A micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion and a method for preparing the same

Abstract

This invention relates to the field of polymer materials technology, specifically to a micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion and its preparation method. The aqueous dispersion is prepared using a two-step method: First, a polyacrylic acid-sodium polyacrylate prepolymer aqueous dispersion with a linear structure and low molecular weight is synthesized by free radical polymerization of aqueous solution, using acrylic acid as the main monomer, and by controlling the amount of initiator and chain transfer agent; Second, one or more post-crosslinking agents selected from polyethylene glycol, polyether diamine, and polyethylene glycol diglycidyl ether are added to the prepolymer aqueous dispersion, and a post-crosslinking reaction is carried out at 40-50℃ to form a stable micro-crosslinked network structure. The aqueous dispersion prepared by this invention has the advantages of low glass transition temperature, low viscosity, excellent cohesive strength and stability, and can be widely used in coatings, adhesives, textiles, pharmaceuticals and personal care fields.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion and its preparation method. Background Technology

[0002] Polyacrylic acid and its salt aqueous dispersions are important water-based polymer materials, widely used in coatings, adhesives, textiles, pharmaceuticals, and personal care products due to their environmental friendliness, non-toxicity, and ease of processing. The glass transition temperature (Tg) and crosslinking density of these materials are key factors determining their film-forming properties, mechanical strength, and water resistance. In practical applications, materials often need to possess both a low film-forming temperature (minimum film-forming temperature, MFFT) and a low glass transition temperature (Tg), along with a suitable crosslinking structure, to ensure the formation of continuous, dense, and sufficiently strong films at room temperature or low temperatures. However, traditional polyacrylic acid aqueous dispersions often face a contradiction: increasing the degree of crosslinking to achieve high strength often leads to an increased glass transition temperature, difficulty in film formation, and material brittleness; while lowering the glass transition temperature and improving flexibility by adding small-molecule plasticizers or reducing molecular weight may cause plasticizer migration, decreased cohesive strength, poor film-forming properties, and poor storage stability. Therefore, developing an aqueous polyacrylic acid dispersion that can precisely balance a low glass transition temperature and a moderate degree of crosslinking, without relying on small molecule plasticizers, has become a critical challenge in this technological field. Patent application CN109608567A prepared polyacrylic acid / sodium polyacrylate with a high glass transition temperature through a pre-polymerization followed by neutralization, reacting at precisely controlled low temperatures (67-70℃) and low monomer concentrations (10-12%). However, its target product has a linear structure and does not introduce any crosslinking agents, resulting in a lack of a crosslinking network. Therefore, it suffers from deficiencies in key application properties such as film formation and mechanical strength, limiting its application in high-end coatings, adhesives, and other fields requiring a three-dimensional network structure.

[0003] To improve the properties of polyacrylic acid / sodium polyacrylate, researchers have conducted extensive studies. Introducing cross-linked structures can improve the polymer's water resistance and mechanical properties, but excessive cross-linking leads to decreased solubility and a sharp increase in viscosity. Lowering the glass transition temperature can improve the polymer's film-forming properties and film strength, but maintaining other polymer properties while lowering the glass transition temperature remains a challenge. Therefore, developing a polyacrylic acid-sodium polyacrylate aqueous dispersion with a micro-cross-linked structure, low glass transition temperature, and excellent overall performance is of significant practical importance. Summary of the Invention

[0004] The purpose of this invention is to provide a micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion and its preparation method, so as to solve the above-mentioned technical problems in the prior art.

[0005] The technical solution provided by this invention is as follows: In a first aspect, the present invention provides a micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion, wherein the micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion is prepared by micro-crosslinking reaction of raw materials comprising the following parts by weight: 110 parts by weight of polyacrylic acid-sodium polyacrylate prepolymer aqueous dispersion, 30-40 parts by weight of 30% crosslinking agent aqueous solution, and 0.02-0.3% (based on the total mass of the final dispersion).

[0006] Preferably, the crosslinking agent is selected from one or more of polyethylene glycol (PEG), polyether diamine, and polyethylene glycol diglycidyl ether, wherein the polyethylene glycol has the structural formula shown in formula (1): Equation (1); The structural formula of polyether diamine is shown in formula (2): Equation (2); The structural formula of polyethylene glycol diglycidyl ether is shown in formula (3): Equation (3).

[0007] Through the above technical solutions, the main chain of the crosslinking agent molecule is a flexible polyether chain (-CH2-CH2-O-), which has an extremely low glass transition temperature.

