Polyurethane-polyamide acid microsphere dispersion liquid and application thereof in epoxy resin composite material
By introducing polyurethane@polyamic acid microspheres into epoxy resin, the problems of decreased tensile strength and uneven dispersion caused by toughening modifiers were solved, and high-toughness and high-strength composite materials of epoxy resin were achieved.
Patent Information
- Application Number
- CN202510869690.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-06-26
- Publication Date
- 2025-09-26
AI Technical Summary
Existing epoxy resin toughening modifiers often lead to a decrease in tensile strength when improving toughness, and the microspheres are unevenly dispersed in the matrix, affecting the mechanical properties.
Polyurethane@polyamic acid microspheres are grafted onto the surface of the polyurethane microspheres through amide bonds. The flexibility of the polyurethane core and the rigidity of the polyamic acid shell are combined to prepare a polyurethane@polyamic acid microsphere dispersion and mix it with epoxy resin to form a composite material.
It significantly improves the toughness and mechanical properties of epoxy resin, avoids interface debonding or brittle fracture caused by insufficient toughness or rigidity of single microspheres, and achieves simultaneous improvement of strength and toughness.
Smart Images
Figure CN120699262A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of toughening modifiers for epoxy resins, and in particular to a polyurethane@polyamic acid microsphere dispersion and application thereof in epoxy resin composite materials. Background Art
[0002] Epoxy resins are oligomers containing epoxy functional groups in their molecular chains. When mixed with a curing agent, they form thermosetting materials with three-dimensional network structures. Cured epoxy resins exhibit excellent bonding, electrical insulation, mechanical, and corrosion resistance properties, making them widely used in many fields. However, epoxy resins have disadvantages such as high crosslink density and high internal stress, resulting in high brittleness and low elongation after curing. These drawbacks significantly limit their application in applications requiring high toughness.
[0003] Toughening epoxy resins not only significantly improves their impact resistance and toughness, but also enhances their strength and durability. Toughened epoxy resins can meet the demands for material durability and reliability in complex environments. Common methods for toughening epoxy resins include rubber elastomer toughening, inorganic nanoparticle toughening, and thermoplastic resin toughening. Rubber elastomers significantly toughen epoxy resins, but significant toughening requires the addition of large amounts of toughening agents, which significantly reduces the strength, modulus, and glass transition temperature of the epoxy resin. Toughening epoxy resins with inorganic rigid nanoparticles can achieve both high strength and high toughness. However, due to the poor interfacial interaction between the inorganic particles and the epoxy resin, they tend to agglomerate within the epoxy resin matrix, significantly impacting the toughening effect and mechanical properties. Using thermoplastic resins with high strength, high modulus, and high performance as epoxy resin toughening agents can effectively improve the toughness of epoxy resins without compromising their mechanical and thermal properties. However, thermoplastic resins have poor solubility, and when they are blended with epoxy resins, the system viscosity is high, making the processing difficult.
[0004] Core-shell microspheres, due to their unique structure, can be used as epoxy resin toughening agents. The core absorbs energy when subjected to stress, while the shell provides support and prevents crack propagation. The two work synergistically to enhance the material's toughness. PU microspheres, due to their excellent elasticity and designability, are often used as the core material for core-shell microspheres. While significant progress has been made in toughening epoxy resins with core-shell microspheres, several challenges remain. These include achieving uniform dispersion of the microspheres within the epoxy resin matrix and enhancing the material's toughness without sacrificing other properties. Summary of the Invention
[0005] A problem with existing technologies is that, while conventional toughening modifiers added to epoxy resin systems improve toughness, their tensile strength significantly decreases. To address this technical issue, the present invention provides a polyurethane@polyamic acid microsphere dispersion. The polyurethane@polyamic acid microspheres in this polyurethane@polyamic acid microsphere dispersion are obtained by chemically grafting polyamic acid (amide bonds) onto the surface of the polyurethane microspheres.
