High-strength coated paper and process for its production

Abstract

The present application relates to the technical field of cellulose laminated paper manufacturing, in particular to a high-strength laminated paper and a preparation process thereof. The present application overcomes the problem of poor strength of laminated paper produced by traditional process. By compounding different types of extracted fibers, optimizing the process, improving the mechanical properties, adding sisal pulp to form a nano skeleton, long fiber of coniferous wood pulp directional interweaving to improve tensile homogeneity, eucalyptus pulp short fiber filling pores and aramid grafting to enhance interface bonding, the obtained paper pulp produces base paper; combined with pre-pressing dewatering to close pores and gradient drying to regulate hydrogen bond density, supplemented by rigid-elastic roller alternate calendering to realize the synergy of surface densification and internal flexibility; using a copolymer-based laminating system, nano material and chitosan composite reinforcement layer bonding, oxidation starch compatibilization and plasticization modification synergistic promotion of compost degradation; large pore closure and surface micropore refinement precise regulation of air permeability, the laminated paper meets the air permeability requirement, while realizing more than 90% biodegradation rate.

Description

Technical Field

[0001] This invention relates to the field of cellulose coated paper manufacturing technology, specifically to a high-strength coated paper and its preparation process. Background Technology

[0002] In the packaging industry, coated paper is a crucial material, its performance directly affecting the quality and effectiveness of packaging. A balance must be struck between mechanical strength, barrier properties, and environmental friendliness. Pulp, as the core raw material of paper, plays a decisive role in paper performance due to its type and characteristics. Traditional coated paper manufacturing often relies on single-fiber raw materials or simple fiber blends to prepare pulp. Using only ordinary wood pulp results in paper with low tensile strength, making it prone to tearing and cracking when packaging heavy objects or subjected to external forces, failing to meet the demands of high-strength packaging. Furthermore, insufficient bonding between fibers leads to poor paper toughness, easily cracking after bending, severely limiting its application in scenarios requiring repeated folding or stretching.

[0003] In the coating process, traditional coated paper typically uses non-degradable plastics such as polyethylene or polypropylene as the coating layer. While these plastics impart good barrier properties and processing performance to the coated paper, they are extremely difficult to decompose in the natural environment, causing serious environmental pollution. With increasingly stringent environmental regulations, bio-based biodegradable materials have gradually become a research hotspot and are beginning to replace traditional plastics in the production of coated paper. Examples include using extracted cellulose as a bioconversion material and adding polylactic acid and polybutylene adipate terephthalate. However, in the preparation of biodegradable coated paper, there is a problem of insufficient interfacial bonding strength between the bio-based coating layer of the bio-based biodegradable material and the base paper obtained from the pulp. Due to the poor compatibility between the biodegradable material and paper fibers, the overall performance of the coated paper declines after the pulp is converted into paper and subjected to coating treatment, making it prone to delamination and resulting in a significant reduction in mechanical properties. In the pulp preparation and paper forming process, it is still difficult to balance the relationship between degradation performance and other paper properties. While ensuring degradability, it is difficult to take into account the strength, flexibility and air permeability of coated paper, so that the overall performance of coated paper cannot meet the diverse needs of the market.

[0004] To address this, a high-strength coated paper and its preparation process were proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a high-strength coated paper and its preparation process. Mechanical properties are improved by blending different types of extracted fibers and optimizing the process. Sisal pulp is added to form a nano-skeleton, long fibers from softwood pulp are oriented and interwoven to improve tensile homogeneity, short fibers from eucalyptus pulp fill pores, and aramid grafting strengthens the interface bonding. The resulting pulp is used to produce base paper. Pre-pressing and dewatering close pores, and gradient drying controls hydrogen bond density. Alternating rigid-elastic roller calendering achieves a synergistic effect of surface density and internal flexibility. A copolymer-based coating system is used, with nanomaterials and chitosan forming a composite reinforcing layer. Oxidized starch composting and plasticizing modification synergistically promote composting degradation. Precise control of air permeability through macropore closure and surface micropore refinement ensures the coated paper meets air permeability requirements while achieving a biodegradability rate of over 90%.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] This invention provides a process for preparing high-strength coated paper, the process of which is as follows:

[0008] Preparation of mixed slurry: Fiber raw materials are subjected to hot water impregnation, extrusion and delamination, first-stage preimpregnation, second-stage preimpregnation, high-consistency refining, retention, low-consistency refining, screening and beating to obtain fiber slurry; aramid pulp raw materials are subjected to ultrasonic treatment, concentrated sulfuric acid treatment, impregnation after passing through a three-stage homogenizing valve, and electric field-assisted treatment to obtain aramid pulp; the fiber slurry and aramid pulp are mixed to obtain mixed slurry;

[0009] Pre-compression dehydration, gradient drying, soft calendering, and sizing and coating solution;

[0010] Solvent evaporation and drying, calendering, cutting and rolling are performed to obtain high-strength coated paper.

[0011] Preferably, the fiber pulp is one of three of the following: sisal pulp, softwood pulp, and eucalyptus pulp.

