Fiber composite reinforcing method of high-strength white kraft paper

Abstract

The invention discloses a fiber composite reinforcing method of high-strength white kraft paper, which comprises the following steps: carrying out pretreatment such as segmented bleaching, biological enzymolysis and surface modification on softwood pulp, composite jute pulp and modified polyester staple fiber according to a specific ratio, controlling the beating degree during mixed pulping, and adding composite additives such as nano cellulose, so as to obtain the high-strength white kraft paper. And then carrying out gradient drying and optional surface sizing processes. Through cooperation of multi-element fibers, microcosmic combination strengthening and performance consolidation, the contradiction between whiteness and strength and between strength and surface performance in a traditional process is solved, the tensile strength, tearing strength and bursting strength of the paper and the environmental stability are improved, and the high-strength paper is suitable for the fields such as packaging and printing which have requirements for high-strength paper.

Description

Technical Field

[0001] This invention relates to the field of white kraft paper production technology, specifically to a fiber composite reinforcement method for high-strength white kraft paper. Background Technology

[0002] In the traditional production of white kraft paper, softwood pulp or a small amount of hardwood pulp is often used as raw material, which presents a problem of difficulty in achieving both strength and whiteness: if high whiteness is pursued, the bleaching process can easily damage the fibers and lead to a decrease in strength; if strength is emphasized, a large amount of long fiber raw materials are required, which is costly and has limited performance.

[0003] Traditional processes often result in insufficient interfiber bonding, limiting the paper's tear resistance and burst strength, making it difficult to meet the demands of packaging, printing, and other industries for high-strength, high-stability paper. Furthermore, some reinforcement methods involve the addition of large amounts of chemical additives, which can easily cause environmental pollution and negatively impact the paper's secondary processing performance. Summary of the Invention

[0004] The purpose of this invention is to provide a fiber composite reinforcement method for high-strength white kraft paper, so as to solve the technical problems of insufficient inter-fiber bonding force, low tear resistance and burst resistance, and insufficient strength and stability of paper in the traditional production process.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A method for fiber composite reinforcement of high-strength white kraft paper includes the following steps:

[0007] S1. Fiber raw material ratio: Mix 60-70 parts by weight of softwood pulp, 15-25 parts by weight of hemp pulp, and 5-10 parts by weight of polyester staple fiber. The hemp pulp is a mixture of sisal pulp and flax pulp in a mass ratio of 3:1. The polyester staple fiber has a length of 3-5 mm and a diameter of 15-20 μm.

[0008] S2. Fiber pretreatment:

[0009] For softwood pulp, a two-stage bleaching process of oxygen bleaching and hydrogen peroxide is adopted to control the post-bleaching brightness to be ≥85% ISO and the fiber length retention rate to be ≥90%.

[0010] The hemp pulp was pretreated with a biological enzyme, wherein the biological enzyme was a mixture of xylanase and pectinase in a mass ratio of 2:1, the amount of enzyme used was 0.3% to 0.5% of the dry hemp pulp mass, the treatment temperature was 50-60℃, and the time was 60 to 90 min.

[0011] Surface modification of polyester staple fiber was carried out by immersing it in a 1%–2% (w / w) ethanol solution of silane coupling agent KH-550 for 30–40 minutes, and then drying it for later use.

[0012] S3. Mixing and pulping: Mix the three types of fibers after step S2, add deionized water to prepare a suspension with a pulp concentration of 3% to 5%, and use a double-disc refiner to pulp, controlling the freeness to 30-45°SR.

[0013] S4. Additives: Add 1.2% to 1.8% nanocellulose, 0.8% to 1.2% cationic polyacrylamide dry strength agent, and 0.5% to 0.8% oxidized starch thickener by weight of oven-dry fiber to the slurry in step S3, and stir evenly.

[0014] S5. Forming: The pulp is formed on a fourdrinier paper machine, and after pressing and drying, the base paper is obtained. The drying adopts a gradient heating method, with the temperature gradually increasing from 60℃ to 110℃, and the total drying time is 8 to 10 minutes.

[0015] The fiber composite reinforcement method for high-strength white kraft paper described above combines a specific ratio of softwood pulp, composite hemp pulp, and modified polyester staple fiber with a segmented fiber pretreatment process (targeted bleaching, bio-enzymatic hydrolysis, and surface modification), along with the synergistic effect of nanocellulose and composite additives and gradient drying technology. This achieves a multi-dimensional improvement in the bonding force between fibers, while also taking into account whiteness and fiber integrity. Ultimately, this results in a comprehensive enhancement of the white kraft paper in terms of tensile strength, tear strength, and bursting strength.