[0006] Preferably, the method for preparing the polyurethane microspheres comprises the following steps: (1) Low molecular weight diol, polymer polyol, and hydrophilic monomer are added to a three-necked flask in sequence. First, the system is dehydrated. Then, diisocyanate is added dropwise to the reaction system while stirring. After the addition is completed, the reaction is stirred at 50-100 °C for 5-10 h. Then, polyisocyanate is added to the system. Then, an organic solvent is added to the reaction system to maintain the flow state of the reaction system. After stirring until the temperature of the reaction system drops to room temperature, a neutralizer is added to the reaction system. The mixture is stirred to make the pH of the reaction system ≥7. Then, deionized water is added to the reaction system for emulsification to obtain a stable polyurethane prepolymer dispersion. (2) Adding polyamine to the polyurethane prepolymer dispersion to carry out chain extension reaction, stirring and reacting at room temperature for 1-2 hours to obtain a polyurethane microsphere emulsion; (3) Adding a dilute hydrochloric acid aqueous solution to the polyurethane microsphere emulsion to break the emulsion, and then collecting the solid product by solid-liquid separation. The obtained solid product is washed and dried to obtain polyurethane microspheres.
[0007] Preferably, the low molecular weight diol includes one or more of ethylene glycol, 1,2-propylene glycol, 1,4-butanediol, neopentyl glycol, and diethylene glycol.
[0008] Preferably, the polymer polyol includes one or more of dihydroxy polyether N210, dihydroxy polyether N220, trihydroxy polyether N330, polytetramethylene ether glycol, and polyethylene adipate glycol.
[0009] Preferably, the hydrophilic monomer includes one or more of 2,2-dihydroxymethylpropionic acid, 2,2-dihydroxymethylbutanoic acid, and 1,2-propylene glycol-3-sulfonate sodium.
[0010] Preferably, the diisocyanate includes one or more of isophorone diisocyanate, toluene diisocyanate, diphenylmethane diisocyanate, and hexamethylene diisocyanate.
[0011] Preferably, the polyisocyanate includes one or more of hexamethylene diisocyanate trimer, polymethylene polyphenyl polyisocyanate, isophorone diisocyanate trimer, and toluene diisocyanate trimer.
[0012] Preferably, the organic solvent is one or more of butanone, acetone, N,N-dimethylformamide, and N-methylpyrrolidone.
[0013] Preferably, the neutralizing agent includes one or more of triethylamine, dimethylethanolamine, and triethanolamine.
[0014] Preferably, the polyamine includes one or more of diethylenetriamine, triethylenetetramine, and tetraethylenepentamine.
[0015] Preferably, the method of grafting polyamic acid onto the surface of polyurethane microspheres through amide bonds comprises the following steps: (1) Add dibasic acid anhydride, diamine and organic solvent into a three-necked flask, stir and dissolve, evacuate and pass argon, circulate evacuate and pass argon three times, stir and react for 3-8 hours at room temperature under argon protection to obtain a polyamic acid solution; (2) Add polyurethane microspheres to the polyamic acid solution and stir the reaction at room temperature for 5-10 hours to obtain a polyurethane@polyamic acid microsphere (PUAA) dispersion.
[0016] Preferably, the dibasic acid anhydride includes one or more of 3,3',4,4'-benzophenone tetracarboxylic dianhydride, pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, and hexafluorodianhydride.
[0017] Preferably, the diamine includes one or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, 1,3-diaminobenzene, 4,4'-diaminodiphenylmethane, and 2,2'-bis(trifluoromethyl)benzidine.
[0018] Preferably, the organic solvent includes one or more of N-methylpyrrolidone, tetrahydrofuran, N, N-dimethylformamide, and N, N-dimethylacetamide.
[0019] A polyurethane @ polyamic acid microsphere / epoxy resin dispersion is provided. The polyurethane @ polyamic acid microsphere dispersion and epoxy resin are added into a three-necked flask, stirred and reacted at 50-75° C. for at least 5 hours, and then the solvent is removed by rotary evaporation to obtain the polyurethane @ polyamic acid microsphere / epoxy resin dispersion.
