Acetylated cellulose, method for preparing acetylated cellulose from poplar and application of acetylated cellulose in preparation of sustained-release drugs

Abstract

The invention relates to the technical field of acetified cellulose production, in particular to acetified cellulose, a method for preparing the acetified cellulose from poplar and application of the acetified cellulose in preparation of sustained-release drugs. According to the method disclosed by the invention, very surplus poplar wood in China is used as a raw material, and the acetified cellulose pulp with high cellulose content, high polymerization degree and high purity is obtained by cooking by adopting an acidic sulfite (magnesium salt base) method added with cooking auxiliaries, namely sodium citrate and methanol, and has a wide application prospect.

Description

Technical Field

[0001] The invention relates to the technical field of acetate-grade cellulose production, and in particular to acetate-grade cellulose and a preparation method thereof using poplar wood and application thereof in the preparation of sustained-release drugs. Background Art

[0002] Cellulose acetate is a high-purity cellulose raw material. With its biocompatibility, plasticity and degradability, it is widely used in the fields of biology and biopharmaceuticals. In the field of biopharmaceuticals, cellulose acetate plays a key role in bioprocessing and purification. Its porous structure and functional groups enable it to effectively adsorb, filter and chromatographically separate biomacromolecules such as proteins, enzymes and nucleobases. Cellulose acetate membranes are widely used in ultrafiltration, dialysis and tangential flow filtration to achieve efficient purification of biopharmaceutical products. Cellulose acetate is also used as a substrate for biosensors to detect and quantify biological analytes. By immobilizing enzymes, antibodies or nucleobases on the surface of cellulose acetate, biosensors can detect target molecules with high sensitivity and specificity, and have good application prospects in medical diagnosis, environmental monitoring and food safety. In the future, it has great potential in the fields of smart drug delivery, 3D printing customized medical devices, regenerative medicine, etc. With the advancement of materials science, new modification technologies (such as copolymerization and nanocomposites) will further optimize its performance, promote its development in the fields of biology and biopharmaceuticals, and benefit human health.

[0003] The raw materials for producing acetate-grade cellulose mainly include the following categories: coniferous wood, such as pine, spruce and fir; broad-leaved wood, such as eucalyptus, rubber wood, and cotton linters. (1) Coniferous wood raw materials have high cellulose content, long fibers and high strength, which are conducive to uniform distribution of pulp. Disadvantages: The growth cycle of coniferous wood is long, and the supply of resources in my country is very limited, resulting in high costs. In addition, due to its high crystallinity, the acetification reaction performance of coniferous wood acetate-grade cellulose is slightly poor. (2) Broad-leaved wood raw materials have a short growth cycle, high yield, sustainable supply, good economic efficiency, and good cellulose acetification reaction performance; its disadvantages: wood fibers are short and have low strength, which may affect the mechanical properties of cellulose derivatives. Some tree species contain high extracts, such as tannins and gums, which may affect the purity and uniformity of cellulose materials. (3) Cotton linter has the highest cellulose content (>95%) and very few impurities, and is the best quality cellulose acetate raw material. However, its sources are limited, and it is expensive. It also has the problem of slightly poor acetification performance due to its high crystallinity. However, there has been no report on the production of cellulose acetate from poplar wood.

[0004] Poplars grow quickly, have strong adaptability, and have a short harvesting cycle. They are important afforestation species that can quickly provide wood. Currently, poplars are only used to produce low-value-added wood-based panels. Therefore, the successful use of poplar wood to produce acetate-grade cellulose pulp will solve the problem of raw material sources for the production of acetate-grade cellulose pulp in my country, especially for papermaking enterprises in the north.

