Copper Naphthenate-Treated Southern Pine Pole Stubs in Field Exposure Part II: Chemical Characterizaion of Full Size Pole Stubs 12 Years After Treatment

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This study examines the influence of pre-treatment and post-treatment steaming on the character and physio-chemical nature of copper naphthenate in hydrocarbon solvent treated pine in larger, pole diameter, pole stub-length samples. This work is the continuation of two projects that began almost a decade ago. Previous reports indicated that certain morphological changes might occur in small laboratory steamed samples of copper naphthenate treated southern pine. Toluene-methanol extraction, UV-Vis spectroscopy, X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM) were used to investigate the nature and properties of the copper naphthenate present in the wood after 12 years of exposure. The formation of solid cuprous oxide occurred regardless of pre- or post-steaming conditioning.

Introduction

Copper naphthenate (CN) has been used as a wood preservative in the USA for more than 50 years for a variety of commodities (Hartford 1973, McIntyre 2000, Nicholas and Freeman 2000). Archer et al. (1990) showed that CN toxic thresholds may vary with the type of solvent and the quality of carboxylic acid used to make the copper carboxylate salt of the relative conjugate acid. Several recent studies have reported efficacy for control of a wide range of decay fungi and termites for both softwood and hardwood (Kamdem et al. 1995, Freeman 1994, De Groot et al. 1988).

Unlike several waterborne preservatives, copper loss from oilborne CN-treated wood is relatively low (Freeman 1994). Generally, water insoluble copper sources are used in its preparation unless the copper naphthenate is produced by double decomposition process (Brient 1992), and the organic components, naphthenic acid and diesel oil, have limited water solubility unless contaminated with other materials. CN is known chemically as a group of cupric cyclopentane carboxylates or cyclohexane carboxylates. Recent changes to the AWPA Standards have clarified the specifications for sources of naturally occurring naphthenic acid may be used to manufacture copper naphthenate, and still conform to AWPA P-8 specifications (Anderson 1998, Freeman 1998).

The color of freshly CN-treated wood is green and the odor of naphthenic acid

is evident. Findings indicate that post-treatment steaming freshly treated wood at 240°F(115°C) will reduce both the green color and the odor resulting in a clean dry surface (DeGroot et al. 1988). The duration of post-treatment steaming may vary from one to 3 hours depending upon initial steaming and the size/species of the treated-wood commodity.

Steam/vacuum treatment can remove up to 80kg/m³ (5 pcf) of water from poles. Barnes and Hein (1988) also reported that the ratio of copper to naphthenic acid remaining in the treating solution after treatment was lower than in the initial treating solution. They suggested that the post-treatment steam conditioning promotes the fixation of copper in wood resulting in the build up of naphthenic acid in the treating solution. This may also be explained by selective absorption of copper during treating. Fixation is also enhanced by a patented post-treatment elevated temperature bath (Hein and Kelso 1987).

Biological performance of CN-treated pole stock was not well documented until recently (Barnes and Freeman 2000, Barnes et al. 2000, Engdahl and Baileys 1992). These studies show that properly conditioned and treated pole stock had extremely low (1%) failure rates. Southern pine stakes treated with copper naphthenate solution in a light aromatic solvent were post-treatment steamed for an hour at a maximum temperature of 126°C (259°F) followed by a one hour vacuum (Gutzmer and Crawford 1995). The service life difference between non-post-treatment steamed and post-treatment steamed stake samples did not appear to be significant. Both post-treatment steamed and non-post-treatment steamed in light aromatic solvent similar to lacolene, failed after about 12 years exposure in Mississippi. It was noted, however, that both penta and copper naphthenate post-steamed samples in this study did perform 8 -14 % better than non-post-treatment steamed samples, but this was not a statistically significant difference.

This study evaluates the physio-chemical effect for initial conditioning post-treatment steaming, and post-treatment fixation of CN-treated southern pine after exposure. The data will be compared to the results found with small, laboratory treated samples of CN-treated wood (Kamdem et al. 1997, 1998a, 1998b).

