SK51092006A3 - Apparatus and treatment method of wood surfaces, wood fibres and wooden materials - Google Patents

Abstract

The present invention relates to an apparatus for surface treatment of wood, wood fibres and wood-based materials using the action of electric plasma. The invention disclosed herein includes at least one electrode system (1) consisting of electrode systems (2) and (3) situated inside of a dielectric body (4). The electrode systems (2) and (3), above which diffuse plasma is generated preferably at atmospheric pressure, are situated on the same side of the surface of wood, wood fibre layer or wood-based materials (5) and are energised by alternating or pulsed electrical voltage applied between them. The invention further relates to a method for surface treatment of wood, wood fibres and wood-based materials, consisting in treating such surfaces with diffuse plasma generated using the apparatus according to the invention, preferably at atmospheric pressure. Such plasma-treated surface is then advantageously coated with a H<SUB>2</SUB>O containing solution, or the surface is exposed to a monomer vapour or that of another organic substance, or is brought into contact with other materials, or is submerged into a liquid.

Description

7Τ

Apparatus and method for surface treatment of wood, wood fibers and wood-based materials

Technical field

The invention relates to an apparatus and method for surface treatment of wood, wood fibers and wood-based materials by electric plasma of surface barrier electrical discharges at pressures close to atmospheric and subsequent surface treatment of the materials so treated.

BACKGROUND OF THE INVENTION

Very often, starting wood materials such as boards, veneers and wood fibers do not have the surface properties needed for demanding applications. Therefore, for the refinement of cheap and readily available starting materials of wood to final products, e.g. adhesion promoters containing volatile organic compounds, surfactants and various other chemicals. However, the use of these chemicals is often problematic in both health and environmental terms. As a result, there has been a continuing need for the development of more environmentally friendly, efficient and less expensive non-traditional methods of wood surface treatment.

As with starting wood materials, it is often necessary to improve the surface properties of wood-based materials, such as surface treated and untreated wood, as well as laminated wood and particle board, plywood, veneer and wood fiber layers.

In the following, as will not be further specified, wood-based materials will be understood to mean surface-treated and untreated wood as well as laminated wood and particle board, plywood, veneer and wood fiber layers.

Application of non-isothermal plasma is a well-tried and widely used technique for etching and surface treatment in the electronics industry. Non-isothermal plasma is increasingly used for polymer surface treatment in the aerospace, automotive, medical, packaging and textile industries. Various surface treatments of wood-based materials with non-isothermic electric plasma are also known. In the generation of non-isothermal plasma, the gas molecules ionize under the influence of a strong electric field, producing electrons and positive ions. Electrons in a strong electric field receive a high energy corresponding to a temperature of the order of up to 10 ⁴ Ka, when they collide with gas molecules, chemically active particles and aroused molecules are formed. The temperature of the working gas does not increase significantly during the generation of the non-isothermal plasma, so that the gas molecules are not in thermal equilibrium with the electrons, so that the wood-based workpiece is not thermally damaged.

A disadvantage of currently available plasma surface treatment devices for polymeric materials is that they only operate at low pressure, which increases equipment cost and costs, as it is necessary to use capital intensive vacuum equipment operated by qualified personnel. Continuous machining of materials is also virtually impossible at low working gas pressures, which is particularly important and critical when plasma is applied at low pressure to the treatment of wood-based materials such as disclosed in Japanese patents JP11042611, JP10305410, JP6106508, JP6099409, JP5069417 , in Russian Patent RU2185283, and in F. Denes, L. Nielsen, and X. Here: Improvement of Biobased FiberPlastic Composite Properties Through Cold Plasma Treatments, 1996. Wood-Fiber Plastic Composites, Proceedings No. 5, pp. 7293, Forest Products Research Soc., Madison, Wl., Pp. 227-234, H. Sabharwal, F. Denes, and L. Nielsen: Free Radical Formation in Lignocellulosics from Argon Plasma Treatment J. Agric. Food Chem. 41 (1993) 2202, and in W. L. E. Magalhaes and M.F. de Souza "Solid softwood coated with plasma-polymer for water repellence" Surface and Coat. Technology 155 (2002) 11-15 and in A.R. Denes, R.A. Young: “Reduction of weathering degradation of wood through plasma-polymer coating” Holzforschung 53 (1999) 632-640.

