Thermally expandable microspheres having a core-shell structure and a method for preparing the same

Abstract

The present application provides a kind of thermal expansion microsphere with core-shell structure and its preparation method, the thermal expansion microsphere includes core and shell layer, the core includes foaming agent, the shell layer includes thermoplastic polymer, and the preparation monomer of the thermoplastic polymer includes conjugated diene monomer;By selecting thermoplastic polymer as the shell layer material of thermal expansion microsphere, and limiting the preparation monomer of the thermoplastic polymer includes conjugated diene monomer, the obtained thermal expansion microsphere with core-shell structure not only has lower initial expansion temperature, higher expansion degree, but also has excellent solvent resistance, and application field is more extensive.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of polymer microspheres, and particularly relates to a thermal expansion microsphere with a core-shell structure and a preparation method thereof. BACKGROUND

[0002] Thermal expansion microspheres refer to microspheres that can foam and expand when heated, because the shell layer softens and the foaming agent contained in the interior volatilizes to cause the internal pressure to rise. Thermal expansion microspheres are used as foaming agents, lightweight fillers, etc. in many different fields, such as in the fields of elastomers, thermoplastic elastomers, polymers, putty, primer, plastisol, printing ink, paper, explosives, and cable insulation.

[0003] At present, in order to prevent the foaming agent in the microspheres from leaking during the foaming expansion process, the shell layer of the thermal expansion microspheres needs to have excellent air tightness, and at the same time, in order to improve the ductility of the shell layer of the thermal expansion microspheres, a multifunctional third monomer is generally introduced into the preparation monomers of the shell layer. In addition, in some specific application fields, people also hope that the thermal expansion microspheres can expand at a lower temperature, which requires obtaining thermal expansion microspheres with a low initial expansion temperature (Tstsrt), and at the same time, the thermal expansion microspheres are also expected to have a higher expansion degree, which requires having a higher expansion degree (maximum expansion ratio).

[0004] CN101378831A discloses a thermally expandable thermoplastic microsphere containing a polymer shell prepared from ethylenically unsaturated monomers containing 40 to 70 wt% of acrylonitrile, 5 to 40 wt% of methacrylonitrile, 10 to 50 wt% of a monomer selected from the group consisting of acrylic acid, esters of methacrylic acid and mixtures thereof, and a propellant comprising at least one of methane, ethane, propane, isobutane, n-butane and neopentane, and further relates to the preparation and use of such microspheres. Although the above-mentioned invention provides a low-temperature foaming microsphere obtained without using halogen monomers, however, the selected preparation monomers of the shell layer are all polar monomers, which in turn leads to the obtained microspheres having poor resistance to polar solvents, and the microspheres cannot maintain the expansion performance in the scene containing polar organic solvents, which limits the application thereof.

[0005] Therefore, it is an urgent technical problem in the field to develop a thermal expansion microsphere with a low initial expansion temperature, a high expansion ratio and excellent solvent resistance. SUMMARY

[0006] In view of the deficiencies of the prior art, the present application aims to provide a thermal expansion microsphere with a core-shell structure and a preparation method thereof, the thermal expansion microsphere having a thermoplastic polymer as a shell layer, and the preparation monomer of the thermoplastic polymer comprising a conjugated diene monomer, so that the obtained thermal expansion microsphere with a core-shell structure has a lower initial expansion temperature and a higher expansion ratio, and also has excellent solvent resistance and excellent thermal expansion performance in a polar organic solvent.

[0007] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0008] In a first aspect, the present application provides a thermal expansion microsphere with a core-shell structure, the thermal expansion microsphere comprising a core and a shell layer, the core comprising a foaming agent;

[0009] The shell layer comprises a thermoplastic polymer, and the preparation monomer of the thermoplastic polymer comprises a conjugated diene monomer.

