Two-layer coating for improved corrosion resistance and hardness

Abstract

The present invention relates to a coating system comprising a first two-component aqueous coating composition comprising a binder component 1A and a curing component 1B, and a second two-component aqueous coating composition comprising a binder component 2A and a curing component 2B. The first two-component aqueous coating composition comprises a first coalescent package comprising at least one coalescent and has a minimum film-forming temperature of less than 5°C. The second two-component aqueous coating composition comprises a second coalescent package comprising at least one coalescent and has a minimum film-forming temperature of between 5°C and 25°C. Methods of preparing the coatings are also disclosed.

Description

[Technical Field]

[0001] In the coatings industry, especially the industrial coatings industry, coatings that are both corrosion-resistant and weather-resistant are often desired. Multilayer coatings were developed to provide a first layer that provides adhesion and corrosion resistance to the substrate. Additional layers were then applied to provide weather resistance, chemical resistance, and / or the desired appearance. Epoxy resins were generally selected for the first layer, and polyurethane or acrylic resins were often used for the top layer. Because it was difficult to find a single resin chemistry that provided the necessary balance between corrosion resistance and weather resistance, different resin chemistries were used.

[0002] Acrylic-epoxy hybrid (AEH) resins, such as those disclosed in U.S. Patent No. 8,658,742, were developed to provide a better balance between corrosion resistance and weather resistance in a single chemical system. These AEH resin systems have enabled a one-coating system that provides good corrosion and weather resistance in many applications, such as direct to metal coatings.

[0003] However, for some applications, the hardness and / or weather resistance provided by existing AEH resin systems is not adequate. Low-cost, simple resin systems, such as single-chemistry systems, that can provide the desired corrosion resistance, weather resistance, and hardness are desired. Summary of the Invention

[0004] In a first aspect, the present invention relates to a coating system comprising: a) a first two-component aqueous coating composition comprising a binder component 1A and a curing component 1B, wherein the binder component 1A comprises a first aqueous dispersion of acrylic polymer particles imbibed with an epoxy compound, and the first two-component aqueous coating composition comprises a first coalescent package comprising at least one coalescent, the first two-component aqueous coating composition having a minimum film-forming temperature (MFFT) of less than 5°C; b) a second two-component coating composition comprising a binder component 2A and a curing component 2B, wherein the binder component 2A comprises a second aqueous dispersion of acrylic polymer particles imbibed with an epoxy compound, and the second two-component aqueous coating composition comprises a second coalescent package comprising at least one coalescent, and the second two-component coating composition has a minimum film formation temperature (MFFT) of 5°C to 25°C.

[0005] In a second aspect, the present invention relates to a method for preparing a coating comprising: (i) providing a coating system according to any one of the preceding claims; (ii) applying a first two-part coating composition to a substrate to form a first coating layer; (iii) at least partially curing the first coating layer at a temperature above room temperature for at least 10 minutes; (iv) applying a second two-component coating composition onto the first coating layer to form a second coating layer; (v) curing the second coating layer at a temperature to form a two-layer coating on the substrate. [Brief explanation of the drawings]

[0006] [Figure 1] 1 is a photograph of coatings according to embodiments of the present invention and comparative examples subjected to salt spray to determine corrosion resistance. [Figure 2] 1 is a photograph of a comparative coating subjected to salt spray to determine corrosion resistance. DETAILED DESCRIPTION OF THE INVENTION

[0007] In a first aspect, the present invention relates to a coating composition comprising a first two-component aqueous coating composition and a second two-component aqueous coating composition.

[0008] The first two-component aqueous coating composition comprises a binder component 1A and a curing (i.e., crosslinking) component 1B. The binder component 1A comprises a first aqueous dispersion of acrylic polymer particles imbibed with an epoxy compound. As used herein, the term "acrylic" includes (meth)acrylic acid, (meth)alkyl acrylate, (meth)acrylamide, (meth)acrylonitrile, and modifications thereof, such as (meth)hydroxyalkyl acrylate. The fragment "(meth)acrylic" refers to both "methacrylic" and "acrylic." For example, (meth)acrylic acid refers to both methacrylic acid and acrylic acid, and methyl (meth)acrylate refers to both methyl methacrylate and methyl acrylate. As used herein, the term "imbibed with an epoxy compound" means that the epoxy compound is at least partially absorbed by the acrylic polymer particles, but does not react with the acrylic polymer particles and is not simply present on the surface of the acrylic polymer particles.

