A method for preparing a solvent-resistant adhesive ink-soluble polyurethane resin

Abstract

The application discloses a preparation method of ink polyurethane resin resistant to solvent-free adhesive ink, which comprises the following steps: (1) mixing polyester diol, polycarbonate diol and diisocyanate, and then performing pre-polymerization, wherein the pre-polymerization temperature is 100-130 DEG C, and a polyurethane prepolymer with NCO end groups is obtained; (2) adding solvent and diisocyanate into the polyurethane prepolymer, mixing uniformly to form a phase-separated prepolymer mixture, wherein the NCO content of the mixture is 0.5%-2.5%; (3) adding the prepolymer mixture into a solution of diamin chain extender, organic amine end-capping agent and solvent, and then performing chain extension reaction, wherein the chain extension reaction temperature is 30-50 DEG C. The ink prepared from the ink polyurethane resin has good stability, resolubility, adhesion and high composite strength, and the resistance to various solvent-free adhesives is obviously improved.

Description

Technical Field

[0001] This invention belongs to the field of polyurethane ink resin technology, specifically relating to a method for preparing a polyurethane resin for inks that is resistant to solvent-free adhesives. Background Technology

[0002] In the domestic plastic flexible packaging printing industry, gravure printing occupies a very important position due to its advantages such as thick ink layers, bright colors, high saturation, high plate durability, stable print quality, and fast printing speed. Among the ink resins used in gravure printing, polyurethane inks have advantages such as wide substrate adaptability, integrated cooking and retorting, and versatility in surface printing and lamination, and occupy an absolute share of gravure ink resins.

[0003] Polyurethane ink resins are generally prepared from polyester or polyether polyols, alicyclic diisocyanates, and diamine / diol chain extenders, with a molecular weight of approximately tens of thousands. The introduction of urea bonds into the PU resin, forming polyurethane-urea resin (PUU), gives it excellent affinity, dispersion, and wetting properties for pigments. The molecular chain segments contain polar groups such as urethane, urethane, ester bonds, and ether bonds, which form hydrogen bonds with polar groups on the surfaces of various polar substrates such as PET and PA plastics, thus forming joints with a certain bonding strength. When this polyurethane resin is made into ink, it exhibits excellent adhesion when printed on polar plastic substrates.

[0004] Because they are more environmentally friendly and do not involve solvents in their preparation and use, solvent-free polyurethane adhesives are increasingly used in the food packaging and printing industry. However, some solvent-free polyurethane adhesives (especially those with lower molecular weight and higher polarity, such as polyester-based solvent-free adhesives) can cause ink dissolution in polyurethane inks used in food printing, affecting print quality.

[0005] The term "ink melting" (or "ink fusion") refers to the phenomenon where ink in localized areas of printed flexible packaging products undergoes lamination, resulting in dissolution, displacement, or disappearance. After solvent-free lamination of the printing substrate film, the solvent-free adhesive remains in a state of creep. Under the pressure of winding, the adhesive layer contacts and penetrates the printing ink layer, potentially leading to ink re-dissolution. This means the ink penetrates through the adhesive layer into a relatively thick ink pigment layer, rendering it a slightly soluble liquid. Additionally, the solvent-free adhesive generates shrinkage stress during curing, and the substrate also experiences shrinkage stress during curing. Furthermore, mismatched lamination tension between the two different substrate films can also lead to relative displacement. Under these various stresses, the cured product exhibits ink melting, typically manifesting as a darkening of the white ink layer and blurred or indistinct outlines of colored or black printed patterns and text.

[0006] Therefore, developing polyurethane ink resins with complete performance that can effectively improve the ink resistance of various polyester solvent-free adhesives has practical application value. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing a polyurethane resin for ink that is resistant to solvent-free adhesives, thereby solving the problem of ink dissolution in existing polyurethane ink resins.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows:

[0009] The present invention discloses a method for preparing a polyurethane resin for ink that is resistant to solvent-free adhesives, characterized by comprising the following steps:

[0010] (1) Polyester diol, polycarbonate diol and diisocyanate are mixed and then subjected to a prepolymerization reaction at a temperature of 100-130°C to obtain an NCO-terminated polyurethane prepolymer.