[0020] An epoxy resin composite material is obtained by uniformly mixing a polyurethane@polyamic acid microsphere / epoxy resin dispersion, an epoxy resin, and a curing agent, vacuum degassing, casting, and then curing at 60°C for 2 hours and then at 120°C for 2 hours.
[0021] Preferably, the curing agent includes one or more of 2-ethyl-4-methylimidazole, diethylenetriamine, dimethylaminophenol, and maleic anhydride.
[0022] Preferably, the amount of curing agent used is 5-200 wt% of the epoxy resin.
[0023] The present invention has the following beneficial effects: (1) The polyurethane@polyamic acid microspheres obtained in the present invention are obtained by grafting polyamic acid onto the surface of polyurethane microspheres through amide bonds. The special structure of the polyurethane@polyamic acid microspheres not only significantly improves the toughness of the epoxy resin material system, but also does not reduce the tensile strength of the epoxy resin material system. On the contrary, it also significantly improves the mechanical properties of the epoxy resin material system. (2) The flexible chain segments of the polyurethane core in the polyurethane@polyamic acid microspheres give the material deformation ability, and the rigid chain segments of the polyamic acid shell provide support. When the material is subjected to external force, the polyurethane@polyamic acid microspheres can dissipate energy through the elastic deformation of the polyurethane. At the same time, the polyamic acid shell layer and the epoxy resin have a strong interfacial bonding effect, which can effectively regulate the crack propagation path. The two work together to avoid the problem of interface debonding or brittle fracture caused by insufficient toughness or rigidity of a single microsphere, thereby achieving a simultaneous improvement in strength and toughness. (3) The toughening effect of the polyurethane@polyamic acid microspheres obtained in the present invention is a technical effect that is difficult to achieve with conventional toughening modifiers currently on the market, and has good market prospects. BRIEF DESCRIPTION OF THE DRAWINGS
[0024] Figure 1 This is a SEM image of the polyurethane@polyamic acid microspheres obtained in Example 5 of the present invention.
[0025] Figure 2 These are mechanical property diagrams of the epoxy resin composite materials obtained in Examples 5-8 of the present invention and Comparative Examples 1-2, respectively. Figure a represents Examples 5-8, Figure b represents Comparative Example 1, and Figure c represents Comparative Example 2.
[0026] Figure 3 These are the DSC curves of the epoxy resins obtained in Examples 4, 8, and 12 of the present invention and Comparative Examples 1-2, respectively. In the figure, PAA represents Comparative Example 2, PU represents Comparative Example 1, E51 represents pure epoxy resin, PUAA11 represents Example 8, PUAA12 represents Example 4, and PUAA21 represents Example 12. DETAILED DESCRIPTION
[0027] The present invention will be described in detail below with reference to the following examples. However, it should be understood that the following examples are merely illustrative of the embodiments of the present invention and are not intended to limit the scope of the present invention.
[0028] Example 1
[0029] (1) Preparation of polyurethane microspheres
[0030] (a) 2.95 g (0.0440 mol) of 2,2-dihydroxymethylpropionic acid (DMPA), 7.00 g (0.0140 mol) of polyether polyol N210 (N210), and 0.63 g (0.0140 mol) of 1,4-butanediol (BDO) were weighed and added to a three-necked flask. The mixture was stirred and dehydrated at 160°C for 10 minutes. After the system was cooled to 90°C, 16.01 g (0.1440 mol) of isophorone diisocyanate (IPDI) was slowly added dropwise to the system. After stirring for four hours, 3.19 g (0.0287 mol) of hexamethylene diisocyanate trimer (HDI trimer) was added to the reaction system. 10 g of butanone was added to the reaction system to reduce the viscosity of the reaction system. After the temperature of the reaction system was cooled to room temperature, 2.23 g (0.1440 mol) of triethanolamine (TEA) was added to the reaction system. g (0.0220 mol), stirred at high speed, then added 82 g of deionized water dropwise into the system, and stirred evenly to obtain a polyurethane prepolymer dispersion; (b) 10.39 g (0.3021 mol) of diethylenetriamine (DETA) dissolved in 51.95 g of deionized water was added dropwise to the polyurethane prepolymer dispersion for chain extension. The mixture was stirred at room temperature for 5 h to obtain a polyurethane microsphere emulsion. (c) Dilute hydrochloric acid with a mass concentration of 5% was added dropwise to the polyurethane microsphere emulsion to neutralize and break the emulsion. The polyurethane microsphere powder was obtained after filtration, washing, and drying.