[0016] As a preferred embodiment of the present invention, the softwood pulp in step S1 is Nordic spruce pulp, the sisal pulp has an average fiber length of 3.5 to 4.2 mm, and the flax pulp has an average fiber length of 2.8 to 3.2 mm.

[0017] The above technical solution limits the type of softwood pulp to Nordic spruce pulp (which has more prominent long fiber characteristics), and clarifies the fiber length range of sisal pulp and flax pulp. By utilizing the synergistic effect of long fiber skeleton and medium and short fiber filling, the fiber interlacing density is further optimized, thereby improving the tensile strength and structural stability of the paper.

[0018] As a preferred embodiment of the present invention, the oxygen bleaching process conditions in step S2 are: oxygen pressure 0.6-0.8 MPa, temperature 90-100℃, time 60-80 min, the amount of H2O2 used in the hydrogen peroxide bleaching process is 3%-5% of the oven-dry pulp mass, and the pH value is controlled at 10-11.

[0019] The aforementioned technology optimizes the pressure and temperature parameters of oxygen bleaching, as well as the dosage and pH value of hydrogen peroxide bleaching, to reduce fiber damage while ensuring high whiteness, avoiding the strength reduction problem caused by traditional bleaching, and achieving a balance between whiteness and strength.

[0020] As a preferred embodiment of the present invention, during the bio-enzyme pretreatment in step S2, the slurry concentration is controlled at 8% to 10%, and the pH value is adjusted to 4.5-5.5.

[0021] The above technical solution provides an optimal reaction environment for the complex enzyme system by controlling the pulp concentration and pH value during bio-enzyme pretreatment, promoting the efficient removal of hemicellulose and pectin in hemp pulp, improving the swelling and flexibility of the fiber, and enhancing its binding ability with other fibers.

[0022] As a preferred embodiment of the present invention, the grinding disc gap of the double-disc refiner in step S3 is 0.15-0.25mm, and the grinding power is 35-45kW.

[0023] The above technical solution controls the degree of fiber fibrillation within a reasonable range by limiting the gap and power of the grinding discs of the double-disc refiner. This avoids fiber breakage caused by over-refining while ensuring sufficient bonding area, thus achieving a balance between the pulp's water filtration performance and the fiber bonding force.

[0024] As a preferred embodiment of the present invention, the nanocellulose in step S4 is cellulose nanofiber with a length of 100-300 nm and a diameter of 5-10 nm, and is prepared by TEMPO oxidation method.

[0025] The above technical solution uses TEMPO oxidation nanocellulose of specific size, which utilizes its high aspect ratio and surface activity to form a nanoscale bridging network between fibers, enhances the hydrogen bonding between fibers, and improves the water retention and molding uniformity of the slurry.

[0026] As a preferred embodiment of the present invention, the cationic polyacrylamide in step S4 has a molecular weight of 8 million to 12 million and an ionicity of 30% to 40%.

[0027] The above technical solution uses cationic polyacrylamide with specific molecular weight and ionicity to form efficient adsorption on the fiber surface. Through charge neutralization and bridging, it promotes fiber flocculation, reduces fiber loss during the papermaking process, and enhances the chemical bonding between fibers.

[0028] As a preferred embodiment of the present invention, the pressing in step S5 adopts a three-roll press with linear pressures of 150kN / m, 200kN / m, and 250kN / m respectively, and the dryness of the base paper after pressing is ≥40%.

[0029] The above technical solution uses a three-roll incremental pressing method to gradually increase the linear pressure, maximizing moisture removal while avoiding damage to the fiber structure. This improves the density of the base paper and the contact density between fibers, laying the foundation for strength formation during the subsequent drying process.

[0030] As a preferred embodiment of the present invention, the method further includes step S6: applying a sizing agent to the base paper obtained in step S5, wherein the sizing agent is a styrene-acrylic emulsion with a mass fraction of 10% to 15%, the sizing amount is 1.5 to 2.5 g / m², and after sizing, drying is performed again at 80-90°C for 2 to 3 minutes.

