A method for preparing a nanofibrillated cellulose-based filter material

Abstract

The application discloses a preparation method of nanofibrillated cellulose-based filter material, and steps are as follows: taking bleached coniferous wood pulp as raw material, carrying out pretreatment through biological enzymolysis and mechanical refining, and preparing nanofibrillated cellulose suspension by using a microjet high-pressure homogenizer; mixing the nanofibrillated cellulose suspension with plant fiber pulp according to an absolute dry mass ratio of 1:9-5:5, and forming a gradient pore structure base paper through a double-layer or multi-layer inclined screen paper machine; preparing modified styrene-acrylic emulsion by using a pre-emulsified semi-continuous emulsion polymerization method with acrylic acid, butyl acrylate and styrene as basic monomers and with functional monomers; impregnating and sizing the base paper, controlling the sizing amount to be 5-20 g / m 2 , and obtaining the filter material after drying and solidification. The application has stable and controllable process, and the obtained product has high strength, high filtration precision, good air permeability and water resistance, and can be completely biodegraded, so that the product is environment-friendly and efficient.

Description

Technical Field

[0001] This invention relates to the field of papermaking and filtration materials technology, and in particular to a method for preparing a nanofibrillated cellulose-based filtration material. Background Technology

[0002] Filter materials are widely used in food, pharmaceutical, environmental protection, and industrial production. Among them, paper-based filter materials have attracted widespread attention due to their advantages such as renewable raw materials, low cost, and biodegradability. Traditional paper-based filter materials, such as coffee filter paper and air filter paper, are usually made from plant fibers and have problems such as loose structure, low physical strength, uneven pore size distribution, and poor filtration accuracy.

[0003] In recent years, nanofibrillated cellulose (CNF) has been used to enhance the performance of filter materials due to its high specific surface area, high aspect ratio, excellent mechanical properties, and biocompatibility. In existing technologies, CNF is mainly prepared via high-pressure homogenization, but its application in filter materials is mostly limited to simple physical blending, failing to fully utilize the gradient filling effect and surface regulation advantages of CNF. Furthermore, the sizing systems of existing filter materials are mostly traditional styrene-acrylic emulsions, which have limited water resistance and reinforcing effects; and the single-layer structure makes it difficult to achieve gradient control of pore size, resulting in a trade-off between filtration accuracy and air permeability. On the other hand, lignin, as a natural component of plant fibers, has good UV absorption and anti-aging properties, but it is rarely used in conjunction with CNF in the gradient structure design of filter materials in existing technologies. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing nanofibrillated cellulose-based filter materials. The filter material obtained by this invention has the advantages of high strength, high filtration accuracy, good air permeability, water resistance, and biodegradability.

[0005] The technical solution of the present invention: a method for preparing a nanofibrillated cellulose-based filter material, comprising the following steps: Step 1: Using bleached softwood pulp as raw material, after pretreatment by bio-enzymatic hydrolysis and mechanical fine grinding, the pulp is homogenized 2-20 times at a pressure of 10000-20000 psi using a micro-jet high-pressure homogenizer to obtain a nano-microfibrillated cellulose suspension. Step 2: Mix the nanofibrillated cellulose obtained in Step 1 with plant fiber pulp with a freeness of 16-30°SR at an oven-dry mass ratio of 1:9 to 5:5, and use a slanted wire paper machine to form double or multiple layers, forming a gradient pore structure in which the surface layer contains a higher proportion of nanofibrillated cellulose than the middle or bottom layer. After pressing and drying, the base paper is obtained. Step 3: Using acrylic acid, butyl acrylate and styrene as basic monomers, and glycidyl methacrylate, methacrylic acid and N-hydroxymethylacrylamide as functional monomers, a modified styrene-acrylic emulsion with a solid content of 20-40% is prepared by pre-emulsification semi-continuous emulsion polymerization. Step 4: Impregnate the base paper obtained in Step 2 into the modified styrene-acrylic emulsion prepared in Step 3, control the sizing amount to be 5-20 g / m², and after drying and curing, obtain the nanofibrillated cellulose-based filter material.

