A water-based coating composition suitable for metal substrates

Abstract

The present application relates to the field of water-based coating technology, and specifically provides a water-based coating composition suitable for metal substrates. The raw material components, by weight, include: 100 parts of polyurethane dispersion, 3-10 parts of film-forming aid, 0.5-1.5 parts of wetting agent, 0.5-1 part of leveling agent, 0.5-1 part of defoamer, 0.5-2 part of thixotropic agent, 0-100 parts of color paste, and 1-10 parts of adhesion aid; the adhesion aid is a polyurethane structure, and its molecular structure contains hydrophilic polyethylene glycol, hydrophobic polysiloxane, and terminal alkoxysilyl. The water-based coating composition of the present application is suitable for coating metals with smooth surfaces and has good and stable adhesion.

Description

Technical Field

[0001] The present application relates to the technical field of water-based coatings, and in particular to a water-based coating composition suitable for metal substrates. Background Art

[0002] Metals, such as stainless steel and aluminum alloys, offer excellent corrosion resistance, workability, fire resistance, moisture resistance, impermeability, and a high surface finish, making them widely used in many industries, including home decoration. Stainless steel and aluminum alloys often have monotonous surfaces, so water-based paints can be used to give them a variety of colors. However, the smooth surfaces of metals like stainless steel and aluminum alloys generally do not adhere well to them. To improve adhesion, a polishing process is often required, but this is time-consuming and labor-intensive. Furthermore, over time, the coating's adhesion decreases, making it susceptible to problems such as peeling and cracking.

[0003] Waterborne polyurethane is a main resin commonly used in waterborne coatings. It has the characteristics of good hand feel, good adhesion, high strength and wear resistance. Waterborne polyurethane dispersions can be divided into polyurethane aqueous solutions (particle size <0.001 micron, transparent appearance), polyurethane microemulsions (particle size 0.001-0.1 micron, translucent appearance) and polyurethane emulsions (particle size >0.1 micron, turbid appearance) according to particle size and appearance. Polyurethane aqueous solutions and polyurethane microemulsions have better adhesion to the substrate due to their low particle size, large contact area with the substrate, and high polar group content. However, relying solely on waterborne polyurethane dispersions, its adhesion to metals with smooth surfaces such as stainless steel and aluminum alloys is not high enough. Waterborne polyurethane coatings still need to continue to improve their adhesion so that metals such as stainless steel and aluminum alloys can be directly coated without polishing and have stable high adhesion. Summary of the Invention

[0004] In the prior art, silane coupling agents, such as methyltrimethoxysilane and 3-aminopropyltrimethoxysilane, are generally added to improve the adhesion of water-based coatings to metal substrates. However, due to the relatively small molecular size of silane coupling agents, when used alone, they have little effect on improving the adhesion of smooth metal surfaces (such as stainless steel and aluminum alloys), and in particular, the adhesion stability is poor. Based on this, the present application provides a water-based coating composition suitable for smooth metal substrates.

[0005] This application adopts the following technical solutions:

[0006] A water-based coating composition suitable for metal substrates with smooth surfaces, comprising, by weight, 100 parts of a polyurethane dispersion, 3-10 parts of a film-forming aid, 0.5-1.5 parts of a wetting agent, 0.5-1 part of a leveling agent, 0.5-1 part of a defoaming agent, 0.5-2 parts of a thixotropic agent, 0-100 parts of a color paste, and 1-10 parts of an adhesion promoter.

[0007] The preparation steps of the adhesion aid include:

[0008] S1, reacting a polyisocyanate monomer and polyethylene glycol at a molar ratio of isocyanate group to hydroxyl group of 1:0.5-0.9 to obtain a polyurethane prepolymer;

[0009] S2, a first amino-terminated polysiloxane and an epoxy silane coupling agent are subjected to a ring-opening reaction at a primary amino group to epoxy group molar ratio of 1:1-3 to obtain a modified polysiloxane;

[0010] S3. The polyurethane prepolymer described in step S1 and the modified polysiloxane described in step S2 are reacted at a molar ratio of isocyanate to secondary amino group of 1:1:1-2 to obtain the adhesion aid.

