Yellowing-resistant water-based table printing ink and preparation method thereof

Abstract

The invention discloses yellowing-resistant water-based surface printing ink and a preparation method thereof, and relates to the technical field of ink, the yellowing-resistant water-based surface printing ink comprises 40-60 parts of a core-shell structure emulsion, 1-3 parts of a dispersant, 15-25 parts of rutile titanium dioxide, 0.3-1 part of an ultraviolet light absorber, and 15-30 parts of a solvent; wherein the core-shell structure emulsion has a structure taking waterborne polyurethane as a core layer and a modified acrylic monomer as a shell layer. The acrylic monomer is modified by a silane coupling agent, and a hydrophobic siloxane group is introduced, so that the hydrophilicity of a shell layer is reduced, and the shell layer is better matched with a hydrophobic interface of a WPU core layer. Adipic dihydrazide (ADH) is added in a shell layer polymerization stage, and is subjected to a cross-linking reaction with carboxyl in acrylate to form a covalent bond network, so that a core-shell interface is further stabilized, the problem that water-based polyurethane (WPU) and acrylic resin are easy to layer when being mixed can be effectively solved, and meanwhile, the water resistance and yellowing resistance of the ink are improved.

Description

Technical Field

[0001] The invention relates to the technical field of inks, and in particular to a yellowing-resistant water-based surface printing ink and a preparation method thereof. Background Art

[0002] Water-based surface printing inks, also known as water-based inks, are primarily composed of water-soluble resins (such as acrylic acid and polyurethane), pigments, additives (such as wetting and dispersing agents and defoamers), and water. Water is the primary solvent, supplemented by a small amount of alcohols (such as ethanol). The low organic solvent content significantly reduces volatile organic compound (VOC) emissions. Water-based surface printing inks made with water-based acrylic resins have poor water resistance and are prone to blistering and shedding due to water absorption. Water-based polyurethanes, while offering excellent performance, are also more expensive.

[0003] Currently, water-based polyurethane and water-based acrylic resin are mixed for use, but water-based polyurethane is highly hydrophobic and acrylic is highly hydrophilic, and they are easily delaminated after mixing. Although water-based polyurethane has better water resistance than acrylic, water absorption channels may form at the interface between the two, reducing water resistance. Summary of the Invention

[0004] The object of the present invention is to provide a yellowing-resistant water-based surface printing ink and a preparation method thereof, to solve the following technical problems: How to improve the water resistance of water-based surface printing inks while avoiding delamination.

[0005] The purpose of the present invention can be achieved through the following technical solutions: In a first aspect, the present invention discloses a yellowing-resistant water-based surface printing ink, comprising the following components in parts by weight: 40–60 parts of core-shell structure emulsion, 1–3 parts of dispersant, 15–25 parts of rutile titanium dioxide, 0.5–2 parts of ultraviolet absorber, 0.3–1 parts of light stabilizer, and 15–30 parts of solvent; The core-shell structure emulsion has a structure with waterborne polyurethane as the core layer and modified acrylic monomer as the shell layer.

[0006] Preferably, the preparation of the core-shell structure emulsion comprises the following steps: Preparation of WPU core layer emulsion; Preparation of the modified acrylic shell layer: 20-40% of the modified acrylic monomer and 50-70% of the emulsifier are stirred at 800-1000 rpm for 40-60 minutes until a pre-emulsion is formed; the WPU core layer emulsion is used as a seed, the pre-emulsion and 30-50% of the initiator are added, the temperature is raised to 70-80°C, and the polymerization reaction is carried out at a stirring speed of 300-500 rpm for 1-2 hours; After mixing the remaining modified acrylic monomer with the emulsifier and initiator, add the mixture dropwise at a constant speed for 2-4 hours using a peristaltic pump, maintaining the reaction temperature and stirring speed. After the addition is completed, keep the mixture warm for 1-2 hours to ensure complete conversion of the monomer. At the beginning of the holding period, add adipic acid dihydrazide, and adjust the stirring speed to 200-300 rpm. The amount of adipic acid dihydrazide added is 1-3% of the total amount of acrylic monomer. The particles were removed by filtering through a 200-mesh filter cloth to obtain a core-shell structure emulsion.

