WOOD PRESERVATION PROCESS

Abstract: A process for preserving wood with either a water-borne or oil-borne preservative including a conditioning step in which the extractives in the wood system are removed prior to the preservation step. The wood is placed in a heated acid bath to diffuse out the water soluble extractives and then subjected to a vacuum so as to reduce the boiling point of the water trapped in the wood whereby the steam so generated will expel the remaining water from the wood cells.

Description: WOOD PBESBRY^DION PROCESS This invention relates to a process for conditioning and preserving wood with a preservative solution. The process applies to both oil-borne and water-borne preservatives. The growth of wood occurs by the photo-synthesis system in which organic substances migrate down the phloem tissue to the εapwood where reactions take place in the Cambian layer to produce cellulose from the sugars and lignin from the aromatics to the wood cell. The sugars proceed from poly-saccharides to he i-cellulose and to πiicrofybrils of cellulose. Esters of benzoic acid and phenolic ethers such as catechol have been isolated from the sap streams of certain hardwoods. The aro atics • condense to form long chain molecules to form the lignin which cements the microfybrils together. During the course of the reactions, cell side reactions take place between the precursors of wood to form such substances as starches and organic acids such as oxalic acid. Many of these substances and precursors are known to be powerful reducing agents than can interfere with the course of preservative reactions. Also present in the sap stream are such substances as ammonium tartrate, potassium di-hydrogen phosphate, magnesium sulphate, zinc sulphate, manganese sulphate, calcium sulphate and trace elements such as iron, sodium that are known' to be basic nutrients in the growth of organic matter. These chemicals and the precursors to the manufacture of the wood substance are generally known as a substrate to the wood system. The effect of extracts or their fractions on the growth of fungi has been studied in growth experiments using a Basal medium containing saccharides and the abovementioned ammonium tartrates and potassium hydrogen phosphates. Stock cultures of fungi grown on Agar and fungus were inocculated directly into the system and at the " end of the experiments the fungi were harvested and the dry weight determined. The effect of a 5% solution of the extract on the growth of Basidiomycetes was to increase the growth rate in the case of collybia velutipes by as much as 18 times the dry weight control. All the white rots tested showed increases in dry weight control varying from 50% upwards. In the case of ascomycetes and fungi imperfecti there was no statistically noticeable increase in the growth rate of the fungi when fed either a 5 or a 20% solution. These type of fungi can be concluded to prosper on the wood cell. In the case of certain mycorrhizal fungi there is a noticeable depressive effect when the extract strength is increased from 5 to 20%. In each case the rate of growth of fungi is depressed. Benzoic acid and catechol are both present in the sap stream and are known to have some toxic properties relating to fungi." A mixture of the two appears to exert a synergistic effect which depresses the rate of growth of fungi to very low levels. The same materials are found in coal tar and possess toxic properties. These phenolics and benzoic acids have the property of being both water soluble and oil soluble, as such they- can set up equilibria between the oil phase and the aqueous phase in any wood system. They tend to wet the wood and to hold the oil in any oil-borne preservative within the wood system reducing the tendency of oil-borne preservatives containing such chemicals to bleed from the wood by syneresis. These abovementioned extractives which exist in the substrate can cause significant problems associated with the fixation of preservatives as they can react with the preservative salts and interfere with the chain of reaction. The treatment of wood with chromated copper arsenate solutions (either hot or cold) is known to have many serious disadvantages - See for example U.S. Patent 3080212. Firstly there is incomplete and non uniform penetration of the cold solution into the cold wood resulting in unsatisfactory treatment of a considerable percentage of the wood. It is believed that this is caused by stearic hindrance of the molecules that arise from the CCA salts due to the premature reduction of the chromium from valency 6 to valency 3. Secondly, a seasoning step is usually required before satisfactory penetration can be achieved. During the seasoning process the decay cycle commences and the destruction of the wood is enevitable and no amount of preservative applied afterwards will correct the situation. Thirdly, reducing sugars are gradually extracted from the wood eventually causing formation of a sludge or precipitate in the solution. These are complexes of copper, chromium and organic substances in the form of chromates. At pH 4 chromates such as basic chromium chromate, basic copper chromate are in stable equilibria. These complexes which are fairly insoluble fix in the solution and in all probability form much of the final fixation