Urea-formaldehyde microcapsule shell surface modification method beneficial to toughening of epoxy composite material

Abstract

A urea-formaldehyde microcapsule shell surface modification method beneficial to toughening of an epoxy composite material. The method comprises: uniformly mixing polyvinyl alcohol powder with deionized water to obtain a surfactant aqueous solution; mixing deionized water with the surfactant aqueous solution, then adding urea, ammonium chloride, and resorcinol, stirring the mixture, dropwise adding a diluted hydrochloric acid solution to adjust the pH, and stirring the mixture to obtain a stable emulsion; adding a formaldehyde-urea mixed solution to the stable emulsion and stirring and heating the mixture; after the heating reaction is completed, cooling the mixture to room temperature, and carrying out vacuum filtration, washing with deionized water, filtration, and then vacuum drying to obtain a urea-formaldehyde microcapsule shell; and mixing 3-aminopropyltriethoxysilane with deionized water, dropwise adding a diluted hydrochloric acid solution to adjust the pH to obtain a modified solution, adding the urea-formaldehyde microcapsule shell to the modified solution, stirring the mixture, and then carrying out vacuum filtration and drying to obtain a urea-formaldehyde microcapsule shell with the surface modified.

Description

A surface modification method for urea-formaldehyde microcapsule shells that is beneficial for toughening epoxy composite materials Technical Field

[0001] The invention belongs to the field of material science and engineering technology, and particularly relates to a surface modification method for a urea-formaldehyde microcapsule shell which is beneficial to toughening epoxy composite materials. Background Art

[0002] In recent years, with the introduction of new smart materials into diverse applications, polymer materials with higher reliability and longer service life have gradually attracted widespread attention. Among them, self-healing materials have broad application prospects due to their ability to autonomously repair damage caused by external conditions (such as ultraviolet radiation, chemical attack, mechanical stress, and thermal decomposition), restore their original function, and avoid material failure caused by damage. This significantly reduces material maintenance and repair costs. In addition to modifying the internal structure of polymer materials, one of the most common practical methods for achieving external self-healing of materials is to prepare functionalized microcapsules containing healing agents based on microencapsulation technology. When mechanical or electrical damage causes internal cracks in the material, the shell of the functionalized microcapsule containing the healing agent ruptures as the crack propagates. The healing agent inside then flows into the crack through capillary action and comes into contact with the catalyst, undergoing a polymerization reaction to bond the crack surface, thereby restoring the integrity of the material structure. Due to the wide variety of core materials, it is important to encapsulate different self-healing core materials using the same wall material and simultaneously evaluate their performance. Therefore, it is necessary to study the preparation methods of the microcapsule shell.

[0003] At present, many scholars have conducted relevant research on microcapsule shell materials such as polyurea formaldehyde, polymelamine formaldehyde, polymethyl methacrylate, polyurethane and epoxy resin. However, polyurea formaldehyde (PUF) is still one of the most commonly used shell materials because it has high mechanical strength, high impermeability, high reactivity, chemical resistance and low cost. It has both appropriate brittleness and can break when cracks appear in the substrate, and can maintain an intact structure when preparing composite materials. Generally, its preparation method is to use in situ polymerization method, through which certain monomers react in the aqueous phase to form UF oligomers, which are then deposited on the droplets to form a cross-linked shell. Its reaction conditions are relatively mild, the method is relatively simple, and the efficiency is relatively high, which can be put into practical production applications on a large scale.

[0004] However, when microcapsules are mixed with epoxy substrates to form composites, weak interfacial adhesion severely compromises the mechanical properties and self-healing efficiency of the composites, limiting their potential for application. Therefore, surface modification of the microcapsule shell is necessary to adapt it to practical applications. In recent years, rigid inorganic nanoparticles have been commonly used as fillers to address the significant decrease in mechanical properties of composites caused by microcapsule doping.

