Coating method and coating system

Abstract

To provide a coating method and a coating system which can reduce carbon dioxide discharge amount.SOLUTION: A coating method for performing coating a surface of a base material 60 with electro-deposition coating 20 and powder coating 30, comprises: an electro-deposition coating process which deposits electro-deposition coating on the surface of the base material; a moisture percentage adjustment process which adjusts moisture percentage at a temperature lower than a hardening temperature of the electro-deposition coating adhered to the base material to form electro-deposition interim coating film 22; a powder coating process which sprays the powder coating on the electro-deposition interim coating film to form powder interim coating film 32 on the electro-deposition interim coating film where the power coating is immersed on a surface layer of the electro-deposition interim coating film to form an uncured multi-phase layer 40 at a lower layer of the powder interim coating film; and a thermosetting process which heats the base material after the power coating process at a temperature higher than the hardening temperature of the power coating to harden the electro-deposition interim coating film, the uncured multi-phase layer and the power interim coating film so as to form a coating system 10.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method and system for coating a substrate. [Background technology]

[0002] Electrodeposition coating is widely used as a method for coating substrates such as agricultural machinery, machine tools, construction machinery, automobiles, etc. Electrodeposition coating makes it easy to apply a uniform and flawless coating to the surface of the substrate or other object to be coated, and can form a coating system with excellent rust prevention properties.

[0003] In electrodeposition coating, the electrodeposition paint applied to the substrate is often heated to a temperature higher than the decomposition temperature of the curing agent contained in the electrodeposition paint to promote a three-dimensional crosslinking reaction and form a coating system. Therefore, in general electrodeposition coating, the substrate needs to be heated to a high temperature of approximately 160°C or higher, which leaves room for improvement in terms of reducing carbon dioxide emissions.

[0004] In view of this problem, Patent Document 1 discloses a cationic electrodeposition coating composition composed of a mixture of two different types of emulsions. According to the technology of Patent Document 1, a coating system can be obtained by inducing a three-dimensional crosslinking reaction without high-temperature baking and drying. In addition, Patent Document 2 discloses a coating method of cationic electrodeposition coating in which drying and crosslinking steps are performed at 10 to 120°C. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-138842 [Patent Document 2] International Publication No. 2020 / 166657 Summary of the Invention [Problem to be solved by the invention]

[0006] However, there is little track record in changing the crosslinking reaction itself from the conventional electrodeposition coating method as in Patent Document 1, or in carrying out the crosslinking reaction at a lower temperature than conventionally as in Patent Document 2, and there have been cases in which the reliability of the rust prevention properties in particular has been poor.

[0007] Therefore, there is a need for improvements in coating methods and coating systems that can reduce carbon dioxide emissions while still using conventional electrodeposition paints. [Means for solving the problem]

[0008] The coating method according to the present invention comprises the steps of: A coating method for coating a surface of a substrate with an electrodeposition paint and a powder paint, comprising the steps of: an electrodeposition coating step of depositing the electrodeposition paint on a surface of the substrate; a moisture content adjusting step of adjusting the moisture content of the electrodeposition paint at a temperature lower than the curing temperature of the electrodeposition paint attached to the substrate to form an electrodeposition temporary coating film; a powder coating process in which the powder paint is sprayed onto the electrodeposition temporary coating film to form a powder coating film on the electrodeposition temporary coating film, and an uncured mixed layer is formed under the powder coating film by the powder paint penetrating into the surface layer of the electrodeposition temporary coating film; a heat curing step of heating the substrate after the powder coating step at a high temperature equal to or higher than the curing temperature of the powder coating material to form a coating system in which the electrodeposition temporary coating film, the uncured mixed layer, and the powder temporary coating film are cured; Includes.

[0009] In the moisture content adjusting step, it is desirable to adjust the moisture content of the electrodeposited temporary coating film to 30% or less.

[0010] In the moisture content adjusting step, it is desirable to adjust the moisture content of the electrodeposited temporary coating film to more than 2%.

[0011] In the heat curing step, it is desirable to increase the temperature to the curing temperature of the powder coating at a temperature gradient of 5° C. / min to 15° C. / min.

[0012] The powder paint may be a pigmented paint.

[0013] The powder coating preferably has an average particle size, as determined by a laser diffraction method, of 10 μm or more and 150 μm or less.

[0014] The electrocoated temporary coating film desirably has a scratch hardness of 5B or more and H or less, as measured in accordance with JIS K 5600-5-4:1999.

[0015] The electrocoating paint may be a crosslinked type.

[0016] The electrocoating paint may be a non-crosslinked type.

[0017] The coating system according to the present invention further comprises: The coating can be formed on the surface of the substrate by the above-mentioned coating method.

