Application method using ultraviolet-curable paint and laminated paint film

Abstract

In order to provide an application method and a laminated paint film capable of using the same material to secure glossiness and adhesiveness, the present invention comprises: a first process (ST11-ST13) of applying a UV-curable paint to a target application surface (3) and emitting UV light to form a first cured paint film layer (261) having porosity of 30%-50% or surface roughness Rz of 0.1-10 μm; and a second process (ST14-ST16) of applying the UV-curable coating to the surface of the first cured paint film layer (261) and emitting the UV light to form a second cured paint film layer (262) having porosity of 0%-20% or surface roughness Rz of less than 0.1 μm.

Description

Coating method using ultraviolet curing paint and multilayer coating film

[0001] The present invention relates to a coating method using an ultraviolet-curable coating material and a multilayer coating film.

[0002] In order to change the appearance of a floor surface, a surface modification structure is known in which an adhesive coating, a mark-containing layer, and a top coating are laminated on the surface (Patent Document 1).

[0003] Special Publication No. 2005-513309

[0004] However, in the above-mentioned prior art, in order to ensure the glossiness of the surface modification structure and the adhesiveness of each layer constituting the structure, it was necessary to use different materials for the top coating and adhesive coating.

[0005] The problem to be solved by the present invention is to provide a coating method and a multilayer coating film that can ensure both gloss and adhesion using the same material.

[0006] In the present invention, an ultraviolet curing paint is applied to a target surface, and ultraviolet light is irradiated to the surface to increase the porosity to P １ % or surface roughness Rz is R １ After forming a first cured coating layer having a thickness of P μm, a solvent-containing ultraviolet curable coating material is applied to the surface of the first cured coating layer, and ultraviolet light is irradiated to the first cured coating layer to increase the porosity to P ２ % (where P ２ <P １ ) or the surface roughness Rz is R ２ μm (where R ２ <R １ The above problem is solved by forming a second cured coating layer of

[0007] According to the present invention, by using ultraviolet-curable coating materials of the same material, adhesion can be ensured by the first cured coating film layer, and gloss can be ensured by the second cured coating film layer.

[0008] 1 is a structural diagram showing an example of an inkjet coating device used in a coating method using an ultraviolet-curable paint according to the present invention; 2 is a structural diagram showing an example of a coating system used in a coating method using an ultraviolet-curable paint according to the present invention; 3 is a cross-sectional view of a coating film for explaining the mechanism of generation of a first cured coating film layer (matte coating film) and a second cured coating film layer (glossy coating film) according to the present invention; 4 is a cross-sectional view of a coating film showing one embodiment of a coating method using an ultraviolet-curable paint according to the present invention; and 5 is a cross-sectional view of a coating film for explaining the mechanism of generation of a first cured coating film layer (matte coating film) and a second cured coating film layer (glossy coating film) according to the present invention.

[0009] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The coating method of this embodiment is a coating method using an ultraviolet-curable paint, and in particular, is a coating method that controls the gloss and unevenness of the dried coating film surface depending on the drying method of the wet coating film of the applied ultraviolet-curable paint. The coating method of this embodiment is not particularly limited, but can be applied to the coating process of automobile parts such as outer and inner panels of automobile bodies and bumpers.

[0010] <<Ultraviolet-Curable Coating of the Present Embodiment>> The ultraviolet-curable coating used in the coating method of the present embodiment can be a coating obtained by blending a known ultraviolet-curable resin composition with, as needed, a colorant and known coating additives (for example, an antifoaming agent such as silicone oil, a fluorine-based surfactant, a silicone-based surfactant, a leveling agent such as an acrylic copolymer, a thickener, a viscosity reducer, etc.). Ultraviolet-curable resin compositions are broadly classified into ultraviolet-radical-curable resin compositions and cation-curable resin compositions, and either type of ultraviolet-curable resin composition can be used in the coating method of the present embodiment.

[0011] The ultraviolet radical curable resin composition contains an ultraviolet curable monomer or prepolymer and a photoradical polymerization initiator. The ultraviolet curable monomer or prepolymer is a monomer or prepolymer having multiple ethylenically unsaturated groups in the molecule, or a mixture thereof, and typical examples thereof include epoxy acrylate resin, urethane acrylate resin, thermosetting acrylic resin, and thermosetting polyester resin.