[0007] Preferably, the raw materials used in the preparation of the WPU core layer emulsion include, by weight: 40-60 parts of polycaprolactone diol, 20-35 parts of isocyanate, 5-15 parts of chain extender, 1-3 parts of neutralizer, 10-25 parts of butanone, and 100-200 parts of deionized water.

[0008] Preferably, the preparation steps of the WPU core layer emulsion include: Add polycaprolactone diol, isocyanate and butanone into the reaction kettle in proportion, heat to 70-80°C, and stir to react for 1-2 hours until the NCO content reaches the theoretical value; Cool down to 50-60℃, add chain extender, and continue the reaction until the NCO content is stable; Add a neutralizing agent to neutralize the carboxylic acid groups to generate ionized WPU prepolymer; The WPU prepolymer is added to deionized water, and dispersed at a high speed of 1000-3000 rpm with stirring for 10-30 minutes. After stirring, butanone is removed by vacuum distillation to form a WPU core layer emulsion.

[0009] Preferably, the raw materials for preparing the modified acrylic shell layer include, by weight: 20-40 parts of modified acrylic monomer, 1-3 parts of emulsifier, and 0.3-0.8 parts of initiator; the mass ratio between the WPU core layer emulsion and the modified acrylic monomer is 3-4:1-2.

[0010] Preferably, the preparation method of the modified acrylic acid monomer is as follows: A silane coupling agent is mixed with deionized water in a mass ratio of 1:20-80, and acetic acid is added as a catalyst at a concentration of 0.1%-1% of the mass of the silane coupling agent. A hydrolysis reaction is carried out at 80° C. for 1-2 hours. The hydrolyzed silane coupling agent solution is mixed with acrylic acid monomers of BA and MMA in a mass ratio of 1-3:1, with the amount of the silane coupling agent being 0.1%-1% of the total amount of the acrylic acid monomer. The mixture is stirred at a stirring speed of 300-500 rpm for 5-15 minutes to obtain a modified acrylic acid monomer.

[0011] Preferably, the solvent is composed of deionized water and propylene glycol methyl ether in a mass ratio of 3:1.

[0012] Preferably, the dispersant is BYK-190 dispersant.

[0013] Preferably, the rutile titanium dioxide is rutile R-902+.

[0014] In a second aspect, the present invention further discloses a method for preparing yellowing-resistant water-based surface printing ink, comprising the following steps: Step 1: Prepare a core-shell structure emulsion in advance, add a solvent accounting for 50% of the total solvent amount to the prepared core-shell structure emulsion, and mix for 5-10 minutes at a stirring speed of 300-500 rpm; Step 2: Add dispersant, increase the speed to 800-1000 rpm, stir for 10-15 minutes, and dropwise add ultraviolet absorber and light stabilizer at a rate of 0.3-0.5 mL / min; Step 3: Add rutile titanium dioxide in 3 batches, with an interval of 5 minutes between each batch, adjust the speed to 1500-2000 rpm, and continue to disperse for 30 minutes. The temperature should be ≤40°C during the stirring process. Step 4: Grind in a horizontal sand mill with a grinding fineness target of ≤10μm and a feed rate of 30–50L / min. Grind in a cycle until the grinding fineness target is reached. Step 5: Add the remaining solvent to reduce the stirring speed to 500-600 rpm, and adjust the pH of the system to 8.5-9.0 with AMP-95 (2-amino-2-methyl-1-propanol); finally, filter to obtain the product.

[0015] Beneficial effects of the present invention: This invention significantly improves yellowing resistance: By selecting rutile titanium dioxide (R-902+), its high weather resistance effectively resists UV damage to ink, reducing yellowing. The addition of a UV absorber (Tinuvin 1130) and a light stabilizer (Chimassorb 119) further absorbs and shields UV rays, protecting the ink from light aging, thereby extending the lifespan of printed products and maintaining long-term aesthetic appeal.

