Flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microspheres and preparation method thereof

Abstract

The present invention relates to the technical field of polymer materials, and specifically to a flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microsphere and a preparation method thereof, wherein the flame-retardant heat-expandable microsphere has a core-shell structure, and the core-shell structure comprises, from the outside to the inside, an outer shell layer, an inner shell layer and a core material; the outer shell layer is formed by mixing thermoplastic polyurethane and an oil-soluble flame retardant, and is attached to the inner shell layer by spray drying to form the outer shell layer; the inner shell layer is composed of a thermoplastic polymer, and the thermoplastic polymer is polymerized from acrylate monomers and nitrile monomers; the core material is composed of a liquid flame retardant and a hydrocarbon. The preparation method prepares flame-retardant heat-expandable microspheres by matching the flame-retardant thermoplastic polyurethane of the outer shell layer with the flame retardant inside the microsphere core material, thereby effectively improving the flame retardant performance and thermal stability of the heat-expandable microspheres while maintaining the foaming size of the heat-expandable microspheres.

Description

Technical Field

[0001] The invention relates to a flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microsphere and a preparation method thereof, belonging to the technical field of polymer materials. Background Art

[0002] Thermal expansion microspheres are particles with a core-shell structure formed by a thermoplastic polymer shell wrapping a hydrocarbon. When the microspheres are heated to a certain temperature, the hydrocarbon inside acts as a foaming agent and vaporizes, generating steam pressure to expand the microspheres. After cooling, the microspheres will still maintain the foaming state when heated. Therefore, thermal expansion microspheres are usually used as foaming agents or lightweight fillers in the construction and infrastructure, packaging, coatings, automotive primers, PVC leather and other industries, playing a role in lightweighting, improving elasticity, reducing costs, sound insulation and heat insulation, and shortening the molding cycle.

[0003] In the prior art, hydrocarbons in general heat-expandable microspheres are flammable substances. As the temperature of the microspheres increases during expansion, the thermoplastic shell becomes thinner, and the gaseous alkanes inside are volatile, which has low safety and limits their application in special fields. Therefore, it is of great significance to improve the flame retardancy and thermal stability of heat-expandable microspheres.

[0004] Chinese patent application CN116970252A discloses a flame-retardant heat-expandable foamed microsphere and a preparation method thereof, wherein a layer of phosphazene flame retardant is attached to the surface of the heat-expandable microsphere, and the microsphere is attached to the surface of the microsphere under the action of cobalt cyclohexanone peroxide and a diluent. This method only performs flame-retardant treatment on the shell material of the microsphere, and does not treat the internal flammable alkane from the polymerization end, and the flame-retardant and safety performance still need to be optimized. Chinese patent application CN108084333A discloses a low-pollution flame-retardant heat-expandable microsphere and a preparation method thereof, wherein an oil-soluble flame retardant is mixed with the core to make the heat-expandable microsphere have flame-retardant properties, and this method only performs flame-retardant treatment on the core material of the microsphere, and does not verify whether the modified microsphere has an effect on the foaming ratio.

[0005] Therefore, how to improve the flame retardant properties of heat-expandable microspheres and further optimize the preparation process of heat-expandable microspheres while maintaining the foaming size of the microspheres so that the resulting product has excellent flame retardant properties and thermal stability is an urgent problem to be solved. Summary of the invention

[0006] The invention aims at the deficiencies in the prior art and provides a flame retardant thermoplastic polyurethane coated flame retardant heat expandable microsphere and a preparation method thereof, which significantly improves the flame retardant performance of the heat expandable microsphere while maintaining the foaming size of the heat expandable microsphere.

[0007] The technical solution of the present invention to solve the above technical problems is as follows: a flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microsphere, the flame-retardant heat-expandable microsphere has a core-shell structure, and the core-shell structure comprises, from the outside to the inside, an outer shell layer, an inner shell layer and a core material; the outer shell layer is formed by mixing thermoplastic polyurethane and an oil-soluble flame retardant, and is attached to the inner shell layer by spray drying to form the outer shell layer; the inner shell layer is composed of a thermoplastic polymer, and the thermoplastic polymer is polymerized by acrylate monomers and nitrile monomers; the core material is composed of a liquid flame retardant and a hydrocarbon.

[0008] Further, the oil-soluble flame retardant is at least one of triphenyl phosphate, tert-butylated triphenyl phosphate, tributyl phosphate, triisobutyl phosphate, tris(2-chloroethyl) phosphate, dimethyl methyl phosphate, pentaerythritol phosphate, resorcinol bis(diphenyl phosphate), tricresyl phosphate, phosphite, tetraphenyl dipropylene glycol diphosphite, melamine cyanurate, and melamine polyphosphate;

[0009] The thermoplastic polyurethane is at least one of Desmopan 2792A, Desmopan 5377A, WANHUA WHT-1570, WANHUAWHT-1190, WANHUA WHT-1198, WANHUA WHT-1164IC, and WANHUA WHT-3385.

[0010] Further, the liquid flame retardant is at least one of triethyl phosphate, tris(1-chloro-2-propyl) phosphate, chlorinated paraffin, 2-ethylhexyl diphenyl phosphate, triisopropylphenyl phosphate, tris(hexafluoroisopropyl) phosphate, tris(2,2,2-trifluoroethyl) phosphate, ethyl polyphosphate, tris(2-chloroethyl) phosphate, and chloroalkyl polyphosphate;

[0011] The hydrocarbon is at least one of n-pentane, isopentane, neopentane, n-butane, isobutane, n-hexane and isooctane.

[0012] Further, the acrylic acid ester monomer is at least one of methyl acrylate, ethyl acrylate, butyl acrylate, isobutyl acrylate, isooctyl acrylate, heptyl acrylate, benzyl acrylate, styrene acrylate, acrylamide, fluorinated acrylate, methyl methacrylate, acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate, and acrylamide;

[0013] The nitrile monomer is at least one of acetonitrile, acrylonitrile, methacrylonitrile, adiponitrile, suberonitrile, 4-aminobenzonitrile, phthalonitrile, benzonitrile, isocyanate nitrile, 1,3-succinonitrile and benzonitrile.

[0014] The present invention also discloses a method for preparing flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microspheres, the preparation method comprising:

[0015] S1. Preparation of aqueous dispersion and oil dispersion:

[0016] Purified water, electrolyte and emulsifier are mixed evenly to obtain a uniform aqueous phase dispersion;

[0017] The acrylate monomer, the nitrile monomer, the liquid flame retardant, the hydrocarbon, the initiator and the cross-linking agent are uniformly mixed to obtain an oil phase dispersion;

[0018] S2. Polymerization reaction:

[0019] Under stirring conditions, the oil phase dispersion is added dropwise to the water phase dispersion to obtain a pre-suspension, and a polymerization reaction is carried out under inert atmosphere and pressure conditions to obtain a suspension containing heat-expandable microspheres;

[0020] S3, post-processing:

[0021] The suspension prepared in step S3 is adjusted to an acidic pH, filtered, washed with water, and dried to obtain flame-retardant heat-expandable microspheres;

[0022] S4. Preparation of outer layer of shell material:

[0023] The solvent, thermoplastic polyurethane, oil-soluble flame retardant and flame-retardant heat-expandable microspheres are mixed by high-speed stirring and then passed through a spray dryer to obtain flame-retardant thermoplastic polyurethane-coated flame-retardant heat-expandable microspheres.

