Ultraviolet-resistant modified waterborne polyurethane coating and preparation method thereof

Abstract

The invention discloses an ultraviolet-resistant modified waterborne polyurethane coating and a preparation method thereof, and belongs to the technical field of coating preparation. Mixing and dispersing nano titanium dioxide, a composite dispersant and a buffer solution to obtain a mixed emulsion; mixing and reacting the mixed emulsion and modified levodopa to obtain a composite dispersion liquid; mixing and reacting the composite dispersion liquid, polyurethane emulsion, diacetone acrylamide and an azo catalyst to obtain modified polyurethane emulsion; and mixing the modified polyurethane emulsion and adipic dihydrazide to obtain the modified waterborne polyurethane coating. A protective coating obtained after curing of the modified waterborne polyurethane coating prepared by the preparation method has good ultraviolet resistance, adhesion and self-cleaning performance.

Description

Technical Field

[0001] The invention belongs to the technical field of coating preparation, and particularly relates to an ultraviolet-resistant modified waterborne polyurethane coating and a preparation method thereof. Background Art

[0002] Polymer coatings are polymer-based coatings widely used in various fields, including architecture, automotive, industrial equipment, and furniture. Polymer coatings occupy a significant position in the modern coatings market and can be categorized into various types based on the polymer type, including acrylic coatings, polyurethane coatings, epoxy coatings, and fluorocarbon coatings. Acrylic coatings, based on acrylic resins, offer excellent weather resistance, adhesion, and transparency, and are primarily used in building exteriors, furniture, and automotive applications. Their advantages include fast drying and ease of application, but they are relatively weak in chemical resistance and are susceptible to corrosion by certain solvents. Polyurethane coatings, based on polyurethane resins, offer excellent abrasion resistance and are widely used in floor coatings, automotive coatings, and wood coatings. However, their disadvantages include long curing times and poor weather resistance. Epoxy coatings, based on epoxy resins, offer excellent adhesion and chemical resistance and are widely used in industrial equipment, marine corrosion protection, and floor coatings. However, their disadvantages include poor weather resistance and yellowing and loss of gloss with prolonged exposure to sunlight. Fluorocarbon coatings are based on fluoropolymers and have excellent weather resistance and stain resistance. They are suitable for exterior wall painting of high-end buildings. Although fluorocarbon coatings have excellent performance, they are expensive and difficult to construct.

[0003] To improve the environmental friendliness of coatings, water-based coatings, which use water as a solvent, are gaining market favor due to their lower volatile organic compound (VOC) content compared to organic solvent-based coatings. Water-based polyurethane coatings are increasingly being used in various fields due to their environmental friendliness, high abrasion resistance, and chemical resistance. However, water-based polyurethane coatings are susceptible to aging and degradation due to exposure to light and heat, especially ultraviolet light, which can damage their protective properties.

[0004] Patent CN109897520A discloses a water-based acrylic polyurethane coating for modified vehicles and its preparation method. The invention improves the acid resistance, alkali resistance and UV resistance of the water-based acrylic polyurethane coating by adding titanium dioxide-modified nanographene to the ingredients and through the synergistic effect between nano-titanium dioxide and nano-graphene.

[0005] Patent CN118374220A discloses a high-adhesion, weather-resistant waterborne polyurethane coating and a preparation method thereof. The invention uses hydrophilic MDT silicone resin to modify polyester polyol. Through the branching of T-segments and the use of hydrophilic polyether end-capping, it contains a large number of branched and end-capped hydrophilic polyether chains, which improves the water solubility of the silicone resin and its compatibility with polyether polyols and polyisocyanate curing agents. Subsequently, through further copolymerization and curing reactions, the polysiloxane and polyurethane are chemically connected, achieving effective modification of the waterborne polyurethane system by polysiloxane, thereby making the prepared waterborne polyurethane coating have good adhesion, strength and weather resistance.

[0006] Patent CN104727150A discloses a waterproof, breathable, antibacterial, UV-resistant water-based polyurethane material and its preparation method. The invention prepares a silver-loaded nano-titanium dioxide and then mixes it with a waterproof, breathable polyurethane resin to obtain a water-based polyurethane material with good UV resistance.