[0018] The coating system according to the present invention comprises: A coating system applied to a surface of a substrate, comprising: The substrate has an electrodeposition coating film formed by electrodeposition coating on the surface side thereof, and a powder coating film formed by powder coating on the electrodeposition coating film, Between the electrodeposition coating film and the powder coating film, a mixed layer coating film is formed in which the powder coating film has penetrated into the electrodeposition paint. Effect of the Invention

[0019] According to the coating method of the present invention, in the moisture content adjustment step, the moisture content of the electrodeposition paint is adjusted at a temperature lower than the curing temperature of the electrodeposition paint, and the electrodeposition paint and the powder paint are cured in a single heat curing step. Therefore, the step requiring a high temperature to cure the electrodeposition paint can be limited to one step, and the amount of energy consumed in the entire coating method can be reduced.

[0020] In addition, according to the coating method and coating system of the present invention, a mixed layer can be formed between the electrodeposition coating film and the powder coating film, in which the powder coating has penetrated into the electrodeposition coating. Since the viscosity of the mixed layer before curing (uncured mixed layer) is higher than that of the powder coating, the paint is less likely to flow away even in places where the coating film is likely to become thin, such as corners of the substrate, and sufficient coating film thickness, i.e., edge coverage, can be ensured. Furthermore, the presence of a mixed layer in which the electrodeposition coating film and the powder coating film are mixed can increase the bonding strength between the electrodeposition coating film and the powder coating film, and can prevent peeling of the coating film. [Brief description of the drawings]

[0021] [Figure 1] FIG. 1 is a cross-sectional view showing the procedure of a coating method according to one embodiment of the present invention, showing a substrate and an electrodeposition paint layer formed by an electrodeposition coating process. [Diagram 2] FIG. 2 is a cross-sectional view including an electrodeposited temporary coating film whose moisture content has been adjusted by the moisture content adjusting step. [Diagram 3] FIG. 3 is a schematic diagram showing a state in which the powder paint has penetrated into the surface of the electrodeposition temporary coating film. [Figure 4] FIG. 4 is a cross-sectional view including an uncured mixed layer and a temporary powder coating film formed on an electrodeposited temporary coating film by a powder coating process. [Diagram 5] FIG. 5 is a schematic diagram showing the path of water evaporating from the electrodeposition temporary coating through the powder temporary coating during the heat curing step. [Figure 6] FIG. 6 is a cross-sectional view of a substrate having a coating system formed thereon according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The coating method and coating system 10 according to the present invention will be described with reference to the drawings. In the following, an embodiment in which the coating method according to the present invention is applied to a substrate 60 having a right-angled corner 61 will be described. However, the coated surface may have a corner other than a right angle, i.e., a corner with an acute angle more acute than a right angle, a corner with an obtuse angle more acute than a right angle, a corner with a radius, etc. In addition, the coated surface may be not only a flat surface, but also a curved surface, or a shape having a convex or concave shape. The coating method and coating system 10 according to the present invention are excellent in edge covering property that can ensure the thickness of the coating film at the corner 61, and are therefore particularly suitable for coating a substrate 60 having a corner 61 of 120° or less.

[0023] The coating method of the present invention includes an electrodeposition coating step, a moisture content adjustment step, a powder coating step, and a heat curing step, and forms a coating system 10 (coating film) on a substrate 60 using an electrodeposition paint 20 and a powder paint 30. As shown in FIG. 6, the coating method of the present invention forms a coating system 10 on the substrate 60, including an electrodeposition coating film 23 derived from the electrodeposition paint 20, a powder coating film 33 derived from the powder paint 30, and a mixed layer coating film 41 in which the powder paint 30 has penetrated into the electrodeposition paint 20 between the electrodeposition coating film 23 and the powder coating film 33. For ease of explanation, the thickness of each layer is exaggerated in the drawings, but it should be understood that in reality, the thickness of each layer is considerably thinner than the thickness of the substrate 60.

[0024] <Base material 60> In this embodiment, the substrate 60 to be coated includes two perpendicular surfaces that form a right-angled corner 61, as shown in Fig. 1. The substrate 60 can be an exterior part of an agricultural machine, a machine tool, a construction machine, an automobile, etc., but is not limited thereto. In the case of cathodic electrodeposition coating, the material of the substrate 60 can be iron, copper, brass, stainless steel, aluminum alloy, zinc die casting, etc.

[0025] <Electrodeposition paint 20> The electrodeposition paint 20 that can be used in the present invention is a known electrodeposition paint, including a general electrodeposition paint. The curing temperature of the electrodeposition paint 20 is lower than the curing temperature of the powder paint 30, which will be described below. The reason will be explained later. The curing temperature of the electrodeposition paint 20 means the temperature that the substrate 60 must reach in order to ensure that the electrodeposition paint is cured, and is the recommended processing temperature when forming a coating film. The curing temperature differs depending on the electrodeposition paint 20, so please refer to the instruction manual or other information.

