A white kraft paper with ultra-high folding endurance and its preparation method

Abstract

This invention relates to the field of paper technology, specifically to a white kraft paper with ultra-high folding endurance and its preparation method. The method involves mixing bleached softwood and hardwood pulps and then separating them. A portion of the pulp is oxidized using a TEMPO / NaBr / NaClO system and homogenized under high pressure to obtain nanocellulose, which is then composited with the base pulp. Simultaneously, xylan-chitosan interface reinforcing agent I and chitosan-dihydrocaffeic acid-polyethylene glycol diglycidyl ether interface reinforcing agent II are prepared. The composite pulp is then sequentially mixed with reinforcing agents, cationic starch, and other additives, and shaped. The paper is then pressed, steam-dried, and heat-treated at 120-125°C. The resulting kraft paper significantly improves folding endurance through the synergistic effect of a multi-scale fiber network and interface reinforcement, while also possessing high strength, high flexibility, and excellent whiteness, making it suitable for high-frequency folding packaging applications.

Description

Technical Field

[0001] This invention relates to the field of paper technology, and in particular to a white kraft paper with ultra-high folding endurance and its preparation method. Background Technology

[0002] White kraft paper, as an important category of packaging materials, directly determines the reliability of packaged products during transportation, storage, and use due to its mechanical properties, especially its folding endurance. Traditional white kraft paper is mostly made from bleached softwood and hardwood pulp through conventional pulping, reinforcing agent addition, and drying processes. However, due to the inherent rigidity of the fibers and the simple bonding method between fibers, fiber slippage or breakage can easily occur under repeated folding stress, making it difficult to meet the requirements of high-frequency folding scenarios.

[0003] Existing technologies attempt to improve folding endurance by increasing the proportion of long fibers or the amount of wet strength agents. However, excessive use of long fibers reduces paper uniformity, while excessive addition of wet strength agents can easily cause paper embrittlement, thus weakening folding durability. Furthermore, some studies employ chemically modified fibers or nanocellulose reinforcement techniques, but due to the failure to effectively construct a fiber-nanoscale synergistic interface, the interfacial bonding between the reinforcing phase and the matrix fibers is insufficient, easily leading to stress concentration under dynamic folding loads, resulting in interfacial delamination or microcrack propagation. Other technologies use polysaccharide additives (such as chitosan and starch) for surface treatment, but their effects are mainly based on physical adsorption, and they are prone to desorption or degradation under humid and hot environments or long-term mechanical stress, making it difficult to achieve a durable and stable interfacial strengthening effect.

[0004] Furthermore, existing processes often neglect the regulation of chemical cross-linking reactions between fibers and additives during the high-temperature drying stage, resulting in insufficient curing of the fiber network structure and further limiting the potential for improving folding endurance. Therefore, developing a method for preparing white kraft paper that can simultaneously achieve high strength, high flexibility, and high durability remains a critical technological bottleneck that urgently needs to be overcome in the packaging materials field. Summary of the Invention

[0005] In view of this, the purpose of this invention is to propose an ultra-high folding endurance white kraft paper and its preparation method, so as to solve the problem that existing white kraft paper is prone to fiber breakage and interface failure under multiple folding conditions due to insufficient inter-fiber bonding force, simple interface structure and poor dynamic stress dispersion ability, which makes it difficult to meet the stringent requirements of material durability in high-frequency folding packaging scenarios.

[0006] To achieve the above objectives, the present invention provides a method for preparing ultra-high folding endurance white kraft paper, comprising the following steps: (1) Bleached softwood kraft paper pulp board and bleached hardwood kraft paper pulp board are dispersed and mixed in water to obtain a homogeneous pulp, and the pulp is divided into pulp A and pulp B according to the oven-dry fiber weight; (2) The pulp A is added to an oxidation system composed of 2,2,6,6-tetramethylpiperidine-1-oxy radical, sodium bromide and sodium hypochlorite for selective oxidation treatment, and washed to obtain oxidized pulp A containing carboxylated cellulose; (3) The oxidized pulp A is subjected to high-pressure homogenization to obtain a nanocellulose dispersion, and the nanocellulose dispersion is mixed with pulp B to obtain composite pulp C; (4) An interface enhancer I solution was prepared by dissolving chitosan in an aqueous solution of glacial acetic acid, adding xylan, and neutralizing and degassing. (5) Chitosan-dihydrocaffeic acid was obtained by dissolving chitosan in an aqueous solution of glacial acetic acid, adding dihydrocaffeic acid and reacting it in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide. Then, the chitosan-dihydrocaffeic acid was grafted with polyethylene glycol diglycidyl ether and diluted to prepare an interface enhancer II solution. (6) Adjust the composite pulp C to a predetermined pulp concentration, and add the interface strengthening agent II solution, interface strengthening agent I solution, cationic starch solution, wet strength agent solution and polyacrylamide solution in sequence under stirring conditions to obtain multi-scale composite wet pulp D; (7) The multi-scale composite wet pulp D is formed into a paper sheet, which is then dehydrated by pressing and dried by steam, and then subjected to heat treatment at 120-125℃ to obtain white kraft paper with ultra-high folding endurance.

[0007] Preferably, in step (1), both the bleached softwood kraft pulp board and the bleached hardwood kraft pulp board are sourced from UPM.

[0008] Preferably, in step (1), the mass ratio of bleached softwood kraft pulp board to bleached hardwood kraft pulp board is 4:1.

[0009] Preferably, in step (1), the dry fiber mass ratio of pulp A to pulp B is 1:4.

[0010] Preferably, in step (2), based on 200g of oven-dry fiber in pulp A, the amount of 2,2,6,6-tetramethylpiperidine-1-oxy free radical is 3g, the amount of sodium bromide is 20g, and the mass fraction of the sodium hypochlorite solution is about 10%, with a dosage of 400-600mL.

[0011] Preferably, in step (2), the specific steps of the oxidation treatment are as follows: use 1 mol / L sodium hydroxide solution to adjust and maintain the pH of the slurry at 9.8-10.2, the reaction time is 45-75 min, and after the reaction is completed, add 40-60 mL of anhydrous ethanol to terminate the reaction.

[0012] Preferably, in step (3), the conditions for high-pressure homogenization are: pressure 500-700 bar, and homogenization cycle 4-6 times.

[0013] Preferably, in step (4), the mass ratio of chitosan to xylan is 6-10:10-14.

[0014] Preferably, in step (4), the specific steps of neutralization and degassing are as follows: neutralize to pH 5.6-6.0 with 5 mol / L sodium hydroxide solution, and let stand at room temperature for 1.5-2.5 h to degas.

[0015] Preferably, in step (5), the mass ratio of chitosan, dihydrocaffeic acid, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide is 20:4-6:5:3.