[0011] (2) Add solvent and diisocyanate to the polyurethane prepolymer and mix them evenly to form a phase-separated prepolymer mixture. At this time, the NCO content of the mixture is 0.5% to 2.5%.

[0012] (3) The prepolymer mixture is added to a solution of a diamine chain extender, an organic amine end-capping agent and a solvent to carry out a chain extension reaction. The chain extension reaction temperature is 30-50°C to obtain the polyurethane resin for ink that is resistant to solvent-free adhesives.

[0013] The raw materials are as follows (by weight): 30-50 parts polyester diol, 8-20 parts polycarbonate diol, 0.01-0.1 parts catalyst, 5-15 parts diisocyanate, 1-7 parts diamine chain extender, 0.1-2 parts organic amine end-capping agent, and 120-180 parts solvent.

[0014] This invention introduces polycarbonate diol into traditional polyester-type polyurethane resins for inks. Because the ester groups in polyester diols have high polarity, they have a strong ability to form hydrogen bonds and strong intermolecular forces. Polycarbonate diols can adjust molecular polarity, creating a difference in polarity between the ink resin and the polyester-type solvent-free adhesive. The combination of these two components can significantly improve the resin's resistance to polyester-type solvent-free adhesives. Furthermore, polycarbonate diols possess excellent mechanical properties, which can improve the resin's composite strength and heat resistance.

[0015] The polyester diol is a hydroxyl-terminated polyester diol formed by the condensation polymerization of a saturated aliphatic diol and an aliphatic or aromatic diacid, with a molecular weight of 1000-5000, preferably 2000-4000; the diol is at least one of 1,2-propanediol, 1,3-propanediol, methylpropanediol, 1,4-butanediol, neopentyl glycol, and 3-methyl-1,5-pentanediol; the diacid is at least one of sebacic acid, adipic acid, azelaic acid, terephthalic acid, and isophthalic acid; preferably, the diol is at least one of 1,3-propanediol, methylpropanediol, and neopentyl glycol, and the diacid is at least one of sebacic acid and adipic acid.

[0016] The polycarbonate diol is a polycarbonate with 1,6-hexanediol and 1,5-pentanediol as end groups, and a molecular weight of 1,500 to 2,500; preferably, the molecular weight is 2,000; for example, the T5652 type polycarbonate from Asahi Kasei Chemicals Co., Ltd. of Japan has a molecular weight of 2,000; compared with the traditional 1,6-HDO polycarbonate, the 1,6-HDO / 1,5-PDO type polycarbonate has low crystallinity, which can improve the gloss of the resin, and its good solubility makes it easy to handle;

[0017] The catalyst is at least one of organic bismuth, tetrabutyl titanate, stannous octanoate, or stannous chloride.

[0018] The diisocyanate is at least one of isophorone isocyanate, toluene diisocyanate, diphenylmethane-4,4'-diisocyanate, and 1,6-hexanediisocyanate; preferably isophorone isocyanate or / and diphenylmethane-4,4'-diisocyanate.

[0019] The diamine chain extender is at least one of 1,3-cyclohexanediamine, isophoronediamine, trimethylhexanediamine, and hexanediamine; preferably 1,3-cyclohexanediamine or / and isophoronediamine; the diamine chain extender acts as a post-chain extender to improve the molecular weight and stability of the polyurethane resin; the urea group obtained by the reaction of NCO group and amine group has strong polarity and strong intermolecular forces, so the resulting polyurethane resin has high cohesive energy, which can effectively improve the ink resistance of the resin to polyester solvent-free adhesives.

[0020] The organic amine capping agent is at least one of ethanolamine, diethanolamine, isopropanolamine, di-n-butylamine, and 2-ethylhexylamine; preferably at least one of diethanolamine and di-n-butylamine.

[0021] The solvent is at least one of ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, and isopropanol; preferably at least one of ethyl acetate, n-propyl acetate, and isopropanol.

