Waterproof printing process for paper printed matter

Abstract

The invention relates to the technical field of paper printing, in particular to a waterproof printing process of a paper printed matter. The method overcomes the problems of poor waterproof effect and poor adhesive force of printed matters caused by imperfect traditional printing processes. According to the method, the printed matter is obtained by pretreating the base material, coating the bottom layer, coating the surface layer and calendering the mirror surface roller; parameters of corona treatment and micron-sized embossing treatment in pretreatment are changed; acrylic resin is modified and then mixed with nano silicon dioxide, and bottom layer coating liquid is prepared; a transition layer is arranged between the bottom layer coating and the surface layer coating; specially-treated hydrophobic nano-alumina is introduced into a surface coating liquid and has a synergistic effect with modified polyurethane, so that the waterproof effect and adhesive force of the printed matter are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of paper printing, and specifically to a waterproof printing process for paper printed matter. Background Art

[0002] As a high-value-added packaging printed matter, cigarette boxes need to have both exquisite visual effects and good functionality. Paper cigarette boxes dominate the market due to advantages such as low cost and good printing adaptability, and more than about 80% of cigarette boxes use paper materials. However, paper cigarette boxes face serious waterproof problems: in a humid environment, traditional paper cigarette boxes are prone to absorb moisture, causing the cigarette boxes to become soft and deformed, which not only affects the appearance but may also cause the internal tobacco products to become damp and deteriorate, reducing the product quality. At the same time, the printed patterns and characters on the surface of the cigarette boxes are prone to blur and fade when exposed to water, affecting the display of the brand image and the identification of product information. Therefore, waterproof treatment technologies are often required in the cigarette box printing process. Traditional cigarette box waterproof processes mainly rely on the following methods:

[0003] 1. Laminating process: Covering a polyethylene (PE) or polypropylene (PP) plastic film on the printed surface can improve the waterproofness, but the laminating material is non-degradable, resulting in poor environmental protection. Moreover, after lamination, the surface of the printed matter is prone to reflection, has a rigid touch, and will cover some details of the printed pattern, affecting the texture of the cigarette box;

[0004] 2. Local UV varnish coating: Spraying ultraviolet curing (UV) varnish locally after printing to form a waterproof layer. However, traditional UV varnish curing requires a high-energy ultraviolet light source, and the coating is prone to crack due to mechanical friction or temperature difference changes, resulting in poor long-term waterproof effect;

[0005] 3. Water-based coating process: Coating with a water-based resin to form a waterproof layer. Although the cost is relatively low, the adhesion and density of a single coating are insufficient, and it is prone to delamination and whitening in a humid environment. Moreover, when the coating thickness increases, it is likely to affect the clarity of the printed pattern.

[0006] In summary, it is of great practical significance to develop a waterproof printing process for paper printed matter with good waterproof effect and strong adhesion. Summary of the Invention

[0007] The purpose of the present invention is to provide a waterproof printing process for paper printed matter, obtaining the printed matter through pre-treatment of the substrate, bottom coating, surface coating, and mirror roll calendering; by changing the parameters of corona treatment and micron-level embossing treatment in the pre-treatment; preparing the bottom coating liquid by modifying acrylic resin and mixing it with nano-silica; providing a transition layer between the bottom coating and the surface coating; introducing specially treated hydrophobic nano-aluminum oxide into the surface coating liquid and synergistically acting with the modified polyurethane to improve the waterproof effect and adhesion of the printed matter.

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

[0009] It should be noted that the parts in the present invention are all parts by mass.

[0010] The present invention provides a waterproof printing process for paper printed matter, and the printing process is as follows:

[0011] S1 Pretreatment: Place the substrate in an environment with a humidity of 50% and a temperature of 25°C for 24 hours, and then perform corona treatment and micron-level embossing treatment to obtain a pretreated substrate;

[0012] S2 Bottom layer coating: Use an anilox roller to coat the bottom layer coating liquid on the surface of the pretreated substrate, and then perform infrared drying to obtain a bottom layer coated substrate; The bottom layer coating liquid is prepared from a modified acrylic resin, nano-silica, and a silane coupling agent; The modified acrylic resin is prepared from acrylic acid, methyl methacrylate, butyl acrylate, 2-hydroxyethyl acrylate, and dodecyl acrylate.

[0013] S3 Top layer coating: Spray the transition layer on the surface of the bottom layer coated substrate and perform UV pre-curing with a wavelength of 395 nm and an energy of 150 mJ / cm 2 , then use a gravure coater to coat the top layer coating liquid on top of the transition layer, first perform UV curing and then hot air curing to obtain a top layer coated substrate; The transition layer is obtained by the reaction of an epoxy-acrylic hybrid resin and a photoinitiator; The top layer coating liquid is obtained by the reaction of hydrophobic nano-aluminum oxide and a modified polyurethane; The modified polyurethane is prepared from polytetrahydrofuran diol, isophorone diisocyanate, dimethylolpropionic acid, perfluorohexylethyl acrylate, and a fluorinated benzoxazine monomer.

[0014] S4 Post-processing: Roll polish the top layer coated substrate with a mirror roller to obtain a printed matter with a line pressure of 50 N / mm, a temperature of 90°C, and a speed of 10 m / min.

[0015] Preferably, in S1, the power of the corona treatment is 50 - 80 W, the speed is 5 - 10 m / min, and the electrode spacing is 1.5 mm; The embossing depth of the micron-level embossing treatment is 10 - 25 μm, the pressure is 25 N / cm 2 , and the embossing speed is 5 m / min.

