Two-component waterborne polyurethane topcoat and method for preparing the same

Abstract

The application discloses a two-component water-based polyurethane finish and a preparation method thereof. The two-component water-based polyurethane finish comprises an A component and a B component. The A component comprises the following components in the following proportions by weight: 40-70 parts of a water-based hydroxyl acrylic acid dispersion, 5-12 parts of an activation period lengthening agent, 1-4 parts of a light stabilizer, 0.2-1 part of a dispersing agent, 0.5-2.5 parts of an anti-bubble assistant and 0.05-0.4 parts of a pH regulator. The B component comprises the following components in the following proportions by weight: 40-70 parts of a polyisocyanate curing agent and 30-60 parts of a solvent. The activation period lengthening agent is a mixture of diacetone alcohol and ethylene glycol butyl ether acetate in a weight ratio of 2:(1-3). The two-component water-based polyurethane finish of the application reduces the reaction speed of the A component and the B component after mixing by using the diacetone alcohol and the ethylene glycol butyl ether acetate in specific amounts and weight ratios, and prolongs the activation period of the two-component water-based polyurethane finish.

Description

Technical Field

[0001] This invention relates to the field of coating technology, and more specifically, to a two-component waterborne polyurethane topcoat and its preparation method. Background Technology

[0002] Polyurethane coatings, developed in the latter half of the 20th century, contain urethane bonds in their structure. Due to their excellent performance in abrasion resistance, oil resistance, corrosion resistance, chemical resistance, heat resistance, water resistance, flexibility, hardness, and other properties, they are widely used for protection in aerospace, automotive, wood, leather, plastics, and metal industries. With increasing environmental awareness and stricter regulations on volatile organic compound (VOC) content in coatings in various countries, higher demands are being placed on polyurethane coatings. Therefore, pollution-free, non-flammable, and non-toxic waterborne polyurethane coatings have attracted attention and become a new research hotspot in the polyurethane coating industry.

[0003] Waterborne polyurethane coatings are coatings based on waterborne polyurethane resin and dispersed in water. They are divided into single-component and two-component waterborne polyurethane coatings. Two-component waterborne polyurethane coatings consist of an isocyanate component containing -NCO and a polyol component containing -OH. The two components are packaged and stored separately and mixed only before application. They cure and crosslink at room temperature, forming a three-dimensional film. While two-component waterborne polyurethane coatings possess good properties such as abrasion resistance, weather resistance, chemical resistance, heat resistance, and water resistance (e.g., the two-component waterborne polyurethane coating prepared using a waterborne hydroxyl acrylic dispersion and an oil-based polyisocyanate curing agent, as described in Chinese patent CN111500177), most of these coatings have a short activation period. This means that the time from mixing the two components to gelation is short, requiring the coating to be used within a short period after mixing. This is detrimental to the actual application of waterborne polyurethane coatings, leading to increased costs and resource waste.

[0004] Therefore, it is of great significance to develop a two-component waterborne polyurethane topcoat with a long activation period. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a two-component waterborne polyurethane topcoat and its preparation method.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] In a first aspect, the present invention provides a two-component waterborne polyurethane topcoat, comprising component A and component B, wherein:

[0008] Component A comprises the following components in parts by weight: 40-70 parts of aqueous hydroxy acrylic dispersion, 5-12 parts of activation period extender, 1-4 parts of light stabilizer, 0.2-1 part of dispersant, 0.5-2.5 parts of anti-bubble agent, and 0.05-0.4 parts of pH adjuster.

[0009] Component B comprises the following components in parts by weight: 40-70 parts of polyisocyanate curing agent and 30-60 parts of solvent;

[0010] The activation period extension agent is diacetone alcohol and ethylene glycol butyl ether acetate in a weight ratio of 2:(1-3).

[0011] Through multiple experiments, this invention has discovered that by using specific amounts and weight ratios of diacetone alcohol and ethylene glycol butyl ether acetate, the activation period of the two-component waterborne polyurethane topcoat after mixing components A and B is extended. This is likely because the special chain structure formed by hydrogen bonds between diacetone alcohol and ethylene glycol butyl ether acetate hinders the reaction between component A (waterborne hydroxyl acrylic dispersion) and component B (polyisocyanate curing agent) through bending and folding, thereby reducing the reaction rate, delaying the gelation time, and extending the activation period of the two-component waterborne polyurethane topcoat. Furthermore, the high boiling point of the diacetone alcohol and ethylene glycol butyl ether acetate mixture makes the effect of diacetone alcohol and ethylene glycol butyl ether acetate in delaying the gelation time of the two-component waterborne polyurethane topcoat more lasting, which is more conducive to extending the activation period of the two-component waterborne polyurethane topcoat.

