Field Performance of Copper Naphthenate Treated Hardwoods
by
H.
M. Barnes1,
T. L. Amburgey1,
M. H. Freeman2,
J. A. Brient3
Abstract
Copper naphthenate (CuNap) is a proven, commercially produced and industry utilized preservative for many wooden commodities including poles, fence posts, lumber, timber and wood shakes/shingles. Historically, most wood preservatives are tested using the sapwood of softwood species like pine (Pinus spp.) and or Douglas-fir (Pseudotsuga menziesii). Although the railroads of the world have commercially utilized hardwoods for the material that bears the load of the gauge, not many preservative evaluations are performed on hardwood species. This paper examines the historical tests of copper naphthenate performance in North American hardwood species previously established by the TVA (Tennessee Valley Authority) and the AREA (American Railroad Engineering Association). This efficacy review includes previously unpublished data of copper naphthenate-treated hardwoods compared to P2 creosote in a high hazard test site. The recent results from the timber bridge-testing program where CuNap is being evaluated in seven hardwood species plus southern pine. The test data presented in this paper supports the potential commercial uses of copper naphthenate to treat hardwoods and provide adequate to excellent service life for those treated members.
Keywords: Copper naphthenate, preservative, efficacy, hardwoods, performance, crossties
Introduction
The use of copper naphthenate (CuNap) as an industrial biocide has been well established since the turn of the century (Freeman 1992, 2000). CuNap is basically the reaction product of a copper compound with naphthenic acids. The naphthenic acids are typically alicyclic acids and are broadly classified as acids of the formula CnH2n-zO2. Chemically speaking, these compounds are known as cupric cyclopentane carboxylates or cyclohexane carboxylates. The physical and chemical characteristics of CuNap and naphthenic acids have been described in detail (Brient et al. 1995). Hartford (1973) discussed their use in wood preservation. A review of the literature cites many applications for use, including field boxes, beehives, benches, flats, fence posts, water tanks, canvas, burlap, ropes, nets, greenhouses, utility poles, crossarms, and wooden structures in ground contact and above ground contact (Thomson 1991). Work presented in this paper will show that CuNap is an excellent preservative for hardwoods and may well be the next preservative for the use of hardwood crosstie material.
Trade names for copper naphthenate in commercial use include Perm-E8, Cop-R-Nap, CuNap8, Cunapsol, and Cuprinol. Of these, the most common name is Cuprinol, dating back to the Danish of over a century ago, meaning, “copper in oil” (Freeman 1992, 2000). Copper naphthenate began its strong leap into the wood preservation business 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. It was determined that copper naphthenate was a likely extender for creosote and did not offer some of the proposed problems that addition of pentachlorophenol (penta) as a phenolic acid would pose in treating plant corrosion.
Minich and Goll (1948) provided a broad background of the technical aspects of copper naphthenate as a wood preserving chemical including solubility in organic solvents, relative vapor pressure, electrical conductivity properties, compatibility with commercially available oils, permanence, ease of application, safety in handling, and the effectiveness against wood decay fungi. A specification was proposed to add copper naphthenate to the AWPA Book of Standards.
Copper naphthenate in the AWPA Standard P-8 (2000) has 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 (see 1999 AWPA Proceedings, p. 133 for a gas chromatographic method for determining conformity with this and following requirement).
- All of the copper present in the concentrate shall be combined as copper naphthenate.
- The copper naphthenate concentrate used to prepare wood preserving solutions shall contain not less than 6 percent, nor more than 8 percent, copper in the form of copper naphthenate.
- The copper naphthenate concentrate shall not contain more than 0.5 percent 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 AWPA Standard P-9.
- The copper naphthenate concentrate shall not contain more than 2 percent (relative) of the total copper in the concentrate as determined by Standard A14.
The values in the current AWPA Standards (2000) vary slightly from the original proposal prepared by Minich and Goll (1948).
Studies with Softwoods
The performance of CuNap in softwood species has been widely reported. In a recent study (Nicholas and Freeman 2000) the performance of CuNap- and penta-treated pine stakes in ground contact was compared at two test sites in Mississippi corresponding to a high hazard zone (AWPA zone 4) and a severe hazard zone (AWPA zone 5). After ten years exposure, the efficacy of CuNap at a retention level of 0.80 kg/m3 Cu was equivalent or slightly better than penta at a retention level of 0.64 kg/m3.
Barnes and Freeman (2000) reported no deleterious effects of pre- or post-treatment steaming on the performance of CuNap-treated pole sections after 12 years of in-ground exposure. A survey of CuNap-treated poles showed that southern pine and Douglas-fir poles in service across all hazard zones in the US demonstrated excellent serviceability (Barnes et al. 2000). McIntyre (2000), in analyzing tests conducted with oilborne preservative systems, concluded that CuNap was an effective preservative equivalent to creosote or penta at comparable retentions. Grace et al. (1993) showed CuNap to be effective against Coptotermes formosanus with less than 2% weight loss for pine treated to 0.64 kg/m3 (as Cu).
