NO854702L - IMPROVING AGENT FOR CELLULOSE-CONTAINING PRODUCTS. - Google Patents

Description

WOOD- PRESERVATIVE

TECHNICAL AREA

The present invention relates to a means and a method for preserving wood and other cellulose-based materials such as paper, fibreboard, textiles, ropes, etc. against decomposing organisms which are responsible for rot and decomposition, namely fungi and insects. More particularly, the invention relates to an organometallic preservative with insecticidal and fungicidal properties, in the form of an aqueous solution of a compound of metal ammonium complexes of certain specified dicarboxylic acids or mono-, di- or tri-carboxylic hydroxy acids.

PRIOR ART

Metal compounds have long been known for their fungicidal properties. Copper sulphate was recommended for use in wood preservation as early as 1767 and patented for this purpose in England in 1937 by Margary. After its initial use in the early 1800s, copper sulfate has played an important role within the wood preservation industry. However, the use of the copper sulphate as a wood preservative has two significant disadvantages. Firstly, the copper sulphate is not permanently fixed in the wood and therefore tends to wash out and secondly, copper alone is not an effective preservative against all forms of wood-destroying organisms.

In the earlier 1940s, a new generation of aqueous preservatives with superior leaching resistance was developed. These new preservation systems were based on copper plus the incorporation of chromium and/or arsenic. These preservative systems are known as chromic copper arsenate (CCA) and ammoniacal copper arsenate (ACA). These systems are effective preservatives and are the predominantly aqueous systems that are currently used within the wood preservation industry.

The use of metal salts and organic acids as wood preservatives has been known since the previous 1900 years. During the creosote shortage in the mid-1940s, mixtures of naphthenic acids, derived from petroleum by-products, were combined with metal salts to form a variety of compounds for wood preservation. One of these compounds was copper naphthenate. Copper naphthenate was formed by reaction between copper salts and a group of organic acids known as cyclopentanecarboxylic acid. Copper naphthenate is an oily preservative and although it is an effective preservative it has a strong odor and due to its waxy nature wood treated with this preservative is difficult to paint.

Additional antifungal aqueous preservatives based on metal salts and fatty acids have been developed later. US patent 4,061,500 describes a wood preservative which is effective against blue fungus, containing a fatty acid with 6 to 11 carbon atoms, boric acid and an alkali compound in stoichiometric excess over the fatty acids. The incorporation of the copper salts with straight-chain fatty acids and fatty alcohols is described in US Patent 4,001,400. Here, copper, zinc, nickel, cadmium and cobalt are combined with an ammoniacal fatty acid salt to produce an aqueous preservative which is claimed to be effective against mold and mildew. The branched-chain fatty acid contains 6 to 12 carbon atoms per molecule.

A method for producing a homogeneous liquid

mixture comprising a cuprammonium complex of one or more monocarboxylic acids containing 1 to 4 carbon atoms per molecule is described in US patent 4,175,090. This special mixture is used as fungicides for the treatment of wood,

paint surfaces, textiles and also to inhibit algae growth. US Patent 4,220,661 describes a preservative suitable for preventing the growth of mold, bacteria and fungi comprising an aqueous solution of a complex salt of an ion selected from NH+4 and a Group I or Group II metal ion and one or more carboxylic acids selected from saturated and unsaturated

aliphatic monocarboxylic acids containing from 2 to 8 carbon atoms.

US Patent 4,193,993 teaches a process for preparing an aqueous fungicide preservative solution comprising a compound of a preservative metal, a branched chain carboxylic acid of 6 to 20 carbon atoms or a dipentene monocarboxylic acid or a dipentene dicarboxylic acid and ammonia and/or an ammonium compound. Correspondingly, US patent document 4,380,561 describes a preservative system comprising aliphatic carboxylic acids with a branched chain containing 6 to 20 carbon atoms or their alkali or ammonium salts. This mixture is particularly suitable for short-term protection of wood against redwood and mould, but not against attack by insects.

