Preparation method of tipping paper

Abstract

The invention relates to the technical field of tipping paper, in particular to a preparation method of tipping paper. The invention solves the problem that the existing tipping paper is difficult to give consideration to both physical strength and moisture resistance. The modified lignin and AKD are adopted in the bottom layer sizing agent for synergistic sizing, the modified chitosan and dialdehyde starch are adopted in the top layer sizing agent for constructing a chemical crosslinking wet strength network, and cellulose nanowhiskers and microcrystalline cellulose are compounded for nanometer enhancement; the top layer of the formed paper sheet is coated with a composite coating composed of modified zein and polylactic acid emulsion, and a physical moisture-resistant barrier layer is formed. Through the synergistic effect of internal enhancement, chemical wet strength and surface barrier, the prepared tipping paper has excellent physical strength and excellent moisture resistance.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of tipping paper, in particular to a preparation method of tipping paper. BACKGROUND

[0002] Tipping paper is an important component of cigarettes, which connects cigarette rods and filters. Its traditional function is to provide physical connection and print carrier for brand information, and to achieve tar and smoke dilution through (static or laser) perforation. With the development of technology, modern tipping paper has carried multiple functions such as selective filtration, odor reduction, and anti-lipstick adhesion. The industry trend is driving tipping paper towards more environmentally friendly, lower grammage, and higher functional integration, which prompts researchers to look beyond lignocellulose to a wider range of biomass conversion materials.

[0003] With the continuous improvement of cigarette production equipment speed, the physical strength of tipping paper is challenged. During the high-speed traction, printing and perforation process, if the tensile strength of the paper sheet is insufficient or the uniformity is poor, paper breakage is likely to occur, leading to unplanned shutdown of the production line, which seriously affects production efficiency. Tipping paper with insufficient stiffness is prone to wrinkling, deformation or misalignment during the rolling process, resulting in appearance defects and increased scrap rate of finished products. Secondly, the challenge of moisture resistance is that when the tipping paper contacts saliva (moisture), water molecules quickly penetrate and destroy the hydrogen bonds between fibers, resulting in a sharp decrease in paper strength. This can cause the paper to soften, fuzz or even break during use, seriously affecting the consumer experience and possibly causing the filter to fall off. At this time, there is an urgent need for a tipping paper that balances physical strength and moisture resistance to meet market demand.

[0004] To this end, a preparation method of tipping paper is provided. SUMMARY

[0005] The purpose of the present application is to design a preparation method of tipping paper. The bottom layer slurry of the present application uses modified lignin and AKD for synergistic sizing, the top layer slurry uses modified chitosan and dialdehyde starch to build a chemical cross-linking wet strength network, and cellulose nanocrystals and microcrystalline cellulose are compounded for nano-enhancement. The composite coating composed of modified zein and polylactic acid emulsion is coated on the top layer of the formed paper sheet to form a physical moisture-resistant barrier layer. The tipping paper prepared by the synergistic effect of internal reinforcement, chemical wet strength and surface barrier has excellent physical strength and outstanding moisture resistance.

