Printing ink for environment-friendly cigarette case packaging paper and preparation method of printing ink

Abstract

The invention relates to the technical field of printing ink for packaging, in particular to printing ink for environment-friendly cigarette case packaging paper and a preparation method of the printing ink. The problems that traditional ink is poor in environmental protection property and comprehensive performance are solved. Deionized water is used as a dispersion medium, so that emission of volatile organic compounds is reduced; through the synergistic effect of a free radical-cation double initiation system, a rigid-flexible interpenetrating network is formed; through chemical bonding modification and nano silicon dioxide enhancement, the adhesive force and the wear resistance of the ink are remarkably improved; casein is subjected to hydrolytic modification and dynamic crosslinking, a hydrophobic-durable composite structure is constructed, and heat resistance and water resistance are enhanced; infrared pre-drying and a dual-curing process are combined to ensure that an ink layer is rapidly and compactly formed; the polyurethane microsphere core-shell structure absorbs thermal stress and inhibits high-temperature deformation; all the components are synergistically optimized, and preparation parameters are reasonably matched, so that the ink has adhesive force, wear resistance, heat resistance and environmental friendliness.

Description

Technical Field

[0001] The present invention relates to the technical field of packaging inks, and specifically to an environmentally friendly printing ink for cigarette box wrapping paper and a preparation method thereof. Background Art

[0002] As the outer packaging of cigarette products, the printing quality and safety of cigarette boxes directly affect the product image and consumer health. At present, cigarette box packaging printing inks are mainly divided into solvent-based inks and water-based inks.

[0003] Although traditional solvent-based inks have advantages such as fast drying, good adhesion, and high gloss, they contain a large amount of volatile organic compounds (VOCs), such as toluene, xylene, ethyl acetate, etc. These VOCs are emitted into the atmosphere during the ink production, printing process, and waste treatment process, polluting the environment and posing a potential threat to the health of production workers and end consumers. In addition, the harmful solvents that may remain in the solvent-based inks may migrate into the cigarette products, posing a safety hazard to human health.

[0004] With the increasingly strict global environmental protection regulations and the improvement of consumers' environmental awareness, the development of environmentally friendly printing materials has become an industry trend. Water-based inks use water as the main solvent, significantly reducing the emission of VOCs, and are considered an ideal choice to replace solvent-based inks; however, when existing water-based inks are applied to substrates such as cigarette box wrapping paper that are usually coated and have a low surface energy, they still face some challenges; after printing, the wetting and penetration ability of the water-based resin on non-absorbent or low-absorbent coated paper is relatively weak, which may lead to insufficient adhesion and is likely to fall off during subsequent processing or use; the evaporation rate of water is slower than that of organic solvents, which may affect the printing efficiency; after the water-based ink film dries, its abrasion resistance and stability after encountering water are sometimes not as good as those of solvent-based inks; the rheology, transferability, etc. of water-based inks need to be precisely controlled to meet the requirements of specific printing.

[0005] Therefore, it is of great practical significance and market demand to develop an environmentally friendly water-based printing ink that not only meets environmental protection requirements but also can achieve excellent printing performance on cigarette box wrapping paper and has good wear resistance and waterproof properties.

[0006] For this reason, an environmentally friendly printing ink for cigarette box wrapping paper and a preparation method thereof are proposed. Summary of the Invention

[0007] The object of the present invention is to provide a printing ink for environmentally friendly cigarette box wrapping paper and a preparation method thereof. By using deionized water as a dispersion medium, the emission of volatile organic compounds is reduced; through the synergistic effect of a free radical-cationic dual-initiating system, an interpenetrating network combining rigidity and flexibility is formed; through chemical bonding modification and nano-silica reinforcement, the adhesion and abrasion resistance of the ink are significantly improved; casein is hydrolytically modified and dynamically crosslinked to construct a hydrophobic-durable composite structure, enhancing heat resistance and water resistance; the combination of infrared pre-drying and dual-curing processes ensures rapid and dense formation of the ink layer; the core-shell structure of polyurethane microspheres absorbs thermal stress and inhibits high-temperature deformation; through the synergistic optimization of each component and the reasonable matching between preparation parameters, the ink has both adhesion, abrasion resistance, heat resistance and environmental friendliness.

[0008] To achieve the above object, the present invention provides the following technical solutions:

[0009] The present invention provides a preparation method of a printing ink for environmentally friendly cigarette box wrapping paper, and the preparation method is as follows:

[0010] S1 Prepolymer synthesis: Add isophorone diisocyanate, polytetrahydrofuran ether diol, dimethylolpropionic acid, and pentaerythritol triacrylate to a reaction kettle. After heating, add a catalyst. After the reaction, add KH550, continue the reaction and then cool down. Dropwise add glycidyl methacrylate, keep warm and react, add triethylamine and then transfer to a disperser, and add deionized water to obtain a prepolymer emulsion; this process is protected by nitrogen and kept away from light throughout; the particle size D50 of the prepolymer emulsion after dispersion is ≤150 nm; the molecular weight of polytetrahydrofuran ether diol is 1000.

[0011] S2 Modified casein preparation: Partially hydrolyze casein and then add cystamine, perfluorooctyltriethoxysilane alcohol, EDC, and NHS, and react to obtain cystamine-modified casein; add cystamine-modified casein to a hydroxymethyldiacetone acrylamide solution, heat up and add N-hydroxymethylacrylamide, reflux and react and then dry to obtain modified casein.

[0012] S3 Nano-silica dispersion: Add acrylated nano-silica to the prepolymer emulsion and disperse to obtain a prepolymer dispersion; the particle size of acrylated nano-silica is 20 - 50 nm.