[0009] The second two-component aqueous coating composition comprises a binder component 2A and a curing component 2B. The binder component 2A comprises a second aqueous dispersion of acrylic polymer particles imbibed with an epoxy compound.

[0010] The first and second aqueous dispersions of acrylic polymer particles imbibed with an epoxy compound may be the same or different, and preferably the first and second aqueous dispersions of acrylic polymer particles imbibed with an epoxy compound are the same.

[0011] Preferably, the first aqueous dispersion of acrylic polymer particles imbibed with an epoxy compound and the second aqueous dispersion comprise an aqueous dispersion of acrylic polymer particles imbibed with a thermosetting compound having at least two oxirane groups. As used herein, the term "thermosetting compound" means a compound that undergoes a chemical reaction to form a thermosetting compound that has distinct chemical and physical properties and does not undergo a reversible thermal transition when exposed to heat.

[0012] The imbibed thermosetting compound preferably has multiple oxirane groups. More preferably, the thermosetting compound is a novolac resin, a di-, tri-, or tetraglycidyl ether, or a di-, tri-, or tetraglycidyl ester.

[0013] Examples of suitable thermosetting compounds include diglycidyl ether of bisphenol A, diglycidyl ether of bisphenol F, 1,4-butanediol diglycidyl ether, 1,6-hexanediol diglycidyl ether, diglycidyl ester of phthalic acid, 1,4-cyclohexanedimethanol diglycidyl ether, 1,3-cyclohexanedimethanol diglycidyl ether, diglycidyl ester of hexahydrophthalic acid, and novolac resins, and combinations thereof. A commercially available thermosetting compound is DER331 liquid epoxy resin (available from Olin Corporation).

[0014] Aqueous dispersions of acrylic polymer particles (i.e., latexes) can be achieved by free radical emulsion or suspension addition polymerization, or by dispersing a preformed polymer in an aqueous medium under shear. Examples of suitable latexes include acrylic latexes and styrene acrylic latexes.

[0015] The acrylic polymer particles may further contain anti-agglomeration functional groups, which refer to hydrophilic groups that are sufficiently unreactive with oxirane groups (and ester groups, if present) so that the latex particles are heat aging stable for 10 days at 60° C. The term “heat aging stable for 10 days at 60° C” is used herein to mean that the particle size of a latex subjected to heat aging stability for 10 days at 60° C. does not increase by more than 30% above the particle size before such heat aging testing.

[0016] Anti-agglomeration functional groups can be incorporated into polymer particles using monomers containing anti-agglomeration functional groups (anti-agglomeration monomers), but such groups can also be incorporated by grafting. Anti-agglomeration groups are considered effective because they are hydrophilic and unreactive with oxirane groups under heat aging conditions. Common classes of such groups include amide groups, acetoacetoxy groups, and strong protonic acids, which are pH-adjusted to form their conjugate bases.

[0017] Specific examples of anti-agglomerating monomers include acrylamide, phosphoethyl methacrylate, sodium styrene sulfonate, acetoacetoxyethyl methacrylate, and acrylamido-methyl-propane sulfonate. If present, the concentration of anti-agglomerating functional groups in the polymer is preferably sufficient to stabilize the thermoplastic polymer under heat aging conditions, and is preferably from 0.5 wt. %, more preferably from 1 wt. %, preferably up to 10 wt. %, more preferably up to 5 wt. %, based on the weight of the polymer.

[0018] Suitable monomers for preparing acrylic latexes include acrylates and methacrylates, such as methyl acrylate, ethyl acrylate, butyl acrylate, glycidyl methacrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, and 2-ethylhexyl methacrylate, as well as combinations thereof. Additionally, acrylic latexes may contain structural units of other monomers, such as styrene and acrylonitrile. The term "structural unit" as used herein refers to the residue of the specified monomer after polymerization. Structural units of one or more acid monomers may also be included, most notably acrylic acid, methacrylic acid, and itaconic acid. Monomers capable of providing co-curable functional groups, such as glycidyl acrylate and methacrylate, may also be included.

[0019] It may be advantageous to include a chain transfer agent in the latex preparation. Examples of chain transfer agents include, but are not limited to, dodecyl mercaptan, butyl mercaptopropionate, methyl mercaptopropionate, mercaptopropionic acid, and the like.

