• Acid Copper Chromate (ACC)
• Alkaline Copper Quat (ACQ)
• Ammoniacal Copper Citrate (CC)
• Copper Azole (CBA-A and CA-B)
• Copper Dimethyldithiocarbamate (CDDC)
• Borate Preservatives
• Other Preservatives

Acid Copper Chromate (ACC)
Acid copper chromate (ACC) has been used for decades for treatment of wood used in cooling towers. It contains 31.8% copper oxide and 68.2% chromium trioxide (AWPA P5). The solid, paste, liquid concentrate, or treating solution can be made of copper sulfate, potassium dichromate, or sodium dichromate. Tests on stakes and posts exposed to decay and termite attack indicate that wood well impregnated with ACC gives acceptable service, but it may be more prone to leaching than wood treated with CCA. It is listed in AWPA standards for a wide range of softwood and hardwood species, with a minimum retention of 4 kg/m³ (0.25 lb/ft³) or 8 kg/m³ (0.5 lb/ft³) for wood used above ground or in ground contact, respectively. It may be difficult to obtain adequate penetration of ACC in some of the more refractory wood species such as white oak or Douglas-fir. This is partially because the AWPA guideline for ACC require treating temperatures at or below 38/49ºC (100/120ºF ) and because rapid reactions of chromium preservative components in the wood can hinder further penetration during longer pressure periods. Elevated temperatures can result in shallower penetration due to faster fixation. ACC is used commercially, although in a limited number of treating facilities.

Alkaline Copper Quat (ACQ)

Alkaline copper quat (ACQ) is one of several wood preservatives that has been developed in recent years because of environmental or safety concerns with CCA. The fungicides and insecticides in ACQ are expressed as copper oxide (67%) and a quaternary ammonium compound (quat, 33%). Multiple variations of ACQ have been standardized or are in the process of standardization. ACQ type B (ACQ-B) is an ammoniacal copper quat formulation, ACQ type D (ACQ-D) is an amine copper quat formulation, and ACQ type C (ACQ-C) is formulated with either ammonia or amine and a slightly different quat compound. Wood treated with ACQ-B wood has a dark greenish brown color that fades to a lighter brown and may have an ammonia odor until the wood dries. Wood treated with ACQ-D has a lighter greenish brown color and little noticeable odor. Wood treated with ACQ-D weathers to a brown color. Wood treated with ACQ-C varies in appearance between that of ACQ-B and ACQ-D, depending on the formulation. Stakes treated with these three formulations have demonstrated efficacy against decay fungi and insects when exposed in ground contact. The ACQ formulations are listed in AWPA standards for a range of applications and many softwood species, although the ACQ-C listings are limited because it is the most recently standardized. Minimum retentions of 4 kg/m³ (0.25 lbs/ft³) or 6.4 kg/m³ (0.4 lbs/ft³) are specified for wood used above ground or in ground contact, respectively.

Ammoniacal Copper Citrate (CC)

Ammoniacal copper citrate (CC) is a recently developed wood preservative that utilizes copper oxide (62%) as the fungicide and insecticide and citric acid (38%) to aid in the distribution of copper within the wood structure. The color of the treated wood varies from light green to dark brown. The wood may have an ammonia odor until it is thoroughly dried after treatment. Exposure tests with stakes and posts placed in ground contact indicate that the treated wood resists attack by both fungi and insects, although CC does not contain a co-biocide to prevent attack by copper-tolerant fungi. CC is listed in AWPA standards for treatment of a range of softwood species and wood products. The minimum CC retention is 4 kg/m³ (0.25 lbs/ft³) or 6.4 kg/m³ (0.4 lb/ft³) for wood used above ground or in ground contact, respectively. As with other preservatives containing ammonia, CC has an increased ability to penetrate into difficult-to-treat wood species such as Douglas-fir. CC treatments do accelerate corrosion of metal fasteners relative to untreated wood, and stainless steel or hot-dipped galvanized fasteners are recommended. Few treating plants currently use CC, and wood treated with this product may not be readily available in most areas.