[0031] (2) Preparation of polyurethane@polyamic acid microsphere dispersion
[0032] (d) 6.57 g (0.0204 mol) of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA) and 87 g of tetrahydrofuran (THF) were weighed and added to a three-necked flask. After the BTDA was completely dissolved, 2.00 g (0.0100 mol) of 4,4'-diaminodiphenyl ether (ODA) was added to the system. The mixture was stirred for 3 h to obtain a polyamic acid solution. (e) 4.29 g of polyurethane microspheres were added to the polyamic acid solution, and the mixture was stirred at room temperature for 5 h to obtain a polyurethane@polyamic acid microsphere dispersion.
[0033] (3) Preparation of polyurethane@polyamic acid microspheres / epoxy resin dispersion
[0034] 40 g of polyurethane@polyamic acid microsphere dispersion and 25 g of E51 epoxy resin (epoxy equivalent weight 190 g / eq, molecular weight 380 g / mol) were added to a three-necked flask. After stirring and reacting at 65 °C for 5 h, tetrahydrofuran was removed by rotary evaporation to obtain a polyurethane@polyamic acid microsphere / epoxy resin dispersion. The microsphere content in the dispersion was 20% of the epoxy resin mass.
[0035] (4) Preparation of epoxy resin composite materials (microsphere content accounts for 1% of the epoxy resin mass)
[0036] 1.75 g of polyurethane@polyamic acid microspheres / epoxy resin dispersion, 33.25 g of E51 epoxy resin (epoxy equivalent weight 190 g / eq, molecular weight 380 g / mol), and 1.75 g of 2-ethyl-4-methylimidazole were weighed and stirred until uniformly mixed. The mixture was then placed in a vacuum oven to remove air bubbles. The mixture was then cast into a mold and cured at 60°C for 120 min and then at 120°C for another 120 min to obtain an epoxy resin composite. The polyurethane@polyamic acid microspheres in the epoxy resin composite accounted for 1% of the total weight of the epoxy resin.
[0037] Example 2 is the same as Example 1, except that, during the preparation of the epoxy resin composite material of Example 2 (the microsphere content accounts for 3% of the mass of the epoxy resin), the amount of polyurethane@polyamic acid microspheres / epoxy resin dispersion used is 5.25 g, and the amount of E51 epoxy resin used is 29.75 g.
[0038] Example 3 is the same as Example 1, except that during the preparation of the epoxy resin composite material of Example 3 (the content of microspheres accounts for 5% of the mass of the epoxy resin), The amount of polyurethane@polyamic acid microspheres / epoxy resin dispersion used is 8.75 g, and the amount of E51 epoxy resin used is 26.25 g.
[0039] Example 4 is the same as Example 1, except that during the preparation of the epoxy resin composite material of Example 4 (the content of microspheres accounts for 7% of the mass of the epoxy resin), The amount of polyurethane@polyamic acid microspheres / epoxy resin dispersion used is 12.25 g, and the amount of E51 epoxy resin used is 22.75 g.
[0040] Example 5
[0041] (1) The preparation of polyurethane microspheres is the same as in Example 1.
[0042] (2) The preparation of polyurethane@polyamic acid microsphere dispersion is basically the same as that in Example 1, except that in step (e), 8.57 g of polyurethane microspheres are added to the polyamic acid solution, and the mixture is stirred at room temperature for 5 h to obtain a polyurethane@polyamic acid microsphere dispersion.
