Preparation method of special bamboo paper plastic pulp

Abstract

This invention provides a method for preparing a special pulp for paper and plastic made from moso bamboo, relating to the field of moso bamboo pulp technology. The invention includes: S1 raw material pretreatment and enzymatic hydrolysis for microporousization; S2 gradient-temperature, low-alkali, deep delignification cooking; S3 multi-stage countercurrent washing and targeted removal of impurities and cells; S4 ECF medium-concentration bleaching; S5 fiber surface charge regulation and point-bridge synergistic enhancement modification; S6 integrated waterproofing and oil-proofing treatment; and S7 molding and drying. This invention is a highly efficient, clean, and intelligent pulping method tailored to the characteristics of moso bamboo. Through physical-biological-chemical gradient penetration and targeted fiber surface modification, it prepares a low-dust, high-strength moso bamboo pulp suitable for hot pressing molding, specifically for paper and plastic production.

Description

Technical Field

[0001] This invention relates to the field of bamboo pulp technology, specifically to a method for preparing bamboo pulp for paper and plastic. Background Technology

[0002] Due to its thick cell walls (average thickness 8-12 μm) and dense structure, traditional sulfate pulping of moso bamboo presents challenges such as uneven chemical penetration, difficulties in cooking (requiring temperatures above 165℃ and NaOH concentration ≥18%), and high chemical consumption. Furthermore, moso bamboo pulp fibers have an average length of only 1.5-2.2 mm, with a high proportion of short, fine fibers (<0.5 mm fibers account for 15%-20%). When directly used in paper-plastic mold production, this results in insufficient bonding strength (tensile index <30 N·m / g), excessive dust during hot pressing (dust content >80 mm² / 500g), and rough product surfaces. Current technologies lack a dedicated preparation process for moso bamboo pulp used in paper-plastic molds, and the lignin in the pulping black liquor is typically burned, failing to achieve high-value utilization. Summary of the Invention

[0003] The purpose of this invention is to develop an efficient, clean, and intelligent pulping method for moso bamboo, which is based on the characteristics of moso bamboo. Through physical-biological-chemical gradient infiltration and directional modification of the fiber surface, a method for preparing moso bamboo paper-plastic pulp with low dust, high strength, and suitable for hot pressing is developed.

[0004] This invention is achieved through the following technical solution:

[0005] A method for preparing a special pulp for paper-plastic composite made of moso bamboo includes:

[0006] S1. Perform raw material pretreatment and enzymatic hydrolysis microporization;

[0007] S2. Perform gradient-heat, low-alkali, deep delignification cooking;

[0008] S3. Perform multi-stage countercurrent washing and targeted removal of impurities and cells;

[0009] S4. Perform ECF medium-concentration bleaching process;

[0010] S5. Perform fiber surface charge regulation and point-bridge synergistic reinforcement modification;

[0011] S6. Integrated waterproof and oil-proof treatment;

[0012] S7. Molding and drying;

[0013] In step S2, the bamboo strips processed in step S1 are sent into a DDS displacement cooking pot. Cooking liquid and cooking penetrant are added according to the weight of the absolutely dry bamboo strips. The cooking liquid and cooking penetrant are added before cooking for room temperature pre-soaking. The cooking process adopts a three-stage gradient heating process and is automatically controlled by the DCS system throughout. After cooking, the black liquor is discharged by displacement.

[0014] In step S3, the coarse pulp obtained in step S2 is sprayed into a spraying pot, and the fiber bundles are further dissociated by flash steam pressure. Then it is sent to a high linear pressure double roller extrusion double roller washing machine for five-stage countercurrent washing. The washing water adopts a closed circulation system. The washing discharges dilute black liquor. The washed pulp is diluted and then pumped to a counter-cone desander to remove light impurities. Then it is sent to a slotted pressure screen to remove impurities. Finally, it is sent to a micro flotation cell to remove bamboo impurities, cells and fine inert components.

[0015] In step S4, the pulp after step S3 is subjected to oxygen delignification, then washed, and then subjected to a first-stage chlorine dioxide bleaching, alkali treatment, and a second-stage chlorine dioxide bleaching. Then, it is reduced with sodium sulfite or sodium thiosulfate to eliminate residual chlorine dioxide. The pulp is then washed in three countercurrent stages and then purified twice by a slotted pressure screen to remove fiber bundles and fine impurities generated during the bleaching process.

[0016] In step S5, the bleached pulp from step S4 is concentrated by a double-net press and then sent to a high-consistency disc mill for pulping. After pulping, the pulp is transferred to a mixing tank, and cationic cellulose nanocrystals and cationic polyacrylamide are added sequentially under stirring conditions.

[0017] In step S6, a waterproof and oil-resistant additive is added online to the slurry modified in step S5, and then cationic colloidal silica is added online and mixed.

[0018] Optionally, the formula of the cooking liquid in step S2 includes 14%-16% NaOH, 3.8%-4.4% Na2S and 0.5%-1.0% Na2SO3, and their mass ratios are all based on absolutely dry bamboo chips.

[0019] The permeasurant accounts for 0.08%-0.12% of the total weight of the cooking liquid. The permeasurant consists of 28-32 parts by weight of fatty alcohol polyoxyethylene ether, 18-22 parts by weight of modified fatty alcohol polyoxyethylene ether phosphate, 8-10 parts by weight of fatty alcohol polyoxyethylene ether sodium sulfate, 6-8 parts by weight of sodium alkylbenzene sulfonate, 4-6 parts by weight of sodium pyrophosphate, 1.5-2.5 parts by weight of tetrasodium ethylenediaminetetraacetate, 0.8-1.2 parts by weight of anthraquinone, 0.2-0.4 parts by weight of polyoxypropylene glycerol ether, and 30-40 parts by weight of deionized water.

