Coating film laminate and method for detecting corrosion under coating film

Abstract

In the present invention, a coating film laminate obtained by laminating at least two types of coating films is such that: each of the coating films is composed of only a substance having a relative dielectric constant of 8 or less; a top coat is formed from a two-component mixed-type coating material containing a main agent and a curing agent; the combination of the main agent and the curing agent is either • main agent = fluororesin, curing agent = polyisocyanate, or • main agent = silicone resin, curing agent = acrylic polyol; a side chain of the curing agent of the top coat has, as a UV absorbing group, at least one type of molecular structure selected from the group consisting of triazine, benzophenone, benzotriazole, and cyanoacrylate; and particles of a crystalline polymer having a crystallinity of 60% or greater are contained in at least one coating film layer.

Description

Paint Stack and Under-Paint Corrosion Detection Methods

[0001] FIELD OF THE DISCLOSURE The present disclosure relates to paint stacks and methods for detecting under-paint corrosion.

[0002] Understanding the corrosion status of steel materials is crucial for ensuring the integrity of steel structures. While methods such as eddy currents exist to detect sub-paint corrosion, these only measure the area where the probe touches the equipment, making them difficult to use for inspecting large areas of the entire steel structure. Furthermore, making contact with equipment such as transmission towers and bridges requires the construction of scaffolding, making it an inconvenient inspection method. In recent years, inspection of steel structures using drones has become widespread, and exposed rust can be detected from images. However, sub-paint corrosion only causes a slight bulge in the paint film, making image detection almost impossible and often overlooked. If sub-paint corrosion progresses and the steel material thins, it can affect the load-bearing capacity of the steel structure, posing a danger. Additionally, if rust removal and repainting maintenance are performed after rust has become apparent, the rust may not be completely removed, leading to rust recurrence in a short period after repainting. Therefore, there is a need for an inspection method that can detect sub-paint corrosion and is applicable to large areas.

[0003] Microwave thermography is a method that can detect corrosion under paint coatings. Microwave thermography is a method that irradiates an object to be measured with microwaves and measures the temperature change behavior of the object using thermography. When the object to be measured is a painted steel plate, microwaves penetrate the resin, while metals mainly reflect them and absorb them slightly. Iron rust has a larger temperature rise when absorbing microwaves than iron, and because it has low thermal conductivity, it takes longer to cool down. By capturing this difference in temperature change behavior between iron and iron rust with thermography, corrosion under paint coatings can be detected (Non-Patent Literature 1). In order to apply microwave thermography to the inspection of steel structures, it is important that the paint coating does not contain components that respond to microwaves in a similar way to iron rust.

[0004] Steel structures are typically coated with multiple layers of paint, including a topcoat with excellent UV resistance and intermediate / undercoats that provide excellent protection against water and salt. The topcoat often contains titanium dioxide or zinc oxide to ensure UV resistance and provide opacity. It may also contain red iron oxide (red: iron oxide) or cobalt blue (blue: cobalt aluminate) for coloring purposes. Furthermore, the intermediate and undercoats may contain mica-like iron oxide (a metallic black pigment mainly composed of flaky ferric oxide) to block corrosive factors such as moisture and salt. These metal oxides exhibit temperature change behavior similar to iron rust on microwave thermography, which can hinder the detection of corrosion beneath the paint film.

[0005] If a coating film that does not contain metal oxides is to be used, then coloring such as red and blue can be done with organic pigments, and the blocking of moisture and salt can be done with other inorganic substances, but it is difficult to replace the functions of titanium dioxide and zinc oxide. These pigments have two roles. One is to reflect and absorb ultraviolet light, preventing the coating resin from decomposing due to ultraviolet rays. This is an important function in ensuring the lifespan of the coating film for equipment that requires long-term durability of 10 to 30 years, such as steel structures. The other is to diffusely reflect not only ultraviolet light but also visible light, ensuring opacity (preventing the underlying material from being visible through the coating). If the opacity is insufficient, the underlying steel material may show through, resulting in poor color development. This is not only aesthetically unpleasing, but also creates a problem where the color cannot be achieved according to regulations such as aviation laws, where red, white, etc., are specified.

[0006] Foudazi, A. Et Al., Ieee Trans. Instrum. Meas. 2015, 64, 2583-2585

[0007] When metal oxides are contained in coatings, their response behavior to microwave thermography is similar to that of iron rust, hindering rust detection. To address this issue, the inventors came up with the idea of ​​using metal oxide-free coatings to enable under-coat corrosion inspection using microwave thermography. When replacing the metal oxides contained in conventional coatings with other substances, a particular challenge is ensuring the UV resistance and hiding power required for coatings for steel structures. Generally, the first choice for imparting UV resistance to steel structure coatings is metallic pigments such as titanium dioxide and zinc oxide. Without metallic pigments, organic UV absorbers can be considered. However, these are less durable than metallic pigments, and their duration of effectiveness depends on the amount added. Even if large amounts of UV absorbers are added, they bleed out of the resin over time, resulting in insufficient effectiveness. Ensuring long-term UV resistance without metallic pigments is difficult. Furthermore, with regard to hiding power, it is common to use metallic pigments such as titanium dioxide (rutile type: refractive index approximately 2.7) and zinc oxide (refractive index approximately 2.0) that have a sufficiently large difference in refractive index from the paint film (refractive index approximately 1.5). These impart hiding power to the paint film by scattering visible light due to the difference in refractive index between the paint film and the pigment, but if metal oxide pigments such as titanium dioxide and zinc oxide are not used, there are almost no substances whose refractive index is sufficiently larger than that of the paint film, and no materials are known that are suitable for imparting hiding power.

