A waterborne polyurethane emulsion, a preparation method thereof and application thereof in gold plating coating

Abstract

This invention provides an aqueous polyurethane emulsion, its preparation method, and its application in hot stamping coatings. The preparation method of the aqueous polyurethane emulsion includes the following steps: S1, 60-70 parts of side-chain polyether polyester and 30-40 parts of hydroxyl-terminated polybutadiene undergo a vacuum dehydration reaction, followed by the addition of isophorone diisocyanate and a heat-maintaining reaction until the -NCO content reaches 8-10%; S2, the system is cooled and a disulfide chain extender, 1,4-butanediol, and acetone are added for a chain extension reaction until the -NCO content decreases to 3-5%; S3, hydroxyethyl acrylate is added to the system for an end-capping reaction until the -NCO disappears, yielding a prepolymer; S4, under shear conditions, water is added to the prepolymer for dispersion and emulsification, followed by vacuum distillation to obtain the aqueous polyurethane emulsion. This invention, through molecular structure design of aqueous polyurethane, introduces nonionic hydrophilic side chains, disulfide bonds, and hydroxyethyl acrylate, resulting in excellent elongation at break, toughness, water absorption, and wash fastness.

Description

Technical Field

[0001] This invention relates to the field of polymer coatings technology, specifically to an aqueous polyurethane emulsion, its preparation method, and its application in hot stamping coatings. Background Technology

[0002] Hot stamping, as a surface decoration technique, is widely used in industries such as textiles, packaging, and printing. By forming a metallic texture or special effects graphic layer on the substrate surface, it significantly enhances the visual quality and added value of products. However, with increasingly stringent environmental regulations, traditional solvent-based hot stamping materials, containing large amounts of volatile organic compounds, are prone to environmental pollution during production and pose safety hazards, failing to meet the requirements of sustainable development. Therefore, developing environmentally friendly hot stamping materials using water as the dispersion medium has become an inevitable trend.

[0003] Waterborne polyurethane, due to its advantages such as using water instead of organic solvents, safety and non-toxicity, good film-forming properties, and strong adhesion, has become a solvent-based alternative and has broad application prospects in hot stamping coatings. However, when conventional waterborne polyurethane is directly applied to hot stamping coatings, the following problems still exist. First, traditional waterborne polyurethane usually uses carboxylate-type ionic hydrophilic groups, which can achieve water dispersion, but the water resistance after film formation is poor, with a water absorption rate of over 48%, causing the hot stamping pattern to easily fade and blur in humid environments, making it difficult to meet the application requirements of medium and dark wet rubbing fastness ≥4. Second, the mechanical properties of polyurethane alone are difficult to balance toughness and resilience. When used for hot stamping on high-elastic base fabrics, it is easy for the coating to crack or the pattern to deform due to stretching deformation. The tensile permanent deformation rate of existing products usually exceeds 20%. In addition, the hot stamping process has strict requirements for the release performance and slitting performance of the coating. Conventional waterborne polyurethane is prone to defects such as gold flyaway, burrs, and unclear graphics during hot pressing and peeling, making it difficult to balance coating uniformity and slitting performance.

[0004] Existing technologies can significantly improve the mechanical properties and resilience of films by introducing flexible segments and cross-linking networks through molecular structure design. However, how to simultaneously achieve nonionic hydrophilic modification, high resilience mechanical properties, controllable release properties, and cross-linkable curing properties in a single molecular system remains a problem to be solved. Summary of the Invention

[0005] Technical problems to be solved To address the shortcomings of existing technologies, this invention provides an aqueous polyurethane emulsion, its preparation method, and its application in hot stamping coatings, thus solving the problems mentioned in the background section.

