WO2003101627A1

WO2003101627A1 - Process and apparatus for uv curing of coating materials with inertization

Info

Publication number: WO2003101627A1
Application number: PCT/IB2003/002254
Authority: WO
Inventors: Fernand Heine; Johannes Beuse
Original assignee: Air Liquide GmbH; Air Liquide SA; LAir Liquide SA a Directoire et Conseil de Surveillance pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: Air Liquide Deutschland GmbH; Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2002-05-31
Filing date: 2003-05-26
Publication date: 2003-12-11
Anticipated expiration: 2004-11-30
Also published as: AU2003228078A8; AU2003228078A1

Abstract

The invention relates to a process for coating a substrate (1) which has a contour (17), in particular 5 an automotive body part, in which a coating material (2) is applied to the substrate (1), the coating material (2) on the substrate (1) is exposed in subregions to an inert gas atmosphere (3) by gas concentration means (5), the coating material (2) which is at the time directly under the inert gas atmosphere (3) is irradiated with UV radiation (4) from a light source (6), the light source (6) and/or the gas concentration means (5) being adaptable at least sectionally to the contour (17) of the substrate (1), and to the apparatus suitable for implementing the process. The process of the invention and the apparatus of the invention for coating a substrate (1) have the feature that the coating material is cured in subregions by UV radiation, the coating material (2) which is at the time irradiated with W radiation being protected by an inert gas atmosphere (3).

Description

PROCESS AND APPARATUS FOR UV CURING OF COATING MATERIALS W_{ITH I}N_ERT_IZATION

The invention relates to a process for coating a substrate, in particular an automotive body part, and 5 to an apparatus for curing coating material on a substrate, in particular coating material on an automotive body part .

In the coating of automotive body parts with coating 0 materials the aim is to ensure that the coating materials are particularly colourfast and durable. For these purposes coating materials have recently been developed which are cured by means of ultraviolet (UV) radiation. One particular advantage of these UV-cured 5 coating materials is their longevity.

A difficulty with the curing of coating materials by ultraviolet radiation, however, is that, on the one hand, the photoinitiator inherent to the process has a 0 high affinity for oxygen and reacts with it first and, on the other hand, UV radiation in an oxygen-containing atmosphere generates ozone, which is a strong oxidizing agent. The ozone attacks the coating material and bodywork parts by oxidizing them. For this reason it 5 has to date been necessary to cure the coating materials in inertized booths. The booths have been flooded with an inert gas such as nitrogen, for example. Inertization protects the photoinitiator against oxygen and prevents the generation of ozone. 0 Applications in the car industry require traversed booths or traversed tunnels through which the vehicle bodies are passed. These tunnel passages occupy a large amount of space, since the thermal treatment necessitates extended preheating and cooling zones for 5 the drying of the coating material.

Because of the low oxygen concentration which must be ensured, of less than 5%, full inertization of a traversed booth of this kind necessitates very large amounts of inert gas, leading to high inertization costs. A further problem is that large amounts of inert gas always pose a safety risk for the operatives.

It is an object of the present invention, therefore, to specify a process for coating a substrate and also an apparatus for curing coating material on a substrate, by means of which coating materials are cured cost- effectively by UV radiation, with a saving in terms of inert gas and with a reduction in the safety problem associated with the use of large amounts of inert gas.

A further object is to specify a particularly efficient cooling system or cooling method for UV light sources.

This object is achieved by a process having the features of Claim 1 and also by an apparatus having the features of Claim 14. Further advantageous embodiments and developments, each of which may be employed individually or combined arbitrarily with others, are subject-matter of each of the dependent claims.

The process of the invention for coating a substrate which has a contour, in particular an automotive body part, comprises the following steps: A coating material is applied to the substrate; the coating material on the substrate is exposed in subregions to an inert gas atmosphere by gas concentration means (5) ; and the coating material which is at the time directly under the inert gas atmosphere is exposed to UV radiation from a light source, the light source and/or the gas concentration means being adaptable at least sectionally to the contour of the substrate.