[0016] The core-shell emulsion of the present invention features a waterborne polyurethane core layer and a modified acrylic monomer shell layer. The WPU core layer is composed of polycaprolactone diol (a hydrophobic segment) and isocyanate, which imparts water resistance to the emulsion. The acrylic monomer is modified with a silane coupling agent (KH-570) to introduce hydrophobic siloxane groups (-Si-O-), reducing the hydrophilicity of the shell layer and improving its hydrophobic interface with the WPU core layer. Adipic acid dihydrazide (ADH) is added during the shell polymerization stage to cross-link with the carboxyl groups in the acrylate, forming a covalent bond network that further stabilizes the core-shell interface. This effectively addresses the issue of easy delamination when mixing waterborne polyurethane (WPU) and acrylic resin, while also improving the ink's water resistance and yellowing resistance. DETAILED DESCRIPTION

[0017] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0018] Unless otherwise specified, the experimental methods in the following examples are conventional methods and were performed according to the techniques or conditions described in the literature in the field or according to the product instructions. The materials and reagents used in the following examples, unless otherwise specified, were all commercially available.

[0019] Preparation Example 1 Preparation of modified acrylic monomer: A silane coupling agent (KH-570) was mixed with deionized water at a mass ratio of 1:60, and acetic acid (0.5% by mass of the silane coupling agent) was added as a catalyst. A hydrolysis reaction was carried out at 80°C for 2 hours. The hydrolyzed silane coupling agent solution was mixed with acrylic monomers of BA and MMA at a mass ratio of 2:1. The amount of the silane coupling agent was 1% (by mass) of the total amount of the acrylic monomer. The mixture was stirred at a stirring speed of 400 rpm for 10 minutes to obtain a modified acrylic monomer.

[0020] Preparation Example 2 Preparation of core-shell structure emulsion: Step S1, preparation of WPU core layer emulsion: In this preparation example, the raw materials used in the preparation of the WPU core layer emulsion include, by weight: 50 parts of polycaprolactone diol, 30 parts of isocyanate (HDI), 10 parts of chain extender (1,4-butanediol (BDO)), 2 parts of neutralizer (triethylamine (TEA)), 15 parts of butanone, and 150 parts of deionized water.

[0021] Add polycaprolactone diol (Mn=2000), isocyanate (HDI), and butanone into the reactor in proportion, heat to 75°C, and stir to react for 1.5 hours until the NCO content reaches the theoretical value; The temperature was lowered to 55°C, and a chain extender (1,4-butanediol (BDO)) was added. The reaction was continued until the NCO content was stable. Add a neutralizing agent (triethylamine) to neutralize the carboxylic acid groups and generate ionized WPU prepolymer; The WPU prepolymer was added into deionized water and dispersed at a high speed of 2000 rpm for 20 minutes. After stirring, butanone was removed by vacuum distillation to form a WPU core layer emulsion.

[0022] In this preparation example, the raw materials used in the preparation of the modified acrylic shell layer include, by weight: 30 parts of the modified acrylic monomer in Preparation Example 1, 2 parts of an emulsifier (sodium lauryl sulfate), and 0.5 parts of an initiator (ammonium persulfate). The ammonium persulfate needs to be prepared into a 10% aqueous solution. The mass ratio between the WPU core layer emulsion and the modified acrylic monomer is 3:1.

[0023] Step S2, Preparation of Modified Acrylic Shell Layer: 30% of the modified acrylic monomer and 60% of the emulsifier were stirred at 900 rpm for 50 minutes until a stable pre-emulsion was formed; the WPU core layer emulsion was used as a seed, the pre-emulsion and 40% of the initiator were added, the temperature was raised to 75°C (heating rate ≤ 2°C / min), and the polymerization reaction was carried out at a stirring speed of 400 rpm for 1.5 hours; The remaining modified acrylic monomer was mixed with the emulsifier and initiator, and then added dropwise at a constant speed for 3 hours using a peristaltic pump, while maintaining the reaction temperature and stirring speed. After the addition was completed, the mixture was kept warm for 1.5 hours to ensure complete conversion of the monomer. At the beginning of the holding period, adipic acid dihydrazide (ADH) was added, and the stirring speed was adjusted to 250 rpm. The amount of adipic acid dihydrazide added was 2% (by weight) of the total amount of acrylic monomer. The particles were removed by filtering through a 200-mesh filter cloth to obtain a core-shell structure emulsion.