[0024] Further, the electrolyte is at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide, sodium chloride, magnesium chloride, potassium chloride, and calcium chloride;

[0025] The emulsifier is at least one of sodium stearate, sodium dodecyl sulfate, sodium hexadecyl sulfate, polyoxyethylene, sodium disproportionate rosin acid, and sodium lauryl sulfate;

[0026] The initiator is at least one of azobisisobutyronitrile, azobisisoheptanenitrile, azobismethylvaleronitrile, benzoyl peroxide, lauroyl peroxide, cumene hydroperoxide, tert-butyl hydroperoxide, di-tert-butyl peroxide, diisopropyl peroxide, tert-butyl perbenzoate, tert-butyl pervalerate, methyl ethyl ketone peroxide, cyclohexanone peroxide, diisopropyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, potassium persulfate, and ammonium persulfate;

[0027] The crosslinking agent is at least one of trimethylolpropane trimethacrylate, 1,4-butanediol dimethacrylate, trimethylolpropane triacrylate, ethylene glycol dimethacrylate, zinc dimethacrylate, zinc diacrylate, divinylbenzene, and divinyl ether;

[0028] The solvent is at least one of DMAC or DMF.

[0029] Furthermore, in step S1, the oil phase dispersion contains 20-40 parts of acrylate monomers, 20-40 parts of nitrile monomers, 0.1-3 parts of liquid flame retardants, 15-40 parts of hydrocarbons, 0.3-2 parts of initiators, and 0.1-1 parts of cross-linking agents, in parts by weight;

[0030] The weight ratio of the aqueous phase dispersion to the oil phase dispersion in step S2 is (70-90):(10-30).

[0031] Furthermore, in step S4, the amount of solvent used is 30-60 parts by weight; the amount of thermoplastic polyurethane used is 10-50 parts by weight; the amount of oil-soluble flame retardant used is 0.1-3 parts by weight; and the amount of flame-retardant thermally expandable microspheres used is 10-50 parts by weight.

[0032] Further, in step S2, the polymerization reaction process is: ultrasonically treat the pre-suspension for 10 to 60 minutes; then put it into a high-pressure reactor, pressurize it to 0.3 to 0.8 MPa, stir it at a speed of 60 to 500 r / min, control the polymerization temperature to 40 to 80° C., and the polymerization reaction time is 16 to 24 hours;

[0033] In step S3, the pH of the suspension is adjusted to 3-6.5 using hydrochloric acid or sulfuric acid.

[0034] Further, in step S4, the high-speed stirring speed is 1500-3000 r / min;

[0035] The spray dryer has an air inlet temperature of 80-230°C, an air outlet temperature of 60-210°C, and a liquid inlet peristaltic pump speed of 10-100 r / min.

[0036] The beneficial effects of the present invention are:

[0037] The method for preparing the flame-retardant heat-expandable microspheres of the present invention is to prepare the heat-expandable microspheres by combining the flame-retardant thermoplastic polyurethane of the outer layer of the shell material with the flame retardant inside the microsphere core material, so that the flame-retardant heat-expandable microspheres are prepared while maintaining the foaming size of the heat-expandable microspheres, thereby effectively improving the flame-retardant performance and thermal stability of the heat-expandable microspheres.

[0038] The liquid flame retardant and oil-soluble flame retardant used in the preparation method of the present invention are halogen-free, low in toxicity, and have high environmental protection. In the process of preparing heat-expandable microspheres, the thermoplastic polyurethane containing the flame retardant is innovatively coated on the inner layer of the shell material by spray drying, which can not only make the thermoplastic polyurethane firmly attached to the microspheres, but also can use polyurethane to increase the compatibility and bonding force of the oil-soluble flame retardant with the shell material of the heat-expandable microspheres. When the corresponding temperature is reached, the thermoplastic polyurethane has plasticity and expands with the foaming of the microspheres. While not affecting the foaming size of the microspheres, the flame retardant effect of the microspheres is improved, and the shell material strength can be improved after the thermoplastic polyurethane is cooled, and the structural stability of the heat-expandable microspheres after expansion is improved. In addition, by adding a liquid flame retardant inside the core material to reduce the risk of gaseous alkane explosion, and cooperating with the flame-retardant thermoplastic polyurethane, a higher flame retardant efficiency and flame retardant stability can be achieved.

[0039] The heat-expandable microspheres of the present invention improve the flame retardant performance of the heat-expandable microspheres, and the mechanism thereof is different from the traditional method of improving the flame retardant effect by relying on a single shell material or core material. The present invention can effectively improve the flame retardant efficiency and thermal stability of the heat-expandable microspheres while retaining the inherent foaming performance of the heat-expandable microspheres, the structural strength of the shell material and other characteristics, and the operation is simple, the reaction conditions are mild, the flame retardants used are all low-toxic or non-toxic substances, the safety is high, and the environmental protection significance is good. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is a scanning electron microscope image of the flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microspheres prepared in Example 1;

[0041] Figure 2 This is a scanning electron microscope image of the heat-expandable microspheres prepared in Comparative Example 1. DETAILED DESCRIPTION

[0042] The specific implementation of the present invention is described in detail below. The present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific embodiments disclosed.

[0043] 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 the present invention belongs. The terms used are only for describing specific embodiments and are not intended to limit the present invention.

[0044] Disclosed is a flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microsphere, the flame-retardant heat-expandable microsphere having a core-shell structure, the core-shell structure comprising, from the outside to the inside, an outer shell layer, an inner shell layer and a core material; the outer shell layer is formed by mixing thermoplastic polyurethane and an oil-soluble flame retardant, and is attached to the inner shell layer by spray drying to form the outer shell layer; the inner shell layer is composed of a thermoplastic polymer, and the thermoplastic polymer is polymerized by acrylate monomers and nitrile monomers; the core material is composed of a liquid flame retardant and a hydrocarbon.

[0045] Specifically, the oil-soluble flame retardant is at least one of triphenyl phosphate, tert-butylated triphenyl phosphate, tributyl phosphate, triisobutyl phosphate, tris(2-chloroethyl) phosphate, dimethyl methyl phosphate, pentaerythritol phosphate, resorcinol bis(diphenyl phosphate), tricresyl phosphate, phosphite, tetraphenyl dipropylene glycol diphosphite, melamine cyanurate, and melamine polyphosphate;

[0046] The thermoplastic polyurethane is at least one of Desmopan 2792A, Desmopan 5377A, WANHUA WHT-1570, WANHUAWHT-1190, WANHUA WHT-1198, WANHUA WHT-1164IC, and WANHUA WHT-3385.