[0011] Preferably, the polyisocyanate in step S1 is selected from any one or a combination of any two or more of TDI, HDI, MDI, IPDI and HMDI.

[0012] Preferably, the average molecular weight of the polyethylene glycol in step S1 is 200-2000.

[0013] Preferably, the general formula of the first amino-terminated polysiloxane in step S2 is R 1 SiOMe2(SiOMeR 2 ) m SiMe2R 3 , where R 1 is a C1-C4 alkyl group, a hydroxyl group or a primary amino group substituted C3-C6 alkyl group, R 2 is selected from C1-C4 alkyl or C6-C12 aromatic groups, R 3 is a primary amino-substituted C3-C6 alkyl group, Me is a methyl group, and m=10-100.

[0014] Preferably, the epoxysilane coupling agent in step S2 is selected from any one of γ-(2,3-epoxypropoxy)propyltrimethoxysilane, γ-(2,3-epoxypropoxy)propyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltriethoxysilane and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, or a combination of any two or more thereof.

[0015] Preferably, the modified polysiloxane in step S2 is obtained by subjecting a second amino-terminated polysiloxane and a vinyl silane coupling agent to a Michael addition reaction at a primary amino group to vinyl group molar ratio of 1:1-2;

[0016] The general formula of the second amino-terminated polysiloxane is R 4 SiOMe2(SiOMeR 5 ) nSiMe2R 6 , where R 4 is a C1-C4 alkyl group, a hydroxyl group or a primary amino group substituted C3-C6 alkyl group, R 5 is selected from C1-C4 alkyl or C6-C12 aromatic groups, R 6 is a primary amino-substituted C3-C6 alkyl group, Me is a methyl group, and n=10-100.

[0017] More preferably, the vinyl silane coupling agent is selected from any one or a combination of any two or more of vinyl trimethoxy silane, vinyl triethoxy silane and vinyl methyl dimethoxy silane.

[0018] Preferably, the polyurethane dispersion can be selected from any one of a polyurethane aqueous solution, a polyurethane microemulsion and a polyurethane emulsion.

[0019] Preferably, the film-forming aid is selected from any one of alcohol ester film-forming aids, alcohol ether film-forming aids and alcohol film-forming aids, or a combination of any two or more.

[0020] Preferably, the raw material components further include: one or a combination of two or more of: 0.5-2 parts of a silane coupling agent, 0.1-1 parts of a preservative, and 3-20 parts of water.

[0021] The technical principle of the water-based coating composition of the present application is as follows: the present application adds an adhesion aid to the water-based coating composition, and the adhesion aid has a hydrophilic polyethylene glycol segment, a hydrophobic polysiloxane segment, a highly polar urethane bond and an alkoxysilyl terminal group in its molecular structure. (1) The hydrophilic polyethylene glycol segment and the hydrophobic polysiloxane segment can promote the adhesion aid to disperse in the water-based coating and form structures such as micelles, and the hydrophobic polysiloxane segment coats and protects the alkoxysilyl terminal group to prevent the alkoxysilyl terminal group from hydrolyzing and condensing during the storage of the water-based coating and losing its adhesion to the substrate; (2) The adhesion aid is a macromolecular The polyurethane structure has good compatibility with the polyurethane polymer chains in the polyurethane dispersion. Therefore, the adhesion aid can also interact with the polyurethane polymer chains in the polyurethane dispersion, and the interaction is strong; (3) On the one hand, the adhesion aid can react with the smooth metal substrate, and the adhesion aid polymer chain is adsorbed on the surface of the metal substrate. On the other hand, the adhesion aid macromolecular chain can interact with the polyurethane polymer chains in the polyurethane dispersion through entanglement, hydrogen bonding, van der Waals force, etc., forming a strong interaction. Therefore, the adhesion aid of the present application can stably adsorb the film-forming polyurethane resin on the surface of the metal substrate.

[0022] In summary, this application has the following beneficial effects:

[0023] 1. After curing, the water-based coating composition of the present application has good adhesion to smooth metal surfaces and good adhesion stability.

