An emulsion type self-crosslinking bio-based itaconate ester pressure sensitive adhesive and a preparation method thereof

Abstract

The application discloses an emulsion type self-crosslinking bio-based itaconate pressure-sensitive adhesive and a preparation method thereof. The emulsion type self-crosslinking bio-based itaconate pressure-sensitive adhesive is prepared from raw materials including water, itaconate monomers, acrylic ester monomers, functional monomers, self-crosslinking monomers, an initiator, a pH buffer and an emulsifier. The application partially replaces acrylic ester-based monomers from non-renewable petrochemical resources with renewable monomers, provides a new scheme for preparing the pressure-sensitive adhesive, and the prepared pressure-sensitive adhesive has high bio-based content, high 180-degree peeling strength and tack retention, simple process, excellent comprehensive bonding performance and good application prospect.

Description

Technical Field

[0001] This invention relates to the field of pressure-sensitive adhesive technology, specifically to an emulsion-type self-crosslinking bio-based itaconic acid ester pressure-sensitive adhesive and its preparation method. Background Technology

[0002] Pressure-sensitive adhesive (PSA) is a pressure-responsive adhesive that can quickly bond smooth surfaces under relatively low pressure.

[0003] Acrylic-based pressure-sensitive adhesives are the most widely used and have the largest production volume among pressure-sensitive adhesives. However, the preparation of acrylic-based pressure-sensitive adhesives is highly dependent on non-renewable petrochemical resources. Therefore, it is of great significance to use renewable bio-based monomers to partially replace non-renewable petroleum-based monomers in the preparation of pressure-sensitive adhesives.

[0004] Acrylic pressure-sensitive adhesives are classified into emulsion-type and solvent-based types. Emulsion-type acrylic pressure-sensitive adhesives, due to their emulsion polymerization process, avoid the use of large amounts of solvents, thus reducing volatile organic compound (VOC) emissions and offering greater environmental advantages. Furthermore, emulsion-type pressure-sensitive adhesives have higher molecular weights, simpler processes, and lower costs. The market share of emulsion-type acrylic pressure-sensitive adhesives has been increasing year by year, indicating broad development prospects.

[0005] Therefore, there is an urgent need for an emulsion-type acrylic pressure-sensitive adhesive that can partially replace non-renewable petroleum-based monomers. Summary of the Invention

[0006] To address the problems existing in current technologies, alleviate the dependence of pressure-sensitive adhesives on non-renewable petroleum resources, reduce VOC pollution during production, and lower carbon emissions, this invention provides an emulsion-type self-crosslinking bio-based itaconic acid ester pressure-sensitive adhesive and its preparation method. Itaconic acid ester can be synthesized from itaconic acid obtained through bio-fermentation and bio-alcohol via esterification. This invention employs a semi-continuous emulsion polymerization process to partially replace acrylate monomers with itaconic acid ester monomers in the preparation of bio-based pressure-sensitive adhesives, providing a solution for the sustainable development of the industry. This invention uses renewable monomers to partially replace acrylate monomers derived from non-renewable petrochemical resources, resulting in a pressure-sensitive adhesive with high bio-based content, high 180° peel strength, and good tackiness. The process is simple, and the overall adhesive performance is excellent, showing promising application prospects.

[0007] One objective of this invention is to provide an emulsion-type self-crosslinking bio-based itaconic acid ester pressure-sensitive adhesive, prepared from raw materials comprising the following components: water, itaconic acid ester monomer, acrylate monomer, functional monomer, self-crosslinking monomer, initiator, pH buffer, and emulsifier.

[0008] In a preferred embodiment of the present invention,

[0009] Each component is based on 100 parts by weight of water.

