A double-esterified starch flame retardant and its preparation method, a starch-based flame retardant finishing agent and its preparation method and application

Abstract

The present invention provides a double-esterified starch flame retardant, a starch-based flame retardant finishing agent, and their preparation methods and applications, belonging to the technical field of flame retardants. The double-esterified starch flame retardant of the present invention is prepared using corn starch, sodium dihydrogen phosphate, disodium hydrogen phosphate, and urea. The raw materials are widely sourced, abundant, easily obtainable, and inexpensive; the preparation process of the flame retardant is simple and requires few equipment; the flame retardant finishing conditions are few, convenient and easy to implement, and the flame retardant effect is good. The limiting oxygen index of the polylactic acid hada fabric after finishing is increased from 21.4 to 24.8; at the same time, the flame retardant can be completely degraded and is not likely to cause environmental pollution during subsequent treatment.

Description

Technical Field

[0001] The invention relates to the technical field of flame retardants, and in particular to a diesterified starch flame retardant, a starch-based flame retardant finishing agent, and a preparation method and application thereof. Background Art

[0002] Polylactic acid fiber has a wide range of sources, and its waste can be degraded into carbon dioxide and water in nature. It is a sustainable ecological fiber. However, its flammability and burning dripping characteristics make it a huge safety hazard in the production and use process. Therefore, it is necessary to study the flame retardant modification of polylactic acid fiber, which is of great significance for its wide application.

[0003] Starch has the advantages of being cheap, easy to obtain, pollution-free, and easy to degrade, and is widely used in flame retardant finishing. Phosphate starch is an esterified starch formed by the esterification reaction between the hydroxyl groups in the glucose residues of starch and the phosphate groups; carbamate starch is a starch in which carbamate groups replace some of the hydroxyl groups in the glucose residues of starch, and amide groups are introduced into the starch macromolecule. When the two esterified starches are used alone, the gelatinization performance and stability of the starch slurry are both low, resulting in poor flame retardant effect, and there are certain limitations in their application. Summary of the invention

[0004] The purpose of the present invention is to provide a diesterified starch, a starch-based flame retardant finishing agent, and a preparation method and application thereof. The obtained diesterified starch has a significant flame retardant effect, and the flame retardant properties of polylactic acid fabrics treated therewith are significantly improved, which can effectively improve the combustion performance of polylactic acid fabrics.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] The present invention provides a method for preparing a diester starch flame retardant, comprising the following steps:

[0007] Dissolve sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium chloride and urea in water, and mix the resulting solution with corn starch to obtain a starch suspension; in parts by mass, the corn starch is 95 to 105 parts; the sodium dihydrogen phosphate is 9.5 to 10.5 parts; the disodium hydrogen phosphate is 9.5 to 10.5 parts; the ammonium chloride is 9.5 to 10.5 parts; and the urea is 9.5 to 10.5 parts;

[0008] filtering the starch suspension to obtain a filter cake;

[0009] The filter cake is dried and then subjected to esterification reaction to obtain a double-esterified starch flame retardant; the temperature of the esterification reaction is 128-130°C.

[0010] Preferably, the esterification reaction time is 110 to 120 minutes.

[0011] Preferably, the water content of the filter cake after drying is 10-12%.

[0012] Preferably, after the esterification reaction, the following steps are further included: washing, drying, pulverizing, and sieving the esterification reaction product in sequence.

[0013] The present invention provides a double-esterified starch flame retardant prepared by the preparation method described in the above solution.

[0014] The present invention provides a preparation method of a starch-based flame retardant finishing agent, including the following steps: mixing 95-105 parts of a double-esterified starch flame retardant, 18-20 parts of a penetrant, 18-20 parts of urea, and 1900-2100 parts of water to obtain a suspension; heating the suspension under reflux condensation to gelatinize the starch to obtain a starch-based flame retardant finishing agent; the double-esterified starch flame retardant is the double-esterified starch flame retardant described above.

[0015] Preferably, the temperature of the reflux condensation heating is 88-90 °C, and the heat preservation time is 15-20 min.

[0016] The present invention provides a starch-based flame retardant finishing agent prepared by the preparation method described in the above solution.