[0007] Titanium dioxide is an inorganic oxide with excellent UV resistance, which it achieves through scattering and absorption. However, due to its small particle size and large specific surface area, titanium dioxide is prone to agglomeration. Agglomeration easily forms stress concentration points in the cured coating protective layer, reducing the coating's mechanical properties and causing cracking and shedding. Therefore, research on modifying titanium dioxide to prepare waterborne polyurethane coatings with excellent UV resistance is of great significance for improving the coating's performance and lifespan in outdoor applications. Summary of the Invention

[0008] In response to the shortcomings of the prior art, the present invention comprises a polyurethane emulsion, a composite dispersion, and diacetone acrylamide, which are mixed to produce a modified polyurethane emulsion. The modified polyurethane emulsion and adipic acid adipic hydrazide are then mixed to form a modified waterborne polyurethane coating, thereby resolving the technical problems identified in the prior art. Specifically, the present invention comprises the following technical solutions:

[0009] One of the purposes of the present invention is to provide a method for preparing a UV-resistant modified waterborne polyurethane coating, the preparation method comprising the following steps:

[0010] Nano-titanium dioxide, a composite dispersant, and a buffer solution are mixed and dispersed in a weight ratio of 3-5:0.5-0.6:80-90 to obtain a mixed emulsion;

[0011] The mixed emulsion and the modified levodopa are mixed in a weight ratio of 500-600:5-7, and reacted at 30° C. for 8-10 hours to obtain a composite dispersion;

[0012] The composite dispersion, polyurethane emulsion, diacetone acrylamide and azo catalyst are mixed in a weight ratio of 15-17:20-30:2-4:0.1-0.2, heated to 60-70° C., and reacted for 4-6 hours to obtain a modified polyurethane emulsion;

[0013] The modified polyurethane emulsion and adipic acid dihydrazide are mixed to obtain a modified waterborne polyurethane coating.

[0014] Furthermore, the particle size of the nano titanium dioxide is 20nm to 30nm.

[0015] Furthermore, the composite dispersant is composed of sodium dodecylbenzenesulfonate and Triton X-100 in a weight ratio of 1:2.

[0016] Furthermore, the buffer solution includes a Tris-hydrochloric acid buffer solution with a pH of 8.5 to 9.0.

[0017] Furthermore, the preparation method of the modified levodopa comprises the following steps:

[0018] Levodopa, an enol compound, EDC hydrochloride and 4-dimethylaminopyridine are mixed in a molar ratio of 1:2-3:1.5:0.4, and reacted at 25° C. for 24 h to 30 h to obtain modified levodopa.

[0019] Furthermore, the enol compound includes 3-butene-1-ol. The function of the enol compound is to introduce the nano-titanium dioxide wrapped by the modified levodopa through oxidative self-polymerization into the polyurethane structure in a polymerization grafting manner.

[0020] Furthermore, the preparation method of the polyurethane emulsion comprises the following steps:

[0021] Diisocyanate, mixed diol, chain extender and dibutyltin dilaurate are mixed in a weight ratio of 1:2.3-2.5:0.4-0.6:0.007, heated to 70°C-80°C and reacted for 2h-3h to obtain a polyurethane prepolymer mixture;

[0022] The polyurethane prepolymer mixture and hydroxy polydimethylsiloxane are mixed in a weight ratio of 1:0.1-0.2 and reacted at 70-80°C for 1h-1.5h, then cooled to 40°C, triethylamine is added and reacted for 25-30min, and finally deionized water is added and stirred to obtain a polyurethane emulsion.

[0023] Furthermore, the diisocyanate includes p-phenylene diisocyanate or toluene diisocyanate.

[0024] Furthermore, the mixed diol is composed of 1,6-hexanediol and 1,4-butenediol in a weight ratio of 1:1. The function of the 1,4-butenediol in the mixed diol is to graft the nano-titanium dioxide and diacetone acrylamide wrapped by the modified levodopa oxidative self-polymerization in the composite dispersion into the polyurethane through a polymerization reaction of carbon-carbon double bonds.

[0025] Furthermore, the chain extender includes dimethylolpropionic acid.

[0026] Furthermore, the usage amount of the triethylamine is 0.8 to 1 times the weight of the chain extender.

[0027] Furthermore, the usage amount of the deionized water is 8 to 9 times the weight of the polyurethane prepolymer mixture.

[0028] Furthermore, the azo catalyst includes azobisisobutyronitrile.

[0029] Furthermore, the usage amount of the adipic acid dihydrazide is 0.6 to 0.8 times the weight of the diacetone acrylamide.