[0010] The thermal expansion microsphere provided by the present application has a core-shell structure, the core of the core-shell structure comprises a foaming agent, and the shell layer of the core-shell structure comprises a thermoplastic polymer. By limiting the preparation monomer of the thermoplastic polymer to comprise a conjugated diene monomer, the obtained thermal expansion microsphere with a core-shell structure not only has a lower initial expansion temperature and a higher expansion ratio without using a halogen-containing monomer, but also has excellent solvent resistance and a wider application field.

[0011] Preferably, the mass percentage content of the shell layer in the thermal expansion microsphere is 70-95%, for example, 72%, 74%, 76%, 78%, 80%, 82%, 85%, 88%, 91% or 94%, and further preferably 70-90%.

[0012] Preferably, the particle size of the thermal expansion microsphere is 1-500 μm, for example, 10 μm, 50 μm, 100 μm, 150 μm, 200 μm, 250 μm, 300 μm, 350 μm, 400 μm or 450 μm, and further preferably 1-200 μm, more preferably 3-100 μm, and still more preferably 5-50 μm.

[0013] Preferably, the foaming agent comprises isobutane.

[0014] Preferably, the foaming agent further comprises an alkane other than isobutane, which has a boiling point of not higher than 120°C (for example, 110°C, 100°C, 90°C, 80°C, 70°C, 60°C or 50°C).

[0015] Preferably, the other alkane having a boiling point not higher than 120℃ includes any one or a combination of at least two of isopentane, n-pentane, n-hexane, cyclohexane, petroleum ether, n-heptane or isooctane.

[0016] Preferably, the mass percentage of isobutane in the foaming agent is greater than 20%, such as 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36% or 38% and the like.

[0017] Preferably, the glass transition temperature of the shell layer is not higher than 120℃, such as 110℃, 100℃, 90℃, 80℃, 70℃, 60℃ or 50℃ and the like, further preferably 50-120℃.

[0018] Preferably, the conjugated diene monomer includes any one or a combination of at least two of 1,3-butadiene, 1,3-pentadiene, isoprene or cyclopentadiene.

[0019] Preferably, the mass percentage of conjugated diene monomer in the monomer for preparing the thermoplastic polymer is greater than 10%, such as 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26% or 28% and the like, further preferably greater than 20%.

[0020] As a preferred technical solution of the present application, the mass percentage of conjugated diene monomer in the monomer for preparing the thermoplastic polymer is greater than 10%, if the content of conjugated diene monomer is too low, it cannot effectively reduce the glass transition temperature of the shell layer, which will further cause the initial expansion temperature of the thermally expandable microspheres to be too high.

[0021] Preferably, the monomer for preparing the thermoplastic polymer further includes other carbon-carbon double bond containing monofunctional monomers and / or multifunctional monomers in addition to the conjugated diene monomer.

[0022] Preferably, the other carbon-carbon double bond containing monofunctional monomer includes any one or a combination of at least two of acrylonitrile monomer, acrylate monomer, vinylpyridine, styrene monomer or vinyl ester monomer.

[0023] Preferably, the acrylonitrile monomer includes any one or a combination of at least two of acrylonitrile, methacrylonitrile, fumaronitrile, crotonitrile, α-chloroacrylonitrile or α-ethoxyacrylonitrile, further preferably acrylonitrile and / or methacrylonitrile.

[0024] As a preferred technical solution of the present application, acrylonitrile and / or methacrylonitrile is selected to be added in combination with the conjugated diene monomer, which can make the shell layer of the obtained thermally expandable microspheres have more excellent barrier property, and can better encapsulate the core foaming agent.

[0025] Preferably, the acrylate monomer comprises any one or a combination of at least two of methyl acrylate, ethyl acrylate, methyl methacrylate, isobornyl methacrylate or ethyl methacrylate.

[0026] Preferably, the vinyl ester monomer comprises vinyl acetate.

[0027] Preferably, the styrene monomer comprises styrene and / or a-methyl styrene.