[0017] The present invention provides an application of the starch-based flame retardant finishing agent described in the above solution in finishing polylactic acid fabrics.

[0018] Preferably, the method for finishing polylactic acid fabrics includes the following steps: immersing and rolling the polylactic acid fabric in the starch-based flame retardant finishing agent, taking out the polylactic acid fabric and performing pre-drying and baking in sequence; the temperature of the pre-drying is 103-105 °C; the temperature of the baking is 125-130 °C.

[0019] The present invention provides a preparation method of a double-esterified starch flame retardant, including the following steps: dissolving sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium chloride, and urea in water, and mixing the obtained solution with corn starch to obtain a starch suspension; calculated by mass parts, the corn starch is 95-105 parts; sodium dihydrogen phosphate is 9.5-10.5 parts; disodium hydrogen phosphate is 9.5-10.5 parts; ammonium chloride is 9.5-10.5 parts; urea is 9.5-10.5 parts; filtering the starch suspension to obtain a filter cake; drying the filter cake and then performing an esterification reaction to obtain a double-esterified starch flame retardant; the temperature of the esterification reaction is 128-130 °C.

[0020] The present invention prepares a double-esterified starch flame retardant by a double-esterification method, and prepares a phosphorus and nitrogen-containing starch-based flame retardant by introducing a phosphate group and an amino group into the starch macromolecule together.

[0021] The present invention uses phosphate-urethane double-esterified starch, namely phospho-urethane starch, which can significantly reduce the gelatinization temperature of starch, improve the stability of the starch paste, and enhance the adhesion performance to polylactic acid fibers, thereby significantly improving the flame retardancy of the polylactic acid hada fabric after finishing.

[0022] The double-esterified starch flame retardant of the present invention combines the advantages of phosphorus-based and nitrogen-based flame retardants. During the combustion degradation process, it can generate acidic substances that promote polymer carbonization, such as pyrophosphoric acid, polyphosphoric acid, etc., thereby forming a dense and continuous carbon layer on the material surface to isolate oxygen and heat from the outside and prevent free radical reactions. And the phosphorus-containing free radicals generated during the combustion process can capture external free radicals, thereby delaying the entire combustion process and achieving the effect of gas-phase flame retardancy. Moreover, during the combustion process, it will decompose under heat and then release a large amount of inert gases such as NH 3 , NO, N 2 O, etc. These gases will dilute the oxygen concentration and the concentration of combustible volatiles on the surface of the matrix to a certain extent, thereby achieving the flame retardant effect. Therefore, the phosphorus-nitrogen double-modified starch flame retardant of the present invention can significantly improve the flame retardant effect of polylactic acid fabrics compared with single phosphorus-based or nitrogen-based flame retardants.

[0023] The double-esterified starch flame retardant prepared by the present invention has good biocompatibility compared with traditional inorganic flame retardants. It not only has good adhesion to polylactic acid fibers but also can be completely biodegradable. It is a brand-new natural polymer flame retardant finishing agent.

[0024] In summary, the double-esterified starch flame retardant of the present invention is prepared using corn starch, sodium dihydrogen phosphate, disodium hydrogen phosphate, and urea. The raw materials are widely sourced, abundant, easily obtainable, and inexpensive; the preparation process of the flame retardant is simple and requires few equipment; the flame retardant finishing conditions are few, convenient and easy to implement, and the flame retardant effect is good. The limiting oxygen index of the polylactic acid hada fabric after finishing is increased from 21.4 to 24.8; at the same time, the flame retardant can be completely degraded and is not likely to cause environmental pollution during subsequent treatment. Description of the Drawings

[0025] Figure 1 Infrared spectrum of the double-esterified starch flame retardant prepared in Example 1 and corn starch;

[0026] Figure 2 Scanning electron micrograph of corn starch;

[0027] Figure 3 Scanning electron micrograph of the urethane starch flame retardant prepared in Comparative Example 1;

[0028] Figure 4 Scanning electron micrograph of the double-esterified starch flame retardant prepared in Example 1. Detailed Description of the Invention

[0029] The present invention provides a preparation method of a double-esterified starch flame retardant, comprising the following steps:

[0030] Dissolve sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium chloride and urea in water, and mix the obtained solution with corn starch to obtain a starch suspension; by mass, the corn starch is 95-105 parts; sodium dihydrogen phosphate is 9.5-10.5 parts; disodium hydrogen phosphate is 9.5-10.5 parts; ammonium chloride is 9.5-10.5 parts; urea is 9.5-10.5 parts;

[0031] Filter the starch suspension to obtain a filter cake;

[0032] Dry the filter cake and then carry out an esterification reaction to obtain a double-esterified starch flame retardant; the temperature of the esterification reaction is 128-130 °C.