[0010]

[0011] In a preferred embodiment of the present invention,

[0012] The itaconic acid ester monomer is one or more selected from dimethyl itaconic acid, diethyl itaconic acid, dibutyl itaconic acid, and diisooctyl itaconic acid, preferably dibutyl itaconic acid; and / or,

[0013] The acrylate monomer is one or more selected from butyl acrylate, n-octyl acrylate, isooctyl acrylate, ethyl acrylate, methyl acrylate, methyl methacrylate, isooctyl methacrylate, isoborneol acrylate, and isoborneol methacrylate, preferably butyl acrylate and / or isooctyl acrylate; and / or,

[0014] The functional monomer is a copolymerizable monomer containing a carboxylic acid group, preferably one or more of acrylic acid, methacrylic acid, itaconic acid, monomethyl itaconic acid, and monobutyl itaconic acid, more preferably acrylic acid and / or methacrylic acid; the functional monomer can undergo a cross-linking reaction in the colloidal particles to form a three-dimensional network structure, enhancing the mechanical properties and stability of the pressure-sensitive adhesive, and can also adjust the adhesion properties of the pressure-sensitive adhesive, and enhance the solvent resistance of the pressure-sensitive adhesive, making it less prone to softening or dissolving when in contact with solvents, thereby improving the stability and durability of the pressure-sensitive adhesive; specific functional monomers can also enhance the oxidation resistance of the pressure-sensitive adhesive, extending its lifespan during storage and use, and reducing performance degradation caused by oxidation; in addition, the introduction of functional monomers can also adjust the rheological properties of the pressure-sensitive adhesive, including viscosity, flowability, and deformation behavior, thereby adapting to different application requirements; and / or,

[0015] The self-crosslinking monomer is one or both of glycidyl methacrylate and glycidyl acrylate; and / or,

[0016] The initiator is a water-soluble thermal initiator, preferably potassium persulfate and / or ammonium persulfate, more preferably ammonium persulfate; and / or,

[0017] The pH buffer is sodium bicarbonate and / or disodium hydrogen phosphate, preferably sodium bicarbonate; and / or,

[0018] The emulsifier is one or more of anionic emulsifiers and nonionic emulsifiers, preferably one or more of sodium dodecyl sulfonate, sodium dodecylbenzene sulfonate, sodium dodecyl sulfate, 1-allyloxy-3-(4-nonylphenol)-2-propanol polyoxyethylene (10) ether ammonium sulfate, and polyoxyethylene octylphenol ether-10, more preferably sodium dodecylbenzene sulfonate and 1-allyloxy-3-(4-nonylphenol)-2-propanol polyoxyethylene (10) ether ammonium sulfate, and even more preferably, the ratio of the two is 1:2.

[0019] In a preferred embodiment of the present invention,

[0020] The raw materials for preparing the emulsion-type self-crosslinking bio-based itaconic acid ester pressure-sensitive adhesive also include a molecular weight regulator; preferably,

[0021] The molecular weight regulator is one or both of n-dodecyl mercaptan and tert-dodecyl mercaptan; and / or

[0022] The amount of the molecular weight regulator is 0-0.15 parts by weight, preferably 0-0.03 parts by weight, and more preferably 0.001-0.03 parts by weight, based on 100 parts by weight of water.

[0023] A second objective of this invention is to provide a method for preparing an emulsion-type self-crosslinking bio-based itaconic acid ester pressure-sensitive adhesive, which is one of the objectives of this invention. The method includes semi-continuous emulsion polymerization of components including water, itaconic acid ester monomer, acrylate monomer, functional monomer, self-crosslinking monomer, initiator, pH buffer, emulsifier and optional molecular weight regulator to obtain the emulsion-type self-crosslinking bio-based itaconic acid ester pressure-sensitive adhesive.

[0024] In a preferred embodiment of the present invention,

[0025] The method includes:

[0026] (a) Using water as a solvent, a pre-emulsion is prepared by mixing itaconic acid monomer, acrylate monomer, functional monomer, self-crosslinking monomer, pH buffer, optional molecular weight regulator and 60-95% emulsifier.

[0027] (b) React 3-20% of the preemulsion obtained in step (a), water, 1-80% of the initiator and the remaining emulsifier until the emulsion turns blue;

[0028] (c) After the remaining pre-emulsion and initiator are added dropwise to the reaction system of step (b) at a uniform rate, the temperature is increased to carry out the reaction. After the reaction, a weak base is added to adjust the pH to a weakly alkaline state, thus obtaining the emulsion-type self-crosslinking bio-based itaconic acid ester pressure-sensitive adhesive.

[0029] In a preferred embodiment of the present invention,

[0030] The mass ratio of the amount of water used in step (a) to the total amount of water used in step (b) is 1:(0.1-10).

[0031] In a preferred embodiment of the present invention,

[0032] In step (b),

[0033] The reaction temperature is 70-80℃, preferably 75-80℃.