[0012] In this invention, the activation period refers to the time from the mixing of the two components of the two-component waterborne polyurethane coating to the occurrence of reaction and gelation. The test method is as follows: after mixing components A and B in a certain proportion, add water or other diluents to adjust to a suitable spraying viscosity, then seal it and start timing until the gloss loss of the paint film produced by spraying is 10%, then stop timing. This period of time is recorded as the activation period of the two-component waterborne polyurethane topcoat.

[0013] In addition, under normal circumstances, in order to achieve the requirements of high gloss and high fullness, the topcoat needs to be sprayed after the primer on the substrate has dried. However, the two-component waterborne polyurethane topcoat prepared by this invention can be sprayed directly onto the substrate without primer and can also achieve the requirements of high gloss and high fullness, especially on substrates such as cold-rolled steel plates, aluminum alloys, and electrophoretic plates, as well as on plastic substrates such as ABS (polyacrylonitrile-butadiene-styrene), ABS / PC (polyacrylonitrile-butadiene-styrene / polycarbonate) or PC (polycarbonate).

[0014] As a preferred embodiment of the two-component waterborne polyurethane topcoat of the present invention, the activation period extender in component A is diacetone alcohol and ethylene glycol butyl ether acetate in a weight ratio of 2:(1.5-2.5).

[0015] In a preferred embodiment of the two-component waterborne polyurethane topcoat of the present invention, the activation period extender in component A is 6.5-10 parts by weight.

[0016] As a preferred embodiment of the two-component waterborne polyurethane topcoat of the present invention, the waterborne hydroxyl acrylic dispersion in component A is a secondary dispersion with a solid content of 40-50%, a pH of 7.0-9.0, and a hydroxyl value of 60-100 mgKOH / g.

[0017] The aqueous hydroxyacrylic acid dispersions suitable for the present invention can be synthesized according to methods known in the art or obtained commercially. Commercially available aqueous hydroxyacrylic acid dispersions suitable for the present invention include, but are not limited to, Uacryl 2120 from LianGu New Materials Technology Co., Ltd.

[0018] Currently, to achieve high gloss and high performance in waterborne polyurethane coatings, most use waterborne hydroxyl acrylic dispersions with medium to high hydroxyl values ​​(hydroxyl value ≥ 105 mg KOH / g). This leads to an increase in the amount of polyisocyanate curing agent used, thus increasing the cost of waterborne polyurethane coatings. However, the waterborne hydroxyl acrylic dispersion of this invention has a hydroxyl value of 60-100 mg KOH / g, belonging to the low hydroxyl value category. The two-component waterborne polyurethane topcoat prepared using this dispersion does not require a large amount of polyisocyanate curing agent, yet still achieves high gloss and high performance in the coating film.

[0019] As a preferred embodiment of the two-component waterborne polyurethane topcoat of the present invention, the polyisocyanate curing agent in component B is hydrophobic, and its -NCO mass fraction is 21.5-22.2%, viscosity (23℃) is 2000-4500 mPa·s, and flash point is >120℃.

[0020] The polyisocyanate curing agents suitable for the present invention can be synthesized according to methods known in the art or obtained commercially. Commercially available polyisocyanate curing agents suitable for the present invention include, but are not limited to, Desmodur N 3300 from Covestro Polymers (China) Co., Ltd.

[0021] In a preferred embodiment of the two-component waterborne polyurethane topcoat of the present invention, the solvent in component B contains an aromatic solvent; more preferably, the solvent in component B contains propylene glycol diacetate and an aromatic solvent, specifically, the solvent in component B is propylene glycol diacetate and an aromatic solvent in a weight ratio of 3:(1-3). More preferably, the aromatic solvent is S100 solvent oil; S100 solvent oil is commercially available from ExxonMobil.

[0022] Through multiple experiments, this invention has discovered that specific amounts and weight ratios of propylene glycol diacetate and aromatic solvents can improve the stability of component B in two-component waterborne polyurethane topcoats. This makes component B less prone to can bulging during long-term storage and helps the paint film of the two-component waterborne polyurethane topcoat maintain high gloss. This may be because a stable equilibrium system is formed between the polyisocyanate curing agent, propylene glycol diacetate, and aromatic solvent, making the properties and state of component B less prone to change and more stable.

