A cigarette frame paper and a preparation process thereof

Abstract

The present application relates to the technical field of coated paperboard manufacturing, in particular to a cigarette frame paper and a preparation process thereof. The present application overcomes the problem that mechanical properties and barrier properties cannot be considered in the prior art. The cigarette frame paper of the present application is prepared by compounding bleached broadleaf wood pulp and coniferous wood pulp, adding calcium carbonate and nanocellulose crystals to prepare a high-uniformity modified paper base; then, a polyurethane emulsion obtained by the reaction of polycaprolactone diol, isophorone diisocyanate, dimethylol propionic acid and 1,4-butanediol and terminated by aminopropyl triethoxysilane is mixed with gelatinized starch and flaky kaolin in steps to obtain a composite coating; the composite coating is coated on the modified paper base on both sides, and is dried, hot-pressed, shaped and aged in steps. Through the multi-stage synergy of paper base-coating-process, the frame paper has excellent mechanical properties, high printing stability and high barrier properties, and is suitable for high-end cigarette packaging fields with strict performance requirements.

Description

Technical Field

[0001] This invention relates to the field of coated paperboard manufacturing technology, specifically to a cigarette frame paper and its preparation process. Background Technology

[0002] Cigarette frame paper is a key structural material in cigarette carton packaging, playing a vital role in protecting the quality of cigarettes during storage, transportation, and sales. As a type of coated paper, existing frame paper technology often faces an inherent contradiction between mechanical properties, barrier properties, and printability.

[0003] First, the frame paper must have excellent mechanical properties, including high tensile strength, high stiffness and high folding endurance, to adapt to the operation of high-speed automated packaging machines and prevent breakage, wrinkling or deformation during processing. Traditional paper often uses thick coatings or high basis weight paper base to improve stiffness, but this leads to a decrease in flexibility and a sharp drop in folding endurance, making it easy for cracks to occur at the folds.

[0004] Secondly, as the first line of defense for cigarettes, the frame paper must possess high barrier properties, especially against water vapor and oxygen, to prevent the tobacco from becoming damp and deteriorating or losing its aroma. Current technologies, in pursuit of high barrier properties, often employ lamination (PE / PP materials), aluminum foil composites, or coatings with polyvinylidene chloride (PVDC). However, lamination and aluminum foil processes are complex and affect subsequent printing. Furthermore, while chlorinated polymers such as PVDC offer good barrier properties, their coating process easily corrodes equipment, and the finished products suffer from serious defects in environmental friendliness, recyclability, and heat-sealing properties, failing to align with the trend of green manufacturing.

[0005] Finally, as a printing substrate, frame paper requires a smooth, uniform surface and good ink receptivity. Many high-barrier or high-stiffness coatings (such as solvent-based coatings) have low surface energy, resulting in poor printing adhesion. Water-based coatings are prone to defects such as blistering and pinholes during the drying process, which seriously affect printing quality and surface strength.

[0006] In summary, although existing technologies have improved the application range of cigarette frame paper by adapting the composition and process, the problem of not being able to simultaneously achieve both mechanical and barrier properties still exists.

[0007] To address this, a cigarette frame paper and its preparation process were proposed. Summary of the Invention

[0008] The purpose of this invention is to provide a cigarette frame paper and its preparation process. The cigarette frame paper of this invention is prepared by compounding bleached hardwood pulp and softwood pulp, adding calcium carbonate and nanocellulose crystals to prepare a highly uniform modified paper base; then, a polyurethane emulsion obtained by reacting polycaprolactone diol, isophorone diisocyanate, dimethylolpropionic acid, and 1,4-butanediol, and capping with aminopropyltriethoxysilane, is mixed stepwise with gelatinized starch and flake kaolin to obtain a composite coating; the composite coating is then applied to both sides of the modified paper base, followed by stepwise drying, hot pressing, and curing. Through the multi-stage synergy of the paper base, coating, and process, the frame paper possesses excellent mechanical properties, high printability, and high barrier properties, making it suitable for high-end cigarette packaging applications with stringent performance requirements.