[0022] Preferably, in step (1), the weight ratio of the polyester diol to the polycarbonate diol is (1.5-2.5):1, and the ratio of the molar number of the diisocyanate to the total molar number of the polyester diol and the polycarbonate diol is (1.50-1.70):1; the mass ratio of the diisocyanate added in step (2) to the diisocyanate in step (1) is 1:(3-8).

[0023] Preferably, in step (3), the ratio of the molar number of amine groups in the diamine chain extender to the molar number of NCO groups in the prepolymer mixture is (0.93 to 1.05):1.

[0024] Preferably, the duration of the prepolymerization reaction is 4 to 10 hours; the duration of the chain extension reaction is 0.5 to 4 hours; in step (2), the prepolymer mixture is uniformly dropped into a solution composed of a diamine chain extender, an organic amine end-capping agent and a solvent within 15 to 30 minutes to carry out the chain extension reaction.

[0025] The polyurethane resin for ink that is resistant to solvent-free adhesives and inks obtained by the preparation method of the present invention has a rotational viscosity of 800-2000 mPa·s / 25℃ and a solid content of 30±2%.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] In the prepolymer reaction stage, diisocyanate is added in two stages with a weight ratio of (4-6):1. Phase separation occurs in the resulting prepolymer mixture, with long-chain molecules being a polymer of diisocyanate and polyester diol, and short-chain molecules being diisocyanate monomers. In the chain extension reaction stage, long-chain molecules react with a chain extender to form the main resin component, providing the resin's primary properties. Short-chain molecules react with the chain extender to form a -NH-NCO-R-NCO-NH- polyurea structure. This structure is highly polar, readily forms hydrogen bonds, and exhibits strong intermolecular forces, effectively increasing molecular density and thus enhancing resistance to polyester-based solvent-free adhesives. The functional end-capping agent can adjust the molecular weight of the polyurethane resin.

[0028] The ink prepared from polyurethane resin using the solvent-resistant adhesive ink of the present invention exhibits excellent stability and resolubility, good adhesion to various plastic films, high composite strength, and significantly improved resistance to various solvent-free adhesives. Detailed Implementation

[0029] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. After reading the contents of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the claims of this application.

[0030] Example 1

[0031] A polyurethane resin for inks that is resistant to solvent-free adhesives is prepared by the following method:

[0032] (1) 34.8 parts by weight (the same below) of poly(1,3-propanediol adipate) (Mn=4000), 17.4 parts of 1,6-HDO / 1,5-PDO polycarbonate diol (Mn=2000), 6.2 parts of isophorone diisocyanate (IPDI) and 0.1 parts of organic bismuth catalyst were added into a container and stirred evenly. The mixture was kept at 115-120℃ for 6 hours to obtain NCO-terminated polyurethane prepolymer.

[0033] (2) After cooling to below 70°C, 40 parts of ethyl acetate and 1.55 parts of isophorone diisocyanate (IPDI) were added and stirred until homogeneous. The mass content of NCO was determined by di-n-butylamine back titration, and the NCO% was found to be 1.46%, thus obtaining a phase-separated prepolymer mixture.

[0034] (3) Add 3.06 parts of isophorone diamine (IPDA), 0.3 parts of di-n-butylamine, 59.1 parts of ethyl acetate and 48.7 parts of isopropanol to a reaction vessel, stir until homogeneous to obtain a mixed solution, and heat to 40-45°C; then add the prepolymer mixture to the mixed solution at a uniform rate over 20 min, and after the dripping is completed, heat to 50°C and keep warm for 3 h to obtain a transparent and stable polyurethane resin for ink, with a solid content of 30.1% and a viscosity of 932 mPa·S / 25°C.

[0035] Comparative Example 1

[0036] Comparative Example 1 is a comparative example of Example 1, the difference being that no diisocyanate monomer is added after prepolymerization.

[0037] (1) 35.8 parts of poly(1,3-propanediol adipate) (Mn = 4000), 17.9 parts of 1,6-HDO / 1,5-PDO polycarbonate diol (Mn = 2000), 6.4 parts of isophorone diisocyanate (IPDI) and 0.1 parts of organic bismuth catalyst were added into a container and stirred evenly. The mixture was kept at 115-120℃ for 6 hours to obtain NCO-terminated polyurethane prepolymer.