[0023] As a preferred embodiment of the two-component waterborne polyurethane topcoat of the present invention, the light stabilizer in component A is at least one of an ultraviolet absorber and a hindered amine light stabilizer; more preferably, the light stabilizer is an ultraviolet absorber and a hindered amine light stabilizer in a weight ratio of 1:(1-2).

[0024] Commonly used ultraviolet absorbers in this field can be used in this invention. The ultraviolet absorbers of this invention can be obtained through commercial channels, including but not limited to Tinuvin 400 from BASF in Germany.

[0025] Commonly used hindered amine light stabilizers in this field can be used in this invention. The hindered amine light stabilizers of this invention can be obtained through commercial channels, including but not limited to Tinuvin 123 from BASF in Germany.

[0026] As a preferred embodiment of the two-component waterborne polyurethane topcoat of the present invention, the dispersant in component A is a polymeric dispersant containing acidic groups; more preferably, the dispersant is a polymeric block copolymer containing acidic groups, including but not limited to Tech-598 from Shanghai Tiger Polymer Technology Co., Ltd.

[0027] The anti-foaming additive of this invention is actually a leveling additive with defoaming properties, which can prevent foaming and boiling marks in water-based paints and avoid surface defects such as pinholes and craters. Commonly used anti-foaming additives in the art can be used in this invention. The anti-foaming additive of this invention can be obtained through commercial channels, including but not limited to the low molecular weight polymer BYK-ETOL-AQ from BYK (Germany).

[0028] Commonly used pH adjusters in this field can be used in this invention. For example, the pH adjuster of this invention can be dimethylethanolamine (DMEA), etc.

[0029] In a preferred embodiment of the two-component waterborne polyurethane topcoat of the present invention, component A further includes at least one of a color paste, a substrate wetting agent, a thickener, and water; the color paste is present in a weight fraction of 15-40 parts; the substrate wetting agent is present in a weight fraction of 0.3-1 parts; the thickener is present in a weight fraction of 0.5-1 parts; the water can be any value and / or any range, for example, its value can be 5, 10, 15, 20, 25, 30, 35, 40, etc., and its range can be 5-40 parts, 10-40 parts, 20-40 parts, 20-35 parts, etc. More preferably, the color paste is present in a weight fraction of 20-35 parts.

[0030] Commonly used pigments in the art can be used in this invention. The pigments of this invention can be synthesized according to methods known in the art or obtained through commercial channels, including but not limited to Hostatint WI (water-based resin-free pigment pigment) and Colanyl Black N 131 (water-based black pigment) from Clariant.

[0031] The substrate wetting agent of the present invention can be at least one of modified siloxanes and acetylenic diols, including but not limited to BYK-348 from BYK (Germany). Generally, modified siloxane wetting agents focus on providing excellent static surface tension, while acetylenic diol wetting agents focus on providing excellent dynamic surface tension; it is worth noting that acetylenic diol wetting agents not only provide wettability but also defoaming ability.

[0032] The thickener of the present invention can be at least one of alkali-swellable thickeners and polyurethane thickeners, including but not limited to Dow's alkali-swellable thickener ASE-60 and Hemings Elementis' polyurethane thickener RHEOLATE 299.

[0033] In a preferred embodiment of the two-component waterborne polyurethane topcoat of the present invention, the weight ratio of component A to component B is 10:(1-2).

[0034] Secondly, the present invention provides a method for preparing a two-component waterborne polyurethane topcoat, the method comprising:

[0035] Preparation method of component A:

[0036] S1. Mix the light stabilizer, dispersant and a portion of ethylene glycol butyl ether acetate to obtain a light stabilizer mixed solution;

[0037] S2. Mix the remaining components of component A with the light stabilizer solution to obtain component A;

[0038] Preparation method of component B:

[0039] Component B is obtained by mixing the polyisocyanate curing agent and solvent.

[0040] Through multiple experiments, this invention has found that by first preparing a light stabilizer mixed solution using a dispersant and a portion of ethylene glycol butyl ether acetate, and then mixing it with other components of component A to prepare component A, it is possible to prevent the migration of the poorly water-soluble light stabilizer on the surface of the two-component waterborne polyurethane topcoat, thus avoiding unevenness such as oily substances on the surface of the paint film.

