Degradable nanofoam filter material for cigarettes and preparation method thereof

Abstract

The application discloses a degradable nanometer foam filter material for cigarettes and a preparation method thereof. The inventor selects whole biomass wood fiber and hydrogel with a nano structure as a filter material matrix to realize complete degradation of the material. The polar functional groups and nano structure of the material are used to realize efficient adsorption of harmful particles in a physical and chemical way, and the original taste of cigarettes is reserved. The wood fiber itself has a natural nanometer tubular structure and shows strong adsorption capacity. The wood fiber is in-situ cross-linked and polymerized with hydrogel with a three-dimensional network structure to form a type, and the wood fiber is wound and interpenetrated to complement each other. The wood fiber is a kind of strong and stable high-efficiency filter material. The toughness and water resistance of the three-dimensional network structure are further improved by boric acid treatment on the wood fiber and the hydrogel. The material in the technical scheme is safer and more environmentally friendly, harmful additives are not added in the processing process, and secondary pollution of the material is avoided.

Description

Technical Field

[0001] This invention relates to the field of new materials technology, specifically to the field of cigarette filter materials technology, and more specifically to a biodegradable nanofoam filter material for cigarettes and its preparation method. Background Technology

[0002] Currently, cigarette filter rods are mainly made of cellulose acetate and polypropylene fibers. Cigarette filter rods made from cellulose acetate and polypropylene fibers have three main drawbacks. First, their filtration efficiency is low. Because cellulose acetate and polypropylene fibers are limited to physical adsorption, they only have a certain adsorption and retention effect on tar and solid substances, while having almost no adsorption and retention effect on gaseous harmful substances, resulting in low filtration efficiency for substances produced by cigarette combustion. Second, the processing of cellulose acetate and polypropylene fibers can introduce new toxic substances, causing secondary pollution. Although cellulose acetate and polypropylene fibers themselves are non-toxic and harmless, various binders and diluents are added during the processing of cigarette filter rods, and plastic particles remain during the fiber forming process, causing secondary pollution. Third, cellulose acetate and polypropylene fibers are non-degradable, causing white pollution. Therefore, developing cigarette filter rod materials with high adsorption rates, greater safety, and biodegradability has become an industry consensus. For example, patent publication number CN115886324A discloses a method for preparing a biodegradable cigarette filter rod. This patent uses biodegradable sodium alginate as a binder for cellulose acetate, which improves the degradation rate of cellulose acetate. However, the addition of a small amount of biodegradable sodium alginate cannot fundamentally change the biodegradability of cigarette filter materials mainly composed of cellulose acetate. Similarly, patent publication number CN105639723B discloses a biodegradable filter adsorption rod and its preparation method. This patent uses cellulose acetate and polybutylene succinate melt-blended at 160-190℃ to produce a biodegradable filter adsorption rod, but this only modifies the cellulose acetate and does not achieve degradation. Patent publication number CN 115005497A discloses a biodegradable cigarette filter masterbatch and a biodegradable cigarette filter. The cigarette filter in this invention uses cellulose diacetate as the base material and adds a porous adsorbent alginate-titanium dioxide biodegradable masterbatch. This type of method belongs to the composite improvement type. Although it enhances the adsorption and degradation capabilities of the material, it still cannot fundamentally solve the problems of low adsorption efficiency and non-degradability of cigarette filters. Researchers have also explored biodegradable pure bio-based materials. For example, patent publication number CN103103633A discloses a modified polylactic acid cigarette tow filter and its preparation method. This is also an improvement type material, changing the filter tow material, but it cannot solve the defect that tow-type filter materials cannot adsorb nano-sized smoke particles. For example, patent publication number CN115652468A discloses a polylactic acid composite cigarette tow, its preparation method, and its application. By composited with nanofibers on a polylactic acid fiber base material, it makes a meaningful attempt to enhance adsorption through nanostructure. However, its nanofibers can only reach a diameter of 600-900nm, and have no adsorption effect on particulate matter in flue gas with a nanodiameter of 0.7nm-300nm, which are more harmful than nitrosamines, polycyclic aromatic hydrocarbons, carbon monoxide, etc.

