A process for the preparation of acetylated distarch phosphate

Abstract

The application belongs to the technical field of starch chemical modification, and particularly relates to a preparation method of acetylated distarch phosphate. The method comprises the following steps: mixing starch with a deep eutectic solvent composed of choline chloride, glycerol and a small amount of water, adding a phosphate crosslinking agent and a delayed-release alkalizing agent for crosslinking reaction after pH adjustment, performing acetylation reaction by adding acetic anhydride or vinyl acetate after cleaning by alcohol solvent replacement, and finally obtaining the product through neutralization, washing, dehydration and drying. The process utilizes the deep eutectic solvent to control water activity and particle structure, and combines the slow-release alkali source to maintain a stable reaction environment, so that the crosslinking and acetylation are uniformly carried out, and the side reactions and residual salt load are effectively reduced. The obtained acetylated distarch phosphate has stable substitution degree, uniform particle structure and excellent performance in freeze-thaw stability, and is suitable for multiple fields such as papermaking, textile and oil field additives.

Description

Technical Field

[0001] This invention belongs to the field of starch chemical modification technology, specifically relating to a method for preparing acetylated distarch phosphate. Background Technology

[0002] Acetylated distarch phosphates possess both cross-linking and esterification modification characteristics, and are widely used in papermaking, oil fields, textile sizing, and other industrial auxiliaries. Current industrial routes typically begin with the preparation of slurry from natural starch. Under alkaline conditions, phosphate cross-linking is first performed, followed by acetylation modification. For example, the pH of the slurry is adjusted to 10.7-11.2 using 2.3% sodium hydroxide, and the temperature is raised to approximately 35-45°C. Sodium trimetaphosphate or phosphorus oxychloride is added as a cross-linking agent and the temperature is maintained for 1-5 hours, with the endpoint being the system viscosity reaching a predetermined value. Subsequently, after acidification, neutralization, and multi-stage cyclone washing, the pH is adjusted to approximately 9.0-9.2, and vinyl acetate or acetic anhydride is added for acetylation for 0.5-3 hours. Finally, the product is obtained through neutralization, washing, pressure filtration, dehydration, and drying.

[0003] However, the above-mentioned process has several pain points in terms of industrial scale-up and quality consistency. First, the crosslinking stage, which uses viscosity as an "indirect endpoint," is easily affected by raw material batches, solid content, shear history, and heat and mass transfer conditions, leading to fluctuations in the degree of crosslinking and gelatinization curve. This amplifies the sensitivity of subsequent acetylation, resulting in the problem of uncontrolled coupling between the degree of substitution (DS) and the degree of crosslinking. Second, each crosslinking agent path has its own constraints: sodium trimetaphosphate exhibits a kinetic competition between hydrolysis and condensation under strong alkali and elevated temperatures. If the pH or temperature window deviates, over- or under-crosslinking can easily occur, causing viscoelastic imbalance in the slurry, increased filtration resistance, and increased drying energy consumption. While the phosphorus oxychloride system has high reactivity, it brings halogenation byproducts and occupational safety management pressures, placing higher demands on equipment sealing, exhaust gas, and wastewater treatment. Third, the process involves multiple acid-base neutralizations and eight-stage cyclone washing, resulting in significant cumulative water consumption and salt load. This not only increases wastewater treatment costs but may also leave inorganic salt residues if washing is insufficient, increasing the ash content of the finished product and affecting the transparency and flavor purity of the paste. Fourth, the acetylation stage typically occurs at pH 9.0-9.2, with vinyl acetate and acetic anhydride each exhibiting different side reactions and exothermic characteristics. Improper control can lead to uneven distribution of acetyl substitution throughout the particle, accumulation of carboxylic acid byproducts, and even the generation of off-odor precursors, ultimately resulting in batch-to-batch fluctuations in freeze-thaw stability, acid shear tolerance, and retrogradation inhibition. Fifth, typical starch slurry systems are non-Newtonian fluids. Scale-up results in insufficient local pH gradients and instantaneous mixing during feeding, easily creating "overreaction" sites in high-shear regions, leading to increased dispersion in product particle size distribution and swelling properties.