[0006] To achieve the above-mentioned purpose, the present application provides the following technical scheme: The present application provides a preparation method of tipping paper, comprising the following steps: Mix 95-105 parts of bleached kraft softwood pulp, beat to 40-45°SR, dilute the beaten pulp to 3% consistency, then add 0.05 parts of cationic polyacrylamide as a retention aid, stir for 5 min; add 0.5-1 parts of modified lignin, stir for 15 min; add 0.3 parts of alkyl ketene dimer, adjust pH to 8.0, to obtain the bottom layer pulp; Mix 35-45 parts of bleached kraft softwood pulp and 60 parts of bleached kraft hardwood pulp, beat to 40-50°SR, dilute the beaten pulp to 2% consistency; add 28-32 parts of cellulose nanowhisker water dispersion (cellulose nanowhisker solid content 10%), treat with a high shear disperser for 10 min; add 3-7 parts of microcrystalline cellulose powder, continue high shear dispersion for 5-15 min; adjust the pH of the pulp to 6.5, slowly add 2 parts of modified chitosan solution (dissolved in deionized water, solid content 30%) under high shear, stir for 10 min; add 1.5 parts of dialdehyde starch, stir for 20 min, to obtain the top layer pulp; Dissolve 8-12 parts of modified zein in 90 parts of 75% ethanol aqueous solution, stir to dissolve at 50°C water bath for 1 h, to obtain the base material; add 0.2 parts of glycerol to the base material, stir at 500 rpm for 10 min; then slowly add 25 parts of polylactic acid emulsion (emulsified with deionized water, solid content 40%) under stirring, dropwise addition time is 30 min, then disperse at 9000 rpm for 15 min, to obtain the composite coating; Use a former with double-layer headboxes, inject the top layer pulp into the top layer headbox and the bottom layer pulp into the bottom layer headbox; use two press sections, gradually increase the linear pressure, the first section is 30 kN / m and the second section is 50 kN / m; use drum drying, control the surface temperature at 98°C, to make the paper dryness reach about 92%; pass the paper through a 120°C oven, residence time is 40 s, to obtain the semi-finished paper; Use air knife coating to coat the composite coating, only on the top layer side, control the dry coating amount to be 0.8 g / m², immediately enter a hot air drying oven after coating, temperature is 70°C, fast dry to form a film; use a soft calender, linear pressure is 40 kN / m, temperature is 60°C, to obtain the tipping paper.

[0007] Preferably, the average fiber length of the bleached sulfate softwood pulp is 2.5-3.5 mm, the ISO brightness is > 88%; the average fiber length of the bleached sulfate hardwood pulp is 0.8-1.2 mm, the ISO brightness is > 88%; the average particle size of the cellulose nanowhisker is 10-20 nm, the aspect ratio is 10-30, and the crystallinity is > 85%; the average particle size of the microcrystalline cellulose is 20-100 μm, and the crystallinity is 70-80%; the cationic degree of the cationic polyacrylamide is 10-20 mol%; the oxidation degree of the dialdehyde starch is 80-90%; and the weight average molecular weight of the polylactic acid is 50,000-150,000 g / mol.

[0008] Preferably, the preparation method of the modified lignin is as follows: 8-12 parts of alkali lignin powder is placed in a vacuum drying oven, vacuum dried at 80°C for 12 h to obtain pretreated alkali lignin; the pretreated alkali lignin is transferred to a three-necked flask (ice water bath), 150 parts of anhydrous pyridine is added, and stirring is performed to obtain a suspension; 18-22 parts of lauroyl chloride is diluted with 20 parts of anhydrous pyridine, then added to a constant-pressure dropping funnel, and slowly added to the suspension, with a dropping time of 30 min; after the addition is completed, the ice water bath is removed, the temperature is raised to 70°C, and reaction is performed for 6-8 h; after the reaction is completed, the reaction liquid is cooled to room temperature, the reaction liquid is slowly poured into a large amount of methanol under vigorous stirring, a vacuum filtration device is used to filter out the precipitate to obtain a crude product filter cake, the filter cake is washed with methanol for 3 times, washed with 0.1M dilute hydrochloric acid for 2 times, and finally washed repeatedly with deionized water until the pH of the filtrate approaches neutral; the washed filter cake is transferred to a vacuum drying oven, dried at 60°C for 24 h to obtain the modified lignin.