[0006] Technical solution To achieve the above objectives, the present invention provides the following technical solution: According to a first aspect of the present invention, a method for preparing an aqueous polyurethane emulsion is provided, comprising the following steps: S1. Vacuum dehydration reaction of 60-70 parts of side-chain polyether polyester and 30-40 parts of hydroxyl-terminated polybutadiene, then add isophorone diisocyanate, keep the reaction at a temperature until the -NCO content reaches 8-10%; S2. Cool the system and add disulfide chain extender, 1,4-butanediol and acetone to carry out chain extension reaction until the -NCO content drops to 3~5%; S3. Add hydroxyethyl acrylate to the system to carry out the end-capping reaction until -NCO disappears, and obtain the prepolymer; S4. Under shear conditions, water is added to the prepolymer for dispersion and emulsification, followed by vacuum distillation to obtain an aqueous polyurethane emulsion.

[0007] This invention addresses the water resistance issue by designing the molecular structure of waterborne polyurethane and introducing nonionic hydrophilic side chains. It also introduces dynamic disulfide bonds to achieve reversible release control, combines hydroxyethyl acrylate to construct a post-crosslinking network, and synergistically enhances resilience with hydroxyl-terminated polybutadiene flexible chains. This allows waterborne polyurethane with a specific structure to be applied to hot stamping coatings, satisfying the combined requirements of coating uniformity, hot stamping clarity, wash fastness, and high and low temperature adaptability.

[0008] Preferably, in step S1, the amount of isophorone diisocyanate added is 105-135 parts.

[0009] Preferably, in step S1, the temperature of the vacuum dehydration reaction is 110~120℃ and the time is 1~2h; The temperature of the heat preservation reaction is 85~90℃, and the time is 2~3h.

[0010] Preferably, in step S1, a dibutyltin dilaurate catalyst is added during the heat preservation reaction, and the amount of the dibutyltin dilaurate catalyst added is 0.03~0.05 parts.

[0011] Preferably, in step S2, the disulfide chain extender is selected from bis(2-hydroxyethyl) disulfide and 3,3'-dithiodipropane-1,2-diol; The amount of the disulfide chain extender added is 3 to 8 parts.

[0012] Preferably, in step S2, the chain extension reaction is carried out at a temperature of 60-65°C for 2-4 hours.

[0013] Preferably, in step S3, the amount of hydroxyethyl acrylate added is 10-15 parts; The end-capping reaction is carried out at a temperature of 65-70°C for 1-2 hours.

[0014] Preferably, in step S4, the shear rate of the dispersion emulsification is 1000~1500 rpm, and the time is 10~15 min.

[0015] It should be noted that, According to a second aspect of the present invention, a water-washable polyurethane emulsion prepared according to the above method is provided, having a solid content of 35-40%.

[0016] According to a third aspect of the present invention, an application of a water-washable polyurethane emulsion in hot stamping coating is provided. The hot stamping fabric, by weight, includes component A and component B, wherein component A includes 80-100 parts of waterborne polyurethane emulsion, 0.1-0.5 parts of silicone wetting and leveling agent, 1-3 parts of waterborne wax emulsion, 0.5-1 parts of photoinitiator, and 0.1-0.5 parts of waterborne defoamer; and component B includes 5-15 parts of hydrophilic isocyanate curing agent. Beneficial effects

[0017] This invention provides an aqueous polyurethane emulsion, its preparation method, and its application in hot stamping coatings. It offers the following advantages: (1) The method for preparing a waterborne polyurethane emulsion provided in this solution introduces nonionic hydrophilic side chains, disulfide bonds and hydroxyethyl acrylate into the waterborne polyurethane molecule, thereby reducing the water absorption rate of the waterborne polyurethane film to below 8%, which greatly improves the wash fastness. At the same time, the elongation at break is ≥500% and the tensile permanent deformation is ≤5%, which can avoid cracking and deformation. In addition, the hydroxyethyl acrylate forms a three-dimensional network after curing, which improves the heat resistance, solvent resistance and cutting smoothness.