[0020] In certain embodiments, it may be advantageous to incorporate copolymerized multi-ethylenically unsaturated monomer groups into the polymer. Multi-ethylenically unsaturated monomers include, for example, allyl (meth)acrylate, diallyl phthalate, 1,4-butylene glycol di(meth)acrylate, 1,2-ethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and divinylbenzene. It may also be advantageous to incorporate such monomer groups heterogeneously into the polymer to form multiphase polymer particles and create core-shell, hemispherical, or closed morphologies.

[0021] An aqueous dispersion of epoxy-imbibed acrylic polymer particles (i.e., an imbibed latex) is advantageously prepared separately from the thermosetting compound using conventional emulsion polymerization techniques and then mixed with the thermosetting compound, which may be neat or in the form of an aqueous emulsion, preferably as an aqueous emulsion, more preferably as a finely divided aqueous emulsion. When the thermosetting compound is added as an aqueous emulsion, the emulsion is stabilized with a stabilizing amount of surfactant, preferably at a concentration ranging from about 0.5 to about 10% by weight. Nonionic surfactants are preferred, including APEO-free nonionic wetting agents such as polyalkylene oxide block copolymers, polyoxyethylene glycol alkyl ethers, glucoside alkyl ethers, fatty acid esters, glycerol alkyl esters, sorbitan alkyl esters, and polyoxyethylene glycol alkyl phenol ethers, as well as commercially available wetting agents such as TRITON™ X-405 octylphenol ethoxylate (a trademark of The Dow Chemical Company or its affiliates). If the thermosetting compound is mixed with the latex as a neat compound, absorption is facilitated by stirring at or above room temperature.

[0022] High solids absorption latex, ie, latex having a solids content of at least 40% by weight, especially in the range of 45 to 60% by weight, based on the total weight of the latex, can be used in the present invention.

[0023] The imbibed latex composition is useful as part of a two-component formulation (i.e., binder component 1A or 2A), with the second component being a curing (i.e., crosslinking) component (1B or 2B) added prior to use to cure or solidify the thermosetting compound. Thus, binder components 1A and 2A of the present invention are substantially free of curing agents, i.e., have insufficient concentrations of compounds that promote oxirane ring opening to destabilize the thermosetting compound. Preferably, the imbibed latex composition contains no more than 0.05 wt. %, more preferably no more than 0.005 wt. %, and most preferably 0 wt. % of a curing agent, based on the total weight of the imbibed latex composition.

[0024] Preferably, the imbibed latex in binder components 1A and 2A is cured with a water-miscible external curing agent comprising curing components 1B and 2B. Preferably, the imbibed latex is cured with a carboxylic acid-based acrylic. Examples of carboxylic acid-based acrylics include, but are not limited to, acrylic polymer emulsions containing structural units derived from carboxylic acids such as methacrylic acid, itaconic acid, and acrylic acid, and acrylic monomers such as methyl acrylate, ethyl acrylate, butyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, octyl acrylate and isooctyl acrylate, n-decyl acrylate, isodecyl acrylate, tert-butyl acrylate, methyl methacrylate, butyl methacrylate, hexyl methacrylate, isobutyl methacrylate, isopropyl methacrylate, 2-hydroxyethyl acrylate, acrylamide, acrylonitrile, methacrylonitrile, isobornyl acrylate, n-propyl methacrylate, sec-butyl methacrylate, cyclohexyl methacrylate, t-butylaminoethyl methacrylate, stearyl methacrylate, glycidyl methacrylate, dicyclopentenyl methacrylate, phenyl methacrylate, and styrene. For example, the acrylic polymer dispersion may include a styrene acrylic polymer.

[0025] Preferably, the acrylic polymer dispersion contains 35 to 70 wt. % acrylic solids, more preferably 40 to 65 wt. % acrylic solids, based on the total weight of the acrylic polymer dispersion. Preferably, the acrylic particles have an average weight particle size in the range of 60 to 450 nm, an acid level of the acid monomer in the range of 0.1 to 15 wt. % based on the weight of the acrylic monomer, a weight average molecular weight in the range of 50,000 to 5,000,000 g / mol, and a glass transition temperature (Tg) in the range of 7 to 100°C, as measured by differential scanning calorimetry (DSC). The acrylic polymer dispersion preferably has a pH in the range of 6 to 10.

