Method for accurately controlling acetyl substitution degree of cellulose acetate

Abstract

The invention relates to a method for accurately controlling the acetyl substitution degree of cellulose acetate, which comprises the following steps: adding ground wood pulp into a reaction kettle, adding acetic acid, stirring at room temperature, then adding acetic acid, acetic anhydride and sulfuric acid, mixing, starting acetylation reaction, and gradually raising the system temperature in the reaction process. After reacting for a period of time, stretching a probe of an on-line infrared instrument into the reaction kettle to collect infrared spectrum data on line, adding a magnesium acetate solution, and starting hydrolysis reaction; adding part of the magnesium acetate solution and water after a period of time, maintaining for a period of time, adding part of the magnesium acetate solution after the relative esterification degree monitored by an online infrared instrument reaches a target value, and terminating the hydrolysis reaction; compared with the prior art, the hydrolysis reaction endpoint can be accurately identified on line, esterification degree fluctuation caused by inaccurate reaction time control is avoided, the production efficiency and economic benefits are improved, and the product quality is favorably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of measuring the degree of substitution (DS value) of cellulose acetate and relates to a method for accurately controlling the degree of substitution of acetyl groups in cellulose acetate. Background Art

[0002] Currently, the DS value, or bound acid value (AV value), of cellulose acetate (hereafter referred to as vinegar tablets) can only be sampled and tested after the tablets exit the dryer. Production departments then adjust the production process based on these test results. Since production is continuous, this leads to a lag in process adjustments, potentially resulting in several consecutive batches with substandard bound acid content. While slurry can be collected for laboratory precipitation and drying during the hydrolysis process, this also takes a significant amount of time. Sample test results are not available in a timely manner after the hydrolysis process is completed, which also leads to a lag in process adjustments.

[0003] Cellulose acetate is a cellulose acetate produced by the esterification reaction of cellulose and acetic anhydride under the action of a catalyst. The hydroxyl groups on the cellulose molecular chain are completely or partially replaced by acetyl groups. The degree of acetyl substitution plays a decisive role in the bound acidity, degradation performance, solubility and optical properties of cellulose acetate. In the actual esterification process, it is impossible for all hydroxyl groups in the cellulose molecule to be esterified. Acetate with different esterification degrees has different properties and uses. The uses of cellulose acetate with different esterification degrees can be roughly divided into three categories: (1) esterification degree (DS value) of 2.30-2.40, corresponding to bound acidity (AV value) of 43-49%, used in the manufacture of coatings and plastics; (2) esterification degree of 2.40-2.60, corresponding to bound acidity of 53-58%, used in the manufacture of rayon and cigarette filters; (3) esterification degree of 2.80-3.00, corresponding to bound acidity of 60-62.5%, used in the manufacture of high-performance electronic films and organic permeable membranes.

[0004] Commonly used methods for testing the degree of acetyl substitution in industry include titration and spectroscopy. These methods often require the cellulose acetate produced by the esterification reaction to be precipitated, washed, dried, and crushed before testing. This is time-consuming and results in a delayed reaction for controlling the degree of acetyl substitution in cellulose acetate, hindering product quality control. Summary of the Invention

[0005] Based on this, in view of the shortcomings of the prior art, the object of the present invention is to provide a method for accurately controlling the degree of acetyl substitution of cellulose acetate, using online infrared real-time monitoring of the cellulose acetate acetyl hydrolysis process to obtain the relative degree of acetyl substitution R in real time. Through the previously established correspondence between the relative degree of substitution R by infrared method and the DS value, the DS value is obtained online in real time. After reaching the target DS value, the hydrolysis reaction is stopped in time, thereby achieving accurate control of the degree of acetyl substitution of cellulose acetate.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for accurately controlling the degree of acetyl substitution of cellulose acetate, the method comprising the following steps:

[0008] S1. Add the ground wood pulp and acetic acid into a reactor and stir for a period of time to fully swell the wood pulp;

[0009] S2, cooling the reactor, sequentially adding a chilled mixture of acetic acid, acetic anhydride, and sulfuric acid to the mixture in the reactor to initiate an acetylation reaction, wherein the system temperature gradually increases during the reaction, reaches a peak temperature, and is maintained for a predetermined time;