[0026] The electrodeposition paint 20 may be a cross-linked electrodeposition paint having a cross-linking curing agent (NCO cross-linking) and requiring heat curing at 140°C or higher, or a non-cross-linked electrodeposition paint that is electrodeposited at a lower temperature than the cross-linked electrodeposition paint. The curing temperature of the cross-linked electrodeposition paint is 10°C or higher, or 20°C or higher, than the cross-linking start temperature at which the electrodeposition paint starts cross-linking, and the upper limit is preferably the cross-linking start temperature + 40°C, or the cross-linking start temperature + 50°C. The curing temperature of the non-cross-linked electrodeposition paint is suitably 100°C or higher, and the upper limit is preferably 140°C or lower, and 110°C or lower.

[0027] The electrodeposition paint 20 is typically in the form of a base resin and a curing agent dispersed in water, and may further contain pigments, organic solvents, curing catalysts, surfactants, etc. as necessary. The base resin is not particularly limited, but may be, for example, an epoxy resin, a modified epoxy resin that is mainly based on this epoxy resin and has been modified in various ways, or a combination of these. The curing agent is also not particularly limited, but may be, for example, an amine complex or a blocked isocyanate-based curing agent.

[0028] <Powder paint 30> The powder paint 30 that can be used in the present invention is a known powder paint including a general powder paint. The curing temperature of the powder paint 30 is higher than the curing temperature of the electrodeposition paint 20.

[0029] The powder coating material 30 typically contains a binder resin and a pigment, and may further contain additives commonly used in the art (such as adhesion promoters, anti-repellent agents, waxes, etc.) as necessary. The binder resin in the powder coating material 30 may be any binder resin commonly used in the art, such as vinyl chloride resin, epoxy resin, polyester resin, polyolefin resin, polyamide resin, cellulose derivative resin, fluororesin, acrylic resin, etc., or a combination thereof.

[0030] The curing temperature of the powder paint 30 is higher than that of the electrodeposition paint 20. The curing temperature of the powder paint 30 refers to the temperature that the substrate 60 must reach in order to ensure curing after the powder paint starts to melt (crosslink) at the melting temperature, and is the recommended processing temperature when forming a coating film. The curing temperature varies depending on the powder paint 30, so please refer to the instruction manual or other information. The curing temperature of the powder paint 30 is 20°C or higher, or 40°C or higher than the melting temperature of the powder paint, and the upper limit is preferably the melting temperature + 70°C, or the crosslinking start temperature + 100°C.

[0031] The powder paint 30 used in the powder coating process has an average particle size, as determined by laser diffraction, of preferably 10 μm or more, and more preferably 20 μm or more. This is to ensure an escape route 50 for moisture from the electrodeposited temporary coating film 22, as will be explained in the heat curing process described later. On the other hand, if the average particle size of the powder paint 30 is large, fine irregularities may remain on the surface of the powder coating film 33 formed, and the smoothness may decrease. Therefore, the average particle size of the powder paint 30 is preferably 150 μm or less, more preferably 100 μm or less, and more preferably 50 μm or less.

[0032] The average particle size of the powder coating 30 can be measured according to JIS K 5600-9-3:2006 General test method for coating materials - Part 9: Powder coating materials. Examples of devices for measuring the average particle size include a particle measuring device Partica LA-960V2 (manufactured by Horiba, Ltd.) and an analytical measuring device SALD-2300 (manufactured by Shimadzu Corporation).

[0033] The powder coating material 30 has a viscosity of 10 -1 Pa s (0.1 Pa s) or more is preferable, and 10 3 It is desirable that the viscosity of the powder paint 30 be in the above range so that the powder coating film 33 formed is smooth and the appearance of the product is good. Here, the predetermined temperature when measuring the viscosity of the powder paint 30 is the temperature that the powder paint 30 reaches in the heat curing process, and can be, for example, the surface temperature of the powder coating film 33 measured by actually carrying out the heat curing process. In addition, in consideration of the thickness of the coating system 10 used in practice, the surface temperature of the powder coating film 33 can be regarded as the same as the temperature that the powder paint 30 reaches.

[0034] An example of a rotational viscometer used to measure the viscosity of the powder coating material 30 is the MCR series manufactured by Anton Paar. In addition, the measurement conditions for this measurement are, for example, a parallel plate shape, a plate spacing of 1 mm, a vibration measurement mode, and a vibration frequency of 1 Hz.

[0035] <Curing temperature of electrodeposition paint 20 and powder paint 30> In the coating method of the present invention, in the moisture content adjusting step described below, the moisture content of the electrodeposition paint 20 applied to the substrate 60 is adjusted at a temperature lower than the curing temperature of the electrodeposition paint 20. In the heat curing step also described below, it is necessary to cure all of the applied paint, i.e., the electrodeposition paint 20 and the powder paint 30 (including the uncured mixed layer 40).

