An expanded composite microsphere and a preparation method and application thereof

Abstract

The application provides an expanded composite microsphere, a preparation method and application thereof. The expanded composite microsphere comprises an expanded microsphere and inorganic nanoparticles coated on the surface of the expanded microsphere. The expanded microsphere is obtained by heat expansion of a heat-expandable microsphere. The expanded composite microsphere has excellent heat resistance, a low volume change rate in a temperature range of 60-120 DEG C, and a low true density, and is suitable for use as a light filler.

Description

Technical Field

[0001] This invention belongs to the field of composite microsphere technology, specifically relating to an expanded composite microsphere, its preparation method, and its application. Background Technology

[0002] When thermally expandable microspheres are heated to a temperature above their softening point, the outer shell softens, and the encapsulated foaming agent expands in volume due to vaporization, resulting in expanded microspheres. Expanded microspheres are typically used as lightweight fillers. Currently, research on thermally expandable microspheres is extensive, primarily focusing on optimizing the composition and proportions of monomers, foaming agents, initiators, and other formulations to improve performance characteristics such as initial expansion temperature and maximum expansion ratio.

[0003] CN104334268A discloses a thermally expandable thermoplastic microsphere comprising a polymer shell encapsulating a propellant, consisting of an olefinically unsaturated monomer. The olefinically unsaturated monomer comprises 21-80% methacrylamide and 20-70% methacrylonitrile, with the total amount of methacrylamide and methacrylonitrile being 70-100% of the olefinically unsaturated monomer. This invention manufactures high-temperature expandable microspheres by adding methacrylamide as a monomer. CN101341227A discloses a thermally foamable microsphere exhibiting stable foaming behavior with excellent heat resistance and high foaming ratio, as well as a method for manufacturing the thermally foamable microsphere and suitable applications. It provides a thermally foamable microsphere wherein the outer shell encapsulating the foaming agent comprises a copolymer having a polymethacrylamide structure. Specifically, it provides a thermally foamable microsphere wherein the monomers forming the polymethacrylamide structure by copolymerization are methacrylonitrile and methacrylic acid. Furthermore, it provides a method for manufacturing the thermally foamable microsphere and its use as an additive. Microspheres capable of high-temperature expansion are provided by using the copolymer of methacrylonitrile and methacrylic acid monomers to form a polymethacrylimide structure at high temperatures. CN104379647A discloses a thermally expandable microsphere, each comprising a shell of thermoplastic resin and a core material encapsulated within the shell. The core material comprises a first foaming agent with a boiling point not higher than the softening point of the thermoplastic resin and a second foaming agent with a boiling point not higher than the softening point of the thermoplastic resin. The second foaming agent is an alcohol compound different from the first foaming agent. This thermally expandable microsphere exhibits excellent foaming performance, particularly a high foaming ratio. A method for its preparation is also provided, along with hollow microspheres, compositions, and molded articles prepared from this thermally expandable microsphere. This invention, by introducing an alcohol as a second foaming agent, yields microsphere products with even higher foaming performance.

[0004] However, to date, research and attention on expanded microspheres have been limited, and the limited research has mainly focused on the development of expansion equipment or processes. For example, CN105396524A discloses an apparatus for preparing expanded microsphere foaming agents. This apparatus includes a cylindrical expansion vessel, the cylinder of which forms an expansion chamber; a hot air inlet is located at the top of the cylinder; a feed port is located on the side wall of the hot air inlet; a discharge port is located at the top of the cylinder; the lower end of the discharge port extends into the expansion chamber, forming a discharge pipe; the lower end of the discharge pipe is located below the hot air inlet; and an arc-shaped converging section is located at the bottom of the cylinder. While using the equipment provided by this invention can improve certain properties of expanded microspheres, it does not significantly improve the volume change rate of expanded microspheres during heat treatment. Furthermore, when expanded microspheres are used as lightweight fillers, they often undergo volume changes during heat treatment, which can alter the shape, density, and properties of the composite material, leading to product defects.

[0005] Therefore, developing an expanded composite microsphere with a low rate of volume change when heated is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide an expanded composite microsphere, its preparation method, and its application. The expanded composite microsphere has excellent heat resistance, a low volume change rate in the temperature range of 60–120°C, and a low true density, making it suitable as a lightweight filler.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides an expanded composite microsphere, the expanded composite microsphere comprising an expanded microsphere and inorganic nanoparticles coated on its surface;

[0009] The expanded microspheres are obtained by thermally expanding thermally expandable microspheres.

