Printing ink and preparation method thereof

Abstract

The invention relates to printing ink and a preparation method thereof, and belongs to the technical field of printing ink. The printing ink is prepared from waterborne polyurethane, a pigment, an auxiliary agent and deionized water, the preparation method of the printing ink comprises the following steps: mixing the waterborne polyurethane, the pigment, the auxiliary agent and the deionized water, and grinding to obtain the printing ink. When the waterborne polyurethane is prepared, polycaprolactone glycol and isophorone diisocyanate are used as initial raw materials, preferably, 2, 2-dimethylolpropionic acid is used as a hydrophilic monomer, 1, 4-butanediol and a modified composite diol monomer are used as chain extenders, and triethylamine is used as a neutralizer; the modified composite diol monomer is a mixture obtained by co-modifying trimethylolpropane monooleate through a composite sulfydryl compound, and the composite sulfydryl compound comprises cyclohexyl mercaptan, mercaptoethylamine and benzyl mercaptan; the printing ink can be well attached to the surfaces of non-polar plastics such as PP, is high in adhesive force and excellent in adhesive property, and improves the printing effect.

Description

Technical Field

[0001] This invention belongs to the field of printing ink technology, specifically relating to a printing ink and its preparation method. Background Technology

[0002] Printing ink is the substance used to form graphic information during the printing process. It is widely used in packaging printing in industries such as food and daily chemicals. Printing ink is mainly composed of four parts: solvent, binder (polymer resin), pigment, and additives. The binder (polymer resin) is the core substance of printing ink and directly determines the performance and quality of the ink. Since traditional printing inks are mainly solvent-based, they have significant drawbacks in terms of environmental protection, safety, and cost. Especially for the food industry, there are high potential dangers and uncertainties. Therefore, existing printing inks are gradually developing towards water-based (i.e., using water as a solvent). For example, water-based polyurethane inks are favored by the food industry due to their advantages such as low VOC content, high film strength after formation, and strong solvent resistance. However, water-based polyurethane inks still have certain drawbacks that limit their effectiveness. The main reason is that the food industry often uses non-polar plastics such as PP as packaging materials. These plastics have low surface energy and stable chemical properties, making it difficult for water-based polyurethane inks to adhere. This results in poor adhesion between the two, leading to mediocre printing results. Even after pretreatment such as corona treatment, the adhesion is still insufficient. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the prior art and provide a printing ink and its preparation method.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0005] In a first aspect, the present invention provides a printing ink, which is prepared from waterborne polyurethane, pigment, additives and deionized water; wherein the mass ratio of the waterborne polyurethane, the pigment, the additives and the deionized water is 100:17-20:2-2.4:28-34.

[0006] Furthermore, the solid content of the waterborne polyurethane is 36-40 wt%.

[0007] Furthermore, the waterborne polyurethane is prepared through the following steps:

[0008] Step A: Add polycaprolactone diol to isophorone diisocyanate, then heat under nitrogen protection, maintain the temperature, and reflux and stir for 1-1.5 h. Add 2,2-dimethylolpropionic acid, reflux and stir for 1.5-2 h, cool down for the first time, maintain the temperature, add 1,4-butanediol, reflux and stir for 20-40 min, then add the modified composite diol monomer dropwise while refluxing and stirring. After the addition is complete, continue refluxing and stirring for 4-5 h. Cool down for the second time, maintain the temperature, add acetone, reflux and stir for 20-40 min, and neutralize with triethylamine to obtain the first component.

[0009] Step B: Add deionized water to the first component, disperse, and distill under reduced pressure to obtain the final product.

[0010] Preferably, in step A, the molar ratio of polycaprolactone diol, isophorone diisocyanate, 2,2-dimethylolpropionic acid, and 1,4-butanediol is 2.5-2.7:7.4-7.8:1.5-1.6:0.8-1.1; the mass ratio of the modified composite diol monomer to the 1,4-butanediol is 0.2-0.25:1; the weight of the acetone is 8-12% of the sum of the weights of the polycaprolactone diol, isophorone diisocyanate, 2,2-dimethylolpropionic acid, 1,4-butanediol, and the modified composite diol monomer; and the molar ratio of triethylamine to the 2,2-dimethylolpropionic acid is 1:1.

[0011] Preferably, in step A, the heating refers to heating to 75-85°C at a rate of 2-4°C / min.

[0012] Preferably, in step A, the first cooling refers to cooling to 60-65°C at a rate of 2-4°C / min.