[0009] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a process for preparing cigarette frame paper, comprising the following steps: Double-sided coating is performed using a film transfer sizing machine. The composite coating is pumped into the circulation tank of the film transfer sizing machine. The paper machine is started, and the modified paper base is led through the main drying unit. The moisture content of the modified paper base before entering the sizing machine is controlled at 8.0%-10.0%. The film transfer sizing machine is started, and the sizing roller contacts the modified paper base to begin coating, allowing the coating to be evenly deposited on both sides of the paper base. The coating amount is set to 2.2-2.8 g / m² / single side, and the coating uniformity is monitored during continuous operation to obtain coated paper base. The coated paper base is then cured in stages, first entering the first curing zone of the post-drying unit, where the surface temperature of the heating unit is 80℃, in one step. The curing time is 10 seconds; then it enters the second curing zone of the post-drying group, where the surface temperature of the drying cylinder is raised to 120°C, and the second curing time is 8 seconds. The high-efficiency ventilation of the drying hood is turned on and maintained to remove the ethanol generated in the reaction and the water vapor generated in the condensation. The final moisture content is controlled at 5.5-7.0% to obtain the cured paper base; the cured paper base is then put into a controllable medium-high pressure calender, with the surface temperature of the calender roller set at 120-130°C and the linear pressure set at 120-180 kN / m. It is then hot-pressed and shaped under high temperature and high pressure; then it is left to stand at 20-30°C for 15-25 hours to mature and obtain the cigarette frame paper; Preferably, the preparation of the modified paper base includes the following steps: 70-80 parts of bleached hardwood pulp are rebeaten to a freeness of 35-40°SR to obtain hardwood pulp; 20-30 parts of bleached softwood pulp are lightly beaten to a freeness of 25-30°SR to obtain softwood pulp, which is then added to the hardwood pulp. The mixing speed is 200-400 rpm, and the mixing time is 10-20 min to obtain a mixed pulp. 15-20 parts of light calcium carbonate, 0.1-0.2 parts of AKD sizing agent, and 1.5-3 parts of nanocellulose crystal suspension are added to the mixed pulp in sequence. A wire mesh forming device is used to ensure high uniformity. Boot press is used with a linear pressure of 800-1000 kN / m and a press dryness of >50% to obtain the modified paper base.

[0010] Preferably, the preparation of the composite coating includes the following steps: adding 30-40 parts of anionic starch to deionized water and stirring to prepare a starch slurry with a solid content of 25%-30%; adding the starch slurry to a steam jet cooker and cooking at 110-130℃ for 10-20 minutes, then flash-cooling to 50-60℃ to obtain gelatinized starch; mixing 10-20 parts of flake kaolin powder with deionized water in a high-shear disperser at a speed of 800-1200 rpm for 20-30 minutes, adding 0.3 parts of sodium polyacrylate dispersant, and stirring evenly to obtain a kaolin slurry with a solid content of 60-70%; and adding the gelatinized starch to the main mixing tank under heat preservation conditions. In the first step, slowly add kaolin slurry and mix at a low speed of 300-500 rpm for 15-20 minutes. Then add 0.5 parts of defoamer and 0.5 parts of wetting agent and mix evenly to obtain a mixture. Slowly add 25% ammonia solution dropwise to the mixture, monitoring the pH in real time with a pH meter, and stir at 200-400 rpm until the pH value stabilizes at 8.0-8.5 to obtain a slurry system. Add 45-55 parts of polyurethane emulsion slowly to the slurry system through a pump while stirring at 100-200 rpm. After the addition is complete, stir at a low speed for 15-30 minutes to mix evenly. Finally, filter through a 200-300 mesh vibrating screen to obtain a composite coating.

[0011] Preferably, the preparation of the polyurethane emulsion includes the following steps: Polycaprolactone diol and isophorone diisocyanate are added to a vacuum drying oven and dried under vacuum at 100°C for 8 hours; 200 parts of polycaprolactone diol, 10.7 parts of dimethylolpropionic acid, and 10.8 parts of 1,4-butanediol are added to a flask, and 150 parts of acetone are added to dissolve them. The mixture is heated to 40-50°C and stirred to remove water for 30 minutes to obtain a mixed solution; the mixed solution is heated to 70°C, and 120 parts of isophorone diisocyanate are slowly added dropwise over 1-2 hours; then the temperature is raised to 80-90°C, and the polymerization reaction is maintained for 2-4 hours to obtain a polymer system; the polymer system is cooled to 50-70°C, and 112 parts of aminopropyltriethoxysilane are slowly added dropwise over 30 minutes; the end-capping reaction is maintained for 60-90 minutes to obtain a reaction system. The reaction system was cooled to 50°C, and 8 parts of triethylamine were added. The mixture was stirred and neutralized for 30 minutes at a stirring speed of 300-500 rpm to obtain polyurethane. The polyurethane was slowly added to 800 parts of deionized ice water at 2000 rpm. After the addition was complete, the mixture was stirred for 30 minutes to obtain an emulsion system. The emulsion system was transferred to a rotary evaporator and vacuum distilled at 50-55°C to completely remove acetone. After cooling, a polyurethane emulsion with a solid content of approximately 35%, a pH of 8.0, and a milky white appearance was obtained. The molar ratio of -NCO groups to -OH groups was 1.6-2:1, the molar ratio of polycaprolactone diol, dimethylolpropionic acid, and 1,4-butanediol was 1:0.8:1.2, and the molar amount of the end-capping agent APTES was equal to the molar amount of the remaining free -NCO groups in the prepolymer.

