Thermally expandable microcapsule, method for producing same, resin composition, and method for producing foam molded body

Abstract

Provided is a thermally expandable microcapsule able to be expanded to a greater degree. The present invention relates to a thermally expandable microcapsule which expands when heated and which has: a shell containing a thermoplastic resin; and a thermal expansion agent housed inside the shell. The shell contains a resin having a carboxyl group and / or a nitrile group. If the expansion initiation temperature is denoted by Ts (ºC) and the temperature at which the displacement in the intensity of a peak that indicates the thermal expansion agent during temperature increase based on the intensity at the start of temperature increase, as measured by gas chromatography, reaches 2% of the maximum intensity of a peak that indicates the thermal expansion agent at 280°C or lower is denoted by Ts_gas(2%), formula 1 is satisfied.　Formula 1: Ts_gas(2%)-Ts≥0ºC

Description

Thermally expandable microcapsules and method for producing the same, resin composition, and method for producing foamed molded articles

[0001] The present invention relates to a thermally expandable microcapsule, a method for producing the same, a resin composition, and a method for producing a foamed molded article.

[0002] Thermally expandable microcapsules are known that have a shell containing a thermoplastic resin and a thermal expansion agent (foaming agent) held inside the shell. When heated, the shell softens, and the thermal expansion agent simultaneously gasifies (volatilizes), increasing the volume of the thermal expansion agent. The expanded volume of the thermal expansion agent then pushes the softened shell outward, causing it to expand.

[0003] Thermally expandable microcapsules are added to a resin composition containing, for example, an elastomer as a base resin, and the thermally expandable microcapsules are foamed to produce a foamed molded article. To achieve satisfactory expansion of the base resin, such as an elastomer, it is desirable for the thermally expandable microcapsules to expand at the molding temperature of the elastomer. Because the molding temperature of elastomers is approximately 150°C to 250°C, thermally expandable microcapsules that begin to expand at such high temperatures are also required.

[0004] It is known that the use of a monomer having a carboxyl group as the shell material can increase the glass transition temperature (Tg) of the shell, thereby raising the softening temperature and increasing the expansion initiation temperature of the thermally expandable microcapsules. It is also known that the use of a monomer having a nitrile group as the shell material can improve the gas barrier properties of the thermally expandable microcapsules (for example, Patent Document 1).

[0005] Japanese Patent Application Laid-Open No. 2023-107172

[0006] In order to expand thermally expandable microcapsules well, it is important to prevent gas leakage before the start of expansion. However, conventional thermally expandable microcapsules such as those described in Patent Document 1 have insufficient gas barrier properties when heated, so gas leakage occurs before the start of expansion, and as a result, sufficient expansion is not possible.

[0007] The present invention has been made in consideration of the above problems, and aims to provide thermally expandable microcapsules that can expand more, a method for producing the same, a resin composition containing the thermally expandable microcapsules, and a method for producing foamed molded articles using the resin composition.

[0008] One embodiment of the present invention for solving the above problems relates to a thermally expandable microcapsule, a method for producing the same, a resin composition, and a method for producing a foamed molded article, as set forth in the following [1] to [6]. [1] A thermally expandable microcapsule that expands upon heating, having a shell containing a thermoplastic resin and a thermally expandable agent held within the shell, wherein the shell contains a resin having a carboxyl group and / or a nitrile group, and the thermally expandable microcapsule satisfies the following formula 1, where Ts (°C) is the expansion initiation temperature, and Ts_gas(2%) (°C) is the temperature at which the change in intensity indicating the thermally expandable agent during heating, based on the intensity at the start of heating, as measured by gas chromatography, is 2% of the maximum intensity of the peak indicating the thermally expandable agent at 280°C or less: Ts_gas(2%) - Ts≧0°C. [2] The thermally expandable microcapsule according to [1], having an average particle size of 10 μm or more and 80 μm or less. [3] A method for producing thermally expandable microcapsules, comprising the steps of: suspending a polymerizable mixture containing a monomer and a thermal expansion agent in an aqueous medium; polymerizing the monomer contained in the suspended polymerizable mixture to obtain microcapsules having a shell containing a thermoplastic resin and a thermal expansion agent held inside the shell; and drying the microcapsules while stirring in a vacuum environment. [4] A resin composition comprising a base resin and the thermally expandable microcapsules described in [1] or [2]. [5] The resin composition described in [4], wherein the base resin contains a thermoplastic elastomer. [6] A method for producing a foamed molded product, comprising the steps of: preparing the resin composition described in [4]; and heating the resin composition to expand the thermally expandable microcapsules.

[0009] According to the present invention, there are provided thermally expandable microcapsules that can be expanded to a greater extent, and a method for producing the same.

[0010] [Thermal Expandable Microcapsules] One embodiment of the present invention relates to thermal expandable microcapsules that expand in volume when heated.

[0011] The thermally expandable microcapsules have a shell containing a thermoplastic resin and a thermal expansion agent held inside the shell. When heated, the shell softens and the thermal expansion agent gasifies (volatilizes), increasing its volume. The expanded volume of the thermal expansion agent then pushes the softened shell outward, causing it to expand.

[0012] The thermally expandable microcapsules contain a resin having a carboxyl group and / or a nitrile group in the shell. The structural unit having a carboxyl group increases the glass transition temperature (Tg) of the resin and the softening temperature of the shell. The structural unit having a nitrile group improves the gas barrier properties of the shell.