[0013] Preferably, in step A, the dripping refers to dripping at a uniform rate and volume; the dripping time is 15-30 minutes.

[0014] Preferably, in step A, the modified composite diol monomer is prepared through the following steps:

[0015] Trimethylolpropane monooleate and a complex mercapto compound were added to tetrahydrofuran, and then refluxed and stirred at 30-40°C for 10-20 min under nitrogen protection. Azobisisobutyronitrile was added, and then refluxed and stirred at 65°C for 8-10 h. After cooling to a constant temperature at room temperature, the mixture was distilled to obtain the final product.

[0016] More preferably, the mass ratio of the trimethylolpropane monooleate, the complex mercapto compound, the tetrahydrofuran, and the azobisisobutyronitrile is 2.8-3.6:1:85-90:0.5-0.6.

[0017] More preferably, the complex thiol compound comprises 1-2 parts by weight of cyclohexanethiol, 3.5 parts by weight of mercaptoethylamine and 2-2.5 parts by weight of benzyl mercaptothiol.

[0018] More preferably, the distillation refers to distillation at 32-35°C under a vacuum of -0.05 MPa until constant weight is achieved.

[0019] Preferably, in step A, the second cooling refers to cooling to 45-50°C at a rate of 2-4°C / min.

[0020] Preferably, in step B, the dispersion conditions are a temperature of 25-30°C, a rotation speed of 1500 rpm, and a time of 10-15 min.

[0021] Preferably, in step B, the vacuum distillation refers to vacuum distillation at 42-45°C under a vacuum of -0.1 MPa until no acetone is distilled off.

[0022] Furthermore, the additive is at least one of defoamer, dispersant and leveling agent.

[0023] Secondly, the present invention provides a method for preparing printing ink, the method comprising the following steps:

[0024] The water-based polyurethane, pigment, additives and deionized water are mixed and ground to obtain the product; the mixing conditions are a temperature of 25°C, a rotation speed of 500-800 rpm and a time of 15-30 min; the grinding refers to grinding to a fineness of 15-25 μm.

[0025] The beneficial effects of this invention are as follows:

[0026] Existing waterborne polyurethane inks often have difficulty adhering to the surface of non-polar plastics such as PP, resulting in poor adhesion and mediocre printing quality. Against this backdrop, this invention technically improves the binder (polymer resin) of waterborne polyurethane inks, thereby overcoming the aforementioned technical defects. Trimethylolpropane monooleate is a compound containing two hydroxyl groups, one olefin, and a hydrophobic alkane. On the one hand, the presence of the two hydroxyl groups allows it to act as a chain extender in the preparation of waterborne polyurethane, incorporating into the polyurethane molecular structure. On the other hand, the presence of the olefin provides a basis for modification, allowing for the addition of other molecular structures. Under this premise, this invention utilizes a complex mercapto compound to co-modify trimethylolpropane monooleate. This complex mercapto compound includes cyclohexanethiol, mercaptoethylamine, and benzyl mercaptothiol. Under the initiation of AIBN, cyclic alkanes, amino groups, and benzene rings are respectively modified to obtain a modified complex diol monomer, which is then incorporated into waterborne inks. In the preparation of polyurethane, the hydrophobic alkane of trimethylolpropane monooleate can enhance the nonpolarity of polyurethane molecules, reduce the surface tension of waterborne polyurethane, and improve affinity and adhesion. The modified amino group can react with -NCO to generate highly polar urea bonds, increase cohesive energy, and enhance the interaction with nonpolar plastics such as PP (the surface is usually corona treated before printing to introduce some polar groups, but the surface will not be completely covered by polar components). The modified benzene ring has π-π stacking effect, which can provide physical crosslinking points and further increase cohesive energy, promoting interfacial adhesion. The modified cyclic alkane can regulate and balance the polarity of polyurethane molecules (the aforementioned modification with amino and benzene rings will lead to a significant change in polarity), better adapting to the interface of nonpolar plastics such as PP. Unlike straight-chain alkanes, cyclic alkanes have stronger intermolecular forces and stronger hydrophobic interactions with other structures, achieving better regulation and balance effects. Detailed Implementation

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0028] In the following examples and comparative examples, the pigment refers to titanium dioxide, the defoamer refers to TEGO902W defoamer from Germany, and the reflux condensation temperature is set at 5°C.

