The present invention relates to a method for the treatment of a wood element by means of heating the wood element followed by de- pressurization and pressurization as well as an apparatus for use in the treatment.

Traditionally, wood is treated for example by staining and/or preservation in order to enhance certain properties of the wood element. Preservation of wood is performed in order to increase the shelf-life of wood, timber, wood structures or engineered wood. Generally, preservation increases the durability and resistance of the wood from destruction by insects, bacteria or fungi. Additionally, wood can be preserved in order to decrease the flammability of the wood.

Dying of wood can increase the shelf-time of the wood, due to the antimicrobial effect of many dyes, as well as preserve the colour and thus improve the appearance of the wood.

Traditionally, treatment of wood can be performed with or without the use of pressure. The most common non-pressure treatments in- volve application of stain or preservative by means of brushing or spraying, dipping, soaking, steeping. Here a wooden product is allowed to soak for a longer period of time or by means of hot and cold baths where the wooden product is prepared in successive baths of hot and cold fluids.

Processes in which pressure is applied are often carried out in closed chambers to which pressure and/or vacuum is applied. Such processes are often adapted to large-scale production and generally comprise the steps of loading the wood element onto special cars and into a cylinder; pressurizing the cylinder in order to enable for example a preservation liquid to enter the wood, optionally in a carrier fluid; and finally applying a vacuum in order to extract excess preservative. Such processes may be repeated to achieve better penetration of the preservation liquid into the wood, and the conditions can be controlled in order to vary the retention and penetration. The critical part of treating wood is the exchange of the natural fluids of the wood with the fluid used to treat the wood, such as a preservation fluid and/or dying fluid this particularly applies in wood species having a low moisture content. Efficient methods and apparatuses for ef- ficiently removing moisture from hard wood do not exist and in the absence of any efficient means for protecting hard wood from for example termites, holes are presently made in the wood in order to be able to supply sufficient insecticide to the wood. The drawback of this method is that additional pores and holes are created in the wood which potentially may cause damage to the wood as the holes create local weak areas in the wood element.

Treatment of wood has been addressed in WO 2009/040656 describing a process for treating wood where the wooden product is treated in an airtight tank with a drying fluid or a preservation fluid un- der vacuum. The wooden product can further be subjected to a heating step within the airtight tank by electromagnetic radiation prior to or subsequent to the addition of the drying fluid or preservation fluid. It is also contemplated that the heating may be performed in the tank prior during or after applying the vacuum. One drawback of this process is that a contact between the dying fluid and/or preservation fluid and the electrodes cannot be avoided. Such contact is a potential safety risk, which could cause severe damage to the apparatus or personnel, due to adverse reactions of the fluids when subjected to heat and thus reduced the options of choice of treatment fluids. This particularly applies when the process is to be used large scale.

Thus, one drawback of the method of WO2009/040656 is that treatment fluid and the electrodes are in contact. Therefore, the method is not suitable for all types of treatment fluids. For example fluids, such as polymers, that will cure during heating are unsuitable for use in the method as the polymerization increases the viscosity of the fluid thus resulting in a poor deposition of treatment fluid in the wood. Another challenge in the process of WO2009/040656 is to achieve a satisfactory degree of deposition of the preservative and or dye within the wood in an efficient an economical way due to the competition with the natural flu- ids of the wood.

It is the object of the present invention to provide a method in which the problems of the prior art are addressed, and to provide a safe and yet efficient method for preservation of all types of wood.

It is a further object of the present invention to provide an apparatus for use in the treatment of a wood element where some or all of the problems of the prior art are alleviated as well as the use of the apparatus. Summary of the Invention

In a first aspect of the invention, this object and further objects is achieved by a method for treating a wood element comprising the steps of i) subjecting the wood element to a heating and drying step by means of electromagnetic radiation to provide a heated and dried wood element from which natural fluids have evaporated; ii) removing evaporated natural fluids of the wood iii) transferring the heated and dried wood element to a preparation tank; iv) performing a preparation process comprising the steps of a) optionally depressurizing said preparation tank to establish a vacuum; b) supplying a treatment fluid, such as a preservation liquid and/or a dying liquid, to the preparation tank; c) optionally pressurizing the preparation tank; and d) optionally depressurizing the tank and evicting the treated wood element. In a particular embodiment the steps a, c and d are mandatory and the wood element is wood. In another embodiment the steps a, c, and d are omitted and the wood element is wood chips or a similar wood structure.