[0005] Acetate-grade cellulose pulp requires high cellulose content (type A cellulose content> 96.5%), high degree of polymerization, high whiteness, and high reactivity. This requires that in the process of chemical pulping and bleaching, non-cellulose substances such as hemicellulose, lignin, extracts, and metal ions must be removed as deeply as possible, and cellulose degradation must be reduced as much as possible. Therefore, the requirements for raw materials and pulping and bleaching technology are very high. However, poplar wood has a low cellulose content, low degree of polymerization, short fiber length, poor strength, high hemicellulose content and non-fibrous impurities, and it is difficult to meet the pulping requirements. Therefore, the production of acetate-grade pulp using poplar wood must optimize and innovate the pulping and bleaching technology to meet the requirements of high type A cellulose content, high intrinsic viscosity (7.0-8.5dL / g), and whiteness of more than 90% for acetate-grade cellulose pulp. This requires that lignin, hemicellulose and non-cellulose substances must be removed as deeply as possible during the pulping and bleaching process; the purification process must reduce cellulose degradation as much as possible; this brings great challenges to the pulping and bleaching production technology.

[0006] Therefore, developing a method for preparing acetate-grade cellulose using poplar wood and further applying it to the field of preparing sustained-release drugs has great application prospects. Summary of the invention

[0007] The purpose of the present invention is to overcome the above technical difficulties and provide a production process of acetified grade fiber pulp with simple process, high efficiency, environmental protection, energy saving and emission reduction, high pulp purity and excellent reaction performance, which is achieved by the following technical scheme: The first aspect of the present invention provides a method for preparing acetate-grade cellulose using poplar wood, comprising the following steps: S1. Using poplar wood chips as raw materials, washing and desanding the raw materials, adding a cooking aid to perform acid sulfite cooking to obtain wood pulp; the cooking aid is sodium citrate and methanol; S2, screening, washing and concentrating the wood pulp of S1 to remove undigested wood knots and fiber pulp agglomerates to obtain pulp A; S3, first stage bleaching: using sodium tungstate aqueous solution to perform oxidative delignification treatment on pulp A, washing until the pulp is no longer viscous and the pH value is neutral, to obtain pulp B; S4, second stage bleaching: add dimethyl dioxirane to pulp B for bleaching at a temperature of 20-30°C for a bleaching time of 60-90 min to obtain pulp C; S5, extracting slurry C with alkali to obtain slurry D; S6. Treat the pulp D with acid to obtain acetate-grade fiber pulp, and dry it to obtain acetate-grade cellulose.

[0008] As a preferred embodiment, the wood pulp of S1 has a cellulose methyl content of ≥90%, a kappa number of 5-8, and an intrinsic viscosity of 9.0-10.5.

[0009] As a preferred embodiment, in S1, a gradient temperature is used for steaming, the first stage is heated to 95-105°C and kept warm for 2-3 hours, and the second stage is heated to a maximum temperature of 135-140°C and kept warm for 1-1.5 hours.

[0010] Furthermore, the cooking conditions are as follows: the free SO 2 6.8-7.7%, combined SO 2 The amount of sodium citrate in the cooking aid is 2.5-3.5% of the raw material mass, and the amount of methanol is 7.5-8.5% of the raw material mass, and the liquid ratio is 1:6.

[0011] As a preferred embodiment, the specific steps of the first stage bleaching include: adding the dried pulp A to a sodium tungstate aqueous solution with a mass concentration of 3-4% and a pH value of 4.5, reacting at 85-95°C for 60-120 minutes, and performing oxidative delignification treatment; after the reaction is completed, washing with clean water until the pulp is no longer viscous and the pH value is measured to be 7±0.5, stopping washing, and obtaining pulp B.

[0012] As a preferred embodiment, the specific steps of the second stage bleaching include: adding dimethyl dioxirane to pulp B for bleaching, the pulp concentration is controlled to be 10wt%, the pH value is 7.0-7.5, the temperature is 20-30°C, the amount of dimethyl dioxirane relative to pulp B is 2.5-3.5wt%, and the bleaching time is 80-120min.

[0013] As a preferred embodiment, the cellulose alpha content in the acetyl fiber pulp of S6 is ≥98.0%, the intrinsic viscosity is 7.0-8.5, the cellulose crystallinity is 55-60, and the whiteness is ≥92%.