[0018] (2) The application of a water-based polyurethane emulsion in hot stamping coating provided by this solution has a water wash fastness of grade 4 or above, a hot stamping resolution of 200 lines / inch, a peel force deviation of <±0.5N / 23mm, a coating speed of 80~120m / min, and VOC emissions close to 0. Detailed Implementation

[0019] To better illustrate the content of this invention, the following description is provided in conjunction with specific embodiments. Example

[0020] A method for preparing an aqueous polyurethane emulsion is as follows: Step 1: Add 65 parts of a side-chain polyether polyester with a molecular weight of 2000 and a hydroxyl content of 56 mg KOH / g and 35 parts of a hydroxyl-terminated polybutadiene with a molecular weight of 2000 to a reaction vessel, and dehydrate under vacuum at 115°C for 1.5 h; cool to 60°C, add 120 parts of isophorone diisocyanate and 0.03 parts of dibutyltin dilaurate, heat to 88°C and maintain the temperature for 2.5 h, and determine that the -NCO content is 9.2%; Step 2: Cool to 55℃, add 4 parts of bis(2-hydroxyethyl) disulfide, 8 parts of 1,4-butanediol and 100 parts of acetone, and perform chain extension reaction at 62℃ for 3 hours. The -NCO content is then measured to be reduced to 4.1%. Step 3: Add 12 parts of hydroxyethyl acrylate, and perform end-capping reaction at 68°C for 1.5 h. FTIR detection is performed at 2270 cm⁻¹. -1 The -NCO characteristic peak disappears, yielding the prepolymer; Step 4: Cool the prepolymer to 35°C, slowly add 550 parts of deionized water under 1200 rpm shear, disperse for 12 min, then add 16 g of ethylenediamine dropwise for post-chain extension, controlling the dropwise addition temperature at 10°C, and finally remove acetone by vacuum distillation at 45°C, adjusting the solid content to 38% to obtain the polyurethane emulsion. Example

[0021] The preparation method of this embodiment is the same as that of Example 1, except that in step 1, the amount of isophorone diisocyanate added is 105 parts. Example

[0022] The preparation method of this embodiment is the same as that of Example 1, except that in step 1, the amount of isophorone diisocyanate added is 135 parts. Example

[0023] The preparation method of this embodiment is the same as that of Example 1, except that in step 1, the amount of side-chain polyether polyester added is 50 parts and the amount of hydroxyl-terminated polybutadiene added is 50 parts. Example

[0024] The preparation method of this embodiment is the same as that of Example 1, except that in step 2, the amount of bis(2-hydroxyethyl) disulfide added is 3 parts. Example

[0025] The preparation method of this embodiment is the same as that of Example 1, except that in step 2, the amount of bis(2-hydroxyethyl) disulfide added is 8 parts. Example

[0026] The preparation method of this embodiment is the same as that of Example 1, except that in step 2, the bis(2-hydroxyethyl) disulfide is replaced with 3,3'-dithiodipropane-1,2-diol. Example

[0027] The preparation method of this embodiment is the same as that of Example 1, except that in step 3, the amount of hydroxyethyl acrylate added is 10 parts. Example

[0028] The preparation method of this embodiment is the same as that of Example 1, except that in step 3, the amount of hydroxyethyl acrylate added is 15 parts.

[0029] Comparative Example 1 The comparative example is prepared in the same way as Example 1, except that in step 2, bis(2-hydroxyethyl) disulfide is not added, and 12 parts of 1,4-butanediol are directly added to carry out the chain extension reaction.

[0030] Comparative Example 2 The preparation method of this comparative example is the same as that of Example 1. The difference is that in step 1, 100 parts of a side-chain polyether polyester with a molecular weight of 2000 and a hydroxyl group of 56 mg KOH / g are added to the reaction vessel, vacuum dehydrated at 115°C for 1.5 h, cooled to 60°C, and then 120 parts of isophorone diisocyanate and 0.03 parts of dibutyltin dilaurate are added. The temperature is raised to 88°C and kept at that temperature for 2.5 h.

[0031] Comparative Example 3 The comparative example is prepared using the same method as Example 1, except that in step 3, hydroxyethyl acrylate is not added, and the emulsification and dispersion treatment in step 4 is carried out directly.

[0032] Comparative Example 4 This comparative example is a traditional carboxylate-type waterborne polyurethane. Using 100 parts of polyester diol, 12 parts of dimethylolpropionic acid, and 120 parts of isophorone diisocyanate as raw materials, triethylamine was added for neutralization and then emulsified to prepare an anionic waterborne polyurethane emulsion with a solid content of 38%.