[0003] Therefore, to fundamentally solve the problems of filter material degradation and adsorption, we can only start from two aspects: complete replacement of the material and nanopore size of the material. We need to invent a foam filter material that is entirely biodegradable and has nanopore size to replace the existing cigarette filter rod material. Summary of the Invention

[0004] This invention provides a novel cigarette filter material with biomass lignocellulose and nanostructured hydrogel as the main matrix. This filter material adsorbs harmful substances in cigarette smoke through both physical and chemical adsorption mechanisms, exhibiting high adsorption efficiency. The processing uses environmentally friendly materials, avoiding secondary pollution. The lignocellulose and hydrogel are biodegradable materials, reducing the environmental pollution caused by cigarette filter materials.

[0005] The first aspect of the present invention provides a biodegradable nanofoam filter material for cigarettes, comprising at least wood fibers and hydrogel, wherein the mass ratio of the wood fibers to the hydrogel is 4 to 3:1, the hydrogel is formed by the reaction of a biodegradable hydrophilic polymer with water, and the solute concentration of the hydrogel is 1 to 10 wt%.

[0006] When the mass ratio of wood fiber to hydrogel is 4 to 3:1, a better three-dimensional network structure can be formed between the wood fiber and the hydrogel.

[0007] As a preferred technical solution, the wood fibers are enhanced and modified by soaking them in a boric acid solution with a concentration of 0.5~0.8mol / L and a temperature of 70~80℃ for 1~3 hours.

[0008] The wood fibers should first be treated with boric acid to make them water-resistant, tough, and reinforced, in order to prevent them from shrinking and deforming.

[0009] As a preferred technical solution, the wood fiber is selected from at least one of broadleaf forest fiber, coniferous forest fiber, straw fiber, bamboo fiber, bagasse fiber and virgin wood pulp fiber.

[0010] As a more preferred technical solution, the wood fiber is selected from at least one of broadleaf forest fiber or virgin wood pulp fiber.

[0011] As a cigarette filter material, lignin, which contains high levels of natural pigments, can negatively affect the appearance and taste of the filter material. Therefore, it is preferable to use broad-leaved wood fibers or virgin wood pulp fibers with low lignin content and high toughness fiber content.

[0012] As a preferred technical solution, the hydrophilic polymer is selected from at least one of alginate, cellulose, chitosan, gelatin, silk protein, and collagen.

[0013] As a preferred technical solution, boric acid is added to the mixture of wood fiber and hydrogel as a crosslinking curing agent, and the amount of boric acid added is 0.2~1wt% of the mass of hydrogel.

[0014] The hydrogel crosslinking curing agent in this technical solution is boric acid. The overall technical solution adds very few non-biomass elements. Boric acid is both a crosslinking curing agent and an antibacterial and preservative agent.

[0015] As a preferred technical solution, the foam filter material formed by the wood fiber and hydrogel has interconnected nanopores, the nanopores being in the range of 0.5~2000nm.

[0016] Different nanopore sizes adsorb smoke particles of different sizes. Studies have shown that mainstream cigarette smoke contains 10... 9 ~10 10 The particles are of various sizes, with diameters ranging from 0.5 nm to 1000 nm. Among them, particles with diameters between 320 nm and 1000 nm account for 70%, particles below 320 nm account for 20%, and particles above 1000 nm account for 10%. The pore size of the filter material is achieved by controlling the content of cross-linking curing agent and wood fiber. The higher the content of cross-linking curing agent and wood fiber, the smaller the pore size of the filter material, and vice versa, thereby achieving the adsorption of flue gas particles of different sizes.