[0010] S3, inserting the probe of the online infrared instrument into the reactor to collect infrared spectrum data online, then quantitatively adding magnesium acetate solution for the first time to start the hydrolysis reaction, and after the temperature of the reactor is raised to a preset temperature, adding magnesium acetate solution and deionized water for a second time, continuing to raise the temperature for a preset time, and after the relative degree of substitution of acetyl groups reaches a target value through online infrared spectrum detection, adding magnesium acetate solution for a third time to terminate the hydrolysis reaction to obtain a cellulose acetate solution;

[0011] S4. Precipitate, wash, and dry the cellulose diacetate solution obtained in S3, test the bound acid value of the dried sample by chemical titration, and calculate the degree of substitution of the product offline.

[0012] Optionally, the stirring time in step S1 is 30-120 minutes, and the stirring speed is 50-500 rpm.

[0013] Optionally, in step S1, the mass ratio of wood pulp to acetic acid is 0.2-5.

[0014] Optionally, in step S2, the polymerization kettle is first cooled to -30-10°C, preferably -20-0°C.

[0015] Optionally, in step S2, the mass ratio of acetic acid to wood pulp in the reactor is 1-10, preferably 2-5; and / or

[0016] The mass ratio of acetic anhydride to wood pulp in the reactor is 1-10, preferably 2-4; and / or

[0017] The mass ratio of sulfuric acid to wood pulp in the reactor is 0.01-1, preferably 0.1-0.3.

[0018] Optionally, in step S2, the peak temperature of the reaction heating is 40-60°C, preferably the peak temperature is 45-55°C.

[0019] Optionally, in step S2, after reaching the peak temperature, the temperature is maintained for a preset time of 10-60 minutes, preferably 20-40 minutes.

[0020] Optionally, in step S3, the mass ratio of the magnesium acetate solution added for the first time to the wood pulp in the reactor is 0.1-10, preferably 0.5-2; and / or

[0021] The concentration of the magnesium acetate solution is 20 wt %.

[0022] Optionally, after the magnesium acetate solution is added for the first time in step S3, the temperature of the reactor is raised to a preset temperature of 60-80°C.

[0023] Optionally, in step S3, the mass ratio of the magnesium acetate solution added for the second time to the wood pulp in the reactor is 0.05-1, preferably 0.1-0.3; the concentration of the magnesium acetate solution is 20wt%; and / or

[0024] The mass ratio of deionized water to wood pulp in the reactor is 0.2-1, preferably 0.3-0.6.

[0025] Optionally, after the magnesium acetate solution is added for the second time in step S3, the temperature of the reactor is raised to 80-100° C. and maintained for 30-180 minutes.

[0026] Optionally, in step S3, the calculation formula (1) of the relative degree of substitution R of acetyl obtained by online infrared is as follows:

[0027]

[0028] Among them, r 1708cm-1 Infrared spectrum 1708cm -1 Corrected absorbance value, r 1051cm-1 Infrared spectrum 1051cm -1 The absorbance value is corrected at , and R is the relative substitution degree of acetyl groups.

[0029] Optionally, after the relative degree of acetyl substitution corresponding to the target esterification degree of the product is reached in step S3, the mass ratio of magnesium acetate solution to the wood pulp in the reactor is 0.1-0.5, and the concentration of magnesium acetate solution is 20wt%, and the hydrolysis reaction is terminated to obtain cellulose acetate with the target esterification degree.

[0030] Due to the adoption of the above technical solution, the present invention can accurately identify the end point of the hydrolysis reaction, avoid fluctuations in the degree of substitution caused by inaccurate control of the reaction time, improve production efficiency and economic benefits, and help improve product quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following drawings are provided for illustration:

[0032] Figure 1 This is the relationship curve between the infrared relative substitution degree R1708cm-1 and the DS value of the present invention.

[0033] Figure 2 This is the infrared spectrum of the cellulose acetylation reaction solution (DS=2.82, R=4.152).