[0012] Preferably, in the three-stage homogenizing valve, the first-stage homogenizing pressure is 80MPa for 15-20 minutes, the second-stage homogenizing pressure is 60MPa for 13-17 minutes, and the third-stage homogenizing pressure is 20MPa for 10-13 minutes.

[0013] Preferably, the preparation process of the coating solution is as follows: polylactic acid and ethylene-vinyl acetate copolymer are added to a mixed solvent of ethyl acetate and γ-butyrolactone, and stirred at room temperature until completely dissolved to form a uniform polymer solution; polyacrylamide is added to deionized water and stirred to dissolve to form a uniform polyacrylamide solution; nano silica suspension is added to the polymer solution, and after continuous stirring, oxidized starch@chitosan complex solution, polyacrylamide solution, tributyl acetylacetic acid, stearic acid and antioxidant 168 are added, and stirred to obtain the coating solution.

[0014] Preferably, the preparation method of the oxidized starch@chitosan complex solution is as follows: Nano-silica is dispersed in anhydrous ethanol to obtain a nano-silica suspension; oxidized starch is added to deionized water and heated and stirred to obtain an oxidized starch solution; chitosan is added to an aqueous acetic acid solution and stirred at room temperature to obtain a chitosan solution; the oxidized starch solution and chitosan solution are mixed, p-toluenesulfonic acid is added, the pH value of the mixture is adjusted, and the reaction is stirred to obtain the oxidized starch@chitosan complex solution.

[0015] Preferably, the mass ratio of nano-silica, oxidized starch, chitosan, polylactic acid, ethylene-vinyl acetate copolymer, polyacrylamide, and tributyl acetyl citrate is 1-1.5:1.5-3:2-2.5:18-22:12-18:1-1.5:4.5-6; and the volume ratio of deionized water, aqueous acetic acid solution, ethyl acetate, and γ-butyrolactone is 100:50:300:100.

[0016] Preferably, the calendering process uses alternating steel rollers and paper pulp rollers, for a total of 8 roller sets.

[0017] The present invention also provides a high-strength coated paper, characterized in that: the tensile strength of the high-strength coated paper is 38.9-47.0 N·m·g. -1 The raw materials for preparing high-strength coated paper include sisal pulp, aramid pulp, softwood pulp, eucalyptus pulp, nano-silica, oxidized starch, chitosan, polylactic acid, ethylene-vinyl acetate copolymer, polyacrylamide, and tributyl acetylacetate.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] 1. Improved mechanical properties of coated paper were achieved through a multi-fiber compounding system and differentiated process design. Multiple methods were employed to optimize the performance of coated paper. In fiber treatment, the proportions and homogenization times of different fibers in the mixed pulp were adjusted to achieve synergistic effects between fibers. Sisal fibers, after alkali treatment and microfiberization, formed a high-specific-surface-area nanofiber network, providing a high-strength skeleton; long fibers of softwood pulp constructed a continuous stress transfer path through directional interweaving, enhancing tensile isotropy; short fibers of eucalyptus pulp filled pores, reducing stress concentration and improving flexibility; aramid pulp, after surface treatment and multi-stage homogenization, formed a uniformly dispersed reinforcing phase, significantly improving interfacial bonding strength; combined with the mechanical densification effect of pre-pressing and dewatering processes and the thermodynamic regulation of gradient drying, the interfiber bonding area was greatly increased; soft calendering technology, through the alternating arrangement of rigid and elastic rollers, maintained the internal flexible structure while densifying the surface, ultimately achieving a balance between high strength and high ductility. The synergistic effect of the process effectively avoids the problems of rigid overload or loose structure of a single raw material, enabling the paper to have both fracture resistance and deformation adaptability when subjected to high tensile loads.

[0020] 2. By employing a biodegradable material system and pore control technology, the coated paper possesses both environmental friendliness and functional advantages. The coating layer is primarily composed of polylactic acid and ethylene-vinyl acetate, incorporating nano-silica and chitosan complexes. Carboxyl-hydroxyl interfacial bonding enhances interlayer adhesion, while the hydrolytic properties of polylactic acid's ester bonds enable controlled degradation. Oxidized starch acts as a compatibilizer to promote fiber-polymer interfacial compatibility, while tributyl acetylacetonate plasticizer inhibits brittle fracture. This allows for efficient biodegradation through a synergistic effect of microbial attack and chemical hydrolysis under composting conditions. Air permeability is precisely controlled through gradient drying and calendering processes. Pre-pressing dehydration closes large pores, while soft calendering refines surface pores, forming a gradient permeable structure. This design ensures oxygen / water vapor exchange requirements, making it suitable for food preservation, while avoiding excessive permeation that leads to loss of barrier properties. Technological innovation achieves an optimal balance between degradation cycle, mechanical strength, and air permeability, guaranteeing the comprehensive performance of the coated paper. Attached Figure Description

[0021] Figure 1 This is a process flow diagram for preparing the high-strength coated paper of the present invention. Detailed Implementation

[0022] 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.