[0017] By selecting different hydrogel monomer matrices and controlling the porosity and pore size, selective adsorption of different smoke particles is achieved. Cigarette smoke contains over 4800 compounds, among which nitrosamines and polycyclic aromatic hydrocarbons (PAHs) are the most harmful to human health. Various fibrous filter materials have extremely low adsorption rates for these compounds. The inventors discovered that the various hydrogel nanopore sizes in this solution are controlled within the range of 0.7 nm to 500 nm, enabling the adsorption of 68% of nitrosamines and PAHs. The type of hydrogel monomer also exhibits selective adsorption. a. Chitosan monomer matrix gel, due to the cationic amino groups on its side chains, selectively adsorbs carbon monoxide, heavy metals, and most tar and irritating rancid valeric acid, but does not adsorb nicotine, solanone and other flavor substances. b. Cellulose monomer matrix gel, due to the presence of hydroxyl and carboxyl functional groups, selectively adsorbs heavy metals, tar and nicotine; c. Alginate monomer matrix gel, containing a large number of carboxyl and hydroxyl groups, is highly polar and selectively adsorbs tar, nicotine and carbon monoxide.

[0018] A second aspect of the present invention provides a method for preparing the above-mentioned biodegradable nanofoam filter material for cigarettes, comprising at least the following steps: Hydrogel prepolymers were prepared using hydrophilic polymers; Strengthening and modifying wood fibers with boric acid; The hydrogel prepolymer and the modified wood fiber are mixed to obtain a mixed slurry; The mixed slurry is fed into the spinning assembly for spinning, and the spun fibers are cross-linked and cured by boric acid. The cross-linked and cured fiber filaments are made into filter rods using a cigarette filter rod forming machine; The filter rod in (5) is dried at a temperature of less than 60°C.

[0019] As a preferred technical solution, the above step (4) specifically involves preparing a boric acid solution with a concentration of 5~7wt% under heating conditions at 80℃, immersing the obtained fiber in the boric acid solution, and maintaining the temperature at 80℃ for 0.5~1h.

[0020] As a preferred technical solution, the method for preparing the above-mentioned biodegradable nanofoam filter material for cigarettes includes at least the following steps: Prepare a hydrogel prepolymer solution by using a hydrophilic polymer. Weigh 1-10 parts by weight of the hydrophilic polymer monomer and add it to 100 parts by weight of water. Heat and stir at 60-80°C for 1-3 hours until the mixture is evenly dispersed and forms a flowing gel. Boric acid is used to strengthen and modify wood fibers. Boric acid strengthening and toughening agent is prepared by taking boric acid and water to prepare 100 parts of boric acid solution with a molar concentration of 0.5~0.8mol / L. Wood fibers are weighed out by mass and added to the boric acid solution. The solution is heated and stirred at 60~80℃ for 0.5~3h. Weigh the modified wood fiber and hydrogel prepolymer at a mass ratio of 4~3:1, heat and stir until uniform, control the temperature at 40~80℃ and the time at 1~5h, to form a semi-gel state and obtain a mixed slurry; The mixed slurry is fed into the spinning assembly for spinning, and the spun fibers are cross-linked and cured by boric acid. The cross-linked and cured filaments are made into filter rods using a cigarette filter rod forming machine; The filter rod in (5) is dried at a temperature of less than 60°C.

[0021] As a preferred technical solution, the preparation method of the biodegradable nanofoam filter material for cigarettes, the step (4) specifically involves: preparing a boric acid solution with a concentration of 5~7wt% under heating conditions at 80℃, immersing the obtained fiber filaments in the boric acid solution, and maintaining the temperature at 80℃ for 0.5~1h.

[0022] In this technical solution, the molding method is in-situ curing molding, that is, the mixed slurry is fed into the spinning assembly, spun into filaments by the spinneret, crosslinked and cured, and then formed into filter rods by filter rod molding machines such as KDF-3 or KDF-4.

[0023] A third aspect of the present invention provides another method for preparing the above-mentioned biodegradable nanofoam filter material for cigarettes, comprising at least the following steps: Hydrogel prepolymers were prepared using hydrophilic polymers; Strengthening and modifying wood fibers with boric acid; The hydrogel prepolymer and the modified wood fiber are mixed to obtain a mixed slurry; The mixed slurry is cross-linked and cured using boric acid; The cross-linked and cured slurry is injected into a filter rod mold to form a filter rod; The filter rod in (5) is dried at a temperature of less than 60°C.