[0028] Preferably, the other carbon-carbon double bond containing multifunctional monomer comprises any one or a combination of at least two of divinyl benzene, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, glyceryl di(meth)acrylate, 1,3-butanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,10-decanediol di(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, pentaerythritol hexa(meth)acrylate, dimethylol tricyclodecane di(meth)acrylate, triallyl formal tri(meth)acrylate, allyl methacrylate, trimethylolpropane tri(meth)acrylate, trimethylolpropane triacrylate, tributylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, 3-acryloyloxy ethylene glycol monoacrylate, triacryloyl formal, or triallyl isocyanurate.

[0029] Preferably, the polyethylene glycol di(meth)acrylate comprises any one or a combination of at least two of PEG#200 di(meth)acrylate, PEG#400 di(meth)acrylate, or PEG#600 di(meth)acrylate.

[0030] Preferably, the mass percentage of the other carbon-carbon double bond containing multifunctional monomer in the preparation monomers of the thermoplastic polymer is 0.1-1%, such as 0.2%, 0.4%, 0.6%, or 0.8%, and further preferably 0.2-0.5%.

[0031] In a second aspect, the present application provides a method for preparing the thermally expandable microspheres as described in the first aspect, the method comprising the following steps:

[0032] (1) mixing a blowing agent, preparation monomers of a thermoplastic polymer, and an initiator to obtain an oil phase mixture;

[0033] (2) adding the oil phase mixture obtained in step (1) into an aqueous phase medium to react, to obtain the thermally expandable microspheres with core-shell structure.

[0034] In the present application, first, the foaming agent, the preparation monomer of the thermoplastic polymer and the initiator are mixed to form an oil phase mixture, and then the oil phase mixture is mixed with an aqueous phase medium to form a stable oil-in-water emulsion with the oil phase mixture as the dispersed phase and the aqueous phase medium as the continuous phase, and then the polymerization occurs under the condition of the initiator, that is, the thermally expandable microspheres with core-shell structure are formed.

[0035] Preferably, the initiator in step (1) includes any one or a combination of at least two of organic peroxide initiators and / or azo initiators, and is further preferably any one of dilauryl peroxide, dicetyl peroxide, diphenyl peroxide, dibenzoyl peroxide, didecanoyl peroxide, tert-butyl peracetate, tert-butyl perlaurylate, tert-butyl benzoin, tert-butyl hydroperoxide, cumene hydroperoxide, diisopropyl hydroxyl dicarboxylate, 2,2'-azobis-2-methylpropionitrile, 2,2'-azobis-2-methylbutyronitrile, 1,1'-azobis(cyclohexane-1-carbonitrile), dimethyl 2,2'-azobis(2-methylpropionate), or 2,2'-azobis(2-methyl-N-(2-hydroxyethyl)propionamide).

[0036] Preferably, the aqueous phase medium includes a combination of water, a solid suspending agent and a water-soluble salt.

[0037] As a preferred technical solution of the present application, in order to reduce the dissolution and diffusion of the preparation monomer of the thermoplastic polymer in the aqueous phase medium, a water-soluble salt needs to be added, and in order to reduce the polymerization of the dissolved preparation monomer in the aqueous phase medium, a water-soluble free inhibitor (sodium nitrite) needs to be added to the aqueous phase to promote the polymerization reaction of the preparation monomer inside the droplets and avoid the consequences of self-polymerization of the preparation monomer free in the aqueous phase, such as wall sticking, agglomeration and slagging. The solid suspending agent is added to ensure good suspension stability, and the solid suspending agent is mainly located on the outer surface of the shell of the thermally expandable microspheres. In some cases, the solid suspending agent can also be washed off during the post-processing stage, so that the final product basically does not contain the solid suspending agent.

[0038] Preferably, the solid suspending agent includes any one or a combination of at least two of silicon dioxide, chalk, bentonite, starch, cross-linked polymer, methyl cellulose, gum agar, hydroxypropyl methyl cellulose, carboxymethyl cellulose, colloidal clay, calcium phosphate, calcium carbonate, magnesium hydroxide, barium sulfate, calcium oxalate, aluminum hydroxide, iron hydroxide, zinc hydroxide, nickel hydroxide or manganese hydroxide.