[0038] (2) After cooling to below 70°C, 40 parts of ethyl acetate were added and stirred evenly. The mass content of NCO was determined by di-n-butylamine back titration. The NCO% was found to be 0.91%, and the prepolymer mixture was obtained.

[0039] (3) Add 1.97 parts of isophorone diamine (IPDA), 0.23 parts of di-n-butylamine, 57.1 parts of ethyl acetate and 47.6 parts of isopropanol to a reaction vessel, stir and mix evenly, and heat to 40-45°C; then add the prepolymer mixture to the mixed solution at a uniform rate over 20 min, and after the dripping is completed, heat to 50°C and keep warm for 3 h to obtain a transparent and stable polyurethane resin with a solid content of 30.0% and a viscosity of 748 mPa·S / 25°C.

[0040] Comparative Example 2

[0041] Comparative Example 2 is a comparative example of Example 1, except that no polycarbonate diol is added after prepolymerization.

[0042] (1) 54 parts of poly(1,3-propanediol adipate) (Mn = 4000), 4.8 parts of isophorone diisocyanate (IPDI) and 0.1 parts of organic bismuth catalyst were added into a container and stirred evenly. The mixture was kept at 115-120℃ for 6 hours to obtain NCO-terminated polyurethane prepolymer.

[0043] (2) After cooling to below 70°C, 40 parts of ethyl acetate and 1.1 parts of isophorone diisocyanate (IPDI) were added and stirred until homogeneous. The mass content of NCO was determined by di-n-butylamine back titration, and the NCO% was found to be 1.13%, thus obtaining a phase-separated prepolymer mixture.

[0044] (3) Add 2.3 parts of isophorone diamine (IPDA), 0.29 parts of di-n-butylamine, 58 parts of ethyl acetate and 48.2 parts of isopropanol to a reaction vessel, stir until homogeneous to obtain a mixed solution, and heat to 40-45°C; then add the prepolymer mixture to the mixed solution at a uniform rate over 20 min, and after the dripping is completed, heat to 50°C and keep warm for 3 h to obtain a transparent and stable polyurethane resin for ink, with a solid content of 30.2% and a viscosity of 1422 mPa·S / 25°C.

[0045] Example 2

[0046] A polyurethane resin for inks that is resistant to solvent-free adhesives is prepared by the following method:

[0047] (1) 42.5 parts by weight of poly(1,3-propanediol adipate) (Mn = 4000), 10.4 parts by weight of 1,6-HDO / 1,5-PDO polycarbonate diol (Mn = 2000), 5.62 parts by weight of isophorone diisocyanate (IPDI) and 0.1 parts by weight of organic bismuth catalyst were added into a container and stirred evenly. The mixture was kept at 115-120℃ for 6 hours to obtain NCO-terminated polyurethane prepolymer.

[0048] (2) After cooling to below 70°C, 40 parts of ethyl acetate and 1.41 parts of isophorone diisocyanate IPDI were added and stirred evenly. The mass content of NCO was determined by di-n-butylamine back titration. The NCO% was found to be 1.33%, and a phase-separated prepolymer mixture was obtained.

[0049] (3) Add 2.79 parts of isophorone diamine (IPDA), 0.3 parts of di-n-butylamine, 58.6 parts of ethyl acetate and 48.6 parts of isopropanol to a reaction vessel, stir until homogeneous to obtain a mixed solution, and heat to 40-45°C; then add the prepolymer mixture to the mixed solution at a uniform rate over 20 min, and after the dripping is completed, heat to 50°C and keep warm for 3 h to obtain a transparent and stable polyurethane resin for ink, with a solid content of 30.0% and a viscosity of 812 mPa·S / 25°C.

[0050] Example 3

[0051] A polyurethane resin for inks that is resistant to solvent-free adhesives is prepared by the following method:

[0052] (1) 35.3 parts by weight of poly(adipate-neopentyl glycol) diol (Mn = 4000), 17.6 parts by weight of 1,6-HDO / 1,5-PDO polycarbonate diol (Mn = 2000), 6.26 parts by weight of isophorone diisocyanate (IPDI) and 0.1 parts by weight of organic bismuth catalyst were added into a container and stirred evenly. The mixture was kept at 115-120℃ for 6 hours to obtain NCO-terminated polyurethane prepolymer.