[0024] 2. The aqueous coating composition of the present application may further be added with an appropriate amount of silane coupling agent to further improve adhesion and adhesion stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Attachment Figure 1 The adhesion test results of the film layer of Example 4 of the present application are as follows;

[0026] Attachment Figure 2 These are the adhesion test results of the film layer of Example 4 of the present application after being soaked in water. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be described in detail below.

[0028] Throughout this specification, unless otherwise specified, the terms used herein should be understood as having the same meaning as commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In the event of any conflict, the present specification shall take precedence.

[0029] The present application provides a water-based coating composition suitable for a metal substrate with a smooth surface. The raw material components include, by weight: 100 parts of a polyurethane dispersion, 3-10 parts of a film-forming aid, 0.5-1.5 parts of a wetting agent, 0.5-1 parts of a leveling agent, 0.5-1 parts of a defoaming agent, 0.5-2 parts of a thixotropic agent, 0-100 parts of a color paste, and 1-10 parts of an adhesion promoter.

[0030] The preparation steps of the adhesion aid include:

[0031] S1, reacting a polyisocyanate monomer and polyethylene glycol at a molar ratio of isocyanate group to hydroxyl group of 1:0.5-0.9 to obtain a polyurethane prepolymer;

[0032] S2, a first amino-terminated polysiloxane and an epoxy silane coupling agent are subjected to a ring-opening reaction at a primary amino group to epoxy group molar ratio of 1:1-2 to obtain a modified polysiloxane;

[0033] S3. The polyurethane prepolymer in step S1 and the modified polysiloxane in step S2 are reacted at a molar ratio of isocyanate to secondary amino group of 1:1-2 to obtain an adhesion aid.

[0034] In the preparation method of the above-mentioned adhesion aid, polyethylene glycol is used in the preparation of the polyurethane prepolymer to introduce a hydrophilic structure, making the adhesion aid amphiphilic; the modified polysiloxane obtained by reacting the first amino-terminated polysiloxane with the epoxysilane coupling agent contains a secondary amino group and a terminal alkoxysilyl group, and the secondary amino group can react with the polyurethane prepolymer. The molecular structure of the obtained adhesion aid contains a hydrophilic polyethylene glycol segment, a hydrophobic polysiloxane segment, a highly polar urethane structure and an alkoxysilyl group at the terminal group. The hydrophilic polyethylene glycol segment and the hydrophobic polysiloxane segment can enable the adhesion aid to form a structure such as micelles in the water-based coating, thereby improving the dispersibility. In addition, the water-sensitive alkoxysilyl group in the terminal group is coated by the hydrophobic polysiloxane in the hydrophobic structure, thereby preventing the alkoxysilyl group from undergoing hydrolysis and condensation, which leads to failure. Highly polar carbamate can not only increase the interaction force with the metal substrate, but also form hydrogen bonds, entanglement, van der Waals forces and other interactions with the polyurethane polymer chains in the polyurethane dispersion, so that the polyurethane polymer chains in the polyurethane dispersion have a better adsorption effect on the metal substrate after forming a film.

[0035] Specifically, the above-mentioned step S1 of preparing the adhesion aid can be: under nitrogen protection, polyisocyanate monomer and polyethylene glycol are added to a reaction container, stirred and mixed evenly, and then a small amount of organic tin catalyst (such as dibutyltin dilaurate) is added, and the temperature is raised to 60-100° C. and reacted for 1-4 hours to obtain a polyurethane prepolymer.

[0036] Specifically, the above-mentioned step S2 of preparing the adhesion aid can be: adding the first amino-terminated polysiloxane and the epoxy silane coupling agent into a reaction container, stirring at room temperature for 1-3 hours, heating to 50-80°C and continuing the reaction for 1-3 hours, and when the epoxy silane coupling agent is excessive, removing the unreacted epoxy silane coupling agent to obtain the modified polysiloxane.

[0037] Specifically, the above-mentioned step S3 of preparing the adhesion aid can be: under nitrogen protection, adding the polyurethane prepolymer and the modified polysiloxane into a reaction container, stirring at room temperature for 1-3 hours, and then heating to 40-70° C. and reacting for 1 hour to 5 hours to obtain the adhesion aid.