[0012] Representative examples of photoradical polymerization initiators used in combination with ultraviolet-curable monomers or prepolymers include benzoin and its alkyl ethers, such as benzoin, benzoin methyl ether, benzoin ethyl ether, and benzoin isopropyl ether; acetophenone, 2,2-dimethoxy-2-phenylacetophenone, 2,2-diethoxy-2-phenylacetophenone, 1,1-dichloroacetophenone, 1-hydroxycyclohexyl phenyl ketone, and 2-hydroxycyclohexyl phenyl ketone. anthraquinones such as 2-methylanthraquinone and 2-amylanthraquinone; thioxanthone such as 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, 2-chlorothioxanthone and 2,4-diisopropylthioxanthone; ketals such as acetophenone dimethyl ketal and benzyl dimethyl ketal; benzophenones or xanthones such as benzophenone; and the like.

[0013] Such a photoradical polymerization initiator is generally used in a proportion of 0.1 to 30 parts by weight, particularly 1 to 25 parts by weight, per 100 parts by weight of the above-mentioned ultraviolet-curable resin component. In addition, at least one of well-known and commonly used photopolymerization accelerators such as benzoic acid-based or tertiary amine-based accelerators may also be used together with the photoradical polymerization initiator.

[0014] In contrast, the cationically curable resin composition contains an ultraviolet-curable epoxy resin as a resin component and a cationic ultraviolet polymerization initiator as a photopolymerization initiator.

[0015] The ultraviolet-curable epoxy resin contains an epoxy resin component having an alicyclic group in the molecule and adjacent carbon atoms of the alicyclic group forming an oxirane ring, and for example, epoxy compounds having at least one epoxycycloalkane group in the molecule, such as an epoxycyclohexane ring or an epoxycyclopentane ring, are used alone or in combination. Suitable examples of the ultraviolet-curable epoxy resin include vinylcyclohexene diepoxide, vinylcyclohexene monoepoxide, 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate, 2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexane-m-dioxane, bis(3,4-epoxycyclohexyl)adipate, limonene dioxide, etc.

[0016] The cationic ultraviolet polymerization initiator used in combination with the epoxy resin is decomposed by ultraviolet light to release a Lewis acid, which has the effect of polymerizing the epoxy group. Suitable examples thereof include aromatic iodonium salts, aromatic sulfonium salts, aromatic selenium salts, and aromatic diazonium salts.

[0017] <<Embodiment of Inkjet Coating Apparatus>> When applying the above-mentioned ultraviolet-curable paint to a target surface, an air atomization paint spray gun, a rotary atomization paint gun (a so-called bell-type paint gun), or the like can be used, or a so-called inkjet coater (a machine that prints by spraying minute droplets of ink from a thin nozzle onto paper) can also be used. Inkjet coaters produce significantly less paint dust than air atomization paint spray guns or rotary atomization paint guns, which has the advantage of significantly improving paint usage efficiency and significantly improving the working environment.

[0018] 1 is a structural diagram showing one embodiment of an inkjet coating apparatus 1 used in a coating method using an ultraviolet-curable paint according to the present invention. The inkjet coating apparatus 1 according to this embodiment is widely known as an "inkjet coating apparatus," and therefore the term "ink" will be used in this specification. However, since the objects to be coated according to the present invention are automobile parts such as the outer and inner panels of an automobile body and bumpers, this ink actually refers to "automotive coating paint" (the above-mentioned ultraviolet-curable paint).

[0019] The inkjet coating device 1 of this embodiment comprises a nozzle 11 having an inlet 111 for the paint 2, a paint chamber 112 and an outlet 113 for the paint 2, a needle 12 having at least a tip 121 arranged in the paint chamber 112 and capable of moving back and forth in the axial direction Y toward the outlet 113, an actuator 13 that moves the needle 12 back and forth in the axial direction Y so that the tip 121 approaches the outlet 113 when the needle 12 advances and moves away from the outlet 113 when the needle 12 retreats, a pressure sensor 14 that detects the pressure of the paint 2 in the paint chamber 112, and a control unit 15 that controls the actuator 13.

