Coating apparatus, and coating method

Abstract

To provide a coating method and a coating apparatus using a mixed fluid of carbon dioxide and an ultraviolet curable paint that can be stably coated regardless of coating conditions.SOLUTION: Disclosed are a coating device and a coating method. The coating device comprises: a mixer for mixing a paint and carbon dioxide; a sprayer for spraying a mixed liquid mixed by the mixer to a member to be coated; and a UV light source which is provided between the sprayer and the member to be coated, and irradiates droplets with ultraviolet light before the droplets sprayed from the sprayer reach the member to be coated.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a coating device and a coating method.

Background Art

[0002] Coating is widely applied for the purpose of suppressing deterioration due to aesthetics, corrosion, etc. of transportation infrastructure facilities such as railways and machine parts.

[0003] In coating technology, in order to obtain a good coating appearance and coating quality, it is common to dilute the paint with a volatile organic compound (VOC: Volatile Organic Compound) and use it after reducing the viscosity. For this reason, a major problem in the coating and coating industry is that the emission of VOC from the process is very large. VOC is a harmful chemical substance that leads to global warming and is also a factor in safety problems (such as ignition, explosion, and health hazards), and efforts are being made to reduce emissions.

[0004] For VOC reduction, a technology using carbon dioxide (CO 2 ) instead of an organic solvent has been proposed. For example, Patent Document 1 discloses a coating method in which a mixed fluid obtained by mixing carbon dioxide at a critical pressure or higher with an ultraviolet curable paint is sprayed onto an object to be coated to form a coating film, and the coating film is irradiated with ultraviolet rays (UV: Ultra-Violet) to form an ultraviolet cured film.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] UV curing using ultraviolet light can cure the coating film in a relatively short time by irradiating the coating film with UV, and since curing hardly progresses before UV irradiation, the time during which the paint can be sprayed and used for painting (referred to as pot life) is also long. Therefore, it is expected to achieve both a long pot life and a reduction in curing temperature. On the other hand, for example, UV-curable paints that cure by radical polymerization have a problem that curing may be insufficient depending on the painting conditions when cured in air because the polymerization is inhibited by oxygen. As a countermeasure, it is conceivable to cure the paint in nitrogen so that it does not come into contact with air, but the technique described in Patent Document 1 does not consider such problems.

[0007] An object of the present invention is to provide a coating apparatus and a coating method using a mixed fluid of carbon dioxide and an ultraviolet-curable paint that enable stable coating regardless of coating conditions.

Means for Solving the Problems

[0008] The configuration of the present invention for achieving the above object is as follows.

[0009] A coating apparatus and a coating method, characterized by comprising a mixer that mixes paint and carbon dioxide, a sprayer that sprays the mixed liquid mixed by the mixer onto a member to be coated, and a UV light source that is provided between the sprayer and the member to be coated and irradiates the droplets sprayed from the sprayer with ultraviolet light before the droplets reach the member to be coated.

Effects of the Invention

[0010] According to the present invention, it is possible to provide a coating method and a coating apparatus using a mixed fluid of carbon dioxide and an ultraviolet-curable paint that enable stable coating regardless of coating conditions.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings. The present invention is not limited to the embodiments described herein, and it is possible to appropriately combine with known technologies or improve based on known technologies without departing from the technical idea of the invention.

Examples

[0013] Referring to FIG. 1, the coating apparatus for an ultraviolet curable paint of the present embodiment will be described. The coating apparatus 1 includes a paint supply unit 10, a carbon dioxide supply unit 20, a mixing unit 30, and a spraying unit 40. It is preferable that the paint supply line 14 of the paint supply unit 10 and the carbon dioxide supply line 25 of the carbon dioxide supply unit 20 are separate lines.

[0014] The paint supply unit 10 includes a paint tank 12 for storing the ultraviolet curable paint 11 and a paint high-pressure pump 13 for pressurizing the ultraviolet curable paint to a predetermined pressure on the paint supply line 14. The carbon dioxide supply unit 20 includes a carbon dioxide cylinder 21 for storing carbon dioxide, a cooler 22 for cooling carbon dioxide to a predetermined temperature, a carbon dioxide high-pressure pump 23 for pressurizing carbon dioxide to a predetermined pressure, and a carbon dioxide pressure regulating valve 24 for adjusting the discharge pressure of the carbon dioxide high-pressure pump on the carbon dioxide supply line 25. The mixing unit 30 includes a mixer 31 for mixing the ultraviolet curable paint and carbon dioxide, and a temperature regulator 32 for adjusting the temperature of the mixer 31.

[0015] The spraying unit 40 includes a spray gun (also referred to as a spray gun) 41. The paint supply unit 10, the carbon dioxide supply unit 20, the mixing unit 30, and the spraying unit 40 may be provided with filters, pressure gauges, flow meters 27, a back pressure valve for the carbon dioxide supply line so that the discharge pressure of the carbon dioxide high-pressure pump is maintained at a predetermined pressure (for example, 1 MPa) or more higher than the pressure of the primary mixer, a differential pressure transmitter for calculating the viscosity of the mixed fluid, and a thermometer, etc., as required.