[0026] The amount of curing agent used generally varies in a stoichiometric ratio of epoxy to carboxylic acid of about 1:1 to 4:1, based on the moles of epoxide groups to the moles of carboxylic acid. Preferably, the curing agent in curing component 1B in the first two-component coating composition is present in an amount of about 1.75:1 to 2.5:1, based on the moles of epoxide groups to the moles of carboxylic acid in the first two-component coating composition, and the curing agent in curing component 2B is present in the second two-component aqueous coating composition in an amount such that the stoichiometric ratio between epoxy groups and acid groups ranges from 1:1 to 1.5:1, based on the moles of epoxide groups to the moles of carboxylic acid in the second two-component aqueous coating composition. More preferably, the curing agent in curing component 1B in the first two-component coating composition is present in an amount of about 2.0:1 to 2.5:1, based on the number of moles of epoxide groups to the number of moles of carboxylic acid in the first two-component coating composition, and the curing agent in curing component 2B is present in the second two-component aqueous coating composition in an amount such that the stoichiometric ratio between epoxy groups and acid groups is in the range of 1:2 to 1.4:1, based on the number of moles of epoxide groups to the number of moles of carboxylic acid in the second two-component aqueous coating composition.

[0027] Preferably, binder components 1A and 2A contain the same acrylic latex imbibed with an epoxy compound, and curing components 1B and 2B contain the same curing agent. In other words, the coating system preferably contains a single chemical resin system.

[0028] The first two-component aqueous coating composition and the second two-component aqueous coating composition further comprise a first coalescent package and a second coalescent package, respectively. Each coalescent package comprises at least one coalescent. As used herein, the term "coalescent" refers to a non-volatile or slow-evaporating solvent that fuses polymer particles into a continuous film under ambient conditions. The at least one coalescent contained in each coalescent package may be the same or different. Preferably, the first coalescent package and the second coalescent package comprise the same coalescent.

[0029] Examples of suitable coalescents include 2-n-butoxyethanol, dipropylene glycol n-butyl ether, propylene glycol n-butyl ether, dipropylene glycol methyl ether, propylene glycol methyl ether, propylene glycol n-propyl ether, diethylene glycol monobutyl ether, ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, triethylene glycol monobutyl ether, dipropylene glycol n-propyl ether, n-butyl ether, or mixtures thereof. Preferred coalescents include dipropylene glycol n-butyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, n-butyl ether, or mixtures thereof. Commercially available coalescents include, for example, OPTIFILM 400 and TEXANOL Coalescent, both available from The Eastman Chemical Company.

[0030] Preferably, the second coalescent package contains a smaller amount of coalescent than the first coalescent package. The at least one coalescent in the first coalescent package is present in an amount ranging from 10 to 30% by weight, preferably from 15 to 25% by weight, based on the total weight of the solids in the first binder component 1A and the first curing component 1B, where the total solids amount does not include any additional components (e.g., pigments present in the first two-component coating composition). The at least one coalescent in the second coalescent package is present in an amount ranging from 2 to 10% by weight, preferably from 4 to 9% by weight, based on the total weight of the solids in the second binder component 2A and the second curing component 2B, where the total solids amount does not include any additional components (e.g., pigments present in the second two-component coating composition).

[0031] The first two-component coating composition and the second two-component coating composition have different minimum film formation temperatures (MFFT). MFFT is the lowest temperature at which polymer particles in an aqueous dispersion coalesce with each other and form a continuous film when the volatile component (e.g., water) evaporates. MFFT can be measured according to GB / T 9267-2008. The first two-component coating composition has a minimum film formation temperature (MFFT) of less than 5°C. Preferably, the MFFT of the first two-component coating composition is in the range of -10°C to 5°C. The second two-component coating composition has a minimum film formation temperature (MFFT) of 5°C to 25°C, preferably 5°C to 15°C.

[0032] The first and second two-component coating compositions according to the present invention may further comprise one or more of the following additives: solvents, fillers, pigments such as titanium dioxide, mica, calcium carbonate, silica, zinc oxide, ground glass, aluminum trihydrate, talc, antimony trioxide, fly ash, clay, etc., polymer-encapsulated pigments such as polymer-encapsulated or partially encapsulated titanium dioxide, zinc oxide, or lithopone, polymers or polymer emulsions that adsorb or bond to the surface of pigments such as titanium dioxide, hollow pigments including pigments with one or more voids, dispersants such as amino alcohols and polycarboxylates, surfactants, defoamers, preservatives such as biocides, fungicides, bactericides, algicides, and combinations thereof, flow agents, leveling agents, and additional neutralizing agents such as hydroxides, amines, ammonia, and carbonates.