[0013] S4 Ink formulation: Mix the prepolymer dispersion, polyethylene glycol, triethylene glycol divinyl ether, diglycidyl ether, polyurethane microspheres, and modified casein, and then add a free radical initiator, iodonium salt, water-based pigment, wetting agent, and defoaming agent, and ball mill to obtain the ink to be cured; the preparation process is carried out in the dark; finally, filter and seal, and the filter bag size used for filtration is 5 μm; the particle size of polyurethane microspheres is 100 - 200 nm; the average molecular weight of polyethylene glycol is 400.

[0014] The water-based pigment can be a red water-based pigment or water-based pigments of other colors such as yellow, blue, and green.

[0015] After printing and coating by the S5 curing process, pre-drying and ultraviolet-thermal dual curing are carried out to obtain the printing ink.

[0016] By first performing ultraviolet curing and then thermal curing, ultraviolet curing rapidly initiates the free radical polymerization of acrylate double bonds to form a rigid network structure, providing initial hardness and surface wear resistance; later thermal curing triggers the ring-opening reaction of epoxy groups, and through catalysis, a flexible cross-linked network is formed, forming an interpenetrating structure with the rigid acrylate network. First performing thermal curing at high temperature initiates the early ring-opening of epoxy groups, consuming the activity of the iodonium salt, resulting in insufficient free radical polymerization during ultraviolet curing, imbalance of the double network structure, side reactions between the cationic initiator of the thermal curing agent and the free radical initiator, and reduction of the cross-linking efficiency; as well as the mismatch of the time scales of ultraviolet curing and thermal curing, resulting in local over-cured or uncured areas. Ultraviolet curing is preferred to rapidly form a rigid skeleton, fix the ink morphology, and avoid structural deformation caused by subsequent thermal curing; thermal curing is used for supplementation, and on the basis of the rigid network, a flexible cross-linking is constructed through the ring-opening of epoxy groups to form a "rigid-flexible combination" network structure and optimize the comprehensive performance.

[0017] Risk of material deformation: The uncured resin is prone to flow during thermal curing, resulting in uneven coating or out-of-control thickness.

[0018] Preferably, in S1, the mass feeding ratio of isophorone diisocyanate, polytetrahydrofuran ether glycol, dimethylolpropionic acid, pentaerythritol triacrylate, glycidyl methacrylate, and triethylamine is 35-50:55-65:5-9:20-30:13-18:4.0-6.5; the temperature is raised to 70-80 °C; the temperature is lowered to 55 °C; and the holding reaction time is 1.5-2.5 h.

[0019] Preferably, the preparation method of the modified casein in S2 is as follows:

[0020] Deionized water is added to hydroxymethyldiacetone acrylamide and stirred until completely dissolved to obtain a hydroxymethyldiacetone acrylamide solution; cystamine-modified casein is added to the hydroxymethyldiacetone acrylamide solution, and after stirring evenly, heating is started. When the temperature is raised to 80 °C, N-hydroxymethylacrylamide is dropped in at a rate of 5 drops / s, and at the same time, an aqueous solution of potassium sulfate with a mass fraction of 5% is dropped in at a rate of 5 drops / s, and the pH of the system is adjusted to 9. The temperature is raised at a rate of 20 °C / min and refluxed for 2.5 h to obtain a modified casein precursor; the modified casein precursor is poured onto the surface of a polytetrafluoroethylene plate and dried to obtain the modified casein.

[0021] Preferably, the preparation method of cystamine-modified casein is as follows: Disperse casein in a phosphate buffer solution preheated to 50 °C with a solid-liquid ratio of 1:10, and the pH of the phosphate buffer solution is 8.0; Stir at 300 rpm for 2 h at 60 °C until completely dissolved to form a homogeneous colloidal solution; Adjust the pH to 8.5 with 1M NaOH solution; Add 2 parts of alkaline protease to the solution, keep the temperature at 50 °C, continuously stir at 200 rpm and maintain pH 8.5, react for 1 h, and cool the reaction vessel in ice water to terminate the reaction, finally obtaining a partially hydrolyzed casein solution; The final degree of hydrolysis is 5%-8%;

[0022] Place the partially hydrolyzed casein solution in a reaction vessel, and slowly add cystamine and perfluorooctyltriethoxysilanol under stirring conditions; Adjust the pH value to 8, and react at 45 °C for 3 h; After the reaction is completed, remove the unreacted cystamine and other impurities by ultrafiltration to obtain cystamine-modified casein.

[0023] Preferably, the preparation method of perfluorooctyltriethoxysilanol is as follows: Add deionized water to perfluorooctyltriethoxysilane; Then dropwise add hydrochloric acid with a mass fraction of 3% to adjust the pH value of the reaction system to 3; Under stirring, heat the reaction mixture to 60 °C and reflux for 4 h to obtain a crude product of perfluorooctyltriethoxysilanol; After the reaction is completed, remove the unreacted substances and water in the crude product by vacuum distillation to obtain perfluorooctyltriethoxysilanol.

[0024] Preferably, the mass dosage ratio of acrylated nano-silica in S3, polyurethane microspheres in S4, and modified casein is 3-5:2-3.5:20-25; For the prepolymer dispersion obtained after dispersing nano-silica, it is confirmed by a laser particle size analyzer that D50≤100 nm.

[0025] Preferably, the mass dosage ratio of the dry weight of the prepolymer dispersion in S4, polyethylene glycol, triethylene glycol divinyl ether, diglycidyl ether, polyurethane microspheres, and modified casein is 60-75:10-15:5-8:3.5-5:3-3.5:1.8-2.2.

[0026] Preferably, the coating amount of printing and coating in S5 is 2.0±0.2 g / m 2 ; The pre-drying time is 1.5-3 s; The energy density of ultraviolet curing in ultraviolet-thermal dual curing is 200-280 mJ / cm 2 .

