EP1681102B1 - Process and apparatus for drying paint layers - Google Patents

Abstract

The method of drying paint containing solvent involves feeding air that is conditioned in respect of moisture and temperature to the workpiece (6) that is coated with the paint (7) while simultaneously feeding energy into the paint coating by the action of electromagnetic radiation (5). The air is conditioned at a temperature in the range of +1 to +18 degrees C and humidity in the range of 50 to 90 per cent. An independent claim is also included for a device for implementing the inventive method.

Description

Field of the invention

The invention relates to a method and a device for drying a solvent-based lacquer layer applied to a workpiece.

State of the art

Each paint consists of a film-forming substance-the so-called paint body or binder-which is dissolved in a volatile solvent or solvent mixture. Depending on the type of paint, there are also pigments, fillers, siccatives, plasticizers, hardeners or other additives.
Drying is the conversion of paint applied to a body, liquid paint layer into a solid film that protects the coated body and beautify.
During this process changes take place in the physical and chemical properties of the lacquer layer, which first give the film the desired characteristic properties.

The drying process takes place by the following steps: physical drying (evaporation of the solvent) and hardening of the layer by colloidal changes and / or chem. Crosslinking reactions (polymerization, polyaddition, polycondensation), which merge seamlessly.

Conventionally, the physical drying is realized as the first step after painting, starting with the Evaporation of the solvent from the paint layer by the coated workpiece is passed through a dust-free area as possible at room temperature or slightly elevated air temperature to a maximum of 30 to 40 ° C. In this evaporation zone, the applied paint layer should be uniform and connect to the workpiece surface. Eventually, the paint pigments should form a specific orientation and layering. In addition, should evaporate in this evaporation, a large part of the volatile constituents of the paint.
Following the evaporation phase, the forced drying or curing process follows. The remaining volatile ingredients are expelled and the crosslinking reactions take place. This can possibly be done with the supply of energy with a temporary increase in the temperature of the workpiece and the coating layer.

During the first stage of the drying, that is, the evaporation of the lacquer layer, it is crucial that the lacquer layer remain open to diffusion, in particular on its surface in contact with the air, since otherwise volatile constituents below the surface can no longer evaporate to a sufficient extent.

If the surface of the varnish layer does not remain sufficiently permeable during the evaporation phase, the volatiles remain partly "trapped" within the varnish.
This proves to be disadvantageous in the subsequent forced drying process.
In fact, the trapped constituents, due to the increased impact of energy there, cause coating defects in the paint such as bubbles (the so-called "cookers"), shrinkage cracks or partial haze.

The WO 2005/023437 describes multi-step processes for drying and curing substrates which are coated with an aqueous basecoat and a topcoat.

The US 5,288,526 describes a ventilated curing oven and a preheat flash zone system for curing coatings on printed circuit boards.

The DE 202 02 512 U1 describes an apparatus for treating a web of material with a gaseous medium and radiant energy.

Object of the invention

It is therefore an object of the present invention to propose a method and an apparatus for drying varnish layers, in which the formation of layer defects during drying can be avoided more reliably. In particular, in the evaporation phase good evaporation should be achieved by the permeability of the paint surface is ensured.

Description of the invention

• Zufuhr von auf Feuchte und Kühle konditionierter Luft auf das Werkstück bei gleichzeitigem Energieeintrag in die Lackschicht durch Einwirkung elektromagnetischer Strahlung, wobei die konditionierte Luft auf eine Temperatur im Bereich von +1°C bis +18°C und/oder eine Luftfeuchte im Bereich von 50 % bis 90 % relative Feuchtigkeit konditioniert ist.

In order to achieve the object just mentioned, the present invention proposes a method for drying a solvent-containing lacquer layer applied to a workpiece, the method comprising the following step:

• Supply of air conditioned to moisture and coolness to the workpiece with simultaneous introduction of energy into the lacquer layer by exposure to electromagnetic radiation, wherein the conditioned air is at a temperature in the range of + 1 ° C to + 18 ° C and / or a humidity in the range of 50 % to 90% relative humidity is conditioned.

The supply of moist and cool air slows down the evaporation process on the surface of the lacquer layer. The paint surface is kept cool, it can not dry out during the evaporation, but remains moist and thus permeable. The formation of a disturbing diffusion-inhibiting surface is avoided. At the same time, the volatiles below the surface are excited by the incident electromagnetic radiation and effectively expelled from the paint. The electromagnetic radiation brings energy into the paint layer, so that the evaporation of the volatile elements contained is promoted over the entire layer cross-section.

In the context of the invention, solvent-containing lacquer is understood to mean all lacquers which contain a liquid solvent or also solvents. It is preferable the solvent around water, but other solvents are also included.