[0012] The present invention also provides a cigarette frame paper, comprising a modified paper base and a composite coating.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention achieves balanced mechanical properties through multi-level synergy. The paper base is made of a blend of softwood and hardwood pulp with added CNC nano-reinforcement, providing a solid mechanical foundation; the introduction of BDO flexible segments into the polyurethane emulsion ensures high toughness of the coating; the silane crosslinking network formed by APTES end-capping and its chemical bonding with fibers enhance the bonding strength between the coating and the paper base, as well as the coating's own strength; compared with existing technologies, this invention combines high stiffness and ultra-high folding endurance, perfectly meeting the needs of high-speed packaging.

[0014] 2. This invention utilizes a precise combination of processes, in which the synergistic effect of flake kaolin, starch, and PU emulsion in the composite coating provides a smooth substrate; the step-by-step curing process allows volatiles to be discharged in an orderly manner, avoiding coating defects; and the final hot-pressing further compacts the coating and fuses the polymer, achieving extremely high surface density and smoothness. Compared with traditional coated paper, which is prone to blistering and pinholes during drying, this invention maintains good levels of surface strength and optical density uniformity, meeting the requirements of high-precision printing.

[0015] 3. This invention constructs a dual physical-chemical barrier system. Physically, the lamellar kaolin in the composite coating is layered and arranged in the coating, forming a labyrinth effect that greatly prolongs the penetration path of oxygen molecules; gelatinized starch fills the micropores between the kaolin and polymer chains; chemically, the end-capped polyurethane undergoes high-temperature curing, during which its silanol groups highly condense and crosslink, forming a dense inorganic network that effectively blocks the penetration of oxygen molecules; hot pressing further eliminates micropores. Compared with existing technologies, this invention achieves excellent barrier performance.

[0016] 4. This invention employs a three-tiered hydrophobic and densification design from the paper base to the coating. AKD sizing agent is added during paper base preparation to achieve internal hydrophobicity, reducing the paper base's affinity for water. Furthermore, the main body of the polyurethane emulsion is hydrophobic, and the coating body also exhibits hydrophobic properties. The resulting dense Si-O-Si cross-linked network, combined with the non-porous surface formed by hot pressing, physically isolates water molecule penetration. Compared to existing technologies, this invention's multiple synergistic barriers result in extremely low water vapor permeability.

[0017] 5. This invention uses a water-based coating system, utilizing polyurethane emulsion with water as the dispersion medium. The composite coating preparation and coating process do not use any additional volatile organic solvents. In addition, the acetone solvent used in polyurethane synthesis can be effectively removed by vacuum distillation after emulsification, and is easy to condense, recover and reuse, forming a closed loop. Compared with existing high-barrier frame paper that relies heavily on PVDC coating or aluminum-plastic composites, the coating components are all chlorine-free, environmentally friendly materials that are easy to degrade or recycle, resulting in significant environmental and economic benefits. Attached Figure Description

[0018] Figure 1 This is a process flow diagram of the cigarette frame paper prepared in Example 1 of the present invention. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] The polycaprolactone diol has a molecular weight of 2000 g / mol and an acid value <0.1 mg KOH / g; DMPA is dimethylolpropionic acid with a molecular weight of 134 g / mol and an acid value of 415-425 mg KOH / g; BDO ​​is 1,4-butanediol with a molecular weight of 90 g / mol; APTES is aminopropyltriethoxysilane; the anionic starch viscosity is 100-300 mPa·s @ 25% solid content, 100℃; the flaky kaolin has an aspect ratio >30:1, a median D50 particle size of 0.5-1.5 μm, and an ISO >85%; the AKD sizing agent has a solid content of 20%, and the mass fraction is dry weight fraction; the nanocellulose crystal suspension... The solid content is 10%, crystallinity >70%, length 100-300nm, diameter 10-20nm, and the mass fraction is the dry weight fraction of nanocellulose crystals; the bleached hardwood pulp is derived from eucalyptus, ISO >88%, and viscosity is 600-800cm³ / g; the bleached softwood pulp is derived from Nordic softwood, ISO >88%, and tensile index >90Nm / g; the sodium polyacrylate dispersant is low molecular weight sodium polyacrylate, CAS: 9003-04-7; the defoamer is poloxamer 188, CAS: 9003-11-6; and the wetting agent is 2,4,7,9-tetramethyl-5-decyn-4,7-diol, CAS: 126-86-3.