[0013] Furthermore, the thermally expandable microcapsules satisfy the following formula 1, where Ts (°C) is the expansion start temperature, and Ts_gas (2%) (°C) is the temperature at which the change in intensity indicating the thermal expansion agent during heating, based on the intensity at the start of heating as measured by gas chromatography, is 2% of the maximum intensity of the peak indicating the thermal expansion agent at 280°C or less. Ts_gas (2%) - Ts ≧ 0°C ... formula 1

[0014] The expansion starting temperature (Ts) is the temperature at which the thermally expandable microcapsules start to expand, and can be measured using a thermomechanical analyzer (TMA).

[0015] Ts_gas(2%) can be used as an index indicating the temperature at which gas begins to leak when the thermally expandable microcapsule is heated. Ts_gas(2%) can be measured using a gas chromatograph. Specifically, the intensity (height) at the start of the temperature rise is set to zero (base), and the thermally expandable microcapsule is heated from 40°C to 330°C at a temperature rise rate of 5°C / min, and the detected gas is continuously detected, and the peak intensity (I) at each temperature is measured.

[0016] The maximum intensity of the peak at 280°C or less is defined as the maximum intensity (Imax), and the temperature at which the displacement calculated using the following formula becomes 2% is defined as the gas leakage start temperature (Ts_gas(2%)): Displacement (%) = I / Imax x 100

[0017] According to the findings of the present inventors, when the gas components detected by the gas chromatograph are qualitatively analyzed by Direct EGA (Evolved Gas Analysis)-MS (Mass Spectrometry), most of the components resulting from the thermal decomposition of the shell of the thermally expandable microcapsules usually appear in a temperature range above 280° C. Therefore, the peak appearing at 280° C. or lower is considered to be the peak representing the thermal expansion agent.

[0018] Thermally expandable microcapsules satisfying Formula 1 hardly leak gas when heated at temperatures lower than the expansion initiation temperature. Therefore, a larger proportion of the gas generated by the volatilization of the thermal expansion agent can contribute to expansion than conventional microcapsules. Therefore, thermally expandable microcapsules satisfying Formula 1 can expand more than conventional microcapsules.

[0019] The expansion rate (volume expansion ratio) of a thermally expandable microcapsule also varies depending on the type of resin constituting the shell, the type of thermal expansion agent, the particle size of the thermally expandable microcapsule, etc. Therefore, in this specification, a thermally expandable microcapsule having a larger expansion rate (volume expansion ratio) than a thermally expandable microcapsule having the same or similar type of resin constituting the shell, the type of thermal expansion agent, and the particle size of the thermally expandable microcapsule.

[0020] According to the findings of the present inventors, the carboxyl and nitrile groups contained in the shell are both hydrophilic groups and have a high affinity for water derived from the aqueous medium used in the production of thermally expandable microcapsules. Therefore, it is difficult to completely remove water molecules held by the carboxyl and nitrile groups deep inside the shell through hydrogen bonds, coordinate bonds, etc., during the drying process performed in the production of thermally expandable microcapsules. Furthermore, it is believed that the small amount of water molecules held by these functional groups remaining in the thermally expandable microcapsules does not sufficiently enhance the gas barrier properties of the shell, making it more susceptible to gas leakage.

[0021] For example, resins containing nitrile groups are thought to enhance the barrier properties of the shell by forming a densely packed structure through aggregation of the nitrile groups. Carboxyl groups often form dimers, which are thought to enhance the barrier properties of the shell by forming a densely packed structure. In contrast, when water molecules in thermally expandable microcapsules coordinate with nitrile or carboxyl groups, the cohesive force between the nitrile or carboxyl groups decreases, causing the packing structure to collapse and reducing the barrier properties.

[0022] In contrast, in this embodiment, as described below, the drying conditions are optimized so that water molecules held by carboxyl groups and nitrile groups can also be removed. This is thought to reduce the amount of water molecules remaining in the thermally expandable microcapsules compared to conventional methods, improving the barrier properties of the shell and suppressing leakage of gas whose volume increases upon heating. Furthermore, in this embodiment, by suppressing gas leakage before the start of expansion, problems such as poor appearance and void formation during foam molding can be made less likely to occur. Note that it is difficult to measure the amount of water molecules held by the carboxyl groups and nitrile groups mentioned above.

[0023] The expansion start temperature (Ts) is preferably 120°C or higher and 250°C or lower, more preferably 130°C or higher and 240°C or lower, and even more preferably 140°C or higher and 230°C or lower. By setting the expansion start temperature (Ts) within the above-mentioned range, the thermally expandable microcapsules can be used as a foaming agent for resins (such as elastomers, engineering plastics, super engineering plastics, etc.) that require processing at higher temperatures, such as kneading with resins or molding of thermally expandable microcapsule-containing resin compositions such as masterbatches, and in this case, a well-foamed foamed molded product can be obtained. The expansion start temperature (Ts) may be adjusted appropriately depending on the application of the thermally expandable microcapsules.

[0024] The difference between Ts_gas (2%) and the expansion start temperature (Ts) calculated by Equation 1 is preferably greater than 0° C. and less than 50° C., more preferably greater than 3° C. and less than 50° C., and even more preferably greater than 5° C. and less than 50° C. The greater the difference, the more gas leakage before the start of expansion is suppressed, and the greater the expansion of the thermally expandable microcapsules can be achieved.

[0025] Furthermore, the maximum expansion temperature (Tmax) of the thermally expandable microcapsules is preferably 170°C or higher and 280°C or lower, more preferably 180°C or higher and 270°C or lower, and even more preferably 190°C or higher and 270°C or lower. By setting the maximum expansion temperature (Tmax) within the above-mentioned range, the thermally expandable microcapsules can be used as a foaming agent for resins (such as engineering plastics and super engineering plastics) that are foam-molded at higher temperatures, and a well-foamed foamed molded product can be obtained. The maximum expansion temperature (Tmax) may be adjusted appropriately depending on the application of the thermally expandable microcapsules. The maximum expansion temperature (Tmax) of the thermally expandable microcapsules can be measured using a thermomechanical analyzer (TMA).

