Alkaline copper quaternary
Alkaline copper quaternary, usually abbreviated ACQ, is a type of water-based wood preservative product containing a soluble copper(II) complex and quaternary ammonium alkyl- or aryl-substituted compounds ("quats"). Thus the product was originally called ammoniacal copper/quaternary ammonium.
The copper in the preparation is in the form of a soluble complex with ammonia NH3 or an amine such as ethanolamine N(C2H5)H2. In any case, the copper content is usually expressed as a weight percentage of the amount of copper oxide CuO that would account for the copper present.
As a copper carrier, ammonia has the advantage that it will penetrate difficult-to-treat Western species better than other waterborne preservatives. Otherwise ethanolamine is preferred as copper carrier.
Quaternary ammonium cations
DDA is commercially produced as the carbonate, under the trade name "Carboquat". The carbonate is used instead of the chloride to reduce corrosion of the treatment equipment.
Both the copper complex and the quats are positive cations. The counterions (the anions that balance their positive charges) are typically hydroxide HO−, chloride Cl−, carbonate CO2−3 or bicarbonate HCO−3.
- ACQ-A: copper-ethanolamine (50% CuO equivalent), and DDA chloride (50%).
- ACQ-B: copper-ammonia (66.7% CuO) and DDA chloride (33.3%). Not currently registered for use in Canada.
- ACQ-C: copper-ammonia and/or copper-ethanolamne (66.7% CuO) and ADBA chloride (30%).
- ACQ-D: copper-ethanolamine (66.7% CuO) and DDA chloride or carbonate (33.3%).
The ammonia carrier ACQ-B improves its ability to penetrate into wood that is difficult to treat, like some US Western lumber. The formulations with ethanolamine, especially ACQ-D, are generally used for easier woods (such as southern pine) because it provides a more uniform surface appearance.
Mechanism of action
During the wood treatment process, the water-soluble copper and the quaternary ammonium cations are immobilized by the formation of stable insoluble compounds with lignin, cellulose, and hemicellulose and other wood components.
ACQ is applied at a timber treatment plants by industrial vacuum-pressure impregnation. The ACQ-C product is normally shipped from manufacturer as a concentrated solution that is diluted at the wood treatment plant. ACQ-D and ACQ-A are shipped as two separate solutions that are mixed and diluted at the latter. 
In the US and other countries, ACQ is registered for use on lumber, timbers, landscape ties, fence posts, building and utility poles, land, freshwater and marine pilings, sea walls, decking, wood shingles, and other wood structures.
Treated wood color
In the treated wood, the copper is relatively harmless and not an environmental or health concern.
ADBA chloride has been used in commercial products in the US since 1947. A registration standard was issued by EPA in 1985.
One disadvantage of ACQ is that significant amounts of air pollution, in the form of ammonia, are released from treatment plants and freshly treated wood in storage yards. While the CCA components are far more toxic in a weight basis, being non-volatile solids they can be effectively contained.
The copper in ACQ treated wood accelerates corrosion of galvanized steel fasteners (such as nails and screws) 10 times or more in comparison with CCA-treated wood. Stainless steel (AISI 316) is not affected. Aluminium and Galvalum fasteners should be avoided. One should use fasteners made of hot-dipped galvanized steel, copper, or stainless steel.
ACQ technology was developed and patented in Canada and improved in the U.S. It has been in commercial production in Europe, Japan and the U.S. since the late 1980s. Wood products treated with ACQ preservative were commercially produced in Canada for the first time in 2004.
ACQ became a widely used wood preservative after concerns were raised about possible environmental contamination by chromium and arsenic from wood treated with chromated copper arsenate (CCA), through contact (especially in playgrounds), leaching, sawing and sanding, or burning. These concerns led to the virtual banning of CCA for residential purposes by the US EPA, in 2003-2004.
- copper azole (CBA), another alternative to CCA.
- Abraham S. C. Chen (1994): "Evaluating ACQ as an Alternative wood Preservative System". Report APA/600/SR-94/036, US EPA, Risk Reduction Engineering Laboratory.
- C A Cushing, R Golden, Y W Lowney, S E Holm (2007): "Human Health Risk Evaluation of ACQ-Treated Wood". Journal Human and Ecological Risk Assessment, volume 13, issue 5, pages 1014-1041. doi:10.1080/10807030701506173
- "Preservative-Treated Wood and Alternative Products in the Forest Service" USDA, United States Forest Service, online report. Accessed on 2019-04-20.
- "Chapter G – Alkaline Copper Quaternaryernary (ACQ) Wood Preservation Facilities" Environment and Climate Change Canada website. Accessed on 2019-04-20.
- Jonathan Chen and others (2006): "Reregistration Eligibility Decision for Alkyl Dimethyl Benzyl Ammonium Chloride (ADBAC)". Report EPA739-R-06-009, US EPA, Dept of Prevention, Pesticides, and Toxic Substances.
- Gareth Kear, Hăi-ZhènWú, Mark S. Jones (2009): "Weight loss studies of fastener materials corrosion in contact with timbers treated with copper azole and alkaline copper quaternary compounds". Corrosion Science, volume 51, issue 2, pages 252-262. doi:10.1016/j.corsci.2008.11.012
- "ACQ (Alkaline Copper Quaternary) Wood Preservative and Fasteners - Are They Compatible?" Fasteners Eurasia website. Accessed on 2019-04-20.
- James (Scott) Groenier, Stan Lebow (2006): "Preservative-treated wood and alternative products in the Forest Service". Missoula, MT : U.S. Dept. of Agriculture, Forest Service, Technology & Development Program, 2006: iv, 44 pages. Accessed on 2021-07-02.