Waterborne polyurethane coating as well as preparation method and application thereof

Abstract

The invention provides a waterborne polyurethane coating as well as a preparation method and application thereof. Through the combined action of the water-based hydroxy acrylic dispersion emulsion, the low-methyl etherified high-imino melamine resin and the blocked isocyanate, and through specific screening of the content of functional groups in the low-methyl etherified high-imino melamine resin, the microscopic cross-linked network structure of the baked coating can be improved; the coating can be firmly attached to the surface of the powder coating, the mechanical performance and durability of a coating film are greatly improved, and meanwhile the requirements for environmental protection and low energy consumption are met.

Description

Technical Field

[0001] The present invention relates to the technical field of coatings, in particular to a waterborne polyurethane coating and a preparation method and application thereof. Background Art

[0002] As a primer for metal surfaces, powder coatings have been widely used in industrial applications due to their excellent mechanical properties, weather resistance, and corrosion resistance. However, due to the chemical inertness and non-polar characteristics of powder coatings, their surface lacks active functional groups, making it difficult for subsequent water-based coatings to effectively adhere. The main method to solve this adhesion problem currently still relies on solvent-based coatings, which can release solvents during the baking process, thereby wetting the powder coating surface and achieving good adhesion. However, the high VOCs emissions of such coatings do not meet environmental protection requirements, so there is an urgent need to develop alternatives to water-based coatings. However, there is a lack of effective interaction between the polar components of water-based coatings and the non-polar surface of powder coatings, making it difficult to form strong adhesion on their surfaces.

[0003] In the research on improving the adhesion of powder coatings, commonly used technologies include epoxy phosphates, acrylate-modified polymers, silane coupling agents, silane oligomers, silanol compounds, etc. These materials improve the adhesion between water-based paints and powder coatings by introducing polar groups or functional groups on the substrate surface. However, these adhesion promoters have the following limitations: (1) Epoxy phosphates: Although their polar groups can produce a certain degree of physical adsorption with metal substrates and powder coating surfaces, their chemical bonding ability with non-polar powder coatings is insufficient, and their adhesion is relatively limited; (2) Phosphate-modified polymers: This type of polymer can provide some polar groups, but it is easily hydrolyzed in aqueous systems, resulting in poor long-term adhesion; (3) Silane coupling agents and silane oligomers: Silane compounds can form silicon-oxygen bonds on the coating surface, but require a wet surface, and their chemical bonding is weak, making it difficult to provide high-strength adhesion effects, poor storage stability, and easy gelation; (4) Silanol compounds: These substances form hydrogen bonds or weak polar interactions on the surface, but the effect is not ideal in the combination of polar water-based coatings and non-polar powder coatings, and the adhesion is not strong enough.

[0004] In addition, it is often necessary to introduce an acid catalyst (such as p-toluenesulfonic acid, phosphoric acid or organic acid compounds). Acid catalysts mainly reduce the activation energy of the cross-linking reaction and accelerate the reaction between hydroxyl groups and cross-linkers during the film formation process of the coating, thereby improving the hardness, adhesion and chemical resistance of the coating. However, the use of such acid catalysts also brings the following problems: (1) Potential side effects: Acid catalysts can easily cause the cross-linking components in the system (such as melamine resin or blocked isocyanate) to react prematurely, shortening the storage life of the coating; (2) Acidic environments may corrode metal substrates or powder coatings, thereby weakening the mechanical properties of the substrate; (3) Environmental issues: Some acid catalysts may produce acidic volatiles during the baking process, which is not conducive to environmental protection. Dependence on process: The activity of acid catalysts is greatly affected by temperature and humidity. The dosage and process parameters may need to be adjusted under different production environments, which increases the complexity of operation.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The purpose of the present invention is to overcome the deficiencies in the above-mentioned prior art and provide a water-based coating having high adhesion to the powder coating, excellent storage stability, and excellent environmental performance.