It has long been desirable to produce wood products that are aesthetically acceptable to customers but still preserved against the wood decay mechanisms. Preserved wood is desirable for homes and is used for panels, fencing and the roofing industry. Unfortunately, many of the fatty acid preservation solutions described above are only effective against fungal and bacterial attacks and have little protective effect for the tree against insect attacks and especially ants. However, it has now been discovered that by changing the organic acid substituent in previously used preservatives from a normal fatty acid to a dicarboxylic acid or to a mono-, di- or tri-carboxylic acid, the resulting preservative will be effective both against attack by fungi and insects .

THE INVENTION

It is an object of the present invention to provide a mixture suitable for use as a wood preservative which can prevent the decomposition of wood or other cellulose-based materials due to fungal decomposition and/or insect attack.

A further object of the invention is to provide a wood preservative with fungicidal and insecticidal properties which is particularly effective against insects such as ants.

A further object of the invention is to provide a method for treating wood and other cellulose-based materials which includes bringing these materials into

contact with the aqueous solution according to the invention.

In order for a special chemical system to be an effective long-term preservative for wood, it must satisfy the following criteria: 1) At least one of the components must be an effective biocide against the organisms responsible for the breakdown of the wood including bacteria, fungi and insects. 2) The conserving components must be capable of easily penetrating the wood.

3) The components must be permanently fixed in the wood.

It is also desirable that the raw materials should be readily available and that their price is not prohibitive for commercial use. It is desirable that the preservative is a water-based system as water is readily available and is a cheap diluent. The resulting treated wood products should also be easily paintable and should not present any possible fire hazard unlike some of the oil-based systems.

The following invention thus provides an aqueous preservative for the treatment of cellulose-based products such as e.g. wood to prevent the degradation of these products by known decay-causing organisms and insects including:

a) a composition that a preservative metal selected from the group consisting of copper, cobalt, cadmium, nickel and zinc in a preservative amount; b) an organic acid, also in a preservative amount, selected from the group consisting of aliphatic dicarboxylic acids containing 2 to 10 carbon atoms per molecule, aliphatic mono-, di- or tri-carboxylic hydroxy acids containing 2 to 6 carbon atoms per molecule, a mixture of these acids and/or their salts, and c) ammonia and/or an ammonium compound such as ammonium compound such as ammonium carbonate, ammonium hydroxide, ammonium bicarbonate, ammonium sulfate or mixtures thereof in an amount sufficient to dissolve the composition in (a) above and neutralizing the organic acid in (b) above.

When cellulose or cellulose by-products such as wood are treated with the above-mentioned aqueous preservative e.g. by dipping, soaking, spraying, brushing, impregnation, etc., the treating material is effectively protected both against attack by fungi and insects.

THE BEST METHOD FOR CARRYING OUT THE INVENTION

The preservative metals suitable for use in this system include copper, cobalt, cadmium, nickel or zinc. Copper is the most preferred metal and can be incorporated into the system as a salt such as copper oxide, copper carbonate, copper sulphate, copper hydroxide or as copper metal, on the condition that a suitable oxidising agent such as air, hydrogen peroxide or nitric acid is present. When cobalt, cadmium, nickel or zinc are used, they can be incorporated into the system as a metal compound or metal salt such as e.g. a metal oxide, metal hydroxide, metal carbonate, etc. or as the metal itself provided that a suitable oxidizing agent is present. These metal compounds are usually soluble in water but can be solubilized in the presence of ammonia and/or ammonia-containing compounds.

The ratio between organic acid and preservative metal compound used in this process will generally depend on simple stoichiometric ratios, i.e. one equivalent metal per equivalent acid. It is preferred to have an amount sufficient to react stoichiometrically with the dicarboxylic acid or with the mono-, di- or tri-carboxylic hydroxy acid. The ratio between metal and organic acid can, however, be varied to less or more than the stoichiometric amount depending on the desired preservative effect.