[0053] (2) After cooling to below 70°C, 40 parts of ethyl acetate and 0.78 parts of isophorone diisocyanate IPDI were added and stirred evenly. The mass content of NCO was determined by di-n-butylamine back titration. The NCO% was found to be 1.18%, and a phase-separated prepolymer mixture was obtained.

[0054] (3) Add 2.49 parts of isophorone diamine (IPDA), 0.3 parts of di-n-butylamine, 58.2 parts of ethyl acetate and 48.3 parts of isopropanol to a reaction vessel, stir until homogeneous to obtain a mixed solution, and heat to 40-45°C; then add the prepolymer mixture to the mixed solution at a uniform rate over 20 min, and after the dripping is completed, heat to 50°C and keep warm for 3 h to obtain a transparent and stable polyurethane resin for ink, with a solid content of 30.2% and a viscosity of 858 mPa·S / 25°C.

[0055] Example 4

[0056] A polyurethane resin for inks that is resistant to solvent-free adhesives is prepared by the following method:

[0057] (1) 33 parts by mass of poly(1,3-propanediol adipate) (Mn = 2000), 16.5 parts by mass of 1,6-HDO / 1,5-PDO polycarbonate diol (Mn = 2000), 8.75 parts by mass of isophorone diisocyanate (IPDI) and 0.1 parts by mass of organic bismuth catalyst were added into a container and stirred evenly. The mixture was kept at 115-120℃ for 6 hours to obtain NCO-terminated polyurethane prepolymer.

[0058] (2) After cooling to below 70°C, 40.3 parts of ethyl acetate and 2.2 parts of isophorone diisocyanate (IPDI) were added and stirred evenly. The mass content of NCO was determined by di-n-butylamine back titration. The NCO% was found to be 2.01%, and a phase-separated prepolymer mixture was obtained.

[0059] (3) Add 3.92 parts of isophorone diamine (IPDA), 0.72 parts of di-n-butylamine, 61.0 parts of ethyl acetate and 49.8 parts of isopropanol to a reaction vessel, stir until homogeneous to obtain a mixed solution, and heat to 40-45°C; then add the prepolymer mixture to the mixed solution at a uniform rate over 20 min, and after the dripping is completed, heat to 50°C and keep warm for 3 h to obtain a transparent and stable polyurethane resin for ink, with a solid content of 29.8% and a viscosity of 964 mPa·S / 25°C.

[0060] The inks obtained in Examples 1-5 and Comparative Examples 1-2 were formulated into inks using polyurethane resin according to the ink formulations described in Table 1, i.e., the material ① in Table 1 was added to 300 cm³ of water. 3 The ink was ground for 2 hours in a fluorinated bottle using an ink mixer; then the material ② in Table 1 was added and ground for 30 minutes using an ink mixer to obtain a printing ink sample.

[0061] Table 1 Ink Formulation

[0062]

[0063]

[0064] The printing ink samples prepared using the above method were subjected to the following performance tests, and the test results are shown in Table 2:

[0065] 1) Ink adhesion test on film: Use a squeegee to apply the ink sample to the surface-treated plastic film, blow dry the ink sample with a hair dryer, and form an ink layer with a thickness of about 2μm after the ink dries; stick 3M tape on the coated surface, peel the tape off at a right angle to the coating at a uniform speed, and observe the coating state.

[0066] Evaluation criteria: × indicates less than 40% coating residue; △ indicates 40% to 80% coating residue; ○ indicates more than 80% coating residue.

[0067] 2) Ink Viscosity Test: Heat the printing ink sample to 25°C. Pour the sample into a #3 Zahn cup until the ink level is flush with the edge of the cup. Use a glass rod to remove air bubbles. The time required for the ink sample to flow from the initial flow to the point where the flow stops in a thread-like pattern and begins to droplet-like is the ink viscosity. For details, refer to the ink viscosity testing method GB / T13217.4-91.