The conditioned air is an air intended for use in the evaporation and accordingly adapted and conditioned in its temperature and humidity. So it is not about simple room or ambient air.

According to the invention, the conditioned air is supplied to the workpiece. This means, for example, that the air is directed onto the workpiece or inflated onto it. There are also other types of air supply conceivable. Thus, e.g. the air is introduced into a separate space in which the coated workpiece is located. It is only essential that an exchange between the supplied air and the surface of the paint layer can take place. The conditioned air should be able to contact the lacquer layer, i. The air should be brought together with the paint layer.

The energy input into the lacquer layer is achieved by irradiating the lacquer layer with electromagnetic radiation. This happens e.g. by appropriate radiation sources whose emissions are directed to the coated workpieces. The electromagnetic waves or rays penetrate into the paint layer and are absorbed by the solvent contained in the paint. Through this supply of energy or heating, the solvent can escape through the permeable surface of the paint.

Further, in the method of the invention, the conditioned air must be conditioned to a temperature in the range of + 1 ° C to + 18 ° C and / or a humidity in the range of 50% to 90% relative humidity.

With these temperature and humidity values, the surface of the lacquer layer can be kept particularly permeable. The temperature range of 1 to 18 ° C guarantees a good cooling and the humidity range of 50 to 90% a good moistening of the surface.

Preferably, the energy input also occurs in the workpiece, i. The electromagnetic radiation also penetrates at least partially directly into the workpiece and is absorbed there by the workpiece. By thus taking place heating of the workpiece, the evaporation of the volatile constituents in the lacquer layer from the contact surface lacquer layer / workpiece is additionally promoted.

Furthermore, the conditioned air can be supplied either in the form of fresh air or in the form of circulating air. In the case of the fresh air supply is constantly new, fresh air supplied to the workpiece. If a recirculation system is provided, only a constantly renewed supply of already supplied air takes place, whereby this air is thereby treated and conditioned again and again. In the case of circulating air, therefore, a circulation of one and the same amount of air occurs, while in the fresh air supply new air is continuously introduced and spent air is disposed of.

Preferably, after the evaporation process, the further, forced drying of the lacquer layer then takes place by means of a jet dryer. Since a large part of the volatile constituents has already escaped below the surface of the lacquer layer as a result of the evaporation according to the invention, there is no risk of blistering or cracking even in the subsequent rapid and intensive drying by the nozzle dryer.
The evaporation process according to the invention thus enables, in conjunction with the subsequent forced drying over conventional processes, shorter drying times with better coating results Significantly reduced occurrence of drying layer defects.

It is advantageous if at least one infrared emitter is used to generate the electromagnetic radiation. Particular preference is given to using an infrared radiator with an emission spectrum tailored specifically to the absorption curve of the volatile constituents of the coating (in the region of emission number> 0.8 by resonance of the radiation frequencies and of the natural vibration frequencies of the molecules of the volatile constituents of the coating). This allows an efficient and low-loss energy transfer into the paint layer, since the vote, a large part of the emitted radiation is absorbed as desired by the solvent in the paint. However, with poorer energy efficiency, conventional IR emitters without a tuned emission spectrum can also be used.

Furthermore, at least one microwave generator, in particular a magnetron, can also be used to generate the electromagnetic radiation. It is also conceivable that the microwave generator is used together with an infrared radiator.
A microwave generator can also be used instead of an infrared radiator. A microwave generator is particularly advantageous if the paint to be evaporated is a paint with water as the solvent. In fact, water molecules in the liquid state can be effectively excited to vibrate by microwave radiation due to their electrical dipole property, releasing heat energy. This allows a particularly efficient energy transfer into the water-containing paint layer. The frequency of the microwave generator is preferably in, released in Europe, range around 2.45 GHz.

But it is also conceivable to apply another shared, higher frequency.
Particularly preferably, the frequency of the microwave generator is in the range between 2.45 GHz and 4.9 GHz.

Finally, in order to achieve the above object, the present invention also proposes a device for carrying out one or more of the methods just described.

Brief description of the figures

The sole figure is a schematic representation of a device with which a method according to the invention can be carried out.

Description of preferred embodiments

The figure shows a device 1 with two boundaries 2a and 2b, which together enclose a evaporation zone 3. Within the evaporation zone 3 a plurality of electromagnetic radiation sources 4 are arranged, which can deliver an electromagnetic radiation 5. The arrangement and number of radiation sources 4 may vary depending on the requirements. Between the radiation sources 4, two workpieces 6a and 6b are shown. The workpiece 6a is coated on all sides with a solvent-containing liquid paint layer 7a. By contrast, the workpiece 6b is only partially coated with a corresponding lacquer layer 7b.

