EP1062053B1 - Method for powder-coating - Google Patents

Abstract

The invention relates to a method for powder-coating a substrate (5), especially temperature-sensitive substrates such as wood, wood-fibre materials, plastic materials, rubber, woven fabric, paper or cardboard. According to said method a heat-sensitive powder is deposited as a base coat (6) on the uncoated surface of the substrate (5). The powder is then either heated continuously to cross-linking temperature by means of infrared radiation having at least fractions in the near and/or short-wave infrared range, and hardened; alternatively, the powder is continuously heated to gelling temperature, cross-linking is completed in a subsequent step and the powder is then hardened. To generate the infrared radiation the invention provides for the use of halogen lamps (7) in combination with a reflector (8) for reflecting the radiation emitted in the direction of the substrate. The halogen lamps (7) are operated in such a way that a maximum radiation flux density of the emitted radiation is in the near infrared region.

Description

The invention relates to a method for powder coating a substrate, especially a temperature sensitive one Substrates such as wood, wood fiber material, plastic, rubber, Fabric, paper or cardboard according to the preamble of claim 1. The invention further relates to the use of a halogen lamp for powder painting. A method of this type is, for example, from W0-A-92 / 0.1517 or from GB-A-2 056 885. When it comes to crosslinking and curing powder coating, it happens decisive for the most homogeneous and rapid warming possible to the curing temperature. This is the only way to melt the powder coating reach the viscosity minimum without being substantial to be prevented from progressing by crosslinking reactions, what an unevenness of the surface due to a non-optimal Course of the powder.

A method for crosslinking thermoreactive powder is known, at which the necessary curing temperature over multi-stage Energy transfers is achieved. First is infrared (IR) radiation or convective the surface of the powder coating heated. Then the warming takes place in the powder layer via heat conduction processes up to Substrate interface. There is the energy, especially at metallic substrates, because of the higher heat conduction dissipated into the substrate much faster. Only at approximately complete warming of the substrate reaches the boundary layer the necessary cross-linking temperature. In this well-known Process provides for heating the coating only the temperature gradient between the coating surface and substrate is the driving process variable. Um homogeneous networking and perfect adhesion on the To ensure substrate, heating times are several minutes necessary.

The crosslinking and curing temperatures are often below Powder coatings between 120 ° C and 300 ° C. Because of this high Temperatures can be temperature sensitive substrates after the known methods not powder-coated or only with restrictions become.

A method for crosslinking and curing is also known a layer of thermoreactive powder on a substrate a primer before applying the thermoreactive powder is applied to the surface of the substrate. The primer consists, for example, of water-based paint. The Priming is particularly useful for substrates made of wood or Wood fiber materials, inhomogeneities on the surface to compensate for the substrate, a moisture barrier form and allow adhesion of thermoreactive powder. The powder can then be irradiated with electromagnetic radiation, especially with medium wave Infrared radiation can be networked and cured. In this known method, the primer also forms a heat conduction barrier that prevents heat transfer during the crosslinking reaction in the powder layer on the substrate with special needs. Especially with temperature sensitive substrates was able to apply a powder coating at all be made possible. However, this is known method limited to the use of thermoreactive powders, their cross-linking temperature is only slightly higher than the damage temperature of the substrate.

In the case of substrates containing or absorbing moisture, in particular with wood or wood fiber material, exists at the known methods also have the problem that a minimum moisture content the substrate is desired on the one hand on the other hand, the application of a uniform powder coating however prevented. Allows moisture in the substrate on the one hand, thermoreactive through electrostatic charging Deposit powder on the loaded surface. on the other hand evaporates the moisture in the subsequent Crosslinking and curing reaction in the substrate because because of the long reaction time at temperatures above the evaporation temperature the substrate at least on its surface is heated to evaporation temperature. It is formed therefore on the surface, under the already cross-linked powder Bubbles that lead to an irregular layer of paint. Also a primer layer does not help here as it does not form a permanently effective heat conduction barrier and there the evaporation temperatures are usually much lower than the crosslinking and curing temperatures of the thermoreactive Are powder. In addition, for example with a primer made of water-based paint, only until the Be primed until the primer is a Powder coating layer can be applied.