[0008] More preferably, the number average molecular weight (Mn) of the PEG is 1000-8000; the Mn of the polyether diamine is 1000-5000; and the Mn of the polyethylene glycol diglycidyl ether is 1000-6000.

[0009] In some preferred embodiments, the molar ratio of the functional groups of the crosslinking agent to the residual carboxyl groups in the prepolymer is 0.01-0.1:1.

[0010] By controlling the molar ratio of active functional groups (such as -OH, -NH2, epoxy groups) in the crosslinking agent to residual carboxyl groups (-COOH) in the prepolymer through the above technical solutions, the extremely low functional group ratio achieves micro-crosslinking, ensuring that the system forms only a sparse three-dimensional network, which can introduce crosslinking points to enhance performance and avoid the risk of gelation caused by excessive crosslinking.

[0011] Preferably, the crosslinking catalyst is one of p-toluenesulfonic acid, xylenesulfonic acid, or concentrated sulfuric acid, and the amount added accounts for 0.02-0.3% of the total system mass.

[0012] Secondly, the present invention also provides a method for preparing a micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion, comprising the following steps: Step (1) Preparation of polyacrylic acid-sodium polyacrylate prepolymer aqueous dispersion 1a. Divide the total initiator into two parts, and dissolve them separately with a portion of deionized water according to the designed ratio to prepare initiator solution A and initiator solution B for later use; 1b. Add the reducing agent, chain transfer agent and remaining deionized water to the reaction vessel, stir until the solid dissolves, and heat to 75-85℃; 1c. Simultaneously add acrylic acid and initiator solution A at 75-85℃; The above process constitutes the main stage of the polymerization reaction.

[0013] 1d. After adding acrylic acid and initiator solution A, add initiator solution B and react at a controlled temperature of 75-85℃; 1e. After all materials have been added, allow the reaction to proceed for 3-6 hours; In the above process, ensure complete monomer conversion to obtain a low molecular weight polyacrylic acid prepolymer dispersion.

[0014] 1f. After the reaction is complete, cool the temperature to below 40°C, and add the neutralizing agent solution dropwise according to the formula design to the required pH value to obtain the polyacrylic acid-sodium polyacrylate prepolymer aqueous dispersion; Step (2) After heating the polyacrylic acid-sodium polyacrylate prepolymer aqueous dispersion, add 30% of the crosslinking agent aqueous solution and crosslinking catalyst dropwise under stirring. After stirring and reacting for 20-30 minutes, cool down to below 40℃ and adjust the pH to 6.0-8.0 to obtain micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion.

[0015] In the above process, the macromolecular crosslinking agent and the prepolymer form a micro-crosslinked structure through chemical bonding and intermolecular interactions.

[0016] Preferably, in step 1a, the initiator is selected from one of sodium persulfate, potassium persulfate, ammonium persulfate, and hydrogen peroxide.

[0017] Preferably, in step 1b, the reducing agent is selected from one or more of sodium sulfite, potassium sulfite, sodium citrate, potassium citrate, and sodium L-ascorbate; the chain transfer agent is selected from one or more of n-dodecyl mercaptan, sodium bisulfite, potassium bisulfite, and isopropanol.

[0018] Preferably, in step 1c, the dripping time is 2.5-4 hours.

[0019] Preferably, in step 1d, the initiator B is added over a period of 0.2-0.3 hours.

[0020] Preferably, in step 1e, the reaction temperature is 75-85℃.

[0021] Preferably, in step 1f, the neutralizing agent is sodium hydroxide or potassium hydroxide; the solid content of the polyacrylic acid-sodium polyacrylate prepolymer aqueous dispersion is 29-31%.

[0022] Preferably, in step (2), the heating temperature is 40-50℃.

[0023] Preferably, in step (2), the total amount of the 30% crosslinking agent aqueous solution is 30-40 parts by weight, based on 110 parts by weight of the polyacrylic acid-sodium polyacrylate prepolymer aqueous dispersion.

