The present invention relates to a material mixture for heat treatment of wood, as well as to a method for treating wood.

BACKGROUND ART

The heat treatment of wood materials is a long known technology of the art. In the course of these treatments, the wood is subjected to a so-called thermic modification, as a result of which the products and elements made therefrom become more resistant to outside impacts and acquire various advantageous properties. Such a solution is described e.g. in Hungarian patent No. HU 199 088.

In prior art heat treatments and thermic modifications, the wood is generally surrounded by a gaseous or liquid media. A disadvantage thereof is that in the course of the treatment, the mass of the wood generally significantly decreases, and the emitted materials (e.g. tar, volatile matters) require separate material treatment and environmental measures.

There are other known solutions, wherein the wood to be heat treated is surrounded by a solid granulate material for the thermic modification; such a solution is disclosed in Hungarian patent document No. P 08 00358. According to this solution, the wood to be treated is covered by mineral salt (NaCI), or by a mixture containing mineral salt of at least 20 percent by weight, and the heat treatment is completed in the covered state of the wood. According to this solution the material mixture requires a minimum mineral salt content of 20 percent by weight in order for it to exhibit advantageous properties. It is a disadvantage of this solution, that due to the high dissociation ability of mineral salt, it outputs numerous free Cl ^" ions into the environment, which are extremely aggressive in terms of environmental impact; as they cannot be utilised or processed by any of the vegetation. Cl ^" has high electronegativity (3.16 on the Pauling scale) and in its water soluble state it corrodes, thereby causing a during the treatment technology significant corrosion problem in the metal (iron) parts of the equipment and containers used by the method as well as in the metal-containing elements of the buildings and vehicles in the vicinity. The necessity of extra corrosion prevention considerably raises the costs of the heat treating equipment. As a further problem, the salt becomes a shapeless salt mass after the heat treatment due to the cementing effects of the magnesium salts and calcium salts contained in the mineral salt. The grinding thereof demands an extra work phase during the reuse. Yet a further disadvantage of this solution is that reinforced holding structure as well as higher heat input is required due to the relatively high specific mass of the mineral salt.

DESCRIPTION OF THE INVENTION

It is an object of the present invention to eliminate the disadvantages of prior art solutions. In the course of my research leading to the present invention, I have recognised that a development direction opposite to the one in the above referred Hungarian patent application P 08 00358 is desirable. I have recognised, that the NaCI-content of the material mixture surrounding the wood used for the heat treatment shall be minimised. In the course of my experiments I have found that by having an NaCI-content not exceeding 10 percent by weight in the material mixture, the harmful effects of NaCI will be negligible, and will not be experienced. I have further recognised that the material mixture shall comprise an anti-caking material of at least 5 percent by weight, as well as a water absorbing/releasing material of at least 20 percent by weight so as to achieve an efficiently processable and functioning solution. According to the present invention, the material mixture further comprises a sufficient amount - at least 5 percent by weight - of a thermally conductive filling material so as to achieve the desired volume of material mixture and for providing thermal conductivity of the material mixture.

The object of the present invention has been achieved by means of the material mixture according to claim 1 and by means of the method according to claim 8. Preferred embodiments of the invention are defined in the dependent claims.

MODES FOR CARRYING OUT THE INVENTION

In creating this invention, I have examined the cell properties of the wood based on diffusion and osmosis pressure. The numb walls of the dead plant cells are to-and- fro permeable for certain solutions and ions of solutions. It is my theory that in case of the material mixture according to the present invention, the moisture removed from the wood in the course of the treatment will first come into contact with the solid salts surrounding the wood and will hydrolyse by breaking their ion binding. In this way it makes it possible that various anions and cations pass into the cells through the walls of the wood cell by means of an osmosis and diffusion process. Here I shall first and foremost refer to the cations of Na ⁺ , K ⁺ , Ca ⁺ , Mg ⁺ as well as to the anions of CI ^" , PO ₄ ^2' , SO ₄ ^2" .

In the course of the treatment, at a temperature of 80 ⁰ C already, the moisture content of the wood starts to pass into the surrounding materials, which hydrolyse with water and then the ions start to pass through the cell wall. In the case of water extraction it is essential for the method to be smooth. Therefore it is important to include in the material mixture in addition to the mineral salts a further water absorbing/releasing material, which first physically absorbs the water, then upon further increase of the temperature it releases the water.

In the course of the heat treatment, therefore, water is first extracted from the wood by means of a water absorbing/releasing (i.e. for example a water-soluble, hygroscopic, and/or water binding) material through diffusion and osmosis. For an effective execution of this method, the material mixture shall contain a water absorbing/releasing material of a minimum of 20 percent by weight.

