Water-Borne Copper Naphthenate: An Emerging Wood Preservative

by

Pascal Nzokou¹
D. Pascal Kamdem¹
Mike H. Freeman²

Abstract

Waterborne copper naphthenate (WB Cu-N) was used to treat southern yellow pine (Pinus spp) and red maple (Acer rubrum) stakes. The treated stakes were exposed in test sites located in Florida, Michigan and Mississippi for 3 to 6 years. Copper Chromium Arsenate (CCA), Ammoniacal Copper Quat (ACQ) and Oil-borne Copper Naphthenate (OB Cu-N) were also used to treat some stakes for comparison. Results obtained showed that decay fungi and termites destroyed untreated samples within 2-3 years on all sites. After 3 years exposure in Florida, red maple stakes treated at retention of 0.04pcf (0.561kg/m³) were destroyed by decay fungi and termites. All red maple treated above 0.07pcf (1.092 kg/m³) and yellow pine treated at retention ranging from 0.04pcf (0.708 kg/m³) of copper to 0.68pcf (10.9 kg/m³) of copper from WB Cu-N performed well. After 6 years exposure in Mississippi and Michigan, 0.07 ± 0.007pcf (0.708 kg/m³) and 0.12 ± 0.01pcf (1.95 kg/m³) of Cu metal from WB Cu-N respectively for yellow pine and red maple, offered a good protection comparable to CCA and ACQ treatments. This study clearly suggests that WB Cu-N can be used as a wood preservative for both above ground and ground contact applications.

Introduction

Wood is used in outdoors applications, where it is susceptible to biological, physical, chemical and environmental degradations. Various chemical formulations have been used to increase the service life of wood products. This usually involves non-pressure and/or pressure treatments with biocides containing arsenic, copper, chromium (CCA), pentachlorophenol (PCP), creosote (Winandy et al., 1993; Smith et al, 1996; Goodell and Pendlebury, 1991; De Groot et al., 1992; Kamdem et al., 1996) or other organic formulations. Recently arsenic based wood preservatives have been under constant scrutiny due their potential migration in the environment (Lebow and Tippie, 2001; Stevanovic-Janezic et al. 2001).

Industrial and academic scientists are looking for environmentally friendly formulations, which are arsenic and chromium free and will protect wood against degradation at relatively low costs. A great deal of research is focused on the development of new formulations with low toxicity to human and without important harmful impact on the environment (Freeman, 1992; Grace et al. 1993; Kamdem et al. 1996). Most of the proposed emerging formulations contain copper as one of the active ingredients.

Copper is well established as an effective ingredient against biodeterioration in waterborne and oilborne formulations. Formulations of oilborne copper naphthenate (OB Cu-N) has been used as a preservative for decades. However, its high cost and a distinctive bad odor have limited the extensive use to protect commodities such as utility poles, piling and boat components. A waterborne formulation of copper naphtenate (WB Cu-N) could be used extensively, due to the relatively low volatile organic compounds emissions and the low cost of the water carrier compared to the oil-borne formulations (Kamdem et al. 1996). In addition WB Cu-N is free of chromium and arsenic, two of the major elements on recent claims of hazardous toxicity of wood preservatives. WB-Cu-N has been reported to successfully protect red oak (Quercus rubra) and red maple (Acer rubrum) against white rot and brown rot cultures under laboratory conditions at retention of 2 ± 0.5kg/m³ (0.12pcf) of copper metal (Kamdem et al. 1995).

Fox and Williams (1994) successfully used waterbone preservatives to protect yellow pine against decay fungi. Their systems contained copper, boron and tebuconazole at various proportions. Shaw (1986) conducted a field study of the biological efficacy of waterborne copper naphtenate treated southern pine. He found that after 6 years exposure, copper retentions of 0.04 to 0.05pcf were adequate to protect against biodegradation. However, Shaw's results were not followed but other substantial work on the field performance of waterborne copper naphtenate. However, with decline of CCA, it is of interest to carry additional testing with different species and conditions, in order to validate the threshold obtained in the laboratory conditions and earlier field tests. These data are urgently needed to confirm the suitability of WB Cu-N as a wood preservative.

The objective of this study was to evaluate the biological performance of stakes treated at various retentions with a formulation of WB Cu-N and exposed at four different locations in the United States.

Materials and Methods

Wood samples

Air-dried defect free red maple and yellow pine sapwood boards were purchased from a local sawmill located at Charlotte, Michigan. Sapwood stakes 0.75" (19 mm) X 0.75" (19 mm) X 20" (400 mm) were cut and kept in the conditioning room at 68 F and 70% relative humidity until they reached an equilibrium moisture content (EMC) of 10 ± 2%. The specific gravity (SG) was determined by using the ovendry weight and the volume at 10% EMC. The average specific gravity for red maple and yellow pine were 0.52 ± 0.04 and 0.58 ± 0.04 respectively.

Treatments with preservatives

Specimens were pressure treated with a WB Cu-N formulation from ISK Biosciences, Memphis, TN. The pH of the treating solution was determined and values ranged from 10 to 13 in function of the solution concentration. The specimens were submitted to a vacuum level of 25 inches of mercury for 30 minutes. The vacuum was followed by a pressure of 150-160 psi applied for one hour. A final vacuum was applied for 30 minutes after removal of the preservative solution. Treated samples were weighed immediately before and after the treatment to determine the amount of treating solution absorbed. To achieve the desired target copper retention of WB Cu-N treated samples (Table 1), treating solutions containing 2, 1.5, 1, 0.5, 0.25 and 0.13 percent weight basis of copper metal were used for treatment. Ammoniacal copper quat (ACQ) type C, chromated copper arsenate (CCA) and oilborne copper naphtenate (OB Cu-N) were used to treat some specimens at commercial treatment facility (Kamdem et al. 1996) and used as reference samples in field tests. Table 2 lists the copper concentrations of the treating solutions and the average copper retentions for CCA, OB Cu-N and ACQ treatments.

Table 1: Average Retention of WB Cu-N Treated Samples
	Treatment Solution Water-Borne Cu-N elemental Cu %
Species	2%	1.5%	1%	0.5%	0.25%	0.13%
YP	10.904* (0.68)**	8.004 (0.50)	5.278 (0.33)	2.581 (0.16)	1.3195 (0.08)	0.70876 (0.04)
SM	8.528 (0.53)	6.708 (0.42)	4.108 (0.26)	1.95 (0.12)	1.092 (0.07)	0.56108 (0.04)
* kg/m3 ** (pcf)

 

Table 2: Average Retention of CCA, OB Cu-N and ACQ Type C Treated Samples
	Treatment Solution
	CCA (Total Oxide)		Oil Borne Cu-N (Elemental Copper)			ACQ Type C
Species	2%	1%	1%	0.5%	0.25%	2%	1%	0.55%
YP	11.37* (0.710)**	5.626 (0.35)	3.944 (0.25)	1.856 (0.12)	0.9715 (0.06)	11.716 (0.73)	8.439 (0.53)	5.51 (0.34)
SM	9.048 (0.565)	4.368 (0.27)	3.12 (0.20)	1.534 (0.10)	0.819 (0.05)	9.256 (0.58)	6.864 (0.43)	4.784 (0.30)
* kg/m3 ** (pcf)

1. Department of Forestry, Michigan State University, East Lansing, Michigan
2. Wood Scientist, 7421 Hunters Tree Cove, Memphis, TN, 38125