[0007] To achieve the above object, the first aspect of the present invention provides a waterborne polyurethane coating, which comprises the following components in parts by weight based on 100 parts by weight of the waterborne polyurethane coating:

[0008] 1-10 parts of low-methyl etherified high-imino melamine resin, 1-5 parts of blocked isocyanate, 40-60 parts of water-based hydroxy acrylic dispersion emulsion, 0.1-0.5 parts of pH adjuster, 0.1-0.3 parts of defoamer, 1-6 parts of film-forming aid, 1-6 parts of cosolvent, 10-20 parts of color paste, 0.1-1.2 parts of thickener, and up to 100 parts of water;

[0009] Wherein, in the low-methyl etherified high-imino melamine resin, the content of methoxymethyl is awt%, and the content of imino is bwt%, satisfying: b>a.

[0010] The waterborne polyurethane-amino baking coating of the present invention overcomes the problem of insufficient adhesion of conventional waterborne coatings to powder coating surfaces and can achieve strong adhesion under existing low-temperature rapid baking processes. Its components include a waterborne hydroxyl acrylic dispersion emulsion, a low-methyl etherified high-imino melamine resin, and a blocked isocyanate, wherein: (1) the waterborne hydroxyl acrylic dispersion emulsion contains polar functional groups, hydroxyl groups, which can intercalate with the microstructure of the powder coating surface and form physical and chemical bonds; (2) the low-methyl etherified high-imino melamine resin can rapidly cross-link at low temperatures, improving the toughness and adhesion of the coating film; and (3) the blocked isocyanate acts as a cross-linking agent, releasing isocyanate groups during the baking process to react with hydroxyl groups in the hydroxyl acrylic dispersion to form a dense cross-linked structure.

[0011] Further research in the present invention found that by jointly acting on the raw materials in the coating and specifically screening the content of individual functional groups in the low-methyl etherified high-imino melamine resin, the microscopic cross-linked network structure of the coating after baking can be improved, which can ensure the strong adhesion of the coating to the surface of the powder coating, greatly improving the mechanical properties and durability of the coating film, while meeting environmental protection and low energy consumption requirements.

[0012] Preferably, the viscosity of the low-methyl etherified high-imino melamine resin at 25° C. is 1000 to 2000 cps.

[0013] Preferably, in the low-methyl etherified high-imino melamine resin, a=10-15.

[0014] Preferably, in the low-methyl etherified high-imino melamine resin, b=38-42.

[0015] Preferably, in the low-methyl etherified high-imino melamine resin, 2.8 ≤ b / a ≤ 3.8. Increasing the imino ratio can accelerate the reaction rate and lower the reaction temperature, but the resulting coating has increased hardness, resulting in reduced impact resistance. Furthermore, excessive activity can reduce storage stability.

[0016] Preferably, the aqueous hydroxylated acrylic dispersion emulsion has a hydroxyl value of 100 to 140 mgKOH / g.

[0017] Preferably, the terminal group of the blocked isocyanate comprises phenol, nonylphenol, methyl ethyl ketoxime, caprolactam, amide, imidazole, 3,5-dimethylpyrazole, ethyl acetoacetate or acetone oxime.

[0018] Preferably, the blocked isocyanate is modified with a polyester polyol having a weight-average molecular weight of 1000 to 2000. The polyester polyol includes, but is not limited to, at least one of polycaprolactone, polycarbonate diol, glycerol polyester polyol, 1,4-butanediol polyester polyol, 1,6-hexanediol polyester polyol, epoxy polyester polyol, and soybean oil-based polyester polyol.

[0019] Preferably, in the polyester polyol-modified blocked isocyanate, the molar ratio of the hydroxyl group in the polyester polyol to the isocyanate in the blocked isocyanate is (1-1.2):1. Modification of the polyester polyol can improve the compatibility of the blocked isocyanate with the coating matrix and enhance its dispersion stability in the matrix.

[0020] Common defoamers, film-forming aids, cosolvents, thickeners, pH regulators, and color pastes in this field can all be used in the present invention.

[0021] Optionally, the film-forming aid includes but is not limited to at least one of dipropylene glycol methyl ether (DPM), dipropylene glycol butyl ether (DPnB), triethylene glycol monoethyl ether (TBET), triethylene glycol monobutyl ether (TBAT), and tripropylene glycol monobutyl ether (TPnB).

