EP1062053A1

EP1062053A1 - Method for powder-coating

Info

Publication number: EP1062053A1
Application number: EP99911798A
Authority: EP
Inventors: Martin Sedlmeyr
Original assignee: Advanced Photonics Technologies AG
Current assignee: Advanced Photonics Technologies AG
Priority date: 1998-03-16
Filing date: 1999-03-16
Publication date: 2000-12-27
Anticipated expiration: 2019-03-16
Also published as: KR20010041912A; DE59902341D1; CN1293598A; CA2324097A1; WO1999047276A1; AU3035299A; CN1203924C; US6436485B1; KR100685477B1; ES2182500T3; JP2002506725A; BR9908843A; EP1062053B1

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

Powder coating process

description

The invention relates to a method for powder coating a substrate, in particular a temperature-sensitive substrate such as wood, wood fiber material, plastic, rubber, fabric, paper or cardboard. The invention further relates to the use of a halogen lamp for powder coating.

When crosslinking and curing powder coating, it is crucial that the heating to the curing temperature is as homogeneous and rapid as possible. This is the only way for the powder coating melt to reach the minimum viscosity without being significantly impeded by crosslinking reactions, which results in an unevenness in the surface due to the powder not running optimally.

A method for crosslinking thermoreactive powder is known, in which the necessary curing temperature is achieved via multi-stage energy transfers. First, the surface of the powder coating is heated via infrared (IR) radiation or convectively. Only then is the soaking in the powder layer carried out via heat conduction processes up to

Substrate boundary layer. There, the energy, particularly with metallic substrates, is dissipated much faster into the substrate via the higher heat conduction. The boundary layer only reaches the necessary crosslinking temperature when the substrate is almost completely warmed up. In this known method, only the temperature gradient between the coating surface and the substrate is the driving process variable for the heating of the coating. In order to ensure homogeneous crosslinking and perfect adhesion to the To ensure substrate, heating times of several minutes are necessary.

The crosslinking and curing temperatures of powder coatings are often between 120 ° C and 300 ° C. Because of these high temperatures, temperature-sensitive substrates cannot be powder-coated or only to a limited extent by the known method.

A method for crosslinking and curing a layer of thermoreactive powder on a substrate is also known, in which a primer is applied to the surface of the substrate before the thermoreactive powder is applied. The primer consists, for example, of water-based paint. The primer is used, in particular in the case of substrates made of wood or wood fiber materials, to compensate for inhomogeneities on the surface of the substrate, to form a moisture barrier and to enable the thereactive powder to adhere. The powder can then be crosslinked and cured by irradiation with electromagnetic radiation, in particular with medium-wave infrared radiation. In this known method, the primer also forms a heat conduction barrier which prevents heat transfer during the crosslinking reaction in the powder layer to the substrate. This made it possible to apply a powder coating in the first place, especially for temperature-sensitive substrates. However, this known method is limited to the use of thermoreactive powders, the crosslinking temperature of which is only slightly higher than the damage temperature of the substrate.

In the case of moisture-containing or absorbing substrates, in particular wood or wood fiber material, the known methods also have the problem that a minimum moisture content of the substrate is desired on the one hand, but on the other hand prevents the application of a uniform powder coating. Moisture in the substrate enables, on the one hand, deposit active powder on the charged surface. On the other hand, the moisture evaporates in the subsequent crosslinking and curing reaction in the substrate, since, because of the long reaction time at temperatures above the evaporation temperature, the substrate is heated to the evaporation temperature at least on its surface. Therefore, bubbles form on the surface, under the already cross-linked powder, which lead to an irregular layer of lacquer. Even a primer layer does not help here because it does not form a heat conduction barrier that is effective in the long term and because the evaporation temperatures are usually significantly lower than the crosslinking and curing temperatures of the thermoreactive powder. In addition, for example in the case of a primer made from water-based lacquer, it is necessary to wait until the primer has completely dried before a powder lacquer layer can be applied to the primer.

In the known methods, there is also the difficulty that, because of the low depth effect of the powder layer heating, a melt connection between the powder layer and the substrate surface or the primer can only be produced after a longer heating period.