[0014] As a preferred embodiment, the alkali extraction method includes: using an alkali amount of 6-8% of the mass of slurry C, an extraction temperature of 50-70°C, an extraction slurry concentration of 8-12wt%, and an extraction time of 40-80min.

[0015] As a preferred embodiment, the acid treatment method comprises: adding HCl to pulp D, based on the absolute dry pulp, the amount of HCl is 0.8-1.2wt%, the amount of sodium hexametaphosphate is 1.5-2.5wt%, stirring and heating to 40-60°C, and the treatment time is 40-80min to obtain acetified grade fiber pulp.

[0016] The second aspect of the present invention provides an acetate-grade cellulose obtained by the above method.

[0017] The third aspect of the present invention provides a use of acetate-grade cellulose in the preparation of a sustained-release drug, wherein the sustained-release drug is a sustained-release capsule.

[0018] As a preferred embodiment, the method for preparing the sustained-release capsule comprises: (1) In a vacuum-sealed polymerization tube, add 1.0 g of polyethylene glycol methyl ether (molecular weight 2000), 9.0 g of acetyl-grade poplar cellulose and 700 ppm of stannous isooctanoate as a catalyst, remove the air from the reactants in the polymerization tube under a vacuum of 0.1 mmHg, seal the polymerization tube under vacuum, place the sealed polymerization tube in a thermostat at 140°C for polymerization for 48 hours, and obtain a white crystalline uniform polymer. Open the polymerization tube and take out the polymerization product.

[0019] (2) The polymer product obtained in step (1) is made into a film with a thickness of 0.25 mm by hot pressing or melt extrusion at 100°C and 1500 Psi pressure with cellulose acetate containing glycol ether. The film is annealed at 55°C on a polytetrafluoroethylene rod with a diameter of 2.3 mm to obtain a hollow tube with an inner diameter of 2.3 mm and an outer diameter of 2.5 mm. A 3.2 cm long hollow tube is cut, one end is sealed by heat melting, 20 mg of the desired drug is loaded from the other end, and the other end is sealed by heat melting to prepare a long-acting sustained-release drug capsule.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. In the present invention, the domestically abundant poplar wood is used as raw material, and the acidic sulfite (magnesium salt-based) cooking method with the addition of cooking aids sodium citrate and methanol is adopted. In an acidic environment, sodium citrate can adjust the pH value, catalyze the aromatic substitution reaction on the benzene ring of lignin, and reduce the acidic degradation of cellulose. In addition, the citrate ion can also chelate with metal ions to reduce the catalytic effect of metal ions on cellulose degradation. Methanol has a strong affinity for lignin, which can prevent lignin from undergoing condensation reaction, and can also promote the dissolution of lignin sulfonate, thereby greatly improving the delignification reaction rate and delignification degree during the acidic sulfite cooking process, and promoting the dissolution of non-cellulose substances and fatty compounds. Methanol has a stabilizing effect on cellulose, which slows down the isomerization rate of cellulose terminal groups. In the acidic sulfite (magnesium-based) cooking with the addition of methanol, the dissolution order of hemicellulose sugars is: arabinose>xylose>glucose; therefore, acidic sulfite (magnesium-based) cooking of poplar wood chips with the addition of cooking aids sodium citrate and methanol is of great significance for obtaining acetified cellulose pulp with high cellulose content, high degree of polymerization and high purity.

