Stain-resistant waterborne polyurethane coating as well as preparation method and application thereof

Abstract

The invention provides a stain-resistant waterborne polyurethane coating as well as a preparation method and application thereof, and belongs to the technical field of chemical materials. The waterborne polyurethane coating is prepared from the following raw materials in parts by weight: 90-110 parts of waterborne polyurethane emulsion and 0.3-1.5 parts of a curing agent, the waterborne polyurethane emulsion is prepared from the following raw materials: dihydric alcohol oligomer, polyisocyanate, a hydrophilic chain extender, a cross-linking agent and a chain extender in a molar ratio of (10-35): (70-100): (15-45): (2-30): (2-30). The invention provides an internal and external double-crosslinking synergistic system, the permeation and adsorption of dye molecules in ink are effectively prevented by balancing the internal and external crosslinking density and reducing the polarity and surface energy of the coating, so that the coating has good stain resistance and adhesive force, and has a wide industrial application prospect.

Description

Technical Field

[0001] This invention belongs to the field of chemical materials technology, specifically relating to a stain-resistant waterborne polyurethane coating, its preparation method, and its applications. Background Technology

[0002] Waterborne polyurethane (WPU) is a high-molecular-weight elastomer with water as the dispersion medium and urethane bonds in its main chain. This material combines the excellent mechanical properties of polyurethane with the environmentally friendly characteristics of a waterborne system. It has advantages such as low volatile organic compound (VOC) emissions, non-toxicity, strong adhesion, and good flexibility, and is widely used in wood coatings, plastic surface coatings, leather finishing, metal protection, and electronic packaging.

[0003] However, traditional waterborne polyurethane coatings generally suffer from poor stain resistance and insufficient graffiti resistance during use. When faced with common oil-based or water-based marker graffiti, the coating surface is easily penetrated by the dye, leaving noticeable marks even after wiping, severely affecting appearance and durability. This is because the WPU system contains numerous polar hydroxyl and carboxyl functional groups, resulting in high surface energy and a tendency to physically adsorb and chemically bond with ink pigment molecules.

[0004] Existing technologies have proposed various modification methods to improve the stain resistance of waterborne polyurethane, such as introducing organosilicon, fluorine monomers, hyperbranched polyethers, highly crosslinked structures, or surface hydrophobic agents. For example, Chinese patent application CN120329844A discloses the introduction of single-terminated dihydroxy polydimethylsiloxane as a low surface energy material into the polyurethane side chain to reduce the surface energy of the coating and the wetting and spreading properties of pollutants. Then, γ-isocyanate-based propyltriethoxysilane is used as a capping agent to prepare solvent-free polyurethane. Then, aziridine crosslinking agent and 3-aminopropyltriethoxysilane crosslinking agent are added to the polyurethane, so that the curing reaction between the polyurethane polar groups and the condensation reaction between -Si-OH generated by the hydrolysis of the capping agent gives the coating a dense crosslinked network, inhibiting the penetration of pollutant molecules. At the same time, the crosslinking reaction between the added crosslinking agent and the polyurethane polar groups reduces the polar components of the coating, weakens the interaction between pollutants and the coating, and makes the coating have only good anti-graffiti and self-cleaning properties.

[0005] However, it only focuses on inhibiting the penetration of pollutants and does not optimize the design for adhesion. At the same time, its antifouling effect mainly relies on the migration of silicone side chains to the coating surface to form a low surface energy layer. Under long-term repeated wiping or local abrasion conditions, the stability of its antifouling performance has a certain degree of uncertainty. Moreover, it uses a solvent-free system and achieves in-situ curing at room temperature. The system has high viscosity and limited leveling process during film formation. The crosslinking reaction and molecular diffusion occur simultaneously, and there is a lack of external heat energy to regulate the curing process. As a result, the crosslinking reaction rate and degree of crosslinking of this system are highly sensitive to environmental humidity, coating thickness and local mixing uniformity. Under thick film or complex substrate conditions, problems such as uneven crosslinking and differences in curing degree between the surface and the interior are prone to occur, which reduces its adhesion.