[0026] The expansion rate (volume expansion ratio) of the thermally expandable microcapsules is preferably 20 times or more and 350 times or less, more preferably 30 times or more and 350 times or less, and even more preferably 40 times or more and 350 times or less. The volume expansion ratio is determined by the density (d ０) and the density (d) of the thermally expandable microcapsules after heating and foaming.

[0027] The expansion rate (volume expansion ratio) of the thermally expandable microcapsules is determined by the density (d ０ The density (d) of the thermally expandable microcapsules after heating and foaming was measured by an immersion method (Archimedes method) using isopropyl alcohol in an environment at a temperature of 25°C. ０ / d (unit: times).

[0028] The average particle diameter (D50) of the thermally expandable microcapsules before expansion is preferably 10 μm or more and 100 μm or less, more preferably 15 μm or more and 80 μm or less, and even more preferably 15 μm or more and 60 μm or less. A moderately large average particle diameter makes the microcapsules less likely to aggregate during drying, thereby enabling more efficient removal of moisture. Furthermore, the larger the average particle diameter, the larger the particle diameter of the thermally expandable microcapsules after expansion, making it easier to reduce the weight of foams obtained from resin compositions containing the microcapsules. A moderately small average particle diameter also makes it possible to appropriately reduce the particle diameter of the thermally expandable microcapsules after expansion, thereby improving the appearance of foamed molded articles. Furthermore, a moderately small average particle diameter makes it possible to appropriately adjust the particle diameter of the thermally expandable microcapsules after expansion, thereby appropriately adjusting the density of the foam and suppressing a decrease in strength of the foam due to a decrease in density. The average particle diameter can be the volume-based median diameter measured using a laser diffraction particle size distribution analyzer.

[0029] (Shell) The shell is a hollow outer shell formed from a thermoplastic resin.

[0030] The type of resin constituting the shell is not particularly limited. For example, the resin constituting the shell may be a (co)polymer having a structural unit having a carboxyl group, a (co)polymer having a structural unit having a nitrile group, or a copolymer having a structural unit having a carboxyl group and a structural unit having a nitrile group.

[0031] A structural unit having a carboxyl group can increase the glass transition temperature (Tg) of the shell and increase the expansion initiation temperature (Ts) of the thermally expandable microcapsules. The structural unit having a carboxyl group can be a structural unit derived from a raw material monomer such as (meth)acrylic acid, a carboxyl group-containing (meth)acrylate such as β-carboxyethyl (meth)acrylate, itaconic acid, crotonic acid, cinnamic acid, maleic acid, maleic anhydride, chloromaleic acid, chloromaleic anhydride, itaconic acid, fumaric acid, or citraconic acid. Among these raw material monomers, (meth)acrylic acid is preferred, and methacrylic acid is more preferred, from the viewpoints of ease of reaction, control of the softening point of the shell, control of the expansion temperature, and the like.

[0032] The structural unit having a nitrile group can improve the gas barrier properties of the shell and increase the expansion rate of the thermally expandable microcapsules. The structural unit having a nitrile group can be a structural unit derived from a raw material monomer such as (meth)acrylonitrile, α-chloro(meth)acrylonitrile, α-ethoxy(meth)acrylonitrile, or fumaronitrile. These raw material monomers may be used alone or in combination of two or more.

[0033] Among these raw material monomers, (meth)acrylonitrile is preferred from the viewpoint of improving the barrier properties of the shell of the thermally expandable microcapsule and increasing the expansion rate at high temperatures, and methacrylonitrile is more preferred from the viewpoint of suppressing yellowing of the thermally expandable microcapsule. Note that, from the viewpoint of more effectively suppressing yellowing of the thermally expandable microcapsule, it is preferable that the resin constituting the shell does not contain a constituent unit derived from acrylonitrile.

[0034] In this specification, (meth)acrylic means acrylic or methacrylic, (meth)acrylate means acrylate or methacrylate, and (meth)acrylo means acrylo or methacrylo, respectively.

[0035] The resin constituting the shell may have structural units derived from other vinyl monomers. Examples of other vinyl monomers include (meth)acrylic acid esters not having a carboxyl group, such as vinylidene chloride, vinyl acetate, divinylbenzene, methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, isobornyl (meth)acrylate, cyclohexyl (meth)acrylate, and benzyl (meth)acrylate; styrene-based monomers, such as styrene sulfonic acid or its salts with sodium or the like, α-methylstyrene, and chlorostyrene; and (meth)acrylamide monomers, such as (meth)acrylamide and its substituted derivatives. These vinyl monomers may be used alone or in combination of two or more.

[0036] From the viewpoint of increasing the expansion initiation temperature (Ts) and further enhancing the gas barrier properties of the shell, the resin constituting the shell is preferably a (co)polymer having a structural unit having a carboxyl group and a structural unit having a nitrile group. In this copolymer, the mass ratio of the structural unit having a carboxyl group to the structural unit having a nitrile group (carboxyl group:nitrile group) can be 1:9 to 9:1, preferably 2:8 to 8:2, and more preferably 4:6 to 6:4. These mass ratios can be determined depending on the expansion initiation temperature (Ts) of the thermally expandable microcapsules. When this copolymer contains other vinyl monomers, the proportion of the structural units derived from the other vinyl monomers can be 0% by mass or more and 30% by mass or less relative to the total mass of the copolymer.