[0022] Optionally, the cosolvent includes but is not limited to at least one of ethylene glycol monomethyl ether (EM), ethylene glycol monoethyl ether (ECS), ethylene glycol monopropyl ether (EP), ethylene glycol butyl ether (BCS), propylene glycol monomethyl ether (PM), propylene glycol monoethyl ether (PE), propylene glycol propyl ether (PnP), propylene glycol monobutyl ether (PnB), and propylene glycol tert-butyl ether (PtB).

[0023] Optionally, the pH adjuster includes but is not limited to at least one of 2-amino-2-methyl-1-propanol, N,N-dimethylethanolamine, N-methylmorpholine, triethylamine, triethanolamine, monoethanolamine, and diethanolamine.

[0024] Optionally, the defoaming agent includes but is not limited to silicone defoaming agents and silicone ether defoaming agents.

[0025] Optionally, the thickener includes but is not limited to nonionic associative polyurethane, and the nonionic associative polyurethane can be selected from at least one of commercially available RHEOLATE 299, RHEOLATE 288, BYK RHEO BYK-H 7625VF, TAFIGEL PUR44, and TAFIGEL PUR 65.

[0026] The second aspect of the present invention provides a method for preparing the waterborne polyurethane coating according to the first aspect of the present invention, comprising the following steps:

[0027] According to the formula of the raw materials, all components are mixed evenly to obtain the water-based polyurethane coating.

[0028] The third aspect of the present invention provides a use of the waterborne polyurethane coating according to the first aspect of the present invention, wherein the waterborne polyurethane coating is provided on the surface of the powder coating and solidified into a waterborne polyurethane coating.

[0029] Specifically, the curing condition of the waterborne polyurethane coating is: curing at 70-140°C.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] The present invention uses a water-based hydroxy acrylic dispersion emulsion, a low-methyl etherified high-imino melamine resin, and a blocked isocyanate to work together. At the same time, the content of individual functional groups in the low-methyl etherified high-imino melamine resin is specifically screened. This can improve the microscopic cross-linked network structure of the coating after baking, ensure the coating's firm adhesion to the powder coating surface, greatly enhance the mechanical properties and durability of the coating, and meet environmental protection and low energy consumption requirements. DETAILED DESCRIPTION

[0032] For better explanation of the purpose, technical scheme and advantage of the present invention, the present invention will be further described below in conjunction with specific embodiment, but embodiment does not limit the present invention in any form.Unless otherwise stated, the reagent, method and equipment adopted in the present invention are conventional reagents, methods and equipment in the art.Unless otherwise stated, the reagents and materials used in the present invention are commercially available.

[0033] The raw material information used in the examples and comparative examples is as follows:

[0034] Table 1 Raw material information

[0035]

[0036]

[0037] In the present invention, the preparation method of polycaprolactone-modified isocyanate YL-WB9100 is as follows:

[0038] The polyol is obtained by reacting glycerol polyester polyol and isocyanate YL-WB9100 at 80°C under nitrogen protection, wherein the molar ratio of the hydroxyl group in the polyol to the isocyanate group in the isocyanate is 1.2:1.

[0039] In the present invention, the methylated iminomelamine resin used in the examples and comparative examples was prepared according to a method comprising the following steps:

[0040] S1. Hydroxymethylation reaction:

[0041] Melamine and formaldehyde solution are mixed and added to a reaction kettle, the pH value is adjusted to a weak alkaline value (pH 8-9) with an alkaline catalyst, and tetramethylolation reaction is carried out under the condition of heating to 40-55°C and keeping the temperature for 60-90 minutes to produce tetramethylol melamine;

[0042] S2. Dehydration and drying:

[0043] The product prepared in step S1 is first centrifuged and dehydrated, and then placed in an oven for drying to control the solid content to be above 85%;

[0044] S3. Etherification reaction:

[0045] Add a certain amount of methanol to a four-necked flask, start stirring and heat to 50-60°C; then add the product of step S2, then add saturated sodium carbonate as a catalyst, adjust the pH to 8-8.5, carry out etherification reaction, and keep warm for 2-3 hours;

[0046] S4. Post-processing:

[0047] After the etherification reaction is completed, the methyl etherified imino melamine resin can be obtained by vacuum distillation and dehydration.