Preparation and Exposure of Southern Pine Pole Stubs

After cooling overnight following the initial treatments in 1987, each of the 4-ft pole stubs was bored to the pith on third points around the circumference of the stub at the mid-point and 1-ft from the end. Borings were segmented into the following zones for analysis: 0.0-0.5, 0.5-2.0, 2.0-3.0, and 3.0-4.0 inches from the surface. Similar zonal segments from all stubs in a charge were combined for copper analysis by X-ray fluorescence spectroscopy (AWPA Standard A9-95). The data were cross-checked by atomic absorption spectrometry (AAS) (AWPA Standard A11-96) using wet ashing procedures (AWPA Standard A7). In December 1987, half of the treated pole stubs were placed 18 inches into the ground while the remainder were placed horizontally on treated 4x4s in above-ground exposure. In 1999, selected pole stubs representing the extremes in the treated population were bored and reassayed using AA spectroscopy. For pole stubs placed in ground contact, four borings were taken at quarter-points mid-way between the ground line and the stub top and four additional borings were taken mid-way between the ground line and the butt end of the stub. For stubs exposed above-ground, four borings were taken at approximately mid-length. One boring from each position was reserved for future testing while the three from each location were separated into the 0-0.5 in, 0.5-2.0 in, and 2.0-3.0-in zones for assay. The three cores for each zone and location were combined for assay. The fourth core section, taken by hand boring, was maintained for further assay and characterization by ESEM, XRD, and /or extraction-chelation with chromophoric reagents.

AAS was used to analyze the elemental copper content of the laboratory treated and non-steamed, or post-steamed copper naphthenate treated samples. AAS was also used to analyze the copper content of the 12-year old field stubs. AAS was performed in accordance with AWPA standard A11-93 (AWPA, 1999).

Environmental scanning electron microscopy (ESEM) coupled with energy dispersive X-rays (EDXA) analysis was performed on an ESEM model 2020 with an accelerating voltage of 20 kV at 77°F (25 °C) and a vacuum level of 2.0 to 3.5 torr. The EDXA detector was equipped with an Oxford Atmospheric thin window capable of detecting elements with atomic number greater than 6 and less than 99. The acquisition time for each spectrum was set at approximately 500 seconds with 1600 to 2500 counts per seconds.

A specific reaction between cuprous ion (Cu⁺) and 2,2'-biquinoline in glacial acetic acid was used to identify and quantify cuprous oxide (Cu₂O) in treated wood. At 540 nm wavelength, the absorbance of Cu⁺ -[2,2'-biquinoline] complex is proportional to the amount of Cu⁺ present in a solution. A Beckman DU 640 B spectrophotometer with a 10-mm light path silica cuvet (Model: S-10C) from Sigma was used to avoid interference with 2,2'-biquinoline solution. All UV-vis scans were performed at a rate of 600 nm/min. Cu⁺-2,2'-biquinoline complex was prepared by dissolving Cu₂O in 2,2'-biquinoline reagent (0.004mol/l). Standard solutions of cuprous ⁺-2,2'-biquinoline were made by dissolving cuprous oxide in -2,2'-biquinoline with copper content varying from 0 to 100 ppm. These standard solutions were used to build the calibration curve. The total amount of copper was determined by AAS. 2,2'-biquinoline reagent was scanned as a blank. The calibration curve was obtained by plotting the maximum UV-VIS absorbance at 540 nm against the copper concentration in the Cu⁺- 2,2'-biquinoline solutions as determined by AAS analysis.

About 25 ml of 2,2'-biquinoline reagent was used to extract about 0.1 g wood sawdust by ultrasonic extraction for 5 minutes at room temperature using ultrasonator Model: ULTRA sonik 57X from Cole-Parmer Instrument Co. samples were purged with nitrogen to prevent air oxidization. After centrifuge, the supernatant solution clear of solid particles was used for UV-VIS analysis.

Results and Discussion