[0008] A coating film laminate according to one embodiment of the present disclosure is a coating film laminate in which at least two types of coating films are laminated together, and each coating film is composed only of a material with a relative dielectric constant of 8 or less; a top coat is formed from a two-component mixed paint containing a base agent and a curing agent, and the combination of the base agent and the curing agent is one of the following: - Base agent: fluororesin, curing agent: polyisocyanate - Base agent: silicone resin, curing agent: acrylic polyol; the curing agent of the top coat has at least one molecular structure selected from the group consisting of triazine, benzophenone, benzotriazole, and cyanoacrylate as a UV-absorbing group on a side chain; and at least one coating film layer contains particles of a crystalline polymer with a crystallinity of 60% or more.

[0009] The paint disclosed herein ensures UV resistance of the coating film at a level comparable to that achieved when using typical amounts of metallic pigments, without causing bleeding of the UV absorber, while also providing sufficient opacity and not affecting microwave reflectivity, thereby enabling microwave-based inspection of corrosion beneath the coating film. Since microwave inspection can be performed non-contact, it can be mounted on drones, etc. Conventionally, in order to prioritize the safety of the equipment, the presence or absence of corrosion beneath the coating film was checked manually using close-range visual inspection or grid testing. However, by enabling inspection with drones, etc., the efficiency of inspections can be expected to increase. By applying the paint of the present invention to steel structures, corrosion beneath the coating film can be inspected over a wide area of ​​the steel structure using microwaves, enabling early detection of material thinning progressing beneath the coating film. Furthermore, if rust removal and repainting maintenance can be performed at an early stage when corrosion beneath the coating film is minor, the life cycle cost (LCC) of the paint can also be reduced.

[0010] Fig. 1 is a schematic diagram showing an example of the layer structure of a coating laminate according to one embodiment. Fig. 2 is a schematic diagram showing another example of the layer structure of a coating laminate according to one embodiment. Fig. 3 is a schematic diagram showing an example of a steel structure having a coating laminate according to one embodiment on its surface.

[0011] In the present disclosure, the relative dielectric constant of a substance is determined by the method described in the Examples.

[0012] In the present disclosure, the crystallinity and solubility parameter (SP) value of the crystalline polymer are determined by the method described in the Examples.

[0013] In the present disclosure, the drawings are given priority to illustrating the configurations of the present disclosure, and the scales of the respective configurations in the drawings are not accurate.

[0014] (Coating Laminate) A coating laminate according to one embodiment of the present disclosure is a coating laminate comprising at least two types of coating films, each coating film composed only of a substance with a relative permittivity of 8 or less, the topcoat being formed from a two-component mixed paint comprising a main agent and a curing agent, the combination of the main agent and the curing agent being one of the following: - Main agent: fluororesin, curing agent: polyisocyanate - Main agent: silicone resin, curing agent: acrylic polyol The coating laminate contains, in at least one coating layer, particles of a crystalline polymer with a degree of crystallinity of 60% or more, having at least one molecular structure selected from the group consisting of triazine, benzophenone, benzotriazole, and cyanoacrylate as a UV absorbing group in the side chain of the curing agent of the topcoat.

[0015] Fig. 1 is a schematic diagram showing an example of the layer structure of a coating laminate according to one embodiment. The coating laminate in Fig. 1 has a two-layer structure of a top coat 10 and a base coat 20.

[0016] Figure 2 is a schematic diagram showing another example of the layer configuration of a coating laminate according to one embodiment. The coating laminate in Figure 2 has a three-layer configuration, consisting of a top coat 10, an intermediate coat 30, and a base coat 20, arranged in order adjacent to each other.

[0017] Figure 3 is a schematic diagram showing an example of a steel structure having a coating laminate on its surface according to one embodiment. The coating laminate 1 shown in Figure 1 is on the surface of the steel material 40 of the steel structure.

[0018] (Topcoat) The topcoat is the uppermost layer of the coating laminate (one of the outermost layers of the coating laminate) and is formed from a two-component mixed paint consisting of a main component and a hardener. The combination of the main component and hardener is either main component C-1: fluororesin, hardener E-1: polyisocyanate; or main component C-2: silicone resin, hardener E-2: acrylic polyol. The hardeners of the topcoat (hardeners E-1 and E-2) have at least one molecular structure selected from the group consisting of triazine, benzophenone, benzotriazole, and cyanoacrylate as a UV absorbing group in their side chains.

[0019] (Main component C1) The fluororesin as the main component is not particularly limited except that the relative dielectric constant is 8 or less, and a fluororesin that is the main component of a known two-component paint containing polyisocyanate as a curing agent can be used. As the fluororesin, a fluororesin having a hydroxyl group is preferred.