[0043] (3) The preparation of polyurethane@polyamic acid microspheres / epoxy resin dispersion is the same as in Example 1.
[0044] (4) The preparation of epoxy resin composite material (microsphere content accounts for 1% of the mass of epoxy resin) is the same as in Example 1.
[0045] Example 6 is the same as Example 5, except that, during the preparation of the epoxy resin composite material of Example 6 (the microsphere content accounts for 3% of the mass of the epoxy resin), the amount of polyurethane@polyamic acid microspheres / epoxy resin dispersion used is 5.25 g, and the amount of E51 epoxy resin used is 29.75 g.
[0046] Example 7 is the same as Example 5, except that, during the preparation of the epoxy resin composite material of Example 7 (the microsphere content accounts for 5% of the mass of the epoxy resin), the amount of polyurethane@polyamic acid microspheres / epoxy resin dispersion used is 8.75 g, and the amount of E51 epoxy resin used is 26.25 g.
[0047] Example 8 is the same as Example 5, except that, during the preparation of the epoxy resin composite material of Example 8 (the microsphere content accounts for 7% of the mass of the epoxy resin), the amount of polyurethane@polyamic acid microspheres / epoxy resin dispersion used is 12.25 g, and the amount of E51 epoxy resin used is 22.75 g.
[0048] Example 9
[0049] (1) The preparation of polyurethane microspheres is the same as in Example 1.
[0050] (2) The preparation of polyurethane@polyamic acid microsphere dispersion is basically the same as that in Example 1, except that in step (e), 17.14 g of polyurethane microspheres are added to the polyamic acid solution, and the mixture is stirred at room temperature for 5 h to obtain a polyurethane@polyamic acid microsphere dispersion.
[0051] (3) The preparation of polyurethane@polyamic acid microspheres / epoxy resin dispersion is the same as in Example 1.
[0052] (4) The preparation of epoxy resin composite material (microsphere content accounts for 1% of the mass of epoxy resin) is the same as in Example 1.
[0053] Example 10 is the same as Example 9, except that, in the preparation process of the epoxy resin composite material of Example 10 (the microsphere content accounts for 3% of the epoxy resin mass), the amount of polyurethane@polyamic acid microspheres / epoxy resin dispersion used is 5.25 g, and the amount of E51 epoxy resin used is 29.75 g.
[0054] Example 11 is the same as Example 9, except that, during the preparation of the epoxy resin composite material of Example 11 (the microsphere content accounts for 5% of the mass of the epoxy resin), the amount of polyurethane@polyamic acid microspheres / epoxy resin dispersion used is 8.75 g, and the amount of E51 epoxy resin used is 26.25 g.
[0055] Example 12 is the same as Example 9, except that, in the preparation process of the epoxy resin composite material of Example 12 (microsphere content accounts for 7% of the epoxy resin mass), the amount of polyurethane@polyamic acid microspheres / epoxy resin dispersion used is 12.25 g, and the amount of E51 epoxy resin used is 22.75 g.
[0056] Comparative Example 1
[0057] (1) The preparation of polyurethane microspheres is the same as in Example 1.
[0058] (2) Preparation of polyurethane microsphere / epoxy resin dispersion
[0059] 40 g of polyurethane microspheres were added to a tetrahydrofuran dispersion to obtain a polyurethane microsphere dispersion with a mass concentration of 12.5 wt%. The polyurethane microsphere dispersion and 25 g of E51 epoxy resin were added to a three-necked flask. After stirring and reacting at 65 °C for 5 h, the tetrahydrofuran was removed by rotary evaporation to obtain a polyurethane microsphere / epoxy resin dispersion. The microsphere content in the dispersion was 20% of the epoxy resin mass.
[0060] (3) The preparation of epoxy resin composite material (microsphere content accounts for 1% of the mass of epoxy resin) is basically the same as Example 1, except that in Comparative Example 1, the polyurethane@polyamic acid microspheres / epoxy resin dispersion in Example 1 is replaced with the same amount of polyurethane microspheres / epoxy resin dispersion.