[0021] Please see Figure 1 This invention provides a cigarette frame paper and its preparation process, the technical solution of which is as follows: Example 1

[0022] 75 parts of bleached hardwood pulp were rebeaten to a freeness of 40°SR to obtain hardwood pulp; 25 parts of bleached softwood pulp were lightly beaten to a freeness of 30°SR to obtain softwood pulp, which was then added to the hardwood pulp. The mixture was stirred at 300 rpm for 20 minutes to obtain a mixed pulp. 20 parts of light calcium carbonate, 0.2 parts of AKD sizing agent, and 2 parts of nanocellulose crystal suspension were added to the mixed pulp in sequence. A wire mesh forming device was used to ensure high uniformity. Boot press was used with a linear pressure of 800 kN / m and a press dryness of >50% to obtain the modified paper base. Polycaprolactone diol and isophorone diisocyanate were added to a vacuum drying oven and dried at 100°C for 8 hours. 200 parts of polycaprolactone diol, 10.7 parts of dimethylolpropionic acid, and 10.8 parts of 1,4-butanediol were added to a flask, dissolved in 150 parts of acetone, and heated to 50°C. The mixture was stirred and dehydrated for 30 minutes to obtain a mixed solution. The mixed solution was heated to 70°C, and 120 parts of isophorone diisocyanate were slowly added dropwise over 1 hour. The temperature was then raised to 80°C, and the polymerization reaction was maintained for 3 hours to obtain a polymer system. The polymer system was cooled to 60°C, and 112 parts of aminopropyltriethoxysilane were slowly added dropwise. The addition was completed within 0 min, and the reaction was kept at the temperature for 75 min to obtain the reaction system. The reaction system was cooled to 50℃, 8 parts of triethylamine were added, and the mixture was stirred and neutralized for 30 min. The pH value was adjusted to 8.0 using ammonia water, and the stirring speed was 400 rpm to obtain polyurethane. The polyurethane was slowly added to 800 parts of deionized ice water at 2000 rpm, and the mixture was stirred for 30 min after the addition was completed to obtain the emulsion system. The emulsion system was transferred to a rotary evaporator and vacuum distilled at 50℃ to completely remove acetone. After cooling and filtration, a polyurethane emulsion with a solid content of about 35%, a pH of 8.0, and a milky white appearance was obtained.

[0023] Preparation of composite coatings: 40 parts of anionic starch were added to deionized water and stirred to prepare a starch slurry with a solid content of 25%. The starch slurry was added to a steam jet cooker and cooked at 120°C for 15 minutes. The starch was then flash-cooled to 50°C to obtain gelatinized starch. 15 parts of flake kaolin powder were mixed with deionized water in a high-shear disperser at 1000 rpm for 30 minutes. 0.3 parts of sodium polyacrylate dispersant were added and stirred until homogeneous to obtain a kaolin slurry with a solid content of 65%. The gelatinized starch was added to the main mixing tank under heat preservation conditions, and the kaolin slurry was slowly added while stirring at low speed. Mix the ingredients at 400 rpm for 20 minutes, then add 0.5 parts of defoamer and 0.5 parts of wetting agent, and stir until homogeneous to obtain a mixture. Slowly add ammonia (25% concentration) dropwise to the mixture, monitoring the pH in real time with a pH meter, and stir at 300 rpm until the pH value stabilizes at 8.0 to obtain a slurry system. Add 50 parts of polyurethane emulsion to the slurry system slowly through a pump under low-speed, low-shear stirring conditions, with a stirring speed of 150 rpm. After the addition is complete, mix evenly at 150 rpm. Finally, filter the mixture through a 250-mesh vibrating screen to obtain a composite coating.

[0024] Double-sided coating is performed using a film transfer sizing machine. The composite coating is pumped into the circulation tank of the film transfer sizing machine. The paper machine is started, and the modified paper base is led through the main drying unit. The moisture content of the modified paper base before entering the sizing machine is controlled at 9%. The film transfer sizing machine is started, and the sizing roller contacts the modified paper base to begin coating. The metering parameters are adjusted to ensure uniform deposition of the coating on both sides of the paper base. The coating amount is set to 2.5 g / m² / single side, and the coating uniformity is monitored during continuous operation to obtain the coated paper base. The coated paper base is immediately subjected to step-by-step curing. It first enters the first curing zone of the post-drying unit, where the surface temperature of the heating unit is... The temperature is 80℃, and the first curing time is 10s. Then it enters the second curing zone of the post-drying group, where the surface temperature of the drying cylinder is raised to 120℃, and the second curing time is 8s. The high-efficiency ventilation of the drying hood is turned on and maintained to remove the ethanol generated in the reaction and the water vapor generated by condensation. The final moisture content is controlled at 6% to obtain the cured paper base. The cured paper base is then put into a controllable medium and high pressure calender, where the surface temperature of the calendering roller is set to 130℃ and the linear pressure is set to 150kN / m. It is then hot-pressed and shaped under high temperature and high pressure. Finally, it is left to stand at 30℃ for 20 hours to mature and obtain cigarette frame paper.