[0036] In order to carry out these steps, it is necessary to know in advance the curing temperature and crosslinking start temperature (in the case of a crosslinked electrodeposition paint) of the electrodeposition paint 20, and the curing temperature and melting temperature (crosslinking start) of the powder paint 30. The curing temperature can be determined by referring to the instruction manual as described above, or by trial and error. In general, the curing temperature of the electrodeposition paint 20 is 120°C or higher and 170°C or lower in the case of a crosslinked electrodeposition paint, and 100°C or higher and 140°C or lower in the case of a non-crosslinked electrodeposition paint, preferably 110°C or lower. The curing temperature of the powder paint 30 is 140°C or higher and 200°C or lower, and the melting temperature of the powder paint 30 is 10°C or higher or 20°C or higher than the curing temperature of the electrodeposition paint 20, and the upper limit is preferably the curing temperature of the electrodeposition paint 20 + 40°C, or the curing temperature of the electrodeposition paint 20 + 50°C.

[0037] On the other hand, if the powder paint 30 melts (starts cross-linking) or hardens before the electrodeposition paint 20 hardens, there is a problem that the moisture contained in the electrodeposition paint 20 cannot evaporate, as described below. Therefore, it is preferable to use a paint whose hardening temperature for the electrodeposition paint 20 is lower than the hardening temperature for the powder paint 30. Desirably, the electrodeposition paint 20 is a paint whose hardening temperature is lower than the melting temperature of the powder paint 30, in other words, the powder paint 30 is a paint whose melting temperature is higher than the hardening temperature for the electrodeposition paint 20.

[0038] <Electrodeposition painting process> The electrodeposition coating process is a process of attaching an electrodeposition paint 20 to a substrate 60 as shown in FIG. 1. The electrodeposition coating process can employ a known electrodeposition coating method, such as cationic electrodeposition coating or an electrodeposition coating system having a UF (ultrafilter) membrane. The cationic electrodeposition coating is a coating method in which the substrate 60 is immersed in a water-soluble electrodeposition paint 20, and a direct current is applied so that the substrate 60 serves as the cathode and the electrode serves as the anode, thereby causing particles of the electrodeposition paint 20 to adhere to the surface of the substrate 60 by electrophoresis. After the electrodeposition coating, the excess paint is washed off with water, forming an electrodeposition paint layer 21 on the surface of the substrate 60. The electrodeposition paint layer 21 contains moisture from the electrodeposition paint 20 and moisture from the water washing. This moisture is adjusted in the next moisture content adjustment process.

[0039] <Moisture content adjustment process> The moisture content adjusting step is a step of adjusting the moisture content of the electrodeposition paint layer 21 (FIG. 1) formed by the electrodeposition coating step. The coating film with the adjusted moisture content of the electrodeposition paint layer 21 is called the temporary electrodeposition coating film 22 (see FIG. 2). In the moisture content adjusting step, the moisture content of the electrodeposition paint layer 21 is adjusted to preferably more than 2%, preferably more than 5%, more preferably 10% or more. In addition, the moisture content of the electrodeposition paint layer 21 is preferably adjusted to 30% or less, preferably 15% or less. The moisture content of the temporary electrodeposition coating film 22 can be determined by the following formula, where W1 is the weight of the substrate 60 before the electrodeposition paint 20 is attached, W2 is the weight of the substrate 60 on which the electrodeposition coating film 23 is formed after the moisture content adjusting step, and W3 is the weight of the substrate 60 after heating (for example, heating at 105° C. for 180 minutes or more) until the moisture content of the electrodeposition coating film 23 becomes zero. (Moisture content:%)=((W2-W3) / (W2-W1))×100

[0040] By not completely drying the electro-deposited temporary coating film 22 and adjusting the moisture content as described above, the powder paint 30 can penetrate into the surface of the electro-deposited temporary coating film 22 in the subsequent powder coating process to form an uncured mixed layer 40 (see Figures 3 and 4).

[0041] The specific method for adjusting the moisture content is not limited, but the moisture content can be adjusted, for example, by blowing air at a temperature lower than the curing temperature of the electrodeposition paint 20 (preferably the crosslinking start temperature in the case of a crosslinking electrodeposition paint) on the substrate 60 on which the electrodeposition paint layer 21 is formed. By employing air blowing, even if the substrate 60 has a complex shape, for example, a shape having a recess in which moisture can accumulate, the moisture can be blown away appropriately to adjust the moisture content. The moisture content can also be adjusted by storing the substrate 60 on which the electrodeposition paint layer 21 is formed in a dry atmosphere lower than the curing temperature of the electrodeposition paint 20 (preferably the crosslinking start temperature in the case of a crosslinking electrodeposition paint) for a predetermined period of time. The temperature lower than the curing temperature of the electrodeposition paint 20 (preferably the crosslinking initiation temperature in the case of a crosslinking-type electrodeposition paint) can be, for example, room temperature (20°C to 35°C, preferably 30°C or less, preferably 25°C or less), and the dry atmosphere can be humidity of 75% or less, preferably 50% or less, but is not limited thereto. The air blow is preferably 0.01 MPa or more and 0.5 MPa or less. Of course, the moisture content may be adjusted by heating to a temperature higher than room temperature, but in order to prevent complete evaporation of moisture, and because rapid evaporation of moisture may cause gas pins to occur on the electrodeposition surface, causing the surface texture to deteriorate, it is preferable to heat the electrodeposition paint to a temperature lower than 100°C, preferably 50°C or less. The conditions of the moisture content adjustment process can be adjusted depending on the composition and solids concentration of the electrodeposition paint 20 and the target moisture content.

