Resin particles for porous polyimide membrane porous

Abstract

Provided are resin particles for use in pore formation of a polyimide porous film, the resin particles being capable of suppressing both an increase in slurry viscosity and a change in the size of the polyimide porous film. According to the present invention, provided are resin particles for pore formation of a polyimide porous film, the resin particles comprising a (meth) acrylic resin, the ratio of (meth) acrylic monomer units in the (meth) acrylic resin being 60-100% by mass, and the ratio of (meth) acrylic monomer units in the (meth) acrylic resin being 10-30% by mass. The resin particles are provided with base particles and a coating portion that coats at least a portion of the surface of the base particles, the base particles are composed of a polymer of a first monomer, the coating portion is composed of a polymer of a second monomer, the content of a polyfunctional (meth) acrylate in the first monomer is 0-10 mass%, and the content of a polyfunctional (meth) acrylate in the second monomer is 0-10 mass%. The content of the polyfunctional (meth) acrylate in the second monomer is 65-100 mass%.

Description

Technical Field

[0001] The present invention relates to resin particles for forming pores in a polyimide porous membrane and a method for producing the same. Background Art

[0002] Polyimide porous membranes are widely used in fields such as lithium-ion battery separators because they have higher heat resistance and higher porosity than polyolefin porous membranes.

[0003] A porous polyimide membrane can be produced, for example, by mixing a polyamic acid solution with resin particles to form a slurry, applying the slurry to form a coated sheet, drying the resulting coated sheet to form a dried sheet 5, and calcining the dried sheet (see, for example, Patent Document 1). The calcination causes the polyamic acid to undergo an imidization reaction to form a polyimide membrane, and the burnout of the resin particles creates numerous pores within the polyimide membrane, making the polyimide membrane porous.

[0004]

Prior art literature

[0005] Generally, the lower the degree of crosslinking of the resin particles, the more likely the viscosity of the slurry containing the resin particles increases. On the other hand, the higher the degree of crosslinking of the resin particles, the more likely the dimensional change of the polyimide porous membrane during the calcination step when forming the polyimide porous membrane. Therefore, it is difficult to simultaneously suppress the increase in slurry viscosity and the dimensional change of the polyimide porous membrane.

[0006] The present invention has been made in view of the above circumstances, and provides resin particles for forming pores in a porous polyimide membrane, which can achieve both suppression of a slurry viscosity increase and suppression of a dimensional change in the porous polyimide membrane.

[0007]

Technical means for solving technical problems

[0008] [1] A resin particle for forming pores in a porous polyimide membrane, the resin particle comprising a (meth)acrylic resin, the (meth)acrylic monomer unit ratio in the (meth)acrylic resin being 60 to 100% by mass, the resin particle comprising a base particle and a coating portion coating at least a portion of the surface of the base particle, the base particle comprising a polymer of a first monomer, the coating portion comprising a polymer of a second monomer, the content of a polyfunctional (meth)acrylate of the first monomer being 0 to 10% by mass, and the content of a polyfunctional (meth)acrylate of the second monomer being 65 to 100% by mass.

[0009] [2] The resin particles according to [1], wherein the first monomer comprises a low-Tg monomer having a homopolymer glass transition temperature of 0° C. or lower in an amount of 10 to 50% by mass, and the low-Tg monomer is a monofunctional (meth)acrylate.

[0010] [3] The resin particles according to [1], wherein the eluted component ratio of the resin particles is 10% by mass or less.

[0011] [4] The resin particles according to [1], wherein the eluted component ratio of the resin particles is 5% by mass or less.

[0012] [5] The resin particles according to any one of [1] to [4], wherein the swelling index of the resin particles is 1.8 or less.

[0013] [6] A resin particle for forming pores in a polyimide porous membrane, the resin particle being composed of a (meth)acrylic resin, wherein the ratio of (meth)acrylic monomer units in the (meth)acrylic resin is 60 to 100% by mass, the slurry viscosity measured by the following measurement method 1 is 25 Pa·s or less, and the polyimide porous membrane dimensional change rate measured by the following measurement method 2 is 40% or less, (Measurement method 1) First, 1.2 g of polyamic acid, 14 g of dimethylacetamide (DMAc), and 4.8 g of the resin particle are weighed, and a rotation / revolution type mixer is used to repeat mixing at 2000 rpm for 5 minutes and degassing at 2200 rpm for 30 seconds three times to obtain a slurry, and then the slurry viscosity of the obtained slurry is measured at 25°C and 5 rpm using an E-type viscometer; (Measurement method 2) First, 1.2 g of polyamic acid, 14 g of dimethylacetamide (DMAc), and 4.8 g of the resin particle are weighed, and a rotation / revolution type mixer is used to repeat mixing at 2000 rpm for 5 minutes and degassing at 2200 rpm for 30 seconds three times to obtain a slurry, and then the slurry viscosity of the obtained slurry is measured using an E-type viscometer at 25°C and 5 rpm. 2 g, 12 g of DMAC, and 1.8 g of the resin particles were mixed using a rotation / revolution type mixer, and mixing was repeated three times at 2000 rpm for 5 minutes and degassing at 200 rpm for 30 seconds to obtain a slurry. The obtained slurry was then applied to a substrate that had been subjected to a peeling treatment so that the film thickness after drying was 30 μm to form a coated sheet. The coated sheet was dried at 100° C. for 2 minutes, and the obtained dried sheet was cut into a size of 2 cm × 5 cm to serve as an evaluation sheet for evaluating the dimensional change rate of the polyimide porous membrane. The evaluation sheet with the substrate peeled off was then calcined in an electric furnace under the calcination conditions of holding at 250° C. for 30 minutes, then raising the temperature and holding at 350° C. for 30 minutes. The dimensional change rate of the polyimide porous membrane was calculated based on the following formula from the area S1 of the evaluation sheet before calcination and the area S2 of the evaluation sheet after calcination.