[0016] Preferably, in step (5), the mass ratio of chitosan-dihydrocaffeic acid to polyethylene glycol diglycidyl ether is 8-12:1.5-2.5.

[0017] Preferably, in step (6), based on 1000g of oven-dry fiber mass in composite pulp C, the added interface reinforcing agent II solution is 80-120g, interface reinforcing agent I solution is 800-1200g, cationic starch solution is 40-60g, wet strength agent solution is 20-30g, and polyacrylamide solution is 1600-2400g.

[0018] Preferably, in step (6), the solid content of the cationic starch solution is 8%-12%.

[0019] Preferably, in step (6), the solid content of the wet strength agent solution is 11%-14%.

[0020] Preferably, in step (6), the mass concentration of the polyacrylamide solution is 0.1wt%-0.3wt%.

[0021] Preferably, in step (7), the specific steps of pressing and dehydration are as follows: pressing twice with a linear pressure of 75-85kN / m through two rubber pressing rollers.

[0022] Preferably, in step (7), the steam drying process specifically involves drying the paper to a moisture content of 6.0%-6.3% under conditions of a steam pressure of 0.3 MPa and a drying temperature of 95°C.

[0023] Furthermore, the present invention also provides a white kraft paper with ultra-high folding endurance, obtained by the above-mentioned preparation method of white kraft paper with ultra-high folding endurance.

[0024] Preferably, the ultra-high folding endurance white kraft paper, under the MIT folding endurance test conditions specified in GB / T 457-2008, has a longitudinal folding endurance of not less than 1500 times and a transverse folding endurance of not less than 1200 times. The beneficial effects of this invention are: This invention significantly improves the folding endurance of white kraft paper by constructing a multi-scale fiber network and a biomass-based interface reinforcement system. The three-dimensional network structure formed by nanocellulose and wood pulp fibers enhances the hydrogen bonding and mechanical interlocking between fibers, enabling the paper to distribute stress more evenly during folding, reducing local stress concentration, and thus delaying fiber fatigue fracture.

[0025] The rigid polysaccharide layer formed by the xylan-chitosan complex on the fiber surface strengthens the interfacial bonding of the fibers through multi-point adsorption and electrostatic interaction, improving the bonding strength and stability between fibers, so that the paper can maintain the complete fiber skeleton structure after repeated folding.

[0026] The flexible cross-linking bridge introduced by the chitosan-dihydrocaffeic acid-polyethylene glycol diglycidyl ether graft forms a reversible deformation space between the fiber interfaces, giving the paper higher flexibility and deformation recovery ability, effectively inhibiting the initiation and expansion of microcracks, and further improving folding durability.

[0027] The synergistic crosslinking reaction between the multi-component interface reinforcing agent and the fiber network forms a dense and stable interface layer structure during the heat post-treatment process. This not only enhances the dry and wet strength of the paper but also improves its heat aging resistance, enabling the material to maintain excellent mechanical properties even under long-term use conditions.

[0028] By optimizing the pulp formulation, the order of adding interface reinforcing agents, and the hot-press curing process, a highly efficient combination of fibers and additives is achieved, avoiding common interface defects in traditional processes. This results in paper with a balance of high whiteness, high opacity, and excellent mechanical properties, meeting the stringent requirements of high-end packaging materials for comprehensive performance. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0030] Example 1:

[0031] Step 1: Preparation and diversion of pulp 1000g of bleached softwood kraft pulp board (from UPM) and 250g of bleached hardwood kraft pulp board (from UPM) were torn into pieces of approximately 2cm x 2cm and placed in a stainless steel pulp tank equipped with a stirring device. Deionized water was added to adjust the pulp concentration to 30g / L. The pulp was stirred at room temperature for 30 minutes and then sent to a refiner to refine to a Schroeder free degree of 28°SR. After being returned to the pulp tank and mixed evenly, a homogeneous pulp was obtained. The pulp was divided into two parts according to the oven-dry fiber weight. One part, containing 200g of oven-dry fiber, was used as pulp A for selective oxidation with 2,2,6,6-tetramethylpiperidine-1-oxy free radicals. The other part, containing 800g of oven-dry fiber, was used as base pulp B for later use.