[0034] Figure 3 This is the infrared spectrum of the cellulose acetylation reaction solution (Example 3, DS=2.44, R=3.3475). DETAILED DESCRIPTION

[0035] In order to make the purpose, technical solutions and beneficial effects of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0036] The object of the present invention is to provide a method for accurately controlling the degree of acetyl substitution of cellulose acetate, comprising the following steps:

[0037] S1. Add the ground wood pulp and acetic acid into a reactor and stir for a period of time to fully wet the wood pulp;

[0038] S2, cooling the reactor, sequentially adding acetic acid, acetic anhydride and sulfuric acid to the mixture in the reactor to start the acetylation reaction, wherein the system temperature gradually increases during the reaction, reaches a peak temperature and is maintained for a period of time;

[0039] S3. Inserting the probe of the online infrared instrument into the reactor to collect infrared spectral data online, then quantitatively adding magnesium acetate solution to start a hydrolysis reaction. After the temperature of the reactor reaches a certain temperature, adding part of the magnesium acetate solution and deionized water, continuing to heat the reactor for a certain period of time, and detecting by online infrared spectroscopy that the relative degree of substitution R of the acetyl groups reaches a target value, adding part of the magnesium acetate solution to terminate the hydrolysis reaction, thereby obtaining a cellulose acetate solution with a target DS;

[0040] S4. The cellulose acetate solution obtained in S3 is precipitated, washed, and dried. The bound acid value of the dried sample is tested by chemical titration, and the degree of substitution DS value of the product is calculated offline.

[0041] Furthermore, the stirring time in step S1 is 30-120 minutes.

[0042] Preferably, the stirring time in step S1 is 50-70 minutes.

[0043] Furthermore, the stirring speed in step S1 is 50-500 rpm.

[0044] Preferably, the stirring speed in step S1 is 200-400 rpm.

[0045] Furthermore, in step S1, the mass ratio of wood pulp to acetic acid is 0.2-5.

[0046] Preferably, in step S1, the mass ratio of wood pulp to acetic acid is 0.5-3.

[0047] Furthermore, in step S2, the reactor is cooled to -30-10°C.

[0048] Preferably, in step S2, the reaction kettle is cooled to -20-0°C.

[0049] Furthermore, in step S2, acetic acid at a mass ratio of 1-10 to S1 wood pulp, acetic anhydride at a mass ratio of 1-10 to S1 wood pulp, and sulfuric acid at a mass ratio of 0.01-1 to S1 wood pulp are added in sequence.

[0050] Preferably, in step S2, acetic acid at a mass ratio of 2-5 to S1 wood pulp, acetic anhydride at a mass ratio of 2-4 to S1 wood pulp, and sulfuric acid at a mass ratio of 0.1-0.3 to S1 wood pulp are added in sequence.

[0051] Furthermore, the peak temperature of the reaction heating in step S2 is 40-60°C.

[0052] Preferably, the peak temperature of the reaction heating in step S2 is 45-55°C.

[0053] Furthermore, the temperature in step S2 is maintained for 10-60 minutes after reaching the peak temperature.

[0054] Preferably, the temperature in step S2 is maintained for 20-40 minutes after reaching the peak temperature.

[0055] Furthermore, in step S3, a magnesium acetate solution (with a concentration of 20 wt%) is added at a mass ratio of 0.1-10 to the wood pulp in S1 to start the hydrolysis reaction.

[0056] Preferably, in step S3, a magnesium acetate solution (concentration of 20 wt%) is added at a mass ratio of 0.5-2 to the wood pulp in step S1 to start the hydrolysis reaction.

[0057] Furthermore, after the magnesium acetate solution is added in step S3, the temperature of the reactor is raised to 60-80°C.

[0058] Furthermore, in step S3, magnesium acetate solution (concentration of 20 wt%) with a mass ratio of 0.05-1 to S1 wood pulp and deionized water with a mass ratio of 0.3-1 to S1 wood pulp are added again.

[0059] Preferably, in step S3, magnesium acetate solution (concentration of 20 wt%) with a mass ratio of 0.1-0.3 to S1 wood pulp and deionized water with a mass ratio of 0.5-0.8 to S1 wood pulp are added again.

[0060] Furthermore, in step S3, the temperature is raised to 80-100° C. for the second time.

