Thermal expansion microsphere as well as preparation method and application thereof

Abstract

The invention relates to the technical field of polymer materials, in particular to thermal expansion microspheres and a preparation method and application thereof. Each thermal expansion microsphere comprises a shell and an inner core coated by the shell, the material of the shell comprises a thermoplastic polymer, and the material of the inner core comprises a foaming agent; the thermoplastic polymer satisfies the following conditions: (1) the total content of residual monomers of the thermoplastic polymer is less than or equal to 1000 ppm; and (2) after the thermal expansion microspheres are heated at 50 DEG C for 2 hours, the loss rate of the foaming agent is less than or equal to 5%. According to the thermal expansion microsphere, the thermoplastic polymer with less residual monomer and less foaming agent wastage rate when the foaming agent is coated is used as the shell, so that the thermal expansion microsphere is safe and environment-friendly, and the thermal expansion microsphere can be ensured to have better air tightness and expansion performance.

Description

Technical Field

[0001] The present application relates to the technical field of polymer materials, and in particular to a thermal expansion microsphere and a preparation method and application thereof. Background Art

[0002] Heat-expandable microspheres are polymer materials with a core-shell structure that uses a thermoplastic polymer as the outer shell and a foaming agent wrapped inside. When heated, the foaming agent core material inside gasifies, generating internal pressure, which increases as the heating temperature rises. When the temperature is higher than the softening point of the thermoplastic polymer, the volume of the heat-expandable microspheres begins to increase under the internal pressure. The expansion characteristics of heat-expandable microspheres can be used in many occasions, such as as lightweight fillers, polishing materials, thermal insulation materials, heat-insulating materials, sound-absorbing materials, packaging materials or elastic materials.

[0003] The thermoplastic polymer shell of heat-expandable microspheres is formed by polymerization of monomers. Many heat-expandable microspheres prepared by many preparation methods have a large amount of residual unreacted monomers, most of which are toxic and harmful monomers, such as acrylonitrile, vinylidene chloride, etc. This has limited the application of heat-expandable microspheres in many fields. Although some reports have disclosed methods for removing residual monomers, many methods have limited ability to remove residual monomers. Even though some residual monomers have been removed, the polymerization of thermoplastic polymers has been affected, the air tightness of thermoplastic polymers has been destroyed, and the expansion performance of heat-expandable microspheres has been adversely affected. Summary of the invention

[0004] Based on this, the first aspect of the present application provides a heat-expandable microsphere, and its technical solution is as follows:

[0005] A heat-expandable microsphere comprises a shell and a core covered by the shell, wherein the shell is made of a thermoplastic polymer and the core is made of a foaming agent;

[0006] The thermoplastic polymer meets the following conditions:

[0007] (1) The total content of residual monomers in the thermoplastic polymer is ≤1000 ppm;

[0008] (2) Ensure that the foaming agent loss rate of the heat-expandable microspheres after heating at 50° C. for 2 hours is ≤5%.

[0009] The heat-expandable microspheres are made of a thermoplastic polymer with less residual monomers and less foaming agent loss rate when coated with a foaming agent as a shell, which is not only safe and environmentally friendly, but also can effectively guarantee the stability of product performance during subsequent storage and use, and ensure that the heat-expandable microspheres have good air tightness and expansion performance. Among them, the expansion performance refers to the fact that when the heat-expandable microspheres are heated, the foaming agent is gasified, the heat-expandable microspheres have a large expansion volume, and a good expansion ratio.

[0010] Optionally, the total content of residual monomers in the thermoplastic polymer is ≤900ppm. Further, the total content of residual monomers is ≤500ppm. Further, the total content of residual monomers is ≤100ppm. Further, the total content of residual monomers is ≤90ppm. Further, the total content of residual monomers is ≤80ppm. Further, the total content of residual monomers is ≤70ppm. The less the content of residual monomers, the more beneficial to environmental protection, and the more suitable the thermally expandable microspheres are for use in areas close to daily life, for example, as lightweight materials and elastic materials used in products such as ultra-light concrete, sealing strips and wine corks that are in close contact with people.

[0011] Optionally, the foaming agent loss rate of the heat-expandable microspheres after being heated at 50° C. for 2 hours is ≤4%. Optionally, the foaming agent loss rate of the heat-expandable microspheres after being heated at 50° C. for 2 hours is ≤3%.

[0012] Optionally, the raw materials of the thermoplastic polymer include polymerization monomers, cross-linking agents and composite initiators.

[0013] Optionally, the polymerizable monomers include ethylenically unsaturated monomers, wherein the ethylenically unsaturated monomers are monomers including alkenyl groups. Different ethylenically unsaturated monomers have different reactivity, and the expansion ratios of the thermoplastic polymers formed by the polymerization reaction are also different.

[0014] Optionally, the ethylenically unsaturated monomers include one or more of acrylonitrile monomers, acrylate monomers, acrylic acid monomers, alkyl vinyl ester monomers, styrene monomers, halogen-containing olefin monomers and acrylamide monomers. Optionally, the ethylenically unsaturated monomers include two or more of acrylonitrile monomers, acrylate monomers, acrylic acid monomers, alkyl vinyl ester monomers, styrene monomers, halogen-containing olefin monomers and acrylamide monomers.

[0015] Optionally, the polymerizable monomers include acrylonitrile monomers, and the content of residual acrylonitrile monomers in the thermoplastic polymer is ≤400 ppm, further ≤200 ppm, further ≤100 ppm, further ≤60 ppm, further ≤50 ppm, further ≤20 ppm.

[0016] Optionally, the polymerizable monomers include acrylate monomers, and the content of residual acrylate monomers in the thermoplastic polymer is ≤400 ppm, further ≤200 ppm, further ≤100 ppm, further ≤60 ppm, further ≤50 ppm, further ≤20 ppm.

[0017] Optionally, the polymerized monomers include acrylic monomers, and the content of residual acrylic monomers in the thermoplastic polymer is ≤400 ppm, further ≤200 ppm, further ≤100 ppm, further ≤60 ppm, further ≤50 ppm, further ≤35 ppm.

[0018] Optionally, the polymerized monomers include halogen-containing olefin monomers, and the content of residual halogen-containing olefin monomers in the thermoplastic polymer is ≤400 ppm, further ≤200 ppm, further ≤100 ppm, further ≤60 ppm, further ≤50 ppm.

[0019] Optionally, the olefinic unsaturated monomers include acrylonitrile monomers and halogen-containing olefin monomers. At this time, the thermoplastic polymer formed by the polymerization reaction has good air tightness and excellent expansibility. Optionally, the mass percentage of acrylonitrile monomers in the polymerized monomer is 20% to 80%, for example, it can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%. It is further preferably 25% to 80%. Optionally, the mass percentage of halogen-containing olefin monomers in the polymerized monomer is 10% to 60%, for example, it can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, and it is further preferably 20% to 50%.