[0033] In the present invention, unless otherwise specified, the raw materials used are all commercially available products well-known in the art.

[0034] In the present invention, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium chloride and urea are dissolved in water, and the obtained solution is mixed with corn starch to obtain a starch suspension.

[0035] In the present invention, by mass, the corn starch is 95-105 parts; sodium dihydrogen phosphate is 9.5-10.5 parts; disodium hydrogen phosphate is 9.5-10.5 parts; ammonium chloride is 9.5-10.5 parts; urea is 9.5-10.5 parts; water is preferably 90-110 parts. Further, the corn starch is preferably 98-102 parts, sodium dihydrogen phosphate is preferably 9.8-10.2 parts, disodium hydrogen phosphate is preferably 9.8-10.2 parts, ammonium chloride is preferably 9.8-10.2 parts, urea is preferably 9.8-10.2 parts, and water is more preferably 95-105 parts. In the present invention, the water is preferably deionized water.

[0036] In the present invention, the mixing of the obtained solution with corn starch is preferably adding corn starch to the solution. In the present invention, after adding corn starch, it is preferably stirred at 48-50 °C for 25-30 min to obtain a starch suspension. The purpose of stirring at 48-50 °C for 25-30 min in the present invention is to swell the starch and mix it fully with the reaction reagents.

[0037] After obtaining the starch suspension, the present invention filters the starch suspension to obtain a filter cake.

[0038] In the present invention, the filtration is preferably suction filtration.

[0039] After obtaining the filter cake, the present invention dries the filter cake and then conducts an esterification reaction to obtain a double-esterified starch flame retardant; the temperature of the esterification reaction is 128 - 130 °C.

[0040] In the present invention, the drying temperature is preferably 48 - 52 °C. The present invention has no special requirements for the drying time, and it is preferably sufficient to make the moisture content of the dried filter cake be 10 - 12%. The present invention controls the moisture content of the filter cake within 10 - 12% with the aim that within this range, the reaction reagents can fully contact the starch, and at the same time, it will not cause hydrolysis side reactions of the esterified starch due to excessive moisture.

[0041] In the present invention, the esterification reaction time is preferably 110 - 120 min. During the esterification reaction of the present invention, the starch macromolecules undergo a phosphoesterification reaction with orthophosphate under the catalysis of urea, and at the same time, they also react with amino groups to produce carbamate starch.

[0042] After completing the esterification reaction, the present invention preferably washes, dries, crushes, and sieves the esterification reaction product in sequence to obtain a double-esterified starch flame retardant.

[0043] In the present invention, the washing is preferably rinsing 2 - 3 times with deionized water; the drying temperature is preferably 48 - 50 °C; the sieving is preferably through a 65-mesh sieve.

[0044] The present invention prepares a double-esterified starch flame retardant by a double-esterification method. By introducing phosphate groups and amino groups into starch macromolecules together, a phosphorus and nitrogen-containing starch-based flame retardant is prepared.

[0045] The present invention provides a double-esterified starch flame retardant prepared by the preparation method described in the above solution. The double-esterified starch flame retardant of the present invention combines the advantages of phosphorus-based and nitrogen-based flame retardants. During the combustion degradation process, it can generate acidic substances that promote polymer carbonization, such as pyrophosphoric acid, polyphosphoric acid, etc., thereby forming a dense and continuous carbon layer on the material surface to isolate oxygen and heat from the outside and prevent free radical reactions; and the phosphorus-containing free radicals generated during the combustion process can capture free radicals from the outside, thereby delaying the entire combustion process and achieving a gas-phase flame retardant effect. Moreover, during the combustion process, it will decompose upon heating and then release a large amount of inert gases such as NH 3 、NO、N 2 O, etc. These gases will dilute the oxygen concentration and the concentration of combustible volatiles on the surface of the matrix to a certain extent, thereby achieving a flame retardant effect. Therefore, the phosphorus and nitrogen-containing double-modified starch flame retardant of the present invention can significantly improve the flame retardant effect of polylactic acid fabrics compared with single phosphorus-based or nitrogen-based flame retardants.