[0004] In summary, there is an urgent need for a preparation process that can balance stable substitution degree, mild exothermic characteristics, low residual salt and good filtration and drying performance while ensuring safety and environmental protection, so as to meet the higher and more stable quality requirements of the industrial sector for functional modified starch. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing acetylated distarch phosphate. Addressing the problems of difficulty in synergistic control of crosslinking and acetylation in existing processes, large fluctuations in product substitution degree and performance, high acid and alkali load and washing load, and high safety, environmental protection and cost, this invention proposes a more stable, scalable, lower residual salt and better quality consistency preparation route to obtain a product with excellent comprehensive performance in terms of freeze-thaw stability, acid shear resistance and storage and transportation stability.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for preparing acetylated distarch phosphate includes the following steps:

[0008] (1) Mix starch with a deep eutectic solvent and adjust the pH to obtain alkaline starch wet material;

[0009] (2) Add phosphate crosslinking agent and delayed-release alkali agent to alkaline starch wet material and carry out crosslinking reaction to obtain crosslinked wet material; wherein, the delayed-release alkali agent is sodium carbonate or sodium bicarbonate that has been coated.

[0010] (3) The cross-linked wet material is cleaned to remove the eutectic solvent. After cleaning, the solid and liquid are separated to obtain the displacement wet starch.

[0011] (4) Add acetic anhydride and / or vinyl acetate to the replaced wet starch to carry out an acetylation reaction to obtain the acetylated product;

[0012] (5) Neutralize, wash, dehydrate and dry the acetylated reactants to obtain acetylated distarch phosphate.

[0013] Furthermore, the deep eutectic solvent is composed of choline chloride, glycerol and water, with a molar ratio of choline chloride to glycerol of 1:1 to 1:2.5, and water accounting for 5 to 10 wt% of the deep eutectic solvent.

[0014] Furthermore, the mass ratio of the starch to the eutectic solvent is 1:0.5 to 1:1.5.

[0015] Further, after adjusting the pH value in step (1), a portion of the sample was taken from the obtained alkaline starch wet material and diluted with deionized water to a 10% solid content suspension. The pH value was measured to be 9.0-9.5.

[0016] This invention incorporates a deep eutectic solvent composed of choline chloride, glycerol, and a specific proportion of water, creating a stable liquid system at room temperature. The deep eutectic solvent, with its low water content, reduces the system's water activity, allowing starch granules to absorb liquid and swell moderately without gelatinization, thus increasing the exposure of hydroxyl sites within the granules. This facilitates the subsequent entry and reaction of phosphate groups and reduces the breakage of starch molecular chains caused by strong alkali and high water activity. Compared to traditional aqueous solutions, the side reactions of the crosslinking agent are suppressed in the deep eutectic solvent, resulting in more uniform crosslinking. Furthermore, this solvent system can be efficiently replaced by ethanol or isopropanol, and the replacement solution can be recycled after separation, improving the environmental friendliness of the process and the utilization rate of raw materials.

[0017] Furthermore, the phosphate crosslinking agent is sodium trimetaphosphate (STMP), sodium tripolyphosphate (STPP), or phosphorus oxychloride (POCl3), and the amount of the phosphate crosslinking agent added is 0.01% to 0.1% of the starch mass.

[0018] Furthermore, the preparation method of the delayed-release alkalizing agent includes:

[0019] (1) Raw material preparation: Weigh sodium carbonate or sodium bicarbonate, dry and sieve to obtain alkaline particles with a particle size of 50-200 mesh; mix stearic acid and paraffin at a mass ratio of 2:(0.8-1.2), heat to 75-85℃ to melt, and obtain a coated melt.