[0010] The expanded composite microspheres provided by this invention include expanded microspheres and inorganic nanoparticles coated on their surface. The expanded microspheres refer to microspheres obtained after thermally expandable microspheres expand upon heating. Because the shell of thermally expandable microspheres softens and becomes somewhat sticky when heated, the microspheres are prone to agglomeration during the expansion process, leading to poor dispersion or shell defects. This can cause the expanded microspheres to collapse or leak foaming agent during application. Although lowering the expansion process temperature can avoid these phenomena to some extent, lowering the expansion temperature will result in a lower degree of expansion and higher density of the expanded microspheres. Furthermore, secondary expansion may occur during application. When the expanded microspheres are used as lightweight fillers, such as during the curing and molding of thermosetting resins like polyurethane, epoxy resin, or phenolic resin, if the expanded microspheres undergo volume changes due to the thermal process, this may alter the shape, density, and properties of the resin material. In severe cases, this could lead to completely unqualified products after molding.

[0011] The expanded composite microspheres provided by this invention include expanded microspheres and inorganic nanoparticles coated on their surface. The expanded microspheres are obtained by thermally expanding thermally expandable microspheres. The inorganic nanoparticles are added during the expansion process of the thermally expandable microspheres. After expansion, they can directly coat the outer shell of the expanded microspheres to form a coating layer. The preparation process is very simple, and the final expanded composite microspheres have excellent heat resistance. Their volume hardly changes within the temperature range of 25-125℃, and they have extremely high stability. They will not undergo secondary expansion or contraction in later applications. At the same time, they also have a low true density, making them very suitable for use as lightweight fillers.

[0012] It should be noted that the present invention does not impose any special restrictions on the source of the thermally expandable microspheres. Commercially available thermally expandable microspheres can be selected directly, or thermally expandable microspheres prepared by the present invention using existing methods can be selected.

[0013] Preferably, the inorganic nanoparticles include any one or a combination of at least two of the following: silicon dioxide, aluminum oxide, mica, calcium carbonate, calcium phosphate, magnesium hydroxide, magnesium phosphate, barium sulfate, titanium dioxide, or zinc oxide.

[0014] Preferably, the silica is hydrophobically modified fumed silica and / or hydrophilic fumed silica, and more preferably hydrophobic fumed silica.

[0015] As a preferred technical solution of the present invention, hydrophobic fumed silica is preferred as inorganic nanoparticles. Its advantages are that the microstructure of hydrophobic fumed silica is loose and porous, with extremely low packing density, which can play a good isolation role. Moreover, its surface hydrophobic modified groups give it low polarity and low surface energy, which can further avoid the occurrence of microsphere surface stickiness and microsphere agglomeration. In this way, expansion can be carried out at a higher expansion process temperature to obtain expanded composite microspheres with low density, no agglomeration, no damage, and no tendency to undergo secondary expansion.

[0016] Preferably, the inorganic nanoparticles have a particle size of 10–200 nm, such as 20 nm, 40 nm, 60 nm, 80 nm, 100 nm, 120 nm, 140 nm, 160 nm, or 180 nm.

[0017] Preferably, the specific surface area of ​​the inorganic nanoparticles is 30–600 m². 2 / g, for example 50m 2 / g, 100m 2 / g, 150m 2 / g、200m 2 / g、250m 2 / g、300m 2 / g, 350m 2 / g、400m 2 / g、450m 2 / g、500m 2 / g or 550m 2 / g etc.

[0018] Preferably, the inorganic nanoparticles in the expanded composite microspheres have a mass percentage content of 5% to 30%, such as 5%, 10%, 15%, 20%, 25%, or 30%, and more preferably 10% to 20%.

[0019] Preferably, the thermally expandable microspheres have a core-shell structure, wherein the shell layer of the core-shell structure comprises a thermoplastic resin, and the core of the core-shell structure comprises a foaming agent.

[0020] Preferably, the monomers used to prepare the thermoplastic resin include nitrile monomers.

[0021] Preferably, the nitrile monomer includes any one or a combination of at least two of acrylonitrile, methacrylonitrile, fumaric acid, crotonyl nitrile, α-chloroacrylonitrile or α-ethoxyacrylonitrile, and more preferably acrylonitrile and / or methacrylonitrile.