[0046] The invention provides a method for preparing a starch-based flame retardant finishing agent, comprising the following steps: mixing 95-105 parts of a diester starch flame retardant, 18-20 parts of a penetrant, 18-20 parts of urea, and 1900-2100 parts of water to obtain a suspension; condensing, refluxing, and heating the suspension to gelatinize the starch to obtain a starch-based flame retardant finishing agent; the diester starch flame retardant is the diester starch flame retardant.

[0047] In the present invention, the temperature of the condensation reflux heating is preferably 88 to 90° C., and the insulation time is preferably 15 to 20 minutes.

[0048] The present invention has no special requirements on the type of the penetrant, and any penetrant known in the art can be used. In the present invention, the role of the urea is to promote the swelling of the polylactic acid fiber in hot water, so as to facilitate the diffusion and penetration of the diesterified starch flame retardant macromolecules into the polylactic acid fiber, increase the loading rate of the flame retardant, and further enhance the flame retardant effect of the polylactic acid Hada fabric.

[0049] The present invention provides a starch-based flame retardant finishing agent prepared by the preparation method described in the above scheme.

[0050] The present invention also provides the use of the starch-based flame retardant finishing agent in the above solution in finishing polylactic acid fabrics. In the present invention, the polylactic acid fabric preferably comprises polylactic acid Hada.

[0051] In the present invention, the method for finishing polylactic acid fabric preferably includes the following steps: placing the polylactic acid fabric in the starch-based flame retardant finishing agent for immersion, taking out the polylactic acid fabric and pre-drying and baking it in sequence; the pre-drying temperature is 103-105°C; the baking temperature is 125-130°C.

[0052] In the present invention, the padding rate of the padding is preferably 200%. In the present invention, the pre-drying time is preferably 8 to 10 minutes; the baking time is preferably 2 to 3 minutes. The pre-drying stage is to remove the side reaction of the esterification reaction caused by water as much as possible, that is, the hydrolysis reaction; and then the baking at 125 to 130°C is to react with the fiber to produce a washable flame retardant effect.

[0053] The diesterified starch flame retardant, starch-based flame retardant finishing agent, and preparation method and application thereof provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0054] Example 1

[0055] Take 10 parts each of sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium chloride, and urea, dissolve them in 100 parts of deionized water to form a solution, and then add corn starch to this solution to make a suspension; place the suspension in a water bath at 50 °C for heating, and stir with a magnetic stirrer for 30 min; then filter with a suction filter to obtain a filter cake and weigh it;

[0056] Place it in an oven at 50 °C and dry it until the water content is 10%, and then react it at 130 °C for 2 h. Rinse the product with deionized water 2 - 3 times, then dry it at a low temperature in an oven at 50 °C, pulverize it and pass it through a 65-mesh sieve to obtain a powdery double-esterified starch flame retardant.

[0057] Comparative Example 1

[0058] Take 10 parts each of ammonium chloride and urea, dissolve them in 100 parts of deionized water to form a solution, and then add corn starch to this solution to make a suspension; place the suspension in a water bath at 50 °C for heating, and stir with a magnetic stirrer for 30 min; then filter with a suction filter to obtain a filter cake and weigh it;

[0059] Place it in an oven at 50 °C and pre-dry it until the water content is 10%, and then react it at 130 °C for 2 h. Rinse the product with deionized water 2 - 3 times, then dry it at a low temperature in an oven at 50 °C, pulverize it and pass it through a 65-mesh sieve to obtain a powdery carbamate starch flame retardant.

[0060] Characterize the flame retardants prepared in Example 1 and Comparative Example 1 above, and analyze their physical and chemical properties.

