Thermal Treatment of Western Red Cedar with Copper Naphthenate
by
Randall T. Baileys1
Mike H. Freeman2
Abstract
A series of thermal treating pilot scale tests were conducted using copper naphthenate (CuNap) preservative solutions produced with base P9-type A oils to evaluate treating and process variations in Western red cedar (WRC) pole stubs. The effect of base oil aromatic content has historically been a major factor in achieving satisfactory pressure treatment results in Western wood species, but no information is available on thermal treatment. Incised Western red cedar (WRC) post-length, typical pole diameter sized material was thermal treated using a hot-bath, cold soak and drain processing sequence in three different base oil solution mixtures containing approximately 1 percent copper (as metal) of copper naphthenate. Penetration, retention and appearance of the thermally treated materials were assessed and reported along with solution analyses for metal content and sludge formation characteristics. Retention and penetration of the preservative solution was acceptable by American Wood Preservers’ Association (AWPA) C35-Thermal Treatment Standard using each of the base oil solutions, however processing characteristics and appearance of the treated posts were altered by the type of base oil in the solution. In these series of tests, the lower aromatic oils formed sludge residue on the heating coils in the hot tank that may prevent this critical processing step using similarly equipped systems. More thorough evaluation of the oil/solution and heat relationship is necessary to fully understand this factor. Penetration of the WRC sapwood and incised zone was very similar to commercially produced thermal treatment oil-penta poles.
Background and Introduction
Background on Copper Naphthenate
The use of copper naphthenate as an industrial biocide/wood preservative has been well established since the turn of the century. Copper naphthenate is basically the metallic salt of a metal ion reacted with naphthenic acids. Naphthenic acids are by-products of petroleum, typically removed from crude petroleum by caustic quenching, then resulting acidification. Typical crude petroleum oils contained 0.5 to 2 percent crude naphthenic acid by weight, with the highest concentrations of crude found in South America, western North America, Romania, Russian, and Central America. The naphthenic acids are typically alicyclic acids. They are broadly classified as acids of the formula CnH2n-zO2 (z=2,4,6,8) .
Chemically speaking, these compounds are typically known as cupric cyclopentane carboxylates or cyclohexane carboxylates. The physical and chemical characteristics of copper naphthenate and naphthenic acids have been described in detail and their use in wood preservation discussed by Hartford et al. Broadly speaking, many naphthenic acids can find their way into wood preservation, since the specifications written for copper naphthenate include a wide variety of acid values, all of which are known to perform extremely well in ground contact. Trade names for copper naphthenate in commercial use include Perm-E8, Cop-R-Nap, CuNap, CuNapSol, Cuprinol, and M-Gard. Of these, the most common name is Cuprinol, dating back to the Danish of over a century ago, meaning “copper in oil”. A review of the literature cites many applications for use, including field boxes, beehives, benches, flats, fenceposts, water tanks, canvas, burlap, ropes, nets, greenhouses, utility poles, crossarms, and wooden structures in ground contact and above ground contact. Copper naphthenate is known to control most decay fungi, molds, mildew, algae, lichens, dry rot, certain marine growths, termites, wood parasites, and bacteria.
Copper naphthenate began its strong leap into the wood preservation business with the need to extend the useful volume of creosote available in the postwar effort. Due to a modification of operating practices of the steel mills, creosote, whose main source is the coking of coal of petroleum products, was in short supply. The American Wood Preservers’ Association (AWPA) began a search for combination biocides that could be added to creosote to effectively extend its service life. Colley et al. determined that copper naphthenate was a likely extender for creosote and did not offer some of the proposed problems that addition of pentachlorophenol (penta) as a phenolic acid would pose in treating plant corrosion.
Resulting papers presented by Minich and Goll included a broad background of the technical aspects of copper naphthenate as a wood preserving chemical, including its solubility in inorganic solvents, relative vapor pressure, electrical conductivity properties, compatibility with commercially available oils, and the effectiveness of copper naphthenate against wood decay fungi. A specification was proposed to add copper naphthenate to the AWPA Book of Standards. The key issues brought about by the proposal by Minich and Goll included copper naphthenate as a chemical compound of uniform performance, its highly effective nature, and as a permanent wood preservative, its easy application, and its safety in handling to workers.
Copper naphthenate exists in the AWPA Wood Preservative Standard, P8-01, with the following specifications:
- The acid used in the manufacture of copper naphthenate shall be naphthenic acid of the group of alicyclic carboxylic acids occurring in petroleum and shall have an acid number of not less than 180 and not more than 250, on an oil-free basis.
- The copper naphthenate concentrate used to prepare wood preserving solutions shall contain not less than 6 percent, nor more than 8 percent, copper in the form of copper naphthenate.