Copper Azole (CBA-A and CA-B)

Copper azole is another recently developed preservative formulation that relies primarily on amine copper, with co-biocides to protect wood from decay and insect attack. The first copper azole formulation developed was copper azole-Type A (CBA-A), which contains 49% copper, 49% boric acid, and 2% tebuconazole. More recently, the copper azole-Type B (CA-B) formulation was standardized. CA-B does not contain boric acid and contains 96% copper and 4% tebuconazole. Wood treated with either copper azole formulation has a greenish-brown color and little or no odor. The copper azole formulations have been evaluated with in-ground stake tests and demonstrated efficacy against attack by decay fungi and insects. The formulations are listed in AWPA standards for treatment of a range of softwood species. Minimum retentions of CBA-A in the wood are 3.3 kg/m³ (0.20 lb/ft³ ) and 6.5 kg/m³ (0.41 lb/ft³) for wood used above ground, or in ground contact, respectively. Minimum retentions of CA-B in the wood are 1.6 kg/m³ (0.10 lb/ft³ or 3.4 kg/m³ (0.21 lb/ft³) for wood used above ground or in ground contact, respectively. Although listed as an amine formulation, copper azole may also be formulated with an amine-ammonia formulation. The ammonia may be included when the copper azole formulations are used to treat refractory species, and the ability of such a formulation to adequately treat Douglas-fir has been demonstrated. The inclusion of the ammonia, however, is likely to have slight affects on the surface appearance and initial odor of the treated wood. The copper azole treatments do increase the rate of corrosion of metal fasteners relative to untreated wood, and hot-dipped galvanized or stainless steel fasteners are recommended. Because copper azole has only been recently developed, relatively few treating facilities are currently using this preservative.

Copper Dimethyldithiocarbamate (CDDC)

Copper dimethyldithiocarbamate (CDDC) is a reaction product formed within the wood after treatment with two different treating solutions. It contains copper and sulfur compounds. Exposure data indicates that CDDC treatment is effective in protecting wood against attack by decay fungi and insects, although a topical preservative finish may be needed to prevent discoloration by mold and mildew. CDDC is standardized for treatment of Southern Pine and some other pine species at copper retentions of 1.6 kg/m³ (0.1 lbs/ft³) or 3.2 kg/m³ (0.2 lb/ft³) for wood used above ground or in ground contact, respectively. CDDC-treated wood has a light brown color and has little or no odor. CDDC has been developed very recently.

Borate Preservatives

Borate preservatives are sodium salts, such as sodium octaborate, sodium tetraborate, and sodium pentaborate, that are dissolved in water. Borate preservatives have received a lot of attention in recent years because they are inexpensive and have low mammalian toxicity. Borate-treated wood is also odorless and colorless and may be painted or stained. Borates are effective preservatives against decay fungi and insects. Borate preservatives are diffusible, and with appropriate treating practices they can achieve excellent penetration in species that are difficult to treat with other preservatives. Borate preservatives are standardized by the AWPA but only for applications that are not exposed to liquid water. The borate in the wood remains water soluble and readily leaches out in soil or rainwater. Borate-treated wood should be used only in applications where the wood is kept free from rainwater, standing water, and ground contact. An example of such a use is in the construction of wooden buildings in areas of high termite hazard. Borates are also used in fire-retardant treatments.

Other Preservatives

Information on CCA and other preservative treatments can be found in the following FPL publications:

Changes in Pressure-Treated Wood for Residential Construction
Wood Preservation, Chapter 14 of the Wood Handbook

Frequently Asked Questions

The first questions address new restrictions on the use of CCA-treated wood and the available alternatives. Then other frequently asked questions on wood preservation and related topics are also addressed. With few exceptions, references in this document were limited to publications of the Forest Products Laboratory. These answers are not intended to represent a complete review of the available literature.

1. What are CCA, ACA, ACZA, etc.?
2. Is it true that EPA has banned CCA-treated wood?
3. What are the alternative types of treated wood for residential use?
4. Has FPL evaluated these treated wood products?
5. Should I have concerns about existing CCA-treated wood?
6. How do I dispose of treated wood?
7. What about naturally durable species?
8. How can I extend the life of untreated wood?
9. Where can I find the climate index map of relative decay hazard?
10. What research is FPL doing on wood preservatives?
11. What experimental preservative treatments for wood has FPL investigated?
12. What test methods are used to evaluate new preservative treatments?
13. What is the impact of preservative treatments on mechanical properties of wood?
14. How can I tell if my wood is properly treated?
15. Where can I find additional information related to wood decks?
16. What is the market for treated wood?
17. Where can I find lumber treated for both flame retardancy and preservation?
18. Does FPL have any information on remedial or field treatment of wood?