The method comprises a separate first step of subjecting the wood to heating by electromagnetic radiation, preferably through at least one electrode arranged on either side of the wood. Heating by means of electromagnetic radiation has the advantage over other types of heating that heating of the wood starts at the core of the wood element. Thus, as the temperature rises the natural fluid of the wood element are forced outwards from the core. In comparison when wood is heated by means of traditional heating, for instance in a drying oven or kiln, the wood is heated from the outside and inwards As a result of the heating from the core and outwards according to the present invention, the heating step carried out by means of electromagnetic radiation is performed more rapidly as compared to heating the wood element by means of traditional heating.

The heating and drying increases the internal temperature and internal pressure in the wood due to evaporation of natural fluids of the wood whereby the natural fluids, e.g. water, volatile organic compounds (VOCs) and air, are expelled from within the wood.

Hereby, natural pathways, pores and vessels within the wood are cleared from obstacles, enabling an easier flow for an external fluid, such as a preservation fluid and/or a dye, into the wood. Further, the increased pressure in the wood caused by evaporated fluids can create microscopic ruptures in the structure of the wood, through which natural fluids can escape.

It was surprisingly found by the inventor of the present invention that natural fluids can be extracted from the wood element without causing damage or deformation to the wood.

A further advantage of the present invention is that the presence of natural fluids within the wood is reduced, why the affinity of the wood to absorb another, external fluid is increased. The evaporated natural fluids are removed from the wood in order to avoid re-entry to the wood for example when pressure is applied in the subsequent preparation process.

Thus, the separate heating and drying step serves two purpos- es. Firstly, all evaporated natural fluids of the wood are removed in order to avoid any competition with the preservation and/or dying fluids to be deposited in the wood. This will ensure an improved preservation and/or dying. In addition, by performing the separate heating and drying step any adverse reactions in a subsequent preparation tank caused by the contacting of the electrodes, the evaporated natural fluids of the wood and the preservation fluid/dye are avoided. Thus, since the electromagnetic heating is performed in a step separate from the preparation process there is no risk of interaction between the electrodes and possible components of the treatment fluid, such as oil. Such an interac- tion in the given environment could cause spontaneous combustion and explosion in particular at high frequencies necessary for industrial application of the method. By the method of the invention any treatment fluid may be used.

Control of the heating and drying step can be made by measuring the temperature of the wood element, such as at the longitudinal ends of the wood elements. A sufficient heating and drying temperature depends on the wood species, in general the temperature increases when the natural moisture content of the wood decreases. The tempera- ture when the heating is sufficient is typically in the range 40 to 180°C, and preferably below 100°C for pine wood and species with the same or higher natural moisture content.

After sufficient heating and drying of the wood, the wood is transferred to the preparation tank in which the preparation step is to be performed. The wood is suitably transferred to the preparation tank immediately after the heating step. However, it is also contemplated that the wood is stored between the step of heating and being transferred to the preparation tank. Storage of the heated and dried wood element require that the temperature of the wood is not decreased critically, in this context critical temperature decrease, is a decrease in the temperature of the wood of 5°C or more, such as 10°C or more and/or the wood takes up moisture from the air. If the wood is to be stored for a longer period of time, the desired temperature may be maintained by supplying heat and/or applying pressure to the storage means.

In one embodiment the preparation tank is a bath comprising the treatment fluid, such as oils. In this particular embodiment the wood element is wood chips and in a further embodiment the oils are bio-oils. In this embodiments waste elements of wood can be converted to valuable fuels for use in power plants etc.

In another embodiment the preparation tank can have the form of a cylinder with convex end-caps. The preparation tank has the ability to sustain both vacuum and a pressurized environment for an extended period of time. In this context a pressurized environment is a pressure of 1-100 bar, such as 5-20 bar, more preferred 10-15 bar, and presently preferred 12 bar. The preparation tank has at least one door, or a contraption with a similar function, for enabling a repeated placing or removal of stacked wood in the tank. As the preparation tank shall sustain a pressurized environment, measures may have to be taken to seal the at least one door to the tank, e.g. by nuts and bolts. It is also contemplated that the preparation tank has two openings, one in each end of the tank. In addition, the wood elements may be transported in and out of the preparation tank by means of a conveyor optionally connected to the upstream heating and drying means.