[0020] The nozzle 11 has a hollow housing 114 made of a metal, resin, or ceramic material, with an inlet 111 formed on one side and an outlet 113 at the tip, and a paint chamber 112 formed inside. Paint 2 is introduced from the inlet 111 into the paint chamber 112 and is ejected (dribbled) from the outlet 113 to the outside by being pushed by the needle 12. The interior of the housing 114 is partitioned liquid-tightly into the paint chamber 112 and an actuator chamber 115 by a seal member 123.

[0021] The needle 12 is a needle-shaped rod made of metal, resin, or ceramic material, with a tip end 121 disposed in the paint chamber 112 and a base end 122 disposed in the actuator chamber 115, with a seal member 123 provided therebetween. The actuator 13 is fixed to the base end 122 of the needle 12. The needle 12 is provided within the housing 114 so as to be movable back and forth in the axial direction Y.

[0022] The actuator 13 is, for example, a stack of multiple piezoelectric elements, and has the property of expanding and contracting in the axial direction Y in response to a voltage applied to the electrodes. The application of voltage to the actuator 13 is executed by the control unit 15, and by applying a voltage to the actuator 13 in response to a command signal from the control unit 15, the needle 12 can be moved back and forth in the axial direction Y. Note that the stroke start position of the needle 12 may also be controlled based on the pressure of the paint 2 in the paint chamber 112 detected by the pressure sensor 14.

[0023] The paint 2 in this embodiment is the ultraviolet curable paint described above, and is stored in a paint tank 21 in a state where it has been adjusted to an appropriate viscosity, and is supplied by a paint pump 23 via a paint pipe 22. Note that a paint pipe may be provided to return the paint 2 introduced into the paint chamber 112 of the nozzle 11 to the paint tank 21, so that the paint 2 is supplied while circulating.

[0024] Fig. 2 is a configuration diagram showing an example of a coating system used in the coating method using an ultraviolet-curable paint according to the present invention. The coating system of this embodiment includes the inkjet coating device 1 described with reference to Fig. 1, an ultraviolet irradiator 4 that irradiates ultraviolet light onto a wet coating film 25 of ultraviolet-curable paint applied to a coating surface 3, a laser displacement meter 5 that measures the thickness of the wet coating film 25, a heater 6 that heats the wet coating film 25 applied to the coating surface 3, and a controller 7 that controls the ultraviolet irradiator 4, the laser displacement meter 5, and the heater 6.

[0025] The embodiment shown in FIG. 2 illustrates an example of an inkjet coating device 1 in which four nozzles 11 are provided for a target coating surface 3. In FIG. 2, if a plane parallel to the coating surface 3 is defined as the X-Y plane and an axis perpendicular to the X-Y plane is defined as the Z axis, the four nozzles 11 are arranged in a row along the Y axis. The four nozzles 11 drop droplets 24 of paint 2 onto the coating surface 3 while moving in a direction (X axis direction) perpendicular to the direction in which the nozzles 11 are arranged (Y axis direction), thereby forming four rows of a wet coating film 25 on the surface of the coating surface 3. The distance between two adjacent nozzles 11 is approximately equivalent to one droplet 24 of paint 2. As shown in FIG. 2, the droplets 24 of paint 2 dropped from the four nozzles 11 form a continuous wet coating film 25 on the coating surface 3. Note that, for the sake of simplicity, an inkjet coating device 1 including four nozzles 11 is exemplified in this specification; however, an appropriate number of nozzles 11 can be arranged in a row depending on the size of the coating surface 3.

[0026] In this specification, the paint 2, droplets 24 of paint 2, wet coating film 25, and cured coating film 26 are used as different technical terms. That is, the liquid material from the paint tank 21 until it adheres to the coating surface 3 is collectively referred to as paint 2, and the granular liquid material from the discharge portion 113 of the nozzle 11 until it adheres to the coating surface 3 is referred to as droplets 24 of paint 2. In contrast, the liquid or semi-cured film from the time when the droplets 24 of paint 2 adhere to the coating surface 3 until it is cured by ultraviolet irradiation is referred to as wet coating film 25, and the coating film obtained by curing the wet coating film is referred to as cured coating film 26.

