EP2095694A1

EP2095694A1 - Apparatus and method for treatment of wood, wood fibres and wood-based materials

Info

Publication number: EP2095694A1
Application number: EP07861188A
Authority: EP
Inventors: Mirko Cernak; Jozef Rahel
Original assignee: Faculty Of Mathematics Physics And Informatics Of Comenius University
Current assignee: Faculty Of Mathematics Physics And Informatics Of Comenius University
Priority date: 2006-12-05
Filing date: 2007-12-04
Publication date: 2009-09-02
Also published as: WO2008085139A1; FIU20100489U0; FI9072U1; SK51092006A3

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 H2O 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

APPARATUS AND METHOD FOR TREATMENT OF WOOD, WOOD FIBRES AND

WOOD-BASED MATERIALS

Technical Field

The invention relates to an apparatus and a method for surface treatment of wood, wood fibres and wood-based materials using surface barrier electric discharge plasma at pressures close to atmospheric pressure and subsequent surface finishing of materials so treated.

Background Art

Source wood materials such as boards, veneer and wood fibres very often lack surface properties necessary for demanding applications. To improve inexpensive and easily accessible source wood materials for final products, use is commonly made of, e.g., adhesion promoters containing volatile organic compounds, surfactants, and various other chemicals. However, the use of such chemicals is often connected with problems with respect to health and the environment. This resulted in a continued need for the development of environmentally friendlier, more effective and less costly untraditional methods of wood surface treatments.

Similarly as in the case of source wood materials, it is often necessary to improve the surface properties of wood-based materials, such as surface-treated and untreated as well as laminated wood boards and chipboards, plywood, veneer, and wood fibre layers.

Hereinafter, unless otherwise specified, the term "wood-based materials" will be understood to mean surface-treated and untreated as well as laminated wood boards and chipboards, plywood, veneer, and wood fibre layers.

The application of cold plasma is a well-tested and widely used technique for etching and surface treatment in electrical engineering industry. Cold plasma is ever more widely used for treatments of polymer surfaces also in aircraft, automotive, health, packaging and textile industries. Various surface treatments of wood-based materials using the action of cold electric plasma are known as well. Upon generation of cold plasma, strong electric field causes ionisation of gas molecules forming electrons and positive ions. In the strong electric field, electrons acquire a high energy corresponding to the temperature of the order of as many as 10⁴ K and, in their collisions with gas molecules, chemically active species and excited molecules are formed. During cold plasma generation, the temperature of the working gas does not increase significantly, that is, gas molecules are not in thermal equilibrium with electrons, and therefore thermal damage of the wood-based material under treatment does not occur.

A disadvantage of the commonly available devices for surface treatment of polymer materials with plasma is that they work only at low pressure, which increases the price of the equipment and costs, as it is necessary to use capital- intensive vacuum equipment operated by skilled personnel. At low pressures of the working gas, it is also virtually impossible to process the materials continuously, which is particularly important and critical when low-pressure plasma is applied to wood-based materials, as described, for example, in Japanese patents JP11042611, JP10305410, JP6106508, JP6099409, JP5069417, in Russian patent RU2185283, and in F. Denes, L. Nielsen and X. Tu: "Improvement of Biobased Fiber-Plastic Composite Properties Through Cold Plasma Treatments", 1996. Wood-Fiber Plastic Composites, Proceedings No. 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 present trend in industrial applications of low-temperature plasma is to replace plasma treatment in vacuum systems with plasma treatment at atmospheric pressure. As described 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) 103-11 , P. Rehn, W.

Viol: "Dielectric barrier discharge treatments at atmospheric pressure for wood surface modification", HoIz als roh-und werkstoff 61 (2003) 145-150, as well as in M.

Bente, G. Avramidis, S. Foerster, E. G. Rohwer and W. Viol: "Wood surface modification in dielectric barrier discharges at atmospheric pressure for creating water repellent characteristics", HoIz Roh Werkst 62 (2004) 157-163, test wood surface treatments were done using plasma technologies originally designed for the treatment of polymer foils and similar materials at atmospheric pressure based on the standard volume barrier discharge or plasma sources of the "industrial corona" type, wherein the treated material is inserted between the electrodes of the device, or is used directly as one of the electrodes, with the displacement electric current lines flowing between the electrodes intersecting both surfaces of the material under treatment. Important disadvantages of the technology based on barrier discharge or "industrial corona" include great energy consumption, non-homogeneous surface treatment, problems with the treatment of dry and thick materials with large electric resistance, the safety risk connected with the use of treated wood material as the discharge electrode, as well as the hazard of spark discharge in dusty and humid environment common in the wood processing industry. Electrodes of such devices are also not safe in accidental contact with the surface of human body.