[0020] As the fluororesin having hydroxyl groups, known fluororesins having hydroxyl groups can be used, for example, the hydroxyl group-containing fluororesin described in Japanese Patent Application Publication No. 2008-081719 can be used. Examples of commercially available fluororesins having hydroxyl groups include the Lumiflon series (manufactured by Asahi Glass Co., Ltd.), such as Lumiflon® LF800 and FE4400; the Cephralcoat series (manufactured by Central Glass Co., Ltd.); the Zeffle series (manufactured by Daikin Industries, Ltd.); and Coatax (manufactured by Toray Industries, Inc.).

[0021] The amount of hydroxyl group-containing fluororesin in the two-component paint can be, for example, about 30 wt % to 70 wt % of the heating residue of the paint film, and preferably about 40 wt % to 60 wt %.

[0022] (Curing agent E1) The ultraviolet absorbing group is any one of triazine, benzophenone, benzotriazole, and cyanoacrylate, which has a hydroxyl group in the side chain that can react with isocyanate (including a hydroxyl group other than the hydroxyl group that acts as the ultraviolet absorbing group), and is obtained by reacting the isocyanate group with the hydroxyl group in the side chain of the ultraviolet absorbing group in advance with an isocyanate monomer having three or more isocyanate groups, such as an isocyanurate skeleton, and then reacting this with various isocyanates and compounds containing two or more hydroxyl groups. The isocyanate may be either an aliphatic or alicyclic isocyanate, and is mainly composed of nurates, biurets, adducts, etc. of diisocyanates such as hexamethylene diisocyanate, lysine diisocyanate, 4,4'-methylenebis(cyclohexyl isocyanate), methylcyclohexane-2,4-diisocyanate, methylcyclohexane-2,6-diisocyanate, 1,3-(isocyanatomethyl)cyclohexane, isophorone diisocyanate, trimethylhexamethylene diisocyanate, dimer acid diisocyanate, methylene diisocyanate, ethylene diisocyanate, butylene diisocyanate, propylene diisocyanate, octadecylene diisocyanate, 1,4-tetramethylene diisocyanate, 1,10-decanemethylene diisocyanate, and 1,3-cyclohexylene diisocyanate. Specific examples of compounds containing two or more hydroxyl groups include ethanediol, propanediol, 1,4-butanediol, 1,6-hexanediol, cyclohexyldimethanol, methylpropanediol, neopentyl glycol, butylethylpropanediol, glycerin, trimethylolethane, trimethylolpropane, polycaprolactone triol, ditrimelylpropane, pentaerythritol, polycaprolactone tetraol, dipentaerythritol, sorbitol, mannitol, etc. A mixture of two or more of these monomers may be used.

[0023] Even if large amounts of low-molecular-weight organic UV absorbers are added to the coating film, there is a problem that they will leach out (bleed out) over time. However, this leaching can be prevented by incorporating (chemically bonding) UV-absorbing groups into the coating resin. By using this method in conjunction with the addition of organic UV absorbers to the paint, the number of UV-absorbing structures in the coating film can be increased, improving UV resistance. A key feature is that the introduction of UV-absorbing structures is limited to the curing agent that constitutes the coating film. Both types of main components of the coating film are mainly composed of C-F bonds or Si-O bonds, which have high bond energy, and therefore have very high UV resistance themselves. If a UV-absorbing structure (mainly composed of C-C bonds and C-H bonds; C-C bonds and C-H bonds have lower bond energy than C-F bonds and Si-O bonds and are easily decomposed by ultraviolet light) were incorporated into the main component, there is a risk that the UV resistance of the main component would decrease as the C-C bonds and C-H bonds would be broken by UV light. Therefore, in this disclosure, UV-absorbing groups are introduced into the curing agent, which originally has many C-C and C-H bonds and lower UV resistance than the main component. This also has the effect of preventing the curing agent from being decomposed by UV by having many UV-absorbing structures near the curing agent, which has relatively weak UV resistance.

[0024] In one embodiment, the main component is a silicone resin, the curing agent is an acrylic polyol, and the total number of carbon, oxygen, or nitrogen atoms between the molecular chain (main chain) of the curing agent and the UV-absorbing aromatic ring is four or more. This is because UV-absorbing groups are bulky, and if they are located close to the molecular chain of the curing agent, they will inhibit the reaction between the main component and the curing agent.

[0025] In a two-component mixed paint, the amount of hardener E1 can be, for example, about 10 to 40 wt% relative to the heat residue of the paint film, with 15 to 30 wt% being preferred.

[0026] (Main component C2) The silicone resin that is the main component is not particularly limited except that its dielectric constant is 8 or less, and any silicone resin that is the main component of a known two-component paint containing an acrylic polyol as a curing agent can be used. A silicone resin having hydroxyl groups is preferred as the silicone resin.