[0034] In a preferred embodiment of the present invention,

[0035] In step (c),

[0036] The reaction temperature is 85-90℃, preferably 85-88℃, and the reaction time is 30-120 min, preferably 50-100 min; and / or,

[0037] The weak base includes ammonia; and / or,

[0038] The term "weakly alkaline" refers to a pH of 7-9, preferably 7-8.

[0039] In a preferred embodiment of the present invention,

[0040] Step (c) also includes the step of adjusting the pH to a slightly alkaline state and then filtering the emulsion.

[0041] The present invention can adopt the following specific technical solutions:

[0042] The method for preparing the bio-based pressure-sensitive adhesive of the present invention includes the following steps:

[0043] (a) Using water as a solvent, itaconic acid ester monomer, acrylate monomer, functional monomer, self-crosslinking monomer, 50-100% pH buffer, optional molecular weight regulator and 60-95% emulsifier are mixed and stirred to prepare a pre-emulsion; wherein the stirring rate is 200-500 r / min and the stirring time is 30-60 min.

[0044] (b) React 3-20% of the pre-emulsion obtained in step (a), water, 1-80% of the initiator, the remaining pH buffer, and the remaining emulsifier at 70-80°C until the emulsion turns blue; wherein the stirring rate is 150-300 r / min.

[0045] (c) The remaining pre-emulsion and initiator are added dropwise to the reaction system of step (b) at a constant rate over a certain period of time; wherein the stirring rate is 150-300 r / min and the dropping time is 2h-4h; after the dropping is completed, the stirring rate is maintained and the reaction temperature is raised to 85-90℃ and reacted for 30-120min. After the reaction is stopped and the temperature is cooled, a weak base is added to adjust the pH value to between 7 and 9; the emulsion is filtered to obtain the emulsion-type self-crosslinking bio-based itaconic acid ester pressure-sensitive adhesive.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] The increased content of bio-based itaconic acid esters (preferably dibutyl itaconic acid) improves the loss modulus of the pressure-sensitive adhesive at a shear frequency of 50 Hz and reduces the storage modulus at a shear frequency of 0.1 Hz under 1% strain conditions, increasing energy loss during the peeling process and thus improving the 180° peel strength. On the other hand, the molecular weight and gel content of the pressure-sensitive adhesive decrease after copolymerizing the acrylate monomer with the itaconic acid ester monomer, which significantly reduces its holding power. Therefore, this invention introduces a self-crosslinking monomer to increase its crosslinking degree, while exploring the appropriate range of self-crosslinking monomer dosage to ensure high initial tack and 180° peel strength while significantly improving holding power. In this invention, the appropriate amount of carboxyl groups reacts with epoxy groups to effectively increase the crosslinking density of the pressure-sensitive adhesive, thereby improving its holding power.

[0048] This invention preferably uses 30-50 wt% (of the total monomer mass) of bio-based itaconic acid ester monomers to reduce the use of petroleum-based acrylate monomers, thereby reducing the dependence of the pressure-sensitive adhesive manufacturing industry on petrochemical resources and reducing carbon emissions. Compared with solvent-based pressure-sensitive adhesive manufacturing processes, the emulsion polymerization process further reduces volatile organic compound (VOC) emissions, reducing environmental pollution. The increased itaconic acid ester content improves the 180° peel strength of the pressure-sensitive adhesive. The self-crosslinking reaction improves the durability and heat resistance of the pressure-sensitive adhesive. The bio-based pressure-sensitive adhesive of this invention preferably has a holding power of 10 hours or more, more preferably 40 hours or more; the initial tack is preferably 5 (representing the ball number of the rolling ball) or more, more preferably 10 or more; and the 180° peel strength is preferably 5 N / 25 mm or more, more preferably 10 N / 25 mm or more. Detailed Implementation

[0049] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are still within the scope of protection of the present invention.

[0050] Unless otherwise specified, all raw materials used in the following examples and comparative examples are commercially available.

[0051] Initial tack was determined according to GB / T 4852-2002; holding tack was determined according to GB / T 4851-1998; 180° peel strength was determined using a computer-controlled ring peel strength tester according to GB / T 2792-2014. Testing was stopped when holding tack exceeded 168 hours.