[0022] Preferably, the monomers for preparing the thermoplastic resin further include any one or a combination of at least two of the following: acrylate monomers, carboxyl-containing monomers, styrene monomers, vinyl acetate, acrylamide monomers, maleimide monomers, or halogenated vinyl monomers.

[0023] Preferably, the acrylate monomers include any one or a combination of at least two of methyl acrylate, ethyl acrylate, methyl methacrylate, isobornyl methacrylate, or ethyl methacrylate.

[0024] Preferably, the carboxyl-containing monomer includes any one or a combination of at least two of acrylic acid, methacrylate, ethyl acrylate, crotonic acid, cinnamic acid, maleic acid, itaconic acid, fumaric acid, or citraconic acid.

[0025] Preferably, the styrene monomers include styrene and / or α-methylstyrene.

[0026] Preferably, the acrylamide monomer includes any one or a combination of at least two of acrylamide, methacrylamide, N-hydroxymethylacrylamide, or N,N-dimethylacrylamide.

[0027] Preferably, the maleimide monomer includes any one or a combination of at least two of N-phenylmaleimide, N-(2-chlorophenyl)maleimide, N-cyclohexylmaleimide or N-laurylmaleimide.

[0028] Preferably, the halogenated ethylene monomers include any one or a combination of at least two of 1,1-dichloroethylene, vinyl chloride, vinyl bromide, vinyl fluoride, or vinylidene fluoride.

[0029] Preferably, the foaming agent comprises alkanes and / or haloalkanes with 1 to 10 carbon atoms (e.g., 2, 3, 4, 5, 6, 7, 8 or 9).

[0030] Preferably, the foaming agent includes any one or a combination of at least two of isopentane, n-pentane, n-hexane, cyclohexane, petroleum ether, n-heptane, or isooctane.

[0031] Preferably, the volume deformation rate of the expanded composite microspheres is less than 5% (e.g., 4%, 3%, 2%, 1% or 0.5%) within a temperature range of 60–120°C (e.g., 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C or 115°C).

[0032] Preferably, the thermally expandable microspheres are prepared by the following method, which includes the following steps:

[0033] (1) Mix the foaming agent, the monomer for preparing the thermoplastic resin and the initiator to obtain an oil phase mixture;

[0034] (2) The oil phase mixture obtained in step (1) is added to the aqueous phase medium for reaction to obtain the thermally expandable microspheres.

[0035] Preferably, the initiator in step (1) includes dodecyl peroxide.

[0036] Preferably, the reaction temperature in step (2) is 50 to 80°C, for example, 53°C, 56°C, 59°C, 62°C, 65°C, 68°C, 71°C, 74°C or 77°C.

[0037] Preferably, the reaction time in step (2) is 5 to 30 hours, such as 10 hours, 15 hours, 20 hours or 25 hours.

[0038] In a second aspect, the present invention provides a method for preparing expanded composite microspheres as described in the first aspect, the method comprising: mixing thermally expandable microspheres and inorganic nanoparticles, and heating the mixture to obtain the expanded composite microspheres.

[0039] Preferably, the mixing temperature is below 60°C, for example, 55°C, 50°C, 45°C, 40°C, 35°C, 30°C, 25°C, or 20°C.

[0040] Preferably, the temperature of the heat treatment is 90 to 150°C, such as 100°C, 110°C, 120°C, 130°C, or 140°C.

[0041] As a preferred embodiment of the present invention, the temperature of the heat treatment is preferably close to its Tmax temperature.

[0042] In this invention, the expansion initiation temperature Tstart and the maximum expansion temperature Tmax are determined as follows: A static thermomechanical analyzer (TMA), model Mettler TMA / SDTA2+, is used for testing at a heating rate of 15°C / min. The testing steps include: ① Adding 1 mg of thermally expandable microspheres to a 150 μL ceramic crucible, placing a matching diameter pad on top of the microsphere layer, and applying a force of 0.06 N to the pad from above using a pressure bar, and measuring the height of the pad; ② Heating from 20°C to 300°C at a heating rate of 15°C / min while the pressure bar applies a force of 0.06 N, and measuring the displacement of the pressure bar in the vertical direction; ③ Setting the temperature at which the displacement in the positive direction begins as the expansion initiation temperature (Tstart), and setting the temperature at which the maximum displacement is displayed as the maximum expansion temperature (Tmax).