In the course of the process the coating material is exposed to an inert gas atmosphere only in one subregion. This avoids the need for complete blanketing of the substrate in a booth with inert gas. Full inertization of a booth is not necessary for the curing of the coating material. As a result, considerable amounts of inert gas are saved and the safety problem which exists when large amounts of inert gas are used is avoided. The adaptability of the light source and/or of the gas concentration means allows local inertization of irregularly shaped substrates, including body parts, for example. In this case there is an advantageous reduction in the consumption of inert gas. The light sources and/or the gas concentration means are advantageously disposed essentially alongside one another or offset diagonally from one another and are held at a specified distance from the substrate.

The inertization ensures that the photoinitiator inherent in the process, which has a high affinity for oxygen and reacts first with it during UV irradiation, is available to the actual reaction. This prevents the formation through photoinitiator oxidation of reaction products which have an adverse effect on the surface quality.

The subregion of the coating material which is protected by the inert gas is exposed to UV radiation. Those regions not protected by inert gas are kept away from UV radiation, so that these regions which are present in an oxygen-containing atmosphere also remain protected against UV radiation and/or ozone. In this way, overall, damage due to reaction with oxygen and/or due to oxidation by ozone during the curing of the coating material is prevented. Residual oxygen concentrations of less than 10%, in particular less than 7%, preferably less than 5%, more preferably less than 1%, are acceptable if reaction products of the oxidation of the photoinitiator with oxygen are minimal and the ozone concentrations generated during the UV treatment are sufficiently small for customary applications in the car industry. The UV radiation is advantageously generated by a gas discharge lamp. Likewise of advantage (but still at the development stage) are photodiodes, which provide particularly precise irradiation of the coating material with UV light. The coating material may be applied by spraying, dipping, spreading or the like.

With the aid of a gas concentration means the inert gas atmosphere is advantageously spatially limited essentially to a gas concentration space. A gas concentration means is, for example, a hood or a housing which is used to create, between the substrate and the light source, a gas concentration space in which the concentration of oxygen is depleted. The coating material on the substrate is protected by the inert gas atmosphere in the gas concentration space . The gas concentration means prevents oxygen penetrating into the gas concentration space and contaminating the inert gas atmosphere. Within the gas concentration space the coating material on the substrate is exposed to UV radiation from the light source . The gas concentration means additionally prevents UV radiation passing outside the gas concentration space and ionizing the surrounding, oxygen-containing air, and thereby prevents the formation of ozone outside the subregion. Accordingly, the gas concentration means brings about, on the one hand, a limitation and thus a concentration of the inert gas to the subregion and, on the other hand, a limitation of the UV radiation to that subregion. The objective here is with the aid of the gas concentration means to seal off the gas concentration space from the surrounding environment as effectively as possible, thereby reducing the amount of inert gas needed for inertizing the gas concentration space.

In one development of the process of the invention the distance of the gas concentration means from the substrate is controlled or regulated. Since on the one hand the gas concentration means must not contact the coating material and on the other hand there should not be any large air gap between gas concentration means and coating material, in order to protect the inert gas atmosphere in the gas concentration space, the control or regulation system maintains a small a distance as possible between gas concentration means and substrate.

In order to prevent penetration of oxygen into the gas concentration space, inert gas is introduced into the gas concentration space, so that within the gas concentration space there is a slight overpressure which causes an outflowing of inert gas, in particular between the gas concentration means and the coating material. This acts against penetration of oxygen from the surrounding environment into the gas concentration space.

The distance of the gas concentration means from the substrate is advantageously regulated as a function of the concentration of the inert gas in the concentration space. This prevents the oxygen concentration in the gas concentration space exceeding a predetermined value which would result in damage to the coating material and/or to the substrate owing to the reaction of the photoinitiator with oxygen and/or owing to the ozone formed as a result of UV radiation.

It is likewise advantageous to regulate the distance of the gas concentration means from the substrate using a distance meter which determines the distance between the gas concentration means and the substrate. By means of a particularly small distance between gas concentration means and substrate the amounts of inert gas required for inertizing the gas concentration space are reduced.