[0041] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0042] The two-component waterborne polyurethane topcoat of the present invention extends the activation period of the mixture of components A and B by using diacetone alcohol and ethylene glycol butyl ether acetate in specific amounts and weight ratios. This may be because the special chain structure formed by diacetone alcohol and ethylene glycol butyl ether acetate through hydrogen bonding hinders the reaction between the waterborne hydroxy acrylic dispersion of component A and the polyisocyanate curing agent of component B through bending and folding, thereby reducing the reaction rate, delaying the gelation time, and extending the activation period of the two-component waterborne polyurethane topcoat. Moreover, the mixture of diacetone alcohol and ethylene glycol butyl ether acetate has a high boiling point, which makes the effect of diacetone alcohol and ethylene glycol butyl ether acetate in delaying the gelation time of the two-component waterborne polyurethane topcoat more lasting and more conducive to extending the activation period of the two-component waterborne polyurethane topcoat. Detailed Implementation

[0043] To better illustrate the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0044] The reagents used in the various embodiments and comparative examples of this invention are as follows:

[0045] Aqueous hydroxy acrylic acid dispersion 1#, LianGu New Materials Technology Co., Ltd., Uacryl 2120, with a solid content of 45±2%, pH of 7.0-9.0, hydroxyl value of 66mgKOH / g, and hydroxyl content of 2.0%;

[0046] Aqueous hydroxy acrylic acid dispersion #2, Covestro (formerly Bayer MaterialScience), Germany, Bayhydrol A 2470, has a solid content of approximately 44-47%, a pH of 7.0-8.0, a hydroxyl value of 129 mg KOH / g, and a hydroxyl content of approximately 3.9%.

[0047] The light stabilizer is a UV absorber and a hindered amine light stabilizer in a weight ratio of 1:1. The UV absorber is Tinuvin 400 from BASF, Germany, and the hindered amine light stabilizer is Tinuvin 123 from BASF, Germany.

[0048] Dispersant, Shanghai Tiger Polymer Technology Co., Ltd., Tech-598;

[0049] Anti-explosion foaming agent, BYK (Germany), BYK-ETOL-AQ;

[0050] The pH adjuster is dimethylethanolamine (DMEA);

[0051] Polyisocyanate curing agent #1, hydrophobic, Covestro (formerly Bayer MaterialScience), Germany, Desmodur N 3300, with a -NCO mass fraction of 21.8±0.3%, viscosity (23℃) of 3000±750 mPa·s, and flash point of approximately 158℃;

[0052] Polyisocyanate curing agent #2, which is hydrophilic, is Bayhydur XP 2655 from Covestro (formerly Bayer MaterialScience), Germany. Its -NCO mass fraction is 20.3-21.3%, viscosity (23℃) is 2500-4500 mPa·s, and flash point is about 192℃.

[0053] The aromatic solvent is S100 solvent oil, ExxonMobil, S100;

[0054] The colorant is a water-based black pigment, Colanyl Black N 131, manufactured by Clariant.

[0055] Substrate wetting agent, BYK (Germany), BYK-348;

[0056] The thickeners are alkali-swellable thickeners and polyurethane thickeners in a weight ratio of 1:1. The alkali-swellable thickener is Dow Chemical Company, ASE-60; the polyurethane thickener is Hemings Elementis Company, RHEOLATE 299.

[0057] Example 1

[0058] This embodiment provides a two-component waterborne polyurethane topcoat, comprising component A and component B, wherein:

[0059] Component A comprises the following components in parts by weight: 60 parts of aqueous hydroxy acrylic dispersion 1#, 8 parts of activation period extender, 1 part of ultraviolet absorber, 1 part of hindered amine light stabilizer, 0.5 parts of dispersant, 1 part of anti-bubble agent, 0.15 parts of pH adjuster, 25 parts of color paste, 0.3 parts of substrate wetting agent, 0.5 parts of thickener, and 20 parts of water;

[0060] Component B comprises the following components in parts by weight: 60 parts of polyisocyanate curing agent 1#, 30 parts of propylene glycol diacetate, and 10 parts of aromatic solvent.

[0061] The activation period extender is a mixture of diacetone alcohol and ethylene glycol butyl ether acetate in a weight ratio of 2:2, with 4 parts diacetone alcohol and 4 parts ethylene glycol butyl ether acetate by weight; and the weight ratio of component A to component B is 10:1.