[0009] Preferably, the preparation method of the modified chitosan is as follows: 9-11 parts of chitosan powder and 100 parts of isopropyl alcohol are added to a three-necked flask, uniformly dispersed to obtain pretreated chitosan; 10 parts of sodium hydroxide is dissolved in 10 parts of deionized water to obtain a concentrated lye; the concentrated lye is slowly added to the pretreated chitosan, the temperature is raised to 50°C, and stirring is performed for 2 h to obtain a reaction system; the reaction system is cooled to 40°C, 23-27 parts of an epoxypropyltrimethylammonium chloride solution (solid content 70%) is weighed, slowly added to the reaction system through a constant-pressure dropping funnel, with a dropping time of 30 min, after the addition is completed, the temperature is slowly raised to 70°C, and reaction is performed for 5-7 h to obtain a reaction liquid; after the reaction is completed, the temperature is cooled to room temperature, glacial acetic acid is used to adjust the pH of the reaction liquid to neutral, then vacuum filtration is performed to obtain a solid crude product, the filter cake is washed with 70% ethanol aqueous solution for 3 times, washed with anhydrous ethanol for 2 times, the washed filter cake is transferred to a vacuum drying oven, dried at 50°C for 18 h to obtain the modified chitosan; and the deacetylation degree of the chitosan is > 90%.

[0010] Preferably, the preparation method of the modified zein is as follows: under stirring, 45-55 parts of zein powder and 48-52 parts of malt dextrin powder are added into 500 parts of 80% ethanol solution, and stirred at room temperature for 30 min until a uniform suspension is formed; the pH value of the suspension is adjusted to 8.5 by using a NaOH solution to obtain a mixture; the mixture is transferred into a constant temperature water bath at 60°C, and sealed and stirred for 5 h; after the reaction is completed, the mixture is quickly cooled to room temperature in an ice water bath; 1M HCl solution is used to adjust the pH value of the solution to 5.0; the above acid-adjusted mixture is slowly poured into a large amount of cold deionized water to obtain a precipitate; the precipitate is allowed to stand for 30 min, and then washed by centrifugation with deionized water for 4 times, and then freeze-dried for 36 h to obtain the modified zein.

[0011] Compared with the prior art, the present application has the following beneficial effects: Cellulose nanocrystals (CNC) have extremely high Young's modulus and rigidity, which form a rigid pin connection between fibers, significantly improving the stiffness and tensile strength of paper; however, pure CNC can easily cause the paper to become brittle, and microcrystalline cellulose (MCC) as a filler with a size between CNC and wood pulp fibers can effectively fill the pores between fibers, improve the density and uniformity of the paper sheet, and at the same time as a stress dispersion point, cooperates with CNC to improve the strength while improving the toughness of the paper, solving the contradiction between high strength and high brittleness.

[0012] The modified chitosan has a permanent strong positive charge due to quaternization, and can be efficiently adsorbed on the negatively charged fibers. More importantly, the amino group on the modified chitosan reacts with the aldehyde group on the dialdehyde starch to form a Schiff base, and at the same time, the aldehyde group on the dialdehyde starch also forms a hemiacetal bond with the hydroxyl group of cellulose. This in-situ formed chemical cross-linking network in the top layer of the pulp locks the fibers together firmly. When the paper is wet (such as saliva), this covalent bond network will not break easily like hydrogen bonds, thereby imparting the paper with extremely high wet strength (moisture resistance).

[0013] Ordinary zein coating is hydrophobic but extremely brittle, and through saccharification modification, a flexible sugar chain is connected to it, solving the problem of brittleness. The PLA emulsion itself is an excellent bio-based barrier material, and after compounding, the flexible modified zein serves as the matrix and cooperates with the PLA particles to form a dense, flexible and firm composite coating. This coating physically prevents the penetration of external moisture (saliva), and at the same time, due to its flexibility, it will not crack when the paper is bent, ensuring excellent moisture resistance and non-sticky lips experience.

[0014] CNC / MCC is concentrated on the top layer (printing surface) which requires high strength and high smoothness, realizing the on-demand distribution of materials and greatly reducing the cost; at the same time, the pulp with strong hydrophobicity is placed on the bottom layer (the surface in contact with tobacco), which is more functional, and this gradient design coordinates the cost, physical strength and functionality.