[0052] Performance test sample preparation: The emulsion-type self-crosslinking bio-based itaconic acid ester pressure-sensitive adhesive prepared in the examples and comparative examples was coated onto a 50 μm PET film using an electric coater, dried in a 105℃ forced-air oven for 8 min, covered with release paper and cut into the required size for later use.

[0053] Example 1

[0054] (a) Preparation of preemulsion: 23 parts by weight of dibutyl itaconic acid, 48 parts by weight of butyl acrylate, 1.53 parts by weight of acrylic acid, 0.38 parts by weight of glycidyl methacrylate, 1.15 parts by weight of 1-allyloxy-3-(4-nonylphenol)-2-propanol polyoxyethylene (10) ether ammonium sulfate, 0.57 parts by weight of sodium dodecylbenzenesulfonate, 0.153 parts by weight of sodium bicarbonate, and 0 parts by weight of tert-dodecyl mercaptan were mixed with 46 parts by weight of deionized water and stirred at 350 r / min for 50 min at 50 °C.

[0055] (b) Semi-continuous emulsion polymerization: 5% of the pre-emulsion obtained in step (a) was placed in a four-necked flask, and then 31 parts by weight of deionized water, 0.38 parts by weight of 1-allyloxy-3-(4-nonylphenol)-2-propanol polyoxyethylene (10) ether ammonium sulfate, 0.19 parts by weight of sodium dodecylbenzenesulfonate and 0.076 parts by weight of sodium bicarbonate were added to the four-necked flask. The mixture was stirred at 230 r / min for 20 min, and the temperature was raised to 78°C. An ammonium persulfate solution (0.15 parts by weight of ammonium persulfate dissolved in 4.6 parts by weight of deionized water) was added to the four-necked flask, and stirring was continued until the emulsion turned blue.

[0056] (c) Using a constant-pressure dropping funnel, the remaining pre-emulsion and ammonium persulfate solution (0.23 parts by weight of ammonium persulfate dissolved in 18.4 parts by weight of deionized water) are added dropwise at a constant rate to the reaction system of step (b). The dropping time is 2 hours. After the dropping is complete, the reaction temperature is increased to 85°C while maintaining the stirring rate and reacted for 90 minutes. Then, the reaction is stopped, the temperature is lowered to 40°C, and ammonia water is added to make the pH of the emulsion 7.5. The emulsion is filtered out, which is the emulsion-type self-crosslinking bio-based itaconic acid ester pressure-sensitive adhesive.

[0057] Example 2

[0058] The formulation and preparation method are the same as in Example 1, except that: 30.5 parts by weight of dibutyl itaconic acid and 40.5 parts by weight of butyl acrylate.

[0059] Example 3

[0060] The formulation and preparation method are the same as in Example 1, except that: 38 parts by weight of dibutyl itaconic acid and 33 parts by weight of butyl acrylate.

[0061] Example 4

[0062] The formulation and preparation method are the same as in Example 1, except that: 0.765 parts by weight of glycidyl methacrylate.

[0063] Example 5

[0064] The formulation and preparation method are the same as in Example 1, except that: 1.15 parts by weight of glycidyl methacrylate.

[0065] Example 6

[0066] The formulation and preparation method are the same as in Example 1, except that: 1.53 parts by weight of glycidyl methacrylate.

[0067] Example 7

[0068] The formulation and preparation method are the same as in Example 1, except that: 0.023 parts by weight of tert-dodecyl mercaptan.

[0069] Example 8

[0070] The formulation and preparation method are the same as in Example 1, except that: 0.038 parts by weight of tert-dodecyl mercaptan.

[0071] Example 9

[0072] The formulation and preparation method are the same as in Example 1, except that: glycidyl methacrylate is used in 0 parts by weight.

[0073] The adhesion test results of pressure-sensitive adhesives in Examples 1-9 are shown in Table 1:

[0074] Table 1. Adhesion Test of Pressure-Sensitive Adhesive

[0075]

[0076]

[0077] Note: The initial tack value represents the ball number of the rolling ball.