When placed in the preparation tank a vacuum environment is established by depressurizing the preparation tank to a pressure of typically 1-0.1 bar, i.e. any pressure below atmospheric. This initially creates a pressure difference between the interior of the wood and the vacuum environment in the tank. By this difference in pressure remain- ing natural fluids, e.g. water, VOCs, resins and air, are expelled from within the wood. Furthermore, the pressure difference may create microscopic ruptures in the structure of the wood, which can enable an external fluid to reach part of the wood otherwise unreachable. The vacuum is applied until the internal pressure in the wood is in equilibrium with the pressure of the vacuum environment of the tank. The period of time depends on the pressure in the tank, the species and type of wood. In general the treatment time is between Vi and 4 hours such as 1-2 hours. In general the period depends on the density of the wood, i.e. the denser the wood species the longer the treatment time. The determina- tion of obtained equilibrium between the tank and the wood is within the skill of the art.

After the pressure equilibrium is obtained the fluid for treating the wood is applied, the fluid is in a particular embodiment a preservation and/or dying fluid.

During the depressurization step evaporated natural fluids, if present, are preferably removed, and in a particular embodiment by means of the pump providing the vacuum in the tank. The removal of the natural fluids will further reduce the competition between these natural fluids and the treatment fluids to be supplied. In certain embodi- merits the additional removal of natural fluids may be omitted provided the affinity of the treatment fluid to the wood is much higher than that of the natural fluids.

After the equilibrium has been established and the remaining natural fluids have optionally been removed, the treatment fluid is supplied to the preparation tank. When the fluid is supplied to the preparation tank in the vacuum environment, the fluid will be able to migrate into the wood and fill cavities of the wood structure otherwise filled by natural gases or liquid.

In the next step the preparation tank is pressurized; the step of pressurizing may be simultaneous to and/or subsequent to supplying the treatment fluid.

When pressure is applied a pressure difference arises between the environment of the preparation tank, now being under pressure, and the wood element initially under a slight under-pressure. This difference in pressure will force the remaining treatment fluid into the cavities of the wood structure whereby a high saturation with the fluid is obtained.

The treatment fluid may be a liquid or a gas, preferably a liquid. The fluid may be a preservation fluid, a dye, or a particular chemical compound or mixture of chemical compounds. An example of a preservation fluid is a solution of dinatriumoctaborat-tetraborat in monoethy- lene glycol; a composition comprising the active ingredients Copper carbonate, 2-aminoethanol, boric acid, Tebuconazole, Propiconazole and Polyethyleneamine, for example sold under the trade name Tanalith E 3492; boric acid; water borne alkyd paints, for example sold under the trade name Teknol Aqua 1410-01; fire retardants comprising ammonium phosphates, ammonium chloride, urea, glycerol, DEQVEST 2066 and water, for example sold under the trade name Flamex C. Dyes may comprise any water or oil based dye, such as a linseed oil based paint. Alter- natively, the fluid may be liquid water, supplied for increasing the water content of the wood. Multiple types of fluids can be used without considering the nature of the fluid e.g. oil- or water-based. In general any oil or water based liquid may be used in the treatment of the wood element according to the method of the present invention. When the treatment fluid is liquid, a second heating step following the preparation process may in a particular embodiment be included. The advantage of an extra heating is that the viscosity of the liquid within the wood decreases, and consequently the liquid can more easily pe- netrate even further into the wood structure. In a further embodiment the treatment liquid may be heated before supplying it during step a) in the preparation step. Such preheating will increase the viscosity of the liquid and facilitate the penetration. This may however also cause an increase of the vapour pressure of the liquid when entering the vacuum in the preparation tank, which requires that means for maintaining the vacuum must be provided.

In a further embodiment the wood element is immersed in the treatment fluid supplied or present in the preparation tank, which has the advantage that the fluid can enter the wood from all sides. When the wood element is machined wood, e.g . sawed, planed, or lathed wood, openings of capillaries and natural pathways for fluids are present at all machined surfaces of the wood elements. Furthermore, the machining may create small or microscopic ruptures at every machined surface of the wood. Hence, more fluid may enter the wood structure through its natural pathways and microscopic ruptures when the wood is completely submerged in the fluid.