[0015] Alkyl ketene dimer (AKD) is a conventional internal sizing agent, but the long carbon chain (C12) of the lignin modified by lauroyl chloride makes it have very strong hydrophobicity, and it has the "similar compatibility" characteristics with the hydrophobic long chain of AKD in the molecular structure. The two work together to form a hydrophobic barrier on the fiber surface that is much more stable and denser than using AKD alone, further assisting to improve the overall wet resistance of the paper. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 The physical strength performance chart of examples 1-5 and comparative examples 1-3 in the present application. DETAILED DESCRIPTION

[0017] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor belong to the scope of protection of the present application.

[0018] The present application provides a preparation method of tipping paper, and the technical scheme is as follows: Example 1

[0019] 10 parts of alkali lignin powder were placed in a vacuum drying oven and vacuum dried at 80℃ for 12h to obtain pretreated alkali lignin; the pretreated alkali lignin was transferred to a three-necked flask (ice water bath), 150 parts of anhydrous pyridine was added, and stirring was performed to obtain a suspension; 20 parts of lauroyl chloride was diluted with 20 parts of anhydrous pyridine, then added to a constant pressure dropping funnel, and slowly added to the suspension, with a dropping time of 30min; after the addition was completed, the ice water bath was removed, and the temperature was raised to 70℃ for reaction for 7h; after the reaction was completed, the reaction liquid was cooled to room temperature, and under vigorous stirring, the reaction liquid was slowly poured into a large amount of methanol, a vacuum filtration device was used to filter out the precipitate to obtain a crude product filter cake, the filter cake was washed with methanol for 3 times, washed with 0.1M dilute hydrochloric acid for 2 times, and finally washed repeatedly with deionized water until the pH of the filtrate was close to neutral; the washed filter cake was transferred to a vacuum drying oven and dried at 60℃ for 24h to obtain modified lignin.

[0020] Ten parts of chitosan powder and 100 parts of isopropanol were added to a three-necked flask and dispersed evenly to obtain pretreated chitosan. Ten parts of sodium hydroxide were dissolved in 10 parts of deionized water to obtain a concentrated alkali solution. The concentrated alkali solution was slowly added dropwise to the pretreated chitosan, and the mixture was heated to 50°C and stirred for 2 hours to obtain a reaction system. The reaction system was cooled to 40°C, and 25 parts of glycidyltrimethylammonium chloride solution (70% solid content) were weighed and slowly added dropwise to the reaction system through a constant pressure dropping funnel over a period of 30 minutes. After the addition was complete, the temperature was slowly raised to 70°C and reacted for 6 hours to obtain a reaction solution. After the reaction was completed, the solution was cooled to room temperature, and the pH of the reaction solution was adjusted to neutral using glacial acetic acid. The solution was then vacuum filtered to obtain a solid crude product. The filter cake was washed three times with 70% ethanol aqueous solution and twice with anhydrous ethanol. The cleaned filter cake was transferred to a vacuum drying oven and dried at 50°C for 18 hours to obtain modified chitosan.

[0021] Under stirring, 50 parts of zein powder and 50 parts of maltodextrin powder were added to 500 parts of 80% ethanol solution and stirred at room temperature for 30 minutes until a uniform suspension was formed. The pH of the suspension was adjusted to 8.5 using NaOH solution to obtain a mixture. The mixture was transferred to a constant temperature water bath at 60°C and stirred under sealed conditions for 5 hours. After the reaction was completed, it was immediately placed in an ice-water bath to cool rapidly to room temperature. The pH of the solution was adjusted to 5.0 using 1M HCl solution. The acidified mixture was slowly poured into a large amount of cold deionized water to obtain a precipitate. After the precipitate was allowed to stand for 30 minutes, it was washed four times by centrifugation with deionized water and then freeze-dried for 36 hours to obtain modified zein.