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] 1. Through the design of the aqueous system and the optimization of the dual-curing process, the environmental friendliness and curing efficiency of the ink have been significantly improved. Using waterborne polyurethane acrylate prepolymer as the base material, deionized water as the main dispersion medium, and combining with low-VOC co-solvents, the emission of volatile organic compounds in the ink has been greatly reduced; by introducing a free radical-cationic dual-initiating system, under the synergistic action of ultraviolet light irradiation and thermal activation, the ink rapidly forms an interpenetrating network structure with both rigidity and flexibility; nitrogen protection and full-process light avoidance operation effectively avoid side reactions, ensuring the stability of prepolymer synthesis. At the same time, through carboxyl neutralization and silane coupling technology, the emulsion dispersion and storage stability are significantly improved; the cured ink surface is dense and defect-free, suitable for printing requirements, and meets strict environmental protection standards.

[0029] 2. Through chemical bonding design and nano-enhancement technology, the adhesion and abrasion resistance of the ink are synergistically improved. Epoxy groups and multi-functional acrylates are introduced into the prepolymer to form a chemical cross-linking network during the dual-curing process, enhancing the chemical bonding force with the substrate; after surface modification, nano-silica particles are evenly embedded in the polyurethane network through high-speed dispersion and ultrasonic treatment to form a rigid reinforcement phase, effectively inhibiting the concentration of friction stress. At the same time, after hydrolysis modification and dynamic cross-linking of casein, its short peptide chains cooperate with components such as fluorosilane and cystamine to construct a hydrophobic and durable composite structure; finally, the cured ink has both excellent adhesion and anti-wear ability.

[0030] 3. Through the dynamic cross-linking network and nano-composite enhancement strategy, the heat resistance and high-temperature stability of the ink are significantly improved. The disulfide bonds and fluorocarbon chains introduced into the modified casein can dissipate heat energy through reversible fracture at high temperatures, delaying thermal degradation; acrylated silica is embedded in the polyurethane network through chemical bonding to improve the thermal conductivity and rigidity of the material; the core-shell structure of polyurethane microspheres absorbs thermal stress to reduce the propagation of microcracks; the interpenetrating network formed by the dual-curing process further balances rigidity and toughness, inhibiting the fracture of molecular chains at high temperatures; the ink still maintains a complete structure after heat treatment, with no discoloration or cracking on the surface, suitable for high-temperature processing environments.

[0031] 4. Through the synergistic design of multi-functional components and the precise control of process parameters, the comprehensive optimization of the ink performance has been achieved. The hydrolysis modification of casein and silane coupling technology enhance its compatibility with the polyurethane matrix, and at the same time, the introduced reactive diluents and initiators optimize the curing performance; the nano-particle dispersion technology and thickener regulation ensure the stability of the ink viscosity; the combination of infrared pre-drying and dual-curing process effectively removes moisture and accelerates film formation, avoiding curing defects; the above preparation methods and parameters work together to balance the adhesion, abrasion resistance, heat resistance and environmental friendliness of the ink, providing a high-performance solution for cigarette box packaging. Description of the Drawings

[0032] Figure 1The heat resistance test results of the ink of the present invention are shown in FIG. DETAILED DESCRIPTION

[0033] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe 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 the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0034] See also Figure 1 The present invention provides an environmentally friendly printing ink for cigarette box packaging paper and a preparation method thereof. The technical scheme is as follows: Unless otherwise specified, the "parts" in the present invention are all "parts by mass".

[0035] The substance information used in the present invention is as follows:

[0036] Isophorone diisocyanate CAS: 4098-71-9; dimethylolpropionic acid CAS: 4767-03-7; pentaerythritol triacrylate CAS: 3524-68-3; glycidyl methacrylate CAS: 106-91-2; γ-aminopropyltriethoxysilane (KH550) CAS: 919-30-2; triethylamine CAS: 121-44-8; zinc acetylacetonate CAS: 14024-63-6; triethylene glycol divinyl ether CAS: 765-12-8; water-dispersible iodonium salt (UVI-6976), diphenyl [4- (phenylthio) phenyl] - hexafluoroantimonate sulfonium CAS: 71449-78-0; casein CAS: 9000-71-9; cystamine CAS: 51-85-4; perfluorooctyl triethoxysilane CAS: 51851-37-7; hydroxymethyl diacetone acrylamide CAS: 255-936-4; tris (N-nitroso-N-phenylhydroxylamine) aluminum salt CAS: 15305-07-4; alkaline protease CAS: 9014-01-1; 1-ethyl-3- (3-dimethylaminopropyl) carbodiimide (EDC) CAS: 1892-57-5; N-hydroxysuccinimide (NHS) CAS: 6066-82-6; acrylated nano-silica was purchased from Xi'an Qiyue Biotechnology Co., Ltd.; free radical photoinitiator (TPO-L) was purchased from Shanghai Kaiyin Chemical Co., Ltd.; water-based pigments were purchased from BASF; wetting agent (BYK Dynwet 800N) was purchased from Guangzhou Haoyi New Materials Technology Co., Ltd.; defoamer (BYK-024) was purchased from BYK Chemical Additives of Germany; HEUR thickener was purchased from Nanjing Kaishitong New Materials Co., Ltd.; polyurethane microspheres were purchased from Suzhou Zhiyi Microsphere Technology Co., Ltd.