[0029] Example 1

[0030] A printing ink, wherein the printing ink is prepared from waterborne polyurethane, pigment, additives and deionized water; wherein the mass ratio of the waterborne polyurethane, the pigment, the additives and the deionized water is 100:17:2:28.

[0031] The solid content of the waterborne polyurethane is 36 wt%.

[0032] The waterborne polyurethane is prepared by the following steps:

[0033] Step A: Polycaprolactone diol (2000 g / mol) was added to isophorone diisocyanate, and then heated and kept at the same temperature under nitrogen protection, and stirred under reflux for 1 h. 2,2-Dimethylolpropionic acid was added, and stirred under reflux for 1.5 h. The temperature was lowered for the first time, kept at the same temperature, and 1,4-butanediol was added. The mixture was stirred under reflux for 20 min. Then, the modified composite diol monomer was added dropwise while stirring under reflux. After the addition was complete, the mixture was stirred under reflux for 4 h. The temperature was lowered for the second time, kept at the same temperature, and acetone was added. The mixture was stirred under reflux for 20 min. Triethylamine was used for neutralization to obtain the first component.

[0034] Step B: Add deionized water to the first component, disperse, and distill under reduced pressure to obtain the final product.

[0035] In step A, the molar ratio of polycaprolactone diol, isophorone diisocyanate, 2,2-dimethylolpropionic acid, and 1,4-butanediol is 2.5:7.4:1.5:0.8; the mass ratio of the modified composite diol monomer to 1,4-butanediol is 0.2:1; the weight of acetone is 8% of the sum of the weights of polycaprolactone diol, isophorone diisocyanate, 2,2-dimethylolpropionic acid, 1,4-butanediol, and the modified composite diol monomer; and the molar ratio of triethylamine to 2,2-dimethylolpropionic acid is 1:1.

[0036] In step A, the heating refers to heating to 75°C at a rate of 2°C / min.

[0037] In step A, the first cooling refers to cooling down to 60°C at a rate of 2°C / min.

[0038] In step A, the dripping refers to the uniform dripping at a constant rate and volume (1 drop / s); the dripping time is 15 minutes.

[0039] In step A, the modified composite diol monomer is prepared through the following steps:

[0040] Trimethylolpropane monooleate and a complex thiol compound were added to tetrahydrofuran, then refluxed and stirred at 30°C for 10 min under nitrogen protection. Azobisisobutyronitrile was added, followed by reflux and stirring at 65°C for 8 h. The mixture was then cooled to a constant temperature at room temperature and distilled to obtain the product. The mass ratio of the trimethylolpropane monooleate, the complex thiol compound, the tetrahydrofuran, and the azobisisobutyronitrile was 2.8:1:85:0.5. The complex thiol compound comprised 1 part by weight of cyclohexanethiol, 3.5 parts by weight of mercaptoethylamine, and 2 parts by weight of benzyl mercaptan. The distillation was performed at 32°C under a vacuum of -0.05 MPa until constant weight was achieved (to remove volatiles).

[0041] In step A, the second cooling refers to cooling down to 45°C at a rate of 2°C / min.

[0042] In step B, the dispersion conditions are: temperature 25°C, rotation speed 1500 rpm, and time 10 min.

[0043] In step B, the vacuum distillation refers to vacuum distillation at 42°C under a vacuum of -0.1 MPa until no acetone is distilled off.

[0044] The additive is an antifoaming agent.

[0045] A method for preparing printing ink, the method comprising the following steps:

[0046] The water-based polyurethane, pigment, additives and deionized water are mixed and ground to obtain the product; the mixing conditions are 25°C, 500 rpm and 15 min; the grinding refers to grinding to a fineness of 15 μm.

[0047] Example 2

[0048] A printing ink is prepared from waterborne polyurethane, pigment, additives and deionized water; the mass ratio of the waterborne polyurethane, the pigment, the additives and the deionized water is 100:20:2.4:34.

[0049] The solid content of the waterborne polyurethane is 40 wt%.

[0050] The waterborne polyurethane is prepared by the following steps:

[0051] Step A: Polycaprolactone diol (2000 g / mol) was added to isophorone diisocyanate, and then heated and kept at the same temperature under nitrogen protection, and stirred under reflux for 1.5 h. 2,2-dimethylolpropionic acid was added, and stirred under reflux for 2 h. The temperature was lowered for the first time, kept at the same temperature, and 1,4-butanediol was added. The mixture was stirred under reflux for 40 min. Then, the modified composite diol monomer was added dropwise while stirring under reflux. After the addition was complete, the mixture was stirred under reflux for another 5 h. The temperature was lowered for the second time, kept at the same temperature, and acetone was added. The mixture was stirred under reflux for 40 min. Triethylamine was used for neutralization to obtain the first component.

