Thermal expansion microsphere as well as preparation method and application thereof

Abstract

The invention relates to the technical field of polymer microspheres, in particular to a thermal expansion microsphere and a preparation method and application thereof. The thermal expansion microsphere comprises a polymer shell and an inner core coated in the shell, the inner core comprises a foaming agent, and the thermal expansion microsphere is characterized in that the polymer shell is formed by polymerization reaction of a monofunctional monomer and a cross-linking agent under the initiation of an initiator; wherein the difference value between the 10-hour half-life period temperature of the initiator and the peak exothermic temperature of the cross-linking agent is 60-81 DEG C. By controlling the decomposition activity of the initiator and the reaction activity of the cross-linking agent to be matched in a specific temperature difference range, construction of a uniform cross-linked network between shell polymer chain segments is achieved, and the obtained thermal expansion microspheres have a wide expansion temperature range, good heat resistance, stable expansion behavior and high expansion rate.

Description

Technical Field

[0001] This invention relates to the field of polymer microsphere technology, specifically to a thermally expandable microsphere, its preparation method, and its application. Background Technology

[0002] Thermally expandable microspheres, as a core-shell structured functional material, consist of a thermoplastic polymer outer shell and a core coated with a low-boiling-point foaming agent. Upon heating, the foaming agent vaporizes and propels the outer shell to expand, thus endowing the material with lightweight, sound-insulating, and heat-insulating properties, making it widely used in aerospace, automotive, construction, and energy fields. Currently, these microspheres are mainly prepared via suspension polymerization, with their expansion performance optimized by controlling monomer composition, foaming agent type, and polymerization process parameters. However, in practical applications, it has been found that the cross-linking structure of the polymer outer shell has a crucial impact on its thermal expansion behavior: if the cross-linking is too high, the outer shell becomes too rigid, delaying the expansion initiation temperature and limiting the expansion ratio; if the cross-linking is too low or unevenly distributed, the microspheres are prone to excessive softening or rupture at higher temperatures, resulting in a narrow expansion temperature range and insufficient heat resistance. Therefore, achieving a balance between a wide expansion temperature range and excellent heat resistance while maintaining a high expansion ratio has become a significant challenge in this field for further improving the overall performance of thermally expandable microspheres.

[0003] While existing technologies have attempted to control the shell structure by selecting different monomer combinations, adding crosslinking agents, or adjusting the initiation system, they often focus on improving a single property, such as reducing residual monomers or improving the stability of the foaming agent coating. Effective control over the uniformity of crosslinking in the polymer shell remains insufficient. In particular, if the reactivity of the initiator and crosslinking agent during polymerization is not well matched, uneven distribution of crosslinking points can easily occur, affecting the stable expansion and heat resistance of the microspheres over a wide temperature range. Therefore, there is an urgent need to develop thermally expandable microspheres capable of constructing a uniform crosslinked network within the shell to synergistically improve their expansion temperature range, expansion ratio, and heat resistance. Summary of the Invention

[0004] This invention provides a thermally expandable microsphere, its preparation method, and its application, to solve the problems of existing thermally expandable microspheres having a narrow expansion temperature range and difficulty in simultaneously achieving both heat resistance and expansion ratio due to uneven cross-linking network in the outer shell.

[0005] In a first aspect, the present invention provides a thermally expandable microsphere comprising a polymer shell and a core encapsulated within the shell, the core comprising a foaming agent, the polymer shell being formed by a polymerization reaction of a monofunctional monomer and a crosslinking agent under the initiation of an initiator; wherein the difference between the 10-hour half-life temperature of the initiator and the peak exothermic temperature of the crosslinking agent is 60°C to 81°C.

[0006] In this invention, the 10-hour half-life temperature of the initiator refers to the time required for the initiator to decompose to half its initial concentration at this temperature, which determines the main temperature range of the polymerization reaction. The peak exothermic temperature of the crosslinking agent can be obtained by differential scanning calorimetry (DSC), reflecting the temperature point at which its polymerization reaction is most intense. By controlling the temperature difference between the two within the range of 60℃ to 81℃, the rate at which the initiator decomposes to generate free radicals at a suitable temperature is well matched with the activity of the crosslinking agent in the crosslinking reaction. If the temperature difference is too small, the crosslinking agent's reactivity is too high, which can easily lead to excessive local crosslinking density in the early stage of polymerization, causing "explosive polymerization," which hinders the subsequent diffusion of monomers, resulting in an uneven crosslinking network in the outer shell, excessive rigidity of the microspheres, and poor expansion performance. If the temperature difference is too large, the crosslinking agent's reactivity is relatively lagging, which may lead to insufficient crosslinking degree in the later stage of polymerization, resulting in a decrease in the heat resistance and barrier performance of the outer shell. The temperature range defined by this invention precisely coordinates the generation of free radicals, the consumption of monofunctional monomers, and the formation rate of the three-dimensional network, thereby gradually and uniformly forming crosslinking points throughout the polymerization process, ultimately constructing a polymer shell with a uniformly distributed degree of crosslinking. This uniform network structure endows the microsphere shell with suitable strength, toughness, and thermal stability, enabling the thermally expandable microspheres to expand smoothly and continuously when heated, exhibiting a wide expansion temperature range, a high expansion ratio, and good heat resistance.