[0005] The existing microcapsule shell prepared by adding rigid inorganic nanoparticles can significantly improve its mechanical strength and hardness, and improve its resistance to chemical substances (acid resistance, alkali resistance, etc.), and can be widely used in scenarios such as electronic packaging and dental restoration. However, the addition of inorganic particles will reduce the toughness and brittle fracture resistance of the composite material, making its ability to prevent crack propagation weaker, and unable to timely suppress the expansion and development of cracks, thereby reducing the repair efficiency of the self-healing material. Especially in the field of electrical insulation, the self-repair of mechanical and electrical damage of high-voltage equipment composite materials is of great significance. Therefore, in order to achieve timely response of microcapsules in damage self-repair, it is necessary to study the improvement of the toughness of the microcapsule shell.

[0006] Summary of the Invention

[0007] The purpose of this section is to summarize some aspects of the embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the abstract and title of this application to avoid obscuring the purpose of this section, the abstract and the title of the invention, and such simplifications or omissions should not be used to limit the scope of the present invention.

[0008] In view of the above problems and / or the problems existing in the prior art, the present invention is proposed.

[0009] Therefore, the purpose of the present invention is to overcome the deficiencies in the prior art and provide a method for modifying the surface of a urea-formaldehyde microcapsule shell that is beneficial to toughening epoxy composite materials.

[0010] In order to solve the above technical problems, the present invention provides the following technical solution: a method for modifying the surface of urea-formaldehyde microcapsule shell for toughening epoxy composite materials, characterized by comprising:

[0011] The polyvinyl alcohol powder is mixed evenly with deionized water to obtain a surfactant aqueous solution;

[0012] Deionized water and surfactant aqueous solution are mixed, urea, ammonium chloride, and resorcinol are added, stirred, diluted hydrochloric acid solution is added dropwise to adjust the pH, and stirred to obtain a stable emulsion;

[0013] Add formaldehyde-urea mixed solution to the stable emulsion and stir and heat;

[0014] After the heating reaction is completed, the product is cooled to room temperature, vacuum filtered, washed with deionized water, filtered, and vacuum dried to obtain the urea-formaldehyde microcapsule shell;

[0015] 3-Aminopropyltriethoxysilane is mixed with deionized water, and diluted hydrochloric acid solution is added dropwise to adjust the pH to obtain a modified solution. Urea-formaldehyde microcapsule shells are added to the modified solution, stirred, vacuum filtered, and dried to obtain surface-modified urea-formaldehyde microcapsule shells.

[0016] As a preferred embodiment of the preparation method of the present invention, the polyvinyl alcohol powder is evenly mixed with deionized water to obtain a surfactant aqueous solution, wherein the mass ratio of polyvinyl alcohol powder to deionized water is 1:19, and the mass fraction of the surfactant aqueous solution is 5%.

[0017] As a preferred embodiment of the preparation method of the present invention, the deionized water and the surfactant aqueous solution are mixed, urea, ammonium chloride, and resorcinol are added and stirred, a diluted hydrochloric acid solution is added dropwise to adjust the pH, and a stable emulsion is obtained by stirring, wherein the volume ratio of deionized water to the surfactant aqueous solution is 10-15:0.4-0.6, and the mass ratio of urea, ammonium chloride, and resorcinol is 10:1:1.

[0018] As a preferred embodiment of the preparation method of the present invention, the diluted hydrochloric acid solution is added dropwise to adjust the pH, and the mixture is stirred to obtain a stable emulsion, wherein the mass fraction of the diluted hydrochloric acid solution is 1wt% to 1.5wt%, the pH is adjusted to 2.5 to 3.5, the stirring rate is 300 to 400 r / min, and the stirring time is 10 to 15 minutes.

[0019] As a preferred embodiment of the preparation method of the present invention, the formaldehyde-urea mixed solution is added to the stable emulsion and stirred and heated, wherein the molar mass ratio of formaldehyde to urea is 2.79-2.83:1.

[0020] As a preferred embodiment of the preparation method of the present invention, the heating temperature is 80-90° C., the heating time is 3.5-4.5 h, and the stirring rate is 100-200 r / min.

