GB2452545A

GB2452545A - Lignocellulose coated with laser fused powder

Info

Publication number: GB2452545A
Application number: GB0717463A
Authority: GB
Inventors: Sue Calver; Peter Kaczmar
Original assignee: Fira International Ltd
Current assignee: Fira International Ltd
Priority date: 2007-09-07
Filing date: 2007-09-07
Publication date: 2009-03-11
Also published as: GB0717463D0; WO2009030935A2; WO2009030935A3; GB201005796D0; GB2465736A

Abstract

A lignocellulose substrate is coated with a powder. Laser radiation is used to fuse and optionally cure the powder to form a coating. Preferably the excess powder is removed. The powder is preferably a resin, especially an epoxy polyester resin. The substrate is preferably paper, cardboard, wood, particle board, oriented strand board or fibreboard. The preferably infrared (IR) laser is focused, so as to produce a design, especially on furniture. The method can be used to produce a Braille sign.

Description

--2452545 Method for coating and applying designs to substrates The present invention relates to a method for applying a coating to, and forming designs on, wood or wood-based materials.

Wood and wood-based materials are commonly used in the building an turniture industries. Synthetic wood-based board materials typically comprise a matrix of to lignocellulosic elements (e.g. in the form of chips, particles or fibres) bonded together by means of an adhesive agent such as, for example, a polyurethane, urea/formaldehyde, melamine-urea or phenolic resin.

Examples of board materials produced in this way include medium density fibreboard (MDF), particleboard and chipboard.

Furniture and items in the building industry are often made from a variety of components made from different substrates, e.g. wood and MDF, each with different visual characteristics. Manufacturers will often try to make a surface of an item more visually appealing either by coating and/or applying designs to the surface, often requiring several different operations. There are many ways of coating substrates known to those skilled in the art, for example, painting and powder coating. The main problem with using the current techniques for applying designs is that they are generally multi-step and complex, which greatly inflates manufacturing costs, slows throughput and increases the likelihood of manufacturing faults.

A common technique for covering MDF is to apply resin-bonded paper overlays. Designs may have been pre-applied to the overlays, thus making the MDF more visually appealing when the overlay covers the MDF.

One particular method of coating a metal substrate is to use a powder coating composition. Such compositions are dvantageouz compare.d co].iqui paint, since they are virtually solvent-free and give off little, if any, volatile material during the application and curing processes. This means powder coatings contribute less to air pollution and are safer to use for workers applying the compositions, since they are not exposed to airborne solvents. Typically, a metal substrate is coated using powder coatings as follows; a dry, finely divided, free flowing, heat fusible powder is deposited on the surface of the substrate. Typical coating processes include electrostatic spray, fluidized bed, and electrostatic fluidized bed techniques, with electrostatic spray principally being used in the metal coating industry today. Following the coating step, the powder is then fused and cured on the substrate by external heating, which may be by placing the metal substrate in an oven and/or applying infrared radiation to the substrate.

Powders that are suitable for electrostatic spray coating of metals are not ideally suitable for use with temperature sensitive materials, such as lignoce].].ulose materials. The reason for this is that the powder coatings generally require melting and curing temperatures in the region of 150 to 190 °C. While it is possible to coat synthetic lignocellulose materials such as MDF (medium density fibreboard) using powder coatings and curing them either by heating the substrate in an oven and/or the application of infrared radiation, such temperatures are detrimental to the physical and chemical properties of the lignocellulose materials.

Powder coating has been used as a finish in the metal industry for over 30 years, but recently thermally cured pcwdrs have L,een used as solvent-free coatings for wood-based materials. This technology is still relatively in its to infancy and there are many common problems associated with thermally cured powder coat finishes (described below).

Blistering may be caused by too much heat in the system, either from infrared lamps or, in the case of ultra-violet cured systems, from both IR and UV lamps.

Bubbles are often found within cured finishes or at the surface of a coating and are generally caused by the presence of air in the powder as it liquefies. The air typically results from outgassing (see below) from the substrate as it is heated to melt the powder.

A milky finish is a phenomenon of clear coatings which have been applied too thickly. This produces an opaque finish. However, this problem can also result from the formulation of the powder itself.

An of f colour may be produced. The final powder finish colour can be influenced by the method of curing, especially by baking durations and curing temperatures. Of f colours can particularly result from baking too long in the final oven cure phase or by curing in an oven that is too hot.