It is of advantage to regulate the flow rate of the inert gas used as a function of the inert gas concentration in the gas concentration space . Regulating the flow rate of the inert gas used is particularly useful in the case of structured substrate surfaces and/or in the case of openings, for example in the case of windows in the bodywork. While planar surfaces require comparatively little inert gas, a greater amount of inert gas is needed in the case of structured surfaces or surfaces having openings . Regulating the flow rate allows this circumstance to be taken into account and hence allows the consumption of inert gas to be tailored to the particular coating film to be cured on the substrate. This saves on inert gas and reduces costs .

In one advantageous embodiment of the invention the substrate is heated, in particular to at least 80°C, preferably to at least 110°C, more preferably to at least 140°C. Heating the substrate results in particularly efficient curing of the coating material on the substrate.

The residual content of oxygen in the inert gas atmosphere is advantageously less than 10%, in particular less than 7%, preferably less than 5%, more preferably less than 3%. Such reduction in the residual content of oxygen in the gas concentration space keeps the formation of ozone by exposure to UV radiation sufficiently low, so that for standard applications there is no damage to the coating material or to the substrate.

The inert gas used is advantageously nitrogen or carbon dioxide .

In one advantageous embodiment of the process of the invention the inert gas is preheated, in particular by 40°C, preferably by 60°C, more preferably by 80°C. Preheating the inert gas results in particularly efficient curing of the coating material . In order to preheat the inert gas it is advantageous to utilize the heat given off by the light source. By this means, energy needed for a thermal treatment, as for a curing operation, for example, is saved. Standard UV lamps convert only a comparatively small fraction of the electrical energy consumed into useful UV radiation, with the greater fraction being lost as heat. The heat of the UV lamps that is developed during operation can be used advantageously to cure the coating material .

It is advantageous if the inert gas exhibits laminar flow in the inert gas atmosphere. This prevents turbulences and eddies which might result in the ambient oxygen-containing air mixing with the inert gas in the gas concentration space. The avoidance of turbulences and/or eddies ensures a sufficiently low oxygen concentration in the gas concentration space, i.e. in particular in the subregion which is exposed to the inert gas atmosphere.

In one development of the process of the invention the light source is cooled by a cooling medium, in particular an inert gas, preferably nitrogen, which is guided in a closed cooling circuit. With the aid of a substantially closed cooling circuit the light source can be cooled efficiently. The heat given off in the closed cooling circuit can be removed with the aid of a heat exchanger. The use of inert gas in the cooling circuit is advantageous since it reduces, or even entirely prevents, the formation of ozone during the cooling of the light source as a result of its UV radiation. The formation of ozone in the course of cooling or in the cooling circuit is suppressed -and the plant and the operatives are no longer exposed or put at risk. Cooling the UV lamp with a closed cooling circuit in this way can find application in principle in applications other than during the curing of coating materials; it is conceivable in principle for all other UV applications where relatively large quantities of heat have to be removed.

It is particularly advantageous if the cooling medium in the cooling circuit has a slight overpressure, in particular an overpressure of between 1 and 1000 mbar, preferably an overpressure of between 10 and 50 mbar. Breaks in the sealing in the region of the hot enclosure of the glass plate through which the UV light emerges are almost impossible to avoid. Losses of cooling medium which occur must be compensated. This is achieved by introducing new cooling medium such as an inert gas, for example, into the cooling circuit. The slight overpressure prevents ambient oxygen from penetrating into the cooling circuit and suppresses the formation of ozone in the cooling circuit.

The apparatus of the invention for curing coating material on a substrate having a contour, in particular coating material on an automotive body part, comprises at least one light source for UV radiation and at least one gas concentration means for inert gas, the light source (6) and/or the gas concentration means (5) being adaptable at least sectionally to the contour (17) of the substrate (1) , and at least one supply device for inert gas, the gas concentration means forming between the light source and the substrate a gas concentration space which can be filled with inert gas by means of the supply device and which can be illuminated by the light source with UV radiation. By means of this apparatus a gas concentration space is formed between the light source and the substrate, and this, by virtue of an inert gas atmosphere, protects the subregion of the coating material that is to be illuminated with UV radiation on the substrate.

