Modified silica sol for thermal expansion microspheres as well as preparation method and application of modified silica sol

Abstract

The invention discloses modified silica sol capable of being used for thermal expansion microspheres and a preparation method and application of the modified silica sol. A silane coupling agent and an anionic polymer are jointly grafted to silicon dioxide under the alkaline condition, and then the modified silica sol is obtained through acidification. The modified silica sol has the effect of Pickering emulsion, when thermoplastic microspheres are prepared, silicon dioxide particles can be spontaneously dispersed on an oil-water interface, after emulsion polymerization reaction is finished, the silicon dioxide particles are mutually gathered and densely arranged on the shell layers of the microspheres, and the shell layer strength and expansion performance of the microspheres are improved. The thermal expansion microspheres prepared by the method are high in expansion rate and low in deslagging rate, and silicon dioxide particles on the surfaces of the microspheres are dispersed compactly.

Description

Technical Field

[0001] This invention belongs to the field of thermally expandable microsphere preparation, specifically relating to a modified silica sol for thermally expandable microspheres, its preparation method, and its application. Background Technology

[0002] Thermally expandable microspheres, also known as thermoplastic microspheres, are a type of foamed material that softens its shell upon heating and expands into spheres using a foaming agent inside. This material has significant application prospects in industries such as automotive interior and exterior trim, building materials, and home appliances. A typical preparation process for thermally expandable thermoplastic microspheres involves internal polymerization within an emulsion system, typically using a silica-based Pickering emulsion to ensure the proper reaction of the droplets. Therefore, a prerequisite for preparing thermally expandable microspheres is selecting a suitable silica dispersion.

[0003] Since the emulsion polymerization reaction for preparing thermally expandable microspheres needs to be carried out under acidic conditions, and silica has a potential approaching 0 under acidic conditions, it is extremely prone to agglomeration. This necessitates modification of silica to improve its stability under acidic conditions. Current research on improving the stability of silica under acidic conditions can be broadly divided into two categories: one uses hydrophobic polymer groups to enhance steric hindrance; for example, patent CN118291095A improves the dispersibility of silica by grafting methacrylate polymers and polyurethane block polymer dispersants onto the silica surface, thus improving the performance of membrane materials prepared by mixing silica and emulsified paraffin in both rigid sealing and tough filling. The other category uses charged groups to increase the charge of silica under acidic conditions, relying on electrostatic repulsion to improve dispersibility; for example, patent CN109071238B uses silica raw materials reacted with cationic silane coupling agents, controlling the reaction conditions to inhibit gelation, and ensuring the stability and high purity of the modified silica dispersion under acidic conditions through the mutual repulsion of positively charged cations.

[0004] However, for the preparation of thermally expandable microspheres, the stability of silica under acidic conditions alone is insufficient. Because silica does not spontaneously form a dense arrangement at the oil-water interface, the droplets in the emulsion used to prepare the microspheres undergo high-speed collisions during suspension polymerization, resulting in a high breakage rate and a high slag yield for the prepared microspheres. Simultaneously, the silica particles on the droplet surface need to possess a certain level of activity to interconnect and form a dense outer shell layer, thereby increasing the shell strength and expansion ratio of the expanded microspheres. Therefore, commercially available modified silica sols are not suitable for direct use in the preparation of thermally expandable microspheres; instead, they need to be custom-made based on specific properties.

[0005] In existing technology, patent CN119799036A directly grafts coupling agents and zwitterionic compounds onto the surface of acidic silica. By selecting appropriate additives, stable modified silica can be obtained. However, the thermal expansion microspheres prepared using this method have a very low slag yield, and thermomechanical analysis (TMA) reveals poor microsphere expansion performance. Carbon content testing shows a low grafting rate of the surface coupling agent and zwitterionic compound, suggesting that some additives undergo self-aggregation. Furthermore, the silica sol itself agglomerates under acidic conditions, making the grafting process unstable. Therefore, a new modification process is urgently needed to achieve a high grafting rate of acidic silica sol, process stability, and high expansion ratio and low slag yield for the thermal microspheres prepared using this process. Summary of the Invention

[0006] To address the aforementioned problems, this invention innovatively proposes a method for preparing modified silica sol for thermally expandable microspheres. The modified silica sol can remain stable for a certain period under acidic conditions while also possessing certain reactivity and hydrophobicity. The thermally expandable microspheres prepared using the silica sol of this invention exhibit high expansion ratios, low slag production, and good surface morphology.