[0038] In a preferred embodiment of the present application, the polyisocyanate in step S1 is selected from any one or a combination of any two or more of toluene diisocyanate TDI, hexamethylene diisocyanate HDI, diphenylmethane diisocyanate MDI, isophorone diisocyanate IPDI and dicyclohexylmethane diisocyanate HMDI.

[0039] In a preferred embodiment of the present application, the average molecular weight of the polyethylene glycol in step S1 is 200-2000. Polyethylene glycol can provide a hydrophilic segment for the adhesion promoter, thereby improving the dispersibility of the adhesion promoter in the water-based coating. Furthermore, the average molecular weight of the polyethylene glycol can be 200-1500, specifically, any value among 200 (for example, it can be represented by PEG-200, and so on), 400, 600, 800, 1000, 1200, 1500, etc.

[0040] In a preferred embodiment of the present application, the first amino-terminated polysiloxane in step S2 can be a single-terminated primary amino-terminated polysiloxane or a double-terminated primary amino-terminated polysiloxane, and the general formula can be R 1 SiOMe2(SiOMeR 2 ) m SiMe2R 3 , where R 1 is a C1-C4 alkyl group, a hydroxyl group or a primary amino group substituted C3-C6 alkyl group, R 2 is selected from C1-C4 alkyl or C6-C12 aromatic groups, R 3 is a primary amino-substituted C3-C6 alkyl, Me is a methyl group, m=10-100. 1 When R is a C1-C4 alkyl or hydroxyl group, it is a single-terminal primary amino polysiloxane, which reacts with an epoxy silane coupling agent to modify the polysiloxane to contain one secondary amino group. Furthermore, in order to prevent the secondary amino group generated by the ring-opening reaction from continuing to react with an excess of epoxy silane coupling agent, in step S2, the molar ratio of the primary amino group to the epoxy group can be 1:1. 1 When the C3-C6 alkyl group is replaced by a primary amino group, a double-terminal primary amino polysiloxane is obtained, which reacts with an epoxy silane coupling agent. Depending on the molar ratio of the amino group to the epoxy group, the modified polysiloxane may contain 1-2 secondary amino groups. Furthermore, in step S2, the molar ratio of the primary amino group to the epoxy group may be 1:1-1.5, and the molar ratio may be any value of 1:1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, etc.

[0041] In a preferred embodiment of the present application, the epoxysilane coupling agent in step S2 can be any one selected from γ-(2,3-epoxypropoxy)propyltrimethoxysilane, γ-(2,3-epoxypropoxy)propyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltriethoxysilane and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, or a combination of any two or more thereof.

[0042] In a preferred embodiment of the present application, the modified polysiloxane in step S2 can also be obtained by subjecting a second amino-terminated polysiloxane and a vinyl silane coupling agent to a Michael addition reaction at a primary amino group to vinyl group molar ratio of 1:1-3;

[0043] The general formula of the second amino-terminated polysiloxane is R 4 SiOMe2(SiOMeR 5 ) n SiMe2R 6 , where R 4 is a C1-C4 alkyl group, a hydroxyl group or a primary amino group substituted C3-C6 alkyl group, R 5 is selected from C1-C4 alkyl or C6-C12 aromatic groups, R 6 is a primary amino-substituted C3-C6 alkyl, Me is a methyl group, and n=10-100. 4 When R is a C1-C4 alkyl or hydroxyl group, it is a single-terminal primary amino polysiloxane, which reacts with a vinyl silane coupling agent. The molar ratio of primary amino group to vinyl group can be 1:1-2. The modified polysiloxane contains one secondary amino group. If the vinyl silane coupling agent is excessive, it can be removed by vacuum distillation, molecular distillation or thin film evaporation. 4 When the C3-C6 alkyl group is replaced by a primary amino group, a double-terminal primary amino polysiloxane is obtained. When reacting with a vinyl silane coupling agent, the molar ratio of the primary amino group to the vinyl group can be 1:2-3. The modified polysiloxane contains two secondary amino groups. If the vinyl silane coupling agent is excessive, it can be removed by reduced pressure distillation, molecular distillation or thin film evaporation.