[0024] As a preferred technical solution, the method for preparing the above-mentioned biodegradable nanofoam filter material for cigarettes includes at least the following steps: Hydrogel prepolymers are prepared by using hydrophilic polymers. Weigh 1 to 10 parts by weight of the hydrophilic polymer monomer and add it to 100 parts by weight of water. Heat and stir at 60 to 80°C for 1 to 3 hours until the mixture is evenly dispersed and forms a flowing gel. Boric acid is used to strengthen and modify wood fibers. Boric acid strengthening and toughening agent is prepared by taking boric acid and water to prepare 100 parts of boric acid solution with a molar concentration of 0.5~0.8mol / L. Wood fibers are weighed out by mass and added to the boric acid solution. The solution is heated and stirred at 60~80℃ for 0.5~3h. Weigh the modified wood fiber and hydrogel prepolymer at a mass ratio of 4~3:1, heat and stir until uniform, control the temperature at 40~80℃ and the time at 1~5h, to form a semi-gel state and obtain a mixed slurry; The mixed slurry is cross-linked and cured using boric acid; The cross-linked and cured slurry is injected into a filter rod mold to form a filter rod; The filter rod in (5) is dried at a temperature of less than 60°C.

[0025] As a preferred technical solution, the drying method is dehydration drying, which is carried out at room temperature for 8 to 24 hours or heating drying, with the temperature controlled at 25 to 60°C and the drying time being 8 to 24 hours.

[0026] This technical solution does not employ freeze drying or supercritical drying, and the process is simple.

[0027] As a preferred technical solution, step (4) is specifically as follows: prepare a 5wt% boric acid solution at room temperature, add it to the mixed slurry in step (3) according to the mass ratio of hydrophilic polymer to boric acid of 100:0.1~0.5, stir evenly, and then stir at 40~70℃ for 30~50min.

[0028] Wood fibers are impregnated in hydrogel, forming an intertwined, interpenetrating three-dimensional network structure. This structure is then cured in situ using physical and chemical cross-linking. After dehydration and drying, the hydrogel undergoes deswelling, forming a dispersed filamentous structure. The cavities created by the separation from the aqueous phase form a permeable nanoporous foam filter material. This filter material achieves selective adsorption. By selecting different hydrogel monomers and controlling the degree of cross-linking and curing, porosity can be regulated. This allows for the adsorption of small amounts or no adsorption of flavor compounds such as nicotine, solanone, and megastigmatrienone in flue gas, while maintaining a high adsorption rate for harmful substances such as tar, nitrosamines, carbon monoxide, and heavy metals, thus achieving selective adsorption. Beneficial effects

[0029] This technical solution selects all-biomass lignocellulose and nanostructured hydrogel as the filter material matrix, achieving complete biodegradability. Utilizing the polar functional groups and nanostructure of the materials, it achieves efficient adsorption of harmful particles through both physical and chemical pathways, while preserving the original flavor of cigarettes. Lignocellulose itself possesses a natural nanotubular structure, exhibiting extremely strong adsorption capacity. Combined with other materials, it possesses strong cross-linking and weaving functions, serving as the skeleton of foam filter materials to ensure their strength. Hydrogel, as a green and multifunctional material, has wide applications in medical, life sciences, smart electronics, energy, and environmental protection fields. Its unique three-dimensional network nanostructure can form molecular sieves for nanofiltration. Combined with lignocellulose, the intertwining and interpenetrating structures provide synergistic effects, resulting in a strong, stable, and highly efficient filter material. Boric acid treatment of the lignocellulose and hydrogel further improves the toughness, strength, and water resistance of the three-dimensional network structure. The materials in this technical solution are safer and more environmentally friendly, with no harmful additives added during processing, avoiding secondary pollution. Attached Figure Description

[0030] Figure 1 A photograph of the cigarette filter material prepared in Example 1.

[0031] Figure 2 SEM microstructure of the filter rod in Example 2 at a magnification of 4000x.

[0032] Figure 3 Example 3: SEM image of filament bundle formation during spinning.