[0020] Among them, acrylonitrile monomers include one or more of acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, α-ethoxyacrylonitrile and fumaronitrile. Acrylate monomers include one or more of methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, butyl acrylate, butyl methacrylate, isobornyl acrylate, isobornyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxypropyl acrylate, allyl acrylate and allyl methacrylate. Acrylic monomers include one or more of methacrylic acid and acrylic acid. Alkyl acid vinyl ester monomers include one or more of vinyl acetate, vinyl laurate and vinyl stearate. Styrene monomers include one or more of styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, ethylstyrene and halogenated styrene. Halogen-containing olefin monomers include halogenated vinyl. Halogenated vinyl includes one or more of vinylidene halide and normal vinyl halide. Vinylidene halide can be vinylidene chloride. The acrylamide monomers include one or more of acrylamide, N-isopropylacrylamide and methacrylamide.

[0021] Many of the above-mentioned polymerizable monomers are toxic and harmful, such as acrylonitrile and vinylidene chloride (low boiling point, extremely flammable and carcinogenic), etc., which will escape into the environment during the subsequent heat foaming process, causing environmental pollution and personal injury. When the polymerizable monomer includes acrylonitrile, the acrylonitrile of the thermoplastic polymer of the present application can be controlled below 100 ppm. Further controlled below 50 ppm. Further controlled below 40 ppm. Further controlled below 30 ppm. Further controlled below 20 ppm. When the polymerizable monomer includes vinylidene chloride, the vinylidene chloride of the thermoplastic polymer of the present application can be controlled below 100 ppm. Further controlled below 80 ppm. Further controlled below 70 ppm. Further controlled below 60 ppm. Further controlled below 50 ppm. Among the methods for removing residual monomers currently reported and disclosed, some methods for removing residual monomers are only applicable to water-soluble monomers, and cannot remove non-water-soluble monomers, for example, vinylidene chloride cannot be removed. In addition, although some methods for removing residual monomers can remove some residual monomers, they affect the polymerization of thermoplastic polymers, destroy the air tightness of thermoplastic polymers, and have an adverse effect on the expansion performance of heat-expandable microspheres. In the present application, both water-soluble monomers (such as acrylonitrile) and water-insoluble monomers (such as vinylidene chloride) have less residues, and at this time, the air tightness of the thermoplastic polymer is good, and the expansion ratio of the heat-expandable microspheres is high.

[0022] Optionally, the crosslinking agent includes one or more of a difunctional crosslinking agent and a trifunctional crosslinking agent. Optionally, the crosslinking agent includes one or more of trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triacrylate, pentaerythritol trimethacrylate, ethylene glycol dimethacrylate, trimethylolpropane triethylene glycol triacrylate, diallyl phthalate, diallyl carbonate, triallyl isocyanate, triallyl isocyanurate, trimethylolpropane diallyl ether, trimethallyl isocyanate, pentaerythritol triallyl ether, 2,2-bis(allyloxymethyl)-1-butanol, 1,6-hexanediol diacrylate and polyethylene glycol dimethacrylate.

[0023] Optionally, the mass ratio of the crosslinking agent to the monomer is (0.01-5):100. For example, it can be 0.01:100, 0.05:100, 0.1:100, 0.3:100, 0.5:100, 0.8:100, 1:100, 1.2:100, 1.5:100, 1.8:100, 2:100, 2.5:100, 3:100, 3.5:100, 4:100, 4.5:100, 5:100. Preferably, the mass ratio is (0.1-2):100. More preferably, the mass ratio is (0.2-1):100.

[0024] Optionally, the composite initiator comprises a first initiator and a second initiator, and a 10 h half-life temperature of the first initiator is at least 10° C. lower than a 10 h half-life temperature of the second initiator.

[0025] Wherein, the half-life temperature of 10h means that at this temperature, the time required for the concentration or quantity of the initiator to be reduced to half of the initial value is 10 hours. Optionally, the 10h half-life temperature of the first initiator is at least 20°C lower than the 10h half-life temperature of the second initiator. Optionally, the 10h half-life temperature of the first initiator is at most 35°C lower than the 10h half-life temperature of the second initiator.

[0026] Optionally, the first initiator and the second initiator independently include a free radical initiator. The free radical initiator includes one or more of an organic peroxide initiator and an azo initiator. Optionally, the peroxide initiator includes tert-amyl peroxypivalate, di(2-ethylhexyl) peroxydicarbonate, isopropyl phenyl neodecane peroxyacid, tert-butyl oxyacetate, tert-butyl peroxybenzoate, tert-butyl peroxyisopropyl monocarbonate, 1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, 1,3-di(2-tert-butylperoxyisopropyl)benzene and other dialkyl peroxides, 1,3-di(2-tert-butylperoxyisopropyl)benzene and other dialkyl peroxides, 1,1-di(tert-butylperoxy)cyclohexane, 1,1 - one or more of di(tert-hexylperoxy)cyclohexane, methyl ethyl ketone peroxide, tert-butyl hydroperoxide, tert-amyl hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, dodecyl peroxide, benzoyl peroxide, diisopropyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, bis(4-tert-butylcyclohexyl) peroxydicarbonate, tert-butyl peroxybenzoate, tert-butyl peroxypivalate, di-tert-butyl peroxide, isopropyl hydroperoxide, diisopropyl hydroperoxide, p-amyl hydroperoxide and bis(2-phenoxyethyl) peroxydicarbonate. Optionally, the azo initiator includes one or more of 2,2'-azobisisobutyronitrile, azobisisovaleronitrile, 2,2'-azobis(2,4-dimethyl)valeronitrile, azobisisoheptanenitrile, 2,2'-azobis(2-methylpropionate), azobisisobutylcyanamide, azobiscyclohexylcarbonitrile and dimethyl azobisisobutyrate.

[0027] The 10h half-life of the following free radical initiators is as follows: tert-amyl peroxypivalate (55°C), 2,2'-azobis(2,4-dimethylvaleronitrile) (51°C), 2,2'-azobis(2-methylpropionate) (66°C), di(2-ethylhexyl) peroxydicarbonate (47°C), isopropyl phenyl neodecanoate (38°C), tert-butyl oxyacetate (110°C), tert-butyl peroxybenzoate (105°C), 2,2'-azobisisobutyronitrile (65°C), tert-butyl peroxyisopropyl monocarbonate (98°C), 1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane (92°C), 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (118°C), 1,3-di(2-tert-butylperoxyisopropyl)benzene and other dialkyl peroxides (118°C~120°C), 1,1-di(tert-butylperoxy)cyclohexane (97°C), 1,1-di(tert-hexylperoxy)cyclohexane (94°C), methyl ethyl ketone peroxide (105°C), tert-butyl hydroperoxide (172°C), tert-amyl hydroperoxide (153°C), 1,1,3,3-tetramethylbutyl hydroperoxide (140°C), cumene hydroperoxide (158°C), diisopropylbenzene hydroperoxide (140°C). The first initiator and the second initiator can be selected according to the above 10h half-life.

[0028] Optionally, the mass ratio of the composite initiator to the monomer is (0.01-5):100. For example, it can be 0.01:100, 0.05:100, 0.1:100, 0.3:100, 0.5:100, 0.8:100, 1:100, 1.2:100, 1.5:100, 1.8:100, 2:100, 2.5:100, 3:100, 3.5:100, 4:100, 4.5:100, 5:100. Preferably, the mass ratio is (0.1-3):100.

[0029] Optionally, the molar ratio of the first initiator to the second initiator is (90-100): 1. For example, it can be 90: 1, 92: 1, 94: 1, 96: 1, 98: 1, 100: 1. Preferably, the molar ratio is (95-100): 1.