[0021] 2. In the present invention, sodium tungstate (Na 2 WO 4 ) solution is used to perform oxidative delignification on pulp A in step S2. In the second stage of the bleaching process, dimethyl dioxirane (DMD) is added to pulp B for bleaching, followed by alkali extraction and acid treatment, thereby obtaining acetyl cellulose pulp with high purity, high whiteness and excellent reaction performance. The sodium tungstate oxidative delignification process is carried out under anaerobic conditions. Sodium tungstate is the only oxidant and will not cause self-oxidation of cellulose free radical chains. Therefore, it has very good delignification selectivity and improves the delignification rate. At the same time, sodium tungstate will catalyze the wet air oxidation (mineralization) of organic matter, making it easy for the oxidative degradation products in the pulp to be converted into CO 2 and H 2 O, thereby achieving closed circulation and zero discharge of bleaching wastewater. In addition, dimethyl dioxirane (DMD) is used to bleach pulp. The electrophilicity of DMD promotes the oxidation of etherified and non-etherified aromatic rings in the lignin structure. It reacts with the electron-rich C=C double bonds on the aliphatic side chains to form epoxides or reacts with aromatic rings to form aromatic oxides. These epoxides are easily hydrolyzed into diols in aqueous solution, and the diols further cause dioxirane to undergo a ring-opening reaction, promoting the degradation and dissolution of residual lignin. The hydrolyzed diols can swell cellulose and relax the pulp fiber structure, improve the accessibility of chemical reagents and pulp cellulose, thereby improving the effects of subsequent alkali extraction and acid treatment, and improving the acetification reaction performance of cellulose pulp. The whiteness of the poplar cellulose material obtained by the acidic sulfite cooking method of this patented technology reached 78% ISO, so this bleaching process adopts a green and environmentally friendly total chlorine-free bleaching technology (TCF).

[0022] 3. In the present invention, the poplar cellulose pulp prepared by the process of this scheme has the following characteristics: cellulose A content ≥ 98.0%, intrinsic viscosity 7.5-8.5, cellulose crystallinity 55-60, whiteness ≥ 92%, and pulp reaction performance index FACTPV (g / cm 2 ) 800-1000, reaching the performance index of imported hardwood acetic cellulose.

[0023] 4. The present invention is characterized in that the "molecular channel" method is used to control the drug release rate. A low molecular weight polyether substance that can control the drug dissolution rate is dissolved in a high molecular weight cellulose acetate hydrogel, and a tube is formed by hot pressing. One end of the hot melt tube is filled with drug powder, and the other end of the hot melt tube is formed into a long-acting sustained-release drug capsule. The method for making this capsule is simple and suitable for automated industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a physical picture of the acetate-grade cellulose pulp prepared in the present invention.

[0025] Figure 2 This is a physical picture of the acetate-grade cellulose prepared in the present invention.

[0026] Figure 3-4 This is a picture of an industrial product prepared using the acetate-grade cellulose prepared by the present invention. DETAILED DESCRIPTION

[0027] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] Example 1 This embodiment provides a method for preparing acetate-grade cellulose using poplar wood, comprising the following steps: S1. Using poplar wood chips (the size of the chips is 15-40 mm in length, 7-8 mm in width, 8-11 mm in thickness, and passes through a 5×5 sieve) as raw materials, washing and desanding the poplar wood chips, adding a cooking aid to perform acid sulfite cooking, and obtaining wood pulp; the cooking aid is sodium citrate and methanol.

[0029] The cooking conditions are: the free SO 2 7.4±0.2%, combined SO 2 It is 1.3±0.2%; the amount of cooking aid sodium citrate is 3.0% of the raw material mass, the amount of methanol is 8.0% of the raw material mass, and the liquid ratio is 1:6.

[0030] The cooking adopts a multi-stage gradient cooking method: the first stage is set to heat up to 100°C for 1.5 hours and keep warm at 100°C for 2 hours; the second stage is set to heat up to the highest temperature of 138°C for 1.5 hours and keep warm for 1 hour.

[0031] After cooking, the poplar pulp has a cellulose content of 92.5%, a kappa number of 6.5, an intrinsic viscosity of 9.7, and a pulp brightness of 78%; the pulp ash content is 0.29% and the iron ion content is 71ppm.

[0032] S2. Use a pressure screen to screen, wash and concentrate the pulp obtained in the above pulping process to remove undigested wood knots and fiber pulp clumps, so that the pH value of the pulp solution is about 7, and obtain pulp A.