The material mixture according to the present invention shall also comprise heat transferring and thermally conductive materials in order to effectuate the best possible heat treatment between the heat source and the cellulose-containing wood materials. Practically, these materials also serve as filling materials, i.e. to increase the amount or volume of the material mixture to an amount or volume necessary for covering the wood material. The amount of this component depends on the actual application and on the applicable volume, however, in order to achieve appropriate heat transfer it shall contain an amount of at least 5 percent by weight.

The material mixture according to the present invention further comprises an anti- caking component so as to prevent caking of the powder-like treating material. My experiments proved that the material mixture shall contain an anti-caking material of at least 5 percent by weight in order to prevent bulking.

In the heat treatment method achieved by means of the material mixture according to the present invention, the wood to be treated is covered with a solid-state, i.e. granulate or powder-form, material so as to seal it from air, oxygen. In the course of - A - the heat treatment, i.e. thermic modification, water is extracted from the cells through the cell walls, thereby reducing its bound water content. The method is made irreversible by next subjecting the wood to a heat treatment of higher temperature, preferably at a temperature of 60 to 240 ⁰ C, for a longer duration, preferably for a period of between 6 to 48 hours.

By way of example, the water absorbing/releasing material according to the above can be a water-soluble hygroscopic and/or water binding material, as well. Preferably, this can be any one of hygroscopic salts, compounds, such as various chlorides, phosphates, or carbonates. Magnesium sulphate, natrium sulphate, calcium sulphate, natrium carbonate, natrium acetate, kalium chloride, magnesium chloride, calcium chloride, zeolites, magnesium oxides, silica gel, as well as any combination thereof proved to be especially preferable.

Although KCI is not hygroscopic, nevertheless, it well dissolves in water, therefore it has a high water binding property. It does not have such aggressive effects on the environment as NaCI (plants can utilise it as a chemical fertiliser), however, its effect of corrosion on metal is yet significant.

MgCI ₂ and CaCI ₂ are both highly hygroscopic materials, dissolve well in water, therefore, have great water binding properties. They do not have such drastic effect on the environment as NaCI (plants can make use of them as chemical fertilisers), however, they also cause corrosion to metal. In their application it shall be considered, that whilst MgCI ₂ acts in water as a mild acid, CaCI ₂ acts as a mild base.

In preparing the material mixture, those compounds containing the Cl ^" ion shall be handled with great care. Owing to the aforementioned reasons, NaCI shall by all means - as much as possible - be avoided. All other similar compounds shall be handled with the utmost care, and their use is advisable only in lower concentrates (5 - 20 percent by weight).

The thermally conductive filling material included in the material mixture according to the present invention is preferably selected from the following group, or is composed of a mixture of one or more members of the following group: natrium sulphate, natrium acetate, calcium chloride, oxide ceramics, aluminium-oxide, clinker minerals, talcum. As thermally conductive filling material preferably such a material shall be used, which is well soluble in water, and possibly is capable to absorb mineral water. In this way, the change in the course of the heat treatment of the water balance of the treatable wood can also be regulated by means of selecting the thermally conductive filling material.

The salts of halogenides, as well as the salts of SO ₄ ^2" , PO ₄ ^2' , CO ₃ ^" , SiO ₃ ^2" roots are less suitable for use as thermally conductive filling materials, as these are not hygroscopic in themselves (do not absorb the moisture content of the air). Na ₂ SO ₄ , or K ₂ SO ₄ , are however highly efficient, as they similarly dissociate as natrium- acetate, or CaSO ₄ , which has a crystal lettice structure similar to that of NaCI.

By effecting the surface, the anti-caking material reduces the caking tendency of the individual particles, and preferably is selected from the following group, or is composed of one member or a combination of members of the following group: oxide ceramics, aluminium oxide, clinker minerals (e.g. AI ₂ O ₃ , Fe ₂ O ₃ +CaO), zeolites, magnesium oxide, talcum (ground soapstone), silica gel. In addition to the various aluminates, the aforementioned silica gel and zeolites also have anti-caking properties.

The major requirements and criteria set for the materials to be used in the material mixture are as follows:

- the compounds and/or mixtures shall have a solid state between the temperatures of -40 C ⁰ and +300 C ⁰ ; and cannot melt or degrade in this range of temperature.

- In preparing the material mixture it shall be considered whether acidic, basic or neutral water solution is required.

- The granule size of the compounds and/or mixtures should be preferably selected between 10 μm and 20 mm.

- The compounds and/or mixtures shall have ideal hygroscopic and moisture retaining abilities in terms of the method in order that the moisture be removed without any damage through the cell walls by means of osmosis pressure having set the appropriate treatment (technological) time. - The compounds and/or mixtures shall remain having no agglomeration after absorbing and releasing the moisture departing the wood. In the course of the treatment method implemented by means of the material mixture according to the invention, the wood pieces are surrounded by the material mixture, thereby creating an oxygen-poor physical environment. The wood to be treated is preferably surrounded with the material mixture on a tray, which is then thrust into the firebox of the furnace. By a gradual increase of the temperature the free moisture in the wood as well as the bound moisture in the cell structure of the wood is removed through the cell wall without causing any damage thereto. Upon completion of this process, the cell walls are sealed by means of a further increase of the temperature in order to bar any later moisture absorption.