[0052] Step B: Add deionized water to the first component, disperse, and distill under reduced pressure to obtain the final product.

[0053] In step A, the molar ratio of polycaprolactone diol, isophorone diisocyanate, 2,2-dimethylolpropionic acid, and 1,4-butanediol is 2.7:7.8:1.6:1.1; the mass ratio of the modified composite diol monomer to 1,4-butanediol is 0.25:1; the weight of acetone is 12% of the sum of the weights of polycaprolactone diol, isophorone diisocyanate, 2,2-dimethylolpropionic acid, 1,4-butanediol, and the modified composite diol monomer; and the molar ratio of triethylamine to 2,2-dimethylolpropionic acid is 1:1.

[0054] In step A, the heating refers to heating to 85°C at a rate of 4°C / min.

[0055] In step A, the first cooling refers to cooling down to 65°C at a rate of 4°C / min.

[0056] In step A, the dripping refers to the uniform dripping at a constant rate and volume (1 drop / s); the dripping time is 30 minutes.

[0057] In step A, the modified composite diol monomer is prepared through the following steps:

[0058] Trimethylolpropane monooleate and a complex thiol compound were added to tetrahydrofuran, then refluxed and stirred at 40°C for 20 min under nitrogen protection. Azobisisobutyronitrile was added, followed by reflux and stirring at 65°C for 10 h. The mixture was then cooled to a constant temperature at room temperature and distilled to obtain the final product. The mass ratio of the trimethylolpropane monooleate, the complex thiol compound, the tetrahydrofuran, and the azobisisobutyronitrile was 3.6:1:90:0.6. The complex thiol compound comprised 2 parts by weight of cyclohexanethiol, 3.5 parts by weight of mercaptoethylamine, and 2.5 parts by weight of benzyl mercaptothiol. The distillation was performed at 35°C under a vacuum of -0.05 MPa until constant weight was achieved (to remove volatiles).

[0059] In step A, the second cooling refers to cooling down to 50°C at a rate of 4°C / min.

[0060] In step B, the dispersion conditions are: temperature 30°C, rotation speed 1500 rpm, and time 15 min.

[0061] In step B, the vacuum distillation refers to vacuum distillation at 45°C under a vacuum of -0.1 MPa until no acetone is distilled off.

[0062] The additive is an antifoaming agent.

[0063] A method for preparing printing ink, the method comprising the following steps:

[0064] The water-based polyurethane, pigment, additives and deionized water are mixed and ground to obtain the product; the mixing conditions are 25°C, 800 rpm and 30 min; the grinding refers to grinding to a fineness of 25 μm.

[0065] Example 3

[0066] A printing ink, wherein the printing ink is prepared from waterborne polyurethane, pigment, additives and deionized water; wherein the mass ratio of the waterborne polyurethane, the pigment, the additives and the deionized water is 100:18:2.2:30.

[0067] The solid content of the waterborne polyurethane is 38 wt%.

[0068] The waterborne polyurethane is prepared by the following steps:

[0069] Step A: Polycaprolactone diol (2000 g / mol) was added to isophorone diisocyanate, and then heated and held under nitrogen protection, refluxed and stirred for 1.3 h. 2,2-Dimethylolpropionic acid was added, and refluxed and stirred for 1.7 h. The temperature was lowered for the first time, held at the same temperature, and 1,4-butanediol was added. The mixture was refluxed and stirred for 25 min. Then, the modified composite diol monomer was added dropwise while refluxing and stirring. After the addition was complete, the mixture was refluxed and stirred for another 4.6 h. The temperature was lowered for the second time, held at the same temperature, and acetone was added. The mixture was refluxed and stirred for 30 min. Triethylamine was used for neutralization to obtain the first component.

[0070] Step B: Add deionized water to the first component, disperse, and distill under reduced pressure to obtain the final product.

[0071] In step A, the molar ratio of polycaprolactone diol, isophorone diisocyanate, 2,2-dimethylolpropionic acid, and 1,4-butanediol is 2.6:7.6:1.53:1; the mass ratio of the modified composite diol monomer to 1,4-butanediol is 0.22:1; the weight of acetone is 10% of the sum of the weights of polycaprolactone diol, isophorone diisocyanate, 2,2-dimethylolpropionic acid, 1,4-butanediol, and the modified composite diol monomer; and the molar ratio of triethylamine to 2,2-dimethylolpropionic acid is 1:1.