[0030] Optionally, the foaming agent is a substance that can be gasified by heat, such as a liquid substance that can be gasified by heat. Optionally, the foaming agent includes a C3-C13 alkane foaming agent, such as an alkane foaming agent of C3, C4, C5, C6, C7, C8, C9, C10, C11, C12, and C13. Optionally, the foaming agent includes one or more of n-butane, isobutane, n-pentane, isopentane, neopentane, n-hexane, n-hexane isomers, n-heptane, n-heptane isomers, n-octane, n-octane isomers, nonane, n-nonane isomers, n-decane, n-decane isomers, n-undecane, n-undecane isomers, n-dodecane, n-dodecane isomers, n-tridecane, and n-tridecane isomers. Preferably, the foaming agent includes one or more of n-butane, isobutane, isopentane, neopentane, n-pentane, n-hexane, n-heptane, isooctane, n-octane and petroleum ether.

[0031] Optionally, the mass percentage of the foaming agent in the heat-expandable microspheres is 20% to 30%, for example, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, or 30%.

[0032] Optionally, the material of the inner core further comprises a gas migration inhibitor. The gas migration inhibitor helps to reduce the escape of the shell of the thermoplastic polymer from the foaming agent during the heating process of the heat-expandable microspheres. Optionally, the boiling point of the gas inhibitor is higher than the glass transition temperature of the thermoplastic polymer. Optionally, the gas inhibitor comprises one or more of hydrocarbon compounds, carboxylic acid compounds, ester compounds and alcohol compounds. Preferably, the gas inhibitor comprises a hydrocarbon compound. Preferably, the gas inhibitor comprises cyclopentane oil.

[0033] Optionally, the mass ratio of the gas inhibitor to the foaming agent is (0-30):100. For example, the mass ratio may be 0:100, 0.1:100, 1:100, 3:100, 5:100, 8:100, 10:100, 12:100, 15:100, 18:100, 20:100, 22:100, 25:100, 28:100. When the mass ratio of the gas inhibitor to the foaming agent is 0:100, it means that no gas inhibitor is added.

[0034] Optionally, the particle size D of the heat-expandable microspheres 50 The diameter is 1 μm to 100 μm. For example, it can be 1 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 80 μm, 90 μm, 100 μm. Preferably, it is 5 μm to 50 μm.

[0035] Optionally, the PDI of the heat-expandable microspheres is 1 to 1.1, wherein PDI=(D 90 -D 10 ) / D 50 For example, it can be 1, 1.01, 1.05, 1.08, 1.0, 1.1.

[0036] Preferably, the polymerizable monomers of the thermoplastic polymer include acrylonitrile, vinylidene chloride and methyl acrylate, and under a pressure of 0.06N, the maximum expansion ratio of the heat-expandable microspheres is ≥50. Further, 60 to 75. The initial expansion temperature of the heat-expandable microspheres is ≥90°C. Further, 95°C to 105°C. The maximum expansion temperature of the heat-expandable microspheres is ≥120°C. Further, 130°C to 140°C.

[0037] Preferably, the polymerizable monomers of the thermoplastic polymer include acrylonitrile, vinylidene chloride and methacrylic acid, and under a pressure of 0.06N, the maximum expansion ratio of the heat-expandable microspheres is ≥50. Further, 85-100; the initial expansion temperature of the heat-expandable microspheres is ≥90°C. Further, 130°C-140°C. The maximum expansion temperature of the heat-expandable microspheres is ≥120°C. Further, 150°C-160°C.

[0038] Preferably, the polymerizable monomers of the thermoplastic polymer include acrylonitrile, vinylidene chloride and methacrylonitrile, and under a pressure of 0.06N, the maximum expansion ratio of the heat-expandable microspheres is ≥50. Further, 90 to 110. The initial expansion temperature of the heat-expandable microspheres is ≥90°C. Further, 140°C to 155°C. The maximum expansion temperature of the heat-expandable microspheres is ≥120°C. Further, 160°C to 180°C.

[0039] The second aspect of the present application provides a method for preparing heat-expandable microspheres, and its technical solution is as follows:

[0040] A method for preparing heat-expandable microspheres comprises the following steps:

[0041] Mixing a polymerizable monomer, a crosslinking agent, a foaming agent and a composite initiator to obtain an oil phase mixture, wherein the composite initiator comprises a first initiator and a second initiator, and a 10-hour half-life temperature of the first initiator is at least 10° C. lower than a 10-hour half-life temperature of the second initiator;

[0042] mixing the oil phase mixture and the aqueous dispersion medium to obtain a suspension;

[0043] The suspension is subjected to a polymerization reaction, and the polymerization reaction comprises the following procedures: a polymerization reaction at a first temperature and then a polymerization reaction at a second temperature; wherein the first temperature is not less than the 10-hour half-life temperature of the first initiator and is less than the 10-hour half-life temperature of the second initiator, and the second temperature is not less than the 10-hour half-life temperature of the second initiator, and the molar ratio of the first initiator to the second initiator and other parameters of the polymerization reaction are controlled to obtain the polymer expanded microspheres comprising a shell and a core coated by the shell, wherein the material of the shell comprises a thermoplastic polymer, and the material of the core comprises a foaming agent; the thermoplastic polymer satisfies the following conditions: (1) the total content of residual monomers of the thermoplastic polymer is ≤1000ppm; and (2) the foaming agent loss rate of the heat-expandable microspheres after heating at 50°C for 2h is ensured to be ≤5%.

[0044] The above method uses a composite initiator, which includes a first initiator and a second initiator, wherein the 10h half-life temperature of the first initiator is at least 10°C lower than the 10h half-life temperature of the second initiator. During the polymerization reaction, the polymerization reaction first occurs at the first temperature and then at the second temperature, wherein the first temperature is not less than the 10h half-life temperature of the first initiator and is less than the 10h half-life temperature of the second initiator, and the second temperature is not less than the 10h half-life temperature of the second initiator. In this design, the first temperature is 10°C lower than the 10h half-life temperature of the first initiator. The first initiator generates active free radicals in the early stage of the polymerization reaction, which initiates the polymerization of the polymerization monomers, realizes chain growth, and forms the main part of the shell of the heat-expandable microspheres. As the polymerization time increases, the content of the first initiator and the polymerization monomers decreases, the polymerization reaction rate decreases and tends to be flat, and the monomer conversion rate approaches the limit. At this time, the temperature is raised to the second temperature, which matches the 10h half-life temperature of the second initiator, and secondary initiation is performed. The active free radicals in the polymerization system increase, and the residual monomers are consumed, thereby achieving the purpose of improving the monomer conversion rate and reducing the residual monomer content. At the same time, the molar ratio of the first initiator and the second initiator and other parameters of the polymerization reaction are controlled to ensure the airtightness of the thermoplastic polymer shell, thereby ensuring the expansion performance of the heat-expandable microspheres.

[0045] In addition, there are reports disclosing several methods for removing acrylonitrile, for example, using sulfur-containing oxygen acids or their salts or derivatives to remove acrylonitrile, using alkali metal sulfites or ammonium sulfites to remove acrylonitrile, and using primary amines, secondary amines, primary alkanolamines, secondary alkanolamines, secondary alkyl alkanolamines and sodium sulfide to remove acrylonitrile by cyanoethylation. However, many methods require the addition of other additives for treatment after the polymerization reaction is completed, and the steps are cumbersome. The above-mentioned and these methods, the present application can fully convert the acrylonitrile monomer in the polymerization reaction, reduce the monomer residue in the thermoplastic polymer, do not introduce other substances, and do not need to post-treat the added substances.