[0020] (2) Spray coating: Spray the coating molten liquid onto the alkaline particles. The mass ratio of the coating molten liquid to the alkaline particles is (1-4):10. Keep stirring during the spraying process. After the spraying is completed, continue stirring for 5-10 minutes to allow the coating layer on the particle surface to cool and solidify, resulting in particles with a fatty acid / paraffin coating layer on the surface. Sieve the particles and collect the portion with a particle size of 150-500 μm as a delayed-release alkalizing agent.

[0021] Furthermore, the amount of the delayed-release alkalizing agent added is 0.2-0.6% of the starch mass.

[0022] Furthermore, the crosslinking reaction described in step (2) is carried out at 30–55°C for 1–5 hours.

[0023] This invention incorporates sodium carbonate or sodium bicarbonate particles coated with stearic acid and paraffin during the crosslinking reaction stage, primarily to slowly release alkaline components. Since the crosslinking reaction requires alkaline conditions, adding a single, fast-acting alkali can easily cause a sudden pH spike in the initial stage, leading to over-crosslinking or localized degradation on the particle surface and insufficient internal reaction. The coated particles gradually dissolve at the reaction temperature, continuously providing an alkali source and maintaining the system pH, allowing the entire reaction to proceed in a relatively constant environment. This slow-release method results in a more uniform reaction of the crosslinking agent, reduces side reactions, and ultimately yields a crosslinked wet material with controllable viscosity and consistent performance. Furthermore, residual particles can be effectively removed by subsequent washing with ethanol or isopropanol and solid-liquid separation. Stearic acid substances also dissolve and are carried away in the alcohol, without affecting product purity.

[0024] Furthermore, the solvent used for cleaning in step (3) is ethanol or isopropanol.

[0025] Furthermore, the acetylation reaction in step (4) is carried out at a temperature of 35–50°C for a reaction time of 0.5–3 hours.

[0026] Further, the mass of the acetic anhydride and / or vinyl acetate in step (4) is 4 to 10% of the mass of starch.

[0027] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0028] 1. This invention introduces a deep eutectic solvent system to achieve moderate expansion and hydroxyl exposure of starch particles under low water activity conditions, thereby improving the uniformity of crosslinking and acetylation, while suppressing side reactions in traditional aqueous systems.

[0029] 2. Using sodium carbonate or sodium bicarbonate coated with stearic acid and paraffin as a delayed-release alkalizing agent can gradually release alkaline components during the reaction process, effectively avoiding excessive cross-linking of particle surface and insufficient internal reaction caused by sudden pH changes, making the entire reaction environment more stable and controllable.

[0030] 3. The eutectic solvent can be efficiently recovered and recycled after alcohol solvent replacement, reducing water consumption and salt load, and significantly improving environmental performance and production costs. The final product not only has a stable degree of substitution and high gelatinized viscosity, but also exhibits significantly better freeze-thaw stability than similar products prepared by traditional processes. This invention balances the feasibility of industrial scale-up with the consistency of product quality, and has high application and promotion value. Detailed Implementation

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Unless otherwise specified, all raw materials used in the examples are commercially available products.

[0033] Example 1

[0034] This embodiment provides a method for preparing acetylated distarch phosphate, comprising the following steps:

[0035] (1) Mix starch with a deep eutectic solvent, which is prepared by mixing choline chloride and glycerol in a molar ratio of 1:2 and contains water at a total mass of 8 wt%. The mass ratio of starch to deep eutectic solvent is 1:1. After stirring evenly, add sodium hydroxide solution to adjust the pH of the system. Take a small amount of sample and dilute it with distilled water to a 10% solids content suspension. The pH measured is 9.0, thus obtaining alkaline starch wet material.

[0036] (2) Add sodium trimephosphate to the above alkaline starch wet material. The amount added is 0.05% of the starch mass. At the same time, add a delayed-release alkalizing agent. The amount added is 0.4% of the starch mass. Carry out a cross-linking reaction. The cross-linking reaction is carried out at 40°C for 3 hours to obtain cross-linked wet material.