[0031] The above technical solution introduces styrene-acrylic emulsion through a surface sizing process to form a continuous film on the paper surface. This not only enhances the surface strength and abrasion resistance of the paper, but also reduces the impact of moisture penetration on the internal fiber bonding force through the sealing effect of the film layer, thereby improving the wet strength stability of the paper.

[0032] As a preferred embodiment of the present invention, the styrene-acrylic emulsion in step S6 is copolymerized from styrene, butyl acrylate and methacrylic acid in a mass ratio of 5:3:2, and has a glass transition temperature of 25-30°C.

[0033] The above technical solution, by limiting the monomer ratio and glass transition temperature of the styrene-acrylic emulsion, enables the sizing film layer to simultaneously possess flexibility and adhesion, avoids film layer cracking, and enhances the tensile strength and folding endurance of the paper surface.

[0034] As a further embodiment of the present invention, after the surface modification of the polyester staple fiber in step S2, it is necessary to perform plasma treatment with a treatment power of 80-100W and a time of 1-2 minutes, using argon as the working gas.

[0035] The above technical solution further improves the surface roughness and polar group content of the polyester staple fiber by adding argon plasma treatment after surface modification, enhances its interfacial compatibility with cellulose fiber, promotes stress transfer between organic and inorganic fibers, and improves the overall tear resistance of the composite fiber network.

[0036] Compared with existing technologies, the fiber composite reinforcement method for high-strength white kraft paper provided by this invention has the following beneficial effects:

[0037] 1. A multi-fiber system consisting of softwood pulp, composite hemp pulp, and modified polyester staple fiber is adopted, combined with targeted pretreatment processes (bleaching, bio-enzymatic hydrolysis, and surface modification). This system retains the skeletal support of long fibers while enhancing toughness through filling with medium and short fibers and chemical fibers, thereby achieving a synergistic improvement in tensile, tear, and burst strength.

[0038] 2. Bio-enzyme pretreatment and surface modification technology reduce fiber damage. Combined with the bridging effect of nanocellulose and composite additives, it enhances the bonding force between fibers and avoids the strength loss caused by excessive pulping in traditional methods.

[0039] 3. The synergistic effect of gradient drying and surface sizing processes ensures high whiteness while improving paper surface properties (such as abrasion resistance and scratch resistance) and environmental stability (such as wet strength and aging resistance), thus resolving the contradiction between whiteness and strength, and strength and surface properties in traditional processes. Attached Figure Description

[0040] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only examples of embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0041] Figure 1 This is a flowchart of an embodiment of the present invention. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0043] In the description of the embodiments of the present invention, it should be understood that the terms "upper", "lower", "front", "rear", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings. They are only for the convenience of describing the embodiments of the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of the present invention.

[0044] In the description of the embodiments of the present invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation", "connection" and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, an integral connection, or a detachable connection; they can refer to the internal connection of two components; they can refer to a direct connection or an indirect connection through an intermediate medium. For those skilled in the art, the specific meaning of the above terms in the embodiments of the present invention should be understood according to the specific circumstances.

[0045] See Figure 1 As shown, the specific embodiments of the present invention provide multiple sets of examples.

[0046] Example 1:

[0047] The formula consists of 65 parts by weight of Nordic spruce softwood pulp, 20 parts by weight of hemp pulp (a 3:1 mixture of sisal and flax pulp), and 15 parts by weight of polyester staple fiber (3mm in length and 15μm in diameter). The softwood pulp is oxygen-bleached at 0.7MPa and 95℃ for 70 minutes, followed by bleaching with 4% hydrogen peroxide at pH 10.5. The hemp pulp is treated with 0.4% compounded biological enzyme at 9% pulp concentration and pH 5 at 55℃ for 75 minutes. The polyester staple fiber is soaked in a 1.5% silane coupling agent KH-550 ethanol solution for 35 minutes, dried, and then treated with 80W argon plasma for 1.5 minutes. After mixing, the pulp concentration is adjusted to 4%, and the pulp is refined to 38°SR using a 40kW dual-disc refiner with a disc gap of 0.2mm. 1.5% nanocellulose, 1% cationic polyacrylamide (molecular weight 10 million, ionic degree 35%), and 0.6% oxidized starch are added. The paper is formed on a long-wire paper machine with three-roll press pressures of 150 kN / m, 200 kN / m, and 250 kN / m respectively. Drying is carried out using a gradient temperature increase of 60℃-90℃-110℃, with a total drying time of 9 minutes. Finally, a styrene-acrylic emulsion copolymerized with 12% styrene, butyl acrylate, and methacrylic acid in a 5:3:2 ratio is applied at a rate of 2 g / m², followed by a secondary drying at 85℃ for 2.5 minutes.