[0022] 100 parts of bleached sulfate softwood pulp were beaten to a freeness of 42°SR. The pulp was then diluted to a concentration of 3%. 0.05 parts of cationic polyacrylamide were added and stirred for 5 minutes. 0.8 parts of modified lignin were added and stirred for 15 minutes. 0.3 parts of alkyl ketene dimer were added and the pH was adjusted to 8.0 to obtain the bottom layer pulp. 40 parts of bleached sulfate softwood pulp and 60 parts of bleached sulfate hardwood pulp were mixed and beaten to 45°SR. The pulp was then diluted to a concentration of 2%. 30 parts of cellulose nanofiber aqueous dispersion were added and treated with a high-shear disperser for 10 min. 5 parts of microcrystalline cellulose powder were added and high-shear dispersion was continued for 10 min. The pH of the pulp was adjusted to 6.5. 2 parts of modified chitosan solution were slowly added under high-speed shear conditions and stirred for 10 min. 1.5 parts of dialdehyde starch were added and stirred for 20 min to obtain the top layer pulp. 10 parts of modified zein was dissolved in 90 parts of 75% ethanol aqueous solution, and stirred at 50°C for 1 hour to obtain a base material; 0.2 parts of glycerol was added into the base material, and stirred at 500 rpm for 10 minutes; then 25 parts of polylactic acid emulsion (emulsified by deionized water, solid content 40%) was slowly added under stirring, the dropping time was 30 minutes, and then dispersed at 9000 rpm for 15 minutes to obtain a composite coating; A former with double-layer headboxes was used, the top layer pulp was injected into the top layer headbox, and the bottom layer pulp was injected into the bottom layer headbox; two presses were used, and the linear pressure was gradually increased, 30 kN / m for the first press and 50 kN / m for the second press; a drum dryer was used, and the surface temperature was controlled at 98°C to make the dryness of the paper sheet reach about 92%; the paper sheet passed through an oven at 120°C, and the residence time was 40 seconds to obtain a semi-finished paper sheet; The composite coating was coated by air knife, only on the top layer side, and the dry coating amount was controlled at 0.8 g / m², and then the coated paper sheet immediately entered a hot air drying oven at 70°C for fast drying to form a film; a soft calender was used, the linear pressure was 40 kN / m, and the temperature was 60°C to obtain a tipping paper.

[0023] Example 2-5 Refer to the parameter conditions in Example 1, and the specific differences are shown in Table 1.

[0024] Table 1 Parameter conditions of Examples 1-5

[0025] Comparative Example 1 Refer to the parameter conditions in Example 1, and the difference is that no cellulose nanowhisker aqueous dispersion is added.

[0026] Comparative Example 2 Refer to the parameter conditions in Example 1, and the difference is that no microcrystalline cellulose powder is added.

[0027] Comparative Example 3 Refer to the parameter conditions in Example 1, and the difference is that the cellulose nanowhisker aqueous dispersion and the microcrystalline cellulose powder are not added in sections during shearing and dispersing.

[0028] Experimental Example 1 Physical strength test According to GB / T 12914-2018, a constant rate of extension tester was used, the clamping length was 180 mm, and the tensile speed was 100 mm / min to test the tensile strength of Examples 1-5 and Comparative Examples 1-3; according to GB / T 22364-2008, a bending stiffness tester was used, the test length was 70 mm, and the bending angle was 15° to test the stiffness of Examples 1-5 and Comparative Examples 1-3. The results are shown in Table 2 and Figure 1 .