[0024] Preparation Example 3 Preparation of core-shell structure emulsion: Step S1, preparation of WPU core layer emulsion: In this preparation example, the raw materials used in the preparation of the WPU core layer emulsion include, by weight: 50 parts of polycaprolactone diol, 30 parts of isocyanate (HDI), 10 parts of chain extender (1,4-butanediol (BDO)), 2 parts of neutralizer (triethylamine (TEA)), 15 parts of butanone, and 150 parts of deionized water.

[0025] Add polycaprolactone diol (Mn=2000), isocyanate (HDI), and butanone into the reactor in proportion, heat to 75°C, and stir to react for 1.5 hours until the NCO content reaches the theoretical value; The temperature was lowered to 55°C, and a chain extender (1,4-butanediol (BDO)) was added. The reaction was continued until the NCO content was stable. Add a neutralizing agent (triethylamine) to neutralize the carboxylic acid groups and generate ionized WPU prepolymer; The WPU prepolymer was added into deionized water and dispersed at a high speed of 2000 rpm for 20 minutes. After stirring, butanone was removed by vacuum distillation to form a WPU core layer emulsion.

[0026] In this preparation example, the raw materials for preparing the modified acrylic shell layer include, by weight, 30 parts of acrylic monomer, 2 parts of emulsifier (sodium lauryl sulfate), and 0.5 parts of initiator (ammonium persulfate), where ammonium persulfate (APS) needs to be prepared into a 10% aqueous solution; the acrylic monomer is a mixture of BA and MMA in a mass ratio of 2:1; and the mass ratio between the WPU core layer emulsion and the modified acrylic monomer is 3:1.

[0027] Step S2, Preparation of a Modified Acrylic Shell Layer: 30% of the total monomer amount of acrylic monomer, 60% of the total emulsifier amount of emulsifier, and 0.6 times the amount of deionized water of the total monomer amount were stirred at 900 rpm for 50 minutes until a stable pre-emulsion was formed. The WPU core layer emulsion was used as a seed, and the pre-emulsion and 40% of the total initiator amount of initiator were added. The temperature was raised to 75°C (heating rate ≤ 2°C / min), and the polymerization reaction was carried out at a stirring speed of 400 rpm for 1.5 hours. The remaining acrylic acid monomer, emulsifier, initiator and deionized water (0.6 times the total amount of monomer) were mixed and added dropwise at a constant speed for 3 hours using a peristaltic pump while maintaining the reaction temperature and stirring speed. After the addition was completed, the mixture was kept warm for 1.5 hours to ensure complete conversion of the monomer. At the beginning of the holding period, adipic acid dihydrazide (ADH) was added, and the stirring speed was adjusted to 250 rpm. The amount of adipic acid dihydrazide added was 2% (by weight) of the total amount of acrylic acid monomer. The particles were removed by filtering through a 200-mesh filter cloth to obtain a core-shell structure emulsion.

[0028] Preparation Example 4 Compared with Preparation Example 2, the only difference is that adipic acid dihydrazide is not added at the beginning of the insulation stage, and the other steps and conditions remain the same, and a core-shell structure emulsion is finally prepared.

[0029] Preparation Example 5 Compared with Preparation Example 3, the only difference is that adipic acid dihydrazide is not added at the beginning of the insulation stage, and the other steps and conditions remain the same, and a core-shell structure emulsion is finally prepared. Example 1

[0030] In this embodiment, a yellowing-resistant water-based surface printing ink comprises the following components in parts by weight: 45 parts of the core-shell structure emulsion prepared in Preparation Example 1, 2 parts of a dispersant, 20 parts of rutile titanium dioxide, 1 part of an ultraviolet absorber, 0.5 parts of a light stabilizer, and 20 parts of a solvent; The solvent is composed of deionized water and propylene glycol monomethyl ether (PM) in a mass ratio of 3:1. The dispersant is BYK-190 dispersant. The rutile titanium dioxide is rutile R-902+. The UV absorber is Tinuvin 1130. The light stabilizer is Chimassorb 119.

