Normal-temperature self-repairing polycarbonate and preparation method thereof

Abstract

The application provides a normal-temperature self-repairing polycarbonate and a preparation method, and has a structural formula as shown in the following formula: wherein R is a group derived from a monophenol; m is 10-120, and n is 5-80. The application introduces a third monomer with a self-repairing function to endow the PC with a normal-temperature self-healing function. The self-repairing PC prepared by the application greatly improves the service life of the PC, and can realize self-repairing of cracks at normal temperature (25 DEG C), and the mechanical repair rate reaches 70%.

Description

Technical Field

[0001] This invention relates to a self-healing polycarbonate material at room temperature and its preparation method, belonging to the field of polymer materials technology. Background Technology

[0002] Polycarbonate (PC), a high-performance thermoplastic engineering plastic, is widely used in various fields, including the automotive industry, instrumentation, electronics, and building materials, due to its high light transmittance, high impact resistance, excellent heat resistance, weather resistance, and good electrical insulation properties. However, as the application fields of polycarbonate materials continue to expand, its defects exposed in actual use are becoming increasingly prominent. Especially in outdoor applications, due to prolonged exposure to the natural environment and external forces, the surface of polycarbonate products is prone to scratches, cracks, and other damage, seriously affecting its performance and service life.

[0003] Currently, the main methods for addressing surface damage in polycarbonate products are surface coatings and surface treatments. However, these methods still have many shortcomings in practical applications. First, while traditional surface coatings can improve the wear resistance and scratch resistance of polycarbonate products to some extent, the damage is often irreversible when the coating is subjected to significant external forces, and it cannot self-repair. Second, although some surface treatment methods can improve the weather resistance and corrosion resistance of polycarbonate products to some extent, they often require specific environmental conditions, making the process complex and costly.

[0004] Furthermore, existing polycarbonate materials often require manual repair or replacement after damage, which not only increases maintenance costs but also affects the continuous performance of the product. This is especially true in specialized fields such as aerospace and marine engineering, where the reliability and durability of materials are extremely critical; damage to these materials can lead to serious consequences.

[0005] To address the shortcomings of existing technologies, developing a self-healing method for preparing polycarbonate has significant practical implications and application value. Firstly, the self-healing function enables the automatic repair of minor surface damage to polycarbonate products, extending their service life, reducing the frequency of replacement and maintenance, and thus lowering operating costs. Secondly, the self-healing function can improve the reliability and durability of polycarbonate products, especially in fields with extremely high material performance requirements, such as aerospace and marine engineering, where it will greatly enhance the safety and reliability of the products.

[0006] A review of existing patent literature revealed that Chinese Patent Publication No. CN108699228A discloses a method of introducing disulfides into the main chain to impart self-healing functionality to polycarbonate. However, the introduction of disulfides significantly reduces the mechanical and heat resistance properties of polycarbonate, and releases toxic gases such as sulfur dioxide during processing. US Patent Publication No. US11136433B2 discloses a method of introducing esters and heterocyclic structures into the main chain to impart self-healing functionality to polycarbonate. However, the introduction of flexible structures leads to a decrease in the strength of polycarbonate, reducing its application range. Furthermore, both methods impart self-healing functionality to polycarbonate based on reversible covalent bonds with high bond energies. Therefore, achieving self-healing requires external energy input (160°C or strong ultraviolet light irradiation), and cannot fundamentally eliminate the artificial processing and repair process after polycarbonate material damage.

[0007] Therefore, it is necessary to develop a room-temperature self-healing polycarbonate material that can simultaneously maintain mechanical properties. Summary of the Invention

[0008] In view of the above-mentioned problems in the prior art, the purpose of this invention is to provide a self-healing polycarbonate (PC) at room temperature and its preparation method. This invention endows polycarbonate with room temperature self-healing function through non-covalent bonds, while enhancing the mechanical properties of polycarbonate.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] This invention provides a room-temperature self-healing polycarbonate having the structural formula shown in formula (1):

[0011]

[0012] Wherein, R is a group derived from a monophenol, preferably a group derived from p-tert-butylphenol, phenol, m-tert-butylphenol, or methylphenol; m takes the value of 10-120, such as 10, 30, 50, 80, 100, 120, etc., and n takes the value of 5-80, such as 5, 10, 30, 50, 70, 80, etc.