[0061] Furthermore, the second temperature increase is maintained for 30-180 minutes.

[0062] Preferably, the second temperature increase is maintained for 60-120 minutes.

[0063] Furthermore, in the online infrared spectrum of step S3, the absorbance of the acetyl group at 1708 cm-1 gradually decreases, indicating that a hydrolysis reaction occurs; 1051 cm-1 is the characteristic peak of the cellulose skeleton C-O-C, and its relative content remains unchanged in the system, serving as the internal standard peak for infrared analysis.

[0064] Furthermore, in step S3, the online infrared instrument is connected to the computer through the I / O interface, and the infrared spectrum data collected by the probe of the online infrared instrument are output on the computer as the absorbance values of 1708 cm-1 and 1051 cm-1. At the same time, the relative substitution degree R1708 cm-1 of the acetyl group is calculated in real time by the software according to formula (1).

[0065] Furthermore, after the relative degree of substitution R of acetyl groups corresponding to the target DS value is reached in step S3, a 20 wt% magnesium acetate solution having a mass ratio of 0.1-0.5 to the S1 wood pulp is added to terminate the hydrolysis reaction and obtain a cellulose acetate product with the target DS value.

[0066] Furthermore, in step S4, the cellulose acetate solution obtained after the hydrolysis reaction in S3 is precipitated, washed, and dried, and the bound acid value (AV value) of the dried sample is tested by chemical titration, and the DS value is obtained by offline calculation using the empirical formula (2).

[0067] Furthermore, the AV value (Formula 2) obtained by the chemical titration method in step S4 is used to calculate the degree of substitution DS value offline using Formula (3). The deviation between the AV value and the degree of substitution DS value obtained by the polynomial fitting formula (3) of the corresponding curves of cellulose acetate samples with different degrees of substitution and infrared relative acetylation degrees established in advance is within 5%. It is believed that the online infrared control of the degree of hydrolysis has a good level and can be applied to production process control.

[0068] The AV value and DS value are determined by chemical titration as follows:

[0069] About 2g of dry powder sample is dissolved in 150ml of acetone. 30ml of sodium hydroxide solution (1mol / L) is added and stirred for several hours at room temperature for saponification. Then, it is titrated with 0.5mol / L sulfuric acid solution, using a small amount of phenolphthalein as an indicator. At the endpoint of the titration, the solution color slowly changes from red to light red or colorless, and the volume of sulfuric acid consumed is A. At the same time, a blank sample is titrated, and the volume of sulfuric acid consumed is B. The calculation formula for combining the acid value (AV value) and the degree of substitution (DS value) is as follows:

[0070] AV=(BA)×CH2SO4×0.06005×100 / m sample mass (2)

[0071] DS=162.14×AV / (6005-42×AV) (3)

[0072] The method for establishing the relationship curve between the relative substitution degree R and DS by infrared method is as follows:

[0073] The same experimental process as in Example 1 was used. After the hydrolysis started, samples were taken at regular intervals and the sampling time and corresponding infrared spectrum data were recorded. The sample solution was added with magnesium acetate solution to terminate the hydrolysis reaction. After precipitation, washing, and drying, the DS value was obtained by chemical titration. The infrared relative substitution degree R and DS value were plotted ( Figure 1 ), and perform polynomial fitting to obtain formula (4).

[0074] The polynomial fitting formula is as follows:

[0075] DS=a+bx+cx 2 (4)

[0076] Where a=8.4987;b=-3.6476;c=0.5492,R 2 =0.9823

[0077] Compared with the prior art, the present invention has the following advantages:

[0078] The method provided by the present invention adopts an online infrared spectroscopy method for the first time to perform in-situ monitoring and control of the hydrolysis step in the cellulose acetate production process. The infrared spectroscopy technology is used to accurately determine the endpoint of the hydrolysis reaction, effectively solving the problem of determining the endpoint of the hydrolysis reaction process, avoiding fluctuations in the degree of substitution of the product, and facilitating improved production efficiency and economic benefits, as well as improved product quality.