[0025] Examples 2-4 follow the same preparation method and parameters as Example 1, with differences shown in Table 1.

[0026] Table 1. Parameter variations in Examples 1-4

[0027] Comparative Example 1: Refer to Example 1, except that 100 parts of bleached hardwood pulp were used instead of bleached softwood pulp.

[0028] Comparative Example 2 is the same as Example 1, except that 100 parts of bleached softwood pulp were used instead of bleached hardwood pulp.

[0029] Comparative Example 3 is the same as Example 1, except that light calcium carbonate is not added during the preparation of the modified paper base, while the amounts of other components remain unchanged.

[0030] Comparative Example 4 is the same as Example 1, except that no nanocellulose crystal suspension is added during the preparation of the modified paper base, while the amounts of other components remain unchanged.

[0031] Comparative Example 5 is the same as Example 1, except that aminopropyltriethoxysilane is not added for end-capping during the preparation of the polyurethane emulsion, while the rest of the process remains unchanged.

[0032] Comparative Example 6 is the same as Example 1, except that 20 parts of dimethylolpropionic acid were used for chain extension in the preparation of the polyurethane emulsion, and 1,4-butanediol was not added.

[0033] Comparative Example 7 is the same as Example 1, except that 20 parts of 1,4-butanediol were used for chain extension in the preparation of the polyurethane emulsion, and dimethylolpropionic acid was not added.

[0034] Comparative Example 8 is the same as Example 1, except that after double-sided coating, step-by-step curing is not used, and it is directly cured in one step for 20 seconds.

[0035] Comparative Example 9 is the same as Example 1, except that after double-sided coating, step-by-step curing is not used, but a second curing is performed directly for 20 seconds.

[0036] Experiment Example 1: Mechanical Property Testing The mechanical properties of the cigarette frame paper prepared in Examples 1-4 and Comparative Examples 1-9 were tested. Tensile strength was tested using a constant-speed tensile tester (GB / T12914-2018), with a sample width of 15 mm, a clamping length of 180 mm, and a tensile speed of 20 mm / min. Stiffness was tested using a bending stiffness tester (GB / T 22364-2008), with a test length of 70 mm and a bending angle of 15°. Folding endurance was tested using GB / T 457-2008, with a sample size of 100 mm × 15 mm, a folding speed of 120 times / min, and a load of 4.9 N. The results are shown in Table 2.

[0037] Table 2 Test Results of Examples and Comparative Examples

[0038] As shown in Table 2, the mechanical properties of the cigarette frame paper obtained in the comparative examples, obtained by adjusting the components and processes, were significantly lower than those in the examples. The results of Comparative Examples 1-2 indicate that while rebeating increased the bonding strength of the short-fibered hardwood pulp, the lack of long-fiber skeletons provided by softwood pulp resulted in insufficient fiber interlacing strength and reduced mechanical properties. Softwood pulp, with its long fibers, provides good tensile strength and folding endurance, but the lack of fine filling and high bonding of short fibers in pure softwood pulp led to a decrease in paper sheet uniformity. The results showed that Comparative Example 2, while achieving the highest stiffness, had significantly lower tensile strength and folding endurance than Example 1, indicating a severe imbalance in mechanical properties that could not meet the demands of high-speed packaging. Comparative Examples 3-4 showed that calcium carbonate, as a filler, provided microscopic bridging and volume filling, enhancing inter-fiber friction and paper densification. Its absence led to increased paper porosity, decreased tensile strength and stiffness, and reduced folding endurance due to increased fiber slippage and weakened synergistic reinforcement. CNC, as a nanoscale reinforcing agent, promoted hydrogen bond cross-linking of the fiber network. The lack of adhesion to the interface leads to uneven stress distribution within the paper substrate, weakening the overall mechanical properties. Comparative Example 5 shows that aminopropyltriethoxysilane, as a capping agent, introduces hydrolyzable triethoxysilane groups, which hydrolyze into silanols during the subsequent curing stage, further crosslinking to form a Si-O-Si network. Simultaneously, it forms chemical bonds with hydroxyl groups on the paper substrate surface, significantly improving mechanical properties. Combined with the results of Comparative Examples 8-9, it is clear that only low-temperature curing is sufficient; the temperature and time are far from enough to trigger a complete condensation and crosslinking reaction of the silanol groups, resulting in incomplete coating curing. Complete crosslinking results in low crosslinking density and insufficient mechanical properties; conversely, only a high-temperature curing process leads to coating blistering, pinholes, or skin effect, damaging the density and uniformity of the coating and causing a decline in mechanical properties; dimethylolpropionic acid provides hydrophilic groups to facilitate emulsification, while BDO provides flexible segments; in Comparative Example 7, the lack of dimethylolpropionic acid prevented the formation of a stable polyurethane emulsion system, making the overall coating process impossible and resulting in the failure to obtain samples and test data; in Comparative Example 6, the lack of BDO led to the introduction of too many rigid groups into the polymer chain, resulting in a brittle coating with insufficient toughness.

