Coating system
By adopting coordinated lower and upper coating characteristics in the coating system, the problems of coating miscibility and appearance requirements in the extra coating and one-baking process are solved, and excellent appearance effects and mechanical properties are achieved, meeting the high requirements of automotive coatings.
Patent Information
- Application Number
- CN202380076667.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Priority Date
- 2022-09-02
- Filing Date
- 2023-09-01
- Publication Date
- 2025-06-10
AI Technical Summary
The extra coating and drying process can easily lead to miscibility between coatings in the application fields of automotive coatings and high appearance requirements, affecting appearance and mechanical properties, and it is difficult to achieve the effect of imitating electroplating silver.
A coating system is adopted, including a lower coating formed from a first coating composition, the upper coating formed from a second coating composition containing an effect pigment, and when cured simultaneously, ensuring that the dry film thickness of the upper coating is between 3 µm and 20 µm, and the difference in wet film solids content of the lower coating and the upper coating is at least 20% by weight.
By coordinating the characteristics of the lower and upper coatings, avoiding miscibility between the coatings, ensuring excellent orientation of the effect pigment, achieving excellent appearance and mechanical properties, and meeting the high requirements of automotive coatings.
Smart Images
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Abstract
Description
Technical Field
[0002] The present disclosure relates to the field of coatings, and more particularly to coating systems, especially coating systems suitable for a multi - coat - one - bake process. Background Art
[0004] In the field of coatings, multi - layer coatings can be formed by a one - coat - one - bake or a multi - coat - one - bake process. Compared with the one - coat - one - bake process in which once a layer is coated, that layer is baked, the so - called multi - coat - one - bake means omitting the intermediate baking step and baking two or more coatings simultaneously after applying two or more coatings.
[0005] Considering energy savings and economy, the multi - coat - one - bake process is increasingly favored by the market. However, the multi - coat - one - bake process is also prone to cause miscibility between coatings, resulting in a poor appearance of the entire coating system and reduced mechanical properties. Therefore, how to meet the requirements for the appearance and performance of coatings while pursuing energy savings and economic benefits is a technical problem, which is particularly challenging for automotive coatings and other application fields with high appearance requirements.
[0006] In addition, automotive manufacturers prefer metallic color effects that mimic electroplated silver and have good aluminum flake orientation and low sparkle. The miscibility between coatings caused by multi - coat - one - bake applications directly affects the arrangement of aluminum flakes, which also has a negative impact on the achievement of such color effects. Summary of the Invention
[0008] Disclosed is a multi - coat - one - bake process, and this multi - coat - one - bake process can meet the requirements for the appearance and mechanical properties of coatings in automotive coatings and other application fields.
[0009] The present disclosure provides a coating system, which includes: a lower coating formed from a first coating composition; and an upper coating formed from a second coating composition containing effect pigments, wherein the first coating composition and the second coating composition are cured simultaneously, the upper coating has a dry film thickness of 3 µm to 20 µm, and the difference between the wet film solid contents of the lower coating and the upper coating is at least 20 wt%.
[0010] The present disclosure further discloses a coated substrate, which includes a substrate and the above - mentioned coating system coated on at least a part of the substrate.
[0011] The present disclosure also discloses a method for coating a substrate, the method comprising:
[0012] Coating the substrate with a first coating composition to form a lower coating;
[0013] Apply a second coating composition over at least a portion of the lower coating to form an upper coating, the second coating composition comprising an effect pigment; and
[0014] Cure the first coating composition and the second coating composition simultaneously, wherein the upper coating has a dry film thickness of 3 µm to 20 µm, and the difference between the wet film solids content of the lower coating and the upper coating is at least 20 wt%.
[0015] The present disclosure also discloses a method for coating a substrate, the method comprising:
[0016] Coat the substrate with a first coating composition to form a lower coating;
[0017] Apply a second coating composition over at least a portion of the lower coating to form an upper coating, the second coating composition comprising an effect pigment;
[0018] Apply a clearcoat coating composition over at least a portion of the upper coating to form a clearcoat layer; and
[0019] Cure the first coating composition, the second coating composition, and the clearcoat coating composition simultaneously, wherein the upper coating has a dry film thickness of 3 µm to 20 µm, and the difference between the wet film solids content of the lower coating and the upper coating is at least 20 wt%.
[0020] The features and advantages of the present disclosure will be shown in detail in the following detailed description. Detailed Description
[0022] In this application, unless otherwise expressly stated, the singular includes the plural and the plural includes the singular in use. For example, although the term "a" resin is mentioned herein, one or more of these substances may be used.
[0023] In this application, the terms "comprising", "including", "containing", etc. are not intended to limit the present disclosure to exclude any variations or additions. Further, although the present disclosure has described coating compositions, preparation methods, etc. using terms such as "comprising", the coating compositions, preparation methods, etc. described in detail herein may also be described as "consisting essentially of" or "consisting of". In such cases, "consisting essentially of" means that any additional ingredients will not substantially affect the properties of the coating formed from the coating composition.
[0024] In this application, unless otherwise expressly stated, "or" is used to mean "and / or", even though "and / or" may be expressly used in some instances. Further, any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of "1 to 10" is intended to include all sub-ranges between and including the recited minimum value of 1 and the recited maximum value of 10, that is, all sub-ranges having a minimum value equal to or greater than 1 and a maximum value equal to or less than 10.
[0025] Except as otherwise indicated in the examples or otherwise expressly stated, all values representing quantities of ingredients, etc. used in the specification and claims are to be construed as variable in accordance with the term "about" in all instances. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and claims are approximations and may vary depending upon the desired properties sought to be obtained by the present disclosure. At the very least, and not as a limitation on the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should be construed in light of at least one significant figure and by applying ordinary rounding techniques.
[0026] Although the numerical ranges and parameters setting forth the broad scope of the present disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. However, any numerical value inherently contains certain errors necessarily resulting from the standard deviation found in their respective measurement techniques.
[0027] The present disclosure relates to a coating system comprising: a lower coating formed from a first coating composition; and an upper coating formed from a second coating composition comprising an effect pigment, the first coating composition and the second coating composition being cured simultaneously, wherein the upper coating has a dry film thickness of 3 µm to 20 µm and the difference between the wet film solids content of the lower coating and the upper coating is at least 20 wt%.
[0028] By the coordination between the upper coating and the lower coating, the miscibility between different coatings in a multi-coat and one-bake process is avoided, and the orientation of the effect pigment is prevented from being unfavorably disturbed by adjacent coatings, thereby ensuring that the coating system has excellent appearance effects and mechanical properties. The upper coating has a lower dry film thickness, which helps to obtain a high FI value (flip-flop index); in addition, the significant difference in the wet film solids content between the lower coating and the upper coating enables moisture to rapidly migrate from the upper coating to the lower coating after application, which helps to rapidly increase the viscosity of the upper coating and assist in the arrangement of the effect pigment, thereby obtaining a higher FI value.