[0042] According to the present invention, the water content adjustment step does not require heating to the curing temperature of the electrodeposition paint 20, and therefore carbon dioxide emissions can be significantly reduced compared to heating the electrodeposition paint 20 to its curing temperature. In particular, in the case of a cross-linking type electrodeposition paint, adjusting the water content at a temperature lower than the cross-linking start temperature prevents the electrodeposition paint from cross-linking and curing, and allows the powder 31 to penetrate suitably into the electrodeposition temporary coating film 22 in the subsequent powder coating step to form the uncured mixed layer 40 (see FIG. 3).

[0043] The electro-deposited temporary coating film 22 formed in the moisture content adjustment step is preferably adjusted so that the lower limit of the scratch hardness measured based on JIS K 5600-5-4:1999 is 5B and the upper limit is H. By setting the hardness of the electro-deposited temporary coating film 22 to 5B or more and H or less, in the next powder coating step, an uncured mixed layer 40 in which the powder paint 30 penetrates the surface of the electro-deposited temporary coating film 22 can be formed. If the hardness of the electro-deposited temporary coating film 22 is less than 5B, the electro-deposited temporary coating film 22 is soft and has fluidity, so there is a risk that the edge covering property will decrease. In addition, if the hardness of the electro-deposited temporary coating film 22 exceeds H, there is a risk that the powder 31 will not penetrate the surface of the electro-deposited temporary coating film 22 even if the powder paint 30 is sprayed. The hardness of the electro-deposited temporary coating film 22 is more preferably 4B or more, and even more preferably 3B or more. The hardness of the electrodeposition temporary coating film 22 is more preferably F or less, further preferably B or less, and desirably 2B or less. The hardness of the electrodeposition temporary coating film 22 can be achieved by adjusting the water content or the composition of the electrodeposition paint 20.

[0044] The moisture content adjustment process removes some of the moisture contained in the electrodeposition paint layer 21, resulting in an uncured electrodeposition temporary coating film 22 (see FIG. 2) with the moisture content and hardness adjusted to the above ranges. The electrodeposition temporary coating film 22 still contains moisture and is at a low temperature, so it does not reach the same complete film formation as when heated at the curing temperature.

[0045] <Powder coating process> The powder coating process is a process in which a powder paint 30 is sprayed and adhered to an electrodeposited temporary coating film 22 on a substrate 60 (FIGS. 3 and 4). As a specific method for carrying out the powder coating process, a known powder coating method can be used, such as electrostatic powder coating (spray coating) or fluidized bed dip coating (dip coating).

[0046] As described above, the electrodeposited temporary coating film 22 contains water and is in an uncured state with the surface hardness adjusted. By carrying out a powder coating process on the electrodeposited temporary coating film 22 in this state, a part of the powder 31 of the sprayed powder paint 30 penetrates into the surface of the electrodeposited temporary coating film 22 as shown in FIG. 3, and an uncured mixed layer 40 in which the electrodeposited paint 20 and the powder paint 30 are mixed is formed. This uncured mixed layer 40 has the effect of increasing the viscosity of the surface of the electrodeposited temporary coating film 22 and reducing the fluidity, thereby improving the edge covering property of the electrodeposited temporary coating film 22 and suppressing the occurrence of pinholes, etc. In addition, by curing the powder paint 30 (the next heat curing process) in a state in which the powder paint 30 penetrates into the electrodeposited temporary coating film 22, the bonding strength between the electrodeposited temporary coating film 22 and the powder temporary coating film 32 can be increased.

[0047] 3 is a schematic diagram showing a state in which powder paint 30 has penetrated into the surface of the electrodeposited temporary coating film 22. When the powder paint 30 hits the surface of the uncured electrodeposited temporary coating film 22, some of the powder 31 penetrates into the electrodeposited temporary coating film 22 to form an uncured mixed layer 40. As the powder coating further progresses, the powder 31 adheres to the uncured mixed layer 40, and an uncured temporary powder coating film 32 is formed.