[0062] A method for preparing a two-component waterborne polyurethane topcoat, the method comprising:

[0063] Preparation method of component A:

[0064] S1. At 600 rpm, the light stabilizer (UV absorber and hindered amine light stabilizer), dispersant and part of ethylene glycol butyl ether acetate are mixed to obtain a light stabilizer mixed solution;

[0065] S2. At 600 rpm, mix the remaining components of component A (aqueous hydroxy acrylic dispersion 1#, anti-bubble agent, pH adjuster, water, diacetone alcohol, and the remaining portion of ethylene glycol butyl ether acetate) with the light stabilizer mixed solution to obtain component A;

[0066] Preparation method of component B:

[0067] Mix polyisocyanate curing agent 1# and solvent evenly to obtain component B.

[0068] Examples 2-5 and Comparative Examples 1-4

[0069] Examples 2-5 and Comparative Examples 1-4 provide different two-component waterborne polyurethane topcoats, differing from Example 1 only in the weight ratio and amount of diacetone alcohol and ethylene glycol butyl ether acetate; all other aspects are consistent with Example 1, as shown in the table below:

[0070] Table 1. Weight ratio of diacetone alcohol and ethylene glycol butyl ether acetate in Examples 1-5 and Comparative Examples 1-4

[0071]

[0072]

[0073] The preparation methods of the two-component waterborne polyurethane topcoats in Examples 2-5 and Comparative Examples 1-4 are the same as those in Example 1.

[0074] Examples 6-9 and Comparative Example 5

[0075] Examples 6-9 and Comparative Example 5 provide different two-component waterborne polyurethane topcoats, differing from Example 1 only in the weight proportion of the activation period extender; all other aspects are identical to Example 1, as shown in the table below:

[0076] Table 2. Parts by weight of activation period extenders in Examples 1, 6-9 and Comparative Example 5

[0077] Parts by weight of activation period extender Example 1 8 Example 6 6.5 Example 7 10 Example 8 5 Example 9 12 Comparative Example 5 25

[0078] The preparation methods of the two-component waterborne polyurethane topcoat in Examples 6-9 and Comparative Example 5 are the same as those in Example 1.

[0079] Example 10

[0080] This embodiment provides a two-component waterborne polyurethane topcoat, comprising component A and component B, wherein:

[0081] Component A comprises the following components in parts by weight: 40 parts of aqueous hydroxy acrylic dispersion 1#, 5 parts of activation period extender, 0.5 parts of ultraviolet absorber, 0.5 parts of hindered amine light stabilizer, 0.2 parts of dispersant, 0.5 parts of anti-bubble agent, 0.05 parts of pH adjuster, 15 parts of color paste, 0.3 parts of substrate wetting agent, 0.5 parts of thickener, and 35 parts of water;

[0082] Component B comprises the following components in parts by weight: 40 parts of polyisocyanate curing agent 1#, 45 parts of propylene glycol diacetate, and 15 parts of aromatic solvent.

[0083] The activation period extender is diacetone alcohol and ethylene glycol butyl ether acetate in a weight ratio of 2:2, with 2.5 parts of diacetone alcohol and 2.5 parts of ethylene glycol butyl ether acetate by weight.

[0084] A method for preparing a two-component waterborne polyurethane topcoat is consistent with that in Example 1.

[0085] Example 11

[0086] This embodiment provides a two-component waterborne polyurethane topcoat, comprising component A and component B, wherein:

[0087] Component A comprises the following components in parts by weight: 70 parts of waterborne hydroxy acrylic dispersion 1#, 12 parts of activation period extender, 2 parts of ultraviolet absorber, 2 parts of hindered amine light stabilizer, 1 part of dispersant, 2.5 parts of anti-bubble agent, 0.4 parts of pH adjuster, 40 parts of color paste, 1 part of substrate wetting agent, 1 part of thickener, and 30 parts of water.

[0088] Component B comprises the following components in parts by weight: 70 parts of polyisocyanate curing agent 1#, 15 parts of propylene glycol diacetate, and 15 parts of aromatic solvent.

[0089] The activation period extender is diacetone alcohol and ethylene glycol butyl ether acetate in a weight ratio of 2:2, with 6 parts diacetone alcohol and 6 parts ethylene glycol butyl ether acetate by weight.

[0090] A method for preparing a two-component waterborne polyurethane topcoat is consistent with that in Example 1.

[0091] Example 12

[0092] This embodiment provides a two-component waterborne polyurethane topcoat, which is consistent with Embodiment 1.