[0039] Preferably, the water-soluble salt includes sodium chloride and sodium nitrite.

[0040] Preferably, the aqueous phase medium further includes a stabilizing aid.

[0041] As a preferred technical scheme of the present application, in order to further improve the suspension effect, a stabilizing aid can also be added to the aqueous medium.

[0042] Preferably, the stabilizing aid comprises any one or a combination of at least two of polyvinylpyrrolidone, sulfonated polystyrene, alginate carboxymethyl fiber, tetramethylammonium hydroxide, tetramethylammonium chloride, water-soluble condensate of diethanolamine and adipic acid, water-soluble condensate of ethylene oxide, urea and formaldehyde, polyaziridine, gelatin, animal glue, casein, albumin, gelatin protein, soap, alkyl sulfate or alkyl sulfonate.

[0043] Preferably, the temperature of the reaction is 40-80℃, such as 45℃, 50℃, 55℃, 60℃, 65℃, 70℃ or 75℃, etc.

[0044] Preferably, the time of the reaction is 5-30h, such as 10h, 15h, 20h or 25h, etc.

[0045] Preferably, the reaction further comprises the steps of filtering and drying after the reaction.

[0046] Preferably, the filtering method comprises any one or a combination of at least two of bed filtration, positive / negative pressure filtration or rotary centrifugal filtration.

[0047] Preferably, the drying method comprises any one or a combination of at least two of spray drying, tunnel drying, rotary drying, drum drying, flash drying, palladium drying or fluidized bed drying.

[0048] Compared with the prior art, the present application has the following beneficial effects:

[0049] The thermally expandable microspheres with core-shell structure provided by the present application comprise a core and a shell layer, the core comprises a foaming agent, and the shell layer comprises a thermoplastic polymer, and the preparation monomer of the thermoplastic polymer comprises a conjugated diene monomer; by selecting the thermoplastic polymer as the shell layer material of the thermally expandable microspheres and limiting the preparation monomer of the thermoplastic polymer to comprise a conjugated diene monomer, the obtained thermally expandable microspheres with core-shell structure not only have a lower initial expansion temperature and a higher expansion degree, but also have excellent solvent resistance, and the application field is more extensive. DETAILED DESCRIPTION

[0050] The technical scheme of the present application will be further described by specific embodiments. Those skilled in the art should understand that the embodiments are only used to help understand the present application and should not be regarded as a specific limitation on the present application.

[0051] Preparation Example 1

[0052] A kind of aqueous medium, its preparation method includes: 100g sodium chloride is added to 500g deionized water, 10g water dispersant of silicon dioxide (the mass content of silicon dioxide is 30%), 0.2g polyvinylpyrrolidone and 0.5g sodium nitrite are added, stirring is uniformly mixed, to obtain the aqueous medium.

[0053] Examples 1-6 and Comparative Examples 1-5

[0054] A thermal expansion microsphere with core-shell structure, the core is isobutane, and the shell is a thermoplastic polymer;

[0055] Its preparation method includes the following steps:

[0056] (1) 50g isobutane, thermoplastic polymer preparation monomer (the specific composition and amount of thermoplastic polymer preparation monomer are shown in Table 1), 0.6g ethylene glycol dimethacrylate and 0.8g dodecanoyl peroxide are mixed to obtain an oil phase mixture;

[0057] (2) The oil phase mixture obtained in step (1) is added to the aqueous medium (Preparation Example 1), and an oil-in-water emulsion is formed after mechanical high-speed dispersion at 5000 rpm. It is transferred to a reaction kettle, nitrogen is filled into the reaction kettle until the pressure of the system is 0.8 MPa, then it is reacted at 60℃ under the condition of 500 rpm stirring for 20h, filtered, washed with deionized water for 3 times, and dried to obtain the thermal expansion microsphere with core-shell structure;

[0058] The specific composition and amount of thermoplastic polymer preparation monomer added in step (1) are shown in Table 1:

[0059] Table 1

[0060]

[0061] Performance test:

[0062] (1) Particle size: the thermal expansion microsphere wet sample is determined by laser light scattering on Bettersize 2000LD laser particle size analyzer, and the average particle size is expressed as the median diameter D50 of volume diameter; the particle size distribution is expressed as span SPAN, and the meaning of SPAN is SPAN=(D90-D10) / D50;

[0063] (2) Expansion: determined on a static thermal mechanical analyzer (TMA), the TMA model is Mettler TMA / SDTA2+, and the test method is 15℃ / min.