[0014] Dimensional change rate of the porous polyimide film (%) = [(S1-S2) / S1]×100.

[0015] [7] A method for producing resin particles for forming pores in a polyimide porous membrane, wherein the resin particles are composed of a (meth)acrylic resin, wherein the ratio of (meth)acrylic monomer units in the (meth)acrylic resin is 60 to 100% by mass, and the method comprises a base particle forming step and a second monomer polymerization step, wherein in the base particle forming step, a first monomer is polymerized in an aqueous medium to form base particles, and in the second monomer polymerization step, a second monomer is polymerized in an aqueous medium in the presence of the base particles, wherein the content of the polyfunctional (meth)acrylate in the first monomer is 0 to 10% by mass, and the content of the polyfunctional (meth)acrylate in the second monomer is 65 to 100% by mass.

[0016] Effects of the invention In the resin particles of the present invention, at least a portion of the surface of the base particles composed of the polymer of the first monomer is coated by the coating portion composed of the polymer of the second monomer. The content of multifunctional (meth)acrylate in the first monomer is less than that of the second monomer. Therefore, the degree of crosslinking of the base particles composed of the polymer of the first monomer is lower than the degree of crosslinking of the coating portion composed of the polymer of the second monomer. According to such a structure, the base particles can maintain a low degree of crosslinking while suppressing the increase in the viscosity of the slurry. In addition, in the resin particles of the present invention, since the degree of crosslinking of the base particles is low, the dimensional change of the polyimide porous membrane is suppressed. Therefore, by using the resin particles of the present invention, it is possible to simultaneously achieve the suppression of the increase in the slurry viscosity and the suppression of the dimensional change of the polyimide porous membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In Figure 1 Figure 1A The dry sheet 5 is composed of resin particles 1 dispersed in a polyamic acid film 5a. Figure 1B represents a state in which the polyamic acid constituting the polyamic acid film 5a is imidized to form the polyimide film 2a, Figure 1C The diagram shows a state in which a large number of pores 2 b are formed in the polyimide membrane 2 a by burning out the resin particles 1 in the polyimide membrane 2 a , thereby forming the porous polyimide membrane 2 .

[0018] Figure 2 It is a cross-sectional view showing the structure of the resin particle 1 .

[0019] Figure 3 These are electron microscope images showing the dispersion state of the resin particles 1 in Examples 1 and 9 to 12. DETAILED DESCRIPTION

[0020] The following describes embodiments of the present invention. The various features described in the embodiments described below can be combined with each other. Furthermore, the present invention can be implemented independently for each feature. In the present invention, "(meth)acrylic acid" refers to a general term for acrylic acid and methacrylic acid. Furthermore, "(meth)acrylate" refers to a general term for acrylate and methacrylate.

[0021] 1. Structure of Resin Particle 1 Use Figure 1~ Figure 2 The resin particles 1 according to one embodiment of the present invention are described below. The resin particles 1 according to this embodiment are used to form pores in a polyimide porous membrane 2. In one example, Figure 1AAs shown, the polyimide porous membrane 2 can be prepared by mixing a polyamic acid solution obtained by dissolving polyamic acid in a solvent with resin particles 1 to prepare a slurry, applying the slurry to form a coated sheet, drying the coated sheet to form a dried sheet 5, and then calcining the dried sheet 5. The dried sheet 5 is composed of the resin particles 1 dispersed in the polyamic acid membrane 5a.

[0022] The calcination preferably includes calcination at an imidization temperature and calcination at a particle burnout temperature. The imidization temperature is a temperature at which the polyamic acid is imidized, and is, for example, 200 to 300° C. By calcining at the above temperature, the polyamic acid constituting the polyamic acid film 5a is imidized, such as Figure 1B As shown, the polyamic acid film 5a is converted into the polyimide film 2a. At this time, it is preferable that the resin particles 1 are not completely burned out. Specifically, the imidization temperature is, for example, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, or 300°C, but may also be within a range between any two of the values exemplified here.