[0007] Another object of the present invention is to provide such a modified silica sol for thermally expandable microspheres and its applications.

[0008] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0009] A method for preparing modified silica sol for thermally expandable microspheres includes the following steps:

[0010] 1) Preparation of hydrolysate: The silane coupling agent is dissolved in a solvent and then added dropwise to an acidic aqueous solution. The hydrolysate is obtained by bathing in a low temperature water bath.

[0011] 2) Preparation of modified silica sol: Take the hydrolysate (denoted as solution A) and surfactant and slowly add them dropwise into the alkaline silica sol. After the reaction is complete, the modified silica sol is obtained. Then, the modified silica sol is acidified.

[0012] In one specific implementation, the silane coupling agent mentioned in step 1) is one or more of γ-glycidoxypropyltrimethoxysilane (KH560), γ-methacryloyloxypropyltrimethoxysilane (KH570), mercaptopropyltrimethoxysilane (KH580), 3-aminopropyltrimethoxysilane (KH540), 3-aminopropyltriethoxysilane (KH550), γ-mercaptopropyltrimethoxysilane (KH590), vinyltrimethoxysilane (A-171), and vinyltriethoxysilane (A-151).

[0013] In one specific embodiment, the solvent in step 1) is a small molecule alcohol with fewer than four carbon atoms, such as methanol, ethanol, isopropanol, etc., preferably ethanol.

[0014] In one specific implementation, in step 1), there is no limitation on the ratio of silane coupling agent to ethanol, but the amount of ethanol is preferably higher than that of silane coupling agent, for example, the mass ratio of silane coupling agent to ethanol can be between 1:1 and 1:2.

[0015] In one specific implementation, in step 1), the mass ratio of the silane coupling agent to water is 5-10:90. If the concentration is too high, the coupling agent is prone to self-condensation, and if the concentration is too low, the hydrolysis rate is slow.

[0016] In one specific implementation, the acid in the acidic aqueous solution in step 1) is an inorganic acid or an organic acid; wherein the inorganic acid is selected from one or more of hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, and perchloric acid, and the organic acid is selected from one or more of formic acid, acetic acid, citric acid, oxalic acid, and malic acid, preferably at least one of hydrochloric acid, nitric acid, acetic acid, and citric acid; the pH of the hydrolysate is controlled at 3-5, preferably around 4.

[0017] In one specific implementation, in step 1), the temperature of the low-temperature water bath is 5-10°C and the duration is 30-60 minutes. The purpose of the low temperature is to prevent the silane coupling agent from self-polymerizing.

[0018] In one specific implementation, in step 2), the mass of the silane coupling agent in solution A is 0.1-2 wt% of the mass of silica in the alkaline silica sol, preferably 0.5-1 wt%. The source of silica is not limited to silica sol prepared by sol-gel method, ion exchange method or silica powder hydrolysis, and the particle size is 10-50 nm with a solid content of 30-40 wt%.

[0019] In one specific implementation, in step 2), it is preferable that the dropping rate of solution A and the surfactant is slower, such as 1-5 ml / min, preferably 1-3 ml / min, the reaction temperature can be 30-80℃, and the reaction time is 6-12 h. The added surfactant can be one or more common anionic copolymers or polymers, such as polyacrylic acid polymers, polyacrylamide copolymers, polysulfonic acid copolymers, polyoxyethylene alkylphenol ethers, and polycaprolactone polyol-polyethyleneimine block copolymers; the amount of surfactant added is preferably 0.5-1 wt% of silica.

[0020] In one specific implementation, during step 2), the inorganic or organic acid mentioned above can be added dropwise to the modified silica sol as quickly as possible to rapidly pass pH 7; alternatively, a cationic resin can be used for acidification, and the pH of the modified silica sol after acidification can be controlled at 2-3.