[0030] A second object of the present invention is to provide a modified waterborne polyurethane coating prepared by a method for preparing a UV-resistant modified waterborne polyurethane coating.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] The invention firstly treats nano-titanium dioxide with a composite dispersant and then disperses it ultrasonically to obtain a mixed emulsion. Then, levodopa is modified, and an enol compound 3-butene-1-ol is esterified and connected with a carboxyl group on levodopa via an alcoholic hydroxyl group, thereby introducing a carbon-carbon double bond into the levodopa structure to obtain modified levodopa. The modified levodopa and the mixed emulsion are stirred and reacted under weak alkaline conditions. The modified levodopa is oxidatively self-polymerized to form a polylevodopa structure, thereby wrapping the nano-titanium dioxide in the mixed emulsion and weakening the agglomeration of the nano-titanium dioxide. Thus, a composite dispersion is obtained. Then, diisocyanate, mixed diol and chain extender are used as raw materials for preparing polyurethane, and a polyurethane prepolymer mixture is reacted under the catalysis of dibutyltin dilaurate, and then a capping reaction is carried out by hydroxyl-terminated polydimethylsiloxane. The hydroxyl-terminated polydimethylsiloxane has low surface energy, and cooperates with the photocatalytic antifouling performance of nano-titanium dioxide to improve the self-cleaning performance of the protective layer after the coating is cured. After neutralization with triethylamine, water is added and dispersed with high-speed stirring to obtain a polyurethane emulsion. After the polyurethane emulsion, composite dispersion and diacetone acrylamide are mixed, Under catalysis, nano-titanium dioxide and diacetone acrylamide wrapped by modified levodopa oxidative self-polymerization in a composite dispersion are grafted onto polyurethane containing carbon-carbon double bonds through a polymerization reaction of carbon-carbon double bonds to obtain a modified polyurethane emulsion; the modified polyurethane emulsion and adipic acid dihydrazide are mixed to form a modified waterborne polyurethane coating, and the adhesion performance of the modified waterborne polyurethane coating is improved through the synergistic coordination of polylevodopa and ketone hydrazide cross-linking. The prepared modified waterborne polyurethane coating not only has good UV resistance, but also has good mechanical properties and self-cleaning functions in the protective coating after curing. DETAILED DESCRIPTION

[0033] The following will clearly and completely describe the technical solutions of the present invention through the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making any creative efforts shall fall within the scope of protection of the present invention.

[0034] Unless otherwise specified, the raw materials and reagents used in the present invention are commercially available or can be prepared by known methods.

[0035] Hydroxypolydimethylsiloxane was purchased from Jiangsu Kexing New Materials Co., Ltd.

[0036] Preparation Example 1:

[0037] The preparation method of modified levodopa specifically includes the following steps:

[0038] Using an electronic balance, 0.1 mol of levodopa, 0.2 mol of 3-butene-1-ol, 0.15 mol of EDC hydrochloride, and 0.04 mol of 4-dimethylaminopyridine were weighed and added to a flask. 150 mL of anhydrous acetone was then added. The flask was then placed in an ultrasonic cleaner and ultrasonically dispersed at 200 W until completely dispersed. Nitrogen was then introduced as a protective gas and the flask was placed at 25°C with stirring at 400 rpm for 24 hours. After the reaction, the mixture was evaporated under reduced pressure and extracted with ethyl acetate to obtain the organic phase. The organic phase was repeatedly rinsed with deionized water until the rinse water was neutral. The ethyl acetate was then removed by rotary evaporation and dried at low temperature to obtain the modified levodopa.

[0039] Preparation Example 2:

[0040] The preparation method of modified levodopa specifically includes the following steps:

[0041] Using an electronic balance, 0.1 mol of levodopa, 0.25 mol of 3-butene-1-ol, 0.15 mol of EDC hydrochloride, and 0.04 mol of 4-dimethylaminopyridine were weighed and added to a flask. 150 mL of anhydrous acetone was then added. The flask was then placed in an ultrasonic cleaner and ultrasonically dispersed at 200 W until completely dispersed. Nitrogen was then introduced as a protective gas and the flask was placed at 25°C with stirring at 400 rpm for 28 hours. After the reaction, the mixture was evaporated under reduced pressure and extracted with ethyl acetate to obtain the organic phase. The organic phase was repeatedly rinsed with deionized water until the rinse water was neutral. The ethyl acetate was then removed by rotary evaporation and dried at low temperature to obtain the modified levodopa.

[0042] Preparation Example 3:

[0043] The preparation method of modified levodopa specifically includes the following steps:

[0044] Using an electronic balance, 0.1 mol of levodopa, 0.3 mol of 3-butene-1-ol, 0.15 mol of EDC hydrochloride, and 0.04 mol of 4-dimethylaminopyridine were weighed and added to a flask. 200 mL of anhydrous acetone was then added. The flask was then placed in an ultrasonic cleaner and ultrasonically dispersed at 200 W until completely dispersed. Nitrogen was then introduced as a protective gas and the flask was placed at 25°C with stirring at 400 rpm for 30 hours. After the reaction, the mixture was evaporated under reduced pressure and extracted with ethyl acetate to obtain the organic phase. The organic phase was repeatedly rinsed with deionized water until the rinse water was neutral. The ethyl acetate was then removed by rotary evaporation and dried at low temperature to obtain the modified levodopa.