[0023] The particle burning temperature is the temperature at which the resin particles 1 are burned out, and is, for example, 310 to 400° C. By calcining at the above temperature, Figure 1C As shown, the resin particles 1 in the polyimide film 2a are burned out, forming a large number of pores 2b in the polyimide film 2a, thereby forming a porous polyimide film 2. Specifically, the particle burnout temperature is, for example, 310, 320, 330, 340, 350, 360, 370, 380, 390, or 400°C, but may also be within a range between any two of the values exemplified here.

[0024] Resin particles 1 are composed of a (meth)acrylic resin. A (meth)acrylic resin is a polymer of monomers containing a (meth)acrylic monomer, and comprises (meth)acrylic monomer units. The monomers may contain only (meth)acrylic monomers or other monomers besides (meth)acrylic monomers. Examples of other monomers include monofunctional monomers such as styrene monomers (styrene, methylstyrene, etc.) and polyfunctional monomers such as divinylbenzene. The proportion of (meth)acrylic monomer units in the (meth)acrylic resin is, for example, 60 to 100% by mass, specifically 60, 65, 70, 75, 80, 85, 90, 95, 96, 97, 98, 99, or 100% by mass, or may be within a range between any two of the values exemplified here. The proportion of other monomer units can be determined by subtracting the (meth)acrylic monomer units from 100% by mass.

[0025] Examples of the (meth)acrylic monomer include monofunctional (meth)acrylates and polyfunctional (meth)acrylates. Examples of the polyfunctional (meth)acrylate include difunctional, trifunctional, tetrafunctional, or higher-functional (meth)acrylates.

[0026] The monofunctional (meth)acrylate is preferably an alkyl (meth)acrylate. In the alkyl (meth)acrylate, the number of carbon atoms of the alkyl group constituting the alkyl ester is preferably 1 to 12, more preferably 1 to 8.

[0027] Examples of the alkyl (meth)acrylate include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, and dodecyl (meth)acrylate.

[0028] Examples of the difunctional (meth)acrylate include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyoxyethylene di(meth)acrylate, polypropylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and 1,9-nonanediol di(meth)acrylate.

[0029] Examples of the trifunctional (meth)acrylate include trimethylolpropane tri(meth)acrylate, trimethylolethane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, propoxylated trimethylolpropane tri(meth)acrylate, and tris(2-(meth)acryloyloxyethyl isocyanurate).

[0030] Examples of tetrafunctional or higher-functional (meth)acrylates include pentaerythritol tetra(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, propoxylated pentaerythritol tetra(meth)acrylate, dipentaerythritol tetra(meth)acrylate, ethoxylated dipentaerythritol tetra(meth)acrylate, propoxylated dipentaerythritol tetra(meth)acrylate, di(trimethylolpropane) tetra(meth)acrylate, ethoxylated di(trimethylolpropane) tetra(meth)acrylate, and tetra(meth)acrylate compounds such as ethoxylated di(trimethylolpropane) tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, and dipentaerythritol hexa(meth)acrylate.

[0031] like Figure 2 As shown, the resin particle 1 includes a base particle 3 and a coating portion 4 that covers at least a portion of the surface of the base particle 3. Figure 2 Although the coating portion 4 is shown as covering the entire base particle 3, it may cover only a portion of the base particle 3. The ratio of the area covered by the coating portion 4 to the surface area of the base particle 3 is, for example, 20 to 100%, specifically, 20, 30, 40, 50, 60, 70, 80, 90, or 100%, and may also be within a range between any two of the values exemplified here.

[0032] The base particles 3 are composed of a polymer of a first monomer, wherein the content of a polyfunctional (meth)acrylate in the first monomer is 10% by mass or less. The coating 4 is composed of a polymer of a second monomer, wherein the content of a polyfunctional (meth)acrylate in the second monomer is 65% to 100% by mass. This configuration reduces the crosslinking degree of the base particles 3 and increases the crosslinking degree of the coating 4. Specifically, at least a portion of the surface of the base particles 3, which have a low crosslinking degree, is coated by the coating 4, which has a high crosslinking degree.

[0033] Generally, the lower the degree of crosslinking of the resin particles 1, the more likely the viscosity of the slurry containing the resin particles 1 will increase. In this embodiment, by coating at least a portion of the surface of the base particles 3 with the coating 4, the degree of crosslinking of the base particles 3 can be suppressed to a low level, and the increase in the viscosity of the slurry can be suppressed. Generally, the higher the degree of crosslinking of the resin particles 1, the greater the dimensional change of the polyimide porous membrane 2 during the calcination step when forming the polyimide porous membrane 2. However, in this embodiment, due to the low degree of crosslinking of the base particles 3, the dimensional change of the polyimide porous membrane 2 is suppressed. Therefore, according to the present invention, it is possible to simultaneously suppress the increase in the viscosity of the slurry and the dimensional change of the polyimide porous membrane 2.

[0034] The content of the polyfunctional (meth)acrylate in the first monomer is, for example, 0 to 10% by mass, specifically 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10% by mass, and may also be within a range between any two of the values exemplified here. To increase the degree of crosslinking of the base particles 3 of the resin particles 1, the content of the polyfunctional (meth)acrylate in the first monomer is preferably 1% by mass or greater, more preferably 2% by mass or greater. The content (mass %) of the monofunctional (meth)acrylate in the first monomer can be calculated as 100% by mass - (content (mass %) of the polyfunctional (meth)acrylate + content (mass %) of the other monomers).

