A method for preparing thermally expandable microspheres having a narrow particle size distribution

Abstract

The application provides a method for preparing thermal expansion microspheres with narrow particle size distribution, and belongs to the field of material preparation.The method comprises the following steps: (1) uniformly stirring monomers, a foaming agent, a crosslinking agent and an initiator to obtain an oil phase; the monomers are composed of acrylonitrile, methacrylonitrile, hydroxyethyl methacrylate and N,N-dimethyl acrylamide; (2) uniformly stirring a suspending agent, a suspending aid, an electrolyte and water to obtain an aqueous phase; (3) emulsifying the oil phase and the aqueous phase after mixing to obtain a suspension; (4) high-pressure reaction; (5) acidification, filtration, washing, drying and thermal expansion microspheres are obtained.The thermal expansion microspheres prepared by the method have narrow particle size distribution, which is beneficial to forming a closed cell structure with uniform pore size in a foam material, is beneficial to improving the strength and impact resistance of the foam material, and has a wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of foamed materials, and specifically relates to a method for preparing thermally expandable microspheres with a narrow particle size distribution. Background Technology

[0002] Thermally expandable microspheres consist of a shell made of thermoplastic polymer and a core containing an alkane as a blowing agent. Normally, the softening temperature of the thermoplastic polymer forming the shell is higher than the boiling point of the blowing agent. Under heating conditions, the shell softens while the propellant evaporates, increasing the internal pressure and causing the microspheres to expand significantly. The initial temperature of expansion is called T. start The temperature at which maximum expansion is achieved is called T. max .

[0003] Thermally expandable microspheres can be used as foaming agents in many applications and are an indispensable raw material in fields such as papermaking, printing, inks, foamed plastics, lightweight automotive materials, building insulation materials, textiles, and artificial leather. The synthesis technology of thermally expandable microspheres is relatively mature. As early as the 1970s, patents researched the synthesis technology of thermally expandable microspheres (US2615972). In the decades since, researchers have conducted in-depth studies on different polymerization and expansion processes for thermally expandable microspheres, such as US4287308, US6235800, US6509381, EP 1054034, EP1408097, and Japanese patent JP1987-286534. However, there are currently few reports on how to prepare thermally expandable microspheres with uniform size.

[0004] Uniformly sized thermally expandable microspheres have a narrow particle size distribution, which is conducive to the formation of closed-cell structures with uniform pore size in foam materials. The closed-cell structure with uniform pore size is beneficial to improving the strength and impact resistance of foam materials ([1] Cheng Yi. Study on the influence of elastomer on the toughness and cell structure of micro-foamed PP composite materials [D]. Wuhan University of Technology, 2021. [2] Sun Kaiqiang. Study on surface coating and cell uniformity improvement of plant fiber / starch-based foam materials [D]. Shandong University, 2021.). Therefore, the development of thermally expandable microspheres with a narrow particle size distribution is of great significance.

[0005] The literature (Preparation and Foaming Behavior Study of Heat-Resistant High-Temperature Thermal Expansion Microspheres, Journal of Zhejiang Sci-Tech University (Natural Science Edition), Vol. 39, No. 3, May 2018) reports a thermal expansion microsphere, prepared by the following method: A certain amount of sodium hydroxide and sodium chloride are dissolved in deionized water to obtain aqueous solution No. 1. A certain amount of magnesium chloride hexahydrate, sodium dodecyl sulfate (SDS), and sodium nitrite are dissolved in deionized water to obtain aqueous solution No. 2. Aqueous solution No. 1 is slowly added dropwise to aqueous solution No. 2 using a peristaltic pump while stirring. After aqueous solution No. 1 is completely added, the pH of the dispersion is adjusted to alkaline, which is used as the aqueous phase of the suspension polymerization system. Acrylonitrile (AN), methacrylic acid (MAA), methyl methacrylate (MMA), N,N-dimethylacrylamide (DMAA), and a crosslinking agent were mixed in proportion to form a monomer solution. A certain amount of benzoyl peroxide (BPO) was then dissolved in the monomer solution. Finally, a certain amount of blowing agents, isooctane and isopentane, were added and mixed thoroughly to obtain the oil phase of the suspension polymerization system. The aqueous dispersion was mixed with the oil phase and dispersed under high-speed shear to obtain a monomer suspension. The obtained suspension was poured into a high-pressure reactor, sealed, and pressurized. Once the preset temperature was reached, timing was started, and the reaction was maintained at a constant temperature for 20 hours. After cooling, depressurization, discharge, washing, filtration, drying, and screening, dried thermally expanded microspheres were obtained. However, the particle size of the thermally expanded microspheres prepared in this literature was mainly distributed in the range of 20-250 μm, with a polydispersity index of approximately 1.7, indicating non-uniform particle size. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing thermally expandable microspheres, and thermally expandable microspheres with a narrow particle size distribution prepared by the method.