The gas concentration means has the effect first that the subregion of the coating material to be illuminated is fed with sufficiently concentrated inert gas, i.e. with an adequately low oxygen content, and second that the UV lamp illuminates only the inertized gas concentration space. By this means, on the one hand, the coating material is protected against ozone formed by UV radiation and, on the other hand, there is prevention of ozone formation outside the gas concentration space, which formation could have the effect of damaging the coating material and/or the substrate outside the inert gas atmosphere. Also prevented is the reaction of the photoinitiator with oxygen, which would have an adverse effect on the surface quality of the coated areas .

The adaptability of the light source and/or of the gas concentration means allows targeted inertization of a subregion of the substrate. It produces in particular inertization which is adapted to the form and/or the contour of the substrate, and which leads to a reduced consumption of inert gas.

The supply device provides inert gas in sufficient amounts: advantageously, the gas concentration space is filled continuously with inert gas, so that the slight overpressure this produces in the gas concentration space prevents oxygen from the surrounding environment penetrating into the gas concentration space, as a result of the inert gas flowing out of the gas concentration space.

In one advantageous embodiment of the apparatus of the invention a positioning means is provided with which the light source and/or the concentration means can be moved. Through the position means both the light source and the gas concentration means can be positioned in relation to the substrate, so making it possible to seal off the gas concentration space in particular. In this way, in particular, a slot between gas concentration means and substrate, through which oxygen from the surrounding environment can penetrate into the gas concentration space, can be minimized, so that the amount of inert gas required for inertizing the gas concentration space is reduced.

The positioning means advantageously comprises a first regulator for regulating the distance of the light source and/or of the gas concentration means from the substrate. In this case it is of advantage if the regulator comprises a distance sensor and/or a gas concentration meter for determining the concentration of a gas in the gas concentration space, so that the distance between light source and/or gas concentration means and substrate and/or the flow rate of the inert gas, i.e. the amount of inert gas fed into the gas concentration space per unit time, can be regulated.

In one development of the invention the apparatus comprises a first preheating means for preheating the substrate. Preheating the substrate produces particularly rapid and efficient curing of the coating material . In a further development of the invention the apparatus comprises a second preheating means for preheating the inert gas. This also produces particularly efficient curing of the coating material. The second preheating means is advantageously in thermal contact with the light source, so that the heat given off by the light source can be used to preheat the inert gas .

In the case of substrates having relatively large geometrical dimensions, such as automative bodies, for example, it is advantageous to dispose two or more apparatuses alongside one another and/or in series with a diagonal offset, in order to cure the coating material on the substrate uniformly over the whole width of the substrate. In that case it is advantageous to carry out individual regulation and/or control of - li the respective distances of each gas concentration means from the coating material .

It is particularly advantageous if the apparatus comprises a closed cooling circuit on the light source. In principle, a UV light source with a closed cooling circuit is advantageous, since it allows removal of heat without emitting ozone to the surrounding environment. The cooling circuit advantageously comprises a feedline for a cooling medium, in particular for an inert gas. Feeding the cooling circuit with an inert gas as cooling medium prevents the formation of ozone in the cooling circuit. The plant is protected against ozone. For the removal of the heat from the cooling circuit the cooling circuit is in communication with a heat exchanger. A UV lamp having such a cooling system can be used not only for curing coating materials but also for other UV applications .

Further embodiments and advantages of the invention will be elucidated with reference to the following drawing, which is intended to illustrate the invention though not to restrict it.

In the drawing, which is diagrammatic,

Fig. 1 shows an apparatus of the invention for curing coating material; and Fig. 2 shows a detailed view of the apparatus of Fig. 1 showing the cooling circuit of a UV lamp.