[0048] As shown in FIG. 4, the powder coating process forms an uncured mixed layer 40 of the electrodeposition paint 20 and the powder paint 30 on top of the temporary electrodeposition coating film 22, and a temporary powder coating film 32 is further formed on top of that.

[0049] <Heat curing process> The heat curing step is a step of heating the substrate 60 on which the uncured mixed layer 40 of the electrodeposition paint 20 and the powder paint 30 and the temporary powder coating film 32 are formed on the upper layer of the electrodeposition temporary coating film 22, to cure each layer. The heat curing step is carried out at a high heating temperature equal to or higher than the curing temperature of the electrodeposition paint 20. For example, when the curing temperature of the powder coating film 30 is in the range of 120°C or higher and 200°C or lower as exemplified above, the heating temperature in the heat curing step is carried out at a temperature equal to or higher than the curing temperature. The heat curing step can be carried out, for example, by a method in which the substrate 60 after the powder coating step is put into a heating furnace whose temperature is adjusted to a predetermined heating temperature. Here, the heating temperature is selected in consideration of the conditions of the heating furnace (output, dimensions, etc.) and the conditions of the substrate 60 (dimensions, material, etc.) so that the temperature of the substrate 60 in the furnace is maintained at a high temperature equal to or higher than the curing temperature of the powder coating film 30.

[0050] The powder temporary coating film 32 is in the form of powder 31 as shown in FIG. 3. The heating temperature in the heat curing step is higher than the boiling point of water, 100°C. Therefore, in the heat processing step, the electrodeposition temporary coating film 22 is heated to 100°C or higher. As a result, the moisture contained in the electrodeposition temporary coating film 22 evaporates and leaves the system through the gaps between the powders 31, 31 formed in the powder temporary coating film 32 as shown in FIG. 5, via a path 50. When the substrate 60 is further heated, the curing temperature of the electrodeposition paint 20 is reached. In the case where the electrodeposition paint 20 is a crosslinking type, the electrodeposition paint 20 reaches a crosslinking initiation temperature by heating before reaching the curing temperature of the electrodeposition paint 20, causing a crosslinking reaction, and the heating continues to form an electrodeposition coating film 23 in which the electrodeposition temporary coating film 22 is cured. In the case where the electrodeposition paint 20 is a non-crosslinking type, the electrodeposition paint 20 is originally a high molecular weight paint resin, and the viscosity increases due to the volatilization of the solution (organic solvent and water), and the finally cured electrodeposition coating film 23 is formed. The powder 31 melts and becomes integrated with the powder 31 to form the temporary powder coating film 32. In this state, the heating is stopped and the work is dried and cooled to form a powder coating film 33 as shown in Fig. 6. The electrodeposition paint 20 and the powder paint 30 in the uncured mixed layer 40 also harden while remaining mixed with each other to form a mixed layer coating film 41.

[0051] The electrodeposition coating film 23, the mixed layer coating film 41 and the powder coating film 33 constitute the coating system 10 of the present invention, as shown in Fig. 6. The coating films 23, 41, 33 are not only cured, but also the electrodeposition coating film 23 is firmly bonded to the substrate 60, and the electrodeposition coating film 23 and the powder coating film 33 are firmly bonded by the mixed layer coating film 41, so that the coating films 23, 41, 33 will not peel off.

[0052] In the present invention, since the coating system 10 consisting of the electrodeposition coating film 23, the mixed layer coating film 41 and the powder coating film 33 is formed, high temperature heating is only required in the final heat curing process, which allows for a significant reduction in carbon dioxide emissions.

[0053] In the heat curing process, if the heating of the substrate 60 proceeds all at once, the powder 31 reaches the melting temperature before the moisture evaporates from the electrodeposition temporary coating film 22, and melting and curing begin. In this case, the gaps between the powders 31, 31 that form the path 50 through which the moisture evaporates from the electrodeposition coating film 23 may be filled. The moisture that cannot be removed may remain in the electrodeposition coating film 23 as air bubbles, causing poor appearance. For this reason, it is desirable to increase the temperature in the heat curing process at a temperature gradient of 5°C / min or more and 15°C / min or less to the curing temperature of the powder paint 30. This allows the removal of moisture from the electrodeposition temporary coating film 22 in the heat curing process to be completed before the powder 31 melts and hardens completely. The temperature increase is not limited to a monotonous increase, and may be a stepwise increase.

[0054] <20 electrocoating paints and 30 powder paint colors> In the present invention, since the electrodeposition paint 20 and the powder paint 30 form the mixed layer coating film 41, it is preferable that the powder paint 30 is a colored paint containing a pigment, rather than a clear one. If a clear powder paint is used, there is a risk that the unevenness of the powder will appear in the mixed layer coating film 41, resulting in a visual effect of being uneven when light hits it.