[0013] The present invention also provides a method for preparing the aforementioned room-temperature self-healing polycarbonate, comprising the following steps:

[0014] 1) Mix bisphenol A, 3,5-dihydroxybenzamide, alkali, sodium dithionite (sodium hydrosulfite) and water to obtain a homogeneous aqueous solution;

[0015] 2) Add an oil-soluble solvent to the homogeneous aqueous solution from step 1) to obtain an oil-water mixture;

[0016] 3) Phosgene and alkaline solution are introduced into the oil-water mixture of step 2) to carry out the reaction;

[0017] 4) Add a catalyst and a monophenol capping agent to the reaction system of step 3), and pass an alkaline solution through to carry out the reaction;

[0018] 5) The reaction solution in step 4) separates into layers. The oil phase is taken and subjected to acid washing, water washing, and removal of oil-soluble solvents to obtain room-temperature self-healing PC polycarbonate.

[0019] In one specific embodiment, the concentration of bisphenol A in the homogeneous aqueous solution in step 1) is 10-20 wt%, for example, 10 wt%, 12 wt%, 14 wt%, 16 wt%, 18 wt%, 20 wt%, etc.

[0020] The concentration of the 3,5-dihydroxybenzamide in the homogeneous aqueous solution is 3-13 wt%, for example, 3 wt%, 5 wt%, 7 wt%, 9 wt%, 11 wt%, 13 wt%, etc.;

[0021] The concentration of the alkali in the homogeneous aqueous solution is 5-8 wt%, for example, 5 wt%, 6 wt%, 7 wt%, 8 wt%, etc.;

[0022] The concentration of sodium hydrosulfite in the aqueous solution is 50-300 ppm, for example, 50 ppm, 100 ppm, 150 ppm, 200 ppm, 250 ppm, 300 ppm, etc.

[0023] Preferably, the homogeneous aqueous solution in step 1) is prepared under an inert atmosphere, such as a nitrogen environment.

[0024] In one specific implementation, the alkali in step 1) is a metal hydroxide, preferably one or more of NaOH and KOH.

[0025] In one specific implementation, the oil-soluble solvent in step 2) is selected from one or more of dimethylacetamide (DMF), chloroform, tetrachloroethane, dichloromethane, chlorobenzene, tetrahydrofuran, dioxane, and pyridine.

[0026] In one specific implementation, the mass ratio of the homogeneous aqueous solution to the oil-soluble solvent in step 2) is 0.5-2, for example, 0.5, 0.8, 1, 1.2, 1.5, 1.8, 2, etc., preferably 0.8-1.5.

[0027] In one specific implementation, the molar ratio of phosgene in step 3) to bisphenol A in step 1) is 1-3, for example, 1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, etc.

[0028] Preferably, the rate at which phosgene is introduced into the reaction system is 0.8-2.5 L / min, for example, 0.8 L / min, 1 L / min, 1.2 L / min, 1.5 L / min, 1.8 L / min, 2 L / min, 2.2 L / min, 2.5 L / min, etc.

[0029] In one specific embodiment, step 3) involves continuously introducing the alkaline solution during the reaction process at a rate of 1-15 g / min, such as 1 g / min, 3 g / min, 5 g / min, 7 g / min, 9 g / min, 11 g / min, 13 g / min, 15 g / min, etc.; the alkaline solution is continuously introduced during the reaction process until the phosgene introduction is completed;

[0030] Preferably, the alkaline solution is an aqueous solution of alkali with a concentration of 5-45 wt%, such as 5 wt%, 15 wt%, 25 wt%, 35 wt%, 45 wt%, etc., and more preferably 32-40%.

[0031] The alkali is a metal hydroxide, preferably one or more strong alkalis such as NaOH and KOH, and more preferably NaOH.

[0032] In one specific implementation, the reaction in step 3) is carried out at a temperature of 20-36°C, such as 20°C, 22°C, 25°C, 28°C, 30°C, 33°C, 35°C, 36°C, etc., and for a time of 5-10 min, such as 5 min, 6 min, 7 min, 8 min, 9 min, 10 min, etc.

[0033] In one specific embodiment, the amount of catalyst added to the reaction solution in step 4) is 50-800 ppm, for example, 50 ppm, 100 ppm, 200 ppm, 400 ppm, 600 ppm, 800 ppm, etc.

