The Performance of Copper Naphthenate

By

Craig R. McIntyre

Abstract

Although copper naphthenate is considered the newest preservative to be used for utility poles, its history dates back to the mid-1930ês. A survey of available data shows continual testing since then either as a stand-alone system or as an adjunct to other preservatives. Thus, the performance of copper naphthenate systems can be easily compared to other common pole preservatives and a rationale for doing this is presented. Some comments on recent premature failures of copper naphthenate treated poles are also given.

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.

At that time, he noted that the Forest Products Laboratory had initiated a field test in 1941 and that copper naphthenate was showing promise

Although 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 performed to adequately develop retention standards for copper naphthenate and now copper naphthenate is readily available and used for pole treatments.

More recently, some performance problems with copper naphthenate treated poles have been reported and although the failures have received some notoriety, a recent nationwide survey² involving 11 utilities found that less than 1% of all copper naphthenate poles have problems. However, the premature failure problems have been investigated by a number of researchers and a number of reasons for the failures have been advanced. This paper reviews the test history of copper naphthenate and comments on the various reasons that have been given for the failures.

Test History

Through the years, there have been numerous efficacy tests done on copper naphthenate. The tests have ranged from laboratory investigations such as agar plate and soil block bioassays with pure monocultures to full-size pole trials installed in test lines. Many of these tests have been published and essentially all show good efficacy for copper naphthenate. A typical test is the Forest Products Laboratory 2x4 field stake trial installed in Mississippi³ and the results from this type of test are usually plotted versus exposure time (Figure 1).

In various tests though, a curious disparity was noted in that often relatively low retentions of copper naphthenate were chosen while the pentachlorophenol controls were at relatively high retentions. For this paper, the retentions have been stated as a percentage of the minimum retentions in AWPA C4 for Southern Pine in order to allow a more reasonable comparison. In effect, this normalizes the retentions to a common basis.

In a review of the test history then, one would expect at this point to see a monotonous repetition of the same sort of data that shows the same general trend of sustained life and then a gradual decrease. However, in reviewing some 27 different exposure tests, it was found that the test history spans several decades and involves a variety of oil-carriers, test plots, sample sizes, rating systems and exposure conditions. It is difficult, if not impossible, to structure all of this data in a rational manner if the typical plots are generated. For example, Figure 2 shows the performance of copper naphthenate in a heavy oil but numerous plots like this would be needed if one wanted to evaluate several oils in several different tests.

A simpler method was found though that allowed a number of different tests to be compared on the same graph. More importantly, this technique dramatically showed the performance differences between copper naphthenate and control preservatives at about the same normalized retentions from the same test. The method was to 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, using these restructured plots allows a comparison of copper naphthenate with the various control preservatives and ?" stake tests, 2x4 stake tests, soil bed test and other exposure tests were reviewed in this manner. Most of the test locations were in the southern USA but one was from Central America. Copper naphthenate was typically tested as a stand-alone oil-borne preservative but it sometimes was incorporated as an additive for creosote or other standard preservative.

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 an equivalent percentage basis. This is borne out by Figure 3 that accumulates the copper naphthenate-pentachlorophenol differences from 25 pairs of similar retentions from several exposure tests. Recall that each data point is the difference in ratings between copper naphthenate and pentachloro-phenol and that values above zero indicate the copper naphthenate rating was higher at that time in that test. As can be seen, in almost all cases, the ratings for copper naphthenate exceeded the standard preservative or the difference stayed within 20-30 per cent.

After reviewing this data, one can only conclude that copper naphthenate is an effective preservative and equivalent to other preservatives at the retentions specified in the AWPA. In short, there is nothing inherently wrong with copper naphthenate that would lead one to believe that a "molecular" shortcoming was at fault for the premature failures.

Premature Failure Background

As background, it is important to consider the nature of the copper naphthenate premature failure problem. Generally, the earliest instance of premature failures occurred in 1992 in the Midwest and knowledge of this soon spread throughout the industry. Instead of failing at the ground line (as oil borne treated poles usually do), these poles exhibited a variety of decay pockets in the aerial portions. The poles involved were only a few years old and two independent consultants investigated and reported that the dominant cause appeared to be inconsistent treating practices.

Since then, a number of other utilities have reported problems of sporadic decay involving some of their copper naphthenate poles. In some cases, the utility purchased several thousand poles while other utilities only bought a truckload or two. Through the years, copper naphthenate has been used at eight different treating plants and three chemical companies have supplied the vast majority of the product. Both southern pine and Douglas fir poles are treated with copper naphthenate but the widespread failure problems have only been reported in southern pine.

The remainder of this paper will review and discuss the various proposals that have been advanced to explain the failure problem.

Chemical Manufacturers

Although there were three major chemical manufacturers in the late 1980ês and early 1990ês (and a total of seven), one had a much larger market share and naturally then this manufacturer is implicated in the failures. Thus, one explanation could be that this particular manufacturerês product was different and caused the problems.

However, if it were simply a "bad product", then one would expect that all treaters using this manufacturerês product would have had problems and that is not the case. As many as five different treaters have used the one manufacturer and problems have been reported with poles from only two of the treaters.

Naphthenic Acid Source and Additives

There was the thought that naphthenic acid is an extremely variable commodity and therefore manufacturers of copper naphthenate have blended synthetic acids with natural naphthenic acids to produce a more uniform product and reduce costs. This thought continues today and at the 1998 AWPA meeting, a paper⁴ was given in which two of eight different naphthenic acids had considerable amounts of low molecular weight compounds, synthetic acids and other undesirable components. However, most of the naphthenic acids in this paper are imports and no US manufacturer of copper naphthenate used imported naphthenic acids. In fact, essentially all of the naphthenic acid used to treat poles came from two chemical plants in the Southeast that used exclusively natural sources.