[0037] Example 1

[0038] S1 Prepolymer Synthesis

[0039] Add 40 parts of isophorone diisocyanate, 60 parts of polytetrahydrofuran ether diol, 8 parts of dimethylolpropionic acid and 25 parts of pentaerythritol triacrylate into the reaction kettle, heat up to 75 °C, add 0.1 part of zinc acetylacetonate, and the stirring rate is 300 rpm; sample and titrate the NCO content every 30 min. After reacting for 30 min, add 4 parts of KH550 and continue to react for 3 h to prepare a polyurethane main chain with an NCO content of 3%; cool down to 55 °C, slowly dropwise add 15 parts of glycidyl methacrylate at a dropping rate of 5 drops / min; keep the temperature and react for 1.5 h, monitor the NCO value until it reaches 0.1%; cool down to 45 °C, add 5.5 parts of triethylamine to neutralize the carboxyl group and adjust the pH to 7.5; transfer the prepolymer to a high-speed disperser, slowly add 200 parts of deionized water, and disperse at 2000 rpm for 1 h to obtain a prepolymer emulsion;

[0040] S2 Modified Casein Preparation

[0041] Preparation of partially hydrolyzed casein solution: Disperse 100 parts of casein in a phosphate buffer preheated to 50 °C with a solid-liquid ratio of 1:10 (w / v), and the pH of the phosphate buffer is 8.0; stir at 300 rpm at 60 °C for 2 h until completely dissolved to form a homogeneous colloidal solution; adjust the pH to 8.5 with 1M NaOH solution; add 2 parts of alkaline protease to the solution and keep the temperature at 50 °C; continuously stir at 200 rpm and maintain the pH at 8.5, react for 1 h, and cool the reaction vessel in ice water to terminate the reaction, finally obtaining a partially hydrolyzed casein solution;

[0042] In a reaction vessel equipped with a stirring device and a reflux condenser, add 5 parts of perfluorooctyltriethoxysilane, and then slowly add 30 parts of deionized water; then dropwise add hydrochloric acid with a mass fraction of 3% to adjust the pH value of the reaction system to 3; under stirring, heat the reaction mixture to 60 °C and reflux for 4 h to obtain a crude product of perfluorooctyltriethoxysilanol; after the reaction, remove the unreacted substances and water in the crude product by vacuum distillation to obtain perfluorooctyltriethoxysilanol;

[0043] Place 142 parts of the partially hydrolyzed casein solution in a reaction vessel, and slowly add 6.0 parts of cystamine, 2 parts of perfluorooctyltriethoxysilanol, 0.1 part of EDC and 0.1 part of NHS under stirring conditions; adjust the pH value of the reaction system to 8 with sodium hydroxide solution, and reflux at 45 °C for 3 h; after the reaction, remove the unreacted cystamine, other impurities and solvents by ultrafiltration to obtain cystamine-modified casein;

[0044] Add 150 parts of deionized water to 19.9 parts of hydroxymethyldiacetone acrylamide, and stir until completely dissolved to obtain a hydroxymethyldiacetone acrylamide solution; transfer the hydroxymethyldiacetone acrylamide solution to a four-necked flask, add 40 parts of casein modified with cystamine, stir evenly and then start heating. When the temperature rises to 80 °C, add 4.5 parts of N-hydroxymethyl acrylamide dropwise into the above solution at a rate of 5 drops / s; at the same time, dropwise add 5 parts of a 5% aqueous potassium sulfate solution at a rate of 5 drops / s, and adjust the pH of the system to 9. Heat up to 110 °C at a rate of 20 °C / min and reflux for 2.5 h to obtain a modified casein precursor; pour the modified casein precursor onto the surface of a polytetrafluoroethylene plate and dry at room temperature for 24 h until there are no bubbles on the film surface, and then dry in an oven at 60.0 °C for 6 h to obtain a cured film, that is, modified casein.

[0045] S3 Nano-silica dispersion

[0046] Nano-particle dispersion, add 3 parts of acrylated nano-silica to the prepolymer emulsion, and disperse at high speed at 2000 rpm for 1 h; under the conditions of a power of 300 W and a frequency of 40 kHz, perform ultrasonic-assisted treatment for 30 min to ensure that there is no visible agglomeration, and obtain a prepolymer dispersion;

[0047] S4 Ink formulation

[0048] Take 60 parts of the dry weight of the prepolymer dispersion, 10 parts of polyethylene glycol, 5 parts of triethylene glycol divinyl ether, 3.5 parts of diglycidyl ether, 2 parts of polyurethane microspheres and 20 parts of modified casein, and stir in the dark for 20 min; sequentially add 3 parts of a radical initiator, 2 parts of a water-dispersible iodonium salt, 30 parts of a red water-based pigment, 0.2 part of a wetting agent and 0.3 part of an antifoaming agent; ball mill for 2 h, the ball mill rotation speed is 500 rpm, and the zirconia bead particle size is 0.5 mm to ensure that the pigment particle size ≤ 1 μm; add 0.5 part of a thickener pre-diluted in 5 parts of deionized water; make up deionized water to a total mass of 160 parts, and store in a sealed manner after filtration.

[0049] S5 Curing process

[0050] Printing and coating: Use a gravure printing machine, the screen mesh number is 250 meshes, and the coating amount is 2.0 g / m 2 ;

[0051] Pre-drying: Perform infrared drying at 85 °C, the infrared wavelength is 5 μm, the curing time is 2 s, and the moisture residue ≤ 5%;

[0052] UV-thermal dual curing: UV curing: Irradiate with a mercury lamp for 3 s, the main peak of the mercury lamp is 365 nm, the light intensity is 80 mW / cm 2 、energy density 240 mJ / cm 2; Thermal curing: Hot air circulation at 90 °C for 2 min to trigger deep cross-linking of epoxy groups, and finally dry at room temperature.

[0053] The preparation methods of Examples 2-4 are generally the same as those of Example 1, but the parameters of some preparation methods are changed during the synthesis of the S1 prepolymer, as shown in Table 1 specifically.

[0054] In Comparative Example 1, polytetrahydrofuran ether diol was not added to S1, and other preparation methods were the same as those of Example 1.