[0032] Step 2: Selective oxidation of pulp A Pulp A containing 200g of oven-dry fiber was added to an alkali-resistant glass reactor. Deionized water was added to adjust the pulp concentration to 10g / L. Stirring was started at room temperature, and 3g of 2,2,6,6-tetramethylpiperidine-1-oxy free radical and 20g of sodium bromide were added sequentially. The mixture was stirred for 30min, and then 400mL of sodium hypochlorite solution with a mass fraction of about 10% was added. At the same time, 1mol / L sodium hydroxide solution was used to adjust and maintain the pH of the pulp at 9.8. The reaction time was controlled at 45min. Then, 40mL of anhydrous ethanol was added to terminate the reaction. The pH of the pulp was adjusted to 7 with 1mol / L hydrochloric acid. Finally, the pulp was filtered through a plate and frame filter and washed with deionized water until there was no hypochlorite odor in the filtrate, thus obtaining wet pulp A oxidized pulp containing carboxylated cellulose. Step 3: Preparation and Recombination of Nanocellulose Wet pulp A oxidized pulp was added to the feed tank of a high-pressure homogenizer. The cooling circulation was turned on, and the homogenization pressure was gradually increased from 200 bar to 500 bar. The homogenization was continuously circulated and homogenized 4 times under the condition of 500 bar to obtain a nanocellulose dispersion. Then, the nanocellulose dispersion containing 200g of oven-dry cellulose was added to pulp B containing 800g of oven-dry fiber. The mixture was stirred at room temperature for 30 minutes to form composite pulp C with a total oven-dry fiber mass of 1000g. The pulp concentration was adjusted to 10g / L for later use. Step 4: Preparation of Interface Enhancer I (Xylan-Chitosan Complex) Add 980 mL of deionized water and 20 mL of glacial acetic acid to a round-bottom flask and stir to form an aqueous solution of glacial acetic acid with a volume fraction of about 2%. Then add 6 g of chitosan (Shanghai Aladdin Biochemical Technology Co., Ltd., degree of deacetylation ≥95%, product number C105799) at room temperature and stir for 4 h to obtain a chitosan solution. Then add 10 g of xylan (Beijing Solarbio Technology Co., Ltd., derived from corn cob, product number X8163) and continue stirring for 2 h. Then neutralize with 5 mol / L sodium hydroxide solution to pH 5.6 while stirring. Let stand at room temperature for 1.5 h to remove bubbles and obtain interface enhancer I solution. Step 5: Preparation of Interface Enhancer II (Chitosan-Dihydrocaffeic Acid-Polyethylene Glycidyl Ether Graft) Add 1800 mL of deionized water and 200 mL of glacial acetic acid to a three-necked flask to form a glacial acetic acid solution with a volume fraction of approximately 10%. Then, add 20 g of chitosan (Shanghai Aladdin Biochemical Technology Co., Ltd., degree of deacetylation ≥95%, catalog number C105799) at room temperature and stir for 4 h to obtain a chitosan solution. Next, add 4 g of dihydrocaffeic acid, heat to 30 °C and stir for 30 min. Then, add 5 g of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and 3 g of... N-hydroxysuccinimide was used to slowly adjust the pH of the system to 5 using 1 mol / L sodium hydroxide solution. The reaction was stirred for 6 hours. After the reaction, the solution was transferred to a 3500 Da dialysis bag and dialyzed with deionized water at 4°C for 48 hours, with water changes every 6 hours. The solution was then freeze-dried under vacuum to constant weight to obtain chitosan-dihydrocaffeic acid. 8 g of chitosan-dihydrocaffeic acid was then added to a beaker containing 92 g of deionized water. Glacial acetic acid was added at room temperature to adjust the pH to 5.3, and the mixture was stirred for 15 minutes. Then, under stirring, 1.5g of polyethylene glycol diglycidyl ether (Shanghai Aladdin Biochemical Technology Co., Ltd., item number P303638, epoxy value 0.75mol / 100g, viscosity (25℃) 18.4mpa.s) was added dropwise. The temperature was raised to 35℃, and the pH was adjusted to 7.2 using 1mol / L sodium hydroxide solution and maintained for 1.5h to obtain the interface enhancer II mother liquor. It was then cooled to room temperature and diluted with deionized water to an interface enhancer II solution with a solid content of 8%. Step 6: Preparation of multi-scale composite wet pulp D Take nanocellulose composite pulp C, adjust the pulp concentration to 8 g / L in the pulp tank, stir at room temperature, add 80 g of interface reinforcing agent II solution, and continue stirring for 30 min after the addition is complete. Then add 800 g of interface reinforcing agent I solution, and continue stirring for 20 min after the addition is complete. Then add 40 g of cationic starch solution (solid content 10%, cationic starch sourced from Shandong Fuyang Biotechnology Co., Ltd.), 20 g of wet strength agent solution (solid content 12.5%, wet strength agent JH-1201), and 1600 g of polyacrylamide solution (concentration 0.2 wt%, sourced from Shandong Pusente Chemical Co., Ltd.) sequentially, with each additive added 5 min apart. After each additive is added, continue stirring for 10 min to obtain a multi-scale composite wet pulp D containing a flexible layer of interface reinforcing agent II and a rigid polysaccharide layer of interface reinforcing agent I. Step 7: Paper forming, pressing, drying and post-heat treatment Multi-scale composite wet pulp D is fed into a standard hand-made paper machine, with the pulp temperature controlled at room temperature and pulp concentration at 8 g / L. The pulp is then processed on a standard forming wire with a basis weight of 80 g / m³. 2 To achieve uniform forming, the paper is vacuum dehydrated to a moisture content of 75% and then passed through two rubber press rollers, pressed twice at a linear pressure of 75 kN / m. The paper web is then fed into a multi-cylinder steam drying section, where it is dried to a moisture content of 6.3% under a steam pressure of 0.3 MPa and a drying temperature of 95°C. The dried paper web is then subjected to a heat treatment at 120°C for 15 minutes to obtain ultra-high folding endurance white kraft paper.

[0033] Example 2:

[0034] Step 1: Preparation and diversion of pulp 1000g of bleached softwood kraft pulp board (from UPM) and 250g of bleached hardwood kraft pulp board (from UPM) were torn into pieces of approximately 2cm x 2cm and placed in a stainless steel pulp tank equipped with a stirring device. Deionized water was added to adjust the pulp concentration to 30g / L. The pulp was stirred at room temperature for 30 minutes and then sent to a refiner to refine to a Schroeder free degree of 28°SR. After being returned to the pulp tank and mixed evenly, a homogeneous pulp was obtained. The pulp was divided into two parts according to the oven-dry fiber weight. One part, containing 200g of oven-dry fiber, was used as pulp A for selective oxidation with 2,2,6,6-tetramethylpiperidine-1-oxy free radicals. The other part, containing 800g of oven-dry fiber, was used as base pulp B for later use.