[0016] Preferably, the preparation steps of the bottom layer coating liquid in S2 are as follows: Mix 80 parts of the modified acrylic resin and 20 parts of nano-silica in a stirring container, add 1.5 parts of the silane coupling agent KH550, stir at a speed of 800 rpm for 2 hours, and then add deionized water to adjust the viscosity to 50 - 80 mPa·s to obtain the bottom layer coating liquid.

[0017] Preferably, the preparation steps of the modified acrylic resin are as follows: Add 3 parts of sodium dodecyl sulfate, 2 parts of alkylphenol polyoxyethylene ether, and 80 parts of deionized water into a reaction kettle and stir until dissolved. Then add acrylic acid, methyl methacrylate, butyl acrylate, 2-hydroxyethyl acrylate, and dodecyl acrylate according to a mass ratio of 8-15:12-20:2-6:5-8:1. Stir at a speed of 500 rpm for 40 min, heat up to 76 °C, add ammonium persulfate, and the dropping rate is 3 mL / min. After the dropping is completed, keep the temperature for reaction for 1.5 h, cool down to 40 °C, add ammonia water to adjust the pH value to 7.5, and filter to obtain the modified acrylic resin.

[0018] Preferably, in S2, the infrared drying temperature is 60-80 °C and the time is 3 min.

[0019] Preferably, the preparation steps of the surface coating solution in S3 are as follows: Perform plasma treatment on nano-aluminum oxide under the conditions of a pressure of 50 Pa and a power of 300 W. The working gas is a mixture of argon and oxygen in a volume ratio of 4:1, and the treatment time is 5 min to obtain pretreated alumina; Add heptadecafluorodecyltrimethoxysilane to an ethanol solution, ultrasonically disperse it at room temperature for 10 min at a power of 40 kHz, then add acetic acid, heat up to 50 °C, and stir for hydrolysis for 30 min to obtain a pre-hydrolyzed solution; Add the pretreated alumina to the pre-hydrolyzed solution, and add the zwitterionic surfactant tetradecyldimethylbetaine, transfer it to a microwave reaction kettle for reaction for 20 min, centrifuge, wash with ethanol, and dry to obtain hydrophobic nano-aluminum oxide; Premix 95 parts of modified polyurethane and 5 parts of hydrophobic nano-aluminum oxide, ultrasonically disperse it at 30 °C for 1 h, add 0.5 part of wetting agent TEGO Wet 270, and stir until the viscosity is 100-150 mPa·s to obtain the surface coating solution.

[0020] Preferably, the preparation steps of the modified polyurethane are as follows: Mix 100 parts of polytetrahydrofuran diol and 75 parts of isophorone diisocyanate evenly and react at 70 °C for 2 h. During the reaction, nitrogen is introduced for protection. Then add 26-36 parts of dimethylolpropionic acid and 15-35 parts of perfluorohexylethyl acrylate, cool down to 60 °C and react for 1 h. Dropwise add 15 parts of fluorinated benzoxazine monomer to it and continue to react with isophorone diisocyanate until the NCO content meets the standard. Then add 10 parts of bis(mercaptomethyl) polyethylene glycol and perform ultraviolet light irradiation to obtain the modified polyurethane.

[0021] Preferably, in S3, the UV curing wavelength is 365 nm and the energy is 500-800 mJ / cm 2 ; the hot air curing temperature is 100-120 °C and the time is 50 s.

[0022] Preferably, the preparation steps of the transition layer in S3 are as follows: Add 60 parts of bisphenol A epoxy resin and 40 parts of propylene glycol methyl ether acetate into a three-necked flask, stir at 50 - 65 °C until the bisphenol A epoxy resin is completely dissolved, add 30 parts of acrylic monomer thereto, and continue stirring for 30 min to obtain a mixed solution; Transfer the mixed solution into a photoreaction device, introduce nitrogen for protection, with the nitrogen flow rate being 8 L / min, add 0.3 - 1.5 parts of photoinitiator 2-hydroxy-2-methyl-1-phenyl-1-propanone at a rotation speed of 120 rpm, and irradiate with ultraviolet light having a wavelength of 365 nm, with the light intensity being 50 - 100 mW / cm 2 , react for 45 min to obtain an oligomer; Remove the oligomer from the photoreaction device, raise the temperature to 75 - 90 °C, add 0.8 part of catalyst tetrabutylammonium bromide and continue reacting for 3.5 h, cool to room temperature and then perform vacuum distillation to obtain an epoxy-acrylic hybrid resin; Stir and mix 1.5 parts of photoinitiator TPO and 0.4 part of leveling agent BYK-333 with the epoxy-acrylic hybrid resin at a rotation speed of 600 rpm at room temperature for 1 h to obtain a transition layer; The acrylic monomer is obtained by mixing acrylic acid and hydroxyethyl acrylate in a mass ratio of 1:2.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] 1. Through the corona treatment and micro-scale embossing treatment in the pretreatment stage, the present invention improves the adhesion of the paper surface to the coating from both the microscopic and macroscopic levels. The corona treatment oxidizes the paper fiber surface, forming a large number of oxygen-containing polar groups, changing the molecular structure, increasing its surface energy, and significantly improving the reaction activity with the coating material; The micro-scale embossing treatment forms tiny grooves on the paper surface by mechanical means, increasing the specific surface area and generating a mechanical anchoring effect between the coating and the paper; The two act synergistically to provide excellent adhesion conditions for the subsequent bottom coating liquid and the entire coating system, ensuring that the coating can firmly adhere to the paper.