[0039] In summary, this invention combines bleached hardwood pulp with bleached softwood pulp, providing excellent fiber bonding strength and smoothness while retaining the skeletal strength of long fibers. CNC machining acts as nano-rivets and bridging between fibers, enhancing the bonding strength between fibers and providing a solid platform for subsequent high-performance coatings. The polyurethane emulsion is end-capped with aminopropyltriethoxysilane to form a dense three-dimensional cross-linked network. Finally, stepwise drying and hot pressing promote the condensation reaction between the silanol groups in the coating and the hydroxyl groups on the paper-based cellulose, forming strong chemical bonds. Through multi-level synergy of fiber mixing, filler bridging, silane end-capping coating, and stepwise hot pressing curing, a highly uniform and highly adhesive composite structure is constructed, achieving a balanced improvement in tensile strength, stiffness, and folding endurance.

[0040] Comparative Example 10 is the same as Example 1, except that the coating amount of the composite coating is 2g / m² / single side.

[0041] Comparative Example 11 is the same as Example 1, except that the coating amount of the composite coating is 3g / m² / single side.

[0042] Comparative Example 12 is the same as Example 1, except that no flake kaolin powder is added during the preparation of the composite coating, while the amounts of the other components remain unchanged.

[0043] Comparative Example 13 is the same as Example 1, except that gelatinized starch is not added in the preparation of the composite coating, while the rest of the process remains unchanged.

[0044] Comparative Example 14 is the same as Example 1, except that hot pressing is not performed, while the rest of the process remains the same.

[0045] Experiment Example 2: Printing Stability Test The printing stability of the cigarette frame paper prepared in Examples 1-4 and Comparative Examples 8-14 was tested. Referring to GB / T12911-2013, an IGT AIC2 surface strength tester was used, with 0.5 mL of ink applied and a printing speed of 0.5 m / s. The critical speed at which the paper roughened was recorded. Following GB / T 17666-2008, an X-Rite eXact spectrophotometer was used to print ink on a standard black and white plate, and the coefficient of variation (CV%) in the 0.4-0.6 optical density region was measured. The test results are shown in Table 3.

[0046] Table 3 Test Results of Examples and Comparative Examples

[0047] As shown in Table 3, the printing stability of the cigarette frame paper obtained by adjusting the components and process in the comparative examples was significantly reduced compared to the examples. Comparative Examples 8-9 show that stepwise curing ensures the gradual condensation of aminopropyltriethoxysilanesilanol, expelling ethanol / water vapor and preventing the formation of bubbles and pores in the coating. Low-temperature curing alone results in residual volatiles, a loose surface, increased deinking length, increased optical density variation, and damage to the coating's uniform ink affinity. The lack of low-temperature curing leads to rapid curing of the coating at high temperatures, insufficient cross-linking, and a tendency to produce surface microcracks and uneven melting. Furthermore, the accumulation of volatiles results in poor local ink penetration, reduced surface strength and uniformity, and affects the smooth synergy after hot pressing. Comparative Examples 10-11 show that a low coating amount leads to insufficient coverage, increased exposure of paper fibers, decreased surface strength, and increased variation in optical density uniformity due to uneven ink penetration, weakening the smooth synergy between fillers and polymers. While a high coating amount increases the coating thickness, it is prone to over-curing. Thick build-up and uneven shrinkage during drying result in a microscopic orange peel texture. While surface strength increases slightly, uniformity is affected, disrupting the synergistic densification of film transfer coating and stepwise curing. Comparative Examples 12-13 show that during the preparation of composite coatings, kaolin, as a sheet-like filler, provides microscopic smoothness and ink adsorption bridging, improving surface flatness. Its absence leads to a rough coating, easy shedding of the gelatinized starch matrix, and a significant reduction in surface strength. The uniformity of optical density is poor due to uneven filler dispersion and poor ink distribution, weakening the synergistic enhancement with polyurethane. Similarly, gelatinized starch, as a binder, works with kaolin to form a stable matrix and promotes uniform dispersion of the polyurethane emulsion. Its absence results in a loose coating structure, and the dispersant cannot effectively anchor the filler, reducing stability. Comparative Example 14 shows that hot pressing drives the condensation and fusion of residual silanol to form a highly dense surface. Its absence leads to a loose coating after curing, residual micropores, decreased surface strength, and uneven ink transfer due to insufficient smoothness, weakening the synergistic optimization of the curing reaction.