[0093] A method for preparing a two-component waterborne polyurethane topcoat differs from Example 1 in that the addition of the light stabilizer and dispersant is different, as detailed below:

[0094] Preparation method of component A:

[0095] At 600 rpm, waterborne hydroxy acrylic dispersion 1#, anti-bubbling agent, pH adjuster, water, diacetone alcohol, ethylene glycol butyl ether acetate, light stabilizer (UV absorber and hindered amine light stabilizer) and dispersant are mixed to obtain component A.

[0096] Preparation method of component B:

[0097] Mix polyisocyanate curing agent 1# and solvent evenly to obtain component B.

[0098] Example 13

[0099] This embodiment provides a two-component waterborne polyurethane topcoat, which differs from Example 1 in that the solvent in component B is only an aromatic solvent (S100 solvent oil), that is, an aromatic solvent (S100 solvent oil) is used instead of propylene glycol diacetate, which is consistent with Example 1.

[0100] A method for preparing a two-component waterborne polyurethane topcoat is consistent with that in Example 1.

[0101] Comparative Example 6

[0102] This comparative example provides a two-component waterborne polyurethane topcoat, which differs from Example 1 in that isophorone is used instead of diacetone alcohol, while the other two are consistent with Example 1.

[0103] A method for preparing a two-component waterborne polyurethane topcoat is consistent with that in Example 1.

[0104] Comparative Example 7

[0105] This comparative example provides a two-component waterborne polyurethane topcoat, which differs from Example 1 in that propylene glycol ethyl ether acetate is used instead of ethylene glycol butyl ether acetate, while the other two are consistent with Example 1.

[0106] A method for preparing a two-component waterborne polyurethane topcoat is consistent with that in Example 1.

[0107] Comparative Example 8

[0108] This comparative example provides a two-component waterborne polyurethane topcoat, which differs from Example 1 in that the solvent in component B is only propylene glycol diacetate, that is, propylene glycol diacetate is used instead of aromatic solvent (S100 solvent oil), which is consistent with Example 1.

[0109] A method for preparing a two-component waterborne polyurethane topcoat is consistent with that in Example 1.

[0110] Comparative Example 9

[0111] This comparative example provides a two-component waterborne polyurethane topcoat, which differs from Example 1 in that waterborne hydroxy acrylic dispersion 2# (hydroxyl value 129 mg KOH / g) is used instead of waterborne hydroxy acrylic dispersion 1# (hydroxyl value 66 mg KOH / g) in component A, while the other components are consistent with Example 1.

[0112] A method for preparing a two-component waterborne polyurethane topcoat is consistent with that in Example 1.

[0113] Comparative Example 10

[0114] This comparative example provides a two-component waterborne polyurethane topcoat, which differs from Example 1 in that in component A, polyisocyanate curing agent 2# (which is hydrophilic, Bayhydur XP 2655, with a -NCO mass fraction of 20.3-21.3%) is used instead of polyisocyanate curing agent 1# (which is hydrophobic, Desmodur N 3300, with a -NCO mass fraction of 21.8±0.3%). The other components are consistent with those in Example 1.

[0115] A method for preparing a two-component waterborne polyurethane topcoat is consistent with that in Example 1.

[0116] Performance testing

[0117] Performance tests were conducted on the two-component waterborne polyurethane topcoats of each embodiment and comparative example, with the weight ratio of component A to component B being 10:1, as detailed below:

[0118] (1) Method for preparing the template

[0119] Sand the tinplate (150mm*70mm) with 600-grit sandpaper. Then, mix and stir the two-component waterborne polyurethane topcoat components A and B of each example and comparative example evenly, and then spray it onto the tinplate to make the film thickness 40-45μm. Next, flash dry it for 30min at 20-25℃ and 30-60%RH, then dry it at 80℃ for 30min, and finally place it at room temperature for 7 days to obtain the sample.

[0120] (2) Initial appearance test

[0121] A sample of the two-component waterborne polyurethane topcoat was made, and the condition of the paint film on the sample was observed directly by eye to obtain the initial appearance test results.

[0122] (3) Odor test

[0123] Two-component waterborne polyurethane topcoat was made into a sample, placed in an odorless bottle and sealed, and heated at 80°C for 2 hours. Then, five testers evaluated the odor in the bottle, and the average value was taken as the odor test result.