[0025] 2. By preparing the bottom coating liquid, the long-chain alkyl-modified acrylic resin and nano-silica are mixed, and then a silane coupling agent is added, laying a solid foundation for improving the adhesion. The modified acrylic resin itself has good affinity for the paper substrate, and the nano-silica fills the paper pores, increasing the contact area; One end of the silane coupling agent reacts with the nano-silica, and the other end binds to the modified acrylic resin, forming a stable chemical bond connection, greatly enhancing the adhesion between the bottom layer and the paper; Moreover, the bottom coating liquid is dried by infrared, which can promote the more sufficient penetration of the nano-silica into the interior of the paper fibers, and together with the modified acrylic resin, form a closer combination with the paper, improving the adhesion between the bottom layer and the paper and making the coating not easily fall off.

[0026] 3. In the present invention, a transition layer is provided between the bottom coating and the top coating. The rigid network of the epoxy component in the transition layer provides a support framework, while the acrylic segments penetrate into the micro-pores of the bottom coating to form molecular entanglements, effectively eliminating the interfacial internal stress. The reactive groups such as epoxy groups and hydroxyl groups in the transition layer react with the carboxyl groups of the bottom coating to generate covalent cross-linking, and the acrylate double bonds enriched on its surface undergo photo-initiated grafting with the polyurethane prepolymer of the top coating to form a gradient chemical bonding structure. Moreover, UV pre-curing can form an open cross-linked network, facilitating the penetration and interlocking of the top coating solution, enhancing the bonding force between the coatings, and making it difficult for the coatings to delaminate.

[0027] 4. In the present invention, hydrophobic nano-aluminum oxide is introduced into the top coating solution and acts synergistically with the modified polyurethane to construct a highly hydrophobic protective layer on the surface of the printed matter. The surface of the plasma-modified nano-aluminum oxide is rich in active sites and forms chemical grafting with the long-chain fluoroalkyl groups in the silane hydrolysis solution, ensuring the stable anchoring of the nano-particles in the coating. The low surface energy characteristics of the perfluorinated segments in the fluorinated polyurethane enable the formation of a densely arranged fluorinated molecular layer on the surface of the coating, which together with the nano-aluminum oxide constitutes a composite structure, imitating the lotus leaf effect, improving the water resistance of the printed matter, and achieving a long-term self-maintenance function. In addition, during the curing process, the thiol-ene reaction initiated by UV fixes the distribution of the nano-particles, and the hot air curing stage optimizes the orientation arrangement of the fluoroalkyl groups through the thermal motion of the molecular chains, further improving the water resistance of the printed matter. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a process flow diagram of a waterproof printing process for a paper printed matter according to the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0030] Please refer to Figure 1 , the present invention provides a waterproof printing process for a paper printed matter, and the printed matter is obtained by corona treatment, micro-embossing treatment, bottom coating, transition layer spraying, top coating, and calendering treatment on the substrate; the technical solutions are as follows:

[0031] The substance information involved in the present invention is as follows:

[0032] Silane coupling agent KH550 CAS: 919-30-2; Sodium dodecyl sulfate CAS: 151-21-3; Acrylic acid CAS: 79-10-7; Methyl methacrylate CAS: 80-62-6; Butyl acrylate CAS: 141-32-2; 2-Hydroxyethyl acrylate CAS: 818-61-1; Dodecyl acrylate CAS: 2156-97-0; Ammonium persulfate CAS: 7727-54-0; Nano-aluminum oxide CAS: 11092-32-3; Heptadecafluorodecyltrimethoxysilane CAS: 83048-65-1; Acetic acid CAS: 64-19-7; Polytetrahydrofuran diol CAS: 25190-06-1; Isophorone diisocyanate CAS: 4098-71-9; Dimethylolpropionic acid CAS: 4767-03-7; Perfluorohexylethyl acrylate CAS: 17527-29-6; Bisphenol A epoxy resin CAS: 25085-99-8; Propylene glycol methyl ether acetate CAS: 108-65-6; 2-Hydroxy-2-methyl-1-phenyl-1-propanone CAS: 7473-98-5; Tetrabutylammonium bromide CAS: 1643-19-2; Photoinitiator TPO CAS: 75980-60-8; Nano-silica was purchased from Xuancheng Jingrui New Materials Co., Ltd.; Alkylphenol polyoxyethylene ether was purchased from Jiangsu Bost Chemical Technology Co., Ltd.; Tetradecyldimethylbetaine was purchased from Guangdong Wengjiang Chemical Reagent Co., Ltd.; Wetting agent TEGO Wet 270 was purchased from Hubei Zhonglong Kangsheng Fine Chemical Co., Ltd.; Bis(mercaptomethyl) polyethylene glycol was purchased from Bis(mercaptomethyl) polyethylene glycol; Leveling agent BYK-333 was purchased from Guangzhou Qianguang Trading Co., Ltd.; Fluorinated benzoxazine was prepared according to the method in "Synthesis of Fluorinated Benzoxazine and Its Application in the Field of Coatings".

[0033] Example 1

[0034] The preparation steps of the bottom coating liquid are as follows: Add 3 parts of sodium dodecyl sulfate, 2 parts of alkylphenol polyoxyethylene ether and 80 parts of deionized water into the reaction kettle and stir until dissolved. Add acrylic acid, methyl methacrylate, butyl acrylate, 2-hydroxyethyl acrylate and 4 parts of dodecyl acrylate according to the mass ratio of 8:12:2:5:1. Stir at a speed of 500 rpm for 40 min, heat up to 76 °C and add ammonium persulfate, with a dropping rate of 3 mL / min. After the dropping is completed, keep the temperature for reaction for 1.5 h, cool down to 40 °C, add ammonia water to adjust the pH value to 7.5, and filter to obtain the modified acrylic resin; Mix 80 parts of the modified acrylic resin and 20 parts of nano-silica in a stirring container, add 1.5 parts of silane coupling agent KH550, stir at a speed of 800 rpm for 2 h, and then add deionized water to adjust the viscosity to 50 mPa·s to obtain the bottom coating liquid;