[0023] Unless otherwise specified, all substance concentrations mentioned are mass concentrations. Please refer to [link / reference]. Figure 1 This invention provides a high-strength coated paper and its preparation process, the technical solution of which is as follows:

[0024] Example 1

[0025] Preparation of S1 mixed slurry: The fiber treatment process is as follows: Sisal fiber bundles are impregnated in hot water at 85℃ for 25 minutes, with a material-to-liquid ratio of 1:15, to obtain heat-treated fibers. Additives are added to the hot water. The fibers are then extruded and decomposed using a twin-screw extruder at a compression ratio of 5:1 and a temperature of 75℃, breaking down the fiber bundles into coarse fibers with a diameter of 0.5-1.5 mm. A pre-impregnation is then performed in an aqueous solution of 1.2% NaOH, 0.5% sodium silicate, and 0.2% diethylenetriaminepentaacetic acid, impregnated at 80℃ for 20 minutes, with an impregnation pH of 12.5. Afterwards, 2... Two-stage pre-impregnation was performed using 0.5% H₂O₂, 0.15% EDTA, and 0.05% MgSO₄ at 85℃ for 30 min, resulting in a residual lignin content of <5%. High-consistency pulping was then performed using a KRK refiner with a disc gap of 1.2 mm and a concentration of 28%, reducing fiber length from 2.1 mm to 1.5 mm. The product of the high-consistency pulp was then held in a 90℃ solution of 4.0% H₂O₂ and 3.0% NaOH for 60 min, followed by low-consistency pulping using a conical refiner with a disc gap of 0.25 mm and a concentration of 12%, reducing the specific surface area from 2.8 m² / s². 2 / g increased to 6.5m 2 / g; under conditions of 4% pulp concentration and 95℃, the pulp was removed for 40 minutes and then screened through a 0.15mm sieve. The screened pulp was dewatered to a concentration of 30%. Before making hand-made sheets, the pulp was first beaten with a PFI mill at a concentration of 10% to obtain sisal pulp.

[0026] The overall process for obtaining sisal pulp, softwood pulp, and eucalyptus pulp from sisal fiber, softwood chips, and eucalyptus chips is the same, but the process parameters are different. The differences in the processing of sisal fiber, softwood chips, eucalyptus chips, and aramid pulp are shown in Tables 1-4. In Table 4, the molecular weight of PEO dispersant, i.e., polyethylene oxide dispersant, is 4.5 million, and Triton X-100 is polyethylene glycol octylphenyl ether.

[0027] Table 1 Specific conditions for heat treatment

[0028]

[0029] Table 2 Pre-impregnation conditions

[0030]

[0031] Table 3. Retention conditions of products from high-consistency pulping

[0032] raw material bleach Bleaching temperature (°C) Duration of stay (min) Softwood pulp <![CDATA[4.0%H2O2+3.5%NaOH]]> 95 120 Eucalyptus pulp <![CDATA[3.5%H2O2+1.2%Na2SiO3]]> 90 90 Sisal paste <![CDATA[4.0% H2O2 and 3.0% NaOH]]> 90 60

[0033] Sisal pulp was beaten to 28-32°SR at a 10% concentration using a PFI mill, retaining a fiber length of 1.2-1.5 mm; softwood pulp was beaten to 35-38°SR at a 10% concentration using a PFI mill, with a fiber cut rate of <15%; eucalyptus pulp was beaten to 45-48°SR at a 10% concentration using a PFI mill to promote fine fiberization. These beating ranges cover ±5% power fluctuations in the equipment, taking into account variations in raw materials between different batches.

[0034] Table 4 Processing conditions for different raw materials

[0035]

[0036] The aramid pulp processing technology is as follows: Aramid pulp raw material is treated in a 20kHz ultrasonic reactor for 30 minutes (power density 50W / L), dispersing the fiber bundles into single fibers with a diameter of 20-50μm. The moisture content of the aramid pulp raw material is 10%, and the length of the aramid fibers in the raw material is 3-6mm. After treatment with 98% concentrated sulfuric acid for 30 seconds, it is immediately washed with water until neutral, increasing the groove depth on the aramid fiber surface by 200nm. The material is then passed through a three-stage homogenization valve: the first stage homogenization pressure is 80MPa for 15 minutes, and the second stage homogenization pressure is 60MPa for 1 minute. The fiber diameter distribution was optimized from 15-50 μm to 8-25 μm by three-stage homogenization at 20 MPa for 3 min, followed by treatment with 300 W argon plasma for 90 s, and then impregnated with 0.5% KH550 silane coupling agent ethanol aqueous solution (ethanol to water volume ratio of 4:1). The impregnated aramid pulp was then removed, washed with deionized water, and then shaped using an electric field assisted at an electric field strength of 1.5 kV / cm to achieve an aramid fiber axial orientation of over 85%, resulting in aramid pulp with a moisture content of 8.5%.