The method according to the present invention is particularly useful for the treatment of hard wood, i.e. wood species having a low moisture content, such as below 10, 8 or 6%, where an effective preser- vation can be obtained thus making the prior art methods of supplying preservative into holes made in the wood, obsolete.

The method according to the present invention also provides an efficient and uniform heating of wood, thereby shortening the time needed for the subsequent preparation process, without any negative structural effects on the final product. The method can be optimized for different wood properties - such as dimensions, water contents and reinforcement spacing - by varying the time and the applied power in each of the heating steps. Further, the frequency of the radiation can be varied to achieve a more favorable heating before the preparation process. In the context of the present invention the wood element may be of any wood species. The wood element may also be constituted by any form of the wood species such as including several pieces, e.g. a baulk, a plank or board, a heartwood for sapwood board, a trimmed or untrimmed board, the slab or the outside board, half or quarter timber, and/or a board with a wane and engineered wood such as wood chips.

Preferably, the wood is a body of stacked wood members such as a package of boards stacked in a package during transportation on trucks from one place to another.

When the wood element is a body of stacked wood members heating according to the present invention is particularly useful as the electrodes may be placed between the individual members of the stack thus ensuring uniform heating. Hereby, the preparation process is highly streamlined since the body of stacked wood members need not be sepa- rated but can be transported directly from the truck to the step of heating, and subsequently transferred further to the preparation tank.

In a presently preferred embodiment the invention describes a method for the treatment of a wood element comprising the further step of applying a mechanical pressure onto the wood element by a mechani- cal compression system. The purpose of applying a slight compression is that deformation of the wood element during the preparation and particularly during heating, is avoided. The mechanical pressure is applied at least before or during the heating step i).

Applying a mechanical pressure on the wood element by a com- pression system for preventing deformation of the wood can be prior to, simultaneously with or subsequent to any of the earlier mentioned steps. Preferably, the mechanical pressure is applied at the same time as the electromagnetic heating of at least step i).

In a second aspect of the invention, an apparatus for the treat- ment of at least one wood element is provided, said apparatus comprising a support structure for said wood element and means for electromagnetic heating of the wood, said means comprising at least a pair of electrodes positioned at opposite sides of said wood element. The provision of electrodes on opposite sides entails that heating occurs from at least two sides of the wood element.

This is particularly advantageous in a preferred embodiment in which the support structure includes a support plate.

In an embodiment, fixation means for effecting mechanical pressure are provided at said opposite sides of said body of stacked wood members. This entails that the wood element, in particular the wood members of a stacked wood element, is fixated during heat treatment and does not deform during heating and drying.

In a mechanically simple further development of the preferred embodiment, the fixation means include a compression structure including a compression plate.

The electrodes may in principle have any suitable form but are preferably formed as plate members. In a particular embodiment, the pair of electrodes is formed in the support plate and the compression plate. As the compression plate and/or the support plate has the additional function of an electrode for heating by electromagnetic radiation, heating is performed throughout an entire body of wood members while keeping the wood element fixated and possibly compressing the wood element in order not to warp.

In a further embodiment, the apparatus comprises means for transferring said wood element to the support structure, means for transferring said wood element from said support structure to a preparation tank comprising means for supplying a preservation liquid and/or a dying liquid, and means for pressurizing and depressurizing said prepa- ration tank. Preferably, means for discharging the wood element from the preparation tank may be provided as well.

In a third aspect of the invention, use of the apparatus for drying a wood element is provided.

Description of the Drawing

Fig. 1 shows a schematic plan view of an embodiment of an apparatus for treatment of wood;

Fig. 2 shows a schematic side view of the apparatus shown in

Fig. 1; Fig. 3 shows a cross-sectional view of the apparatus of Figs 1 and 2, along the section III-III in Fig. 1;

Fig. 4 shows a perspective view of an embodiment of a section of the apparatus in an embodiment of the invention, with a wood ele- ment before entry into the first section;

Fig. 5 shows a perspective view corresponding to Fig. 4, with the wood element after discharge from the first section;

Fig. 6 shows a side view of the section of the apparatus shown in Fig. 4; and

Fig. 7 shows a cross-sectional view of the section shown in Fig .