[0044] In a more preferred embodiment of the present application, the vinyl silane coupling agent is selected from any one or a combination of any two or more of vinyl trimethoxy silane, vinyl triethoxy silane and vinyl methyl dimethoxy silane.

[0045] In a preferred embodiment of the present application, the polyurethane dispersion can be selected from any one of a polyurethane aqueous solution, a polyurethane microemulsion, and a polyurethane emulsion. Further, the polyurethane dispersion can be a polyurethane aqueous solution or a polyurethane microemulsion, both of which can be directly obtained from the market.

[0046] In a preferred embodiment of the present application, the film-forming aid is selected from any one of alcohol ester film-forming aids, alcohol ether film-forming aids, and alcohol film-forming aids, or a combination of any two or more. The film-forming aids can be directly obtained from the market. For example, the alcohol ester film-forming aid can be alcohol ester dodecahydrate, the alcohol ether film-forming aid can be propylene glycol butyl ether, propylene glycol phenyl ether, etc., the alcohol ether ester film-forming aid can be hexanediol butyl ether acetate, propylene glycol methyl ether acetate, etc., and the alcohol film-forming aid can be benzyl alcohol.

[0047] In a preferred embodiment of the present application, the raw material components further include: one or a combination of 0.5-2 parts of silane coupling agent, 0.1-1 parts of preservative and 3-20 parts of water. For example, the silane coupling agent can be methyltrimethoxysilane coupling agent, 3-aminopropyltrimethoxysilane, etc., which can further improve the adhesion of the water-based coating composition to the metal substrate after curing. The silane coupling agent can be pre-mixed with the adhesion aid and then added to the polyurethane dispersion for dispersion. The preservative can prevent the water-based coating composition from spoiling and moldy for a certain period of time, and can be benzisothiazoline, etc. Adding water can further adjust the solid content and viscosity of the water-based coating composition.

[0048] In the present application, there is no particular limitation on the wetting agent, which may be a polyether-modified silicone oil wetting agent; there is no particular limitation on the leveling agent, which may be a polyether-modified silicone oil wetting agent; there is no particular limitation on the defoaming agent, which may be a dimethyl silicone oil defoaming agent; there is no particular limitation on the thixotropic agent, which may be fumed silica, attapulgite, organic bentonite, hydrogenated castor oil, polyurethane water-based thickener, etc.; there is no particular limitation on the color paste, which may be a water-based white color paste, a water-based yellow color paste, a water-based blue color paste, a water-based green color paste, etc., which can be directly obtained from the market.

[0049] The water-based coating composition of the present application can be used in conjunction with an isocyanate curing agent. N3600, N3900, XP2730, Bayhydur304, Bayhydur305, Bayhydur XP 2655, etc.) are added to the aqueous coating composition of the present application, mixed evenly, and then applied to the surface of the substrate to be coated.

[0050] The technical solution of the present application is described in detail below with reference to preparation examples and comparative examples.

[0051] Preparation Example 1-3 Preparation of Adhesion Aid

[0052] Preparation Example 1

[0053] Under nitrogen protection, IPDI and PEG-400 were added to a reaction vessel in a molar ratio of 1:0.8, stirred and mixed evenly, and then 0.2% by weight of dibutyltin dilaurate of PEG-400 was added. The temperature was raised to 80° C. and the reaction was carried out for 2 hours to obtain a polyurethane prepolymer.

[0054] The amino-terminated polysiloxane SiOMe3 (SiOMe2) 31.8 SiMe2(CH2)3NH2 and γ-(2,3-epoxypropoxy)propyltrimethoxysilane were added into the reaction vessel in a molar ratio of 1:1, stirred at room temperature for 1 hour, heated to 60°C and reacted for 2 hours to obtain modified polysiloxane.

[0055] Under nitrogen protection, the polyurethane prepolymer and modified polysiloxane were added to a reaction vessel at a molar ratio of isocyanate to secondary amino group of 1:1, stirred at room temperature for 2 hours, and then heated to 60° C. for reaction for 1.5 hours to obtain an adhesion promoter.