[0048] Technical effects: By precisely proportioning fiber raw materials and utilizing the characteristics of each fiber, combined with optimized pretreatment, pulping, papermaking and sizing processes, the paper strength is comprehensively improved, including tensile, tear and burst strength, while ensuring good whiteness and paper forming quality, and improving paper surface properties.

[0049] Working principle: Nordic spruce softwood pulp provides a long fiber skeleton to enhance tensile strength; hemp pulp is enzymatically treated and interweaves better with softwood pulp; modified polyester staple fiber enhances overall toughness; each stage of treatment optimizes fiber performance and bonding strength; additives enhance inter-fiber interaction; sizing forms a protective film.

[0050] Experimental data: Whiteness reached 88% ISO, tensile index increased to 90 N·m / g, tear index was 12 mN·m² / g, bursting index reached 7.5 kPa·m² / g, and paper surface roughness was reduced to 3 μm.

[0051] Example 2:

[0052] The formula consists of 60 parts by weight of softwood pulp, 25 parts by weight of hemp pulp (sisal pulp to flax pulp ratio 3:1), and 15 parts by weight of polyester staple fiber. Softwood pulp is oxygen-bleached at 0.6 MPa and 90℃ for 60 min, with hydrogen peroxide bleaching using 3% H2O2 at pH 10. Hemp pulp is treated with enzymes at 0.3%, pulp concentration at 8%, pH 4.5, temperature at 50℃, and time at 60 min. Polyester staple fiber is soaked in a 1% silane coupling agent solution for 30 min, dried, and then treated with 90W argon plasma for 1 min. The mixed pulp concentration is 3%, and the pulping is done at a grinding gap of 0.15 mm and a power of 35 kW to a final SR of 30°. 1.2% nanocellulose, 0.8% cationic polyacrylamide (molecular weight 8 million, ionic degree 30%), and 0.5% oxidized starch are added. During papermaking, the pressing line pressure was 150 kN / m, 180 kN / m, and 220 kN / m, and the drying temperature was increased from 60°C to 100°C for a total time of 8 minutes. Surface sizing was performed using 10% styrene-acrylic emulsion (monomer ratio as in Example 1), with an application rate of 1.5 g / m², followed by a second drying at 80°C for 2 minutes.

[0053] Technical effects: Strengthens the bond between fibers, improves the overall strength of paper, enhances paper flexibility, increases tensile strength and tear resistance, and strengthens paper surface strength and abrasion resistance.

[0054] Working principle: A specific ratio of fiber combination, pretreatment to optimize fiber performance, pulping to control fiber state, additives to promote bonding, and sizing to improve surface properties.

[0055] Experimental data: whiteness 86% ISO, tensile index 85 N·m / g, tear index 11 mN·m² / g, bursting index 7 kPa·m² / g, paper flexibility increased by 15%, and surface abrasion resistance increased to 500 cycles.

[0056] Example 3:

[0057] Formula: 70 parts by weight of softwood pulp, 15 parts by weight of hemp pulp (sisal and flax mixed in a 3:1 ratio), and 15 parts by weight of polyester staple fiber. Softwood pulp was oxygen-bleached at 0.8 MPa, 100℃, and 80 min, with hydrogen peroxide bleaching using 5% H2O2 at pH 11. Hemp pulp was treated with a 0.5% enzyme at 10% pulp concentration and pH 5.5 at 60℃ for 90 min. Polyester staple fiber was soaked in a 2% silane coupling agent solution for 40 min, dried, and then treated with 100W argon plasma for 2 min. The mixed pulp concentration was 5%, and the pulping was beaten to 45°SR at a grinding gap of 0.25 mm and a power of 45 kW. 1.8% nanocellulose, 1.2% cationic polyacrylamide (molecular weight 12 million, degree of ionization 40%), and 0.8% oxidized starch were added. The pressing line operates at pressures of 180 kN / m, 220 kN / m, and 280 kN / m, with drying performed at a gradient temperature of 60℃-100℃-110℃, completed in 10 minutes. A 15% styrene-acrylic emulsion is used for sizing at a rate of 2.5 g / m², followed by a second drying at 90℃ for 3 minutes.