[0037] The preparation method of the delayed-release alkalizing agent is as follows: Sodium carbonate is sieved and particles with a diameter of 50-100 mesh are selected. Stearic acid and paraffin are mixed at a mass ratio of 2:1 and heated to 80℃ to melt, so as to obtain a coated molten liquid. The molten liquid is sprayed onto the surface of sodium carbonate particles at a mass ratio of 1:5. After spraying, the mixture is stirred and cooled. Particles with a diameter of less than 200 μm are collected by sieving as delayed-release alkalizing agents.

[0038] (3) The cross-linked wet material was washed twice with ethanol. The mass ratio of the cross-linked wet material to ethanol was 1:1 each time. After washing, the wet starch after displacement was obtained by solid-liquid separation.

[0039] (4) Add the replaced wet starch to the reactor, adjust the reaction temperature to 40°C, add acetic anhydride to the system, the amount of which is 6% of the starch mass, and react for 1.5 hours under stirring to obtain the acetylated product.

[0040] (5) The acetylated reactants were neutralized with dilute hydrochloric acid solution. A small amount of sample was taken and diluted with distilled water to a 10% solid content suspension. The pH was measured to be 6.0. The sample was then washed twice with deionized water, with a material-to-deionized water mass ratio of 1:2 each time. The washing liquid was removed by solid-liquid separation. The filter cake was dehydrated and dried in an airflow dryer to a moisture content of about 10 wt%. It was then pulverized and passed through a 200-mesh sieve to obtain the acetylated distarch phosphate product.

[0041] Example 2

[0042] This embodiment provides a method for preparing acetylated distarch phosphate, comprising the following steps:

[0043] (1) Starch was mixed with a deep eutectic solvent, which was prepared by mixing choline chloride and glycerol in a molar ratio of 1:1.2 and contained 6 wt% water by mass of the total mass of the deep eutectic solvent. The mass ratio of starch to deep eutectic solvent was 1:0.8. After stirring evenly, sodium hydroxide solution was added to adjust the pH of the system. A small amount of sample was taken and diluted with distilled water to a 10% solids content suspension. The pH measured was 9.0, thus obtaining alkaline starch wet material.

[0044] (2) Add sodium trimephosphate to the above alkaline starch wet material. The amount added is 0.05% of the starch mass. At the same time, add a delayed-release alkalizing agent. The amount added is 0.4% of the starch mass. Carry out the cross-linking reaction at 35°C for 4 hours to obtain the cross-linked wet material.

[0045] The preparation method of the delayed-release alkalizing agent is as follows: sodium carbonate is sieved to select particles with a particle size of 50-100 mesh, stearic acid and paraffin are mixed at a mass ratio of 2:1, heated to 80℃ to melt, and a coated molten liquid is obtained; the molten liquid is sprayed onto the surface of sodium carbonate particles, the mass ratio of molten liquid to sodium carbonate is 1:5, and after spraying, it is stirred and cooled, and particles with a particle size of 200 μm are collected by sieving as delayed-release alkalizing agents.

[0046] (3) The cross-linked wet material was washed twice with ethanol. The mass ratio of the cross-linked wet material to ethanol was 1:1 each time. After washing, the wet starch after displacement was obtained by solid-liquid separation.

[0047] (4) Add the replaced wet starch to the reactor, adjust the reaction temperature to 40°C, add acetic anhydride to the system, the amount of which is 5% of the starch mass, and react for 2 hours under stirring to obtain the acetylated product.

[0048] (5) The acetylated reactants were neutralized with dilute hydrochloric acid solution. A small amount of sample was taken and diluted with distilled water to a 10% solid content suspension. The pH was measured to be 6.0. The sample was then washed twice with deionized water, with a material-to-deionized water mass ratio of 1:2 each time. The washing liquid was removed by solid-liquid separation. The filter cake was dehydrated and dried in an airflow dryer to a moisture content of about 10 wt%. It was then pulverized and passed through a 200-mesh sieve to obtain the acetylated distarch phosphate product.