[0006] Existing technologies for improving the stain resistance of waterborne polyurethane often suffer from drawbacks such as reduced adhesion and loss of flexibility, failing to simultaneously achieve both adhesion and stain resistance. Therefore, a waterborne polyurethane system that combines both adhesion and stain resistance is still needed. Summary of the Invention

[0007] The purpose of this invention is to provide a stain-resistant waterborne polyurethane coating, its preparation method, and its applications.

[0008] This invention provides an aqueous polyurethane coating, which is prepared from raw materials comprising the following parts by weight: 90-110 parts of aqueous polyurethane emulsion and 0.3-1.5 parts of curing agent; wherein the aqueous polyurethane emulsion comprises the following raw materials: a diol oligomer in a molar ratio of 10-35:70-100:15-45:2-30:2-30, a polyisocyanate, a hydrophilic chain extender, a crosslinking agent, and a chain extender.

[0009] Further, the waterborne polyurethane coating is prepared from raw materials comprising the following parts by weight: 100 parts waterborne polyurethane emulsion and 0.9 parts curing agent; wherein the waterborne polyurethane emulsion comprises the following raw materials: diol oligomers in a molar ratio of 25:80:30:15:7.5, polyisocyanate, hydrophilic chain extender, crosslinking agent, and chain extender.

[0010] Furthermore, the waterborne polyurethane coating is prepared from raw materials comprising the following parts by weight: 90-110 parts waterborne polyurethane emulsion, 0.3-1.5 parts curing agent, and 0.02-0.08 parts defoamer.

[0011] Furthermore, the waterborne polyurethane coating is prepared from raw materials comprising the following parts by weight: 100 parts waterborne polyurethane emulsion, 0.9 parts curing agent, and 0.05 parts defoamer.

[0012] Further, the curing agent is an aziridine-based curing agent; the diol oligomer includes at least one of polytetrahydrofuran, polyethylene glycol, polycarbonate, and polycaprolactone; the polyisocyanate includes at least one of toluene diisocyanate, isoflurane diisocyanate, and dicyclohexylmethane-4,4'-diisocyanate; the hydrophilic chain extender includes at least one of dimethylolpropionic acid, dimethylolbutyric acid, sodium ethylenediaminoethanesulfonate, sodium 1,2-propanediol 3-sulfonate, and sodium 1,4-butanediol 2-sulfonate; the crosslinking agent includes at least one of trimethylolpropane, trimethylolethane, glycerol, and pentaerythritol; and the chain extender includes at least one of 1,4-butanediol, ethylene glycol, propylene glycol, and neopentyl glycol.

[0013] Further, the aziridine-based curing agent includes at least one of aziridine, 2-methylaziridine, 1-phenylaziridine, N-hydroxymethylaziridine, trimethylolpropane-tris[3-(2-methylaziridine)propionate], and trimethylolpropane-tris(β-aziridine)propionate.

[0014] This invention also provides a method for preparing an aqueous polyurethane coating, which is prepared by the following method: (1) The diol oligomer, polyisocyanate and catalyst are reacted; then the hydrophilic chain extender and crosslinking agent are added and reacted; then the chain extender and neutralizing agent are added and reacted; finally, water is added to disperse and emulsify to obtain an aqueous polyurethane emulsion; (2) Add the curing agent to the water-based polyurethane emulsion, disperse it evenly, spray it on the surface of the product, and then cure it into a film.

[0015] Furthermore, a defoamer is also added when the curing agent is added to the waterborne polyurethane emulsion.