[0031] The method for preparing the yellowing-resistant water-based surface printing ink in this embodiment comprises the following steps: Step 1: Add the core-shell structure emulsion prepared in Preparation Example 1 to a solvent accounting for 50% of the total solvent amount, and mix at a stirring speed of 400 rpm for 10 minutes; Step 2: Add dispersant, increase the speed to 800 rpm, stir for 15 minutes, and dropwise add ultraviolet absorber and light stabilizer at a rate of 0.4 mL / min; Step 3: Add rutile titanium dioxide in 3 batches, with an interval of 5 minutes between each batch, adjust the speed to 1500 rpm, and continue dispersing for 30 minutes. The temperature should be ≤40°C during the stirring process. Step 4: Grind in a horizontal sand mill with a grinding fineness target of ≤10μm and a feed rate of 40L / min. Grind in a cycle until the grinding fineness target is reached. Step 5: Add the remaining solvent to reduce the stirring speed to 500 rpm, and adjust the pH of the system to 8.5–9.0 with AMP-95 (2-amino-2-methyl-1-propanol); finally, filter to obtain the product. Example 2

[0032] In this embodiment, a yellowing-resistant water-based surface printing ink comprises the following components in parts by weight: 55 parts of the core-shell structure emulsion prepared in Preparation Example 1, 2 parts of a dispersant, 25 parts of rutile titanium dioxide, 1.5 parts of an ultraviolet absorber, 0.5 parts of a light stabilizer, and 25 parts of a solvent; The solvent is composed of deionized water and propylene glycol monomethyl ether (PM) in a mass ratio of 3:1. The dispersant is BYK-190 dispersant. The rutile titanium dioxide is rutile R-902+. The UV absorber is Tinuvin 1130. The light stabilizer is Chimassorb 119.

[0033] The method for preparing the yellowing-resistant water-based surface printing ink in this embodiment comprises the following steps: Step 1: Add the core-shell structure emulsion prepared in Preparation Example 1 to a solvent accounting for 50% of the total solvent amount, and mix at a stirring speed of 400 rpm for 10 minutes; Step 2: Add dispersant, increase the speed to 800 rpm, stir for 15 minutes, and dropwise add ultraviolet absorber and light stabilizer at a rate of 0.4 mL / min; Step 3: Add rutile titanium dioxide in 3 batches, with an interval of 5 minutes between each batch, adjust the speed to 1500 rpm, and continue dispersing for 30 minutes. The temperature should be ≤40°C during the stirring process. Step 4: Grind in a horizontal sand mill with a grinding fineness target of ≤10μm and a feed rate of 40L / min. Grind in a cycle until the grinding fineness target is reached. Step 5: Add the remaining solvent to reduce the stirring speed to 600 rpm, and adjust the pH of the system to 8.5-9.0 with AMP-95 (2-amino-2-methyl-1-propanol); finally, filter and can the product. Example 3

[0034] In this embodiment, a yellowing-resistant water-based surface printing ink comprises the following components in parts by weight: 50 parts of the core-shell structure emulsion prepared in Preparation Example 1, 2 parts of dispersant, 20 parts of rutile titanium dioxide, 1.2 parts of ultraviolet absorber, 0.4 parts of light stabilizer, and 20 parts of solvent; The solvent is composed of deionized water and propylene glycol monomethyl ether (PM) in a mass ratio of 3:1. The dispersant is BYK-190 dispersant. The rutile titanium dioxide is rutile R-902+. The UV absorber is Tinuvin 1130. The light stabilizer is Chimassorb 119.