The current trend in industrial low temperature plasma applications is to replace plasma treatment in vacuum systems with plasma treatment at atmospheric pressure. As disclosed in German Patent DE 199 578 775 and in Podgorski L., Chevet B., Onic L., Merlin A .: "Modification of Wood Wettability by Plasma and Corona Treatments" Inter. Journal of Adhesion and Adhesives (2000) 10311, P Rehn, W. Viol: "Dielectric barrier discharge treatment at atmospheric pressure for wood surface modification" Holz als Horn- und Werkstoff "61 (2003) 145150, as well as M. Bente, G Avramidis, S. Foerster, EG Rohwer and W. Viol: "Wood surface modification in dielectric barrier discharges at atmospheric pressure for creating water repellent characteristics" Holz Roh Werkst 62 (2004) 157-163 for surface treatment of wood was tested by plasma technology initially Designed for atmospheric pressure treatment of polymeric films and similar materials based on standard volumetric barrier discharges or plasma sources of 'industrial corona' type, where the material to be treated is sandwiched between the inclusion electrodes or used directly as one of the electrodes, with a displacement current flow flowing between electrodes pass through both surfaces of the workpiece. Significant disadvantages of barrier-based or "industrial corona" technology are high energy consumption, inhomogeneous surface treatment, the problem of treating dry and coarse materials with high electrical resistance, the safety risk associated with the use of treated wood material as a discharge electrode, and the risk of sparking. discharges in dusty and humid environments common in the woodworking industry. Also, the electrodes of such devices are not safe in accidental contact with the surface of the human body.

A method and apparatus for treating wood using an improved barrier discharge is described in P. Rehn, A. Wolkenhauer, M. Bente, S. Forster, W. Viol: "Wood surface modification in dielectric barrier discharges at atmospheric pressure" Surface and Coating Technology, 174-175 (2003) 515-518, where the homogeneity of plasma and thus of machining is improved by the rapid flow of working gas. Even in this method, however, the electrical barrier discharge burns against the surface of the treated wood, so that the feed current streams pass through both surfaces of the treated material, causing a problem of machining dry and coarse materials of high electrical resistance. Another disadvantage of this process is the need for a large flow of working gas, which results in a large consumption of gas and in the generation of large amounts of exhaust gas. Another disadvantage is the considerable energy consumption of 0.1 kWh per m ^{2 of} activated wood surface. Also, the device electrodes are not safe when accidentally touching the surface of the human body.

SUMMARY OF THE INVENTION

The drawbacks of the above methods and apparatuses are overcome by the apparatus of the invention, wherein the surface of the wood-based material is treated with a thin, preferably thinner than 1 mm and thicker than 0.05 mm, diffuse layer of strongly non-isothermic electric plasma generated on the surface portion of the dielectric material. preferably a ceramic material or glass, preferably above the surface of the conductive electrodes embedded in the dielectric material. The plasma treated surface is located near, preferably more than 0.05 mm and less than 1 mm, from the surface of the dielectric material on which the thin plasma layer is generated.

The plasma is generated in any working gas, preferably in the working gas not containing He and containing molecules of N 2, O 2, H ₂ O, CO 2, halogen, hydrocarbon, or a few monomers. The device according to the invention can operate over a wide range of working gas pressures of the order of 1 kPa to 1000 kPa, preferably at atmospheric pressure and at a gas flow velocity of less than 10 m / s.

Diffuse plasma is generated on the surface of the dielectric material separating the electrically conductive electrodes of the device disposed in the volume of the dielectric, preferably in a thin layer above the conductive electrodes, wherein the conductive electrodes are not in contact with the plasma. An alternating or pulsed electrical voltage of 50 Hz to 1 GHz with an amplitude of 100 V to 100 kV is applied between the electrodes. The minimum electrode gap to which the AC voltage is applied is less than 2 mm and greater than 0.05 mm.

The electrodes are arranged such that a significant portion of the field field flux that is greater than 50% of the total field field flux between the electrodes separated by a layer of dielectric material that is powered by an alternating electrical voltage does not pass through the plasma treated surface of the material. In such an arrangement, the electrodes on which the electrical voltage is applied to generate the plasma are located on the same side of the workpiece or the wood fiber layer to be treated.

Surprisingly, it has been found that by the method according to the invention it is possible to generate a diffusion plasma with a high volumetric power up to the order of 100 W / cm ³ above the surface of the conductive electrodes embedded in the described method in a dielectric material. 0.1 s to 1 s. An advantage of the solution according to the invention is that such a diffusion plasma can be generated even without a rapid flow of the working gas and without using a working gas containing helium or argon. Surprisingly, it has been found that the homogeneity of the plasma thus generated, in contrast to all known plasma devices tested so far for the above purpose, increases with increasing electrical power supplied to the plasma.