[0016] Further, the curing temperature is 80~120℃, and the time is 40~80min; the amount of the neutralizing agent is 15~25% of the total molar amount of the diol oligomer, polyisocyanate, hydrophilic chain extender, crosslinking agent, and chain extender; the neutralizing agent includes at least one of triethylamine, N,N-dimethylethanolamine, and triethanolamine; the amount of the catalyst is 0.05~0.2wt% of the sum of the mass of the diol oligomer, polyisocyanate, hydrophilic chain extender, crosslinking agent, and chain extender; the catalyst includes at least one of bismuth neodecanoate, stannous octoate, dibutyltin dilaurate, and zinc 2-ethylhexanoate; the defoamer includes at least one of Zhonglianbang B-112, Zhonglianbang B-04, Zhonglianbang B-104, and Zhonglianbang B-3095.

[0017] Use of the waterborne polyurethane coating according to any one of claims 1 to 6 in elastomer protection, wood coating, plastic surface coating, leather finishing, metal protection or electronic packaging.

[0018] This invention proposes a synergistic system of internal and external dual crosslinking. An internal crosslinking agent regulates the molecular skeleton structure, increasing the internal density of the coating. Simultaneously, an aziridine-based curing agent is introduced as an external crosslinking agent, reacting with the waterborne polyurethane during the film-forming stage to form a highly crosslinked surface network. This allows the waterborne polyurethane coating to achieve higher crosslinking density and a denser chain segment arrangement both on the surface and internally, reducing the coating's polarity and surface energy. This effectively prevents the penetration and adsorption of dye molecules in the ink, exhibiting excellent stain resistance. Furthermore, by balancing the internal and external crosslinking densities, the coating also possesses good adhesion. Experiments demonstrate that the waterborne polyurethane coating prepared with appropriate amounts of trimethylolpropane and aziridine-based curing agents exhibits both excellent stain resistance and adhesion.

[0019] This invention constructs a stable cross-linked network through synergistic internal and external cross-linking. Its anti-marker graffiti performance is independent of surface component migration, exhibiting higher stability and good adhesion, making it more suitable for repeated writing and erasing scenarios. Furthermore, the waterborne polyurethane coating of this invention does not contain organic solvents, has stable raw material sources, and moderate costs, showing good prospects for industrial application.

[0020] Obviously, based on the above description of the present invention, and according to common technical knowledge and conventional methods in the field, various other modifications, substitutions or alterations can be made without departing from the basic technical concept of the present invention.

[0021] The following detailed embodiments further illustrate the above-described content of the present invention. However, this should not be construed as limiting the scope of the present invention to the following embodiments. All technologies implemented based on the above-described content of the present invention fall within the scope of the present invention. Detailed Implementation

[0022] The raw materials and equipment used in this invention are all known products, obtained by purchasing commercially available products.

[0023] In the specific embodiments of the present invention, the polytetrahydrofuran used has a molecular weight of 2000 and a degree of polymerization of 27-28.

[0024] Example 1: Preparation of waterborne polyurethane coating (1) 50g (0.025mol) of polytetrahydrofuran was dehydrated under vacuum at 110℃ for 2h, cooled to 80℃ and added 22.3g (0.085mol) of 4,4'-diisocyanate dicyclohexylmethane and 0.1wt% of neodecanoate bismuth, which is the sum of the masses of all the reaction raw materials in step (1), and reacted for 2h; then 4.44g (0.03mol) of dimethylolbutyric acid and 2g (0.015mol) of trimethylolpropane were added and reacted for 2h. Subsequently, 0.67g (0.0075mol) of 1,4-butanediol and 3.03g (0.03mol) of triethylamine were added at room temperature, and 30mL of tetrahydrofuran was added to reduce viscosity. The reaction was carried out for 15min, and then 300mL of deionized water was added under rapid stirring to disperse and emulsify for 30min. Finally, tetrahydrofuran was evaporated to obtain an aqueous polyurethane emulsion. (2) By weight, 0.05 parts of Zhonglian B-112 (purchased from Guangdong Zhongke Hongtai New Materials Co., Ltd.) and 0.9 parts of trimethylolpropane-tris[3-(2-methylacrylidinyl)propionate] were added to 100 parts of water-based polyurethane emulsion. The mixture was quickly stirred and dispersed evenly. The mixture was then sprayed onto the surface of hydrogenated nitrile rubber and placed in an oven to cure into a film. The curing temperature was 100℃ and the curing time was 60 min.