[0037] The resin constituting the shell may have structural units derived from a crosslinkable monomer. The crosslinkable monomer may be a monomer having two or more functional groups (e.g., polymerizable double bonds) within the molecule that react with other monomers. Examples of crosslinkable monomers include aromatic divinyl compounds including divinylbenzene, divinylnaphthalene, and derivatives thereof; polyfunctional (meth)acrylates including ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, butane di(meth)acrylate, nonane di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and tri(meth)acrylformal; divinyl compounds such as N,N-divinylaniline and divinyl ether; and polyfunctional isocyanate compounds such as triallyl cyanurate and triallyl isocyanurate. These crosslinkable monomers may be used alone or in combination of two or more.

[0038] From the viewpoint of increasing the expansion coefficient of the thermally expandable microcapsules, it is preferable that the proportion of the structural units derived from the crosslinkable monomer in the thermally expandable microcapsules is small. Specifically, the proportion of the structural units derived from the crosslinkable monomer is preferably 0% by mass or more and 5% by mass or less, more preferably 0.02% by mass or more and 3% by mass or less, and even more preferably 0.03% by mass or more and 2% by mass or less, relative to the total mass of the resin constituting the shell.

[0039] The presence or absence of a nitrile group can be confirmed by observing the IR spectrum of the shell at 2260 to 2240 cm －１ The presence or absence of a peak at 1730 to 1700 cm in the IR spectrum of the shell indicates the presence or absence of a carboxyl group. －１ This can be confirmed by the presence or absence of a peak (a peak based on a C═O bond) appearing nearby.

[0040] (Thermal expansion agent) The thermal expansion agent is an additive enclosed inside the shell that volatilizes and increases in volume when heated. The thermal expansion agent is preferably a substance that is liquid at room temperature and volatilizes at a temperature lower than the softening temperature of the shell.

[0041] Examples of such thermal expansion agents include hydrocarbons or isomers thereof such as methane, ethane, ethylene, propane, propene, n-butane, isobutane, butene, isobutene, n-pentane, isopentane, neopentane, n-hexane, isohexane, n-heptane, isoheptane, n-octane, isooctane, n-nonane, isononane, n-decane, isodecane, n-dodecane, isododecane, petroleum ether, isoparaffin mixtures, CCl ３ F, CCl ２ F ２ , CClF ３ , CClF ２ -CClF ２ Examples of suitable thermal expansion agents include chlorofluorocarbons such as methyl silane, trimethylethyl silane, trimethylisopropyl silane, and trimethyl-n-propyl silane, as well as tetraalkyl silanes such as tetramethylsilane, trimethylethyl silane, trimethylisopropyl silane, and trimethyl-n-propyl silane. These thermal expansion agents may be used singly or in combination of two or more. Among these, isobutane, n-butane, n-pentane, isopentane, n-hexane, isooctane, isododecane, and petroleum ether are preferred. The type and combination of thermal expansion agents can be determined based on the expansion initiation temperature (Ts) of the thermally expandable microcapsules. From the perspective of achieving a high expansion initiation temperature, isooctane, which has a high boiling point, is preferred. Furthermore, from the perspective of achieving a desired expansion initiation temperature, a combination of isooctane and isopentane is preferred.

[0042] The content of the thermal expansion agent in the thermally expandable microcapsules is preferably 5 to 50 parts by mass, more preferably 12 to 40 parts by mass, and even more preferably 15 to 35 parts by mass, per 100 parts by mass of the shell. By setting the amount of the thermal expansion agent within the above range, the expansion coefficient of the thermally expandable microcapsules can be sufficiently increased. Furthermore, by setting the amount of the thermal expansion agent to an appropriate amount, not too much, gas leakage from the thermally expandable microcapsules can be suppressed.

[0043] [Method for producing thermally expandable microcapsules] The method for producing thermally expandable microcapsules is not particularly limited, and they may be produced by any known method. For example, thermally expandable microcapsules can be produced by suspending a polymerizable mixture containing a monomer that is a raw material for the resin that constitutes the shell and a thermal expansion agent in an aqueous medium, polymerizing the monomer contained in the suspended polymerizable mixture to produce microcapsules that contain a shell and a thermal expansion agent held therein, and drying the produced microcapsules.

[0044] (Polymerizable mixture and suspension thereof) The polymerizable mixture containing the monomer and the thermal expansion agent may be a mixture containing the above-mentioned monomer and the above-mentioned thermal expansion agent. The polymerizable mixture may also contain a polymerization initiator for initiating polymerization of the monomer.

[0045] The polymerization initiator is preferably an oil-soluble polymerization initiator that dissolves in the monomer. Examples of polymerization initiators include dialkyl peroxide compounds, diacyl peroxide compounds, peroxyester compounds, peroxydicarbonate compounds, and azo compounds. Examples of dialkyl peroxide compounds include methyl ethyl peroxide, di-t-butyl peroxide, and dicumyl peroxide. Examples of diacyl peroxide compounds include diisobutyryl peroxide, dibenzoyl peroxide, di(2,4-dichlorobenzoyl)peroxide, and di(3,5,5-trimethylhexanoyl)peroxide. Examples of peroxyester compounds include t-butyl peroxypivalate, t-hexyl peroxypivalate, t-butyl peroxyneodecanoate, t-hexyl peroxyneodecanoate, 1-cyclohexyl-1-methylethyl peroxyneodecanoate, 1,1,3,3-tetramethylbutyl peroxyneodecanoate, cumyl peroxyneodecanoate, and (α,α-bis-neodecanoylperoxy)diisopropylbenzene. Examples of peroxydicarbonate compounds include bis(4-t-butylcyclohexyl)peroxydicarbonate, di-n-propyl-oxydicarbonate, di-isopropyl peroxydicarbonate (hereinafter sometimes referred to as "IPP"), di(2-ethylethylperoxy)dicarbonate, di-methoxybutyl peroxydicarbonate, and di(3-methyl-3-methoxybutylperoxy)dicarbonate. Examples of the azo compound include 2,2'-azobisisobutyronitrile, 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), and 1,1'-azobis(1-cyclohexanecarbonitrile). These polymerization initiators may be used alone or in combination of two or more.