[0047] Specifically, the liquid flame retardant is at least one of triethyl phosphate, tris(1-chloro-2-propyl) phosphate, chlorinated paraffin, 2-ethylhexyl diphenyl phosphate, triisopropylphenyl phosphate, tris(hexafluoroisopropyl) phosphate, tris(2,2,2-trifluoroethyl) phosphate, ethyl polyphosphate, tris(2-chloroethyl) phosphate, and chloroalkyl polyphosphate;

[0048] The hydrocarbon is at least one of n-pentane, isopentane, neopentane, n-butane, isobutane, n-hexane and isooctane.

[0049] Specifically, the acrylic acid ester monomer is at least one of methyl acrylate, ethyl acrylate, butyl acrylate, isobutyl acrylate, isooctyl acrylate, heptyl acrylate, benzyl acrylate, styrene acrylate, acrylic acid amide, fluorinated acrylic acid ester, methyl methacrylate, acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate, and acrylamide;

[0050] The nitrile monomer is at least one of acetonitrile, acrylonitrile, methacrylonitrile, adiponitrile, suberonitrile, 4-aminobenzonitrile, phthalonitrile, benzonitrile, isocyanate nitrile, 1,3-succinonitrile and benzonitrile.

[0051] A method for preparing flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microspheres, the preparation method comprising:

[0052] S1. Preparation of aqueous dispersion and oil dispersion:

[0053] Purified water, electrolyte and emulsifier are mixed evenly to obtain a uniform aqueous phase dispersion;

[0054] The acrylate monomer, the nitrile monomer, the liquid flame retardant, the hydrocarbon, the initiator and the cross-linking agent are uniformly mixed to obtain an oil phase dispersion;

[0055] S2. Polymerization reaction:

[0056] Under stirring conditions, the oil phase dispersion is added dropwise to the water phase dispersion to obtain a pre-suspension, and a polymerization reaction is carried out under inert atmosphere and pressure conditions to obtain a suspension containing heat-expandable microspheres;

[0057] S3, post-processing:

[0058] The suspension prepared in step S3 is adjusted to an acidic pH, filtered, washed with water, and dried to obtain flame-retardant heat-expandable microspheres;

[0059] S4. Preparation of outer layer of shell material:

[0060] The solvent, thermoplastic polyurethane, oil-soluble flame retardant and flame-retardant heat-expandable microspheres are mixed by high-speed stirring and then passed through a spray dryer to obtain flame-retardant thermoplastic polyurethane-coated flame-retardant heat-expandable microspheres.

[0061] Specifically, the electrolyte is at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide, sodium chloride, magnesium chloride, potassium chloride, and calcium chloride;

[0062] The emulsifier is at least one of sodium stearate, sodium dodecyl sulfate, sodium hexadecyl sulfate, polyoxyethylene, sodium disproportionate rosin acid, and sodium lauryl sulfate;

[0063] The initiator is at least one of azobisisobutyronitrile, azobisisoheptanenitrile, azobismethylvaleronitrile, benzoyl peroxide, lauroyl peroxide, cumene hydroperoxide, tert-butyl hydroperoxide, di-tert-butyl peroxide, diisopropyl peroxide, tert-butyl perbenzoate, tert-butyl pervalerate, methyl ethyl ketone peroxide, cyclohexanone peroxide, diisopropyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, potassium persulfate, and ammonium persulfate;

[0064] The crosslinking agent is at least one of trimethylolpropane trimethacrylate, 1,4-butanediol dimethacrylate, trimethylolpropane triacrylate, ethylene glycol dimethacrylate, zinc dimethacrylate, zinc diacrylate, divinylbenzene, and divinyl ether;

[0065] The solvent is at least one of DMAC or DMF.

[0066] Specifically, in step S1, the oil phase dispersion contains 20-40 parts of acrylate monomers, 20-40 parts of nitrile monomers, 0.1-3 parts of liquid flame retardants, 15-40 parts of hydrocarbons, 0.3-2 parts of initiators, and 0.1-1 parts of cross-linking agents, in parts by weight;

[0067] The weight ratio of the aqueous phase dispersion to the oil phase dispersion in step S2 is (70-90):(10-30).

[0068] Specifically, in step S4, the amount of solvent used is 30-60 parts by weight; the amount of thermoplastic polyurethane used is 10-50 parts by weight; the amount of oil-soluble flame retardant used is 0.1-3 parts by weight; and the amount of flame-retardant thermally expandable microspheres used is 10-50 parts by weight.

[0069] Specifically, in step S2, the polymerization reaction process is: subjecting the pre-suspension to ultrasonic treatment for 10 to 60 minutes; then placing it in a high-pressure reactor, pressurizing it to 0.3 to 0.8 MPa, stirring at a speed of 60 to 500 r / min, controlling the polymerization temperature to 40 to 80° C., and the polymerization reaction time to 16 to 24 hours;

[0070] In step S3, the pH of the suspension is adjusted to 3-6.5 using hydrochloric acid or sulfuric acid.

[0071] More specifically, a hydrochloric acid aqueous solution with a volume concentration of 10 to 36% or a sulfuric acid aqueous solution with a volume concentration of 10 to 36% is used to adjust the pH.

[0072] Specifically, in step S4, the high-speed stirring speed is 1500-3000 r / min;

[0073] The spray dryer has an air inlet temperature of 80-230°C, an air outlet temperature of 60-210°C, and a liquid inlet peristaltic pump speed of 10-100 r / min.

[0074] Example 1

[0075] A method for preparing flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microspheres comprises the following steps:

[0076] (1) Preparation of aqueous phase:

[0077] Purified water, sodium hydroxide and sodium lauryl sulfate were weighed in a mass ratio of 80:19:1, added into a beaker in sequence and stirred evenly to obtain a milky white uniform aqueous phase dispersion.

[0078] (2) Oil phase preparation:

[0079] Methyl methacrylate, methacrylonitrile, triethyl phosphate, isobutane, azobisisobutyronitrile and trimethylolpropane triacrylate were weighed in a mass ratio of 40:20:3:34:2:1, added into a beaker in sequence and stirred evenly to obtain an oil phase dispersion.

[0080] (3) Polymerization reaction: under stirring conditions, slowly add dropwise the oil phase dispersion of the system in step (2) to the aqueous phase dispersion of the system in step (1) (the mass ratio of the oil phase dispersion to the aqueous phase dispersion is 30:70), and stir evenly to obtain a pre-suspension;

[0081] The obtained pre-suspension was ultrasonically treated for 60 min and then dried under N 2 Under protection, the prepared solution was added into a high-pressure reactor, pressurized to 0.6 MPa, the polymerization temperature was controlled at 80°C, the stirring speed was 500 r / min, the reaction time was 24 h, and a suspension containing heat-expandable microspheres was obtained.