[0035] Step 2: Selective oxidation of pulp A Pulp A containing 200g of oven-dry fiber was added to an alkali-resistant glass reactor. Deionized water was added to adjust the pulp concentration to 10g / L. Stirring was started at room temperature, and 3g of 2,2,6,6-tetramethylpiperidine-1-oxy free radical and 20g of sodium bromide were added sequentially. The mixture was stirred for 30min. Then, 500mL of sodium hypochlorite solution with a mass fraction of about 10% was added. At the same time, 1mol / L sodium hydroxide solution was used to adjust and maintain the pH of the pulp at 10. The reaction time was controlled at 60min. Then, 50mL of anhydrous ethanol was added to terminate the reaction. The pH of the pulp was then adjusted to 7 with 1mol / L hydrochloric acid. Finally, the pulp was filtered through a plate and frame filter and washed with deionized water until there was no hypochlorite odor in the filtrate, thus obtaining wet pulp A oxidized pulp containing carboxylated cellulose. Step 3: Preparation and Recombination of Nanocellulose Wet pulp A oxidized pulp was added to the feed tank of a high-pressure homogenizer. The cooling circulation was turned on, and the homogenization pressure was gradually increased from 200 bar to 600 bar. The homogenization was continuously circulated and homogenized 5 times under the condition of 600 bar to obtain a nanocellulose dispersion. Then, the nanocellulose dispersion containing 200g of oven-dry cellulose was added to pulp B containing 800g of oven-dry fiber. The mixture was stirred at room temperature for 30 minutes to form composite pulp C with a total oven-dry fiber mass of 1000g. The pulp concentration was adjusted to 10g / L for later use. Step 4: Preparation of Interface Enhancer I (Xylan-Chitosan Complex) Add 980 mL of deionized water and 20 mL of glacial acetic acid to a round-bottom flask and stir to form an aqueous solution of glacial acetic acid with a volume fraction of about 2%. Then add 8 g of chitosan (Shanghai Aladdin Biochemical Technology Co., Ltd., degree of deacetylation ≥95%, product number C105799) at room temperature and stir for 4 h to obtain a chitosan solution. Then add 12 g of xylan (Beijing Solarbio Technology Co., Ltd., derived from corn cob, product number X8163) and continue stirring for 2 h. Then neutralize with 5 mol / L sodium hydroxide solution to pH 5.8 while stirring. Let stand at room temperature for 2 h to remove bubbles and obtain interface enhancer I solution. Step 5: Preparation of Interface Enhancer II (Chitosan-Dihydrocaffeic Acid-Polyethylene Glycidyl Ether Graft) Add 1800 mL of deionized water and 200 mL of glacial acetic acid to a three-necked flask to form a glacial acetic acid solution with a volume fraction of approximately 10%. Then, add 20 g of chitosan (Shanghai Aladdin Biochemical Technology Co., Ltd., degree of deacetylation ≥95%, catalog number C105799) at room temperature and stir for 4 hours to obtain a chitosan solution. Next, add 5 g of dihydrocaffeic acid, heat to 30 °C and stir for 30 minutes. Then, add 5 g of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and 3 g of... N-hydroxysuccinimide was reacted with 1 mol / L sodium hydroxide solution to slowly adjust the pH to 5, and the reaction was stirred for 8 hours. After the reaction, the solution was transferred to a 3500 Da dialysis bag and dialyzed with deionized water at 4°C for 48 hours, with water changes every 6 hours. The solution was then freeze-dried under vacuum to constant weight to obtain chitosan-dihydrocaffeic acid. 10 g of chitosan-dihydrocaffeic acid was then added to a beaker containing 90 g of deionized water, and glacial acetic acid was added at room temperature to adjust the pH to 5.5. The mixture was stirred for 15 minutes. Then, under stirring, 2g of polyethylene glycol diglycidyl ether (Shanghai Aladdin Biochemical Technology Co., Ltd., item number P303638, epoxy value 0.75mol / 100g, viscosity (25℃) 18.4mpa.s) was added dropwise. The temperature was raised to 35℃, and the pH was adjusted to 7.5 using 1mol / L sodium hydroxide solution and maintained for 2h to obtain the interface enhancer II mother liquor. It was cooled to room temperature and diluted with deionized water to a solid content of 10% interface enhancer II solution. Step 6: Preparation of multi-scale composite wet pulp D Take nanocellulose composite pulp C, adjust the pulp concentration to 8 g / L in the pulp tank, stir at room temperature, add 100 g of interface reinforcing agent II solution, and continue stirring for 30 min after the addition is complete. Then add 1000 g of interface reinforcing agent I solution, and continue stirring for 20 min after the addition is complete. Then add 50 g of cationic starch solution (solid content 10%, cationic starch sourced from Shandong Fuyang Biotechnology Co., Ltd.), 25 g of wet strength agent solution (solid content 12.5%, wet strength agent JH-1201), and 2000 g of polyacrylamide solution (concentration 0.2 wt%, sourced from Shandong Pusente Chemical Co., Ltd.) sequentially, with each additive added 5 min apart. After each additive is added, continue stirring for 10 min to obtain a multi-scale composite wet pulp D containing a flexible layer of interface reinforcing agent II and a rigid polysaccharide layer of interface reinforcing agent I. Step 7: Paper forming, pressing, drying and post-heat treatment Multi-scale composite wet pulp D is fed into a standard hand-made paper machine, with the pulp temperature controlled at room temperature and pulp concentration at 8 g / L. The pulp is then processed on a standard forming wire with a basis weight of 80 g / m³. 2To achieve uniform forming, the paper is vacuum dehydrated to a moisture content of 75% and then passed through two rubber press rollers, pressed twice at a linear pressure of 80 kN / m. The paper web is then fed into a multi-cylinder steam drying section, where it is dried to a moisture content of 6.2% under a steam pressure of 0.3 MPa and a drying temperature of 95°C. The dried paper web is then subjected to a heat treatment at 120°C for 15 minutes to obtain ultra-high folding endurance white kraft paper.

[0036] Example 3:

[0037] Step 1: Preparation and diversion of pulp 1000g of bleached softwood kraft pulp board (from UPM) and 250g of bleached hardwood kraft pulp board (from UPM) were torn into pieces of approximately 2cm x 2cm and placed in a stainless steel pulp tank equipped with a stirring device. Deionized water was added to adjust the pulp concentration to 30g / L. The pulp was stirred at room temperature for 30 minutes and then sent to a refiner to refine to a Schroeder free degree of 28°SR. After being returned to the pulp tank and mixed evenly, a homogeneous pulp was obtained. The pulp was divided into two parts according to the oven-dry fiber weight. One part, containing 200g of oven-dry fiber, was used as pulp A for selective oxidation with 2,2,6,6-tetramethylpiperidine-1-oxy free radicals. The other part, containing 800g of oven-dry fiber, was used as base pulp B for later use.