[0046] The amount of the polymerization initiator in the polymerizable mixture can be 0.01% by mass or more and 7% by mass or less, preferably 0.1% by mass or more and 5% by mass or less, and more preferably 0.3% by mass or more and 3% by mass or less, based on the total mass of the monomers.

[0047] This polymerizable mixture is added to an aqueous medium and stirred. As a result, the polymerizable mixture, which is the oil phase, is dispersed in the aqueous medium in the form of droplets. Stirring may be carried out so as to form droplets of a size corresponding to the average particle size of the thermally expandable microcapsules to be produced.

[0048] The aqueous medium may be water to which a dispersion stabilizer, a co-stabilizer, a polymerization aid, etc. is added. The aqueous medium is preferably adjusted to a pH suitable for the polymerization reaction (about pH 2 to 5) by adding an acid.

[0049] Examples of the dispersion stabilizer include silica, colloidal silica, calcium phosphate, magnesium hydroxide, aluminum hydroxide, ferric hydroxide, barium sulfate, calcium sulfate, sodium sulfate, calcium oxalate, calcium carbonate, barium carbonate, and magnesium carbonate.

[0050] The dispersion stabilizer may be a colloid of a poorly water-soluble metal hydroxide (e.g., magnesium hydroxide) obtained by reacting a water-soluble polyvalent metal compound (e.g., magnesium chloride) with an alkali metal hydroxide (e.g., sodium hydroxide) in an aqueous phase. The calcium phosphate may be a reaction product of sodium phosphate and calcium chloride in an aqueous phase.

[0051] The amount of the dispersion stabilizer in the aqueous medium can be 0.1% by mass or more and 20% by mass or less based on the total mass of the added monomers.

[0052] In addition to the dispersion stabilizer, a co-stabilizer may be used. Examples of the co-stabilizer include condensation products of diethanolamine and aliphatic dicarboxylic acids, condensation products of urea and formaldehyde, polyvinylpyrrolidone, polyethylene oxide, polyethyleneimine, tetramethylammonium hydroxide, gelatin, methylcellulose, polyvinyl alcohol, dioctyl sulfosuccinate, and sorbitan ester.

[0053] The amount of the co-stabilizer in the aqueous medium can be 0.05% by weight or more and 2% by weight or less, based on the total weight of the added monomers.

[0054] Examples of the polymerization aid include alkali metal nitrite, stannous chloride, stannic chloride, water-soluble ascorbic acids, boric acid, etc. These polymerization aids can efficiently remove heat generated by polymerization, suppress the aggregation of polymer particles, and suppress the adhesion of polymerized products to the walls of the polymerization vessel.

[0055] The amount of the polymerization aid in the aqueous medium can be 0.001% by mass or more and 1% by mass or less based on the total mass of the added monomers.

[0056] The aqueous medium may also contain inorganic salts such as sodium chloride, sodium sulfate, sodium phosphate, etc. These inorganic salts can make the particle shape of the resulting microcapsules uniform.

[0057] The amount of these inorganic salts in the aqueous medium can be from 0 to 100% by mass, preferably from 0.1 to 50% by mass, based on the total mass of the added monomers.

[0058] The aqueous medium may contain an anionic surfactant such as a dialkyl sulfosuccinate or a phosphate ester of polyoxyethylene alkyl (allyl) ether as an emulsifier, but may not necessarily contain an emulsifier.

[0059] (Polymerization) The resulting dispersion of the polymerizable mixture dispersed in an aqueous medium is poured into a reaction vessel, where the monomer is polymerized. Polymerization can be carried out, for example, by stirring the dispersion in a pressurized atmosphere of an inert gas such as nitrogen at 40°C to 80°C for 5 to 50 hours. This produces a slurry containing microcapsules having a shell containing a thermoplastic resin and a thermal expansion agent held within the shell.

[0060] During polymerization, an organosilicon compound such as a silane coupling agent may be added to the dispersion liquid, which can suppress fusion of the thermally expandable microcapsules during expansion and sharpen the expansion behavior.

[0061] Examples of organosilicon compounds include 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltris(trimethylsiloxy)silane, vinyltrichlorosilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(2-methoxyethoxy)silane, vinyltris(2-methoxyethoxy)silane, vinyltriacetoxysilane, vinyltris(2-trimethylsiloxy)silane, allyltrimethylsilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldi Examples of suitable compounds include methoxysilane, 3-[N-allyl-N-(2-aminoethyl)]aminopropyltrimethoxysilane, 3-(N-allyl-N-glycidyl)aminopropyltrimethoxysilane, 3-(N-allyl-N-methacryloyl)aminopropyltrimethoxysilane, N,N-bis[3-(methyldimethoxysilyl)propyl]methacrylamide, N,N-bis[3-(trimethoxysilyl)propyl]methacrylamide, 1-(3-methacryloxypropyl)-1,1,3,3,3-pentamethyldisiloxane, and trimethoxysilylvinylbicyclo[2,2,1]heptane. Among these, compounds having a polymerizable group are preferred, and 3-methacryloxypropyltrimethoxysilane is more preferred, from the viewpoint of more effectively suppressing fusion during expansion of the thermally expandable microcapsules.