[0072] In step A, the heating refers to heating to 80°C at a rate of 3°C / min.

[0073] In step A, the first cooling refers to cooling down to 63°C at a rate of 3°C / min.

[0074] In step A, the dripping refers to the uniform dripping at a constant rate and volume (1 drop / s); the dripping time is 25 minutes.

[0075] In step A, the modified composite diol monomer is prepared through the following steps:

[0076] Trimethylolpropane monooleate and a complex thiol compound were added to tetrahydrofuran, then refluxed and stirred at 33°C for 14 min under nitrogen protection. Azobisisobutyronitrile was added, followed by reflux and stirring at 65°C for 9 h. The mixture was then cooled to a constant temperature at room temperature and distilled to obtain the product. The mass ratio of the trimethylolpropane monooleate, the complex thiol compound, the tetrahydrofuran, and the azobisisobutyronitrile was 3:1:88:0.56. The complex thiol compound comprised 1.6 parts by weight of cyclohexanethiol, 3.5 parts by weight of mercaptoethylamine, and 2.4 parts by weight of benzyl mercaptothiol. The distillation was performed at 34°C under a vacuum of -0.05 MPa until constant weight was achieved (to remove volatiles).

[0077] In step A, the second cooling refers to cooling down to 48°C at a rate of 3°C / min.

[0078] In step B, the dispersion conditions are: temperature 27°C, rotation speed 1500 rpm, and time 14 min.

[0079] In step B, the vacuum distillation refers to vacuum distillation at 43°C under a vacuum of -0.1 MPa until no acetone is distilled off.

[0080] The additive is an antifoaming agent.

[0081] A method for preparing printing ink, the method comprising the following steps:

[0082] The water-based polyurethane, pigment, additives and deionized water are mixed and ground to obtain the product; the mixing conditions are 25°C, 700 rpm and 23 min; the grinding refers to grinding to a fineness of 20 μm.

[0083] Comparative Example 1

[0084] Based on Example 3, the modified composite diol monomer in step A was replaced with an equal weight of trimethylolpropane monooleate, while the other steps and parameters remained the same as in Example 3.

[0085] Comparative Example 2

[0086] Based on Example 3, the composite thiol compound comprising 1.6 parts by weight of cyclohexanethiol, 3.5 parts by weight of mercaptoethylamine and 2.4 parts by weight of benzylthiol is replaced with the composite thiol compound comprising 1.6 parts by weight of cyclohexanethiol and 3.5 parts by weight of mercaptoethylamine, while other steps and parameters remain the same as in Example 3.

[0087] Comparative Example 3

[0088] Based on Example 3, the composite thiol compound comprising 1.6 parts by weight of cyclohexanethiol, 3.5 parts by weight of mercaptoethylamine and 2.4 parts by weight of benzylthiol is replaced with the composite thiol compound comprising 1.6 parts by weight of cyclohexanethiol and 2.4 parts by weight of benzylthiol, while other steps and parameters remain the same as in Example 3.

[0089] Comparative Example 4

[0090] Based on Example 3, the composite thiol compound comprising 1.6 parts by weight of cyclohexanethiol, 3.5 parts by weight of mercaptoethylamine and 2.4 parts by weight of benzylthiol is replaced with the composite thiol compound comprising 3.5 parts by weight of mercaptoethylamine and 2.4 parts by weight of benzylthiol, while other steps and parameters remain the same as in Example 3.

[0091] Comparative Example 5

[0092] Based on Example 3, the composite thiol compound comprising 1.6 parts by weight of cyclohexanethiol, 3.5 parts by weight of mercaptoethylamine and 2.4 parts by weight of benzylthiol is replaced with the composite thiol compound comprising 1.6 parts by weight of 1-hexanethiol, 3.5 parts by weight of mercaptoethylamine and 2.4 parts by weight of benzylthiol, while other steps and parameters remain the same as in Example 3.

[0093] Performance testing:

[0094] The adhesion strength was tested according to standard GB / T 13217.7-91. The specific method was as follows: the printing inks prepared in Example 3 and Comparative Examples 1-5 were coated on the surface of PP film (after corona treatment, the surface free energy was measured to be 35.3 mN / m) using a bar coater. Then, the film was dried at 50°C for 6 hours and left to stand at room temperature for 24 hours. Then, adhesive tape was applied and peeled off. Finally, a 20 mm wide square grid paper was placed on the film, and the adhesion strength was calculated. The formula was: adhesion strength = number of grids of ink layer / (number of grids of ink layer + number of grids of peeled ink layer), rounded to two decimal places.