[0024] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: 1. A two-step method, involving the initial synthesis of the prepolymer and subsequent micro-crosslinking, enables precise control over the crosslinking density and network structure. The initial reaction generates a linear, low-molecular-weight polymer backbone, providing a basis for subsequent crosslinking at residual carboxyl sites. The subsequent introduction of a flexible macromolecular crosslinking agent constructs a sparse yet uniform three-dimensional network on the prepolymer carboxyl groups, controlling the functional group molar ratio to avoid gelation risks while imparting appropriate crosslinking strength and water resistance to the material. The polyether-based crosslinking agent itself has a low Tg; its flexible long chains insert between the sodium polyacrylate backbone, effectively weakening the ionic interactions and hydrogen bond network between the strongly polar sodium carboxylate groups, increasing the molecular chain spacing, and enhancing chain segment mobility, thereby reducing the overall Tg and achieving good film-forming properties at room temperature and even low temperatures.

[0025] 2. Traditional methods rely on small molecule plasticizers to reduce Tg, but these are prone to migration and precipitation, leading to performance degradation. This invention achieves structural plasticization through an internally plasticizing macromolecular crosslinking agent, which has no migration risk and excellent long-term storage stability.

[0026] 3. The prepolymer stage has a linear low molecular weight structure, resulting in low dispersion viscosity and good flowability. Even after micro-crosslinking, it maintains good water dispersion without gelation or sedimentation, making it suitable for various application methods such as spraying, dipping, and scraping. It has a wide range of applications. Furthermore, the entire process is an aqueous reaction, requiring no organic solvents. The crosslinking reaction is carried out under mild conditions of 40-50℃, resulting in low energy consumption, high safety, and clear parameters for each step with good repeatability, making it suitable for large-scale continuous production. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is the infrared spectrum of the micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion of the present invention. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0030] Unless otherwise specified, all reagents and raw materials used in the embodiments and comparative examples of this invention are purchased from the market.

[0031] The micro-crosslinked network structure described in this invention refers to a sparse network composed of chemical bonds introduced between prepolymer segments by a post-crosslinking agent and associative connections formed through intermolecular interactions such as ionic bonds and hydrogen bonds. The post-crosslinking reaction can include chemical bonding reactions between the post-crosslinking agent and the carboxyl groups of the prepolymer, such as ring-opening reactions of epoxy groups, or associative micro-crosslinking processes formed by intermolecular interactions such as ionic bonds and hydrogen bonds. When the post-crosslinking agent is polyethylene glycol diglycidyl ether, ring-opening reactions between epoxy groups and carboxyl, hydroxyl, and amino groups are preferred, thereby forming chemical crosslinking points. The formation of this network allows the system to maintain good fluidity and aqueous stability while achieving enhanced cohesive strength and water resistance.

[0032] The molar number of residual carboxyl groups in the prepolymer can be determined by acid value titration or potentiometric titration. A certain mass of the prepolymer sample is taken, acidified, and then subjected to solvent replacement treatment. The acid value is calculated by titration with a standard alkaline solution and converted to the molar number of carboxyl groups. The molar number of active functional groups in the crosslinking agent is converted according to the end-group equivalent; for example, polyether diamine is converted to the amino equivalent, polyethylene glycol diglycidyl ether to the epoxy equivalent, and polyethylene glycol to the hydroxyl equivalent. The molar ratio of active functional groups to carboxyl groups is determined according to the above equivalent calculations. Example 1

[0033] This embodiment discloses a method for preparing a micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion, including the following steps: Step (1) Preparation of polyacrylic acid-sodium polyacrylate prepolymer aqueous dispersion 1a. Preparation of initiator solution: Mix 1.75g ​​potassium persulfate with 15g deionized water and stir until completely dissolved to obtain initiator solution A; mix 0.07g potassium persulfate with 5g deionized water and stir until completely dissolved to obtain initiator solution B; 1b. Add 0.05g of reducing agent sodium bisulfite, 0.05g of chain transfer agent isopropanol and 45g of deionized water to the reaction vessel, and start stirring to dissolve the solid; 1c. Heat the reaction system to 80°C, and at this temperature, simultaneously and uniformly add 35g of acrylic monomer and all of the initiator solution A through two constant pressure dropping funnels, controlling the dropping rate to maintain the total dropping time at 2.5-3h. 1d. After acrylic acid and initiator solution A have been added dropwise, initiator solution B is added dropwise, and the addition time is controlled at 15-20 minutes. 1e. After the initiator solution B is completely added, maintain the reaction temperature at 80°C and continue stirring for 3.5 hours to ensure that the monomer is fully polymerized; 1f. Neutralization: After the reaction is complete, cool the system to below 40°C. While stirring, slowly add 30% sodium hydroxide aqueous solution to adjust the pH of the system to 2.5. Continue stirring for 15-30 minutes to ensure complete neutralization and obtain an aqueous dispersion of polyacrylic acid-sodium polyacrylate prepolymer with a solid content of about 30%.

