Water-Borne Copper Naphthenate: A Potential New Preservative for Softwoods, Hardwoods and Composites

by

D. Pascal Kamdem¹
Mike H. Freeman²
Elmer Schmidt³
Pascal Nzokou⁴

Abstract

Copper naphthenate is a well known commercially used wood preservative used for the preservative treatment of wood poles, fence posts, lumber, glulam beams, timbers, and wooden shakes/shingles. In recent years, researchers have begun to investigate alternative formulations that transform the typically oil-borne preservative into new, environmentally friendly water borne systems. Included in the research reported here is the six-year stake test efficacy data comparing water borne copper naphthenate in Southern Pine (SYP) at multiple exposure sites in the USA with oil borne copper naphthenate, ACQ and CCA -Type C. This paper will also compare some other alternate formulations of water borne copper naphthenate using alternative coupling systems to make the oil-borne preservative water borne in nature. Additional work indicates that water borne copper naphthenate may be a good biocide candidate for the protection of wood composites. This emerging preservation technology offers a potential new lumber and composite treatment offering benefits to treaters and consumers over that of conventional wood preservatives in performance and environmental matters. Future reports will evaluate the performance of water borne copper naphthenate (i.e., efficacy) in the lesser-studied softwood species of Red Pine and Ponderosa Pine and in Maple, Oak, Beech, and Y. Poplar.

Specifically in this study, Waterborne copper naphthenate (WB Cu-N or WB CuNap) was used to treat southern yellow pine (Pinus spp) 19 mm x 19 mm x 400 mm stakes. The treated stakes were exposed in test sites located in Gainesville, Florida, Augusta and East Lansing, Michigan and Aberdeen, Mississippi. CCA, ACQ and oil-borne copper naphthenate (CuNap) were also used to treat some stakes for comparison. Decay fungi and termites destroyed untreated samples within 2 years on all sites. After 3 years exposure in Florida, the rating of stakes with 2.0 kg/m³ copper retention from WB Cu-N is still equivalent to CCA at 5 kg/m³ retention, ACQ at 6.4 kg/m³ retention and oil borne copper naphthenate at 1.6 kg/m³ copper retention. After 6 years exposure in Mississippi and 5 years in Michigan, 0.76 ± 0.07 kg/m³ Cu metal from WB Cu-N 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

As major structural material, wood is used in many outdoor applications, where it is susceptible to biological and environmental degradation. Various chemical preservatives have been used to protect wood against biodegradation. This usually involves impregnation or pressure treatment with highly toxic chemicals such as copper chromium arsenate (CCA), pentachlorophenol (PCP) and creosote (Winandy et al., 1993; Smith et al, 1996; Goodell and Pendlebury, 1991; De Groot et al., 1992; Kamdem et al., 1996). However, in the recent years many wood preservatives have been under constant scrutiny due to their negative impact on the environment. For example, there is increasing concern about the leaching of arsenic from CCA treated wood into ground water sources and streams (Lebow and Tippie, 2001; Stevanovic-Janezic et al. 2001) and in human exposure in playground equipment and on decking materials.

Both field and administrative personnel are often asked to justify the choice of treated wood in constructions structures, and there is sometimes pressure to reduce or eliminate the use of treated wood in favor of products that are perceived more environmentally friendly (Lebow and Tippie, 2001). That increased social pressure is mainly directed against a large range of wood preservatives such as creosote, pentachlorophenol, and arsenic based formulations (Kamdem et al., 1996). For those reasons combined with the VOC compliance, the wood preservative industry is looking for alternative more environmentally friendly wood preservatives. A great deal of research is focused on the development of new chemicals with low toxicity and less harmful impact on the environment (Freeman, 1992; Grace et al. 1993; Kamdem et al. 1996).