[0021] As a preferred embodiment of the preparation method of the present invention, the urea-formaldehyde microcapsule shell is obtained by washing with deionized water, filtering and vacuum drying, wherein the washing times are 3 to 4 times, the vacuum drying temperature is 30 to 40° C., and the time is 48 to 72 hours.

[0022] As a preferred embodiment of the preparation method of the present invention, the 3-aminopropyltriethoxysilane is mixed with deionized water, and a diluted hydrochloric acid solution is added dropwise to adjust the pH to obtain a modified liquid. The urea-formaldehyde microcapsule shell is added to the modified liquid, stirred, vacuum filtered, and dried to obtain a surface-modified urea-formaldehyde microcapsule shell. The volume ratio of 3-aminopropyltriethoxysilane to deionized water is 2:98, and the mass fraction of the modified liquid is 2-5%.

[0023] As a preferred embodiment of the preparation method of the present invention, the modified liquid is obtained by adding a diluted hydrochloric acid solution to adjust the pH, wherein the mass fraction of the diluted hydrochloric acid solution is 1wt% to 1.5wt%, and the pH is adjusted from 10 to 11 to 7.

[0024] As a preferred embodiment of the preparation method of the present invention, the urea-formaldehyde microcapsule shell is added to the modified liquid, stirred, vacuum filtered, and dried to obtain the surface-modified urea-formaldehyde microcapsule shell, wherein the stirring temperature is 80-90°C, the speed is 100-200 r / min, and the time is 1-1.5 h.

[0025] Beneficial effects of the present invention:

[0026] (1) The present invention modifies the shell of urea-formaldehyde microcapsules to reveal the toughening mechanism of the microcapsule composite material after doping and modification, provides a surface modification method of urea-formaldehyde microcapsules shell that is beneficial to the toughening of epoxy composite materials, and provides a feasible solution for damage inhibition and self-repair efficiency improvement of composite materials.

[0027] (2) The present invention prepares a urea-formaldehyde microcapsule shell by in-situ polymerization and performs surface modification to functionalize it. The silane coupling agent used in the preparation process increases the surface roughness of the microcapsule by bonding with the microcapsule surface, thereby improving the interfacial interaction between the microcapsule and the polymer matrix, further enhancing the fracture toughness of the composite material.

[0028] (3) The present invention adopts an in-situ polymerization method to prepare a urea-formaldehyde microcapsule shell sample and performs surface modification treatment on it. The basic principle is to uniformly mix the urea-formaldehyde resin mixture with the aqueous phase, and add an appropriate amount of emulsifier to form a stable emulsion, and then adjust the temperature, pH and other conditions to cause the above emulsion to undergo a cross-linking reaction, thereby forming a solid shell layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive effort. Among them:

[0030] FIG1 is a schematic diagram of the urea-formaldehyde (UF) dimer reaction according to Example 1 of the present invention.

[0031] FIG2 is a schematic diagram of the preparation and surface modification of the urea-formaldehyde microcapsule shell in Example 1 of the present invention.

[0032] FIG3 is an SEM image of the urea-formaldehyde microcapsule shell before and after functionalization with a silane coupling agent in Example 1 of the present invention.

[0033] FIG4 is a graph showing the particle size distribution of urea-formaldehyde microcapsules before and after functionalization with a silane coupling agent in Example 1 of the present invention.

[0034] FIG5 is an FT-IR image of urea-formaldehyde microcapsules before and after functionalization with a silane coupling agent in Example 1 of the present invention.

[0035] FIG6 is a fracture toughness curve of the urea-formaldehyde microcapsule composite material before and after modification with different mass fractions of doping in Example 2 of the present invention.

[0036] FIG7 is a fracture toughness curve of the modified urea-formaldehyde microcapsule composite material doped with different mass fractions according to Example 2 of the present invention. DETAILED DESCRIPTION

[0037] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the embodiments of the specification.

[0038] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0039] Secondly, the term "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in various places throughout this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive of other embodiments.