[0035] The preparation steps of the surface coating solution are as follows: 12 parts of nano-aluminum oxide are subjected to plasma treatment under the conditions of a pressure of 50 Pa and a power of 300 W. The working gas is a mixture of argon and oxygen in a volume ratio of 4:1, and the treatment time is 5 min to obtain pretreated alumina; 2 parts of heptadecafluorodecyltrimethoxysilane are added to 30 parts of an ethanol solution, ultrasonically dispersed at room temperature for 10 min at a power of 40 kHz, then 0.4 part of acetic acid is added, and the temperature is raised to 50 °C and stirred for hydrolysis for 30 min to obtain a pre-hydrolyzed solution; the pretreated alumina is added to the pre-hydrolyzed solution, and 0.8 part of zwitterionic surfactant tetradecyldimethylbetaine is added, transferred to a microwave reaction kettle for reaction for 20 min, centrifuged, washed with ethanol and dried to obtain hydrophobic nano-aluminum oxide; 100 parts of polytetrahydrofuran diol and 75 parts of isophorone diisocyanate are mixed evenly and reacted at 70 °C for 2 h. Nitrogen is introduced for protection during the reaction. Then 26 parts of dimethylolpropionic acid and 15 parts of perfluorohexylethyl acrylate are added, the temperature is lowered to 60 °C and reacted for 1 h. 15 parts of fluorinated benzoxazine monomer are added dropwise thereto and continue to react with isophorone diisocyanate until the -NCO content reaches the standard (the content is 1.2%). Then 10 parts of bis(mercaptomethyl)polyethylene glycol are added for ultraviolet irradiation to obtain a modified polyurethane; 95 parts of the modified polyurethane and 5 parts of hydrophobic nano-aluminum oxide are premixed and ultrasonically dispersed at 30 °C for 1 h, 0.5 part of wetting agent TEGOWet270 is added, and stirred until the viscosity reaches 100 mPa·s to obtain the surface coating solution.

[0036] The preparation steps of the transition layer are as follows: 60 parts of bisphenol A epoxy resin and 40 parts of propylene glycol methyl ether acetate are added to a three-necked flask, stirred at 50 °C until the bisphenol A epoxy resin is completely dissolved, 30 parts of acrylic acid monomer are added thereto, and stirred continuously for 30 min to obtain a mixed solution; the mixed solution is transferred to a photoreaction device, nitrogen is introduced for protection, the nitrogen flow rate is 8 L / min, 0.3 part of photoinitiator 2-hydroxy-2-methyl-1-phenyl-1-propanone is added at a rotation speed of 120 rpm, and irradiated with ultraviolet light with a wavelength of 365 nm, and the light intensity is 50 mW / cm 2 ², and reacted for 45 min to obtain an oligomer; the oligomer is removed from the photoreaction device, the temperature is raised to 75 °C, 0.8 part of catalyst tetrabutylammonium bromide is added and reacted for another 3.5 h, and after cooling to room temperature, epoxide-acrylic hybrid resin is obtained by vacuum distillation; the epoxide-acrylic hybrid resin, 1.5 parts of photoinitiator TPO and 0.4 part of leveling agent BYK-333 are stirred and mixed at a rotation speed of 600 rpm at room temperature for 1 h to obtain the transition layer; the acrylic acid monomer is obtained by mixing acrylic acid and hydroxyethyl acrylate in a mass ratio of 1:2.

[0037] The printing process is as follows:

[0038] S1 Pretreatment: Place the substrate (white cardboard) in an environment with a humidity of 50% and a temperature of 25°C for 24 hours, then perform corona treatment and micro-scale embossing treatment to obtain a pretreated substrate; the power of the corona treatment is 50 W, the speed is 5 m / min, and the electrode spacing is 1.5 mm; the embossing depth of the micro-scale embossing treatment is 10 μm, and the pressure is 25 N / cm 2 , and the embossing speed is 5 m / min;

[0039] S2 Bottom Coating: Use a gravure roll to coat the bottom coating liquid on the surface of the pretreated substrate, and then perform infrared drying at 60°C for 3 minutes to obtain a bottom-coated substrate, with an infrared wavelength of 10 μm; the wet film thickness of the bottom coating is 8 μm;

[0040] S3 Top Coating: Spray the transition layer (spraying amount 2 g / m 2 ) on the surface of the bottom-coated substrate and perform UV pre-curing for 40 s, with a wavelength of 395 nm and an energy of 150 mJ / cm 2 , then use a gravure coater to coat the top coating liquid on top of the transition layer, first perform UV curing and then hot air curing to obtain a top-coated substrate; the wavelength of UV curing is 365 nm and the energy is 500 mJ / cm 2 ; the temperature of hot air curing is 100°C and the time is 50 s; the wet film thickness of the top coating is 12 μm;

[0041] S4 Post-processing: Roll polish the top-coated substrate with a mirror roll to obtain a printed product, with a line pressure of 50 N / mm, a temperature of 90°C, and a speed of 10 m / min.

[0042] Examples 2 - 4

[0043] Refer to the preparation method and parameter conditions of Example 1, and the specific differences are shown in Table 1.

[0044] Comparative Example 1

[0045] Refer to the preparation method and parameter conditions of Example 1, except that corona treatment is not performed in S1 pretreatment.

[0046] Comparative Example 2

[0047] Refer to the preparation method and parameter conditions of Example 1, except that micro-scale embossing treatment is not performed in S1 pretreatment.

[0048] Comparative Example 3

[0049] Refer to the preparation method and parameter conditions of Example 1, except that the substrate is not pretreated.