[0007] In one optional embodiment, the crosslinking agent comprises a bifunctional and / or multifunctional monomer; the bifunctional or multifunctional crosslinking agent can form effective crosslinking points between polymer chains to construct a three-dimensional network structure. And / or, based on 100 parts by weight of the total mass of the monofunctional monomers, the amount of the crosslinking agent is 0.01 to 5 parts by weight; for example, it can be 0.05 parts, 0.1 parts, 0.3 parts, 0.5 parts, 0.8 parts, 1 part, 2 parts, 3 parts, 4 parts, or 4.5 parts. Within this range, it is possible to ensure the formation of a sufficiently dense crosslinked network to provide the necessary mechanical strength and heat resistance, while avoiding excessive crosslinking of the shell due to excessive crosslinking agent, resulting in decreased elasticity and affecting the ability of the foaming agent to expand the shell through vaporization. Preferably, the amount of the crosslinking agent is 0.1 to 2 parts by weight, more preferably 0.2 to 1 part by weight; And / or, the initiator includes organic peroxide initiators and / or azo initiators; these are conventional free radical initiators suitable for suspension polymerization of olefinic unsaturated monomers; And / or, based on 100 parts by weight of the total mass of the monofunctional monomers, the amount of the initiator is 0.01 to 5 parts by weight. For example, it can be 0.05 parts, 0.1 parts, 0.3 parts, 0.5 parts, 0.8 parts, 1 part, 2 parts, 3 parts, 4 parts, or 4.5 parts. A suitable amount of initiator ensures that the polymerization reaction proceeds at an appropriate rate, avoiding runaway reaction due to excessive speed or insufficient conversion due to excessively slow reaction. Preferably, the amount of the initiator is 0.1 to 3 parts by weight.

[0008] In one optional embodiment, the monofunctional monomer includes at least two selected from acrylonitrile monomers, (meth)acrylate monomers, (meth)acrylate monomers, alkyl vinyl ester monomers, styrene monomers, olefin monomers, and acrylamide monomers; preferably, the monofunctional monomer includes at least two selected from acrylonitrile monomers; more preferably, the monofunctional monomer includes acrylonitrile and at least one selected from methacrylonitrile, α-chloroacrylonitrile, α-ethoxyacrylonitrile, and fumaric acid. By copolymerizing at least two monofunctional monomers, the comprehensive properties of the final polymer shell, such as glass transition temperature, toughness, film-forming properties, and chemical resistance, can be adjusted. Acrylonitrile monomer copolymers typically provide good heat resistance, solvent resistance, and film-forming properties, making them a preferred raw material combination for preparing high-performance thermally expandable microsphere shells.

[0009] In one optional embodiment, the acrylonitrile content in the monofunctional monomer is 20% to 80% by mass. This acrylonitrile content helps to impart high heat resistance and strength to the shell while maintaining a certain degree of polymer flexibility, which is crucial for achieving stable expansion at high temperatures. Preferably, the acrylonitrile content is 30% to 80% by mass.

[0010] In one optional embodiment, the (meth)acrylate monomers include, but are not limited to, 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.

[0011] In one alternative embodiment, the (meth)acrylic monomers include, but are not limited to, methacrylic acid and acrylic acid.

[0012] In one alternative embodiment, the alkyl vinyl ester monomers include, but are not limited to, vinyl acetate, vinyl laurate, and vinyl stearate.

[0013] In one alternative embodiment, the styrene monomers include, but are not limited to, styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, ethylstyrene, and halostyrene.

[0014] In one alternative embodiment, the olefin monomer includes, but is not limited to, halogenated ethylene, unsymmetrical ethylene, and dihalogenated ethylene.

[0015] In one optional embodiment, the acrylamide monomer includes, but is not limited to, acrylamide, N-isopropylacrylamide, and methacrylamide.

[0016] In an optional embodiment, the crosslinking agent includes, but is not limited to, at least one of trimethylolpropane triacrylate, ethylene glycol dimethacrylate, diallyl phthalate, pentaerythritol triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triacrylate, pentaerythritol trimethacrylate, trimethylolpropane triacrylate, 1,6-hexanediol diacrylate, polyethylene glycol di(meth)acrylate, diallyl carbonate, triallyl isocyanate, triallyl isocyanurate, trimethylolpropane diallyl ether, trimethylallyl isocyanate, pentaerythritol triallyl ether, and 2,2-bis(allyloxymethyl)-1-butanol.

[0017] In one optional embodiment, the initiator includes, but is not limited to, peroxide carboxylic acid esters such as tert-butyl peroxyacetate, di(2-ethylhexyl) peroxydicarbonate, benzoyl peroxide, tert-amyl peroxypentanoate, tert-butyl peroxybenzoate, dilauroyl peroxide, and di-tert-butyl peroxyphthalate; tert-butylperoxyisopropyl monocarbonate, tert-butylperoxyisopropyl carbonate, tert-amylperoxyisopropyl carbonate, and 1,6-bis(tert-butylperoxycarbonyloxy) Hexane and other peroxide carbonates; dialkyl peroxides such as 2,5-dimethyl-2,5-di-tert-butylperoxy-3-hexyne, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, and 1,3-bis(tert-butylperoxyisopropyl)benzene; 2,2-di(tert-butylperoxy)butane, 1,1-di-tert-butylperoxycyclohexane, ethyl 3,3-di(tert-butylperoxy)butyrate, and 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane. Hexane, 1,1-di-tert-hexylcyclohexane peroxide, butyl-4,4-bis(tert-butylperoxy)valerate, 1,1-bis(tert-hexylperoxy)-3,3,5-trimethylcyclohexane, 2,2-di-[4,4-di(tert-butylperoxy)cyclohexyl]propane, etc.; ketal peroxides such as ketal peroxide and methyl ethyl ketone peroxide; tert-butyl hydroperoxide, tert-pentyl hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, cumene hydroperoxide, allyl tert-butyl... Hydroperoxides such as peroxycarbonate, dicumyl peroxide, and 3,3',4,4'-tetra(tert-butylperoxycarbonyl)benzophenone; and at least one of azo initiators such as 2,2'-azobis(4-methoxy-2,4-dimethylpentanonitrile), 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylpentanonitrile), 2,2'-azobis(2-methylpropionic acid) dimethyl ester, and 2,2'-azobis(2-methylbutyronitrile).