[0033] The volatile organic compound (VOC) content of the first and second two-component coating compositions can also be different, for example, the VOC content of the first two-component coating composition can be in the range of 0.75 to 1.25 lb / gal, and the VOC content of the second two-component coating composition can be in the range of 0.25 to 0.5 lb / gal.

[0034] Similarly, the pigment volume concentration (PVC) of the first two-component coating composition and the second two-component coating composition may be different. The PVC can be calculated by the following formula:

[0035]

number

[0036] For example, the PVC of the first two-component coating composition may be in the range of 15-20%, and the PVC of the second two-component coating composition may be in the range of 7.5-12.5%.

[0037] A second aspect of the invention relates to a method of preparing a coating, the method comprising the steps of providing a first aqueous coating composition and a second aqueous coating composition as described above.

[0038] The first coating layer is formed by applying a first two-component coating composition to a substrate. The first coating layer is at least partially cured at a temperature above room temperature for at least 10 minutes. Preferably, the curing temperature is at least 50°C, more preferably at least 60°C, and the curing time is preferably at least 15 minutes. Prior to curing, the first layer may be allowed to flash at room temperature.

[0039] After the first coating layer is at least partially cured, a second layer is applied over the first layer using a second two-component coating composition. The second layer is then cured at a temperature above room temperature for at least 10 minutes. Preferably, the curing temperature is at least 50°C, more preferably at least 60°C, and the curing time is preferably at least 15 minutes. The second layer can be flashed at room temperature before curing to allow volatile components to evaporate.

[0040] Preferably, the substrate is a metal substrate, which can be either a bare metal (e.g., the coating is a direct-to-metal coating) or a primed metal substrate (e.g., a metal substrate primed with a zinc-rich primer).

[0041] EXAMPLES: The following examples are included for illustrative purposes only and are not intended to limit the scope of the invention. The following materials are used in the examples. AEH-1 is an AEH dispersion having a solids content of 53.9%, an epoxy equivalent weight of 312 g / eq based on solids weight, and a pH of 7.

[0042] AEH-2 is an AEH dispersion having a solids content of 52.2%, an epoxy equivalent weight of 468 g / eq based on solids weight, and a pH of 7.

[0043] SAP is a carboxylic acid functional styrene acrylic polymer dispersion with 49.5% solids and an acid equivalent weight of 2457 g / eq on polymer solids, used as a curing agent.

[0044] DOWANOL™ DPnB is dipropylene glycol mono n-butyl ether available from The Dow Chemical Company and is used as a coalescent.

[0045] OPTIFILM 400 is a coalescing agent available from Eastman Chemical Company.

[0046] OROTAN™ 681 is a polymethacrylic acid with a hydrophobic comonomer available from The Dow Chemical Company and is used as a dispersing agent.

[0047] ACRYSOL™ RM-12W is a nonionic urethane rheology modifier available from The Dow Chemical Company.

[0048] TEGO Airex 902W is a polyether siloxane copolymer emulsion available from Evonik Corporation and is used as a defoamer.

[0049] Ti-PURE R-706 is a titanium dioxide pigment available from The Chemours Company.

[0050] TAMOL™ 681 is a hydrophobic copolymer dispersant available from The Dow Chemical Company.

[0051] TRITON™ HW-1000 is a nonionic surfactant available from The Dow Chemical Company.

[0052] ANCAMINE K-54 is a tris-(dimethylaminomethyl)phenol curing agent available from Evonik Corporation.

[0053] XIAMETER™ OFS-6020 is an aminoethylaminopropyltrimethoxysilane coupling agent available from The Dow Chemical Company.