[0062] The amount of the organosilicon compound is preferably 0.005% by weight or more and 5% by weight or less, more preferably 0.01% by weight or more and 5% by weight or less, and even more preferably 0.015% by weight or more and 2% by weight or less, relative to the total weight of the thermally expandable foam microcapsules.

[0063] Thereafter, the microcapsules are separated and recovered from the aqueous medium by methods such as filtration, centrifugation, sedimentation, etc. The recovered microcapsules are preferably washed with washing water such as ion-exchanged water.

[0064] (Drying) Finally, the microcapsules are dried.

[0065] As described above, by sufficiently removing the water molecules held by the carboxyl groups and nitrile groups, including the water molecules held by these functional groups deep in the shell, it is possible to suppress the deterioration of the barrier properties due to the water molecules in the shell and increase the expansion rate of the thermally expandable microcapsules. Therefore, in this embodiment, the microcapsules are dried using a method that removes water molecules more efficiently than conventional methods.

[0066] Specifically, the microcapsules are dried while stirring under a vacuum environment. Because it is difficult to sufficiently remove the aqueous medium from the slurry by filtration or other methods, the dried microcapsules are typically cake-like and aggregated into clumps via the aqueous medium. Conventional drying methods involving heating while standing at atmospheric pressure and drying with vibration as described in Patent Document 1 are unable to sufficiently break down the aggregated microcapsules, and are unable to fully remove the water molecules held deep within the shells. In contrast, drying with stirring can fully break down the aggregated microcapsules, removing not only the free water present on the surface of the microcapsules but also the bound water held by carboxyl groups and nitrile groups deep within the shells. The stirring conditions are not particularly limited. The stirring conditions can be adjusted depending on the size of the container used for drying with stirring, the shape and material of the stirring blades, the amount of wet cake, the water content, etc., so that the aggregated microcapsules are broken down while the microcapsules are not destroyed by stirring.

[0067] The degree of vacuum (gauge pressure) during drying is preferably −100 kPa or more and −10 kPa or less, more preferably −100 kPa or more and −30 kPa or less, even more preferably −100 kPa or more and −50 kPa or less, and particularly preferably −100 kPa or more and −70 kPa or less. By setting the degree of vacuum within the above range, bound water can be removed more efficiently.

[0068] The temperature of the microcapsules during drying is preferably 30°C or higher and 80°C or lower, more preferably 40°C or higher and 75°C or lower, and even more preferably 45°C or higher and 70°C or lower. By setting the lower limit of the temperature of the microcapsules during drying to the above value, bound water can be removed more efficiently. By setting the upper limit of the temperature of the microcapsules during drying to the above value, fusion of the microcapsules due to heat can be suppressed.

[0069] The drying time is preferably 2 to 20 hours, more preferably 3 to 18 hours, and even more preferably 4 to 15 hours. By setting the drying time to 4 hours or more, bound water can be more thoroughly removed. By setting the drying time to 15 hours or less, fusion of microcapsules due to heat can be suppressed.

[0070] Drying is preferably carried out in one go. Furthermore, drying is preferably carried out while continuously increasing the temperature. By carrying out drying a few times while continuously increasing the temperature, free water and bound water can be efficiently removed, and adjacent functional groups can be immediately aggregated after the water removal, facilitating the formation of a densely packed structure of nitrile groups and a structure due to the dimerization of carboxyl groups.

[0071] [Uses of Thermally Expandable Microcapsules] Thermally expandable microcapsules can be mixed with resins, fibers, etc. to use as a foaming agent to turn them into foams, or can be added to inks, paints, etc. to use as a foaming agent to foam them.

[0072] The resin composition may contain a thermosetting resin as a base resin and thermally expandable microcapsules, or may contain a thermoplastic resin as a base resin and thermally expandable microcapsules. Examples of thermoplastic resins include polyvinyl chloride (PVC), polypropylene (PP), ethylene-vinyl acetate copolymer (EVA), thermoplastic elastomers (TPE), ethylene-methyl methacrylate copolymer (EMMA), ethylene-α-olefin copolymer (LLDPE), and low-density polyethylene (LDPE). Examples of thermosetting resins include ethylene-propylene-diene rubber (EPDM), silicone rubber, and epoxy resin.

[0073] Among these, thermoplastic resins are preferred from the viewpoint of ease of molding, and thermoplastic elastomers (TPEs) are more preferred from the viewpoint of obtaining foamed molded articles that are lightweight and have excellent properties such as impact absorption. Thermoplastic elastomers refer to substances that exhibit the properties of an elastomer, i.e., vulcanized rubber, at room temperature and thermoplastic properties at high temperatures. The thermoplastic elastomer is not particularly limited, and examples that can be used include styrene-based thermoplastic elastomers (abbreviated as TPS; hereinafter, the abbreviations in parentheses represent abbreviations), olefin-based thermoplastic elastomers (TPO, TPV), urethane-based thermoplastic elastomers (TPU), ester-based thermoplastic elastomers (TPEE), and amide-based thermoplastic elastomers (TPAE). These thermoplastic elastomers may be used alone or in combination of two or more.

[0074] It is preferable that the thermoplastic resin has a softening temperature in a range lower than the expansion initiation temperature (Ts) of the thermally expandable microcapsules, and it is preferable that the thermosetting resin has a hardening temperature in a range higher than the expansion initiation temperature (Ts) of the thermally expandable microcapsules.