[0049] Example 3

[0050] This embodiment provides a method for preparing acetylated distarch phosphate, comprising the following steps:

[0051] (1) Mix starch with a deep eutectic solvent, which is prepared by mixing choline chloride and glycerol in a molar ratio of 1:2.5 and contains 10 wt% water by mass of the total mass of the deep eutectic solvent. The mass ratio of starch to deep eutectic solvent is 1:1. After stirring evenly, add sodium hydroxide solution to adjust the pH of the system. Take a small amount of sample and dilute it with distilled water to a 10% solids content suspension. The pH measured is 9.0, thus obtaining alkaline starch wet material.

[0052] (2) Add sodium trimephosphate to the above alkaline starch wet material at an amount of 0.08% of the starch mass, and add a delayed-release alkalizing agent at an amount of 0.4% of the starch mass to carry out a cross-linking reaction. The cross-linking reaction is carried out at 40°C for 3 hours to obtain cross-linked wet material.

[0053] The preparation method of the delayed-release alkalizing agent is as follows: Sodium carbonate is sieved and particles with a particle size of 50-100 mesh are selected. Stearic acid and paraffin are mixed at a mass ratio of 2:1 and heated to 80℃ to melt, so as to obtain a coated molten liquid. The molten liquid is sprayed onto the surface of sodium carbonate particles at a mass ratio of 1:3. After spraying, the mixture is stirred and cooled. Particles with a particle size of 200 μm are collected by sieving as the delayed-release alkalizing agent.

[0054] (3) The cross-linked wet material was washed twice with ethanol. The mass ratio of the cross-linked wet material to ethanol was 1:1 each time. After washing, the wet starch after displacement was obtained by solid-liquid separation.

[0055] (4) Add the replaced wet starch to the reactor, adjust the reaction temperature to 50°C, add acetic anhydride to the system, the amount of which is 10% of the starch mass, and react for 0.8 hours under stirring to obtain the acetylated product.

[0056] (5) The acetylated reactants were neutralized with dilute hydrochloric acid solution. A small amount of sample was taken and diluted with distilled water to a 10% solid content suspension. The pH was measured to be 6.0. The sample was then washed twice with deionized water, with a material-to-deionized water mass ratio of 1:2 each time. The washing liquid was removed by solid-liquid separation. The filter cake was dehydrated and dried in an airflow dryer to a moisture content of about 10 wt%. It was then pulverized and passed through a 200-mesh sieve to obtain the acetylated distarch phosphate product.

[0057] Comparative Example 1

[0058] This comparative example provides a method for preparing acetylated distarch phosphate, which differs from Example 1 in that the eutectic solvent in step (1) is replaced with a 40wt% aqueous ethanol solution.

[0059] Comparative Example 2

[0060] This comparative example provides a method for preparing acetylated distarch phosphate, which differs from Example 1 in that the eutectic solvent in step (1) is prepared by mixing choline chloride and glycerol in a molar ratio of 1:2 and contains 20 wt% water of the total mass of the eutectic solvent.

[0061] Comparative Example 3

[0062] This comparative example provides a method for preparing acetylated distarch phosphate, which differs from Example 1 in that no deep eutectic solvent is added in step (1).

[0063] Comparative Example 4

[0064] This comparative example provides a method for preparing acetylated distarch phosphate, which differs from Example 1 in that: in step (1), sodium hydroxide solution is used to adjust the pH of the system, and when a small amount of sample is taken and diluted with distilled water to a 10% solid content suspension, the pH is measured to be 10.5, and no delayed-release alkalizing agent is added in step (2).