[0040] The manufacturers and purities of the chemical reagents used in the preparation of urea-formaldehyde microcapsule shell samples and their modification treatments in the method of the present invention are shown in Table 1:

[0041] Table 1:

[0042] The models and manufacturers of the main experimental instruments for preparing urea-formaldehyde microcapsule shell samples in the method of the present invention are shown in Table 2:

[0043] Table 2:

[0044] Example 1

[0045] The present invention provides a method for modifying the surface of a urea-formaldehyde microcapsule shell, which is beneficial for toughening epoxy composite materials:

[0046] (1) Preparation of surfactant solution

[0047] 190 g of deionized water and 10 g of polyvinyl alcohol (PVA) powder were weighed and mixed in a 400 mL beaker to obtain a surfactant aqueous solution with a mass fraction of 5%, so as to promote better mixing and dispersion of the urea-formaldehyde resin and the aqueous phase.

[0048] (2) Forming a stable emulsion

[0049] 130 mL of deionized water and 5 mL of the surfactant aqueous solution prepared in (1) were added to a 400 mL beaker, and the speed of the CNC magnetic stirrer was set to 200 r / min at room temperature to mix them thoroughly. 3.5 g of urea (as a reaction monomer), 0.35 g of ammonium chloride (as an initiator) and 0.35 g of resorcinol (as a cross-linking agent) were weighed. Under stirring conditions, the weighed solid shell-forming material was slowly added to the above aqueous solution, and after stirring evenly, 1 wt% hydrochloric acid solution (diluted) was added dropwise thereto. The pH value of the mixed solution was adjusted to 3, and the speed was set to 350 r / min. The mixture was stirred for 15 minutes to stabilize the mixed solution into an emulsion under vigorous stirring.

[0050] (3) Reaction to form shell

[0051] Weigh 93 mL of 37% formaldehyde solution, add formaldehyde solution to form a stable emulsion in (2), set the temperature of the CNC magnetic stirrer to 85°C and the speed to 200 r / min, and slowly heat for 4 h to allow it to fully react to form a shell.

[0052] (4) Washing and drying

[0053] After the reaction, the suspension was cooled to room temperature and vacuum filtered. The shells of the filtered urea-formaldehyde microcapsules were repeatedly rinsed with deionized water three times to remove impurities and stored in a vacuum drying oven at 35°C for 48 hours.

[0054] (5) Surface modification

[0055] Weigh 2 mL of 3-aminopropyltriethoxysilane (silane coupling agent) and 98 mL of deionized water and mix them evenly to prepare a modified solution with a mass fraction of 2% and a pH value of 11. Add 1 wt% hydrochloric acid solution (diluted) to adjust the pH value of the modified solution to 7. Then, take the dried microcapsules in (4) and add them to the prepared modified solution. Set the temperature of the CNC magnetic stirrer to 80°C and the speed to 200 r / min and stir for 1 hour. After stirring, vacuum filter and dry the suspension again to obtain the urea-formaldehyde microcapsule shell after surface modification.

[0056] The dried urea-formaldehyde microcapsule shell sample (Example 1) is placed on a sample table, sprayed with platinum to prepare a sample, and then a scanning electron microscope (SEM) is used to test and observe its micromorphology. The SEM images of the urea-formaldehyde microcapsule shell before and after the functionalization of the silane coupling agent are obtained as shown in Figure 3. As can be seen from the figure, the entire microcapsule presents a relatively full spherical shape before and after surface modification, with a relatively uniform particle size, a relatively complete shape, and no damage. By comparing the SEM images before and after modification, it can be seen that after the microcapsule shell is surface-modified, the silane coupling agent adheres to the microcapsule surface and forms a connected structure with its surface, so that the surface roughness increases and covers a thin layer. The increase in the surface roughness of the microcapsule is conducive to forming a covalent bond when doped into a polymer matrix to improve the interfacial adhesion with the matrix, and improves the mechanical properties such as the toughness of the composite material. Figure 1 is a schematic diagram of the urea-formaldehyde (UF) dimer reaction of Example 1 of the present invention. Figure 2 is a schematic diagram of the preparation and surface modification of the urea-formaldehyde microcapsule shell of Example 1 of the present invention.