[0018] In one alternative embodiment, the foaming agent is a liquid substance that can be vaporized under heating conditions.

[0019] In one optional embodiment, the blowing agent is a C3-C13 alkane blowing agent, such as C3, C4, C5, C6, C7, C8, C9, C10, C11, C12, and C13 alkane blowing agents; the blowing agent includes, but is not limited to, pentane, hexane, heptane, octane, nonane, decane, undecane, dodecane, and tridecane, etc., and the aforementioned alkanes include all feasible isomers; for example, pentane can be n-pentane, isopentane, neopentane, etc. Optionally, the C3-C13 alkane blowing agent includes, but is not limited to, at least one of isopentane, neopentane, n-pentane, n-hexane, heptane, isooctane, n-octane, and petroleum ether; these alkanes have suitable boiling points and are easily vaporized within the softening temperature range of the polymer shell to generate sufficient expansion internal pressure.

[0020] In one optional embodiment, the median volumetric particle size D50 of the thermally expanded microspheres is 1~100μm; for example, it can be 5μm, 10μm, 20μm, 30μm, 40μm, 50μm, 60μm, 80μm, or 90μm. This particle size range is suitable for most applications and is easy to disperse and use. Preferably, the particle size D50 is 5~50μm; And / or, the initial expansion temperature of the thermally expanding microspheres is not lower than 125°C, preferably not lower than 130°C; a higher initial expansion temperature means that the microspheres have good dimensional stability during processing and are suitable for processes that require a certain preheating step. And / or, the maximum expansion temperature of the thermally expandable microspheres is not less than 180°C. A higher maximum expansion temperature indicates that the polymer of the microsphere shell has good heat resistance, can maintain structural integrity and continue to expand at higher temperatures, thereby broadening its applicable processing temperature window.

[0021] Secondly, the present invention also provides a method for preparing thermally expandable microspheres as described above, comprising the following steps: A monofunctional monomer, a crosslinking agent, a foaming agent, and an initiator are mixed to form an oil phase mixture; wherein the difference between the 10-hour half-life temperature of the initiator and the peak exothermic temperature of the crosslinking agent is 60℃~81℃. The oil phase mixture is dispersed in an aqueous dispersion medium to form a suspension; The suspension is heated to undergo a polymerization reaction, forming the thermally expanded microspheres.

[0022] The key to this invention lies in the use of a combination of initiator and crosslinking agent in the oil phase mixture that meets specific temperature difference requirements. During the subsequent suspension polymerization process, by controlling the polymerization temperature within the suitable activity range of the initiator (typically near its 10-hour half-life temperature), the rate of free radical generation matches the reactivity of the crosslinking agent, thereby achieving uniform crosslinking of the polymer shell. This invention employs a one-step suspension polymerization method to prepare the thermally expandable microspheres. During polymerization, monofunctional monomers and crosslinking agents copolymerize under the action of the initiator. The resulting polymer exhibits reduced solubility in the foaming agent (low-boiling-point alkanes), leading to phase separation within the tiny oil-phase droplets dispersed in the aqueous phase. The precipitated polymer tends to accumulate and deposit at the oil-water interface. As the polymerization reaction continues, it gradually solidifies on the droplet surface, forming a dense crosslinked polymer shell, while simultaneously encapsulating the foaming agent within, ultimately forming thermally expandable microspheres with a core-shell structure in situ.

[0023] In one optional embodiment, the foaming agent in the oil phase mixture has a mass percentage content of 20% to 30%; for example, it can be 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, or 29%. This content range ensures that there is sufficient foaming agent to provide expansion power, while avoiding the difficulty of coating during the preparation process or affecting the integrity of the shell due to excessive content; And / or, the oil phase mixture further comprises a gas migration inhibitor, preferably, the gas migration inhibitor comprising at least one of hydrocarbon compounds, carboxylic acid compounds, ester compounds, and alcohol compounds; adding a gas migration inhibitor helps reduce the permeation rate of the blowing agent through the polymer shell, improving the storage stability of the microspheres; optionally, the amount of gas migration inhibitor is no more than 30 parts by weight per 100 parts by weight of the blowing agent. An appropriate amount of gas migration inhibitor can effectively improve the long-term encapsulation stability of the microspheres without significantly affecting the function of the blowing agent; And / or, the aqueous dispersion medium comprises water, a stabilizer, an electrolyte, and a polymerization inhibitor.

[0024] In one optional embodiment, the stabilizer includes at least one 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; the stabilizer is used to adsorb onto the surface of the oil phase droplets to prevent them from coalescing during the polymerization process. And / or, based on 100 parts by weight of water, the amount of the stabilizer is 1 to 21 parts by weight; for example, it can be 2 parts, 5 parts, 8 parts, 10 parts, 15 parts or 19 parts, preferably 5 to 21 parts by weight. And / or, the electrolyte is a metal salt; optionally, the metal salt includes at least one of sodium chloride, potassium chloride, magnesium chloride, calcium chloride, sodium sulfate, potassium sulfate, magnesium sulfate, sodium carbonate, potassium carbonate, sodium nitrate, and potassium nitrate; the addition of the electrolyte can reduce the solubility of the monomer in the aqueous phase and contribute to the dispersion stabilization of the stabilizer. And / or, based on 100 parts by weight of water, the amount of the electrolyte is 1 to 50 parts by weight; for example, it can be 5 parts, 10 parts, 15 parts, 20 parts, 25 parts, 30 parts, 40 parts, or 48 parts. Preferably, it is 11 to 35 parts by weight; And / or, the polymerization inhibitor includes at least one of sodium nitrite, potassium nitrite, ferric chloride, sodium sulfide, potassium dichromate, cuprous chloride, copper sulfate, titanium trichloride, sodium sulfate, and ammonium thiocyanate; the polymerization inhibitor is used to inhibit the homopolymerization of monomers in the aqueous phase; And / or, based on 100 parts by weight of water, the amount of the polymerization inhibitor is 0.01 to 5 parts by weight. For example, it can be 0.05 parts, 0.1 parts, 0.5 parts, 1 part, 2 parts, 3 parts or 4.5 parts. Preferably, it is 0.05 to 1 part by weight.