[0037] A mixed pulp was prepared by mixing four types of pulps—sisal pulp, aramid pulp, softwood pulp, and eucalyptus pulp—at a mass ratio of 15:3:50:32. The mixture was then delaminated using a standard delaminator for 15,000 revolutions at 25°C and 50% relative humidity, with a pulp addition rate of 80 g / m³. 2 Under certain conditions, handwritten copies were made.

[0038] S2 Pre-compression Dehydration

[0039] Linear pressure 90kN / m, roller temperature 60℃, speed 13m / min; pre-dehydrated mixed slurry is obtained by pressing with a double roller press.

[0040] S3 gradient drying (thermodynamic control)

[0041] The paper is dried in a tunnel dryer. The initial drying section is at 80°C for 8 minutes, the main drying section is at 110°C for 15 minutes, and the shaping section is at 70°C for 5 minutes. The humidity is reduced from 90% to 15%. The paper is then cooled to 25°C at a rate of 5°C / min and held for 10 minutes. Finally, it is rapidly cooled to 5°C to obtain high-strength base paper.

[0042] S4 Soft Calendering

[0043] A combination of a hard roll with a Shore hardness of 90° and a soft roll with a Shore hardness of 30° is used to perform soft calendering on high-strength base paper to obtain calendered base paper. In the calender, the diameter of both the hard roll side and the soft roll side is 0.5m, the wrap angle is 12°, the linear speed is 80m / min, the contact time is 0.12s, the electric heating temperature of the hard roll side is 100℃, the heat transfer oil circulation temperature of the soft roll side is 80℃, and the linear pressure is 50kN / m.

[0044] S5 sizing coating liquid

[0045] 1 g of 20 nm nano-silica was dispersed in anhydrous ethanol to obtain a 2% (w / w) nano-silica suspension; 2 g of oxidized starch was slowly added to 50 mL of deionized water, heated to 60 °C and stirred for 60 min to obtain an oxidized starch solution; 2 g of chitosan with a degree of deacetylation of 80% was slowly added to 50 mL of 1% acetic acid aqueous solution and stirred at room temperature for 2 h to obtain a chitosan solution; the oxidized starch solution and chitosan solution were mixed, 0.1 g of p-toluenesulfonic acid was added, the pH of the mixture was adjusted to 4.5, and the mixture was stirred at 50 °C for 3 h to obtain an oxidized starch@chitosan complex solution; 20 g of polylactic acid and 15 g of ethylene-vinyl acetate copolymer were added to 3... In a mixed solvent of 0.0 mL ethyl acetate and 100 mL γ-butyrolactone, the mixture was stirred at room temperature for 4 h until completely dissolved to form a homogeneous polymer solution. 1 g of polyacrylamide was slowly added to 50 mL of deionized water and stirred for 2 h to form a homogeneous polyacrylamide solution. The obtained nano-silica suspension was slowly added to the polymer solution and stirred continuously at 500 rpm for 30 min. Then, an oxidized starch@chitosan complex solution was added and stirred for 20 min. After stirring, the polyacrylamide solution was added and stirred evenly. 5 g of acetylthiocitrate tributyl ester was added and stirred for 30 min. 0.2 g of stearic acid and 0.2 g of antioxidant 168 were added and stirred for 15 min to obtain the coating solution.

[0046] Gravure coating is performed, with a coating amount of 6 g / m². 2 Double-sided, roller depth 50μm, the angle between the doctor blade and the roller surface is 70°, the linear speed is 100m / min, and the base paper coated with the coating liquid is obtained after coating.

[0047] S6 Solvent Evaporation and Drying

[0048] The process involves preheating at 60℃ for 3 seconds, main drying at 90℃ for 15 seconds, and post-drying at 130℃ for 6 seconds; followed by vacuum-assisted drying at 60℃ to remove excess solvent.

[0049] S7 calendering, cutting, and rolling

[0050] Under a total pressure of 300 kN / m and a speed of 800 m / min, an 8-roll calendering system is used to produce coated paper, which is then cut and rolled. The roll diameter is 0.5 m, the roll spacing is 10 mm, and the contact time between each pair of rolls is 0.094 s, matching the linear speed of 800 m / min. The steel rolls (120℃) and the paper pulp rolls (room temperature) alternate. The pressure on the first four rolls is 180 kN / m, and the pressure on the last four rolls (5-8) is 120 kN / m. The steel rolls are mirror-polished with a surface finish of Ra ≤ 0.05 μm, while the paper pulp rolls are frosted with a surface finish of Ra ≈ 0.8 μm. The total processing time for the 8 rolls is 0.75 s, and the coated paper thickness is 0.19 mm. The 8-roll calendering system consists of four alternating groups of steel and paper pulp rolls: steel roll-paper pulp roll-steel roll-paper pulp roll-steel roll-paper pulp roll-steel roll-paper pulp roll. Afterward, the paper is cut and rolled to obtain the coated paper.

[0051] Examples 2-4

[0052] The preparation process of S1 mixed slurry and the pre-compression dewatering process of S2 have been changed, as shown in Table 5.