[0027] The ultraviolet irradiator 4 is provided in a process subsequent to the application process of the ultraviolet-curable paint. For convenience, one ultraviolet irradiator 4 is shown in Fig. 2, but when the object to be coated has a large surface area 3, such as an automobile body or automobile part, multiple ultraviolet irradiators 4 may be provided inside a tunnel-type oven, such as a coating and drying oven for bake-curable paint, and ultraviolet light may be irradiated onto the automobile body or automobile part while it is being transported. The ultraviolet irradiator 4 controls the ultraviolet irradiation intensity, irradiation time, etc., by control signals from a controller 7.

[0028] The laser displacement meter 5 irradiates the surface of the wet coating film 25 coated by the inkjet coating device 1 with laser light and detects the displacement from the state before coating, thereby measuring the film thickness of the wet coating film 25 in real time. The detected displacement of the wet coating film 25 is output to the controller 7, which then calculates the wet coating film 25. The controller 7 then outputs a control signal to the ultraviolet irradiator 4 according to the calculated film thickness of the wet coating film 25. For example, the controller 7 outputs a control signal to the ultraviolet irradiator 4 to increase the irradiation intensity of ultraviolet light or lengthen the irradiation time as the film thickness of the wet coating film 25 increases.

[0029] The heater 6 is a heating means for heating the wet coating film 25 applied to the coating surface 3, and is arranged on the back surface of the coating surface 3 as well as in the vicinity of the wet coating film 25 as shown in the figure. Alternatively, heat emitted from the ultraviolet irradiator 4 may be utilized. The heater 6 of this embodiment, or an alternative ultraviolet irradiator 4, is provided to evaporate a volatile solvent contained in the wet coating film 25 before curing the wet coating film 25 by irradiating it with ultraviolet light. Here, the technical significance of evaporating a volatile solvent contained in the wet coating film 25 of an ultraviolet-curable paint before curing the wet coating film 25 by ultraviolet light will be explained. Note that typical ultraviolet-curable paints contain a volatile solvent to improve the dispersibility of the resin component and the photopolymerization initiator, and this volatile solvent evaporates before and after curing by ultraviolet light.

[0030] 3 is a cross-sectional view of a coating film for explaining the mechanism of formation of the first cured coating film layer (matte coating film) and the second cured coating film layer (gloss coating film) according to the present invention. In FIG. 3, the cross-section of a coating film in which a volatile solvent contained in a wet coating film 25 of an ultraviolet-curable paint applied to a coating surface 3 is dried over time and then irradiated with ultraviolet light to form a cured coating film 26 is shown in steps ST1 → ST2 → ST3, and the cross-section of a coating film in which a volatile solvent contained in a wet coating film 25 of an ultraviolet-curable paint applied to a coating surface 3 is irradiated with ultraviolet light while drying the volatile solvent is shown in steps ST1 → ST4 → ST5 to form a cured coating film 26.

[0031] As shown in step ST1 of Figure 3, an ultraviolet-curable paint is applied to the coating surface 3 to form a wet coating film 25. Then, as shown in step ST2, the wet coating film 25 is left at room temperature for a sufficiently long time. The volatile solvent slowly evaporates before the resin components inside the wet coating film 25 harden, resulting in almost no stirring inside the coating film. When ultraviolet light is applied in this state, the resin components are photo-cured after the volatile solvent has evaporated, resulting in a cured coating film 26 with a smooth, glossy surface, as shown in step ST3. In this embodiment, the cured coating film 26 thus obtained is also referred to as a second cured coating film layer 262 or a glossy coating film.

[0032] In contrast, as shown in step ST1 of FIG. 3 , an ultraviolet-curable paint is applied to the coating surface 3 to form a wet coating film 25. Then, as shown in step ST4, when the wet coating film 25 is heated by a heater 6, the volatile solvent inside the wet coating film 25 rapidly evaporates, causing agitation within the coating film. When ultraviolet light is irradiated in this state, the volatile solvent evaporates while the resin component photo-cures, resulting in a cured coating film 26 with an uneven surface and no gloss, as shown in step ST5. Even if the wet coating film 25 is cured with ultraviolet light while evaporating the volatile solvent with the heat of an ultraviolet irradiator 4 instead of the heater 6, agitation occurs within the wet coating film 25, resulting in a cured coating film 26 with an uneven surface and no gloss, as shown in steps ST4 and ST5. In this embodiment, the cured coating film 26 with an uneven surface and no gloss obtained in this manner is also referred to as a first cured coating film layer 261 or a matte coating film. A matte coating film is also called a matte coating film.