[0050] Experimental Example 1 Adhesion Test

[0051] The adhesion was detected according to GB / T13217.7-2009. During the test, the tape method was adopted. The tape was pasted on the surface of the substrate, and the rolling machine was rolled back and forth 3 times, then left for 5 min. According to the method disclosed in GB / T7707-2008, the tape was peeled off at a speed of 0.8 m / s, and the adhesion fastness was observed and calculated; the obtained results are shown in Table 1.

[0052] Table 1 Adhesion test of Examples 1-4 and Comparative Examples 1-3

[0053]

[0054]

[0055] As can be seen from Table 1, in Examples 1-4, through corona treatment and micron-level embossing treatment of the substrate, the adhesion of the paper surface to the coating was improved simultaneously from the microscopic and macroscopic levels. Corona treatment oxidizes the surface of paper fibers, forming a large number of oxygen-containing polar groups, changing the molecular structure, increasing its surface energy, and significantly improving the reactivity with coating materials; micron-level embossing treatment forms tiny grooves on the paper surface by mechanical means, increasing the specific surface area and generating mechanical anchoring between the coating and the paper; the synergistic effect of the two provides excellent adhesion conditions for the subsequent bottom coating liquid and the entire coating system, ensuring that the coating can firmly adhere to the paper and significantly improving the durability of printed products. In Example 2, when the corona power was 60 W, the corona speed was 7 mm / min, and the embossing depth was 15 μm, the adhesion of the coating was the best, and the adhesion fastness was 96.3%. In Comparative Example 1, no corona treatment was carried out in the S1 pretreatment. The fibers on the surface of the paper substrate showed a low-energy inert state, the reactivity with coating materials decreased, and the porosity of the substrate increased, resulting in a decrease in the coating adhesion. In Comparative Example 2, no micron-level embossing treatment was carried out in the S1 pretreatment, destroying the mechanical anchoring effect between the coating and the substrate, resulting in a significant reduction in the tolerance of adhesion to dynamic stress. In Comparative Example 3, no pretreatment was carried out on the substrate, and the combination of the coating and the paper base relied on simple physical adsorption, and the adhesion deteriorated severely. The polar activation of corona treatment and the mechanical reinforcement of embossing were both absent. It was difficult for the bottom coating liquid to penetrate the fiber pores, and the active groups in its resin could not form a stable bond with the substrate either.

[0056] Examples 5-7

[0057] Referring to the preparation method and parameter conditions of Example 2, the specific differences are shown in Table 2; the mass ratio of the four in Table 2 is the mass ratio of acrylic acid, methyl methacrylate, butyl acrylate, 2-hydroxyethyl acrylate, and dodecyl acrylate.

[0058] Comparative Example 4

[0059] Referring to the preparation method and parameter conditions of Example 2, the difference is that when preparing the bottom coating solution, the acrylic resin was not modified, but an aqueous acrylic resin emulsion (Shandong Zouping Dazhan New Materials Co., Ltd.) was directly used.

[0060] Comparative Example 5

[0061] Referring to the preparation method and parameter conditions of Example 2, the difference is that acrylic acid was not added when preparing the modified acrylic resin.

[0062] Comparative Example 6

[0063] Referring to the preparation method and parameter conditions of Example 2, the difference is that methyl methacrylate was not added when preparing the modified acrylic resin.

[0064] Comparative Example 7

[0065] Referring to the preparation method and parameter conditions of Example 2, the difference is that butyl acrylate was not added when preparing the modified acrylic resin.

[0066] Comparative Example 8

[0067] Referring to the preparation method and parameter conditions of Example 2, the difference is that 2-hydroxyethyl acrylate was not added when preparing the modified acrylic resin.

[0068] Comparative Example 9

[0069] Referring to the preparation method and parameter conditions of Example 2, the difference is that in step S2 of the bottom coating, infrared drying was replaced by hot air drying.

[0070] Experimental Example 2 Adhesion Test

[0071] Refer to the method of Experimental Example 1 to test the adhesion fastness; the obtained results are shown in Table 2.

[0072] Table 2 Adhesion Test of Example 2, Examples 5 - 7 and Comparative Examples 4 - 9

[0073]