[0029] Table 2 Physical strength of Examples 1-5 and Comparative Examples 1-3

[0030] From Table 2 and Figure 1 It can be found that, without cellulose nanocrystals in Comparative Example 1, the tensile strength and stiffness are significantly lower than those of Example 1, because CNC has extremely high Young's modulus, acting as a nano-steel bar in the top layer of pulp, which is a key functional material to provide high strength and high stiffness. Without the nano-enhancing effect of CNC, the physical strength of the paper sheet is greatly reduced, which proves that the addition of CNC is a necessary condition to achieve high physical strength. In Comparative Example 2, without microcrystalline cellulose, the tensile strength and stiffness are significantly lower than those of Example 1, but slightly higher than those of Comparative Example 1, which proves the synergistic effect of MCC. In Example 1, MCC acts as a micro-sized filler to fill the gap between the fibers and the nanomaterials that CNC fails to cover, improving the density and uniformity of the paper sheet. Without MCC, this multi-scale physical enhancement synergistic effect is destroyed, resulting in a decrease in the overall strength of the paper sheet. In Comparative Example 3, CNC and MCC are mixed and added at once without segmentation and high shear, the tensile strength and stiffness are lower than those of Example 1, but higher than those of Comparative Examples 1 and 2, which proves that the process of segmented addition and stepwise shearing in Example 1 is necessary. In Comparative Example 3, CNC and MCC are added together, which may lead to the agglomeration of nanomaterials and micromaterials due to hydrogen bonding, rather than effective dispersion and anchoring of each on the wood pulp fibers. This poor dispersion results in a significant reduction in the enhancement effect.

[0031] Example 6-9 Refer to the parameter conditions in Example 1, with specific differences as shown in Table 3.

[0032] Table 3 Parameter conditions of Example 1 and Examples 6-9

[0033] Comparative Example 4 Refer to the parameter conditions in Example 1, with the difference that the alkali lignin is not modified.

[0034] Comparative Example 5 Refer to the parameter conditions in Example 1, with the difference that no modified lignin is added.

[0035] Comparative Example 6 Refer to the parameter conditions in Example 1, with the difference that no AKD is added.

[0036] Experimental Example 2 Wet resistance test The water vapor transmission rate was tested according to GB / T 1037-2021, using the cup method, a test area of 50 cm², a humidity gradient of 0-90% RH, and examples 1, 6-9 and comparative examples 4-6 were tested; using reagents containing glyceryl trivalerate, drop on the bottom layer of paper, keep at 60℃, 95% RH for 24h, observe whether there are transparent oil spots with a diameter greater than 0.5mm on the outer surface of examples 1, 6-9 and comparative examples 4-6. The results are shown in Table 4.

[0037] Table 4 Moisture resistance of examples 1, 6-9 and comparative examples 4-6

[0038] From Table 4, it can be found that comparative example 4 uses unmodified alkali lignin. Its water vapor transmission rate is much higher than that of example 1, and penetration occurs in the penetration resistance test. This is because unmodified lignin contains a large number of hydrophilic groups (such as phenolic hydroxyl groups), which not only cannot cooperate with AKD to hydrophobize, but also may compete with AKD for the reaction sites of fibers in the pulp, and become "wet guide points" inside the paper, which proves that the modification of lignin by lauryl chloride esterification (making it highly hydrophobic) in the present application is a necessary prerequisite for realizing the internal defense line synergistic effect. Comparative example 5 does not add modified lignin, but only relies on 0.3 parts of AKD for internal sizing, and its water vapor transmission rate is significantly higher than that of example 1, and a small amount of penetration occurs in the penetration resistance test, which shows that although AKD alone provides a certain basic moisture resistance, it is not enough to form a dense hydrophobic network in the entire paper matrix as in example 1, which inversely proves that the addition of modified lignin can produce a synergistic effect with AKD, and together improve the performance of the internal defense line to the level of no penetration. Comparative example 6 does not add AKD, but only relies on modified lignin, and its water vapor transmission rate and penetration resistance are poorer than those of example 1 and comparative example 5, which reveals another key of the synergistic system. The core role of AKD is to esterify with the hydroxyl groups of cellulose and chemically bond to the fibers, while the modified lignin is hydrophobic, but without the assistance of AKD, it mainly exists in the form of physical adsorption, which is easy to fall off during the papermaking process or when it meets moisture, which proves that AKD is the necessary anchor point for realizing the synergy, and without it, the hydrophobic advantage of modified lignin cannot be stably exerted.