[0045] Preparation Example 4:

[0046] The preparation method of polyurethane emulsion specifically includes the following steps:

[0047] 10 parts by weight of p-phenylene diisocyanate, 23 parts by weight of a mixed diol (1,6-hexanediol and 1,4-butenediol in a 1:1 weight ratio), and 4 parts by weight of dimethylolpropionic acid (DMPA) as a chain extender were added to a reactor. 0.07 parts by weight of dibutyltin dilaurate were then added. The reactor was heated to 70°C and allowed to react for 2 hours. After the reaction, the resulting polyurethane prepolymer mixture was incubated. Hydroxypolydimethylsiloxane (0.1 times the weight of the polyurethane prepolymer mixture) was added as a capping agent and the reaction continued at 70°C for 1 hour. After the reaction, the reactor temperature was lowered to 40°C. Triethylamine (0.8 times the weight of the DMPA) was added as a salt-forming agent and neutralized for 25 minutes. After the neutralization reaction, deionized water (8 times the weight of the polyurethane prepolymer mixture) was added, and the mixture was emulsified at a high stirring speed of 5000 rpm for 30 minutes to produce a polyurethane emulsion.

[0048] Preparation Example 5:

[0049] The preparation method of polyurethane emulsion specifically includes the following steps:

[0050] 10 parts by weight of p-phenylene diisocyanate, 24 parts by weight of a mixed diol (1,6-hexanediol and 1,4-butenediol in a 1:1 weight ratio), and 5 parts by weight of dimethylolpropionic acid (DMPA) as a chain extender were added to a reactor. 0.07 parts by weight of dibutyltin dilaurate were then added. The reactor was heated to 75°C and allowed to react for 2.5 hours. After the reaction, the resulting polyurethane prepolymer mixture was incubated. Hydroxypolydimethylsiloxane (0.15 times the weight of the polyurethane prepolymer mixture) was added as a capping agent and the reaction continued at 75°C for 1 hour. After the reaction, the reactor temperature was lowered to 40°C, and triethylamine (0.9 times the weight of the DMPA) was added as a salt-forming agent to neutralize the mixture for 28 minutes. After the neutralization reaction, deionized water (9 times the weight of the polyurethane prepolymer mixture) was added, and the mixture was emulsified at a high stirring speed of 5000 rpm for 30 minutes to produce a polyurethane emulsion.

[0051] Preparation Example 6:

[0052] The preparation method of polyurethane emulsion specifically includes the following steps:

[0053] 10 parts by weight of toluene diisocyanate, 25 parts by weight of a mixed diol (1,6-hexanediol and 1,4-butenediol in a 1:1 weight ratio), and 6 parts by weight of dimethylolpropionic acid (DMPA) as a chain extender were added to a reactor. 0.07 parts by weight of dibutyltin dilaurate were then added. The reactor was heated to 80°C and allowed to react for 3 hours. After the reaction, the resulting polyurethane prepolymer mixture was incubated. Hydroxypolydimethylsiloxane (0.2 times the weight of the polyurethane prepolymer mixture) was added as a capping agent and the reaction continued at 80°C for 1.5 hours. After the reaction, the reactor temperature was lowered to 40°C. Triethylamine (1 times the weight of the DMPA) was added as a salt-forming agent to neutralize the mixture for 30 minutes. After the neutralization reaction, deionized water (9 times the weight of the polyurethane prepolymer mixture) was added, and the mixture was emulsified at a high stirring speed of 5000 rpm for 30 minutes to produce a polyurethane emulsion.

[0054] Preparation Example 7:

[0055] The preparation method of polyurethane emulsion specifically includes the following steps:

[0056] The 1,4-butenediol in the mixed diol in Preparation Example 6 was replaced with 1,4-butanediol, and the rest of the preparation process was consistent with Preparation Example 6.

[0057] Preparation Example 8:

[0058] The preparation method of polyurethane emulsion specifically includes the following steps:

[0059] The toluene diisocyanate in Preparation Example 6 was replaced by hexamethylene diisocyanate, and the rest of the preparation process was consistent with Preparation Example 6.

[0060] Preparation Example 9:

[0061] The preparation method of polyurethane emulsion specifically includes the following steps:

[0062] The amount of toluene diisocyanate in Preparation Example 6 was increased from 10 parts by weight to 20 parts by weight, and the rest of the preparation process was the same as that in Preparation Example 6.

[0063] Preparation Example 10:

[0064] The preparation method of polyurethane emulsion specifically includes the following steps:

[0065] The hydroxypolydimethylsiloxane in Preparation Example 6 was replaced by phenol, and the rest of the preparation process was consistent with Preparation Example 6.

[0066] Preparation Example 11:

[0067] The preparation method of polyurethane emulsion specifically includes the following steps:

[0068] The amount of hydroxy polydimethylsiloxane in the polyurethane prepolymer mixture in Preparation Example 6 was increased from 0.2 times the weight to 1 times, and the rest of the preparation process was consistent with Preparation Example 6.