[0043] Preferably, the heating treatment time is 0.5 to 20 minutes, such as 1 minute, 2 minutes, 4 minutes, 6 minutes, 8 minutes, 10 minutes, 12 minutes, 14 minutes, 16 minutes, or 18 minutes.

[0044] Thirdly, the present invention provides an application of expanded composite microspheres as described in the first aspect as a lightweight filler.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] The expanded composite microspheres provided by the present invention include expanded microspheres and inorganic nanoparticles coated on their surface. The expanded microspheres are obtained by thermally expanding thermally expandable microspheres. The expanded composite microspheres have excellent heat resistance, low volume change rate in the temperature range of 25 to 125°C, and low true density, making them suitable as lightweight fillers. Detailed Implementation

[0047] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention.

[0048] Preparation Example 1

[0049] An aqueous medium is prepared by mixing 10,000g of water, 3,300g of sodium chloride and 200g of colloidal silica (SiO2 mass percentage of 30%) to obtain the aqueous medium.

[0050] Preparation Example 2

[0051] A thermally expandable microsphere is prepared by the following steps:

[0052] (1) Mix 1400g acrylonitrile, 500g methyl methacrylate, 600g vinylidene chloride, 500g isobutane, 10g dodecyl peroxide and 50g ethylene glycol dimethacrylate to obtain an oil phase mixture;

[0053] (2) The oil phase mixture obtained in step (1) and the aqueous phase medium (preparation example 1) were emulsified at 7000 rpm for 5 min, polymerized at 65°C for 5 h, and then polymerized at 75°C for 5 h. After drying, the thermally expandable microspheres were obtained.

[0054] Example 1

[0055] An expanded composite microsphere comprises an expanded microsphere and hydrophobic fumed silica coated on its surface;

[0056] The preparation method of the expanded composite microspheres provided in this embodiment includes: adding 500g of thermally expandable microspheres (Preparation Example 2) to a powder mixing tank, followed by adding 50g of hydrophobic fumed silica (… H15 (Wacker), after being stirred and mixed evenly, is transferred to a container with a heating function, and the solid mixture powder is quickly stirred to suspend it in the container. It is then heated at 125°C for 3 minutes to obtain the expanded composite microspheres.

[0057] Example 2

[0058] An expanded composite microsphere, differing from Example 1 only in that it uses hydrophilic fumed silica (… S13 (Wacker) replaces the hydrophobic fumed silica, and the other materials, structures and preparation methods are the same as in Example 1.

[0059] Example 3

[0060] An expanded composite microsphere is different from Example 1 only in that magnesium hydroxide is used instead of hydrophobic fumed silica. All other materials, structures and preparation methods are the same as in Example 1.

[0061] Example 4

[0062] An expanded composite microsphere is different from Example 1 only in that the amount of hydrophobic fumed silica added is 10g, while the other materials, structures and preparation methods are the same as in Example 1.

[0063] Example 5

[0064] An expanded composite microsphere is different from Example 1 only in that the amount of hydrophobic fumed silica added is 20g, while the other materials, structures and preparation methods are the same as in Example 1.

[0065] Example 6

[0066] An expanded composite microsphere is different from Example 1 only in that the amount of hydrophobic fumed silica added is 30g, while the other materials, structures and preparation methods are the same as in Example 1.

[0067] Example 7

[0068] An expanded composite microsphere is different from Example 1 only in that the heat treatment time is 7 minutes, while the other substances, conditions and parameters are the same as in Example 1.

[0069] Example 8

[0070] An expanded composite microsphere is different from Example 1 only in that the heat treatment time is 10 min, while the other substances, conditions and parameters are the same as in Example 1.

[0071] Example 9

[0072] An expanded composite microsphere is different from Example 1 only in that the heat treatment temperature is 130°C, while the other substances, conditions and parameters are the same as in Example 1.

[0073] Comparative Example 1

[0074] An expanded microsphere, which differs from Example 1 only in that hydrophobic fumed silica is not added, while the other materials, structures and preparation methods are the same as in Example 1.

[0075] Comparative Example 2

[0076] An expanded microsphere, which differs from Example 1 only in that hydrophobic fumed silica is not added and the heat treatment time is increased to 7 minutes, while the other substances, conditions and parameters are the same as in Example 1.