[0064] The test procedure is as follows: 1 mg of heat-expandable microspheres is added to a 150 μL ceramic crucible, a matching pad is added above the microsphere layer, the sample is prepared, the sample height is measured under the condition that a presser is applied to the sample from above at a force of 0.06 N, the presser is heated from 20 °C to 300 °C at a temperature increase rate of 15 °C / min under the condition that the presser is applied at a force of 0.06 N, the displacement of the presser in the vertical direction is measured; the temperature at which the displacement starts to increase is set as the expansion start temperature (Tstart), the temperature at which the maximum displacement is shown is set as the maximum expansion temperature (Tmax), and the ratio of the maximum height of the expansion process to the initial sample height is the expansion ratio.

[0065] (3) Solvent resistance (expansion retention rate): after the heat-expandable microspheres are completely immersed in dimethyl carbonate solvent and then soaked at 60 °C for 24 h, the microspheres are taken out and dried. 1 mg of the microspheres is subjected to the above expansion property test on a TMA instrument, the obtained expansion ratio is N0, the same test is performed using heat-expandable microspheres that have not been subjected to solvent immersion treatment, the expansion ratio is N1, and the expansion performance retention rate of the microspheres before and after solvent immersion = (N0 / N1) x 100%, the greater the value, the higher the expansion retention rate of the microspheres after immersion in the organic solvent, and the better the solvent resistance.

[0066] The heat-expandable microspheres obtained in Examples 1 to 6 and Comparative Examples 1 to 5 are tested according to the above test method, and the test results are shown in Table 2.

[0067] Table 2

[0068]

[0069]

[0070] According to the data in Table 2, it can be seen that the D50 of the heat-expandable microspheres provided in Examples 1 to 4 is 9 to 12 μm, the SPAN is 1.21 to 1.35, the Tstart is 83 to 95 °C, the Tmax is 120 to 135 °C, the expansion ratio is 90 to 96, and the expansion retention rate is 25 to 49%.

[0071] Comparing the data of Examples 1 to 4 and Comparative Examples 1 to 5, it can be seen that the expansion retention rate of the heat-expandable microspheres prepared without introducing conjugated diene as the shell layer monomer is 0, indicating that the solvent resistance of the shell layer is poor. This is because the heat-expandable microspheres prepared from shell layer monomers including vinylidene chloride and methyl methacrylate have a low initial expansion temperature, but the shell layer has poor resistance to the polar solvent dimethyl carbonate, so that the heat-expandable microspheres no longer have the heat-expandable property after being immersed in dimethyl carbonate at high temperature.

[0072] It can also be seen from the comparison of the data of Example 1 and Examples 5-6 that too low an amount of the conjugated diene added can also result in the thermal expansion microspheres obtained having a higher initial foaming temperature and poor solvent resistance.

[0073] The applicant states that the thermal expansion microspheres having a core-shell structure and the preparation method thereof are illustrated by the above examples, but the present application is not limited to the above examples, i.e. it does not mean that the present application must rely on the above examples to be implemented. It should be understood by those skilled in the art that any improvement on the present application, equivalent replacement of each raw material of the product of the present application, addition of auxiliary ingredients, selection of specific modes, etc. all fall within the protection scope and disclosure scope of the present application.