[0016] Also, when performing automatic painting, a robot for holding and transporting the spray gun 41 is installed (not shown). Further, as shown in FIG. 2, in order to perform UV irradiation on the paint droplets 51 being sprayed from the spray gun 41, a UV irradiator 52 is installed between the spray gun and the object (workpiece) 50 to be painted. As the light source of the UV irradiator, a "high-pressure mercury type" that emits ultraviolet rays with a wavelength of 365 nm, a "metal halide type" that emits ultraviolet rays of 200 to 400 nm, a "low-pressure mercury type lamp" that oscillates at wavelengths of 254 nm and 185 nm, or an LED light source is preferably appropriately selected according to the specifications of the ultraviolet curable paint to be used.

[0017] When using a paint that is not suitable for dilution with only carbon dioxide, a conventional organic solvent (thinner) can be used in combination as a diluent, and the paint, carbon dioxide, and thinner can be mixed in the mixer 31. 〔Film formation in painting〕 In film formation in painting, in addition to drying the solvent component contained in the paint and physically forming the non-volatile component into a film, there is a chemical film formation in which the paint components are chemically polymerized and cured to improve the durability of the formed paint film. In the present invention, UV curing that irradiates ultraviolet rays to promote the polymerization reaction of the paint components is used among chemical film formations. UV curing cures in a relatively short time, and the energy required for UV irradiation is relatively small.

[0018] UV curable coatings can be broadly classified into two types: a radical polymerization system mainly composed of acrylate and a cationic polymerization system mainly composed of epoxy, depending on the difference in the photopolymerization system. Among them, it is known that the UV curing of the radical polymerization system has a drawback that the polymerization reaction is inhibited by oxygen, resulting in insufficient curing. As will be described later, when spraying a paint diluted with carbon dioxide for coating, the paint will cure in a carbon dioxide atmosphere (an atmosphere with very little oxygen), so the polymerization reaction is not inhibited, and the paint can be sufficiently cured. 〔UV Curing during CO2 Coating Process〕 Generally, spray coating is carried out in the atmosphere. However, when using CO 2 as a solvent component, the periphery of the sprayed paint will be in an inert gas atmosphere of CO 2 . Therefore, when irradiating the droplets of the sprayed paint with UV light, the polymerization reaction is not inhibited by oxygen. When the sprayed paint droplets are completely cured, no bonding occurs between the droplets, so coating cannot be performed. However, by stopping at semi-curing, for example, a shortening of the two-component polymerization reaction time by the curing agent after coating can be expected. By shortening the two-component polymerization reaction time after spraying, pot life can be ensured while enabling curing in a short time.

[0019] Fig. 3 shows the coating flow according to the present invention. It consists of a step (S1) of mixing the paint and CO 2 , a step (S2) of spraying the mixed paint with a spray gun, a step (S3) of irradiating the sprayed paint droplets before coating with UV light, and a step (S4) of forming a film.

[0020] Here, UV irradiation on the sprayed paint and the two-component polymerization reaction after coating are exemplified, but other polymerization reactions can be combined in the same way. 〔UV Irradiation Method〕 The UV irradiation in the present invention is carried out before the sprayed paint adheres to the workpiece. The UV irradiation position and UV illuminance (ultraviolet intensity) at this time can be arbitrarily set in consideration of the amount of the paint to be sprayed and the required curing amount. Also, a part of the UV may be irradiated on the workpiece surface.

[0021] Figure 4 shows a graph demonstrating the concept of the present invention, with the UV irradiation dose (arbitrary scale) on the horizontal axis, the coating film curing time reduction (arbitrary scale) on the right vertical axis, and the surface roughness of the coating film (arbitrary scale) on the left vertical axis. When the UV irradiation dose increases, the droplets harden before reaching the workpiece, and the hardened droplets remain on the workpiece surface as they are, resulting in an increase in surface roughness (the surface becomes uneven). This is not a problem in the case of textured coating, but when a smooth coating film is required, it is desirable to reduce the UV irradiation dose and adjust it so that the droplets reach the workpiece surface in a liquid state before hardening.

[0022] On the other hand, when the UV irradiation dose is low, the hardening of the droplets is less, so the coating film curing time reduction becomes longer. In the present invention, by irradiating the droplets with UV light in a carbon dioxide atmosphere (inert atmosphere), the radical reaction is promoted, and it is characterized by forming a coating film in a shorter time compared to the case of irradiating with UV light in the atmosphere. Therefore, in order to maximize this effect, the UV light irradiation dose that allows the coating film to cure in a shorter time should be selected. The optimal UV light irradiation dose depends on factors such as the type of paint and UV curing agent, the blending ratio of carbon dioxide as a solvent to the paint, the room temperature and humidity during painting, etc. Therefore, it is desirable to first experimentally determine the optimal value by changing the UV light irradiation dose, and then paint the actual product with that optimal value. For this reason, it is desirable for the coating device to have a function of adjusting the light amount of the UV light source, and it is preferable to be equipped with a dial, button, etc. that can adjust the light amount on the spray gun so that the light amount can be easily adjusted.