Between the wood materials, a sheet of 2 - 3 mm in width with high thermal conductivity (e.g. aluminium) can preferably be placed along the full length of the material to be treated. By doing so, any local overheating can be prevented and a more even heat distribution can be effectuated. This solution significantly improves the efficiency of energy usage.

In the course of the heat treatment, temperature is varied between 60 to 240 ⁰ C and the treatment is executed for a duration of 6 to 48 hours. It can therefore be a periodic or a continuous method.

My experience shows that based on the temperature, various mineral compounds (salts) and their combinations have varying effects on the cellulose-based wood materials during the treatment. While beech treated with a material mixture mainly containing NaCI resulted a relatively high quality, pine only gave results of medium value. Oak and acacia had especially low results. In respect of the natrium acetate - silica gel combination, the treatment of pine and oak yielded good results at a low temperature (150 - 170 ⁰ C), while beech yielded poorer results. With regard to beech, better results were achieved when adding aluminium oxide of 30 percent by weight to the aforementioned compound together with increasing the silica gel ratio as well as the temperature.

The effects achieved by certain material mixtures can be modified in such a way that the surface of the material mixture surrounding the wood is sprayed with a liquid (water). Subjected to heat, it forms a thin layer on the surface of the material mixture in contact with the salts therein (e.g. CaSO ₄ ), and intensifies sealing of the wood from its surroundings, and thereby reduces the specific weight loss of the wood.

Table 1 herebelow contains the exemplary materials preferably applied in the material mixture according to the invention.

Table 1

It is clearly evident that Table 1 contains a number of materials exhibiting at least two of the properties as follows: anti-caking, water absorbing/releasing and thermal conductivity. These double-property materials can be used in the material mixture according to the present invention in a particularly preferred manner.

Made up of the materials indicated in Table 1 , a plurality of preferred material mixtures according to the invention have been prepared, which exhibited higher efficiency and more environmentally friendly properties as compared to the treatment with mineral salt. In preparing the preferred material mixtures it is advisable to use the following basic compounds alone or in combination:

- Natrium acetate, silica gel, talcum, aluminium oxide, calcium sulphate, magnesium sulphate, natrium carbonate, natrium sulphate.

- Halogenide salts, kalium chloride, calcium chloride, natrium bromide, natrium iodide.

Example 1 Natrium acetate 20 - 80 percent by weight Silica gel 10 - 40 percent by weight Talcum 10 - 50 percent by weight Natrium carbonate 5 - 20 percent by weight

Example 2 Calcium sulphate 10 - 40 percent by weight Natrium carbonate 10 - 40 percent by weight Talcum 10 - 30 percent by weight Silica gel 20 - 40 percent by weight

Example 3 Magnesium sulphate 30 - 50 percent by weight Aluminium oxide 10 - 50 percent by weight Silica gel 10 - 70 percent by weight

Example 4 Natrium carbonate 10 - 40 percent by weight Magnesium sulphate 15 - 50 percent by weight Calcium sulphate 10 - 40 percent by weight Talcum 5 - 25 percent by weight Aluminium oxide 5 - 25 percent by weight

Example 5 Kalium chloride 40 - 80 percent by weight Aluminium oxide 25 - 45 percent by weight Magnesium chloride 2 - 15 percent by weight Example 6

Natrium acetate 20 - 50 percent by weight

Silica gel 20 - 30 percent by weight

Aluminium oxide 20 - 30 percent by weight Kalium chloride 5 - 20 percent by weight

The following exemplary material mixture proved to be especially advantageous in terms of each and every object of the invention:

Example 7

Aluminium oxide 10 - 20 percent by weight Natrium acetate 25 - 30 percent by weight

Silica gel 20 - 30 percent by weight

Calcium sulphate 5 - 10 percent by weight

Natrium sulphate 20 - 30 percent by weight

The specific mass of this latter material mixture is 686.25 kg/m ³ which is essentially half of the specific mass of 1 ,259.63 kg/m ³ of NaCI, therefore, it requires a less expensive holding and actuating structure, as well as less heat input.

With regard to the compounds of the above examples it shall be noted that they are mainly non-toxic, nonetheless, a number of them are also used in the food industry as food additives. In terms of their environmental effect, it shall be noted that in small quantities they are not harmful.

In order to achieve the material treatment objectives, the optimal material mixture shall be selected in light of the type of wood, the intensity of treatment, the moisture content, and the desired properties of the treated wood.

The invention is, of course, not limited to the preferred embodiments detailed hereabove, but further material mixtures and methods are also possible within the scope defined by the claims. Additive and thermic distribution enhancer components, used by way of example in a manner as known in the field of thermic modification, can be added to the material mixture as well.