Figure 1 shows diagrammatically an apparatus for curing coating material 2 on a substrate 1, which can be a automative body, for example. First of all the substrate 1 is coated with coating material 2, advantageously by spraying. The coating material 2 is then cured by means of UV radiation from a light source 6, the particular coating material under irradiation being protected by an inert gas atmosphere 3. This protection prevents unwanted reaction of the photoinitiator with oxygen or ozone. The inert gas atmosphere 3 is formed by nitrogen as inert gas, which is provided by a supply device 7. The coating material

2 is exposed in subregions to the inert gas atmosphere 3 , a purpose served by a gas concentration space 8 which is formed with the aid of a gas concentration means 5 between the light source 6 and the substrates 1. The light source 6 and the gas concentration means 5 are sectionally adapted to a contour 17 of the substrate 1, allowing the gaps 16 to be kept narrow and the distances A of the gas concentration means 5 from the substrate 1 to be kept small. The respective light sources 6 and/or the gas concentration means 5 can advantageously be moved individually, so that adaptation can also be made to a substrate 1 contour 17 which changes over time, if, for example, a body part is moved by an apparatus for curing. This lowers the consumption of inert gas.

The gas concentration means 5 is formed by a hood which spatially limits both the UV radiation of the light source 6 and the inert gas of the inert gas atmosphere

3 to the gas concentration space.

Between the gas concentration means 5 and the substrate 1 there is a gap 16. The gas concentration means 5 is guided at a distance A from the substrate 1. This distance is preferably less than 10 cm, in particular less than 5 cm. The distance A is detected by a distance sensor 11 and regulated with the aid of a positioning means 9.

A gas concentration sensor 12 detects the concentration of the inert gas in the gas concentration space 8. With the aid of the gas concentration sensor 12 the supply device 7 regulates the flow rate of the inert gas. With the aid of a second preheating means 15 the heat given off by the light source 6 is utilized to warm up the inert gas in the inert gas atmosphere 3 , so that curing of the coating material 2 is rapid. A first preheating means 14 heats the substrate 1 to 100°C, likewise producing particularly efficient curing of the coating material 2 on the substrate 1.

The light sources 6 with the respective gas concentration means 5 are held at a small distance from the substrate 1 with the aid of the respective positioning means 9, a purpose served by a regulator 10. This ensures that the coating material 2 on the substrate 1 is exposed in subregions to an inert gas atmosphere 3 and in these subregions is cured in each case with UV radiation. The light source 6 is cooled by means of a cooling circuit 19 in which a cooling medium 18 circulates. The heat given off is removed via a heat exchanger 20. In order to prevent ambient oxygen penetrating the cooling circuit owing to leaks in the cooling circuit 19, the cooling medium 18 in the cooling circuit 19 is subjected to a slight overpressure of 30 mbar.

Fig. 2 shows by way of example the cooling circuit 19 of a UV light source of an apparatus according to Fig. 1, the cooling medium 18 used being nitrogen which is supplied via the supply line 21 with the aid of a valve, which advantageously is pressure-regulated. The heat of the cooling medium 18 is removed by means of the heat exchanger 20 and passed to a cooling means 24. The cooling medium 18 is moved in the cooling circuit by means of a circulating fan 22.

The invention relates to a process for coating a substrate 1 which has a contour 17, in particular an automotive body part, in which a coating material 2 is applied to the substrate 1, the coating material 2 on the substrate 1 is exposed in subregions to an inert gas atmosphere by gas concentration means 5 , the coating material 2 which is at the time directly under the inert gas atmosphere 3 is irradiated with UV radiation 4 from a light source 6, the light source 6 and/or the gas concentration means being adaptable at least sectionally to the contour 17 of the substrate 1, and to the apparatus suitable for implementing the process .

The process of the invention and the apparatus of the invention for curing coating material have the feature that the coating material is cured in subregions by UV radiation, the coating material which is at the time irradiated with UV radiation being protected by an inert gas atmosphere.

List of reference symbols

Substrate Coating material Inert gas atmosphere Radiation Gas concentration means Light source Supply device Gas concentration space Positioning means Regulator Distance sensor Gas concentration sensor Window First preheating means Second preheating means Gap Contour of the substrate 1 Cooling medium Cooling circuit Heat exchanger Supply line Circulating fan Valve Cooling means Distance of gas concentration means 5 from substrate 1

Claims

1. Process for coating a substrate (1) which has a contour (17), in particular an automotive body part, which comprises the following steps: a coating material (2) is applied to the substrate (1) ; the coating material (2) on the substrate (1) is exposed in subregions to an inert gas atmosphere (3) by gas concentration means

(5) ; the coating material (2) which is at the time directly under the inert gas atmosphere (3) is irradiated with UV radiation (4) from a light source (6) , the light source (6) and/or the gas concentration means (5) being adaptable at least sectionally to the contour (17) of the substrate (1) .