[0020] Optionally, in step S3, the washed slurry is diluted to a concentration of 2.5% ± 0.3%, and a reverse cone desander is used to remove light impurities with a density <1g / cm³. Then, it is sent to a 0.20mm slotted pressure screen to remove impurities with a size >0.2mm. Finally, it is sent to a micro flotation cell to remove bamboo impurities and fine inert components with a particle size <0.2mm and a density of 1.05-1.2g / cm³. The dust content of the slurry is ≤20mm² / 500g.

[0021] Optionally, in step S4, oxygen delignification is carried out at a temperature of 95–100°C and oxygen is introduced to a pressure of 0.6–0.8 MPa, with 2%–3% NaOH and 0.3% MgSO4 added by weight of the oven-dried pulp, and the reaction time is 60 min.

[0022] The first stage of chlorine dioxide bleaching is carried out at a temperature of 70-75℃, with 1.5%-2.0% ClO2 added by weight of oven-dry pulp, pH adjusted to 3.5-4.0, and kept at this temperature for 90 minutes.

[0023] Alkali treatment is carried out at a temperature of 60-65℃ by adding 1%-1.5% NaOH by weight of the oven-dry pulp and keeping it at that temperature for 60 minutes.

[0024] The two-stage chlorine dioxide bleaching process involves adding 0.5%–1.0% ClO2 by weight of the oven-dried pulp at a temperature of 70–75℃, adjusting the pH to 4.0, and bleaching for 90 minutes.

[0025] Optionally, in step S5, the amount of cationic cellulose nanocrystals added accounts for 0.25%-0.35% of the weight of the oven-dry fiber, and the addition method is in the form of a 0.5% concentration aqueous dispersion, which is slowly added over 5 minutes and stirred for 15 minutes.

[0026] The cationic polyacrylamide is added at a rate of 0.4%-0.5% of the weight of the oven-dry fiber. It is added in the form of a 0.1% aqueous solution, and then added evenly over 10 minutes, followed by stirring for another 30 minutes.

[0027] Optionally, in step S5, the method for preparing cationic cellulose nanocrystals includes:

[0028] Cellulose nanocrystals with a length of 100-300 nm and a diameter of 5-20 nm were obtained by hydrolyzing bleached bamboo pulp with 64 wt% sulfuric acid for 60 min. The nanocrystals were then reacted with 2,3-epoxypropyltrimethylammonium chloride in a mixed solvent of isopropanol and water at 60 °C for 4 h to obtain a quaternized modified product with a degree of substitution of 0.08 ± 0.02 and a zeta potential of +28 to +35 mV. The zeta potential was determined by a nanoparticle size analyzer.

[0029] Optionally, in step S6, the waterproof and oil-repellent additive formulation includes 0.9%-1.1% of a bio-based wax emulsion and 0.25%-0.3% of an alkyl ketene dimer by weight of the oven-dry fiber.

[0030] The bio-based wax emulsion has a solid content of 30%±2%, an average particle size of ≤0.5μm, and a melting point of 55-60℃. This emulsion is made from carnauba wax and rice bran wax, and has a bio-based content of ≥90%.

[0031] The alkyl ketene dimer has a solid content of 30% ± 2%, is emulsified with cationic starch, has a particle size ≤ 1 μm, and undergoes esterification reaction with hydroxyl groups on the fiber under neutral conditions.

[0032] Optionally, in step S7, the pulp processed in step S6 is diluted and fed into a paper-plastic forming machine. The forming section uses a special double-layer composite forming mesh, and the wet paper blank is vacuum-fled into the pressing section, where a two-roll press is used.

[0033] The drying process utilizes a three-layer mesh belt hot air drying line with zoned temperature control, including:

[0034] First drying zone, temperature 95-105℃, residence time 2min;

[0035] The second drying zone has a temperature of 115-125℃ and a residence time of 3 minutes.

[0036] The temperature in the third drying zone is 100-110℃, and the residence time is 2 minutes.

[0037] After drying, the pulp board is slit and packaged by an automatic paper cutter. The moisture content of the finished product is controlled at 9%±2%. Samples of each batch of products are taken for testing six key indicators, including tensile strength, burst strength, tear strength, viscosity, whiteness, and dust content, and the results are recorded in the quality traceability system.

[0038] Optionally, in step S1, the freshly cut bamboo is lightly de-greened and the surface cuticle is removed by mechanical brushing or soaking in 1% NaOH solution at room temperature for 20 minutes.

[0039] After shaving, the bamboo chips are monitored using an online moisture content sensor. They are then stored for 48-72 hours using a combination of spray moisturizing and shading film covering to maintain a stable moisture content of 45% ± 5%.

[0040] After the storage is completed, the bamboo strips are sent into a sealed reaction vessel. Acetic acid-sodium acetate buffer solution with pH 5.0-5.5 is added at a ratio of 1:3-4 based on the dry weight of the bamboo strips to completely submerge the bamboo strips. Then, a compound enzyme solution is added, which is a mixture of laccase and xylanase at a weight ratio of 1:2, with a total enzyme dosage of 30-40 U / g. The mixture is treated at 45±2℃ using an intermittent stirring mode with stirring for 5 minutes every 30 minutes for 6-8 hours. During the treatment, the pH is maintained constant using an online pH meter and acetic acid or NaOH is added dropwise.