[0069] Example 1:

[0070] A method for preparing a UV-resistant modified waterborne polyurethane coating comprises the following steps:

[0071] 30 parts by weight of nano-titanium dioxide with a particle size of 20 nm, 5 parts by weight of a composite dispersant (composed of sodium dodecylbenzenesulfonate and Triton X-100 in a weight ratio of 1:2), and 800 parts by weight of a Tris-hydrochloric acid buffer solution with a pH of 8.5 were added to a container, and then subjected to an ultrasonic mixing and dispersion treatment at a power of 300 W for 15 minutes to obtain a mixed emulsion;

[0072] Subsequently, 500 parts by weight of the mixed emulsion and 5 parts by weight of the modified levodopa obtained in Preparation Example 1 were weighed and mixed, and then placed in a water bath at 30° C. and stirred at 400 r / min for 8 hours to obtain a composite dispersion. An appropriate amount of the composite dispersion was measured using a laser particle size analyzer, and the average particle size of the particles encapsulated in the composite dispersion was 265.2 nm. The surface tension of the composite dispersion was measured using a surface tension meter, and the result was 32.1 mN / m, indicating no demulsification.

[0073] 15 parts by weight of the composite dispersion, 20 parts by weight of the polyurethane emulsion obtained in Preparation Example 4, 2 parts by weight of diacetone acrylamide, and 0.1 parts by weight of azobisisobutyronitrile were added to a reactor. Nitrogen was then introduced as a protective gas to replace the air in the reactor. The reactor was then heated to 60°C and a timed reaction was performed for 4 hours. After cooling to room temperature, adipic acid dihydrazide (0.6 times the weight of diacetone acrylamide) was added to the resulting modified polyurethane emulsion, mixed and stirred, and the pH was adjusted to 7.0 to obtain a modified waterborne polyurethane coating.

[0074] Example 2:

[0075] A method for preparing a UV-resistant modified waterborne polyurethane coating comprises the following steps:

[0076] 40 parts by weight of nano-titanium dioxide with a particle size of 30 nm, 5.5 parts by weight of a composite dispersant (composed of sodium dodecylbenzenesulfonate and Triton X-100 in a weight ratio of 1:2), and 850 parts by weight of a Tris-hydrochloric acid buffer solution with a pH of 8.8 were added to a container, and then subjected to an ultrasonic mixing and dispersion treatment at a power of 300 W for 15 minutes to obtain a mixed emulsion;

[0077] Subsequently, 550 parts by weight of the mixed emulsion and 6 parts by weight of the modified levodopa obtained in Preparation Example 2 were weighed and mixed, and then placed in a 30°C water bath and stirred at 400 r / min for 9 hours to obtain a composite dispersion. An appropriate amount of the composite dispersion was measured using a laser particle size analyzer, and the average particle size of the particles encapsulated in the composite dispersion was 247.8 nm. The surface tension of the composite dispersion was measured using a surface tension meter, and the result was 28.3 mN / m, indicating no demulsification.

[0078] 16 parts by weight of the composite dispersion, 25 parts by weight of the polyurethane emulsion obtained in Preparation Example 5, 3 parts by weight of diacetone acrylamide, and 0.15 parts by weight of azobisisobutyronitrile were added to a reactor. Nitrogen was then introduced as a protective gas to replace the air in the reactor. The reactor was then heated to 65°C and the reaction was timed for 5 hours. After cooling to room temperature, adipic acid dihydrazide (0.7 times the weight of diacetone acrylamide) was added to the resulting modified polyurethane emulsion, the mixture was mixed and stirred, and the pH was adjusted to 7.0 to obtain a modified waterborne polyurethane coating.

[0079] Example 3:

[0080] A method for preparing a UV-resistant modified waterborne polyurethane coating comprises the following steps:

[0081] 50 parts by weight of nano-titanium dioxide with a particle size of 30 nm, 6 parts by weight of a composite dispersant (composed of sodium dodecylbenzenesulfonate and Triton X-100 in a weight ratio of 1:2), and 900 parts by weight of a Tris-hydrochloric acid buffer solution with a pH of 9.0 were added to a container, and then subjected to an ultrasonic mixing and dispersion treatment at a power of 400 W for 20 minutes to obtain a mixed emulsion;

[0082] Subsequently, 600 parts by weight of the mixed emulsion and 7 parts by weight of the modified levodopa obtained in Preparation Example 3 were weighed and mixed, and then placed in a 30°C water bath and stirred at a speed of 500 r / min for 10 hours to obtain a composite dispersion. An appropriate amount of the composite dispersion was measured using a laser particle size analyzer to obtain an average particle size of 211.7 nm for the particles encapsulated in the composite dispersion. The surface tension of the composite dispersion was measured using a surface tension meter to obtain a surface tension of 30.5 mN / m, indicating no demulsification.

