Abstract
Ground-contact stakes, made from defect-free southern yellow pine sapwood, were treated with water-borne copper(II) [ammoniacal copper carbonate, ACC, Cu(II)] alone, oil-borne oxine copper (Cu-8) alone, or a dual treatment of ACC followed by drying and then Cu-8. The treated stakes were installed at two locations in Mississippi (Dorman Lake and Saucier) in high- or severe-deterioration hazard zones and inspected after 69 months of exposure. Stakes treated with Cu(II) alone at the highest retention of 5.1 kg m−3 [copper retention is based on the copper oxide (CuO) weight, as is standard in the US] performed moderately well at the Saucier test site, with decay and termite ratings of 8.0 out of 10.0, but unsatisfactory at the Dorman Lake test plot, with decay and termite ratings both below 7.0. Samples treated with Cu-8 alone at the highest retention of 1.92 kg m−3 had poor termite protection, with ratings below 7 at both sites, and only fair decay ratings of 7.0 (Dorman Lake) and 7.6 (Saucier). In contrast, all stakes treated with Cu(II)/Cu-8 combinations, with at least 2.4 kg m−3 Cu(II) and 0.2 kg m−3 Cu-8, performed satisfactorily, with average decay and termite ratings all above 8.0 at both locations. The addition of Cu(II) to Cu-8 likely causes the di-ligand form [bis(8-hydroxyquinolinolate)Cu(II)] to equilibrate principally to the mono form. Thus, the combination of Cu(II) and Cu-8 appears to form a new compound with greater efficacy than either of the two starting “reagents” and, consequently, this mixture is not literally synergistic. Possible advantages of mono Cu-8 include: (1) the mono form may be easier to formulate in a water- or solvent-borne system than the commercial bis-Cu-8 biocide; (2) the preservative would have a relatively low metal content, which may make future disposal of treated wood easier; (3) this system would likely have much fewer metal corrosion problems than preservative systems formulated with uncomplexed copper(II); and (4) the two copper coordination sites that are not complexed with the ligand could bind to the carboxylic or phenolic groups in wood to make the mono form relatively leach-resistant.