[0057] Example 2

[0058] In order to study the enhancing effect of the prepared modified urea-formaldehyde microcapsule shell on the toughness of the composite material when doped into the polymer matrix, it can be embedded in the epoxy matrix at different loading amounts (0.2wt%, 0.5wt%, 1.0wt%) to form an epoxy composite material, and the fracture toughness of the composite material is determined by a three-point bending test using an electronic universal testing machine.

[0059] By plotting the critical stress intensity factor curves at different microcapsule loadings, the changes in the fracture toughness of the microcapsule composites before and after doping and modification can be determined, as shown in Figure 6. As can be seen, the fracture toughness of the composites first increases and then decreases with increasing microcapsule concentration, reaching a peak at lower doping concentrations. Furthermore, compared to pure epoxy resin and epoxy composites doped with unmodified microcapsules, composites doped with functionalized microcapsules exhibit higher fracture toughness values, indicating a stronger resistance to brittle fracture. This prevents the composites from rapidly breaking with increasing load when cracks are caused by various internal and external factors, thus laying the foundation for self-healing of the material.

[0060] Comparative Example 1

[0061] CN 110511727 A A method for preparing a double-layer shell phase-change microcapsule comprises the following steps:

[0062] 1) 3 g of gum arabic was evenly dispersed in 100 g of water to prepare an emulsifier solution. 25 g of paraffin PCM35 (phase transition temperature 35°C) and 3 g of HDI were then mixed and added to the emulsifier solution. The mixture was stirred at 1200 rpm and constant temperature of 40°C for 20 min to obtain an oil-in-water emulsion.

[0063] 2) 6 g of tetraethylenepentamine was dispersed evenly in water and then added to the oil-in-water emulsion. The stirring speed was controlled at 600 rpm and the mixture was stirred at a constant temperature of 60° C. for 50 min. The mixture was then filtered and washed with water to obtain single-shell phase change microcapsules.

[0064] 3) Add 8 g of urea to 20 g of a 37% formaldehyde solution, adjust the pH to 8.5, control the stirring speed to 400 rpm, and stir at 65° C. for 50 min to obtain a urea-formaldehyde oligomer;

[0065] 4) 3 g of polyethylene-maleic anhydride copolymer and 3 g of resorcinol were dispersed evenly in 150 g of water and then added to urea-formaldehyde oligomer. The pH was adjusted to 3.3, and then single-shell phase change microcapsules were added. The stirring speed was controlled to 400 rpm and the mixture was stirred at a constant temperature of 52°C for 100 min. The mixture was then filtered, washed with water, and dried to obtain double-shell phase change microcapsules.

[0066] Comparative Example 2

[0067] CN1298420C Preparation method of polyurea-urea-formaldehyde resin double-layer microcapsules

[0068] Add 1.46g of triethylenetetramine to a 100mL three-necked flask, then add 50mL of water and heat to 90°C. Dilute 1.00g of 50% glutaraldehyde to 40mL and add dropwise to the flask. Stir and react for 3 hours to obtain a dialdehyde-modified triethylenetetramine aqueous solution.

[0069] Take 10 mL of tetrachloroethylene, add 0.1 g of OP-10 and 0.32 g of 1,4-toluene diisocyanate, add the tetrachloroethylene oil phase into a 250 mL three-necked flask containing 40 mL of water, stir at 800 rpm for 1 minute, add 20 mL of glutaraldehyde-modified triethylenetetramine aqueous solution, react for 5 minutes, reduce the stirring speed to 400 rpm, stop stirring after 10 minutes, filter, and wash with deionized water until neutral to obtain polyurea microcapsules.

[0070] Take 6g of polyurea microcapsules and add them to a 100mL three-necked flask containing 40mL of water. Add 1.5g of urea, dissolve it, add 1M hydrochloric acid to adjust the pH to 2.0, heat it to 70℃ with stirring at 40rpm, add 3g of 37% formaldehyde, react for 3 hours, filter it, wash it with deionized water until it is neutral, and dry it at room temperature to obtain polyurea-urea-formaldehyde resin double-layer microcapsules.