The known methods mentioned above continue to exist the difficulty that because of the low depth effect of the Powder coating heating only after a longer heating period a fusion bond between the powder layer and the Substrate surface or the primer are produced can.

GB-A-2 056 885 describes a process for powder coating of substrates made of wood, cellulose, paper, cardboard or similar known in which a thermoreactive powder on the applied uncoated surface of the substrate and by means of infrared radiation, preferably in the wavelength range between 1 µm and 5 µm, is cured. The in the Irradiation times specified in the publication are in the range between 20 s and 2 min.

From the publication "Infrared (IR) and its industrial Application "RUBBER AND RUBBER - PLASTICS., Vol. 36, No. 10, October 1983 (1983-10), pages 899-901, XP002110442 DR. ALFRED HUTHIG VERLAG GMBH. HEIDELBERG., DE ISSN: 0948-3276 it is known, so-called light emitters, d. H. with a Reflector provided in the area of the near or short-wave Infrared radiating quartz halogen lamps, in industrial thermal processes. The publication describes advantages of these heat sources and certain Fields of application.

WO 92/01517 A describes a method for coating a heat-sensitive material described with powder coating, which a two-stage procedure for infrared treatment of the applied paint. In a first, short step becomes a physical one at high temperature State change, while in a second, much longer step at reduced temperature Paint is cured.

The Patent Abstracts of Japan vol. 095, no. 007, August 31 1995 (1995-08-31) & JP 07 092831 A (KIYOTSUKOU SEIKO KK), 7. April 1995 (1995-04-07) describes simultaneous radiation of a body with radiation from far away and the near infrared associated with toner fixation in an electrophotographic printer.

The invention has for its object a method for Powder coating of a substrate, especially a temperature sensitive one Substrates such as wood, wood fiber material, Plastic, rubber, fabric, paper or cardboard to indicate that a powder coating of the uncoated surface of the substrate allowed without damaging it, and that to one uniform, completely cross-linked and well adhering lacquer layer leads.

The task is accomplished through a process with the characteristics of Claim 1 and by the use of a halogen lamp for Fuherlackier according to one of claims 1 to 9 according to solved the claim 10.

Further training is the subject of the dependent Expectations.

An essential idea in the method according to the invention for powder coating is that for crosslinking necessary and targeted energy throughout the entire Powder layer thickness in the as the base layer on the uncoated Amount of powder applied to the surface of the substrate is introduced. The gelation or crosslinking energy is at least in the form of radiation energy Base layer introduced and absorbed there. The one used Radiation has at least radiation components in the near and / or short-wave infrared. Preferably the Powder layer and the substrate surface through near infrared radiation (NIR radiation) homogeneous and in a matter of seconds the required gelling or crosslinking temperature is heated. Under near infrared, the wavelength range becomes more electromagnetic Radiation between the visible range and about 2 µm Understood wavelength.

According to the invention, the infrared radiation makes the thermoreactive Powder either warmed to cross-linking temperature and cured, or warmed to gel temperature and only crosslinked in a later process step and cured. In the latter case, gelling results a composite of the powder material without a complete Crosslinking or curing to form a layer of lacquer. Here the powdery base layer and 99fs. line second layer no longer than 12 s, in particular irradiated for no longer than 8 s until hardening. After applying a second layer, however due to radiation penetrating into the base layer Irradiation of the base layer continued, so that the total irradiation time the base layer is longer than 12 or 8 s can.

The targeted, preferably homogeneous across the thickness of the base layer distributed introduction of energy using infrared radiation, especially NIR radiation, speeds up the process the connection or crosslinking of the powder particles considerably compared to the known method in which the energy input due to the depth of the base layer essentially of heat conduction takes place. This is also an excellent one Controllability of the connection or networking process given, especially because of a control of the Radiation flux density, the spectral distribution of radiation energy and / or the duration of radiation exactly the desired Process progress can be controlled. Is cheap it if the aforementioned process parameters depend on the absorption properties of the thermoreactive powder, on the reflective properties the substrate surface and the thermal conductivity of the substrate can be adjusted.

