WO2009063134A2

WO2009063134A2 - Coating an object

Info

Publication number: WO2009063134A2
Application number: PCT/FI2008/050652
Authority: WO
Inventors: Mikael Knif; Kari Soljamo
Original assignee: Tikkurila Oy
Priority date: 2007-11-12
Filing date: 2008-11-12
Publication date: 2009-05-22
Also published as: FI20070857A0; FI124379B; FI20070857A; EP2222415A2; WO2009063134A3

Abstract

The invention relates to a method and apparatus for coating an object (4) in such a way that a composition curable by UV radiation is spread onto the surface of the object, the object is conveyed under an irradiation device (2.1), wherein an inert gas heavier than oxygen is first delivered onto the surface of the object and then a second inert gas is delivered and the object is exposed to radiation.

Description

COATING AN OBJECT

Description

Technical field

The invention relates to the coating of objects, wherein a composition spread onto the surface of an object is cured by UV irradiation. The invention concerns a method and apparatus, wherein an inert gas is delivered onto the surface of an object during the course of irradiation.

Technical background

The coating or sizing of objects can be performed by using compositions suitable for curing by UV irradiation. Thus, onto the surface of an object is spread a composition, which is then cured by irradiation. The composition contains a photoinitiator, which is activated in response to radiation and which sets off a curing reaction. Coatings apt for curing by irradiation are particularly useful when the coating is desired to have a good chemical and wear resistance.

The curing reaction is generally inhibited by oxygen, which is why the curing is often performed in a gas phase more inert than air. Useful as this inert gas is for example nitrogen, carbon dioxide, argon or helium or a mixture of one of these with air. One way is to place the object in an inert gas-filled chamber and to perform the curing there. However, this practice is not always applicable.

Methods have also been proposed, in which a flow of inert gas is delivered onto the surface of an object to be irradiated. For example, publication EP 1 804 243 A2 discloses a method for curing a web-like material. The method involves conveying a web of material under an irradiation device whose width is equal to that of the web. At the same time, an inert gas is delivered from an upstream edge of the device in between the irradiation device and the web and drawn away at a downstream edge of the irradiation device. What is achieved by the arrangement is a laminar flow of inert gas along the surface of a web. General description of the invention

Now, as defined in the independent claims, there has now been invented a method and apparatus for coating an object, and the use thereof for coating especially an object with depressions or protrusions on its surface.

According to the invention, an object is conveyed under an irradiation device, wherein a composition spread onto the object is cured by irradiation. The irradiation device is provided with means for delivering an inert gas heavier than oxygen onto the surface of the object, followed by means for delivering a second inert gas onto the surface of the object. Oxygen is displaced effectively from the surface of the object by the gas heavier than oxygen, even in the presence of depressions. Thus, the second gas can be more appropriate, for example cheaper or easier to use or affecting the irradiation as little as possible.

The heavy gas stays well in depressions despite a turbulence caused by surface irregularities. Consequently, the oxygen concentration in depressions is less than in higher layers, in which oxygen mixes more easily with the inert gas because of turbulence. After all, the depressions are further away from a radiation source, the amount of radiation in the depressions being respectively smaller. Thus, the invention also contributes to a steady curing process.

Typically, the composition to be cured contains monomers, oligomers, or polymers, which polymerize by a radical reaction or a cation mechanism. Hence, the composition contains a photoinitiator, becoming active in response to UV radiation and setting off the reaction. The composition to be spread out can be for example a UV-curable lacquer, a tinted lacquer or paint.

By virtue of the invention, it is possible to use a smaller radiation dose or to accelerate a curing process.

The method according to the invention enables the curing of a composition to be accelerated further by raising the temperature of a composition meted onto an object for achieving a higher reaction rate. This cannot be done in oxygen-laden conditions as the number of oxidation reactions would increase adversely along with the rising temperature. The composition can be heated for example by heating an inert gas to be delivered onto the surface, particularly the first inert gas. Another option for heating the composition is to expose it to IR radiation just before the object proceeds under the irradiation device. The target temperature can be for example 20-50 ⁰C, such as 30-40 ⁰C. Heating improves especially the deep-curing of a composition, i.e. the curing of its core.

