WO1998048989A1

WO1998048989A1 - Method for modifying wooden surfaces

Info

Publication number: WO1998048989A1
Application number: PCT/DE1998/000953
Authority: WO
Inventors: Michael Panzner; Günther WIEDEMANN; Andreas Lenk
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 1997-04-30
Filing date: 1998-04-02
Publication date: 1998-11-05
Also published as: EP0991511A1; DE19718287C1; AU8009198A

Abstract

The invention relates to a method for modifying wooden surfaces, especially by removal of layers. According to the invention, the wooden surfaces are modified thermally using high-energy radiation. This modification occurs by means of thermal absorption. During a short period of exposure of an area of the surface of the wood to the high-energy radiation, a thin layer of said wood is heated abruptly, so strongly that most of the volume of wood thus heated vaporizes or is converted into plasma just as abruptly.

Description

Process for modifying wooden surfaces

The invention relates to the field of processing or wood finishing of materials or components made from the ecological and renewable raw material wood. One area of application of the invention is, for example, the gluing and coating of wood, where the use of the invention leads to higher strengths and durability. The general area of application is all branches of industry that deal in any way with wood and wood processing and processing. In addition to conventional woodworking and processing industries, the restoration of components, building and works of art etc. made of wood or with a wood surface, for example by removing damaged wood surface layers or parts thereof, is also counted.

The durability of protective coatings on wooden surfaces and the strength of adhesive bonds between wooden parts can be significantly increased if the coating material or the adhesive can penetrate deeper into the wooden surface. This is prevented in the case of mechanically processed wood surfaces by a layer of destroyed wood structure, which mainly consists of compressed wood fibers. There was therefore a need to remove this layer with destroyed wood structure.

To remove these layers, it is proposed in patent application WO 95/25621 to use, depending on the type of wood, pulsed light of such high intensity and wavelength for irradiating the surface that covalent bonds can be broken with a sufficient number of pulses, and thereby the layer described can be removed so far that the cell structure becomes visible. The method has the disadvantage that the mechanism of removal by breaking covalent bonds only by means of pulse lasers of extremely high pulse power (breaking of the bonds by multiphoton processes) or high pulse power and UV wavelengths (where the photon energy is in the order of magnitude of the binding energies of the molecules) and with Pulse numbers of approximately 100 shots per unit area can be triggered. Such lasers are currently only available with limited average powers and are very expensive to operate and difficult to integrate into mechanical woodworking machines.

In addition, this method is inconceivable, for example, for the restoration of components, buildings and works of art made of wood or with a wooden surface. The invention is based on the object of proposing a method for modifying, for example machining, in particular for removing or removing layers of wood surfaces, by means of which the disadvantages of the prior art are eliminated.

Accordingly, it is an object of the invention to provide a method of the type mentioned, which is considerably cheaper in operation, which can also be easily integrated into mechanical woodworking machines, and which can also be used for the restoration of components, building and works of art made of wood or with a wooden surface .

According to the invention, these objects are achieved with a method as described in claim 1. Claims 2 to 20 represent advantageous embodiments.

The process for modifying wooden surfaces, in particular for removing layers of wood on the wooden surface, is carried out using high-energy electromagnetic radiation.

According to the invention, this modification of wooden surfaces takes place through thermal absorption. With the help of a powerful energy source, preferably a laser source (e.g. C0 ₂ -cw laser), with which high-energy electromagnetic radiation is generated, focused and directed onto the surface of the wood surface, so that during a short exposure time, preferably <1 ms , the wooden surface is suddenly heated so strongly in a thin layer that the essential part of the volume thus heated is also evaporated suddenly or converted into plasma. In this case, the partial active surface of the energy source on the wooden surface, which is small compared to the processing surface, is continuously guided over this or the desired segments or contours thereof.

In the method according to the invention, a laser ablation process is used to remove the layer on the wood surface, for example the layer with a destroyed wood structure. In contrast to the prior art (WO 95/25621), in which the removal takes place by photochemical decomposition (direct breaking / breaking of the covalent bonds) of the material wood or wood molecules on the surface, a thermal process is carried out in the method according to the invention. In the method according to the invention, the removal is carried out by thermal absorption via the excitation of phonons in the different wood molecules and subsequent thermal removal process. The coupling of the radiation, in particular of laser radiation, causes the surface to heat up rapidly, which mainly leads to partially explosive evaporation with plasma formation. Prerequisites for a removal free of damage to the remaining wood surface are short ones Exposure times to this radiation, which prevents a deep thermal effect with disadvantageous phenomena, such as partial melting and especially charring. Such short exposure times can be achieved either by a rapid relative movement between the laser beam and the workpiece and / or by using lasers with short pulses. The rapid relative movement can be achieved by rapid movement of the wood under a laser beam, for example a cw laser beam, or by a rapid movement of the laser beam, for example a cw laser beam, over the wooden surface or by a combined movement. The lasers used should preferably work in the infrared spectral range, since there is a high optical absorption of the wood and the absorbed laser radiation is largely converted into thermal energy, which is used for evaporation. The laser radiation can be focused or unfocused depending on the respective application.

