retention standards for copper naphthenate and now copper naphthenate is readily available and extensively used for pole treatments in the USA. Recently some of the test history that had not been published before became available and this paper reviews the test history and provides a rationale for summarizing and comparing the data from the widely differing test protocols.

Protocol Differences

As one might expect with a long test history, the various protocols have many differences. Some of the significant variations are different specimen sizes ranging from 19 mm stakes to 150 mm diameter posts, different rating schemes varying from percent showing decay to log ratings (0-10) and different carriers such as heavy oils to mineral spirits.

In all, over 20 tests of copper naphthenate were available and almost all show good efficacy for copper naphthenate. A typical test example is the one Blew mentioned and it is shown in Figure 1². For this plot, the rating scheme is the percent still serviceable and the pentachlorophenol and creosote information comes from tests installed approximately one year earlier than the copper naphthenate. In this and other tests though, it was noted in that often relatively low retentions of copper naphthenate were chosen while the controls were at relatively high retentions. In order to allow a more logical comparison, the retentions have been normalized to the minimum retentions in AWPA Standard C4 for Southern Pine poles, which are:

• Copper Naphthenate 0.96 kg/m³ (0.06 pcf)
• Creosote 96.0 kg/m³ (6 pcf)
• Pentachlorophenol 4.8 kg/m³ (0.30 pcf)

Another southern Mississippi compared creosote, pentachlorophenol and copper naphthenate in the same test (Figure 2). In this early test, preservative performance fell within a reasonably narrow band even though there was a 10-fold range of copper naphthenate retentions and the pentachlorophenol and creosote retentions spanned 1.5 and 2.5-fold ranges, respectively. Even considering this range of retentions, most of the stakes failed within several years.

Similarly, a plot of copper naphthenate in three different carrier oils from a different test site can be drawn (Figure 3). At approximately equal nominal retentions in light oil, the copper naphthenate gave similar performance to pentachlorophenol.

In volatile solvents, such as kerosene and mineral spirits, the preservatives give essentially the same performance in either solvent (Figure 4). Also, both preservatives have greatly decreased lives.

Another solvent system of interest for oil-bornes at one time was the liquefied petroleum gas (LPG) treatment. In this treatment, the preservative was deposited by the evaporation of the liquefied gas and one could speculate that there may be differences between the residual crystalline pentachlorophenol and amorphous copper naphthenate. As shown in Figure 5, this seems to be the case since data from two southern sites shows about half of the nominal copper naphthenate retention matches pentachlorophenol while a higher retention of copper naphthenate does much better. In commercial use though, a small amount of oil was incorporated in the pentachlorophenol treatment to address the ability of decay fungi to attack areas unprotected by crystalline pentachlorophenol.

Methodology

So far, comparisons have been made in the classical manner in that a graph has been generated for each test series. A simpler method was found though that allowed a number of different tests to be compared on the same graph. This technique dramatically showed the performance differences between copper naphthenate and control preservatives at about the same normalized retentions.

The method was to simply restructure the data by plotting the differences in the ratings for the copper naphthenate stakes and the control stakes. For example, if the copper naphthenate rating was 80 and the pentachlorophenol rating was 60 at a particular time in a particular test, then the rating difference of 20 would be shown on the plot. In effect, a positive value indicates that the copper naphthenate rating was higher than the control rating and vice versa. Thus, data from Figure 2 and Figure 3 was used to generate Figure 6.

A quick review of Figure 6 shows that in one test, copper naphthenate outperformed pentachlorophenol at approximately equal retentions while in the other test, a higher retention of copper naphthenate did not fare as well as pentachlorophenol or creosote.

These seemingly conflicting results from just two tests confuse the issue of the relative performance of the preservatives.

However, it was found that these restructured plots allow a comparison of copper naphthenate with the various control preservatives in all of the available tests. In this manner, 19 mm stake tests, 50 x 200 stake tests, 150 mm post tests and other exposure tests were reviewed. Most of the test locations were in the southern USA but one was from Central America.

The review showed overwhelmingly that there was little, if any, difference in the performance of copper naphthenate when it was compared to standard preservatives on a roughly equivalent percentage basis. This is borne out by Figure 7 that accumulates the copper naphthenate-pentachlorophenol differences from the exposure tests.

For ease of analysis, the graph was divided into quadrants. Most of the data points lie in quadrants I where copper naphthenate outperformed pentachlorophenol. Quadrant III represents the values where pentachlorophenol fared better than copper naphthenate and the bulk of these points come from tests with light solvents or oils or the pentachloro-phenol retention is much higher. Even then, the differences stayed within 20-30 per cent. Quadrants II and IV show that copper naphthenate and pentachlorophenol have essentially the same performance in tests lasting over 20 years.

Indeed if one wanted to say that rating differences of ± 20% are insignificant, then only a few of the negative points would exceed that value while a considerable number of the positive values would. Thus, it seems reasonable to conclude after reviewing this data that copper naphthenate is an effective preservative and equivalent to other preservatives at the retentions specified in the AWPA.

Addendum

It would be remiss to not note that in spite of the very good test performance history, there have been performance problems with copper naphthenate in commercial use. In New Zealand, light solvent retentions were eventually raised significantly after problems with above ground treatments³ and there were questions regarding the nature of the naphthenic acid used in the manufacture of copper naphthenate⁴. In the USA, a survey⁵ showed less than 1% of the copper naphthenate treated poles were considered failures when inspected and a recent paper⁶ showed that it is reasonable to believe that the learning curve for handling a new preservative was to blame for these problems. As well, AWPA Standards⁷ now preclude use of any synthetic acids in copper naphthenate.

References

1. Blew, J.O., 1946, Preservatives for Wood Poles—Emergency Alternative Methods for the Standard Coal-tar Creosote Treatment, USDA For. Prod. Lab. Rep. No. R1693, 9p.
2. Gutzmer, D. I. and D. M. Crawford, 1995, Comparison of Wood Preservatives in Stake Tests—1995 Progress Report, USDA For. Prod. Lab Res. Note FPL-RN-02 and previous reports in series.
3. Drysdale, J., 2000, Private communication.
4. Archer, K., J. van der Waals, M. Hedley, 1990, The Comparative Performance of Copper Naphthenate Formulations in Laboratory Decay Tests, Proc. Amer. Wood-Pres. Assoc., 86:78-95.
5. Barnes, H.M., M.H. Freeman, J.A. Brient and C.N. Kerr, 1999, A Preliminary Evaluation of Copper Naphthenate-Treated Poles in Service, Proc. Amer. Wood-Pres. Assoc., 95: 52.
6. McIntyre, C.R., 2000, The Performance of Copper Naphthenate, Inter. Conf. On Utility Line Structures, Ft. Collins, CO, March 20-22, 2000.
7. Amer. Wood-Pres. Assoc., 1999, Annual Book of Standards, P8—Standard for Oil-Borne Preservatives.

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