[0095] Table 1. Adhesion strength

[0096] Adhesion strength Example 3 0.96 Comparative Example 1 0.77 Comparative Example 2 0.89 Comparative Example 3 0.84 Comparative Example 4 0.87 Comparative Example 5 0.92

[0097] As can be seen from Table 1 above, the printing ink prepared by the present invention has strong adhesion to PP film, excellent bonding performance, and improves printing effect.

[0098] The above description is merely an example and illustration of the concept of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described or use similar methods to replace them, as long as they do not deviate from the concept of the invention or exceed the scope defined in the claims, they should all fall within the protection scope of the present invention.

Claims

1. A printing ink, characterized in that: The printing ink is prepared from water-based polyurethane, pigment, additives and deionized water; the mass ratio of the water-based polyurethane, the pigment, the additives and the deionized water is 100:17-20:2-2.4:28-34.

2. The printing ink according to claim 1, characterized in that: The solid content of the waterborne polyurethane is 36-40 wt%.

3. The printing ink according to claim 1, characterized in that: The waterborne polyurethane is prepared by the following steps: Step A: Add polycaprolactone diol to isophorone diisocyanate, then heat under nitrogen protection, maintain the temperature, and reflux and stir for 1-1.5 h. Add 2,2-dimethylolpropionic acid, reflux and stir for 1.5-2 h, cool down for the first time, maintain the temperature, add 1,4-butanediol, reflux and stir for 20-40 min, then add the modified composite diol monomer dropwise while refluxing and stirring. After the addition is complete, continue refluxing and stirring for 4-5 h. Cool down for the second time, maintain the temperature, add acetone, reflux and stir for 20-40 min, and neutralize with triethylamine to obtain the first component. Step B: Add deionized water to the first component, disperse, and distill under reduced pressure to obtain the final product.

4. The printing ink according to claim 3, characterized in that: In step A, the molar ratio of polycaprolactone diol, isophorone diisocyanate, 2,2-dimethylolpropionic acid, and 1,4-butanediol is 2.5-2.7:7.4-7.8:1.5-1.6:0.8-1.1; the mass ratio of the modified composite diol monomer to 1,4-butanediol is 0.2-0.25:1; the weight of acetone is 8-12% of the sum of the weights of polycaprolactone diol, isophorone diisocyanate, 2,2-dimethylolpropionic acid, 1,4-butanediol, and the modified composite diol monomer; and the molar ratio of triethylamine to 2,2-dimethylolpropionic acid is 1:

1.

5. The printing ink according to claim 3, characterized in that: In step A, the heating refers to raising the temperature to 75-85°C at a rate of 2-4°C / min.

6. The printing ink according to claim 3, characterized in that: In step A, the first cooling refers to cooling down to 60-65℃ at a rate of 2-4℃ / min.

7. The printing ink according to claim 3, characterized in that: In step A, the modified composite diol monomer is prepared through the following steps: Trimethylolpropane monooleate and a complex mercapto compound were added to tetrahydrofuran, and then refluxed and stirred at 30-40°C for 10-20 min under nitrogen protection. Azobisisobutyronitrile was added, and then refluxed and stirred at 65°C for 8-10 h. After cooling to a constant temperature at room temperature, the mixture was distilled to obtain the final product.

8. The printing ink according to claim 7, characterized in that: The mass ratio of the trimethylolpropane monooleate, the complex mercapto compound, the tetrahydrofuran, and the azobisisobutyronitrile is 2.8-3.6:1:85-90:0.5-0.

6.

9. The printing ink according to claim 7, characterized in that: The complex thiol compound comprises 1-2 parts by weight of cyclohexanethiol, 3.5 parts by weight of mercaptoethylamine, and 2-2.5 parts by weight of benzyl mercaptothiol.

10. A method for preparing printing ink as described in any one of claims 1-9, characterized in that: The preparation method includes the following steps: The water-based polyurethane, pigment, additives and deionized water are mixed and ground to obtain the product; the mixing conditions are a temperature of 25°C, a rotation speed of 500-800 rpm and a time of 15-30 min; the grinding refers to grinding to a fineness of 15-25 μm.