[0034] The amount of monophenol added to the reaction solution is 1500-3000 ppm, for example, 1500 ppm, 1700 ppm, 2000 ppm, 2200 ppm, 2500 ppm, 2800 ppm, 3000 ppm, etc.

[0035] In one specific embodiment, the monophenol in step 4) is used as a capping agent and is selected from one or more of p-tert-butylphenol, phenol, m-tert-butylphenol, methylphenol, etc., preferably p-tert-butylphenol and / or phenol.

[0036] In one specific embodiment, the catalyst in step 4) is triethylamine or a quaternary ammonium salt compound, preferably one or more of triethylamine, N,N,N,N-trimethylbenzylammonium chloride, and N,N,N,N-dimethylethyloctadecylammonium sulfate, more preferably triethylamine.

[0037] In one specific implementation, the amount of alkaline solution introduced in step 4), calculated based on the alkali content, is 0.4-1% of the mass of the reaction system, for example, 0.4%, 0.6%, 0.8%, 1%, etc.

[0038] Preferably, the alkaline solution is an aqueous solution of alkali with a concentration of 25-45 wt%, such as 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, etc., and preferably 30-40 wt%.

[0039] The alkali is a metal hydroxide, preferably one or more strong alkalis such as NaOH and KOH, and more preferably NaOH.

[0040] In one specific implementation, the reaction in step 4) is carried out at a temperature of 25-38°C, such as 25°C, 28°C, 30°C, 33°C, 35°C, 36°C, 38°C, etc., and for a time of 5-40 min, such as 5 min, 10 min, 20 min, 30 min, 40 min, etc.

[0041] In one specific implementation, step 5) includes operations such as layering, acid washing, water washing, and solvent removal, all of which are conventional processing methods in the field and are not specifically required by this invention. Specifically, the acid washing involves washing the oil phase with an acid-water solution, wherein the acid can be one or more strong acids such as hydrochloric acid, sulfuric acid, and nitric acid, preferably hydrochloric acid; preferably, the concentration of the acid-water solution is 0.5wt%-10wt%, preferably 0.9wt%; the water washing continues until the aqueous phase after washing is neutral and has a conductivity of less than 100μS / cm.

[0042] The room-temperature self-healing PC polycarbonate solid product prepared by the method described above in this invention has a catalyst content of less than 0.5 ppm, a chloride ion content of less than 8 ppm, and an oil-soluble solvent content of less than 200 ppm.

[0043] Compared with the prior art, the present invention has the following advantages:

[0044] 1) A self-healing PC material at room temperature was prepared, which can achieve self-repair of cracks at room temperature (25℃) with a mechanical repair rate of 70%, which greatly improves the service life of PC and significantly enhances the safety, reliability and service life of PC.

[0045] 2) The addition of 3,5-dihydroxybenzamide monomer to this PC material imparts room temperature self-healing properties through bidentate hydrogen bonds (the self-healing mechanism is shown in the following formula). After a crack appears, only pressure needs to be applied to both sides of the fracture to achieve efficient self-healing.

[0046]

[0047] 3) Compared with existing technologies, this invention is the first to produce a room-temperature self-healing PC. The molecular structure of PC has been optimized. Compared with self-healing PC prepared using monomers containing reversible covalent bonds, the advantages are as follows: First, the conditions for self-healing are more extensive, and efficient self-healing can be achieved without external energy input (heating or light exposure); second, the mechanical properties of PC are improved, and the tensile strength of PC is improved through structural optimization; third, the heat resistance of PC is improved, and the high-density bidentate hydrogen bonds absorb heat during PC processing, preventing PC from overheating and degrading. Detailed Implementation

[0048] The present invention will be further described in detail below through specific embodiments. The embodiments described in this invention are only for illustration and do not limit the scope of the invention.

[0049] The main raw materials used in the various embodiments and comparative examples of this invention are sourced as follows. Unless otherwise specified, other raw materials and reagents were obtained through commercially available channels:

[0050] Bisphenol A: Purchased from Maclean's, item number B802575-500g;

[0051] 3,5-Dihydroxybenzamide: Purchased from Maclean's, product number D859848-25g;

[0052] Sodium hydroxide: purchased from Maclean's, product number S832169;

[0053] Phosgene: Refer to Zhou Yiping's "Improvement of Phosgene Preparation Method";

[0054] p-tert-butylphenol: purchased from Maclean's, product number B802759;

[0055] Triethylamine: Purchased from Maclean's, product number T818774;

[0056] Dichloromethane: Purchased from Maclean's, product number D807826;

[0057] Sodium hydrosulfite: Purchased from Maclean's, product number S817916.