[0006] In the above preparation method, the pressure of microjets for high-pressure homogenization in step one is 15000 psi, and the number of homogenization cycles is 5-10. The resulting nanofibrillated cellulose has an average diameter of 20-100 nm and an average length of 500-2000 nm.

[0007] In the aforementioned preparation method, the plant fiber pulp in step two includes a mixed pulp of bleached softwood pulp and bleached hardwood pulp, wherein the oven-dry mass ratio of softwood pulp to hardwood pulp is 2:8 to 5:5, and the freeness of the softwood pulp is 16-25°SR, while the freeness of the hardwood pulp is 21-30°SR.

[0008] In the aforementioned preparation method, in the double-layer or multi-layer papermaking process described in step two, the amount of nanofibrillated cellulose added to the surface layer is 30-50% of the total amount of oven-dry fibers, and the amount added to the middle or bottom layer is 1-10%, forming a gradient structure with increasing porosity and pore size from top to bottom.

[0009] In the aforementioned preparation method, the functional monomers in step three account for 5-15% of the total mass of the monomers, wherein the mass ratio of glycidyl methacrylate, methacrylic acid and N-hydroxymethylacrylamide is 2:1:1 to 4:2:1.

[0010] In the aforementioned preparation method, the impregnation and sizing in step four is carried out by double-sided coating or impregnation extrusion. The quantitative amount of the material after sizing is 30-70 g / m², and the thickness is 0.10-0.15 mm.

[0011] In the aforementioned preparation method, the nanofibrillated cellulose suspension in step one is subjected to cationization modification: a cationic etherifying agent is added, and the mixture is reacted for 1-3 hours at pH 10-12 and temperature 60-80℃ to obtain cationic nanofibrillated cellulose. When the cationic nanofibrillated cellulose is mixed with plant fiber pulp, the fiber binding force is enhanced through electrostatic adsorption, and the surface of the filter material is positively charged to adsorb negatively charged microparticles.

[0012] In the aforementioned preparation method, the multilayer papermaking in step two consists of a three-layer structure: a surface layer, a core layer, and a bottom layer. The surface layer is composed of nanofibrillated cellulose and softwood pulp in a mass ratio of 4:6 to 6:4, with an average pore size ≤10μm. The core layer is composed of hardwood pulp and nanofibrillated cellulose in a mass ratio of 9:1 to 7:3, with an average pore size of 15-30μm. The bottom layer is composed of unbleached softwood pulp and nanofibrillated cellulose in a mass ratio of 8:2 to 6:4, with an average pore size of 30-50μm. Furthermore, the lignin in the bottom layer imparts UV blocking and anti-aging properties to the filter material.

[0013] The aforementioned preparation method further includes a hot pressing and calendering step after step four: the impregnated and sizing material is hot pressed and calendered at 80-120℃ and a linear pressure of 10-30 kN / m to improve the surface smoothness of the material to ≥100 s and reduce the air permeability by no more than 15%, while improving the filtration accuracy and tensile strength.

[0014] In the aforementioned preparation method, the filter material is a coffee filter material with a quantitative content of 50-70 g / m² and a longitudinal wet tensile strength ≥0.2 kN / m.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention, through multi-layer fabrication and the layered gradient addition of CNF (Chemical Fluoride), forms a pore size gradient structure with a dense surface layer and a loose bottom layer. This ensures high filtration accuracy (the surface layer intercepts fine particles) while maintaining good air permeability and filtration flux. Furthermore, this invention cationically modifies the CNF, enabling it to form electrostatic adsorption with negatively charged plant fibers, significantly improving the inter-fiber bonding force and CNF retention rate. Simultaneously, the positively charged filter material surface exhibits highly efficient adsorption capacity for negatively charged PM2.5 particles in the air and negatively charged colloidal particles in water.

[0016] 2. This invention utilizes a multifunctional monomer copolymer modified with epoxy, carboxyl, and hydroxymethyl groups to form a dense hydrophobic network on the fiber surface after cross-linking and curing. This significantly improves the material's wet tensile strength and water resistance without affecting air permeability. This invention leverages the naturally occurring lignin in unbleached softwood pulp, eliminating the need for additional UV absorbers to impart excellent UV blocking properties to the filter material and extend its lifespan in outdoor or sun-exposed environments. This invention employs gentle hot-pressing and calendering after sizing, improving surface smoothness (facilitating printing or heat sealing) while controlling air permeability loss to within 15%, ensuring stable filtration performance.