product. The toxic thresholds of these chromates than can exist and precipitate out of chromated copper arsenate solutions have been shown to be from 5 to 10 times higher than when basic copper arsenate is fixed out of a chrome-free solution. They form a sludge that is suspended in the solution deposits on the wood and can be toxic as well as a skin irritant. There are some practical implications from these phenomenon and on theoretical grounds there exists a health hazard to of humans and animals having high stomach acidity. Amounts of elemental arsenic as high as 6.99 milligrams per 100 square centimetres of surface have been removed from wood treated with chromated copper arsenate salts and 6.74 milligrams per 100 centimetres squared of copper have also been removed from intact end areas of OMPI timber posts so treated. For round surfaces the arsenic can be 1.22 milligrams per 100 centimetres squared and the copper 1.46 milligrams per 100 centimetres squared. No tests are available for chromium but it is known to be a skin irritant. The disproportioning of preservative elements with penetration depth as happen in this situation leads to the solution becoming unbalanced as it moves into the wood system, becoming arsenic rich and copper lean. Much of the arsenic is lost by leaching. Primary fixation of the chemicals in the wood can take up to 500 hours. This means that fixation is often incomplete and treated timber must be protected from rain to avoid loss of preservative from superficial layers of the timber and so to avoid contamination of the soil in the storage area. There is also a medical aspect in that soluble hexavalent chromium can produce skin allergies. It is commonknowledge that wood freshly treated with CCA preservative and then exposed to sun-shine acquires a deep tone of green. Variability in colour intensity of treated wood can occur which is readily visible. The existance of reducing agents in the wood substrate causes a premature reduction of the chromium from Valency 6 to Valency 3. Previous attempts to treat wood with hot chromated copper arsenate solutions have been unsuccessful, as the poly-saccharides found in the wood are hydrolysed to reducing sugars which are rapidly extracted into the solution. These react with the chromated copper arsenate to form difficultly soluble salts as a sludge or precipitate, which coat the surface of the wood clogging the pores and preventing further penetration. In accordance with this invention wood is treated with a hot solution of copper chrome and arsenic whereby the precipitation of difficulty soluble salts by reducing - 5 - sugars and other organic agents in the wood is prevented and the wood is completely and uniformly penetrated by infusion and diffusion with copper chrome and arsenic ions so that the chromium ion is maintained at Valency 6. The presence of these extractives in the case of hardwoods and certain resins in the case of softwoods has in the past prevented the preservation of wood while green. One of the purposes of the conditioning step of this invention is to remove these extractives to enable unseasoned green wood, particularly sapwood, to be subjected to preservation processes. U.S. Patent 3,957,494 of Oberley describes a chromated copper arsenate wood treating solutions in which the ratio of hexavalent chromium to trivalent chromium is between 4:1 to about 1:1 6 to improve the penetration of wood to ensure better retention of preservative in the wood and to improve stability of -treating solutions. The solution contain an acid selected to maintain the desired pH while further improving solution stability and preventing corrosion. Typical formulations of widely used CCA compositions of the prior art are listed in U.S. Patent 3,957,494 which when adapted to standardised American formulations have the following compositions in percent by weight. CCA TYPE A CCA TYPE B CCA TYPE C Cr0 3% 65.5 35.3 47.5 CuO% 18.1 19.6 18.5 As 2 o 3 % 16.4 45.1 34.0 According to U.S. Patent 3,957,494 wood commonly treated with chromium copper arsenated solutions in which the chromium was present in hexavalent form has several drawbacks. For example, there is incomplete penetration of chromated copper arsenate solution in the wood. This is particularly true of refractory species of wood such as OMPI Douglas Fir. In addition the initial retention of the chromated copper arsenate compositions is not as high as is desired. Another drawback of the prior art CCA treating solutions is their instability in the presence of reducing sugars normally found in most untreated wood. CCA solutions react with reducing sugars to form sludge and precipitates. These precipitates interfere with the penetration retention greatly reduces the usable life of the treating solutions and causes corrosion of treating equipment. This results in a large chemical and monetary loss. The conditioning step as specified with acid, preferably sulphuric acid with a pH of less than 2.8, effectively removes by hydrolysis reducing sugars which are rapidly extracted into the solution, releases resins and oils in refractory species such as Douglas Fir and enables the wood to be treated green without seasoning. Further more, should the treating solution be of the copper chrome arsenate hot diffusion type in which the CrO^ content is of the