[0074] As can be seen from Table 2, in Examples 2 and 5-7, by preparing the bottom coating solution, the modified acrylic resin and nano-silica are mixed, and then a silane coupling agent is added, laying a solid foundation for improving the adhesion. The modified waterborne acrylic resin itself has good affinity for the paper substrate. Nano-silica fills the pores of the paper, increasing the contact area. One end of the silane coupling agent reacts with nano-silica, and the other end binds to the modified acrylic resin, forming a stable chemical bond connection, greatly enhancing the adhesion between the bottom layer and the paper. Moreover, the bottom coating solution is dried by infrared, which can promote the more sufficient penetration of nano-silica into the interior of the paper fibers, forming a closer combination with the modified acrylic resin and the paper together, improving the adhesion between the bottom layer and the paper and making the coating not easy to fall off. In Example 6, when the mass ratio of the four is 12:16:6:8:1, the viscosity of the bottom coating solution is 70 mPa·s, and the infrared drying temperature is 70 °C, the coating adhesion is the best, and the adhesion fastness reaches 97.5%. In Comparative Example 4, when preparing the bottom coating solution, the acrylic resin was not modified, but the waterborne acrylic resin emulsion was directly used. The coating lacked the directional bonding ability of the functional monomer, and the base resin could not effectively bind to the corona-treated paper-based oxide layer through hydrogen bonds or covalent bonds. At the same time, the silane coupling agent lost the interfacial bridging effect of nano-silica due to the lack of sufficient carboxyl or hydroxyl groups to react with it, resulting in a significant reduction in adhesion. In Comparative Example 5, acrylic acid was not added when preparing the modified acrylic resin, and the carboxyl content in the resin chain segment decreased sharply, making it difficult to form stable ionic bonds or amide bond connections with the silane coupling agent of nano-silica, resulting in weakened interfacial bonding between the nano-particles and the resin. In addition, the lack of carboxyl groups reduced the ability of the bottom coating solution to wet the polar surface of the matrix, and the resin could not tightly bite the anchor points formed by the micron embossing during drying. In Comparative Example 6, methyl methacrylate was not added when preparing the modified acrylic resin. The bottom coating film softened severely during the hot air drying stage and could not effectively fill the concave areas of the micron embossing. The loose resin structure weakened the mechanical interlocking effect and the adhesion decreased. In Comparative Example 7, butyl acrylate was not added when preparing the modified acrylic resin. The bottom coating film was too rigid to buffer the deformation stress of the substrate fibers during calendering or humidity changes, resulting in the expansion of interfacial microcracks. In Comparative Example 8, 2-hydroxyethyl acrylate was not added when preparing the modified acrylic resin. The bottom resin could not form stable silicon-oxygen bonds with the silane coupling agent of nano-silica, and the nano-particles migrated to the surface during drying to form a weak interface layer. In Comparative Example 9, in the bottom coating in step S2, infrared drying was replaced by hot air drying. Compared with infrared radiation drying, the heat transfer rate was low and the temperature distribution was uneven. The slow evaporation of the solvent caused the surface layer of the bottom coating film to form a film quickly but there was residual solvent inside, resulting in the concentration of volume shrinkage stress and the formation of micron-sized holes, reducing the adhesion.

[0075] Examples 8-10

[0076] Referring to the preparation method and parameter conditions of Example 6, the specific differences are shown in Table 3; in Table 3, Temperature 1 is the temperature for stirring bisphenol A epoxy resin until it is completely dissolved when preparing the epoxy-acrylic hybrid resin; the light intensity is the ultraviolet light irradiation intensity after adding the photoinitiator when preparing the epoxy-acrylic hybrid resin; Temperature 2 is the temperature for heating up after removing the oligomer from the photoreaction device when preparing the epoxy-acrylic hybrid resin.

[0077] Comparative Example 10

[0078] Referring to the preparation method and parameter conditions of Example 6, the difference is that no transition layer is added between the bottom coating and the surface coating.

[0079] Comparative Example 11

[0080] Referring to the preparation method and parameter conditions of Example 6, the difference is that when preparing the epoxy-acrylic hybrid resin, the acrylic monomer is obtained by mixing acrylic acid and hydroxyethyl acrylate in a mass ratio of 1:1.

[0081] Comparative Example 12

[0082] Referring to the preparation method and parameter conditions of Example 6, the difference is that the amount of photoinitiator used is 3 parts when preparing the epoxy-acrylic hybrid resin.

[0083] Experimental Example 3 Adhesion Test

[0084] Referring to the method of Experimental Example 1, the adhesion fastness was tested; the obtained results are shown in Table 3.

[0085] Table 3 Adhesion Test of Example 6, Examples 8 - 10 and Comparative Examples 10 - 12

[0086]

[0087]

[0088] As can be seen from Table 3, in Example 6, Examples 8 - 10, by providing a transition layer between the bottom coating and the surface coating, the rigid network of the epoxy component in the transition layer provides a support framework, and the acrylic segments penetrate into the tiny pores of the bottom coating to form molecular-level entanglement, effectively eliminating the internal stress at the interface; the active groups such as epoxy groups and hydroxyl groups in the transition layer react with the carboxyl groups of the bottom coating to form covalent crosslinks, and the acrylate double bonds enriched on its surface undergo photoinitiated grafting with the polyurethane prepolymer of the surface coating to form a gradient chemical bonding structure; and UV pre-curing can form an open crosslinked network, facilitating the penetration and interlocking of the surface coating solution, enhancing the bonding force between the coatings, making it difficult for delamination to occur between the coatings, and avoiding water entering the layer interface to erode the bottom coating and cause a decrease in adhesion. In Example 9, when Temperature 1 is 60 °C, the amount of photoinitiator used is 1 part, and the light intensity is 75 mW / cm2 When the second temperature is 85 °C, the adhesion fastness is 98.5, and the adhesion is the best. In Comparative Example 10, no transition layer was added. Due to the polarity difference between the bottom acrylic resin and the surface fluorinated polyurethane, it was difficult for the surface coating solution to spread evenly on the bottom layer. Moreover, the lack of a transition layer made the layers only rely on weak van der Waals forces for bonding, and the penetration of interfacial water molecules accelerated delamination, reducing the adhesion. In Comparative Example 11, when preparing the epoxy-acrylic hybrid resin, the acrylic monomer was obtained by mixing acrylic acid and 2-hydroxyethyl acrylate in a mass ratio of 1:1. The imbalance in the mass ratio of the two significantly weakened the cross-layer reaction activity of the epoxy hybrid resin, and delamination easily occurred between the layers, resulting in a decrease in adhesion. In Comparative Example 12, when preparing the epoxy-acrylic hybrid resin, the amount of photoinitiator was 3 parts. The excessive photoinitiator caused an out-of-control reaction in ultraviolet curing, destroying the structural uniformity of the transition layer. Moreover, the undepleted photoinitiator decomposed to produce gas during the hot air curing stage, which would exacerbate the interfacial porosity and reduce the adhesion.

[0089] Examples 11 - 13

[0090] Referring to the preparation method and parameter conditions of Example 9, the specific differences are shown in Table 4.