[0053] Table 5 Preparation processes of S1 and S2

[0054]

[0055]

[0056] In Comparative Example 1S1, the processing technology for eucalyptus chips and softwood chips is consistent with that for sisal pulp, while other process steps are consistent with those in Example 1.

[0057] In Comparative Example 2, the pulp was not mixed in the same way as the four pulps in Example 1, but only sisal pulp was used as the raw material for the high-strength base paper.

[0058] In Comparative Example 3, the pulp was not mixed in the same way as the four pulps in Example 1, but only softwood pulp was used as the raw material for the high-strength base paper.

[0059] In Comparative Example 4, the pulp was not mixed in the same way as the four pulps in Example 1, but only eucalyptus pulp was used as the raw material for the high-strength base paper.

[0060] Comparative Example 5 did not undergo the S2 pre-compression dehydration process.

[0061] Experimental Example 1

[0062] The coated papers obtained in the above examples and comparative examples were tested for tensile strength and flexibility. The tests were conducted according to GB / T12914-2018 "Determination of Tensile Strength of Paper and Paperboard - Constant Speed ​​Tensile Test (20 mm / min)", with a sample width of 15 mm, an ambient temperature of 23 ± 1 ℃, and a relative humidity of 50 ± 2%. The bending stiffness was tested using a LAW bending stiffness tester according to GB / T457-2008 "Determination of Folding Endurance of Paper and Paperboard", with a bending angle of 15° and a loading rate of 1° / s. The elongation at break was also tested. The final test results are shown in Table 6.

[0063] Table 6. Tests for tensile strength and flexibility

[0064]

[0065]

[0066] By adjusting the mixing ratio of the four pulps in the S1 mixed pulp and the homogenization time in the S1 stage, the aim is to optimize the synergistic effect between fibers and improve the strength of the base paper. The S2 pre-pressing and dewatering process increases the bonding area between fibers through mechanical densification, further improving the tensile strength of the base paper. The subsequent gradient drying and soft calendering processes also help to improve the strength and surface properties of the paper. The coated paper prepared by the process in Examples 1-4 exhibits high tensile strength and good flexibility. Sisal fiber provides a high-strength skeleton. After microfiberization and subsequent thermal drying and curing, the hydrogen bond density is high. The long fibers of softwood pulp interweave to form a continuous network. After S2 pre-dewatering, the pores are closed, increasing the fiber bonding area. The short fibers of eucalyptus pulp fill the pores, reducing stress concentration and increasing the elongation at break. The interfacial bonding strength after plasma grafting of aramid pulp is also improved. After multi-stage homogenization, the fibers are refined with a moderate diameter distribution range, enhancing fiber interweaving. The synergistic effect of the above raw materials improves the hydrogen bond density and fiber orientation, thereby jointly improving the strength and flexibility of the coated fabric. In Comparative Example 1, the processing techniques for eucalyptus chips, aramid pulp, and sisal fibers were kept consistent. The tensile strength of the coated fabric decreased significantly, and its flexibility was also reduced compared to the Example 1. High temperature softened the lignin in the sisal fibers, and the high compression ratio promoted fiber fibrillation. Triton... X-100 removes surface wax and enhances hydrophilicity. Insufficient temperature, such as 80℃ for coniferous wood, results in low pectin dissolution, reduced inter-fiber bonding area, and decreased tensile strength. Diethylenetriaminepentaacetic acid added to coniferous wood chips chelates metal ions, preventing lignin condensation. Moderate compression preserves the long fiber skeleton; excessively high compression ratios, such as 5:1 for sisal, lead to excessive fiber cutting, reducing breaking elongation to below 2%. Eucalyptus chips protect hemicellulose at low temperatures, while sodium silicate stabilizes pH and prevents cellulose hydrolysis. Excessively high temperatures, such as 85℃ for sisal, result in hemicellulose dissolution >40%, increased fiber rigidity, and decreased flexibility. Aramid pulp is kept at room temperature to prevent thermal degradation, and PEO dispersant prevents fiber agglomeration. Excessively high processing temperatures cause aramid molecular chain breakage, leading to a decline in the overall performance of the coated fabric. Strong alkali in sisal fibers dissolves pectin, while sodium silicate inhibits cellulose... Degradation, insufficient NaOH, high pectin residue, and reduced inter-fiber hydrogen bond density; the addition of diethylenetriaminepentaacetic acid to coniferous wood chips chelates metal ions in lignin, promoting selective removal; without the addition of diethylenetriaminepentaacetic acid, lignin condensation occurs, reducing fiber swelling and flexibility; eucalyptus wood chip processing conditions are relatively mild, MgSO4 protects the hemicellulose structure, and excessive NaOH leads to greater hemicellulose dissolution, resulting in deterioration of flexibility; silane coupling agents form chemical bonds on the aramid surface, enhancing interfacial bonding with different fibers; high gaps in coniferous wood pulp retain long fibers, while low concentrations avoid over-brooming; excessively small gaps reduce fiber length, thus decreasing the tensile index of coated paper; narrow sieve openings in eucalyptus pulp remove thin-walled cells, but excessively wide openings increase ash content from 0.5% to 1.5%, porosity from 12% to 18%, and reduce tensile strength.Comparative Examples 2-4 were coated papers prepared using only one raw material. Comparative Example 2 used only sisal pulp, which increased tensile strength but decreased flexibility. The tensile index and flexibility of Comparative Examples 3 and 4 were significantly lower than those of Example 1. Comparative Example 5 did not undergo the S2 pre-pressing and dewatering process, so it could not achieve the effect of mechanical densification. Through pre-pressing and dewatering, the dryness of the wet paper sheet was increased from 20% to 45% by double-roll pressing, the surface pores were closed, the pore size was reduced from 50μm to 10μm, and the wet paper sheet density was increased from 0.35 to 0.45 g / cm. 3 The inter-fiber bonding area increases by 50%; without pre-compression dewatering, the wet paper sheet has low dryness, high porosity, and insufficient fiber slip resistance. Even if gradient drying is carried out subsequently, it cannot compensate for the loose structure, and the hydrogen bond density is greatly reduced. Therefore, the tensile index of the coated paper decreases and its flexibility decreases.