[0082] (4) Post-treatment: Add a 36% by volume hydrochloric acid aqueous solution to the suspension containing the heat-expandable microspheres prepared in step (3), adjust the pH of the system to 3.5, and obtain flame-retardant heat-expandable microspheres by filtering, washing with water, and drying.

[0083] (5) Preparation of outer layer of shell material: DMAC, Desmopan 5377A, tri(2-chloroethyl) phosphate and flame retardant heat expandable microspheres prepared in step (4) were weighed in a mass ratio of 40:30:1:29. During the stirring process, the weighed materials were added into a high-speed mixer in sequence, and the stirring speed was controlled to 2000 r / min to stir evenly. The prepared solution was then passed through a spray dryer, and the air inlet temperature was controlled to 200°C, the air outlet temperature was controlled to 170°C, and the speed of the liquid inlet peristaltic pump was controlled to 30 r / min to obtain flame retardant thermoplastic polyurethane coated flame retardant heat expandable microspheres.

[0084] Example 2

[0085] A method for preparing flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microspheres comprises the following steps:

[0086] (1) Preparation of aqueous phase:

[0087] Purified water, magnesium chloride and sodium hexadecyl sulfate were weighed in a mass ratio of 85:14.5:0.5, added into a beaker in sequence and stirred evenly to obtain a milky white uniform aqueous phase dispersion.

[0088] (2) Oil phase preparation:

[0089] Methyl acrylate, acrylonitrile, tri(2,2,2-trifluoroethyl) phosphate, n-pentane, lauroyl peroxide, and trimethylolpropane trimethacrylate were weighed in a mass ratio of 40:20:3:34:2:1, added into a beaker in sequence, and stirred evenly to obtain an oil phase dispersion.

[0090] (3) Polymerization reaction: under stirring conditions, slowly add dropwise the oil phase dispersion of the system in step (2) to the aqueous phase dispersion of the system in step (1) (the mass ratio of the oil phase dispersion to the aqueous phase dispersion is 20:80), and stir evenly to obtain a pre-suspension;

[0091] The obtained pre-suspension was ultrasonically treated for 30 min and then dried under N 2 Under protection, the prepared solution was added into a high-pressure reactor, pressurized to 0.5 MPa, the polymerization temperature was controlled at 70°C, the stirring speed was 400 r / min, the reaction time was 24 h, and a suspension containing heat-expandable microspheres was obtained.

[0092] (4) Post-treatment: adding sulfuric acid with a volume concentration of 36% to the suspension containing heat-expandable microspheres prepared in step (3), adjusting the pH of the system to 5, filtering, washing with water, and drying to obtain flame-retardant heat-expandable microspheres.

[0093] (5) Preparation of outer layer of shell material: DMF:WANHUA WHT-1570, melamine polyphosphate and flame retardant heat expandable microspheres prepared in step (4) were weighed in a mass ratio of 30:50:1:19. During the stirring process, the weighed materials were added into a high-speed mixer in sequence, and the stirring speed was controlled to 3000 r / min for uniform stirring. The prepared solution was then passed through a spray dryer with the air inlet temperature controlled to 200°C, the air outlet temperature controlled to 170°C, and the speed of the liquid inlet peristaltic pump controlled to 30 r / min to obtain flame retardant thermoplastic polyurethane coated flame retardant heat expandable microspheres.

[0094] Example 3

[0095] A method for preparing flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microspheres comprises the following steps:

[0096] (1) Preparation of aqueous phase:

[0097] Purified water, potassium chloride and sodium dodecyl sulfate were weighed in a mass ratio of 95:4.9:0.1, added into a beaker in sequence and stirred evenly to obtain a milky white uniform aqueous phase dispersion.

[0098] (2) Oil phase preparation:

[0099] Ethyl acrylate, 1,3-succinonitrile, chlorinated paraffin, n-butane, di-tert-butyl peroxide and divinylbenzene were weighed in a mass ratio of 30:30:0.1:39:0.8:0.1, added into a beaker in sequence and stirred evenly to obtain an oil phase dispersion.

[0100] (3) Polymerization reaction: under stirring conditions, slowly add dropwise the oil phase dispersion of the system in step (2) to the aqueous phase dispersion of the system in step (1) (the mass ratio of the oil phase dispersion to the aqueous phase dispersion is 10:90), and stir evenly to obtain a pre-suspension;

[0101] The obtained pre-suspension was ultrasonically treated for 60 min and then dried under N 2 Under protection, the prepared solution was added into a high-pressure reactor, pressurized to 0.8 MPa, the polymerization temperature was controlled at 80°C, the stirring speed was 60 r / min, the reaction time was 16 h, and a suspension containing heat-expandable microspheres was obtained.

[0102] (4) Post-treatment: Add 36% by volume hydrochloric acid to the suspension containing heat-expandable microspheres prepared in step (3), adjust the pH value of the system to 3.5, and obtain flame-retardant heat-expandable microspheres by filtering, washing with water, and drying.

[0103] (5) Preparation of outer layer of shell material: DMAC:WANHUA WHT-1198, tricresol phosphate and flame retardant heat expandable microspheres prepared in step (4) were weighed in a mass ratio of 40:30:1:29. During the stirring process, the weighed materials were added into a high-speed mixer in sequence, and the stirring speed was controlled to 3000 r / min to stir evenly. The prepared solution was then passed through a spray dryer, and the air inlet temperature was controlled to 150°C, the air outlet temperature was controlled to 120°C, and the speed of the liquid inlet peristaltic pump was controlled to 10 r / min to obtain flame retardant thermoplastic polyurethane coated flame retardant heat expandable microspheres.

[0104] Example 4

[0105] A method for preparing flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microspheres comprises the following steps:

[0106] (1) Preparation of aqueous phase:

[0107] Purified water, sodium chloride and polyoxyethylene were weighed in a mass ratio of 90:9:1, added into a beaker in sequence and stirred evenly to obtain a milky white uniform aqueous phase dispersion.

[0108] (2) Oil phase preparation:

[0109] Heptyl acrylate, adiponitrile, tri(1-chloro-2-propyl) phosphate, isopentane, azobisisoheptanenitrile and zinc dimethacrylate were weighed in a mass ratio of 40:20:3:34:2:1, added into a beaker in sequence and stirred evenly to obtain an oil phase dispersion.

[0110] (3) Polymerization reaction: under stirring conditions, slowly add dropwise the oil phase dispersion of the system in step (2) to the aqueous phase dispersion of the system in step (1) (the mass ratio of the oil phase dispersion to the aqueous phase dispersion is 20:80), and stir evenly to obtain a pre-suspension;

[0111] The obtained pre-suspension was ultrasonically treated for 10 min and then dried under N 2 Under protection, the prepared solution was added into a high-pressure reactor, pressurized to 0.3 MPa, the polymerization temperature was controlled at 40°C, the stirring speed was 60 r / min, the reaction time was 16 h, and a suspension containing heat-expandable microspheres was obtained.