Examples of aliphatic dicarboxylic acids which are suitable for use in this preservative system are oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, femilic acid, coric acid, azelaic acid and sebacic acid. Aliphatic dicarboxylic acids containing 2 to 10 carbon atoms per molecule is preferred, based on solubility considerations. Even more preferred acids are those containing 2 to 4 carbon atoms per molecule. Dicarboxylic acids with a branched chain are not considered suitable for use when carrying out this process as it is part of the theory that such compounds do not have sufficient mobility to allow good penetration into the wood. It has been found that only straight chain dicarboxylic acids combined with metal preservatives exhibit both fungicidal and insecticidal properties.

Examples of suitable aliphatic mono-, di- or tri-carboxylic hydroxy acids are glycolic acid, lactic acid, alpha-hydroxybutyric acid, glycerine acid, malic acid, tartaric acid, mesotartaric acid and citric acid. It is preferred to have an aliphatic mono-, di- or tri-carboxylic hydroxy acid containing from 2 to 6 carbon atoms per molecule.

Combinations of two or more organic acids and/or the salts of these acids can be used in the practice of this invention and it can be accepted to use any known commercially available product. Isomers of these acids or mixtures of isomers are also usable within the scope of the invention.

The presence of ammonia and/or ammonia and carbon dioxide in this system is necessary for solubilization of the metal compounds and/or neutralization of the organic acid. The ammonia can be incorporated into the system as ammonia or as an ammonium salt such as ammonium hydroxide, ammonium carbonate, ammonium bicarbonate and/or some combination of such a salt and ammonia. It is desirable that the ammonium hydroxide (30% NH3) constitutes at least 1.75 times the weight of the copper expressed as metal and that the ammonium compounds such as ammonium carbonate and/or ammonium bicarbonate, if necessary, constitute at least 0.5 times the weight of the copper expressed as the metal. These conditions may vary, but there must be sufficient ammonia present for complete complex formation of the metal and complete neutralization of the organic acid so that the organic acid will be sufficiently solubilized. If sufficient ammonia is not present, the preservative will be exposed to precipitation during storage or during the treatment process.

The preservative can be prepared by first mixing the organic acid with water and allowing as much dissolution as possible based on the inherent solubility of the acid. The metal or metal compound is then added to this solution and allowed to react. The reaction can take a few minutes or several hours depending on the reactivity of the organic acid. The ammonia and/or ammonium compounds are then added to dissolve the product that forms from the reaction between the metal and the organic acid.

It is also permitted to first mix the preservative metal compound with an aqueous solution of the ammonia and/or the ammonium compound. The organic acid can then be added to the solution at any time prior to treatment. Alternatively, the ammonia and/or ammonium compounds can initially be added to the organic acid. This solution is then mixed with the preservative metal.

As a further alternative, it is also possible to mix the preservative metal (or metal compounds), the organic acid and the ammonia and/or ammonia compounds in a single operation. A further alternative manufacturing method will allow carbon dioxide to replace the ammonium carbonate and/or the ammonium bicarbonate in any of the manufacturing methods described above. However, a minimum amount of ammonia will still be necessary to ensure sufficient solubility.

The preservative solution can be formulated under varied temperature conditions, although the preferred temperature is from about 15 to 30°C.

The limiting factors for selection of an appropriate temperature are the freezing point of the preservative and loss of ammonia at high temperature due to evaporation. Such ammonia losses can be controlled by maintaining the system under suitable pressure.

The treatment solution can be applied to the wood by immersion, soaking, spraying, brushing or in any other well-known way. Negative pressure and/or pressure techniques can also be used to impregnate the wood in accordance with the invention including both the so-called "Empty Cell" process and "Full Cell" which are well known to those skilled in the field.