[0017] 3. The raw materials used in this invention are all natural plant fibers and biodegradable modified styrene-acrylic emulsion (acrylate copolymers can be gradually degraded in the natural environment). The material has a degradation rate of ≥90% in soil within 90 days, which meets environmental protection requirements. Attached Figure Description

[0018] Figure 1 The surface morphology of nanofibrillated cellulose under different homogenization pressures and homogenization cycles is shown, where (a) is not homogenized; (b) homogenized twice at 10000 psi; (c) homogenized five times at 15000 psi; and (d) homogenized 20 times at 20000 psi.

[0019] Figure 2 The effect of homogeneous pressure on the average length and width of nanofibrillated cellulose was demonstrated.

[0020] Figure 3 The surface morphology of filter materials with different CNF addition amounts is shown, where (a) no CNF added; (b) 10% CNF added; (c) 30% CNF added; and (d) 50% CNF added.

[0021] Figure 4 The graph shows the effect of CNF addition on air permeability.

[0022] Figure 5 The effect of CNF addition on the average and maximum pore size of the filter material is shown. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments, but this should not be construed as limiting the present invention.

[0024] Example 1: This example describes the preparation of a material for high-end coffee filtration, comprising the following steps: Step 1: Raw material preparation Softwood pulp: Select bleached softwood pulp (beating degree of 20°SR, used for surface and core layers) and unbleached softwood pulp (used for the underlayer), and dry them for later use.

[0025] Hardwood pulp: Select bleached hardwood pulp (beating degree 25°SR) and oven dry for later use.

[0026] Chemical reagents: acrylic acid, butyl acrylate, styrene (basic monomers); glycidyl methacrylate (GMA), methacrylic acid (MAA), N-hydroxymethylacrylamide (NMA) (functional monomers); cationic etherifying agents (such as epichlorohydrin trimethylamine); biological enzyme preparations.

[0027] Preparation of cationic nanofibrillated cellulose (CNF) suspension: Pretreatment: Take 30g of oven-dried bleached softwood pulp and 70g of oven-dried bleached hardwood pulp, tear them into pieces, and soak them in water until they swell. Add 0.6g of neutral cellulase and enzymatically hydrolyze at 50℃ for 30 minutes.

[0028] Mechanical refining: The enzymatically hydrolyzed pulp is placed in a PFI refiner and pulped at 47,000 rpm for about 35 minutes until the freeness is about 90°SR.

[0029] High-pressure homogenization and modification: The slurry was diluted to a concentration of 2%, a cationic etherifying agent was added, and the pH was adjusted to 11. The mixture was then reacted at 70°C for 2 hours to undergo cationization modification, yielding cationic nanofibrillated cellulose. When mixed with plant fiber slurry, the cationic nanofibrillated cellulose enhances fiber bonding through electrostatic adsorption and imparts a positive charge to the surface of the filter material, allowing it to adsorb negatively charged microparticles. Subsequently, the slurry was injected into a microfluidic high-pressure homogenizer and homogenized 10 times at a pressure of 15000 psi until the fibers were completely microfibrillated, resulting in a cationic CNF suspension with an average diameter of approximately 50 nm.

[0030] Figure 1 The surface morphology of nanofibrillated cellulose under different homogenization pressures and homogenization cycles is shown, where (a) is unhomogenized; (b) homogenized twice at 10000 psi; (c) homogenized five times at 15000 psi; and (d) homogenized 20 times at 20000 psi. Figure 1 As can be seen from the figure, with the increase of homogenization pressure and homogenization times, the fibers are gradually sheared and refined, and the fiber splitting is obvious, eventually forming a uniform nanoscale fiber structure, proving that the micro-jet high-pressure homogenization process used in this invention can effectively realize the nano-sizing of cellulose.