[0046] The polymerizable monomers, cross-linking agents, foaming agents and composite initiators in the oil phase mixture are mentioned above and will not be described in detail here.

[0047] Optionally, the step of mixing the polymerization monomer, the crosslinking agent, the foaming agent and the composite initiator also includes adding a gas migration inhibitor. In this case, the oil phase mixture also includes a gas migration inhibitor. The gas migration inhibitor is described above and will not be described in detail here.

[0048] Optionally, the aqueous dispersion medium includes water, an electrolyte, a stabilizer and an inhibitor.

[0049] Optionally, the electrolyte comprises a metal salt. Optionally, the metal salt comprises one or more of sodium chloride, potassium chloride, magnesium chloride, calcium chloride, sodium sulfate, potassium sulfate, magnesium sulfate, sodium carbonate, potassium carbonate, sodium nitrate and potassium nitrate.

[0050] Optionally, the mass ratio of the electrolyte to water is (1-50):100. For example, the mass ratio can be 1:100, 2:100, 5:100, 8:100, 10:100, 12:100, 15:100, 18:100, 20:100, 22:100, 25:100, 28:100, 30:100, 32:100, 35:100, 38:100, 40:100, 42:100, 45:100, 48:100, 50:100. Preferably, the mass ratio is (10-30):100.

[0051] Optionally, the stabilizer includes one or more of colloidal silicon dioxide, colloidal calcium carbonate, magnesium hydroxide, calcium hydroxide, aluminum hydroxide, aluminum hydroxide sol, iron hydroxide, calcium sulfate, calcium oxalate, calcium carbonate, barium sulfate, barium carbonate, magnesium carbonate and calcium phosphate. Preferably, the stabilizer includes one or more of colloidal silicon dioxide, magnesium hydroxide and calcium hydroxide. Optionally, in the colloidal silicon dioxide, the mass percentage of silicon dioxide is 20% to 30%. For example, the mass percentage is 20%, 25%, 30%.

[0052] Optionally, the mass ratio of the stabilizer to water is (1-20):100. For example, the mass ratio may be 1:100, 2:100, 3:100, 4:100, 5:100, 6:100, 7:100, 8:100, 10:100, 11:100, 13:100, 15:100, 17:100, 19:100, 20:100. Preferably, the mass ratio is (5-15):100.

[0053] Optionally, the polymerization inhibitor is a water-phase polymerization inhibitor. Since some polymerization monomers (e.g., acrylonitrile monomers) have a certain water solubility, water-phase polymerization is an undesirable polymerization reaction during the polymerization process, so the polymerization inhibitor is added to inhibit polymerization through charge transfer, thereby preventing the polymerization monomer from polymerizing in the water phase. Optionally, the polymerization inhibitor includes one or more of sodium nitrite, potassium nitrite, ferric chloride, sodium sulfide, potassium dichromate, cuprous chloride, copper sulfate, titanium trichloride, sodium sulfate, and ammonium thiocyanate.

[0054] Optionally, the mass ratio of the polymerization inhibitor to water is (0.01-5):100. For example, the mass ratio may be 0.01:100, 0.1:100, 0.3:100, 0.5:100, 0.5:100, 1:100, 1.2:100, 1.5:100, 1.8:100, 2:100, 2.5:100, 3:100, 3.5:100, 4:100, 4.5:100, 5:100. Preferably, the mass ratio is (0.05-1):100.

[0055] Optionally, the aqueous dispersion medium further includes a dispersion stabilizing agent to improve the dispersion effect. Optionally, the dispersion stabilizing agent includes one or more of carboxymethyl cellulose, hydroxyethyl cellulose, polyvinyl pyrrolidone, tetramethyl ammonium chloride, polyvinyl alcohol, a water-soluble condensation product of diethanolamine and adipic acid, polyethyleneimine, gelatin, casein and albumin.

[0056] Optionally, the mass ratio of the dispersion stabilization aid to water is (0-1):100. For example, it can be 0:100, 0.001:100, 0.005:100, 0.01:100, 0.05:100, 0.1:100, 0.2:100, 0.3:100, 0.4:100, 0.5:100, 0.6:100, 0.7:100, 0.8:100, 0.9:100, 1:100. When the mass ratio of the dispersion stabilization aid to water is 0:100, it means that no dispersion stabilization aid is added.

[0057] Optionally, the pH value of the aqueous dispersion medium is 1 to 7. For example, it can be 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7. Preferably, it is 3 to 5. The pH value of the aqueous dispersion medium can be adjusted by adding an acidic medium, for example, by adding hydrochloric acid to adjust the pH value of the aqueous dispersion medium to the above value.

[0058] Optionally, the mass of the oil phase mixture accounts for 15% to 30% of the total mass of the oil phase mixture and the aqueous dispersion medium. For example, 15%, 16%, 18%, 20%, 21%, 23%, 25%, 27%, 29%, 30%. Preferably, the mass of the oil phase mixture accounts for 18% to 28% of the total mass of the oil phase mixture and the aqueous dispersion medium.

[0059] Understandably, the oil phase mixture and the aqueous dispersion medium are mixed, and the oil phase mixture can be dispersed in the aqueous dispersion medium by high-speed shearing. The oil phase mixture is dispersed into fine droplets in the aqueous dispersion medium by high-speed shearing, and each fine droplet is coated with a stabilizer to prevent it from aggregating. Optionally, the speed of dispersion is 500rpm to 8000rpm, for example, 500rpm, 1000rpm, 1500rpm, 2000rpm, 2500rpm, 3000rpm, 3500rpm, 4000rpm, 4500rpm, 5000rpm, 6000rpm, 7000rpm, 8000rpm.

[0060] It is understood that the polymerization of the suspension is carried out in an inert atmosphere. Optionally, the inert atmosphere includes one or more of a nitrogen atmosphere, an argon atmosphere and a helium atmosphere. Optionally, the polymerization process is accompanied by a stirring step.

[0061] Wherein, the first temperature is the same as or higher than the 10h half-life temperature of the first initiator, lower than the 10h half-life temperature of the second initiator, and the second temperature is the same as or higher than the 10h half-life temperature of the second initiator. The first temperature matches the 10h half-life temperature of the first initiator. At the first temperature, the first initiator preferentially decomposes, the polymerization reaction starts quickly, the polymerization monomer gradually polymerizes to form a polymer, and the polymer and the foaming agent form a phase separation and accumulate in the outer layer of the oil phase fine droplets, gradually forming a core-shell structure in which the polymer wraps the foaming agent. In this process, as the polymerization reaction proceeds, the content of the first initiator and the polymerization monomer decreases, the degree of polymerization slows down, and the monomer conversion rate tends to the limit. At this time, the temperature is raised to the second temperature, and the second temperature matches the 10h half-life temperature of the second initiator. The activity of the second initiator increases, a large number of active free radicals are generated, and secondary initiation is performed to improve the monomer conversion rate and reduce the residual monomer content.