The invention has for its object to provide a method for powder coating a substrate, in particular a temperature sensitive substrate such as wood, wood fiber material, plastic, rubber, fabric, paper or cardboard, which allows powder coating of the uncoated surface of the substrate without damaging it, and which leads to an even, completely cross-linked and well-adhering lacquer layer.

The object is achieved by a method having the features of claim 1. Further developments are the subject of the dependent claims.

An essential idea in the process for powder coating according to the invention is that the required energy is introduced in a targeted and continuous manner over the entire thickness of the powder layer into the amount of powder applied as a base layer to the uncoated surface of the substrate. The gelation or crosslinking energy is at least in the form of radiation energy

Base layer introduced and absorbed there. The radiation used here has at least radiation components in the near and / or short-wave infrared. The powder layer and the substrate surface are preferably heated homogeneously by near infrared radiation (NIR radiation) and in a matter of seconds to the required gelling or crosslinking temperature. Near infrared is understood to mean the wavelength range of electromagnetic radiation between the visible range and 1.2 μm wavelength. Short-wave infrared is understood to mean the wavelength range between 1.2 μm wavelength and 2 μm wavelength.

According to the invention, the thermo-reactive powder is either heated to the crosslinking temperature and cured by the infrared radiation, or heated to the setting temperature and only crosslinked and cured in a later process step. In the latter case, gelation creates a composite of the powder material without complete crosslinking or curing to form a lacquer layer.

The targeted introduction of energy by means of infrared radiation, in particular NIR radiation, which is preferably distributed homogeneously over the thickness of the base layer, significantly accelerates the process of connecting or crosslinking the powder particles compared to the known method in which the energy input into the depth of the base layer mainly due to heat conduction. This also provides excellent controllability of the connection or crosslinking process, in particular since the desired process progress can be controlled precisely by controlling the radiation flux density, the spectral distribution of the radiation energy and / or the radiation duration. It is advantageous if the process parameters mentioned above are based on the absorption tion properties of the thermoreactive powder, on the reflection properties of the substrate surface and on the thermal conductivity of the substrate.

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

A second layer of a thermoreactive powder is preferably applied to the hardened or pre-gelled base layer, and the entire not yet fully crosslinked coating is crosslinked and cured by means of the infrared radiation. In a further development of the method, after the hardening or gelling of the base layer, the latter is cooled below the gelling temperature or hardening temperature, preferably by compressed air which flows against the surface or flows along it. In an alternative embodiment, the second layer is applied immediately after curing or pre-gelling.

The second layer, with the application and curing of which the painting process is particularly ended, can produce a uniform paint surface that meets the highest quality requirements. In particular, irregularities in the base layer are compensated for by the second layer, as a result of which, for example, a uniformly glossy or matt lacquer surface can be achieved. In contrast to known powder coatings with UV powder coatings, matt powder coating surfaces can be achieved with both the first and the second. Compared to methods in which a primer layer and a second lacquer layer formed from powder consist of different materials, a particularly homogeneous lacquer layer which is crosslinked evenly over the depth of the overall lacquer can form, in particular when using the same type of powder for the base layer and the second layer. The advantages of this powder coating system are therefore particularly evident the robustness, abrasion resistance and chemical resistance of the paint.

With the two-layer variant of the method according to the invention, substrates such as wood and wood fiber-containing materials (in short: wood fiber materials) can be powder-coated with a high coating quality. On the one hand, the targeted control of the crosslinking and curing process described above can prevent bubbles of moisture from producing irregularities in the lacquer layer. On the other hand, the problem of uneven adhesion of powder particles to an uncoated surface, at least partially formed by wood fibers, is overcome. An adhesive layer is formed through the base layer, which may still have an irregular surface or even consist of individual, island-like lacquer spots that are not connected to one another. After the base layer has hardened or pre-gelled, however, much better starting conditions exist for the second layer. Adhesion is improved and, as a rule, more material is applied when the powder of the second layer is applied. During the subsequent crosslinking and curing of the entire coating material which has not yet been crosslinked or only partially crosslinked, the coating material then runs to form a uniform lacquer layer.