[0021] In another aspect of the present invention, a modified silica sol for thermally expandable microspheres prepared by the aforementioned preparation method is provided.

[0022] In another aspect of the present invention, a modified silica sol prepared by the aforementioned preparation method is used in the preparation of thermally expandable microspheres.

[0023] Specifically, the steps for preparing thermally expandable microspheres include, in addition to the aforementioned steps for preparing modified silica sol for thermally expandable microspheres, the steps also include: mixing the modified silica sol with a mixture of water, saturated salt, catalyst, monomer, and foaming agent to polymerize and prepare thermally expandable microspheres.

[0024] In this invention, there are no particular limitations on the process and materials for preparing thermally expandable microspheres using modified silica sol. Existing technologies can be referenced, such as the applicant's patent CN 115197519 B, and the relevant prior art is incorporated into this invention.

[0025] In one specific embodiment, the salt is selected from one or more of sodium chloride, potassium chloride, calcium chloride, sodium sulfate, and sodium nitrate, preferably one or more of sodium chloride, potassium chloride, and sodium nitrate, and the mass of the salt tends to be saturated.

[0026] In one specific embodiment, the blowing agent may be selected from low-boiling-point alkanes, preferably one or more of n-butane, isobutane, cyclohexane, isopentane, and chloromethane; the amount of the blowing agent is 5-50 wt% of the total mass of the monomer and the blowing agent, preferably 20-40 wt%.

[0027] In one specific embodiment, the catalyst can be mainly selected from organic peroxides and / or azo compounds; for example: one or more of azobisisobutyronitrile, azobisisovalerate, azobisisoheptanenitrile, dilauryl peroxide, benzoyl peroxide, tert-butyl peroxide, bis(4-tert-butylcyclohexyl peroxide), tert-butyl peroxypentanoate, diisopropyl peroxide, and di-tert-butyl peroxide, preferably one or more of azobisisobutyronitrile, dilauryl peroxide, and benzoyl peroxide, and the amount of catalyst used is 0.1-5 wt% of the monomer material, preferably 1-4 wt%.

[0028] In one specific embodiment, the monomer is an organic compound capable of free radical polymerization containing double bonds, comprising at least one monomeric compound containing one double bond and at least one monomeric compound containing multiple double bonds, preferably acrylonitrile, methacrylonitrile, styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, ethylstyrene, halostyrene, methyl acrylate, methyl methacrylate, methacrylic acid, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, butyl acrylate, butyl methacrylate, isobornyl methacrylate, hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxypropyl methacrylate, hydroxypropyl acrylate, vinyl acetate, vinyl laurate, vinyl stearate, haloethylene, unsymmetrical dihaloethylene. One or more of the following: olefins, dihaloethylene, acrylamide, N-isopropylacrylamide, methacrylamide, hydroxyethyl methacrylate, diallyl phthalate, allyl methacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triacrylate, pentaerythritol trimethacrylate, ethylene glycol dimethacrylate, trimethylolpropane triethylene glycol triacrylate, 1,6-hexanediol diacrylate, 2,2-bis(allyloxymethyl)-1-butanol, pentaerythritol triallyl ether, and diallyl phthalate; wherein a monomer compound containing multiple double bonds is added to improve the mechanical properties of the polymer shell, and the amount of monomer containing multiple double bonds added is 0.1-10 wt% of the total monomers, preferably 2-8 wt%.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] 1) This invention grafts surfactants and silane coupling agents onto silica particles, which can make silica relatively stable under acidic conditions while ensuring its reactivity.

[0031] 2) In the modified silica sol prepared by this invention, the silica particles are arranged very densely at the emulsion interface and are not prone to agglomeration, which ensures the strength of the thermal expansion microsphere shell. At the same time, the prepared microspheres have regular or even smooth morphology. Attached Figure Description

[0032] Figure 1 The TEM morphology of the modified silica sol particles prepared in Example 1 of this invention is shown.

[0033] Figure 2 The SEM morphology of the thermally expanded microspheres prepared from the modified silica sol in Example 1 of this invention is shown.

