Copper
Naphthenate:
An Analysis of the Materials Found in the Worldwide Marketplace
Using
a New Analytical Technique
By
J.A.
Brient1
R. E. Moyer2
M.H. Freeman3
and H. Jiang4
Abstract
Naphthenic acid has been recovered from products of petroleum refining for well over 100 years. The metal soaps, specifically copper and zinc naphthenate, have been evaluated and used as commercial wood preservatives for in excess of 60 years. Most of the early data supporting the continued use of copper naphthenate as a wood preservative was based on materials manufactured from naturally occurring naphthenic acid derived from petroleum. Recent work investigating pesticide label claims for materials asserting to be naphthenic acid salts of copper has in fact shown the presence of many synthetic carboxylic acids as substitutes for naphthenic acid. Additionally, some earlier efficacy studies indicated that non-naphthenate salts of copper actually seemed to promote decay in laboratory tests. This work investigates several sources of copper naphthenate in commercial markets and investigates a novel analytical technique and a modification of this technique to characterize the copper naphthenates found in commercial use worldwide. The basic analytical technique was recently published as an appendix to the American Wood Preservers’ Association P-5 (Methods for Chemical Analysis) Sub-Committee annual report to assure compliance with the AWPA P8 Standard.Keywords: Copper naphthenate; Naphthenic acid; Analysis
Introduction
Copper naphthenate is a widely used, broad-spectrum wood preservative. It has several properties which make it a highly valued commodity, including effective control of decay fungi and excellent control or mitigation of wood destroying insects, including termites, beetles carpenter ants and other host organisms. Extensive references and citations illustrating this fact are found in the literature. In the United States, the Environmental Protection Agency also classifies copper naphthenate as a general-use (unrestricted) pesticide, thus making it available for over-the-counter (OTC) sales and distribution without Certified Pesticide Applicators licensing required for purchase or use.
One common fact emerging from the historical long-term efficacy and performance data on copper naphthenate is that all of the copper soaps evaluated were based on naphthenic acid. The term naphthenic acid, as commonly used in the petroleum industry, refers collectively to the naturally occurring carboxylic acid component of petroleum. Although of variable and incompletely characterized composition, naphthenic acids are generally classified as monobasic carboxylic acids, composed predominantly of alicyclic (cycloaliphatic) acids containing single or multiply fused rings. The naphthene moiety consists of alkylated cylcopentane and cyclohexane derivatives, as shown below.
Other carboxylic acid components include acyclic fatty acids and aromatic carboxylic acids. These acids are present to varying degrees, depending on the crude oil origin. Commercial naphthenic acids recovered from kerosene, jet fuel, and diesel fractions of petroleum are composed of C10-C25+ carboxylic acids. These acids are extracted from the petroleum fractions to reduce corrosion and improve fuel quality.
A study (Seifert, 1975) by Chevron in the early 70’s identified about 1500 different organic acids in a single California crude oil, varying in molecular weight between 200 and 700 and peaking in the 300-400 range. The gas chromatograms shown in Figure 1 and Figure 2 show the "hump" of non-resolvable peaks that hinders the identification of any individual isomers found in naphthenic acid. Advanced mass spectral analyses shown in Figure 3 illustrate the complex composition of naphthenic acid, including carbon number and ring content distribution.
Only limited data are found in the literature describing the preservative efficacy of copper soaps of other carboxylic acids. Metal soaps of naturally occurring plant-based acids such as oleic, stearic, and tall oil acids were inferior to naphthenates when used to protect cellulosic material (Marsh, 1944). Hilditch (1983) observed that copper and zinc soaps of "acypetacs", their term for synthetic acyclic branched carboxylic acids, were also slightly less effective than the corresponding naphthenates in ground contact studies. Metal soaps of synthetic acids are frequent substitutes for naphthenate soaps in paint driers and other applications because of their lower cost and higher metal content. Table 1 shows that acid value, defined as mg KOH/g, is inversely proportional to molecular weight. The lower MW synthetic acids have higher acid value and therefore can carry higher metal content.