[0031] Figure 2 The effect of homogenization pressure on the average length and width of nanofibrillated cellulose (CNF) was demonstrated. It is evident that as the homogenization pressure increases, both the length and width of CNF gradually decrease, resulting in finer filaments and a higher aspect ratio. This indicates that high-pressure homogenization can significantly regulate the microstructure of CNF, providing a fundamental basis for the subsequent preparation of gradient pore structure filter materials.

[0032] Step 2: Multi-layer fabrication (gradient porosity structure) A three-layer structure is fabricated using a slanted wire paper machine. The proportions and processes for each layer are as follows: Surface layer: Cationic CNF and bleached softwood pulp are mixed at a mass ratio of 5:5. The pulp freeness is controlled at 60-70°SR. The on-line concentration is controlled at 0.8% to form a dense layer.

[0033] Core layer: Cationic CNF and bleached hardwood pulp (freezing degree 25°SR) are mixed at a mass ratio of 2:8.

[0034] Bottom layer: Unbleached softwood pulp (with lignin retained) is mixed with cationic CNF at a mass ratio of 7:3.

[0035] Forming: The three layers of pulp are sequentially dewatered on a wire mesh, then pressed (linear pressure 5 kN / m) and dried to obtain a base paper with a basis weight of 60 g / m². At this point, the material has formed a gradient structure with a surface layer pore size ≤10 μm and a bottom layer with higher porosity.

[0036] Figure 3 The images show the surface morphology of filter materials with different CNF addition amounts: (a) no CNF added; (b) 10% CNF added; (c) 30% CNF added; and (d) 50% CNF added. It is evident that as the CNF addition amount increases, the pores between fibers are gradually filled by nanofibers, resulting in a denser and more uniform material surface. This demonstrates that the present invention achieves a gradient pore structure design with a dense surface layer and a loose bottom layer through the gradient addition of CNF.

[0037] Figure 4 The graph shows the effect of CNF addition on air permeability. As the amount of CNF added increases, air permeability gradually decreases. However, within the limit of addition specified in this invention, the decrease in air permeability is controllable and does not exceed 15%, proving that this invention can effectively balance air permeability and filtration flux while improving filtration accuracy.

[0038] Figure 5 The effect of CNF addition on the average and maximum pore size of the filter material is demonstrated. It is evident that CNF addition significantly reduces the pore size, creating a gradient structure with increasing pore size from top to bottom. This achieves a balance between high-precision interception and high air permeability, supporting the technical effectiveness of the gradient pore structure of this invention.

[0039] Step 3: Preparation of modified styrene-acrylic emulsion Formulation: The basic monomer (butyl acrylate: styrene = 3:1) accounts for 85% of the total monomers, and the functional monomer (GMA: MAA: NMA = 3:1:1) accounts for 15%.

[0040] Polymerization: A pre-emulsified semi-continuous emulsion polymerization method was adopted. First, the emulsifier and a portion of the monomer were pre-emulsified. Then, the initiator was added to the reactor, and the pre-emulsion was added dropwise in batches. The reaction temperature was controlled at 80-85℃. After the reaction was completed, the temperature was lowered to 40℃, and the pH was adjusted to 7-8 to obtain a modified styrene-acrylic emulsion with a solid content of 30%.

[0041] Step 4: Impregnation and Post-treatment Impregnation: The base paper is impregnated with the above-mentioned modified styrene-acrylic emulsion through a two-roll impregnation machine, and the impregnation pressure is controlled to achieve an application rate of 10 g / m².

[0042] Curing and drying: Dry at 120°C for 60 seconds to allow the epoxy groups in the emulsion to crosslink and cure with the hydroxyl groups of cellulose.

[0043] Hot calendering: After curing, the paper enters the hot calendering machine and is subjected to hot calendering treatment at 100℃ and a linear pressure of 20 kN / m.

[0044] The filter material prepared in this embodiment has a basis weight of 60 g / m², a longitudinal wet tensile strength of 0.35 kN / m, and a particle interception rate of 99.5% for particles with a diameter ≥5 μm. The material exhibits high surface smoothness and excellent UV blocking performance due to the presence of lignin in the unbleached pulp at the bottom layer. Hot pressing and calendering treatment reduced air permeability by only 12%, perfectly balancing strength and filtration flux.