Furthermore, the rapid continuous heating of the Base layer ensures good adhesion to the substrate surface.

Preferably, the cured or pre-gelled Base layer a second layer of a thermoreactive powder is applied and the whole is not yet fully cross-linked Coated by means of infrared radiation and cured. In a further development of the method after the base layer has hardened or gelled Gel temperature or curing temperature cooled, preferably by compressed air that flows towards the surface this flows along. In an alternative embodiment is the second layer immediately after curing or Pre-gelling applied.

Through the second layer, with its application and curing the painting process is terminated, in particular uniform paint surface are generated, the highest quality requirements equivalent. In particular, the second layer compensated for irregularities in the base layer, which, for example, ensures a consistently uniform can achieve glossy or matt lacquer surface. in the Difference to known powder coatings with UV powder coatings can be used with both the first and the second also achieve matt powder coating surfaces. Across from Processes in which a primer coat and a second, Powder coating made of different materials can exist, especially when using similar Powder for the base layer and the second layer a particularly homogeneous and across the depth of the overall finish form a uniformly crosslinked lacquer layer. Advantages of this Powder coating systems are therefore particularly included robustness, abrasion resistance and chemical resistance the paint job.

With the two-layer variant of the method according to the invention can especially substrates such as wood and wood fiber Materials (in short: wood fiber materials) with high coating quality be powder coated. On the one hand, by the targeted control of networking and Curing process can be prevented that moisture bubbles Generate irregularities in the paint layer. on the other hand the problem of uneven adhesion of powder particles on an uncoated, at least partially overcome surface formed by wood fibers. Through the Base layer, an adhesive layer is formed, which may still has an irregular surface or even from individual, not connected island-like Paint stains exist. After curing or pre-gelling the The base layer then has a lot for the second layer better starting conditions. Liability is improved and it is therefore usually when the powder is applied second layer applied more material. In the subsequent Networking and curing of the whole not yet networked or only partially cross-linked coating material then the coating material runs to a uniform Paint layer.

In particular, the controllability of the process progress is to be further increased if the process is further developed the surface temperature of the thermoreactive powder a pyrometer is measured and controlled by the radiation flux density regulated the infrared radiation. Thus defined temporal temperature profiles of the powder coating run be, e.g. B. with steep temperature rise and subsequent Phase of constant temperature over time Crosslinking process just above the minimum crosslinking temperature continue until fully cured.

Preference is given to generating infrared radiation a high-performance halogen lamp with a beam temperature of more than 2500 K. Such sources of radiation generate very high electromagnetic radiation Radiation flux densities, which in particular allow the crosslinking temperature can be reached within a few seconds. Preferably are incandescent in the halogen lamp, in particular Heating coils, used with low mass, so that the radiation flux density is responsive controllable. Especially a preferred embodiment is the halogen lamp with a reflector to reflect the emitted radiation in the direction of the substrate combined and the halogen lamp is operated in such a way that a radiation flux density maximum of the emitted Radiation is in the near infrared. The surface temperature of the filament can be set up to 3500 K. preferred dimensions line-like halogen lamps in combination with channel-like ellipsoidal or parabolic reflectors used.

The uncoated surface of the substrate, especially made of plastic, a pretreatment for Improve conductivity for an electrostatic Application of the thermoreactive powder subjected. In particular Design is done on the surface of the substrate an electrically conductive liquid is applied.

In particular for powder coating a moisture-containing or receiving substrate by drying and / or moistening the substrate before applying the Base layer generates a defined moisture content. Consequently can achieve particularly uniform powder coating can and can determine the process parameters in certain Limits vary without reducing the coating quality.