[0079] Example 1

[0080] 200g of ground wood pulp and 100g of acetic acid (with a mass ratio of 0.5:1 to wood pulp) were added to the reactor and stirred at room temperature for 30 minutes at a stirring speed of 200rpm. After the mixing was completed, the reactor was cooled to -10°C. Subsequently, 400g of acetic acid (with a mass ratio of 2:1 to wood pulp), 400g of acetic anhydride (with a mass ratio of 2:1 to wood pulp), and 20g of sulfuric acid (with a mass ratio of 0.1:1 to wood pulp) were added and mixed. During the reaction, the system temperature gradually increased, reaching a peak temperature of about 45°C and then maintained for 20 minutes.

[0081] The online infrared instrument is connected to the computer through the I / O interface, and the infrared spectrum data collected by the probe of the online infrared instrument is output to the computer at 1708 cm -1 、1051cm-1 The absorbance value is measured, and the relative substitution degree R of acetyl groups is obtained in real time by the software according to formula (1). 1708cm-1 , when hydrolysis does not occur, R 1708cm-1 The corresponding DS value is 2.82 (infrared spectrum as shown in Figure 2 100 g of magnesium acetate solution (20 wt% concentration, 0.5:1 mass ratio to wood pulp) was added to start the hydrolysis reaction. The reactor was heated to 60° C., 20 g of magnesium acetate solution (20 wt% concentration, 0.1:1 mass ratio to wood pulp) and 100 g of deionized water (0.5:1 mass ratio to wood pulp) were added, and the temperature was continued to rise to 80° C. and maintained for 60 minutes. When R reached 4.0487, corresponding to a DS value of 2.73, 10 g of magnesium acetate solution (20 wt% concentration, 0.1:1 mass ratio to wood pulp) was added to terminate the reaction.

[0082] After precipitation, washing and drying, the sample was subjected to off-line chemical analysis of DS value, and the obtained DS value was 2.71, which deviated from the DS value obtained by online infrared analysis by 0.70%.

[0083] Example 2

[0084] 200g of ground wood pulp and 200g of acetic acid (1:1 mass ratio with wood pulp) were added to the reactor and stirred at room temperature for 60 minutes at a stirring speed of 400rpm. After the mixing was completed, the reactor was cooled to -5°C. Subsequently, 600g of acetic acid (3:1 mass ratio with wood pulp), 600g of acetic anhydride (3:1 mass ratio with wood pulp), and 40g of sulfuric acid (0.2:1 mass ratio with wood pulp) were added and mixed. During the reaction, the system temperature gradually increased, reaching a peak temperature of about 50°C and maintaining it for 30 minutes.

[0085] The online infrared instrument is connected to the computer through the I / O interface, and the infrared spectrum data collected by the probe of the online infrared instrument is output to the computer at 1708 cm -1 、1051cm -1 The absorbance value is measured, and the relative substitution degree R of acetyl groups is obtained in real time by the software according to formula (1). 1708cm-1 . 200g of magnesium acetate (20wt% concentration, 1:1 mass ratio with wood pulp) solution was added to start the hydrolysis reaction. When the hydrolysis reaction occurred, the reactor was heated to 65°C, 40g of magnesium acetate solution (20wt% concentration, 0.2:1 mass ratio with wood pulp) and 120g of deionized water (0.6:1 mass ratio with wood pulp) were added, and the temperature was continued to rise to 85°C and maintained for 80 minutes. When R reached 3.9836, corresponding to a DS value of 2.68, 20g of magnesium acetate (20wt% concentration, 0.2:1 mass ratio with wood pulp) solution was added to terminate the reaction.

[0086] After precipitation, washing and drying, the sample was subjected to off-line chemical analysis of DS value, and the obtained DS value was 2.63, which deviated from the DS value obtained by online infrared analysis by 1.90%.

[0087] Example 3

[0088] 200g of ground wood pulp and 600g of acetic acid (3:1 mass ratio with wood pulp) were added to the reactor and stirred at room temperature for 70 minutes at a stirring speed of 500rpm. After the mixing was completed, the reactor was cooled to -5°C. Subsequently, 1000g of acetic acid (5:1 mass ratio with wood pulp), 800g of acetic anhydride (4:1 mass ratio with wood pulp), and 40g of sulfuric acid (0.3:1 mass ratio with wood pulp) were added and mixed. During the reaction, the system temperature gradually increased, reaching a peak temperature of about 55°C and maintaining it for 40 minutes.

