That Pressure-Treated Wood

What’s in That Pressure-Treated Wood?

By Black Locust Lumber team

Wood in contact with the ground, or above ground thatoften gets wet, will eventually be attacked by decayfungi and insects. With the exception of naturally du-rable species, such as redwood and cedar, wood usedin these applications should be pressure treated withpreservatives if it is expected to last more than a fewyears. Several new wood preservatives developed inrecent years have added to the variety of preservativesavailable.The following descriptions are intended to help theconsumer understand what is in the pressure-treatedwood or what type of pressure-treated wood theyshould choose. Wood preservatives are broadly classi-fied as either water based or oil type, depending on thechemical composition of the preservative and the car-rier used during the treating process. Several preserva-tives that are not currently available, but werepreviously in use or discussed in the literature are alsodescribed.Water-Based Wood Preservatives

Water-based preservatives react with or precipitate inthe wood substrate, thus becoming fixed and resistantto leaching. Water-based preservatives leave a dry,paintable surface; therefore, they are commonly usedto treat wood for residential applications such as decksand fences. They are primarily used to treat softwoodsbecause hardwoods treated with these preservativesmay not be well protected from soft-rot attack. Water-based wood preservatives can increase susceptibility tocorrosion, so all metal fasteners used with the treatedwood should be hot-dipped galvanized or made ofstainless steel. Borates, another type of water-basedpreservative, do not become fixed in the wood andreadily leach if exposed to rain or wet soil.Chromated copper arsenate contains chromium,copper, and arsenic. Wood treated with CCA, com-monly called “green-treated” wood, dominated theresidential market for several decades and was sold atUnited States Forest Forest Products Phone: (608) 231–9200; FAX: (608) 231–9592Department of Service Laboratory Website: under a variety of trade names. However,as the result of the voluntary label changes submittedby CCA registrants, labeling of CCA by the Environ-mental Protection Agency (EPA) now permits theproduct to be used primarily for industrial applica-tions. Use of CCA-treated wood prior to 2004 is notaffected by this change. CCA protects against attackby decay fungi, insects, and most types of marine bor-ers. CCA-treated wood is now primarily used in poles,piling, and bridge timbers.1Obtaining adequate treat-ment with CCA can be a problem with difficult-to-treat species such as Douglas-fir. Wood treated withCCA has no odor and is typically light green in color.It can be readily painted or stained.Ammoniacal copper zinc arsenate (ACZA) containscopper, zinc, and arsenic and is a refinement of theoriginal formulation ammoniacal copper arsenate(ACA).

ACZA protects against attack by decay fungi,insects, and most types of marine borers. It is used totreat poles, piling, and timbers. Because of its abilityto penetrate Douglas-fir and other difficult-to-treatwood species, it is most widely used on the West Coast. The color of the treated wood is olive to bluish-green. The wood initially has a slight ammonia odor,but this dissipates soon after treatment. ACZA-treatedwood can be painted or stained.Alkaline copper quat (ACQ) is one of several re-cently developed wood preservatives. It contains cop-per and a quaternary ammonium compound. ACQ pro-tects against decay fungi and insects but has not beenstandardized for use in marine applications. Multiplevariations of ACQ have been or are in the process ofbeing standardized.

ACQ-B is an ammoniacal copper quat formulation; ACQ-D is an amine copper quat formulation; and ACQ-C is formulated with either ammonia or amineand a slightly different quat compound. CurrentlyACQ-D is the most commonly used formulation. LikeACZA, ACQ-B is able to penetrate Douglas-fir andother difficult-to-treat wood species and is used pri-marily on the West Coast. Wood treated with ACQ-Bhas a dark greenish-brown color. ACQ-D is manufac-tured with amine copper, which gives the treated wooda light brown color. ACQ-D is not as effective asACQ-B in penetrating difficult-to-treat woods. BothACQ-B- and ACQ-D-treated wood can be painted orstained.

Copper azole (CBA) is another recently developedpreservative formulation that relies on amine copper,with co-biocides, to protect wood from decay and in-sect attack. The first copper azole formulation devel-oped was CBA-A, which contains copper, boric acid,and tebuconazole. Recently the CA-B formulation wasstandardized and has largely replaced CBA-A. CA-Bdoes not contain boric acid, but contains more copperand tebuconazole. Copper azole formulations can beused to treat a wide range of wood species used inaboveground or ground-contact applications but arenot standardized for use in seawater. In some cases,additional ammonia may be added to the treating solu-tion to improve penetration of difficult-to-treat speciessuch as Douglas-fir. Wood treated with either copperazole formulation has a light brown color and little orno odor.

The treated wood can be painted or stained.Borate preservatives are salts such as sodium octabo-rate, sodium tetraborate, and sodium pentaborate thatare dissolved in water. Borates are effective preserva-tives against decay fungi and insects. Borate preserva-tives are diffusible, and with appropriate treatingpractices, can achieve excellent penetration in speciesthat are difficult to treat with other preservatives.However, the borate in the wood remains water solu-ble and readily leaches out in soil or rainwater. Borate-treated wood should be used only in applicationswhere the wood is kept free from rainwater, standingwater, and ground contact. An example of such a useis in the construction of wooden buildings in areas ofhigh termite hazard. Borate-treated wood is odorlessand colorless and can be painted or stained.Oil-Type Wood PreservativesThe most common oil-type preservatives are creosote,pentachlorophenol, and copper naphthenate.