[0065] Comparative Example 5

[0066] This comparative example provides a method for preparing acetylated distarch phosphate, which differs from Example 1 in that the delayed-release alkalizing agent in step (2) is replaced with uncoated sodium carbonate.

[0067] Comparative Example 6

[0068] This comparative example provides a method for preparing acetylated distarch phosphate, comprising the following steps:

[0069] (1) Mix starch with distilled water to prepare a starch emulsion with a mass concentration of 30 wt%;

[0070] (2) The pH of the starch emulsion was adjusted to 10 with 2 wt% sodium hydroxide solution, and sodium trimetaphosphate, accounting for 7 wt% of the starch content, was added as a crosslinking agent. The crosslinking reaction was carried out at 40°C for 2 h.

[0071] (3) After the cross-linking reaction is completed, the pH of the reaction solution is adjusted to 8.0 with 0.4 mol / L hydrochloric acid solution. Then, acetic anhydride accounting for 3 wt% of the starch content is added to the reaction solution as an esterifying agent. The esterification reaction is carried out at 25°C for 1.5 h. During the reaction, 2% sodium hydroxide solution is added dropwise to control the pH of the reaction solution to about 8.0.

[0072] (4) After the reaction is complete, add 0.4 mol / L hydrochloric acid solution dropwise through the nozzle to adjust the pH of the reaction solution to 6.5 for neutralization to stop the reaction. Then, centrifuge the reaction solution to separate the solid and liquid components.

[0073] (5) Take the solid part and wash it to remove unreacted impurities. After solid-liquid separation, take the solid part and place it in a 45°C oven for drying. After drying, crush it and pass it through a 200-mesh sieve to obtain the product.

[0074] Performance testing

[0075] Test samples: Acetylated distarch phosphates prepared in Examples 1-3 and Comparative Examples 1-6.

[0076] Test method:

[0077] 1. Degree of Substitution (DS): Take 1.00 g of sample and place it in a 250 mL Erlenmeyer flask. Add 50 mL of 75% ethanol solution to dissolve and stir thoroughly. Add 50 mL of 0.5 mol / L sodium hydroxide solution to the system and react for 30 min with stirring to completely hydrolyze the acetyl groups. Then, back-titrate the excess sodium hydroxide with 0.5 mol / L hydrochloric acid solution and record the volume of hydrochloric acid consumed. V 1. Perform a blank experiment using the same method and record the volume of hydrochloric acid consumed. V 0.

[0078] The formula for calculating the acetyl content A (%) is:

[0079]

[0080] in, V 0: Volume of hydrochloric acid consumed in the blank (mL);

[0081] V 1: Volume of hydrochloric acid consumed by the sample (mL);

[0082] C: Concentration of hydrochloric acid solution (mol / L);

[0083] m: Sample mass (mg);

[0084] 43.04: Molecular weight of acetyl (CH3CO);

[0085] Calculate the degree of substitution (DS) based on the acetyl group content:

[0086]

[0087] Where A is the acetyl content (%) in the sample; 162 is the relative molecular mass of the unsubstituted glucose unit; 42 is the relative molecular mass of the acetyl group; and 4200 is a constant.

[0088] 2. Gelatinized viscosity (mPa·s): The peak viscosity was recorded by heating to 95°C and holding for 15 minutes under 10% solids conditions using a Brabender starch gelatinizer.

[0089] 3. Freeze-thaw stability (centrifugal water separation rate, %): Prepare a 5% paste, freeze (-18℃, 24h) and thaw (25℃, 4h) three times, centrifuge at 4000 rpm for 10 min, and measure the percentage of water separated relative to the initial water volume.

[0090] The test results are shown in Table 1.