[0033] The coating method of this embodiment utilizes the agitation phenomenon that occurs during the evaporation of the volatile solvent in the ultraviolet-curable paint, and is characterized by applying a second cured coating film layer 262 having a smooth and glossy surface, as shown in steps ST2 to ST3 (where the agitation phenomenon is not generated), and a first cured coating film layer 261 having an uneven and non-glossy surface, as shown in steps ST4 to ST5 (where the agitation phenomenon is generated). The surface of the first cured coating film layer 261, which has an uneven and non-glossy surface, has a large surface area at the adhesive interface, which is expected to have an anchoring effect. Therefore, by laminating the second cured coating film layer 262 on the surface of this first cured coating film layer 261, the adhesion between the first cured coating film layer 261 and the second cured coating film layer 262 is enhanced. Furthermore, since the smooth and glossy second cured coating film layer 262 is present as the outermost layer, a smooth and glossy coating film can be provided as the final laminated coating film.

[0034] In the coating method and laminated coating film of this embodiment, the second cured coating film layer 262 (glossy coating film) having a smooth and glossy surface as shown in step ST3 refers to a cured coating film 26 having a surface roughness (ten-point average roughness Rz according to the Japanese Industrial Standards) of less than 0.1 μm, a porosity of 0 to 20%, and a gloss (20° specular gloss according to the Japanese Industrial Standards) of 90 or more.

[0035] The surface roughness Rz in this embodiment refers to the ten-point average roughness as defined in Japanese Industrial Standards (JIS) B 0601. A reference length is extracted from the roughness curve in the direction of the mean line, and the sum of the average absolute values ​​of the elevations (Yp) of the top five highest peaks and the average absolute values ​​of the elevations (Yv) of the bottom five lowest valleys is calculated from the average line in the longitudinal direction of the roughness curve. This sum is expressed in micrometers (μm). Furthermore, the porosity in this embodiment refers to the percentage of the area of ​​voids (e.g., voids) per unit area of ​​the cross section of the cured coating film 26. This porosity can be obtained by enlarging the cross section of the cured coating film 26 and measuring the area of ​​the voids visually or otherwise.

[0036] The glossiness in this embodiment is a numerical representation of the intensity ratio between incident light and specularly reflected light when an object is irradiated with light, and refers to the 20° specular glossiness measured in accordance with the Japanese Industrial Standards Specular Gloss Measurement Method (JIS Z8741-1997). The glossiness can be measured, for example, using a gloss measuring device (MICRO-GLOSS, manufactured by BYK Gardner).

[0037] In contrast, in the coating method and laminated coating film of this embodiment, the first cured coating film layer 261 having an uneven, non-glossy surface as shown in step ST5 refers to a cured coating film 26 having a surface roughness (ten-point average roughness Rz according to the Japanese Industrial Standards) of 0.1 to 10 μm, a porosity of 30 to 50%, and a gloss (20° specular gloss according to the Japanese Industrial Standards) of 10 or less.

[0038] Next, a coating method of this embodiment will be described. In the coating method of this embodiment, an ultraviolet curing paint is applied to a target coating surface 3 to form a wet coating film 25, and ultraviolet light is irradiated onto the wet coating film 25 to increase the porosity to P １ % or surface roughness Rz is R １ The first step is to form a first cured coating layer 261 having a thickness of P μm, and the second step is to apply an ultraviolet curable paint to the surface of the first cured coating layer 261 to form a wet coating film 25, which is then irradiated with ultraviolet light to form a wet coating film having a porosity of P ２ % (where P ２ <P １ ) or the surface roughness Rz is R ２ μm (where R ２ <R １ and a second step of forming a second cured coating layer 262 having a relatively large porosity or surface roughness Rz on the coating surface 3. That is, after forming a first cured coating layer 261 having a relatively large porosity or surface roughness Rz on this surface, a second cured coating layer 262 having a relatively small porosity or surface roughness Rz is formed on this surface.

[0039] A more specific example will be described below. Figure 4 is a cross-sectional view of a coating film showing one embodiment of a coating method using an ultraviolet curable coating material according to the present invention.