[0058] Technical effects: Greatly enhances fiber bonding, significantly improves paper strength, increases whiteness, improves paper stiffness and resistance to deformation, and enhances paper's water resistance and wet strength.

[0059] Working principle: Reasonable process parameters allow the fiber performance to be fully utilized, additives enhance the bonding, and sizing forms a waterproof and wear-resistant layer.

[0060] Experimental data: whiteness 90% ISO, tensile index 95 N·m / g, tear index 13 mN·m² / g, bursting index 8 kPa·m² / g, paper stiffness increased by 20%, and wet strength retention rate reached 70%.

[0061] Example 4:

[0062] Formula: 63 parts by weight of softwood pulp, 18 parts by weight of hemp pulp (sisal and flax 3:1), and 19 parts by weight of polyester staple fiber. Softwood pulp oxygen bleaching process: 0.7 MPa, 93℃, 75 min; hydrogen peroxide bleaching (H2O2): 4%; pH: 10.3. Hemp pulp bio-enzyme treatment: 0.4% enzyme, 9% pulp concentration, pH: 5.2; 58℃: 80 min. Polyester staple fiber: soaked in 1.3% silane coupling agent solution for 33 min, dried, and treated with 85W argon plasma for 1.2 min. Mixed pulp concentration: 4%; grinding disc gap: 0.18 mm; power: 38 kW; beating to 35°SR. Additions: 1.4% nanocellulose, 0.9% cationic polyacrylamide (molecular weight 9 million, ionic degree 32%), and 0.65% oxidized starch. The pressing line operates at pressures of 160 kN / m, 210 kN / m, and 260 kN / m, and is dried at 60℃-95℃-110℃ for 9 minutes. A 13% styrene-acrylic emulsion is used for sizing at a rate of 2.2 g / m², followed by a secondary drying at 88℃ for 2.3 minutes.

[0063] Technical effects: Achieves good fiber composite, enhances the overall strength of paper, improves paper uniformity and smoothness, optimizes paper appearance quality, and improves the scratch resistance of paper surface.

[0064] Working principle: Precise control of each step ensures uniform fiber distribution and bonding, additives improve performance, and sizing enhances surface quality.

[0065] Experimental data: whiteness 87% ISO, tensile index 88 N·m / g, tear index 11.5 mN·m² / g, bursting index 7.2 kPa·m² / g, paper uniformity improved by 10%, and surface scratch resistance grade improved to 4H.

[0066] Example 5:

[0067] Formula: 68 parts by weight of softwood pulp, 16 parts by weight of hemp pulp (sisal and flax 3:1), and 16 parts by weight of polyester staple fiber. Softwood pulp was oxygen-bleached at 0.75 MPa, 98℃, for 78 min, with hydrogen peroxide bleaching (H2O2) dosage of 4.5% and pH 10.8. Hemp pulp was treated with a bio-enzyme at 0.45% enzyme dosage, pulp concentration of 9.5%, pH 5.3, and treated at 56℃ for 85 min. Polyester staple fiber was soaked in a 1.8% silane coupling agent solution for 38 min, dried, and then treated with 95W argon plasma for 1.8 min. The mixed pulp concentration was 4.5%, with a grinding disc gap of 0.22 mm and a power of 42kW to beating to 42°SR. 1.6% nanocellulose, 1.1% cationic polyacrylamide (molecular weight 11 million, ionic degree 38%), and 0.7% oxidized starch were added. The pressing line operates at pressures of 170 kN / m, 230 kN / m, and 270 kN / m, and is dried at 60℃-98℃-110℃ for 9.5 minutes. A 14% styrene-acrylic emulsion is used for sizing at a rate of 2.3 g / m², followed by a secondary drying at 87℃ for 2.8 minutes.

[0068] Technical effects: Effectively enhances fiber synergy, strengthens paper, improves paper stability and durability, enhances paper's resistance to environmental interference, and improves paper's aging resistance.

[0069] Working principle: Optimized processes enable complementary fiber properties, while additives and sizing enhance the overall performance of the paper.

[0070] Experimental data: whiteness 89% ISO, tensile index 92 N·m / g, tear index 12.5 mN·m² / g, bursting index 7.8 kPa·m² / g, and the paper retains 85% of its strength after accelerated aging test.