[0027] As the silicone resin having hydroxyl groups, known silicone resins having hydroxyl groups can be used, for example, the hydroxyl group-containing silicone resin described in Japanese Patent Application Publication No. 2006-045339 can be used. Examples of commercially available silicone resins having hydroxyl groups include "22-160AS", "KF-6001", "KF-6002", "KF-6003", "X-22-4015", "X-22-170DX", "Lighthobe 5000", and "Granol 410" manufactured by Shin-Etsu Chemical Co., Ltd.

[0028] The amount of silicone resin having hydroxyl groups in the two-component paint can be, for example, about 30 to 70 wt % relative to the heating residue of the coating film, and preferably about 40 to 60 wt %.

[0029] The paint liquid containing the main component (C1 or C2) of the two-component paint contains additives commonly found in top coats, such as UV absorbers, light stabilizers, and antifoaming agents, as well as extender pigments such as talc and calcium carbonate (but does not contain metal oxides such as titanium dioxide).

[0030] (Curing agent E-2) The ultraviolet absorbing group is obtained by copolymerizing a monomer constituting the acrylic polyol with a benzophenone or benzotriazole having a functional group copolymerizable with a vinyl monomer. Examples of monomers containing a hydroxyl group include 2-hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, 6-hexanolide addition polymer of 2-hydroxyethyl methacrylate (degree of polymerization 1 to 6), glyceryl (meth)acrylate, glyceryl-1-methacryloyloxyethyl urethane, etc. Examples of monomers having a vinyl group include esters of acrylic acid or methacrylic acid: for example, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, hexyl (meth)acrylate, octyl (meth)acrylate, lauryl (meth)acrylate, cyclohexyl (meth)acrylate, octyl methacrylate, lauryl methacrylate, cyclohexyl methacrylate, etc. These monomers may be mixed in plural kinds.

[0031] The amount of the curing agent E2 in the two-component mixed paint can be, for example, about 10 to 40 wt % relative to the heating residue of the coating film, and about 15 to 30 wt % is preferably used.

[0032] (Crystalline Polymer) The two-component topcoat paint may contain particles of a crystalline polymer having a crystallinity of 60% or more.

[0033] Generally, titanium dioxide (rutile type: refractive index approximately 2.7) or zinc oxide (refractive index approximately 2.0), which have a higher refractive index than the coating resin, are used to diffusely reflect visible light due to the refractive index difference at the boundary between the coating resin (refractive index approximately 1.5) and the pigment, thereby providing opacity. However, since there are almost no substances with such a high refractive index when not using metal oxides, the inventors have developed a method to provide opacity without using the refractive index difference between the coating film and the pigment. The simplest approach would be to add a substance that appears white in the air (refractive index approximately 1.0) to the coating film, but extender pigments commonly found in paints, such as calcium carbonate (refractive index approximately 1.56-1.59) and talc (refractive index approximately 1.57), have refractive indices close to those of the coating resin, and when added to paint, they become semi-transparent and cannot be used to provide opacity. Therefore, the inventors conceived of providing opacity by adding a crystalline polymer that appears white due to the difference in refractive index between its crystalline and amorphous parts. Specifically, polyethylene or polyethylene terephthalate fine particles (the shape does not need to be particle size; flake-like shapes are also acceptable) can be used. In paints, pigments suitable for imparting opacity, such as titanium dioxide and zinc oxide, exist; therefore, the addition of these crystalline polymer materials to paints for the purpose of imparting opacity is not common and not an easily conceivable method. Furthermore, it has been newly discovered that to impart opacity, both crystalline and amorphous portions must be present in appropriate amounts, and a crystallinity of 60% or higher is required. (While crystalline polymer fine particles appear nearly white in the atmosphere due to their fine particle size and light scattering, when added to a paint film, crystalline polymers with few crystalline / amorphous boundaries become semi-transparent and cannot impart opacity. The threshold crystallinity required to impart opacity in a paint film was identified during the development process of this disclosure.)

[0034] The crystallinity of the crystalline polymer is 60% or more, preferably 60 to 80%.

[0035] Examples of crystalline polymers include polyolefins (polyethylene, polypropylene, etc.), thermoplastic polyesters (polyethylene terephthalate, polybutylene terephthalate, etc.), aliphatic polyesters (polybutylene succinate, etc.), copolymers thereof, etc. A mixture of multiple types of crystalline polymers may be used.

[0036] In one embodiment, the crystalline polymer (including partially crystalline polymers) is at least one selected from the group consisting of polyolefins, thermoplastic polyesters, and aliphatic polyesters. These crystalline polymers have relatively good UV resistance and low water absorption.

[0037] A compatibilizing agent or a silane coupling agent may be used to make the crystalline polymer compatible with the coating resin.

[0038] In one embodiment, the SP value of the crystalline polymer is between the SP value of the main component constituting the coating film and the SP value of the curing agent. In another embodiment, the SP value of the crystalline polymer is 6 to 12. If the SP values ​​are far apart, the coating resin and the fine particles of the crystalline polymer will separate.

[0039] The shape of the crystalline polymer particles is not particularly limited, and may be spherical, scaly, cubic, or the like.