[0029] As used herein, "simultaneous curing" means that the first coating composition and the second coating composition can be cured simultaneously to form a film, and the first coating composition is not cured when the second coating composition is applied. The term "curing" means that at least a portion of the components in the coating composition are polymerized and / or crosslinked, or dried to form a hardened coating film.
[0030] Suitably, the first coating composition and the second coating composition, and optionally one or more other coating compositions, can be cured simultaneously. The other coating compositions can form a coating below the lower coating and / or a coating above the upper coating.
[0031] Suitably, the first coating composition and the second coating composition can be cured by heating. For example, the first coating composition and the second coating composition can be cured at 140 °C for 20 minutes. The term "curable" means that after baking at 140 °C for 20 minutes, the resulting coating has a MEK double rub value of at least 50 times, suitably even at least 100 times.
[0032] As will be understood by those skilled in the art, the multiple coat and one bake process can include flash drying each applied coating. For example, the lower coating formed from the first coating composition is flash dried after the first coating composition is cured and before the second coating composition is applied. The term "flash drying" is the process of evaporating the solvent in the coating composition. Suitably, the first coating composition, the second coating composition, and / or optionally one or more other coating compositions can be flash dried at room temperature (e.g., 23 °C) for 1 to 15 minutes independently.
[0033] As used herein, "dry film thickness" means the thickness of the coating formed from the coating composition after complete curing. The "dry film thickness" can be obtained by measurement with a commercially available film thickness gauge.
[0034] The upper coating can have a dry film thickness of 3 µm to 20 µm. For example, the upper coating can have a dry film thickness of 3 µm or greater, 4 µm or greater, 5 µm or greater, and / or 20 µm or less, 19 µm or less, 18 µm or less, 17 µm or less, 16 µm or less, 15 µm or less, 14 µm or less, 13 µm or less, 12 µm or less, such as 11 µm, 10 µm, 9 µm, 8 µm, 7 µm, or 6 µm.
[0035] The lower coating may have a dry film thickness of 5 µm to 20 µm. For example, the lower coating may have a dry film thickness of 5 µm or greater, 6 µm or greater, 7 µm or greater, 8 µm or greater, and / or 20 µm or less, 18 µm or less, 16 µm or less, 14 µm or less, 12 µm or less, 10 µm or less.
[0036] As used herein, the "wet film solids content" refers to the solids content in a coating formed from a coating composition applied to a surface and that has undergone flash drying. The "wet film solids content" is associated with the content of solids contained in the coating composition and the drying rate of the coating. The test method for the "wet film solids content" is described in detail in the Examples section below.
[0037] The difference in wet film solids content between the lower coating and the upper coating may be at least 20 wt%, i.e., the difference between the wet film solids content of the lower coating minus the wet film solids content of the upper coating is at least 20 wt%. Suitably, the difference in wet film solids content between the lower coating and the upper coating may be at least 25 wt%. Suitably, the difference in wet film solids content between the lower coating and the upper coating may be at least 30 wt%. Suitably, the difference in wet film solids content between the lower coating and the upper coating may be at least 35 wt%. Suitably, the difference in wet film solids content between the lower coating and the upper coating may be at least 40 wt%. Suitably, the difference in wet film solids content between the lower coating and the upper coating may be at least 45 wt%. Suitably, the difference in wet film solids content between the lower coating and the upper coating may be at least 50 wt%.
[0038] For example, the lower coating may have a wet film solids content of 50 wt% or higher, such as 55 wt% or higher, 60 wt% or higher, 65 wt% or higher, or 70 wt% or higher. As used herein, the wet film solids content of the lower coating is determined and obtained at a dry film thickness of 5 µm to 20 µm.
[0039] For example, the upper coating may have a wet film solids content of 30 wt% or lower, such as 25 wt% or lower, or 20 wt% or lower. As used herein, the wet film solids content of the upper coating is determined and obtained at a dry film thickness of 3 µm to 20 µm.
[0040] Suitably, compared with the second coating composition that forms the upper coating, the first coating composition that forms the lower coating may have a higher solids content. The "solids content" refers to the content of solids in the coating composition, which can be obtained by determining the ratio of the weight of the coating composition after the solvent has been completely evaporated to the total weight of the coating composition.
[0041] Suitably, the first coating composition may have a solids content of 30 wt% to 45 wt%, such as a solids content of 30 wt% to 40 wt%. Suitably, the second coating composition may have a solids content of 8 wt% to 20 wt%, such as a solids content of 8 wt% to 16 wt%.
[0042] The second coating composition that forms the upper coating may include effect pigments. Effect pigments refer to pigments that provide a metallic effect to the coating, and these pigments may include aluminum powder pigments and / or mica powder pigments. The effect pigments may include surface-treated and / or non-surface-treated effect pigments. The non-surface-treated effect pigments may be pre-coated with a passivating agent before use.
[0043] Suitably, the effect pigments may include silver dollar type effect pigments, such as silver dollar type aluminum powder. The silver dollar type effect pigment refers to a plate-shaped effect pigment having the form of a silver dollar. Suitably, the effect pigments may have an aspect ratio of 50:1 to 200:1, such as an aspect ratio of 70:1 to 200:1, for example, an aspect ratio of 100:1 to 200:1. The "aspect ratio" refers to the ratio of the diameter to the thickness of the pigment.
[0044] Suitably, the effect pigments may have a WCA greater than 30,000 cm 2 / g. WCA refers to the area of water covered by aluminum powder, that is, the area per gram of aluminum powder when the aluminum powder is located on the surface of water via a monolayer of particles and no gaps are visible between the particles. The WCA can be determined according to the GB / T 3173-1982 standard.
[0045] In order to obtain a higher FI value, the effect pigments may include pigments having a narrow particle size distribution. The particle size distribution can be represented in a metric form as a cumulative particle size distribution curve, where the percentage of particles smaller than a determined size is obtained respectively. For example, D10 (corresponding to the particle size at which the cumulative particle size distribution percentage reaches 10%), D50 (corresponding to the particle size at which the cumulative particle size distribution percentage reaches 50%), and D90 (corresponding to the particle size at which the cumulative particle size distribution percentage reaches 90%). Herein, the narrow particle size distribution can be characterized by calculating the particle size span, that is, (D90 - D10) / D50. Suitably, the effect pigments may have a particle size span of less than 1.5.
[0046] To achieve the desired flashing effect, the effect pigment may have an average particle size of 5 µm to 25 µm, suitably an average particle size of 5 µm to 20 µm, such as an average particle size of 5 µm to 15 µm, and even an average particle size of 5 µm to 10 µm. In this context, the particle size can be obtained by measurements using a commercially available particle size analyzer.