[0124] The odor assessment criteria are divided into six levels: 1, 2, 3, 4, 5, and 6. The higher the number, the higher the level and the stronger the odor of the sample. Specifically, 1 - no odor; 2 - slight odor; 3 - odor, but not irritating; 4 - irritating odor; 5 - strong irritating odor; 6 - unbearable odor.

[0125] (4) Activation period test

[0126] After mixing components A and B at a weight ratio of 10:1, deionized water is added to adjust the viscosity to a suitable spraying viscosity, specifically 35 seconds. The mixture is then sealed, and a timer is started. Timing is stopped when the gloss loss of the sprayed paint film reaches 10%. This period is recorded as the activation period of the two-component waterborne polyurethane topcoat. The formula for calculating gloss loss is as follows:

[0127] Gloss loss (%) = (S0 - S) n ) / S0×100%

[0128] S0 refers to the 60° mirror gloss value of the two-component waterborne polyurethane topcoat obtained by mixing components A and B, adding deionized water to adjust the spray viscosity to 35S, and then spraying immediately. n This refers to the 60° mirror gloss value of the two-component waterborne polyurethane topcoat obtained after spraying for n hours.

[0129] (5) Gloss (60°) test

[0130] Two-component waterborne polyurethane topcoat was made into samples. According to GB / T 9754-2007 standard, the samples of each embodiment and comparative example were measured using a BYK AG-4446 gloss meter. The 60° specular gloss value was used as the judgment standard.

[0131] During testing, a five-point sampling method was used to measure five different parts of the paint film on the sample, and then the average gloss value was calculated.

[0132] (6) Adhesion test

[0133] Two-component waterborne polyurethane topcoat samples were prepared using electrophoresis plates and ABS (polyacrylonitrile-butadiene-styrene) plastic plates instead of tinplate plates. The samples were subjected to a cross-cut adhesion test according to GB / T 9286-2021 and graded on a scale of 0-5. The higher the number, the more severe the paint film peeling and the worse the adhesion of the two-component waterborne polyurethane topcoat. The adhesion tested using ABS (polyacrylonitrile-butadiene-styrene) plastic plates as the substrate is called "ABS adhesion," and the adhesion tested using electrophoresis plates as the substrate is called "electrophoresis plate adhesion."

[0134] (7) Bubble-free film thickness test

[0135] Two-component waterborne polyurethane topcoat components A and B are mixed and stirred evenly, then sprayed onto a tinplate. Seven coats are applied, flash-drying is allowed for 30 minutes, and the coating is then baked in a 60℃ oven for 60 minutes. The film thickness at which prickly heat begins to appear is the bubble-free film thickness. The test environment temperature and humidity are 23℃ and 60%RH.

[0136] (8) Hardness test

[0137] Hardness tests were conducted on two-component waterborne polyurethane topcoats according to GB / T 6739-2006.

[0138] (9) Impact resistance test

[0139] Impact resistance tests were conducted on two-component waterborne polyurethane topcoats according to GB / T 1732-2020.

[0140] (10) Acid resistance test

[0141] Impact resistance tests were conducted on two-component waterborne polyurethane topcoats according to GB / T 9274-1988, using Method A (immersion method), with a 10% sulfuric acid solution as the test liquid and a test duration of 48 hours.

[0142] (11) Alkali resistance test

[0143] Impact resistance tests were conducted on two-component waterborne polyurethane topcoats according to GB / T 9274-1988, using Method A (immersion method), with a 10% sodium hydroxide solution as the test liquid and a test duration of 48 hours.

[0144] (12) Curing agent expansion test

[0145] A curing agent bulging test was conducted on component B of the two-component waterborne polyurethane topcoat: component B of the two-component waterborne polyurethane topcoat was stored at 50°C for 30 days, and then the appearance changes of the iron cans containing component B were observed and evaluated. "No bulging" means that no noticeable expansion appeared on the surface of the iron can; "slight bulging" means that noticeable expansion appeared on the surface of the iron can, affecting the appearance of the can but not affecting continued use; "severe bulging" means that severe expansion appeared on the surface of the can, affecting the appearance of the can and making it unusable.

[0146] (13) Thermal storage stability test

[0147] Unmixed two-component waterborne polyurethane topcoat components A and B were baked at 50℃ for 30 days, cooled to room temperature, and then made into samples. The condition of the paint film on the samples was then observed directly by eye to obtain the thermal stability test results. Thermal stability can be used to demonstrate the migration of light stabilizers on the paint film surface. When uneven phenomena such as oily substances appear on the paint film surface, it indicates that the light stabilizer has migrated to the paint film surface.