[0062] Optionally, the polymerization reaction is carried out at a first temperature until the monomer conversion rate is 80% to 90%, and then the polymerization reaction is carried out at a second temperature. For example, the polymerization reaction is carried out at a first temperature until the monomer conversion rate is 80%, 85%, or 90%, and then the polymerization reaction is carried out at a second temperature.

[0063] Optionally, the first temperature is 0°C to 10°C higher than the 10h half-life temperature of the first initiator, for example, 0°C, 1°C, 3°C, 5°C, 7°C, or 10°C higher.

[0064] Optionally, the second temperature is 0°C to 20°C higher than the 10h half-life temperature of the second initiator, for example, 0°C, 1°C, 3°C, 5°C, 7°C, 10°C, 12°C, 15°C, 18°C, 20°C higher.

[0065] By introducing the first initiator and the second initiator with different half-lives of 10 hours and adjusting the temperature of the polymerization reaction, more active free radicals are generated in the polymerization process, thereby improving the monomer conversion rate and reducing the residual monomer content.

[0066] In some examples, according to the 10h half-life temperature of the first initiator, the first temperature may be 40°C to 70°C, for example, 40°C, 42°C, 45°C, 48°C, 50°C, 52°C, 55°C, 58°C, 60°C, 70°C. The second temperature may be 60°C to 180°C, for example, 60°C, 62°C, 65°C, 68°C, 70°C, 72°C, 75°C, 78°C, 80°C, 100°C, 150°C, 180°C.

[0067] Optionally, the molar ratio of the first initiator to the second initiator is controlled to be (90-100): 1. The molar amount of the first initiator is much greater than the molar amount of the second initiator, ensuring that the main body of the shell of the thermoplastic polymer is fully formed in the early stage of the polymerization reaction, preventing the late polymerization reaction from violently affecting the inner wall of the shell, and ensuring the airtightness of the thermoplastic polymer shell.

[0068] Optionally, other parameters of the polymerization reaction are controlled to ensure that the particle size D50 expansion rate of the heat-expandable microspheres caused by the foaming agent does not foam or does not exceed 20%. Due to the low boiling point of the foaming agent, if it is uncontrollably foamed during the polymerization reaction, it is manifested as an increase in the particle size D50 of the heat-expandable microspheres formed, which will have an adverse effect on the air tightness and expansion performance of the heat-expandable microspheres. The controller can foam by controlling other parameters of the polymerization reaction, for example, controlling the pressure and time of the polymerization reaction. Wherein, the expansion rate of the particle size D50 of the heat-expandable microspheres caused by foaming refers to the rate of change of the particle size D50 of the heat-expandable microspheres after foaming relative to the particle size D50 of the heat-expandable microspheres formed without foaming.

[0069] In some examples, the second temperature is relatively high, and the foaming agent is prone to uncontrollable foaming. The foaming of the foaming agent can be controlled by controlling the pressure of the polymerization reaction at the second temperature.

[0070] Optionally, the reaction pressure of the polymerization reaction at the first temperature is 0.3 MPa to 0.8 MPa. For example, the reaction pressure is 0.3 MPa, 0.35 MPa, 0.4 MPa, 0.45 MPa, 0.5 MPa, 0.55 MPa, 0.6 MPa, 0.65 MPa, 0.7 MPa, 0.75 MPa, 0.8 MPa.

[0071] Optionally, the reaction time of the polymerization reaction at the first temperature is 15 h to 20 h. For example, the reaction time is 15 h, 16 h, 17 h, 18 h, 19 h, or 20 h.

[0072] Optionally, the reaction pressure of the polymerization reaction at the second temperature is 1 MPa to 1.5 MPa. For example, the reaction pressure is 1 MPa, 1.1 MPa, 1.2 MPa, 1.3 MPa, 1.4 MPa, 1.5 MPa. Compared with the reaction pressure of the polymerization reaction at the first temperature, the reaction pressure of the polymerization reaction at the second temperature is greater, which is beneficial to suppress the foaming expansion of the foaming agent.

[0073] Optionally, the reaction time of the polymerization reaction at the second temperature is 1 hour to 4 hours. For example, the reaction time is 1 hour, 2 hours, 3 hours, or 4 hours. Compared with the polymerization reaction time at the first temperature, the polymerization reaction time at the second temperature is shorter, which is beneficial to suppressing the foaming expansion of the foaming agent.

[0074] The present application designs the composite initiator and the polymerization reaction procedure so that there are ideal numbers of active free radicals in both the early and late stages of the polymerization reaction, thereby increasing the conversion rate of the polymerization monomers and achieving the purpose of reducing the residual monomer content in the residual thermoplastic polymer. At the same time, the polymer shell has better air tightness and better expansion performance.

[0075] It can be understood that after the polymerization reaction is completed, a post-treatment step is also included. Optionally, the post-treatment includes filtration and drying.

[0076] The heat-expandable microspheres are prepared by the suspension polymerization method. The preparation method is simple. A composite initiator is used in the oil phase system. The corresponding temperature is matched in the suspension polymerization reaction procedure, so that the monomer conversion rate reaches more than 99%, which can reduce the residual monomer content in the thermoplastic polymer. The obtained heat-expandable microspheres are safe and environmentally friendly. At the same time, the air tightness of the thermoplastic polymerization is not affected, and the expansion performance of the heat-expandable microspheres is guaranteed.

[0077] A third aspect of the present application provides the use of the heat-expandable microspheres as described above or the heat-expandable microspheres prepared by the preparation method as described above in lightweight fillers, polishing materials, thermal insulation materials, heat-insulating materials, sound-absorbing materials, packaging materials or elastic materials.

[0078] For example, heat-expandable microspheres are added to ultra-light clay as lightweight fillers, heat-expandable microspheres are added to sealing strips as elastic materials, and heat-expandable microspheres are added to wine corks as elastic materials. The above-mentioned heat-expandable microspheres have a good residual monomer content, good air tightness, high expansion ratio, safety and environmental protection, and can meet the application fields close to daily life and a large number of human contacts. DETAILED DESCRIPTION

[0079] The present application will be further described in detail below in conjunction with specific embodiments. The present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thoroughly understood.

[0080] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0081] the term

[0082] Unless otherwise specified or incompatible herewith, the terms and phrases used herein shall have the following meanings:

[0083] In the present application, "plurality", "multiple", "multiple times", "plurality", etc., unless otherwise specified, refer to a number greater than 2 or equal to 2. For example, "one or more" means one or greater than or equal to two.

[0084] In this application, "optionally", "optional" and "optional" refer to optional, that is, to any one of the two parallel schemes of "yes" or "no". If multiple "options" appear in a technical solution, unless otherwise specified and there is no contradiction or mutual restriction, each "optional" is independent.

[0085] In this application, in the "first aspect", "second aspect", "third aspect", "fourth aspect", etc., the terms "first", "second", "third", "fourth", etc. are used only for descriptive purposes and cannot be understood as indicating or implying relative importance or quantity, nor can they be understood as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first", "second", "third", "fourth", etc. only serve the purpose of non-exhaustive enumeration and description, and it should be understood that they do not constitute a closed limitation on quantity.

[0086] In the present application, when referring to a numerical interval (i.e., a numerical range), unless otherwise specified, the optional numerical distribution within the above numerical interval is deemed to be continuous and includes the two numerical endpoints of the numerical range (i.e., the minimum value and the maximum value), and every numerical value between the two numerical endpoints.