[0047] The second coating composition may further include other pigments, and the other pigments refer to pigments other than the effect pigments mentioned above. Pigments suitable for use in the second coating composition may include inorganic pigments such as carbon black, titanium white, and iron red, and / or organic pigments such as azo pigments, phthalocyanine pigments, and perylene pigments.
[0048] In the second coating composition, the content of the pigments (effect pigments and optional other pigments) can be represented by the pigment-binder ratio. The "pigment-binder ratio (P / B)" refers to the solid weight ratio of the pigments (effect pigments and optional other pigments) in the second coating composition to the binder in the second coating composition. Suitably, the second coating composition has a pigment-binder ratio of 0.02:1 to 0.3:1, such as a pigment-binder ratio of 0.02:1 to 0.1:1.
[0049] The coating system may have a high FI value. Suitably, the coating system may have an FI value of at least 20. The method for determining the FI value is described in detail in the section of the following examples.
[0050] The coating system may have a low G value. The G value represents the flashing intensity of the coating, where: the smaller the G value, the weaker the flashing; and the larger the G value, the stronger the flashing. Suitably, the coating system may have a G value not exceeding 6. The method for determining the G value is described in detail in the section of the following examples.
[0051] The coating system may have a low VOC content. Suitably, each of the first coating composition and the second coating composition in the coating system may have a VOC content of less than 420 g / L. The VOC value is measured without water, which can be obtained by detecting the content of each organic compound component in the first coating composition or the second coating composition by gas chromatography and then adding up the contents of all components.
[0052] In addition, the structural properties of the lower coating may enable the lower coating to form a stable coating film, which reduces the intrusion of organic solvents in the upper coating into the lower coating, thereby further avoiding the miscibility between different coatings in the multi-coat and one-bake process, such that the lower coating does not interfere with the arrangement of aluminum powder in the upper coating and does not unfavorably reduce the mechanical properties of each coating. Meanwhile, the structural properties of the upper coating also enable the upper coating to be free from the interference with the arrangement of effect pigments caused by the intrusion of organic solvents in the coating above it (such as a clear coat), thereby obtaining a high FI value and achieving qualified mechanical properties.
[0053] First coating composition
[0054] The lower coating in the coating system can be formed from a first coating composition. The first coating composition can be an aqueous coating composition. As used herein, "aqueous" means that the solvent of the coating composition contains at least 50% by weight of water.
[0055] The first coating composition can be a one-component coating composition. The term "one-component" means that all film-forming resins (i.e., binders), pigments and fillers, solvents, and / or additives, etc. of the coating composition are packaged in one container, thus having advantages such as convenient storage and use.
[0056] In the first coating composition, the viscosity of the film-forming resin increases rapidly after curing, thereby minimizing the interference between the lower coating formed from the first coating composition and other coatings (such as the upper coating formed from a second coating composition). The film-forming resin affects the structural properties of the formed coating and provides the required mechanical properties to the coating system, such as stone chip resistance, impact resistance, cupping resistance, moisture resistance, UV radiation resistance, etc.
[0057] In the first coating composition, the film-forming resin may include an acrylic resin, an acrylic-grafted polyurethane resin, a polyurethane resin, and / or a melamine formaldehyde resin, wherein the acrylic resin, the acrylic-grafted polyurethane resin, the polyurethane resin, and / or the melamine formaldehyde resin are different from each other. Suitably, the film-forming resin at least includes a polyurethane resin. For example, the film-forming resin may include a polyurethane resin, an acrylic resin, and / or an acrylic-grafted polyurethane resin and a melamine formaldehyde resin. The acrylic resin and / or the acrylic-grafted polyurethane resin cooperate with the polyurethane resin to form a stable coating structure and enhance the mechanical properties of the coating. Suitably, the weight ratio of the polyurethane resin to the acrylic resin and / or the acrylic-grafted polyurethane resin (i.e., the weight of the polyurethane resin / (the weight of the acrylic resin and / or the acrylic-grafted polyurethane resin)) may be from 0.1 to 0.9.
[0058] A polyurethane resin refers to a polymer whose repeating units contain polyurethane groups. The polyurethane may include a polymer having at least 50% by weight of organic units (e.g., at least 70% by weight, such as at least 90% by weight of organic units) connected by urethane bonds. In this context, organic units include one or more residues selected from the following: simple diols, polyester diols, polyether diols, and polycarbonate diols.
[0059] The polyurethane resin can be prepared from polyisocyanates and polyols. Suitably, the polyisocyanate includes one or more of the following: aromatic isocyanates, aliphatic isocyanates, and cycloaliphatic isocyanates.
[0060] Suitably, the polyurethane resin can have a weight-average molecular weight (Mw) of 2,000 to 200,000, such as a weight-average molecular weight of 10,000 to 150,000. The weight-average molecular weight can be determined by gel permeation chromatography using a suitable standard such as a polystyrene standard. Suitably, the polyurethane resin can have an average particle size of 100 nm to 2,000 nm, such as an average particle size of 200 nm to 1,500 nm. The particle size can be measured by laser diffraction.
[0061] Suitably, the polyurethane resin can be in the form of a dispersion or an emulsion having a solids fraction of 20% to 60% by weight. The "solids fraction" refers to the percentage by mass of the mass remaining after evaporation of the solvent in the dispersion / emulsion relative to the mass of the original dispersion / emulsion. The polyurethane dispersion / emulsion has good stability and film-forming properties and has good chemical resistance and flexibility after film formation.
[0062] Based on the total weight of the first coating composition, the polyurethane resin can be at least 1% by weight, suitably at least 2% by weight, such as at least 3% by weight and / or at most 10% by weight, suitably at most 8% by weight, such as at most 5% by weight. Based on the total weight of the first coating composition, the polyurethane resin can be present in the coating composition in a range of 1% to 10% by weight, suitably 2% to 8% by weight, such as 3% to 5% by weight or any other combination using these end values.
[0063] An acrylic resin refers to a polymer having (meth)acrylic monomers as the basic component. The "basic component" means that the (meth)acrylic monomers in the polymer account for at least 70% by weight of all polymer monomers, for example at least 80% by weight, such as at least 90% by weight.
[0064] Suitably, the acrylic resin may have a hydroxyl value of 50 mgKOH / g to 150 mgKOH / g, such as a hydroxyl value of 70 mgKOH / g to 120 mgKOH / g. The hydroxyl value refers to the milligrams of potassium hydroxide (KOH) equivalent to the hydroxyl groups in 1 gram of the resin. Suitably, the acrylic resin may have an acid value of 1 mgKOH / g to 50 mgKOH / g, such as an acid value of 10 mgKOH / g to 40 mgKOH / g. The acid value refers to the milligrams of potassium hydroxide required to neutralize the free acids in 1 gram of the resin. Suitably, the acrylic resin may have an average particle size of 100 nm to 1000 nm, such as an average particle size of 100 nm to 800 nm. The particle size can be measured by laser diffraction method.