The so-called "Full Cell" or Bethell process is used for creosote treatment of railway sleepers and timber for piers and the like and is the usual treatment method for all types of timber with water-containing preservatives and can be used with the treatment solution in accordance with the invention. The method has been in continuous use since 1838 and consists in first subjecting the timber in a cylinder to a negative pressure of up to approx. 70 cm for 1/2 to 1 hour after which the cylinder is filled with the treatment solution and a pressure of up to 12.5 to 13.8 kg/cm 2 is applied until the required amount of treatment solution has been injected into the timber. The cylinders are then emptied of treatment solution and the treated timber is optionally subjected to a short final negative pressure to clean the surface of the timber. It is common to heat the treatment solution during the treatment, e.g. to a temperature of 66 to 93°C, the penetration being better during heating.

As in all printing processes, the printing period is by far the most important factor affecting the degree and depth of impregnation. In practice, it is the height and duration of the pressure that controls the absorption of the treatment solution in the timber. In the earlier stages of the pressure period, the absorption in the timber is quite uniform but it takes place gradually more slowly until the absorption is too slow to be easily observed. When this point is reached, the timber is said to have been treated to the rejection point. The rate of absorption varies greatly with the different types of wood and timber such as e.g. beech or Corsican pine will be completely impregnated within a few minutes, while other types of wood such as Douglas fir, larch or oak heartwood are not completely penetrated even when under pressure for several days.

The aforementioned "Emty Cell" treatment which uses an initial air pressure is also known as the rueping process and is the standard method for creosote treatment of power transmission poles. It is also used for wood for pavements, enclosures and building timber and can be used with the treatment solution in accordance with the invention. The treatment procedure aims to achieve complete penetration of all sapwood present. The Rueping treatment was carried out around 1912 and is different from the aforementioned full-cell method in that the timber is initially subjected to compressed air instead of negative pressure. The cylinder is then filled with the treatment solution while this pressure is maintained and the pressure is then increased with a hydraulic pump until the desired amount of treatment solution is injected into the timber. The pressure is then relieved and the air compressed in the interior of the timber is allowed to escape and during this the excess liquid is squeezed out, leaving the cell walls coated with treatment solution. This treatment method allows a deep impregnation of the timber without strong absorption. The compression of the air initially in the wood serves to recover a small amount of the injected treatment solution when the pressure is relieved. A prolonged final negative pressure is also used to help with this.

Before the timber is impregnated with any wood treatment solution, it is necessary that it is first stored so that at least all free water has been removed from the cell spaces. This storage stage represents a moisture content of approximately 25 to 30% and varies somewhat with different species. There are two very good reasons for this: firstly, it is not possible to inject another liquid into the wood which contains a lot of water and secondly, cracks develop as a result of the subsequent drying of the timber, which would almost certainly occur for untreated timber. It is also desirable to carry out all cutting, machining and drilling, etc. of the timber before the treatment is carried out, as all these operations, if carried out after treatment, will expose untreated wood. When these operations cannot be carried out until after treatment, all exposed untreated timber should be given a generous application of treatment solution and holes should preferably be treated with a hole treatment device of the push-bolt type.

The following examples serve to further illustrate the invention.

EXAMPLE 1

A preservative solution was prepared by dissolving 87 g of citric acid in 500 g of water. To this solution was added 100 g of basic copper carbonate and this was allowed to react until the development of CC^ and was complete. After completion of the reaction, 446 g of 30% ammonium hydroxide was added to solubilize the product. The resulting solution was diluted to a concentration of 1.5% and used to treat strips of southern yellow pine (approximately 0.3 x 1.2 x 25 cm) using the above Empty Cell method in which the wood was exposed to the treatment solution and the system was then pressurized for 30 min. at a pressure of 7.6 kg/cm 2 . The resulting strips were air dried and found to be resistant to fungal and insect attack.

EXAMPLE 2

A preservative solution was prepared by dissolving 114 g of oxalic acid in 500 g of water. After dissolving the oxalic acid, 100 g of basic copper carbonate was added to the solution. The solution was then heated to 49°C to ensure complete reaction between the copper carbonate and the oxalic acid. After completion of the reaction, 446 g of 30% ammonium hydroxide was added to dissolve the product. The product was then diluted to a 2% working solution and used to treat strips of southern yellow pine (approximately 0.3 x 1.2 x 25 cm) using the Full Cell method in which the wood was initially placed under a negative pressure of approximately 75 cm Hg for 30 min. followed by the addition of the treatment solution. The system was then pressurized for 30 min. at a pressure of about 7.6 kg/cm 2 . The resulting strips were air dried and found to be resistant to fungal and insect attack.