[0061] Following the same experimental steps, the ratio of polyurethane microspheres to epoxy resin was adjusted to obtain epoxy resin composite materials with polyurethane microsphere contents of 3%, 5%, and 7%, respectively.
[0062] Comparative Example 2
[0063] (1) Synthesis of polyamic acid microspheres
[0064] (a) Weigh 4.00 g (20.0000 mmol) of 4,4'-diaminodiphenyl ether, 6.57 g (20.4000 mmol) of 3,3',4,4'-benzophenone tetracarboxylic dianhydride, and 51.00 g of N-methylpyrrolidone into a three-necked flask and stir to dissolve. Vacuum the flask and purge with argon for three cycles. Cool the reaction system to 5°C. Stir the reaction for 6 h before stopping the reaction to obtain a polyamic acid solution. (b) The polyamic acid solution was diluted with N-methylpyrrolidone to a solid content of 1 wt%, 50.00 g (1 wt%) of the diluted polyamic acid solution, 0.0282 g (0.04787 mmol) of tetraethylammonium hydroxide and 0.05 g of octylphenol polyoxyethylene ether were weighed and added to a three-necked flask and stirred to dissolve completely. Then, 200 g of deionized water containing 0.5 g of polyvinylpyrrolidone was added dropwise to the solution. After stirring evenly, the solution was centrifuged and washed three times in sequence to remove N-methylpyrrolidone on the surface of the solid product. The product was then dried at 60°C to obtain light yellow solid powder of polyamic acid ammonium salt microspheres.
[0065] (2) Preparation of polyamic acid microspheres / epoxy resin dispersion
[0066] A tetrahydrofuran dispersion containing 40 g of polyamic acid microspheres (12.5 wt%) and 25 g of E51 epoxy resin (the same epoxy resin as in Example 1) were added to a three-necked flask. The mixture was reacted at 65°C for 5 h. The tetrahydrofuran was then removed by rotary evaporation to obtain a polyamic acid microsphere / epoxy resin dispersion. The microsphere content was 20% of the epoxy resin by weight.
[0067] (3) Polyamic acid microsphere epoxy resin composite material
[0068] Weigh 1.75 g of polyamic acid microsphere / epoxy resin dispersion, 33.25 g of E51 epoxy resin, and 1.75 g of 2-ethyl-4-methylimidazole, stir and mix thoroughly, then place in a vacuum oven to remove air bubbles. The mixture is then cast into a mold and cured at 60°C for 120 min and then at 120°C for 120 min to obtain a polyamic acid microsphere-epoxy resin composite. The polyamic acid microspheres account for 1% of the total weight of the epoxy resin.
[0069] Following the same experimental steps, by adjusting the ratio of polyamic acid microspheres to epoxy resin, epoxy resin composite materials with polyamic acid microsphere contents of 3%, 5%, and 7% were obtained.
[0070] The SEM image of the polyurethane@polyamic acid microspheres obtained in Example 5 of the present invention is shown in the attached specification. Figure 1 shown.
[0071] Instructions attached Figure 2 These are mechanical property diagrams of the epoxy resin composite materials obtained in Examples 5-8 of the present invention and Comparative Examples 1-2, respectively. Figure a represents Examples 5-8, Figure b represents Comparative Example 1, and Figure c represents Comparative Example 2.
[0072] Instructions attached Figure 3 These are the DSC curves of the epoxy resins obtained in Examples 4, 8, and 12 of the present invention and Comparative Examples 1-2, respectively. In the figure, PAA represents Comparative Example 2, PU represents Comparative Example 1, E51 represents pure epoxy resin, PUAA11 represents Example 8, PUAA12 represents Example 4, and PUAA21 represents Example 12.
[0073] With the above-described preferred embodiments of the present invention as a guide, and with reference to the above description, relevant personnel are fully capable of making various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the contents of the specification and must be determined according to the scope of the claims.
Claims
1. A polyurethane@polyamic acid microsphere dispersion, characterized in that: The polyurethane@polyamic acid microspheres in the polyurethane@polyamic acid microsphere dispersion are obtained by chemically grafting polyamic acid onto the surface of the polyurethane microspheres.