[0083] 17 parts by weight of the composite dispersion, 30 parts by weight of the polyurethane emulsion obtained in Preparation Example 6, 4 parts by weight of diacetone acrylamide, and 0.2 parts by weight of azobisisobutyronitrile were added to a reactor. Nitrogen was then introduced as a protective gas to replace the air in the reactor. The reactor was then heated to 70°C and a timed reaction was carried out for 6 hours. After cooling to room temperature, adipic acid dihydrazide (0.8 times the weight of diacetone acrylamide) was added to the resulting modified polyurethane emulsion, mixed and stirred, and the pH was adjusted to 7.0 to obtain a modified waterborne polyurethane coating.

[0084] Comparative Example 1:

[0085] A method for preparing a UV-resistant modified waterborne polyurethane coating comprises the following steps:

[0086] The polyurethane emulsion in Example 3 was replaced by the polyurethane emulsion obtained in Preparation Example 7, and the rest of the preparation process remained the same as in Example 3.

[0087] Comparative Example 2:

[0088] A method for preparing a UV-resistant modified waterborne polyurethane coating comprises the following steps:

[0089] The polyurethane emulsion in Example 3 was replaced by the polyurethane emulsion obtained in Preparation Example 8, and the rest of the preparation process remained the same as in Example 3.

[0090] Comparative Example 3:

[0091] A method for preparing a UV-resistant modified waterborne polyurethane coating comprises the following steps:

[0092] The polyurethane emulsion in Example 3 was replaced by the polyurethane emulsion obtained in Preparation Example 9, and the rest of the preparation process was consistent with that in Example 3.

[0093] Comparative Example 4:

[0094] A method for preparing a UV-resistant modified waterborne polyurethane coating comprises the following steps:

[0095] The polyurethane emulsion in Example 3 was replaced by the polyurethane emulsion obtained in Preparation Example 10, and the rest of the preparation process remained the same as in Example 3.

[0096] Comparative Example 5:

[0097] A method for preparing a UV-resistant modified waterborne polyurethane coating comprises the following steps:

[0098] The polyurethane emulsion in Example 3 was replaced by the polyurethane emulsion obtained in Preparation Example 11, and the rest of the preparation process was consistent with that in Example 3.

[0099] Comparative Example 6:

[0100] A method for preparing a UV-resistant modified waterborne polyurethane coating comprises the following steps:

[0101] The composite dispersant in Example 3 was replaced by sodium dodecylbenzenesulfonate, and the rest of the preparation process was consistent with that of Example 3.

[0102] Comparative Example 7:

[0103] A method for preparing a UV-resistant modified waterborne polyurethane coating comprises the following steps:

[0104] 50 parts by weight of nano-titanium dioxide with a particle size of 100 nm, 6 parts by weight of a composite dispersant (composed of sodium dodecylbenzenesulfonate and Triton X-100 in a weight ratio of 1:2), and 900 parts by weight of a Tris-hydrochloride buffer solution with a pH of 9.0 were added to a container, and then subjected to an ultrasonic mixing and dispersion treatment at a power of 400 W for 20 minutes to obtain a mixed emulsion;

[0105] Subsequently, 600 parts by weight of the mixed emulsion and 7 parts by weight of the modified levodopa obtained in Preparation Example 3 were weighed and mixed, and then placed in a water bath at 30°C and stirred at a speed of 500 r / min for 10 hours to obtain a composite dispersion. It was found that the composite dispersion was demulsified and the preparation failed. The reason was analyzed as follows: it may be that in this system, the use of nano-titanium dioxide with larger particle size for emulsion wrapping is not conducive to the stability of the emulsion system.

[0106] Comparative Example 8:

[0107] A method for preparing a UV-resistant modified waterborne polyurethane coating comprises the following steps:

[0108] 17 parts by weight of the composite dispersion prepared in Example 3, 30 parts by weight of the polyurethane emulsion prepared in Preparation Example 6, and 0.2 parts by weight of azobisisobutyronitrile were added to a reactor. Nitrogen was then introduced as a protective gas to replace the air in the reactor. The reactor was then heated to 70°C and reacted for 6 hours. The mixture was cooled to room temperature to obtain a modified waterborne polyurethane coating.

[0109] The tinplate surface was polished with sandpaper, then rinsed with deionized water to remove impurities. The surface was then dried. The modified waterborne polyurethane coatings prepared in Examples 1-3, Comparative Examples 1-6, and Comparative Example 8 were applied to the tinplate. The coatings were allowed to stand at room temperature until they solidified to form a protective coating. The coatings were then placed at 200°C for 4 hours. After the high-temperature treatment, the coatings were cooled to room temperature and observed for cracking and shedding. The adhesion levels were then tested according to the "GB / T 9286-2021 Cross-cut Test for Paints and Varnishes." The results are shown in Table 1 below.