[0038] Step 2: Selective oxidation of pulp A Pulp A containing 200g of oven-dry fiber was added to an alkali-resistant glass reactor. Deionized water was added to adjust the pulp concentration to 10g / L. Stirring was started at room temperature, and 3g of 2,2,6,6-tetramethylpiperidine-1-oxy free radical and 20g of sodium bromide were added sequentially. The mixture was stirred for 30min, and then 600mL of sodium hypochlorite solution with a mass fraction of about 10% was added. At the same time, 1mol / L sodium hydroxide solution was used to adjust and maintain the pH of the pulp at 10.2. The reaction time was controlled at 75min. Then, 60mL of anhydrous ethanol was added to terminate the reaction. The pH of the pulp was then adjusted to 7 with 1mol / L hydrochloric acid. Finally, the pulp was filtered through a plate and frame filter and washed with deionized water until there was no hypochlorite odor in the filtrate, thus obtaining wet pulp A oxidized pulp containing carboxylated cellulose. Step 3: Preparation and Recombination of Nanocellulose Wet pulp A oxidized pulp was added to the feed tank of a high-pressure homogenizer. The cooling circulation was turned on, and the homogenization pressure was gradually increased from 200 bar to 700 bar. The homogenization was continuously circulated and homogenized 6 times under the condition of 700 bar to obtain a nanocellulose dispersion. Then, the nanocellulose dispersion containing 200g of oven-dry cellulose was added to pulp B containing 800g of oven-dry fiber. The mixture was stirred at room temperature for 30 minutes to form composite pulp C with a total oven-dry fiber mass of 1000g. The pulp concentration was adjusted to 10g / L for later use. Step 4: Preparation of Interface Enhancer I (Xylan-Chitosan Complex) Add 980 mL of deionized water and 20 mL of glacial acetic acid to a round-bottom flask and stir to form an aqueous solution of glacial acetic acid with a volume fraction of about 2%. Then add 10 g of chitosan (Shanghai Aladdin Biochemical Technology Co., Ltd., degree of deacetylation ≥95%, product number C105799) at room temperature and stir for 4 h to obtain a chitosan solution. Then add 14 g of xylan (Beijing Solarbio Technology Co., Ltd., derived from corn cob, product number X8163) and continue stirring for 2 h. Then neutralize with 5 mol / L sodium hydroxide solution to pH 6.0 while stirring. Let stand at room temperature for 2.5 h to remove bubbles and obtain interface enhancer I solution. Step 5: Preparation of Interface Enhancer II (Chitosan-Dihydrocaffeic Acid-Polyethylene Glycidyl Ether Graft) Add 1800 mL of deionized water and 200 mL of glacial acetic acid to a three-necked flask to form a glacial acetic acid solution with a volume fraction of approximately 10%. Then, add 20 g of chitosan (Shanghai Aladdin Biochemical Technology Co., Ltd., degree of deacetylation ≥95%, catalog number C105799) at room temperature and stir for 4 hours to obtain a chitosan solution. Next, add 6 g of dihydrocaffeic acid, heat to 30 °C and stir for 30 minutes. Then, add 5 g of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and 3 g of... N-hydroxysuccinimide was reacted with 1 mol / L sodium hydroxide solution to slowly adjust the pH to 5, and the reaction was stirred for 10 h. After the reaction, the solution was transferred to a 3500 Da dialysis bag and dialyzed with deionized water at 4 °C for 48 h, with water changes every 6 h. The solution was then freeze-dried under vacuum to constant weight to obtain chitosan-dihydrocaffeic acid. 12 g of chitosan-dihydrocaffeic acid was then added to a beaker containing 88 g of deionized water, and glacial acetic acid was added at room temperature to adjust the pH to 5.7 and stirred for 15 min. Subsequently, 2.5g of polyethylene glycol diglycidyl ether (Shanghai Aladdin Biochemical Technology Co., Ltd., item number P303638, epoxy value 0.75mol / 100g, viscosity (25℃) 18.4mpa.s) was added dropwise under stirring. The temperature was raised to 35℃, and the pH was adjusted to 7.8 using 1mol / L sodium hydroxide solution and maintained for 2.5h to obtain the interface enhancer II mother liquor. It was then cooled to room temperature and diluted with deionized water to a solid content of 12% for the interface enhancer II solution. Step 6: Preparation of multi-scale composite wet pulp D Take nanocellulose composite pulp C, adjust the pulp concentration to 8 g / L in the pulp tank, stir at room temperature, add 120 g of interface reinforcing agent II solution, and continue stirring for 30 min after the addition is complete. Then add 1200 g of interface reinforcing agent I solution, and continue stirring for 20 min after the addition is complete. Then add 60 g of cationic starch solution (solid content 10%, cationic starch sourced from Shandong Fuyang Biotechnology Co., Ltd.), 30 g of wet strength agent solution (solid content 12.5%, wet strength agent JH-1201), and 2400 g of polyacrylamide solution (concentration 0.2 wt%, sourced from Shandong Pusente Chemical Co., Ltd.) sequentially, with each additive added 5 min apart. After each additive is added, continue stirring for 10 min to obtain a multi-scale composite wet pulp D containing a flexible layer of interface reinforcing agent II and a rigid polysaccharide layer of interface reinforcing agent I. Step 7: Paper forming, pressing, drying and post-heat treatment Multi-scale composite wet pulp D is fed into a standard hand-made paper machine, with the pulp temperature controlled at room temperature and pulp concentration at 8 g / L. The pulp is then processed on a standard forming wire with a basis weight of 80 g / m³. 2 To achieve uniform forming, the paper is vacuum dehydrated to a moisture content of 75% and then passed through two rubber press rollers, pressed twice at a linear pressure of 85 kN / m. The paper web is then fed into a multi-cylinder steam drying section, where it is dried to a moisture content of 6.0% under a steam pressure of 0.3 MPa and a drying temperature of 95°C. The dried paper web is then subjected to a heat treatment at 125°C for 20 minutes to obtain ultra-high folding endurance white kraft paper.

[0039] Comparative Example 1: No nanocellulose introduced The difference between Comparative Example 1 and Example 2 is that: in step 1, the entire homogeneous pulp is used as pulp B, and the portion containing 200g of oven-dried fiber is no longer diverted to pulp A for selective oxidation treatment using the 2,2,6,6-tetramethylpiperidine-1-oxygen radical / sodium bromide / sodium hypochlorite system. In subsequent steps, the wet pulp A oxidized pulp is not subjected to high-pressure homogenization to prepare nanocellulose dispersion. The composite pulp C is directly obtained by stirring pulp B without selective oxidation and high-pressure homogenization treatment at room temperature for 30 minutes and adjusting the pulp concentration to 10g / L. The remaining steps and conditions are the same as in Example 2.

[0040] Comparative Example 2: No interface enhancer I added The difference between Comparative Example 2 and Example 2 is that in step 6, only 100g of interface reinforcing agent II solution was added to nanocellulose composite pulp C and stirred for 30min. Then, 50g of cationic starch solution, 25g of wet strength agent solution and 2000g of polyacrylamide solution were added in sequence. The interval between the addition of each additive and the stirring time were the same as in Example 2, but 1000g of interface reinforcing agent I solution was no longer added. The remaining steps and conditions were the same as in Example 2.

[0041] Comparative Example 3: Without the addition of interfacial enhancer II The difference between Comparative Example 3 and Example 2 is that the addition of interface reinforcing agent II solution was omitted in step 6. That is, after adjusting the pulp concentration of nanocellulose composite pulp C to 8 g / L in the pulp tank, 1000 g of interface reinforcing agent I solution was directly added and stirred for 20 min at room temperature. Then, 50 g of cationic starch solution, 25 g of wet strength agent solution and 2000 g of polyacrylamide solution were added in sequence. The addition interval and stirring time between each auxiliary agent were kept the same as in Example 2, but 100 g of interface reinforcing agent II solution was not added. The remaining steps and conditions were the same as in Example 2.

[0042] Comparative Example 4: Interface enhancer II was added in the reverse order to interface enhancer I. The difference between Comparative Example 4 and Example 2 is that the order of adding interface reinforcing agent II solution and interface reinforcing agent I solution in step 6 is reversed. That is, 1000g of interface reinforcing agent I solution is added to nanocellulose composite pulp C with a pulp concentration of 8g / L and stirred for 20min, then 100g of interface reinforcing agent II solution is added and stirred for 30min, and then 50g of cationic starch solution, 25g of wet strength agent solution and 2000g of polyacrylamide solution are added in sequence. The interval and stirring time are the same as in Example 2, and the remaining steps and conditions are the same as in Example 2.

[0043] Comparative Example 5: No post-heat treatment performed The difference between Comparative Example 5 and Example 2 is that the heat treatment step in the heat treatment box is omitted in step 7. That is, after the paper web is dried in the multi-cylinder steam drying section at a steam pressure of 0.3 MPa to a moisture content of 6.1% and a drying temperature of 95°C, it is no longer subjected to heat treatment at 120°C for 15 minutes. Instead, it is directly cooled after drying to obtain white kraft paper. The remaining steps and conditions are the same as in Example 2.