[0035] The content of the polyfunctional (meth)acrylate in the second monomer is, for example, 65 to 100% by mass, specifically 65, 70, 75, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, or 100% by mass, or may be within a range between any two of the values exemplified here. From the perspective of suppressing an increase in slurry viscosity, the content of the polyfunctional (meth)acrylate in the second monomer is preferably 80% by mass or greater, more preferably 90% by mass or greater. The content (mass %) of the monofunctional (meth)acrylate in the second monomer can be calculated as 100% by mass - (content (mass %) of the polyfunctional (meth)acrylate + content (mass %) of the other monomers).

[0036] From the perspective of facilitating particle formation, the monofunctional (meth)acrylate in the first monomer or the second monomer preferably contains methyl methacrylate. The methyl methacrylate content in the monofunctional (meth)acrylate in the first monomer is, for example, 60 to 100% by mass, specifically 60, 65, 70, 75, 80, 85, 90, 95, or 100% by mass, and may also be within a range between any two of the values exemplified here.

[0037] From the perspective of facilitating particle formation, the polyfunctional (meth)acrylate of the first monomer or the second monomer preferably includes ethylene glycol di(meth)acrylate. The content of ethylene glycol di(meth)acrylate in the polyfunctional (meth)acrylate is, for example, 80 to 100% by mass, specifically 80, 85, 90, 95, or 100% by mass, and may also be within a range between any two of the values exemplified here.

[0038] In the first monomer, the content of the low-Tg monomer whose homopolymer has a glass transition temperature (Tg) of 0°C or less is preferably 10% to 50% by mass, specifically, for example, 10, 15, 20, 25, 30, 35, 40, 45, or 50% by mass, and may also be within the range between any two of the values exemplified here. In this case, the dimensional change of the polyimide porous membrane 2 is particularly small. Examples of low-Tg monomers include ethyl acrylate, isobutyl acrylate, n-butyl acrylate, n-pentyl (meth)acrylate, n-hexyl (meth)acrylate, n-octyl (meth)acrylate, n-nonyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-dodecyl (meth)acrylate, n-tetradecyl (meth)acrylate, isopropyl acrylate, isobutyl acrylate, isooctyl (meth)acrylate, isodecyl (meth)acrylate, isononyl (meth)acrylate, and tridecyl (meth)acrylate. Among them, n-butyl acrylate is preferred from the perspective of easy particle formation. The Tg of a low-Tg monomer is, for example, -100 to 0°C, specifically -100, -90, -80, -70, -60, -50, -40, -30, -20, -10, or 0°C, and may also be within a range between any two of the values listed here. The glass transition temperature (Tg) of the homopolymer is the value listed in the Polymer Handbook (Second Edition). If the glass transition temperature (Tg) of a homopolymer is not listed in the aforementioned literature, the value listed in other literature or product catalogs, or the value actually measured during homopolymer production, may be used.

[0039] The average particle size of the base particles 3 is, for example, 0.01 to 10.0 μm, preferably 0.05 to 1.0 μm, and more preferably 0.1 to 0.5 μm. Specifically, this average particle size is, for example, 0.01, 0.05, 0.1, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1.0, 5.0, or 10.0 μm, and may also be within a range between any two of the values exemplified here. The average particle size can be measured by the method described in the Examples.

[0040] The resin particles 1 may include a base particle 3 and a coating 4 that covers at least a portion of the surface of the base particle 3. The average particle size is, for example, 0.01 to 10.0 μm, preferably 0.05 to 1.0 μm, and more preferably 0.1 to 0.5 μm. Specifically, the average particle size is, for example, 0.01, 0.05, 0.1, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1.0, 5.0, or 10.0 μm, and may also be within a range between any two of the values exemplified herein. The average particle size can be measured by the method described in the Examples.

[0041] The particle size ratio defined by [average particle size of resin particles 1 / average particle size of base particles 3] is, for example, 1.00 to 1.50, preferably 1.00 to 1.30, and more preferably 1.001 to 1.30 from the perspective of suppressing dimensional change. Theoretically, the particle size ratio is greater than 1, but due to calculation errors caused by rounding, it may be 1.00. Specifically, the particle size ratio is, for example, 1.00, 1.001, 1.005, 1.01, 1.02, 1.03, 1.04, 1.05, 1.10, 1.15, 1.20, 1.25, 1.30, 1.35, 1.40, 1.45, or 1.50, and may also be within the range between any two of the values exemplified here.

[0042] The coefficient of variation (CV value) of the average particle size of the resin particles 1 is preferably 10% or less, and more preferably 5% or less. The coefficient of variation (CV value) is calculated by the following formula.