[0078] As can be seen from Examples 1 and 2-3, when itaconic acid monomers partially replace acrylate monomers to prepare bio-based pressure-sensitive adhesives in this invention, the content of itaconic acid monomers should not be too high. Although adding too much bio-based itaconic acid monomers will significantly improve the 180° peel strength, it will reduce the initial tack and holding power (as in Example 3). In order to ensure that the bio-based pressure-sensitive adhesive has both high holding power and high initial tack and 180° peel strength, itaconic acid monomers preferably account for 30-50 wt% of the total monomer mass in this invention, more preferably 30-40 wt%, and even more preferably 30-35 wt%.

[0079] As can be seen from Examples 1 and 9, if only itaconic acid ester monomers are used to partially replace acrylate monomers in the preparation of bio-based pressure-sensitive adhesives without adding self-crosslinking monomers, the holding power will be significantly reduced (Example 9). Therefore, the present invention specifically introduces self-crosslinking monomers to ensure initial tack and 180° peel strength while significantly improving holding power (Example 1). Furthermore, as can be seen from Examples 1 and Examples 4-6, although the introduction of self-crosslinking monomers can improve holding power, excessive introduction of self-crosslinking monomers will cause a decrease in initial tack and 180° peel strength. Therefore, in order to ensure that the bio-based pressure-sensitive adhesive has both high holding power and high initial tack and 180° peel strength, the amount of self-crosslinking monomers used in the present invention is preferably 0.1-3 parts by weight, more preferably 0.16-0.38 parts by weight.

[0080] It should be noted that the embodiments described above are only for explaining the present invention and do not constitute any limitation on the present invention. The present invention has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory terms, not limiting terms. Modifications can be made to the present invention within the scope of the claims, and revisions can be made to the present invention without departing from the scope and spirit of the present invention. Although the present invention described herein relates to specific methods, materials, and embodiments, it does not mean that the present invention is limited to the specific examples disclosed herein; on the contrary, the present invention can be extended to all other methods and applications with the same function.

[0081] All publications, patent applications, patents, and other references mentioned in this specification are incorporated herein by reference. Unless otherwise defined, all technical and scientific terms used in this specification have the meanings commonly understood by those skilled in the art. In case of conflict, the definitions in this specification shall prevail.

[0082] When this specification uses the prefixes “known to those skilled in the art,” “prior art,” or similar terms to derive materials, substances, methods, steps, apparatus, or components, the objects derived from such prefixes cover those commonly used in the art at the time of this application, but also include those that are not currently commonly used but will become generally recognized in the art as suitable for similar purposes.

[0083] The endpoints and any values ​​of the ranges disclosed in this application are not limited to the precise ranges or values; such ranges or values ​​should be understood to include values ​​close to them. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. In principle, various technical solutions can be combined with each other to obtain new technical solutions, which should also be considered as specifically disclosed herein.

[0084] In the context of this specification, except where expressly stated otherwise, any matters or issues not mentioned shall apply directly to those known in the art without any modification.

[0085] Furthermore, any implementation described herein can be freely combined with one or more other implementations described herein, and the resulting technical solutions or technical ideas shall be regarded as part of the original disclosure or original record of the present invention, and should not be regarded as new content not disclosed or anticipated herein, unless those skilled in the art consider the combination to be obviously unreasonable.

Claims

1. An emulsion type self-crosslinking bio-based itaconate pressure sensitive adhesive prepared from raw materials comprising water, itaconate monomer, acrylate monomer, functional monomer, self-crosslinking monomer, initiator, pH buffer, and emulsifier; the itaconate monomer is one or more of dimethyl itaconate, diethyl itaconate, dibutyl itaconate, diisooctyl itaconate; the acrylate monomer is one or more of butyl acrylate, n-octyl acrylate, isooctyl acrylate, ethyl acrylate, methyl acrylate, methyl methacrylate, isooctyl methacrylate, isobornyl acrylate, isobornyl methacrylate; the functional monomer is a copolymerizable monomer containing a carboxylic acid group; the self-crosslinking monomer is one or both of glycidyl methacrylate and glycidyl acrylate; each component is calculated based on 100 parts by weight of water, the itaconate monomer is 5-50 parts by weight; the acrylate monomer is 20-70 parts by weight; the functional monomer is 0.1-4 parts by weight; the initiator is 0.01-1 part by weight; the self-crosslinking monomer is 0.1-3 parts by weight; the pH buffer is 0.01-0.4 parts by weight; and the emulsifier is 0.1-15 parts by weight; the itaconate monomer accounts for 30-50 wt% of the total mass of monomers.