"Orange peel" is a surface effect which occurs where cured powder finishes resemble, as the name suggests,, the texture of the skin of an orange. The main reason for this 3 phenomenon is poor flow of the powder prior to curing, i.e. the powder has not been able to melt and create a smooth finish before passing into the crosslinking stage of the process. Reasons for this poor flow include passing the finish into the curing stage too soon. I0

Outgassing is an occurrence resulting from the release of gases from the substrate and/or the powder due to heating during the flowing and curing phases. Outgassing is evidenced by bubbles and pin holing.

Pin holing is a crater effect caused by bubbles bursting prior to the surface being cured.

Scorching occurs where a surface finish has been burnt during the process. This can occur within the ovens used to melt and flow the powders (both thermal and UV systems) or during the curing phase. The scorching can occur under both IR and UV lamps.

e25 Warping of temperature sensitive substrates, such as wood-based panels, may occur when the heat required to melt, flow and (in the case of thermal powders) cure the coating is concentrated on one surface.

Recently, powders with lower curing temperatures have been developed specifically for application to MDF materials. An example of such a powder coating may be found in EP 0 806 459 A2, which discloses a coating composition comprising imidazole and a polyamine with a curing temperature in the region of about 95 to about 110°C. Other references can be found in European Patent 371528 and United States Patent 5124387. It has been found that the low-curing-temperature compositions generally do not form a coating that is as hardwearing as the high-temperature-curing compriticn. Additioally, the use of a powder coating on natural].ignocellulose materials such as wood has to not generally been successful due to outgassing.

A further disadvantage of the use of current powder coating techniques is that it is only possible to Coat surfaces in a uniform colour. The use of powder coatings to form even simple designs on products has not been feasible on a commercial scale.

The present invention aims to overcome or at least mitigate at least some of the problems associated with the

prior art.

The present invention will now be further described. In the following passages different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

The present invention provides a method of forming a coating on a lignocellulose substrate, the method comprising providing a lignocellulose substrate with a fusible powder disposed on a surface thereof and applying laser radiation to fuse and, optionally, cure the powder to form a coating.

Figure 1, provided by way of example, shows a lignocellulose substrate provided with a coating and a design according to the method of the present invention.

The present inventors have found that the use of laser radiation to fuse and, optionally, cure a powder coating on a lignocellulose substrate has number of surprising advantages over simply heating the substrate in an oven and/or the application of infrared radiation. Because of the high intensity of the laser radiation, it has been found that the heat-curable powder requires only a very short period of exposure to the radiation for it to fuse and optionally cure, in the order of seconds or milliseconds, rather than minutes. Additionally, it has been found that is possible to focus the laser radiation so that only the surface is heated, in contrast with conventional powder coating methods, thus preventing deterioration of the interior of the substrate.

Furthermore, because the laser radiation can be highly focussed, it is possible to apply the laser radiation to a specific pre-determined part of the surface to form a design in the coating, i.e. cure only selected portions ot the powder on the surface with the laser. Very sharp lines can be produced in this manner. Following creation of the design, the powder that has not been exposed to laser radiation, and hence has not been fused, can be easily removed from the surface, leaving the fixed design formed from the fused powder on the surface.

The laser radiation may be applied to a specific pre-determined part of the surface to form a design in the coating. The designs are created using computer software packages. The machine producing the laser may be programmed using known techniques to scan the surface in a certain design. I0

The method may further comprise removal of non-fused powder from the surface following application of the laser radiation. This removal may be performed by conventional means.

The fusible powder may comprise a thermosetting material and/or a thermoplastic material. Thermosetting materials are known in the art and are materials in which the constituent molecules crosslink on the application of heat or radiation. Thermoplastic materials are known in the art and are materials, which melt, but do not undergo a chemical reaction, on the application of heat. Particles of the powder made from a thermoplastic material will, on application of the laser in the method of the present *25 invention, melt and fuse together, thus forming the layer.

The layer formed may be then be allowed to harden, either by allowing the substrate to cool, by applying further heating or laser radiation, or other techniques known in the art.

The powder may comprise any powder suitable for forming a coating on the application of heat or radiation. For example, the powder may be curable by heat and/or ultraviolet radiation. The powder may comprise a thermosetting material selected from, but not limited to, polyester urethane, polyester trig].ycidyl isocyanurate (TOIC), acrylic urethanes, acrylic hybrids (acidic functional resins), glycidyl methacrylate acrylate (GMA), epoxy functional resins, epoxy-based materials and epoxy/polyester coating powders. The powder may comprise a thermoplastic material selected from, but not limited to, polyamides, polyolef ins, polyvinylchloride-based materials, polyester materials and polyvinylidene fluoride. The powder may optionally comprise a blend of two or more of the above materials. Most preferably, the powder comprises epoxy polyester resins.