[0048] By adjusting the amount of raw materials, reaction temperature and time, a series of methyl etherified imino melamine resins can be prepared. The relevant parameters are detailed in Table 2:

[0049] Table 2 Parameters of methyl etherified imino melamine resin

[0050]

[0051]

[0052] Example 1

[0053] A series of waterborne polyurethane coatings are provided, which are prepared according to the formulations in Tables 3 and 4 by a method comprising the following steps:

[0054] The aqueous hydroxy acrylic emulsion is dispersed in water at 550 rpm using a disperser, a pH adjuster is added to a pH value within the range of 8-9, and the mixture is dispersed for 30 minutes to obtain a milky white liquid. Subsequently, low-methyl etherified high-imino melamine resin, blocked isocyanate, film-forming aid, cosolvent, defoamer, color paste, and water are added, and the mixture is stirred at 400 rpm for 20 minutes. A thickener is then added to adjust the viscosity. After uniform dispersion, a uniformly mixed dispersion is obtained, which is the aqueous polyurethane coating.

[0055] Table 3 Formulation of waterborne polyurethane coating

[0056]

[0057] Table 4 Formulation of waterborne polyurethane coating

[0058]

[0059]

[0060] Comparative Example 1

[0061] This comparative example 1 provides a waterborne polyurethane coating, which is prepared according to the method of Example 1. The difference from Example 3 is that the methyl etherified iminomelamine resin MF-1 in the formula is replaced by an equal mass of MF-8.

[0062] Comparative Example 2

[0063] This comparative example 1 provides a waterborne polyurethane coating, which is prepared according to the method of Example 1. The difference from Example 3 is that the methyl etherified iminomelamine resin MF-1 in the formula is replaced by an equal mass of waterborne amino resin 303LF.

[0064] Comparative Example 3

[0065] This comparative example 1 provides a waterborne polyurethane coating, which was prepared according to the method of Example 1. The difference from Example 3 is that the low-methyl etherified high-imino melamine resin MF-1 is not added to component A, and 6 parts by weight of a waterborne acrylic resin (DSM's Neocryl XK-52) is added to component B.

[0066] Comparative Example 4

[0067] This comparative example 1 provides a water-based polyurethane coating, which is prepared by referring to the method of Example 1. The difference from Example 1 is that low-methyl etherified high-imino melamine resin is not added to component A, but is replaced by an equal weight portion of water.

[0068] Comparative Example 5

[0069] This comparative example 1 provides a waterborne polyurethane coating, which is prepared according to the method of Example 1. The difference from Example 1 is that low-methyl etherified high-imino melamine resin MF-1 is added to component B.

[0070] Performance Testing

[0071] The waterborne polyurethane coatings prepared in the above examples and comparative examples were sprayed onto powder coatings (fused bonded epoxy (FBE) heavy-duty anti-corrosion powder coatings, purchased from Qingtian Materials Technology Co., Ltd., applied using high-voltage electrostatic spraying and cured at 160-230°C for 3-5 minutes). The coatings were leveled for 1 minute, and the oven temperature was set to 70°C. The sample was placed in the oven and heated to 140°C within 6 minutes. Finally, the high temperature of 140°C was maintained for 6 minutes. The resulting film thickness was 12.5±2.5 μm. After cooling to room temperature, the following tests were performed:

[0072] (1) Appearance of the paint film: Visual inspection is used to determine whether the surface is smooth and whether there are orange peel-like lines, granular protrusions, etc.

[0073] (2) Impact resistance: Tested in accordance with GB / T 1732-2020, with a drop weight of 1 kg and a drop height of 50 cm;

[0074] (3) Pencil hardness: Tested in accordance with GB / T6739-2022;

[0075] (4) Adhesion: Tested according to GB / T 9286-2021, with a grid of 1 mm × 1 mm;

[0076] (5) Storage stability: Place at 50°C for 7 days to observe whether gelation occurs.

[0077] (6) VOC (g / L): According to GB 24409-2020, a VOC content of ≤120g / L is considered qualified;

[0078] The test results are shown in Table 5.

[0079] Table 5 Performance test results

[0080]

[0081] It can be seen from the above results that the coating obtained by the coating of the present invention has good adhesion, impact resistance, mechanical properties, storage stability, and good appearance.