[0045] Example 2: This example provides a method for preparing an economical filter material with low CNF addition.

[0046] This embodiment explores the effect of CNF addition at the lower limit (1:9) in order to reduce costs.

[0047] Preparation: Same as in Example 1, but homogenized 5 times at 10000 psi.

[0048] Copying process: The structure is a double-layer structure, and the quantitative control is 50 g / m².

[0049] Surface layer: Nanofibrillated cellulose and plant fiber pulp are mixed at an oven-dry weight ratio of 1:9.

[0050] Bottom layer: Plant fiber pulp (without CNF).

[0051] Sizing process: Modified styrene-acrylic emulsion with a solid content of 20%, sizing amount of 5 g / m².

[0052] Hot pressing and polishing conditions: 80℃, linear pressure 10 kN / m.

[0053] The filter material prepared in this embodiment has a basis weight of 50 g / m². Due to the low CNF addition, the reinforcing effect of the material is limited. Its longitudinal wet tensile strength is approximately 0.18 kN / m, and its interception rate for particles ≥5 μm is approximately 97% (refer to Table 1). This material retains basic filtration functions and has a low preparation cost, making it suitable for cost-sensitive low-end markets. Example 3: This embodiment provides a method for preparing a high-homogeneity, high-fineness filter material.

[0054] This embodiment explores the effect of high voltage (20,000 psi) on CNF performance.

[0055] Preparation: Homogenized 20 times at 20,000 psi. The average diameter of the obtained CNF was reduced to 20 nm, and the average length was 500 nm.

[0056] Forming and sizing: A three-layer structure is adopted, with CNF addition of 50% in the surface layer, solid content of 40% in the modified styrene-acrylic emulsion, and sizing amount of 20 g / m².

[0057] Hot pressing and polishing conditions: 120℃, linear pressure 30 kN / m.

[0058] The filter material prepared in this embodiment has a basis weight of 60 g / m². Due to the use of an ultra-high pressure homogenization process and a high proportion of CNF filling, the material exhibits excellent mechanical properties, with a longitudinal wet tensile strength as high as 0.40 kN / m. In terms of filtration accuracy, it achieves a 99.9% interception rate for particles ≥5μm in diameter, almost achieving complete interception. Although the highly dense structure results in relatively low air permeability (increased Gurley value), the material surface is extremely smooth, making it suitable for high-precision filtration in the medical or electronics industries.

[0059] Example 4: This example provides a method for preparing a heat-free, pressure-pressed air filter material.

[0060] This embodiment focuses on maintaining maximum air permeability and is suitable for air filtration.

[0061] Papermaking process: 30% CNF added to the top layer, 10% to the middle layer. Unbleached softwood pulp is used for the bottom layer.

[0062] Post-treatment: After impregnation and sizing, no hot pressing or calendering is performed; only conventional drying is carried out.

[0063] The filter material prepared in this embodiment has a basis weight of 60 g / m². Although the longitudinal wet tensile strength (0.28 kN / m) is slightly lower than that of the optimal embodiment, the material achieves excellent air permeability through structural control, exhibiting superior air permeability. It achieves a 90% interception rate for particles ≥5 μm in diameter, maintaining effective filtration while allowing for a certain degree of permeability. The material surface is smooth, and the lignin in the unbleached pulp at the bottom layer provides good UV protection. Hot-pressing and calendering improves surface smoothness while minimizing the impact on air permeability, successfully achieving a synergy between high strength and high throughput. The material in this embodiment has high air permeability (low Gurley value) and minimal pressure drop, making it suitable for ventilation systems. However, due to the lack of hot-pressing and calendering, the surface smoothness is relatively low, making it unsuitable for printing high-precision patterns.

[0064] Example 5: This example provides a method for preparing a lignin-based filter material.

[0065] This embodiment verifies the role of lignin (see comparative analysis).

[0066] Papermaking process: The bottom layer is made of unbleached softwood pulp, and lignin is not removed. Other processes are the same as in Example 1.