Several copper amine based formulation are under development. Copper naphthenate (Cu-N or CuNap) is a well known commercially used wood preservative used for the treatment of wood poles, fence posts, lumber, glulam beams, timbers, and wooden shakes/shingles. Recently, researchers have begun to investigate alternative formulations of the typically oil-borne preservative into new, environmentally friendly water-borne formulations. Oil-borne Cu-N has been used as a preservative since the beginning of the century, but it cost and odor have limited its extensive used. Water borne formulation of copper naphthenate (WB Cu-N) could be more extensively used because of its low volatile organic compounds emissions and its relatively low cost compared to the oil-borne formulations (Kamdem et al. 1996, Kamdem and Freeman 2001). However, before a new preservative is fully accepted, it must undergo a series of tests including standard field test to demonstrate its suitability for the intended purpose. In North America, the appropriate test is the AWPA E7-93 (AWPA, 1999). The standard AWPA E7-93 provides guidance on methods to presents data from field tests as decay rating over time for a range of preservative retention (AWPA, 1999). The method has been qualified as practical and simple as a tool for deriving conclusions from results of stakes tests on water borne preservatives (Cook and Morris, 1995).

Other Background Information

Presented in 1992 was an extensive discussion by Jonathon Shaw of Bell Labs of efficacy and performance data on decade old wood stakes in a high hazard decay site in Florida on amine solubulized water borne CuNap. In this study, WB CuNap performed well as compared to CCA-C and Oil Borne CuNap. In comparison, performance was equivalent between CCA-c at retentions of 0.40-0.60 pcf oxides to WB CuNap at retentions of 0.15 pcf (Cu as metal) to oil-borne CuNap at 0.13 pcf (Cu as metal).

Other methods of solubulizing oil-borne CuNap have been used in the past with varying results. The most common solubulizing agents for CuNap have historically been either ammonia or amines. In two different independent studies, where ammonia was the primary solubulizing agent for CuNap, results in test plots in both Central Florida and in Dorman Lake, MS (Hazard zones 5, and 4, respectively) indicate in SYP (pinus spp.) in 19 mm x 19 mm sapwood stakes in plots where both termites and decay fungi will destroy untreated controls in less than 2 years, stakes continue to perform well after 10+ years with retentions of 0.21 pcf in Fl. Or retentions of 0. pcf (Cu as metal) in MS. In Ms, retentions of 0.045 pcf Cu (WB CuNap) compared favorably with CCA-C retentions of 0.274 pcf total oxides after 8+ years. This data can be seen graphically in figure 1. The other ammonical CuNap system data will be read out later this year and presented in the follow-up paper at the FPS Annual Meeting. This data indicates that even the slightly lesser performing Ammonically solubulized WB CuNap could be a very functional replacement product for the arsenic and chromium containing product CCA-Type C.

Additionally, the formulation being studied extensively in this report (CuNap amine- formulation) of WB CuNap has been used extensively for over the counter brush on and cold soak application for a number of years. Tade names that are readily and commercially available include Behr Products #1, Zinsser No. 1 and No.2, Fields Copper Nap, Henry Green Wood Preservative, Jasco #1, and End-Cut Solutions recommended in AWPA Standard M-4 are also manufactured by Arch and CSI containing WB CuNap. . Furthermore, mine timbers of red oak and other mixed hardwoods have performed well after either a 24 or a 72 hour cold soak in a 2% Cu (as metal) formulation of WB CuNap for in excess of 15 years in moist, high humidity, and decay prone environments in coal and mineral mines when used as support timbers for in excess of 15 years. One mine timber user reported that he had better performance with WB CuNap that he had with Penta in Mineral spirits when comparing the life span of hardwood mine timbers in KY and W. VA.

When WB CuNap was applied to Western Red Cedar (WRC) machine or hand split shakes and machine grade shingles, either with or without a minor water-repellent dye added, shingles and shakes in Palm Beach, FL and Seattle, WA performed well with no moss, algae, or lichens growing on them for a period of a two year inspection. Samples were removed with a standard 1-sq.inch wood surface sampling punch at time intervals of o months (initial treatment), 6 months, 12 months, and 24 months after treatment. Initial treatment had retention levels of 270 ug/cm² of Cu (as metal), and samples taken at 1 year and at 2 years showed a resultant loss in Cu (as metal) of 17% and 24% respectively. The trend for Cu loss from the wood surface was slightly greater in the Seattle, WA test roof structures, but not significantly different. Wood samples from these same test roves were also sent to Michigan Technological University in Houghton, MI. for isolation and characterization of any fungal infestation at 6 months, 12 months, and 24 months after treatment. No basidiomycete (decay) fungi were found on any of the test samples during any of the sampling intervals. In fact, only zygomycete fungi were isolated from the three Seattle test roof structures at the 24-month sampling interval. This test indicates that WB CuNap is an excellent protectant for wood roofing materials, and superior to many other pesticidal and non-biocidal treatments.