- All of the copper present in the concentrate shall be combined as copper naphthenate.
- The copper naphthenate concentrate shall not contain more than 0.5 percent water. The foregoing tests shall be made in accordance with the standard methods of the AW
- PA Standards A5-00.
- Solvents used to prepare solutions of copper naphthenate shall comply with the standards of the AWPA Standard P9-01.
- The copper naphthenate concentrate shall not contain more than 2 percent (relative) of the total copper in the concentrate as being water extractable as determined by the analytical method A14-97 on Page 299 of the 2001 AWPA Book of Standards.
These values in the current AWPA Standards vary slightly from the original proposal prepared by Minich and Goll, including an upper limitation of the naphthenic acid value to preclude the use of synthetic carboxylic acids in preparing copper carboxylate solutions. Today there are over 25 Standards in the AWPA Book of Standards that relate to copper naphthenate and the treated wood commodities that have been treated with it. These are also reflected in the dozen or so UCS Processing and Commodity Standards reflecting the proper use of copper naphthenate. As a wood preservative, copper naphthenate continues to grow in both use and product flexibility into expanding roles throughout North America.
Many discussions have shown that CuNap performed equivalent to or better than other oil-borne wood preservatives. In a recent publication, 0.05 pcf of oil-borne CuNap performed equivalent or better than 0.40 pcf pentachlorophenol in various P9-Type A oil in AWPA hazard zone 4. Additionally, there has been some concern in AWPA over the use of steam to condition poles and to post-steam poles. Recent documents have shown no deleterious effects either on efficacy or on CuNap chemical composition, including no soft rot, no decay or insect attack after 12 years of pole stubs in service in hazard zone 4 after 12 years.
Background on the Thermal and Thermal Butt-Treating Processes
The thermal process, originally patented by C.A. Seely, is also known as the open-tank treatment, the boiling-and-cooling method, and more recently the thermal process. It is also possible to perform the thermal process on only the butts of durable heartwood species, like WRC. The process involves immersion of seasoned wood or unseasoned wood, for a matter of hours, in successive baths of hot and relatively cool preservative. There may also be a hot expansion bath used as a final clean-up process to surface dry the treated wooden commodity and clean to wood surface of any residues. The function of the initial hot bath is to expand the surface of the wood and expand the air entrained in the wood in the wood cells and open cavities; the duration of the bath and temperature of the preservative will largely determine the extent to which air and water vapor leave the wood, and how long a sterilization cycle is to be on the size and original MC of the treated pole or pole butt (usually with an incised groundline area).
The cold bath, in turn, causes the air and vapor remaining in the outer shell of the wood to contract, thus forming a partial vacuum. To satisfy this vacuum, atmospheric pressure forces the surrounding preservative into the wood. Some penetration takes place during the hot bath in unusually absorptive wood, but most of the absorption and penetration occurs during the cold bath. Transferring materials in the baths may be accomplished in several ways:
- Moving the heated wood to a separate tank of relatively cool preservative;
- Withdrawing the hot liquid from the tank, replacing it with non-warmed preservative and not moving the timber;
- Merely discontinuing the heating and allowing the wood and preservative to cool together.
In the first two cases, it is imperative that the change is made without delay; otherwise, the heat differential of the subsequent cold bath will be impaired. The timber to be treated should be rather thoroughly dried initially, not only to facilitate penetration but also to eliminate subsequent extension of seasoning checks through the impregnated shell of wood. Although green WRC has been successfully treated with sapwood moisture contents above 150%, it is not possible to achieve the necessary level of protection in all wood species.
Both preservative oils and water-soluble salts may be applied by the hot- and cold-bath process, but the great bulk of treatment is done with coal-tar creosote and other oils, and oil-borne preservatives. Preservative oils have a definite advantage in affording more permanent protection to poles, posts, and other forms of timber that will be exposed to the weather and because they can be heated to the desired temperatures in open tanks without excessive loss by evaporation. When water solutions are employed in the hot bath, temperatures must be kept low enough to maintain the solution at proper strength. Water-borne preservatives that cannot be heated to high temperatures without danger of precipitating part of the salts out of solution are not suitable, nor are water-borne solutions that use volatile carrier solvents, like amines and ammonia.