[0033] Figure 4 SEM image of the filter media in Example 3 undergoing natural degradation. Detailed Implementation

[0034] The following are non-limiting examples illustrating embodiments of the present invention, and the scope of protection of the present invention is not limited to these embodiments. The scanning electron microscope (SEM) testing method in the present invention is performed in accordance with techniques known in the art. Example 1

[0035] (1) Preparation of chitosan hydrogel prepolymer solution: Weigh 30g of chitosan and slowly add it to 1000ml of 60℃ hot water while stirring. Keep the temperature constant for 1.5h until the chitosan is completely dissolved and a transparent flowable gel is formed. (2) Prepare boric acid solution: Weigh boric acid and prepare 1000 ml of boric acid aqueous solution with a molar concentration of 0.6 mol / L; (3) Wood fiber strengthening modification: Weigh 30g of finished wood fiber from broad-leaved forests, break it up, and add it to the boric acid solution prepared in step (2) while stirring. The stirring speed is 2000 R / min, the temperature is controlled at 60℃, and the time is controlled at 2 hours. Take it out and set it aside. (4) Mixing and pulping: Weigh 30g of modified wood fiber and 10g of chitosan prepolymer solution, control the temperature at 60℃, mix and stir evenly, and form a semi-gel state after 1 hour. (5) Pour the mixed slurry prepared in step (4) into a boric acid solution with a mass concentration of 5wt%, wherein the volume ratio of the mixed slurry to the boric acid solution is 1:1, stir rapidly at a speed of 3000 R / min, control the temperature at 40℃, control the time at 30 min, inject it into the cigarette filter rod forming mold, and let it stand for 1 hour to solidify and form. (6) Dehydration and drying: The filter rods obtained in step (5) are dried and dehydrated at 50°C to obtain the finished filter rods.

[0036] The filter rods in Example 1 were subjected to flue gas filtration experiments according to international standards ISO4387-2000, YC / T255-2008, and GB / T23228-2008. In this context, A is the filter rod in Example 1, B is a commercially available cellulose diacetate filter rod, and C is a commercially available polypropylene fiber filter rod. A / B represents the residual ratio of harmful components in A and B, and A / C represents the residual ratio of harmful components in A and C. The test results are shown in Table 1.

[0037] Table 1 - Results of flue gas filtration experiment NNN / (ng / stick) 9. 2 17 .8 16 .9 0. 5 0. 5 NAT / (ng / stick) 17 .1 37 . 5 36 .6 0. 5 0.5 NAB / (ng / stick) 1. 5 3. 5 4. 2 0.4 0.4 NNK / (ng / stick) 2. 1 5. 1 5.6 0. 4 0.4 BaA / (ng / stick) 6. 2 11.8 12.2 0. 5 0.5 Chr / (ng / stick) 8. 6 16 .9 18.8 0.5 0.5 BaP / (ng / stick) 4. 1 7. 5 7. 6 0. 5 0. 5 Formaldehyde / (μg / stick) 20 . 5 41. 1 53. 5 0. 5 0.4 Acetaldehyde / (μg / stick) 98.1 640. 5 725. 8 0. 2 0. 1 Acetone / (μg / stick) 87 . 2 289. 2 303. 5 0. 3 0. 3 Acrolein / (μg / stick) 30 . 3 55. 3 68. 1 0. 5 0. 4 Propionaldehyde / (μg / stick) 20 . 5 44 .6 51. 2 0. 5 0. 4 Crotonaldehyde / (μg / stick) 10 .1 20.5 26.1 0.5 0.4 2-Butanone / (μg / stick) 32 .6 73. 5 76 .8 0. 4 0.4 Butyraldehyde / (μg / stick) 12.3 31. 5 35. 6 0.4 0.3 Hydrogen cyanide / (μg / stick) 86.5 138. 5 140.6 0. 6 0. 6 Ammonia / (μg / stick) 4. 0 8.8 9. 5 0. 5 0. 4 Hydroquinone / (μg / stick) 18. 3 40 . 3 43. 5 0. 5 0. 4 Resorcinol / (μg / stick) 0.7 1. 7 1.9 0. 4 0.4 Catechol / (μg / stick) 25. 5 48. 6 50.2 0. 5 0. 5 Phenol / (μg / stick) 5.8 8.8 16.5 0.7 0. 4 Resorcinol / (μg / stick) 5.8 8.5 10.8 0.7 0.5 o-Cresol / (μg / stick) 1.7 1.8 3.3 0.9 0.5 Hazard Index 4.8 8.6 9.8 0.6 0.5 Example 2