[0043] Coefficient of variation (CV value) = (standard deviation of the particle size distribution of the resin particles ÷ average particle size of the resin particles) × 100 The average particle size and particle size distribution can be measured by the method described in Examples. Within the above range, when a porous polyimide membrane is formed using the resin particles of the present invention, the mechanical strength of the porous polyimide membrane is further improved.

[0044] The elution rate of the resin particles 1 is preferably 20% by mass or less, more preferably 10% by mass or less, and particularly preferably 5% by mass or less. In addition, the swelling index of the resin particles 1 is preferably 4.0 or less, more preferably 1.8 or less. The elution rate and the swelling index can be measured by the method shown in the examples. When the elution rate and the swelling index meet the above range values, the dispersibility of the resin particles 1 can be improved. The elution rate is, for example, 0 to 20% by mass, specifically, for example, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20% by mass, and can also be within the range between any two values exemplified here. The swelling index is, for example, 0.0 to 4.0, specifically 0.0, 0.5, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.5, 3.0, 3.5, or 4.0, and may be within the range between any two of the values exemplified here.

[0045] Resin particles 1, from the viewpoint of making the coating film uniform, the slurry viscosity measured by the following measurement method 1 is 25Pa s or less, and the dimensional change rate of the polyimide porous membrane measured by the following measurement method 2 is preferably 40% or less. In this case, the increase in slurry viscosity and the dimensional change of the polyimide porous membrane are particularly suppressed. The slurry viscosity is, for example, 1 to 25 Pa s, specifically, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 Pa s may also be within the range between any two of the values exemplified herein. The dimensional change rate of the porous polyimide membrane is, for example, 0 to 40%, specifically, for example, 0, 5, 10, 15, 20, 25, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, or 40, and may also be within the range between any two of the values exemplified herein.

[0046] (Measurement method 1) First, 1.2g of polyamic acid, 14g of dimethylacetamide (DMAc), and 4.8g of the aforementioned resin particles were weighed and mixed using a rotary / orbital mixer at 2000rpm for 5 minutes. Degassing was repeated three times at 2200rpm for 30 seconds to prepare a slurry. The resulting slurry's viscosity was then measured using an E-type viscometer at 25°C and 5rpm.

[0047] (Measurement method 2) First, 1.2 g of polyamic acid, 12 g of DMAc, and 1.8 g of the resin particles were weighed, mixed at 2000 rpm × 5 minutes using a rotation / revolution type mixer, and mixed by repeating degassing at 2200 rpm × 30 seconds three times to obtain a slurry. Then, the obtained slurry was coated on a substrate that had been subjected to a peeling treatment in such a way that the film thickness after drying was 30 μm to form a coating sheet, and the coating sheet was dried at 100 ° C and 2 minutes to obtain a dry sheet cut into a size of 2 cm × 5 cm as an evaluation sheet for evaluating the dimensional change rate of the polyimide porous membrane. Then, the evaluation sheet with the substrate peeled off was calcined in an electric furnace. The calcination conditions were to keep it at 250 ° C for 30 minutes, then raise the temperature and keep it at 350 ° C for 30 minutes. The dimensional change rate of the polyimide porous membrane was calculated based on the following formula from the area S1 of the evaluation sheet before calcination and the area S2 of the evaluation sheet after calcination.

[0048] Dimensional change rate of porous polyimide membrane (%) = [(S1-S2) / S1] × 100 2. Method for producing resin particles 1 The method for producing the resin particles 1 according to one embodiment of the present invention includes a base particle forming step and a second monomer polymerization step.

[0049] In the base particle forming step, the first monomer is polymerized in an aqueous medium to form base particles 3. In the second monomer polymerization step, the second monomer is polymerized in an aqueous medium in the presence of base particles 3 to form resin particles 1. The first monomer and the second monomer are described above.

[0050] After the second monomer polymerization step, the resin particles 1 can be made into a powder state by drying and crushing them. The drying conditions are appropriately adjusted according to the ability and capacity of the dryer used. General-purpose hot air drying, reduced pressure drying, vacuum drying, etc. can be used as appropriate. The crushing is preferably carried out at 10 to 40°C, and the crushing pressure is preferably 0.1 to 0.5 MPa. The resin particles 1 can be classified as needed to keep the particle size, coefficient of variation, etc. within a predetermined range. Classification can be wet classification or dry classification. Wet classification can be carried out, for example, by passing the polymerization solution through a metal mesh after polymerization, and dry classification can be carried out by using an appropriate classification device on the particles that have been further dried and crushed after polymerization.

[0051] Examples of the polymerization in an aqueous medium include soap-free emulsion polymerization, emulsion polymerization, suspension polymerization, and seed polymerization.

[0052] Examples of polymerization initiators that can be used in the polymerization include persulfates such as potassium persulfate and ammonium persulfate; peroxides such as benzoyl peroxide and lauryl peroxide; and azo compounds such as azobisisobutyronitrile. These polymerization initiators can be used alone or in combination of two or more. The amount of polymerization initiator used is preferably 0.1 to 10 parts by mass per 100 parts by mass of the monomer component.