order of 20% a bath of the solution containing copper ions, arsenic acid ions and chromic acid ions with the mole ratio of the copper to arsenic being such as to establish the acid copper arsenate equilibrium at a pH less than 2.8 and a temperature greater than 40°C in the wood system. The solution has a sufficient concentration of chromic acid ions to form tertiary chromium arsenate in the wood to be treated and to provide a mole ratio of chromium to arsenic which will subsequently ensure that all or substantially all arsenic is precipitated and fixed as tertiary chromium arsenate in the wood to be treated. The concentration of the chromic acid ions is such that pH of the aquaeous phase in the wood is kept below 4 and the precipitation and fixation reactions are substantially completed to prevent formation of chromates within and/or on the surface of the wood. It has been shown by experiment that the toxic threshold measured on the basis of retention of copper at a 0.2% /W % copper is from 5 to 10 times less when copper and arsenic are fixed from the acid copper arsenic solution as is the case when fixed from chromated copper arsenate solutions. This in effect means that the toxicity of the treatment when the copper chrome arsenate hot diffusion system is used is at least five times that when chromated copper arsenate solutions are used. This will result in a greatly increased life of the wood so treated or a lower level of application for a required service. In addition, the treating solutions when using the CCAHD process after conditioning can be kept stable continuously by adjusting the pH down to below 2.8. The control of the equilibrium within the wood system (provided the pH in the wood system is below 2.8} is automatic as the equilibrium set up will be that of acid copper arsenate. This is relatively independent of the formulation in the solution outside the wood system. It has been found, however, that should CCA Type C salts with 47.5% of Cr0 3 be used, the chromic ion absorbs onto the wood system and is reduced causing a rise in the pH and the penetration is somewhat limited to the outer sapwood. In the case of oil-borne preservatives it has been found that the conditioning step by removing the extraneous organic resins and oils from the substrate greatly improves the penetration of the oil-based preservative and permits it to move freely throughout the wood. The conditioning process thoroughly wets the wood cell walls and the capillary passages particularly the mid lamella. When an oil based preservative is used and applied to the wood at a temperature above 100°C the water volatilises and is ejected from the vessels and lumen as steam. On cooling, the oil carrier infuses into the wood system and the tar acids and bases, which are both oil and water soluble, will partition between the aqueous phase in the wood system and the oil phase thus fixing the preservative more securely to the wood system with little tendency for the oil carrier to bleed from the pole by syneresis. The tar acid and tar base content of the oil-borne preservative have to be in excess of 20% for best results. Oil-borne preservatives of the creosote high temperature type and containing tar acid contents of less than 10% are known to exude from the pole and also may be lost by evaporation. High phenol content may reduce evaporative losses. Taking into account the tar acid content, the rate of decay can be related to a combination of boiling range and tar acid content. It has been shown that in the case of Australian Vertical Retort creosotes with a tar acid content of 15% and above, a reasonably good correlation is achieved between threshold retention of the preservative and the "Residual Phenolic Value", the latter being the product of the initial phenolic content of the creosote and the percentage of creosote remaining after weathering. Because tar acids contribute so much to toxicity particularly of low temperature creosotes, it follows that a minimum tar acid content of at least 15% should be specified for reasonable protection to be attained. It has been found that when the oil-based preservative used are an extract from the lignite deposits in the Latrobe Bailey of Victoria the penetration of the sapwood is complete and the dispersion is even throughout. This provides an envelope of preservative that would be almost impenetrable. It further permits water to move freely in and out of the sapwood system, preventing conditions that could lead to destruction of the heartwood by basidiomycetes and other fungi. Further it has been found that when wood treated with an oil of a high content of tar acids that on exposure to fire of a flash nature, the water contained in the sapwood boils and includes in it the tar acid content. This suppresses the tendency to burn and the wood merely chars. The conditioning step has been shown to improve the physical condition of sapwood particularly of young trees that are fast grown under plantation conditions. Large tensile stresses are known to build up in the sapwood of young hardwood, particularly Eucalypt species that give rise to spring and bow in the freshly sawn timber. This greatly detracts from their application as sawn lumber. The consequent radial strain gives rise to compressive stresses that causes the collapse of the wood