4.

Detailed Description of the Invention

In the Figures of the drawing, one embodiment of an apparatus for the treatment of a wood element is shown. The apparatus is generally designated 1 and comprises a support structure 2 to be described in further detail below, on which the wood element 3 is positioned such that the bottom side of the wood element 3 abuts on the support structure 2. The wood element 3 may be a single piece of wood or a body of stacked wood members. On the opposite side of the wood element 3, i.e. on the top side of the wood element 3, fixation means for effecting mechanical pressure are provided. In principle, the wood element 3 may be compressed in any suitable manner, for instance by means of cramps or clamps applied to the wood element 3, particularly in the case in which the wood element 3 comprises a stack of wood members. In the embodiment shown, however, the fixation means include a compression structure 4.

In the apparatus 1, heating and drying of the wood element 3 is carried out by means of electromagnetic radiation in a first section 11 of the apparatus 1. To this end, the first section 11 of the apparatus comprises a pair of electrodes 5, 6 which as shown in the schematic view of Fig. 3 are positioned at opposite sides of the wood element 3. The pair of electrodes 5, 6 may be provided in any suitable shape, including point- shaped, linear and plate shaped electrodes, and are powered by a high- frequency generator 7. The electromagnetic radiation may have a frequency in the range of approximately 10 to approximately 30 MHz, or any frequency range found suitable to the purpose. In particular, industry standard frequencies may be applied, i.e. 13.56 MHz or 27.12 MHz.

Referring now in particular to the cross-sectional view of Fig. 7, the configuration of the support structure 2, the compression structure 4 and the means for applying high frequency to the wood element 3 will be described in further detail. In the embodiment shown, the first section 11 of the apparatus 1 comprises a frame 10, in which the support struc- ture 2 and the compression structure 4 are located. The support structure 2 comprises an insulating member 21 of any suitable material, e.g . a plastic material, on top of which a support plate 22 is positioned. A conveying belt 23 surrounds the insulating element 21 and the support plate 22, and forms the abutment surface of the support structure 2 rel- ative to the wood element 3. The conveying belt 23 is in a manner well- known in the art driven by rollers to move the wood element 3 into, through and out of the first section 11 of the apparatus. The compression structure 4 on the opposite side of the wood element 3 comprises an insulating element 41 and a compression plate 42.

In the preferred embodiment described and shown in the drawings, the electrodes 5, 6 are plate-shaped and provided within the same plate members as the support plate 22 and the compression plate 42, respectively. In addition to providing electrodes below and on top of the wood element 3, electrodes may also be placed between the individual members of the stack thus ensuring uniform heating. Means for applying a variable mechanical pressure on the compression plate 42 and in turn the wood element 3 are provided as pressure cylinders 8 along the first section 11 of the apparatus.

The fixation means comprising the compression structure 4 and the support structure 2 keep the wood element 3 in place during heating of the wood element 3 in the first section 11. The compression structure 4 including the compression plate 42 is placed in abutment with the upper side of the wood element 3. Possibly, the compression structure 4 also compresses the wood element 3 during the electromagnetic prepa- ration of the wood element 3 slightly in order to prevent warping of the wood during the heating and drying taking place in the first section 11 of the apparatus. As the compression plate 42 and/or the support plate 22 in the embodiment shown has the additional function of an electrode for heating by electromagnetic radiation, heating is performed throughout an entire body of wood members while keeping the wood element 3 fixated and possibly compressed in order not to warp. This feature can present an advantage when heating from both the top and bottom of the wood members. The wood element is heated for a period of time suffi- cient to allow the liquid to disappear/evaporate, suitable process times being for instance approximately 10 to 20 minutes. Normally, the liquid is heated to a temperature of 60 to 110°C. If possible, the wooden material is not heated or only heated to a limited extent during this process.

The electrodes generally designated 5 and 6 in the schematic view of Fig. 3 and which in the preferred embodiment form part of the support plate 22 and the compression plate 42, respectively, are connected to the frame 10 of the first section 11 by means of electrically conducting sheet elements 51, 52 and 61, 62, respectively. The high frequency applied by means of generator 7 is thus two-sided, which allows for admitting wood elements of a substantially larger height than if the high frequency had been applied on one side only. Typical dimensions of wood elements 3 treatable in the apparatus according to the invention are for instance but not limited to wood elements being 6000 mm long, having a width of 1200 and a height of 600 mm.