[0071] In summary, the fiber composite reinforcement method for high-strength white kraft paper provided in this embodiment has the following advantages:

[0072] This invention achieves a comprehensive improvement in the performance of white kraft paper through a multi-dimensional synergistic approach:

[0073] 1. A multi-fiber system consisting of softwood pulp, composite hemp pulp, and modified polyester staple fiber is adopted, combined with targeted pretreatment processes (bleaching, bio-enzymatic hydrolysis, and surface modification). This system retains the skeletal support of long fibers while enhancing toughness through filling with medium and short fibers and chemical fibers, thereby achieving a synergistic improvement in tensile, tear, and burst strength.

[0074] 2. Bio-enzyme pretreatment and surface modification technology reduce fiber damage. Combined with the bridging effect of nanocellulose and composite additives, it enhances the bonding force between fibers and avoids the strength loss caused by excessive pulping in traditional methods.

[0075] 3. The synergistic effect of gradient drying and surface sizing processes ensures high whiteness while improving paper surface properties (such as abrasion resistance and scratch resistance) and environmental stability (such as wet strength and aging resistance), thus resolving the contradiction between whiteness and strength, and strength and surface properties in traditional processes.

[0076] This invention constructs a three-stage synergistic system of "fiber optimization - bonding reinforcement - performance consolidation": 1. Fiber optimization stage: Through raw material ratio design, long fibers from softwood pulp form a network skeleton, medium and short fibers from hemp pulp fill the gaps, and the high strength of polyester staple fiber compensates for the lack of toughness in natural fibers; targeted pretreatment (such as segmented bleaching of softwood pulp to reduce damage, bio-enzymatic hydrolysis of hemp pulp to remove impurities, and surface modification of polyester staple fiber to improve compatibility) maximizes the performance of various fibers, laying the foundation for subsequent bonding. 2. Bonding reinforcement stage: During mixed pulping, the fiber fracturing is moderately achieved by controlling the beating degree, increasing the contact area; nano-cellulose bridging, cationic polyacrylamide charge adsorption, and oxidized starch adhesion enhance the hydrogen and chemical bonds between fibers at the microscopic level, forming a dense and tough fiber network. 3. Performance consolidation stage: Gradient drying avoids fiber thermal damage and gradually solidifies the fiber bonding structure; surface sizing forms a continuous protective film, which enhances surface strength and reduces the damage of moisture to the internal fiber bonding force, ultimately achieving a comprehensive improvement in paper strength, whiteness, and stability.

[0077] How to use:

[0078] 1. Raw material preparation and pretreatment: Select a mixture of softwood pulp, sisal pulp and flax pulp, and polyester staple fiber of a specific specification according to the proportion; perform two-stage bleaching on the softwood pulp, bio-enzyme treatment on the flax pulp, and silane coupling agent soaking and plasma treatment on the polyester staple fiber (optional).

[0079] 2. Mixing and pulping: Mix the pretreated fibers, adjust to the appropriate concentration, and pulp to the target freeness using a double-disc refiner. Add auxiliaries such as nanocellulose, cationic polyacrylamide, and oxidized starch and stir evenly.

[0080] 3. Papermaking: The pulp is formed on a fourdrinier paper machine, and the moisture is removed by multi-roll incremental pressing. Then, the base paper is obtained by gradient temperature drying.

[0081] 4. Post-treatment (optional): The base paper is sizing the surface using a specific ratio of styrene-acrylic emulsion, and the amount of sizing and secondary drying parameters are controlled to finally obtain high-strength white kraft paper.

[0082] 5. The entire process can be adjusted according to actual needs, including raw material ratios, processing parameters, and post-processing steps, to adapt to different scenarios and their emphasis on paper performance.