[0055] In Comparative Example 2, dimethylolpropionic acid was not added to S1, and other preparation methods were the same as those of Example 1.

[0056] In Comparative Example 3, glycidyl methacrylate and pentaerythritol triacrylate were not added to S1, and other preparation methods were the same as those of Example 1.

[0057] In Comparative Example 4, triethylamine was not used for carboxyl neutralization in S1, and other preparation methods were the same as those of Example 1.

[0058] In Comparative Example 5, after the polyurethane main chain was prepared in S1, no temperature reduction treatment was carried out, and the temperature remained at 75 °C.

[0059] In Comparative Example 6, the total mass was 600 parts after deionized water was supplemented in S4.

[0060] Table 1 Preparation parameters for the synthesis of the S1 prepolymer

[0061]

[0062] Experimental Example 1

[0063] The viscosity of the inks obtained from the above examples and comparative examples was tested before the S5 curing process, specifically according to the method of GB / T 14623-2009 "Determination of Rheological Properties of Inks"; the adhesion was tested after the S5 curing process, according to the method of GB / T 13217.7—2023 "Test Method for Ink Adhesion", and the adhesion substrate used was cigarette box wrapping paper. The test results are shown in Table 2.

[0064] Table 2 Test results of ink viscosity and adhesion

[0065] Group Viscosity (mPa·s) Adhesion (%) Example 1 1320 95 Example 2 1300 90 Example 3 1380 98 Example 4 1310 88 Comparative Example 1 1250-1390 72 Comparative Example 2 1400 64 Comparative Example 3 1250 44 Comparative Example 4 1320 51 Comparative Example 5 1450 59 Comparative Example 6 859 37

[0066] The printing ink for conformal cigarette box wrapping paper prepared by the present invention has a viscosity of 1300 - 1380 mPa·s and an adhesion of 88% - 95% under the conditions of Examples 1 - 4. The prepolymer is prepared under nitrogen protection and full light avoidance. The isocyanate group in isophorone diisocyanate and the hydroxyl group in polytetrahydrofuran ether diol undergo a stepwise addition polymerization reaction to form a polyurethane main chain. The content of isocyanate groups is controlled by the reaction process. The hydroxyl group in dimethylolpropionic acid reacts with the isocyanate group to introduce a carboxyl group into the polyurethane main chain, providing conditions for subsequent emulsification and water-based treatment. At the same time, during the reaction process, the amino group of KH550 undergoes a condensation reaction with the groups in the system, and the epoxy group of glycidyl methacrylate partially opens under heating conditions and reacts with the isocyanate group and carboxyl group in the system. Triethylamine neutralizes the carboxyl group to form carboxylate, making the prepolymer hydrophilic. The introduced pentaerythritol triacrylate and glycidyl methacrylate provide chemically bonded reaction active sites for subsequent crosslinking and curing reactions, so that the ink has a moderate viscosity before curing and good adhesion to the wrapping paper after curing. The ink obtained in Comparative Example 1 has unstable viscosity. The lack of polytetrahydrofuran ether diol leads to excessive rigidity. The absence of PETA reduces the crosslinking density, resulting in a significant decrease in adhesion. And the failure to form a stable prepolymer and uneven dispersion lead to viscosity fluctuations, and the ink viscosities obtained from different sampling points in the same container are different. In Comparative Example 2, the lack of carboxyl group leads to unstable emulsion, poor film-forming property of the ink, reduced adhesion, and easy agglomeration of the unneutralized prepolymer, with too high viscosity. In Comparative Example 3, glycidyl methacrylate and pentaerythritol triacrylate are not added, and the subsequent thermal curing and photocuring processes cannot proceed smoothly, relying only on physical adsorption, with insufficient adhesion and a significant decrease in viscosity. In Comparative Example 4, the unneutralized carboxyl group leads to emulsion stratification, the ink cannot form a uniform film, the adhesion is reduced, and the viscosity does not change significantly. In Comparative Example 5, after the polyurethane main chain is prepared, no temperature reduction treatment is carried out. At high temperature, glycidyl methacrylate self-polymerizes and side reactions destroy the prepolymer structure, resulting in a decrease in adhesion, molecular chain breakage or uneven crosslinking, and abnormally high viscosity. In Comparative Example 6, excessive dilution leads to low solid content, incomplete film formation, extremely poor adhesion, and a significant decrease in viscosity.

[0067] In Examples 5 - 6, the ink formulation of S4 and the process parameters of S5 curing are changed, as shown in Table 3 specifically. Other preparation steps are the same as those in Example 3. The adjustment of the ultraviolet energy density is achieved through the irradiation time. The irradiation time in Example 5 is 2.5 s, and the irradiation time in Example 6 is 3.5 s.

[0068] Table 3 Ink formulation of S4 and process parameters of S5 curing

[0069]

[0070]

[0071] In Comparative Example 7, the dry weight of the prepolymer dispersion was 100 parts, and the other preparation steps were the same as those in Example 3.

[0072] In Comparative Example 8, no prepolymer dispersion was added, and the other preparation steps were the same as those in Example 3.

[0073] In Comparative Example 9, no polyethylene glycol was added, and the other preparation steps were the same as those in Example 3.

[0074] In Comparative Example 10, no triethylene glycol divinyl ether was added, and the other preparation steps were the same as those in Example 3.

[0075] In Comparative Example 11, no diglycidyl ether was added, and the other preparation steps were the same as those in Example 3.

[0076] In Comparative Example 12, no free radical initiator and water-dispersible iodonium salt were added, and the other preparation steps were the same as those in Example 3.

[0077] In Comparative Example 13, the coating amount was 4.0 g / m 2 , and the other preparation steps were the same as those in Example 3.