[0033] After preparing coatings from the aqueous polyurethane emulsions of Examples 1 to 9 and Comparative Examples 1 to 4, tests were conducted, and the test results are shown in Table 1.

[0034] The film was soaked in deionized water for 24 hours, weighed, and its water absorption was tested. The elongation at break of the adhesive film was tested using a universal testing machine at a tensile speed of 200 mm / min. According to the test standard of GB / T 528-2009, the film was stretched to 100% and held for 5 minutes, and the residual deformation was measured after relaxation. The glass transition temperature of the film was tested using a differential scanning calorimeter at a heating rate of 10℃ / min. Aqueous polyurethane emulsion was coated onto a PET film, and the T-peel force was tested after hot pressing it with a substrate. Test the change of hot stamping pattern and the wash fastness under standard washing conditions in accordance with the guidance of ISO 105-C06.

[0035] Table 1 Test case Water absorption rate (%) Elongation at break (%) Permanent tensile deformation (%) Tg (°C) Peel force (N / 25mm) Wash fastness (grade) Storage stability Example 1 7.2 580 3.8 -42.3 5.6 4.5 ≥6 months Example 2 6.8 510 4.2 -40.1 4.8 4.5 ≥6 months Example 3 8.1 620 3.5 -44.5 6.2 4.0 ≥6 months Example 4 7.5 750 2.8 -48.2 5.2 4.0 ≥6 months Example 5 7.0 540 4.5 -41.5 3.8 4.5 ≥6 months Example 6 8.5 610 3.2 -43.8 8.5 4.0 5 months Example 7 7.8 560 3.6 -41.9 5.9 4.5 ≥6 months Example 8 10.2 530 4.8 -40.5 5.3 3.5 ≥6 months Example 9 7.0 595 3.5 -42.8 5.7 4.5 4 months Comparative Example 1 8.5 520 4.8 -40.5 12.3 3.0 ≥6 months Comparative Example 2 6.5 320 18.5 -18.6 4.5 4.5 ≥6 months Comparative Example 3 7.8 590 4.2 -42.5 5.8 3.5 ≥6 months Comparative Example 4 48.6 450 22.5 -25.3 7.2 2.0 3 months According to the data comparison of Examples 1 to 3, Example 1 has the best overall performance with a water absorption rate of 7.2%, an elongation at break of 580%, a permanent deformation of 3.8%, and a washability rating of 4.5.

[0036] In Example 2, the low amount of isophorone diisocyanate resulted in insufficient chain extension, leading to slightly lower elongation at break and exfoliation transition temperature compared to Example 1, while the tensile permanent deformation was slightly higher, indicating insufficient crosslinking density. In Example 3, the high amount of isophorone diisocyanate resulted in more residual -NCO groups in the molecular chain, leading to the formation of more urea bonds in subsequent reactions with water. This increased the elongation at break to 620%, lowered the glass transition temperature to -44.5°C, increased water absorption to 8.1%, and reduced wash fastness. Therefore, to ensure sufficient chain extension while avoiding excessive urea bond formation, an addition of 120-125 parts of isophorone diisocyanate is preferable. Based on the comparison of Examples 1, 4 and Comparative Example 2, it can be seen that the flexible long chain of hydroxyl-terminated polybutadiene can effectively reduce the glass transition temperature of the molecular chain, giving the coating excellent low-temperature flexibility and enabling the film to quickly rebound even with an elongation at break of over 500%. The optimal amount of hydroxyl-terminated polybutadiene is 30-40 parts, while excessive amounts of hydroxyl-terminated polybutadiene will reduce the water resistance.

[0037] Based on the comparison of Examples 1, 5-7 and Comparative Example 1, it can be seen that the peel force of Comparative Example 1 is as high as 12.3 N / 25 mm, and the peeling process is unstable. Gold flying is easy to appear at the edge of the hot stamping pattern, and the water wash fastness is only 3.0 grade. It can be seen that the disulfide bond undergoes a reversible exchange reaction during the hot pressing process, releasing the internal pressure and making the peel force precise and controllable. At the same time, the dynamic disulfide bond and the main chain covalent structure work together to ensure the release performance without sacrificing the overall mechanical strength of the film.