Preference is given as a, in particular exclusive, pretreatment before powder application to dry damp Substrates, such as wood or wood composite materials, the substrate surface with the same or higher than for the irradiation necessary energy input irradiated, especially by NIR radiation. Through this energy input a surface temperature is reached which is above is the melting point of the powder system. Then will then the thermoreactive powder as a base layer on the substrate surface applied. The thermoreactive powder melts immediately and is continued if necessary Radiation cross-linked. By pretreating the substrate surface the order efficiency during the Powder application increased many times over. At the same time prevents that during the actual networking process expelled moisture deposited on the substrate surface that could disrupt homogeneous film formation.

Fig. 1

eine mitteldichte Faserplatte (MDF) mit zwei Pulverlackschichten und

Fig. 2

eine Anordnung zur Vernetzung von Pulverlack auf in sich geschlossen umlaufenden Oberflächen eines Kunststoffsubstrats.

Exemplary embodiments of the invention will now be described with reference to the accompanying drawing. However, the invention is not restricted to these exemplary embodiments. The individual figures of the drawings show:

Fig. 1

a medium density fibreboard (MDF) with two layers of powder coating and

Fig. 2

an arrangement for crosslinking powder paint on self-contained circumferential surfaces of a plastic substrate.

The substrate shown in Fig. 1 consists of a medium density Fiberboard (MDF) 1 with a base layer of thermoreactive Powder and also from a second layer was coated from thermoreactive powder. This was the MDF 1 grounded on the side not to be coated and it the thermoreactive powder of first lacquer layer 2 on the uncoated surface of the MDF 1 applied. Then the base layer was coated with Infrared radiation from a radiation source whose Radiation flux density maximum is about 1 µm wavelength, Irradiated for 5 s until the temperature of the powder rises Setting temperature has increased. This, about the thickness of the first Lacquer layer 2 approximately homogeneous temperature was maintained for approximately 1 s. The irradiation process was then stopped.

The substrate was only on during the gelling process its surface and only slightly warmed, so that in the MDF 1 bound water did not leak on the surface is and the uniformity of the paint coating is not was disturbed.

After cooling, the MDF 1 was on the uncoated Grounded side and it was thermoreactive via the tribo process Powder for the second lacquer layer 3 on the surface the first lacquer layer 2 applied. Then were for about 6 s the first 2 and the second 3 layers of paint with the Infrared radiation at a radiation flux density maximum with irradiated at a wavelength of about 1 µm until the crosslinking temperature was reached. Through continued radiation with lower radiation flux density over about 3 s was the crosslinking reaction until complete curing both layers of paint continued. After that, the radiation canceled and waited a few seconds until the layers of paint cooled significantly below the crosslinking temperature had. Also through the second irradiation process No vapor or gas bubbles are formed, leading to an irregularity of the paint coating could have resulted.

In further experiments, MDF (not shown) was also used Surface contours immediately after pre-drying coated by NIR radiation. Here too even with single-layer powder coatings achieved with uniform thickness and smooth surface.

2, a hollow cylinder 5 made of plastic is shown, which is irradiated by a total of 3 halogen tube lamps 7. The hollow cylinder 5 consists for example of acrylonitrile-butadiene-styrene (ABS), made of polypropylene (PP) or polyetylene (PE). For the powder coating of its outer cylinder surface For example, as for MDF, polyester resin powder, Epoxy or an epoxy / polyester powder used.

2 are the halogen tube emitters 7 and a reflector 8 combined with them can be seen. By the reflector geometry is one over the length of the hollow cylinder 5 ensures uniform radiation. In one variant 2 extend the Halogen tube spotlights and the channel profiles of the reflector approximately parallel to the axis of rotation of the hollow cylinder.

The hollow cylinder 5 has a lacquer layer 6 made of thermoreactive Powder on. To apply the lacquer layer 6 was the The surface of the hollow cylinder 5 is first sprayed with isopropanol. The isopropanol layer was then grounded and applied the thermoreactive powder. Then started the irradiation with infrared radiation from the halogen tube lamps 7, the hollow cylinder 5 with a rotation frequency of about one revolution in six seconds was rotated. In a variant of the method, the Hollow cylinder five with a higher rotation frequency, in particular with a rotation frequency of five revolutions per Second, rotates. The radiation was on after about six Seconds canceled. The paint layer 6 was complete cross-linked and cured. Applying a second Paint layer on the hollow cylinder 5 was not necessary because even the first coat of paint is an even and homogeneous one Appearance.