[0025] Example 2: Preparation of waterborne polyurethane coating Referring to Example 1, the only difference is: 0.3 parts of trimethylolpropane-tris[3-(2-methylacridinyl)propionate].

[0026] Example 3: Preparation of waterborne polyurethane coating Referring to Example 1, the only difference is: 1.5 parts of trimethylolpropane-tris[3-(2-methylacridinyl)propionate].

[0027] Example 4: Preparation of waterborne polyurethane coating Referring to Example 1, the only difference is that: 0.67 g (0.005 mol) of trimethylolpropane and 2.03 g (0.0225 mol) of 1,4-butanediol.

[0028] Example 5: Preparation of waterborne polyurethane coating Referring to Example 1, the only difference is that 2.68 g (0.02 mol) of trimethylolpropane was used and 1,4-butanediol was not added.

[0029] Example 6: Preparation of waterborne polyurethane coating Referring to Example 1, the only difference is that: 0.67 g (0.005 mol) of trimethylolpropane, 2.03 g (0.0225 mol) of 1,4-butanediol, and no addition of trimethylolpropane-tris[3-(2-methylacridinyl)propionate].

[0030] Example 7: Preparation of waterborne polyurethane coating Referring to Example 1, the only difference is that: 2g (0.015 mol) of trimethylolpropane, 0.67g (0.0075 mol) of 1,4-butanediol, and no addition of trimethylolpropane-tris[3-(2-methylacridinyl)propionate].

[0031] Example 8: Preparation of waterborne polyurethane coating Referring to Example 1, the only difference is that: 2.68 g (0.02 mol) of trimethylolpropane was used, butanediol was not added, and trimethylolpropane-tris[3-(2-methylacridinyl)propionate was not added].

[0032] Comparative Example 1: Preparation of Waterborne Polyurethane Coating Referring to Example 1, the only difference is that: 2.7 g (0.03 mol) of 1,4-butanediol was used, trimethylolpropane was not added, and trimethylolpropane-tris[3-(2-methylacridinyl)propionate was not added].

[0033] Comparative Example 2: Preparation of Waterborne Polyurethane Coating Referring to Example 1, the only difference is that 2g (0.015mol) of trimethylolpropane, 0.67g (0.0075mol) of 1,4-butanediol, and trimethylolpropane-tris[3-(2-methylacridinyl)propionate] are replaced with polyisocyanate curing agent DurnateWL72-100 (Asahi Kasei).

[0034] Experimental Example 1: Performance Testing of Waterborne Polyurethane Coatings 1. Testing Method (1) Water contact angle test: The wettability of the sample surface was tested using a contact angle measuring instrument. Before the test, the sample was placed in a room temperature environment to ensure that the sample surface was clean and dry. During the test, a predetermined volume of deionized water droplets was added to the sample surface using a micro-injection device. After the droplets spread naturally on the sample surface and reached a stable state, the droplet contour image was acquired. The contact angle value formed between the droplet and the sample surface was calculated using image analysis software.

[0035] (2) Stain resistance test: Doodle on the coating surface with a marker, then wipe it with alcohol, and calculate the residual area after the doodle is wiped.

[0036] (3) Adhesion test: A cross-cutting tool is used to cut multiple parallel cuts on the coating surface under uniform pressure along the first direction, so that the cuts penetrate the coating to the substrate; then the above operation is repeated along the second direction perpendicular to the first direction to form a regular square cutting area; loose debris in the cutting area is removed; tape is pasted on the surface of the square cutting area and uniform pressure is applied to make it fully adhered, and the tape is pressed with a rubber nail (note that the tape should not be damaged) to remove air bubbles between the tape and the coating, so as to increase the contact area and force between the tape and the test area; after standing for 90±30s, one end of the tape is held by hand and the tape is pulled off in 0.5s~1s at a 60° angle. The test is repeated once, and the paint coating is checked for peeling off with a 5x magnifying glass after the test.