[0055] Therefore, it is preferable that the powder paint 30 contains a pigment. On the other hand, the electrodeposition paint 20 can also contain a pigment. The pigment may be any pigment that is normally used in the art, for example, a general organic pigment or inorganic pigment, or a pigment that can impart a distinctive appearance, such as an aluminum pigment or a pearl pigment that has a lustrous appearance. The type of pigment used in each paint may be one or more types.

[0056] In this case, it is preferable that the electrodeposition paint 20 and the powder paint 30 have the same or a similar color. This is because by keeping the colors of the two paints close to each other, even if the balance of the film thicknesses of the coating films (electrodeposition coating film 23 and powder coating film 33) derived from each paint changes, the difference in appearance is less likely to be apparent. This increases the reproducibility of the appearance of the coated product, making it easier to produce it as an industrial product. In order for the electrodeposition paint 20 and the powder paint 30 to have the same or a similar color, for example, the electrodeposition paint 20 and the powder paint 30 may contain the same pigment. EXAMPLES

[0057] The test panels were painted and various evaluations were carried out.

[0058] <Test conditions> A commercially available 3.2 mm thick SPCC (cold rolled steel sheet) was bent 90° as shown in Fig. 1 to form a corner 61 to obtain a test plate. The test plate was painted according to the above embodiment and the procedure in Table 1 below. The painted test plate was cut, and the corner 61 and its neighboring cross section were observed under a microscope to observe the state of the paint film.

[0059] The paints used are as follows:

[0060] [Electrodeposition paint 20] A: Cationic electrocoating paint (Power Float (registered trademark) 1200, manufactured by Nippon Paint Industrial Coatings Co., Ltd.) B: Cationic electrocoating paint (Power Float 510, manufactured by Nippon Paint Industrial Coatings Co., Ltd.) C: Cationic electrocoating paint (Insullead (registered trademark) 3000, manufactured by Nippon Paint Industrial Coatings Co., Ltd.) In addition, A, B, and C are all cross-linked electrocoating paints.

[0061] [Powder paint 30] D: Thermosetting powder paint (Vilyusia PL5000, manufactured by Nippon Paint Industrial Coatings Co., Ltd.) E: Thermosetting powder paint (Vilyusia PL5100, manufactured by Nippon Paint Industrial Coatings Co., Ltd.) F: Thermosetting powder paint (Vilyusia PL1000, manufactured by Nippon Paint Industrial Coatings Co., Ltd.) G: Thermosetting powder paint (Powdax (registered trademark) E200, manufactured by Nippon Paint Industrial Coatings Co., Ltd.) H: Thermosetting powder paint (Taiwai Bilyusia (registered trademark) Hammer Tone, manufactured by Nippon Paint Industrial Coatings Co., Ltd.)

[0062] The conditions for applying the electrodeposition paint 20 were as follows: an electrolytic bath (where the electrodeposition paint is stored) in which an electrode that serves as the counter electrode of the conductive workpiece is immersed, a hanger for immersing the workpiece in the electrolytic bath while transporting it, and a power source (rectifier having a function of applying a predetermined DC voltage) for applying a DC voltage for electrodeposition between the workpiece immersed in the electrolytic bath and the electrode; the voltage was set to 200 V and electricity was passed for a predetermined time. In the electrodeposition coating, a coating film is deposited on the workpiece as electricity is passed, and the surface current decreases, completing the film formation.

[0063] The powder coating material 30 was sieved to adjust the particle size so that the average particle size (measured by the method described above) was as shown in Table 1. The coating conditions for the powder coating material 30 were as follows: the powder coating material was applied to an earthed workpiece to charge it (negatively charged electrostatic coating, -50 KV to -90 KV), and then electrostatic air spray coating was performed. The average particle size of the powder coating material used here was 8 μm to 160 μm.

[0064] As shown in Table 1, for Examples 1 to 14 and Comparative Examples 1 to 7, the combination of electrodeposition paint 20 and powder paint 30, the moisture content of the electrodeposition temporary coating film 22, the average particle size of the powder paint 30, the temperature gradient and surface temperature reached in the heating processing step, and the holding time were changed to form a coating system 10, and the thicknesses of the mixed layer and film 41 were measured.

[0065] As shown in Table 1, for Examples 1 to 14 and Comparative Examples 1 to 7, the type of paint used, other conditions, conditions for carrying out the moisture content adjusting step and the heat curing step, measurement results for the electrodeposition temporary coating film 22 formed after the moisture content adjusting step, and the thickness of the mixed layer coating film 41 formed after the heat curing step are each shown in Table 1. It should be noted that Example 4 and Comparative Example 1 are test examples in which air blowing was performed in the moisture content adjusting step, and the rest are test examples in which the sample was left in a space at a set temperature for the retention time. The retention time in the moisture content adjusting step and the heat curing step refers to the time after the set temperature was reached.