[0039] Examples 10-13 Refer to the parameter conditions in example 1, the specific differences are shown in Table 5.

[0040] Table 5 Parameter conditions of examples 1 and examples 10-13

[0041] Comparative example 7 Refer to the parameter conditions in example 1, the difference is that the chitosan is not modified.

[0042] Comparative Example 8 Reference is made to the parameter conditions in Example 1, except that no modified chitosan is added.

[0043] Comparative Example 9 Reference is made to the parameter conditions in Example 1, except that no dialdehyde starch is added.

[0044] Comparative Example 10 Reference is made to the parameter conditions in Example 1, except that no modification treatment is performed on the zein.

[0045] Comparative Example 11 Reference is made to the parameter conditions in Example 1, except that no modified zein is added.

[0046] Comparative Example 12 Reference is made to the parameter conditions in Example 1, except that no composite coating is coated on the semi-finished paper, and the tipping paper is directly processed.

[0047] Comparative Example 13 Reference is made to the parameter conditions in Example 1, except that the bottom layer slurry and the top layer slurry are interchanged.

[0048] Experimental Example 3 Test of moisture resistance Reference is made to the test method of Experimental Example 2 to test Example 1, Examples 10-13 and Comparative Examples 7-13, and the results are shown in Table 6.

[0049] Table 6 Moisture resistance of Example 1, Examples 10-13 and Comparative Examples 7-13

[0050] From Table 6, it can be found that Comparative Example 7 uses unmodified ordinary chitosan, its water vapor transmission rate is higher than Example 1, and penetration occurs, because the solubility of ordinary chitosan (acidic) does not completely match the pH 6.5 environment of the top layer slurry, and its positive charge is weaker than the quaternary ammonium modified chitosan, which leads to low efficiency and uneven chemical cross-linking network formed by it and dialdehyde starch, and the internal wet strength of the paper is insufficient, which proves that quaternary ammonium modification is a key step to achieve efficient and uniform wet strength network. Comparative Example 8 does not add modified chitosan, which completely destroys the chemical cross-linking wet strength system of the top layer slurry, without modified chitosan as a bridge to react with dialdehyde starch, the top layer of the paper completely relies on hydrogen bonding of the fibers when wet, and the paper substrate quickly loses strength. Comparative Example 9 does not add dialdehyde starch, and the result is similar to Comparative Example 8, which proves the synergy of the chemical cross-linking wet strength system. If there is only modified chitosan without dialdehyde starch, the modified chitosan can only play a role in retention and drainage, and cannot form a cross-linking network, which reversely proves the wet resistance of the present application, which is derived from the synergistic chemical reaction between the amino group of the modified chitosan and the aldehyde group of the dialdehyde starch. Comparative Example 10 uses unmodified zein, and its water vapor transmission rate and barrier performance are severely reduced, because the unmodified zein is extremely brittle after film formation, and during the drying, winding and calendering processes of the semi-finished paper, the coating layer will produce micro-cracks that are not visible to the naked eye, and during the barrier test, moisture and oil will penetrate through these cracks, causing the barrier to completely fail. Comparative Example 11 does not add modified zein, and only has PLA emulsion in the coating, and its water vapor transmission rate is higher than Example 1, and penetration occurs, which shows that the modified zein plays the role of a flexible matrix and film-forming skeleton in the composite coating, and without the synergistic effect of the modified zein, the PLA emulsion particles alone cannot form a complete, uniform, and pore-free film, resulting in a decrease in barrier performance. Comparative Example 12 does not coat the composite coating at all, and its water vapor transmission rate is the highest among all the examples. This comparative example strongly proves the dual-line design concept of the present application, which shows that relying only on internal sizing and internal wet strength in the pulp is absolutely insufficient to resist the penetration of harsh external moisture and oil, and it must rely on the physical barrier provided by this surface composite coating. Comparative Example 13 exchanges the top layer and the bottom layer slurry, and its wet resistance is poor, which proves that the functional gradient structure of the top layer (hydrophilic, wet strength) + bottom layer (hydrophobic) designed in the present application is a necessary process design to achieve the final performance.