Claims

1. A thermally expandable microsphere having a core-shell structure, characterized by, The thermal expansion microspheres comprise a core and a shell layer, the core comprises a foaming agent; The shell layer comprises a thermoplastic polymer, and the glass transition temperature of the shell layer is not higher than 120℃; The preparation monomers of the thermoplastic polymer comprise a conjugated diene monomer, and the mass percentage of the conjugated diene monomer in the preparation monomers of the thermoplastic polymer is greater than 20%; the preparation monomers of the thermoplastic polymer further comprise other carbon-carbon double bond containing monofunctional monomers and / or multifunctional monomers except the conjugated diene monomer; The particle size of the thermal expansion microspheres is 5-12 μm; The thermal expansion microspheres are prepared by the following method, which comprises the following steps: (1) mixing the foaming agent, the preparation monomers of the thermoplastic polymer and the initiator to obtain an oil phase mixture; (2) adding the oil phase mixture obtained in step (1) into an aqueous phase medium to carry out reaction, to obtain the thermal expansion microspheres with a core-shell structure; the aqueous phase medium is composed of water, a solid suspending agent, a water-soluble salt and a stabilizing aid.

2. The heat-expandable microspheres according to claim 1, wherein The mass percentage of the shell layer in the thermal expansion microspheres is 70-95%.

3. The heat-expandable microspheres according to claim 2, wherein The mass percentage of the shell layer in the thermal expansion microspheres is 70-90%.

4. The heat-expandable microspheres according to claim 1, wherein The foaming agent comprises isobutane.

5. The heat-expandable microspheres according to claim 4, wherein The foaming agent further comprises other alkanes with a boiling point not higher than 120℃ except isobutane.

6. The heat-expandable microspheres according to claim 5, wherein The other alkanes with a boiling point not higher than 120℃ comprise any one or a combination of at least two of isopentane, n-pentane, n-hexane, cyclohexane, petroleum ether, n-heptane or isooctane.

7. The heat-expandable microspheres according to claim 4, wherein The mass percentage of isobutane in the foaming agent is not less than 20%.

8. The heat-expandable microspheres according to claim 1, wherein The glass transition temperature of the shell layer is 50-120℃.

9. The heat-expandable microspheres according to claim 1, wherein The conjugated diene monomer comprises any one or a combination of at least two of 1,3-butadiene, 1,3-pentadiene, isoprene or cyclopentadiene.

10. The heat-expandable microspheres according to claim 1, wherein The other carbon-carbon double bond containing monofunctional monomers comprise any one or a combination of at least two of acrylonitrile monomers, acrylate monomers, vinylpyridine, styrene monomers or vinyl ester monomers.

11. The heat-expandable microspheres according to claim 10, wherein The acrylonitrile monomers comprise any one or a combination of at least two of acrylonitrile, methacrylonitrile, fumaronitrile, crotonitrile, α-chloroacrylonitrile or α-ethoxyacrylonitrile.

12. The heat-expandable microspheres according to claim 11, wherein The acrylonitrile monomers are acrylonitrile and / or methacrylonitrile.

13. The heat-expandable microspheres according to claim 10, wherein The acrylate monomers comprise any one or a combination of at least two of methyl acrylate, ethyl acrylate, methyl methacrylate, isobornyl methacrylate or ethyl methacrylate.

14. The heat-expandable microspheres according to claim 10, wherein The vinyl ester monomers comprise vinyl acetate.

15. The heat-expandable microspheres according to claim 10, wherein The styrene monomers comprise styrene and / or α-methylstyrene.

16. The heat-expandable microspheres according to claim 1, wherein The other multi-functionality monomer containing carbon-carbon double bond includes any one of divinylbenzene, di( meth) acrylate ethylene glycol, di( meth) acrylate diethylene glycol, di( meth) acrylate triethylene glycol, di( meth) acrylate propylene glycol, di( meth) acrylate-1, 4-butanediol, di( meth) acrylate-1, 6-hexanediol, di( meth) acrylate glycerol, di( meth) acrylate-1, 3-butanediol, di( meth) acrylate neopentyl glycol, di( meth) acrylate-1, 10-decanediol, tri( meth) acrylate pentaerythritol, tetra( meth) acrylate pentaerythritol, hexa( meth) acrylate pentaerythritol, dimethylol tricyclodecane di( meth) acrylate, allyl methacrylate, trimethylol propane tri( meth) acrylate, di( meth) acrylate tributylene glycol or triacryloyl formaldehyde or a combination of at least two thereof.