[0040] In one embodiment, the thickness of the crystalline polymer particles is 1 / 3 or less of the dry film thickness of the coating film containing the crystalline polymer particles. If the crystalline polymer is too large relative to the film thickness, problems such as separation from the coating resin and loss of surface smoothness occur. The particle thickness is the diameter for spherical particles, the thickness for scaly particles, and the length of the average longest side for cubic particles.

[0041] The size of the crystalline polymer particles is, for example, 5 to 30 μm.

[0042] When a two-component topcoat paint contains particles of crystalline polymer with a degree of crystallinity of 60% or more, the amount can be, for example, 5 to 30 wt% relative to the heat residue of the paint film. However, if the amount added is too small, the opacity cannot be ensured, and if the amount added is too large, the strength of the paint film decreases. Therefore, 10 to 20 wt% is preferably used.

[0043] At least one coating of the coating laminate contains particles of a crystalline polymer having a crystallinity of 60% or more.

[0044] The dry film thickness of the top coat can be, for example, about 20 to 100 μm.

[0045] (Intermediate Coat) The coating laminate may have an intermediate coat. The intermediate coat is also composed only of materials with a relative permittivity of 8 or less. The intermediate coat is usually adjacent to the topcoat.

[0046] The paint forming the intermediate coating is not particularly limited except that the relative dielectric constant is 8 or less, and known intermediate coating resins, curing agents, etc. can be used.

[0047] In one embodiment, the second coating layer from the top of the coating laminate is an acrylic polyurethane coating containing a UV-absorbing group (at least one selected from the group consisting of triazine, benzophenone, benzotriazole, and cyanoacrylate) as the main component. When the UV-absorbing group is consumed, UV rays may reach the undercoat, thus increasing UV resistance. In another embodiment, the resin of the paint forming the intermediate coat is an acrylic polyurethane coating containing a UV-absorbing group as the main component (a combination of main component C-1 and curing agent E-1).

[0048] When the intermediate coating contains particles of crystalline polymer with a degree of crystallinity of 60% or more, the amount can be, for example, 5 to 30 wt% relative to the heat residue of the coating film. However, if the amount added is too small, the opacity cannot be ensured, and if the amount added is too large, the strength of the coating film decreases. Therefore, 10 to 20 wt% is preferably used.

[0049] The dry film thickness of the intermediate coating is, for example, about 30 to 500 μm.

[0050] (Undercoat) The coating laminate may have an undercoat, which is also composed only of materials having a relative dielectric constant of 8 or less.

[0051] The paint forming the undercoat is not particularly limited except that it has a relative dielectric constant of 8 or less, and known undercoat paint resins, curing agents, etc. can be used.

[0052] When the primer paint contains crystalline polymer particles with a crystallinity of 60% or more, the amount can be, for example, 5 to 30 wt % relative to the heating residue of the paint film. However, if the amount added is too small, hiding power cannot be ensured, and if the amount added is too large, the strength of the paint film decreases, so 10 to 20 wt % is preferably used.

[0053] The dry film thickness of the undercoat is, for example, about 30 to 500 μm.

[0054] (Method for manufacturing coating film laminate) The coating film laminate of the present disclosure can be manufactured, for example, by applying the coating material of each of the above-mentioned coating films in order from the bottom layer side and curing them. Any known coating method can be used as the coating method. The curing method is not particularly limited, and each layer may be cured individually, or multiple uncured layers may be cured at once.

[0055] (Method for detecting corrosion under a coating) A method for detecting corrosion under a coating according to one embodiment of the present disclosure is characterized by measuring the coating laminate of the present disclosure with microwave thermography.

[0056] The coating laminate is placed on the surface of the steel material of the steel structure, with the top coat being located at the position farthest from the steel material.

[0057] In one embodiment, the microwave wavelength range used is 2 to 10 GHz, which is a wavelength range in which rust can be easily detected.

[0058] The subject performing the microwave thermography measurements may be a human or a drone.

[0059] The steel structure is not particularly limited, and examples thereof include steel towers and bridges.

[0060] The following examples will further clarify the effects of this disclosure. However, this disclosure is not limited to the following examples and may be modified as appropriate without altering its essence.

[0061] The raw materials used in the examples are as follows:

[0062] (Main component C) Fluororesin C1: Lumiflon (registered trademark) LF800, SP value 7.5 Silicone resin C2: Manufactured by Shin-Etsu Silicone Co., Ltd., product name "KR-517", SP value 8.5

[0063] (Curing Agent E) Polyisocyanate E1 In a reaction vessel under a nitrogen atmosphere, 1,3,5-tris(6-isocyanatehexyl)-1,3,5-triazinane-2,4,6-trione (50 g) and 2-[4,6-bis(2,4-dimethylphenyl)-1,3,5-triazin-2-yl]-5-[3-[(2-ethylhexyl)oxy]-2-hydroxypropoxy]phenol (5.8 g) having a UV absorbing group were reacted in toluene (70 g) as a solvent at 80°C for 4 hours, and then hexamethylene diisocyanate (168 g), methylcyclohexane-2,4-diisocyanate (35 g), ethylene glycol (31 g), and 1,4-butanediol (23 g) were added, and the reaction was continued at 80°C for an additional 6 hours to obtain Polyisocyanate E1. The SP value was measured to be 11.