[0067] Examples 5 and 6

[0068] The conditions for the S3 gradient drying process and the S5 sizing and coating solution process have been changed, while the remaining process steps remain the same as in Implementation 1. The specific differences are shown in Table 7. The solid substances in Table 7 are nano-silica, oxidized starch, chitosan, polylactic acid, ethylene-vinyl acetate copolymer, polyacrylamide, and tributyl acetylacetonate.

[0069] Table 7. Process conditions for S3 gradient drying process and S5 sizing and coating solution.

[0070]

[0071] Comparative Example 6S3 was not subjected to gradient drying; it was directly dried at 80℃ for 60 min.

[0072] The comparative example 7S3 was not subjected to a rapid cooling process after gradient drying, but was allowed to cool naturally to room temperature.

[0073] In the process of applying the sizing and coating solution in Comparative Example 8S5, no nano-silica, chitosan, oxidized starch, and tributyl acetyl citrate were added, and other conditions were kept the same as those in Example 1.

[0074] The linear speed of the gravure coating in Comparative Example 9 was 150 m / min.

[0075] In Comparative Example 10S5, after preparing the coating solution, the solvent was first removed to obtain the polymer. Then, hot-melt coating was performed. The average polymer melt temperature was 150℃, the extrusion speed was 250 m / min, the die gap was 1.0 mm, the cooling roller temperature was 20℃, and the pressure roller pressure was 3.5 kg / cm. The polymer coating amount on both sides of the base paper was 0.5 g / m. 2 .

[0076] Experiment Example 2

[0077] The coated paper obtained under the above process conditions was subjected to tensile index and flexibility tests, as well as biodegradation rate tests under simulated composting conditions of 58℃, 55% humidity and 180d, in accordance with GB / T19277.1-2011. The final test results are shown in Table 8.

[0078] Table 8 Results of Tensile Index, Flexibility and Biodegradability Tests

[0079]

[0080] According to the preparation process of the present invention, the tensile strength under the conditions of Examples 1, 5 and 6 is 40.5-46.8 N·m·g. -1 The elongation at break is 3.9-4.8%, the flexural stiffness is 78-92 mN, and the biodegradability is 88%-92%. By optimizing the S3 gradient drying conditions, the fiber rebound is slowed down in the low-temperature section (<100℃), hydrogen bond recombination is promoted in the high-temperature section, internal stress is eliminated in the shaping section, and rapid cooling inhibits fiber rebound; and by optimizing the composition ratio of the coating layer raw materials in the S5 sizing coating liquid process, hydrogen bond recombination is optimized, pre-compression dehydration closes the pores, the fiber bonding is tight, silica and chitosan in the coating layer enhance the interfacial bonding, and polylactic acid and ethylene-vinyl acetate copolymer are added to the coating layer. The carboxyl groups and other groups contained therein combine with the amino and hydroxyl groups on the surface of the base paper, which on the one hand provides toughness to the coated paper and on the other hand improves the biodegradability performance; In Example 5, the main drying temperature is increased to 120℃ to accelerate hydrogen bond formation and increase tensile strength, but the high temperature causes some cellulose chains to break and the elongation at break to decrease; In Example 6, the initial drying temperature of 90℃ quickly removes free water, but the fiber shrinks too early and the tensile strength decreases slightly. By rapidly cooling, the β crystal form is retained, and the flexibility is better than that of Example 5.

