FIELD OF THE INVENTION

The invention relates to a method for manufacturing a wood board as defined in the preamble of claim 1 and a wood board as defined in the preamble of claim 10.

BACKGROUND OF THE INVENTION

Known from prior art are various plywoods, veneer boards or the like. Known from prior art is the gluing of veneers by various glues, e.g. polyurethane or phenolic glue, for providing a plywood or the like.

Further, known from prior art is different types of methods for manufacturing plywoods.

In some industrial areas flat plywood is desired, e.g. die boards. However, phenol formaldehyde or melamine-urea-formaldehyde bonded plywood is prone to swelling, shrinking, twisting and warping. This undesirable effect is resulting in customers investigating alternative materials.

OBJECTIVE OF THE INVENTION

The objective of the invention is to disclose a new type of method for manufacturing a wood board. Further, the objective of the invention is to disclose a new type of a wood board.

SUMMARY OF THE INVENTION

A method and a wood board according to the invention are characterized by what is presented in the claims.

The invention is based on a method for manufacturing a wood board, wherein a wood board comprising multiple veneer layers is formed from a number of veneers in such manner that the veneers are laid one above the other and combined by means of glue. In accordance with the invention, at least a part of the veneers are heat-treated at temperature between 160 to 215 °C prior to the gluing for reducing moisture in the wood board, and the veneers is glued with the thermoplastic glue including maleic anhydride, and the glue is activated by means of the heat of veneers in connection with the wood board manufacturing.

Further, the invention is based on a wood board, which comprises a number of veneers which are laid one above the other and combined by means of glue. In accordance with the invention, at least a part of the veneers are heat-treated at temperature between 160 to 215 °C prior to the gluing for reducing moisture in the wood board, and the veneers is glued with the thermoplastic glue including maleic anhydride, and the glue is activated by means of the heat of veneers in connection with the wood board manufacturing .

The invention is specifically based on the wood board production which considerations for utilisation of heating the veneers prior to the forming of the wood board with focus on reducing moisture absorption in the veneers and wood board and obtaining a product that can be flat or formable. Further, by means of the heating the veneers the glue can be activated.

In this context, a veneer refers to any veneer of a wood board. The wood board can be a wood panel product, plywood product, composite product, beam, pressed panel product or the like, formed of a number of layers, preferably veneer layers, and principally of wood-based materials, in which the layers are laid one upon the other and glued together. The veneer can be formed of any material, e.g. wood-based material, fiber material, composite material or the like. In this context, the veneer refers to any layer of the wood board. Typically the veneer is a thin layer of the wood board. The thicknesses of the veneer layers can vary.

In one embodiment of the invention all the veneers are heat-treated. In one embodiment only a part of the veneers are heat-treated. It is not necessary for all applications to heat-treat all the veneers to be used in the wood board construction. It may be in the case of humidity changes sufficient to just heat-treat the outer veneers to reduce warpage and twisting.

In one embodiment, during the heat-treatment the wood is modified at temperatures between 160 - 215 °C, e.g. 0.5 to 4 hours. The process involves heating the wood to temperatures exceeding its spontaneous combustion temperature with water vapour functioning as a protective gas and heat conductor. During the heat-treatment there are physical, mechanical and chemical changes to the wood. The physical changes involve a change in colour. The colour change is from light brown to very dark brown and the treated samples can resemble in colour more exotic woods, e.g. walnut and oak. The chemical changes that take place significantly reduce the swelling and shrinking of the wood with variation in moisture. Owing to the break down of hemicelluloses wood no longer contains enough nutritional substances as food for decay fungi. In addition to this many harmful extractive agents are removed, such as formaldehyde resulting in low formaldehyde emissions. Bonding the heat-treated veneers with self-adhesive films (no formaldehyde added or formed) results in an extremely low formaldehyde emission product.

The heat-treatment changes the structure and composition of wood. The hemicelluloses are the most effected components at high temperatures followed by cellulose and then lignin. Between 195 - 205 °C hemicelluloses are decomposed or changed considerably. There are less accessible free hydroxyl groups for bonding but the treatment lowers the polarity of the wood resulting in better compatibility with the polymer. In fact the combination of maleic anhydride, lower moisture content and reduced polarity results in better bonding. In addition to this it is expected that the improved dimensional stability improves the bonding performance, especially after soaking and boiling, because the stresses due to swelling and shrinking on the glue-line are reduced.

In one embodiment of the invention the veneer is heat-treated by IR- or microwave-radiation.

In one embodiment the veneers can be heat- treated in a hot-press or oven but the treatment times need to be much shorter than 4 hours to avoid combustion of the wood. The industrial processes can be applied for longer periods as they are usually operated in the absence of oxygen or use steam as a means of preventing combustion of the wood. Obviously, if shorter heat-treatment times are used the modification will only be moderate and may for lower temperatures not be permanent.