EXAMPLE 3

A preservative solution was prepared by dissolving 45 g of tartaric acid in 150 g of water. To this solution 38 g of CufOH 3 were added followed by the addition of 37 g of ammonium carbonate and 75 g of 30% ammonium hydroxide. The solution was then diluted to a 1-1/2% concentration and used to treat strips of southern yellow pine using the aforementioned Full Cell method. The treated wood was found to be resistant to attack by fungi and insects.

EXAMPLE 4

To a mixture of 150 g of water and 150 g of 30% ammonium hydroxide was added 30 g of basic copper carbonate. The solution was stirred until the copper carbonate dissolved and then 44 g of adipic acid was added and stirred until dissolved. The resulting solution was diluted to a concentration of 2.0% and used for the treatment of strips of southern yellow pine using the aforementioned Full Cell method. The resulting moldings were air-dried and found to be resistant to rot and insect attack.

EXAMPLE 5

A solution was prepared by adding 61 g of copper metal to 150 g of water containing 100 g of 30% ammonium hydroxide and 70 g of ammonium bicarbonate. The mixture was stirred and aerated until all copper metal was dissolved. Another solution was prepared by adding 31 g of malonic acid to 100 g of water. The second solution was mixed with the first and the resulting product diluted with water to 2.5%. This solution was used to treat strips of southern yellow pine using the aforementioned Full Cell method. The strips were then air-dried and found to be resistant to attack by rot and insects.

EXAMPLE 6

To 500 g of water containing 450 g of ammonium hydroxide was added 132 g of basic copper carbonate. After dissolving the copper carbonate, 59 g of citric acid was added to the solution. The product was stirred until a clear solution was obtained. This solution was then diluted with water to a concentration of 1.5% and used for treating strips of Douglas fir using the aforementioned Full Cell method. The moldings were then oven-dried and found to be resistant to attack by rot and insects.

EXAMPLE 7

A solution was prepared by dissolving 75 g of oxalic acid in 250 g of water. 75 g of zinc carbonate were added to this solution. The mixture was allowed to react until the development of CG^ was complete. After completion of the reaction, 200 g of 30% ammonium hydroxide was added to solubilize the product. This solution was then diluted to 2.0% and used for the treatment of strips of southern yellow pine using the aforementioned Empty Cell method. The resulting strips were air-dried and found to be resistant to fungal and insect attack.

EXAMPLE 8

A preservative solution was prepared by dissolving 54 g of sodium oxalate in 150 g of water. Another solution was prepared by adding 100 g of copper sulfate pentahydrate to 150 g of water containing 42 g of 30% ammonium hydroxide and 29 g of ammonium bicarbonate. Both solutions were mixed and the resulting product diluted with water to a 1.5% concentration. This solution was used to treat test strips of southern yellow pine using the aforementioned Full Cell method. The resulting strips were then air-dried and found to be resistant to rot and insect attack.

EXAMPLE 9

LIST TEST DATA IN USE

Sapwood strips of southern yellow pine (approximately 3.8 x 0.3 x 25 cm) were impregnated with various preservative solutions using the aforementioned Full Cell method. The test lists were preserved with copper naphthenate, copper citrate and copper oxalate with three different uptake (concentration) levels measured as kilograms per cubic meters (kg/m<3>). Ten strips were treated with each preservative at each concentration level for a total of 90 treated strips. Ten untreated lists were also included as a comparison. The test site was located in Florida in an environment that provided accelerated test conditions. The strips were removed from the ground at 12-month intervals to determine performance. Even at the lowest impregnation levels, it turned out that the conservation system according to the present invention worked perfectly well after 36 months of testing.