[0053] Examples of emulsifiers that can be used in the polymerization include quaternary ammonium salts such as dodecyltrimethylammonium chloride, stearyltrimethylammonium chloride, cetyltrimethylammonium chloride, and alkylbenzyldimethylammonium chloride; alkyl sulfonates such as sodium dodecylsulfonate; alkylbenzenesulfonates such as sodium dodecylbenzenesulfonate; α-sulfo fatty acid ester salts such as sodium 1-methyl 2-sulfotetradecanoate; polyethylene glycol alkylaryl ethers such as polyethylene glycol nonylphenyl ether; polyoxyethylene alkyl ethers such as polyoxyethylene lauryl ether; polyoxyethylene polycyclic phenyl ethers, allyl ethers, and salts of their sulfates. Quaternary ammonium salts are preferred. These emulsifiers may be used alone or in combination of two or more. The amount of emulsifier used is preferably 0.01 to 20 parts by mass per 100 parts by mass of the monomer component.

[0054] A chain transfer agent may be used during the polymerization. Examples include β-mercaptopropionic acid, methyl mercaptan, n-dodecyl mercaptan, and α-methylstyrene dimer. These mercaptan-based chain transfer agents may be used alone or in combination of two or more. The amount of chain transfer agent used depends on the type of chain transfer agent and the polymerization conditions and is not particularly limited, but is preferably 0.1 to 10 parts by mass, more preferably 0.1 to 5 parts by mass, per 100 parts by mass of the monomer component.

[0055] In the above polymerization, a polymer dispersion stabilizer, an inorganic dispersion stabilizer, a polymerization inhibitor, etc. may be further included as needed.

[0056] Examples of aqueous media that can be used in the polymerization include water and mixtures of water and hydrophilic organic solvents. Examples of water include purified water (e.g., ion-exchanged water, distilled water), groundwater, and tap water. Examples of hydrophilic organic solvents include lower alcohols such as methanol, ethanol, and isopropanol; polyols such as ethylene glycol, propylene glycol, butylene glycol, diethylene glycol, and triethylene glycol; cellosolves such as methyl cellosolve and ethyl cellosolve; ketones such as acetone; ethers such as tetrahydrofuran; and esters such as methyl formate. Hydrophilic organic solvents can be used alone or in combination of two or more. The amount of hydrophilic organic solvent added is generally 10 parts by mass or less relative to 100 parts by mass of water.

[0057] In the above polymerization method, the polymerization temperature is usually 40 to 100° C., preferably 55 to 85° C., and the polymerization time is usually 1 to 24 hours, preferably 1 to 10 hours.

[0058] [Example] 1. Production of resin particles The resin particles of Examples and Comparative Examples were produced by the following methods. The meanings of the abbreviations and products in the following description are as follows.

[0059] MMA: Methyl methacrylate (homopolymer Tg: 105°C) BA: Butyl acrylate (homopolymer Tg: -54°C) EGDMA: ethylene glycol dimethacrylate TMPTMA: trimethylolpropane trimethacrylate St:Styrene DVB: divinylbenzene Emulsifier A: Dodecyltrimethylammonium chloride V-50: Polymerization initiator, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name "V-50", water-soluble azo polymerization initiator, 2,2'-azobis(2-methylpropionamidine) dihydrochloride βMPA: Chain transfer agent, SC Organic Chemical Co., Ltd., trade name "BMPA", β-mercaptopropionic acid Table 1 summarizes the compositions of the compounds used in the base particle formation step and the second monomer polymerization step in Examples and Comparative Examples. Compositions are expressed in parts by mass. Table 1 shows the total parts by mass of monomers added during the initial polymerization and additional polymerization in the base particle formation step. In both the initial and additional polymerizations, the compounds listed in Table 1 were added in a mass ratio of 1:19.

[0060]

Table 1

[0061] Base particle formation process (initial polymerization) First, a reaction apparatus equipped with a stirrer, reflux condenser, thermometer, and nitrogen inlet was charged with 3.9 parts by mass of MMA, 0.5 parts by mass of BA, 0.14 parts by mass of EGDMA, 212 parts by mass of ion-exchanged water, and 0.007 parts by mass of emulsifier A. The temperature was raised to 68°C while nitrogen was introduced. Subsequently, 0.25 parts by mass of V-50 was added to initiate the polymerization reaction.

[0062] Base particle formation process (additional polymerization) Then, 30 minutes after the start of the reaction, an emulsion of 73.4 parts by mass of MMA, 9.5 parts by mass of BA, 2.57 parts by mass of EGDMA, 132 parts by mass of ion-exchanged water, and 0.326 parts by mass of emulsifier A was added dropwise over 90 minutes. The mixture was then maintained at 68°C for 30 minutes to obtain base particles.

[0063] Second monomer polymerization step Next, an emulsion consisting of 10 parts by mass of EGDMA, 50 parts by mass of ion-exchanged water, and 0.125 parts by mass of emulsifier A was added dropwise over 10 minutes and held for 30 minutes. The temperature was then raised to 90°C and held for 60 minutes to polymerize the particles. The reaction solution was cooled, filtered through a 400 mesh filter, dried using a spray dryer, and pulverized using a jet mill to obtain resin particles.