[0041] After treatment, the bamboo strips are sprayed and rinsed with water at 80-85℃ for 15 minutes to completely inactivate the enzyme activity and wash away soluble degradation products and residual buffer solution. The strips are then drained until there are no obvious droplets on the surface.

[0042] Optionally, the black liquor discharged from the cooking process in step S2 and the dilute black liquor produced from the washing process in step S3 are used for lignin extraction and utilization. Specific steps include:

[0043] A. Black liquor pretreatment: After removing fiber debris by passing the black liquor and dilute black liquor through a vibrating screen, they are sent to a multi-effect evaporator for concentration to a solid content of 40%-45%;

[0044] B. Perform lignin acid precipitation. Transfer the concentrated black liquor to an acid precipitation tank equipped with a stirrer and pH meter. Add 10wt% dilute sulfuric acid at room temperature to adjust the pH to 2.0-2.5. Stir at 60 rpm for 30 min. The acid precipitation tank is equipped with a sealed gas collection hood and an alkaline absorption tower to absorb and treat H2S gas to meet emission standards. Lignin precipitates in the form of particles. After standing for 1 h, the supernatant is sent to a nanofiltration and reverse osmosis system to recover Na2SO4 concentrate. Na2SO4 concentrate is used to prepare cooking aid or to supplement sodium sulfate. Fresh water is reused for pulp washing.

[0045] C. Purify and dry the lignin. Wash the precipitate three times with a plate and frame filter press to obtain crude lignin, which is then dried by spray drying or vacuum oven drying, and then pulverized through a 100-mesh sieve to obtain refined lignin powder.

[0046] The beneficial effects of this invention are:

[0047] This invention features a physical-biological-chemical gradient permeation pretreatment system, which for the first time organically combines de-greening, precise moisture content control, and enzymatic hydrolysis in a buffer system. De-greening removes the dense waxy layer on the surface of moso bamboo, and enzymatic hydrolysis opens microporous channels under controllable pH and temperature, reducing the amount of chemicals used in subsequent cooking by 15%-20%, and lowering the maximum cooking temperature from the conventional 165℃ to 153±2℃, thus reducing fiber viscosity loss. This system forms a gradient permeation path from the outside to the inside, solving the problem of "external ripening and internal aging" caused by the thick wall of moso bamboo.

[0048] Based on research on the softening kinetics of bamboo cell walls, a three-stage gradient heating cooking process controlled by DCS was designed, consisting of three temperature platforms: "low-temperature wetting (115℃) - medium-temperature main delignification (140℃) - high-temperature deep delignification (153℃)". The residence time of each stage was optimized according to the characteristics of the bamboo species. With the addition of Na2SO3 selective additive, the pulp yield was increased by about 2.5-3.0 percentage points compared with the conventional process.

[0049] The reverse slag removal + dual-cationic "point-bridge" reinforcement fiber-oriented modification first applied the reverse conical slag remover to the directional removal of impurities and cells in bamboo paper-plastic pulp, solving the dust problem during hot pressing of paper-plastic products (dust amount reduced by ≥45%). At the same time, a dual-cationic synergistic reinforcement strategy of "C-CNC + CPAM" was proposed. C-CNC anchors short and fine fibers (<0.5mm), and CPAM bridges long fibers with anchor points to form a three-dimensional network. After testing, the tensile index of pulp with a high proportion of short and fine fibers (<0.5mm accounts for 20%) after this modification can be increased from 28 N·m / g to more than 42 N·m / g.

[0050] The matching design of the forming mesh and the three-stage hot air drying curve is designed to address the characteristics of bamboo short and fine fibers that are prone to clogging the mesh and the need for high-temperature curing of AKD. A double-layer composite forming mesh (open area ratio 32%-35%, Ra≤0.6μm) and a three-stage drying temperature curve (95-105℃ preheating → 115-125℃ curing → 100-110℃ equilibration) are designed. The temperature of the second zone is precisely controlled between 115-125℃ to ensure AKD curing (≥115℃) and avoid wax emulsion oxidation (≤125℃).

[0051] This invention enables the high-value extraction and closed-loop application of lignin from black liquor. In the traditional sulfate pulping process, black liquor is usually directly burned in the alkali recovery furnace. This invention extracts high-purity lignin (≥92%) from it and reuses it as a bio-based polyol substitute or wet-end additive in the process. This realizes a closed-loop circular economy model from bamboo, pulping, lignin extraction to modified paper and plastic materials. This not only reduces the load on the alkali recovery furnace, but also provides a new type of bio-based additive for paper and plastic materials, significantly improving the utilization value of all components. Detailed Implementation

[0052] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of the invention. Therefore, the following description is to be considered exemplary in nature and not restrictive.

[0053] The embodiments of the present invention will be described in detail below.

[0054] This invention discloses a method for preparing a special pulp for paper-plastic composite made of moso bamboo, comprising the following steps:

[0055] S1. Raw material pretreatment and enzymatic hydrolysis microporization

[0056] Freshly cut bamboo is lightly de-greened and then soaked in a 1% NaOH solution at room temperature for 20 minutes to remove the surface cuticle (only the hard bamboo green cuticle is removed, about 0.1 mm thick), in order to improve the penetration of the medicine without damaging the fiber layer.