[0071] The idea of ​​the present invention is to first add a surfactant aqueous solution to activate the microcapsule shell during the shell formation process, and then treat it with a modifier after the microcapsule shell is successfully formed. This is a different technical route from the second point of modifying first and then activating.

[0072] Comparative Example 1 only involves the reaction of urea-formaldehyde oligomers and polyphenols, without the addition of a surfactant solution. The addition of new materials results in different reaction conditions and scenarios. The modification method in Comparative Example 2 involves treating the microcapsule surface, while the modification is achieved through a chemical reaction of the internal core material.

[0073] Comparative Example 3

[0074] The difference from Example 1 is that no modification treatment is performed in step (5).

[0075] Because microcapsule size significantly influences its doping performance, it was necessary to investigate the particle size distribution of the samples. Figure 4 shows the particle size distribution before and after modification. The results indicate that functionalized surface modification increases microcapsule size. However, due to the diffusion of silane bonds, microcapsule dispersion is improved, and interactions between microcapsules effectively control agglomeration.

[0076] Figure 5 shows the absorption FT-IR spectra of urea-formaldehyde microcapsules before and after functionalization. Characteristic absorption peaks for groups such as -CH, -C, =O, and -CN are visible, indicating the formation of urea-formaldehyde resin. After the addition of the silane coupling agent, the peak around 2900 cm⁻¹ broadens, indicating that the silane coupling agent has bound to the surface of the UF microcapsules. Chemical elemental analysis of the samples further confirmed that Si was absent in the unfunctionalized microcapsules, while Si was detected in the functionalized microcapsules. These data strongly confirm that the silane molecules have successfully attached to the surface of the urea-formaldehyde microcapsules, modifying their properties.

[0077] Comparative Example 4

[0078] The difference from Example 1 is that the concentration of the modified solution in step (5) (the concentration of the mixture of 3-aminopropyltriethoxysilane (silane coupling agent) and deionized water) is 1%.

[0079] Comparative Example 5

[0080] The difference from Example 1 is that the concentration of the modified solution in step (5) (the concentration of the mixture of 3-aminopropyltriethoxysilane (silane coupling agent) and deionized water) is 6%.

[0081] Modifying fluid concentration affects the fracture toughness of epoxy composites. As the silane coupling agent content increases, the impact energy of the epoxy composite gradually increases, but the microcapsule loading rate, which leads to the highest fracture toughness, also increases accordingly. As shown in Figure 7, for composites with functionalization rates of 1wt%, 2wt%, 5wt%, and 6wt%, the impact energy of the composite filled with 0.2wt% microcapsules is 1.4 J / m, 2.8 J / m, 3.7 J / m, and 4.0 J / m, respectively. The higher the impact energy, the less susceptible the material is to brittle fracture. Therefore, it can be concluded that functionalization of urea-formaldehyde microcapsules improves impact behavior. However, the improvement in fracture toughness is relatively small at a functionalization rate of 1wt%, while a functionalization rate of 6wt% results in a significant increase in microcapsule loading, which affects the mechanical properties. As shown in Example 2 and Comparative Examples 4 / 5, a modifying fluid concentration range of 2-5wt% functionalization rate and the addition of a small amount of silane coupling agent can improve the fracture toughness of the composite while minimizing the impact on mechanical properties. This toughening effect is due to the surface interaction between the modified urea-formaldehyde microcapsules and the matrix, which increases the energy required for fracture and improves the impact strength.

[0082] Comparative Example 6

[0083] The difference from Example 1 is that no surfactant solution is added in step (1).

[0084] In the final experimental results, the lack of surfactant solution resulted in uneven dispersion or local excessive aggregation of urea-formaldehyde resin, which reduced the stability of the formed emulsion. On the one hand, this led to uneven size distribution of microcapsules and large size differences; on the other hand, the wall thickness of the microcapsules was unstable, relatively weak, and easily broken or dissolved, thus affecting its mechanical properties.

[0085] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, and all of these should be included in the scope of the present invention.