cells in the inner region and because of the high intensity of the compression stresses and the low strength of the heartwood across the grain in comparison to along the grain invariably some radial splitting occurs at the ends. As the wood dies out the sapwood comes under increasing pressure and end-checks often open up right through the sapwood. The conditioning step appears to make the sapwood more elastic and enable to expand to absorb the increased load placed upon it, thus containing end-checking. Conditioned wood that is stood on end to drain dries out quite rapidly. The conditioning step releases bordered pits that would otherwise be cemented to the cell wall blocking the valves. The removal of extractives lessens the tendency of the torus to fix to the valve and much better ventilation takes place through the wood during the drying phase. It has been found that within days of conditioning wood stood on end dries out into what can be described as kiln dried condition and removes the necessity for that process. Phenols extracted from coal have been used as a natural base for manufacture of oil-based detergents. Sulphuric acid sulphonates hydroxy benzene radicals known to be present in the sapstrea . Alkyl olephics also present in the sap system condense to form alkyl-aryl molecules that are soluble and which cleanse the fibres of residual oils and resins that hinder the application of the preservation of the sapwood. These alkyl-aryl molecules act as wetting agents which induce better penetration by both oil-based and water-based preservatives and permit the wood system to be thoroughly wetted before the application of preservative. The following examples illustrate embodiments of the present invention: EXAMPLE 1 Green, unseasoned wood was placed in a bath containing sulphuric acid preferably with the vessels or trachieds in the vertical plane. The bath contained sulphuric acid which was added to water to reduce the pH to 2, and was heated to boiling point. Convention currents set up in the wood system purged out the sap and replaced it with a sulphuric acid solution. The wood was left in the bath until the temperature of the wood rose to between 60°C and 70°C, the period depends, of course, on the size and configuration of the cross-section of the wood. The less soluble material such as poly-saccharides were hydrolysed to sugars and removed along with starches from the system. Residual oils floated out of the wood system and alkyl-aryl compounds formed by condensation of precursors of the lignin manufacture formed and being soluble moved out of the system. The wood was thoroughly wetted and became completely waterlogged and was allowed to stand and cool. The water containing the extracts from the substrate of the wood being treated was removed and replaced by a preserving solution comprising a bath of a solution containing copper ions, arsenic acid ions, chromic acid ions such that the acid copper arsenate equilibrium became established. The bath was once again brought to the boil and allowed to cool. Convection currents set up within the wood system replace the sulphuric acid solution with preservative solution which then diffused throughout the wood system. The wood was allowed to stand and cool so that the ions diffused throughout the wood system. EXAMPLE 2 Following the conditioning process described above the wood was removed from the sulphuric acid bath and placed in a vessel and whi'le still hot and a vacuum applied. The boiling point of the contained water was lowered and the remaining sulphuric acid solution in the wood system ejected by the steam formed in the wood. The wood was then subjected to a bath of a preserving solution containing copper ions, arsenic acid ions, chromic acid ions as described above and a pressure of 2 atmospheres applied to help the infusion of the carrier. The diffusion of the ions continued throughout the wood system and as the temperature drops below 40°C the Copper Chrome Arsenate Hot Diffusion chain became dominant as the reactions continued. The treating solution was then removed and a vacuum applied to remove the last of the carrier. The wood was removed and stood in a vertical plane to dry out. In three days the wood was in kiln dried condition and treated ready for use. EXAMPLE 3 Following the conditioning process as described above, the wood was inserted into a cylinder and oil-borne preservative containing a high level of tar acids and bases at least 20% was introduced to the system. A pressure of five atmospheres was applied order to assist the infusion. A vacuum was applied after a period of time and the oil carrier removed from the system. Alternatively the butt end of the post or pole can be stood upright in a bath of an oil-borne carrier such as a lignin tar or high tar acid creosote and brought to the boil. The water in the butt of the post or pole is evaporated and ejected from the system, on cooling the atmospheric pressure forces the oil carrier into the wood system and thoroughly encapsulates the wood. In the treatment process the chromic acid ion absorbs into the wood-substance, preferentially the lignin. The pH in the system inside the wood rises and the chromium of Valency 6, is reduced to Valency 3 to permit the fixation of basic copper arsenate. The preliminary step is to establish