The apparatus according to the invention conveniently comprises conveyor-lining elements for precluding leakage of an inert gas. The elements preferably include a minor flow barrier, which is in contact with the irradiation device and fits tightly against the side of the conveyor.

The invention finds application for example in the coating of shaped utensils and upholstery materials and fixtures, such as doors, door frames, window frames, furniture components, and panels. Unless the delivery of a first gas is necessary, like for example whenever the object has a smooth surface (such as floor coverings, smooth panels and fixtures), the first gas delivery can be switched off.

In particular, the first gas can be carbon dioxide and the second gas can be for example nitrogen, argon or helium, especially nitrogen. Sufficiently pure nitrogen can be obtained in situ by removing oxygen from air with membrane filtration.

In the past, UV radiation has been commonly produced by using mercury-arc lamps. The energy consumption of these lamps is very high indeed, and the temperature is also very high (typically about 800 ⁰C). A significant portion of the energy ends up in lost heat, which causes problems with uncontrolled heating of the coating and the substrate. In the presence of oxygen, the mercury-arc lamps produce also ozone. The elimination of ozone and the demand for cooling require a large amount of air.

At present, UV light can be produced by using solid-state emitters, particularly light-emitting diodes (LED), whose energy consumption is considerably less than that of traditional arc lamps. The temperature of emitters is low (typically not higher than about 70 ⁰C), whereby irradiation can also be more easily provided. As opposed to arc lamps, there is no ozone hazard. The service life of emitters is also very long (typically about 20 000 hours) as compared to arc lamps (whose longevity is typically about 2000 hours). Emitters also retain a consistent performance level over the entire longevity. Neither does the emitter experience hardly any wearing as a result of being switched on and off (contrary to arc lamps), thus enabling a further reduction of energy consumption without compromising longevity. Because thermal radiation only develops in a small amount, there are more options in terms of using also materials adversely affected by thermal radiation. The output of such emitters is quite low, which is why a plurality of those have been assembled for larger units, especially so-called panels, comprising hundreds or even thousands of emitters. For example, the above-cited publication EP 1 804 243 A2 and the publication WO 2006/072071 A2 describe such irradiation technology in more detail.

Drawings

The accompanying drawings constitute a part of the written specification and are related to the subsequent detailed description of some embodiments of the invention. In the drawings, fig. 1 shows one irradiation apparatus for implementing the invention, fig. 2 shows a vertical adjustment and a sealing system for the irradiation apparatus, and fig. 3 shows another irradiation apparatus.

Detailed description of some embodiments of the invention

The apparatus shown in fig. 1 includes a mat conveyor 1 and thereabove an irradiation device 2.1 , featuring a UV-LED unit 3 with a multiplicity of light-emitting diodes. Onto an object 4 to be coated is spread a curable composition, after which the object is conveyed under the irradiation device and exposed to radiation for curing the composition.

The irradiation device 2.1 is first provided with carbon dioxide delivery means 5.1 , which are used for meting out carbon dioxide onto the surface of the object 4 arriving under the irradiation device. Being heavier than oxygen, carbon dioxide displaces oxygen effectively from the surface of the object, even when there are depressions on the surface. Still upstream of the UV-LED unit 3 there are nitrogen delivery means 6.1 , which are used for delivering nitrogen onto the surface of the object arriving under the UV-LED unit to establish a flow as smooth as possible, most preferably a laminar flow. As long as the flow remains laminar, the escape of nitrogen is reduced and there may even be some oxygen above the nitrogen layer without affecting the curing reaction. The nitrogen is delivered onto the surface diagonally to the traveling direction. An ellipse in the figure indicates the area for inertiation and irradiation to take place.

The carbon dioxide delivery means 5.1 are also preferably provided with a heater for heating the gas to be delivered onto the surface of an object. The irradiation apparatus 2.1 comprises a sealed surrounding enclosure 7.1 , having its side edges sealed against the conveyor 1. The distance of the irradiation device from the conveyor is adjustable (e.g. up to 200 mm). The irradiation device is set at an exact height which just allows the object to pass thereunder. This enables minimizing escape of the inert gas.

Fig. 2 shows a vertical adjustment and edge sealing for the irradiation device 2.1. The device has on either side a threaded upright post 8, which has been engaged by a threaded fit with a laterally extending bar 9 included in the irradiation device. Each side edge of the irradiation device is provided over its entire length with a panel-like minor flow barrier 10, which extends to below the conveyor surface and mates as tightly as possible with the edge of the conveyor.