[0061] Figure 1 The infrared spectrum of the double-esterified starch flame retardant prepared for Example 1 and corn starch. As Figure 1 shown, compared with the original starch, in the range of 3500 - 3200 cm -1 region, ν O-H and near 1600 cm -1 the δOH band becomes narrower and the peak intensity weakens. This change is easily observable in the infrared spectrum and is due to the consumption of some hydroxyl groups in the starch molecule during the esterification reaction, resulting in a decrease in the number of O - H bonds. In addition, the carbonyl stretching vibration absorption peak ν -1 appearing near 1750 cm C＝O may be due to the conversion of a large number of hydroxyl groups into C = O bonds during the grafting process. In addition, at 1175 cm -1 and 1116 - 1007 cm -1Peak groups of stretching vibrations of C-N bonds and P=O and P-O bonds also appeared respectively at this position. The above changes indicate that after double esterification modification, amino and phosphate flame retardant functional groups have been introduced into the natural starch macromolecules.

[0062] Figure 2 Figure 4 is the scanning electron micrograph of corn starch. Figure 3 Figure 5 is the scanning electron micrograph of the urethane starch flame retardant prepared in Comparative Example 1. Figure 4 Figure 6 is the scanning electron micrograph of the double esterified starch flame retardant prepared in Example 1. It can be seen from Figure 2 Figure 4 that most of the natural corn starch granules are polygonal, with a relatively smooth surface, regular morphology, and smaller particle sizes, mostly in the range of 10 - 15 μm. It can be seen from Figure 3 Figure 5 that there is no significant difference between the nitrogen-containing single esterified starch granules prepared in Comparative Example 1 of the present invention and natural starch, indicating a relatively low degree of denaturation. In contrast, Figure 4 for the phosphorus and nitrogen-containing double esterified starch flame retardant prepared in Example 1 of the present invention in Figure 6, its granules have more significant changes compared with natural starch. Some particle surfaces show damage and pores, causing certain damage to the morphological structure of the surface chemically modified starch granules. At the same time, there is a certain degree of adhesion between the granules. This is because chemical modification leads to an increase in the surface polarity of starch granules, so the interaction between granules is enhanced. The above changes are beneficial to the gelatinization of starch granules, forming a stable finishing solution to improve the adhesion effect and flame retardant finishing effect on polylactic acid fibers.

[0063] Application Example 1

[0064] The mass composition in parts is: 100 parts of double esterified starch flame retardant; 20 parts of penetrant; 20 parts of urea; 2000 parts of deionized water.

[0065] Dissolve the double esterified starch flame retardant, penetrant, and urea in deionized water to form a suspension, transfer it to a three-necked flask, place the flask on a constant temperature magnetic stirrer, connect the condensation reflux device, and stir at 90 °C for 20 min to gelatinize the starch and prepare a flame retardant finishing agent.

[0066] Take out the product, carry out flame retardant finishing on the polylactic acid fabric, adopt the padding treatment method with a padding rate of 200%, put the treated fabric into an oven at 105 °C and dry it for 10 min, and then put it in an oven at 130 °C and bake it for 3 min to obtain the flame retardant treated polylactic acid fabric.

[0067] Comparative Application Example 1

[0068] Figure 7 is the polylactic acid fabric without flame retardant finishing.

[0069] Comparative Application Example 2

[0070] The parts by mass of the raw materials are as follows: 100 parts of the urethane starch flame retardant in Comparative Example 1; 20 parts of penetrant; 20 parts of urea; 2000 parts of deionized water.

[0071] Dissolve the urethane starch flame retardant, penetrant and urea in deionized water to form a suspension, transfer it to a three-necked flask, place the flask in a constant-temperature magnetic stirrer, connect the condensation reflux device, and stir at 90 °C for 20 min to gelatinize the starch and prepare the flame retardant finishing agent.

[0072] Take out the product, conduct flame retardant finishing on the polylactic acid fabric, use the padding treatment method, the padding rate is 200%, put the treated fabric into an oven at 105 °C and dry it for 10 min, and then put it into an oven at 130 °C and bake it for 3 min to obtain the flame retardant-treated polylactic acid fabric.

[0073] Conduct a limiting oxygen index test on the polylactic acid fabrics treated with the flame retardants in each application example and comparative application example. The determination of the limiting oxygen index refers to "GB / T5454-1997 Textiles - Burning performance - Oxygen index method", and the results are shown in Table 1.