Laboratory Studies with Hardwoods
Historically, hardwoods have not been used for many efficacy tests with the exception of some white rot tests and some limited efficacy tests of lesser-used species. In a laboratory soil-block study on northern red oak (Quercus rubra), Kamdem et al. (1995a) found that at 1.28 kg/m3 (as Cu) of an oilborne CuNap preservative, weight loss of northern red oak exposed to brown rot (Gloeophyllum trabeum) or white rot (Pleurotus ostreatus or Trametes versicolor) fungi was ≤3%. A higher elemental Cu retention of 1.6 kg/m3 was needed to protect red oak against the Cu-tolerant brown rot Poria placenta.
Monoculture tests by Kamdem et al. (1996) showed that 3.2 ± 0.5 kg/m3 or 4.2 ± 0.5 kg/m3 of a waterborne CuNap system was needed to protect northern red oak (Quercus rubra) or red maple (Acer rubrum), respectively. Results from AWPA standard leaching tests indicated that the treated samples retained 85 to 95% of the copper initially absorbed depending on the initial retention. In agar plate tests, Smith et al. (1996) showed that the oilborne formulation was more effective than the waterborne formulation at equivalent copper loading.
Field Studies with Hardwoods
In a study partially funded by the US Timber Bridge Initiative, the pressure treatment of keruing, maple (heartwood and sapwood), yellow birch, and southern pine is being investigated (Crawford et al. 2000). The most recent progress report indicates wood treated with CuNap in oil at retentions as low as 0.26 kg/m3 is performing satisfactorily at a South Mississippi site (AWPA zone 5). Some attack at the low level was noted after four years of exposure. The stakes were treated full cell with toluene dilutions of CuNap in diesel using the following treatment solution strengths: 0.04%, 0.08%, 0.30 % and 1.2 % copper (as metal). Based on the average uptake of treating solution, the approximate retentions were 0.26, 0.51, 1.92, and 7.7 kg/m3 copper (as metal). At all except the lowest solution strengths, ratings were perfect with no termite attack or decay after four years exposure. Untreated controls after four years were completely destroyed or rated at a level of 1 on a scale of 0 to 10, with 10 being perfect with no attack.
Smith et al. (1996) evaluated the treatability of red maple with oilborne and waterborne CuNap systems. Variability between oilborne and waterborne systems was minimal. Kamdem and others (1995b, 1999) investigated the treatability and effect of pressure on CuNap absorption in red maple, sugar maple, beech, and red oak. For applied pressures of 1.03 MPa or higher, the authors found no difference in the preservative gradient for a given species. Absorption followed the trend beech < red oak < sugar maple < red maple.
Kresyzewski (1977) compared preservative-treated crossties in service in Canadian railways for more than 40 years. Checking, splitting, plate or rail cutting, and decay were the main defects noted. In most of the species in this test, the performance of CuNap compares favorably with that of creosote. Yellow birch ties treated with CuNap had an average service life of 25 years in secondary track. Creosoted aspen/poplar ties had an average life of 23 years in low speed main track. Red oak in this study was mechanically superior to other hardwood species evaluated with service life exceeding 25 years minimum.
DeGroot and Stroukoff (1986) evaluated five oilborne and nine waterborne preservatives with 5% penta in petroleum solvent for protecting nailed pine boxes, wire-bound gum (Nyssa sylvatica and Liquidambar styraciflua) boxes and wire bound plywood boxes. Boxes were exposed above ground for 36 months in a jungle and in an open field in Panama and for 47 months in a partially shaded field in South Mississippi. Oil- and water-based formulations of CuNap performed better than penta in the jungle. All treatments protected pine and plywood boxes, and both oil-borne and water-borne CuNap were effective on gum boxes in Mississippi. Results for the open-field site in Panama were intermediate between those from the jungle and those from Mississippi.
The US Forest Products Lab maintains test plots on the Mississippi Gulf Coast (AWPA zone 5) and other sites that contain CuNap-treated stakes of various species treated with different treatment methods (Gutzmer and Crawford 1995). Red oak stakes treated with CuNap in high boiling point aromatic oil ranging in retention from 2.2 – 5.1 kg/m3 have yet to fail after 30 years in ground contact. Only one similarly treated stake (1.2 kg/m3) exposed in Florida (AWPA zone 5) had failed. Performance in both plots for CuNap-treated stakes (Cu as metal) was equivalent to that for creosote at ≥96 kg/m3. In analyzing these data, DeGroot et al. (1988) noted that at comparable retentions, red oak stakes performed substantially better than pine stakes in Florida.
Laks et al. (1997) compared seven hardwood species and southern pine treated with eleven different wood preservative systems, including oilborne CuNap. In this study there was good correlation between soil bed and field stake results. The more highly developed preservative systems and those in an AWPA P9 Type A oil carrier, like oilborne CuNap, tended to perform better (Laks et al. 1997). Updated information shows that CuNap is performing well in all species tested, with only one species, cottonwood, having limited performance after 54 months of testing in Hawaii. Dose response curves from this study by species for CuNap and creosote can be seen in Figure 1 and Figure 2.
- Professor, Forest Products Laboratory, Mississippi State University
- Wood Scientist, Memphis, TN
- Manager, Naphthenic Acid Technology, Merichem, Houston, TX
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