[0057] The bamboo slices are shaved to a length of 15-25mm and a thickness of 3-5mm. After shaving, the bamboo slices are monitored by an online moisture sensor. They are then stored for 48-72 hours by spraying to keep them moist and by covering them with a shading film to control the moisture content of the bamboo slices to be stable at 45%±5% in order to avoid surface crusting and internal hardening, and to prevent mold growth.

[0058] After the stacking is completed, the bamboo chips are sent to a sealed reaction vessel equipped with an automatic temperature control and intermittent stirring device. A breather valve and an exhaust device are installed on the top of the sealed reaction vessel to control the pressure inside the vessel to ≤0.05MPa. Acetic acid-sodium acetate buffer solution with pH=5.0-5.5 is added at a ratio of 1:3-4 based on the oven-dry weight of the bamboo chips to completely submerge the bamboo chips. Then, a compound enzyme solution is added. This compound enzyme solution is a mixture of laccase (activity ≥1000 U / mL) and xylanase (activity ≥2000 U / mL) at a weight ratio of 1:2, with a total enzyme dosage of 30-40 U / g (oven-dry bamboo chips). The mixture is treated at 45±2℃ using an intermittent stirring mode with stirring for 5 minutes every 30 minutes for 6-8 hours. During the treatment, the pH is kept constant by an online pH meter, and 0.1M acetic acid or 0.1M NaOH is automatically added dropwise.

[0059] After treatment, the bamboo strips are rinsed with water at 80-85℃ for 15 minutes to completely inactivate the enzyme activity and wash away soluble degradation products and residual buffer solution. They are then drained until there are no obvious droplets on the surface and set aside for later use.

[0060] S2. Gradient temperature increase - low-alkali deep delignification cooking

[0061] The bamboo chips processed in step S1 are fed into the DDS super displacement cooking pot. Cooking solution is added according to the weight of the oven-dried bamboo chips, with a solution ratio of 1:3.5-4.0 (mass ratio). The cooking solution formula includes 14%-16% NaOH, 3.8%-4.4% Na2S, and 0.5%-1.0% Na2SO3 (as an auxiliary delignification agent to improve selectivity). The mass ratios of NaOH, Na2S, and Na2SO3 are all based on the oven-dried bamboo chips, that is, their mass accounts for 14%-16%, 3.8%-4.4%, and 0.5%-1.0% of the oven-dried bamboo chips, respectively.

[0062] A cooking and penetrating agent is also added, accounting for 0.08%-0.12% of the total weight of the cooking liquid.

[0063] The penetrant composition includes 28-32 parts by weight of fatty alcohol polyoxyethylene ether, 18-22 parts by weight of modified fatty alcohol polyoxyethylene ether phosphate, 8-10 parts by weight of fatty alcohol polyoxyethylene ether sodium sulfate, 6-8 parts by weight of sodium alkylbenzene sulfonate, 4-6 parts by weight of sodium pyrophosphate, 1.5-2.5 parts by weight of tetrasodium ethylenediaminetetraacetate, 0.8-1.2 parts by weight of anthraquinone, 0.2-0.4 parts by weight of polyoxypropylene glycerol ether, and 30-40 parts by weight of deionized water.

[0064] The cooking solution and cooking penetrant are added for room temperature pre-soaking before cooking. The cooking process employs a three-stage gradient heating process, and the entire process is automatically controlled by a DCS system, including:

[0065] First stage: Raise the temperature from room temperature to 115±2℃ within 30 minutes and maintain the temperature for 40±5 minutes. This stage allows the medicinal solution to fully wet the bamboo strips and initially penetrate them, avoiding a violent reaction that could lead to overcooking of the surface.

[0066] Second stage: Raise to 140±2℃ within 30 minutes and hold for 50±5 minutes; This stage initiates the main lignin removal reaction, and about 60%-70% of the lignin is removed in this stage;

[0067] The third stage involves raising the temperature to 153±2℃ within 20 minutes and holding it at that temperature for 75±15 minutes. This stage completes deep delignification and controls the residual lignin content at the endpoint to be 4%-6% (for oven-dried pulp).

[0068] The residual alkali (calculated as Na2O) at the end of the cooking process is controlled to be 5-8 g / L, and the pH of the black liquor is ≥12. After cooking, the black liquor is discharged to the black liquor storage tank by displacement for subsequent lignin extraction.

[0069] S3. Multi-stage countercurrent washing and targeted removal of contaminants and cells

[0070] The coarse pulp obtained in step S2 is sprayed into a spraying pot, and the fiber bundles are further dissociated by flash steam pressure of 0.6-0.8MPa. Then it is sent into a high linear pressure double roller extrusion double roller washing machine for five-stage countercurrent washing. The washing water adopts a closed circulation system, and the fresh water consumption is ≤8m³ / ton pulp. The dilute black liquor discharged from the washing is used for subsequent lignin extraction.