[0065] Suitably, the acrylic resin may be in the form of a dispersion or an emulsion, which has a solids fraction of 20 wt% to 60 wt%. The "solids fraction" refers to the percentage by mass of the remaining mass after evaporation of the solvent in the dispersion / emulsion relative to the mass of the original dispersion / emulsion. Suitably, the acrylic dispersion / emulsion has a viscosity of 100 mPa to 1,000 mPa at room temperature, where room temperature refers to 15°C to 30°C. In this context, the viscosity can be obtained by measurement using a commercially available viscometer. The acrylic resin dispersion / emulsion provides good viscoelasticity, film-forming properties and stability.
[0066] Based on the total weight of the first coating composition, the acrylic resin may be at least 3 wt%, suitably at least 4 wt%, such as at least 5 wt% and / or at most 15 wt%, suitably at most 12 wt%, such as at most 8 wt%. Based on the total weight of the first coating composition, the acrylic resin may be present in the coating composition in the range of 3 wt% to 15 wt%, suitably 4 wt% to 12 wt%, such as 5 wt% to 8 wt% or any other combination using these end values.
[0067] The acrylic grafted polyurethane resin refers to a polyurethane resin having branched chains containing acrylic groups, and at least 50 wt% of the organic units in the polyurethane resin are connected by urethane bonds.
[0068] Suitably, the acrylic grafted polyurethane resin may have a weight average molecular weight (Mw) of from 2,000 to 200,000, such as a weight average molecular weight of from 10,000 to 150,000. The weight average molecular weight can be determined by gel permeation chromatography using a suitable standard such as a polystyrene standard. Suitably, the acrylic grafted polyurethane resin may have a hydroxyl value of from 10 mgKOH / g to 100 mgKOH / g, such as a hydroxyl value of from 20 mgKOH / g to 80 mgKOH / g. The hydroxyl value refers to the milligrams of potassium hydroxide (KOH) equivalent to the hydroxyl groups in 1 gram of the resin. Suitably, the acrylic grafted polyurethane resin may have an acid value of from 1 mgKOH / g to 50 mgKOH / g, such as an acid value of from 10 mgKOH / g to 40 mgKOH / g. The acid value refers to the milligrams of potassium hydroxide required to neutralize the free acids in 1 gram of the resin. Suitably, the acrylic grafted polyurethane resin may have an average particle size of from 100 nm to 2,000 nm, such as an average particle size of from 500 nm to 1,500 nm. The average particle size can be measured by laser diffraction method.
[0069] Suitably, the acrylic grafted polyurethane resin may be in the form of a dispersion or an emulsion, which has a solids fraction of from 20 wt% to 60 wt%. The "solids fraction" refers to the percentage by mass of the mass remaining after evaporation of the solvent in the dispersion / emulsion relative to the mass of the original dispersion / emulsion. The acrylic grafted polyurethane dispersion / emulsion has good compatibility with other components and has good stability and chemical resistance after film formation.
[0070] Based on the total weight of the first coating composition, the acrylic grafted polyurethane resin may be at least 5 wt%, suitably at least 8 wt%, such as at least 10 wt% and / or at most 30 wt%, suitably at most 20 wt%, such as at most 15 wt%. Based on the total weight of the first coating composition, the acrylic grafted polyurethane resin may be present in the coating composition in the range of from 5 wt% to 30 wt%, suitably from 8 wt% to 20 wt%, such as from 10 wt% to 15 wt% or any other combination using these end values.
[0071] The film-forming resin in the first coating composition may further include a melamine formaldehyde resin. The melamine formaldehyde resin can crosslink with the above-mentioned polyurethane resin, acrylic resin, and / or acrylic grafted polyurethane resin to improve the film-forming properties of the first coating composition and accelerate curing. The melamine formaldehyde resin may be a single melamine formaldehyde resin or a combination of multiple different melamine formaldehyde resins. In this melamine formaldehyde resin or the combination of melamine formaldehyde resins, the functional group ratio of the sum of imino and hydroxymethyl in the melamine formaldehyde resin to alkoxy is from 5:80 to 50:50.
[0072] Based on the total weight of the first coating composition, the melamine formaldehyde resin may be at least 5% by weight, suitably at least 10% by weight and / or at most 25% by weight, such as at most 20% by weight. Based on the total weight of the first coating composition, the melamine formaldehyde resin may be present in the coating composition in the range of 5% to 25% by weight, suitably 10% to 20% by weight or any other combination using these end values.
[0073] The first coating composition may further comprise a pigment. The pigment provides the desired color and / or hiding power to the lower coating, thereby affecting the appearance of the coating system. The pigment may be any pigment suitable for an aqueous coating composition. Suitably, the pigment includes carbon black, titanium white, iron oxide red and / or perylene red, as well as other inorganic or organic pigments to form a dark lower coating with high hiding power. In this context, "dark color" means that based on the L*a*b color system defined in JIS Z 8729, the brightness value (45° L value) of the lower coating is not more than 80.
[0074] Suitably, the pigment may be in the form of a color paste having a solid fraction of 10% to 45% by weight. Generally, the first coating composition may comprise 5% to 35% by weight of the color paste based on the total weight of the first coating composition.
[0075] The first coating composition may also comprise 30% to 50% by weight of water based on the total weight of the first coating composition.
[0076] The first coating composition may further comprise one or more of the following other auxiliary components: organic co-solvent, wetting additive, pinhole additive, curing catalyst, defoamer, ultraviolet absorber, light stabilizer, adhesion promoter, surface leveling agent, pH regulator, substrate wetting agent, thickener, filler, etc. When these auxiliary components are present, each auxiliary component is present in an amount of at most 10% by weight based on the total weight of the first coating composition.
[0077] Second coating composition
[0078] The upper coating in the coating system may be formed from a second coating composition. The second coating composition may be an aqueous coating composition. The second coating composition may be a one-component coating composition.
[0079] The film-forming resin in the second coating composition may include a high-Tg acrylic resin and a high-Tg polyurethane resin, where the high-Tg acrylic resin and the high-Tg polyurethane resin are different from each other. Suitably, the weight ratio of the high-Tg acrylic resin to the high-Tg polyurethane resin may be 10-80:5-50, such as 30-50:10-30. The combination of the high-Tg acrylic resin and the high-Tg polyurethane resin forms a dense and stable coating structure, which reduces the penetration of organic solvents of other coatings; and the selection of the film-forming resin will endow the coating with mechanical properties, so that various application requirements can be met even at a low film thickness. At the same time, it facilitates the orientation of the effect pigments in the second coating composition.
[0080] Acrylic resin refers to a polymer with (meth)acrylic monomers as the basic components. The "basic components" means that the (meth)acrylic monomers in the polymer account for at least 70% by weight of all polymer monomers, for example, at least 80% by weight, such as at least 90% by weight.
