1. Field of the Invention
The invention relates to a method of preserving wood with lanthanide derivatives.
2. Discussion of the Prior Art
Wood preservatives known in the art are either oil-based or water-based. Oil-based preservatives fall into two main classes, namely (i) coal tar creosote and solutions of creosote with coal tar or petroleum oils and (ii) solutions of preservative chemicals, such as pentachlorophenol dissolved in a suitable organic carrier.
One disadvantage of oil based preservatives is that they exude from the wood. Thereafter they may wash from the surface or evaporate. In order to compensate for the loss of the oil-based preservative, high initial retentions are required. In tropical and high rainfall areas, the use of oil-based preservatives has been found to be uneconomical. Another disadvantage of certain oil-based preservatives is that they are regarded as a skin irritants and can cause burns. Furthermore, oil-based preservatives such as creosote cannot be painted and do not have attractive appearances. These preservatives often have toxic side effects.
Water based preservatives are those containing chemical preservatives in the form of aqueous solutions. Such preservatives react within the wood to form compounds, the solubility of which may be increased by adjustment of pH. When chemical changes occur within the wood resulting in compounds with very low solubility, the compounds are designated as leach-resistant. Those which form soluble compounds are designated as leachable.
Leach-resistant water-based preservatives in commercial use include acid copper chromate solution (ACC), chromated copper arsenate solution (CAA) and ammoniacal copper arsenate solution (ACA). CCA solutions are commonly used. They form, in the wood, compounds which are toxic to both fungi and insects. Leachable water-based preservatives include chromated zinc chloride and fluoride-chromium-arsenate-phenol mixtures and boron compounds. The leachable water-based preservatives can only be used in treating timber to be used internally or where leaching conditions are not severe.
Use of water-based preservatives has many advantages: cleanliness, paintability of the treated wood, freedom from odor, and when correctly applied, longer protection of the wood.
The American Wood Preserver's Association "Book of Standards" (1986) defines and describes on pages 1978 and 1979 well known techniques for applying wood preservative compositions. Among these are mentioned:
Brushing
Butting
Diffusion
Dipping
Double Pressure
Dual
Empty-Cell
End Pressure
Full-Cell
Internal
Non-Pressure
Pressure
Spray
Surface
Thermal
Vacuum
These methods achieve the desired result of applying compositions having wood preservation properties to wood. The descriptions of wood preservation methods described on these pages and throughout the Book of Standards are hereby incorporated by reference.
Essential features of the pressure method are that (1) the wood is surrounded by a preservative solution in a closed vessel; and (2) hydrostatic pressure is applied by mechanical means to force the solution into the wood fibers by replacing air or water already there, or going into any voids. It is conventional to evacuate the sytem to about 26" of mercury vacuum to remove air from cells within the wood. When a solution of CCA is used to impregnate wood, the CCA reacts inside the wood with reducing sugars found therein to form a mixture of insoluble salts.
U.S. Pat. No. 2,565,175 to Hager describes a method of preserving wood using specific types of preservatives in combination with specific methods and conditions of penetration and distribution of these preservatives within the wood. One specific type of preservative is CCA, to which ammonia is added to render the preservative solution alkaline. The ammonia addition prevents rapid fixation of the preservative in the wood. According to the method described by Hager, the preservative is introduced into the wood and the wood is kept in an undried condition for a period of time during which no fixation of the preservative occurs, and the preservative diffuses through the cell walls. Thereafter, the wood is dried.
U.S. Pat. No. 4,303,705 to Kelso, Jr. describes a process for preserving wood against attack by living organisms, e.g., fungi and insects. The process may comprise one or two steps. In the two step process, there is a fungicidal step comprising introducing a copper solution into wood, and an insecticidal step comprising introducing a chromium and arsenic solution into wood.
One disadvantage of using CCA is that not all fixation of the preservative takes place in the wood. Sludging may occur in the working solution due to pickup of wood or wood extractives, corrosion, or impurities in the chemicals used to make up the solution. Sludging causes a deposit of solids on the surface of the wood. These deposits contain varying percentages of arsenic and thus are a matter of environmental concern. Recent treatment standards (AWPA 1982) have recognized this (see Hartford. W., "The Practical Chemistry of CCA is Service", American Wood Preservers' Association Annual Meeting, Apr. 28, 29 and 30, 1986, pp. 1-16).
Lanthanide derivatives are used in glass, ceramic, paint, plastics, and rubber manufacture. Compositions comprising cerium compounds are known to have bacteriocidal effects, e.g. compositions comprising cerium nitrate and silver sulfadiazine (Boeckx, et al., Burns vol. 11, no. 5 (1985) pp. 337-342; Monafo, 3rd International Congress on Pharmacological Treatment of Burns, Milan, Italy, May 12-15 1980, Panmainerva Med., vol. 25, no. 3 (1983) pp. 151-154; Bowser, et al., J. Trauma, vol. 21, no. 7 (1981) pp. 558-563; Monafo, et al., Arch. Surg. vol. 113, no. 4 (1978) pp. 397-401; Monafo, et al. Surgery (St. Louis) vol. 80, no. 4 (1976) pp. 465-473), and compositions containing electrically activated silver and cerium stearate (Colmano, et al., 23rd Annual Meeting of the Biophysical Society (New York), Atlanta Ga., Feb. 26-28, 1979, Biophys. J. vol. 25, no. 2, part 2 (1979) p. 217A). Cerium derivatives are also used as additives in plastics for food packaging.