[0025] In one alternative embodiment, the pH of the aqueous dispersion medium is 1 to 7; for example, it can be 2, 3, 4, 5, or 6. Adjusting the pH (e.g., with hydrochloric acid) to an acidic range facilitates the optimal function of certain stabilizers (such as colloidal silica) and affects the polymerization rate. Preferably, the pH is 3 to 5. And / or, in the suspension, the mass of the oil phase mixture accounts for 20% to 35% of the total mass of the oil phase mixture and the aqueous dispersion medium, for example, it can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, or 35%. This solid content range takes into account both production efficiency and dispersion stability, and is preferably 25% to 30%. And / or, the polymerization reaction is carried out under an inert atmosphere (such as nitrogen or argon) to eliminate oxygen interference; And / or, the pressure of the polymerization reaction is 0.3~0.8 MPa; for example, it can be 0.4 MPa, 0.5 MPa, 0.6 MPa or 0.7 MPa. Maintaining a certain pressure helps prevent the low-boiling-point blowing agent from vaporizing prematurely at the polymerization temperature; And / or, the polymerization reaction temperature is 50~80°C, preferably 52~76°C; for example, it can be 55°C, 60°C, 65°C, 70°C, or 75°C. This reaction temperature range ensures that polymerization takes place within a suitable activity range of the initiator. Preferably, the polymerization reaction temperature is 5~15°C higher than the 10-hour half-life temperature of the selected initiator. By correlating the reaction temperature with this half-life temperature, the rate of free radical generation can be effectively controlled, thereby matching the reactivity of the crosslinking agent, ensuring a stable and controllable polymerization process, and ultimately forming a uniform crosslinked network on the polymer shell.

[0026] And / or, the polymerization reaction takes 15 to 25 hours; for example, it can be 16 hours, 18 hours, 20 hours, 22 hours or 24 hours to ensure sufficient conversion of monomers; And / or, the polymerization reaction may further include a post-processing step of filtering and drying the product to obtain a dried thermally expanded microsphere product.

[0027] In one alternative embodiment, the oil phase mixture is dispersed in an aqueous dispersion medium by high-speed shearing (e.g., rotation speed of 500-8000 rpm). The polymerization reaction is carried out under stirring conditions.

[0028] Thirdly, the present invention also provides the application of the thermally expanding microspheres described above in printing and dyeing, coatings, inks, polyurethane polishing materials, shoe soles, sound-absorbing materials or thermal insulation materials.

[0029] The thermally expandable microspheres provided by this invention possess a wide expansion temperature range, good high-temperature dimensional stability, and a high expansion ratio due to their uniform cross-linked network structure on the outer shell. They can be used as 3D printing or foaming coating agents in printing and dyeing and coating industries; for preparing Braille or anti-counterfeiting patterns in inks; as lightweight fillers in polyurethane polishing materials; to reduce weight and improve cushioning comfort in shoe soles; and to prepare lightweight, porous, high-performance composite materials in sound-absorbing and thermal insulation materials. Their excellent comprehensive performance meets the higher requirements of these fields for lightweight, functional, and processable materials.

[0030] The technical solution of this invention has the following advantages: The thermally expandable microspheres provided by this invention achieve precise matching between the initiator decomposition kinetics and the crosslinking agent reactivity during polymerization by controlling the difference between the 10-hour half-life temperature of the initiator and the peak exothermic temperature of the crosslinking agent within a specific range of 60℃ to 81℃. This matching allows for synergistic generation of active free radicals and spatial distribution of crosslinking points, thereby constructing a uniform and stable three-dimensional crosslinked network throughout the polymer shell. This uniform network structure effectively overcomes the defects of localized brittleness or weakness caused by uneven crosslinking, enabling the thermally expandable microspheres to obtain stable and continuous expansion force when heated, while maintaining suitable strength and ductility of the shell over a wide temperature range. Its direct technical effects are: significantly widening the expansion temperature window of the microspheres (expansion temperature range ≥ 60℃), while maintaining high volume expansion capacity (expansion ratio ≥ 150), and endowing the microspheres with excellent heat resistance stability. Therefore, this invention successfully solves the technical problems of existing thermally expandable microspheres, which suffer from narrow expansion temperature range and difficulty in simultaneously achieving heat resistance and high expansion ratio due to uneven crosslinking network in the shell. Detailed Implementation

[0031] The following embodiments are provided to better understand the present invention, but the following embodiments do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the scope of protection of the present invention.

[0032] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0033] All materials and reagents used in the following embodiments and comparative examples are commercially available. The raw materials and testing methods involved are conventionally chosen in the art. To facilitate understanding of the key thermodynamic parameters of the initiator and crosslinking agent in this invention, the relevant characteristic temperatures of the main initiators and crosslinking agents used in this section are listed below: The initiator di(2-ethylhexyl) peroxide dicarbonate (EHP) has a 10-hour half-life temperature of 47°C.

[0034] The initiator azobisisobutyronitrile (AIBN) has a 10-hour half-life temperature of 65°C.

[0035] The initiator benzoyl peroxide (BPO) has a 10-hour half-life temperature of 71°C.

[0036] The initiator dilauroyl peroxide (LPO) has a 10-hour half-life temperature of 61°C.