[0112] (4) Post-treatment: Add 10% by volume hydrochloric acid to the suspension containing heat-expandable microspheres prepared in step (3), adjust the pH value of the system to 6.5, and obtain flame-retardant heat-expandable microspheres by filtering, washing with water, and drying.

[0113] (5) Preparation of outer layer of shell material: DMF:WANHUA WHT-3385, triphenyl phosphate and flame retardant heat expandable microspheres prepared in step (4) were weighed in a mass ratio of 60:20:0.1:19.9. During the stirring process, the weighed materials were added into a high-speed mixer in sequence, and the stirring speed was controlled to 1500 r / min for uniform stirring. The prepared solution was then passed through a spray dryer with the air inlet temperature controlled to 80°C, the air outlet temperature controlled to 60°C, and the liquid inlet peristaltic pump speed controlled to 10 r / min to obtain flame retardant thermoplastic polyurethane coated flame retardant heat expandable microspheres.

[0114] Comparative Example 1

[0115] Heat-expandable microspheres were prepared by the same method as in Example 1, except that no liquid flame retardant, thermoplastic polyurethane, or oil-soluble flame retardant was added, and no high-speed mixer or spray dryer was used to prepare heat-expandable microspheres without flame retardant effect. The specific preparation process is as follows:

[0116] (1) Preparation of aqueous phase:

[0117] Purified water, sodium hydroxide and sodium lauryl sulfate were weighed in a mass ratio of 80:19:1, added into a beaker in sequence and stirred evenly to obtain a milky white uniform aqueous phase dispersion.

[0118] (2) Oil phase preparation:

[0119] Methyl methacrylate, methacrylonitrile, isobutane, azobisisobutyronitrile and trimethylolpropane triacrylate were weighed in a mass ratio of 40:20:34:2:1, added into a beaker in sequence and stirred evenly to obtain an oil phase dispersion.

[0120] (3) Polymerization reaction: under stirring conditions, slowly add dropwise the oil phase dispersion of the system in step (2) to the aqueous phase dispersion of the system in step (1) (the mass ratio of the oil phase dispersion to the aqueous phase dispersion is 30:70), and stir evenly to obtain a pre-suspension;

[0121] The obtained pre-suspension was ultrasonically treated for 60 min and then dried under N 2 Under protection, the prepared solution was added into a high-pressure reactor, pressurized to 0.6 MPa, the polymerization temperature was controlled at 80°C, the stirring speed was 500 r / min, the reaction time was 24 h, and a suspension containing heat-expandable microspheres was obtained.

[0122] (4) Post-treatment: Add 36% by volume hydrochloric acid to the suspension containing heat-expandable microspheres prepared in step (3), adjust the pH of the system to 3.5, and obtain the heat-expandable microspheres by filtering, washing with water, and drying.

[0123] Comparative Example 2

[0124] The heat-expandable microspheres were prepared by the same method as in Example 1, except that no thermoplastic polyurethane and oil-soluble flame retardant were added, and no high-speed mixer and spray dryer were used. The heat-expandable microspheres with only a core material having a flame retardant effect were prepared. The specific preparation process is as follows:

[0125] (1) Preparation of aqueous phase:

[0126] Purified water, sodium hydroxide and sodium lauryl sulfate were weighed in a mass ratio of 80:19:1, added into a beaker in sequence and stirred evenly to obtain a milky white uniform aqueous phase dispersion.

[0127] (2) Oil phase preparation:

[0128] Methyl methacrylate, methacrylonitrile, triethyl phosphate, isobutane, azobisisobutyronitrile and trimethylolpropane triacrylate were weighed in a mass ratio of 40:20:3:34:2:1, added into a beaker in sequence and stirred evenly to obtain an oil phase dispersion.

[0129] (3) Polymerization reaction: under stirring conditions, slowly add dropwise the oil phase dispersion of the system in step (2) to the aqueous phase dispersion of the system in step (1) (the mass ratio of the oil phase dispersion to the aqueous phase dispersion is 30:70), and stir evenly to obtain a pre-suspension;

[0130] The obtained pre-suspension was ultrasonically treated for 60 min and then dried under N 2 Under protection, the prepared solution was added into a high-pressure reactor, pressurized to 0.6 MPa, the polymerization temperature was controlled at 80°C, the stirring speed was 500 r / min, the reaction time was 24 h, and a suspension containing heat-expandable microspheres was obtained.

[0131] (4) Post-treatment: Add 36% by volume hydrochloric acid to the suspension containing heat-expandable microspheres prepared in step (3), adjust the pH value of the system to 3.5, and obtain flame-retardant heat-expandable microspheres by filtering, washing with water, and drying.

[0132] Comparative Example 3

[0133] The heat-expandable microspheres were prepared by the same method as in Example 1, except that no liquid flame retardant was added, and only the outer layer of the shell material had a flame retardant effect. The specific preparation process was as follows:

[0134] (1) Preparation of aqueous phase:

[0135] Purified water, sodium hydroxide and sodium lauryl sulfate were weighed in a mass ratio of 80:19:1, added into a beaker in sequence and stirred evenly to obtain a milky white uniform aqueous phase dispersion.

[0136] (2) Oil phase preparation:

[0137] Methyl methacrylate, methacrylonitrile, isobutane, azobisisobutyronitrile and trimethylolpropane triacrylate were weighed in a mass ratio of 40:20:34:2:1, added into a beaker in sequence and stirred evenly to obtain an oil phase dispersion.

[0138] (3) Polymerization reaction: under stirring conditions, slowly add dropwise the oil phase dispersion of the system in step (2) to the aqueous phase dispersion of the system in step (1) (the mass ratio of the oil phase dispersion to the aqueous phase dispersion is 30:70), and stir evenly to obtain a pre-suspension;

[0139] The obtained pre-suspension was ultrasonically treated for 60 min and then dried under N 2 Under protection, the prepared solution was added into a high-pressure reactor, pressurized to 0.6 MPa, the polymerization temperature was controlled at 80°C, the stirring speed was 500 r / min, the reaction time was 24 h, and a suspension containing heat-expandable microspheres was obtained.

[0140] (4) Post-treatment: Add 36% by volume hydrochloric acid to the suspension containing heat-expandable microspheres prepared in step (3), adjust the pH of the system to 3.5, and obtain the heat-expandable microspheres by filtering, washing with water, and drying.