Two recent events related to the question of naphthenic acid source should also be noted. The first is that new analytical techniques have been published by the AWPA that readily analyze naphthenic acid⁵ and standardization of the techniques is being sought. The second is that the AWPA preservative standard P8 was reworded in 1998 to reflect that only acids from the naphthenic acid group can be used.⁶

One factor that has involved copper naphthenate is that some formulations and/or batches appear to have a greater propensity to form stable emulsions and the handling of these emulsions is difficult at the treating plant. While this may be, a number of remedies were developed through the years and solution maintenance is part of every preservative treatment. It should also be noted that addition of various emulsion breakers was found to be an effective way to combat this problem and water readily separates from todayês formulations.

To summarize, if the problem lay with one manufacturers product, then one would expect a widespread failure problem involving all users of that product and that is not what is found. Instead, there are concentrations of problems involving a few years at a few treaters.

Retention Distribution and Threshold

The theory has been advanced that there is something different about the treatment of copper naphthenate that results in a different distribution of retentions. The thought is that there is a larger percentage of the poles at the extremely low retention poles end due to an extremely wide range of retentions.

To investigate this possibility, the distribution of retentions found in about 1000 copper naphthenate poles was compared with that for 900 creosote poles⁷. For both cases (Figure 4), slightly skewed "bells" were obtained but the curves are essentially the same. Thus, one can only conclude that there is nothing unique about copper naphthenate treatment compared to creosote treatment. Since other work⁸ shows that the retention distributions for creosote and pentachlorophenol in oil are also the same, one can extend the conclusion to include pentachlorophenol as well.

The narrow distribution is in agreement with the data from the recent survey mentioned previously that also showed a narrow retention range2. In that work, a random sampling of 150 southern pine poles from 11 utilities nationwide showed that less than 5 percent of the poles had retentions below 50% of the mean.

There has also been discussion that the threshold for fungal attack for copper naphthenate is too close to the minimum AWPA retention value and the normal "margin of safety" was not being maintained. Although there is some variation, the weathered threshold for copper naphthenate soil blocks generally falls around 0.05 pcf Cu while the unweathered threshold is about 0.03 pcf Cu^9-13. The author has difficulty rationalizing these values with a 0.06 pcf Cu minimum pole retention but the above distributions argue that there is an adequate margin. The table below presents the percentage of poles expected at the different retention values and one can see that something like 42% of the poles would be expected to fail if the threshold were truly 0.05 pcf. This is not the case.

Species

As mentioned earlier, both southern pine and Douglas fir poles are treated with copper naphthenate but failure problems have been reported only in southern pine. It does not appear that there is a species differentiation in this regard but rather that there are significant differences in the retention values (0.075 pcf Cu for Douglas fir vs. 0.06 pcf Cu for southern pine) and processing methods used. Typically, the Douglas fir charges were kiln-dried and longer processing times were used.

Failure Distributions

Although much of the premature failure information is not available, two utilities purchased about 18,000 poles of all sizes from one treater. When one considers the failure pattern for this combined group, an interesting pattern emerges. Just three years, 1991-93, account for 97% of the 878 failures but most importantly, this is only 5% of the 18,000 total poles (Figure 5). This graph clearly shows that the premature failures are concentrated in the early years of production at that plant. After 1993, the failure rate drops off to essentially zero.

Admittedly, one has to be somewhat careful in interpreting the above data. Some utilities have experienced failure rates higher than 5% of the total purchased but typically these utilities have bought a much smaller number of poles and their purchases have been concentrated in the early years. On the other hand, other utilities have bought much larger quantities of poles but have not reported any failures. It seems that on a larger scale, the report of a failure rate of less than 1% could be used to represent the entire population of several hundred thousand copper naphthenate treated poles since it was based on a random sampling.²

Post Treatment Steaming

Recently, there have been indications that post-treatment steaming may inactivate copper naphthenate¹⁴. Specifically, it appears that elevated temperatures for extended periods in laboratory tests can reduce a significant portion of copper naphthenate to biologically inactive copper oxide. However, it is uncertain that the conditions necessary to effect this transformation in the laboratory are available in full-size treatment to any appreciable extent. Also, since the pole surfaces would be hottest, one would expect that the inactive copper oxide would be on the surface of the poles and thus a uniform surface decay would result. This is not the pattern seen in the field.

Other Considerations

There have been assertions of other problems leading to premature failures of copper naphthenate as well. Specifically, incipient decay, improper pretreatment conditioning, poor post treatment inspections and field boring have been cited. It is important to note that none of these have any thing at all to do with the efficacy of copper naphthenate. In fact, all of these reasons could lead to early failures with any preservative and it is reasonable to believe that some of the premature failures attributed to copper naphthenate are indeed due to some of these causes.

Learning Curve Conclusion

When one considers all of the available data, one inescapably concludes that the premature failures that occurred with copper naphthenate treated poles are mostly due to simply learning how to handle and apply the preservative in a treating plant. In short, the failure problem is a demonstration of a learning curve and this learning curve is not unlike those associated with either pentachloro-phenol¹⁵ or creosote^16-18 treatment modifications when they were first introduced. In fact, it seems that it is a typical learning curve in that there were a relatively large number of failures in early years that rapidly decreased as familiarity with the product grew.

As time continues, one would expect that the use history of copper naphthenate would be the same as other oil borne preservatives. Indeed the long stake test history predicts that it should be so.

Acknowledgements

The assistance of Colin McCown, Thomasson Lumber Co. and Merichem Chemical Co. is gratefully acknowledged.

References

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