[0077] Comparative Example 3

[0078] An expanded microsphere is different from Example 1 only in that hydrophobic fumed silica is not added and the heat treatment temperature is 130°C. All other substances, conditions and parameters are the same as in Example 1.

[0079] Comparative Example 4

[0080] An expanded microsphere is different from Example 1 only in that hydrophobic fumed silica is not added and the heat treatment temperature is 115°C. All other substances, conditions and parameters are the same as in Example 1.

[0081] Performance testing:

[0082] (1) Morphology: Microspheres were observed under a microscope to see if they were agglomerated and if their shells were damaged. The microscope model was JSM-7900F from Japan Electronics.

[0083] (2) Volume change rate: The test was conducted using a static thermomechanical analyzer (TMA), model Mettler TMA / SDTA2+. The test procedure was as follows: 1 mg of expanded microspheres was added to a 150 μL ceramic crucible. A matching diameter pad was placed on the expanded microsphere layer. A force of 0.06 N was applied to the pad from above using a pressure bar, and the height h0 of the pressure bar was measured. Under the applied force of 0.06 N, the temperature was increased from 60 °C to 170 °C at a heating rate of 15 °C / min (the volume change rate at 125 °C was selected as representative, indicating that the microspheres do not show significant volume change (shrinkage or expansion) within the room temperature range of 25–125 °C). The displacement Δh of the pressure bar in the vertical upward direction was measured. If the displacement direction was downward, Δh was negative. The volume change rate was defined as the percentage of the displacement at that temperature relative to the initial sample height. The calculation formula is as follows:

[0084] (3) True density: The true density was determined using the isopropanol displacement method (Archimedes method) under ambient temperatures of 25°C and relative humidity of 50%. The specific test steps are as follows: A 100mL volumetric flask was washed and dried, and its weight was recorded as WB1. Isopropanol was added to the flask, and after accurate volumetric filling to the mark, the weight of the flask after filling with 100mL of isopropanol was recorded as WB2. In addition, another 100mL volumetric flask was washed and dried, and its weight was recorded as WS1. Approximately 50mL of the already... Expanded microspheres were added to the volumetric flask, and the weight of the volumetric flask containing the thermally expanded microspheres was weighed as WS2. Then, isopropanol was added to the volumetric flask containing the thermally expanded microspheres to accurately bring it to the mark (ensuring no air bubbles were introduced during the process), and the weight after bringing it to the mark was weighed as WS3. Finally, the obtained WB1, WB2, WS1, WS2, and WS3 were imported into ρ=(WS2-WS1)×(WB2-WB1) / 100] / [(WB2-WB1)-(WS3-WS2) to calculate the true density of the expanded microspheres.

[0085] The expanded microspheres provided in Examples 1-9 and Comparative Examples 1-4 were tested according to the above test methods. The test results are shown in Table 1.

[0086] Table 1

[0087]

[0088]

[0089] According to the data in Table 1:

[0090] The expanded composite microspheres provided in Examples 1-9 showed no agglomeration or damage, and their volume change rate at 125°C was only -8% to -1%, indicating no significant secondary expansion or contraction. Furthermore, their true density was 22-30 kg / m³. 3 .

[0091] Comparing the data of Example 1 and Comparative Examples 1-4, it can be seen that the expanded composite microspheres provided by Comparative Examples 1-3 have a higher volume change rate at 125°C, indicating that secondary expansion or volume shrinkage occurred (negative numbers represent shrinkage, and positive numbers represent expansion). Furthermore, the expanded composite microspheres obtained by Comparative Examples 1-3 all exhibited aggregation and breakage.

[0092] The applicant declares that this invention illustrates an expanded microsphere, its preparation method, and its application through the above embodiments. However, this invention is not limited to the above embodiments, meaning that this invention does not necessarily rely on the above embodiments for implementation. Those skilled in the art should understand that any improvements to this invention, equivalent substitutions of raw materials, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection and disclosure scope of this invention.

Claims

1. An expanded composite microsphere, characterized in that, The expanded composite microspheres include expanded microspheres and inorganic nanoparticles coated on their surface; The expanded microspheres are obtained by thermally expanding thermally expandable microspheres; The inorganic nanoparticles are hydrophobic fumed silica; The expanded composite microspheres contain 5-15% inorganic nanoparticles by mass. The thermal expansion occurs at a temperature of 90~130℃ for a time of 0.5~8 minutes. The expanded composite microspheres exhibit a volume deformation rate of less than 5% within a temperature range of 60~120℃.