[0141] (5) Preparation of outer layer of shell material: DMAC, Desmopan 5377A, tri(2-chloroethyl) phosphate and heat-expandable microspheres prepared in step (4) were weighed in a mass ratio of 40:30:1:29. During the stirring process, the weighed materials were added to a high-speed mixer in sequence, and the stirring speed was controlled to 2000 r / min to stir evenly. The prepared solution was then passed through a spray dryer with the air inlet temperature controlled to 200°C, the air outlet temperature controlled to 170°C, and the speed of the liquid inlet peristaltic pump controlled to 30 r / min to obtain flame-retardant thermoplastic polyurethane coated heat-expandable microspheres.

[0142] Comparative Example 4

[0143] The heat-expandable microspheres were prepared by the same method as in Example 1, except that the amount of the liquid flame retardant in the core material was increased. The specific preparation process was as follows:

[0144] (1) Preparation of aqueous phase:

[0145] Purified water, sodium hydroxide and sodium lauryl sulfate were weighed in a mass ratio of 80:19:1, added into a beaker in sequence and stirred evenly to obtain a milky white uniform aqueous phase dispersion.

[0146] (2) Oil phase preparation:

[0147] Methyl methacrylate, methacrylonitrile, triethyl phosphate, isobutane, azobisisobutyronitrile and trimethylolpropane triacrylate were weighed in a mass ratio of 40:20:7:30:2:1, added into a beaker in sequence and stirred evenly to obtain an oil phase dispersion.

[0148] (3) Polymerization reaction: under stirring conditions, slowly add dropwise the oil phase dispersion of the system in step (2) to the aqueous phase dispersion of the system in step (1) (the mass ratio of the oil phase dispersion to the aqueous phase dispersion is 30:70), and stir evenly to obtain a pre-suspension;

[0149] The obtained pre-suspension was ultrasonically treated for 60 min and then dried under N 2 Under protection, the prepared solution was added into a high-pressure reactor, pressurized to 0.6 MPa, the polymerization temperature was controlled at 80°C, the stirring speed was 500 r / min, the reaction time was 24 h, and a suspension containing heat-expandable microspheres was obtained.

[0150] (4) Post-treatment: Add 36% by volume hydrochloric acid to the suspension containing heat-expandable microspheres prepared in step (3), adjust the pH value of the system to 3.5, and obtain flame-retardant heat-expandable microspheres by filtering, washing with water, and drying.

[0151] (5) Preparation of outer layer of shell material: DMAC, Desmopan 5377A, tri(2-chloroethyl) phosphate and flame retardant heat expandable microspheres prepared in step (4) were weighed in a mass ratio of 40:30:1:29. During the stirring process, the weighed materials were added into a high-speed mixer in sequence, and the stirring speed was controlled to 2000 r / min to stir evenly. The prepared solution was then passed through a spray dryer, and the air inlet temperature was controlled to 200°C, the air outlet temperature was controlled to 170°C, and the speed of the liquid inlet peristaltic pump was controlled to 30 r / min to obtain flame retardant thermoplastic polyurethane coated flame retardant heat expandable microspheres.

[0152] Comparative Example 5

[0153] The heat-expandable microspheres were prepared by the same method as in Example 1, except that the amount of the oil-soluble flame retardant in the outer layer of the shell material was increased. The specific preparation process is as follows:

[0154] (1) Preparation of aqueous phase:

[0155] Purified water, sodium hydroxide and sodium lauryl sulfate were weighed in a mass ratio of 80:19:1, added into a beaker in sequence and stirred evenly to obtain a milky white uniform aqueous phase dispersion.

[0156] (2) Oil phase preparation:

[0157] Methyl methacrylate, methacrylonitrile, triethyl phosphate, isobutane, azobisisobutyronitrile and trimethylolpropane triacrylate were weighed in a mass ratio of 40:20:3:34:2:1, added into a beaker in sequence and stirred evenly to obtain an oil phase dispersion.

[0158] (3) Polymerization reaction: under stirring conditions, slowly add dropwise the oil phase dispersion of the system in step (2) to the aqueous phase dispersion of the system in step (1) (the mass ratio of the oil phase dispersion to the aqueous phase dispersion is 30:70), and stir evenly to obtain a pre-suspension;

[0159] The obtained pre-suspension was ultrasonically treated for 60 min and then dried under N 2 Under protection, the prepared solution was added into a high-pressure reactor, pressurized to 0.6 MPa, the polymerization temperature was controlled at 80°C, the stirring speed was 500 r / min, the reaction time was 24 h, and a suspension containing heat-expandable microspheres was obtained.

[0160] (4) Post-treatment: Add 36% by volume hydrochloric acid to the suspension containing heat-expandable microspheres prepared in step (3), adjust the pH value of the system to 3.5, and obtain flame-retardant heat-expandable microspheres by filtering, washing with water, and drying.

[0161] (5) Preparation of outer layer of shell material: DMAC, Desmopan 5377A, tri(2-chloroethyl) phosphate and flame retardant heat expandable microspheres prepared in step (4) were weighed in a mass ratio of 40:20:11:29. During the stirring process, the weighed materials were added into a high-speed mixer in sequence, and the stirring speed was controlled to 2000 r / min to stir evenly. The prepared solution was then passed through a spray dryer, and the air inlet temperature was controlled to 200°C, the air outlet temperature was controlled to 170°C, and the speed of the liquid inlet peristaltic pump was controlled to 30 r / min to obtain flame retardant thermoplastic polyurethane coated flame retardant heat expandable microspheres.

[0162] Comparative Example 6

[0163] The heat-expandable microspheres were prepared by the same method as in Example 1, except that the amount of thermoplastic polyurethane in the outer layer of the shell material was increased. The specific preparation process was as follows:

[0164] (1) Preparation of aqueous phase:

[0165] Purified water, sodium hydroxide and sodium lauryl sulfate were weighed in a mass ratio of 80:19:1, added into a beaker in sequence and stirred evenly to obtain a milky white uniform aqueous phase dispersion.

[0166] (2) Oil phase preparation:

[0167] Methyl methacrylate, methacrylonitrile, triethyl phosphate, isobutane, azobisisobutyronitrile and trimethylolpropane triacrylate were weighed in a mass ratio of 40:20:3:34:2:1, added into a beaker in sequence and stirred evenly to obtain an oil phase dispersion.

[0168] (3) Polymerization reaction: under stirring conditions, slowly add dropwise the oil phase dispersion of the system in step (2) to the aqueous phase dispersion of the system in step (1) (the mass ratio of the oil phase dispersion to the aqueous phase dispersion is 30:70), and stir evenly to obtain a pre-suspension;

[0169] The obtained pre-suspension was ultrasonically treated for 60 min and then dried under N 2 Under protection, the prepared solution was added into a high-pressure reactor, pressurized to 0.6 MPa, the polymerization temperature was controlled at 80°C, the stirring speed was 500 r / min, the reaction time was 24 h, and a suspension containing heat-expandable microspheres was obtained.