[0078] In Comparative Example 14, no pre-drying process was carried out, and the other preparation steps were the same as those in Example 3.

[0079] In Comparative Example 15, nano-silica was added in S3 instead of acrylated nano-silica, and the other preparation steps were the same as those in Example 3.

[0080] In Comparative Example 16, no acrylated nano-silica was added in S3, and no polyurethane microspheres were added in S4, and the other preparation steps were the same as those in Example 3.

[0081] Experimental Example 2

[0082] The abrasion resistance of the inks obtained from the above examples and comparative examples was tested. A standard eraser with a hardness of 70 Shore A was used to rub the ink layer at a fixed pressure of 500 g and a speed of 40 times / min. The number of rubs before the ink layer peeled off was recorded. The ink for cigarette box packaging needs to be ≥1500 times (without showing the bottom layer) to be qualified. The final test results are shown in Table 4.

[0083] Table 4 Abrasion resistance test results of Example 3, Examples 5-6, Comparative Example 3 and Comparative Examples 7-16

[0084] Group Wear resistance (friction times) Group Wear resistance (friction times) Example 3 1800 Comparative Example 10 1100 Example 5 2100 Comparative Example 11 900 Example 6 1950 Comparative Example 12 700 Comparative Example 3 800 Comparative Example 13 900 Comparative Example 7 1150 Comparative Example 14 600 Comparative Example 8 600 Comparative Example 15 1500 Comparative Example 9 1300 Comparative Example 16 450

[0085] For the ink prepared by the preparation method of the present invention, the friction times are 1800 - 2100 times under the conditions of Example 3, Example 5 and Example 6, and the wear resistance is qualified. During the dispersion process of S3 nano-silica, high-speed dispersion and ultrasonic treatment are used to ensure that the nano-particles are evenly dispersed in the prepolymer emulsion. The surface modification of the introduced nano-silica enables it to form chemical bonds with the polyurethane matrix. The silica particles are embedded in the polyurethane network through chemical bonds and physical adsorption to form a rigid reinforcing phase, enhancing the wear resistance; when formulating the ink in step S4, the prepolymer emulsion, active diluent, initiator and other additives are mixed; polyethylene glycol diacrylate is used as the active diluent to reduce the viscosity and participate in free radical polymerization. The added prepolymer provides double curing sites, including acrylate and epoxy groups. Triethylene glycol divinyl ether and epoxy groups synergistically carry out cationic curing to form a flexible network. The free radical initiator TPO-L decomposes into free radicals under ultraviolet light irradiation to initiate the polymerization of acrylate double bonds. The water-dispersible iodonium salt ultraviolet I-6976 releases hydrogen ions under the condition of thermal activation at 90 °C, catalyzing the ring-opening polymerization of epoxy groups to form an interpenetrating network structure, which synergistically improves the wear resistance of the ink with nano-silica; in the curing process of S5, infrared pre-drying is first carried out to remove moisture, and then ultraviolet light is used to initiate free radical polymerization, and thermal curing promotes the ring-opening polymerization of epoxy groups. The double curing mechanism ensures that the ink layer is quickly formed and has a dense structure; the rigid acrylate network and the flexible epoxy network penetrate each other, and synergistically with nano-silica and polyurethane microspheres, the wear resistance of the ink is improved.

[0086] In Comparative Example 3, glycidyl methacrylate and pentaerythritol triacrylate were not added, lacking chemical cross-linking sites, and the curing network was loose, resulting in a significant decrease in wear resistance. In Comparative Example 7, the prepolymer dispersion was excessive, and the high solid content led to uneven dispersion and a decrease in wear resistance. In Comparative Example 8, the prepolymer dispersion was not added, lacking film-forming substances, and an effective protective layer could not be formed, resulting in a significant decrease in wear resistance. In Comparative Example 9, the absence of the active diluent led to insufficient cross-linking and a decrease in wear resistance. In Comparative Example 10, triethylene glycol divinyl ether was not added, resulting in blocked cationic curing and insufficient cross-linking of epoxy groups, reducing the performance of the ink. In Comparative Example 11, diglycidyl ether was not added, lacking epoxy cross-linking points, and the curing network was loose. In Comparative Example 12, the initiator was not added, the ink could not be cured, and the film-forming effect became poor. Only the single modified casein played a film-forming role, thus leading to a decrease in wear resistance. In Comparative Example 13, the coating amount was too high, and the too thick ink layer led to uneven internal stress and a decrease in wear resistance. In Comparative Example 14, pre-drying was not carried out, and the residual moisture caused curing defects, and the ink layer was easily peeled off. In Comparative Example 15, nano-silica was added instead of acrylated nano-silica, and there was no substance in the surface of the nano-particles that could cross-link with the system, and the wear resistance decreased compared with Example 3. In Comparative Example 16, acrylated nano-silica and polyurethane microspheres were not added, and the friction times decreased significantly, and the wear resistance decreased.

[0087] In Comparative Example 17S2, casein was directly modified without hydrolysis, that is, 142 parts of the partially hydrolyzed casein solution was replaced with 142 parts of a casein dispersion. In the 142 parts of the casein dispersion, the casein content was 12.9 parts, and the balance was phosphate buffer solution. Other preparation steps were the same as those in Example 3.

[0088] In Comparative Example 18S2, cysteamine was not added, and other preparation steps were the same as those in Example 3.

[0089] In Comparative Example 19S2, perfluorooctyltriethoxysilanol was not added, and other preparation steps were the same as those in Example 3.

[0090] In Comparative Example 19S2, hydroxymethyldiacetoneacrylamide was not added, and other preparation steps were the same as those in Example 3.

[0091] In Comparative Example 20S2, N-hydroxymethylacrylamide was not added, and other preparation steps were the same as those in Example 3.