The oil-type preservatives are commonly used for applicationssuch as utility poles, railroad ties, piling, and lami-nated beams. Wood treated with oil-type preservativesis not usually used for applications that involve fre-quent human skin contact or inside dwellings becausethey can be visually oily, oily to touch, or have astrong odor. Because of their oily nature, these pre-servatives also act as water repellants and can help toprevent checking and splitting.Creosote is made from coal tar, which is a byproductof the carbonization of coal during steel production.Unlike other oil-type preservatives, creosote is not dis-solved in oil, but it does have properties that make itlook and feel oily. Creosote contains a chemicallycomplex mixture of organic molecules, most of whichare polycyclic aromatic hydrocarbons (PAHs).Creosote is effective in preventing attack by decayfungi, insects, and most marine borers. Creosote iswidely used in railroad ties, utility poles, bridge tim-bers, and piling.

Creosote-treated wood has a darkbrown-black color with an oily surface and strongodor. It is very difficult to paint, stain, or seal. It is notrecommended for use inside dwellings or areas whereit could come into frequent contact with human hands,such as handrails.Pentachlorophenol is a crystalline solid that can bedissolved in different types of oils. Pentachlorophenolis very effective against fungi and insects but does notprotect well against ocean marine borers. It is widelyused to treat utility poles, bridge timbers, laminatedbeams, and foundation and fresh-water piling. Theappearance of pentachlorophenol-treated wooddepends greatly on the type of oil in which it is dis-solved. The wood may have a very light brown colorand dry surface if a light oil is used, or a dark browncolor and somewhat oily surface if a heavy oil is used.

Pentachlorophenol-treated wood is generally moredurable if a heavy oil is used, so light oil is most oftenused for aboveground applications.Pentachlorophenol is odorless, but the odor of the oilin which it is dissolved may be noticeable near thetreated wood. Pentachlorophenol-treated wood shouldnot be used inside dwellings, and it is not recom-mended for areas where it could come into frequentcontact with human hands, such as handrails. Penta-chlorophenol-treated wood is difficult to paint or stainunless it was pressure treated using a light oil.Copper naphthenate is a mixture of napthenic acidsand copper salts dissolved in oil. It is effective againstdecay fungi and insects but is not recommended foruse in marine applications. Copper naphthenate is notas widely used as creosote or pentachlorophenol, but itis used for the treatment of utility poles and in high-way construction.

As with pentachlorophenol, theproperties of copper naphthenate are dependent on thetype of oil in which it is dissolved. The most com-monly used oils are fuel oil and mineral spirits. Thecolor of the treated wood varies from light brown todark green, depending on the type of oil and treatingprocess. The odor of the oil may be noticeable near thetreated wood. The treated wood is difficult to paint orstain unless pressure treated using a light oil. Coppernaphthenate is not a restricted-use pesticide, and theliquid preservative can be purchased at retail lumber-yards and hardware stores. It is widely used to treatfield cuts of pressure-treated wood that are madeduring construction.Preservatives Not Commercially AvailableSeveral preservative formulations that have been usedin the past or were discussed in previous publicationsare not currently available. These preservatives areincluded here to inform the reader that they are notcurrently available or because they may become avail-able in the future.

Ammoniacal copper arsenate (ACA) is an older for-mulation of ACZA that does not contain zinc. It hasnot been available in the United States for many yearsand is not likely to be produced again in the future.ACA wording should be replaced with ACZA in olderguideline and specification literature.Acid copper chromate (ACC) has been used as awood preservative in Europe and the United Statessince the 1920s. ACC contains copper oxide andchromium trioxide. The treated wood has a lightgreenish-brown color and little noticeable odor. Testson stakes and posts exposed to decay and termite at-tack indicate that wood well-impregnated with ACCgives acceptable service, although it may be suscepti-ble to attack by some species of copper-tolerant fungi.As with CCA, it may be difficult to obtain adequateACC penetration in some of the more refractory woodspecies such as white oak or Douglas-fir. However, thehigh chromium content of ACC has the benefit of pre-venting much of the corrosion that might otherwiseoccur with an acidic copper preservative.

Ammoniacal copper citrate uses copper oxide as thefungicide and insecticide and citric acid to aid in the distribution of copper within the wood structure. In2004, ammoniacal copper citrate was withdrawn fromthe American Wood Preservers’ Association standardsdue to lack of use.Copper dimethyldithiocarbamate (CDDC) is a reac-tion product formed within the wood after treatmentwith two different treating solutions. It contains copperand sulfur compounds. CDDC protects against decayfungi and insects but has not been standardized for usein seawater. CDDC-treated wood has a light browncolor and little or no odor.AvailabilitySuppliers of treated wood can be found by contacting locallumberyards or trade associations that work with treatedwood manufacturers. A few such trade associations includethe following:American Wood Preservers’ Association;
P.O. Box 388
Selma; AL 36702-0388;

(334) 874-9800;
fax (334) 874-9008;

Western Wood Preservers Institute; 7017 NE Highway 99;
Suite 108; Vancouver, WA 98665
(360) 693-9958; Fax (360) 693-9967;

Southern Pressure Treaters Association;
P.O. Box 3219; Pineville, LA 71361-3219

(318) 619-8589; fax 318-767-1388;

Southern Pine Council; 2900 Indiana Avenue;
Kenner, LA 70065-4605
(504) 443-4464; fax: (504) 443-6612;

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