[0083] The foregoing has shown and described the basic principles of the present invention. The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. The above embodiments and descriptions in the specification are only illustrative of the principles of the present invention. Any modifications, equivalent substitutions, and improvements made within the scope of the present invention without departing from the scope of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for fiber composite reinforcement of high-strength white kraft paper, characterized in that... Includes the following steps: S1. Fiber raw material ratio: Mix 60-70 parts by weight of softwood pulp, 15-25 parts by weight of hemp pulp, and 5-10 parts by weight of polyester staple fiber. The hemp pulp is a mixture of sisal pulp and flax pulp in a mass ratio of 3:

1. The polyester staple fiber has a length of 3-5 mm and a diameter of 15-20 μm. S2. Fiber pretreatment: For softwood pulp, a two-stage bleaching process of oxygen bleaching and hydrogen peroxide is adopted to control the post-bleaching brightness to be ≥85% ISO and the fiber length retention rate to be ≥90%. The hemp pulp was pretreated with a biological enzyme, wherein the biological enzyme was a mixture of xylanase and pectinase in a mass ratio of 2:1, the amount of enzyme used was 0.3% to 0.5% of the dry hemp pulp mass, the treatment temperature was 50-60℃, and the time was 60 to 90 min. Surface modification of polyester staple fiber was carried out by immersing it in a 1%–2% (w / w) ethanol solution of silane coupling agent KH-550 for 30–40 minutes, and then drying it for later use. S3. Mixing and pulping: Mix the three types of fibers after step S2, add deionized water to prepare a suspension with a pulp concentration of 3% to 5%, and use a double-disc refiner to pulp, controlling the freeness to 30-45°SR. S4. Additives: Add 1.2% to 1.8% nanocellulose, 0.8% to 1.2% cationic polyacrylamide dry strength agent, and 0.5% to 0.8% oxidized starch thickener by weight of oven-dry fiber to the slurry in step S3, and stir evenly. S5. Forming: The pulp is formed on a fourdrinier paper machine, and after pressing and drying, the base paper is obtained. The drying adopts a gradient heating method, with the temperature gradually increasing from 60℃ to 110℃, and the total drying time is 8 to 10 minutes.

2. The fiber composite reinforcement method for high-strength white kraft paper according to claim 1, characterized in that: The softwood pulp mentioned in step S1 is Nordic spruce pulp, the sisal pulp has an average fiber length of 3.5-4.2 mm, and the flax pulp has an average fiber length of 2.8-3.2 mm.

3. The fiber composite reinforcement method for high-strength white kraft paper according to claim 1, characterized in that: The oxygen bleaching process conditions described in step S2 are: oxygen pressure 0.6-0.8 MPa, temperature 90-100℃, time 60-80 min, H2O2 dosage in the hydrogen peroxide bleaching process is 3%-5% of the oven-dry pulp mass, and pH value is controlled at 10-11.

4. The fiber composite reinforcement method for high-strength white kraft paper according to claim 1, characterized in that: During the bio-enzyme pretreatment described in step S2, the slurry concentration is controlled at 8%–10%, and the pH value is adjusted to 4.5–5.

5.

5. The fiber composite reinforcement method for high-strength white kraft paper according to claim 1, characterized in that: The grinding disc gap of the double-disc refiner described in step S3 is 0.15-0.25mm, and the grinding power is 35-45kW.

6. The fiber composite reinforcement method for high-strength white kraft paper according to claim 1, characterized in that: The nanocellulose mentioned in step S4 is cellulose nanofibers with a length of 100-300 nm and a diameter of 5-10 nm, which are prepared by TEMPO oxidation.

7. The fiber composite reinforcement method for high-strength white kraft paper according to claim 1, characterized in that: The cationic polyacrylamide mentioned in step S4 has a molecular weight of 8 million to 12 million and an ionicity of 30% to 40%.

8. The fiber composite reinforcement method for high-strength white kraft paper according to claim 1, characterized in that: The pressing in step S5 uses a three-roll press with linear pressures of 150kN / m, 200kN / m, and 250kN / m respectively, and the dryness of the base paper after pressing is ≥40%.

9. The fiber composite reinforcement method for high-strength white kraft paper according to claim 1, characterized in that: The process also includes step S6: applying a sizing agent to the base paper obtained in step S5. The sizing agent is a styrene-acrylic emulsion with a mass fraction of 10% to 15%, and the sizing amount is 1.5 to 2.5 g / m². After sizing, the paper is dried again at 80-90°C for 2 to 3 minutes.

10. The fiber composite reinforcement method for high-strength white kraft paper according to claim 9, characterized in that: The styrene-acrylic emulsion mentioned in step S6 is copolymerized from styrene, butyl acrylate, and methacrylic acid in a mass ratio of 5:3:2, with a glass transition temperature of 25-30℃. After the surface modification of the polyester staple fiber in step S2, it also needs to be subjected to plasma treatment with a treatment power of 80-100W and a time of 1-2 minutes, using argon as the working gas.

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