[0066] [Table 1]

[0067] Referring to Table 1, it can be seen that a mixed layer coating film 41 is formed in all of the examples, but the thickness of the mixed layer coating film is zero in all of the comparative examples, that is, no mixed layer coating film is formed.

[0068] Comparing the Examples, the mixed layer coating film 41 can be formed thicker by making the pencil hardness of the electrodeposition temporary coating film 22 equal to or less than B. This is because the powder coating material 30 penetrates more easily as the surface of the electrodeposition temporary coating film 22 becomes softer.

[0069] It can also be seen that the smaller the average particle diameter of the powder coating material 30, the thicker the mixed layer coating film 41 could be formed. This is because the smaller the powder 31, the deeper the powder 31 penetrates into the electrodeposited temporary coating film 22.

[0070] On the other hand, the main reason why the mixed layer coating film was not formed in the comparative examples is considered to be as follows. In the comparative examples 1 and 2, the moisture content of the electrodeposition temporary coating film was low, and the pencil hardness of the base temporary coating film was H and 2H, so that the powder could not penetrate into the electrodeposition temporary coating film. In the comparative examples 3 to 5, the curing temperature of the electrodeposition paint was higher than that of the powder paint, and even if the powder paint was cured, moisture remained in the electrodeposition paint layer and it did not harden, so that the coating system was not formed. In the comparative example 3, the moisture content of the electrodeposition temporary coating film was also high. Similarly, in the comparative example 6, the moisture content of the electrodeposition temporary coating film was high, so that moisture remained in the electrodeposition paint layer and the coating system was not formed. In the comparative example 7, the moisture content of the electrodeposition temporary coating film was appropriate, but the temperature gradient in the heat curing process was high, and the powder coating film was formed before the electrodeposition temporary coating film was dried, so that a path for the moisture in the electrodeposition temporary coating film to escape could not be secured, so that moisture remained in the electrodeposition coating film and the coating system was not formed.

[0071] The above description is for the purpose of explaining the present invention, and should not be interpreted as limiting the invention described in the claims or restricting the scope of the invention. Furthermore, the configuration of each part of the present invention is not limited to the above embodiment, and various modifications are possible within the technical scope described in the claims. [Explanation of symbols]

[0072] 10 Painting 20 Electrodeposition paint 21 Electrodeposition paint layer 22 Electrochemical temporary coating 23 Electrodeposition coating film 30 Powder coating 31 Powder 32 Powder temporary coating 33 Powder Coating 40 Uncured mixed layer 41 Mixed layer coating 50 routes 60 Base material 61 Corner

Claims

1. A coating method for coating a surface of a substrate with an electrodeposition paint and a powder paint, comprising the steps of: an electrodeposition coating step of depositing the electrodeposition paint on a surface of the substrate; a moisture content adjusting step of adjusting the moisture content of the electrodeposition paint at a temperature lower than the curing temperature of the electrodeposition paint attached to the substrate to form an electrodeposition temporary coating film; a powder coating process in which the powder paint is sprayed onto the electrodeposition temporary coating film to form a powder coating film on the electrodeposition temporary coating film, and an uncured mixed layer is formed under the powder coating film by the powder paint penetrating into the surface layer of the electrodeposition temporary coating film; a heat curing step of heating the substrate after the powder coating step at a high temperature equal to or higher than the curing temperature of the powder coating material to form a coating system in which the electrodeposition temporary coating film, the uncured mixed layer, and the powder temporary coating film are cured; A painting method comprising:

2. The moisture content adjusting step adjusts the moisture content of the electrodeposition temporary coating film to 30% or less. The coating method according to claim 1.

3. The moisture content adjusting step adjusts the moisture content of the electrodeposited temporary coating film to more than 2%. The coating method according to claim 2.

4. The heat curing step includes increasing the temperature to the curing temperature of the powder coating at a temperature gradient of 5° C. / min to 15° C. / min. The coating method according to claim 3.

5. The powder paint is a pigmented paint. The coating method according to claim 4.

6. The powder coating has an average particle size, as determined by a laser diffraction method, of 10 μm or more and 150 μm or less. The coating method according to claim 5.

7. The electrocoated temporary coating film has a scratch hardness of 5B or more and H or less, as measured based on JIS K 5600-5-4:1999. The coating method according to claim 6.

8. The electrodeposition paint is a crosslinked system. The coating method according to any one of claims 1 to 7.

9. The electrodeposition paint is a non-crosslinked system. The coating method according to any one of claims 1 to 7.

10. A coating system formed on a surface of a substrate by the coating method according to any one of claims 1 to 7.

11. A coating system applied to a surface of a substrate, comprising: The substrate has an electrodeposition coating film formed by electrodeposition coating on the surface side thereof, and a powder coating film formed by powder coating on the electrodeposition coating film, Between the electrodeposition coating film and the powder coating film, a mixed layer coating film in which the powder coating film has penetrated into the electrodeposition paint is formed. Paint system.

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