[0081] The high-Tg acrylic resin may have a glass transition temperature (Tg) greater than 40 °C, suitably a glass transition temperature of 40 °C to 150 °C. The glass transition temperature can be measured by dynamic thermomechanical analysis (DMA) using a TA Instruments Q800 instrument. The measurement parameters are: a frequency of 10 Hz, an amplitude of 5 mm, a temperature ramp of -100 °C to 250 °C, and the Tg is determined as the peak of the tanδ curve according to ASTM D7028.
[0082] Suitably, the high-Tg acrylic resin may have a hydroxyl value of 10 mgKOH / g to 100 mgKOH / g, such as a hydroxyl value of 20 mgKOH / g to 80 mgKOH / g. The hydroxyl value refers to the milligrams of potassium hydroxide (KOH) equivalent to the hydroxyl groups in 1 gram of resin. Suitably, the high-Tg acrylic resin may have an acid value of 1 mgKOH / g to 70 mgKOH / g, such as an acid value of 10 mgKOH / g to 50 mgKOH / g. The acid value refers to the milligrams of potassium hydroxide required to neutralize the free acid in 1 gram of resin. Suitably, the high-Tg acrylic resin may have an average particle size of 100 nm to 1,000 nm, such as an average particle size of 100 nm to 800 nm. The particle size can be measured by laser diffraction.
[0083] Suitably, the high-Tg acrylic resin may be in the form of a dispersion or an emulsion, which has a solid fraction of 20% to 60% by weight. The "solid fraction" refers to the percentage of the mass remaining after the evaporation of the solvent in the dispersion / emulsion relative to the mass of the original dispersion / emulsion. The dispersion / emulsion of the high-Tg acrylic resin has good film-forming properties and dries quickly.
[0084] Based on the total weight of the second coating composition, the high-Tg acrylic resin can be at least 2% by weight, suitably at least 3% by weight, such as at least 4% by weight and / or at most 15% by weight, suitably at most 10% by weight, such as at most 8% by weight. Based on the total weight of the second coating composition, the high-Tg acrylic resin can be present in the coating composition in the range of 2% to 15% by weight, suitably 3% to 10% by weight, such as 4% to 8% by weight or any other combination using these end values.
[0085] The polyurethane resin refers to a polymer whose repeating units contain polyurethane groups. The polyurethane can include a polymer having at least 50% by weight of organic units (e.g., at least 70% by weight, such as at least 90% by weight of organic units) connected by urethane bonds. In this context, the organic units include one or more residues selected from the following: simple diols such as butanediol, polyester diols, polyether diols, and polycarbonate diols.
[0086] The high-Tg polyurethane resin can have a glass transition temperature (Tg) greater than 30 °C, suitably a glass transition temperature of 30 °C to 70 °C. The glass transition temperature can be measured by dynamic thermomechanical analysis (DMA) using a TA Instruments Q800 instrument, with the measurement parameters: a frequency of 10 Hz, an amplitude of 5 mm, a temperature ramp of -100 °C to 250 °C, and the Tg is determined as the peak of the tanδ curve according to ASTM D7028.
[0087] Suitably, the high-Tg polyurethane resin can have a weight average molecular weight (Mw) of 2,000 to 15,000, such as a weight average molecular weight of 5,000 to 12,000. The weight average molecular weight can be determined by gel permeation chromatography using a suitable standard such as a polystyrene standard.
[0088] Suitably, the high-Tg polyurethane resin can be in the form of a dispersion or an emulsion, which has a solids fraction of 20% to 60% by weight. The "solids fraction" refers to the percentage of the mass remaining after evaporation of the solvent in the dispersion or emulsion relative to the mass of the original dispersion or emulsion. The dispersion or emulsion of the high-Tg polyurethane has a high drying rate and good chemical resistance after film formation.
[0089] Based on the total weight of the second coating composition, the high-Tg polyurethane resin can be at least 1% by weight, suitably at least 2% by weight, such as at least 3% by weight and / or at most 18% by weight, suitably at most 15% by weight, such as at most 10% by weight. Based on the total weight of the second coating composition, the high-Tg polyurethane resin can be present in the coating composition in the range of 1% to 18% by weight, suitably 2% to 15% by weight, such as 3% to 10% by weight or any other combination using these end values.
[0090] The film-forming resin in the second coating composition may further comprise a polyester resin. The polyester resin refers to a polymer produced by the poly-esterification of a polyol and a polyacid, wherein the polyol is a compound containing two or more hydroxyl groups per molecule, and the polyacid is a compound containing two or more carboxyl groups per molecule.
[0091] Suitably, the polyester resin may have a hydroxyl value of 10 mgKOH / g to 100 mgKOH / g, such as a hydroxyl value of at least 20 mgKOH / g to 80 mgKOH / g. The hydroxyl value refers to the milligrams of potassium hydroxide (KOH) equivalent to the hydroxyl groups in 1 gram of the resin. Suitably, the polyester resin may have an acid value of 1 mgKOH / g to 100 mgKOH / g, such as an acid value of 10 mgKOH / g to 70 mgKOH / g. The acid value refers to the milligrams of potassium hydroxide required to neutralize the free acid in 1 gram of the resin.
[0092] Based on the total weight of the second coating composition, the polyester resin may be at least 0.1% by weight, suitably at least 0.5% by weight and / or at most 5% by weight, suitably at most 3% by weight. Based on the total weight of the second coating composition, the polyester resin may be present in the coating composition in a range of 0.1% by weight to 5% by weight, suitably 0.5% by weight to 3% by weight or any other combination using these end values.
[0093] The film-forming resin in the second coating composition may further include a melamine formaldehyde resin. The melamine formaldehyde resin may crosslink with the above-mentioned high-Tg acrylic resin, high-Tg polyurethane resin and / or polyester resin to improve the film-forming properties of the second coating composition and accelerate curing. The melamine formaldehyde resin may be a single melamine formaldehyde resin or a combination of multiple different melamine formaldehyde resins. In the melamine formaldehyde resin or the combination of melamine formaldehyde resins, the functional group ratio of the sum of imino and hydroxymethyl groups to alkoxy groups in the melamine formaldehyde resin is 5:80 to 50:50.
[0094] Based on the total weight of the second coating composition, the melamine formaldehyde resin may be at least 1% by weight, suitably at least 3% by weight and / or at most 15% by weight, for example at most 10% by weight. Based on the total weight of the second coating composition, the melamine formaldehyde resin may be present in the coating composition in a range of 1% by weight to 15% by weight, suitably 3% by weight to 10% by weight or any other combination using these end values.
