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. Composition Copper complex The copper in the preparation is in the form of a soluble complex with ammonia or an amine such as ethanolamine . In any case, the copper content is usually expressed as a weight percentage of the amount of copper oxide 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 Quaternary ammonium cations DDA The quaternary ammonium cation in some formulations is didecyldimethylammonium (DDA) . 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. DDA carbonate is non-volatile and highly soluble in water, with near zero octanol-water partition coefficient. ADBA Another quaternary ammonium cation used in some formulations is alkyldimethylbenzylammonium (ADBA). The formula is , where n varies between 8 and 18. Counterions Both the copper complex and the quats are positive cations. The counterions (the anions that balance their positive charges) are typically hydroxide , chloride , carbonate or bicarbonate . Standard formulations Formulations of ACQ differ in the "carrier" (complexing agent for copper), either ammonia or ethanolamine; and on the quaternary ammonium cation present. Types registered in the US and Canada: 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 ACQ Type D formulation also contains two moldicides: 2-methyl-4- isothiazolin-3-one and 5-chloro-2-methyl-4-isothiazolin-3-one. 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. The copper is the primary bactericide