[0091] Table 1 Performance Test Results

[0092]

[0093] As can be seen from the above test results, the performance of the embodiments of the present invention is significantly better than that of the comparative examples, indicating that the preparation method of the present invention can achieve more efficient and uniform crosslinking and acetylation reactions, thereby obtaining acetylated distarch phosphate products with more stable performance and better overall performance. In contrast, comparative examples 1 and 3, due to the lack of a suitable deep eutectic solvent system, resulted in insufficient modification of the particle structure, low degree of substitution, and a significantly increased water separation rate after freeze-thaw. In comparative example 2, the water content of the deep eutectic solvent was too high, which weakened the advantage of low water activity and led to a decrease in product performance. Comparative examples 4 and 5, due to the lack of a delayed-release alkalizing agent or insufficient coating treatment, experienced large pH fluctuations during the crosslinking process, resulting in poor reaction uniformity and ultimately lower performance than the embodiments. Comparative example 6, using a traditional aqueous phase process, although able to obtain a product, had significantly insufficient degree of substitution and stability, demonstrating the advantages of the process route proposed in the present invention in terms of stability and overall performance.

[0094] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing acetylated distarch phosphate, comprising the following steps: (1) Mix starch with a deep eutectic solvent and adjust the pH to obtain alkaline starch wet material; (2) Add phosphate crosslinking agent and delayed-release alkalizing agent to alkaline starch wet material to carry out crosslinking reaction to obtain crosslinked wet material; wherein, The delayed-release alkalizing agent is coated sodium carbonate or sodium bicarbonate; (3) The cross-linked wet material is cleaned to remove the eutectic solvent. After cleaning, the solid and liquid are separated to obtain the displacement wet starch. (4) Acetic anhydride and / or vinyl acetate are added to the replaced wet starch to carry out an acetylation reaction to obtain the acetylated product; (5) The acetylated reactants are neutralized, washed, dehydrated and dried to obtain acetylated distarch phosphate; The deep eutectic solvent is composed of choline chloride, glycerol and water, with a molar ratio of choline chloride to glycerol of 1:1 to 1:2.5, and water accounting for 5 to 10 wt% of the deep eutectic solvent.

2. The preparation method according to claim 1, characterized in that, The mass ratio of starch to deep eutectic solvent is 1:0.5 to 1:1.

5.

3. The preparation method according to claim 1, characterized in that, The phosphate crosslinking agent is sodium trimetaphosphate, sodium tripolyphosphate, or phosphorus oxychloride, and the amount of the phosphate crosslinking agent added is 0.01% to 0.1% of the starch mass.

4. The preparation method according to claim 1, characterized in that, The method for preparing the delayed-release alkalizing agent includes: (1) Raw material preparation: Weigh sodium carbonate or sodium bicarbonate, dry and sieve to obtain alkaline particles with a particle size of 50-200 mesh; mix stearic acid and paraffin at a mass ratio of 2:(0.8-1.2), heat to 75-85℃ to melt, and obtain a coated melt. (2) Spray coating: Spray the coating molten liquid onto the alkaline particles. The mass ratio of the coating molten liquid to the alkaline particles is (1-4):

10. Keep stirring during the spraying process. After the spraying is completed, continue stirring for 5-10 minutes to allow the coating layer on the particle surface to cool and solidify, resulting in particles with a fatty acid / paraffin coating layer on the surface. Sieve the particles and collect the portion with a particle size of 150-500 μm as a delayed-release alkalizing agent.

5. The preparation method according to claim 1, characterized in that, The amount of the delayed-release alkalizing agent added is 0.2-0.6% of the starch mass.

6. The preparation method according to claim 1, characterized in that, The crosslinking reaction in step (2) is carried out at 30–55°C for 1–5 hours.

7. The preparation method according to claim 1, characterized in that, The solvent used for cleaning in step (3) is ethanol or isopropanol.

8. The preparation method according to claim 1, characterized in that, The acetylation reaction in step (4) is carried out at a temperature of 35–50°C for 0.5–3 hours.

9. The preparation method according to claim 1, characterized in that, The mass of the acetic anhydride and / or vinyl acetate in step (4) is 4 to 10% of the mass of starch.

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