[0064] Acrylic polyol E2 Under a nitrogen atmosphere, methyl ethyl ketone (300 g) as a solvent was used to polymerize methyl methacrylate (100 g), butyl methacrylate (14 g), 2-hydroxyethyl methacrylate (13 g), cyclohexyl methacrylate (8 g), 2-[2-hydroxy-5-[2-(methacryloyloxy)ethyl]phenyl]-2H-benzotriazole (32 g) having a UV absorbing group, and 2,2'-azobis-methylbutyronitrile (5 g) as a polymerization initiator by heating under reflux for 10 hours to obtain acrylic polyol E2. The SP value was 11.

[0065] Polyisocyanate E3: 1,3,5-tris(6-isocyanatehexyl)-1,3,5-triazinane-2,4,6-trione (50 g), hexamethylene diisocyanate (168 g), methylcyclohexane-2,4-diisocyanate (35 g), ethylene glycol (31 g), and 1,4-butanediol (23 g) were reacted in a reaction vessel under a nitrogen atmosphere at 80°C for 6 hours to obtain Polyisocyanate E3. The SP value was 11.

[0066] Acrylic polyol E4: Methyl methacrylate (100 g), butyl methacrylate (14 g), cyclohexyl methacrylate (8 g), 2-hydroxyethyl methacrylate (13 g), and 2,2'-azobis-methylbutyronitrile (5 g) as a polymerization initiator were polymerized under reflux in a nitrogen atmosphere for 10 hours using methyl ethyl ketone (300 g) as a solvent, to obtain acrylic polyol E4. The SP value was 11.

[0067] (UV absorber) 1,3,5-tris(6-isocyanatehexyl)-1,3,5-triazinane-2,4,6-trione (manufactured by Tokyo Chemical Industry Co., Ltd., B6601) 2-[2-hydroxy-5-[2-(methacryloyloxy)ethyl]phenyl]-2H-benzotriazole (manufactured by Tokyo Chemical Industry Co., Ltd., H1261) (for Comparative Example 37)

[0068] (Crystalline polymer particles D) Polyethylene: manufactured by Corefront Co., Ltd., product name "PENS-0.95", indicated as PE in the table Polyethylene terephthalate: manufactured by Techno Polymer Co., Ltd., product name "TP-MG", indicated as PET in the table Polypropylene: manufactured by Seishin Enterprise Co., Ltd., product name "PPW-5J", indicated as PP in the table Polybutylene terephthalate: manufactured by Toray Industries, Inc., product name "Toraycon 1401 X06", freeze-pulverized and used as particulates, indicated as PBT in the table Polytetrafluoroethylene: manufactured by Techno Chemical Co., Ltd., Microdispers-3000, indicated as PTFE in the table Polyoxymethylene: manufactured by Goodfellow Co., Ltd., OX316810, freeze-pulverized and used as particulates, indicated as POM in the table

[0069] (Diluent F) Paint thinner A, manufactured by Dainippon Paint Co., Ltd.

[0070] (Comparative Additive G) Titanium dioxide: Sakai Chemical Industry Co., Ltd. R-5N Alumina: Daido Paint, Alumina Aggregate #60 Mica: Yamaguchi Mica Co., Ltd., General Purpose Mica Powder A-21S Glass: Nippon Sheet Glass Co., Ltd., RCF-600 Calcium carbonate: Maruo Calcium Co., Ltd., Product name: Heavy Calcium Carbonate Talc: Nippon Talc Co., Ltd., Product name: MS-K

[0071] Measurement method for relative dielectric constant: Measured using a material impedance analyzer.

[0072] Method for Measuring Crystallinity: Using X-ray diffraction, the proportion of crystalline portions was calculated from the X-ray diffraction pattern.

[0073] The SP value was calculated by measuring the surface tension of the main component C and the hardener E using the Wihelmy method. The surface tension was calculated using a test specimen prepared by drying the sample on a polyvinyl chloride board at 20°C for one week to a dry film thickness of 150 μm, and measuring the contact angle when water and paraffin were dropped onto it in a 20°C atmosphere using a contact angle meter "FACE" (product name, manufactured by Kyowa Interface Science Co., Ltd.).

[0074] (Painting) Two layers of primer A were applied to the SS400 steel plate, with a dry film thickness of 50 μm per layer. Topcoat B was then applied on top of the primer to a dry film thickness of 30 μm. The primer used was a commercially available epoxy resin paint that did not contain metallic oxides and had a dielectric constant of 5 or less. The topcoat was prepared by adding 20 g of crystalline polymer particles D or 20 g of comparative additive G to 100 g of main component C, or by adding 15 g of hardener E and 3 g of diluent F, and mixing them together. In addition, a portion of the test piece was masked with commercially available high-weather-resistant tape to evaluate the amount of wear under ultraviolet irradiation.

[0075] (Evaluation Method) The following items were evaluated.