Another surprising finding is that the diffusion and homogeneity of the plasma is higher when the surface of the workpiece to be treated is at a distance of 0.05 mm to 1 mm, preferably at 0.1 to 0.3 mm, from the surface of the dielectric material on which the plasma is generated. wood-based material. Another surprising finding is that the plasma thus generated is safe in contact with the surface of the human body.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of electrode systems according to the invention are shown schematically in the attached figures. Only planar-shaped electrode systems are shown in the figures.

Fig. 1 is a cross-sectional view of an electrode system as part of an apparatus for plasma treatment of a surface of wood-based materials without an auxiliary electrode. The material to be processed is at most 1 mm from the electrode system.

Fig. 2 shows a component for plasma treatment of the surface of wood-based materials with an auxiliary electrode.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

The device and method of the invention have been used to hydrophilize a wood surface in order to obtain suitable surface properties for subsequent surface treatments, for example with water-soluble chemicals without the use of surfactants.

The pinewood was dried in a conventional dryer at 100 ° C for 6 hours. After drying, the wood was cooled to room temperature. The wetting angle of the wood surface immediately after application of the drop onto the wood surface was 90 °, i.e. the surface was hydrophobic. A portion of the dried samples was treated by the method of the invention in plasma generated in laboratory air without air flow at a power of 5 W / cm ² for 1 s. The sample surface was located at a mean distance of 0.2 mm from the surface of the electrode system. The electrodes of the electrode system were loaded with an alternating voltage of 25 kHz and a peak-to-peak voltage of 15 kW. After exposure for 1 s, the wetting angle was less than 5 ° so that the sample surface was perfectly hydrophilic. The electricity consumption was 0.0135 kWh / m ² .

Example 2

The device and method of the invention have been used to improve the adhesion properties of the wood surface.

Chembond® CB 303 phenol-formaldehyde adhesive was applied to the samples of pine wood dried as in Example 1 at a rate of 200 g / m ² . After one minute, the samples were glued in a laboratory press at 200 ° C and a pressure of 2 N / mm ² for 12 minutes. After gluing, the resulting sample was stored at room temperature in room air for 1 day. The adhesive strength of the sample measured by ASTM D3433 1997 as the energy required to create two new surfaces by tearing off the bonded parts was 52 J / m ²

The dried wood samples were plasma treated as described in Example 1 and tested for adhesion strength as above for untreated samples. Plasma activation resulted in a 158% increase in adhesion strength.

Example 3

The apparatus and method of the invention have been used to hydrophobize a wood surface by a plasma polymerization method.

The 21 cm x 50 cm x 5 cm poplar wood board was dried for 24 hours at 75 ° C and sanded with fine 150 grid sandpaper. The surface of such a material was highly hydrophilic since a drop of water was absorbed by the material in less than 1 second.

The plate was surface treated in plasma generated at atmospheric pressure in a mixture of N ₂ + 4% hexamethyldisiloxane (HMDSO). The sample surface was located at a mean distance of 0.35 mm from the surface of the electrode system. The electrodes of the electrode system were loaded with an alternating voltage of 50 kHz and a peak-to-peak voltage of 15 kW. The working gas flow rate above the sample surface was about 2 m / s, the area power was 7 W / cm ^2, and the exposure time was 30 s. In this treatment, a highly hydrophilic polymer layer having a surface energy of 15 mJ / m ^{2 was} formed on the wood surface by plasma polymerization.

Example 4

The device and method of the invention were used to activate the wood surface prior to subsequent inoculation with a UV absorber from the solution.

Pine wood samples were activated in plasma as described in Example 1. A 5% solution of 2-Hydroxy-4- (2,3-epoxypropoxy) benzophenone in ethanol was prepared and sprayed onto the surface of plasma-activated and non-plasma-activated samples. After drying, the samples were analyzed by FTIR-PAS. HEPBP was found to be covalently bound to hydroxyl groups and free radicals generated by plasma activation. Such desirable binding was not detected on plasma non-activated samples.

Example 5

The device and method of the invention were used to activate the wood surface prior to subsequent acetylation from acetic anhydride vapors.

Pine wood samples of 21 cm x 10 cm and 0.5 cm thickness were plasma-activated as described in Example 1. Subsequently, plasma-activated and non-activated samples at 120 ° C were exposed to saturated acetic anhydride vapor for 2 hours.

Example 6

The apparatus and method of the invention have been used to increase the efficiency and stability of the hydrophobic treatment of the wood surface.

The dried pinewood samples were immersed for 10 minutes in a 0.5 mol.dm &^lt; ^{3 &gt;} solution of chlorotrimethylsilane (Merck) in chloroform. They were then rinsed in chloroform and air-dried. A portion of the silanized samples were treated according to the invention in plasma generated in a mixture of nitrogen with saturated water vapors at a power of 5 W / cm ² for 2 s. After 12 hours, the wetting angles of the plasma treated samples and the plasma unexposed samples were measured. The wetting angle of the plasma-treated samples was 105 ° and the plasma of the untreated samples was 83 °, thus the plasma-treated samples were more hydrophobic. Plasma treated samples also showed increased resistance to hydrophobic treatment against aging and biogegradation.