The powdery base layer and / or the second layer is preferably not irradiated for longer than 12 s, in particular not longer than 8 s, until it gels or hardens. After the application of a second layer, however, the radiation of the base layer is continued by radiation penetrating into the base layer, so that the total radiation duration of the base layer can be longer than 12 or 8 s.

In particular in order to increase the controllability of the process progress, the surface temperature of the thermoreactive powder is determined in a further development of the method a pyrometer measured and controlled by controlling the radiation flux density of the infrared radiation. Thus defined temporal temperature profiles of the powder coating can be run, e.g. B. with steep temperature rise and subsequent phase constant temperature over time

Continue the crosslinking process just above the minimum crosslinking temperature until it has fully hardened.

A high-performance halogen lamp with a beam temperature of more than 2500 K is preferably used to generate the infrared radiation. Such radiation sources generate electromagnetic radiation with very high radiation flux densities, which in particular allow the crosslinking temperature to be reached within a few seconds. Incandescent bodies, in particular heating coils, with a low mass are preferably used in the halogen lamp, so that the radiation flux density can be controlled quickly. In a particularly preferred embodiment, the halogen lamp is combined with a reflector for reflecting the emitted radiation in the direction of the substrate and the halogen lamp is operated in such a way that a maximum radiation flux density of the emitted radiation is in the near infrared. The surface temperature of the filament can be adjusted up to 3500 K. Line-like halogen lamps are preferably used in combination with trough-like ellipsoidal or parabolic reflectors.

The uncoated surface of the substrate, in particular of plastic, is expediently pretreated to improve the conductivity for an electrostatic

Application of the thermoreactive powder subjected. In a special embodiment, an electrically conductive liquid is applied to the surface of the substrate.

In particular for powder coating a substrate containing or absorbing moisture, a defined moisture content is generated by drying and / or moistening the substrate before applying the base layer. Consequently Particularly uniform powder coating can be achieved and the process parameters can vary within certain limits without reducing the coating quality.

Preferably, as a, in particular exclusive, pre-treatment before the powder application for drying moist substrates, such as wood or wood composite materials, the substrate surface is irradiated with the same or higher energy input than is necessary for the actual crosslinking process, in particular by NIR radiation. This energy input achieves a surface temperature that is above the melting point of the powder system. The thermoreactive powder is then applied to the substrate surface as a base layer. The thermoreactive powder melts immediately and is optionally crosslinked by continued irradiation. The pretreatment of the substrate surface increases the application efficiency many times over during powder application. At the same time, it is prevented that during the actual crosslinking process, moisture accumulated on the substrate surface is expelled, which could disrupt homogeneous film formation.

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

Fig. 2 shows an arrangement for crosslinking powder paint on self-contained peripheral surfaces of a plastic substrate.

The substrate shown in FIG. 1 consists of a medium-density fiberboard (MDF) 1, which also has a base layer made of thermally active powder and a second layer was coated from thermoreactive powder. For this purpose, the MDF 1 was grounded on the side not to be coated and the thermoreactive powder of the first lacquer layer 2 was applied to the uncoated surface of the MDF 1 using the tribo process. The base layer was then irradiated for 5 s by means of infrared radiation from a radiation source, the maximum radiation flux density of which is about 1 μm wavelength, until the temperature of the powder has risen to the setting temperature. This temperature, which was approximately homogeneous over the thickness of the first lacquer layer 2, was held for approximately 1 s. The irradiation process was then stopped.

During the gelling process, the substrate had warmed only on its surface and only slightly, so that the water bound in the MDF 1 did not escape at the surface and the uniformity of the coating was not disturbed.

After cooling, the MDF 1 was grounded on the uncoated side and thermoreactive powder for the second lacquer layer 3 was applied to the surface of the first lacquer layer 2 using the tribo process. The first 2 and the second 3 lacquer layers were then irradiated with the infrared radiation at a maximum radiation flux density with a wavelength of approximately 1 μ until the crosslinking temperature was reached. The crosslinking reaction was continued until both coating layers had completely hardened by continued irradiation with a lower radiation flux density for about 3 s. Thereafter, the radiation was stopped and a few seconds waited until the layers of paint had cooled significantly below the crosslinking temperature. The second irradiation process also did not generate any steam or gas bubbles that could have caused the paint coating to be irregular.