[0037] 2. Experimental Results As shown in Table 1, compared with Example 7, the addition of aziridine curing agents reduced the graffiti residue area from 45% to 35% in Examples 1-3, demonstrating that aziridine curing agents can improve the stain resistance of waterborne polyurethane coatings. Compared with Example 7, the cross-cut adhesion rating of Example 1 increased from 1B to 2B, demonstrating that an appropriate amount of aziridine curing agent can simultaneously improve the stain resistance and adhesion of waterborne polyurethane coatings. Meanwhile, with the increase of aziridine curing agent, the water contact angle of the coating gradually increased, and the hydrophobicity of the coating surface improved, but the graffiti residue area decreased with the addition of more aziridine curing agent. After reaching 35%, the concentration remained unchanged, and the cross-cut adhesion rating first increased to 2B and then decreased to 0B. In Example 4, compared with Example 1, the decrease in the molar ratio of trimethylolpropane and butanediol led to a decrease in the stain resistance and adhesion of the waterborne polyurethane coating. In Example 5, compared with Example 1, the amount of trimethylolpropane was further increased, but butanediol was not added. Although the hydrophobicity and stain resistance were further improved, the adhesion was significantly reduced. The above experimental results prove that only waterborne polyurethane coatings prepared by adding appropriate amounts of trimethylolpropane, butanediol, and aziridine curing agents can simultaneously have good stain resistance and adhesion.

[0038] In Examples 6 and 7, without the addition of aziridine curing agent, the hydrophobicity, stain resistance, and adhesion of the waterborne polyurethane coating were improved with the increase of the molar ratio of trimethylolpropane and butanediol. In Example 8, the amount of trimethylolpropane was further increased, but without the addition of butanediol and aziridine curing agent, the hydrophobicity and stain resistance of the waterborne polyurethane coating were further improved, but the adhesion was no longer improved. This indicates that adding only trimethylolpropane is insufficient to improve the stain resistance and adhesion of the waterborne polyurethane coating.

[0039] Comparative Example 2 used isocyanate curing agent instead of aziridine curing agent. Its reaction byproducts were highly polar, the surface cross-linking was insufficient, and the effect of reducing interfacial energy was limited, resulting in poor hydrophobicity, stain resistance and adhesion of the waterborne polyurethane coating.

[0040] Table 1. Water contact angle, graffiti residue area, and cross-graffiti adhesion level of waterborne polyurethane coatings. In summary, this invention introduces trimethylolpropane and aziridine-based curing agents to form an internal and external double cross-linked structure, allowing the waterborne polyurethane coating to simultaneously achieve higher cross-linking density and denser chain segment arrangement on both the surface and interior. This reduces the polarity and surface energy of the coating, effectively preventing the penetration and adsorption of dye molecules in the ink, exhibiting excellent stain resistance while also possessing good adhesion. Experiments have shown that adding appropriate amounts of trimethylolpropane can simultaneously improve the stain resistance and adhesion of waterborne polyurethane coatings, but the improvement in stain resistance and adhesion is insufficient. Adding aziridine-based curing agents can further enhance the stain resistance and adhesion of waterborne polyurethane coatings. Only waterborne polyurethane coatings prepared with appropriate amounts of trimethylolpropane, butanediol, and aziridine-based curing agents can simultaneously possess good stain resistance and adhesion. In Example 1, the waterborne polyurethane coating prepared with 0.015 mol of trimethylolpropane, 0.0075 mol of butanediol, and 0.9 parts of aziridine-based curing agent exhibited the best performance, with a graffiti residue area of ​​35% and a cross-cut adhesion rating of 2B, demonstrating both the best stain resistance and adhesion.

Claims

1. A waterborne polyurethane coating, characterized in that, It is prepared from raw materials comprising the following parts by weight: 90-110 parts of waterborne polyurethane emulsion and 0.3-1.5 parts of curing agent; wherein the waterborne polyurethane emulsion comprises the following raw materials: diol oligomers, polyisocyanates, hydrophilic chain extenders, crosslinking agents, and chain extenders in a molar ratio of 10-35:70-100:15-45:2-30:2-30.