The halogen tube emitters 7 in Fig. 2 have a filament 10 low mass in a quartz glass tube 11. The two Ends of the filament 10 are each flowing through Compressed air cooled to the life of the halogen tube lamp 7 increase. Likewise, the reflector 8 is by means of Compressed air or liquid cooled to constant conditions for the reflection of the halogen tube spotlights 7 to create emitted radiation.

With this is a powder coating process Substrate are clear compared to known methods shorter cycle times when networking and curing the Powder coating available. It is also possible to apply powder coating crosslink on heat sensitive substrates. Focusing arrangements using reflectors allow a targeted, the geometry of the substrate adapted radiation. So one can both extend the surface of the substrate or the coating and more homogeneous over the depth or thickness of the coating Energy input can be effected. When using halogen lamps the crosslinking process is with low inertia also controllable in time so that even powder coatings are baked can be, their cross-linking temperatures higher than the damage temperature of the heat sensitive substrate are.

LIST OF REFERENCE NUMBERS

1

MDF

2

first layer of paint

3

second layer of paint

5

hollow cylinder

6

paint layer

7

Halogen-tube radiators

8th

reflector

9

axis of rotation

10

filament

11

quartz glass tube

Claims (10)

1. Method for powder coating a substrate (1, 5), in particular a temperature-sensitive substrate (1, 5) such as wood, fibreboard, plastic, rubber, textile, paper or cardboard, whereby a thermo-setting powder is applied as a base coat (2, 6) to the non-coated surface of the substrate (1, 5) and the powder is consistently heated up to cross-linking temperature by means of infrared radiation at least with radiation components in the near infrared and as a result hardened through, or is consistently heated up to gelling temperature and only fully cross-linked and hardened through in a later process stage, characterised in that the powder coat (2, 6, 3) is irradiated for no more than 12 seconds in total, in particular no more than 8 seconds, until it has fully hardened.
2. Method according to claim 1,
   whereby a second coat (3) of thermo-setting powder is applied to the hardened or pre-gelled base coat (2) and the entire coating which is not yet fully cross-linked is cross-linked and through hardened by infrared radiation.
3. Method according to claim 2,
   whereby after the base coat (2) has hardened or gelled this is cooled down below the hardening or gelling temperature.
4. Method according to one of claims 1 to 3,
   whereby the surface temperature of the thermo-setting powder is measured with a pyrometer and regulated by controlling the radiation flow density of the infrared radiation.
5. Method according to one of claims 1 to 4,
   whereby at least one high performance halogen lamp (7) with a radiation temperature of more than 2500 K is used to produce the infrared radiation.
6. Method according to one of claims 1 to 5,
   whereby the non-coated surface of the substrate (5) is pre-treated to improve adhesion for the thermo-setting powder, in particular by applying an electrically conductive liquid.
7. Method according to one of claims 1 to 6,
   whereby to powder coat a moisture-containing or absorbing substrate (1) a defined moisture content is produced by drying and/or wetting the substrate before applying the base coat.
8. Method according to claim 7,
   whereby to dry the moisture-containing substrate (1) the substrate surface is irradiated with the same or higher amount of energy necessary for the actual cross-linking.
9. Method according to claim 8,
   whereby the surface of the substrate is heated by the radiation to a temperature which lies above the fusion point of the thermo-setting powder so that at least some of the thermo-setting powder fuses immediately after being applied to the substrate surface.
10. Use of a halogen lamp (7) in the powder coating process according to one of claims 1 to 9,
    whereby the halogen lamp (7) is combined with a reflector (8) to reflect the emitted radiation in the direction of the substrate (1, 5) and is operated in such a way that a radiation flow density maximum of the emitted radiation lies in the near infrared.

Images