[0071] Main parameters of the double roller washing machine:

[0072] Linear voltage: 35-40kN / m;

[0073] Speed: 8-12 rpm;

[0074] Slurry dryness after washing: ≥36%;

[0075] Residual alkali content: ≤0.25%;

[0076] Electrical conductivity: ≤800μS / cm (a decrease of ≥65% compared to before washing);

[0077] The washed slurry is diluted to a concentration of 2.5% ± 0.3% and pumped to a reverse cone separator to remove light impurities with a density <1g / cm³. It is then sent to a 0.20mm slotted pressure screen to remove impurities >0.2mm in size, and finally to a micro-flotation tank to remove bamboo debris cells and fine inert components with a particle size <0.2mm and a density of 1.05-1.2g / cm³. After this treatment, the dust content of the slurry (tested according to GB / T10740, 0.5-5.0mm²) is ≤20mm² / 500g, a decrease of more than 45% compared to the untreated slurry. Separator parameters:

[0078] Slurry inlet pressure: 270±20kPa;

[0079] Pressure difference: 100±10kPa;

[0080] Bottom flow rate: 12%-15%;

[0081] Single-unit throughput: 8-12 m³ / h.

[0082] S4.ECF medium-concentration bleaching process

[0083] After step S3, the pulp undergoes oxygen delignification. Under conditions of 95–100℃ and oxygen purging to a pressure of 0.6–0.8 MPa, 2%–3% NaOH and 0.3% MgSO4 by weight of the oven-dry pulp are added. The reaction time is 60 min, the delignification rate is about 40%–50%, and the brightness increases to 55%–60%.

[0084] Then, after washing, a first-stage chlorine dioxide bleaching process is carried out. At a temperature of 70-75℃, 1.5%-2.0% of the weight of oven-dried pulp ClO2 is added, the pH is adjusted to 3.5-4.0, and the mixture is kept at this temperature for 90 minutes, which increases the whiteness to 75%-80%.

[0085] Next, after washing, an alkali treatment is performed. At a temperature of 60-65℃, 1%-1.5% NaOH by weight of the oven-dried pulp is added and kept at this temperature for 60 minutes to dissolve the residual lignin.

[0086] The pulp is washed and then subjected to a two-stage chlorine dioxide bleaching process. At a temperature of 70–75℃, 0.5%–1.0% of the dry weight of ClO2 is added, the pH is adjusted to 4.0, and the time is 90 minutes. The final whiteness is controlled at 85%–87%.

[0087] After bleaching, the pulp is reduced with sodium sulfite or sodium thiosulfate to remove residual chlorine dioxide. The pulp is then washed in three countercurrent stages. After washing, the residual chlorine is ≤5mg / L and the conductivity is ≤500μS / cm. It is then purified a second time by a 0.15mm slotted pressure screen to remove fiber bundles and fine impurities generated during the bleaching process.

[0088] S5. Fiber surface charge regulation and "point-bridge" synergistic reinforcement modification

[0089] The bleached pulp from step S4 is concentrated to a concentration of 24%–28% using a double-mesh press, and then fed into a high-consistency disc mill (the discs are made of high-chromium cast iron with sparse teeth) for light beating. The beating parameters are as follows:

[0090] Grinding disc clearance: 0.20-0.25mm;

[0091] Pulping energy consumption: 40-60 kWh / ton of pulp;

[0092] Freeness: 30±2°SR (Schober process);

[0093] Wet weight: 2.5-3.5g (to reflect fiber length retention);

[0094] Pulping principles: Focus on fiber splitting and avoid excessive cutting; fiber length retention rate ≥85%;

[0095] After pulping, the pulp is transferred to a mixing tank (equipped with a slow-speed agitator and a temperature-controlled jacket). The temperature is adjusted to 35±2℃, and the pH is left to stand naturally (approximately 6.5-7.5, no adjustment required). Cationic cellulose nanocrystals (C-CNC) and cationic polyacrylamide (CPAM) are added sequentially under stirring conditions. The preparation method of cationic cellulose nanocrystals is as follows:

[0096] Cellulose nanocrystals (CNC, length 100-300 nm, diameter 5-20 nm) were obtained by hydrolyzing bleached bamboo pulp with 64 wt% sulfuric acid for 60 min. Then, they were reacted with 2,3-epoxypropyltrimethylammonium chloride (EPTMAC) in isopropanol / water mixed solvent at 60 °C for 4 h to obtain a quaternized modified product with a degree of substitution (DS) of 0.08 ± 0.02 and a zeta potential of +28 to +35 mV. The zeta potential was determined by a nanoparticle size analyzer. The addition amount was 0.25%-0.35% of the oven-dry fiber weight. The addition method was to add the product slowly over 5 minutes in the form of a 0.5% concentration aqueous dispersion and stir for 15 min.

[0097] The cationic polyacrylamide (CPAM) has the following specifications: molecular weight 9,000,000,000, degree of ionization 20% ± 3%, solid content ≥ 90%, and the addition amount accounts for 0.4%-0.5% of the weight of the oven-dry fiber. The addition method is to add it evenly within 10 minutes in the form of a 0.1% concentration aqueous solution and continue stirring for 30 minutes.

[0098] Due to its high aspect ratio (>20) and strong positive charge, C-CNC preferentially electrostatically adsorbs onto the surface of negatively charged short and fine fibers (length <0.5mm) and microfibers, forming spatially stable "anchor points." CPAM, with its long molecular chains, builds flexible "bridges" between these "anchor points" and between long fibers, forming a three-dimensional reinforcing network. This "point-bridge" synergistic structure increases the hydrogen bond density in the fiber bonding region by about 30%, which macroscopically manifests as a significant improvement in tensile index and bursting index.

[0099] S6. Waterproof and oil-proof integrated treatment

[0100] Add a waterproof and oil-resistant additive online to the slurry modified in step 5, and then add 0.05%–0.08% cationic colloidal silica (such as Eka NP series) online. The addition point is located on the outlet pipe of the slurry mixing tank, and the mixture is achieved by an online high-shear mixer (stator-rotor gap 0.5mm).