[0095] The second coating composition may further comprise a pigment. The pigment includes an effect pigment and optional other pigments. Suitably, the pigment may be in the form of a color paste having a solids fraction of 10 wt% to 45 wt%. Generally, the second coating composition may contain 1 wt% to 30 wt% of the color paste based on the total weight of the second coating composition.
[0096] The second coating composition may further comprise a thickener. The thickener can effectively improve the low-shear rate viscosity such that the formed coating has high-shear thinning characteristics, thereby enabling excellent storage stability and processability and avoiding a fat edge. Suitably, the thickener may comprise one or more of the following: a silicate thickener, an acrylic thickener, a polyurethane thickener, a polyamide wax thickener, and a polyurea thickener. Suitably, the thickener comprises a silicate thickener. When a silicate thickener and other thickeners are used simultaneously, the weight ratio of the silicate thickener to the other thickeners may be from 0.2:1 to 5:1, suitably from 0.5:1 to 2:1. Based on the total weight of the second coating composition, the thickener may be from 0.01 wt% to 5 wt%.
[0097] The second coating composition may further comprise 40 wt% to 70 wt% of water based on the total weight of the second coating composition.
[0098] The second coating composition may further comprise one or more other auxiliary components such as an organic co-solvent, a wetting additive, a pinhole additive, a curing catalyst, an antifoaming agent, an ultraviolet absorber, a light stabilizer, an adhesion promoter, a surface leveling agent, a pH regulator, a substrate wetting agent, a filler, etc. When these auxiliary components are present, each auxiliary component is present in an amount of up to 10 wt% based on the total weight of the second coating composition.
[0099] The present disclosure further provides a coated substrate comprising a substrate and a coating system as described above coated on at least a portion of the substrate. Suitably, the substrate may comprise a metal. The substrate may be pretreated, untreated, pre-coated, and / or uncoated.
[0100] The lower coating in the coating system can be obtained by applying the first coating composition via any standard method known in the art. The upper coating in the coating system can be obtained by applying the second coating composition via any standard method known in the art. Standard methods known in the art include but are not limited to spraying, dipping, roll coating, brush coating, etc.
[0101] The present disclosure also provides a method for coating a substrate, the method comprising:
[0102] coating the substrate with the first coating composition to form a lower coating;
[0103] A second coating composition is applied over at least a portion of the lower coating to form an upper coating, the second coating composition comprising an effect pigment; and
[0104] The first coating composition and the second coating composition are cured simultaneously, wherein the upper coating has a dry film thickness of 3 µm to 20 µm, and the difference between the wet film solids content of the lower coating and the upper coating is at least 20 wt%.
[0105] Suitably, the substrate may comprise metal. The substrate may be pretreated, untreated, pre-coated and / or uncoated. Suitably, curing is carried out by heating (e.g., baking at 140 °C for 20 to 60 minutes).
[0106] Suitably, the method for coating a substrate according to the present disclosure further comprises: applying one or more coating compositions over at least a portion of the upper coating to form an additional coating or additional coatings. The one or more coating compositions may comprise a clearcoat composition.
[0107] Thus, the method for coating a substrate comprises:[[]]END]]
[0108] Coating the substrate with a first coating composition to form a lower coating;
[0109] Applying a second coating composition over at least a portion of the lower coating to form an upper coating, the second coating composition comprising an effect pigment;
[0110] Applying one or more coating compositions over at least a portion of the upper coating to form an additional coating or additional coatings; and
[0111] Curing the first coating composition, the second coating composition and the one or more coating compositions simultaneously, wherein the upper coating has a dry film thickness of 3 µm to 20 µm, and the difference between the wet film solids content of the lower coating and the upper coating is at least 20 wt%.
[0112] Suitably, the method for coating a substrate according to the present disclosure further comprises: flash drying the lower coating, the upper coating and optionally an additional coating or additional coatings separately. Suitably, flash drying may be carried out at room temperature (e.g., 23 °C) for 1 to 15 minutes. Suitably, the method for coating a substrate according to the present disclosure further comprises: pre-baking after forming the upper coating. Suitably, the pre-baking may be carried out at 70 °C to 90 °C for 1 to 10 minutes, e.g., 80 °C * 5 minutes.
[0113] Examples
[0114] The following examples are provided to further illustrate the present disclosure and should not be construed as limiting the present disclosure to the details described in the examples. Unless otherwise specified, all parts and percentages in the following examples are by weight.
[0115] First coating composition
[0116] The components and contents listed in the following table are used to prepare the first coating composition of the examples (Examples 1 to 3) and comparative examples (Comparative Examples 1 to 2), and the specific steps are as follows: (1) Under stirring conditions at room temperature (e.g., 23 °C), an acrylic resin, an acrylic-grafted polyurethane resin, and a polyurethane resin are sequentially added to a reactor; (2) A portion of the pH regulator is added to adjust the pH value of the mixture described in step (1) to 8.2 to 8.7, and an appropriate amount of deionized water is added to adjust the system viscosity to 150 mPa·s to 200 mPa·s at 1000 s -1 ; (3) An organic solvent, a wetting additive, a pinhole additive, a melamine formaldehyde resin, a color paste, and a filler paste are added to the mixture described in step (2) under stirring; (4) The remaining pH regulator is added to adjust the pH of the system to 8.2 to 8.7, and the remaining deionized water is added to adjust the viscosity to 60 mPa·s to 80 mPa·s at 1000 s -1 .
[0117] Table 1. First coating composition
[0118]
[0119] a Having a solid fraction of 40 wt% to 50 wt%, a weight average molecular weight (Mw) of 2,000 to 200,000, a hydroxyl value of 10 mgKOH / g to 100 mgKOH / g, an acid value of 1 mgKOH / g to 50 mgKOH / g, and an average particle size of 100 nm to 2,000 nm;
[0120] b Having a solid fraction of 30 wt% to 40 wt%, a weight average molecular weight (Mw) of 2,000 to 200,000, a hydroxyl value of 10 mgKOH / g to 100 mgKOH / g, an acid value of 1 mgKOH / g to 50 mgKOH / g, and an average particle size of 100 nm to 2,000 nm;
[0121] c Having a solid fraction of 35 wt% to 45 wt%, a weight average molecular weight (Mw) of 10,000 to 150,000, and an average particle size of 200 nm to 1,500 nm;
[0122] d having a solid fraction of 35 wt% to 45 wt%, a hydroxyl value of 50 mgKOH / g to 150 mgKOH / g, an acid value of 1 mgKOH / g to 50 mgKOH / g, and an average particle size of 100 nm to 1,000 nm;
[0123] e having a solid fraction of 20 wt% to 35 wt%, a hydroxyl value of 50 mgKOH / g to 150 mgKOH / g, an acid value of 1 mgKOH / g to 50 mgKOH / g, and an average particle size of 100 nm to 1,000 nm;
[0124] f having a solid fraction of 75 wt% to 85 wt% and a functional group ratio of the sum of imino and hydroxymethyl to alkoxy of 5:80 to 50:50;
[0125] g alcohol and alcohol ether solvents;
[0126] h including titanium white, carbon black, and / or perylene red and other pigments;
[0127] i barium sulfate;
[0128] j commercially available from BYK;
[0129] k commercially available from BYK;
[0130] l commercially available from DOW.