[0038] (1) Preparation of chitosan-sodium alginate composite hydrogel prepolymer solution: Weigh 20g of chitosan and 10g of sodium alginate, slowly add them to 1000ml of 60℃ hot water while stirring, keep the temperature constant for 1.5h until chitosan and sodium alginate are completely dissolved to form a transparent flowable hydrogel. (2) Prepare boric acid solution: Weigh boric acid and prepare 1000 ml of boric acid aqueous solution with a molar concentration of 0.6 mol / L; (3) Wood fiber strengthening modification: Weigh 40g of commercial pulp fiber, add water and disperse it, and add it to the boric acid solution prepared in step (2) while stirring. The volume ratio of pulp fiber to boric acid solution is 1:1. The stirring speed is 2000R / min, the temperature is controlled at 60℃, and the time is controlled at 2 hours. Take it out for later use. (4) Mixing and pulping: Weigh 40g of modified pulp fiber and 10g of chitosan sodium alginate prepolymer solution, control the temperature at 60℃, mix and stir evenly, and form a semi-gel state after 1 hour; (5) Pour the mixed slurry prepared in step (4) into a boric acid solution with a mass concentration of 5wt% and stir rapidly at a speed of 3000 R / min, a temperature of 40℃ and a time of 30 min. Then pour it into the filter rod forming mold and let it stand for one hour to solidify and form. (6) Dehydration and drying: The filter rod obtained in step (5) is dried and dehydrated at 50°C to obtain the finished filter rod. The obtained filter rod is observed under a SEM electron microscope at 4000x magnification. Figure 2 . Example 3

[0039] (1) Preparation of chitosan hydrogel prepolymer solution: Weigh 30g of chitosan and slowly add it to 1000ml of 60℃ hot water while stirring. Keep the temperature constant for 1.5h until the chitosan is completely dissolved and a transparent flowable gel is formed. (2) Preparation of boric acid strengthening modifier: Weigh boric acid and prepare 1000 ml of boric acid aqueous solution with a molar concentration of 0.6 mol / L; (3) Wood fiber strengthening modification: Weigh 30g of finished wood fiber from broad-leaved forests, break it up, and add it to the boric acid solution prepared in step (2) while stirring. The stirring speed is 2000 R / min, the temperature is controlled at 60℃, and the time is controlled at 2 hours. Take it out and set it aside. (4) Mixing and pulping: Weigh 30g of modified wood fiber and 10g of chitosan prepolymer solution, control the temperature at 60℃, mix and stir evenly, and form a semi-gel state after 1 hour. (5) Forming method: The slurry obtained in step (4) is fed into the spinning assembly and spun into filaments by the spinneret. The resulting filament bundle is observed under a SEM electron microscope. Figure 3 It can be seen that the filament bundles have a mesh-like structure.

[0040] (6) The filamentous gel obtained in step (5) is subjected to secondary cross-linking and curing by immersion in boric acid solution. A boric acid solution with a concentration of 5wt% is prepared under heating at 80°C. The obtained fiber is immersed in the boric acid solution and kept at 80°C for 0.5~1h. The cross-linked and cured fiber is then formed into a filter rod by KDF-3 filter rod forming machine.

[0041] (7) Drying treatment: The filter rod obtained in step (6) is dried at 60°C to obtain the finished filter rod. The filter rod in this embodiment was tested according to the national standard GB / T19277 test and testing methods, and the test results are as follows. Figure 4 As shown in Figures a, b, c, d, e, and f, SEM images were observed according to the extension of degradation time. It can be seen that the filter rod gradually degrades to complete degradation over time.