In one embodiment of the invention the heat- treatment of the veneer comprises a drying stage, a heating stage and/or a cooling stage.

In one embodiment the heat-treatment comprises at least one heating stage, e.g. hot-pressing, for heating and drying the veneers. Various heat devices and methods can be used.

In one embodiment the heat-treatment comprises at least one cooling stage for cooling the veneers. Various cooling devices and methods can be used.

In one embodiment the heat-treatment comprises at least one heating stage and at least one cooling stage. Preferably the heating stage should be followed by the cooling stage.

In a preferable embodiment the veneers, e.g. green veneers, is heat-treated prior to the lay-up stage. In one embodiment the method comprises at least one heating unit for heat-treating the veneers prior to the lay-up stage. In a preferable embodiment the veneers are fed directly to the lay-up stage after the heat-treating as hot veneers. In fact hot veneers can be an advantage in speeding the adhesion process up by reducing the hot-pressing time. Also, using hot veneers is a cost saving.

In one embodiment the green veneers are pre- treated, e.g. in steam or gas heated dryers, prior the heat-treatment stage.

The veneers can be glued with usual wood board or plywood glues.

In one embodiment of the invention the glue is a polymer film modified maleic anhydride.

In one embodiment of the invention the glue is a self-adhesive film including maleic anhydride

In one embodiment the glue is a film is formed of polyolefin containing reactive groups with -OH groups of the wood for forming bonds via esterification between the veneer and the film. In one embodiment the film contains maleic anhydride grafted polyolefin. In one embodiment the film is 3-layer co- extruded modified polyolefin film. In one embodiment the adhesive film can be in the form of a 3-layer co- extruded film where the outer layers contain maleic anhydride grafted polyethylene or polypropylene.

In one embodiment the film is the self-adhesive polyolefin film, as defined in patent applications PCT/FI2009/050130 and PCT/FI2009/050662, to create the wood board that has strength properties and fibre breakage percentage that exceeds that of phenol formaldehyde bonded wood board. The heat-treating of the veneers and the use of the self-adhesive films including maleic anhydride as glue improve especially dimensional stability of the wood board.

In one embodiment the self-adhesive film is a hot melt film.

In one embodiment of the invention the glue is a foamed glue-line. In one embodiment, the foamed glue-line can be used alone or as combination with e.g. 3-layer film. The 3-layer film can be extruded so only the middle layer is foamed. The foaming glue-line can reduce the density by > 30% which will result also in a much lighter product, especially if thicker films are utilised. In order to avoid the foamed structure collapsing during the hot-pressing the foamed part cannot soften or melt.

In one embodiment of the invention the glue is a film containing glue material provided on the surface of the film. In one embodiment the glue material can be hot-melt, resin adhesive, maleic anhydride composition or the like. The glue material can be in liquid or molten form.

In one embodiment the veneers can be joined together using any adhesive or glue, e.g. phenol formaldehyde, melamine urea formaldehyde, bio-adhesive or other adhesive.

In one embodiment of the invention the glue, e.g. film, polymer film, self-adhesive film, foamed glue-line or glue material, is activated by means of the heat of veneers in connection with the wood board manufacturing. In one embodiment the glue is heated separately at temperatures of between 120 - 170 °C in the manufacturing of the glue film or glue material. The glue layer or film is heated again in connection with the wood board manufacturing so that the glue becomes adhesive.

The veneers can be joined together using the usual methods, e.g. the hot pressing technique or hot- cold pressing technique.

The drying, heating, laying-up, gluing and pressing steps and some typical steps can be performed in any manner known per se in the art.

In the case where the wood board is formable the wood board can be reheated in an oven or microwave. Pressure is not required during the re ^¬ heating but it is desired that the wood board product is formed under pressure and cooled in order not to damage the bonds formed during the initial hot- pressing .

By the means of the heat-treatment the moisture absorption is reduced in the veneers, preferably by about 25 - 50 %. Further, the heat- treatment makes bonding with the maleic anhydride better .

An advantage of the invention is to provide dimensionally stable, flat and/or formable wood board products. Thanks to the invention the veneers are less polar. Owing to this adhesion between the glue, e.g. the maleated modified glue-line, and the wood is improved. The improved dimensional stability of the wood board improves the bonding performance, because the stresses due to swelling and shrinking on the glue-line are reduced. In addition to the improved dimensional stability it can be to retain the mechanical properties of the wood board.