[0075] The resin composition may be in the form of pellets to be added to other resins during foam molding of a resin to be foamed for the purpose of weight reduction, etc. In this case, the resin composition may be a built-in pellet (BIP) in which the same type of resin as the molding resin that is the main component of the foam molded product (for example, a resin that accounts for 50% by mass or more of the resin components contained in the foam molded product) is used as the base resin, or may be a masterbatch in which any other type of resin is used as the base resin.

[0076] The amount of the thermally expandable microcapsules in the resin composition may be 0.01% by mass or more and 10% by mass or less, preferably 0.05% by mass or more and 8% by mass or less, and more preferably 0.1% by mass or more and 5% by mass or less, based on the total mass of the resin composition in the case of built-in pellets, and 10% by mass or more and 70% by mass or less, preferably 20% by mass or more and 65% by mass or less, and more preferably 30% by mass or more and 60% by mass or less, based on the total mass of the resin composition in the case of master batches.

[0077] The resin composition may contain additives such as fillers, coloring materials such as pigments and dyes, rust inhibitors, and antioxidants.

[0078] The resin composition can be produced by kneading a resin and any additives with thermally expandable microcapsules at a temperature above the softening temperature of the resin and below the expansion initiation temperature (Ts) of the thermally expandable microcapsules.

[0079] The resin composition can be heated to a temperature equal to or higher than the melting temperature and the expansion initiation temperature (Ts) of the base resin to expand the thermally expandable microcapsules while melting the base resin, and then simultaneously molded to produce a foamed molded article. Therefore, the resin composition can be heated and molded as is. Alternatively, the resin composition (built-in pellets or masterbatch) can be added to the molding resin to be foamed, and the molding resin and base resin can be melted and kneaded while expanding the thermally expandable microcapsules, and then simultaneously molded to produce a foamed molded article. In this case, using built-in pellets with the same type of resin as the molding resin as the base resin can prevent unwanted resin from being mixed into the foamed molded article and ensure sufficient compatibility between the molding resin and the base resin. This allows the properties of the molding resin to be fully expressed, resulting in a foamed molded article with a good appearance.

[0080] The foamed molded article thus obtained is lightweight and has high impact absorption properties. The foamed molded article can be used, for example, as a cushioning material, a float material, an acoustic absorbing material, a heat insulating material, a decorative material, etc.

[0081] [Other Embodiments] It goes without saying that the above-described embodiments are exemplary embodiments of the present invention, and the present invention may include embodiments other than the above-described embodiments within the scope of its core technical concept.

[0082] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0083] 1. Preparation of thermally expandable microcapsules (1) Preparation of aqueous medium An aqueous dispersion medium was prepared by mixing 6.5 parts by mass of colloidal silica (20% by mass aqueous dispersion), 0.65 parts by mass of a diethanolamine-adipic acid condensation product (50% by mass aqueous solution, acid value 78 mgKOH / g), 0.48 parts by mass of sodium nitrite, 80 parts by mass of sodium chloride, and 278 parts by mass of water. Hydrochloric acid (5% by mass aqueous solution) was added to this aqueous dispersion medium to adjust the pH to 3.5.

[0084] (2) Preparation of Polymerizable Mixture Methacrylonitrile (MAN), methacrylic acid (MAA), and methyl acrylate (MA) were prepared as monomers. Hydrocarbons, isopentane (IP) and isooctane (IO), were prepared as thermal expansion agents. 2,2'-azobisisobutyronitrile (AIBN) was prepared as a polymerization initiator. These were mixed in the ratios (units: parts by mass) shown in Table 1 to prepare a polymerizable mixture. The amount of AIBN was 0.8 parts by weight when the amount of monomer was 100 parts by weight.

[0085] (3) Suspension Polymerization: The aqueous medium and polymerizable mixture prepared above were stirred and mixed using a homogenizer to prepare a dispersion in which minute droplets of the polymerizable mixture were dispersed in the aqueous medium. This dispersion was charged into a polymerization vessel equipped with a stirrer and heated at 60°C for 13.5 hours using a hot water bath, and then heated at 70°C for 10.5 hours to react the monomers. During heating, 0.4 parts by weight of 3-methacryloxypropyltrimethoxysilane, a silane coupling agent having a polymerizable reactive group (13.7% by weight aqueous solution, adjusted to pH 3.0 with 5% by weight aqueous hydrochloric acid), was added to the polymerization system.

[0086] (4) After the drying polymerization, the resulting slurry containing the microcapsules was sieved, filtered, and washed with a sufficient amount of water. The washed microcapsules were then dried under a vacuum environment of a gauge pressure (vacuum degree) of −80 kPa or less and a temperature of 30 to 70° C. while stirring at 45 to 75 rpm and continuously increasing the temperature, to obtain thermally expandable microcapsules-1.

[0087] Thermally expandable microcapsules 2 to 6 were obtained in the same manner as for thermally expandable microcapsule 1, except that the amounts of each material and production conditions were changed as shown in Table 1. When producing thermally expandable microcapsules 2, 4, and 6, the amounts of trisodium phosphate anhydrous and trisodium phosphate dodecahydrous shown in Table 1 were added at the time of preparing the aqueous medium. When producing thermally expandable microcapsule 4, the microcapsules were dried by instantaneously flowing them using a flash jet dryer (Seishin Enterprise Co., Ltd., continuous flash dryer, inlet temperature 250-280°C, outlet temperature 100-130°C, feed rate 70-100 kg / h). When producing thermally expandable microcapsule 5, the heating during monomer polymerization was carried out for 24 hours at 60°C. When producing thermally expandable microcapsule 6, the microcapsules were dried by applying vibration during vacuum drying.