[0131] Second coating composition
[0132] The components and contents listed in the following table are used to prepare the second coating composition of the examples and comparative examples, and the specific steps are: (1) sequentially add acrylic resin, acrylic grafted polyurethane resin, and polyester resin to the reactor under stirring conditions at room temperature (e.g., 23 °C); (2) add part of the pH regulator to adjust the pH value of the mixture described in step (1) to 8.2 to 8.7, and add appropriate deionized water to adjust the system viscosity to 1000 s -1from 150 mPa.s to 200 mPa.s; (3) adding a portion of organic solvents, wetting additives, pinhole additives, melamine formaldehyde resin, color paste, and a portion of thickeners to the mixture described in step (2) under stirring; (4) mixing and stirring aluminum powder, passivator, and the remaining solvents in another stirring tank to uniformly disperse the aluminum powder; (5) adding the mixture in step (4) to the mixture prepared in step (3) sequentially under stirring conditions; (6) adding the remaining thickeners to the mixture in step (5), adjusting the pH of the system to 8.2 to 8.7, and adding the remaining deionized water to adjust the viscosity to -1 from 60 mPa.s to 80 mPa.s at
[0133] Table 2. Second coating composition
[0134]
[0135] a having a solid fraction of 20 wt% to 35 wt%, a glass transition temperature (Tg) above 40 °C, a hydroxyl value of 10 mgKOH / g to 100 mgKOH / g, an acid value of 1 mgKOH / g to 70 mgKOH / g, and an average particle size of 100 to 1,000;
[0136] b having a solid fraction of 30 wt% to 40 wt%, a glass transition temperature (Tg) of 30 °C to 70 °C, and a weight average molecular weight (Mw) of 2,000 to 15,000;
[0137] c having a solid fraction of 35 wt% to 45 wt% and a glass transition temperature (Tg) of -10 °C to 10 °C;
[0138] d having a solid fraction of 35 wt% to 75 wt%, a hydroxyl value of 10 mgKOH / g to 100 mgKOH / g, and an acid value of 1 mgKOH / g to 100 mgKOH / g;
[0139] e having a solid fraction of 75 wt% to 85 wt% and a functional group ratio of the sum of imino and hydroxymethyl to alkoxy of 5:80 to 50:50;
[0140] f alcohol and alcohol ether solvents;
[0141] g including titanium white, carbon black, and / or perylene red, and other pigments;
[0142] hSilver dollar-shaped aluminum powder from Toyal;
[0143] i Commercially available from BYK;
[0144] j Commercially available from BYK;
[0145] k Having an acid value of 20 mgKOH / g to 200 mgKOH / g;
[0146] l Including a silicate thickener and other thickeners selected from the following: acrylic thickeners, polyurethane thickeners, polyamide wax thickeners, and polyurea thickeners, and the weight ratio of the silicate thickener to the other thickeners is 0.2:1 to 5:1;
[0147] m Commercially available from DOW;
[0148] n Commercially available from BASF, Elementis, and BYK.
[0149] The wet film solid contents of the above-mentioned first coating composition and second coating composition are obtained by testing with the following method:
[0150] 1. Prepare a foil;
[0151] 2. Spray the foil in the specified area with STD spraying parameters, spraying environment: temperature is 23°C, humidity is 65%, and the coating thickness is 10 µm (for the first coating composition) or 6 µm (for the second coating composition);
[0152] 3. Flash dry at 23°C for 3 minutes;
[0153] 4. Bake at 140°C for 30 minutes; and
[0154] 5. Measure the weight after baking and calculate the weight ratio of the weight after baking to the total weight of the coating composition used for spraying.
[0155] Coating system
[0156] The coating system includes a lower coating formed from the first coating composition and an upper coating formed from the second coating composition, and the specific preparation method includes:
[0157] Spray the first coating composition onto a metal substrate (pretreated and pre-coated) to form a lower coating; flash dry the lower coating at room temperature for at least 3 minutes; apply a second coating composition onto at least a portion of the lower coating to form an upper coating; pre-bake at 80 °C for at least 5 minutes; apply a clearcoat composition (a commercially available two-component clearcoat composition) onto at least a portion of the upper coating; and simultaneously cure the first coating composition, the second coating composition, and the clearcoat composition at 140 °C * 20 minutes.
[0158] The following table shows coating systems comprising a first coating composition and a second coating composition having different wet film solid contents.
[0159] Table 3. Coating systems
[0160]
[0161] Note: The difference in wet film solid content is the wet film solid content of the lower coating (first coating composition) minus the wet film solid content of the upper coating (second coating composition).
[0162] Property testing:
[0163] The following performance tests were conducted on the coating systems obtained in Examples 6 to 11 and Comparative Examples 5 to 8:
[0164] 1 - FI value:
[0165] It is related to the color brightness at each angle: the larger the FI value, the greater the difference in color brightness between the small angle and the large angle, and the closer the color is to a similar mirror-like effect.
[0166] In this article, the brightness values (L) at 15°, 45°, and 110° were measured using a commercially available BYK colorimeter, and then the FI value was calculated by the following formula.
[0167]
[0168] 2 - G value:
[0169] It represents the flashing intensity of the color under the light source. The smaller the G value, the lower the flashing, and the closer the color is to a similar delicate mirror-like effect. In this article, the G value was obtained by testing with a commercially available BYK colorimeter.
[0170] Table 4. Property tests of coating systems - FI value and G value
[0171]
[0172] Examples 1 to 3 have a high wet film solids content, while Comparative Examples 1 to 2 have a low wet film solids content. Therefore, Examples 1 to 3 and Examples 4 to 5 produce a large difference in wet film solids content (at least 20%), while Comparative Examples 1 to 2 and Comparative Examples 3 to 4 produce a relatively small difference in wet film solids content. From the test results in the above table, it can be seen that the difference in wet film solids content between the upper coating and the lower coating in Examples 6 to 11 is relatively large, so the final FI value obtained is at least 20 and the G value is low; while the difference in wet film solids content in Comparative Examples 5 to 8 is relatively small, so the color obtained has a low FI value and a high G value.
[0173] 3 - VOC Content
[0174] In this article, the VOC (excluding solvents, g / L) content of the first coating composition and the second coating composition is calculated by the following method:
[0175] 1. Measure the mass m of each component in the composition i , the mass m of water in the composition w and the density ρ of the composition s ;
[0176] 2. Calculate according to the following formula
[0177]
[0178] where ρ w is the density of water at 23 °C.