[0110] Table 1 Adhesion properties

[0111]

[0112]

[0113] The tinplate surface was polished with water sandpaper, and then the impurities generated after polishing were rinsed with deionized water and then dried. The modified waterborne polyurethane coatings prepared in Examples 1 to 3, Comparative Examples 1 to 6 and Comparative Example 8 were applied to the tinplate and allowed to stand at room temperature until they were cured to form a protective coating. Subsequently, the tinplate was irradiated under 365 nm ultraviolet light for 48 h, and the irradiance was controlled to be 0.5 W / m 2 After irradiation, mechanical properties tests were carried out in accordance with GB / T1732-2020 Determination of impact resistance of paint films and GB / T 16421-1996 Test method for small specimens of plastic tensile properties. The results are shown in Table 2 below.

[0114] Table 2 Mechanical properties

[0115]

[0116]

[0117] The tinplate surface was polished with water sandpaper, and then the impurities generated after polishing were rinsed with deionized water and then dried. The modified waterborne polyurethane coatings prepared in Examples 1 to 3, Comparative Examples 1 to 6 and Comparative Example 8 were applied to the tinplate and allowed to stand at room temperature until they were cured to form a protective coating. Subsequently, a methyl red solution with a concentration of 1 g / L was applied to the surface of the protective coating. The irradiance was controlled at 0.5 W / m 2 After standing for 2 days under ultraviolet irradiation, the samples were rinsed with water to observe whether the methyl red color faded. The results are shown in Table 3 below.

[0118] Table 3 Self-cleaning performance

[0119] Sources Is there any methyl red residue on the surface of the protective coating? Example 1 none Example 2 none Example 3 none Comparative Example 1 none Comparative Example 2 have Comparative Example 3 have Comparative Example 4 have Comparative Example 5 have Comparative Example 6 have Comparative Example 8 none

[0120] The following conclusions can be drawn from Tables 1 to 3 above:

[0121] (1) It can be found from Examples 1 to 3 that the protective coating obtained after curing of the modified waterborne polyurethane coating prepared by the present invention has good UV resistance, adhesion and self-cleaning properties.

[0122] (2) It can be found from Comparative Example 1 that the protective coating of the prepared modified waterborne polyurethane coating after curing has poor anti-ultraviolet ability, resulting in poor mechanical properties and poor adhesion performance. This may be because during the preparation of polyurethane, although 1,4-butanediol can react with isocyanate through hydroxyl groups, the prepared polyurethane emulsion does not contain carbon-carbon double bonds. Therefore, the composite dispersion obtained by the self-polymerization of modified levodopa and nano-titanium dioxide and diacetone acrylamide are difficult to be grafted into the polyurethane structure through polymerization reaction, resulting in the nano-titanium dioxide not being effectively introduced into the polyurethane, which in turn leads to poor anti-ultraviolet ability. In addition, since diacetone acrylamide is difficult to be connected to the polyurethane, the ketone hydrazine cross-linking effect is lacking, resulting in poor adhesion.

[0123] (3) It can be found from Comparative Example 2 that the protective coating of the prepared modified waterborne polyurethane coating after curing has poor UV resistance, resulting in poor mechanical properties, and poor adhesion and self-cleaning effects. This may be because during the preparation of polyurethane, hexamethylene diisocyanate does not contain a benzene ring structure with steric hindrance and rigidity compared to toluene diisocyanate, while the carbon chain in the mixed diol is longer and more flexible, without the improvement of the benzene ring. Under the nano-titanium dioxide used in this system, the UV resistance is poor and it is not impact-resistant, which ultimately makes the performance of the protective coating after curing poor.

[0124] (4) Comparative Example 3 shows that the protective coating of the prepared modified waterborne polyurethane coating after curing has poor UV resistance, resulting in poor mechanical properties, and poor adhesion and self-cleaning effects. This may be because, during the polyurethane preparation process, although the rigid benzene ring has the function of regulating the UV resistance and self-cleaning of the coating after curing, in this system, due to excessive use, the content of the rigid benzene ring in the coating is too high, and the coating is brittle after curing, which in turn weakens the performance of the protective coating after curing of the modified waterborne polyurethane coating.

[0125] (5) It can be found from Comparative Example 4 that the self-cleaning effect of the protective coating after curing of the prepared modified water-based polyurethane coating is poor. This may be because when the capping agent is replaced with phenol, phenol does not have low surface energy properties compared to hydroxypolydimethylsiloxane. It only relies on the photocatalytic anti-fouling effect of nano-titanium dioxide, and the self-cleaning effect is poor.

[0126] (6) Comparative Example 5 shows that the protective coating of the prepared modified waterborne polyurethane coating after curing has poor UV resistance, and poor adhesion and self-cleaning effects. This may be because, during the preparation of polyurethane, although hydroxy polydimethylsiloxane can increase the self-cleaning ability of the protective coating after curing through low surface energy, in this system, excessive use may weaken the polymerization grafting of the polyurethane emulsion, the composite dispersion and diacetone acrylamide due to the low surface energy, thereby affecting the ketone hydrazine cross-linking process of the modified polyurethane emulsion and adipic acid dihydrazide in the prepared modified waterborne polyurethane coating, which is not conducive to improving the performance of the protective coating after curing of the modified waterborne polyurethane coating.