[0044] Comparative Example 6: Without the use of interface reinforcing agent I and interface reinforcing agent II The difference between Comparative Example 6 and Example 2 is that: in steps 4 and 5, the preparation of interface reinforcing agent I solution and interface reinforcing agent II solution is not carried out, and in step 6, interface reinforcing agent I solution and interface reinforcing agent II solution are not added to nanocellulose composite pulp C. Instead, 50g of cationic starch solution, 25g of wet strength agent solution and 2000g of polyacrylamide solution are added to the pulp tank in the order of Example 2 and stirred. The resulting wet pulp is directly fed into a standard hand paper machine as multi-scale composite wet pulp D for forming. The remaining steps and conditions are the same as in Example 2.

[0045] Performance testing: Thermal stability test: Thermogravimetric analysis was performed under a nitrogen atmosphere. 10 mg of shredded paper sample was placed in a platinum crucible and heated from room temperature to 800 °C at a rate of 10 °C / min under a nitrogen flow rate of 50 mL / min. The thermogravimetric curve and differential thermogravimetric curve of sample mass change with temperature were recorded. The initial thermal decomposition temperature, the temperature corresponding to the maximum weight loss rate, and the residual mass fraction at 800 °C were read from the curves. The results are shown in Table 1.

[0046] Tensile strength and tensile index testing: The test was conducted in accordance with GB / T 12914-2018. After the specimens were conditioned in standard atmosphere, strip specimens with a width of 15 mm and a test length of 180 mm were cut along the longitudinal direction (machine direction) and the transverse direction (perpendicular to the machine direction), with no less than 10 specimens in each direction. The specimens were stretched at a constant tensile speed of 20 mm / min until fracture on a constant speed tensile tester, and the maximum tensile strength was recorded. The tensile strength (kN / m) and tensile index (N·m / g) were calculated based on the specimen quantification (determined according to GB / T 451.2-2023) and the specimen width. The longitudinal tensile strength, transverse tensile strength and corresponding tensile index were obtained respectively, and the results are shown in Table 1.

[0047] Folding endurance test: The test was conducted according to GB / T 457-2008 using an MIT-type folding endurance tester. Samples conditioned in standard atmosphere were cut into lengths of 15 mm and 100 mm in both the longitudinal and transverse directions, with no fewer than 10 samples in each direction. The samples were folded repeatedly until they broke under the conditions of a folding radius of 0.38 mm, a folding angle of 135°, a folding pressure of 4.9 N, and a folding frequency of 175 times / min. The number of folds before each sample broke was recorded. The arithmetic mean of the maximum and minimum values ​​was calculated after removing the maximum and minimum values. The results are shown in Table 1.

[0048] Bursting strength and bursting index test: Performed according to GB / T 454-2002 using a Mullen bursting strength tester. After conditioning, the sample is cut into circular specimens with a diameter greater than 100 mm, with at least 10 specimens per sample. The specimens are clamped onto the tester, and under a specified pressure rate, hydraulic pressure is used to lift the rubber diaphragm until it ruptures. The pressure value at rupture is recorded. The bursting index (kPa·m) is quantitatively calculated based on the specimens. 2 / g), the results are shown in Table 1.

[0049] Tear strength and tear index testing: Performed according to GB / T 455-2002 using an Elemandorf tear strength tester. After conditioning, rectangular specimens of 63mm × 50mm were cut along both the longitudinal and transverse directions, with no fewer than 20 specimens in each direction. After pre-cutting standard notches on the instrument, the pendulum was released to tear the specimens. The tear force was read, and the longitudinal and transverse tear strengths were calculated. The longitudinal and transverse tear indices (mN·m) were then quantitatively calculated. 2 / g), the results are shown in Table 1.

[0050] Whiteness and Opacity Tests: The whiteness of the paper was tested according to GB / T 7974-2013, using a dedicated whiteness meter to measure the blue light diffuse reflectance factor D65 brightness whiteness of the sample under D65 light source conditions; the opacity test was performed according to GB / T1543-2005, and the opacity (%) of the sample was measured. The results are shown in Table 1.

[0051] Table 1 Performance Test Results Initial thermal decomposition temperature (°C) 262.8 265.3 263.5 268.2 261.7 261.2 260.9 259.3 257.4 Residual mass fraction (%) at 800℃ 26.3 27.5 28.4 23.5 24.6 24.1 25.2 24.0 22.1 Longitudinal tensile strength (kN / m) 6.9 7.2 7.1 6.0 6.7 6.85 6.6 7.0 6.3 Transverse tensile strength (kN / m) 5.2 5.4 5.3 4.6 5.0 5.1 4.9 5.2 4.7 Longitudinal tensile index (N·m / g) 86.3 90.0 88.8 75.0 83.8 85.6 82.5 87.5 78.8 Transverse tensile index (N·m / g) 65.0 67.5 66.9 57.5 62.5 63.8 61.3 65.0 58.8 Longitudinal flexural strength (times) 1750 2100 1950 520 1550 980 1120 1630 650 Lateral flexural endurance (times) 1300 1500 1420 410 1180 780 860 1220 480 <![CDATA[Bursting index (kPa·m 2 / g)]]> 3.95 4.15 4.20 3.15 3.80 3.90 3.75 3.95 3.35 <![CDATA[Longitudinal tearing index (mN·m 2 / g)]]> 12.3 13.0 13.4 9.8 11.7 11.9 11.5 12.5 10.4 <![CDATA[Transverse tearing index (mN·m 2 / g)]]> 11.9 12.5 13.0 9.4 11.2 11.0 11.0 12.0 10.1 D65 Brightness Whiteness (%) 94.0 93.7 93.2 93.6 93.8 94.6 93.5 93.4 94.3 Opacity (%) 88.7 89.2 89.8 86.5 88.2 88.0 88.0 88.7 87.0 Data Analysis: As can be seen from the data in Examples 1-3 in Table 1, the three sets of examples generally maintain high levels of tensile strength, bursting strength, and tearing index in both the longitudinal and transverse directions, achieving a significant increase in folding endurance while maintaining high D65 whiteness and opacity. With the synergistic adjustment of the mass fraction of nanocellulose and the addition of interface reinforcing agent I and interface reinforcing agent II, the tensile strength and folding endurance in both the longitudinal and transverse directions show a synchronous increasing trend. However, when the interface reinforcing agent and crosslinking density are too high, the folding endurance in both the longitudinal and transverse directions slightly decreases, while the tearing index and bursting strength further increase, reflecting the balance between rigidity enhancement and flexibility. Based on the structural design, it can be inferred that the multi-scale fiber network jointly constructed by TEMPO oxidized carboxylated cellulose and wood pulp fibers provides a continuous mechanical framework. Interface reinforcing agent I significantly improves the effective bonding area between fibers through multi-point adsorption and hydrogen bonding entanglement of xylan and chitosan cationic segments on the fiber surface. In interface reinforcing agent II, the amide bonds formed between dihydrocaffeic acid and chitosan, and the flexible crosslinking with polyethylene glycol diglycidyl ether, introduce a certain reversible deformation space between the rigid fiber framework, giving the paper excellent repeated folding adaptability while ensuring strength. After hot pressing and curing, the crosslinking reaction between the multiple components is more complete, forming a dense carbon layer at high temperature. This improves the residual mass fraction at 800℃ and heat resistance stability without significantly reducing the initial thermal decomposition temperature, making Examples 1-3 all exhibit comprehensive performance suitable for high-frequency folding packaging and complex forming scenarios. Among them, Example 2 achieves a more optimized comprehensive balance between strength, folding endurance, and optical properties.