[0074] Table 1 Limiting oxygen index of application examples and comparative application examples

[0075] Test Example Limiting Oxygen Index (%) Application Example 1 24.8 Comparative Application Example 1 21.4 Comparative Application Example 2 22.5

[0076] It can be seen from the test results in Table 1 that in Comparative Application Example 1, the limiting oxygen index of the pure-spun polylactic acid fiber hada fabric was relatively low at 21.4%, indicating relatively high flammability. In Comparative Application Example 2, for the polylactic acid hada finished with urethane monoesterified starch, the limiting oxygen index increased to 22.5%, indicating an improvement in flame retardancy. This is because in the monoesterification modification, nitrogen-containing groups were introduced, which would decompose upon heating during combustion and then release a large amount of inert gases such as NH 3 , NO, N 2 O, etc. These gases would dilute the oxygen concentration on the surface of the matrix and the concentration of flammable volatiles to a certain extent, thus achieving a flame retardant effect. However, in Comparative Application Example 2, the increase in the limiting oxygen index was not significant, indicating that the improvement in its flame retardancy was not significant. This is because although the above gases would dilute the oxygen concentration on the surface of the matrix and the concentration of flammable volatiles to a certain extent, thus achieving a flame retardant effect, their flame retardant efficiency is not high when used alone.

[0077] In Application Example 1, for the polylactic acid hada fabric treated with the double-esterified starch flame retardant, its limiting oxygen index reaches 24.8%, which is further improved compared with Comparative Application Example 2. This is because the double-esterified starch flame retardant contains both nitrogen-containing and phosphorus-containing flame retardant groups. During the combustion and degradation process, acidic substances that can promote the carbonization of the polymer, such as pyrophosphoric acid, polyphosphoric acid, etc., can be generated, thereby forming a dense and continuous carbon layer on the surface of the material to isolate the external oxygen and heat and prevent the reaction of free radicals. In addition, the phosphorus-containing free radicals generated during the combustion process can capture the external free radicals, thereby delaying the entire combustion process. Therefore, the double-esterified starch flame retardant of the present invention can have both the functions of condensed-phase flame retardancy and gas-phase flame retardancy, which is the main reason for its good flame retardancy for the polylactic acid hada fabric.

[0078] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. Application of starch-based flame retardant finishing agent in finishing polylactic acid fabric; The preparation method of the starch-based flame retardant finishing agent, comprising the following steps: Mix 95 - 105 parts of double-esterified starch flame retardant, 18 - 20 parts of penetrant, 18 - 20 parts of urea, and 1900 - 2100 parts of water to obtain a suspension; heat the suspension under reflux condensation to gelatinize the starch to obtain the starch-based flame retardant finishing agent; The preparation method of the double-esterified starch flame retardant comprises the following steps: Dissolve sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium chloride, and urea in water, and mix the obtained solution with corn starch to obtain a starch suspension; by mass, the corn starch is 95 - 105 parts; sodium dihydrogen phosphate is 9.5 - 10.5 parts; disodium hydrogen phosphate is 9.5 - 10.5 parts; ammonium chloride is 9.5 - 10.5 parts; urea is 9.5 - 10.5 parts; Filter the starch suspension to obtain a filter cake; Dry the filter cake and then carry out an esterification reaction to obtain the double-esterified starch flame retardant; the moisture content of the dried filter cake is 10 - 12%; the temperature of the esterification reaction is 128 - 130 °C, and the time of the esterification reaction is 110 - 120 min.

2. The application according to claim 1, characterized in that the method for finishing the polylactic acid fabric comprises the following steps: place the polylactic acid fabric in the starch-based flame retardant finishing agent for padding, take out the polylactic acid fabric and carry out pre-drying and baking in sequence; the temperature of the pre-drying is 103 - 105 °C; the temperature of the baking is 125 - 130 °C.

3. The application according to claim 1, characterized in that the temperature of the reflux condensation heating is 88 - 90 °C, and the heat preservation time is 15 - 20 min.

4. The application according to claim 1, characterized in that after the esterification reaction, it further includes: washing, drying, pulverizing, and sieving the esterification reaction product in sequence.

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