2. Process according to Claim 1, characterized in that the inert gas exhibits laminar flow in the inert gas atmosphere (3).

3. Process according to Claim 1 or 2 , characterized in that the inert gas atmosphere is spatially limited essentially to a gas concentration space

(8) with the aid of the gas concentration means

(5) .

4. Process according to Claim 3, characterized in that a distance (A) of the gas concentration means (5) from the substrate (1) is controlled or regulated.

5. Process according to Claim 4, characterized in that the distance (A) of the gas concentration means (5) from the substrate (1) is regulated as a function of the concentration of the inert gas in the gas concentration space (8) .

6. Process according to any of the foregoing Claims 3 to 5, characterized in that the flow rate of the inert gas used is regulated as a function of the inert gas concentration in the gas concentration space (8) .

7. Process according to any of the foregoing claims, characterized in that the substrate (1) is heated, in particular to at least 80°C, preferably to at least 110°C, more preferably to at least 140°C.

8. Process according to one of the foregoing claims, characterized in that the residual oxygen content in the inert gas atmosphere (3) is less than 10%, in particular less than 7%, preferably less than 5%, more preferably less than 3%.

9. Process according to any of the foregoing claims, characterized in that the inert gas used is nitrogen.

10. Process according to any of the foregoing claims, characterized in that the inert gas is preheated, in particular by 40°C, preferably by 60°C, more preferably by 80°C.

11. Process according to Claim 10, characterized in that the light source (6) heats the inert gas.

12. Process according to any of the foregoing claims, characterized in that the light source (6) is cooled by a cooling medium (18) , in particular an inert gas, preferably nitrogen, which is guided in a closed cooling circuit (19) .

13. Process according to Claim 12, characterized in that the cooling medium (18) in the cooling circuit (19) has a slight overpressure, in particular an overpressure of between 1 and 1000 mbar, preferably an overpressure of between 10 and 50 mbar.

14. Apparatus for curing coating material (2) on a substrate (1) having a contour (17) , in particular coating material on an automotive body part, comprising at least one light source (6) for UV radiation (4) and at least one gas concentration means (5) for inert gas, the light source (6) and/or the gas concentration means (5) being adaptable at least sectionally to the contour (17) of the substrate (1) , and comprising at least one supply device (7) for inert gas, the gas concentration means (5) forming between the light source (6) and the substrate (1) a gas concentration space (8) which can be filled with inert gas by means of the supply device (7) and which can be illuminated by the light sources (6) with UV radiation.

15. Apparatus according to Claim 14, characterized by positioning means (9) with which the light sources

(6) and/or the gas concentration means (5) can be moved.

16. Apparatus according to Claim 14 or 15, characterized by a regulator (10) for regulating the respective distances (A) of the light sources (6, 6') and/or the gas concentration means (5) from the substrate (1) .

17. Apparatus according to Claim 16, characterized in that the regulator (10) comprises a distance sensor (11) and/or a gas concentration sensor (12) for determining the concentration of a gas in the gas concentration space (8).

18. Apparatus according to any of the foregoing Claims 14 to 17, characterized by a first preheating means (14) for preheating the substrate (1) .

19. Apparatus according to any of the foregoing Claims 14 to 18, characterized by a second preheating means (15) for preheating the inert gas.

20. Apparatus according to Claim 19, characterized in that the second preheating means (15) is in thermal contact with the light source (6) for the purpose of utilizing the heat it gives off.

21. Apparatus according to any of the foregoing Claims 14 to 20, characterized in that the light source

(6) is arranged in a substantially closed cooling circuit (19) .

22. Apparatus according to Claim 21, characterized in that the cooling circuit comprises a supply line

(21) for a cooling medium, in particular for an inert gas .

23. Apparatus according to either of Claims 21 and 22, characterized in that the cooling circuit comprises a heat exchanger (20) for the removal of heat from the cooling circuit.