[0007] This invention provides a method for preparing thermally expandable microspheres, the method comprising the following steps:

[0008] (1) The monomer, foaming agent, crosslinking agent and initiator are stirred evenly to obtain an oil phase; the monomer is composed of acrylonitrile, methacrylonitrile, hydroxyethyl methacrylate and N,N-dimethylacrylamide;

[0009] (2) Stir the suspending agent, the co-suspending agent, the electrolyte and water evenly to obtain an aqueous phase;

[0010] (3) The oil phase and the aqueous phase are mixed and emulsified to obtain a suspension;

[0011] (4) High-pressure reaction;

[0012] (5) Acidification, filtration, washing, and drying are performed to obtain thermally expanded microspheres.

[0013] Further, in step (1), the stirring time is 20-40 min; in step (2), the stirring time is 35-50 min.

[0014] Further, in step (1), the stirring time is 30 min; in step (2), the stirring time is 44 min.

[0015] Further, in step (3), the emulsification time is 2-5 min; in step (4), the high-pressure reaction temperature is 56-65℃, the time is 22-26 h, and the pressure is 0.39-0.50 MPa; in step (5), the pH value after acidification is 2-4.

[0016] Further, in step (3), the emulsification time is 3 min; in step (4), the high-pressure reaction temperature is 59℃, the time is 24 h, and the pressure is 0.48 MPa; in step (5), the pH value after acidification is 3.

[0017] Further, the mass ratio of acrylonitrile, methacrylonitrile, hydroxyethyl methacrylate and N,N-dimethylacrylamide is (54-81):(20-30):(13-24):(12-24);

[0018] The mass ratio of the monomer, foaming agent, crosslinking agent, initiator, suspending agent, co-suspending agent, electrolyte and water is (110-140):(10-40):(0.03-0.3):(1-3):(50-52):(0.5-0.7):(65-75):(450-550).

[0019] Further, the mass ratio of acrylonitrile, methacrylonitrile, hydroxyethyl methacrylate and N,N-dimethylacrylamide is (54-48):30:(18-24):(12-24);

[0020] The mass ratio of the monomer, foaming agent, crosslinking agent, initiator, suspending agent, co-suspending agent, electrolyte and water is 120:30:(0.2701-0.2643):(1.99-2.03):50.86:0.56:70:500.

[0021] Further, the foaming agent is one or a mixture of two of isopentane and n-hexane, the crosslinking agent is 1,6-hexanediol dimethacrylate, the initiator is azobisisobutyronitrile, the electrolyte is sodium chloride, the suspending agent is one or a mixture of two of magnesium chloride hexahydrate and sodium hydroxide, and the co-suspending agent is one or a mixture of two of polyvinylpyrrolidone and sodium dodecyl sulfate.

[0022] Furthermore, in the mixture of isopentane and n-hexane, the mass ratio of isopentane to n-hexane is 1:1;

[0023] In the mixture of magnesium chloride hexahydrate and sodium hydroxide, the mass ratio of magnesium chloride hexahydrate to sodium hydroxide is 3:1;

[0024] In the mixture of polyvinylpyrrolidone and sodium dodecyl sulfate, the mass ratio of polyvinylpyrrolidone to sodium dodecyl sulfate is (3.2-8.3):1.

[0025] The present invention also provides thermally expandable microspheres prepared by the above method.

[0026] Furthermore, the particle size distribution of the thermally expanded microspheres is 0.78-0.99.

[0027] The method provided by this invention produces thermally expandable microspheres with a narrow particle size distribution, which is beneficial for the formation of closed-cell structures with uniform pore size in foam materials, thereby improving the strength and impact resistance of foam materials and showing broad application prospects.

[0028] Obviously, based on the above description of the present invention, and according to common technical knowledge and conventional methods in the field, various other modifications, substitutions or alterations can be made without departing from the basic technical concept of the present invention.

[0029] The following detailed embodiments further illustrate the above-described content of the present invention. However, this should not be construed as limiting the scope of the present invention to the following examples. All technologies implemented based on the above-described content of the present invention fall within the scope of the present invention. Detailed Implementation

[0030] Unless otherwise specified, the raw materials and equipment used in this invention are all known products, obtained by purchasing commercially available products.

[0031] Examples 1-6: Methods for preparing thermally expandable microspheres with narrow particle size distribution

[0032] The formulation of the thermally expandable microspheres is shown in Table 1.

[0033] Table 1. Formulation of thermally expandable microspheres (unit: g)

[0034]

[0035]

[0036] The preparation method of thermally expandable microspheres is as follows:

[0037] (1) Oil phase preparation: Add monomers (acrylonitrile, methacrylonitrile, hydroxyethyl methacrylate and N,N-dimethylacrylamide), crosslinking agent, initiator and foaming agent in sequence, and stir for 30 min under ice bath conditions to obtain the oil phase.

[0038] (2) Aqueous phase preparation: Deionized water, sodium chloride, suspending agent (magnesium chloride hexahydrate and sodium hydroxide), and co-suspending agent (polyvinylpyrrolidone (K30) and sodium dodecyl sulfate) are mixed and stirred in an ice bath for 40-44 min to obtain the aqueous phase.