[0179] Table 5. VOC Values of the First Coating Composition
[0180]
[0181] Table 6. VOC Values of the Second Coating Composition
[0182]
[0183] The above VOC content data shows that for the first coating composition, Examples 1 to 3 can maintain the same VOC level as Comparative Examples 1 to 2, but achieve a higher wet film solids content and obtain a better color effect (FI value and G value). However, for the second coating composition, Comparative Examples 3 to 4 cannot obtain a good color effect regardless of whether the VOC content is high or low. While Examples 4 to 5 can obtain a satisfactory color effect while ensuring a low VOC content.
[0184] 4 - Other Properties
[0185] Table 7. Property Tests of the Coating System - Other Properties
[0186]
[0187] As shown in the above performance data, Examples 6 to 11 can not only obtain relatively good color effects, but also meet the standard requirements for coating applications in terms of other appearance and mechanical properties.
[0188] Although specific aspects of the present disclosure have been illustrated and described, it will be apparent to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the present disclosure. Accordingly, the appended claims are intended to cover all such changes and modifications that fall within the scope of the present disclosure.
Claims
1. A coating system, which comprises: a lower coating formed from a first coating composition; and an upper coating formed from a second coating composition comprising an effect pigment, the first coating composition and the second coating composition being cured simultaneously, wherein the upper coating has a dry film thickness of 3 µm to 20 µm, and the difference between the wet film solid contents of the lower coating and the upper coating is at least 20 wt%.
2. The coating system according to claim 1, wherein the difference between the wet film solid contents of the lower coating and the upper coating is at least 30 wt%.
3. The coating system according to claim 1, wherein the difference between the wet film solid contents of the lower coating and the upper coating is at least 40 wt%.
4. The coating system according to any one of claims 1 to 3, wherein the lower coating has a wet film solid content of at least 50 wt%.
5. The coating system according to any one of claims 1 to 4, wherein the upper coating has a wet film solid content of 30 wt% or less.
6. The coating system according to any one of claims 1 to 5, wherein the first coating composition has a solid content of 30 wt% to 45 wt%.
7. The coating system according to any one of claims 1 to 6, wherein the second coating composition has a solid content of 8 wt% to 20 wt%.
8. The coating system according to any one of claims 1 to 7, wherein the lower coating has a dry film thickness of 5 µm to 20 µm.
9. The coating system according to any one of claims 1 to 8, wherein the effect pigment comprises a silver dollar type pigment.
10. The coating system according to any one of claims 1 to 9, wherein the effect pigment has an aspect ratio of 50:1 to 200:
1.
11. The coating system according to any one of claims 1 to 10, wherein the second coating composition has a pigment-binder ratio of 0.02:1 to 0.3:
1.
12. The coating system according to any one of claims 1 to 11, wherein the first coating composition and / or the second coating composition is an aqueous coating composition.
13. The coating system according to any one of claims 1 to 12, wherein each of the first coating composition and the second coating composition has a VOC content of less than 420 g / L.
14. The coating system according to any one of claims 1 to 13, wherein the first coating composition comprises a polyurethane resin, an acrylic resin, and / or an acrylic grafted polyurethane resin.
15. The coating system according to claim 14, wherein the weight ratio of the polyurethane resin to the acrylic resin and / or the acrylic grafted polyurethane resin is 0.1:1 to 0.9:
1.
16. The coating system according to claim 14 or 15, wherein the polyurethane resin has a weight average molecular weight of 2,000 to 200,000 and an average particle size of 200 nm to 2,000 nm.
17. The coating system according to any one of claims 14 to 16, wherein the acrylic resin has a hydroxyl value of 50 mgKOH / g to 150 mgKOH / g, an acid value of 1 mgKOH / g to 50 mgKOH / g, and an average particle size of 100 nm to 1,000 nm.
18. The coating system according to any one of claims 14 to 17, wherein the acrylic grafted polyurethane resin has a hydroxyl value of 10 mgKOH / g to 100 mgKOH / g, an acid value of 1 mgKOH / g to 50 mgKOH / g, an average particle size of 100 nm to 2,000 nm, and a weight average molecular weight of 2,000 to 200,000.
19. The coating system according to any one of claims 14 to 18, wherein the first coating composition further comprises a pigment, and the pigment comprises carbon black and optionally titanium white.
20. The coating system according to any one of claims 1 to 19, wherein the second coating composition comprises a high Tg acrylic resin and a high Tg polyurethane resin, the high Tg acrylic resin has a Tg greater than 40 °C, and the high Tg polyurethane resin has a Tg greater than 30 °C.
21. The coating system according to claim 20, wherein the weight ratio of the high Tg acrylic resin to the high Tg polyurethane resin is 10 - 80:5 - 50.
22. The coating system according to claim 20 or 21, wherein the high Tg acrylic resin has a hydroxyl value of 10 mgKOH / g to 100 mgKOH / g, an acid value of 1 mgKOH / g to 70 mgKOH / g, and an average particle size of 100 nm to 1,000 nm.
23. The coating system according to any one of claims 20 to 22, wherein the high Tg polyurethane resin has a weight average molecular weight of 2,000 to 15,000.
24. The coating system according to any one of claims 20 to 23, wherein the second coating composition further comprises a thickener, and the thickener comprises a silicate thickener, an acrylic thickener, a polyurethane thickener, a polyamide wax thickener, a polyurea thickener, or a combination thereof.
25. The coating system according to any one of claims 14 to 24, wherein the first coating composition and / or the second coating composition further comprises a melamine formaldehyde resin, and the functional group ratio of the sum of imino and hydroxymethyl to alkoxy in the melamine formaldehyde resin is 5:80 to 50:
50.
26. A coated substrate, which comprises a substrate and the coating system according to any one of claims 1 to 25 coated on at least a part of the substrate.
27. The coated substrate according to claim 26, wherein the substrate comprises a metal.
28. A method for coating a substrate, the method comprises: Coat the substrate with a first coating composition to form a lower coating, coat a second coating composition on at least a portion of the lower coating to form an upper coating, the second coating composition comprising an effect pigment, and simultaneously cure the first coating composition and the second coating composition, wherein the upper coating has a dry film thickness of from 3 µm to 20 µm and the difference between the wet film solids content of the lower coating and the upper coating is at least 20% by weight.
29. A method for coating a substrate, the method comprising: coating the substrate with a first coating composition to form a lower coating; coating a second coating composition on at least a portion of the lower coating to form an upper coating, the second coating composition comprising an effect pigment; coating a clearcoat coating composition on at least a portion of the upper coating to form a clearcoat layer; and simultaneously curing the first coating composition, the second coating composition and the clearcoat coating composition, wherein the upper coating has a dry film thickness of from 3 µm to 20 µm and the difference between the wet film solids content of the lower coating and the upper coating is at least 20% by weight.