The grading system used to assess infestations of rot and insects is as follows: 10 - healthy, 9 - weak infestation, 7 - moderate infestation, 4 - severe infestation, 0 - failure.

As can be seen from the test data in the table, copper citrate and copper oxalate work well after 36 months of testing, just like the copper naphthenate used in well-known oily preservatives. The untreated comparison strips showed moderate to severe infestation after 12 months of testing and complete failure due to ant infestation after 24 months of testing. In addition, various known additives can be combined with the preservatives composed according to the present invention without significantly affecting the preservation ability of the agent according to the invention. E.g. other preservative compounds including those containing arsenic may be added to the agents. Colorants, waxes, resins, aqueous solution, various emulsions and other ingredients can be added to the mixture where such additional properties are desirable.

A wide range of different types of wood can be preserved in accordance with the invention, including hardwoods and/or conifers. Many other types of cellulose-based materials including paper, fibreboard, textiles, ropes and other such well-known cellulose-containing products can also be treated with the preservative if only the material can withstand the treatment process.

In addition, the aqueous solutions produced by the invention can also be used for the treatment of live plants and seeds to prevent attacks by fungi and/or insects.

For this purpose, the treatment solution will most simply be applied by spraying, but these solutions can also be applied using any method that is commonly used for applying known insecticides to plants and agricultural crops.

All the examples mentioned are of an illustrative type and do not limit the present invention which is stated in the subsequent patent claims.

Claims (32)