[0064] 1-2. Example 2 Resin particles were produced in the same manner as in Example 1 except that the monomer composition in the second monomer polymerization step was changed as shown in Table 1.

[0065] 1-3. Example 3 Resin particles were produced in the same manner as in Example 1, except that TMPTMA was used instead of EGDMA in the base particle forming step and the second monomer polymerization step.

[0066] 1-4. Examples 4 to 8 Resin particles were produced in the same manner as in Example 1, except that the monomer composition in the base particle formation step was changed as shown in Table 1.

[0067] 1-5. Example 9 Resin particles were produced in the same manner as in Example 1, except that the base particle forming step was performed by the following method and the amount of emulsifier A added in the second monomer polymerization step was 0.005 parts by mass.

[0068] Basic particle formation process First, a reactor equipped with a stirrer, reflux condenser, thermometer, and nitrogen inlet was charged with 96 parts by mass of MMA, 3 parts by mass of EGDMA, 384 parts by mass of ion-exchanged water, and 0.01 parts by mass of emulsifier A. While introducing nitrogen, the temperature was raised to 68°C. Next, 0.25 parts by mass of V-50 was added to initiate the polymerization reaction. Ten minutes after the addition of V-50, 0.5 parts by mass of β-MPA was added. The mixture was then maintained at 68°C for 120 minutes to prepare base particles.

[0069] 1-6. Example 10 Resin particles were produced in the same manner as in Example 9, except that the amount of emulsifier A added in the second monomer polymerization step was 0.015 parts by mass.

[0070] 1-6. Examples 11-12 Resin particles were produced in the same manner as in Example 9 except for the following points.

[0071] As shown in Table 1, the monomer compositions in the base particle forming step and the second monomer polymerization step were changed.

[0072] In the second monomer polymerization step, the amount of emulsifier A added was 0.05 parts by mass.

[0073] 1-7. Comparative Example 1 Resin particles were produced in the same manner as in Example 1 except that the second monomer polymerization step was not performed.

[0074] 1-8. Comparative Example 2 Resin particles were produced in the same manner as in Example 1 except for the following points.

[0075] As shown in Table 1, the monomer composition in the base particle formation step was changed.

[0076] The second monomer polymerization step was not performed.

[0077] 2. Measurement of Particle Size, Slurry Viscosity, and Dimensional Change of Polyimide Porous Membranes For the resin particles shown in Table 2, the particle size, slurry viscosity, and dimensional change rate of the polyimide porous membrane were measured according to the method shown below, and the average particle size, particle size ratio, and CV value were calculated. The results are shown in Table 2. In addition, the average particle size of the resin particles 1 and the average particle size of the base particles 3 in Table 2 are rounded to the third decimal place, and the particle size ratio is also rounded to the third decimal place. The particle size ratio of 1.00 in Examples 9 and 10 is a calculation error caused by the rounding mentioned above. This does not mean that the average particle size of the resin particles 1 = the average particle size of the base particles 3. In fact, the average particle size of the resin particles 1 is considered to be larger than the average particle size of the base particles 3 only in the coating layer.

[0078]

Table 2

[0079] Furthermore, when Example 1 is compared with Examples 4 and 5, it is found that the larger the amount of BA, which is a low Tg monomer, the smaller the dimensional change rate of the porous polyimide membrane.

[0080] Determine particle size From an image of powdered base particles or resin particles taken at 20,000x magnification using a scanning electron microscope, 200 particles of average size are selected and the diameter of each particle is measured to obtain the particle size distribution. The average particle diameter is taken as the average particle size, and the particle size ratio is calculated by dividing the average particle size of the resin particles by the average particle size of the base particles. The CV value is calculated by dividing the standard deviation of the resin particle size distribution by the average particle size of the resin particles.

[0081] Determination of slurry viscosity The slurry viscosity was measured according to the above-mentioned measurement method 1. As a rotation / revolution type mixer, an Awatori Rentaro ARE-310 (manufactured by THINKY) was used. Determination of dimensional change rate of porous polyimide membranes The dimensional change rate of the porous polyimide film was measured according to the above-mentioned measurement method 2. As a rotation / revolution type mixer, Awatori Rentaro ARE-310 (manufactured by THINKY Corporation) was used.

[0082] 3. Determination of gel fraction, dissolution rate, and swelling index The gel fraction, elution fraction, and swelling index of the resin particles shown in Table 3 were measured by the following methods, and the dispersibility of the resin particles was evaluated. The results are shown in Table 3.

[0083]

Table 3

[0084] Determination of gel fraction and dissolved component rate Resin particles were dispersed in DMAc to a concentration of 10% by mass to prepare a dispersion. The dispersion was then allowed to stand in a thermostatic bath at 25°C for one day. Then, 10 g of the dispersion was weighed into a centrifuge tube as a measurement sample and subjected to solid-liquid separation using a centrifuge. The supernatant fraction was collected and dried. The weight after drying was used as the weight of the eluted fraction. The gel fraction (insoluble fraction) weight, gel fraction, and eluted component ratio were calculated using the following formulas.