[0068] 3) Ink stability test: Place the prepared printing ink sample at room temperature (about 25°C) for 30 days and observe the ink layering and sedimentation.

[0069] Evaluation criteria: × indicates ink sedimentation and layering; △ indicates ink not layered, with a small amount of sediment; ○ indicates ink with no sedimentation and no layering.

[0070] 4) Ink resolubility test: Use a squeegee to apply the printing ink sample to the plexiglass and allow the ink to dry to form a printing layer with a thickness of about 2μm. After drying at room temperature (about 25℃) for 2 to 3 minutes, immerse it in a mixed solution of n-propyl acetate / isopropanol with a mass ratio of 8 / 2 and observe the resolubility of the ink layer.

[0071] Evaluation criteria: × indicates that the ink layer is completely undissolved; △ indicates that the ink layer is dissolved but not completely dissolved; ○ indicates that the ink layer is completely dissolved.

[0072] 5) Ink resistance test to polyester solvent-free adhesive: A printing ink sample was applied to a surface-treated PET plastic film using a squeegee. The ink sample was then dried with a blower until it formed an ink layer approximately 2 μm thick. A two-component solvent-free adhesive was then applied to the ink layer using a squeegee at an application rate of 2–2.5 g / m². 3The two-component solvent-free adhesives used were Asahikawa Chemical's XCNA-2020A / B (main agent is polyester type), XCNA-3030A / B (main agent is polyether type), and XCNA-5030A / B (main agent is polyester polyether mixed type). After being laminated with surface-treated VMCPP, the adhesives were placed in an oven at 45℃ for 24 hours to mature. After being removed, the ink dissolution was observed.

[0073] Evaluation criteria: × indicates severe ink dissolution; △ indicates slight ink dissolution; ○ indicates no ink dissolution.

[0074] 6) Ink Composite Strength Test: Using a squeegee, a printing ink sample is coated onto a surface-treated PET plastic film. The ink sample is then dried with a blower until it forms an ink layer approximately 2 μm thick. A two-component solvent-free adhesive is then applied onto the ink layer using a squeegee, with an application rate of 2–2.5 g / m². 3 The two-component solvent-free adhesive used is Asahikawa Chemical XCNA-2020A / B; then, after lamination with surface-treated VMCPP, it is placed in an oven at 45℃ for 48 hours to mature. The composite film is cut into 15mm wide pieces, and the peel strength is measured and read using an electronic tensile testing machine; the higher the reading, the higher the composite strength.

[0075] Table 2 Test results of Examples 1-5 and Comparative Examples 1-2

[0076]

[0077]

[0078] As shown in Table 2, Comparative Example 1 did not include diisocyanate in segments, and the highly polar polyurea molecular chain -NH-NCO-R-NCO-NH- was not formed in the molecular chain. Therefore, the ink formulated with this example exhibited severe ink solubility in polyester-based solvent-free adhesives. Comparative Example 2 did not include polycarbonate diol, and the ink formulated with this example also showed poor ink solubility in polyester-based solvent-free adhesives. Furthermore, due to the absence of polycarbonate diol, the composite strength of the ink formulated with this example also decreased significantly. Examples 1-4, by adding polycarbonate diol and segmenting diisocyanate to form the highly polar polyurea molecular chain -NH-NCO-R-NCO-NH-, not only improved the ink's ink solubility resistance to polyester-based solvent-free adhesives but also resulted in superior properties such as composite strength.

[0079] Therefore, it can be seen that the ink prepared by the polyester-resistant solvent-free adhesive polyurethane resin obtained by the present invention has good adhesion to PET, OPP and other films, high composite strength, and is resistant to polyester-resistant solvent-free adhesive ink dissolution, and is suitable for use in combination with polyester-resistant solvent-free adhesive for lamination.