[0076] (Determination of Corrosion Detection Possibility) Topcoat B was prepared with the formulations shown in Tables 1 and 2. In preliminary studies (Comparative Examples 1 to 16), the following were measured using microwave thermography: - Coated test specimens on which an accelerated corrosion test had not been conducted and on which the steel plate was free of rust; - Coated test specimens subjected to an accelerated corrosion test and on which under-film corrosion had occurred. The results were judged as follows: - If no corrosion was present but a response behavior similar to that of iron rust was detected, there was a risk of false detection, so the rating was "FP." - If no corrosion was present and no response behavior similar to that of iron rust was detected, there was no risk of false detection, so the rating was "N." - If corroded areas were detected, corrosion was detected, so the rating was "S." The accelerated corrosion test involved making a cross-cut in the test specimen and performing the JIS K 5700-7-8 accelerated corrosion test (cycle A) for 3,000 hours. Corroded areas under the paint film located approximately 10 mm away from the cut were measured using microwave thermography to determine whether they could be detected. Unless otherwise specified, the microwave thermography frequency used was 2.45 GHz.

[0077]

[0078]

[0079] Comparative Examples 17 to 34 A three-layer coating laminate was obtained on a steel plate in the same manner as in the "Coating" above, with the exception that topcoat B was prepared using the formulations shown in Tables 3 and 4. The coated test specimens were subjected to an accelerated corrosion test to evaluate whether corrosion could be detected and for each of the following items.

[0080] (Bleed-out evaluation method) The test specimen was heated at 100°C for 15 minutes to check for the presence or absence of bleed-out of the UV absorber.

[0081] (Amount of UV-induced Depletion) The boundary between the masked and unmasked areas was measured using a laser microscope, and the amount of reduction in the thickness of the coating film in the unmasked area relative to the thickness of the coating film in the masked area was calculated.

[0082] (Concealment) A coating film was applied to a PTFE plate in advance, and after drying, the coating film was peeled off from the PTFE plate. Using UV-vis 2550, the transmittance of light at a wavelength of 410 nm was measured relative to a blank (air) and evaluated according to the following criteria: A: transmittance of 5% or less B: transmittance of more than 5%

[0083]

[0084]

[0085] (Examples 1-34, Comparative Examples 35-36) Except for preparing topcoat B with the formulations shown in Tables 5-7, a three-layer coating laminate was obtained on a steel plate with two layers of undercoat A and one layer of topcoat B on top, similar to the "painting" described above. Accelerated corrosion tests were performed on the coated test pieces, and the following items were evaluated: detection of corrosion, bleed-out, amount of UV wear, and opacity.

[0086] (Compatibility) When mixing and applying each component, visual inspection was performed to check whether any components clumped together or separated from the resin. Compatibility was evaluated according to the following criteria: A: If no abnormalities were observed, it was determined that the mixture was uniform. B: When applied to a test piece, if the painted surface was not smooth and bumps were observed, it was determined that crystalline polymer particles had clumped together.

[0087] (Accelerated Weathering Test) An accelerated weathering test was carried out for 5000 hours in accordance with JIS K 5700-7-7A.

[0088] (Chalkation Measurement) For samples that underwent accelerated weathering testing for 5000 hours, the chalkation grade was evaluated according to the chalkation measurement method of JIS K 5600-8-6:2014. A chalkation measurement tape was attached to the surface of the coating film, then peeled off, and compared with the standard illustration of the said standard. The chalkation grade was evaluated according to the following criteria: 0: No chalking 1: Chalkation has progressed to the extent of standard illustration 1 2: Chalkation has progressed to the extent of standard illustration 2 If a piece of coating film, rather than powder, adhered to the chalkation measurement tape, it was judged that the coating film was brittle, and "E" was marked.

[0089]

[0090]

[0091]

[0092] (Comparative Examples 37-38) Paints were prepared according to the formulations shown in Table 8. The paints were applied to obtain coating films. The coating films were then subjected to a coating film strength evaluation using JIS K5600 General Test Methods for Paints 4.4 Scratch Hardness (Pencil Method). The results are shown in Table 8. The structures of the UV absorbers used in Comparative Examples 37 and 38 are as follows.

[0093]

[0094] (Examples 35-42) For the painted test pieces of Examples 1 and 4, the detection of corrosion was evaluated by changing the microwave wavenumber. The results are shown in Table 9. A: The corrosion area and the areas that were actually corroded generally coincided, and good detection results were obtained. (Since the corroded areas had fine paint blistering or bulging of the paint film due to rust, the fine surface irregularities were measured using a laser microscope OLS5100, and the convex areas were assumed to be the true corrosion areas, and evaluated by comparing them with the measurement results of microthermography.) B: The areas where corrosion was detected were slightly wider than the actual corrosion area. Generally, the amount of heat absorbed by microwave heating of a dielectric material is proportional to the microwave frequency, so at a high frequency of 10 GHz, the heat absorbed due to corrosion was large, and it is thought that this heat diffused into the metal of the substrate, causing a temperature change to be detected over a slightly wider area. C: The temperature difference between the corrosion area and its surroundings was small, making it difficult to detect minute corrosion. Generally, the amount of heat absorbed by a dielectric due to microwave heating is proportional to the microwave frequency. Therefore, at the low frequency of 0.915 GHz, the heat absorption in the corroded area was small, and it is thought that detection was difficult when the corroded area was small.