Claims

1. A method for surface modification of the shell of urea-formaldehyde microcapsules for toughening epoxy composites, characterized in that: including, A surfactant aqueous solution is obtained by mixing polyvinyl alcohol powder and deionized water evenly. After mixing deionized water and the surfactant aqueous solution, urea, ammonium chloride, and resorcinol are added and stirred, and then a diluted hydrochloric acid solution is dropped in to adjust the pH, and a stable emulsion is obtained by stirring. A formaldehyde-urea mixed solution is added to the stable emulsion and stirred and heated. After the heating reaction ends, it is cooled to room temperature, vacuum filtered, washed with deionized water, and then vacuum dried to obtain the urea-formaldehyde microcapsule shell. 3-aminopropyltriethoxysilane is mixed with deionized water, and a diluted hydrochloric acid solution is dropped in to adjust the pH to obtain a modified solution. The urea-formaldehyde microcapsule shell is added to the modified solution, stirred, then vacuum filtered and dried to obtain the surface-modified urea-formaldehyde microcapsule shell.

2. The preparation method according to claim 1, characterized in that: The surfactant aqueous solution is obtained by mixing polyvinyl alcohol powder and deionized water evenly, wherein the mass ratio of polyvinyl alcohol powder to deionized water is 1:19, and the mass fraction of the surfactant aqueous solution is 5%.

3. The preparation method according to claim 1, characterized in that: After mixing deionized water and the surfactant aqueous solution, urea, ammonium chloride, and resorcinol are added and stirred, and then a diluted hydrochloric acid solution is dropped in to adjust the pH, and a stable emulsion is obtained by stirring, wherein the volume ratio of deionized water to the surfactant aqueous solution is 10-15:0.4-0.6, and the mass ratio of urea, ammonium chloride, and resorcinol is 10:1:

1.

4. The preparation method according to claim 3, wherein: When dropping in the diluted hydrochloric acid solution to adjust the pH and stirring to obtain a stable emulsion, the mass fraction of the diluted hydrochloric acid solution is 1wt% - 1.5wt%, the pH is adjusted to 2.5 - 3.5, the stirring rate is 300 - 400r / min, and the time is 10 - 15min.

5. The preparation method according to claim 1, characterized in that: When adding the formaldehyde-urea mixed solution to the stable emulsion and stirring and heating, the molar mass ratio of formaldehyde to urea is 2.79 - 2.83:

1.

6. The preparation method according to claim 5, characterized in that: The heating temperature is 80 - 90°C, the heating time is 3.5 - 4.5h, and the stirring rate is 100 - 200r / min.

7. The preparation method according to claim 1, characterized in that: When washing with deionized water, filtering, and then vacuum drying to obtain the urea-formaldehyde microcapsule shell, the number of washing times is 3 - 4 times, the vacuum drying temperature is 30 - 40°C, and the time is 48 - 72h.

8. The preparation method according to claim 1, characterized in that: 3-aminopropyltriethoxysilane is mixed with deionized water, and a diluted hydrochloric acid solution is dropped in to adjust the pH to obtain a modified solution. The urea-formaldehyde microcapsule shell is added to the modified solution, stirred, then vacuum filtered and dried to obtain the surface-modified urea-formaldehyde microcapsule shell, wherein the volume ratio of 3-aminopropyltriethoxysilane to deionized water is 2:98, and the mass fraction of the modified solution is 2 - 5%.

9. The preparation method according to claim 8, characterized in that: When dropping in the diluted hydrochloric acid solution to adjust the pH to obtain a modified solution, the mass fraction of the diluted hydrochloric acid solution is 1wt% - 1.5wt%, and the pH is adjusted from 10 - 11 to 7.

10. The preparation method according to claim 8, characterized in that: When adding the urea-formaldehyde microcapsule shell to the modified solution, stirring, then vacuum filtering and drying to obtain the surface-modified urea-formaldehyde microcapsule shell, the stirring temperature is 80 - 90°C, the rate is 100 - 200r / min, and the time is 1 - 1.5h.

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