[0038] Based on the comparison of Examples 1, 8-9, and Comparative Example 3, it can be seen that without the addition of hydroxyethyl acrylate in Comparative Example 3, a cross-linked network cannot be formed, and the wash fastness decreases to 3.5. However, adding too much hydroxyethyl acrylate, although maintaining the wash fastness at 4.5, leads to decreased storage stability, with gelation occurring after 4 months. This indicates that the three-dimensional cross-linked network formed by the curing of hydroxyethyl acrylate, combined with the dynamic disulfide bonds of the main chain and the flexible hydroxyl-segmented polybutadiene chains, forms a composite structure that ensures both wash fastness and solvent resistance without affecting the reversible release function of the dynamic disulfide bonds.

[0039] A comparison of Example 1 and Comparative Example 4 shows that Comparative Example 4 has a water absorption rate as high as 48.6%, which is 6.7 times that of Example 1; its tensile permanent deformation is 22.5%, which is 5.9 times that of Example 1; its wash fastness is only grade 2.0; and its storage stability is only 3 months. The overall performance of the waterborne polyurethane emulsion of this application is far superior to that of the carboxylate-type waterborne polyurethane in the prior art.

[0040] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing an aqueous polyurethane emulsion, characterized in that: Includes the following steps: S1. Vacuum dehydration reaction of 60-70 parts of side-chain polyether polyester and 30-40 parts of hydroxyl-terminated polybutadiene, then add isophorone diisocyanate, keep the reaction at a temperature until the -NCO content reaches 8-10%; S2. Cool the system and add disulfide chain extender, 1,4-butanediol and acetone to carry out chain extension reaction until the -NCO content drops to 3~5%; S3. Add hydroxyethyl acrylate to the system to carry out the end-capping reaction until -NCO disappears, and obtain the prepolymer; S4. Under shear conditions, water is added to the prepolymer for dispersion and emulsification, followed by vacuum distillation to obtain an aqueous polyurethane emulsion.

2. The method for preparing an aqueous polyurethane emulsion according to claim 1, characterized in that: In step S1, the amount of isophorone diisocyanate added is 105-135 parts.

3. The method for preparing an aqueous polyurethane emulsion according to claim 1, characterized in that: In step S1, the temperature of the vacuum dehydration reaction is 110~120℃, and the time is 1~2h; The temperature of the heat preservation reaction is 85~90℃, and the time is 2~3h.

4. The method for preparing an aqueous polyurethane emulsion according to claim 3, characterized in that: In step S1, a dibutyltin dilaurate catalyst is added during the heat preservation reaction, and the amount of the dibutyltin dilaurate catalyst added is 0.03~0.05 parts.

5. The method for preparing an aqueous polyurethane emulsion according to claim 1, characterized in that: In step S2, the disulfide chain extender is selected from bis(2-hydroxyethyl) disulfide and 3,3'-dithiodipropane-1,2-diol; The amount of the disulfide chain extender added is 3 to 8 parts.

6. The method for preparing an aqueous polyurethane emulsion according to claim 1, characterized in that: In step S2, the chain extension reaction is carried out at a temperature of 60-65°C for 2-4 hours.

7. The method for preparing an aqueous polyurethane emulsion according to claim 1, characterized in that: In step S3, the amount of hydroxyethyl acrylate added is 10-15 parts; The end-capping reaction is carried out at a temperature of 65-70°C for 1-2 hours.

8. The method for preparing an aqueous polyurethane emulsion according to claim 1, characterized in that: In step S4, the shear rate of the dispersion emulsification is 1000~1500 rpm, and the time is 10~15 min.

9. An aqueous polyurethane emulsion obtained by the preparation method according to any one of claims 1 to 8.

10. The application of the waterborne polyurethane emulsion of claim 9 in hot stamping coating.