Claims

1. A method for preparing a polyurethane resin for inks resistant to solvent-free adhesives, characterized in that, Includes the following steps: (1) A prepolymer is prepared by mixing polyester diol, polycarbonate diol and diisocyanate at a temperature of 100-130°C to obtain an NCO-terminated polyurethane prepolymer; the weight ratio of polyester diol to polycarbonate diol is (1.5-2.5):1; the molar ratio of diisocyanate to the total molar ratio of polyester diol and polycarbonate diol is (1.50-1.70):1; the polycarbonate diol is a polycarbonate with 1,6-hexanediol and 1,5-pentanediol as end groups and a molecular weight of 1500-2500. (2) Add solvent and diisocyanate to the polyurethane prepolymer and mix them evenly to form a phase-separated prepolymer mixture. At this time, the NCO content of the mixture is 0.5% to 2.5%. The mass ratio of the diisocyanate added in step (2) to the diisocyanate in step (1) is 1:(3 to 8). (3) The prepolymer mixture is added to a solution of a diamine chain extender, an organic amine end-capping agent and a solvent to carry out a chain extension reaction at a temperature of 30-50°C to obtain the polyurethane resin for ink that is resistant to solvent-free adhesives; the ratio of the molar number of amine groups in the diamine chain extender to the molar number of NCO groups in the prepolymer mixture is (0.93-1.05):1; The raw materials are in the following proportions by weight: 30-50 parts polyester diol, 8-20 parts polycarbonate diol, 0.01-0.1 parts catalyst, 5-15 parts diisocyanate, 1-7 parts diamine chain extender, 0.1-2 parts organic amine end-capping agent, and 120-180 parts solvent.

2. The method for preparing polyurethane resin for ink according to claim 1, characterized in that, The polyester diol is a hydroxyl-terminated polyester diol formed by the condensation polymerization of a saturated aliphatic diol and an aliphatic or aromatic dicarboxylic acid, with a molecular weight of 1000-5000; the diisocyanate is at least one selected from isophorone isocyanate, toluene diisocyanate, diphenylmethane-4,4'-diisocyanate, and 1,6-hexanediisocyanate; the diamine chain extender is at least one selected from 1,3-cyclohexanediamine, isophorone diamine, trimethylhexanediamine, and hexanediamine.

3. The method for preparing polyurethane resin for ink according to claim 2, characterized in that, The polyester diol has a molecular weight of 2000-4000, the polycarbonate diol is T5652 type polycarbonate from Asahi Kasei Chemicals Co., Ltd. of Japan, with a molecular weight of 2000; the diisocyanate is isophorone isocyanate or / and diphenylmethane-4,4'-diisocyanate; the diamine chain extender is 1,3-cyclohexanediamine or / and isophoronediamine.

4. The method for preparing polyurethane resin for ink according to claim 2, characterized in that, The saturated aliphatic diol is at least one of 1,2-propanediol, 1,3-propanediol, methylpropanediol, 1,4-butanediol, neopentyl glycol, and 3-methyl-1,5pentylene glycol; the aliphatic or aromatic dicarboxylic acid is at least one of sebacic acid, adipic acid, azelaic acid, terephthalic acid, and isophthalic acid.

5. The method for preparing polyurethane resin for ink according to claim 2, characterized in that, The saturated aliphatic diol is at least one of 1,3-propanediol, methylpropanediol, and neopentyl glycol, and the aliphatic or aromatic dicarboxylic acid is at least one of sebacic acid and adipic acid.

6. The method for preparing polyurethane resin for ink according to claim 1, characterized in that, The catalyst is at least one of organic bismuth, tetrabutyl titanate, stannous octoate, or stannous chloride; the organic amine end-capping agent is at least one of ethanolamine, diethanolamine, isopropanolamine, di-n-butylamine, or 2-ethylhexylamine; and the solvent is at least one of ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, or isopropanol.

7. The method for preparing polyurethane resin for ink according to claim 1, characterized in that, The duration of the prepolymerization reaction is 4 to 10 hours; the duration of the chain extension reaction is 0.5 to 4 hours; in step (2), the prepolymer mixture is uniformly dropped into a solution composed of a diamine chain extender, an organic amine end-capping agent and a solvent within 15 to 30 minutes to carry out the chain extension reaction.

8. The polyurethane resin for ink prepared by the method of any one of claims 1 to 7, which is resistant to solvent-free adhesives, has a rotational viscosity of 800 to 2000 mPa·s / 25℃ and a solid content of 30 ± 2%.

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