[0056] Preparation Example 2

[0057] Under nitrogen protection, HMDI and PEG-800 were added to a reaction vessel in a molar ratio of 1:0.85, stirred and mixed evenly, and then 0.25% by weight of dibutyltin dilaurate of PEG-800 was added. The temperature was raised to 80° C. and the reaction was carried out for 2 hours to obtain a polyurethane prepolymer.

[0058] The amino-terminated polysiloxane NH2(CH2)3SiOMe2(SiOMe2) 25.4 SiMe2(CH2)3NH2 and γ-(2,3-epoxypropoxy)propyltrimethoxysilane were added into the reaction vessel in a molar ratio of 1:2.4, stirred at room temperature for 1 hour, heated to 60°C and continued to react for 2 hours to obtain modified polysiloxane.

[0059] Under nitrogen protection, the polyurethane prepolymer and modified polysiloxane were added to a reaction vessel at a molar ratio of isocyanate to secondary amino group of 1:1, stirred at room temperature for 2 hours, and then heated to 60° C. for reaction for 2 hours to obtain an adhesion promoter.

[0060] Preparation Example 3

[0061] The amino-terminated polysiloxane NH2(CH2)3SiOMe2(SiOMe2) 40.1 SiMe2(CH2)3NH2 and vinyltrimethoxysilane were added to the reaction vessel in a molar ratio of 1:3, the temperature was raised to 100°C and the reaction was continued for 96 hours, the pressure was reduced to below -0.099 MPa, the unreacted vinyltrimethoxysilane was removed, and the temperature was lowered to obtain modified polysiloxane.

[0062] Under nitrogen protection, the polyurethane prepolymer and modified polysiloxane in Preparation Example 1 were added to a reaction vessel at a molar ratio of isocyanate to secondary amino group of 1:2, stirred at room temperature for 2 hours, and then heated to 60° C. for reaction for 2 hours to obtain an adhesion promoter.

[0063] Example 1

[0064] The raw materials of the water-based coating composition include: 100 parts of polyurethane dispersion (Covestro Bayhydrol U XP 2750), 2 parts of propylene glycol butyl ether, 3 parts of propylene glycol methyl ether acetate, 0.6 parts of polyether-modified silicone oil wetting agent, 0.8 parts of polyether-modified silicone oil leveling agent, 1 part of dimethyl silicone oil defoamer, 1 part of polyurethane water-based thickener, 30 parts of water-based white color paste and 5 parts of the adhesion aid of Preparation Example 1.

[0065] Add adhesion aid, propylene glycol butyl ether and propylene glycol methyl ether acetate to the polyurethane dispersion, stir at a stirring speed of 600 rpm for 10 minutes, add a wetting agent, a leveling agent and a defoaming agent in sequence, continue stirring for 15 minutes, add a water-based white color paste, increase the speed to 1200 rpm and stir and disperse for 10 minutes, add a polyurethane water-based thickener, and stir at a stirring speed of 500 rpm for 15 minutes to obtain a water-based coating composition.

[0066] Example 2

[0067] The difference between Example 2 and Example 1 is that in Example 1, the adhesion promoter in Preparation Example 1 is adjusted from 5 parts to 1 part. The other steps remain unchanged.

[0068] Example 3

[0069] The difference between Example 3 and Example 1 is that in Example 1, the adhesion promoter in Preparation Example 1 is adjusted from 5 parts to 10 parts. The other steps remain unchanged.

[0070] Example 4

[0071] The difference between Example 4 and Example 1 is that in Example 1, the adhesion aid in Preparation Example 1 is adjusted to an equal weight portion of the adhesion aid in Preparation Example 2. The remaining steps remain unchanged.

[0072] Example 5

[0073] The difference between Example 5 and Example 1 is that in Example 1, the adhesion aid in Preparation Example 1 is adjusted to an equal weight portion of the adhesion aid in Preparation Example 3. The remaining steps remain unchanged.

[0074] Example 6

[0075] The raw materials of the water-based coating composition include: 100 parts of polyurethane dispersion (Covestro Bayhydrol U XP 2750), 2 parts of propylene glycol butyl ether, 3 parts of propylene glycol methyl ether acetate, 0.6 parts of polyether-modified silicone oil wetting agent, 0.8 parts of polyether-modified silicone oil leveling agent, 1 part of dimethyl silicone oil defoamer, 1 part of polyurethane water-based thickener, 30 parts of water-based white color paste, 1 part of 3-aminopropyltrimethoxysilane and 5 parts of the adhesion aid of Preparation Example 1.