[0101] Additive formulation (as a percentage of oven-dry fiber weight):

[0102] Bio-based wax emulsion: 0.9%-1.1%, solid content 30%±2%, average particle size ≤0.5μm, melting point 55-60℃. This emulsion is prepared by high-pressure homogenization using carnauba wax and rice bran wax as raw materials, and has a bio-based content ≥90%.

[0103] Alkyl ketene dimer (AKD): 0.25%–0.3%, solid content 30%±2%, emulsified with cationic starch, particle size ≤1μm. AKD undergoes esterification reaction with hydroxyl groups on fibers under neutral conditions to form stable β-keto ester bonds, giving the paper internal water resistance.

[0104] High shear mixer parameters: rotation speed 3500–4500 rpm, residence time 5~10s, outlet slurry temperature ≤45℃, and additive retention rate in the treated slurry ≥85%.

[0105] S7. Molding and Drying

[0106] The pulp processed in step S6 is diluted to a pulp consistency of 0.8% ± 0.2% and fed into a paper-plastic forming machine (or a long-wire paper machine, selected according to the shape of the paper-plastic substrate). The forming section uses a specially made double-layer composite forming wire. Technical parameters:

[0107] Material: Polyester (PET) and Nylon 66 composite material, warp 0.25mm, weft 0.20mm;

[0108] Weaving structure: Double-layered weave, with a fine mesh (60 mesh) on the surface and a coarse mesh (30 mesh) on the bottom.

[0109] Open area ratio: 32%-35%, which is about 7 percentage points higher than that of ordinary papermaking wire (25%-28%);

[0110] Surface smoothness: Ra≤0.6μm (measured by optical profilometer);

[0111] Breathability: 2500–3500 CFM (cubic feet per square foot per minute);

[0112] This forming mesh can effectively reduce the loss of short and fine fibers and mesh clogging, thus improving the uniformity of wet paper sheet forming;

[0113] The wet paper blank is transferred under vacuum (vacuum degree -40 to -50 kPa) and then enters the press section. It is pressed using a two-roll press with linear pressure of 45±5 kN / m for the first press and 60±5 kN / m for the second press. The dryness after pressing is ≥42%.

[0114] The drying process utilizes a three-layer mesh belt hot air drying line (18m in length, 2.5m in effective width), with zoned temperature control, including:

[0115] First drying zone (preheating zone): temperature 95-105℃, residence time about 2 minutes, mainly removes most of the free water, and the paper dryness increases from 42% to about 70%;

[0116] Second drying zone (curing zone): temperature 115-125℃, residence time about 3 minutes. In this stage, AKD undergoes esterification reaction with the fiber at high temperature to complete curing. The wax emulsion melts and spreads to form a continuous hydrophobic film. The temperature is controlled at ≥115℃ to ensure the AKD reaction rate, and the temperature is ≤125℃ to avoid wax oxidation.

[0117] The third drying zone (equilibrium zone): temperature 100-110℃, residence time about 2 minutes, to balance the moisture of the whole width, eliminate internal stress and prevent curling;

[0118] After drying, the pulp board is cut and packaged by an automatic paper cutter. The moisture content of the finished product is controlled at 9%±2%. Samples of each batch of products are taken to test six key indicators (tensile strength, burst strength, tear strength, viscosity, whiteness, and dust content) and recorded in the quality traceability system.

[0119] By-product resource utilization

[0120] The black liquor discharged from the cooking process in step S2 and the dilute black liquor produced from the washing process in step S3 are used for lignin extraction and high-value utilization. The specific steps are as follows:

[0121] A. Black liquor pretreatment

[0122] After removing fiber debris by passing black liquor (pH≈13) and dilute black liquor through a vibrating screen, they are sent to a multi-effect evaporator to concentrate to a solid content of 40%-45%.

[0123] B. Lignin acid precipitation

[0124] The concentrated black liquor was transferred to an acid precipitation tank equipped with a stirrer and a pH meter. 10wt% dilute sulfuric acid was slowly added at room temperature to adjust the pH to 2.0-2.5. The stirring speed was 60 rpm and the sedimentation time was 30 min. The acid precipitation tank was equipped with a sealed gas collection hood and an alkaline absorption tower to absorb and treat the H2S gas to meet emission standards. Lignin was precipitated in the form of particles. After standing for 1 hour, the supernatant (mainly containing Na2SO4 and residual alkali) was sent to a nanofiltration (NF) and reverse osmosis (RO) system to recover the Na2SO4 concentrate. The Na2SO4 concentrate was used to prepare cooking aids or to supplement sodium sulfate. The fresh water was reused for pulp washing to achieve zero discharge.

[0125] C. Lignin purification and drying

[0126] The precipitate was washed three times by a plate and frame filter press to obtain crude lignin with a moisture content of about 60%. It was then dried by spray drying or vacuum oven to a moisture content of ≤5%, and pulverized through a 100-mesh sieve to obtain refined lignin powder.

[0127] This lignin possesses high purity (≥92%, determined by Klason method), low ash content (≤2%), and abundant active groups, and can be used for:

[0128] Refined lignin is reacted with ethylene glycol at 120°C under the action of a catalyst for 2 hours to increase the hydroxyl content to 6%-8%, and then used to replace polyols.