[0087] The temperature parameters in this application, unless otherwise specified, are allowed to be either constant temperature treatment or to vary within a certain temperature range. It should be understood that the constant temperature treatment allows the temperature to fluctuate within the accuracy range controlled by the instrument. Fluctuations within the range of ±5°C, ±4°C, ±3°C, ±2°C, and ±1°C are allowed.

[0088] In the present application, the percentage content, unless otherwise specified, refers to mass percentage for solid-liquid mixing and solid-solid mixing, and volume percentage for liquid-liquid mixing.

[0089] In this application, when it comes to percentage concentration, unless otherwise specified, it refers to the final concentration. The final concentration refers to the proportion of the added component in the system after the addition of the component.

[0090] The following is further described in conjunction with specific embodiments and comparative examples. The raw materials involved in the following specific embodiments and comparative examples, unless otherwise specified, can all be commercially available, the instruments used, unless otherwise specified, can all be commercially available, and the processes involved, unless otherwise specified, are all routinely selected by those skilled in the art.

[0091] (1) Ethylenically unsaturated monomers

[0092] Acrylonitrile, AN, Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%.

[0093] Methacrylonitrile, MAN, Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%

[0094] Vinylidene chloride, VDC, Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%.

[0095] Methyl acrylate, MA, Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%.

[0096] Methacrylic acid, MAA, Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%.

[0097] (2) Cross-linking agent

[0098] Ethylene glycol dimethacrylate, EGDMA, Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%.

[0099] Trimethylolpropane triacrylate, TMPTA, Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%.

[0100] (3) First initiator

[0101] Tert-amyl peroxypivalate, TAPV, 10h half-life temperature 54℃, molecular weight 188.3, ​​Shanghai Aladdin Biochemical Technology Co., Ltd.

[0102] Di(2-ethylhexyl) peroxydicarbonate, EHP, 10h half-life temperature 47℃, molecular weight 346.4, Shanghai Aladdin Biochemical Technology Co., Ltd.

[0103] (4) Second initiator

[0104] Azobisisobutyronitrile, AIBN, 10h half-life temperature 65℃, molecular weight 164.2, Shanghai Aladdin Biochemical Technology Co., Ltd.

[0105] Dibenzoyl peroxide, BPO, 10h half-life temperature 71℃, molecular weight 242.2, Shanghai Aladdin Biochemical Technology Co., Ltd.

[0106] (5) Foaming agent

[0107] Isobutane, Aladdin Group Co., Ltd., reagent grade 99%.

[0108] Isopentane, Aladdin Group Co., Ltd., reagent grade 99%.

[0109] (6) Gas migration inhibitors

[0110] Naphthenic oil, Aladdin Group Ltd., reagent grade 99%.

[0111] (7) Stabilizer

[0112] Colloidal silica, the mass percentage of silica is 25%, Guangdong Huierte Nano Technology Co., Ltd.

[0113] (8) Electrolytes

[0114] Sodium chloride, Aladdin Group Co., Ltd., reagent grade 99%.

[0115] (9) Inhibitor

[0116] Sodium nitrite, Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%.

[0117] Example 1

[0118] This embodiment provides a heat-expandable microsphere and a preparation method thereof. Referring to Table 1 and Table 2, the steps are as follows:

[0119] (1) Preparation of aqueous dispersion medium: Add 150 parts of sodium chloride, 80 parts of colloidal silicon dioxide (containing 25% silicon dioxide) and 2 parts of sodium nitrite to 800 parts of deionized water, by weight, stir well, and adjust the pH value of the system to 3.5 to obtain an aqueous dispersion medium.

[0120] (2) preparing an oil phase mixture: by weight, 190 parts of AN, 80 parts of VDC, 45 parts of MA, 40 parts of isobutane, 1 part of EGDMA, 1.883 parts of TAPV (molar weight 0.01), 0.0164 parts of AIBN (molar weight 0.0001) and 6 parts of cyclohexane oil were fully mixed to obtain an oil phase mixture;

[0121] (3) dispersing the oil phase mixture obtained in step (2) into the aqueous dispersion medium obtained in step (1), and dispersing at a high speed of 1600 rpm for 20 min in a closed container to obtain a suspension;

[0122] (4) Purging with nitrogen, in a nitrogen atmosphere and stirring at 100 rpm, polymerizing the suspension obtained in step (3), wherein the polymerization procedure is as follows: the initial reaction pressure is controlled at 0.5 MPa, the polymerization reaction is carried out at 55° C. for 18 hours, and the monomer conversion rate is measured to be 85%; then the temperature and pressure are increased, the reaction pressure is increased to 1.2 MPa, and the polymerization reaction is carried out at 80° C. for 3 hours. After the polymerization reaction is completed, the product is filtered and dried to obtain the heat-expandable microspheres.

[0123] Embodiment 2-6

[0124] Example 2-6 provides a heat-expandable microsphere and a preparation method thereof, which is basically the same as Example 1, and the main difference is that the composition of the oil phase mixture and the polymerization reaction procedure are different. The specific differences are shown in Table 1 and Table 2.

[0125] Table 1

[0126]

[0127] Note: “--” means not added.

[0128] Table 2

[0129]

[0130] Comparative Examples 1-7

[0131] Comparative Examples 1-7 provide a heat-expandable microsphere and a preparation method thereof, which are basically the same as Example 1, and the main difference lies in the composition of the oil phase mixture and the polymerization reaction procedure. The specific differences are shown in Tables 3 and 4.

[0132] Table 3

[0133]

[0134] Note: “--” means not added.

[0135] Table 4

[0136]

[0137] The performance of the heat-expandable microspheres prepared in Examples 1-6 and Comparative Examples 1-7 was tested, and the specific method included:

[0138] (1) Particle size test

[0139] The particle size of the heat-expandable microspheres was tested using a laser particle size analyzer (Bettersize 2600). The test method was a wet method, the shading rate was 5-20%, and the test medium was water. The median particle size D 50 , D 90 and D 10 , particle size distribution PDI = (D 90 -D 10 ) / D 50 ;

[0140] (2) Expansion performance test

[0141] The test was performed using a static thermomechanical analyzer (TMA, Mettler TMA / SDTA2+) with a heating rate of 15°C / min. The specific test steps are as follows:

[0142] Add 1 mg of the heat-expandable microspheres to be tested into a 150 μL ceramic crucible, add a gasket of matching diameter on top of the microsphere layer to prepare the sample; measure the sample height while applying a force of 0.06 N to the sample from above using a pressurizer; while applying a force of 0.06 N using a pressurizer, increase the temperature from 20°C to 300°C at a rate of 15°C / min, and measure the displacement of the pressurizer in the vertical direction; the temperature at which the displacement in the positive direction starts is recorded as the starting expansion temperature (Tstart), the temperature at which the maximum displacement is displayed is recorded 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.

[0143] (3) Residual monomer content

[0144] The test was performed using a gas chromatograph (Shimadzu GC-2010 Pro). The test steps are as follows:

[0145] Chromatographic conditions: initial temperature 40°C, constant temperature for 2 min, heated to 100°C at a rate of 5°C / min, constant temperature for 2 min, then heated to 200°C at a rate of 6°C / min, maintained for 5 min; nitrogen was used as the carrier gas with a flow rate of 2.0 mL per minute; when water was used as the solvent, the headspace bottle equilibrium temperature was 70~85°C, and the headspace bottle equilibrium time was 30~60 minutes; the injection port temperature was 200°C.