[0091] Table 4 Specific preparation parameters of Examples 11 - 13

[0092]

[0093]

[0094] Comparative Example 13

[0095] Referring to the preparation method and parameter conditions of Example 9, the difference is that when preparing the surface coating solution, hydrophobic nano-alumina was replaced with nano-alumina.

[0096] Comparative Example 14

[0097] Referring to the preparation method and parameter conditions of Example 9, the difference is that when preparing hydrophobic nano-alumina, the nano-alumina was not subjected to plasma treatment.

[0098] Comparative Example 15

[0099] Referring to the preparation method and parameter conditions of Example 9, the difference is that when preparing the surface coating solution, hydrophobic nano-alumina was not added.

[0100] Comparative Example 16

[0101] Referring to the preparation method and parameter conditions of Example 9, the difference is that when preparing the surface coating solution, the modified polyurethane was replaced with ordinary polyurethane (purchased from Guangdong Wengjiang Chemical Reagent Co., Ltd., model RM-12W).

[0102] Comparative Example 17

[0103] Referring to the preparation method and parameter conditions of Example 9, except that the fluorinated benzoxazine monomer was not added during the preparation of the modified polyurethane.

[0104] Comparative Example 18

[0105] Referring to the preparation method and parameter conditions of Example 9, except that in step S3, the surface coating solution was coated on the transition layer, and then only UV curing was carried out.

[0106] Comparative Example 19

[0107] Referring to the preparation method and parameter conditions of Example 9, except that in step S3, the surface coating solution was coated on the transition layer, and then only hot air curing was carried out.

[0108] Experimental Example 4 Waterproof Performance Test

[0109] The water contact angle was measured using an automatic contact angle tester. 5 μl of water droplets were quantitatively injected onto the surface of the printed products prepared in Example 9, Examples 11 - 13, and Comparative Examples 13 - 19. After the shape of the water droplets no longer changed, the contact angle was measured.

[0110] The printed substrate was cut into samples of 5 cm × 10 cm, immersed in deionized water for 24 h, taken out, dried the surface moisture, weighed the samples before and after immersion, and calculated the water absorption rate of the samples. The results are shown in Table 5.

[0111] Table 5 Waterproof Performance Test of Example 9, Examples 11 - 13, and Comparative Examples 13 - 19

[0112]

[0113]

[0114] As can be seen from Table 4 and Table 5, in Examples 9, 11 - 13, by introducing hydrophobic nano-aluminum oxide into the surface coating solution and synergistically acting with modified polyurethane, a highly hydrophobic protective layer was constructed on the surface of the printed matter. The surface of the plasma-modified nano-aluminum oxide is rich in active sites, which form chemical grafting with the long-chain fluoroalkyl groups in the silane hydrolysis solution to ensure the stable anchoring of the nanoparticles in the coating; while the low surface energy characteristics of the perfluorinated chain segments in the fluorinated polyurethane enable the formation of a densely arranged fluorinated molecular layer on the coating surface, which together with the nano-aluminum oxide constitutes a composite structure, mimicking the lotus leaf effect, improving the water resistance of the printed matter, and realizing the long-term self-maintenance function; in addition, during the curing process, the thiol-ene reaction initiated by UV fixes the distribution of the nanoparticles, and in the hot air curing stage, the thermal motion of the molecular chains optimizes the orientation arrangement of the fluoroalkyl groups, further improving the water resistance of the printed matter. In Example 12, when the dosage of dimethylolpropionic acid is 33 parts, the dosage of perfluorohexylethyl acrylate is 25 parts, the viscosity of the surface coating solution is 130 mPa·s, and the UV curing energy is 700 mJ / cm 2 , the waterproof performance of the printed matter is the best, the water contact angle is 168.5°, and the water absorption rate is 2.18%. In Comparative Example 13, when preparing the surface coating solution, the hydrophobic nano-aluminum oxide was replaced with nano-aluminum oxide, which was not modified with fluorosilane and had a high surface energy. When water contacts the surface of the printed matter, the nano-aluminum oxide cannot repel water like the hydrophobic nano-aluminum oxide, resulting in water being more likely to adhere to the coating surface and penetrate into the coating interior, thus reducing the waterproof performance of the printed matter. In Comparative Example 14, when preparing the hydrophobic nano-aluminum oxide, the nano-aluminum oxide was not subjected to plasma treatment, and there were hydroxyl passivation layers and organic pollutants on its surface, resulting in a decrease in the reaction activity with heptadecafluorodecylsilane and a decline in the waterproof performance. In Comparative Example 15, when preparing the surface coating solution, the hydrophobic nano-aluminum oxide was not added, and it was difficult to form an effective hydrophobic network only relying on the hydrophobicity of the modified polyurethane itself, and water was more likely to break through the coating defense line and penetrate into the interior of the printed matter. In Comparative Example 16, when preparing the surface coating solution, the modified polyurethane was replaced with ordinary polyurethane, and the surface energy was relatively high, and it could not form an efficient waterproof coating synergistically with the hydrophobic nano-aluminum oxide like the modified polyurethane. In Comparative Example 17, when preparing the modified polyurethane, the fluorinated benzoxazine monomer was not added, and the stability and durability of the waterproof barrier of the printed matter were insufficient. In Comparative Example 18, in Step S3, the surface coating solution was coated on the transition layer, and then only UV curing was carried out, and the surface curing was incomplete. When the coating faced water, the molecular gaps inside were relatively large, and water was more likely to penetrate into the coating through these gaps, reducing the waterproof performance of the printed matter; in Comparative Example 19, in Step S3, the surface coating solution was coated on the transition layer, and then only hot air curing was carried out, resulting in an insufficiently dense overall structure of the coating and the waterproof performance being affected.