[0040] First, in step ST11, an ultraviolet-curable paint is applied to the coating surface 3 to form a wet coating film 25 with a predetermined thickness. The predetermined thickness is not particularly limited, but is, for example, 5 to 50 μm. In the subsequent step ST12, ultraviolet light is irradiated onto the wet coating film 25 in a state where the amount of evaporation of the volatile solvent contained in the wet coating film 25 is less than a predetermined value, in other words, the wet coating film 25 contains a sufficient amount of volatile solvent, so that the wet coating film 25 applied to the coating surface 3 becomes a first cured coating film layer 261 (matte coating film) with no gloss. Specifically, the wet coating film formed on the coating surface 3 is irradiated with ultraviolet light without being subjected to heat treatment, or is irradiated with ultraviolet light after being subjected to a short heat treatment less than the first predetermined time. In either case, the time between the formation of the wet coating film 25 and the irradiation with infrared light is not set long. This causes a stirring phenomenon when the resin component of the wet coating film 25 is photocured by irradiation with infrared light, resulting in a first cured coating film layer 261 (matte coating film) with an uneven surface, as shown in the subsequent step ST13.

[0041] In the subsequent step ST14, an ultraviolet-curable paint is applied to the surface of the first cured coating layer 261 to form a wet coating film 25 with a predetermined thickness. The predetermined thickness is not particularly limited, but is, for example, 5 to 50 μm. In the subsequent step ST15, ultraviolet light is irradiated onto the wet coating film 25 in a state where the amount of evaporation of the volatile solvent contained in the wet coating film 25 is equal to or greater than a predetermined value, in other words, the wet coating film 25 is almost free of volatile solvent, so that the wet coating film 25 applied to the surface of the first cured coating layer 261 becomes a glossy second cured coating film 262 (glossy coating). Specifically, the wet coating film formed on the coating surface 3 is subjected to a heat treatment for a first predetermined time or more before being irradiated with ultraviolet light, or ultraviolet light is irradiated after a predetermined time has elapsed since the application of the ultraviolet-curable paint. In either case, the time between the formation of the wet coating film 25 and the irradiation with infrared light is set long. This prevents stirring when the resin component of the wet coating film 25 is photocured by irradiation with infrared rays, and a second cured coating film layer 262 (glossy coating film) with a smooth surface is obtained, as shown in the subsequent step ST16.

[0042] In the embodiment shown in Fig. 4, the laminated coating film is composed of two layers, the first cured coating film layer 261 and the second cured coating film layer 262, but the laminated coating film may be composed of three or more layers. That is, although not shown, an ultraviolet curable paint is applied to the target coating surface 3 to form a wet coating film 25, which is then irradiated with ultraviolet light to form a coating film with a porosity of P １ % or surface roughness Rz is R １ Next, a UV-curable paint is applied to the surface of the first cured coating layer to form a wet coating film 25, which is then irradiated with UV light to form a first cured coating film with a porosity of P ２ % (where P ２ <P １ ) or the surface roughness Rz is R ２ μm (where R ２ <R １ Next, a UV-curable paint is applied to the surface of the second cured coating layer to form a wet coating film 25, which is then irradiated with UV light to form a second cured coating film with a porosity of P ３ % (where P ３ <P ２ ) or the surface roughness Rz is R ３ μm (where R ３ <R ２ A multi-layer coating film consisting of four or more layers can be formed by repeating this process.

[0043] As described above, according to the coating method using the ultraviolet curing coating material of this embodiment, the ultraviolet curing coating material is applied to the target coating surface 3, and ultraviolet rays are irradiated to the coating surface 3, so that the porosity becomes P １ % or surface roughness Rz is R １ The first step ST11 to ST13 is to form a first cured coating layer 261 having a thickness of P μm, and the second step ST13 is to apply an ultraviolet curable coating material to the surface of the first cured coating layer 261 and irradiate it with ultraviolet light to form a layer having a porosity of P ２ % (where P ２ <P １ ) or the surface roughness Rz is R ２ μm (where R ２ <R １) and second steps ST14 to ST16 of forming the second cured coating layer 262. Therefore, by using the same material of ultraviolet curable paint, adhesion can be ensured by the first cured coating layer 261, and gloss can be ensured by the second cured coating layer 262.