[0170] (4) Post-treatment: Add a 36% by volume hydrochloric acid aqueous solution to the suspension containing the heat-expandable microspheres prepared in step (3), adjust the pH of the system to 3.5, and obtain flame-retardant heat-expandable microspheres by filtering, washing with water, and drying.

[0171] (5) Preparation of outer layer of shell material: DMAC, Desmopan 5377A, tri(2-chloroethyl) phosphate and flame retardant heat expandable microspheres prepared in step (4) were weighed in a mass ratio of 40:60:1:29. During the stirring process, the weighed materials were added into a high-speed mixer in sequence, and the stirring speed was controlled to 2000 r / min to stir evenly. The prepared solution was then passed through a spray dryer, and the air inlet temperature was controlled to 200°C, the air outlet temperature was controlled to 170°C, and the speed of the liquid inlet peristaltic pump was controlled to 30 r / min to obtain flame retardant thermoplastic polyurethane coated flame retardant heat expandable microspheres.

[0172] Comparative Example 7

[0173] The heat-expandable microspheres were prepared by the same method as in Example 1, except that the outer layer of the shell material was prepared directly by stirring in a reactor instead of spray drying in step (5). The specific preparation process is as follows:

[0174] (1) Preparation of aqueous phase:

[0175] Purified water, sodium hydroxide and sodium lauryl sulfate were weighed in a mass ratio of 80:19:1, added into a beaker in sequence and stirred evenly to obtain a milky white uniform aqueous phase dispersion.

[0176] (2) Oil phase preparation:

[0177] Methyl methacrylate, methacrylonitrile, triethyl phosphate, isobutane, azobisisobutyronitrile and trimethylolpropane triacrylate were weighed in a mass ratio of 40:20:3:34:2:1, added into a beaker in sequence and stirred evenly to obtain an oil phase dispersion.

[0178] (3) Polymerization reaction: under stirring conditions, slowly add dropwise the oil phase dispersion of the system in step (2) to the aqueous phase dispersion of the system in step (1) (the mass ratio of the oil phase dispersion to the aqueous phase dispersion is 30:70), and stir evenly to obtain a pre-suspension;

[0179] The obtained pre-suspension was ultrasonically treated for 60 min and then dried under N 2 Under protection, the prepared solution was added into a high-pressure reactor, pressurized to 0.6 MPa, the polymerization temperature was controlled at 80°C, the stirring speed was 500 r / min, the reaction time was 24 h, and a suspension containing heat-expandable microspheres was obtained.

[0180] (4) Post-treatment: Add a 36% by volume hydrochloric acid aqueous solution to the suspension containing the heat-expandable microspheres prepared in step (3), adjust the pH of the system to 3.5, and obtain flame-retardant heat-expandable microspheres by filtering, washing with water, and drying.

[0181] (5) Preparation of outer layer of shell material: DMAC, Desmopan 5377A, tri(2-chloroethyl) phosphate and flame-retardant heat-expandable microspheres prepared in step (4) were weighed in a mass ratio of 40:30:1:29. During the stirring process, the weighed materials were added into a high-speed mixer in sequence. The stirring speed was controlled to 2000 r / min and the mixture was stirred evenly. The mixture was heated to 70° C. and the reaction time was 1.5 h to obtain a composite block sample of thermoplastic polyurethane and heat-expandable microspheres.

[0182] The heat-expandable microspheres prepared in the above examples and comparative examples were tested for performance.

[0183] 1. Scanning electron microscope test:

[0184] The flame retardant thermoplastic polyurethane coated flame retardant heat expandable microspheres prepared in Example 1 and the heat expandable microspheres prepared in Comparative Example 1 were tested by scanning electron microscope. The test results are as follows: Figure 1 , Figure 2 ,from Figure 1 and Figure 2It can be seen that after coating with thermoplastic polyurethane and oil-soluble flame retardant, the particle size of the heat-expandable microspheres increased significantly, proving the effective preparation of the outer layer of the flame-retardant shell material.

[0185] 2. Foaming performance test:

[0186] Take the heat-expandable microspheres prepared in the above Examples 1-4 and Comparative Examples 1-7, and take the unexpanded and expanded heat-expandable microspheres respectively for testing by a particle size analyzer, with the stirring rate set to 2500 r / min, the refractive index to 1.48, and ultrasound before the test. The test results are shown in Table 1 below.

[0187] Among them, the formula for the expansion ratio of thermally expandable microspheres is: expansion ratio = expanded particle size / unexpanded particle size.

[0188] Table 1 Thermal expansion microsphere foaming performance parameters

[0189]

[0190] 3. Flame retardant performance test:

[0191] The heat-expandable microspheres prepared in the above Examples 1-4 and Comparative Examples 1-7 were used in textiles. The specific test methods involved were: GB / T 5454-1997 (Textile Combustion Performance Test Oxygen Index Method); GB / T 5455-2014 (Textile Combustion Performance Test - Determination of Vertical Damage Length, Smoldering and Afterflaming Time). The specific test results are shown in Table 2 below.

[0192] Table 2 Limiting oxygen index parameters of thermal expansion microspheres

[0193]

[0194] It can be seen from the above data that the heat-expandable microspheres prepared by the preparation method of the present invention in Examples 1-4 effectively improve the flame retardant performance and thermal stability of the heat-expandable microspheres while maintaining the foaming size of the heat-expandable microspheres.

[0195] From the comparison of the data of Example 1 and Comparative Example 1, it can be seen that the heat-expandable microspheres prepared by the preparation method of the present invention have very good flame retardant properties and can maintain good foaming properties.

[0196] From the comparison of the data of Example 1 and Comparative Example 2, it can be seen that if thermoplastic polyurethane and oil-soluble flame retardant are not added, the flame retardant performance of the heat-expandable microspheres is greatly reduced.

[0197] From the data comparison of Example 1 and Comparative Example 3, it can be seen that if no liquid flame retardant is added, the flame retardant performance of the heat-expandable microspheres will also be significantly reduced. Therefore, using both liquid flame retardant and oil-soluble flame retardant when preparing heat-expandable microspheres is more conducive to obtaining flame-retardant heat-expandable microspheres with excellent performance.

[0198] From the comparison of the data of Example 1 and Comparative Example 4, it can be seen that if the proportion of the liquid flame retardant in the core material is increased, the proportion of the foaming agent is reduced, the foaming ratio of the heat-expandable microspheres is reduced, and the flame retardant performance is not significantly improved.

[0199] From the data comparison of Example 1 and Comparative Example 5, it can be seen that if the usage ratio of the oil-soluble flame retardant in the outer layer of the shell material is increased, the bonding strength between the thermoplastic polyurethane and the heat-expandable microspheres will be affected, and the thermoplastic polyurethane / oil-soluble flame retardant will be easily peeled off from the heat-expandable microspheres, so the flame retardant performance of the prepared microspheres will be significantly reduced.