[0037] The peak exothermic temperature of the crosslinking agent pentaerythritol triacrylate (PETA) is 121°C (obtained by DSC testing).

[0038] The peak exothermic temperature of the crosslinking agent ethylene glycol dimethacrylate (EGDMA) is 142°C (obtained by DSC testing).

[0039] The peak exothermic temperature of the crosslinking agent diallyl phthalate (DAP) is 187°C (obtained by DSC testing).

[0040] Specific information about some of the raw materials is as follows: (1) Alkene unsaturated monomers Methacrylonitrile, MAN; Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%; Acrylonitrile, AN, Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%; Methyl methacrylate (MMA), Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%.

[0041] (2) Crosslinking agent Ethylene glycol dimethacrylate, EGDMA, Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%; diallyl phthalate (DAP), Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%; Pentaerythritol triacrylate, PETA, Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%.

[0042] (3) Initiator Di(2-ethylhexyl) percarbonate, EHP, Nouryon Chemicals Ltd., reagent grade 50%; Azobisisobutyronitrile (AIBN), Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%; Benzoyl peroxide, BPO, Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%; Dilauroyl peroxide, LPO, Anhui Zesheng Technology Co., Ltd., reagent grade 99%.

[0043] (4) Foaming agent Isopentane, Aladdin Group Co., Ltd., reagent grade 99%; Isooctane, Aladdin Group Ltd., reagent grade 99%.

[0044] (5) Stabilizer Colloidal silica, containing 25 wt% silica as the active ingredient, Shandong Kehan ​​Silicon Products Co., Ltd.

[0045] (6) Electrolytes Sodium chloride, Shenghai Chemical Co., Ltd., industrial grade 99%.

[0046] (7) Polymerization inhibitor Sodium nitrite, Shanghai Aladdin Biochemical Technology Co., Ltd., reagent grade 99%.

[0047] In the following specific embodiments of the present invention, unless otherwise specified, "parts" and "%" are based on mass.

[0048] Example 1 This embodiment provides a thermally expandable microsphere and its preparation method, the specific steps of which are as follows: (1) Preparation of aqueous dispersion medium: Add 150 parts by weight of sodium chloride, 80 parts by weight of colloidal silica (the effective content of silica is 25wt%) and 2 parts by weight of sodium nitrite to 800 parts by weight of deionized water. After stirring and mixing thoroughly, adjust the pH of the system to 4 with hydrochloric acid to obtain the aqueous dispersion medium. (2) Preparation of the oil phase mixture: 200 parts by weight of acrylonitrile (AN), 85 parts by weight of methacrylonitrile (MAN), 40 parts by weight of methyl methacrylate (MMA), 30 parts by weight of isopentane, 50 parts by weight of isooctane, 2 parts by weight of crosslinking agent pentaerythritol triacrylate (PETA), and 1.5 parts by weight of initiator di(2-ethylhexyl) peroxide dicarbonate (EHP) were mixed and stirred evenly to obtain the oil phase mixture. The 10-hour half-life temperature of the initiator EHP was 47℃, and the peak exothermic temperature of the crosslinking agent PETA was 121℃, with a temperature difference of 74℃ between the two. (3) Suspension and polymerization: The aqueous dispersion medium obtained in step (1) and the oil phase mixture obtained in step (2) (the oil phase accounts for 28% of the total mass of the system) were transferred to a closed reaction vessel and dispersed at high speed of 1600 rpm for 20 minutes to form a suspension. After replacing the air in the reactor with nitrogen, the mixture was continuously stirred at 100 rpm under a nitrogen atmosphere, and the initial reaction pressure was controlled at 0.5 MPa. The polymerization reaction was carried out at 52℃ (5℃ above the 10h half-life temperature of the initiator EHP of 47℃) for 20 hours. (4) Post-processing: After the polymerization reaction is completed, the reaction product is filtered, washed and vacuum dried at 50°C for 12 hours to obtain thermally expanded microspheres.

[0049] Example 2 This embodiment provides a thermally expandable microsphere and its preparation method, the specific steps of which are as follows: (1) Preparation of aqueous dispersion medium: Add 150 parts by weight of sodium chloride, 80 parts by weight of colloidal silica (the effective content of silica is 25wt%) and 2 parts by weight of sodium nitrite to 800 parts by weight of deionized water. After thorough mixing, adjust the pH of the system to 3.5 with hydrochloric acid to obtain the aqueous dispersion medium. (2) Preparation of the oil phase mixture: 200 parts by weight of acrylonitrile (AN), 85 parts by weight of methacrylonitrile (MAN), 40 parts by weight of methyl methacrylate (MMA), 50 parts by weight of isopentane, 50 parts by weight of isooctane, 2 parts by weight of crosslinking agent ethylene glycol dimethacrylate (EGDMA), and 1.5 parts by weight of initiator benzoyl peroxide (BPO) were mixed and stirred evenly to obtain the oil phase mixture. The 10-hour half-life temperature of initiator BPO was 71℃, and the peak exothermic temperature of crosslinking agent EGDMA was 142℃, with a temperature difference of 71℃ between the two. (3) Suspension and polymerization: The aqueous dispersion medium obtained in step (1) and the oil phase mixture obtained in step (2) (the oil phase accounts for 28% of the total mass of the system) were transferred to a closed reaction vessel and dispersed at high speed at 2000 rpm for 15 minutes to form a suspension. After replacing the air in the reactor with nitrogen, the mixture was continuously stirred at 150 rpm under a nitrogen atmosphere, and the initial reaction pressure was controlled at 0.5 MPa. The polymerization reaction was carried out at 76°C (5°C above the 10-hour half-life temperature of initiator BPO of 71°C) for 20 hours. (4) Post-processing: After the polymerization reaction is completed, the reaction product is filtered, washed and vacuum dried at 50°C for 12 hours to obtain thermally expanded microspheres.