The method and wood board in accordance with the invention are suitable for various applications. The heat-treated wood board products can be used to form flatter plywood, e.g. die boards, more dimensionally stable plywood product, musical instruments and sound insulated plywood for buildings and stairs. Heat-treated wood board can be utilised in formwork applications as a means of reducing rippling of the face veneer. This application does not require all of the veneers to be heat-treated just the face veneers. However, the whole construction can be bonded with self-adhesive films.

An application of the wood board is musical instruments. With increasing demand for high quality musical instruments at economical prices there is a need for more cost effective materials. Tropical woods are usually used after a process of aging. For certain types of musical instruments heat-treated veneers bonded with self-adhesive films could be used. Wood used in musical instruments needs to be dimensionally stable and also to have specific sound characteristics. The desired sound characteristics are usually obtained from wood exotic woods stored for a long time. The heat-treated wood exhibits improved acoustic properties for creating similar sound characterized to naturally aged wood. In addition to this the colour change that results from the heat- treatment also resembles the colour of more exotic woods .

LIST OF FIGURES

In the following, the invention is described by means of detailed embodiment example with reference to accompanying figures 1 - 2, in which

Fig. 1 shows a schedule of a process according to the invention, and

Fig. 2 shows a reaction of conversion of maleic acid to maleic anhydride and its reaction with heat- treated wood hydroxide groups .

DETAILED DESCRIPTION OF THE INVENTION

In this example, the plywood is manufactured by the method of the invention. In figure 1 it is shown a process for manufacturing the plywood.

The process comprises heat-treatment unit 1, lay-up system 2, feeding of adhesive 3 and pressuring unit 4.

First stage is heat-treating veneers 1. Green veneers are heat-treated. The wood of the veneers is modified at temperatures between 160 - 215 °C between 0.5 to 4 hours. The process involves heating the wood to temperatures exceeding its spontaneous combustion temperature with water vapour functioning as a protective gas and heat conductor.

From the heat-treating 1 the hot veneers are fed automatically to the lay-up system 2. Also the adhesive film 3 is fed to the lay-up system 2.

The film is self-adhesive film formed of polyolefin containing maleic anhydride. The film is 3- layer coextruded film where the outer layers contain maleic anhydride polyethylene. In figure 2 it is shown a reaction of conversion of maleic acid to maleic anhydride and its reaction with heat-treated wood hydroxide groups. The film is attached to the heat- treated veneers where the heat activates the adhesive of the film and the active maleic anhydride groups of the adhesive react with the hydroxide groups of the woods cellulose material forming a covalent bond via esterification . The degree of hydroxyl groups at the surface of the wood are considerably reduced but this results in better compatibility between the wood and polymer. It was expected that the improved dimensional stability improves the bonding performance, because the stresses due to swelling and shrinking on the glue-line are reduced. However, the heat-treatment was expected to cause changes related to the adhesion of the maleic anhydride to the wood. The adhesion improved .

The film is heated by means of the hot veneers so that it becomes self-adhesive. At the lay- up system 2 the veneers are transferred onto the lay- up station and the film 3 is supplied between each two veneers. The veneers are glued together by the film.

The adherence of the co-extruded film to the veneers is carried out by hot-pressing or high frequency pressing. The film is laid between each veneers and the film is hot-pressed typically at a pressure of 0.9 - 2.0 N/mm ² and temperature 120 - 170 °C for 12 minutes. The hot-pressing conditions depend on the wood type, e.g. spruce, birch, pine or combination, and polymer melting temperature. It is important in order to induce plastic melt flow that the hot-pressing temperature is set to a temperature 20 - 50 °C above the melting temperature of the polymer. Further, the temperature and time depends on the thickness of the plywood and grade of the polymer used. Alternatively, the film is pressed by high frequency method between the veneers. Typical conditions of the high frequency are: frequency 13.10 MHz and time 5 - 8 minutes. In one embodiment the hot press is followed by a cold press, typically 0 - 7 minutes at 0.9 - 2.0 N/mm ² , in order to cool the polyolefin film to a temperature below its crystalline temperature, typically, 80 - 100 °C depending on the polymer grade. The plywood can be birch, spruce or any of their combinations.

Further, the process comprises the pressing unit 4 for pressing the veneers together to form the plywood product. The pressing unit 4 comprises hot and cold pressing stages. The hot pressing stage is followed by the cold pressing stage.

From the tests it was discovered that a method of the invention is suitable to be used for manufacturing plywoods easily and cost effectively.

Further from the tests it was discovered reduction in water absorption of the plywood of the invention. The plywood of the invention absorbs 25 - 50 % of the moisture absorbed by prior known plywood.

A method and wood board according to the invention are suitable in its different embodiments for different types of applications.

The embodiments of the invention are not limited to the examples presented rather many variations are possible within the scope of the accompanying claims .