[0067] The filter material prepared in this embodiment has a basis weight of 60 g / m² and a longitudinal wet tensile strength of 0.32 kN / m. Thanks to the high lignin content retained in the unbleached pulp at the bottom layer, the material exhibits excellent UV blocking performance, with a strength retention rate as high as 95% after aging tests. The interception rate for particles ≥5 μm remains at a high level of 99.0%. The material surface is smooth, and hot-pressing and calendering further optimizes its surface properties, enabling it to maintain excellent filtration function while possessing superior environmental resistance.

[0068] Example 6: This example provides a method for preparing filter material, which differs from Example 1 in that it uses untreated virgin CNF (anionic type). Other processes are the same.

[0069] The filter material prepared in this embodiment has a basis weight of 60 g / m². Due to the omission of the cationization modification step, the retention rate of nanofibrillated cellulose (CNF) on the plant fibers is reduced, resulting in a longitudinal wet tensile strength of 0.22 kN / m. Although this strength is lower than that of Example 1, its strength is still significantly better than that of traditional filter materials due to the cross-linking and sizing effect of the modified styrene-acrylic emulsion. In terms of filtration performance, it achieves a 92% interception rate for particles ≥5 μm, demonstrating good filtration effect. The material retains the UV blocking properties provided by the lignin in the unbleached pulp at the bottom layer, and the preparation process is simplified and the cost is slightly reduced due to the absence of cation modification. After hot pressing and calendering, the material exhibits moderate air permeability, meeting the flux requirements of general filtration scenarios.

[0070] To demonstrate the superiority of the technical solution of this invention, the following comparative examples are provided: Comparative Example 1: Traditional single-layer coffee filter paper (blank control); Formula: Use only bleached softwood pulp and bleached hardwood pulp in a 3:7 ratio, without adding CNF, without using modified emulsion sizing, and only perform conventional drying and calendering.

[0071] Performance test results: Wet tensile strength: 0.08 kN / m (extremely low, easily broken when exposed to water).

[0072] Filtration accuracy: The interception rate of 5μm particles is only 85% (high permeability, resulting in cloudy coffee liquid).

[0073] Degradability: Although it is degradable, its low strength limits its application scenarios.

[0074] Comparative Example 2: Ordinary CNF blended filter paper (physical blending only); Formulation: CNF is simply physically blended with plant fiber (without separation) and without cationic modification.

[0075] Process: Single-layer papermaking, without emulsion impregnation, only surface coating with ordinary starch.

[0076] Performance test results: Uniformity: Poor; CNF agglomeration causes spots on the paper surface.

[0077] Wet strength: 0.12 kN / m (low CNF retention rate, and the reinforcing effect is not obvious).

[0078] Table 1 presents detailed comparative data between Examples 1-6 of the present invention and Comparative Examples 1-2: Table 1 Evaluation indicators Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Comparative Example 1 Comparative Example 2 Wet tensile strength (kN / m) 0.35 0.18 0.40 0.28 0.32 0.22 0.08 0.12 Filtering interception rate (≥5μm) 99.5% 97% 99.9% 90% 99.0% 92% 85% 88% Breathability (Gurley s) 300 250 500 150 320 200 400 350 Biodegradation rate >90% >90% >90% >90% >90% >90% >90% >90% UV aging strength retention rate 90% 85% 90% 88% 95% 90% 70% 75% As can be seen, this invention solves the problems of low strength and poor precision of traditional paper (Comparative Example 1) by using a high proportion of CNF and cationic modification in the surface layer. This invention employs modified emulsion crosslinking and a mild hot-pressing and calendering process, which improves strength and surface properties while also exhibiting good environmental friendliness. Furthermore, this invention achieves anti-aging properties without adding chemical additives by retaining the underlying lignin.

[0079] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the invention without departing from the spirit and scope of the present invention.

