Synergy and Diffusion with a Borax–Copper Hydroxide Groundline Preservative: 20 Year Update

Authors: Stan Lebow and Bessie Woodward USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin Bill Abbott, Copper Care Wood Preservatives, Inc, Columbus, Nebraska Mike West, Chemist (retired), Senatobia, Mississippi

A groundline remedial treatment containing 3.1% copper hydroxide (2% elemental copper) and 40% sodium tetraborate decahydrate (borax) was applied to unseasoned pine posts prior to placement in a test site in southern Mississippi. The soundness of the posts was periodically evaluated using a push test. After 3.5, 6.5, 10, 15 and 20 years, sections were taken from two posts to determine retention of borax and copper hydroxide as a function of vertical position above ground and distance from the wood surface. Within 3.5 years of exposure all untreated control posts had failed. The upper portion of the treated posts became severely decayed after 6.5 – 10 years, but assay sections removed from posts were sound for 15 years. After 20 years one post remained sound through-out the assay area, while the other post was exhibiting some decay at groundline and above. Borax retentions were greatest in the above-ground sampling zones, and generally declined over time. The overall overage borax retention in the assay zones was 21.3, 15.0, 9.7, 11.5 and 1.6 kg/m3 (1.33, 0.93, 0.60, 0.72 and 0.10 lb/ft3) after 3.5, 6.5, 10, 15 and 20 years, respectively. Copper hydroxide retentions were less affected by vertical position in the post and remained more stable over time. Although copper retentions were greatest in the outer assay zone, copper was also detected in the inner assay zones. The overall average copper hydroxide retention across all assay zones was 3.0, 3.5, 2.9, 2.5 and 2.9 kg/m3 (0.19, 0.22, 0.18, 0.16 and 0.18 lb/ft3) after 3.5, 6.5,10, 15 and 20 years, respectively. Although retentions varied among posts, after 20 years above-ground borax retentions were still above the threshold needed for protection, while copper remained above the protection threshold in the groundline and below ground areas.

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Updated on Wed, 28 Jul 2021 by SuperUser Account