[0082] In Comparative Example 1, the imino content in the methyl etherified iminomelamine resin was higher than the methoxy content, and in Comparative Example 2, ordinary water-based amino resin was added. The adhesion, impact resistance and pencil hardness of the obtained coating were all deteriorated.

[0083] It can be seen from Comparative Example 3 and Example 3 that when the organic acid catalyst is added, the thermal stability of the coating obtained in Comparative Example 3 becomes poor and gels easily.

[0084] It can be seen from Comparative Example 4 and Example 3 that when the low-methyl etherified high-imino melamine resin is not added, the adhesion, impact resistance and pencil hardness of the coating obtained in Comparative Example 4 are all deteriorated.

[0085] Comparison of Comparative Example 5 with Example 3 shows that when the low-methyl etherified high-imino melamine resin is dispersed in different components, the storage stability will deteriorate, and during the curing process, the dispersion will be uneven, resulting in defects such as orange peel and particles after film formation.

[0086] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A waterborne polyurethane coating, characterized in that: Based on 100 parts by weight of the waterborne polyurethane coating, the composition comprises the following components in parts by weight: 1-10 parts of low-methyl etherified high-imino melamine resin, 1-5 parts of blocked isocyanate, 40-60 parts of water-based hydroxy acrylic dispersion emulsion, 0.1-0.5 parts of pH adjuster, 0.1-0.3 parts of defoamer, 1-6 parts of film-forming aid, 1-6 parts of cosolvent, 0-20 parts of color paste, 0.1-1.2 parts of thickener, water to 100 parts; Among them, in the low-methyl etherified high-imino melamine resin, the content of methoxymethyl is awt%, and the content of imino is bwt%, satisfying: b>a.

2. The waterborne polyurethane coating according to claim 1, characterized in that: The low-methyl etherified high-imino melamine resin satisfies at least one of the following characteristics: (1)a＝10～15； (2)b＝38～42； (3)2.8≤b / a≤3.

8.

3. The waterborne polyurethane coating according to claim 1, characterized in that: The viscosity of the low-methyl etherified high-imino melamine resin at 25° C. is 1000-2000 cps.

4. The waterborne polyurethane coating according to claim 1, characterized in that: The hydroxyl value of the aqueous hydroxy acrylic dispersion emulsion is 100 to 140 mgKOH / g.

5. The waterborne polyurethane coating according to claim 1, characterized in that: The terminal group of the blocked isocyanate includes phenol, nonylphenol, methyl ethyl ketoxime, caprolactam, amide, imidazoles, 3,5-dimethylpyrazole, ethyl acetoacetate or acetone oxime.

6. The waterborne polyurethane coating according to any one of claims 1 or 5, characterized in that: The blocked isocyanate is modified by polyester polyol, and the weight average molecular weight of the polyester polyol is 1000-2000.

7. The waterborne polyurethane coating according to claim 1, characterized in that: Satisfy at least one of the following characteristics: (1) The defoaming agent includes at least one of an organosilicon defoaming agent and a silicone ether defoaming agent; (2) the film-forming aid includes at least one of dipropylene glycol methyl ether, dipropylene glycol butyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether and tripropylene glycol monobutyl ether; (3) The cosolvent comprises at least one of ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol butyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol propyl ether, propylene glycol monobutyl ether and propylene glycol tert-butyl ether; (4) The thickener comprises a nonionic associative polyurethane; (5) The pH adjuster includes at least one of 2-amino-2-methyl-1-propanol, N,N-dimethylethanolamine, N-methylmorpholine, triethylamine, triethanolamine, monoethanolamine, and diethanolamine.

8. The method for preparing the waterborne polyurethane coating according to any one of claims 1 to 7, characterized in that: The steps include: According to the formula of the raw materials, all components are mixed evenly to obtain the waterborne polyurethane coating.

9. Use of the waterborne polyurethane coating according to any one of claims 1 to 7, characterized in that: The waterborne polyurethane coating is disposed on the surface of the powder coating and solidified into a waterborne polyurethane coating.

10. The use of the waterborne polyurethane coating according to claim 9, characterized in that: The curing condition of the waterborne polyurethane coating is: curing at 70-140°C.

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