17. The heat-expandable microspheres according to claim 1, wherein The other multi-functionality monomer containing carbon-carbon double bond includes polyethylene glycol di( meth) acrylate.

18. The heat-expandable microspheres according to claim 1, wherein The mass percentage of the other multi-functionality monomer containing carbon-carbon double bond in the preparation monomer of the thermoplastic polymer is 0.1-1%.

19. The heat-expandable microspheres according to claim 18, wherein The mass percentage of the other multi-functionality monomer containing carbon-carbon double bond in the preparation monomer of the thermoplastic polymer is 0.2-0.5%.

20. A process for the preparation of thermally expandable microspheres as claimed in any one of claims 1 to 19, characterized in that, The preparation method comprises the following steps: (1) mixing a foaming agent, a preparation monomer of a thermoplastic polymer and an initiator to obtain an oil phase mixture; (2) adding the oil phase mixture obtained in step (1) into an aqueous phase medium to react, to obtain the thermally expandable microspheres with core-shell structure; the aqueous phase medium is composed of water, a solid suspending agent, a water-soluble salt and a stabilizing aid.

21. The method of claim 20, wherein, The initiator in step (1) includes an organic peroxide initiator and / or an azo initiator.

22. The method of claim 21, wherein, The initiator in step (1) is any one of dodecyl peroxydicarbonate, di(4-tert-butylcyclohexyl) peroxydicarbonate, dioctanoyl peroxide, dibenzoyl peroxide, dilauroyl peroxide, didecanoyl peroxide, tert-butyl peracetate, tert-butyl hydroperoxide, cumene hydroperoxide, 2, 2'-azobis-isoheptane nitrile, 2, 2'-azobis-iso-butyl nitrile, 1, 1'-azobis(cyclohexane-1-carbonitrile), dimethyl 2, 2'-azobis(2-methylpropionate) or 2, 2'-azobis[2-methyl-N-(2-hydroxyethyl) propionamide] or a combination of at least two thereof.

23. The preparation method according to claim 20, characterized in that, The solid suspending agent includes any one of silicon dioxide, bentonite, starch, methyl cellulose, gum agar, hydroxypropyl methyl cellulose, carboxymethyl cellulose, colloidal clay, calcium phosphate, calcium carbonate, magnesium hydroxide, barium sulfate, calcium oxalate, aluminum hydroxide, iron hydroxide, zinc hydroxide, nickel hydroxide or manganese hydroxide or a combination of at least two thereof.

24. The method of claim 20, wherein, The water-soluble salt includes sodium chloride and sodium nitrite.

25. The method of claim 20, wherein, The stabilizing aid includes any one or a combination of at least two of polyvinylpyrrolidone, sulfonated polystyrene, tetramethylammonium hydroxide, tetramethylammonium chloride, diethanolamine / adipic acid water-soluble condensate, urea / formaldehyde water-soluble condensate, polyaziridine, animal glue, casein, albumin, soap, alkyl sulfate or alkyl sulfonate.

26. The method of claim 20, wherein, The temperature of the reaction in step (2) is 40-80℃.

27. The method of claim 20, wherein, The time of the reaction is 5-30 h.

28. The method of claim 20, wherein, After the reaction in step (2) ends, the process further includes steps of filtering and drying.

29. The method of claim 28, wherein, The filtering method includes any one or a combination of at least two of bed filtration, positive / negative pressure filtration or rotary centrifugal filtration.

30. The method of claim 28, wherein, The drying method includes any one or a combination of at least two of spray drying, tunnel drying, rotary drying, drum drying, flash drying or fluidized bed drying.

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