[0092] In Comparative Example 21S2, casein was not modified, and other preparation steps were the same as those in Example 3.

[0093] In Comparative Example 22S4, modified casein was not added, and other preparation steps were the same as those in Example 3.

[0094] In Comparative Example 23S1, KH550 was not added during the prepolymer synthesis, and other preparation steps were the same as those in Example 3.

[0095] Experimental Example 3

[0096] The abrasion resistance and hydrophilicity of the inks prepared in Example 3, Example 5 and Comparative Examples 17 - 22 were tested. The abrasion resistance was tested according to the test method in Experimental Example 2; the hydrophilicity was tested according to the test method of water contact angle. The test results are shown in Table 5.

[0097] Table 5 Test Results of Abrasion Resistance and Hydrophilicity

[0098] Group Wear resistance (friction times) Water contact angle (°) Example 3 1800 100 Example 5 2100 102 Comparative Example 17 1500 60 Comparative Example 18 1000 64 Comparative Example 19 1750 45 Comparative Example 20 1100 77 Comparative Example 21 1800 51 Comparative Example 22 450 40 Comparative Example 23 1800 73

[0099] By modifying casein and using it as a raw material for ink preparation, the number of friction times in Example 3 and Example 5 are 1,800 times and 2,100 times respectively, and the water contact angles are 100° and 102° respectively. In S2, by hydrolyzing and modifying casein, the hydrolysis process is carried out first. Alkaline protease selectively hydrolyzes the peptide bonds of casein to generate short peptide chains, reducing the molecular weight, improving solubility and fluidity. At the same time, more amino and hydroxyl groups are exposed in the short peptide chains, providing active sites for subsequent chemical modification. Perfluorooctyltriethoxysilane is hydrolyzed into perfluorooctylsilanol under acidic conditions and condenses with the amino or hydroxyl groups of casein to form Si-O-C / N bonds, introducing fluorocarbon chains. Cystamine is introduced for modification. The amino group of cystamine (containing disulfide bonds) condenses with the carboxyl or amino group of casein short peptides to form a dynamic disulfide bond crosslinking network, enhancing the mechanical properties and improving water resistance, thereby increasing the water contact angle. The hydroxymethyl group of N-hydroxymethylacrylamide condenses with the amino group of casein, and the hydroxyl group contained in hydroxymethyldiacetoneacrylamide condenses with the groups of casein. At the same time, its vinyl group participates in subsequent free radical polymerization, providing crosslinking active sites, improving compatibility with acrylate monomers and other substances containing unsaturated double bonds in the ink, forming a reinforcing phase, and at the same time improving the hydrophobic effect. In addition, the introduction of KH550 in S1 also improves the water resistance of the cured ink.

[0100] In Comparative Example 17, the unhydrolyzed casein has a long molecular chain, poor solubility, insufficient exposure of modification sites, low crosslinking density, weak hydrophobicity, and a decrease in the content of crosslinkable and curable groups, resulting in a reduction in wear resistance. In Comparative Example 18, cystamine is not added, lacking dynamic disulfide bond crosslinking, with poor water resistance, and reduced wear resistance and hydrophobicity. In Comparative Example 19, no fluorocarbon chain is introduced, resulting in a significant decrease in hydrophobicity and a sudden drop in wear resistance due to water penetration. In Comparative Example 20, hydroxymethyldiacetoneacrylamide is not added, lacking crosslinking active sites, insufficient binding of acrylate, and limited wear resistance and hydrophobicity. In Comparative Example 21, casein is not modified. The original casein has poor solubility and cannot participate in crosslinking. The ink film is loose, with extremely low wear resistance and hydrophobicity. In Comparative Example 22, modified casein is not added, lacking the casein reinforcing phase and film-forming substances, resulting in a fragile ink structure and a significant decrease in wear resistance and hydrophobicity. In Comparative Example 23, KH550 is not added during the synthesis of the S1 prepolymer. The wear resistance remains unchanged, but the water contact angle is lower than that in Example 3.

[0101] Example 7

[0102] In S3, the addition amount of acrylated nano-silica is 5 parts, the addition amount of polyurethane microspheres is 3 parts, and the addition amount of modified casein is 25 parts. Other preparation steps are the same as those in the preparation method of Example 5.

[0103] Example 8

[0104] The addition amount of acrylated nano-silica in S3 is 4.5 parts, the addition amount of polyurethane microspheres is 3.5 parts, and the addition amount of modified casein is 22 parts. Other preparation steps are the same as those in the preparation method of Example 5.

[0105] Experimental Example 4

[0106] The inks obtained in Example 5, Example 7, Example 8, Comparative Example 16 and Comparative Example 22 were subjected to heat resistance tests. According to GB / T 1735-2009 "Determination of Heat Resistance of Paints and Varnishes", the heat resistance test method of the ink is as follows. The cured ink coating is placed in a constant temperature oven, heated to the set temperature of 120 °C, held for 1 h, and after cooling, the surface state of the coating (such as discoloration, blistering, cracking) is observed, and the adhesion retention rate is tested. The test results are shown in Table 6 and Figure 1 as shown.