[0048] In summary, this invention coats a highly uniform paper substrate with an organic-inorganic hybrid coating composed of silane-crosslinked polyurethane, gelatinized starch, and flake kaolin. It employs a stepwise curing process involving low-temperature dehydration / pre-hydrolysis and high-temperature condensation / crosslinking to control the formation of the coating's chemical network. Finally, it achieves physical compaction, polymer fusion, and final chemical curing of the coating through high-temperature and high-pressure hot pressing. This results in a highly dense, ink-receptive, and uniformly ink-receptive surface structure, meeting the high-precision printing requirements of cigarette frame paper.

[0049] Examples 5-7 follow the same preparation method and parameter conditions as Example 1, with differences shown in Table 4.

[0050] Table 4. Parameter changes in Examples 1 and 5-7

[0051] Comparative Example 15 is the same as Example 1, except that the composite coating is prepared by direct mixing instead of stepwise mixing.

[0052] Comparative Example 16 is the same as Example 1, except that AKD sizing agent is not added for internal sizing during the preparation of the modified paper base.

[0053] Experiment Example 3 Barrier Performance Test The tobacco frame paper prepared in Examples 1 and 5-7 was subjected to barrier performance tests. The water vapor transmission rate was tested according to GB / T 1037-2021 using the cup method, with a test area of ​​50 cm² and a humidity gradient of 0-90% RH. The oxygen transmission rate was tested according to GB / T 19789-2021 using the coulometric method, with a test area of ​​50 cm² and an oxygen partial pressure of 1 atm. The test results are shown in Table 5.

[0054] Table 5 Test Results of Examples and Comparative Examples

[0055] As shown in Table 5, the barrier properties of the cigarette frame paper obtained in the comparative examples, through adjustments to the components and processes, were significantly reduced compared to the examples. Referring to Table 3, the results of Comparative Examples 8-9 indicate that stepwise curing ensured the gradual condensation of APTES silanol, sealing the coating pores and preventing volatile residues from creating diffusion channels. The lack of initial curing led to uneven cross-linking during high-temperature rapid curing, increasing gas permeation paths due to surface microcracks, thus reducing barrier properties and weakening the synergistic sealing effect of ventilation and drying. The results of Comparative Examples 12-13 show that the kaolin sheet structure provides a layered barrier, synergistically filling micro-pores with starch and enhancing gas torsional diffusion paths. Its absence resulted in a loose coating, increased exposure of hydrophilic groups, poorer barrier properties, and weakened dispersion synergy with the polyurethane emulsion. The absence of gelatinized starch rendered the dispersant ineffective. The method of stabilizing the slurry results in a loose coating structure with high porosity, which disrupts the synergistic stability of the slurry under pH control. In Comparative Example 14, hot pressing drives the condensation and fusion of residual silanol coating, forming a gapless barrier on the compacted surface. The absence of this barrier leads to relaxation of the coating after curing, residual microchannels, increased permeability, and weakens the synergistic optimization of the curing reaction. In Comparative Example 15, direct mixing leads to incompatibility between kaolin and polyurethane, resulting in localized looseness of the coating and increased water vapor and oxygen permeability, which disrupts the synergistic homogenization of the vibrating screen filtration. Stepwise mixing ensures compatibility and uniform dispersion, avoiding agglomeration and defects. In Comparative Example 16, internal sizing of AKD enhances the hydrophobicity of the paper base, and the synergistic coating reduces the affinity for water vapor. The absence of this barrier causes the paper base fibers to absorb moisture and expand, increasing the channels at the coating-substrate interface. Although the barrier is still relatively good, the synergistic barrier is weakened, and the overall barrier performance decreases.