[0023] FIG. 5 shows a modified example of the UV light source. In order to uniformly UV-cure the droplets 51 sprayed from the spray gun 41, a ring-shaped light source 60 is used as the UV light source. The ring-shaped light source 60 is fixed to the spray gun 41 via the support columns 61. Thus, even if the orientation of the spray gun 41 changes, the UV light irradiation conditions for the droplets 51 remain unchanged, and a stable coating film can be obtained. Although the support columns 61 are provided at 120° intervals, only two support columns are depicted in this figure because one support column is hidden behind the spray gun. When fixing the UV light source to the spray gun 41, it is desirable to use a light source that is as lightweight as possible from the perspective of workability. As such a light source, ultraviolet LEDs can also be provided at equal intervals on a ring-shaped support column.

[0024] When using an ultraviolet LED as the light source, it is preferable to arrange the light source so that the light is irradiated toward the object to be coated, taking advantage of the directivity of the LED. This is because the droplets 51 sprayed from the spray gun 41 spread in a conical shape during flight toward the object to be coated. That is, the droplets 51 just sprayed from the spray gun 41 are in the shadow of other droplets 51, and even if UV light is irradiated, the UV light is blocked by the presence of other droplets 51, making it difficult for the polymerization reaction to proceed. By irradiating the droplets with UV light in a region close to the object to be coated, UV curing can be efficiently performed.

[0025] Particularly when the paint is a color such as black that is difficult to transmit light, it is preferable to advance the UV curing as much as possible during flight as droplets within a range where the coated surface does not become matte. This is because once the paint adheres to the surface of the object to be coated as a coating film, it becomes difficult for the UV light to reach the inner surface of the coating film (the side close to the object to be coated).

[0026] Note that in the above, an example of diluting the paint using carbon dioxide has been described, but nitrogen (N 2 ) gas, Ar gas, etc. can also be used as the diluting solvent.

[0027] As the paint, it is most preferable to use a radical polymerization-based paint mainly composed of acrylate as described above. However, the effects of the present invention can also be achieved with a paint containing at least any one of urethane resins, unsaturated polyester resins, vinyl ester resins, and epoxy resins.

Explanation of symbols

[0028] 1 Coating apparatus, 10 Paint supply section, 20 Carbon dioxide supply section, 30 Mixing section, 40 Spraying section, 14 Paint supply line, 25 Carbon dioxide supply line, 11 UV curable paint, 12 Paint tank, 13 Paint high-pressure pump, 21 Carbon dioxide cylinder, 22 Cooler, 23 Carbon dioxide high-pressure pump, 24 Carbon dioxide pressure regulating valve, 31 Mixer, 32 Temperature regulator, 41 Spraying gun.

Claims

1. A mixer for mixing paint and carbon dioxide, a sprayer for spraying the mixed liquid mixed by the mixer onto a member to be painted, a UV light source provided between the sprayer and the member to be painted, for irradiating ultraviolet light onto the droplets before the droplets sprayed from the sprayer reach the member to be painted, A painting apparatus characterized by comprising the above.

2. In the painting apparatus according to Claim 1, A painting apparatus characterized by comprising an adjustment mechanism for adjusting the light quantity of the UV light source.

3. In the painting apparatus according to Claim 1 or 2, The UV light source is a ring-shaped light source, and the ring-shaped light source is fixed to the sprayer. A painting apparatus characterized by this.

4. In the painting apparatus according to Claim 3, The ring-shaped light source is one in which a plurality of ultraviolet light LEDs are attached to a ring-shaped support column. A painting apparatus characterized by this.

5. In the painting apparatus according to Claim 4, The optical axis of the ultraviolet light LED is directed toward the member to be painted. A painting apparatus characterized by this.

6. In the painting apparatus according to Claim 1 or 2, A painting apparatus characterized in that part or all of the carbon dioxide is replaced with N2 or Ar.

7. In the painting apparatus according to Claim 1 or 2, The paint is a paint containing at least one of urethane resin, unsaturated polyester resin, vinyl ester resin, and epoxy resin. A painting apparatus characterized by this.

8. In the painting apparatus according to Claim 1 or 2, The sprayer has a shield nozzle for ejecting an inert gas. A painting apparatus characterized by this.

9. In the painting apparatus according to Claim 1 or 2, The mixer mixes an organic solvent in addition to the paint and carbon dioxide. A painting apparatus characterized by this.

10. A mixing step of mixing paint and carbon dioxide to produce a mixed liquid, a spraying step of spraying the produced mixed liquid onto a member to be painted, a UV light irradiation step of irradiating ultraviolet light onto the droplets before the sprayed droplets reach the member to be painted, A painting method characterized by including the above.

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