[0034] Figure 3 The SEM morphology of the thermally expanded microspheres prepared from the modified silica sol in Example 3 of this invention is shown. Detailed Implementation

[0035] To better understand the technical solution of the present invention, the following embodiments will further illustrate the method provided by the present invention. However, the present invention is not limited to the listed embodiments, but should also include any other known modifications within the scope of the claims of the present invention.

[0036] To enhance the comparison of the effects of silica sol, the preparation and testing of thermally expanded microspheres were uniformly carried out using the preparation process of Example 2 in the applicant's prior patent CN 115197519B.

[0037] To compare with the process of patent CN119799036A, Comparative Examples 4 and 5 also investigated the effects of first acidifying the silica sol and then using the coupling agent of this patent to prepare acidic silica sol, as well as the effect of using zwitterionic compounds on modified silica sol.

[0038] The main raw materials used in the following examples and comparative examples are shown in Table 1 below:

[0039]

[0040]

[0041] Detection method:

[0042] The morphology of the modified silica sol particles was observed using TEM, and the morphology of the thermally expanded microspheres was observed using SEM.

[0043] The expansion performance of the thermal expansion microspheres was determined by placing them in an oven at 60°C for expansion, and the expansion ratio was calculated by measuring the height difference before and after expansion using a TMA test.

[0044] The amount of residue discharged from the thermally expanded microspheres is the mass of the filter residue obtained by passing the microsphere slurry through a 100-mesh filter screen / the total mass of the slurry before filtration.

[0045] Example 1

[0046] The pH of 90g water after adding hydrochloric acid is 3.25; 5g KH560 is dissolved in 5g ethanol and then poured into the hydrochloric acid aqueous solution. The solution is placed in a low-temperature constant temperature bath at 5℃ and stirred for 30min to obtain solution A.

[0047] 18g of solution A and 0.8g of polyacrylic acid copolymer-BYK154 were added to 81.2g of water and then added to 250g of silica sol at a rate of 1ml / min using a peristaltic pump. The mixture was then placed in a 50℃ water bath and stirred for 6 hours to obtain modified silica sol. Nitric acid was then added in one go to adjust the pH of the silica sol to 2.5.

[0048] Example 2

[0049] The pH of 90g water after adding phosphoric acid is 3.7; 6g KH540 is dissolved in 8g ethanol and then poured into the phosphoric acid aqueous solution. The solution is placed in a low temperature constant temperature bath at 5℃ and stirred for 40min to obtain solution A.

[0050] 15g of solution A and 0.6g of polyacrylamide copolymer BYK-2003FB were added to 84.4g of water and then added to 250g of silica sol at a rate of 1.5ml / min using a peristaltic pump. The mixture was then placed in a water bath at 40℃ and stirred for 8 hours to obtain modified silica sol. Phosphoric acid was then added in one go to adjust the pH of the silica sol to 2.3.

[0051] Example 3

[0052] The pH of 90g water after adding nitric acid is 3.4; 7g KH550 is dissolved in 9g ethanol and then poured into the nitric acid aqueous solution. The solution is placed in a low-temperature constant temperature bath at 8℃ and stirred for 50min to obtain solution A.

[0053] Take 10g of solution A and 0.8g of polysulfonic acid copolymer Dispers752W and pour them into 89.2g of water. Add the mixture to 250g of silica sol at a rate of 2ml / min using a peristaltic pump. Place the mixture in a 60℃ water bath and stir for 8 hours to obtain modified silica sol. Then, add hydrochloric acid all at once to adjust the pH of the silica sol to 2.7.

[0054] Example 4

[0055] The pH of 90g water after adding acetic acid is 4.1; 9g vinyltrimethoxysilane is dissolved in 14g ethanol and then poured into an aqueous solution of acetic acid. The solution is placed in a low-temperature constant temperature bath at 10℃ and stirred for 55min to obtain solution A.

[0056] Take 8g of solution A and 0.5g of polyacrylic acid copolymer-BYK154 and pour them into 81.2g of water. Add the mixture to 250g of silica sol at a rate of 2.5ml / min using a peristaltic pump. Place the mixture in a water bath at 70℃ and stir for 10h to obtain modified silica sol. Then, add sulfuric acid all at once to adjust the pH of the silica sol to 2.9.