[0042] The filter rods from Examples 1, 2, and 3 were used to make cigarettes with the same tobacco composition and numbered 1, 2, and 3, respectively. Sensory evaluation was conducted using the method in GB5606.4-2005, where "↑" indicates slight improvement or enhancement, "↑↑" indicates significant improvement or enhancement, "↑↑↑" indicates substantial improvement or enhancement, "↓" indicates slight deterioration, "↓↓" indicates significant deterioration, and "==" indicates no change. The results are shown in Table 2.

[0043] Table 2 - Cigarette Sensory Evaluation Record Form

Claims

1. A degradable nanofiber filter material for a cigarette, characterized by comprising a nanofiber web having a nanofiber diameter of 100 nm or less and a nanofiber length of 100 μm or more. The mixture comprises at least wood fibers and hydrogel, wherein the mass ratio of wood fibers to hydrogel is 4 to 3:1, the hydrogel is formed by the reaction of a biodegradable hydrophilic polymer with water, the solute concentration of the hydrogel is 1 to 10 wt%, the wood fibers are reinforced and modified by soaking in a boric acid solution with a concentration of 0.5 to 0.8 mol / L and a temperature of 70 to 80°C for 1 to 3 hours, and boric acid is added to the mixture of wood fibers and hydrogel as a crosslinking curing agent, wherein the amount of boric acid added is 0.2 to 1 wt% of the mass of the hydrogel.

2. The biodegradable nanofoam filter material for cigarettes according to claim 1, characterized in that, The wood fiber is selected from at least one of broadleaf forest fiber, coniferous forest fiber, straw fiber, bamboo fiber, bagasse fiber, and virgin wood pulp fiber.

3. The biodegradable nanofoam filter material for cigarettes according to claim 2, characterized in that, The hydrophilic polymer is selected from at least one of alginate, cellulose, chitosan, gelatin, silk protein, and collagen.

4. The biodegradable nanofoam filter material for cigarettes according to any one of claims 1-3, characterized in that, The foam filter material formed by the wood fiber and hydrogel has interconnected nanopores, the nanopores being in the range of 0.5~2000nm.

5. A method for preparing the biodegradable nanofoam filter material for cigarettes as described in claims 1-4, characterized in that, At least the following steps are included: Hydrogel prepolymers were prepared using hydrophilic polymers; Strengthening and modifying wood fibers with boric acid; The hydrogel prepolymer and the modified wood fiber are mixed to obtain a mixed slurry; The mixed slurry is fed into the spinning assembly for spinning, and the spun fibers are cross-linked and cured by boric acid. The cross-linked and cured fiber filaments are made into filter rods using a cigarette filter rod forming machine; The filter rod is dried at a temperature below 60°C.

6. A method for preparing the biodegradable nanofoam filter material for cigarettes as described in claims 1-4, characterized in that, At least the following steps are included: Hydrogel prepolymers were prepared using hydrophilic polymers; Strengthening and modifying wood fibers with boric acid; The hydrogel prepolymer and the modified wood fiber are mixed to obtain a mixed slurry; The mixed slurry is cross-linked and cured using boric acid; The cross-linked and cured slurry is injected into a filter rod mold to form a filter rod; The filter rod is dried at a temperature below 60°C.

7. The method for preparing biodegradable nanofoam filter material for cigarettes according to claim 5, characterized in that, The process involves feeding the mixed slurry into the spinning assembly for spinning, and then cross-linking and curing the spun fibers with boric acid. Specifically, a boric acid solution with a concentration of 5-7 wt% is prepared under heating conditions at 80°C, and the resulting fibers are immersed in the boric acid solution at 80°C for 0.5-1 h.

8. The method for preparing biodegradable nanofoam filter material for cigarettes according to claim 6, characterized in that, The cross-linking and curing of the mixed slurry by boric acid is specifically carried out as follows: a 5wt% boric acid solution is prepared at room temperature, and added to the mixed slurry at a mass ratio of hydrophilic polymer to boric acid of 100:0.1~0.

5. After stirring evenly, the mixture is stirred at 40~80℃ for 30~50 minutes.

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