[0044] Furthermore, according to the coating method using the ultraviolet-curable paint of this embodiment, the first cured coating layer 261 has a porosity of 30 to 50% and a surface roughness Rz of 0.1 to 10 μm, and the second cured coating layer 262 has a porosity of 0 to 20% and a surface roughness Rz of less than 0.1 μm. Therefore, even when using ultraviolet-curable paint of the same material, adhesion can be further ensured by the first cured coating layer 261, and gloss can be further ensured by the second cured coating layer 262.

[0045] Furthermore, according to the coating method using ultraviolet-curable paint of this embodiment, the ultraviolet-curable paint used in the first step and the ultraviolet-curable paint used in the second step are supplied from the same paint supply system (paint tank 21, paint piping 22, and paint pump 23), thereby reducing equipment costs.

[0046] Furthermore, according to the coating method using the ultraviolet-curable paint of this embodiment, in the first step, ultraviolet rays are irradiated when the amount of evaporation of the solvent contained in the wet coating film 25 applied to the coating surface 3 is less than a predetermined value, and in the second step, ultraviolet rays are irradiated when the amount of evaporation of the solvent contained in the wet coating film 25 applied to the surface of the first cured coating film layer 261 is equal to or greater than the predetermined value. Therefore, even when using ultraviolet-curable paint of the same material, it is possible to further ensure adhesion by the first cured coating film layer 261 and gloss by the second cured coating film layer 262.

[0047] Furthermore, according to the coating method using the ultraviolet-curable paint of this embodiment, in the first step, ultraviolet rays are irradiated to the wet coating film 25 without heat treatment, or ultraviolet rays are irradiated after heat treatment for less than a first predetermined time, so that the amount of evaporation of the solvent contained in the wet coating film 25 applied to the coating surface 3 is less than a predetermined value, thereby making it possible to generate the first cured coating film layer 261 more reliably.

[0048] Furthermore, according to the coating method using the ultraviolet-curable paint of this embodiment, in the second step, the wet coating film 25 applied to the surface of the first cured coating layer 261 is subjected to a heat treatment for a first predetermined time or more before being irradiated with ultraviolet light so that the amount of evaporation of the solvent contained in the wet coating film 25 is equal to or greater than a predetermined value, or ultraviolet light is irradiated after a time exceeding a second predetermined time has elapsed since the application of the ultraviolet-curable paint, thereby making it possible to generate the second cured coating layer 262 even more reliably.

[0049] According to the coating method using the ultraviolet curing coating material of this embodiment, the ultraviolet curing coating material is applied to the target coating surface, and ultraviolet rays are irradiated to the coating material to increase the porosity to P. １ % or surface roughness Rz is R １ a first step of forming a first cured coating film layer having a thickness of P μm; a step of applying an ultraviolet curable coating material to the surface of the first cured coating film layer and irradiating ultraviolet light thereon to form a first cured coating film layer having a porosity of P μm; ２ % (where P ２ <P １ ) or the surface roughness Rz is R ２ μm (where R ２ <R １ a second step of forming a second cured coating layer of the above-mentioned second cured coating layer, and a second step of applying an ultraviolet curable coating material to the surface of the second cured coating layer and irradiating the coating with ultraviolet light to increase the porosity to P ３ % (where P ３ <P ２ ) or the surface roughness Rz is R ３ μm (where R ３ <R ２ and a third step of forming a third cured coating layer of the same material. Therefore, even for a laminated coating film of three or more layers, the same ultraviolet-curable paint can be used, ensuring adhesion through the first and second cured coating layers, and gloss through the third cured coating layer.