WB CuNap has also been used to successfully treat Aspen composites bound with Phenol-Formaldehyde (PF) resin systems. (Schmidt; 1991). In the study with WB CuNap incorporated into the PF resin system and then sprayed and tumbled onto the Aspen flakes, OSB produced from those same flakes had significant resistance to decay from basisiomycete attack and no reduction in any of it mechanical properties like IB, MOR, MOE, and WPL. The treated composite was light brown in color, and the color darkened with increasing loadings of WB CuNap. Interestingly, no mold or surface fungi attacked these boards when exposed to mold room conditions of >90°F and > 90% RH for 90 days. This preliminary study demands further investigation as WB CuNap may be an excellent candidate for the protection of wood and wood-plastic composites for the future and may even be able to protect these same composites from in ground exposure or extended wetting conditions.

Objective of this Study

The objective of this study is to evaluate the biological performance of a formulation of WB Cu-N treated wood by using a series of field tests in different hazard and geographical zones. For that purpose, soft maple and yellow pine stakes were pressure treated with various concentrations of copper in WB Cu-N and installed for decay testing in the ground in Michigan, Mississippi and Florida. Alternate formulations using ammonia as the coupling agent rather than amine were also pressure treated and installed in locations in Mississippi and Florida to compare performance in SYP using alternate formulation technologies.

Material and Methods

Soft maple (Acer rubrum) and southern yellow pine (Pinus spp) stakes were selected for this project. Stakes were cut from air-dried defect-free boards and surfaced to 0.75" (19 mm) X 0.75" (19 mm) X 20" (400 mm).

Stakes were treated with waterborne copper naphthenate (WB Cu-N), oilborne copper naphthenate (OB Cu-N), ammoniacal copper quat (ACQ) and chromated copper arsenate (CCA) to the target retentions presented in table 1.

Table 1: Decay rating scheme according to AWPA E
Rating	Description of Condition
10	Sound, suspicion of decay permitted
9	Trace decay to 3% of cross section
8	Decay from 3 to 10% of cross section
7	Decay from 10 to 30% of cross section
6	Decay from 30 to 50% of cross section
4	Decay from 50 to 75% of cross section
0	Failure

A full cell laboratory process including an initial vacuum of 25 inches of mercury for 30 minutes, One-hour pressure at 180-200 psi at room temperature and a final vacuum of 25 inches of mercury for 30 minutes was used for WB Cu-N. ACQ, CCA and OB Cu-N were employed to treat the samples at commercial treatment facility (Kamdem et al. 1996).

An average of 10 stakes for each species and chemical were exposed in test sites in Augusta and East Lansing, Michigan, Gainesville, Florida and Aberdeen, Mississippi. They were randomly installed vertically to half their length in rows of holes made with an iron bar.

Augusta is located in southwest Michigan about 50 km east of Lake Michigan (42° 24' N, 85° 24' W, elevation 288 m). Annual rainfall averages 890 mm/yr and mean annual temperature is 9.7°C. Most regional soils are sandy loam and silky clay loam of moderate fertility. The East Lansing test site is located at the Tree Research Center south of the Michigan State University campus. The site receives an average rainfall of 750 mm/yr, and the mean annual temperature is 7.9°C. The Aberdeen site is located in "Hardwood" forest in Aberdeen, MS. The average annual rainfall is 1416 mm and mean temperatures are above 17°C. The site is located on a poorly drained silty clay soil, occasionally subject to floods. The Gainesville site is located at the Austin Cary memorial forest of the University of Florida in Gainesville. The climate can be characterized as warm subtropical, and the site receives an average of 1310 mm of rainfall each year with temperature averages above 15°C during most of the year. The site soils are well-drained sandy soils and are infested with termites.

1. Professor, Department of Forestry, Michigan State University, East Lansing, Michigan 38824
2. Wood Scientist, 7421 Hunters Tree Cove, Memphis, Tennessee, 38125
3. Professor, University of Minnesota, Department of Forestry Products, St. Paul, Minnesota 55108
4. Graduate Research Assistant, Department of Forestry, Michigan State University, East Lansing, Michigan 38824