Example 7

The device and method of the invention have been used to improve the impregnation of wood with melamine resin.

A saturated aqueous melamine solution was prepared by ultrasonication. Fir trees were sampled at 5 cm x 10 cm x 0.5 cm and treated on one surface with a plasma generated nitrogen-saturated water vapor mixture at 5 W / cm ² for 3 s. After treatment, they were immersed in a melamine resin solution for 6 hours at room temperature. The samples were rinsed in distilled water and dried for 3 days under laboratory conditions. Subsequently, the samples were heat treated for 10 minutes at 180 ° C. The surface concentration of melamine was determined by UV spectroscopy at a wavelength of 240 nm. In plasma treated samples, the melamine concentration was increased by 250%.

Claims (12)

PATENT CLAIMS Apparatus for surface treatment of wood, wood fibers and wood-based materials, characterized in that it comprises an electrode system (1) consisting of an array of electrically conductive electrodes (2) and (3) housed inside a body of dielectric material (4) at a minimum distance between the electrodes (2) and (3) of less than 2 mm and greater than 0.05 mm, located on the same side of the surface of the wood material, wood fiber or wood-based material (5), so that of plasma (6) is generated on a portion of the surface of the dielectric body (4) preferably above the surface of the electrically conductive electrodes (2) and (3), wherein a significant portion of the field flux is greater than 50% the electrodes (2) and (3) do not cross the surface of the machined material (5) which is in contact with the plasma (6), the distance of The electric body (4) on which the plasma is generated from the surface (5) to be treated is less than 1 mm and wherein the plasma layer (6) is not in contact with the electrically conductive electrodes (2) and (3). Device according to claim 1, characterized in that the frequency of the voltage applied between the conductive electrode assemblies (2) and (3) is in the range of 50 Hz to 1 MHz. Device according to claim 1, characterized in that the amplitude of the voltage applied between the conductive electrode assemblies (2) and (3) is in the range of 0.5 kV to 100 kV. Device according to claim 1, characterized in that it comprises a further auxiliary electrode assembly (7) housed inside a body of dielectric material (4) which is part of the electrode system (1) and which is at a potential different from other electrodes (7). 2) and (3). Device according to claim 1, characterized in that the surface of the dielectric body (4) on which the plasma layer (6) is generated is present in the working gas at a pressure of 1 kPa to 500 kPa. Device according to claim 1, characterized in that the wood material, the wood fiber layer or the wood-based material (5) moves relative to the surface of the dielectric body (4) on which the plasma layer (6) is generated, in a minimum distances of less than 1 mm. 7. A method for surface treatment of wood, wood fibers and wood-based materials by an electric plasma, characterized in that the surface of wood, wood fibers and wood-based materials is treated with an electric plasma generated by an array of electrically conductive electrodes embedded within the body of dielectric material and on the same side of wood, wood fibers and wood-based materials, wherein a significant portion of the field field flux greater than 50% of the total field field flux between the electrodes does not cross the surface of the plasma-treated material; is an electric plasma layer generated on a portion of the surface of the dielectric body without contact with the electrically conductive electrodes, at a minimum distance from the surface of the dielectric body on which the plasma is generated, from the surface of wood, wood fibers and bait materials Wood of less than 1 mm. A method for the surface treatment of wood, wood fibers and wood-based materials, characterized in that a water-containing solution, a water-containing suspension or a water-containing emulsion of another material in the form of an aerosol is subsequently applied to the surface of the material treated according to the method. electrically charged aerosol, foam, printing, or coating. Method for the surface treatment of wood, wood fibers and wood-based materials, characterized in that the surface of the material treated according to the method of claim 7 is subsequently treated with a gaseous medium containing monomer vapors or vapors of another organic substance. Method for the surface treatment of wood, wood fibers and wood-based materials, characterized in that a layer of polymeric material is subsequently applied to the surface of the material treated by the method of claim 7 by extrusion, lamination, printing, coating, spraying or electrostatically in powder form. . Method for the surface treatment of wood, wood fibers and wood-based materials, characterized in that the surface of the material treated by the method of claim 7 is subsequently brought into contact with the surface of another solid material. Method for the surface treatment of wood, wood fibers and wood-based materials, characterized in that the surface of the material treated by the method according to claim 7 is subsequently immersed in a liquid.