In contrast to the method shown in WO 95/25621, the removal according to the invention is brought about by a thermal process (heating, evaporation). Preferred lasers are therefore C0 ₂ lasers (cw lasers), which can also work continuously (not pulsed), since the method according to the invention opens up the possibility of realizing the short exposure times required and thus also in continuous operation by means of rapid relative movements work without damage in continuous operation.

The method according to the invention thus enables the use of C0 ₂ -cw lasers which are available with a high average power, are considerably cheaper to operate and can be easily integrated into a mechanical woodworking machine. This significantly improves the process described in patent application WO 95/25621 and enables the process to be used economically in the wood industry.

With the method according to the invention, all mentioned disadvantages of the prior art are eliminated.

The method according to the invention is to be explained in more detail with reference to the exemplary embodiment below.

Embodiment

The variant of the method according to the invention shown in this exemplary embodiment is outlined in FIG. 1. The focused beam of a C0 ₂ -cw laser 1 is guided by means of a scanner mirror 2 so quickly perpendicular to the longitudinal axis of the board over the wooden surface of a board 3 to be processed that the short exposure times arise. The laser beam is focused by a focusing mirror 4 in such a way that the power densities required for removal are achieved over the entire width of the board. In the longitudinal direction of the board, the board feed 5 takes place at such a speed that removal by multiple scanning with the laser beam with track overlap takes place, which frees the wooden surface of residues of the mechanical processing.

List of the reference symbols and terms used

1 laser beam

2 scanner mirrors

3 wooden surface to be processed (board)

4 focusing mirrors

5 board feed

Claims

claims

1. A method for modifying wood surfaces, in particular for layer-by-layer removal of wood on the wood surface, by means of high-energy radiation, characterized in that this modification is carried out by thermal ablation, the high-energy radiation being generated with the aid of a powerful energy source, bundled and focused on the active surface is directed onto the wood surface and absorbed in it, so that during a short exposure time the wood surface is suddenly heated so strongly in a thin layer that the essential part of the volume thus heated is also evaporated suddenly or converted into plasma, the opposite of which to be machined, the small surface of the energy source on the wooden surface is guided over this or the desired segments or contours.

2. The method according to claim 1, so that the thermal absorption takes place via the excitation of phonons in the different wood molecules and the subsequent thermal removal process.

3. The method according to claim 1 or 2, characterized in that the guidance of the small partial surface of action of the energy source on the wood surface or segments or contours is carried out by the beam of the energy source itself and / or the wood surface are moved accordingly.

4. The method according to one or more of claims 1 to 3, characterized geke nn ze ichn et that high-energy electromagnetic radiation of such a wavelength is generated, bundled and directed with the powerful energy source that it by the components of the wood very well, that means is already absorbed in a very thin layer.

5. The method according to claim 4, so that the energy-rich radiation is an infrared (IR) radiation.

6. The method of claim 5, d ad u rch g e ke n n ze i ch n et that the high-energy radiation has a wavelength λ> 5 microns.

7. The method according to claim 5 or 6, characterized in that a powerful IR source is used as the energy source.

8. The method of claim 7, d ad u rc h g e ke n n ze i c h n et that the powerful IR source is an IR laser.

9. The method according to claim 6 and 8, characterized in that an IR laser with a wavelength λ> 5 microns is used.

10. The method according to one or more of claims 1 to 9, d a d u r c h geke n n I n et that the exposure time of the energy-rich, focused and directed radiation on the wood surface is <1 ms.

11. The method according to one or more of claims 1 to 10, characterized in that the energy provided during the contact time per partial contact area is> 0.5 Jcm ^"2 .

12. The method according to one or more of claims 1 to 11, characterized in that the powerful energy source generates a pulsed radiation.

13. The method according to claim 12, d ad u rch geken nzeich n et that the powerful energy source is a pulse laser.

14. The method according to claim 12 or 13, characterized in that the pulse duration of a pulse corresponds to the desired exposure time of the radiation on the wood surface.

15. The method according to claim 14, d ad u rch g eke n n I n et that the pulse duration of a pulse is <1 ms.

16. The method according to one or more of claims 12 to 15, characterized in that the continuous guidance of the active surface of the radiation on the wooden surface via this or segments or contours thereof takes place by the energy beam over the wooden surface and / or the wooden surface in each case be moved alone or with each other in such a way that with each radiation pulse a new surface is created on the wooden surface, which directly adjoins the previous one or overlaps it so that there is a fixed number of pulses per unit area, and the entire desired period during the entire processing period Wood surface or the segment thereof or the contour is traced with the energy beam.

17. The method according to one or more of claims 1 to 11, characterized in that the powerful energy source generates continuous radiation.

18. The method according to claim 17, so that the energy source is a continuously radiating laser (cw laser).

19. The method according to claim 17 or 18, characterized in that the short exposure times, preferably> 1 ms, are achieved on the wooden surface in that the energy beam is moved one or more times over the wooden surface or segments or contours thereof and / or the wood surface itself, or if necessary the wood surface according to the desired segments or contours to be machined, the respective movement speeds being determined by the required duration of action of the energy beam in the action surface on the wood surface.

20. The method according to one or more of claims 1 to 19, characterized in that a pulsed or continuously radiating C0 ₂ laser is used as the energy source.