2. The emulsion type self-crosslinking bio-based itaconate pressure sensitive adhesive according to claim 1, wherein: each component is calculated based on 100 parts by weight of water, the itaconate monomer is 23-38 parts by weight; the acrylate monomer is 38-53 parts by weight; the functional monomer is 1.5-2.3 parts by weight; the initiator is 0.24-0.4 parts by weight; the self-crosslinking monomer is 0.16-0.38 parts by weight; the pH buffer is 0.24-0.32 parts by weight; and the emulsifier is 1.5-4.6 parts by weight.

3. The emulsion type self-crosslinking bio-based itaconate pressure sensitive adhesive according to claim 1, wherein: the functional monomer is one or more of acrylic acid, methacrylic acid, itaconic acid, monomethyl itaconate, monobutyl itaconate; and / or, the initiator is a water-soluble thermal initiator; and / or, the pH buffer is sodium bicarbonate and / or disodium hydrogen phosphate; and / or, the emulsifier is one or more of an anionic emulsifier, a non-ionic emulsifier.

4. The emulsion type self-crosslinking bio-based itaconate pressure sensitive adhesive according to claim 3, wherein: the initiator is potassium persulfate and / or ammonium persulfate; and / or, the emulsifier is one or more of sodium dodecyl sulfonate, sodium dodecyl benzene sulfonate, sodium dodecyl sulfate, 1-allyloxy-3-(4-nonylphenol)-2-propanol polyoxyethylene (10) ether ammonium sulfate, polyoxyethylene octyl phenol ether-10.

5. The emulsion type self-crosslinking bio-based itaconate pressure sensitive adhesive according to claim 1, wherein: the raw materials for preparing the emulsion type self-crosslinking bio-based itaconate pressure sensitive adhesive further comprise a molecular weight regulator.

6. The emulsion type self-crosslinking bio-based itaconate pressure sensitive adhesive according to claim 5, wherein: the molecular weight regulator is one or both of n-dodecyl mercaptan and tert-dodecyl mercaptan; and / or, ​ ​ ​ wherein ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ The amount of the molecular weight regulator is 0-0.15 parts by weight based on 100 parts by weight of water. 7.The emulsion type self-crosslinking bio-based itaconate pressure sensitive adhesive according to claim 6, wherein: The amount of the molecular weight regulator is 0-0.03 parts by weight based on 100 parts by weight of water. 8.A method for preparing the emulsion type self-crosslinking bio-based itaconate pressure sensitive adhesive according to any one of claims 1-7, comprising semi-continuous emulsion polymerization of components including water, itaconate monomers, acrylic monomers, functional monomers, self-crosslinking monomers, an initiator, a pH buffer, an emulsifier, and optionally a molecular weight regulator to obtain the emulsion type self-crosslinking bio-based itaconate pressure sensitive adhesive.

9. The production method according to claim 8, characterized by The method comprises: (a) mixing itaconate monomers, acrylic monomers, functional monomers, self-crosslinking monomers, a pH buffer, optionally a molecular weight regulator, and 60-95% of an emulsifier to obtain a pre-emulsion with water as a solvent; (b) reacting 3-20% of the pre-emulsion obtained in step (a), water, 1-80% of an initiator, and the remaining emulsifier until the emulsion turns blue; (c) after adding the remaining pre-emulsion and initiator to the reaction system of step (b) at a constant rate, increasing the temperature for reaction, and then adding a weak base to adjust the pH to weak alkaline, the emulsion type self-crosslinking bio-based itaconate pressure sensitive adhesive is obtained. 10.The method according to claim 9, wherein: The mass ratio of the amount of water in step (a) to the total amount of water in step (b) is in the range of 1: (0.1-10). 11.The method according to claim 9, wherein: In step (b), The reaction temperature is 70-80℃. 12.The method according to claim 9, wherein: In step (c), The reaction temperature is 85-90℃, and the reaction time is 30-120 min; and / or, The weak base comprises ammonia; and / or, The weak alkaline refers to a pH of 7-9. 13.The method according to claim 9, wherein: In step (c), a step of filtering the emulsion after adjusting the pH to weak alkaline is further included.

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