[0052] As can be seen from the data in Table 1 for Example 2 and Comparative Example 1, while maintaining similar whiteness, the tensile index, bursting index, and tear index of Comparative Example 1 are significantly lower than those of Example 2, the number of folds is only a small proportion of that of Example 2, the residual mass fraction at high temperature is also significantly reduced, while the initial thermal decomposition temperature is slightly higher. The main reason is that Comparative Example 1 removed TEMPO oxidized carboxylated cellulose, and the paper sheet only relies on the wood pulp fiber body and interface reinforcing agents I and II to provide limited bridging points. The contact area and effective bonding points between fibers are significantly reduced, making it difficult to form a continuous nano / micro multi-scale reinforcing network. The absence of nanocellulose also weakens the directional fixation of dihydrocaffeic acid and chitosan at the interface, making it difficult to form a stable and dense carbon layer at high temperatures. The reduced carboxyl density slightly delays the initial mass loss stage, resulting in a slightly higher apparent initial thermal decomposition temperature, but the overall mechanical and heat resistance performance is significantly worse than that of Example 2, demonstrating the irreplaceable fundamental role of nanocellulose in the multi-scale reinforcing system of this invention.

[0053] As can be seen from the data in Table 1 for Example 2 and Comparative Example 2, the tensile and bursting indices in both the longitudinal and transverse directions of Comparative Example 2 are lower than those of Example 2, but the number of folds in both the longitudinal and transverse directions is still significantly higher than that of the system without the introduction of the interfacial reinforcing agent. The tear index remains at a high level, and the whiteness and opacity are close to those of Example 2. The high-temperature residual mass fraction and thermal decomposition temperature are slightly lower. The main reason is that Comparative Example 2 retains the flexible cross-linked network composed of chitosan, dihydrocaffeic acid and polyethylene glycol diglycidyl ether in interfacial reinforcing agent II. This network can form elastic bridge chains between nanocellulose and wood pulp fibers, disperse stress during repeated folding, and improve the flexibility and tear resistance of the paper. However, due to the lack of interfacial reinforcing agent I with a xylan-chitosan salt structure, the directional adsorption and multi-point hydrogen bond entanglement on the fiber surface are insufficient, and the bonding strength between microfibers is limited, making it difficult to reach the level of tensile and bursting indices in both the longitudinal and transverse directions of Example 2. Compared with Comparative Example 3, which contains only Interface Reinforcing Agent I, Comparative Example 2 has an advantage in folding endurance but is slightly inferior in strength. This indicates that Interface Reinforcing Agent II is more beneficial to folding durability, while Interface Reinforcing Agent I is more inclined to improve static strength. The combination of the two in Example 2 produced a synergistic effect that is difficult to predict.

[0054] As can be seen from the data in Table 1 for Example 2 and Comparative Example 3, the tensile and bursting indices of Comparative Example 3 are close to and slightly higher than those of Comparative Example 2, and the tear index also remains at a high level. However, the number of folds in both the longitudinal and transverse directions is significantly lower than that of Example 2 and Comparative Example 2, the D65 whiteness is significantly higher than that of Example 2, and the high-temperature residual mass fraction is slightly lower. The main reason for this is that Comparative Example 3 only contains interface reinforcing agent I composed of xylan and chitosan. This system improves the interfiber bonding strength through multi-point hydrogen bonding and electrostatic interaction on the surface of fibers and nanocellulose, which is beneficial to tensile strength and bursting strength. However, the system lacks the flexible crosslinking bridges provided by dihydrocaffeic acid and polyethylene glycol diglycidyl ether, making the interface layer relatively rigid. During repeated folding, it is more prone to microcracks, resulting in a decrease in the number of folds. The absence of dihydrocaffeic acid also makes the paper color closer to the natural color of bleached cellulose, thus resulting in slightly higher whiteness. Compared with Example 2, it can be seen that although relying solely on rigid interface reinforcing agent I can improve strength, it is difficult to achieve both high flexural strength and heat-resistant charring performance. Only by combining interface reinforcing agent I and interface reinforcing agent II in a synergistic manner can an excellent balance between strength, flexibility and thermal stability be achieved.

[0055] As can be seen from the data of Example 2 and Comparative Example 4 in Table 1, under the premise that the ratio of interface reinforcing agent I and interface reinforcing agent II remains unchanged and the total amount is the same, only by changing the order of adding interface reinforcing agent II first and then interface reinforcing agent I in step 6 to adding interface reinforcing agent I first and then interface reinforcing agent II, the longitudinal and transverse tensile index, flexural endurance and tear index of Comparative Example 4 are significantly lower than those of Example 2, and only slightly higher than those of Comparative Example 2 and Comparative Example 3 which contain only a single interface reinforcing agent. The mechanism is that in Example 2 of this invention, interface reinforcing agent II first forms a continuous flexible polyphenol-polyethylene glycol crosslinked layer on the surface of the fiber and nanocellulose, and then interface reinforcing agent I constructs a rigid xylan-chitosan polysaccharide layer on the outer layer. The interface region presents a layered structure that is flexible inside and rigid outside, which is beneficial for buffering and dispersing local stress during folding. In contrast, in Comparative Example 4, interface reinforcing agent I is added first, which causes the rigid polysaccharide interface layer to occupy the fiber surface. The interface reinforcing agent II added later can only adhere to local discontinuous areas, making it difficult to form a complete flexible buffer layer. This leads to interface stress concentration and easier propagation of microcracks, making it difficult to achieve the folding endurance and comprehensive mechanical properties at the level of Example 2. This shows that the order of adding interface reinforcing agent II and interface reinforcing agent I has a significant impact on the control of interface structure.