[0057] Example 5

[0058] The pH of 90g water after adding citric acid is 4.5; 8g vinyltriethoxysilane is dissolved in 10g ethanol and then poured into hydrochloric acid aqueous solution, and placed in a low temperature constant temperature bath at 5℃ and stirred for 60min to obtain solution A.

[0059] Take 10g of solution A and 0.8g of polyacrylic acid copolymer-BYK154 and pour them into 89.2g of water. Add the mixture to 250g of silica sol at a rate of 3ml / min using a peristaltic pump. Place the mixture in a water bath at 80℃ and stir for 6 hours to obtain modified silica sol. Then, add nitric acid all at once to adjust the pH of the silica sol to 2.1.

[0060] Comparative Example 1

[0061] Compared to Example 1, liquid A was not added, but all other conditions were exactly the same.

[0062] Comparative Example 2

[0063] Compared to Example 1, the polyacrylic acid copolymer BYK154 was not added, but all other conditions were exactly the same.

[0064] Comparative Example 3

[0065] Compared to Example 1, without adding silane coupling agent KH560 and surfactant polyacrylic acid copolymer, nitric acid was added directly in one step to adjust the pH of 250g of silica sol to 2.5.

[0066] Comparative Example 4

[0067] The pH of 90g water after adding hydrochloric acid is 3.25; 5g KH560 is dissolved in 5g ethanol and then poured into the hydrochloric acid aqueous solution. The solution is placed in a low temperature constant temperature bath at 5℃ and stirred for 30min to obtain solution A.

[0068] Add hydrochloric acid to 250g of silica sol, adjust the pH to 2.3, and stir for 30 minutes to obtain acidic silica sol.

[0069] Take 18g of solution A and 0.8g of polyacrylic acid copolymer BYK154 and pour them into 81.2g of water. Add the mixture to the acidic silica sol at a rate of 1ml / min using a peristaltic pump. Place the mixture in a water bath at 50℃ and stir for 6 hours to obtain the modified silica sol (pH 2.5).

[0070] Comparative Example 5

[0071] The pH of 90g water after adding hydrochloric acid is 3.25; 5g KH560 is dissolved in 5g ethanol and then poured into the hydrochloric acid aqueous solution. The solution is placed in a low temperature constant temperature bath at 5℃ and stirred for 30min to obtain solution A.

[0072] 18g of solution A and 0.8g of polymethyl methacrylate carboxybetaine (PCBMA) were added to 81.2g of water and then added to 250g of silica sol at a rate of 1ml / min using a peristaltic pump. The mixture was then placed in a 50℃ water bath and stirred for 6 hours to obtain modified silica sol. Nitric acid was then added in one go to adjust the pH of the silica sol to 2.5.

[0073] To evaluate the stability of the modified silica sol, the modified and acidified silica sol was placed in an oven at 60℃ for 30 days for aging observation (according to industry experience, the aging effect of the 60℃ oven is equivalent to 15 times the aging time at room temperature), and the results are shown in Table 2.

[0074] Example State change Example 1 clarify Example 2 clarify Example 3 clarify Example 4 clarify Example 5 clarify Comparative Example 1 clarify Comparative Example 2 gel Comparative Example 3 gel Comparative Example 4 clarify Comparative Example 5 clarify

[0075] Table 2. Stability Test of Modified Silica Sol

[0076] Evaluation of the application of modified silica sol in the preparation of thermoplastic expandable microspheres:

[0077] The specific process is as follows:

[0078] S1: Weigh 110g of water, 6.5g of potassium chloride, and 5g of modified silicon dioxide, and mix them to prepare mixture A.

[0079] S2: Weigh 10g acrylonitrile, 2g methacrylonitrile, 8g methyl acrylate, 5g methyl methacrylate, 0.25g diallyl phthalate, 1.5g foaming agent isobutane, and 0.375g polymerization initiator dilauryl peroxide, and mix them to prepare mixture B.