[0033] S3, first stage bleaching: add pulp A into Na2O3 with a mass concentration of 3.5% and a pH of 4.5. 2 WO 4 In the aqueous solution, the mass ratio of pulp A to sodium tungstate is 1:4.5, and the mixture is reacted at 90°C for 90 minutes to perform oxidative delignification treatment. After the reaction is completed, the pulp is washed 4 times with 40°C clean water until the pulp is no longer viscous and the pH is measured at 7±0.5, then washing is stopped to obtain pulp B.

[0034] S4. Second stage bleaching: dimethyl dioxirane (DMD) is added to pulp B for bleaching: the pulp concentration is controlled to be 10wt%, the amount of DMD relative to pulp B is 3.0%, pH is 7.0-7.5, the temperature is 25°C, the bleaching time is 90min, and pulp C is obtained.

[0035] S5. Perform alkali extraction on slurry C: use alkali in an amount of 7% of the mass of slurry C, temperature of 60°C, extraction concentration of 10%, time of 60 min, to obtain slurry D, the alkali is sodium hydroxide.

[0036] S6. Treat pulp D with acid: add 1.0% HCl (based on absolute dry pulp) and 2.0% sodium hexametaphosphate to pulp D with a pulp concentration of 5%. Heat to 50°C with stirring for 60 minutes to obtain acetified poplar cellulose pulp. Dry to obtain acetified cellulose.

[0037] Example 2 This embodiment provides a method for preparing acetate-grade cellulose using poplar wood, comprising the following steps: S1. Using poplar wood chips (the size of the chips is 15-40 mm in length, 7-8 mm in width, 8-11 mm in thickness, and passes through a 5×5 sieve) as raw materials, washing and desanding the poplar wood chips, adding a cooking aid to perform acid sulfite cooking, and obtaining wood pulp; the cooking aid is sodium citrate and methanol.

[0038] The cooking conditions are: the free SO 2 6.9±0.2%, combined SO 2 It is 1.0±0.2%; the amount of cooking aid sodium citrate is 2.5% of the raw material mass, the amount of methanol is 7.5%, and the liquid ratio is 1:6.

[0039] The cooking adopts a multi-stage gradient cooking method: the first stage is set to heat up to 100°C for 1.5 hours and keep warm at 100°C for 2 hours; the second stage is set to heat up to the highest temperature of 138°C for 1.5 hours and keep warm for 1 hour.

[0040] After cooking, the poplar pulp has a cellulose content of 91.8%, a kappa number of 7.2, an intrinsic viscosity of 10.1, and a pulp brightness of 77%; the pulp ash content is 0.29% and the iron ion content is 71ppm.

[0041] S2. Use a pressure screen to screen, wash and concentrate the pulp obtained in the above pulping process to remove undigested wood knots and fiber pulp clumps, so that the pH value of the pulp solution is about 7, and obtain pulp A.

[0042] S3, the first stage of bleaching: the dried pulp A is added into a Na solution with a mass concentration of 3.5% and a pH of 4.5. 2 WO 4 In the aqueous solution, the mass ratio of pulp A to sodium tungstate is 1:3.5, and the mixture is reacted at 90°C for 90 minutes to perform oxidative delignification treatment. After the reaction is completed, the pulp is washed 4 times with 40°C clean water until the pulp is no longer viscous and the pH is measured at 7±0.5, then washing is stopped to obtain pulp B.

[0043] S4. Second stage bleaching: dimethyl dioxirane (DMD) is added to pulp B for bleaching: the pulp concentration is controlled to be 10wt%, the amount of DMD relative to pulp B is 2.5%, pH is 7.0-7.5, the temperature is 25°C, the bleaching time is 90min, and pulp C is obtained.

[0044] S5. Perform alkali extraction on slurry C: use alkali in an amount of 6% of the mass of slurry C, temperature of 60°C, extraction concentration of 10%, time of 60 min, to obtain slurry D, the alkali is sodium hydroxide.

[0045] S6. Treat pulp D with acid: add 1.0% HCl (based on absolute dry pulp) and 2.0% sodium hexametaphosphate to pulp D with a pulp concentration of 5%. Heat to 50°C with stirring for 60 minutes to obtain acetified poplar cellulose pulp. Dry to obtain acetified cellulose.