[0054] [Preparation of base coat composition] Base 1, Base 2, and Topcoat pigment grinds were prepared according to Table 1 below. The following ingredients were added to a 1-L stainless steel container and mixed at low shear (1000 rpm) in a disperser with a 2-inch cowl blade until uniform (approximately 5 minutes): 299.56 g deionized water, 11.68 g ammonia (28%), 7.49 g Foamex 1488, 48.13 g Tamol 681, and 9.99 g Triton HW1000. After approximately 5 minutes, 1123.15 g TiPure R706 was slowly added under shear. The mixing speed was gradually increased to achieve a good vortex. After all the TiO2 had been added, the disperser was stopped so that the blade, shaft, and sides of the container could be scraped with a metal spatula. The disperser was then started and the speed increased to approximately 2000 rpm. Mixing was complete after approximately 15 minutes when the Hegman gauge indicated 7-8 units. The pigment grind was then ready to be added to the formulation in Table 2, as listed below.

[0055] [Table 1]

[0056] Two basecoat compositions (Base 1 and Base 2) were prepared using two different acrylic-epoxy hybrids (AEH-1 and AEH-2, respectively). The Part B side mixture was prepared in a plastic container of appropriate size (8 oz. to 32 oz.) for the contents using a metal paddle blade (1-inch diameter) on a laboratory mixer set at an appropriate speed (500-1000 rpm) to maintain a good vortex. The first component was added with a laboratory balance, and additional components were added while mixing to ensure good incorporation. After the last component was added, the mixture was mixed at approximately 1100 rpm for an additional 15 minutes. The Part B side was allowed to equilibrate overnight before being mixed with a tongue depressor to create the coating.

[0057] [Table 2]

[0058] The properties of Base 1, Base 2, and the topcoat are shown in Table 3.

[0059] [Table 3]

[0060] Double coated metal sample The specimens were prepared by coating iron phosphate (BONDERITE 1000) cold-rolled steel substrates with two-layer coatings. In Example 1, a first coat of Base 1 was applied, followed by a topcoat. Example 2 was prepared by coating the substrate with a first layer of Base 2 and a second layer of Topcoat. Comparative Example 1 was prepared by coating the substrate with a first layer of Base 2 and a second layer of Base 1. To prepare Comparative Example 2, two layers of Topcoat were applied. For each of the two-layer coated specimens, the first layer was applied, flashed off at room temperature for 30 minutes, and then cured at 80°C for 20 minutes. The specimens were then allowed to cool to room temperature for 1-2 hours before applying the second layer. The second coating was flashed off at room temperature for 30 minutes, and then cured at 80°C for 20 minutes. After curing, the coated metal specimens were kept at room temperature for 7 days before testing.

[0061] The corrosion resistance of the samples was determined by subjecting the samples to salt spray for 504 hours followed by scrubbing. As shown in Figure 1, Examples 1, 2, and Comparative Example 1 exhibited good corrosion resistance, while Comparative Example 2, which consisted of two top coat layers, exhibited poor corrosion resistance.

[0062] The hardness of each of the coated metal samples was tested using a Persoz pendulum in accordance with ASTM-D4366. The results of the Persoz hardness test are shown in Table 4 below, which further demonstrates that each sample had relatively similar thickness and gloss when tested with a micro-TRI-gloss machine (BYK Company). The hardness of Examples 1 and 2 was significantly improved compared to the two-layer coating of Comparative Example 1, which lacked the layer containing the topcoat.

[0063] [Table 4]

[0064] Single-layer coated metal sample Single-layer coated metal samples were prepared on iron phosphate (BONDERITE 1000) coated cold-rolled steel substrates. Comparative Example 3 was prepared by coating Base 2 onto a metal substrate. Comparative Example 4 was prepared by blending Base 2 and Topcoat 1:1 to form a single layer on the substrate. Comparative Example 5 was prepared by coating a metal substrate with a single layer of Topcoat. Each sample was prepared by coating a single layer onto the substrate, flashing off at room temperature for 30 minutes, curing at 80°C for 20 minutes, and then curing at room temperature for 7 days before testing.

[0065] The corrosion resistance and hardness tests were repeated for the single-layer coated metal samples. For the corrosion resistance test, the samples were exposed to salt spray for 552 hours before being scraped. As shown in Figure 2, Comparative Examples 4 and 5 had inferior corrosion resistance compared to Comparative Example 3. The hardness test results are shown in Figure 5.

[0066] [Table 5]

[0067] Although Comparative Example 4 exhibited slightly better hardness than Comparative Example 3, the hardness of Comparative Example 4 was significantly lower than the two-layer coated metal sample of Example 2.