[0076] The polyurethane dispersion was added into a container, and a mixed solution of 3-aminopropyltrimethoxysilane, an adhesion aid, propylene glycol butyl ether and propylene glycol methyl ether acetate was added dropwise at a stirring speed of 600 rpm. After the addition, stirring was continued for 10 minutes. A wetting agent, a leveling agent and a defoaming agent were added in sequence and stirring was continued for 15 minutes. An aqueous white color paste was added, and the speed was increased to 1200 rpm and stirred and dispersed for 10 minutes. A polyurethane aqueous thickener was added and stirred at a stirring speed of 500 rpm for 15 minutes to obtain an aqueous coating composition.

[0077] Example 7

[0078] The difference between Example 7 and Example 6 is that 3-aminopropyltrimethoxysilane is replaced by an equal weight of methyltrimethoxysilane. The other steps remain unchanged.

[0079] Comparative Example 1

[0080] The difference between Comparative Example 1 and Example 1 is that in Example 1, the adhesion promoter is replaced by 3-aminopropyltrimethoxysilane in an equal weight portion, and the remaining steps remain unchanged.

[0081] Comparative Example 2

[0082] The difference between Comparative Example 2 and Example 1 is that in Example 1, the adhesion promoter is replaced by 1.5 parts of 3-aminopropyltrimethoxysilane, and the remaining steps remain unchanged.

[0083] Performance Testing

[0084] Stability of waterborne coating compositions: The waterborne coating compositions of Examples 1-7 and Comparative Examples 1-2 were sealed and placed at 50° C. for 3 months to observe whether any abnormalities such as precipitation, stratification, and oil separation occurred.

[0085] The aqueous coating compositions of Examples 1-7 and Comparative Examples 1-2 were respectively mixed with Bayhydur 305 curing agent in a weight ratio of 10:1 and stirred at a stirring speed of 1000 rpm for 5 min to obtain a waterborne polyurethane coating. The coating was applied to a clean 202 stainless steel surface (the stainless steel was degreased and not polished) and cured at room temperature for 24 hours to form a film with an average thickness of 30 μm.

[0086] Film adhesion test: Tested according to ISO2409 cross-cut test for paints and varnishes, with grade 0 being the best and grade 5 being the worst.

[0087] Water immersion test: Soak the film-coated stainless steel in water at room temperature for 72 hours. Remove the film and wipe away any moisture. Leave the film at room temperature for 24 hours. Test the film adhesion using the above-mentioned film adhesion test method.

[0088] Thermal Shock Test: Subject the film-coated stainless steel to 10 cycles of thermal shock. One thermal shock test consists of two hours at -20°C and four hours at room temperature. Test the film adhesion using the same method as above.

[0089] The results are shown in Table 1 below. The adhesion test results of the film layer of Example 4 and the test results after water immersion are shown in Table 1 below. Figure 1 and 2 shown.

[0090] Table 1

[0091] stability Adhesion / Grade Water immersion / level Thermal shock / level Example 1 normal 4 4 4 Example 2 normal 3-4 3 3 Example 3 normal 5 4 5 Example 4 normal 4-5 4 4 Example 5 normal 4-5 4 4-5 Example 6 normal 5 5 5 Example 7 normal 5 4-5 4 Comparative Example 1 A small amount of precipitation / / / Comparative Example 2 normal 4 1 2

[0092] It can be seen from the data results in Table 1 that the water-based coating composition of the present application has high adhesion after curing on the stainless steel surface, and the adhesion performance is stable.

[0093] This specific embodiment is merely an explanation of the present application and is not a limitation of the present application. After reading this specification, those skilled in the art may make non-creative modifications to the present embodiment as needed, but as long as they are within the scope of the claims of the present application, they are protected by the patent law.