[0080] S3: Add mixture A and mixture B into sealed containers and stir at 1200 rpm for 25 min to form an emulsion.

[0081] S4: The formed emulsion was polymerized at 62℃ and 500rpm for 20h. After drying in an oven at 60℃, expandable microspheres were obtained.

[0082] The obtained microspheres were passed through a 100-mesh sieve, and the mass of the filter residue remaining on the sieve was tested. The results are shown in Table 3.

[0083] Table 3 Performance Tests of Thermally Expanding Microspheres

[0084] magnification slag discharge % Thermal expansion microsphere morphology Example 1 85 0.05 dense Example 2 77 0.16 rough Example 3 70 0.20 rough Example 4 76 0.07 Dense and smooth Example 5 69 0.33 rough Comparative Example 1 38 24 rough Comparative Example 2 76 28 Dense and smooth Comparative Example 3 42 79 rough Comparative Example 4 55 0.08 relatively dense Comparative Example 5 64 0.07 rough

[0085] As shown in the table above, in the embodiments of the present invention, modified silica sol that is stable under acidic conditions can be prepared by adding different types of silane coupling agents and surfactants. The silica sol remains clear and transparent after aging in a 60°C oven for 1 month (according to industry experience, this is equivalent to standing at room temperature for 15 months). In contrast, silica sol without the addition of anionic surfactants is unstable and gels. Figure 1 The TEM morphology of the modified silica sol is shown. Figure 2 The SEM morphology of the thermally expanded microspheres prepared by the modified silica sol in Example 1 is shown. Figure 3 The SEM morphology of the thermally expanded microspheres prepared by the modified silica sol in Example 3 is shown. It can be seen that the particle morphology is regular, especially the thermally expanded microspheres prepared in Example 1 have a dense morphology.

[0086] Meanwhile, thermal expansion experiments showed that the microspheres prepared from the modified silica sol grafted with silane coupling agents exhibited a high expansion ratio, with higher expansion ratios indicating better performance. In Comparative Example 4, the modified silica sol obtained by directly grafting coupling agents onto acidic silica sol produced microspheres with a lower thermal expansion ratio and poorer performance. Furthermore, the microspheres prepared from the modified silica sol grafted with zwitterionic compounds (Comparative Example 5) also showed only moderate expansion performance, with the expansion ratio decreasing from 85 in Example 1 to 64, significantly worse than the modified silica sol using anionic surfactants and polymeric surfactants.

[0087] Although the present invention has been described in detail through the preferred embodiments described above, it should be understood that the above description should not be considered as limiting the present invention. Those skilled in the art will understand that modifications or adjustments can be made to the present invention based on the teachings of this specification. These modifications or adjustments should also be limited by the claims of the present invention.

Claims

1. A method for preparing modified silica sol for thermally expandable microspheres, characterized in that, Includes the following steps: 1) Preparation of hydrolysate: The silane coupling agent is dissolved in a solvent and then added dropwise to an acidic aqueous solution. The hydrolysate is obtained by bathing in a low temperature water bath. 2) Preparation of modified silica sol: Take the hydrolysate (denoted as solution A) and surfactant and slowly add them dropwise into the alkaline silica sol. After the reaction is complete, the modified silica sol is obtained. Then, the modified silica sol is acidified.

2. The preparation method according to claim 1, characterized in that, The silane coupling agent mentioned in step 1) is one or more of the following: γ-glycidoxypropyltrimethoxysilane (KH560), γ-methacryloyloxypropyltrimethoxysilane (KH570), mercaptopropyltrimethoxysilane (KH580), 3-aminopropyltrimethoxysilane (KH540), 3-aminopropyltriethoxysilane (KH550), γ-mercaptopropyltrimethoxysilane (KH590), vinyltrimethoxysilane (A-171), and vinyltriethoxysilane (A-151); The solvent is a small molecule alcohol with fewer than four carbon atoms, preferably ethanol; Preferably, in step 1), the mass ratio of silane coupling agent to ethanol is 1:1 to 1:2; More preferably, in step 1), the mass ratio of silane coupling agent to water is 5-10:

90.