[0034] Step (2) Preparation of micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion The prepolymer aqueous dispersion obtained in step 1f was heated to 45°C. Under continuous stirring, 33.34g of 30% polyethylene glycol aqueous solution and 0.2g of crosslinking catalyst p-toluenesulfonic acid were added dropwise. After the addition was completed, the reaction was continued at 45°C for 25min. After the reaction was completed, the system was cooled to below 40°C, and the pH was adjusted to 6.5 with sodium hydroxide aqueous solution or potassium hydroxide aqueous solution. Then the product was discharged to obtain a micro-crosslinked, low glass transition temperature polyacrylic acid-sodium polyacrylate aqueous dispersion.

[0035] The number-average molecular weight (Mn) of the polyethylene glycol used is approximately 1000.

[0036] like Figure 1 As shown, Figure 1 The image shows the infrared spectrum of the micro-crosslinked, low glass transition temperature polyacrylic acid-sodium polyacrylate aqueous dispersion prepared in Example 1. Specifically, the spectrum is at 3460.75 cm⁻¹. -1 The peak represents the hydroxyl stretching vibration in acrylic polymers, at 1627.84 cm⁻¹. -1 The carboxylate group (-COO) in acrylic polymers - Stretching vibration peak, 1450.42 cm⁻¹ -1The peak represents the stretching vibration of the methylene group, at 1008.49 cm⁻¹. -1 The infrared spectrum shows the characteristic absorption peaks of the polyether segments (C–O–C) obtained by reacting acrylic polymers with the crosslinking agent polyethylene glycol. The infrared spectrum confirms the successful preparation of Example 1. Example 2

[0037] The only difference between Example 2 and Example 1 is that the crosslinking agent used in step (2) is 33.34g of 30% polyethylene glycol aqueous solution; the number average molecular weight Mn of the polyethylene glycol is about 8000. Example 3

[0038] The only difference between Example 3 and Example 1 is that the crosslinking agent used in step (2) is 33.34g of 30% polyether diamine aqueous solution; the number average molecular weight Mn of the polyether diamine is 1000. Example 4

[0039] The only difference between Example 4 and Example 1 is that the crosslinking agent used in step (2) is 33.34g of 30% polyethylene glycol diglycidyl ether aqueous solution; the number average molecular weight Mn of polyethylene glycol diglycidyl ether is 1000. Example 5

[0040] The only difference between Example 5 and Example 1 is that the crosslinking agent used in step (2) is a combination of 16.67g of 30% polyethylene glycol aqueous solution, 8.34g of 30% polyether diamine aqueous solution and 8.34g of 30% polyethylene glycol diglycidyl ether aqueous solution.

[0041] The Mn of polyethylene glycol is 1000; the Mn of polyether diamine is 1000; and the Mn of polyethylene glycol diglycidyl ether is 1000.

[0042] Comparative Example 1 The only difference between Comparative Example 1 and Example 1 is that the crosslinking agent used in step (2) is 22.22g of 30% polyethylene glycol aqueous solution, 22.22g of 30% polyether diamine aqueous solution and 22.22g of 30% polyethylene glycol diglycidyl ether aqueous solution.

[0043] The Mn of polyethylene glycol is 1000; the Mn of polyether diamine is 1000; and the Mn of polyethylene glycol diglycidyl ether is 1000.

[0044] Comparative Example 2 The only difference between Comparative Example 2 and Example 1 is that no crosslinking agent is added.

[0045] This comparative example only performs step (1) of Example 1, that is, after preparing the polyacrylic acid-sodium polyacrylate prepolymer aqueous dispersion, the micro-crosslinking reaction in step (2) is not carried out, and the prepolymer aqueous dispersion is directly used as the final product.