Claims

1. A water-based coating composition suitable for a metal substrate with a smooth surface, characterized in that: The raw material components include, by weight: 100 parts of polyurethane dispersion, 3-10 parts of film-forming aid, 0.5-1.5 parts of wetting agent, 0.5-1 part of leveling agent, 0.5-1 part of defoaming agent, 0.5-2 parts of thixotropic agent, 0-100 parts of color paste and 1-10 parts of adhesion promoter; The preparation steps of the adhesion aid include: S1, reacting a polyisocyanate monomer and polyethylene glycol at a molar ratio of isocyanate group to hydroxyl group of 1:0.5-0.9 to obtain a polyurethane prepolymer; S2, a first amino-terminated polysiloxane and an epoxy silane coupling agent are subjected to a ring-opening reaction at a primary amino group to epoxy group molar ratio of 1:1-2 to obtain a modified polysiloxane; S3. The polyurethane prepolymer described in step S1 and the modified polysiloxane described in step S2 are reacted at a molar ratio of isocyanate to secondary amino group of 1:1-2 to obtain the adhesion aid.

2. The water-based coating composition suitable for a metal substrate with a smooth surface according to claim 1, characterized in that: The polyisocyanate monomer in step S1 is selected from any one of TDI, HDI, MDI, IPDI and HMDI, or a combination of any two or more.

3. The water-based coating composition suitable for a metal substrate with a smooth surface according to claim 1, characterized in that: The average molecular weight of the polyethylene glycol in step S1 is 200-2000.

4. The aqueous coating composition suitable for a metal substrate with a smooth surface according to claim 1, characterized in that: The general formula of the first amino-terminated polysiloxane in step S2 is R 1 SiOMe2(SiOMeR 2 ) m SiMe2R 3 , where R 1 is a C1-C4 alkyl group, a hydroxyl group or a primary amino group substituted C3-C6 alkyl group, R 2 is selected from C1-C4 alkyl or C6-C12 aromatic groups, R 3 It is a primary amino-substituted C3-C6 alkyl group, Me is a methyl group, and m=10-100.

5. The water-based coating composition suitable for a metal substrate with a smooth surface according to claim 1, characterized in that: The epoxysilane coupling agent in step S2 is selected from any one of γ-(2,3-epoxypropoxy)propyltrimethoxysilane, γ-(2,3-epoxypropoxy)propyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltriethoxysilane and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, or a combination of any two or more thereof.

6. The water-based coating composition suitable for a metal substrate with a smooth surface according to claim 1, characterized in that: The modified polysiloxane in step S2 is obtained by subjecting a second amino-terminated polysiloxane and a vinyl silane coupling agent to a Michael addition reaction at a primary amino group to vinyl group molar ratio of 1:1-3; The general formula of the second amino-terminated polysiloxane is R 4 SiOMe2(SiOMeR 5 ) n SiMe2R 6 , where R 4 is a C1-C4 alkyl group, a hydroxyl group or a primary amino group substituted C3-C6 alkyl group, R 5 is selected from C1-C4 alkyl or C6-C12 aromatic groups, R 6 It is a primary amino-substituted C3-C6 alkyl group, Me is a methyl group, and n=10-100.

7. The aqueous coating composition suitable for a metal substrate with a smooth surface according to claim 6, characterized in that: The vinyl silane coupling agent is selected from any one of vinyl trimethoxy silane, vinyl triethoxy silane and vinyl methyl dimethoxy silane, or a combination of any two or more thereof.

8. The aqueous coating composition suitable for a metal substrate with a smooth surface according to any one of claims 1 to 7, characterized in that: The polyurethane dispersion is selected from any one of a polyurethane aqueous solution, a polyurethane microemulsion and a polyurethane emulsion.

9. The aqueous coating composition suitable for a metal substrate with a smooth surface according to any one of claims 1 to 7, characterized in that: The film-forming aid is selected from any one of alcohol ester film-forming aids, alcohol ether film-forming aids, alcohol ether ester film-forming aids and alcohol film-forming aids, or a combination of any two or more.

10. The water-based coating composition suitable for a metal substrate with a smooth surface according to any one of claims 1 to 7, characterized in that: The raw material components further include: one or a combination of two or more of 0.5-2 parts of a silane coupling agent, 0.1-1 parts of a preservative and 3-20 parts of water.

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