[0046] Test example This test example provides the use of the acetate-grade cellulose prepared in Example 1 in the preparation of sustained-release capsules. The preparation method of the sustained-release capsules comprises: (1) In a vacuum-sealed polymerization tube, add 1.0 g of polyethylene glycol methyl ether (molecular weight 2000), 9.0 g of acetyl-grade poplar cellulose and 700 ppm of stannous isooctanoate as a catalyst, remove the air from the reactants in the polymerization tube under a vacuum of 0.1 mmHg, seal the polymerization tube under vacuum, place the sealed polymerization tube in a thermostat at 140°C for polymerization for 48 hours, and obtain a white crystalline uniform polymer. Open the polymerization tube and take out the polymerization product.

[0047] (2) The polymer product obtained in step (1) is subjected to hot pressing or melt extrusion at 100°C and 1500 Psi to form a film with a thickness of 0.25 mm with cellulose acetate containing glycol ether. The film is annealed at 55°C on a polytetrafluoroethylene rod with a diameter of 2.3 mm to obtain a hollow tube with an inner diameter of 2.3 mm and an outer diameter of 2.5 mm. A 3.2 cm long hollow tube is cut, one end is sealed by heat melting, 20 mg of the desired drug is loaded from the other end, and the other end is sealed by heat melting to obtain a long-acting sustained-release drug capsule.

[0048] Comparative Example 1 The difference between this comparative example and Example 1 is that in Comparative Example 1, the poplar wood chips are cooked by the conventional acidic sulfite (magnesium salt-based) method without adding a cooking aid.

[0049] The cooking conditions are: the free SO 2 7.5±0.2%, combined SO 2 The liquid ratio is 1.3±0.2%, and the liquid ratio is 1:6. The multi-stage gradient cooking method is used for steaming: the first stage is set to heat up to 100℃ for 1.5h and keep it at 100℃ for 2h; the second stage is set to heat up to the highest temperature of 138℃ for 1.5h and keep it at 100℃ for 1h.

[0050] After cooking, the poplar wood pulp is obtained, whose cellulose A content is 87.5%, Kappa number is 9.3, intrinsic viscosity is 10.8, pulp brightness is 62.5%; pulp ash content is 0.33%, and iron ion content is 85ppm.

[0051] The prepared poplar wood pulp was treated according to steps S2-S6 of Example 1 to prepare poplar cellulose pulp, which had a cellulose alpha content of 93.6%, an intrinsic viscosity of 6.74, a whiteness of 91.5%, an ash content of 0.03%, and an iron ion content of 8 ppm. The cellulose crystallinity was 56.86%. The Fock reaction performance of the pulp was 410 g / cm 2 (The Fock reaction performance of the pulp does not reach the standard of acetified grade pulp).

[0052] Comparative Example 2 The difference between this comparative example and Example 1 is that the cooking aid sodium citrate is not added in step S1 of Example 1, and the other steps are the same as Example 1. Poplar cellulose pulp is prepared, and its cellulose alpha content is 95.8%, the intrinsic viscosity is 7.53, and the whiteness is 91.8%; the ash content is 0.04%, and the iron ion content is 7ppm. The cellulose crystallinity is 60.79%. The Fock reaction performance of the pulp is 610g / cm 2 (The Fock reaction performance of the pulp does not reach the standard of acetified grade pulp).

[0053] Comparative Example 3 The only difference between this comparative example and Example 1 is that in step S1 of Example 1, methanol, a cooking aid, is not added. The remaining steps are the same as Example 1. Poplar cellulose pulp is prepared, and its cellulose alpha content is 96.0%, its intrinsic viscosity is 7.93, and its whiteness is 91.8%; its ash content is 0.03%, and its iron ion content is 7ppm. The cellulose crystallinity is 59.79%. The Fock reaction performance of the pulp is 580g / cm 2 (The Fock reaction performance of the pulp does not reach the standard of acetified grade pulp).