2. The waterborne polyurethane coating according to claim 1, characterized in that, It is prepared from the following raw materials in parts by weight: 100 parts of waterborne polyurethane emulsion and 0.9 parts of curing agent; the waterborne polyurethane emulsion includes the following raw materials: diol oligomers in a molar ratio of 25:80:30:15:7.5, polyisocyanate, hydrophilic chain extender, crosslinking agent, and chain extender.

3. The waterborne polyurethane coating according to any one of claims 1 to 2, characterized in that, It is made from the following raw materials in parts by weight: 90-110 parts of waterborne polyurethane emulsion, 0.3-1.5 parts of curing agent, and 0.02-0.08 parts of defoamer.

4. The waterborne polyurethane coating according to claim 3, characterized in that, It is made from the following raw materials in parts by weight: 100 parts waterborne polyurethane emulsion, 0.9 parts curing agent, and 0.05 parts defoamer.

5. The waterborne polyurethane coating according to any one of claims 1 to 4, characterized in that, The curing agent is an aziridine-based curing agent; the diol oligomer includes at least one of polytetrahydrofuran, polyethylene glycol, polycarbonate, and polycaprolactone; the polyisocyanate includes at least one of toluene diisocyanate, isoflurane diisocyanate, and dicyclohexylmethane-4,4'-diisocyanate; the hydrophilic chain extender includes at least one of dimethylolpropionic acid, dimethylolbutyric acid, sodium ethylenediaminoethanesulfonate, sodium 1,2-propanediol 3-sulfonate, and sodium 1,4-butanediol 2-sulfonate; the crosslinking agent includes at least one of trimethylolpropane, trimethylolethane, glycerol, and pentaerythritol; and the chain extender includes at least one of 1,4-butanediol, ethylene glycol, propylene glycol, and neopentyl glycol.

6. The waterborne polyurethane coating according to claim 5, characterized in that, The aziridine-based curing agent includes at least one of aziridine, 2-methylaziridine, 1-phenylaziridine, N-hydroxymethylaziridine, trimethylolpropane-tris[3-(2-methylaziridine)propionate], and trimethylolpropane-tris(β-aziridine)propionate.

7. The method for preparing the waterborne polyurethane coating according to any one of claims 1 to 6, characterized in that, Prepared by the following method: (1) The diol oligomer, polyisocyanate and catalyst are reacted; then the hydrophilic chain extender and crosslinking agent are added and reacted; then the chain extender and neutralizing agent are added and reacted; finally, water is added to disperse and emulsify to obtain an aqueous polyurethane emulsion; (2) Add the curing agent to the water-based polyurethane emulsion, disperse it evenly, spray it on the surface of the product, and then cure it into a film.

8. The preparation method according to claim 7, characterized in that, The curing agent is added to the waterborne polyurethane emulsion along with a defoamer.

9. The preparation method according to any one of claims 7-8, characterized in that, The curing temperature is 80~120℃, and the time is 40~80min; the amount of the neutralizing agent is 15~25% of the total molar amount of the diol oligomer, polyisocyanate, hydrophilic chain extender, crosslinking agent, and chain extender; the neutralizing agent includes at least one of triethylamine, N,N-dimethylethanolamine, and triethanolamine; the amount of the catalyst is 0.05~0.2wt% of the sum of the mass of the diol oligomer, polyisocyanate, hydrophilic chain extender, crosslinking agent, and chain extender; the catalyst includes at least one of bismuth neodecanoate, stannous octoate, dibutyltin dilaurate, and zinc 2-ethylhexanoate; the defoamer includes at least one of Zhonglianbang B-112, Zhonglianbang B-04, Zhonglianbang B-104, and Zhonglianbang B-3095.

10. Use of the waterborne polyurethane coating according to any one of claims 1 to 6 in elastomer protection, wood coating, plastic surface coating, leather finishing, metal protection or electronic packaging.

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