[0050] Example 3 This embodiment provides a thermally expandable microsphere and its preparation method, the specific steps of which are as follows: (1) Preparation of aqueous dispersion medium: Add 150 parts by weight of sodium chloride, 80 parts by weight of colloidal silica (the effective content of silica is 25wt%) and 2 parts by weight of sodium nitrite to 800 parts by weight of deionized water. After thorough mixing, adjust the pH of the system to 4.5 with hydrochloric acid to obtain the aqueous dispersion medium. (2) Preparation of the oil phase mixture: 200 parts by weight of acrylonitrile (AN), 85 parts by weight of methacrylonitrile (MAN), 40 parts by weight of methyl methacrylate (MMA), 30 parts by weight of isopentane, 50 parts by weight of isooctane, 2 parts by weight of crosslinking agent ethylene glycol dimethacrylate (EGDMA), and 1.5 parts by weight of initiator azobisisobutyronitrile (AIBN) were mixed and stirred evenly to obtain the oil phase mixture. The 10-hour half-life temperature of the initiator AIBN was 65℃, and the peak exothermic temperature of the crosslinking agent DAP was 142℃, with a temperature difference of 77℃ between the two. (3) Suspension and polymerization: The aqueous dispersion medium obtained in step (1) and the oil phase mixture obtained in step (2) (the oil phase accounts for 28% of the total mass of the system) were transferred to a closed reaction vessel and dispersed at high speed of 1800 rpm for 18 minutes to form a suspension. After replacing the air in the reactor with nitrogen, the mixture was continuously stirred at 120 rpm under a nitrogen atmosphere, and the initial reaction pressure was controlled at 0.5 MPa. The polymerization reaction was carried out at 70°C (i.e., 5°C above the 10-hour half-life temperature of the initiator AIBN of 65°C) for 20 hours. (4) Post-processing: After the polymerization reaction is completed, the reaction product is filtered, washed and vacuum dried at 50°C for 12 hours to obtain thermally expanded microspheres.

[0051] Example 4 This embodiment provides a thermally expandable microsphere and its preparation method, the specific steps of which are as follows: (1) Preparation of aqueous dispersion medium: Add 100 parts by weight of potassium chloride, 50 parts by weight of magnesium hydroxide and 0.1 parts by weight of potassium nitrite to 900 parts by weight of deionized water. After thorough mixing, adjust the pH of the system to 3 using hydrochloric acid to obtain the aqueous dispersion medium.

[0052] (2) Preparation of the oil phase mixture: 300 parts by weight of acrylonitrile (AN), 50 parts by weight of methyl acrylate (MA), 0.5 parts by weight of crosslinking agent pentaerythritol triacrylate (PETA), 90 parts by weight of isopentane, and 0.1 parts by weight of initiator dilauryl peroxide (LPO) were mixed and stirred until homogeneous to obtain the oil phase mixture. The difference between the 10-hour half-life temperature of the initiator and the peak exothermic temperature of the crosslinking agent was 60°C. The foaming agent (isopentane) in the oil phase mixture comprised approximately 25% by weight.

[0053] (3) Suspension and polymerization: The aqueous dispersion medium obtained in step (1) and the oil phase mixture obtained in step (2) (the oil phase accounts for 25% of the total mass of the system) were transferred to a closed reaction vessel and dispersed at 500 rpm for 30 minutes to form a suspension. After replacing the air in the reactor with argon, the mixture was stirred under an argon atmosphere, and the reaction pressure was controlled at 0.3 MPa. The polymerization reaction was carried out at 66°C for 25 hours.

[0054] (4) Post-processing: After the polymerization reaction is completed, the reaction product is filtered, washed and dried to obtain thermally expanded microspheres.

[0055] Example 5 This embodiment provides a thermally expandable microsphere and its preparation method, the specific steps of which are as follows: (1) Preparation of aqueous dispersion medium: Add 250 parts by weight of sodium sulfate, 150 parts by weight of calcium carbonate and 35 parts by weight of copper sulfate to 700 parts by weight of deionized water. After thorough mixing, adjust the pH of the system to 5 with hydrochloric acid to obtain the aqueous dispersion medium.

[0056] (2) Preparation of the oil phase mixture: 150 parts by weight of methacrylonitrile (MAN), 50 parts by weight of styrene (St), 4 parts by weight of crosslinking agent ethylene glycol dimethacrylate (EGDMA), 110 parts by weight of n-octane, 10 parts by weight of naphthenic oil (gas migration inhibitor), and 6 parts by weight of initiator dilauryl peroxide (LPO) were mixed and stirred until homogeneous to obtain the oil phase mixture. The difference between the 10-hour half-life temperature of the initiator and the peak exothermic temperature of the crosslinking agent was 81℃. The foaming agent (n-octane) in the oil phase mixture comprised approximately 30% by weight.

[0057] (3) Suspension and polymerization: The aqueous dispersion medium obtained in step (1) and the oil phase mixture obtained in step (2) (the oil phase accounts for 30% of the total mass of the system) were transferred to a closed reaction vessel and dispersed at high speed of 8000 rpm for 5 minutes to form a suspension. After replacing the air in the reactor with nitrogen, the mixture was stirred under a nitrogen atmosphere, and the reaction pressure was controlled at 0.8 MPa. The polymerization reaction was carried out at 66°C for 15 hours.

[0058] (4) Post-processing: After the polymerization reaction is completed, the reaction product is filtered, washed and dried to obtain thermally expanded microspheres.