[0148] The test results are shown below:

[0149] Table 3 Performance test results of two-component waterborne polyurethane topcoat

[0150]

[0151]

[0152] In the table above, the initial appearance of the paint film of Comparative Example 5 has a large number of particles, and its gloss value (60°) is only 55, so it loses its significance to conduct activation period testing and bubble-free film thickness testing.

[0153] Table 4. Results of Curing Agent Expansion Test for Component B of Two-Component Waterborne Polyurethane Topcoat

[0154] Example 1 Example 10 Example 11 Example 13 Comparative Example 8 Comparative Example 10 Gloss (60°) 97 94 97 82 97 97 Curing agent expansion No bulging can No bulging can No bulging can No bulging can Slight swelling Severe tank bulging

[0155] Table 5. Results of thermal storage stability test for two-component waterborne polyurethane topcoat

[0156] Example 1 Example 10 Example 11 Example 12 Initial appearance smooth smooth smooth smooth thermal storage stability smooth smooth smooth More oily substances

[0157] From Table 3-5, we can see that:

[0158] The two-component waterborne polyurethane topcoat of the present invention extends the activation period of the A and B components after mixing by using diacetone alcohol and ethylene glycol butyl ether acetate in specific amounts and weight ratios.

[0159] The B component of the two-component waterborne polyurethane topcoat of this invention improves its stability by using propylene glycol diacetate and aromatic solvents in specific amounts and weight ratios, making the B component less prone to can bulging during long-term storage and helping the paint film of the two-component waterborne polyurethane topcoat maintain high gloss.

[0160] This invention first uses a dispersant and a portion of ethylene glycol butyl ether acetate to prepare a light stabilizer mixed solution, and then mixes it with other components of component A to prepare component A. This can prevent the migration of light stabilizers with poor water solubility on the surface of the paint film formed by the two-component waterborne polyurethane topcoat, and avoid uneven phenomena such as oily substances on the paint film surface.

[0161] 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 modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A two-component waterborne polyurethane topcoat, characterized in that, It includes component A and component B, wherein: Component A comprises the following components in parts by weight: 40-70 parts of aqueous hydroxy acrylic dispersion, 5-12 parts of activation period extender, 1-4 parts of light stabilizer, 0.2-1 part of dispersant, 0.5-2.5 parts of anti-bubble agent, and 0.05-0.4 parts of pH adjuster. Component B comprises the following components in parts by weight: 40-70 parts of polyisocyanate curing agent and 30-60 parts of solvent; The activation period extension agent is diacetone alcohol and ethylene glycol butyl ether acetate in a weight ratio of 2:(1-3); The solvent in component B contains propylene glycol diacetate and aromatic solvents.

2. The two-component waterborne polyurethane topcoat as described in claim 1, characterized in that, The activation period extender in component A is diacetone alcohol and ethylene glycol butyl ether acetate in a weight ratio of 2:(1.5-2.5).

3. The two-component waterborne polyurethane topcoat as described in claim 1, characterized in that, The activation period extender in component A is 6.5-10 parts by weight.

4. The two-component waterborne polyurethane topcoat as described in claim 1, characterized in that, The aqueous hydroxy acrylic acid dispersion in component A is a secondary dispersion with a solid content of 40-50%, a pH of 7.0-9.0, and a hydroxyl value of 60-100 mg KOH / g.

5. The two-component waterborne polyurethane topcoat as described in claim 1, characterized in that, The polyisocyanate curing agent in component B is hydrophobic, and its -NCO mass fraction is 21.5-22.2%, its viscosity at 23°C is 2000-4500 mPa·s, and its flash point is >120°C.

6. The two-component waterborne polyurethane topcoat as described in claim 1, characterized in that, Component A further includes at least one of color paste, substrate wetting agent, thickener, and water.

7. The two-component waterborne polyurethane topcoat as described in claim 1, characterized in that, The weight ratio of component A to component B is 10:(1-2).

8. A method for preparing the two-component waterborne polyurethane topcoat according to any one of claims 1-7, the method comprising: Preparation method of component A: S1. Mix the light stabilizer, dispersant and a portion of ethylene glycol butyl ether acetate to obtain a light stabilizer mixed solution; S2. Mix the remaining components of component A with the light stabilizer solution to obtain component A; Preparation method of component B: Component B is obtained by mixing the polyisocyanate curing agent and solvent.

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