[0081] Comparative Example 6, without gradient drying, resulted in uneven fiber structure, decreased hydrogen bond density, and reduced tensile strength; increased porosity from 25% to 35%, leading to deteriorated flexibility; decreased fiber density, but polylactic acid retained its complete chain structure, with a moderate degradation rate. Comparative Example 7, without quenching, resulted in increased brittleness due to residual stress, increased flexural stiffness, and decreased elongation at break; the biodegradation rate decreased. Comparative Example 8, without the addition of nano-silica and chitosan, without modification of the polylactic acid / ethylene-vinyl acetate copolymer, and without the addition of tributyl acetylacetic acid for toughening, showed deteriorated flexibility, decreased interfacial bonding energy, and reduced tensile strength. Simultaneously, the added oxidized starch molecules contained more polar groups such as carboxyl and aldehyde groups, which could form hydrogen bonds with the hydroxyl groups on the paper fiber surface, significantly enhancing the interfiber bonding force. In the preparation of mixed pulp for coated paper, oxidized starch can act as a bridge between different fibers such as sisal pulp, aramid pulp, softwood pulp, and eucalyptus pulp, making the fibers more tightly interwoven, thereby improving the overall strength of the paper and enhancing its tensile strength and other properties. It also increases the internal flexibility of the paper, allowing it to deform better under external forces and resist breakage. During the preparation of the coating solution, oxidized starch can be uniformly dispersed in the coating solution system, preventing the agglomeration or stratification of other components. Oxidized starch reacts with chitosan and other components to form an oxidized starch@chitosan complex. This complex can improve the compatibility between different components in the coating solution, allowing polymers such as polylactic acid and ethylene-vinyl acetate copolymer to mix better with other additives, forming a stable coating solution, thus ensuring the smooth progress of the coating process and the stability of the coated paper quality. Comparative Example 8 shows the synergistic effect between the coating layer raw materials, which jointly improves the performance of the coated paper. Without chitosan, microbial activity increases; the lack of nano-SiO2 hinders the hydrolysis of polylactic acid; and oxidized starch cannot perform cross-linking, resulting in a reduced degradation rate. Comparative Example 9 showed that a linear speed of 150 m / min resulted in uneven coating with a thickness deviation of ±15%, leading to localized stress concentration and decreased tensile strength. Excessive thickness in some areas also resulted in a lower degradation rate compared to the previous example. Comparative Example 10 demonstrated that, compared to the hot-melt coating process, the composition of the coating solution in this invention is more compatible with the gravure coating process, resulting in coated paper with good tensile strength and flexibility. In the hot-melt coating process, some raw materials, polylactic acid, undergo thermal degradation, increasing the brittleness of the coating layer and decreasing tensile strength. The hot-melt process also disrupts the polylactic acid molecular chains, broadening the molecular weight distribution and increasing resistance to biodegradation, making it suitable for use in food packaging materials.

[0082] Example 7

[0083] S6 Solvent evaporation and drying: First stage preheating: 40℃, 6s; Second stage main drying: 75℃, 18s; Third stage post-drying: 100℃, 8s. Other processes are the same as in Example 1.

[0084] Example 8

[0085] S6 Solvent evaporation and drying: First stage preheating: 50℃, 5s; Second stage main drying: 60℃, 25s; Third stage post-drying: 80℃, 9s. Other processes are the same as in Example 1.

[0086] Comparative Example 11 did not perform the S4 soft calendering process step, and the other processes were the same as in Example 1.

[0087] Comparative Example 12S6 underwent a curing process during solvent evaporation and drying, and was preheated at 60°C for 1 minute. Other processes were consistent with those in Example 1.

[0088] In Comparative Example 13S7, the calendering, cutting, and rolling processes do not include the calendering process; the other processes are consistent with those in Example 1.

[0089] In Comparative Example 14S5, except that no nano-silica, chitosan, oxidized starch and acetylated tributyl citrate were added, no calendering was performed in step S7.

[0090] Experimental Example 3

[0091] The coated papers obtained from the above examples and comparative examples were subjected to tensile strength and flexibility tests, and the air permeability of the coated papers was tested in accordance with GB / T458-2008 "Determination of air permeability of paper and paperboard". The final test results are shown in Table 9.

[0092] Table 9 Results of tensile strength, flexibility, and air permeability tests for coated paper.

[0093]

[0094]