An irradiation device 2.2 shown in fig. 3 is likewise provided upstream of a UV- LED unit 3 with carbon dioxide delivery means 5.2 and nitrogen delivery means 6.2. In this case, the carbon dioxide delivery means are housed in a specific tight enclosure 7.2' of their own and the nitrogen delivery means and the UV-LED unit in a specific enclosure 7.2" of their own. The surface of the object 4 features a depression 11 , having its bottom marked with a dashed line.

The distance of the object 4 to be irradiated from the UV-LED unit's 3 light- emitting diodes is for example 2-10 mm.

The UV-LED units 3 are advisably provided with cooling, most preferably by means of a water circulation. Thereby, the curing conditions remain as stable as possible.

The assemblies are preferably provided with an automatic control for deactivating the UV-LED units 3, the cooling and the inertiation in the event that no object to be cured has been present in the curing zone for a certain period of time.

The conveyor 1 has preferably coupled therewith a limit switch, for example an optical one, which stops the conveyor in case the arriving object is too large. A required speed of the conveyor depends on the required amount of radiation. The speed is typically 4-20 m/min. Example

The effect on the curing of a coating composition obtained by the delivery of inerting gases was tested in the apparatus of fig. 1 as follows: On a parquet, used as a specimen and having dimensions: width 20 cm, length 30 cm, and thickness 14 mm, was spread Uvipar Top 2D lacquer in the amount of 10 g/m² and the specimens were fed into the apparatus at two different rates of speed, 4 m/min and 5 m/min. The applied inerting gas was lighter than oxygen nitrogen, which was produced by two nitrogen generators and the flow of which was maintained constant (40 + 40 l/min) throughout the test. The applied inerting gas heavier than oxygen was carbon dioxide and its delivery rate fluctuated within the range of 0- 150 l/min. The gas heavier than oxygen had its delivery unit positioned 120 mm upstream of that of the gas lighter than oxygen and 180 mm upstream of the LED unit. The LED unit had its distance to the top surface of an object to be coated fluctuated within the range of 4-10 mm. The coating was tested for successful curing by scraping the object with a spatula in its forward, central and rear parts. If scratching was observed, the coating process had not been truly successful. The results are presented in table 1.

Table 1

Claims

1. A method for coating an object (4), said method comprising

- spreading onto the surface of an object a composition curable by UV radiation, - conveying the object under an irradiation device (2.1 ; 2.2), wherein an inert gas is delivered onto the surface of the object and the object is exposed to UV radiation for curing the composition, characterized in that

- in the irradiation device (2.1 ; 2.2) there is first delivered an inert gas heavier than oxygen onto the surface of the object, and

- then a second inert gas is delivered onto the surface and the object is exposed to UV radiation.

2. A method as set forth in claim 1 , wherein the UV radiation is produced by solid-state emitters.

3. A method as set forth in claim 2, wherein the UV radiation is produced by light-emitting diodes.

4. A method as set forth in any of claims 1-3, wherein the inert gas heavier than oxygen is carbon dioxide.

5. A method as set forth in any of claims 1-5, wherein the second inert gas is nitrogen.

6. A method as set forth in any of claims 1-5, wherein the object has depressions or protrusions on its surface.

7. A method as set forth in any of claims 1-6, wherein the composition meted onto the object is heated prior to irradiation, preferably by means of an inert gas.

8. An apparatus for the curing by irradiation of a UV-radiation curable composition spread onto the surface of an object (4), said apparatus comprising - a conveyor (1 ), - an irradiation device (2.1 ; 2.2) above the conveyor, including inert gas delivery means and a UV radiation source (3), characterized in that - the irradiation device is provided with delivery means (5.1 ; 5.2) for an inert gas heavier than oxygen, and

- downstream thereof, delivery means (6.1 ; 6.2) for a second inert gas.

9. An apparatus as set forth in claim 8, which is provided with elements (10) for precluding escape of the inert gas across the side of the conveyor.

10. The use of an apparatus as set forth in claim 8 or 9 for curing a coating on an object which has depressions or protrusions on its surface.