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Leaching of copper from amine-copper treated softwood decking Chung, Pablo Antonio

Abstract

The leaching of copper from softwood lumber treated with three different copper-amine preservative systems (ACQ, CAz and CX™) was studied. The lumber samples were exposed to natural conditions in a field test, which was designed to simulate the use of treated wood as decking. A variety of species were analysed in the experiment. ACQ treated samples were composed of hem-fir (a commercial mixture of western hemlock (Tsuga heterophylla Raf.) and amabilis fir (Abies amabilis Forb.)) and spruce (Picea sp.). CAz treated samples were composed of jack pine (Pinus banksiana Lamb.), hem-fir, and lodgepole pine (Pinus contorta Dougl.), while CX™ treated samples were only hem-fir. Monthly observations of copper leachate were analysed. It was found that the leaching of copper from the treated wood in natural conditions can be divided in two periods. During the first period a rapid increase in copper leaching was observed, as unfixed copper from the surface of the board was leached out. The amount of leaching during this period was dependent on the amount of rainfall and lasted for 4 to 8 months, depending on the season when the boards were installed. A linear model was fit to the observations collected during this initial time period. During the second period a slow decrease in leaching was observed, though some increases occurred after a drying period prior to the beginning of the rainy season. Copper was found to be leached from the inner portion of the samples through a process of diffusion, which was affected by the wetting and drying of the sample. This period is dependent on the diffusion of copper and could be extended if the unfixed copper in the treated wood is not depleted. The observations from this period were fitted to a power or a logarithm model depending on the species and treatment type. An accelerated laboratory leaching test was conducted on reference samples to determine the total amount of leachable copper. A comparison was made with the values obtained from the field leaching test. For hem-fir samples it was found that the amount of copper leached after 26 months of exposure was significantly lower than the total amount leached during the laboratory test due to the extreme conditions in latter one. For the pine and spruce samples, the amount leached during the laboratory test was significantly lower than the amount leached during the field test. This is possibly due to the lower penetration of preservative into these species, which made it difficult to calculate the total amount of leachable copper. To decrease copper loss due to leaching, two post-treatments were designed: a water pressure wash and a water repellent finish. While the water pressure wash designed to dissolve surface mobile copper and remove did not provide any positive results, the application of water repellent was found to decrease the amount of copper leached. Finally, a study on the migration of copper into the checks (formed in the wood surface during exposure) was conducted. It was observed that unfixed copper relocated to the untreated surfaces of the checks during rainfall as leached copper from the surface of the board was deposited into these checks.

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