[0127] (7) It can be found from Comparative Example 6 that the protective coating of the prepared modified waterborne polyurethane coating after curing has poor UV resistance, and its adhesion performance and self-cleaning effect are also poor. This may be because the composite dispersant plays an important role in the pre-dispersion of nano-titanium dioxide in the early stage. The use of a single emulsifying dispersant may not be able to achieve a good dispersion effect on nano-titanium dioxide in this system, which is not conducive to improving the performance of the protective coating after the modified waterborne polyurethane coating is cured.

[0128] (8) Comparative Example 8 shows that the adhesion performance of the protective coating after curing of the modified waterborne polyurethane coating is poor. This may be because when esterifying levodopa, the introduced structure may have a certain steric hindrance effect on the oxidative autopolymerization of levodopa, resulting in a poor oxidative autopolymerization effect. Relying solely on the adhesion performance of the oxidative autopolymerization of levodopa is not sufficient to support the adhesion protection of the protective coating obtained after curing of the modified waterborne polyurethane coating to the substrate material.

[0129] The embodiments described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention, and such changes and modifications fall within the scope of the present invention as claimed.

Claims

1. A method for preparing a UV-resistant modified waterborne polyurethane coating, characterized in that: The preparation method comprises the following steps: Nano-titanium dioxide, a composite dispersant, and a buffer solution are mixed and dispersed in a weight ratio of 3-5:0.5-0.6:80-90 to obtain a mixed emulsion; The mixed emulsion and the modified levodopa are mixed in a weight ratio of 500-600:5-7, and reacted at 30° C. for 8-10 hours to obtain a composite dispersion; The composite dispersion, polyurethane emulsion, diacetone acrylamide and azo catalyst are mixed in a weight ratio of 15-17:20-30:2-4:0.1-0.2, heated to 60-70° C., and reacted for 4-6 hours to obtain a modified polyurethane emulsion; The modified polyurethane emulsion and adipic acid dihydrazide are mixed to obtain a modified waterborne polyurethane coating.

2. The method for preparing a UV-resistant modified waterborne polyurethane coating according to claim 1, characterized in that: The composite dispersant is composed of sodium dodecylbenzenesulfonate and Triton X-100 in a weight ratio of 1:

2.

3. The method for preparing a UV-resistant modified waterborne polyurethane coating according to claim 1, characterized in that: The preparation method of the modified levodopa comprises the following steps: Levodopa, an enol compound, EDC hydrochloride and 4-dimethylaminopyridine are mixed in a molar ratio of 1:2-3:1.5:0.4, and reacted at 25° C. for 24 h to 30 h to obtain modified levodopa.

4. The method for preparing a UV-resistant modified waterborne polyurethane coating according to claim 3, characterized in that: The enol compound includes 3-butene-1-ol.

5. The method for preparing a UV-resistant modified waterborne polyurethane coating according to claim 1, characterized in that: The preparation method of the polyurethane emulsion comprises the following steps: Diisocyanate, mixed diol, chain extender and dibutyltin dilaurate are mixed in a weight ratio of 1:2.3-2.5:0.4-0.6:0.007, heated to 70°C-80°C and reacted for 2h-3h to obtain a polyurethane prepolymer mixture; The polyurethane prepolymer mixture and hydroxy polydimethylsiloxane are mixed in a weight ratio of 1:0.1-0.2 and reacted at 70-80°C for 1h-1.5h, then cooled to 40°C, triethylamine is added and reacted for 25-30min, and finally deionized water is added and stirred to obtain a polyurethane emulsion.

6. The method for preparing a UV-resistant modified waterborne polyurethane coating according to claim 5, characterized in that: The diisocyanate includes p-phenylene diisocyanate or toluene diisocyanate.

7. The method for preparing a UV-resistant modified waterborne polyurethane coating according to claim 5, characterized in that: The mixed diol consists of 1,6-hexanediol and 1,4-butenediol in a weight ratio of 1:

1.

8. The method for preparing a UV-resistant modified waterborne polyurethane coating according to claim 5, characterized in that: The chain extender includes dimethylolpropionic acid.

9. The method for preparing a UV-resistant modified waterborne polyurethane coating according to claim 1, characterized in that: The usage amount of the adipic acid dihydrazide is 0.6 to 0.8 times the weight of the diacetone acrylamide.

10. A modified waterborne polyurethane coating prepared by the method for preparing a UV-resistant modified waterborne polyurethane coating according to any one of claims 1 to 9.

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