[0068] A direct comparison of Example 2 and Comparative Example 4 clearly demonstrates that by using different formulations for two separate layers that are at least partially cured between each layer, both the corrosion resistance and hardness of the resulting coating can be significantly improved.

Claims

1. 1. A coating system comprising: a) a first two-component aqueous coating composition comprising a binder component 1A and a curing component 1B, wherein the binder component 1A comprises a first aqueous dispersion of acrylic polymer particles imbibed with an epoxy compound, the first two-component aqueous coating composition comprising a first coalescent package comprising at least one coalescent, and the first two-component aqueous coating composition has a minimum film-forming temperature (MFFT) of less than 5°C; b) a second two-component coating composition comprising a binder component 2A and a curing component 2B, wherein the binder component 2A comprises a second aqueous dispersion of acrylic polymer particles imbibed with an epoxy compound, and the second two-component aqueous coating composition comprises a second coalescent package comprising at least one coalescent, and the second two-component coating composition has a MFFT of 5°C to 25°C.

2. 10. The coating system of claim 1, wherein the first aqueous dispersion of acrylic polymer particles imbibed with an epoxy compound is the same as the second aqueous dispersion of acrylic polymer particles imbibed with an epoxy compound.

3. 3. The coating system according to claim 1 or claim 2, wherein the curing component 1B and the curing component 2B are selected from carboxylic acid-based acrylic curing agents.

4. 4. The coating system of claim 3, wherein said curing component 1B is present in said first two-component aqueous coating composition in an amount such that the molar ratio of epoxy groups to carboxylic acid groups in said first two-component aqueous coating composition is in the range of 1.75:1 to 4:1, and said curing component 2B is present in said second two-component aqueous coating composition in an amount such that the molar ratio of epoxy groups to carboxylic acid groups in said second two-component aqueous coating composition is in the range of 1:1 to 1.5:

1.

5. Coating system according to any one of claims 1 to 4, wherein said curing component 1B is the same as said curing component 2B.

6. 6. The coating system of any one of claims 1 to 5, wherein the first coalescent package is present in an amount comprising 10 to 30 wt. %, based on the total weight of solids in the first binder component 1A and the first curing component 1B in the first two-component coating composition, and the second coalescent package is present in an amount comprising 2 to 10 wt. %, based on the total weight of solids in the second binder component 2A and the second curing component 2B in the second two-component coating composition.

7. 7. The coating system of claim 6, wherein the first coalescent package is present in an amount comprising 15 to 25 wt. %, based on the total weight of solids in the first binder component 1A and the first curing component 1B in the first two-component coating composition, and the second coalescent package is present in an amount comprising 4 to 9 wt. %, based on the total weight of solids in the second binder component 2A and the second curing component 2B in the second two-component coating composition.

8. 8. The coating system according to any one of claims 1 to 7, wherein the second two-component coating composition has a MFFT of 5°C to 15°C.

9. 9. The coating system of claim 1, wherein the first two-component coating composition has a VOC content in the range of 0.75 to 1.25 lb / gal and the second two-component coating composition has a VOC content in the range of 0.25 to 0.5 lb / gal.

10. 10. Coating system according to any one of the preceding claims, wherein the first two-component coating composition has a pigment volume concentration (PVC) of 15 to 20% and the second two-component coating composition has a PVC of 7.5 to 12.5%.

11. 1. A method for preparing a coating, comprising: (i) providing a coating system according to any one of claims 1 to 10; (ii) applying the first two-part coating composition to a substrate to form a first coating layer; (iii) at least partially curing the first coating layer at a temperature above room temperature for at least 10 minutes; (iv) applying the second two-component coating composition onto the first coating layer to form a second coating layer; (v) curing the second coating layer at a temperature above room temperature for at least 10 minutes to form a two-layer coating on the substrate.

12. The method of claim 11 , wherein the substrate comprises a metal substrate.

13. The method of claim 12 , wherein the metal substrate comprises a bare metal substrate or a primed metal substrate.

14. 14. The method of any one of claims 11 to 13, wherein step (iii) of at least partially curing the first coating layer comprises at least partially curing the first coating layer at a temperature of at least 50°C for at least 15 minutes.

15. 15. The method of any one of claims 11 to 14, wherein step (v) of curing the second coating layer comprises curing the second coating layer at a temperature of at least 50°C for at least 15 minutes.

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