[0046] Performance testing: To verify the performance of the micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion described in this invention, the samples obtained in Examples 1-5 and Comparative Examples 1-2 were subjected to the following tests: the glass transition temperature (Tg) test method was performed according to GB / T 19466.2-2004, and the test instrument used was a differential scanning calorimeter (DSC); the viscosity test method was performed according to GB / T 2794-2022, and the test instrument used was a rotational viscometer. The crosslinking index (CI) test method is as follows: Take appropriate amounts of the products from Examples 1-5 and Comparative Examples 1-2 into a polytetrafluoroethylene mold, and dry them in a vacuum drying oven at 40℃ until constant weight is achieved, obtaining a dried sample. Accurately weigh the sample (recorded as m0). Place the dried film sample into a sealed container containing sufficient aqueous solution of 0.1 mol / L NaCl, and allow it to swell at a constant temperature of 25℃ for 24 hours to ensure swelling equilibrium is reached. Remove the swollen sample, quickly absorb the surface moisture with filter paper, and immediately and accurately weigh the sample (recorded as m1). Crosslinking characterization is performed using the swelling method. The crosslinking index (CI,%) = (m0 / m1) × 100%. A higher value indicates a lower degree of swelling and stronger network constraint.

[0047] Gel content (GF) test method: Prepare a dry film from a separate sample and vacuum dry it to constant weight at 40℃. Cut off a sample of the dry film and accurately weigh its mass (recorded as m0,G). Place the dry film sample in a sufficient amount of extraction solution and extract it at 25℃ for 48 hours by standing or shaking (changing the extraction solution every 24 hours) to remove soluble components. The extraction solution is an aqueous solution containing 0.1 mol / L NaCl. If necessary, the pH of the extraction system can be adjusted to 9-10 with NaOH to promote the dissolution of linear components. After extraction, remove the residue, vacuum dry it to constant weight at 40℃, and weigh its mass (recorded as m2). The gel content is calculated as: GF (%) = (m2 / m0,G) × 100%. The larger the GF, the higher the proportion of insoluble network structure in the system.

[0048] The low-temperature film-forming temperature was determined through a gradient cooling film-forming experiment; storage stability was observed by storing at room temperature for 6 months. The test results are shown in Table 1; meanwhile, to highlight the advantages of the present invention, the sample of Example 5 was compared with commercially available and literature-reported similar products, and the results are listed in Table 2.

[0049] Table 1. Performance test results of products from Examples 1-5 and Comparative Examples 1-2

[0050] Table 2 Comparison of Core Performance with Existing Technologies

[0051] Note: Comparative sample 1 (Wanhua Chemical Unicap K45 sodium polyacrylate / 2-sodium acrylate homopolymer) is a commercially available partially neutralized polyacrylate-sodium polyacrylate copolymer (uncrosslinked); Comparative sample 2 was prepared according to the method disclosed in patent CN109608567; Comparative sample 3 (Jiangsu Sanmu Chemical EA1621A waterborne acrylic resin) is a commercially available conventional crosslinked polyacrylate aqueous dispersion (high degree of crosslinking).

[0052] As shown in Table 1, the polyacrylic acid-sodium polyacrylate aqueous dispersions prepared in Examples 1-5 of this invention all exhibit low glass transition temperatures and low viscosity, along with excellent film-forming properties and strength. In particular, the type and amount of crosslinking agent significantly affect the product performance. When the amount of crosslinking agent is low, the glass transition temperature decreases with increasing molecular weight. This is because the ether bond oxygen atoms on the polyether chain compete with the sodium carboxylate groups for interaction, effectively weakening the strong ionic and hydrogen bond network originally formed between the polyacrylate prepolymer chains. Simultaneously, the long-chain polyether molecules themselves have volume; when they insert into the polyacrylic acid-sodium polyacrylate prepolymer chains, they physically increase the average distance between molecular chains. The longer the molecular chain, the weaker the interaction between molecular chains, and the enhanced chain segment mobility, allowing them to begin moving at lower temperatures, thus lowering the polymer's glass transition temperature. Since different types of crosslinking agents react with polyacrylic acid-sodium polyacrylate prepolymers to produce products with different structures, their properties also differ. Crosslinking agents with different structures react with the prepolymer to form networks with different topological structures. Due to the high reactivity of its two end amino groups, polyether diamine can react with multiple carboxyl groups to form a network with relatively more crosslinking points, resulting in a slightly higher viscosity than other embodiments. Polyethylene glycol diglycidyl ether, on the other hand, mainly undergoes linear chain extension or forms more relaxed crosslinks, thus exhibiting the lowest viscosity.

[0053] In Comparative Example 1, a high amount of crosslinking agent resulted in direct gelation, indicating that its performance was unstable and unusable. Comparative Example 2, without any crosslinking agent, exhibited poor film-forming properties, was prone to pulverization, and had a film strength of only 2.1 MPa. This demonstrates that crosslinking agents are crucial for forming a complete and high-strength film structure; appropriate crosslinking can enhance the interaction between molecular chains, improving the stability and strength of the film.