[0095]

[0096] The paint disclosed herein ensures UV resistance of the coating film at a level comparable to that achieved when using typical amounts of metallic pigments, without causing bleeding of the UV absorber, while also providing sufficient opacity and not affecting microwave reflectivity, thereby enabling microwave-based inspection of corrosion beneath the coating film. Since microwave inspection can be performed non-contact, it can be mounted on drones, etc. Conventionally, in order to prioritize the safety of the equipment, the presence or absence of corrosion beneath the coating film was checked manually using close-range visual inspection or grid testing. However, by enabling inspection with drones, etc., the efficiency of inspections can be expected to increase. By applying the paint of the present invention to steel structures, corrosion beneath the coating film can be inspected over a wide area of ​​the steel structure using microwaves, enabling early detection of material thinning progressing beneath the coating film. Furthermore, if rust removal and repainting maintenance can be performed at an early stage when corrosion beneath the coating film is minor, the life cycle cost (LCC) of the paint can also be reduced.

[0097] The following additional notes are provided regarding the above-described embodiments.

[0098] (Note 1) A coating laminate comprising at least two types of coating films, wherein each coating film is composed only of a substance with a relative permittivity of 8 or less, and the topcoat is formed from a two-component mixed paint containing a main component and a curing agent, the combination of the main component and the curing agent being one of the following: - Main component: fluororesin, curing agent: polyisocyanate - Main component: silicone resin, curing agent: acrylic polyol The side chain of the curing agent in the topcoat has at least one molecular structure selected from the group consisting of triazine, benzophenone, benzotriazole and cyanoacrylate as a UV absorbing group, and at least one coating layer contains particles of a crystalline polymer with a degree of crystallinity of 60% or more. (Note 2) The coating laminate according to Note 1, wherein the crystalline polymer has an SP value between the SP value of the main component constituting the coating film and the SP value of the curing agent. (Note 3) The coating laminate according to Note 1 or 2, wherein the crystalline polymer (including partially crystalline polymer) is at least one selected from the group consisting of polyolefins, thermoplastic polyesters, and aliphatic polyesters. (Note 4) The coating laminate according to any one of Notes 1 to 3, wherein the thickness of the particles of the crystalline polymer is 1 / 3 or less of the dry film thickness of the coating containing the particles of the crystalline polymer. (Note 5) The coating laminate according to any one of Notes 1 to 4, wherein the main component is a silicone resin, the curing agent is an acrylic polyol, and the total number of carbons, oxygens, or nitrogens between the molecular chain of the curing agent and the UV-absorbing aromatic ring is 4 or more. (Note 6) The coating laminate according to any one of Notes 1 to 5, wherein the second coating from the top is an acrylic polyurethane coating containing a UV-absorbing group as the main component. (Note 7) A method for detecting corrosion under a coating, characterized by measuring the coating laminate according to any one of Notes 1 to 6 with microwave thermography. (Appendix 8) The method for detecting under-film corrosion according to appendix 7, wherein the microwave used has a wavelength band of 2 to 10 GHz.

[0099] 1: Coating film laminate 10: Top coat 20: Primer coat 30: Intermediate coat 40: Steel material

Claims

1. A coating laminate comprising at least two types of coating films, each coating film composed solely of a substance with a relative permittivity of 8 or less, wherein the topcoat is formed from a two-component mixed paint containing a main component and a curing agent, and the combination of the main component and the curing agent is one of the following: - Main component: fluororesin, curing agent: polyisocyanate - Main component: silicone resin, curing agent: acrylic polyol The side chain of the curing agent in the topcoat contains at least one molecular structure selected from the group consisting of triazine, benzophenone, benzotriazole, and cyanoacrylate as a UV absorbing group, and at least one coating layer contains particles of a crystalline polymer with a degree of crystallinity of 60% or more.

2. The coating laminate according to claim 1, wherein the crystalline polymer has an SP value between the SP value of the base agent constituting the coating and the SP value of the curing agent.

3. The coating film laminate according to claim 1, wherein the crystalline polymer (including partially crystalline polymer) is at least one selected from the group consisting of polyolefins, thermoplastic polyesters, and aliphatic polyesters.

4. The coating film laminate according to claim 1, wherein the thickness of the crystalline polymer particles is 1 / 3 or less of the dry film thickness of the coating film containing the crystalline polymer particles.

5. The coating laminate according to claim 1, wherein the base resin is a silicone resin, the curing agent is an acrylic polyol, and the total number of carbon atoms, oxygen atoms, or nitrogen atoms between the molecular chain of the curing agent and the UV-absorbing aromatic ring is 4 or more.

6. The coating film laminate according to claim 1, wherein the second coating film from the top is an acrylic polyurethane coating film containing a UV-absorbing group as the main component.

7. A method for detecting under-film corrosion, comprising measuring the coating laminate of claim 1 using microwave thermography.

8. The method for detecting under-film corrosion according to claim 7, wherein the microwave used has a wavelength band of 2 to 10 GHz.

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