[0058]

Example 1

[0059] Preparation of room temperature self-healing polycarbonate (sample 1)# -4 # ):

[0060] 1) Under nitrogen protection, add 700ml of pure water to a 3L jacketed reactor. After stirring, add 40g of solid NaOH until completely dissolved. After cooling to room temperature, add 1.5g of sodium hydrosulfite, 100g of bisphenol A solid, and add 30g, 40g, 50g, and 60g of 3,5-dihydroxybenzamide solid according to samples 1#-4# respectively. Continue stirring until the solids are completely dissolved to obtain homogeneous aqueous solutions of samples 1#-4#.

[0061] 2) respectively to 1 # -4 # Add 850g of dichloromethane to a homogeneous aqueous solution and stir to mix the aqueous and oil phases evenly to obtain an oil-water mixture.

[0062] 3) Introduce 12.3L of phosgene into the reactor at a rate of 1.23L / min, and simultaneously add a 32wt% NaOH aqueous solution into the reactor at a rate of 2.4g / min until the phosgene introduction is completed. During this period, control the reaction temperature at 35.5℃ and react for 10min.

[0063] 4) Add 94 ppm of triethylamine and 1800 ppm of p-tert-butylphenol to the reaction solution in the reactor. At the same time, add 32 wt% of NaOH aqueous solution to the reactor. The amount of NaOH is 0.6% of the mass of the reaction solution. Stir the reaction continuously for 30 min, and control the reaction temperature at 36-38℃ during the reaction.

[0064] 5) After separating the water and oil phases, the oil phase was first washed with a 1.5 wt% HCl aqueous solution at an appropriate water-to-oil ratio, followed by washing with pure water until the aqueous phase was neutral and had a conductivity below 100 μS / cm. Then, dichloromethane was removed and the mixture was dried to obtain a room-temperature self-healing polycarbonate solid (i.e., sample 1). # -4 # );

[0065] The self-healing polycarbonate prepared in this embodiment has the following structural formula:

[0066] Add 30g of 3,5-dihydroxybenzamide (sample 1) # ):

[0067]

[0068] Add 40g of 3,5-dihydroxybenzamide (sample 2) # ):

[0069]

[0070] Add 50g of 3,5-dihydroxybenzamide (sample 3) # ):

[0071]

[0072] Add 60g of 3,5-dihydroxybenzamide (sample 4) # ):

[0073]

[0074] Characterization: The prepared room-temperature self-healing polycarbonate was analyzed by 1H NMR spectroscopy, which confirmed that 3,5-dihydroxybenzamide participated in the polymerization and entered the polymer backbone; the specific analytical results are as follows. 1 H NMR (CDCl3, 500MHz): δ7.89-8.11(m,2H),7.55(s,1H),7.50(s,2H),7.09-7.35(m,8H),1.66(s,6H).

[0075] Self-healing polycarbonate at room temperature (sample 1) # -4 # The performance test results are shown in Table 1 below:

[0076] Sample plates were prepared according to GB / T 35513.2-2017, and their heat resistance (ΔYI) was analyzed.

[0077] Prepare tensile specimens according to GB / T 1040-1992. Use a blade to make a 1mm deep scratch in the middle of the narrow neck and immediately test the tensile strength. Then, manually compress the tensile specimens that have undergone the same scratching treatment at room temperature (25℃) for 15 minutes and test the tensile strength. The repair rate is calculated based on the tensile strength of the un-scratched tensile specimens.

[0078] Table 1 Sample 1 # -4 # Performance test results

[0079]

[0080]

Example 2

[0081] Preparation of room temperature self-healing polycarbonate (sample 5) # -7 # ):

[0082] 1) Under nitrogen protection, add 700ml of pure water to a 3L jacketed reactor. After stirring, add 40g of solid NaOH until completely dissolved. After cooling to room temperature, add 1.5g of sodium hydrosulfite, 100g of bisphenol A solid, and 30g of 3,5-dihydroxybenzamide solid. Continue stirring until the solids are completely dissolved to obtain a homogeneous aqueous solution.