When coal-tar creosote is used, hot-bath temperatures of 210-230°F are usually adequate for general purposes. However, old AWPA Standards C7-58 and C8-58 of the American Wood-Preservers’ Association, covering treatment of cedar poles by the thermal process, stipulate a temperature range of 190-235°F. The higher temperatures tend to improve penetration but also cause somewhat greater evaporation of oil from the treating tank; this loss may be considerable with creosotes of relatively low boiling range and also other oil borne systems. The cold bath should be as cool as is consistent with keeping the preservative thoroughly liquid; temperatures of around 100°F are about right for coal-tar creosote. Former AWPA Standard C7-58 required that the cold bath shall be “between 150°F and the temperature at which solids form in preservative.” In practice, both maximum and minimum temperatures vary more widely than the specification limits. When water solutions are used, the cold bath can be maintained at atmospheric temperatures, as long as they remain above freezing. The new AWPA standard, C35-97 or the UCS system does not cite these limitations.
Treating time may vary considerably depending upon such factors s the species of wood, type of product, extent to which the timber has been seasoned, weather conditions, and often the opinion of the person in charge of the operation. Each bath may last 1-12 hr or even longer. Former American Wood-Preservers’ Association Standards C7-58 and C8-58 provide for a hot bath of not less that 6hr and cold bath of at least 2 hr.
Woods that are easily impregnated may require special treatment if the operator is to obtain reasonable penetration without excessive retention of preservative. To this end, work by the Texas Forest Service in treating small shortleaf and loblolly pine pole showed the advantage of reheating such material, following the cold bath. When these poles were treated by the usual hot-and-cold bath method, excellent penetrations (averaging 2 in.) were obtained, but the amount of creosote absorbed (18-20 lb per cu ft of wood) was excessive for the class of material involved, and heavy bleeding resulted. Absorption could be reduced by decreasing the time the wood was held in the regular baths, but only at a very marked sacrifice in depth or penetration. It was determined the desired results could be obtained if, following a 1-hr hot bath (200-225°F), the poles were allowed to stand in the cold bath preservative (100°F) only long enough to receive a penetration of about 1 in. (as indicated by borings). Following this cold soak, they were returned to the hot bath for 30 minutes. Timbers treated in this way had an average penetration of 1.83 in. and an average retention of 12.8 lb of creosote per cubic foot, which was considered sufficient for protection and did not induce bleeding in the poles.
An adequate hot-and cold-bath treatment with preservative oils is the best substitute for pressure impregnation. The process finds extensive commercial use in the butt treatment of poles, especially lodgepole pine and western red cedar, and to an increasing extent for full-length treatment of these timbers. In the drier parts of the country, the aboveground parts of cedar poles are sufficiently durable to give long service in their natural condition, so it is necessary to treat them only to a height about 1 ft above the groundline. They will give longer service with full-length treatment however, and this is especially important in areas where sapwood decay (shell rot) in aboveground sections of cedar poles is common. Round timbers apt to decay rather rapidly above ground, such as pine because of their high content of volatile organics. By proper control of humidity in the kiln during the heating process, the wood can be warmed satisfactorily without decreasing its moisture content, thus avoiding any danger of shrinking and checking the material. This is especially important with finished products or finished parts of unassembled products, including window sash and frames, doors, millwork, flooring blocks, and furniture.
In another variation (Boardman) the wood is placed in an open tank, the tank covered with a tarpaulin or other suitable material, and steam at atmospheric or slightly higher pressure admitted for several hours. When the wood has been heated sufficiently, steaming is discontinued and a cold solution of water-borne preservative admitted to the tank. This method has found commercial use on a small scale. Heating in steam can be accomplished, of course, in a closed cylinder either at or above atmospheric pressure; this procedure is also in limited commercial use, mainly for treatment of mine timbers with a water-borne preservative.
Still another modification of the hot and cold bath (Hammond) provides for heating wood in water at temperatures of 120-212°F and then replacing the water with a water-borne solution of a wood-preserving or fire-retardant chemical. The stated purpose of the water treatment is to increase the moisture content of the wood so diffusion of the chemical will take place more rapidly in the second bath.
In the Conger method (U.S. 2,308,491 to E. F. Conger, Jan. 19, 1943), poles that had received full-length pressure impregnation with preservative of the zinc-meta-arsenite type were subsequently given a hot-and-cold-bath butt treatment in coal-tar creosote. The supplement creosote treatment was said to hasten formation of relatively insoluble zinc-meta-arsenite in the wood by quickly driving off the volatile acid in the solution. Absorption of creosote during the cold bath provided extra protection to the underground portions of the poles, while the aboveground section remained clear and free from bleeding. Full-length creosote treatment could be given if desired. Although several thousand poles were treated by one or another variation of Conger method, it did not find extensive use.
- Technical Services Manager, J. H. Baxter & Co.. P.O. Box 10797, Eugene, OR 97448-2797.
- Wood Scientist, 7421 Hunters Tree Cove, Memphis, TN, 38125
 Contact us by e-mail |
 Inquiries by form |
|
| HOME All Information are ©Copyright 1999 of Merichem Chemicals & Refinery Services LLC |
||