A method and an apparatus for wood surface treatment using an improved barrier discharge is described in P. Rehn, A. Wolkenhauer, M. Bente, S. Fόrster, 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 the plasma and thus also of the treatment is improved by a fast flow of the working gas. However, with this method as well, electric barrier discharge burns against the surface of the treated wood, therefore the displacement electric current lines intersect both surfaces of the treated material, which causes problems with the treatment of dry and thick materials with large electric resistance. Another disadvantage of this method is the need for a strong flow of the working gas, which results in its great consumption and causes significant air pollution. Another disadvantage is a significant energy consumption of 0.1 kWh per m² of activated wood surface. Electrodes of the device are also not safe in accidental contact with the surface of human body. Disclosure of Invention

The above-discussed disadvantages are solved by the present invention, where the wood based material surface is exposed to a thin layer non-equilibrium plasma, preferably with a thickness ranging from 0.05 mm to 1 mm. The plasma layer is generated on a portion of a dielectric body surface, advantageously the body made from a ceramics or glass, preferably on the dielectric body surface above the surfaces of conductive electrodes situated inside of the dielectric body. The plasma exposed surface is situated in a vicinity of the dielectric body surface on which the plasma layer is generated, preferably closer than 1 mm and farther than 0.05 mm, from the dielectric body surface on which the plasma layer is generated.

The plasma is generated in any working gas, preferably in the working gas not containing helium and containing molecules of N₂, O2, H2O, CO₂, or monomer vapours. The plasma is generated at gas pressures ranging from 1 kPa to 1000 kPa, preferably at atmospheric pressure and, preferably, using working gas flow velocity less than 10 m/s.

The plasma layer is generated on the surface of a dielectric body, which is separating conductive electrodes situated inside of the dielectric body, in such a way that the electrodes surfaces are not in contact with the plasma. The electrodes are energised by an alternating or pulsed electrical voltage with a frequency ranging from 50 Hz to 1 GHz and a magnitude from 100 V to 100 kV. The minimum interelectrode distance is less than 2 mm and more than 0.05 mm.

The electrodes are situated in such a way that a significant portion of the electric field lines flux, which is larger than 50% of the total electric field lines flux flowing between the electrodes separated by a layer of the dielectric material and supplied with alternating electric voltage, is not intersecting the plasma-treated material surface. At such an arrangement, the electrodes supplied with electrical voltage to generate plasma are placed on the same side of the material under treatment or the wood fibres under treatment.

It was found surprisingly that using the method in accordance with the invention, it is possible to generate, above the surface of conductive electrodes positioned in a dielectric material in the above-described manner, visually diffuse strongly nonequilibrium plasmas with high power densities reaching the order of 100 W/cm³ suitable for fast treatment of wood-based material surface at exposure times of the order of 0.1 to 1 s. An advantage of the solution in accordance with the invention is that such diffuse plasma can be generated even without a high working gas flow and without using a helium- or argon-containing working gas. It was found surprisingly that the homogeneity of plasma so generated, as opposed to all known plasma devices tested previously for the above-mentioned purpose, increases with growing plasma power density.

Another surprising finding is that the plasma uniformity, diffusivity and power density is increased by situating the treated wood-based material surface at a distance from 0.05 to 1 mm, preferably from 0.1 to 0.3 mm, from the dielectric body surface on which the plasma layer is generated. Another surprising finding is that plasma so generated is safe in contact with the surface of human body.

Brief Description of Drawings Examples of the electrode systems according to the invention are described schematically in the attached figures. The illustrations in the figures are confined to the planar electrode systems.

Figure 1 is a schematic cross-sectional view illustrating the electrode system that can be an essential part of the apparatus for the plasma treatment of wood- based material surface without an auxiliary electrode. The treated substrate surface is situated at a distance of not more than 1 mm from the electrode system.

Figure 2 illustrates part of the apparatus for the plasma treatment of wood- based material surface with an auxiliary electrode.