[0059] Comparative Example 1 This comparative example provides a method for preparing thermally expandable microspheres. The only difference between this method and Example 1 is that diallyl phthalate (DAP) is used as the crosslinking agent in the oil phase mixture. All other conditions are exactly the same as in Example 1. Specifically, the 10-hour half-life temperature of the initiator EHP is 47°C, and the peak exothermic temperature of the crosslinking agent DAP is 187°C, with a temperature difference of 140°C.

[0060] Comparative Example 2 This comparative example provides a method for preparing thermally expandable microspheres. The only difference from Example 1 is that in the oil phase mixture, ethylene glycol dimethacrylate (EGDMA) is used as the crosslinking agent, and di(2-ethylhexyl) peroxide dicarbonate (EHP) is used as the initiator. All other conditions are identical to those in Example 1. Specifically, the 10-hour half-life temperature of the initiator EHP is 47°C, and the peak exothermic temperature of the crosslinking agent EGDMA is 142°C, with a temperature difference of 95°C between the two.

[0061] Comparative Example 3 This comparative example provides a method for preparing thermally expandable microspheres. The only difference from Example 1 is that, in the oil phase mixture, the crosslinking agent is pentaerythritol triacrylate (PETA), and the initiator is benzoyl peroxide (BPO). Correspondingly, to ensure effective decomposition of the initiator, the polymerization temperature is adjusted to 76°C (i.e., 5°C higher than the 10-hour half-life temperature of the selected initiator BPO). All other conditions are exactly the same as in Example 1. Specifically, the 10-hour half-life temperature of the initiator BPO is 71°C, and the peak exothermic temperature of the crosslinking agent PETA is 121°C, with a temperature difference of 50°C between the two.

[0062] Comparative Example 4 This comparative example provides a method for preparing thermally expandable microspheres. The only difference from Example 1 is that, in the oil phase mixture, diallyl phthalate (DAP) is used as the crosslinking agent, and benzoyl peroxide (BPO) is used as the initiator. Correspondingly, to ensure effective decomposition of the initiator, the polymerization temperature is adjusted to 76°C (i.e., 5°C higher than the 10-hour half-life temperature of the selected initiator BPO). All other conditions are exactly the same as in Example 1. Specifically, the 10-hour half-life temperature of the initiator BPO is 71°C, and the peak exothermic temperature of the crosslinking agent DAP is 187°C, with a temperature difference of 116°C.

[0063] Comparative Example 5 This comparative example provides a method for preparing thermally expandable microspheres. The only difference from Example 1 is that the crosslinking agent used is pentaerythritol triacrylate (PETA), and the initiator is azobisisobutyronitrile (AIBN). Correspondingly, to ensure effective decomposition of the initiator, the polymerization temperature is adjusted to 70°C (i.e., 5°C higher than the 10-hour half-life temperature of the selected initiator AIBN). All other conditions are exactly the same as in Example 1. Specifically, the 10-hour half-life temperature of the initiator AIBN is 65°C, and the peak exothermic temperature of the crosslinking agent PETA is 121°C, with a temperature difference of 56°C.

[0064] Comparative Example 6 This comparative example provides a method for preparing thermally expandable microspheres. The only difference from Example 1 is that, in the oil phase mixture, diallyl phthalate (DAP) is used as the crosslinking agent, and azobisisobutyronitrile (AIBN) is used as the initiator. Correspondingly, to ensure effective decomposition of the initiator, the polymerization temperature is adjusted to 70°C (i.e., 5°C higher than the 10-hour half-life temperature of the selected initiator AIBN). All other conditions are exactly the same as in Example 1. Specifically, the 10-hour half-life temperature of the initiator AIBN is 65°C, and the peak exothermic temperature of the crosslinking agent DAP is 187°C, with a temperature difference of 122°C.

[0065] Test case The performance of the thermally expanded microspheres provided in the above embodiments and comparative examples was tested. The specific test methods are as follows, and the test results are shown in Table 1 below: Particle size testing: The particle size of the thermally expanded microspheres was tested using a laser particle size analyzer (Bettersize2600). The test method was wet, with a light-blocking rate of 5-20% and water as the test medium. The median particle size D50, D90, and D10 were measured respectively, and the particle size distribution PDI = (D90-D10) / D50.

[0066] Expansion and heat resistance tests: Static thermomechanical analyzer (TMA, Mettler TMA / SDTA2+) was used for testing at a heating rate of 15℃ / min. The specific test steps are as follows: 1 mg of the microspheres to be tested were added to a 150 μL ceramic crucible. A matching diameter shim was placed above the microsphere layer to prepare the sample. A force of 0.06 N was applied to the sample from above using a pressure bar, and the sample height was measured. With the pressure bar still applying a force of 0.06 N, the temperature was increased from 20℃ to 300℃ at a heating rate of 15℃ / min, and the displacement of the pressure bar in the vertical direction was measured. The temperature at which the displacement in the positive direction began was recorded as the initial expansion temperature (Tstart), and the temperature at which the maximum displacement was displayed was recorded as the maximum expansion temperature (Tmax). The ratio of the maximum height during the expansion process to the initial sample height is the expansion ratio. The expansion temperature range (ΔT) was calculated using the formula ΔT = Tmax - Tstart, and is used to comprehensively characterize the wide-temperature-range expansion behavior and heat resistance of the microspheres.

[0067] Table 1

[0068] Based on the above test results, the thermally expandable microspheres prepared in Examples 1 to 5 of this invention exhibit significant advantages in terms of expansion performance and heat resistance. The expansion temperature range (ΔT) of each example is greater than 50°C, the expansion ratio is greater than 150, and the initial expansion temperature and maximum expansion temperature are both at relatively high levels, indicating that their outer shell has a uniform cross-linked network, enabling stable and sufficient expansion over a wide temperature range. In contrast, the overall performance of each comparative example shows varying degrees of decline. Specifically, the temperature differences between the initiator and cross-linking agent in Comparative Examples 1, 2, 4, and 6 (140°C, 95°C, 116°C, and 122°C, respectively) significantly exceed the scope of this invention, resulting in microspheres with expansion temperature ranges (ΔT ≤ 21°C) and expansion ratios (≤ 69) far lower than those in the examples. The temperature differences between the initiator and cross-linking agent in Comparative Examples 3 and 5 (50°C and 56°C, respectively) are below the lower limit of the scope of this invention, with expansion ratios of only 20 and 22, respectively, and narrower expansion temperature ranges.