[0146] (4) Determination of foaming agent loss rate

[0147] Weigh the weight of the heat-expandable microspheres, record it as W1, put the heat-expandable microspheres in a container, weigh the total weight of the heat-expandable microspheres and the container, record it as W2, put the container containing the heat-expandable microspheres in an oven, heat it at 50°C for 2h, take it out and measure the total weight of the two as W3. The loss rate of the foaming agent is calculated by the following formula:

[0148] Foaming agent loss rate (%) = (W3-W2) / W1×100%

[0149] The test results are shown in Table 5 and Table 6:

[0150] Table 5

[0151]

[0152] Table 6

[0153]

[0154] Note: “--” means not added.

[0155] According to the test data in Table 5 and Table 6:

[0156] (1) In Examples 1-6, under different polymerization monomers, the first initiator and the second initiator were added, and the polymerization reaction procedure was followed. An ideal number of active free radicals were present in both the early and late stages of the polymerization reaction, the monomer conversion rate was high, the residual monomer content was low, and the loss rate of the thermally expandable microsphere foaming agent was low, indicating good air tightness and excellent expansion performance.

[0157] (2) By comparing Examples 1-2 and Comparative Examples 1-2, it can be seen that using a single type of initiator, whether a single type of first initiator or a single type of second initiator, will result in a higher residual monomer content and poor air tightness.

[0158] (3) By comparing Examples 3-4 and Comparative Examples 3-4, it can be seen that when two initiators with similar 10h half-life temperatures are used, the problem of high residual monomer content will also occur. At the same time, the two initiators initiate together in the early stage of polymerization, resulting in deviations in the formation of the polymer shell, which ultimately affects the air tightness of the microspheres, and the expansion ratio decreases, resulting in poor expansion performance.

[0159] (4) By comparing Examples 5-6 and Comparative Examples 5-6, it can be seen that when the molar ratio of the first initiator to the second initiator is 50:1, the content of the second initiator is relatively high, the number of active free radicals in the later polymerization is relatively large, the molecular chain segments are relatively short, and the reaction is violent, which affects the formation of the inner wall of the polymer shell, resulting in reduced air tightness, and the expansion ratio is reduced, and the expansion performance is deteriorated.

[0160] (5) By comparing Example 5 and Comparative Example 7, it can be seen that when the reaction pressure of the second temperature polymerization is low, the foaming agent will be vaporized to a certain extent during the reaction process, resulting in an increase in the D50 of the formed microspheres (the change range is (28.66-20.51) / 20.51×100%=39.7%), affecting the air tightness and expansion performance of the final microspheres.

[0161] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0162] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims.

Claims

1. A heat-expandable microsphere, characterized in that: It comprises a shell and a core covered by the shell, wherein the material of the shell comprises a thermoplastic polymer, and the material of the core comprises a foaming agent; The thermoplastic polymer meets the following conditions: (1) The total content of residual monomers in the thermoplastic polymer is ≤1000 ppm; (2) Ensure that the foaming agent loss rate of the heat-expandable microspheres after heating at 50° C. for 2 hours is ≤5%.

2. The heat-expandable microspheres according to claim 1, characterized in that: Includes at least one of the following features: (1) The raw materials of the thermoplastic polymer include polymerizable monomers, crosslinking agents and composite initiators; optionally, the polymerizable monomers include ethylenically unsaturated monomers; optionally, the ethylenically unsaturated monomers include one or more of acrylonitrile monomers, acrylate monomers, acrylic acid monomers, alkyl vinyl ester monomers, styrene monomers, halogen-containing olefin monomers and acrylamide monomers; optionally, the crosslinking agent includes trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triacrylate, pentaerythritol trimeth ... One or more of tetraol trimethacrylate, ethylene glycol dimethacrylate, trimethylolpropane triethylene glycol triacrylate, diallyl phthalate, diallyl carbonate, triallyl isocyanate, triallyl isocyanurate, trimethylolpropane diallyl ether, trimethallyl isocyanate, pentaerythritol triallyl ether, 2,2-bis(allyloxymethyl)-1-butanol, 1,6-hexanediol diacrylate and polyethylene glycol dimethacrylate; optionally, the mass ratio of the crosslinking agent to the monomer is (0.01~5): 100; Optionally, the composite initiator includes a first initiator and a second initiator, and the 10h half-life temperature of the first initiator is at least 10°C lower than the 10h half-life temperature of the second initiator; Optionally, the molar ratio of the first initiator to the second initiator is (90~100): 1; Optionally, the first initiator and the second initiator each independently include tert-amyl peroxypivalate, di(2-ethylhexyl) peroxydicarbonate, neodecyl peroxyacid Isopropyl benzene ester, tert-butyl oxyacetate, tert-butyl peroxybenzoate, tert-butyl peroxyisopropyl monocarbonate, 1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, dialkyl peroxides such as 1,3-di(2-tert-butylperoxyisopropyl)benzene, 1,3-di(2-tert-butylperoxyisopropyl)benzene, 1,1-di(tert-butylperoxy)cyclohexane, 1,1-di(tert-butylperoxy) Hexyl peroxy) cyclohexane, methyl ethyl ketone peroxide, tert-butyl hydroperoxide, tert-amyl hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, dodecyl peroxide, benzoyl peroxide, diisopropyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, bis(4-tert-butylcyclohexyl) peroxydicarbonate, tert-butyl peroxybenzoate, tert-butyl peroxypivalate, di-tert-butyl peroxide, cumene hydroperoxide, hydrogen peroxide One or more of diisopropylbenzene, paraben hydroperoxide and bis(2-phenoxyethyl) peroxydicarbonate, 2,2'-azobisisobutyronitrile, azobisisovaleronitrile, 2,2'-azobis(2,4-dimethyl)valeronitrile, azobisisoheptanenitrile, 2,2'-azobis(2-methylpropionate), azobisisobutylcyanoformamide, azobiscyclohexylcarbonitrile and dimethyl azobisisobutyrate; optionally, the mass ratio of the composite initiator to the monomer is (0.01-5):100;. (2) The foaming agent includes a C3-C13 alkane foaming agent; (3) The mass percentage of the foaming agent in the thermally expandable microspheres is 20% to 30%; (4) The material of the inner core further comprises a gas migration inhibitor; optionally, the boiling point of the gas inhibitor is higher than the glass transition temperature of the thermoplastic polymer; optionally, the mass ratio of the gas inhibitor to the foaming agent is (0-30):

100.

3. The heat-expandable microspheres according to any one of claims 1 to 2, characterized in that: Includes at least one of the following features: (1) The particle size D of the heat-expandable microspheres 50 1 μm~100 μm; preferably 5 μm~50 μm; (2) The PDI of the heat-expandable microspheres is 1 to 1.1, wherein PDI = (D 90 -D 10 ) / D 50 .