Modes for Carrying Out the Invention Example 1

The apparatus and the method according to the present invention were used for hydrophilisation of wood surface to obtain desired surface properties for subsequent surface treatment, for example, with water-soluble chemicals without using surfactants. Pinewood was dried for 6 hours in an ordinary drying kiln at the temperature of 100⁰C. After drying, wood was cooled to ambient temperature. The wetting angle of wood surface immediately after application of a drop on the wood surface was 90°, that is, the surface was hydrophobic. Part of the dried samples was treated using the method according to the present invention in ambient-air plasma without airflow at a power density of 5 W/cm² for 1 s. The treated sample surface was situated at a mean distance of 0.2 mm from the electrode system. The electrodes of the electrode system were supplied with alternating voltage at the frequency of 25 kHz and the peak-peak magnitude of 15 kW. After 1 s of exposure, the wetting angle was less than 5°, that is, the sample surface was perfectly hydrophilic. Electric energy consumption was 0.0135 kWh/m².

Example 2

The apparatus and method according to the invention were used to improve adhesion properties of wood surface. Pinewood samples dried as in Example 1 were coated, using a roller, with phenol formaldehyde adhesive Chembond® CB 303 in the amount of 200 g/m². After one minute, the samples were cured in a laboratory press at 200⁰C and at the pressure of 2 N/mm² for 12 min. After binding, the resulting sample was stored for 1 day at ambient temperature and ambient air. The adhesion strength of the sample was measured using the ASTM D3433 1997 method as the energy necessary to create two new surfaces by tearing off the bonded parts, which was 52 J/m².

Dried wood samples were treated with plasma using the method described in Example 1 and tested for adhesion strength as mentioned above for untreated samples. Plasma activation resulted in a 158% increase of adhesion strength. Example 3

The apparatus and method according to the invention were used to make wood surface hydrophobic using the method of plasma polymerisation.

Poplar wood board with the dimensions of 21 cm x 50 cm x 5 cm was dried for 24 hours at the temperature of 75°C and ground with a fine 150 grid sandpaper. The surface of such material was strongly hydrophilic, as a drop of water was absorbed by the material in less than 1 s.

The board was surface-treated in atmospheric-pressure plasma generated in the mixture of N₂ + 4% of hexamethyldisiloxane (HMDSO). The treated sample surface was situated at a mean distance of 0.35 mm from the electrode system. The electrodes of the electrode system were supplied with alternating voltage at the frequency of 50 kHz and the peak-peak magnitude of 15 kW. The working gas flow velocity at the sample surface was about 2 m/s, the power density was 7 W/cm² and the exposure time was 30 s. Such treatment resulted in the formation, by plasma polymerisation, of a strongly hydrophilic polymer layer with the surface energy of 15 mJ/m² on the wood surface.

Example 4

The apparatus and method according to the invention were used to activate wood surface before subsequent treatment with UV radiation absorber from a solution.

Pinewood samples were activated in plasma using the method described in Example 1. A 5% solution of 2-hydroxy-4-(2,3-epoxypropoxy)benzophenone in ethanol was prepared and, in the form of an aerosol, applied to the surface of plasma-activated and plasma-unactivated samples. After drying, the samples were analysed using the FTIR-PAS method. It was found that HEPBP was covalently bound to hydroxy! groups and free radicals formed by plasma activation. On plasma- unactivated samples, such desirable bonds were not found.

Example 5

The apparatus and method according to the invention were used to activate wood surface before subsequent acetylation with acetanhydride vapours.

Pinewood samples with the dimensions of 21 cm x 10 cm x 0.5 cm were activated in plasma using the method described in Example 1. Subsequently, the plasma activated and unactivated samples were exposed to saturated acetanhydride vapours at 120⁰C for 2 hours. Example 6 The apparatus and method according to the invention were used to increase the efficiency and stability of hydrophobic wood surface treatment.

Dried samples of pinewood were submerged for 10 minutes in a 0.5 mol.dm^"3 solution of chlorotrimethylsilane (Merck) in chloroform. Subsequently they were rinsed with chloroform and dried in a stream of air. Part of the samples so silanised was treated using the method according to the present invention with plasma generated in a mixture of nitrogen and saturated water vapour at a power density of 5

W/cm² for 2 s. After 12 hours, wetting angles of plasma treated samples and plasma unexposed samples were measured. The wetting angle of plasma treated samples was 105° and that of plasma untreated samples was 83°, that is, the plasma treated samples were more hydrophobic. The plasma treated samples also showed an increased resistance of the hydrophobic treatment to ageing and biodegradation.

Example 7

The apparatus and method according to the invention were used to improve wood impregnation with melamine resin.