[0129] As a rubber reinforcing agent or asphalt modifier;

[0130] The refined lignin is modified into cationic lignin (quaternization modification) and then used as a wet-end additive.

[0131] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the technical solutions of the present invention. Any technical solution that can be implemented based on the above embodiments without creative effort should be considered to fall within the scope of protection of the patent of the present invention.

Claims

1. A method for preparing a special bamboo paper pulp, characterized in that, include: S1. Perform raw material pretreatment and enzymatic hydrolysis microporization; S2. Perform gradient-heat, low-alkali, deep delignification cooking; S3. Perform multi-stage countercurrent washing and targeted removal of impurities and cells; S4. Perform ECF medium-concentration bleaching process; S5. Perform fiber surface charge regulation and point-bridge synergistic reinforcement modification; S6. Integrated waterproof and oil-proof treatment; S7. Molding and drying; In step S2, the bamboo strips processed in step S1 are sent into a DDS displacement cooking pot. Cooking liquid and cooking penetrant are added according to the weight of the absolutely dry bamboo strips. The cooking liquid and cooking penetrant are added before cooking for room temperature pre-soaking. The cooking process adopts a three-stage gradient heating process and is automatically controlled by the DCS system throughout. After cooking, the black liquor is discharged by displacement. In step S3, the coarse pulp obtained in step S2 is sprayed into a spraying pot, and the fiber bundles are further dissociated by flash steam pressure. Then it is sent to a high linear pressure double roller extrusion double roller washing machine for five-stage countercurrent washing. The washing water adopts a closed circulation system. The washing discharges dilute black liquor. The washed pulp is diluted and then pumped to a counter-cone desander to remove light impurities. Then it is sent to a slotted pressure screen to remove impurities. Finally, it is sent to a micro flotation cell to remove bamboo impurities, cells and fine inert components. In step S4, the pulp after step S3 is subjected to oxygen delignification, then washed, and then subjected to a first-stage chlorine dioxide bleaching, alkali treatment, and a second-stage chlorine dioxide bleaching. Then, it is reduced with sodium sulfite or sodium thiosulfate to eliminate residual chlorine dioxide. The pulp is then washed in three countercurrent stages and then purified twice by a slotted pressure screen to remove fiber bundles and fine impurities generated during the bleaching process. In step S5, the bleached pulp from step S4 is concentrated by a double-net press and then sent to a high-consistency disc mill for pulping. After pulping, the pulp is transferred to a mixing tank, and cationic cellulose nanocrystals and cationic polyacrylamide are added sequentially under stirring conditions. In step S6, a waterproof and oil-resistant additive is added online to the slurry modified in step S5, and then cationic colloidal silica is added online and mixed.

2. The method for preparing bamboo pulp for paper and plastic as described in claim 1, characterized in that, The formula for the cooking liquid in step S2 includes 14%-16% NaOH, 3.8%-4.4% Na2S and 0.5%-1.0% Na2SO3, and their mass ratios are all based on absolutely dry bamboo chips. The permeasurant accounts for 0.08%-0.12% of the total weight of the cooking liquid. The permeasurant consists of 28-32 parts by weight of fatty alcohol polyoxyethylene ether, 18-22 parts by weight of modified fatty alcohol polyoxyethylene ether phosphate, 8-10 parts by weight of fatty alcohol polyoxyethylene ether sodium sulfate, 6-8 parts by weight of sodium alkylbenzene sulfonate, 4-6 parts by weight of sodium pyrophosphate, 1.5-2.5 parts by weight of tetrasodium ethylenediaminetetraacetate, 0.8-1.2 parts by weight of anthraquinone, 0.2-0.4 parts by weight of polyoxypropylene glycerol ether, and 30-40 parts by weight of deionized water.

3. The method for preparing bamboo pulp for paper and plastic as described in claim 1, characterized in that, In step S3, the washed slurry is diluted to a concentration of 2.5% ± 0.3%, and a reverse cone desander is used to remove light impurities with a density <1g / cm³. The slurry is then sent to a 0.20mm slotted pressure screen to remove impurities with a size >0.2mm, and finally sent to a micro flotation cell to remove bamboo impurities and fine inert components with a particle size <0.2mm and a density of 1.05-1.2g / cm³. The dust content of the slurry is ≤20mm² / 500g.

4. The method for preparing bamboo-based paper-plastic pulp according to claim 1, characterized in that, In step S4, oxygen delignification is carried out at a temperature of 95–100℃ and oxygen is introduced to a pressure of 0.6–0.8 MPa, with 2%–3% NaOH and 0.3% MgSO4 added by weight of the oven-dried pulp, and the reaction time is 60 min. The first stage of chlorine dioxide bleaching is carried out at a temperature of 70-75℃, with 1.5%-2.0% ClO2 added by weight of oven-dry pulp, pH adjusted to 3.5-4.0, and kept at this temperature for 90 minutes. Alkali treatment is carried out at a temperature of 60-65℃ by adding 1%-1.5% NaOH by weight of the oven-dry pulp and keeping it at that temperature for 60 minutes. The two-stage chlorine dioxide bleaching process involves adding 0.5%–1.0% ClO2 by weight of the oven-dried pulp at a temperature of 70–75℃, adjusting the pH to 4.0, and bleaching for 90 minutes.