[0054] Performance Testing The physical and chemical properties of the poplar cellulose pulp prepared in the examples and comparative examples were tested by the following method: The properties of pulp are tested using the national standard GB / T1548-2016.

[0055] The test results are shown in Table 1.

[0056] Table 1

[0057] From the above data, it can be seen that the acetate-grade cellulose prepared by the present invention has high cellulose content, high degree of polymerization, and high purity, and also has good whiteness, cellulose crystallinity and reaction performance. In Comparative Examples 1-3, the types of cooking aids were adjusted, and the corresponding effects decreased to varying degrees, indicating that acid sulfite (magnesium salt-based) cooking of poplar wood chips with the addition of cooking aids sodium citrate and methanol is of great significance for obtaining acetate-grade cellulose pulp with high cellulose content, high degree of polymerization and high purity.

Claims

1. A method for preparing acetate-grade cellulose using poplar wood, characterized in that: The steps include: S1. Using poplar wood chips as raw materials, washing and desanding the raw materials, adding a cooking aid for cooking, and obtaining wood pulp; the cooking aid is sodium citrate and methanol; S2, screening, washing and concentrating the wood pulp of S1 to remove undigested wood knots and fiber pulp agglomerates to obtain pulp A; S3, first stage bleaching: using sodium tungstate aqueous solution to perform oxidative delignification treatment on pulp A, washing until the pulp is no longer viscous and the pH value is neutral, to obtain pulp B; S4, second stage bleaching: add dimethyl dioxirane to pulp B for bleaching at a temperature of 20-30°C for a bleaching time of 60-90 min to obtain pulp C; S5, extracting slurry C with alkali to obtain slurry D; S6. Treat the pulp D with acid to obtain acetate-grade fiber pulp, and dry it to obtain acetate-grade cellulose.

2. The method according to claim 1, characterized in that The wood pulp of S1 has a cellulose alpha content of ≥90%, a kappa number of 5-8, and an intrinsic viscosity of 9.0-10.

5.

3. The method according to claim 2, characterized in that The amount of sodium citrate in the cooking aid is 2.5-3.5% of the mass of the raw material, and the amount of methanol is 7.5-8.5% of the mass of the raw material.

4. The method according to claim 3, characterized in that The specific steps of the first stage bleaching include: adding the dried pulp A to a sodium tungstate aqueous solution with a mass concentration of 3-4% and a pH value of 4.5, reacting at 85-95°C for 60-120 minutes to perform oxidative delignification treatment; after the reaction is completed, washing with clean water until the pulp is no longer viscous and the pH value is measured to be 7±0.5, stopping washing, and obtaining pulp B.

5. The method according to claim 4, characterized in that The specific steps of the second stage bleaching include: adding dimethyl dioxirane to pulp B for bleaching, the pulp concentration is controlled to be 10wt%, the pH value is 7.0-7.5, the temperature is 20-30°C, the amount of dimethyl dioxirane relative to pulp B is 2.5-3.5wt%, and the bleaching time is 80-120min.

6. The method according to claim 5, characterized in that The S6 acetified fiber pulp has a cellulose content of ≥98.0%, an intrinsic viscosity of 7.0-8.5, a cellulose crystallinity of 55-60, and a whiteness of ≥92%.

7. The method according to claim 6, characterized in that The alkali extraction method includes: using an alkali amount of 6-8% of the mass of the slurry C, an extraction temperature of 50-70° C., an extraction slurry concentration of 8-12wt%, and an extraction time of 40-80min.

8. The method according to claim 1, characterized in that The acid treatment method comprises: adding HCl to pulp D, with the amount of HCl being 0.8-1.2wt% and the amount of sodium hexametaphosphate being 1.5-2.5wt% based on the absolute dry pulp, stirring and heating to 40-60° C. for a treatment time of 40-80 minutes to obtain acetified fiber pulp.

9. An acetate-grade cellulose, characterized in that: Obtained by the method described in any one of claims 1 to 8.

10. Use of acetate-grade cellulose in the preparation of sustained-release drugs, characterized in that: The sustained-release medicine is a sustained-release capsule.

Images