[0050] REFERENCE SIGNS LIST 1... Inkjet coating device 11... Nozzle 111... Introduction section 112... Paint chamber 113... Discharge section 114... Housing 12... Needle 121... Tip section 122... Base section 123... Sealing member 13... Actuator 14... Pressure sensor 15... Control section 2... Paint 21... Paint tank 22... Paint piping 23... Paint pump 24... Droplet 25... Wet coating film 26... Cured coating film 261... First cured coating film layer (matte coating film) 262... Second cured coating film layer (glossy coating film) 3... Coating surface 4... Ultraviolet irradiator 5... Laser displacement meter 6... Heater 7... Controller

Claims

1. Apply ultraviolet curing paint to the target surface, irradiate with ultraviolet light, and the porosity is increased to P １ % or surface roughness Rz is R １ a first step of forming a first cured coating layer having a thickness of 1 μm; An ultraviolet curing paint is applied to the surface of the first cured coating film layer, and ultraviolet light is irradiated to the surface to make the porosity P ２ % (where P ２ <P １ ) or the surface roughness Rz is R ２ μm (where R ２ <R １ and a second step of forming a second cured coating film layer of the ultraviolet-curable paint.

2. the first cured coating layer has a porosity of 30 to 50% and a surface roughness Rz of 0.1 to 10 μm; 2. A coating method using an ultraviolet-curable coating material according to claim 1, wherein the second cured coating layer has a porosity of 0 to 20% and a surface roughness Rz of less than 0.1 μm.

3. 3. The coating method using ultraviolet curable paint according to claim 1, wherein the ultraviolet curable paint used in the first step and the ultraviolet curable paint used in the second step are supplied from the same paint supply system.

4. In the first step, ultraviolet light is irradiated in a state where the amount of evaporation of the solvent contained in the wet coating film applied to the coating surface is less than a predetermined value, 3. A coating method using an ultraviolet-curable paint according to claim 1 or 2, wherein in the second step, ultraviolet light is irradiated in a state where the amount of evaporation of the solvent contained in the wet coating film applied to the surface of the first cured coating film layer is equal to or greater than the predetermined value.

5. In the first step, Irradiating the wet coating film with ultraviolet light without heat treatment so that the amount of evaporation of the solvent contained in the wet coating film applied to the coating surface becomes less than a predetermined value, or 5. The coating method using an ultraviolet curable coating material according to claim 4, wherein the coating is subjected to a heat treatment for less than the first predetermined time before irradiating with ultraviolet light.

6. In the second step, or, after subjecting the wet coating film to heat treatment for the first predetermined time or more so that the amount of evaporation of the solvent contained in the wet coating film applied to the surface of the first cured coating film layer becomes equal to or greater than the predetermined value, irradiating the wet coating film with ultraviolet light; 6. The coating method using ultraviolet curable paint according to claim 5, wherein the ultraviolet ray is irradiated after a time exceeding a second predetermined time has elapsed since the application of the ultraviolet curable paint.

7. Apply ultraviolet curing paint to the target surface, irradiate with ultraviolet light, and the porosity is increased to P １ % or surface roughness Rz is R １ a first step of forming a first cured coating layer having a thickness of 1 μm; An ultraviolet curing paint is applied to the surface of the first cured coating film layer, and ultraviolet light is irradiated to the surface to make the porosity P ２ % (where P ２ <P １ ) or the surface roughness Rz is R ２ μm (where R ２ <R １ a second step of forming a second cured coating layer of An ultraviolet curable paint is applied to the surface of the second cured coating film layer, and ultraviolet light is irradiated to the surface to make the porosity P ３ % (where P ３ <P ２ ) or the surface roughness Rz is R ３ μm (where R ３ <R ２ and a third step of forming a third cured coating film layer of the ultraviolet curable paint.

8. 8. The coating method using ultraviolet curable paint according to claim 7, wherein the ultraviolet curable paint used in the first step, the ultraviolet curable paint used in the second step, and the ultraviolet curable paint used in the third step are supplied from the same paint supply system.

9. 8. The coating method according to claim 1, wherein the surface to be coated is an automobile body or an automobile part.

10. The upper and lower layers laminated on the target coating surface are made of cured coating films of ultraviolet curing paint, The porosity of the lower coating film is greater than the porosity of the upper coating film; or A multilayer coating film in which the surface roughness Rz of the lower coating film is greater than the surface roughness Rz of the upper coating film.

11. The multilayer coating film according to claim 10, wherein the lower coating film has a porosity of 30 to 50% and a surface roughness Rz of 0.1 to 10 μm, and the upper coating film has a porosity of 0 to 20% and a surface roughness Rz of less than 0.1 μm.

12. 12. The multilayer coating film according to claim 10, wherein the surface to be coated is an automobile body or an automobile part.