[0035] The method for preparing the yellowing-resistant water-based surface printing ink in this embodiment comprises the following steps: Step 1: Add the core-shell structure emulsion prepared in Preparation Example 1 to a solvent accounting for 50% of the total solvent amount, and mix at a stirring speed of 300 rpm for 10 minutes; Step 2: Add dispersant, increase the speed to 800 rpm, stir for 15 minutes, and add ultraviolet absorber and light stabilizer dropwise at a rate of 0.5 mL / min; Step 3: Add rutile titanium dioxide in 3 batches, with an interval of 5 minutes between each batch, adjust the speed to 1500 rpm, and continue dispersing for 30 minutes. The temperature should be ≤40°C during the stirring process. Step 4: Grind in a horizontal sand mill with a grinding fineness target of ≤10μm and a feed rate of 40L / min. Grind in a cycle until the grinding fineness target is reached. Step 5: Add the remaining solvent to reduce the stirring speed to 500 rpm, and adjust the pH of the system to 8.5-9.0 with AMP-95 (2-amino-2-methyl-1-propanol); finally, filter and can the product.

[0036] Comparative Example 1 Compared with Example 3, the only difference is that the core-shell structure emulsion of Preparation Example 2 is replaced by the core-shell structure emulsion prepared in Preparation Example 3; the other steps and conditions remain the same, and finally a yellowing-resistant water-based surface printing ink is prepared.

[0037] Comparative Example 2 Compared with Example 3, the only difference is that the core-shell structure emulsion of Preparation Example 2 is replaced by the core-shell structure emulsion prepared in Preparation Example 4; the other steps and conditions remain the same, and finally a yellowing-resistant water-based surface printing ink is prepared.

[0038] Comparative Example 3 Compared with Example 3, the only difference is that the core-shell structure emulsion of Preparation Example 2 is replaced by the core-shell structure emulsion prepared in Preparation Example 5; the other steps and conditions remain the same, and finally a yellowing-resistant water-based surface printing ink is prepared.

[0039] The yellowing-resistant water-based surface printing inks prepared in Examples 1-3 and Comparative Examples 1-3 were used for performance testing of plastic film substrates. The test results are shown in Table 1:

[0040] Analysis of the data in Table 1 shows that, compared with Comparative Examples 1-3, the water resistance, adhesion, yellowing resistance, and friction resistance of the yellowing-resistant water-based surface printing inks prepared in Examples 1-3 are significantly improved. In the embodiments of the present invention, the core-shell structure emulsion prepared with water-based polyurethane as the core layer and modified acrylic monomer as the shell layer as the raw material can effectively improve the performance of the ink. In addition, in the embodiments of the present invention, the modification of the acrylic monomer and the addition of adipic acid dihydrazide can effectively synergize to avoid the delamination of water-based polyurethane and acrylic acid, thereby effectively improving the performance of the ink.

[0041] The above is a detailed description of an embodiment of the present invention. However, the content described is only a preferred embodiment of the present invention and should not be considered to limit the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the patent coverage of the present invention.

Claims

1. A yellowing-resistant water-based surface printing ink, characterized in that: Calculated by weight, it includes the following components: 40–60 parts of core-shell structure emulsion, 1–3 parts of dispersant, 15–25 parts of rutile titanium dioxide, 0.3–1 parts of ultraviolet absorber, and 15–30 parts of solvent; The core-shell structure emulsion has a structure with waterborne polyurethane as the core layer and modified acrylic monomer as the shell layer.

2. The yellowing-resistant water-based surface printing ink according to claim 1, characterized in that: The preparation of the core-shell structure emulsion comprises the following steps: Preparation of WPU core layer emulsion; Preparation of the modified acrylic shell layer: 20-40% of the modified acrylic monomer and 50-70% of the emulsifier are stirred at 800-1000 rpm for 40-60 minutes until a pre-emulsion is formed; the WPU core layer emulsion is used as a seed, the pre-emulsion and 30-50% of the initiator are added, the temperature is raised to 70-80°C, and the polymerization reaction is carried out at a stirring speed of 300-500 rpm for 1-2 hours; After mixing the remaining modified acrylic monomer with the emulsifier and initiator, add the mixture dropwise at a constant speed for 2-4 hours using a peristaltic pump, maintaining the reaction temperature and stirring speed. After the addition is completed, keep the mixture warm for 1-2 hours to ensure complete conversion of the monomer. At the beginning of the holding period, add adipic acid dihydrazide, and adjust the stirring speed to 200-300 rpm. The amount of adipic acid dihydrazide added is 1-3% of the total amount of acrylic monomer. The particles were removed by filtering through a 200-mesh filter cloth to obtain a core-shell structure emulsion.