[0054] As shown in Table 2, compared with the prior art, the product of the present invention has significant advantages in terms of glass transition temperature, viscosity, film strength, low temperature film formation and storage stability. In particular, it solves the technical problem that traditional products cannot achieve both low glass transition temperature and high strength, and has broad application prospects.

[0055] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

[0056] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to specific implementations. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for preparing a micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion, characterized in that, Includes the following steps: Step (1) Preparation of polyacrylic acid-sodium polyacrylate prepolymer aqueous dispersion: 1a. Divide the total initiator into two parts, and dissolve them separately with a portion of deionized water to prepare initiator solution A and initiator solution B for later use; 1b. Mix the reducing agent, chain transfer agent and remaining deionized water, stir to dissolve and heat to 75-85℃; 1c. Simultaneously add acrylic acid and the initiator solution A at 75-85°C; 1d. After the addition is complete, add the initiator solution B dropwise over 0.2-0.5 hours and control the temperature at 75-85℃ for the reaction; 1e. After all materials have been added, continue the reaction at 75-85℃ for 3-6 hours; 1f. After the reaction is complete, cool the temperature to below 40°C, add a neutralizing agent solution dropwise to the desired pH value, and obtain a polyacrylic acid-sodium polyacrylate prepolymer aqueous dispersion; Step (2) Heat the prepolymer aqueous dispersion obtained in step 1f to 40-50°C, add 30% crosslinking agent aqueous solution and crosslinking catalyst dropwise while stirring, stir and react for 20-30 min, then cool down to below 40°C and adjust the pH to 6.0-8.0 to obtain micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion; wherein, based on 110 parts by weight of the polyacrylic acid-sodium polyacrylate prepolymer aqueous dispersion obtained in step 1f, the total amount of the 30% crosslinking agent aqueous solution is 30-40 parts by weight.

2. The method for preparing the micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion according to claim 1, characterized in that, In step 1a, the initiator is selected from one of sodium persulfate, potassium persulfate, ammonium persulfate, and hydrogen peroxide.

3. The method for preparing the micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion according to claim 1, characterized in that, In step 1b, the reducing agent is selected from one or more of sodium sulfite, potassium sulfite, sodium citrate, potassium citrate, and sodium L-ascorbate; the chain transfer agent is selected from one or more of n-dodecyl mercaptan, sodium bisulfite, potassium bisulfite, and isopropanol; in step 1c, the dropping time is 2.5-4 hours.

4. The method for preparing the micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion according to claim 1, characterized in that, In step 1f, the neutralizing agent is sodium hydroxide or potassium hydroxide; the solid content of the resulting polyacrylic acid-sodium polyacrylate prepolymer aqueous dispersion is 29-31%; and the pH value is 2.0-8.

0.

5. The method for preparing the micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion according to claim 1, characterized in that, In step (2), the crosslinking agent is selected from one or more of polyethylene glycol (PEG), polyether diamine, and polyethylene glycol diglycidyl ether.

6. The method for preparing the micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion according to claim 5, characterized in that, In step (2), the number average molecular weight (Mn) of PEG is 1000-8000; the Mn of polyether diamine is 1000-5000; and the Mn of polyethylene glycol diglycidyl ether is 1000-6000.

7. The method for preparing the micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion according to claim 1, characterized in that, In step (2), the crosslinking catalyst is selected from p-toluenesulfonic acid, xylenesulfonic acid or concentrated sulfuric acid, and the amount added is 0.02-0.3% of the total mass of the final dispersion.

8. The method for preparing the micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion according to claim 5, characterized in that, In step (2), the crosslinking agent is a compound crosslinking agent, which is obtained by compounding polyethylene glycol, polyether diamine, and polyethylene glycol diglycidyl ether, wherein the structural formula of polyethylene glycol is shown in formula (1): Equation (1); The structural formula of polyether diamine is shown in formula (2): Equation (2); The structural formula of polyethylene glycol diglycidyl ether is shown in formula (3): Equation (3).

9. The method for preparing the micro-crosslinked polyacrylic acid-sodium polyacrylate aqueous dispersion according to claim 5, characterized in that, The molar ratio of the active functional group of the crosslinking agent to the residual carboxyl group in the prepolymer obtained in step 1f is 0.01-0.1:1.

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