[0083] 2) Add 850g of dichloromethane to the homogeneous aqueous solution and stir to mix the aqueous and oil phases evenly to obtain an oil-water mixture;

[0084] 3) Introduce 10.3 L of phosgene into the reactor at a rate of 1.23 L / min, and simultaneously add 32 wt% NaOH solution into the reactor at a rate of 2.4 g / min until the phosgene introduction is completed. During this period, control the reaction temperature at 35.5℃ and react for 8 min.

[0085] 4) Add 94 ppm of triethylamine to the reaction solution, and repeat the above steps to add the solutions to the three reaction vessels according to sample 5. # -7 # Add 1800 ppm, 2400 ppm and 3000 ppm of p-tert-butylphenol, and simultaneously add 32 wt% NaOH aqueous solution to the reactor. The amount of NaOH added is 0.6% of the mass of the reaction solution. Stir the reaction continuously for 30 min, and control the reaction temperature at 36-38℃ during the process.

[0086] 5) After separating the water and oil phases, the oil phase was first washed with a 1.5% HCl solution at an appropriate water-to-oil ratio, followed by washing with pure water until the aqueous phase was neutral and had a conductivity below 100 μS / cm. Then, dichloromethane was removed and the mixture was dried to obtain a room-temperature self-healing polycarbonate solid (i.e., sample 5). # -7 # );

[0087] The self-healing polycarbonate prepared in this embodiment has the following structural formula:

[0088] Add 1800 ppm p-tert-butylphenol (sample 5) # ):

[0089]

[0090] Add 2400 ppm p-tert-butylphenol (sample 6) # ):

[0091]

[0092] Add 3000 ppm p-tert-butylphenol (sample 7) # ):

[0093]

[0094] Characterization: 5 samples of the prepared room-temperature self-healing polycarbonate were... # -7 # GPC analysis showed that the weight-average molecular weight of the polymer decreased with increasing amounts of p-tert-butylphenol; specific data are shown in Table 2 below:

[0095] Table 2 Sample 5 # -7 # weight average molecular weight

[0096] sample p-tert-butylphenol addition amount weight average molecular weight <![CDATA[5 # ]]> 1800ppm 42600 <![CDATA[6 # ]]> 2400ppm 31850 <![CDATA[7 # ]]> 3000ppm 24960

[0097] The room-temperature self-healing polycarbonate (sample 5) was treated using the same method as in Example 1. # -7 # Performance tests were conducted, and the results are shown in Table 3 below:

[0098] Table 3 Sample 5 # -7 # Performance test results

[0099]

[0100] The data in Tables 2 and 3 demonstrate that the self-healing ability of the room-temperature self-healing polycarbonate of this invention does not change with the change of molecular weight.

[0101] Example 3

[0102] Following the method for sample 1# in Example 1, p-tert-butylphenol was replaced with an equal amount of phenol, with other operations and conditions remaining unchanged, to obtain a room-temperature self-healing polycarbonate solid (i.e., sample 8). # )

[0103] The polycarbonate was tested using the same method as in Example 1, and the results are shown in Table 4 below:

[0104] Table 4 Sample 8 # Performance test results

[0105]

[0106] Comparative Example 1

[0107] Following the method of Example 1, the only difference is that 3,5-dihydroxybenzamide is not added in step 1), while other operations and conditions remain unchanged, to obtain polycarbonate.

[0108] The performance of polycarbonate was tested using the same method as in Example 1, and the results are shown in Table 5 below:

[0109] Table 5 Performance Test Results

[0110]

[0111] Comparative Example 2

[0112] The method of Example 1 was followed, except that in step 1), 3,5-dihydroxybenzamide was replaced with 3,5-dihydroxyacetophenone (Maclean, catalog number D829137), while other operations and conditions remained unchanged, to obtain polycarbonate.

[0113] The polycarbonate was tested using the same method as in Example 1, and the results are shown in Table 6 below:

[0114] Table 6 Performance Test Results

[0115]

[0116]

Claims

1. A self-healing polycarbonate at room temperature, having the structural formula shown in formula (1): （1） in, R is a group derived from monophenol; m takes values ​​of 10-120, and n takes values ​​of 5-80.

2. The room-temperature self-healing polycarbonate according to claim 1, characterized in that, R is a group derived from p-tert-butylphenol, phenol, m-tert-butylphenol, or methylphenol.

