Update On Copper Naphthenate Tie Research

by

Jim Brient¹
David A. Webb²

Abstract

The North American railroad industry has been served since its earliest days by the wood crosstie. These ties have been the very foundation on which the rail and track structure have been placed. The performance of this wood component with its dependability and service life has been exemplary. However it has been the application of preservatives, particularly creosote, to the wood crosstie that has given the significant durability and service life enhancement.

Due to a shortage of creosote during the mid-part of the 20^th century, there was a focus on extending the availability of the preservative through the use of additive materials. Heavy petroleum was used as an extender, which made more material available but with a somewhat reduced effectiveness. However, other active preservative products were also used as creosote extenders, including copper naphthenate.

Copper naphthenate is produced by the reaction of various copper compounds with naphthenic acid, which is the naturally occurring acidic component of petroleum. The term naphthenic acid, as commonly used in the petroleum industry, refers collectively to the carboxylic acid components found in petroleum (Brient et al., 1995). Naphthenic acids are generally classified as monobasic carboxylic acids, composed predominantly of cycloaliphatic acids containing single or multiply fused rings, as shown in Figure 1. The naphthene moiety consists of alkylated cylcopentane and cyclohexane derivatives. Naphthenic acid is recovered commercially from kerosene, jet fuel, and diesel fractions during petroleum refining. Standard P8 of the American Wood Preservers’ Association specifies the grades of naphthenic acid applicable to production of copper naphthenate, and by definition excludes the use of synthetic and other non-naphthenic acids in copper naphthenate.

US EPA classifies copper naphthenate as a registered, non-restricted use pesticide under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), meaning no certified pesticide applicator’s license is required for its use. Under FIFRA, copper naphthenate is not listed as an acute toxicity category I chemical, and has no significant sub-chronic, chronic, or delayed toxic effects. Like naphthenic acid, copper naphthenate is not a carcinogen, mutagen, or reproductive/developmental toxicant.

Copper naphthenate is classified as a non-hazardous waste under the Resource Conservation and Recovery Act (RCRA). Copper naphthenate wood preservation wastes are neither “listed” nor “characteristic” hazardous wastes under RCRA guidelines. Copper and copper naphthenate are also not subject to the Toxicity Characteristic. No reportable quantity (RQ) for copper naphthenate spills is required under Comprehensive Environmental Reporting, Compensation and Liability Act (CERCLA, or “Superfund”) regulations. Releases must be cleaned up but do not have to be reported under CERCLA.

Copper naphthenate has been used since the late 1800’s in wood poles, timber, shingles, and lumber. It has also been used in preserving cordage and textiles. Copper naphthenate was first widely used in the US to extend creosote during World War II. Minich and Goll (1948) summarized the physical properties of copper naphthenate, including water and solvent solubility, vapor pressure, and electrical properties. Copper naphthenate was moved to AWPA Standard P8 in 1949. AWPA retention standards for poles (Standard C4) include 0.06, 0.08, and 0.13 pcf (as copper) in southern pine and 0.075, 0.095, and 0.150 pcf (as copper) for the outer assay zone of coastal Douglas fir, depending on the AWPA hazard zone. Land or freshwater piles under AWPA Standard C3 have retention standards of 0.100 and 0.140 pcf (as copper) for southern pine and Douglas fir, respectively. There are currently no standards for copper naphthenate treatment of oak and other hardwoods under AWPA Standard C6 for crossties and switch ties.

Copper naphthenate is currently used in pressure and non-pressure treatment of wood. Pressure treatment markets include utility poles, cross arms, bridge timbers, lumber, posts, and glue-lam beams. Greater than 2 million cubic feet of wood were pressure treated with copper naphthenate in 1997, representing about 70,000 poles. Non-pressure treatments include remedial treatment of poles, shingles, pallets, millwork, and ammo boxes. In addition to heavy-duty applications such as pressure treatment of utility poles, copper naphthenate is sold over-the-counter for consumer use.

Copper naphthenate-treated wood is not conductive; resistance of copper naphthenate-treated southern pine at 315 kohms is greater than the 275 ohms of untreated southern pine. Southern pine treated with chromated copper arsenic (salt formulation) is much more conductive at 35 kohms . Since copper naphthenate is insoluble in water, leaching of preservative from treated wood is minimal.