With the aid of ultrasound, a saturated aqueous solution of melamine resin was prepared. Fir wood samples with the dimensions of 5 cm x 10 cm x 0.5 cm were prepared and treated, on one surface, with plasma generated in a mixture of nitrogen and saturated water vapour at a power density of 5 W/cm² for 3 s. After the treatment, they were submerged for 6 hours into a melamine resin solution at ambient temperature. The samples were rinsed with distilled water and dried for 3 days at ambient conditions. Subsequently, the samples were thermally treated for 10 minutes at 18O⁰C. UV spectroscopy at the wavelength of 240 nm was used to determine the surface concentration of melamine. In plasma-treated samples, the melamine concentration was found to be increased by 250%.

List of the reference symbols used

1 - electrode system 2 - system of electrically conductive electrodes 3 - system of electrically conductive electrodes - dielectric body - wood, wood fibre layer, or wood-based material - electric plasma layer - auxiliary electrode structure

Claims

1. An apparatus for surface treatment of wood, wood fibres and wood-based materials characterised in that it includes the electrode system (1) comprising systems of electrically conductive electrodes (2) and (3) that are situated inside of the dielectric body (4) at a minimum relative distance of electrodes (2) and (3) less than 2 mm and more than 0.05 mm, located on the same side of the surface of wood, wood fibre layer or wood-based materials (5), so that the diffuse electric plasma layer (6) is generated on the part of the dielectric body surface (4), preferably above the surface of the electrically conductive electrodes (2) and (3), whereby a significant portion of the electric field lines flux, which is larger than 50% of the total electric field lines flux flowing between the electrodes (2) and (3), is not intersecting the surface of the treated material (5) that is in contact with plasma (6), the distance of the part of the dielectric body surface (4), on which plasma is generated, from the treated surface (5) is less than 1 mm, and where the plasma layer (6) is not in contact with the electrically conductive electrodes (2) and (3).

2. The apparatus according to claim 1, wherein the voltage of a frequency from 50 Hz to 1 MHz is applied between the electrodes (2) and (3) of the electrode system.

3. The apparatus according to claim 1, wherein the voltage of a magnitude from 0.5 kV to 100 kV is applied between the electrodes (2) and (3) of the electrode system.

4. The apparatus according to claim 1 , wherein the apparatus contains the auxiliary electrode structure (7) that is situated inside the dielectric body (4), the structure being part of the electrode system (1 ) and being at a potential different from that of the electrodes (2) and (3).

5. The apparatus according to claim 1 , wherein the dielectric body (4) surface, where the plasma layer (6) is generated, is situated in a working gas with the gas pressure from 1 kPa to 500 kPa.

6. The apparatus according to claim 1 , wherein the wood material, wood fibre layer, or wood-based material (5) is moving relative to the surface of the dielectric body (4), on which the plasma layer (6) is generated, at a minimum distance of less than 1 mm.

7. A method for surface treatment of wood, wood fibres and wood-based materials using the action of electric plasma, characterised in that the surface of wood, wood fibres and wood-based materials is affected by electric plasma generated using systems of electrically conductive electrodes situated inside of a dielectric body on the same side of the wood, wood fibres and wood-based materials, whereby a significant portion of the electric field lines flux, which is larger than 50% of the total electric field lines flux flowing between the electrodes, is not intersecting the surface of the plasma-treated material, where the electric plasma layer is generated on the part of this dielectric body's surface without contact with the electrically conductive electrodes, and where the minimum distance between the portion of the dielectric body surface coated with the plasma layer and the surface of wood, wood fibres and wood-based materials is less than 1 mm.

8. A method for surface treatment of wood, wood fibres and wood-based materials characterised in that a water containing solution, water containing suspension, or water containing emulsion is deposited on the surface treated according to claim 7 in the form of an aerosol, electrically charged aerosol, foam, by printing or painting.

9. A method for surface treatment of wood, wood fibres and wood-based materials characterised in that the surface of material treated according to claim 7 is subsequently exposed to a gaseous atmosphere containing monomer vapours or vapours of another organic substance.

10. A method for surface treatment of wood, wood fibres and wood-based materials characterised in that the surface of material treated according to claim 7 is subsequently coated with a layer of a polymer material using extrusion, lamination, printing, painting, spraying, or electrostatically using a powder.

11. A method for surface treatment of wood, wood fibres and wood-based materials characterised in that the surface of material treated according to claim 7 is subsequently brought in contact with the surface of another solid material.

12. A method for surface treatment of wood, wood fibres and wood-based materials characterised in that the surface of material treated according to claim 7 is subsequently submerged in a liquid.