Claims

1. A method for preparing a nanofibrillated cellulose-based filter material, characterized in that, Includes the following steps: Step 1: Using bleached softwood pulp as raw material, after pretreatment by bio-enzymatic hydrolysis and mechanical fine grinding, the pulp is homogenized 2-20 times at a pressure of 10000-20000 psi using a micro-jet high-pressure homogenizer to obtain a nano-microfibrillated cellulose suspension. Step 2: Mix the nanofibrillated cellulose obtained in Step 1 with plant fiber pulp with a freeness of 16-30°SR at an oven-dry mass ratio of 1:9 to 5:5, and use a slanted wire paper machine to form double or multiple layers, forming a gradient pore structure in which the surface layer contains a higher proportion of nanofibrillated cellulose than the middle or bottom layer. After pressing and drying, the base paper is obtained. Step 3: Using acrylic acid, butyl acrylate and styrene as basic monomers, and glycidyl methacrylate, methacrylic acid and N-hydroxymethylacrylamide as functional monomers, a modified styrene-acrylic emulsion with a solid content of 20-40% is prepared by pre-emulsification semi-continuous emulsion polymerization. Step 4: Impregnate the base paper obtained in Step 2 into the modified styrene-acrylic emulsion prepared in Step 3, control the sizing amount to be 5-20 g / m², and after drying and curing, obtain the nanofibrillated cellulose-based filter material.

2. The preparation method according to claim 1, characterized in that, The pressure of the microjets high-pressure homogenization in step one is 15000 psi, and the number of homogenizations is 5-10 times. The average diameter of the obtained nanofibrillated cellulose is 20-100 nm, and the average length is 500-2000 nm.

3. The preparation method according to claim 1, characterized in that, The plant fiber pulp mentioned in step two includes a mixture of bleached softwood pulp and bleached hardwood pulp, wherein the oven-dry weight ratio of softwood pulp to hardwood pulp is 2:8 to 5:5, and the freeness of softwood pulp is 16-25°SR, while the freeness of hardwood pulp is 21-30°SR.

4. The preparation method according to claim 1, characterized in that, In the double-layer or multi-layer papermaking process described in step two, the amount of nanofibrillated cellulose added to the surface layer is 30-50% of the total amount of oven-dry fibers, and the amount added to the middle or bottom layer is 1-10%, forming a gradient structure with increasing porosity and pore size from top to bottom.

5. The preparation method according to claim 1, characterized in that, The functional monomers mentioned in step three account for 5-15% of the total mass of the monomers, wherein the mass ratio of glycidyl methacrylate, methacrylic acid and N-hydroxymethylacrylamide is 2:1:1 to 4:2:

1.

6. The preparation method according to claim 1, characterized in that, The impregnation and sizing process described in step four involves double-sided coating or impregnation extrusion. After sizing, the quantitative amount of the material is 30-70 g / m², and the thickness is 0.10-0.15 mm.

7. The preparation method according to claim 1, characterized in that, The nanofibrillated cellulose suspension described in step one is subjected to cationization modification: a cationic etherifying agent is added, and the mixture is reacted for 1-3 hours at pH 10-12 and temperature 60-80℃ to obtain cationic nanofibrillated cellulose. When the cationic nanofibrillated cellulose is mixed with plant fiber pulp, the fiber binding force is enhanced through electrostatic adsorption, and the surface of the filter material is positively charged to adsorb negatively charged microparticles.

8. The preparation method according to claim 1, characterized in that, The multi-layer fabrication described in step two consists of a three-layer structure: a surface layer, a core layer, and a bottom layer. The surface layer is composed of nanofibrillated cellulose and softwood pulp in a mass ratio of 4:6 to 6:4, with an average pore size ≤10 μm. The core layer is composed of hardwood pulp and nanofibrillated cellulose in a mass ratio of 9:1 to 7:3, with an average pore size of 15-30 μm. The bottom layer is composed of unbleached softwood pulp and nanofibrillated cellulose in a mass ratio of 8:2 to 6:4, with an average pore size of 30-50 μm. Furthermore, the lignin in the bottom layer imparts UV blocking and anti-aging properties to the filter material.

9. The preparation method according to claim 1, characterized in that, Step four is followed by a hot-pressing and calendering process: the impregnated and sizing material is hot-pressed and calendered at 80-120℃ and a linear pressure of 10-30 kN / m to improve the surface smoothness of the material to ≥100 s and reduce the air permeability by no more than 15%, while improving the filtration accuracy and tensile strength.

10. The preparation method according to claim 1, characterized in that, The filter material is a coffee filter material with a quantitative content of 50-70 g / m².

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