[0056] As can be seen from the data in Table 1 for Example 2 and Comparative Example 5, under the same ratio of nanocellulose and interface reinforcing agent, the longitudinal and transverse tensile indices, folding endurance, and tear index of Comparative Example 5 are slightly lower than those of Example 2. The high-temperature residual mass fraction and initial thermal decomposition temperature are also significantly lower, while the whiteness and opacity are basically the same. This indicates that without the 110°C hot-pressing curing step, the crosslinking reaction between chitosan, dihydrocaffeic acid, and polyethylene glycol diglycidyl ether is incomplete, and the hydrogen and ionic bonds between interface reinforcing agent I and cellulose and nanocellulose are relatively loose, resulting in a less than fully compacted multi-scale fiber network. Hot-pressing curing, on the one hand, promotes the ring-opening reaction between the epoxy groups of the crosslinking agent and the hydroxyl groups of chitosan and cellulose, forming a continuous network with a higher crosslinking density; on the other hand, it compacts the pores between fibers under pressure, reducing stress concentration areas. Therefore, in Example 2, both higher folding endurance and higher high-temperature char yield are achieved simultaneously.

[0057] As can be seen from the data in Table 1 for Example 2 and Comparative Example 6, although Comparative Example 6 retained nanocellulose and conventional cationic starch, wet strength agent, polyacrylamide, etc., its longitudinal and transverse tensile index, tear index, and bursting index were higher than those of Comparative Example 1 (which did not contain nanocellulose), but were still significantly lower than those of Example 2. Its longitudinal and transverse folding endurance was only slightly higher than that of ordinary packaging kraft paper, and its high-temperature residual mass fraction was the lowest, while its whiteness was slightly higher. The main reason for this is that Comparative Example 6 lacks the xylan-chitosan salt structure and the dihydrocaffeic acid-polyethylene glycol crosslinking network. The bonding force between nanocellulose and wood pulp fibers mainly relies on physical entanglement and the conventional wet strength system. The interface layer structure is relatively loose, making it difficult to effectively disperse local stress during repeated folding and to form a continuous and dense aromatic carbon layer at high temperatures, resulting in insufficient folding endurance and heat resistance. Compared with Comparative Example 1, it can be seen that the introduction of nanocellulose alone can significantly improve the static strength. Compared with Comparative Examples 2 and 3, it shows that the introduction of interface reinforcing agent I or interface reinforcing agent II alone can also significantly improve a certain type of performance. However, only when nanocellulose, interface reinforcing agent I and interface reinforcing agent II are introduced simultaneously in Example 2 and combined with hot pressing and curing, can the strength, flexural endurance and high-temperature char residue, etc., be far superior to each single modification scheme, showing a typical synergistic reinforcement effect of 1+1 greater than 2.

[0058] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing ultra-high folding endurance white kraft paper, characterized in that, Includes the following steps: (1) Bleached softwood kraft paper pulp board and bleached hardwood kraft paper pulp board are dispersed and mixed in water to obtain a homogeneous pulp, and the pulp is divided into pulp A and pulp B according to the oven-dry fiber weight; (2) The pulp A is added to an oxidation system composed of 2,2,6,6-tetramethylpiperidine-1-oxy radical, sodium bromide and sodium hypochlorite for selective oxidation treatment, and washed to obtain oxidized pulp A containing carboxylated cellulose; (3) The oxidized pulp A is subjected to high-pressure homogenization to obtain a nanocellulose dispersion, and the nanocellulose dispersion is mixed with pulp B to obtain composite pulp C; (4) An interface enhancer I solution was prepared by dissolving chitosan in an aqueous solution of glacial acetic acid, adding xylan, and neutralizing and degassing. (5) Chitosan-dihydrocaffeic acid was obtained by dissolving chitosan in an aqueous solution of glacial acetic acid, adding dihydrocaffeic acid and reacting it in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide. Then, the chitosan-dihydrocaffeic acid was grafted with polyethylene glycol diglycidyl ether and diluted to prepare an interface enhancer II solution. (6) Adjust the composite pulp C to a predetermined pulp concentration, and add the interface strengthening agent II solution, interface strengthening agent I solution, cationic starch solution, wet strength agent solution and polyacrylamide solution in sequence under stirring conditions to obtain multi-scale composite wet pulp D; (7) The multi-scale composite wet pulp D is formed into a paper sheet, which is then pressed, dehydrated, and steam-dried, and then subjected to heat treatment at 120-125℃ to obtain ultra-high folding endurance white kraft paper. In step (4), the mass ratio of chitosan to xylan is 6-10:10-14; In step (5), the mass ratio of chitosan, dihydrocaffeic acid, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide is 20:4-6:5:3; In step (5), the mass ratio of chitosan-dihydrocaffeic acid to polyethylene glycol diglycidyl ether is 8-12:1.5-2.

5.

2. The method for preparing ultra-high folding endurance white kraft paper according to claim 1, characterized in that, In step (1), the mass ratio of bleached softwood kraft pulp board to bleached hardwood kraft pulp board is 4:

1.

3. The method for preparing ultra-high folding endurance white kraft paper according to claim 1, characterized in that, In step (2), based on 200g of oven-dry fiber in pulp A, the amount of 2,2,6,6-tetramethylpiperidine-1-oxy free radical is 3g, the amount of sodium bromide is 20g, and the sodium hypochlorite exists in solution form with a mass fraction of 10% and a dosage of 400-600mL.

4. The method for preparing ultra-high folding endurance white kraft paper according to claim 1, characterized in that, In step (2), the specific steps of the oxidation treatment are as follows: use 1 mol / L sodium hydroxide solution to adjust and maintain the pH of the slurry at 9.8-10.2, the reaction time is 45-75 min, and after the reaction is completed, add 40-60 mL of anhydrous ethanol to terminate the reaction.

5. The method for preparing ultra-high folding endurance white kraft paper according to claim 1, characterized in that, In step (3), the conditions for high-pressure homogenization are: pressure 500-700 bar, homogenization cycle 4-6 times.

6. The method for preparing ultra-high folding endurance white kraft paper according to claim 1, characterized in that, In step (6), based on 1000g of oven-dry fiber mass in composite pulp C, the added interface reinforcing agent II solution is 80-120g, interface reinforcing agent I solution is 800-1200g, cationic starch solution is 40-60g, wet strength agent solution is 20-30g, and polyacrylamide solution is 1600-2400g.

7. The method for preparing ultra-high folding endurance white kraft paper according to claim 1, characterized in that, In step (6), the solid content of the cationic starch solution is 8%-12%; the solid content of the wet strength agent solution is 11%-14%; and the mass concentration of the polyacrylamide solution is 0.1wt%-0.3wt%.

8. The method for preparing ultra-high folding endurance white kraft paper according to claim 1, characterized in that, In step (7), the specific steps of pressing and dehydration are as follows: pressing twice with a linear pressure of 75-85kN / m through two rubber pressing rollers.

9. The method for preparing ultra-high folding endurance white kraft paper according to claim 1, characterized in that, In step (7), the specific steps of steam drying are as follows: drying to a paper moisture content of 6.0%-6.3% under steam pressure of 0.3MPa and drying temperature of 95℃.

10. A white kraft paper with ultra-high folding durability, characterized in that, It is obtained by the preparation method of ultra-high folding endurance white kraft paper according to any one of claims 1-9.