[0115] Although embodiments of the present invention have been shown and described, those of ordinary skill in the art will appreciate that various changes, modifications, substitutions and variations can be made to these embodiments 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 waterproof printing process for paper printed matter, characterized in that: The printing process is as follows: S1 pretreatment: subjecting the substrate to corona treatment and micron-level embossing treatment to obtain a pretreated substrate; S2 bottom coating: coating the bottom coating liquid on the surface of the pretreated substrate, and then performing infrared drying to obtain a bottom coating substrate; the bottom coating liquid is prepared from modified acrylic resin, nano silicon dioxide and silane coupling agent; the modified acrylic resin is prepared from acrylic acid, methyl methacrylate, butyl acrylate, hydroxyethyl acrylate and dodecyl acrylate; S3 surface coating: spraying a transition layer on the surface of the bottom coating substrate and curing it, then coating the surface coating liquid on the transition layer, first UV curing and then hot air curing to obtain a surface coating substrate; the transition layer is obtained by reacting an epoxy-acrylic hybrid resin and a photoinitiator; the surface coating liquid is obtained by reacting hydrophobic nano-alumina and modified polyurethane; the modified polyurethane is prepared from polytetramethylene glycol, isophorone diisocyanate, dimethylol propionic acid, perfluorohexyl ethyl acrylate and fluorinated benzoxazine monomer; S4 post-processing: calendering the surface-coated substrate to obtain a printed product.

2. The waterproof printing process for paper printed matter according to claim 1, characterized in that: The corona treatment power in S1 is 50-80W, the speed is 5-10m / min, and the electrode spacing is 1.5mm; the micron-level embossing treatment has an embossing depth of 10-25μm and a pressure of 25N / cm 2 , embossing speed 5m / min.

3. The waterproof printing process for paper printed matter according to claim 1, characterized in that: The preparation steps of the bottom coating liquid in S2 are as follows: the modified acrylic resin and the nano-silicon dioxide are mixed in a stirring container, the silane coupling agent is added and stirred, and then deionized water is added to adjust the viscosity to 50-80 mPa·s to obtain the bottom coating liquid.

4. The waterproof printing process for paper printed matter according to claim 1, characterized in that: The modified acrylic resin is prepared as follows: sodium dodecyl sulfate, alkylphenol polyoxyethylene ether and deionized water are added into a reaction kettle and stirred until dissolved; acrylic acid, methyl methacrylate, butyl acrylate, hydroxyethyl acrylate and lauryl acrylate are added into the reaction kettle in a mass ratio of 8-15:12-20:2-6:5-8:1; the temperature is raised to 76°C and ammonium persulfate is added; after the addition is completed, the reaction is kept warm for 1.5 hours; the temperature is lowered to 40°C; ammonia water is added to adjust the pH value; and the modified acrylic resin is obtained by filtering.

5. The waterproof printing process for paper printed matter according to claim 1, characterized in that: The S2 medium infrared drying temperature is 60-80℃.

6. The waterproof printing process for paper printed matter according to claim 1, characterized in that: The steps for preparing the surface coating liquid in S3 are as follows: subjecting nano-alumina to plasma treatment to obtain pre-treated alumina; adding heptadecafluorodecyltrimethoxysilane to an ethanol solution, adding acetic acid after ultrasonic dispersion, heating to 50° C. and stirring to obtain a pre-hydrolyzed solution; adding the pre-treated alumina to the pre-hydrolyzed solution, adding a zwitterionic surfactant, transferring to a microwave reactor for reaction for 20 minutes, centrifuging, washing with ethanol and drying to obtain the hydrophobic nano-alumina; The modified polyurethane and the hydrophobic nano-alumina are premixed and then ultrasonically dispersed, a wetting agent is added, and the mixture is stirred until the viscosity is 100-150 mPa·s to obtain the surface coating liquid.

7. The waterproof printing process for paper printed matter according to claim 1, characterized in that: The preparation steps of the modified polyurethane are as follows: the polytetrahydrofuran diol and the isophorone diisocyanate are uniformly mixed and reacted at 70° C. for 2 hours, then 26-36 parts of the dimethylol propionic acid and 15-35 parts of the perfluorohexyl ethyl acrylate are added, the temperature is lowered to 60° C. and reacted for 1 hour, the fluorinated benzoxazine monomer is dripped therein, and then double-terminated mercapto polyethylene glycol is added and ultraviolet light is irradiated to obtain the modified polyurethane.

8. The waterproof printing process for paper printed matter according to claim 1, characterized in that: The UV curing wavelength described in S3 is 365nm and the energy is 500-800mJ / cm 2 ; The hot air curing temperature is 100-120°C and the curing time is 50s.

9. The waterproof printing process for paper printed matter according to claim 1, characterized in that: The preparation steps of the transition layer in S3 are as follows: add bisphenol A epoxy resin and propylene glycol methyl ether acetate into a three-necked flask, stir at 50-65° C., add acrylic acid monomer thereto and stir to obtain a mixed solution; transfer the mixed solution to a photoreaction device, introduce nitrogen protection, add 0.3-1.5 parts of the photoinitiator, and irradiate with ultraviolet light at an illumination intensity of 50-100 mW / cm 2 , react to obtain a oligomer; remove the oligomer from the photoreaction device, heat to 75-90°C, add a catalyst to continue the reaction for 3.5 hours, cool to room temperature and then perform reduced pressure distillation to obtain the epoxy-acrylic hybrid resin; mix and stir the epoxy-acrylic hybrid resin, the photoinitiator and the leveling agent to obtain the transition layer; the acrylic monomer is obtained by mixing the acrylic acid and the hydroxyethyl acrylate in a mass ratio of 1:2.

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