[0095] The coated paper obtained by the process of this invention has a tensile strength of 40.8-43.5 N·m·g under the conditions of Examples 1, 7 and 8. -1 The elongation at break is 4.0%-4.8%, the flexural stiffness is 80-90 mN, and the air permeability is 0.70×10⁻⁶. -1 Up to 0.95×10 -1By combining the segmented drying process (S6 solvent evaporation and drying), the calendering process (S7), the cutting and rolling process with the steps of S1-S5, fiber bonding is optimized, the fiber bonding interface is strengthened, pores are closed, the ability of hydrogen bonding to accelerate bonding is improved, the uniformity of fiber shrinkage is improved, and the fiber bonding strength is increased. Example 1: Pre-pressing dehydration and soft calendering close the surface pores, with a pore size of 10 μm, and nano-SiO2 fills the gaps. Combined with gradient drying, soft calendering, sizing coating, solvent evaporation and drying, and calendering, cutting and rolling processes, the barrier properties are increased. Example 7: Low-temperature main drying retains some pores, with a pore size of 15 μm, and the air permeability is slightly higher. Example 8: High-temperature drying further densifies the material, reducing the porosity to 12% and decreasing the air permeability. Comparative Example 11 (without soft calendering): Increased surface porosity, looser fiber bonding, decreased tensile strength, increased rigidity, and improved flexural stiffness. In soft calendering, the soft roller elastically deforms to fill fiber gaps, while the hard roller provides thermoplastic shaping. Without soft calendering, surface pores remain unclosed with a pore size of 30 μm, increasing air permeability. Comparative Example 12: Uneven solvent evaporation leads to localized stress concentration and decreased tensile strength; inconsistent pore distribution results in fluctuating air permeability. Comparative Example 13 (without calendering): Fibers do not exhibit plastic flow. Alternating between high-temperature steel rollers and room-temperature paper pulp rollers creates a thermal shock-cooling relaxation cycle, optimizing crystallinity. High pressure in the front rollers and low pressure in the rear rollers gradually refine the surface structure. Dynamic hot pressing induces plastic flow in the fibers, embedding nano-SiO2 into fiber gaps. Without calendering, tensile strength decreases, rigidity increases, and interfiber pores are retained with a pore size of 25 μm, increasing air permeability. Comparative Example 14 was neither calendered nor had any key substances added to the coating solution, resulting in a decrease in interfacial bonding energy, an increase in porosity from 20% to 35%, a deterioration in tensile strength and flexibility, and a significant increase in air permeability.

[0096] 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 the preparation of high strength coated paper, characterized by: The preparation process is as follows: Preparation of the mixed pulp: the fiber raw material is immersed in hot water, extruded and defibered, pre-impregnated in the first stage, pre-impregnated in the second stage, high-concentration ground, stopped, low-concentration ground, screened and beaten to obtain a fiber pulp; the aramid pulp raw material is treated by ultrasonic, concentrated sulfuric acid, impregnated after passing through a three-stage homogenizing valve and electric field assisted treatment to obtain aramid pulp; the fiber pulp and the aramid pulp are mixed to obtain the mixed pulp; Pre-pressing dewatering, gradient drying, soft calendering and sizing film solution; Solvent evaporation and drying, calendering, cutting and rolling to obtain the high-strength film-coated paper; The preparation process of the sizing solution is as follows: polylactic acid and ethylene-vinyl acetate copolymer are added to a mixed solvent of ethyl acetate and gamma-butyrolactone, stirred at room temperature until completely dissolved to form a uniform polymer solution; polyacrylamide is added to deionized water, stirred and dissolved to form a uniform polyacrylamide solution; nano-silica suspension is added to the polymer solution, and after continuous stirring, oxidized starch@chitosan complex solution, the polyacrylamide solution, acetyl tri-butyl citrate, stearic acid and antioxidant 168 are added, and stirred to obtain the sizing solution; The nano-silica is dispersed in anhydrous ethanol to obtain the nano-silica suspension; the preparation method of the oxidized starch@chitosan complex solution is as follows: oxidized starch is added to deionized water, heated and stirred to obtain an oxidized starch solution; chitosan is added to an aqueous acetic acid solution, stirred at room temperature to obtain a chitosan solution; the oxidized starch solution and the chitosan solution are mixed, p-toluenesulfonic acid is added, the pH value of the mixed solution is adjusted, and stirring is carried out to obtain the oxidized starch@chitosan complex solution; The mass fraction ratio of the nano-silica, the oxidized starch, the chitosan, the polylactic acid, the ethylene-vinyl acetate copolymer, the polyacrylamide and the acetyl tri-butyl citrate is 1-1.5:1.5-3:2-2.5:18-22:12-18:1-1.5:4.5-6; the volume fraction ratio of the deionized water, the aqueous acetic acid solution, the ethyl acetate and the gamma-butyrolactone is 100:50:300:

100.

2. A process for the preparation of high strength coated paper as claimed in claim 1, wherein: The fiber pulp is one of sisal pulp, coniferous wood pulp and eucalyptus pulp.

3. The process for preparing high strength coated paper as claimed in claim 1, wherein: In the three-stage homogenizing valve, the first-stage homogenizing pressure is 80 MPa for 15-20 min, the second-stage homogenizing pressure is 60 MPa for 13-17 min, and the third-stage homogenizing pressure is 20 MPa for 10-13 min.

4. The process for preparing high strength coated paper as claimed in claim 1, wherein: The calendering adopts steel rollers and pulp rollers alternately, a total of 8 groups of calendering treatment; that is, 4 groups of steel rollers and pulp rollers alternately, steel roller-pulp roller-steel roller-pulp roller-steel roller-pulp roller-steel roller-pulp roller-steel roller-pulp roller calendering mode.

5. A high-strength coated paper according to any one of claims 1 to 4, characterized in that: The preparation raw materials of the high-strength film-coated paper include sisal pulp, aramid pulp, coniferous wood pulp, eucalyptus pulp, nano-silica, oxidized starch, chitosan, polylactic acid, ethylene-vinyl acetate copolymer, polyacrylamide and acetyl tri-butyl citrate.

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