The device 1 also has an air treatment system with supply air 8a and exhaust air 8b. This can be a fresh air system. Alternatively, a recirculation system may be provided, as indicated by the dashed arrow 9.

The mode of operation of the device 1 will now be explained below.

First of all, the work piece 6a coated on all sides is assumed. This workpiece 6a has been provided on all sides with a lacquer layer in a coating process I not shown. Then, the workpiece 6 a, as indicated by the arrow A, introduced into the device 1. In device 1, the evaporation II of the applied lacquer layer takes place. For this purpose, the workpiece 6a is passed through the evaporation zone 3.

In the evaporation zone 3, a moist and cool climate prevails due to the supply 8a and the removal 8b, corresponding to conditioned air. The movement of the conditioned air in the evaporation zone 3 takes place opposite to the direction of movement of the workpiece 6a through the evaporation zone 3. This is achieved in that the supply air 8a is blown in at the rear end of the device 1 in the direction of the workpiece 6a and consumed at the front end Air in the form of the exhaust air 8b is sucked away from the workpiece 6a.

At the same time, the radiation sources 4 irradiate the lacquer layer 7a with electromagnetic radiation 5, which may be infrared and / or microwave radiation. Thanks to the moist and cool climate, when the varnish layer 7a evaporates, its surface is prevented from drying out. The surface of the lacquer layer 7a remains permeable. Therefore, the lower volatiles of the lacquer layer can freely exit the lacquer when excited by the radiation 5.

After the lacquer layer 7a is well evaporated by the interaction of the moist and cool air and the irradiation, the workpiece exits the evaporation zone 3 at the rear end of the device 1. Then it will Workpiece, as indicated by arrow B, promoted to the actual drying process III.

The evaporation of the partially coated workpiece 6b is similar to the just described evaporation of the workpiece 6a. However, the workpiece 6b is not irradiated at those locations where there is no paint. In order to achieve this, certain radiation sources 4 can simply remain switched off during the evaporation phase II. Alternatively, the device 1 can be designed specifically for the evaporation of the workpieces 6b, so that the arrangement of the radiation sources 4 is such that irradiation of uncoated workpiece parts is avoided.

With the method and the device according to the invention a much better and more thorough evaporation of solvent-based paints is achieved. Thus, the formation of layer defects can be effectively avoided in the subsequent drying process with increased energy.

LIST OF REFERENCE NUMBERS

1

Abdunsteinrichtung

2a

Upper delimitation

2 B

Lower demarcation

3

evaporation zone

4

radiation source

5

radiation

6a

All-over coated workpiece

6b

Partially coated workpiece

7a, 7b

paint layer

8a

supply air

8b

exhaust

9

circulating air

I

coating

II

flash-off

III

desiccation

Claims (9)

1. Method for the drying of a solvent-containing lacquer coating (7) applied to a workpiece (6), characterized by the following step:

   - feed (8a) of moisture- and chill-conditioned air onto the workpiece (6) with simultaneous energy input into the lacquer coating (7) by exposure to electromagnetic radiation (5), wherein the conditioned air is conditioned to a temperature in the range of from +1°C to +18°C and an air humidity in the range of from 50% to 90% relative humidity.
2. Method according to claim 1, characterized in that the energy is also input into the workpiece (6).
3. Method according to one of the previous claims, characterized in that the conditioned air is supplied in the form of fresh air or circulating air (9).
4. Method according to one of the previous claims, characterized by the further step:

   - drying (III) of the lacquer coating (7) by means of a nozzle drier.
5. Method according to one of the previous claims, characterized in that at least one infrared radiator is used to generate the electromagnetic radiation (5).
6. Method according to claim 5, characterized in that an infrared radiator with an emission spectrum adapted in particular to the absorption curve of the volatile lacquer constituents is used.
7. Method according to one of the previous claims, characterized in that at least one microwave generator, in particular a magnetron, is used to generate the electromagnetic radiation.
8. Method according to claim 7, characterized in that the frequency range of the microwave generator lies at 2.45 GHz or higher.
9. Device for carrying out a method according to one of claims 1 to 8, comprising an apparatus for feeding moisture- and chill-conditioned air onto a workpiece with a solvent-containing lacquer coating applied thereto with simultaneous energy input into the lacquer coating by exposure to electromagnetic radiation, further comprising an apparatus for the emission of electromagnetic radiation and an air-conditioning unit for conditioning the air humidity in the range of from 50% to 90% relative air humidity and to a temperature in the range of from +1° to +18°.

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