[0089] The online infrared instrument is connected to the computer through the I / O interface, and the infrared spectrum data collected by the probe of the online infrared instrument is output to the computer at 1708 cm -1 、1051cm -1 The absorbance value is measured, and the relative substitution degree R of acetyl groups is obtained in real time by the software according to formula (1). 1708cm-1 . Add 400g of magnesium acetate (20wt% concentration, 2:1 mass ratio with wood pulp) solution to start the hydrolysis reaction. When the hydrolysis reaction occurs, the reactor is heated to 80℃, 60g of magnesium acetate solution (20wt% concentration, 0.3:1 mass ratio with wood pulp) and 160g of deionized water (0.8:1 mass ratio with wood pulp) are added, and the temperature is continued to rise to 100℃ and maintained for 120 minutes. When R reaches 3.3475, the corresponding DS value is 2.44 (infrared spectrum as shown in FIG. Figure 3 When the reaction mixture was stirred for 1 h, 50 g of magnesium acetate (20 wt% concentration, 0.5:1 mass ratio to wood pulp) solution was added to terminate the reaction.

[0090] After precipitation, washing and drying, the sample was subjected to off-line chemical analysis of DS value, and the obtained DS value was 2.39, which deviated from the DS value obtained by online infrared analysis by 2.05%.

[0091] Example 4

[0092] 200g of ground wood pulp and 400g of acetic acid (with a mass ratio of 2:1 to wood pulp) were added to the reactor and stirred at room temperature for 60 minutes at a stirring speed of 500rpm. After the mixing was completed, the reactor was cooled to -7°C. Subsequently, 800g of acetic acid (with a mass ratio of 4:1 to wood pulp), 600g of acetic anhydride (with a mass ratio of 3:1 to wood pulp), and 40g of sulfuric acid (with a mass ratio of 0.3:1 to wood pulp) were added and mixed. During the reaction, the system temperature gradually increased, reaching a peak temperature of about 50°C and maintaining it for 30 minutes.

[0093] The online infrared instrument is connected to the computer through the I / O interface, and the infrared spectrum data collected by the probe of the online infrared instrument is output to the computer at 1708 cm -1 、1051cm -1 The absorbance value is measured, and the relative substitution degree R of acetyl groups is obtained in real time by the software according to formula (1). 1708cm-1 . 200g of magnesium acetate (20wt% concentration, 1:1 mass ratio with wood pulp) solution was added to start the hydrolysis reaction. When the hydrolysis reaction occurred, the reactor was heated to 75°C, 40g of magnesium acetate solution (20wt% concentration, 0.2:1 mass ratio with wood pulp) and 120g of deionized water (0.6:1 mass ratio with wood pulp) were added, and the temperature was continued to rise to 90°C and maintained for 90 minutes. When R reached 3.7984, corresponding to a DS value of 2.57, 40g of magnesium acetate (20wt% concentration, 0.4:1 mass ratio with wood pulp) solution was added to terminate the reaction.

[0094] After precipitation, washing and drying, the sample was subjected to off-line chemical analysis of DS value, and the obtained DS value was 2.60, which deviated from the DS value obtained by online infrared analysis by 1.10%.

[0095] The experimental conditions and reaction results of the above examples are shown in Table 1.

[0096]

[0097] The above description of the embodiments is intended to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above-described embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for accurately controlling the degree of acetyl substitution of cellulose acetate, characterized in that: The method comprises the following steps: S1. Add the ground wood pulp and acetic acid into a reactor and stir for a period of time to fully swell the wood pulp; S2, cooling the reactor, sequentially adding a chilled mixture of acetic acid, acetic anhydride, and sulfuric acid to the mixture in the reactor to initiate an acetylation reaction, wherein the system temperature gradually increases during the reaction, and maintains the peak temperature for a predetermined time after reaching the peak temperature; S3, inserting the probe of the online infrared instrument into the reactor to collect infrared spectrum data online, then quantitatively adding magnesium acetate solution for the first time to start the hydrolysis reaction, and after the temperature of the reactor is raised to a preset temperature, adding magnesium acetate solution and deionized water for a second time, continuing to raise the temperature for a preset time, and after the relative degree of substitution of acetyl groups reaches a target value through online infrared spectrum detection, adding magnesium acetate solution for a third time to terminate the hydrolysis reaction to obtain a cellulose acetate solution; S4. Precipitate, wash, and dry the cellulose diacetate solution obtained in S3, test the bound acid value of the dried sample by chemical titration, and calculate the degree of substitution of the product offline.