5. The method for preparing bamboo pulp for paper and plastic as described in claim 1, characterized in that, In step S5, the amount of cationic cellulose nanocrystals added accounts for 0.25%-0.35% of the weight of the oven-dry fiber. The addition method is in the form of a 0.5% concentration aqueous dispersion, which is added within 5 minutes and stirred for 15 minutes. The cationic polyacrylamide is added at a rate of 0.4%-0.5% of the weight of the oven-dry fiber. It is added in the form of a 0.1% aqueous solution, and then added evenly over 10 minutes, followed by stirring for another 30 minutes.

6. The method for preparing bamboo pulp for paper and plastic as described in claim 1, characterized in that, In step S5, the method for preparing cationic cellulose nanocrystals includes: Cellulose nanocrystals with a length of 100-300 nm and a diameter of 5-20 nm were obtained by hydrolyzing bleached bamboo pulp with 64 wt% sulfuric acid for 60 min. The nanocrystals were then reacted with 2,3-epoxypropyltrimethylammonium chloride in a mixed solvent of isopropanol and water at 60 °C for 4 h to obtain a quaternized modified product with a degree of substitution of 0.08 ± 0.02 and a zeta potential of +28 to +35 mV. The zeta potential was determined by a nanoparticle size analyzer.

7. The method for preparing bamboo pulp for paper and plastic as described in claim 1, characterized in that, In step S6, the waterproof and oil-repellent additive formulation includes 0.9%-1.1% of bio-based wax emulsion and 0.25%-0.3% of alkyl ketene dimer by weight of the oven-dry fiber. The bio-based wax emulsion has a solid content of 30%±2%, an average particle size of ≤0.5μm, and a melting point of 55-60℃. This emulsion is made from carnauba wax and rice bran wax, and has a bio-based content of ≥90%. The alkyl ketene dimer has a solid content of 30% ± 2%, is emulsified with cationic starch, has a particle size ≤ 1 μm, and undergoes esterification reaction with hydroxyl groups on the fiber under neutral conditions.

8. The method for preparing bamboo-based paper-plastic pulp according to claim 1, characterized in that, In step S7, the pulp processed in step S6 is diluted and fed into a paper-plastic forming machine. The forming section uses a special double-layer composite forming mesh. The wet paper blank is vacuum-fed and then enters the pressing section, where it is pressed by two rollers. The drying process utilizes a three-layer mesh belt hot air drying line with zoned temperature control, including: First drying zone, temperature 95-105℃, residence time 2min; The second drying zone has a temperature of 115-125℃ and a residence time of 3 minutes. The temperature in the third drying zone is 100-110℃, and the residence time is 2 minutes. After drying, the pulp board is slit and packaged by an automatic paper cutter. The moisture content of the finished product is controlled at 9%±2%. Samples of each batch of products are taken for testing six key indicators, including tensile strength, burst strength, tear strength, viscosity, whiteness, and dust content, and the results are recorded in the quality traceability system.

9. The method for preparing bamboo-based paper-plastic pulp according to claim 1, characterized in that, In step S1, the freshly cut bamboo is lightly de-greened and the surface cuticle is removed by mechanical brushing or soaking in 1% NaOH solution at room temperature for 20 minutes. After shaving, the bamboo chips are monitored using an online moisture content sensor. They are then stored for 48-72 hours using a combination of spray moisturizing and shading film covering to maintain a stable moisture content of 45% ± 5%. After the storage is completed, the bamboo strips are sent into a sealed reaction vessel. Acetic acid-sodium acetate buffer solution with pH 5.0-5.5 is added at a ratio of 1:3-4 based on the dry weight of the bamboo strips to completely submerge the bamboo strips. Then, a compound enzyme solution is added, which is a mixture of laccase and xylanase at a weight ratio of 1:2, with a total enzyme dosage of 30-40 U / g. The mixture is treated at 45±2℃ using an intermittent stirring mode with stirring for 5 minutes every 30 minutes for 6-8 hours. During the treatment, the pH is maintained constant using an online pH meter and acetic acid or NaOH is added dropwise. After treatment, the bamboo strips are sprayed and rinsed with water at 80-85℃ for 15 minutes to completely inactivate the enzyme activity and wash away soluble degradation products and residual buffer solution. The strips are then drained until there are no obvious droplets on the surface.

10. The method for preparing bamboo-based paper-plastic pulp according to any one of claims 1 to 9, characterized in that, The black liquor discharged from the cooking process in step S2 and the dilute black liquor produced from the washing process in step S3 are used for lignin extraction and utilization. The specific steps include: A. Black liquor pretreatment: After removing fiber debris by passing the black liquor and dilute black liquor through a vibrating screen, they are sent to a multi-effect evaporator for concentration to a solid content of 40%-45%; B. Perform lignin acid precipitation. Transfer the concentrated black liquor to an acid precipitation tank equipped with a stirrer and pH meter. Add 10wt% dilute sulfuric acid at room temperature to adjust the pH to 2.0-2.

5. Stir at 60 rpm for 30 min. The acid precipitation tank is equipped with a sealed gas collection hood and an alkaline absorption tower to absorb and treat H2S gas to meet emission standards. Lignin precipitates in the form of particles. After standing for 1 h, the supernatant is sent to a nanofiltration and reverse osmosis system to recover Na2SO4 concentrate. Na2SO4 concentrate is used to prepare cooking aid or to supplement sodium sulfate. Fresh water is reused for pulp washing. C. Purify and dry the lignin. Wash the precipitate three times with a plate and frame filter press to obtain crude lignin, which is then dried by spray drying or vacuum oven drying, and then pulverized through a 100-mesh sieve to obtain refined lignin powder.