[0085] Gel component weight (g) = resin particle weight (g) - eluted component weight (g) Gel fraction (mass %) = [Gel component weight (g) / Resin particle weight (g)] × 100 Dissolution rate (mass %) = [dissolution weight (g) / resin particle weight (g)] × 100 Determination of swelling index The amount of solvent that could not be recovered in the centrifugation step during the measurement of the gel fraction was calculated, and the ratio of the solvent to the weight of the gel fraction was calculated to calculate the swelling index of the particles.

[0086] Swelling index = {weight of dispersion medium (g) - (weight of supernatant component (g) - weight of dissolved component (g))} / weight of gel component (g) Dispersibility The resin particles were dispersed in DMAc to a concentration of 10% by mass, and the dispersibility of the dispersion was observed using a scanning electron microscope at a magnification of 5000 times. The results were judged based on the following criteria. The electron microscope images of Example 1 and Examples 9 to 12 are shown in FIG. Figure 3 shown.

[0087] ◯: The resin particles are highly dispersed, and no aggregation of the resin particles is observed.

[0088] Δ: The resin particles are highly dispersed, but regions where the resin particles aggregate are observed.

[0089] ×: Resin particles are clearly aggregated.

[0090]

Explanation of symbols

Claims

1. A resin particle for forming pores in a polyimide porous membrane, characterized in that: The resin particles are composed of (meth)acrylic resin, The ratio of the (meth)acrylic acid monomer unit in the (meth)acrylic acid resin is 60 to 100% by mass. The resin particles include a base particle and a coating portion that covers at least a portion of a surface of the base particle. The base particles are composed of a polymer of a first monomer, The coating portion is composed of a polymer of a second monomer, The content of the polyfunctional (meth)acrylate of the first monomer is 0 to 10% by mass. The content of the polyfunctional (meth)acrylate of the second monomer is 65 to 100% by mass.

2. The resin particles according to claim 1, wherein In the first monomer, the content of the low Tg monomer whose homopolymer has a glass transition temperature of 0° C. or lower is 10 to 50% by mass. The low Tg monomer is a monofunctional (meth)acrylate.

3. The resin particles according to claim 1, wherein The eluted component ratio of the resin particles is 10% by mass or less.

4. The resin particles according to claim 1, wherein The eluted component ratio of the resin particles is 5% by mass or less.

5. The resin particles according to any one of claims 1 to 4, wherein The swelling index of the resin particles is 1.8 or less.

6. A resin particle for forming pores in a polyimide porous membrane, characterized in that: The resin particles are composed of (meth)acrylic resin, The ratio of the (meth)acrylic acid monomer unit in the (meth)acrylic acid resin is 60 to 100% by mass. The slurry viscosity measured by the following measurement method 1 is less than 25 Pa·s, The dimensional change rate of the porous polyimide membrane measured by the following measurement method 2 is 40% or less. Determination method 1: First, 1.2 g of polyamic acid, 14 g of dimethylacetamide (DMAc), and 4.8 g of the resin particles were weighed and mixed using a rotation / revolution mixer. Mixing was repeated three times at 2000 rpm for 5 minutes and degassing was repeated at 2200 rpm for 30 seconds to obtain a slurry. The viscosity of the obtained slurry was then measured using an E-type viscometer at 25°C and 5 rpm. Determination method 2: First, 1.2 g of polyamic acid, 12 g of DMAC, and 1.8 g of the resin particles were weighed and mixed three times using a rotation / revolution type mixer at 2000 rpm for 5 minutes and degassing at 200 rpm for 30 seconds to obtain a slurry. The slurry was then applied to a substrate that had been subjected to a peeling treatment so that the film thickness after drying reached 30 μm to form a coating sheet. The coating sheet was dried at 100° C. for 2 minutes. The obtained dried sheet was cut into a size of 2 cm × 5 cm. The polyimide was calcined in an electric furnace under the conditions of maintaining the film at 250° C. for 30 minutes, then heating the film and maintaining the film at 350° C. for 30 minutes. The dimensional change rate of the porous polyimide membrane was calculated based on the area S1 of the evaluation sheet before calcination and the area S2 of the evaluation sheet after calcination based on the following formula: Dimensional change rate of the porous polyimide film (%) = [(S1-S2) / S1]×100.

7. A method for producing resin particles for forming pores in a porous polyimide membrane, characterized in that: The resin particles are composed of (meth)acrylic resin, The ratio of the (meth)acrylic acid monomer unit in the (meth)acrylic acid resin is 60 to 100% by mass. The method includes a base particle forming step and a second monomer polymerization step. In the base particle forming step, a first monomer is polymerized in an aqueous medium to form base particles. In the second monomer polymerization step, the second monomer is polymerized in an aqueous medium in the presence of the base particles. The content of the polyfunctional (meth)acrylate of the first monomer is 0 to 10% by mass. The content of the polyfunctional (meth)acrylate of the second monomer is 65 to 100% by mass.

Images