IS The heat-curable powder may be applied to the substrate by any of the processes known in the art. This may be as described in EP 0 806 459 A2. This document describes the use of electrostatic spraying of powders using the principle of electrostatic charging, using either the corona method or the triboelectric method. In the corona method, a charge is passed to the powder particles from ionized discharged air within the corona spray gun and the particles are then ejected from the gun onto the target object. In the triboelectric method, the powder particles are charged using friction by passing them over a surface, such as polytetrafluoroethylene, and then they are ejected from an electrostatic spray gun -this is generally the preferred method. The substrate may be preheated as required to increase adhesion of the particles to the surface.

Alternatively the particles may also be applied using the fluidised bed method in which the substrate is passed into a fluidized bed containing the powder.

"Lignocellulose substrate" means a substrate that comprises a lignocellulose material. The lignocel].u].ose substrate may comprise a material selected from paper, cardboard, wood, particleboard, OSB (Oriented Strand Board) and fibreboard. Preferably, the material comprises Medium Density Fibreboard.

The powder may be disposed directly on the surface of the lignocellulose material of the substrate or, alternatively, other (non-lignocellu].ose) material may be disposed between the lignocellulose material and the powder.

The other material may be in the form of a layer and may, for example, be produced using powder coatings, optionally is by using the method of the present invention or using conventional techniques, or they may be produced using painting techniques or a surfacing foil The wavelength of the laser is preferably selected so that it is readily absorbed by the powder being employed, causing it to fuse and optionally cure. Typically commercially available sources are employed e.g. 355nm frequency tripled YAG lasers in the UV band, l.O6um wavelength YAG lasers that are absorbed by dark coloured coatings and lO.6um wavelength CO2 lasers are absorbed by nearly all polymers. The focussing of the laser radiation is within the skill and knowledge of the skilled person.

The intensity of the laser beam can be varied by varying the laser power and/or by varying the size of the laser beam on the powder. Typically a smaller beam size, suitable for creating fine detail, is obtained by moving the -10 -focus close to the powder surface while a larger size, suitable for covering larger areas, is obtained by moving away from this position by a controlled amount. In addition the interaction of the laser beam and powder can be varied by altering the speed of the beam's traverse. Combinations of these parameters are selected so that the powder is preferably fused and optionally cured to the required standard whilc avoiding overexposure and resultant material degredat ion. I0

Optionally a stationary laser beam can be projected over an area and used to fuse and optionally cure the powder. The use of appropriate power levels allows this to be achieved rapidly, normally in a sub-second interval.

Controlling the beam shape, by the use for example of masks, preferably projected, enables designs to be produced.

The present invention further provides a lignocellulose substrate having on a surface thereof at least one coating formed by the application of laser radiation to a fusible powder disposed on the surface. The present invention further provides a lignocellulose substrate having a design on a surface thereof, the design having been formed from at least one coating formed by the application of laser radiation to a fusible powder disposed on the surface. The at least one coating may have been formed using a method of the present invention.

The present invention further provides an item of furniture, a wall or floor panel or a Braille sign comprising a lignocellulose substrate having on a surface thereof at least one coating formed as herein described in -1]. -accordance with the present invention. The present invention still further provides an item of furniture, a wall or floor panel or a Braille sign comprising a lignocellulose substrate having a design on a surface thereof, the design having been formed as herein described in accordance with the present invention.

An item cf furniture or a wail or tloor panel according to the present invention may be preferably visually appealing and preferably hardwearing. Furthermore, it can be cheap to produce due to the reduced number of manufacturing steps compared to known methods. A Braille sign made according to the present invention may be easily formed from several layers of a specific design. Such signs may be preferably cheap to produce and preferably hard wearing. Such signs could be produced quickly, the design pattern being controlled by a computer program.

The process of the present invention allows intricate designs to be applied directly onto wood-based substrates using the coating polymer as the "ink" medium and has potential application in adding value to wood commodities through the application of designs which can be purely decorative or functional (Braille, barcoding etc) in nature.

The present invention will now be described further with reference to the following non-limiting example.

Example

One embodiment of the invention utilises a solid state YAG laser capable of producing up to 70 watts of output -12 -power at a wavelength of l.O6um. The output beam is steered over the electrostatically powder coated wood, MDF or other lignoce].lulose substrate by a pair of computer controlled mirrors and a focussing system. When fine detail is required the focus is arranged to be at the surface and the beam moves at typically 50 to 220mm/sec, depending upon the powder absorption characteristics. After each line the beam stepc cvcr by typically 0.1mm and trie next line produced, the process being repeated until the required shape is created. For area coverage the focus is raised by 60mm to defocus the beam to produce a smoother finish, lowering the focus producing similar results. The laser power is adjusted to produce the best visible results and depends upon the powder absorption characteristics being typically in the 20 IS to 35W range. These parameters are examples and there are many other combinations of parameters and laser types possible.

