Release of Copper from Pressure Treated Wood

IRG/WP 16-20584

Jun Zhang, J Horton

Micronized copper based wood preservatives including micronized copper quat (MCQ) and micronized copper azole (MCA) have been introduced commercially to the North American market since 2006 as alternatives to alkaline copper quat (ACQ) or soluble copper azole (CA) preservatives. Unlike ACQ or CA, MCQ and MCA use dispersed particulate copper particles rather than soluble ionic copper to make treating solutions for pressure treating wood (PTW). The use of particulate copper for PTW raises questions about the potential exposure of copper or copper particles to the environment. Common pathways of exposure include leaching during contact with rain water, physical contact during handling, and airborne contact during sawing of PTW at job site. This paper reviews and reports a few studies on the release of copper from MCA or MCQ treated wood during water leaching and wiping. In addition, a sawing study of PTW to determine airborne copper particles is also reported. In all the studies, the soluble copper preservatives, ACQ-D or CA-B were used as reference control. The water leaching studies indicated that wood treated with micronized copper preservative systems has substantially less copper leached than wood treated with soluble copper preservatives, resulting in a much lower potential impact on the environment. For the micronized copper treated wood, greater than 95% copper leached from PTW is in ionic form. Only small amount of copper leached is in particulate form. In addition, the particulate copper released is not in free copper form, but rather attached to wood cellulose substrate. The results from wipe test indicate that both MCA and its soluble counterpart CA released approximately the same amount of copper with each contact. In both cases, the level of copper released high initially and then levelled off after 2-3 wipes. The sawing study indicated that the total copper release from PTW during sawing at job site is negligible. Majority of copper particles in the collected air samples during sawing were embedded in wood matrix, and only occasional free copper particles were found.

Keywords: micronized copper, soluble copper, particulate copper, MCQ, MCA, wipe, leaching, sawing

Conference: 16-05-15/19 Lisbon, Portugal

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