As the temperature of the wood element 3 rises during heating in the first section 11 of the apparatus, the power supplied to the wood element 3 may be adjusted during the process in order to maintain a stable level of the frequency. In the embodiment shown, the adjustment is carried out by adjustment means 71 and 72 positioned at the outside of the frame 10 of the first section 11 of the apparatus 1. The adjustment may be carried out either at pre-defined intervals during the processing, or in response to temperature measurement. The temperature of the wood element 3 may be measured at any suitable location of the wood element 3, for instance at the longitudinal ends.

In the embodiment shown, the apparatus comprises two further sections, namely a second section 12, in which heating without the use of electromagnetic radiation optionally takes place, and a third section 13 comprising a preparation tank 14. In the preparation tank 14, the wood element 3 is subjected to a preparation process. The embodiment applies equally to both aspects of the invention and further details of the process are given in the introductory portion of the description. In the embodiment shown, means for supplying a preservation liquid and/or a dying liquid are provided. Furthermore, means for pressurizing and de- pressurizing said preparation tank 14 are provided as well. The preparation tank 14 may have the form of a cylinder with convex end-caps or any suitable shape. The preparation tank has the ability to sustain both vacuum and a pressurized environment for an extended period of time. In this context a pressurized environment is a pressure of 1-180 bar, such as 5-20 bar, more preferred 10-15 bar, and presently preferred 12 bar. In a manner known per se, the preparation tank is provided with a door, or a contraption with a similar function, for enabling a repeated placing or removal of stacked wood in the tank. As the preparation tank shall sustain a pressurized environment, measures may have to be taken to seal the door to the tank, e.g. by nuts and bolts, especially if the door opens outwards from the interior of the preparation tank.

In order to charge and discharge the wood element 3 into and out of the apparatus 1, and to move the wood element 3 between the individual sections of the apparatus, in the direction of arrow A, a conveyor 9 is provided. The conveyor 9 may be any suitable conveyor such as a chain conveyor, a conveyor belt, a roller conveyor etc. Means for transferring the wood element 3 to and from the conveyor 9 are also foreseen, although not shown in the Figures. Furthermore, suitable sto- rage means may be provided upstream and downstream, respectively, of the apparatus to store wood elements before and after treatment.

The invention should not be regarded as being limited to the embodiment shown and described in the above, but various modifications and combinations are conceivable within the frame of the claims. The method of the invention will now be illustrated by the following non limiting example.

Example 1

A test was performed with pine wood having a moisture content of 4-6% and the following dimensions.

Ten pieces of wood were weighed individually before treatment. The pieces were stacked in a stack of totally 40 pieces having the following dimensions 30 mm X 220 mm X 1500 mm.

The stacked wood elements were placed in a heating and drying apparatus according to the invention and were heated until the temperature measured at the ends of the stack was 55°C. This temperature was obtained using two electrodes connected to a generator having an effect of 200 kW and operated at 13.56 MHz.

After the heating step was finalized the extracted natural fluids were allowed to evaporate. The stack was conveyed to a preparation tank according to the invention. In the preparation tank a slight vacuum was applied for 15 minutes. During the vacuum the treatment fluid Bo- racol20 (a 20 w/w % solution of disodiumoctaborate tetrahydrate in gly- col and water) was added to the preparation tank.

After the supply of treatment fluid during the vacuum was completed a pressure of 12 bar was applied for 30 minutes where after the pressure was released and the stack was ejected. Excess visible treatment fluid was removed, and the wood was allowed to dry for 14 days.

The same ten pieces were weighed again after the treatment process to assess the difference in weight. The results are given in table 1. Table 1 : Wood elements before and after treatment according to the invention

Weight before treatment/kg Weight after treatment/ kg Relative weight gain

4.3 8.8 104%

4.6 8.9 93%

4.5 8.4 86%

4.1 9.7 136%

4.4 6.1 38%

5.4 8.3 53%

3.9 7.4 89%

4.7 7.8 65%

4.0 9.2 130%

4.8 8.2 70%