[0056] In summary, suitable coating substrates are prepared through refined paper base modification; an organic-inorganic hybrid physicochemical dual barrier system is constructed through ingenious coating compounding; and finally, a controlled multi-stage process (stepwise mixing, stepwise curing, and hot pressing) ensures that the coating has a dense and non-porous microstructure and a high degree of integrity in chemical cross-linking, thereby constructing a low-permeability, multi-layer barrier structure and achieving excellent barrier performance, which is significantly better than single-component or simplified processes, and meets the high barrier requirements of cigarette frame paper.

[0057] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for preparing cigarette frame paper, characterized in that, Includes the following steps: The modified paper base is coated on both sides with a composite coating to obtain a coated paper base; the coated paper base is cured in steps to obtain a cured paper base; the cured paper base is hot-pressed and then allowed to mature to obtain the cigarette frame paper; the modified paper base is obtained by mixing bleached hardwood pulp, bleached softwood pulp, light calcium carbonate, nano-cellulose crystal suspension and AKD sizing agent; the composite coating includes anionic starch, kaolin powder and polyurethane emulsion; the polyurethane emulsion is obtained by polymerizing polycaprolactone diol, isophorone diisocyanate and chain extender, and then reacting with end-capping agent; The stepwise curing process includes the following steps: the coated paper base is first placed into the first curing zone for a first curing; then it is placed into the second curing zone for a second curing, and the moisture content is controlled to obtain the cured paper base. The preparation of the modified paper base includes the following steps: 70-80 parts of the bleached hardwood pulp are rebeaten to a freeness of 35-40°SR to obtain hardwood pulp; 20-30 parts of the bleached softwood pulp are lightly beaten to a freeness of 25-30°SR to obtain softwood pulp, which is then added to the hardwood pulp. The mixing speed is 200-400 rpm, and the mixing time is 10-20 min to obtain a mixed pulp. 15-20 parts of the light calcium carbonate, 0.1-0.2 parts of the AKD sizing agent, and 1.5-3 parts of the nanocellulose crystal suspension are added sequentially to the mixed pulp. A mesh forming device is used to ensure high uniformity, and a boot press is used with a linear pressure of 800-1000 kN / m. The press dryness is >50% to obtain the modified paper base. The preparation of the composite coating includes the following steps: adding 30-40 parts of the anionic starch to deionized water and stirring to prepare a starch slurry with a solid content of 25%-30%; adding the starch slurry to a steam jet cooker and cooking at 110-130℃ for 10-20 minutes, then flash-cooling to 50-60℃ to obtain gelatinized starch; mixing 10-20 parts of the kaolin powder with deionized water in a high-shear disperser at a speed of 800-1200 rpm for 20-30 minutes, adding 0.3 parts of sodium polyacrylate dispersant, and stirring evenly to obtain a kaolin slurry with a solid content of 60-70%; adding the gelatinized starch to the main mixing tank under heat preservation conditions, and slowly... Slowly add the kaolin slurry and mix at a low speed of 300-500 rpm for 15-20 minutes. Then add 0.5 parts of defoamer and 0.5 parts of wetting agent and mix evenly to obtain a mixture. Slowly add 25% ammonia solution dropwise to the mixture, monitoring the pH in real time with a pH meter, and stir at 200-400 rpm until the pH value stabilizes at 8.0-8.5 to obtain the slurry system. Slowly add 45-55 parts of the polyurethane emulsion to the slurry system by pumping under stirring conditions of 100-200 rpm. After the addition is complete, stir at a low speed for 15-30 minutes to mix evenly. Then filter through a 200-300 mesh vibrating screen to obtain the composite coating. The preparation of the polyurethane emulsion includes the following steps: adding polycaprolactone diol and a chain extender to a flask, adding acetone to dissolve, stirring and dehydrating to obtain a mixed solution; adding isophorone diisocyanate dropwise to the mixed solution, and performing a polymerization reaction to obtain a polymer system; adding an end-capping agent dropwise to the polymer system, and performing an end-capping reaction to obtain a reaction system; adding triethylamine to the reaction system, and stirring to neutralize to obtain polyurethane; adding the polyurethane to deionized ice water under stirring conditions, and stirring to obtain an emulsified system; and then obtaining the polyurethane emulsion by vacuum distillation, cooling and filtration.

2. A process for the preparation of a cigarette frame paper according to claim 1, characterized in that, The chain extender is dimethylolpropionic acid and 1,4-butanediol; the end-capping agent is aminopropyltriethoxysilane.

3. A cigarette frame paper, characterized in that, The invention includes a modified paper base and a composite coating; the modified paper base raw materials include a mixed pulp and a nano-cellulose crystal suspension; the composite coating includes a polyurethane emulsion, anionic starch and kaolin powder; the cigarette frame paper is prepared by the preparation process described in any one of claims 1-2.

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