[0039] (3) Emulsification: Mix the oil phase and water phase, and emulsify in a homogenizer for 3-5 minutes to obtain a suspension.

[0040] (4) High pressure reaction: Add the suspension to the high pressure reactor, react at a temperature of 56-59℃ for 22-24 hours and at a pressure of 0.39-0.48MPa.

[0041] (5) Post-treatment: The wet material after reaction is acidified with hydrochloric acid to pH 3, filtered, washed and dried to obtain thermally expandable microspheres.

[0042] Table 2. Parameters of the preparation method of thermally expanded microspheres

[0043]

[0044] The following experimental examples demonstrate the beneficial effects of the thermally expanding microspheres of the present invention.

[0045] Experimental Example 1: Performance Testing of the Thermally Expanding Microspheres of the Present Invention

[0046] 1. Experimental Methods

[0047] The initial temperature of expansion (T) start ) and the temperature at which maximum expansion is achieved (T) max Characterization methods for )

[0048] A thermomechanical analyzer (TMA) (model: Q400EM, manufacturer: TA Instruments, USA) was used. A small amount of sample was spread in a single layer in an aluminum crucible, the crucible lid was closed, and the instrument was placed under the probe. The instrument was in compression mode, the heating rate was set to 10℃ / min, the heating range was 30-250℃, and the probe load was 0.06N. The first point in the temperature-size change curve where the size change shows a significant increase was denoted as T. start The maximum value of the size change is denoted as T. max .

[0049] Characterization methods for particle size and particle size distribution span:

[0050] A wet-dry laser particle size analyzer (LPSA) (model: HELOS KR, manufacturer: Neupatek, Germany) was used. Test mode: dry method, pressure: 4 bar, injection rate: 45%, span calculation formula: span = (D90 - D10) / D50.

[0051] 2. Experimental Results

[0052] Table 3. Performance test results of the thermal expansion microspheres of the present invention

[0053]

[0054] As can be seen from Table 3, the particle size distribution Span of the thermally expandable microspheres prepared by the methods in Examples 1-6 of the present invention is all below 0.99, and the particle size distribution Span of the thermally expandable microspheres prepared in Examples 5 and 6 is as low as 0.78.

[0055] In summary, this invention provides a method for preparing thermally expandable microspheres. The thermally expandable microspheres prepared by this method have a narrow particle size distribution, which is beneficial for the formation of a closed-cell structure with uniform pore size in foam materials. This is beneficial for improving the strength and impact resistance of foam materials and has broad application prospects.

Claims

1. A method for preparing thermally expandable microspheres with a narrow particle size distribution, characterized in that, The method is as follows: (1) The monomer, foaming agent, crosslinking agent and initiator are stirred evenly to obtain an oil phase; the monomer is composed of acrylonitrile, methacrylonitrile, hydroxyethyl methacrylate and N,N-dimethylacrylamide; (2) Stir the suspending agent, the co-suspending agent, the electrolyte and water evenly to obtain the aqueous phase; (3) The oil phase and the aqueous phase are mixed and emulsified to obtain a suspension; (4) High-pressure reaction; (5) Acidification, filtration, washing, and drying are performed to obtain thermally expanded microspheres; In step (1), the stirring time is 20-40 min; in step (2), the stirring time is 35-50 min; in step (3), the emulsification time is 2-5 min; in step (4), the high-pressure reaction temperature is 56-65℃, the time is 22-26 h, and the pressure is 0.39-0.50 MPa; in step (5), the pH value after acidification is 2-4. The mass ratio of acrylonitrile, methacrylonitrile, hydroxyethyl methacrylate, and N,N-dimethylacrylamide is (54-48):30:(18-24):(12-24). The mass ratio of the monomer, foaming agent, crosslinking agent, initiator, suspending agent, co-suspending agent, electrolyte, and water is 120:30:(0.2701-0.2643):(1.99-2.03):50.86:0.56:70:500; The foaming agent is isopentane, the crosslinking agent is 1,6-hexanediol dimethacrylate, the initiator is azobisisobutyronitrile, the electrolyte is sodium chloride, the suspending agent is a mixture of magnesium chloride hexahydrate and sodium hydroxide, wherein the mass ratio of magnesium chloride hexahydrate to sodium hydroxide in the mixture is 3:1, and the co-suspending agent is a mixture of polyvinylpyrrolidone and sodium dodecyl sulfate, wherein the mass ratio of polyvinylpyrrolidone to sodium dodecyl sulfate in the mixture is 8.3:

1.

2. The method according to claim 1, characterized in that: In step (1), the stirring time is 30 min; in step (2), the stirring time is 44 min.

3. The method according to claim 1, characterized in that: In step (3), the emulsification time is 3 min; in step (4), the high-pressure reaction temperature is 59℃, the time is 24 h, and the pressure is 0.48 MPa; in step (5), the pH value after acidification is 3.

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