A working example of providing a design onto a standard grade MDF substrate will now be described. This example is shown in figure 1.

A wasp emblem was produced by first laying down a yellow background layer on standard grade MDF and then as superimposing a black wasp outline layer. Both layers were produced using an electrostatic gun to apply the appropriate coloured powder coating followed by exposure to a computer controlled scanned YAG laser beam of l.O6um (near infrared) wavelength. However, the parameters used for the two layers were different as will be explained.

-13 -Absorption of the YAG laser is relatively low in paler powders making the production of a defect free yellow base layer difficult as the laser beam tends to be preferentially absorbed by surface features on the MDF. To overcome this problem the base laser processing was done with the laser focused 60mm above the sample surface. The resultant wider beam offered a less concentrated heat input which reduced the tendency tc thtaract WILIi surface features. The laser beam was moved at 100 mm/s with a step-over distance of 0.1 mm between adjacent passes. Laser lamp current was 18.6 A (equivalent to a power of about 30 Watts) in continuous mode.

After application of the continuous base layer the black powder was applied by electrostatic "spray" gun and the black wasp outline was fused into the black layer (see figure 1).

The powder that formed the black wasp outline absorbed the laser light strongly. As the design required detailed features to be produced, the processing was carried out with the laser beam focused at the surface of the MDF. The laser power was reduced to limit burning caused by the intense energy density of the resultant approximately 0.2 mm diameter focussed spot. The laser beam was moved at 100 mm/s with a step-over distance of 0.1 mm between adjacent passes but the lamp current was reduced to 14 A (equivalent to a power of about 12 Watts) in continuous mode.

Unf used powder was removed from the surface of the specimen using a compressed air line. Unfused powder -14 -collected from the surface in this manner was recovered for re-use.

Claims

-15 -CLAIMS: 1. A method of forming a coating on a lignocellulose substrate, the method comprising: providing a lignocellulose substrate with a fusible powder disposed on a surface thereof, and applying laser radiation to fuse and, optionally, cure the powder to form a coating.
2. A method according to claim 1, wherein the laser radiation is applied to a particular region of the surface, to fuse or optionally cure the powder in this region.

IS
3. A method according to claim 1, wherein the laser radiation is applied to a specific pre-determined part of the surface to form a design in the coating.
4. A method according to any of the preceding claims, further comprising removal of non-fused powder from the surface following application of the laser radiation.
5. A method according to any of the preceding claims, wherein the heat-curable powder comprises a ?25 thermoplastic material and/or a thermosetting material.
6. A method according to claim 5, wherein the thermosetting material comprises an epoxy resin.
7. A method according to claim 5, wherein the thermoplastic material comprises a polyester resin.

-16 -
8. A method according to any of claims 1 to 5, wherein the heat-curable powder is. an epoxy polyester resin.
9. A method according to any one of the preceding claims, wherein the lignocellulose substrate comprises a material selected from paper, cardboard, wood, particleboard, OSB (Oriented Strand Board) and fibreboard.
10. A method according to claim 9, wherein the lignocellulose substrate comprises Medium Density Fibreboard.
11. A method according to any of the preceding claims, wherein the laser radiation has a frequency in the infrared range of the electromagnetic spectrum.
12. A method according to any of the previous claims, wherein the laser radiation is focussed on the surface.
13. A method according to any of claims 1 to 12, wherein the laser radiation is focussed above or below the surface.
14. A lignocel].ulose substrate having on a surface thereof at least one coating formed by the application of laser radiation to a fusible powder disposed on the surface.
15. A lignocellulose substrate having a design on a surface thereof, the design having been formed from at least one coating formed by the application of laser radiation to a fusible powder disposed on the surface.

-17 -
16. A lignocellulose substrate according to claim 14 or 15, wherein the at least one coating has been formed using a method as defined in any one of claims 1 to 13.
17. An item of furniture comprising a lignocellulose substrate as defined in any of claims 14 to 16.
18. A Braille sign comprising a].ignocel].ulose substrate having a design on a surface thereof, the design having been formed using a method as defined in any one of claims 1 to 13.
19. A wall or floor panel comprising a lignocellulose is substrate as defined in any of claims 14 to 16.
20. A method for forming a coating on a lignocellulose substrate as substantially described herein.