2. The expanded composite microspheres according to claim 1, characterized in that, The inorganic nanoparticles have a particle size of 10~200 nm.

3. The expanded composite microspheres according to claim 1, characterized in that, The specific surface area of ​​the inorganic nanoparticles is 30~600 m². 2 / g.

4. The expanded composite microspheres according to claim 1, characterized in that, The thermally expandable microspheres have a core-shell structure, with the shell layer comprising a thermoplastic resin and the core comprising a foaming agent.

5. The expanded composite microspheres according to claim 4, characterized in that, The monomers used to prepare the thermoplastic resin include nitrile monomers.

6. The expanded composite microspheres according to claim 5, characterized in that, The nitrile monomers include any one or a combination of at least two of acrylonitrile, methacrylonitrile, fumaric acid, crotonyl nitrile, α-chloroacrylonitrile, or α-ethoxyacrylonitrile.

7. The expanded composite microspheres according to claim 6, characterized in that, The nitrile monomers are acrylonitrile and / or methacrylonitrile.

8. The expanded composite microspheres according to claim 5, characterized in that, The monomers used in the preparation of the thermoplastic resin also include any one or a combination of at least two of the following: acrylate monomers, carboxyl-containing monomers, styrene monomers, vinyl acetate, acrylamide monomers, maleimide monomers, or halogenated vinyl monomers.

9. The expanded composite microspheres according to claim 8, characterized in that, The acrylate monomers include any one or a combination of at least two of methyl acrylate, ethyl acrylate, methyl methacrylate, isobornyl methacrylate, or ethyl methacrylate.

10. The expanded composite microspheres according to claim 8, characterized in that, The carboxyl-containing monomers include any one or a combination of at least two of the following: acrylic acid, methacrylic acid, ethylacrylic acid, crotonic acid, cinnamic acid, maleic acid, itaconic acid, fumaric acid, or citraconic acid.

11. The expanded composite microspheres according to claim 8, characterized in that, The styrene monomers include styrene and / or α-methylstyrene.

12. The expanded composite microspheres according to claim 8, characterized in that, The acrylamide monomers include any one or a combination of at least two of acrylamide, methacrylamide, N-hydroxymethylacrylamide, or N,N-dimethylacrylamide.

13. The expanded composite microspheres according to claim 8, characterized in that, The maleimide monomers include any one or a combination of at least two of N-phenylmaleimide, N-(2-chlorophenyl)maleimide, N-cyclohexylmaleimide or N-laurylmaleimide.

14. The expanded composite microspheres according to claim 8, characterized in that, The halogenated ethylene monomers include any one or a combination of at least two of 1,1-dichloroethylene, vinyl chloride, vinyl bromide, vinyl fluoride, or vinylidene fluoride.

15. The expanded composite microspheres according to claim 4, characterized in that, The foaming agent includes alkanes and / or haloalkanes with 1 to 10 carbon atoms.

16. The expanded composite microspheres according to claim 4, characterized in that, The foaming agent includes any one or a combination of at least two of isopentane, n-pentane, n-hexane, cyclohexane, petroleum ether, n-heptane, or isooctane.

17. The expanded composite microspheres according to claim 1, characterized in that, The thermally expandable microspheres are prepared by the following method, which includes the following steps: (1) Mix the foaming agent, the monomer for preparing the thermoplastic resin and the initiator to obtain an oil phase mixture; (2) The oil phase mixture obtained in step (1) is added to the aqueous phase medium for reaction to obtain the thermally expandable microspheres.

18. The expanded composite microspheres according to claim 17, characterized in that, The initiator in step (1) includes dodecyl peroxide.

19. The expanded composite microspheres according to claim 17, characterized in that, The reaction temperature in step (2) is 50~80℃.

20. The expanded composite microspheres according to claim 17, characterized in that, The reaction time in step (2) is 5 to 30 hours.

21. A method for preparing expanded composite microspheres as described in any one of claims 1 to 20, characterized in that, The preparation method includes: mixing thermally expandable microspheres and inorganic nanoparticles, and heating them to obtain the expanded composite microspheres.

22. The preparation method according to claim 20, characterized in that, The mixing temperature is below 60°C.

23. An application of the expanded composite microspheres as described in any one of claims 1 to 20 as a lightweight filler.

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