the acid copper arsenate equilibrium within the wood system, and is achieved by water-logging the wood in acid solution to reduce the pH within the wood to less than 2.8, at which point copper chrome arsenate is the stable equilibrium. The equations are as follows:- 2 Cu H As 0 4 < .8 »3. >Cu (OH) Cu As 0 4 + H3AS 0 4 Acid Copper Arsenate Basic Copper Arsenate 2 H 3 AS ) 4 + Na Cr 2 O^ 2 ' Cr As 0 4 + 2 Na O H Tertiary Chromium Arsenate Sufficient chromic acid ion is included in the formulation to ensure that the arsenic acid ion, freed by the fixation of the basic copper arsenate, is fixed as tertiary chromium arsenate which is a recognised insecticide. A typical formulation is as follows:- ( a) 4.CuSo 4 .5H 2 ) + As 2 0 5 .5H 2 0 (1000 gram) + (320 gram) 2 CuOHCuA 5 Q 4 + H 2 S0 4 + 20H 2 O (b) Na 2 Cr 2 O 7 + As 2 0 5 , 4H 2 0 (262 gram) + (302 gram) 2CrAs0 4 + 2NaOH However, it is preferred to include an excess of chromate by using:- 1000 gram Copper sulphate pentahydrate 620 gram Arsenic pentoxide pentahydrate 300 gram Sodium dichromate Conveniently, the solution can be prepared by adding to 1000 litres of water:- 70kg Copper Sulphate pentahydrate 45kg Arsenic pentoxide pentahydrate 20kg Sodium dichromate Provided the pH in the wood system is below 2.8, the fixation reactions are self controlled. The pH of the solution must be kept below 2 by adding copper sulphate and arsenic pentoxide. The objectives of the Condition for Preservation Process are as follows: (i) To reduce the pH in the wood system below pH 2.8. (ii) To remove sugars and starches than can act as a source of sustenance for invading fungi or insects, (iii) To sterilise the wood and eliminate pre-seasoning decay or organisms. (i ' v) To remove fast reacting reducing agents than can cause the premature reduction of the chromium from Valency 6 to Valency 3 with a consequent formation of poisonous sludges that settle on the wood after treatment with CCA slats. (v) To remove reducing agents that can cause the • premature reduction of the chrome to 3 with the establishment of the basic chromium chromate or basic copper chromate equilibrium in the solution at a pH of about 4. Such a solution when impregnated into wood would cause the immediate reaction chain that would give rise to high levels of chromates in the wood system. (vi) To wet thoroughly the wood to ensure complete saturation of the wood fibres and so assist the hot diffusion process that follows. (vii) To remove residual precursors of lignin in the substrate that can polymerise and condense in the capillary passage cementing fibres together during drying. Thus improving the elasticity of the outer layers of sapwood and the ability to relieve growth stresses as they are released. (viii) To heat the wood through and to produce a kiln dried product within days of felling. (ix) To remove resins and oils that exist in wood, that give rise to refactory species rejecting the application of preservative solutions. (x) To remove sugars and other extractives that can be hydrolysed and removed that can act as a source of nutriment to invading fungi. The conditioning step in itself should give an extra 20 years of life to wood so treated. (xi) To relieve stresses in the sapwood of trees and so reduce the tendency to spring and bow in sawn green timber. OMPI - V/1FO

Claims: - 15 - CLAIMS 1. A process for treating wood comprising the steps of:- (i) placing the wood in an acidic water bath the temperature of which at least 50°C so as to sterililize the sapwood, to diffuse out from the sapwood the water soluble extractures and to establish the liquid phase throughout the wood system, (ii) subjecting the wood to a vacuum so as to lower the boiling point of the water trapped in the wood system so that the steam pressure generated internally in the wood will force the wter trapped in the cells and vessels out of the wood but will leave the cell walls saturated with water, and, (iii) introducing a preservative solution into the wood system at a temperature of about 50°C so that the solvent infuses into the wood ststem as it cools and the toxic ions diffuse into the capillary passages. 2. A process according to claim 1, wherein the preservative solution is a copper chrome arsenic solution and the process further includes the additional step of allowing the system to cool so that the chrome acid ion is adsorbed into the wood system, the copper arsenic and chrome arsenic equilibria is established and the less soluble and highly toxic basic copper arsenate and chromium arsenate are precipitated. 3. A process for treating wood wherein the wood to be treated is placed in a bath of hot copper/chrome/arsenic solution to prevent precipitation of salts by the reducing sugars and other organic agents in the wood and to provide uniform penetration by infusion and diffusion of the copper, chrome and arsenic ions so that the chromium ions are maintained at valency 6. 4. A process according to claim 3 wherein the bath contains sulphuric acid sufficient to keep the pH below 2.8. - 16 - 5. A process according to claim 4 wherein the temperature of the bath is at least 50°C. 6. A process according to claim 4 wherein the temperature of the bath is between 50° and 70°C.

Applicants: SCHRODER JOHN G
Application year: 1981
Main classification: B27K3
IPC codes: B27K3