[0051] Although embodiments of the present application have been shown and described, it is to be understood that various modifications, substitutions, replacements and changes can be made to these embodiments without departing from the principles and spirit of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for preparing tipping paper, characterized in that, The preparation method includes: A bottom layer pulp is obtained by beating and diluting bleached sulfate softwood pulp, adding cationic polyacrylamide, modified lignin, and alkyl ketene dimer; a top layer pulp is obtained by mixing and beating the bleached sulfate softwood pulp and bleached sulfate hardwood pulp, adding cellulose nanofibers, microcrystalline cellulose, modified chitosan, and dialdehyde starch; the bottom layer pulp and top layer pulp are shaped and dried to obtain a semi-finished paper; a composite coating is applied to the semi-finished paper, and after drying, the tipping paper is obtained; the modified lignin is obtained through esterification modification; the modified chitosan is obtained through glycidyltrimethylammonium chloride modification; the composite coating is obtained by mixing modified zein and polylactic acid emulsion, and the modified zein is obtained through saccharification modification.

2. The method for preparing tipping paper according to claim 1, characterized in that, The specific preparation method of the underlying pulp is as follows: after beating the bleached sulfate softwood pulp, dilute it, add the cationic polyacrylamide, stir, add the modified lignin, continue stirring, then add alkyl ketene dimer, and adjust the pH to obtain the underlying pulp.

3. The method for preparing tipping paper according to claim 1, characterized in that, The specific preparation method of the top layer slurry is as follows: the bleached sulfate softwood pulp and the bleached sulfate hardwood pulp are mixed, pulped, and diluted. Cellulose nanofiber aqueous dispersion is added, and the microcrystalline cellulose is added after dispersion treatment. The dispersion is continued. After adjusting the pH value, the modified chitosan is added. After stirring, the dialdehyde starch is added. After stirring, the top layer slurry is obtained.

4. The method for preparing tipping paper according to claim 1, characterized in that, The specific preparation method of the modified lignin is as follows: alkali lignin powder is dried to obtain pretreated alkali lignin; anhydrous pyridine is added to the pretreated alkali lignin and stirred to obtain a suspension; lauroyl chloride is diluted with the anhydrous pyridine and added dropwise to the suspension to carry out the reaction; after the reaction is completed, the reaction solution is poured into methanol, vacuum filtered, and then washed and vacuum dried to obtain the modified lignin.

5. The method for preparing tipping paper according to claim 1, characterized in that, The modified chitosan is prepared by mixing chitosan powder and isopropanol to obtain pretreated chitosan; adding concentrated alkali solution dropwise to the pretreated chitosan, followed by adding glycidyltrimethylammonium chloride solution to react; adjusting the pH after cooling, filtering, washing and drying to obtain the modified chitosan.

6. The method for preparing tipping paper according to claim 1, characterized in that, The composite coating is prepared by dissolving the modified zein in an aqueous ethanol solution and stirring to form a base material; adding glycerol to the base material, stirring, and then adding the polylactic acid emulsion dropwise, and dispersing to obtain the composite coating.

7. The method for preparing tipping paper according to claim 6, characterized in that, The modified zein is prepared as follows: zein powder and maltodextrin powder are added to an ethanol solution and stirred to obtain a suspension; the pH value is adjusted to obtain a mixture; the mixture is stirred in a water bath and reacted; after the reaction is completed, it is cooled to room temperature, the pH value is adjusted to acidic, and then poured into deionized water to obtain a precipitate; the precipitate is allowed to stand, centrifuged and washed, and freeze-dried to obtain the modified zein.

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