1. Aqueous impregnating agent for the treatment of cellulose-based products including wood to prevent degradation of these products caused by known degradation-causing organisms and insects, characterized in that it includes: a) a composition of a preservative metal selected from the group consisting of copper, cobalt, cadmium, nickel and zinc in a preservative amount; b) an organic acid in a preservative amount selected from the group consisting of aliphatic dicarboxylic acids containing 2 to 10 carbon atoms per molecule, aliphatic monocarboxylic hydroxy acids containing 2 to 6 carbon atoms per molecule, aliphatic dicarboxylic hydroxy acids containing 2 to 6 carbon atoms per molecule, aliphatic tricarboxylic hydroxy acids containing 2 to 6 carbon atoms per molecule, a mixture of the aforementioned organic acids and/or the salts of the aforementioned organic acids, and c) an ammonia-containing compound capable of providing sufficient ammonia to solubilize said composition under (a) above and neutralize said organic acid under (b) above. 2. Means as stated in claim 1, characterized in that the organic acid is a straight-chain aliphatic dicarboxylic acid containing 2 to 10 carbon atoms per molecule. 3. Means as specified in claim 1, characterized in that the organic acid is a straight-chain aliphatic dicarboxylic acid containing 2 to 4 carbon atoms per molecule. 4. Agent as stated in claim 1, characterized in that the organic acid is an aliphatic monocarboxylic hydroxy acid containing 2 to 6 carbon atoms per molecule. 5. Agent as stated in claim 1, characterized in that the organic acid is an aliphatic dicarboxylic hydroxy acid containing 2 to 6 carbon atoms per molecule. 6. Agent as stated in claim 1, characterized in that the organic acid is an aliphatic tricarboxylic hydroxy acid containing 2 to 6 carbon atoms per molecule. 7. Agent as stated in claim 1, characterized in that the organic acid is oxalic acid. 8. Agent as stated in claim 1, characterized in that the organic acid is citric acid. 9. Agent as stated in claim 1, characterized in that the ammonia-containing compound is ammonia. 10. Agent as stated in claim 1, characterized in that the ammonia-containing compound is an ammonium salt. 11. Agent as stated in claim 1, characterized in that the ammonia-containing compound is selected from the group consisting of ammonia, ammonium carbonate, ammonium bicarbonate and ammonium sulfate. 12. Means as stated in claim 1, characterized in that the ammonia-containing compound is a mixture of ammonia and an ammonium salt. 13. Means as specified in claim 1, characterized in that the ammonium compound is a mixture of ammonia and an ammonium salt selected from the group consisting of ammonium carbonate and ammonium bicarbonate. 14. Agent as stated in claim 1, characterized in that the preservative metal is copper. 15. Procedure for treating wood and woody and cellulose-containing products to prevent decomposition caused by known decomposition-causing organisms and insects, characterized in that the wood is brought into contact with an aqueous solution comprising: a) a composition of a preservative metal selected from the group consisting of copper, cobalt, cadmium, nickel and zinc in a preservative amount b) an organic acid in a preservative amount selected from the group consisting of aliphatic dicarboxylic acids containing 2 to 10 carbon atoms per molecule, aliphatic monocarboxylic hydroxy acids containing 2 to 6 carbon atoms per molecule, aliphatic dicarboxylic hydroxy acids containing 2 to 6 carbon atoms per molecule, aliphatic tricarboxylic hydroxy acids containing 2 to 6 carbon atoms per molecule, a mixture of the said organics and/or their salts with the said organic acids, and c) an ammonia-containing compound which can provide sufficient ammonia to solubilize the composition under (a) above and neutralize the said organic acid under (b) above. 16. Method as stated in claim 15, characterized in that a straight-chain aliphatic dicarboxylic acid containing 2 to 10 carbon atoms per molecule. 17. Method as stated in claim 15, characterized in that a straight-chain aliphatic dicarboxylic acid containing 2 to 4 carbon atoms per molecule. 18. Method as stated in claim 15, characterized in that an aliphatic monocarboxylic hydroxy acid containing 2 to 6 carbon atoms per molecule. 19. Method as stated in claim 15, characterized in that the organic acid is an aliphatic dicarboxylic hydroxy acid containing 2 to 6 carbon atoms per molecule. 20. Process as stated in claim 15, characterized in that the organic acid is an aliphatic dicarboxylic hydroxy acid containing 2 to 6 carbon atoms per molecule. 21. Method as stated in claim 15, characterized in that oxalic acid is used as organic acid. 22. Method as stated in claim 15, characterized in that citric acid is used as organic acid. 23. Method as stated in claim 15, characterized in that ammonia is used as the ammonia-containing compound. 24. Method as stated in claim 15, characterized in that an ammonium salt is used as the ammonia-containing compound. 25. Method as set forth in claim 15, characterized in that a compound selected from the group consisting of ammonia, ammonium carbonate, ammonium bicarbonate and ammonium sulfate is used as the ammonia-containing compound. 26. Method as stated in claim 15, characterized in that the ammonia compound is a mixture of ammonia and ammonium salt. 27. Method as stated in claim 15, characterized in that a mixture of ammonia and an ammonium salt selected from the group consisting of ammonium carbonate and ammonium bicarbonate is used as ammonium compound. 28. Method as stated in claim 15, characterized in that copper is used as the preserving metal. 29. Method as stated in claim 15, characterized in that a solution with the following composition on a weight percentage basis is used as the aqueous solution: citric acid approximately 11.0%, copper approximately 7.0%, ammonia approximately 16.3%, carbon dioxide approximately 2.5% and water up to 63.2%. 30. Method as stated in claim 15, characterized in that the aqueous solution is used as a solution with the following composition on a weight percentage basis: oxalic acid approximately 13.9%, copper approximately 6.7%, ammonia approximately 15.8%, carbon dioxide approximately 2 .4% and water up to approximately 61.2%. 31. Preservative, as stated in claim 1, characterized in that it comprises the following components on a weight percent basis: citric acid approximately 11.0%, copper approximately 7.0%, ammonia approximately 16.3%, carbon dioxide approximately 2.5% and water up to approximately 63.2%. 32. Agent as stated in claim 1, characterized in that it comprises the following components on a weight percent basis: oxalic acid about 13.9%, copper about 6.7%, ammonia about 15.8, carbon dioxide about 2.4% and water up to about 61 .2%.