2. The method for accurately controlling the degree of acetyl substitution of cellulose acetate according to claim 1, characterized in that: The stirring time in step S1 is 30-120 minutes, and the stirring speed is 50-500 rpm.

3. The method for accurately controlling the degree of acetyl substitution of cellulose acetate according to claim 1, wherein: In step S1, the mass ratio of wood pulp to acetic acid is 0.2-5.

4. The method for accurately controlling the degree of acetyl substitution of cellulose acetate according to claim 1, wherein: Step S2: First, cool the polymerization kettle to -30-10°C, preferably -20-0°C.

5. The method for accurately controlling the degree of acetyl substitution of cellulose acetate according to claim 1, wherein: In step S2, the mass ratio of acetic acid to wood pulp in the reactor is 1-10, preferably 2-5; and / or The mass ratio of acetic anhydride to wood pulp in the reactor is 1-10, preferably 2-4; and / or The mass ratio of sulfuric acid to wood pulp in the reactor is 0.01-1, preferably 0.1-0.

3.

6. The method for accurately controlling the degree of acetyl substitution of cellulose acetate according to claim 1, wherein: In step S2, the peak temperature of the reaction temperature rise is 40-60°C, preferably 45-55°C.

7. The method for accurately controlling the degree of acetyl substitution of cellulose acetate according to claim 1, wherein: In step S2, after reaching the peak temperature, the temperature is maintained for a preset time of 10-60 minutes, preferably 20-40 minutes.

8. The method for accurately controlling the degree of acetyl substitution of cellulose acetate according to claim 1, wherein: In step S3, the mass ratio of the magnesium acetate solution added for the first time to the wood pulp in the reactor is 0.1-10, preferably 0.5-2; and / or The concentration of the magnesium acetate solution is 20 wt %.

9. The method for accurately controlling the degree of acetyl substitution of cellulose acetate according to claim 1, wherein: After the magnesium acetate solution is added for the first time in step S3, the temperature of the reactor is raised to a preset temperature of 60-80°C.

10. The method for accurately controlling the degree of acetyl substitution of cellulose acetate according to claim 1, wherein: The mass ratio of the magnesium acetate solution added for the second time in step S3 to the wood pulp in the reactor is 0.05-1, preferably 0.1-0.3; the concentration of the magnesium acetate solution is 20wt%; and / or The mass ratio of deionized water to wood pulp in the reactor is 0.2-1, preferably 0.3-0.

6.

11. The method for accurately controlling the degree of acetyl substitution of cellulose acetate according to claim 1, wherein: After the magnesium acetate solution is added for the second time in step S3, the reactor temperature is raised to 80-100° C. and maintained for 30-180 minutes.

12. The method for accurately controlling the degree of acetyl substitution of cellulose acetate according to claim 1, wherein: In step S3, the calculation formula (1) of the relative substitution degree R of acetyl obtained by online infrared is as follows: Among them, r 1708cm-1 Infrared spectrum 1708cm -1 Corrected absorbance value, r 1051cm-1 Infrared spectrum 1051cm -1 The absorbance value is corrected at , and R is the relative substitution degree of acetyl groups.

13. The method for accurately controlling the degree of acetyl substitution of cellulose acetate according to claim 1, wherein: After reaching the relative degree of acetyl substitution corresponding to the target degree of esterification of the product in step S3, a magnesium acetate solution is added for the third time at a mass ratio of 0.1-0.5 to the wood pulp in the reactor, and the concentration of the magnesium acetate solution is 20 wt %, and the hydrolysis reaction is terminated to obtain cellulose acetate with the target degree of esterification.

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