3. The yellowing-resistant water-based surface printing ink according to claim 2, characterized in that: The raw materials used in the preparation of the WPU core layer emulsion include, by weight: 40-60 parts of polycaprolactone diol, 20-35 parts of isocyanate, 5-15 parts of chain extender, 1-3 parts of neutralizer, 10-25 parts of butanone, and 100-200 parts of deionized water.

4. The yellowing-resistant water-based surface printing ink according to claim 2, characterized in that: The preparation steps of the WPU core layer emulsion include: Add polycaprolactone diol, isocyanate and butanone into the reaction kettle in proportion, heat to 70-80°C, and stir to react for 1-2 hours until the NCO content reaches the theoretical value; Cool down to 50-60℃, add chain extender, and continue the reaction until the NCO content is stable; Add a neutralizing agent to neutralize the carboxylic acid groups to generate ionized WPU prepolymer; The WPU prepolymer is added to deionized water, and dispersed at a high speed of 1000-3000 rpm with stirring for 10-30 minutes. After stirring, butanone is removed by vacuum distillation to form a WPU core layer emulsion.

5. The yellowing-resistant water-based surface printing ink according to claim 2, characterized in that: By weight, the raw materials used in the preparation of the modified acrylic shell layer include: 20-40 parts of modified acrylic monomer, 1-3 parts of emulsifier, and 0.3-0.8 parts of initiator; the mass ratio between the WPU core layer emulsion and the modified acrylic monomer is 3-4:1-2.

6. The yellowing-resistant water-based surface printing ink according to claim 5, characterized in that: The preparation method of the modified acrylic acid monomer is as follows: A silane coupling agent is mixed with deionized water in a mass ratio of 1:20-80, and acetic acid is added as a catalyst at a concentration of 0.1%-1% of the mass of the silane coupling agent. A hydrolysis reaction is carried out at 80° C. for 1-2 hours. The hydrolyzed silane coupling agent solution is mixed with acrylic acid monomers of BA and MMA in a mass ratio of 1-3:1, with the amount of the silane coupling agent being 0.1%-1% of the total amount of the acrylic acid monomer. The mixture is stirred at a stirring speed of 300-500 rpm for 5-15 minutes to obtain a modified acrylic acid monomer.

7. The yellowing-resistant water-based surface printing ink according to claim 1, characterized in that: The solvent is composed of deionized water and propylene glycol methyl ether in a mass ratio of 3:

1.

8. The yellowing-resistant water-based surface printing ink according to claim 1, characterized in that: The dispersant is BYK-190 dispersant.

9. The yellowing-resistant water-based surface printing ink according to claim 1, characterized in that: The rutile titanium dioxide is rutile R-902+.

10. A method for preparing the yellowing-resistant water-based surface printing ink according to any one of claims 1 to 9, characterized in that: The steps include: Step 1: Prepare a core-shell structure emulsion in advance, add a solvent accounting for 50% of the total solvent amount to the prepared core-shell structure emulsion, and mix for 5-10 minutes at a stirring speed of 300-500 rpm; Step 2: Add dispersant, increase the speed to 800-1000 rpm, stir for 10-15 minutes, and dropwise add ultraviolet absorber and light stabilizer at a rate of 0.3-0.5 mL / min; Step 3: Add rutile titanium dioxide in 3 batches, with an interval of 5 minutes between each batch, adjust the speed to 1500-2000 rpm, and continue to disperse for 30 minutes. The temperature should be ≤40°C during the stirring process. Step 4: Grind in a horizontal sand mill with a grinding fineness target of ≤10μm and a feed rate of 30–50L / min. Grind in a cycle until the grinding fineness target is reached. Step 5: Add the remaining solvent to reduce the stirring speed to 500-600 rpm, adjust the pH of the system to 8.5-9.0 with 2-amino-2-methyl-1-propanol; and finally filter to obtain the product.

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