[0107] Table 6 Heat Resistance Test Results

[0108] Adhesion (%) Adhesion after heat treatment (%) Surface state Example 5 95 92 No discoloration, no cracks Example 7 97 95 No discoloration, no cracks Example 8 93 89 No discoloration, no cracks Comparative Example 16 60 45 Obvious cracking and blistering Comparative Example 22 45 28 Severe discoloration and peeling

[0109] The ink prepared by the present invention has good heat resistance. Under the conditions of Example 5, Example 7 and Example 8, the adhesion before heat treatment is 93%-97%, and the adhesion after heat treatment is 89%-95%. There is no discoloration or crack on the ink surface. Modified casein, casein after hydrolysis and fluorosilane modification form a dynamic crosslinking network (disulfide bond and fluorocarbon chain), and the structure remains stable at high temperature; hydroxymethylacrylamide grafting provides heat-resistant crosslinking sites; acrylated silica is embedded in the polyurethane network through chemical bonds to improve thermal stability. When used in combination with polyurethane microspheres, the core-shell structure absorbs thermal stress, reduces crack propagation, and the interpenetrating network formed by infrared-ultraviolet-thermal dual curing balances rigidity and toughness, inhibits the breakage of molecular chains at high temperature, and jointly improves the heat resistance of the ink. In Comparative Example 16, acrylated silica and polyurethane microspheres were not added, lacking reinforcing phases, and uneven heat conduction led to cracking. In Comparative Example 22, modified casein was not added, and the ink relied on a single polyurethane network, softening and deforming at high temperature, resulting in a decrease in heat resistance.

[0110] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made therein without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A preparation method of printing ink for environment-friendly cigarette box wrapping paper, characterized in that: The preparation method is as follows: Ink formulation: Mix the prepolymer dispersion, polyethylene glycol, triethylene glycol divinyl ether, diglycidyl ether, polyurethane microspheres and modified casein, then add a radical initiator, iodonium salt, water-based pigment, wetting agent and defoaming agent. After ball milling, add deionized water and thickener to obtain the ink to be cured; the modified casein is obtained by enzymatically hydrolyzing casein and then treating it with cysteamine, perfluorooctyltriethoxysilanol, hydroxymethyldiacetone acrylamide and N-hydroxymethylacrylamide; Curing process: After the ink to be cured is printed and coated, pre-drying and ultraviolet-thermal dual curing are carried out to obtain the printed ink.

2. The preparation method of a printing ink for an environment-friendly cigarette box wrapping paper according to claim 1, characterized in that: The preparation method of the prepolymer dispersion is as follows: Add 35-50 parts of isophorone diisocyanate, 55-65 parts of polytetrahydrofuran ether glycol, 5-9 parts of dimethylolpropionic acid and 20-30 parts of pentaerythritol triacrylate to a reaction kettle. After heating, add a catalyst. After the reaction, add γ-aminopropyltriethoxysilane, continue the reaction and then cool down. Dropwise add 13-18 parts of glycidyl methacrylate, keep the temperature for reaction, add 4.0-6.5 parts of triethylamine and transfer it to a disperser, and add the deionized water to obtain a prepolymer emulsion; add acrylated nano-silica to the prepolymer emulsion and disperse to obtain the prepolymer dispersion; The heating temperature is 70-80 °C; the temperature is cooled to 55 °C; the holding reaction time is 1.5-2.5 h.

3. The preparation method of a printing ink for an environment-friendly cigarette case wrapping paper according to claim 1, characterized in that: The preparation method of the modified casein is as follows: Add the deionized water to the hydroxymethyldiacetone acrylamide and stir until completely dissolved to obtain a hydroxymethyldiacetone acrylamide solution; add cysteamine-modified casein to the hydroxymethyldiacetone acrylamide solution, stir evenly and then start heating. When the temperature rises to 80 °C, dropwise add the N-hydroxymethylacrylamide, and at the same time dropwise add an aqueous potassium sulfate solution. After heating, reflux to obtain a modified casein precursor; pour the modified casein precursor onto the surface of a polytetrafluoroethylene plate and dry to obtain the modified casein.

4. The preparation method of a printing ink for an environment-friendly cigarette box wrapping paper according to claim 3, characterized in that: The preparation method of the cysteamine-modified casein is as follows: Disperse the casein in a preheated phosphate buffer solution to completely dissolve it to form a homogeneous colloidal solution; add alkaline protease to the homogeneous colloidal solution and continuously stir at a speed of 200 rpm. After the reaction, put the reaction vessel into ice water to cool down to terminate the reaction, and finally obtain a partially hydrolyzed casein solution; Place the partially hydrolyzed casein solution in a reaction vessel, and add the cysteamine and the perfluorooctyltriethoxysilanol under stirring conditions; after the reaction is completed, ultrafilter to obtain the cysteamine-modified casein.

5. The preparation method of a printing ink for an environment-friendly cigarette box wrapping paper according to claim 3, characterized in that: The preparation method of the perfluorooctyltriethoxysilanol is as follows: Add the deionized water to the perfluorooctyltriethoxysilane; under stirring, heat the reaction mixture and reflux to obtain a crude product of perfluorooctyltriethoxysilanol; after the reaction is completed, remove the unreacted substances and water in the crude product by vacuum distillation to obtain the perfluorooctyltriethoxysilanol.

6. The preparation method of a printing ink for an environment-friendly cigarette case wrapping paper according to claim 1, wherein: The mass dosage ratio of the acrylated nano-silica, the polyurethane microspheres and the modified casein is 3-5: 2-3.5: 20-25.

7. The preparation method of a printing ink for an environment-friendly cigarette box wrapping paper according to claim 1, characterized in that: The mass dosage ratio of the dry weight of the prepolymer dispersion liquid, the polyethylene glycol, the triethylene glycol divinyl ether, the diglycidyl ether, the polyurethane microspheres and the modified casein is 60-75:10-15:5-8:3.5-5:3-3.5:1.8-2.

2.

8. The preparation method of a printing ink for an environment-friendly cigarette case wrapping paper according to claim 1, characterized in that: In the curing process, the coating amount of the printing coating is 1.8 - 2.2 g / m 2 ; the pre-drying time is 1.5 - 3 s; the energy density of ultraviolet curing in the ultraviolet-thermal dual curing is 200 - 280 mJ / cm 2 .

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