3. The preparation method according to claim 1 or 2, characterized in that, The acid in the acidic aqueous solution mentioned in step 1) is an inorganic acid or an organic acid; Preferably, the inorganic acid is selected from one or more of hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, and perchloric acid, and the organic acid is selected from one or more of formic acid, acetic acid, citric acid, oxalic acid, and malic acid, preferably at least one of hydrochloric acid, nitric acid, acetic acid, and citric acid; More preferably, the pH of the hydrolysate is controlled at 3-5.

4. The preparation method according to any one of claims 1 to 3, characterized in that, In step 1), the temperature of the low-temperature water bath is 5-10℃ and the duration is 30-60 minutes.

5. The preparation method according to claim 1, characterized in that, In step 2), the mass of the silane coupling agent in solution A is 0.1-2 wt% of the mass of silica in the alkaline silica sol. Preferably, the silica particles in the alkaline silica sol have a particle size of 10-50 nm and a solid content of 30-40 wt%.

6. The preparation method according to claim 1 or 5, characterized in that, The surfactant mentioned in step 2) is at least one of anionic copolymers or polymers, preferably one or more of polyacrylic acid polymers, polyacrylamide copolymers, polysulfonic acid copolymers, polyoxyethylene alkylphenol ethers, and polycaprolactone polyol-polyethyleneimine block copolymers. Preferably, the amount of surfactant added is 0.5-1 wt% of silica.

7. The preparation method according to claim 6, characterized in that, In step 2), the dropping rate of solution A and surfactant is 1-5 ml / min, preferably 1-3 ml / min, the reaction temperature is 30-80℃, and the reaction time is 6-12 h.

8. The preparation method according to claim 7, characterized in that, In step 2), the modified silica sol is acidified by adding inorganic or organic acids dropwise to the modified silica sol or by using a cationic resin for acidification. Preferably, the pH of the acidified modified silica sol is controlled at 2-3.

9. The modified silica sol for thermally expandable microspheres prepared by the preparation method according to any one of claims 1 to 8.

10. The modified silica sol for thermally expandable microspheres prepared by the preparation method according to any one of claims 1 to 8, or the modified silica sol for thermally expandable microspheres according to claim 9, is used in the preparation of thermally expandable microspheres. Preferably, the method includes the step of polymerizing the modified silica sol with a mixture of water, saturated salt, catalyst, monomer, and foaming agent to prepare thermally expandable microspheres; More preferably, the salt is selected from one or more of sodium chloride, potassium chloride, calcium chloride, sodium sulfate, and sodium nitrate; and / or The foaming agent is a low-boiling-point alkane, preferably one or more of n-butane, isobutane, cyclohexane, isopentane, and chloromethane; and / or The catalyst is an organic peroxide and / or an azo compound, preferably one or more of the following: azobisisobutyronitrile, azobisisovalerate, azobisisoheptanenitrile, dilauroyl peroxide, benzoyl peroxide, tert-butyl peroxide, bis(4-tert-butylcyclohexyl peroxide), tert-butyl peroxypentanoate, diisopropyl peroxide, and di-tert-butyl peroxide; and / or The monomer is an organic compound containing double bonds that can undergo free radical polymerization, preferably acrylonitrile, methacrylonitrile, styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, ethylstyrene, halostyrene, methyl acrylate, methyl methacrylate, methacrylic acid, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, butyl acrylate, butyl methacrylate, isobornyl methacrylate, hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxypropyl methacrylate, hydroxypropyl acrylate, vinyl acetate, vinyl laurate, vinyl stearate. One or more of the following: vinylidene halide, unsymmetrical dihaloethylene, dihaloethylene, acrylamide, N-isopropylacrylamide, methacrylamide, hydroxyethyl methacrylate, diallyl phthalate, allyl methacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triacrylate, pentaerythritol trimethacrylate, ethylene glycol dimethacrylate, trimethylolpropane triethylene glycol triacrylate, 1,6-hexanediol diacrylate, 2,2-bis(allyloxymethyl)-1-butanol, pentaerythritol triallyl ether, and diallyl phthalate.

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