3. A method for preparing the room-temperature self-healing polycarbonate as described in claim 1 or 2, characterized in that, Includes the following steps: 1) Mix bisphenol A, 3,5-dihydroxybenzamide, alkali, sodium hydrosulfite and water to obtain a homogeneous aqueous solution; 2) Add an oil-soluble solvent to the homogeneous aqueous solution from step 1) to obtain an oil-water mixture; 3) Phosgene and alkaline solution are introduced into the oil-water mixture of step 2) to carry out the reaction; 4) Add a catalyst and a monophenol capping agent to the reaction system of step 3), and pass an alkaline solution through to carry out the reaction; 5) The reaction solution in step 4) separates into layers. The oil phase is taken and subjected to acid washing, water washing, and removal of oil-soluble solvent to obtain room-temperature self-healing PC polycarbonate.

4. The preparation method according to claim 3, characterized in that, Step 1) The concentration of bisphenol A in the homogeneous aqueous solution is 10-20 wt%; The concentration of the 3,5-dihydroxybenzamide in the homogeneous aqueous solution is 3-13 wt%. The concentration of the alkali in the homogeneous aqueous solution is 5-8 wt%; The concentration of sodium hydrosulfite in the aqueous solution is 50-300 ppm; and / or The alkali mentioned in step 1) is a metal hydroxide.

5. The preparation method according to claim 4, characterized in that, The alkali is one or more of NaOH and KOH.

6. The preparation method according to claim 3, characterized in that, Step 2) The oil-soluble solvent is selected from one or more of dimethylacetamide, chloroform, tetrachloroethane, dichloromethane, chlorobenzene, tetrahydrofuran, dioxane, and pyridine; and / or Step 2) The mass ratio of the homogeneous aqueous solution to the oil-soluble solvent is 0.5-2.

7. The preparation method according to claim 6, characterized in that, The mass ratio of the homogeneous aqueous solution to the oil-soluble solvent is 0.8-1.

5.

8. The preparation method according to claim 3, characterized in that, The molar ratio of phosgene in step 3) to bisphenol A in step 1) is 1-3.

9. The preparation method according to claim 3, characterized in that, In step 3), the rate at which phosgene is introduced into the reaction system is 0.8-2.5 L / min.

10. The preparation method according to claim 3, characterized in that, Step 3) During the reaction, the alkaline solution is continuously introduced at a rate of 1-15 g / min into the reaction system; the alkaline solution is continuously introduced until the phosgene introduction is completed.

11. The preparation method according to claim 3, characterized in that, Step 3) The alkaline solution is an aqueous solution of alkali with a concentration of 5-45 wt%. The alkali mentioned is a metal hydroxide.

12. The preparation method according to claim 11, characterized in that, The concentration of the alkaline solution is 32-40%.

13. The preparation method according to claim 11, characterized in that, The alkali is one or more of NaOH and KOH.

14. The preparation method according to claim 3, characterized in that, The reaction described in step 3) is carried out at a temperature of 20-36℃ for 5-10 minutes.

15. The preparation method according to claim 3, characterized in that, Step 4) The amount of catalyst added to the reaction solution is 50-800 ppm; The amount of the monophenol added to the reaction solution is 1500-3000 ppm; and / or Step 4) The monophenol is selected from one or more of p-tert-butylphenol, phenol, m-tert-butylphenol, and methylphenol; and / or Step 4) The catalyst is triethylamine or a quaternary ammonium salt compound.

16. The preparation method according to claim 15, characterized in that, The catalyst is one or more of triethylamine, N,N,N,N-trimethylbenzylammonium chloride, and N,N,N,N-dimethylethyloctadecylammonium sulfate.

17. The preparation method according to claim 3, characterized in that, Step 4) The amount of alkaline solution introduced, calculated based on the alkali content, is 0.4-1% of the mass of the reaction system.

18. The preparation method according to claim 3, characterized in that, Step 4) The alkaline solution is an aqueous solution of alkali with a concentration of 25-45 wt%. The alkali mentioned is a metal hydroxide.

19. The preparation method according to claim 18, characterized in that, The concentration of the alkali solution is 30-40 wt%.

20. The preparation method according to claim 18, characterized in that, The alkali is one or more of NaOH and KOH.

21. The preparation method according to claim 3, characterized in that, The reaction described in step 4) is carried out at a temperature of 25-38℃ for 5-40 minutes.

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