2. The polyurethane@polyamic acid microsphere dispersion according to claim 1, characterized in that: The preparation method of the polyurethane microspheres comprises the following steps: (1) Low molecular weight diol, polymer polyol, and hydrophilic monomer are added to a three-necked flask in sequence. First, the system is dehydrated. Then, diisocyanate is added dropwise to the reaction system while stirring. After the addition is completed, the reaction is stirred at 50-100°C for 5-10 hours. Then, polyisocyanate is added to the system. Then, an organic solvent is added to the reaction system to maintain the flow state of the reaction system. After stirring until the temperature of the reaction system drops to room temperature, a neutralizer is added to the reaction system. The mixture is stirred and mixed until the pH of the reaction system is ≥7. Then, deionized water is added to the reaction system for emulsification to obtain a stable polyurethane prepolymer dispersion. (2) Adding polyamine to the polyurethane prepolymer dispersion to carry out chain extension reaction, stirring and reacting at room temperature for 1-2 hours to obtain a polyurethane microsphere emulsion; (3) Adding a dilute hydrochloric acid aqueous solution to the polyurethane microsphere emulsion to break the emulsion, and then collecting the solid product by solid-liquid separation. The obtained solid product is washed and dried to obtain polyurethane microspheres.
3. The polyurethane@polyamic acid microsphere dispersion according to claim 2, characterized in that: The polymer polyol includes one or more of dihydroxy polyether N210, dihydroxy polyether N220, trihydroxy polyether N330, polytetramethylene ether glycol, and polyethylene adipate glycol.
4. The polyurethane@polyamic acid microsphere dispersion according to claim 2, characterized in that: The hydrophilic monomer includes one or more of 2,2-dihydroxymethylpropionic acid, 2,2-dihydroxymethylbutanoic acid, and 1,2-propylene glycol-3-sulfonate sodium.
5. The polyurethane@polyamic acid microsphere dispersion according to claim 2, characterized in that: The polyamine includes one or more of diethylenetriamine, triethylenetetramine and tetraethylenepentamine.
6. The polyurethane@polyamic acid microsphere dispersion according to claim 1, characterized in that: The method for chemically grafting polyamic acid onto the surface of polyurethane microspheres comprises the following steps: (1) Add dibasic acid anhydride, diamine and organic solvent into a three-necked flask, stir and dissolve, evacuate and pass argon, circulate evacuate and pass argon three times, stir and react for 3-8 hours at room temperature under argon protection to obtain a polyamic acid solution; (2) Add polyurethane microspheres to the polyamic acid solution and stir the reaction at room temperature for 5-10 hours to obtain a PUAA microsphere dispersion.
7. The polyurethane@polyamic acid microsphere dispersion according to claim 6, characterized in that: The dibasic acid anhydride includes one or more of 3,3',4,4'-benzophenone tetracarboxylic dianhydride, pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, and hexafluorodianhydride.
8. The polyurethane@polyamic acid microsphere dispersion according to claim 6, characterized in that: The diamine includes one or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, 1,3-diaminobenzene, 4,4'-diaminodiphenylmethane, and 2,2'-bis(trifluoromethyl)benzidine.
9. A polyurethane@polyamic acid microsphere / epoxy resin dispersion, characterized in that: The polyurethane@polyamic acid microsphere dispersion obtained according to any one of claims 1 to 8 and epoxy resin are added to a three-necked flask, stirred and reacted at 50-75° C. for at least 5 hours, and then the solvent is removed by rotary evaporation to obtain a polyurethane@polyamic acid microsphere / epoxy resin dispersion.
10. An epoxy resin composite material, characterized in that: The composite material is obtained by uniformly mixing the polyurethane@polyamic acid microsphere / epoxy resin dispersion described in claim 9, epoxy resin and curing agent, vacuum degassing, casting, and then curing at 60°C for 2 hours and then at 120°C for 2 hours.