In further experiments, MDF (not shown) was coated with surface contours immediately after pretreatment by NIR radiation. Here too, <_o LO to to h- ¹

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The halogen tube radiators 7 in FIG. 2 have a filament 10 of low mass in a quartz glass tube 11. The two ends of the filament 10 are each cooled by inflowing compressed air in order to increase the service life of the halogen tube beam 7. Likewise, the reflector 8 is cooled by means of compressed air or liquid in order to create constant conditions for the reflection of the radiation emitted by the halogen tube emitters 7.

With the method according to the invention for powder coating a substrate, significantly shorter cycle times can be achieved in the crosslinking and curing of the powder coating compared to known methods. It is also possible to cross-link powder coating on heat-sensitive substrates. Focusing arrangements using reflectors allow targeted radiation that is adapted to the geometry of the substrate. In this way, a homogeneous energy input can be achieved both by the extent of the surface of the substrate or the coating and by the depth or thickness of the coating. When using halogen lamps with low inertia, the crosslinking process can also be controlled precisely, so that even powder coatings can be burned in, the crosslinking temperatures of which are higher than the damage temperature of the heat-sensitive substrate.

Reference list

I MDF 2 first coat of paint

3 second coat of paint

5 hollow cylinders

6 lacquer layer

7 halogen tube spotlights 8 reflector

9 axis of rotation

10 filament

II quartz glass tube

Claims

claims

1. Process for powder coating a substrate (1, 5), in particular a temperature-sensitive substrate (1, 5) such as wood, wood fiber material, plastic, rubber, fabric, paper or cardboard, with a thermoreactive powder as the base layer (2, 6) on the uncoated surface of the substrate (1, 5) is applied, and wherein the powder is continuously heated to crosslinking temperature and brought to curing by means of infrared radiation, at least with radiation components in the near and / or short-wave infrared, or is continuously heated to gelling temperature and only in a later one Process step is fully cross-linked and cured.

2. The method according to claim 1, wherein a second layer (3) of thermoreactive powder is applied to the hardened or pre-gelled base layer (2) and the entire not yet fully crosslinked coating is crosslinked and cured by means of infrared radiation.

3. The method according to claim 2, wherein after curing or gelling of the base layer (2) this is cooled below the curing temperature or gelling temperature.

4. The method according to any one of claims 1 to 3, wherein the powder layer (2, 6) and / or the second

Layer (3) in each case not longer than 12 s, in particular not longer than 8 s, until the gelation or curing is irradiated.

5. The method according to any one of claims 1 to 4, wherein the surface temperature of the thermoreactive powder is measured by means of a pyrometer and by control 13

tion of the radiation flux density of the infrared radiation is regulated.

6. The method according to any one of claims 1 to 5, wherein at least one high-power halogen lamp (7) with a beam temperature of more than 2500 K is used to generate the infrared radiation.

7. The method according to any one of claims 1 to 6, wherein the uncoated surface of the substrate (5) is subjected to a pretreatment to improve the adhesiveness for the thermoreactive powder, in particular by applying an electrically conductive liquid.

8. The method according to any one of claims 1 to 7, for powder coating a moisture-containing or absorbing substrate (1), wherein a defined moisture content is generated by drying and / or moistening the substrate before applying the base layer.

9. The method according to claim 8, wherein for drying the moisture-containing substrate (1), the substrate surface is irradiated with the same or higher energy input than is necessary for the crosslinking.

10. The method according to claim 9, wherein the substrate surface is heated by the radiation to a temperature which is above the melting temperature of the thermoreactive powder, so that at least part of the thermoreactive powder melts immediately after application to the substrate surface.

11. Use of a halogen lamp (7) for powder coating according to one of claims 1 to 10, wherein the halogen lamp (7) is combined with a reflector (8) for reflecting the emitted radiation in the direction of the substrate (1, 5), and wherein the halogen lamp (7) is operated such that the radiation flux density maximum of the emitted radiation in the near Infrared lies.