4. A method for preparing heat-expandable microspheres, characterized in that: The following steps are involved: Mixing a polymerizable monomer, a crosslinking agent, a foaming agent and a composite initiator to obtain an oil phase mixture, wherein the composite initiator comprises a first initiator and a second initiator, and a 10-hour half-life temperature of the first initiator is at least 10° C. lower than a 10-hour half-life temperature of the second initiator; mixing the oil phase mixture and the aqueous dispersion medium to obtain a suspension; The suspension is subjected to a polymerization reaction, and the polymerization reaction comprises the following procedures: a polymerization reaction at a first temperature and then a polymerization reaction at a second temperature; wherein the first temperature is not less than the 10-hour half-life temperature of the first initiator and is less than the 10-hour half-life temperature of the second initiator, and the second temperature is not less than the 10-hour half-life temperature of the second initiator, and the molar ratio of the first initiator to the second initiator and other parameters of the polymerization reaction are controlled to obtain the polymer expanded microspheres comprising a shell and a core coated by the shell, wherein the material of the shell comprises a thermoplastic polymer, and the material of the core comprises a foaming agent; the thermoplastic polymer satisfies the following conditions: (1) the total content of residual monomers of the thermoplastic polymer is ≤1000ppm; and (2) the foaming agent loss rate of the heat-expandable microspheres after heating at 50°C for 2h is ensured to be ≤5%.

5. The method for preparing heat-expandable microspheres according to claim 4, characterized in that: At least one of the following conditions is met: (1) controlling the molar ratio of the first initiator to the second initiator to be (90-100):1; (2) Controlling other parameters of the polymerization reaction to ensure that the foaming agent does not foam or that the particle size D50 expansion rate of the heat-expandable microspheres caused by foaming does not exceed 20%.

6. The method for preparing heat-expandable microspheres according to claim 4, comprising at least one of the following features: (1) The reaction pressure of the polymerization reaction at the second temperature is 1 MPa to 1.5 MPa; (2) the reaction time of the polymerization reaction at the second temperature is 1 h to 4 h; (3) the reaction pressure of the polymerization reaction at the first temperature is 0.3 MPa to 0.8 MPa; (4) The reaction time of the polymerization reaction at the first temperature is 15 h to 20 h; (5) After polymerizing at a first temperature until the monomer conversion rate reaches 80% to 90%, polymerizing at a second temperature.

7. The method for preparing heat-expandable microspheres according to claim 4, characterized in that: At least one of the following conditions is met: (1) The first temperature is 0°C to 10°C higher than the 10h half-life temperature of the first initiator; (2) The second temperature is 0°C to 20°C higher than the 10h half-life temperature of the second initiator.

8. The method for preparing heat-expandable microspheres according to any one of claims 4 to 7, characterized in that: Includes at least one of the following features: (1) The polymerizable monomer includes an ethylenically unsaturated monomer; optionally, the ethylenically unsaturated monomer includes one or more of acrylonitrile monomers, acrylate monomers, acrylic acid monomers, alkyl vinyl ester monomers, styrene monomers, halogen-containing olefin monomers and acrylamide monomers; (2) The first initiator and the second initiator each independently include tert-amyl peroxypivalate, di(2-ethylhexyl) peroxydicarbonate, isopropyl neodecane peroxyacid, tert-butyl oxyacetate, tert-butyl peroxybenzoate, tert-butyl peroxyisopropyl monocarbonate, 1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, 1,3-di(2-tert-butylperoxyisopropyl)benzene and other dialkyl peroxides, 1,3-di(2-tert-butylperoxyisopropyl)benzene and other dialkyl peroxides, 1,1-di(tert-butylperoxy)cyclohexane, 1,1-di(tert-hexylperoxy)cyclohexane, methyl ethyl ketone peroxide, tert-butyl hydroperoxide, tert-amyl hydroperoxide, 1,1, One or more of 3,3-tetramethylbutyl hydroperoxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, dodecyl peroxide, benzoyl peroxide, diisopropyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, bis(4-tert-butylcyclohexyl) peroxydicarbonate, tert-butyl peroxybenzoate, tert-butyl peroxypivalate, di-tert-butyl peroxide, isopropyl hydroperoxide, diisopropyl hydroperoxide, p-hydroxybenzene hydroperoxide and bis(2-phenoxyethyl) peroxydicarbonate, 2,2'-azobisisobutyronitrile, azobisisovaleronitrile, 2,2'-azobis(2,4-dimethyl)valeronitrile, azobisisoheptanenitrile, 2,2'-azobis(2-methylpropionate), azobisisobutylcyanoformamide, azobiscyclohexylcarbonitrile and dimethyl azobisisobutyrate; (3) The mass ratio of the composite initiator to the monomer is (0.01-5):100; (4) The crosslinking agent includes one or more of trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triacrylate, pentaerythritol trimethacrylate, ethylene glycol dimethacrylate, trimethylolpropane triethylene glycol triacrylate, diallyl phthalate, diallyl carbonate, triallyl isocyanate, triallyl isocyanurate, trimethylolpropane diallyl ether, trimethallyl isocyanate, pentaerythritol triallyl ether, 2,2-bis(allyloxymethyl)-1-butanol, 1,6-hexanediol diacrylate and polyethylene glycol dimethacrylate; (5) The mass ratio of the cross-linking agent to the monomer is (0.01-5):100; (6) The foaming agent includes a C3-C13 alkane foaming agent; (7) The mass percentage of the foaming agent in the thermally expandable microspheres is 20% to 30%; (8) mixing a polymerizable monomer, a crosslinking agent, a foaming agent and a composite initiator, further comprising the step of adding a gas migration inhibitor; optionally, the boiling point of the gas inhibitor is higher than the glass transition temperature of the thermoplastic polymer; optionally, the mass ratio of the gas inhibitor to the foaming agent is (0-30):100; (9) The aqueous dispersion medium comprises water, an electrolyte, a stabilizer and an inhibitor; optionally, the electrolyte comprises a metal salt; optionally, the metal salt comprises one or more of sodium chloride, potassium chloride, magnesium chloride, calcium chloride, sodium sulfate, potassium sulfate, magnesium sulfate, sodium carbonate, potassium carbonate, sodium nitrate and potassium nitrate; optionally, the mass ratio of the electrolyte to water is (1-50):100; optionally, the stabilizer comprises one or more of colloidal silica, colloidal calcium carbonate, magnesium hydroxide, calcium hydroxide, aluminum hydroxide, aluminum hydroxide sol, iron hydroxide, calcium sulfate, calcium oxalate, calcium carbonate, barium sulfate, barium carbonate, magnesium carbonate and calcium phosphate; optionally, the mass ratio of the stabilizer to water is (1-20):100; optionally, the inhibitor comprises one or more of sodium nitrite, potassium nitrite, ferric chloride, sodium sulfide, potassium dichromate, cuprous chloride, copper sulfate, titanium trichloride, sodium sulfate and ammonium thiocyanate; optionally, the mass ratio of the inhibitor to water is (0.01-5):100; (10) The pH value of the aqueous dispersion medium is 1 to 7, preferably 3 to 5.

9. The method for preparing heat-expandable microspheres according to any one of claims 4 to 7, characterized in that: The mass of the oil phase mixture accounts for 15% to 30% of the total mass of the oil phase mixture and the aqueous dispersion medium.

10. Use of the heat-expandable microspheres according to any one of claims 1 to 3 or the heat-expandable microspheres prepared by the preparation method according to any one of claims 4 to 9 in lightweight fillers, polishing materials, thermal insulation materials, heat-insulating materials, sound-absorbing materials, packaging materials or elastic materials.

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