For the past several decades there has been an active effort by both the preservative manufacturers and the wood treating industry to develop new preservatives for the wood crosstie. Copper naphthenate has been evaluated in lab and field stake test studies as a preservative for oak and other hardwood species. Soil block studies (Kamdem, et al.,1995) found that 0.08 pcf (as Cu metal) was the minimum retention necessary for protection against G. trabeum, P. ostreatus, and T. versicolor, while 0.10 pcf Cu was required for copper-tolerant fungi such as P. placenta. A follow-up study (Kamdem, et al. 1999) showed that oak was readily treatable with copper naphthenate, with no adverse effects on bending strength of the treated wood.

McIntyre (2000) recently summarized test data originally reported in 1975 by the American Railway Engineering Association (AREA) Committee 3 - Ties and Wood Preservation. AREA recommended minimum retentions for copper naphthenate (as Cu) and creosote of 0.10 and 8.0 pcf, respectively. Retentions of 50% & 100% of the AREA recommendations for copper naphthenate, creosote and 60:40 creosote:coal tar gave comparable decay ratings over 15 years in oak stakes exposed in Florida, as shown in Figure 2 and Figure 3. A stake test study was also recently reported on various hardwood species treated with copper naphthenate or creosote (Barnes et al., 2001). Those data showed that copper naphthenate (Mooney Chemical’s T500) compares favorably with creosote or creosote - coal tar systems after 7.5 years in test in Mississippi and Florida, as shown in Figure 4 and Figure 5. That study reported 4.0 pcf creosote retention is roughly equivalent to 0.10 pcf (Cu as metal) copper naphthenate in red oak.

At the fall 2000 RTA convention, railroads challenged the industry to improve tie performance against biological degradation in Southern, wet, and certain high decay hazard areas. This brings into focus the subject of this paper, which is an inspection of copper naphthenate and borate preservatives used in the treatment of oak crossties. This crosstie test was installed on the old Conrail system, now Norfolk-Southern, near Lewistown, Pennsylvania. Along with the railroad company, the original participants were the Burke-Parsons-Bowlby Corporation (BPB), Spencer, WV; Mooney Chemical Company, Cleveland, OH; and U. S. Borax, Los Angeles, CA.

Mooney Chemical Company developed a copper naphthenate formulation known as M-Guard™ T500 specifically for the wood tie industry using a heavy #6 fuel oil carrier and containing a water repellant. This particular formulation contained 10% copper naphthenate (~0.9% as Cu) having a density of 7.0 lb/gal, 240°F flash point, 65°F pour point, and 72 cP viscosity at 80°F. This product was prepared from Merichem Company’s naphthenic acid but is not currently being produced commercially by Mooney (now OM Group).

Methods And Materials

A total of 923 oak crossties were treated for the test by BPB at their wood treating plant in Spencer, WV during February of 1988. Two tank trucks were used as the work tank during the treatment process. Pour point of the M-Guard™ T500 copper naphthenate treating solution was higher than that for creosote and caused some difficulty in transferring the liquid treating solution through the lines during the treating cycle due to the sub-freezing ambient temperature. Stream tracing the transfer lines and an adjustment in the diluting solvent would solve the pour point problem. Charge data from treating with copper naphthenate are summarized in Table I; the ties were treated to a retention of 0.031-0.046 pcf (as Cu). Copper naphthenate is not currently listed in AWPA Standard C6 for crossties or other commodities using oak or mixed hardwoods. The minimum retention for southern pine poles under AWPA C4 is 0.06 pcf as Cu, greater than the retention used in this crosstie test.

Creosote ties were treated to 7.8 pcf retention, slightly above the AWPA C6 required retention of 7.0 pcf. Satisfactory and comparable penetration and retention of both the copper naphthenate and creosote preservatives were achieved in mixed red and white oak ties. Borings taken during treatment show >3 inches of penetration in red oak ties. Poorer penetration was seen in white oak ties, as expected.

Within the test track layout in Lewistown, shown in Figure 6, Sections 1, 2, 6 and 7 contain the copper naphthenate and creosote oak crossties that were conditioned by either air seasoning or Boulton drying methods (Table II). Boulton seasoning involves heating the green ties in a solution of the preservative under vacuum to remove water much faster than obtained with air seasoning.

1. Manager, Naphthenic Acid Technology, Merichem, Houston, TX
2. David A. Webb, Inc.

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