[0200] From the comparison of the data of Example 1 and Comparative Example 6, it can be seen that if the usage ratio of the thermoplastic polyurethane in the outer layer of the shell material is increased, the foaming ratio of the heat-expandable microspheres will be seriously affected due to the excessive thickness of the outer layer of the shell material.

[0201] From the comparison of the data of Example 1 and Comparative Example 7, it can be seen that if the outer layer of the shell material is not prepared by spray drying, the heat-expandable microspheres and the thermoplastic polyurethane form a block sample, the heat-expandable microspheres cannot be separated from each other, the particle size and foaming ratio of a single microsphere cannot be effectively measured, and the flame retardant performance cannot be measured.

[0202] 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 exhaustively listed. 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.

[0203] For those skilled in the art, several modifications and improvements may be made without departing from the concept of the present invention, all of which belong to the protection scope of the present invention. The protection scope of the present invention shall be based on the attached claims.

Claims

1. A method for preparing flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microspheres, characterized in that: The preparation method is: S1. Preparation of aqueous dispersion and oil dispersion: Purified water, electrolyte and emulsifier are mixed evenly to obtain a uniform aqueous phase dispersion; The acrylate monomer, the nitrile monomer, the liquid flame retardant, the hydrocarbon, the initiator and the cross-linking agent are uniformly mixed to obtain an oil phase dispersion; S2. Polymerization reaction: Under stirring conditions, the oil phase dispersion is added dropwise to the water phase dispersion to obtain a pre-suspension, and a polymerization reaction is carried out under inert atmosphere and pressure conditions to obtain a suspension containing heat-expandable microspheres; S3, post-processing: The suspension prepared in step S3 is adjusted to an acidic pH, filtered, washed with water, and dried to obtain flame-retardant heat-expandable microspheres; S4. Preparation of outer layer of shell material: The solvent, thermoplastic polyurethane, oil-soluble flame retardant, and flame-retardant heat-expandable microspheres are mixed by high-speed stirring, and then passed through a spray dryer to obtain flame-retardant thermoplastic polyurethane-coated flame-retardant heat-expandable microspheres; In step S1, the oil phase dispersion contains 20-40 parts of acrylate monomers, 20-40 parts of nitrile monomers, 0.1-3 parts of liquid flame retardants, 15-40 parts of hydrocarbons, 0.3-2 parts of initiators, and 0.1-1 parts of cross-linking agents, in parts by weight; The weight ratio of the aqueous dispersion to the oil dispersion in step S2 is (70-90):(10-30); In step S4, the amount of solvent used is 30 to 60 parts by weight; the amount of thermoplastic polyurethane used is 10 to 50 parts by weight; the amount of oil-soluble flame retardant used is 0.1 to 3 parts by weight; and the amount of flame-retardant thermally expandable microspheres used is 10 to 50 parts by weight.

2. The method for preparing flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microspheres according to claim 1, characterized in that: The oil-soluble flame retardant is at least one of triphenyl phosphate, tert-butylated triphenyl phosphate, tributyl phosphate, triisobutyl phosphate, tris(2-chloroethyl) phosphate, dimethyl methyl phosphate, pentaerythritol phosphate, resorcinol bis(diphenyl phosphate), tricresyl phosphate, phosphite, tetraphenyl dipropylene glycol diphosphite, melamine cyanurate, and melamine polyphosphate; The thermoplastic polyurethane is at least one of Desmopan 2792A, Desmopan 5377A, WANHUA WHT-1570, WANHUAWHT-1190, WANHUA WHT-1198, WANHUA WHT-1164IC, and WANHUA WHT-3385.

3. The method for preparing flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microspheres according to claim 1, characterized in that: The liquid flame retardant is at least one of triethyl phosphate, tris(1-chloro-2-propyl) phosphate, chlorinated paraffin, 2-ethylhexyl diphenyl phosphate, triisopropylphenyl phosphate, tris(hexafluoroisopropyl) phosphate, tris(2,2,2-trifluoroethyl) phosphate, ethyl polyphosphate, tris(2-chloroethyl) phosphate, and chloroalkyl polyphosphate; The hydrocarbon is at least one of n-pentane, isopentane, neopentane, n-butane, isobutane, n-hexane and isooctane.

4. The method for preparing flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microspheres according to claim 1, characterized in that: The acrylic acid ester monomer is at least one of methyl acrylate, ethyl acrylate, butyl acrylate, isobutyl acrylate, isooctyl acrylate, heptyl acrylate, benzyl acrylate, styrene acrylate, acrylamide, fluorinated acrylate, methyl methacrylate, acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate, and acrylamide; The nitrile monomer is at least one of acetonitrile, acrylonitrile, methacrylonitrile, adiponitrile, suberonitrile, 4-aminobenzonitrile, phthalonitrile, benzonitrile, isocyanate nitrile, 1,3-succinonitrile and benzonitrile.

5. The method for preparing flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microspheres according to claim 1, characterized in that: The electrolyte is at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide, sodium chloride, magnesium chloride, potassium chloride, and calcium chloride; The emulsifier is at least one of sodium stearate, sodium dodecyl sulfate, sodium hexadecyl sulfate, polyoxyethylene, sodium disproportionate rosin acid, and sodium lauryl sulfate; The initiator is at least one of azobisisobutyronitrile, azobisisoheptanenitrile, azobismethylvaleronitrile, benzoyl peroxide, lauroyl peroxide, cumene hydroperoxide, tert-butyl hydroperoxide, di-tert-butyl peroxide, diisopropylbenzene peroxide, tert-butyl perbenzoate, tert-butyl pervalerate, methyl ethyl ketone peroxide, cyclohexanone peroxide, diisopropyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, potassium persulfate, and ammonium persulfate; The crosslinking agent is at least one of trimethylolpropane trimethacrylate, 1,4-butanediol dimethacrylate, trimethylolpropane triacrylate, ethylene glycol dimethacrylate, zinc dimethacrylate, zinc diacrylate, divinylbenzene, and divinyl ether; The solvent is at least one of DMAC or DMF.

6. The method for preparing flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microspheres according to claim 1, characterized in that: In step S2, the polymerization reaction process is: subjecting the pre-suspension to ultrasonic treatment for 10 to 60 minutes; then placing the pre-suspension in a high-pressure reactor, pressurizing it to 0.3 to 0.8 MPa, stirring at a speed of 60 to 500 r / min, controlling the polymerization temperature to 40 to 80° C., and the polymerization reaction time to 16 to 24 hours; In step S3, the pH of the suspension is adjusted to 3-6.5 using hydrochloric acid or sulfuric acid.

7. The method for preparing flame-retardant thermoplastic polyurethane coated flame-retardant heat-expandable microspheres according to claim 1, characterized in that: In step S4, the high-speed stirring speed is 1500-3000 r / min; The spray dryer has an air inlet temperature of 80-230°C, an air outlet temperature of 60-210°C, and a liquid inlet peristaltic pump speed of 10-100 r / min.

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