[0069] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A thermally expandable microsphere comprising a polymer shell and a core encapsulated within the shell, the core containing a foaming agent, characterized in that, The polymer shell is formed by a polymerization reaction of a monofunctional monomer and a crosslinking agent under the initiation of an initiator; wherein the difference between the 10-hour half-life temperature of the initiator and the peak exothermic temperature of the crosslinking agent is 60℃~81℃.

2. The thermally expanding microspheres according to claim 1, characterized in that, The crosslinking agent includes a bifunctional and / or multifunctional monomer; And / or, based on 100 parts by weight of the total mass of the monofunctional monomers, the amount of the crosslinking agent is 0.01 to 5 parts by weight, preferably 0.1 to 2 parts by weight; And / or, the initiator includes organic peroxide initiators and / or azo initiators; And / or, based on 100 parts by weight of the total mass of the monofunctional monomers, the amount of the initiator is 0.01 to 5 parts by weight, preferably 0.1 to 3 parts by weight.

3. The thermally expanding microspheres according to claim 1 or 2, characterized in that, The monofunctional monomer includes at least two of acrylonitrile monomers, (meth)acrylate monomers, (meth)acrylate monomers, alkyl vinyl ester monomers, styrene monomers, olefin monomers, and acrylamide monomers; preferably, the monofunctional monomer includes at least two of acrylonitrile monomers; more preferably, the monofunctional monomer includes acrylonitrile and at least one selected from methacrylonitrile, α-chloroacrylonitrile, α-ethoxyacrylonitrile, and fumaric acid.

4. The thermally expanding microspheres according to any one of claims 1 to 3, characterized in that, The foaming agent is a C3-C13 alkane foaming agent. Optionally, the C3-C13 alkane foaming agent includes at least one of isopentane, neopentane, n-pentane, n-hexane, heptane, isooctane, n-octane, and petroleum ether.

5. The thermally expanding microspheres according to any one of claims 1 to 4, characterized in that, The median particle size D50 of the thermally expandable microspheres is 1~100μm, preferably 5~50μm; And / or, the initial expansion temperature of the thermally expandable microspheres is not lower than 125°C, preferably not lower than 130°C; And / or, the maximum expansion temperature of the thermally expanding microspheres is not less than 180°C.

6. A method for preparing thermally expandable microspheres as described in any one of claims 1 to 5, characterized in that, Includes the following steps: A monofunctional monomer, a crosslinking agent, a foaming agent, and an initiator are mixed to form an oil phase mixture; wherein the difference between the 10-hour half-life temperature of the initiator and the peak exothermic temperature of the crosslinking agent is 60℃~81℃. The oil phase mixture is dispersed in an aqueous dispersion medium to form a suspension; The suspension is heated to undergo a polymerization reaction, forming the thermally expanded microspheres.

7. The method according to claim 6, characterized in that, In the oil phase mixture, the mass percentage of the foaming agent is 20% to 30%. And / or, the oil phase mixture further comprises a gas migration inhibitor, optionally including at least one of hydrocarbon compounds, carboxylic acid compounds, ester compounds, and alcohol compounds; preferably, the amount of gas migration inhibitor is no more than 30 parts by weight per 100 parts by weight of the foaming agent. And / or, the aqueous dispersion medium comprises water, a stabilizer, an electrolyte, and a polymerization inhibitor.

8. The method according to claim 7, characterized in that, The stabilizer includes at least one 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. And / or, based on 100 parts by weight of water, the amount of the stabilizer is 1 to 21 parts by weight, preferably 5 to 21 parts by weight; And / or, the electrolyte is a metal salt; optionally, the metal salt includes at least one of sodium chloride, potassium chloride, magnesium chloride, calcium chloride, sodium sulfate, potassium sulfate, magnesium sulfate, sodium carbonate, potassium carbonate, sodium nitrate, and potassium nitrate. And / or, based on 100 parts by weight of water, the amount of the electrolyte is 1 to 50 parts by weight, preferably 11 to 35 parts by weight; And / or, the polymerization inhibitor includes at least one of sodium nitrite, potassium nitrite, ferric chloride, sodium sulfide, potassium dichromate, cuprous chloride, copper sulfate, titanium trichloride, sodium sulfate, and ammonium thiocyanate; And / or, based on 100 parts by weight of water, the amount of the polymerization inhibitor is 0.01 to 5 parts by weight.

9. The method according to any one of claims 6 to 8, characterized in that, The pH value of the aqueous dispersion medium is 1~7, preferably 3~5; And / or, in the suspension, the mass of the oil phase mixture accounts for 20% to 35% of the total mass of the oil phase mixture and the aqueous dispersion medium, preferably 25% to 30%; And / or, the polymerization reaction is carried out under an inert atmosphere; And / or, the pressure of the polymerization reaction is 0.3~0.8 MPa; And / or, the polymerization reaction is carried out at a temperature of 50-80°C, preferably 52-76°C; And / or, the polymerization reaction takes 15 to 25 hours; And / or, the polymerization reaction may further include a post-processing step of filtering and drying the product.

10. The application of thermally expandable microspheres as described in any one of claims 1 to 5 in printing and dyeing, coatings, inks, polyurethane polishing materials, shoe soles, sound-absorbing materials, or thermal insulation materials.

Images