[0088] 2. Evaluation of Thermally Expandable Microcapsules (1) Average Particle Diameter (D50) The average particle diameter (D50) of each thermally expandable microcapsule was measured using a laser diffraction particle size distribution measuring device (SALD2300, manufactured by Shimadzu Corporation).

[0089] (2) Expansion Start Temperature (Ts) and Maximum Expansion Temperature (Tmax) The expansion start temperature (Ts) and maximum expansion temperature (Tmax) of each thermally expandable microcapsule were measured using a thermomechanical analyzer (TMA / SDTA840, manufactured by Mettler Toledo Co., Ltd.). Specifically, a sample of 0.2 mg to 0.4 mg of thermally expandable microcapsules was placed in a 7.5 mm diameter, 2.3 mm high SUS container, and an aluminum lid was placed on it. The temperature was increased under conditions of a load of 0.1 N and a temperature increase rate of 5 ° C. / min, and the height displacement of the portion occupied by the sample was continuously measured under a nitrogen atmosphere. The temperature at which the height displacement began was taken as the expansion start temperature (Ts), and the temperature at which the height displacement was greatest was taken as the maximum expansion temperature (Tmax).

[0090] (3) Gas Leakage Evaluation The gas leakage initiation temperature of each thermally expandable microcapsule was measured using a gas chromatograph (GC-14A, manufactured by Shimadzu Corporation). Specifically, a measurement sample was prepared by placing 1 mg of a thermally expandable microcapsule sample in a capillary tube and plugging both ends of the capillary tube with glass wool. This measurement sample was placed in an SUS column attached to a gas chromatograph, and after zero correction, the temperature was raised from 40°C to 330°C at a heating rate of 5°C / min, and the detected gas was continuously detected. Nitrogen was used as the carrier gas, and the gas flow rate was 6.4±0.05 mL / min. The intensity (I) of the peak indicating the thermal expansion agent at each temperature was measured. The peak intensity at 40°C at the start of measurement after zero correction was set to zero (I ０ The maximum intensity of the peak indicating the thermal expansion agent detected at 280°C or less was taken as the maximum intensity (Imax), and the temperature at which the displacement calculated using the following formula reached 2% was taken as the gas leakage initiation temperature (Ts_gas(2%)): Displacement (%) = I / Imax x 100

[0091] (4) Evaluation of Expansion Ratio The expansion ratio of the foam of the thermally expandable microcapsule was measured by the immersion method (Archimedes method) using isopropyl alcohol in an environment at a temperature of 25°C, and the density (d ０ ) and the density (d) of the thermally expandable microcapsules after heating and foaming were measured, and d ０ The volume expansion ratio of the thermally expandable microcapsules was calculated in units of 1 / d (unit: times). 0.1 g of each thermally expandable microcapsule was weighed into an aluminum cup and heated for 3 minutes in an oven heated to a predetermined temperature (190°C, 200°C, 240°C, or 250°C) to obtain a foamed thermally expandable microcapsule. The foamed thermally expandable microcapsule was placed in a 100 ml measuring flask, diluted with isopropanol, and the sample weight was measured. The density of the foam was calculated using the weight of the isopropanol alone.

[0092] (5) Preparation of Resin Composition 98 parts by mass of a urethane elastomer (BASF, Elastollan 1180A) was added to a Labo Plastomill (Toyo Seiki Seisakusho, 4C150-01), followed by 2 parts by mass of each thermally expandable microcapsule. Then, the mixture was melt-kneaded for 5 minutes at a set temperature of 160°C and a screw rotation speed of 25 rpm. After kneading, the kneaded product was recovered from the mixer and cooled at room temperature to obtain a resin composition.

[0093] Table 1 shows the materials and manufacturing conditions used to prepare each thermally expandable microcapsule, as well as the evaluation results.

[0094]

[0095] As shown in Table 1, by drying the microcapsules while stirring in a vacuum environment, it was possible to obtain thermally expandable microcapsules satisfying Ts_gas(2%)-Ts≧0°C. Furthermore, the thermally expandable microcapsules satisfying Ts_gas(2%)-Ts≧0°C were able to expand more greatly when heated.

[0096] This application claims priority from Japanese Patent Application No. 2024-117046, filed July 22, 2024. The entire disclosure and claims of that application as originally filed are incorporated herein by reference.

[0097] According to the present invention, there is provided a thermally expandable microcapsule that expands well.

Claims

1. A thermally expandable microcapsule that has a shell containing a thermoplastic resin and a thermal expansion agent held inside the shell and expands when heated, wherein the shell contains a resin having a carboxyl group and / or a nitrile group, and the thermally expandable microcapsule satisfies the following formula 1, where Ts (°C) is the expansion start temperature, and Ts_gas (2%) (°C) is the temperature at which the change in strength indicating the thermal expansion agent during heating, based on the strength at the start of heating, as measured by gas chromatography, is 2% of the maximum peak strength indicating the thermal expansion agent at 280°C or less: Ts_gas (2%) - Ts ≧ 0°C ... formula 1 2. The thermally expandable microcapsules according to claim 1, having an average particle size of 10 μm or more and 80 μm or less.

3. A method for producing thermally expandable microcapsules, comprising the steps of: suspending a polymerizable mixture containing a monomer and a thermal expansion agent in an aqueous medium; polymerizing the monomer contained in the suspended polymerizable mixture to obtain microcapsules having a shell containing a thermoplastic resin and a thermal expansion agent held inside the shell; and drying the microcapsules while stirring in a vacuum environment.

4. A resin composition comprising a base resin and the thermally expandable microcapsules according to claim 1 or 2.

5. The resin composition according to claim 4, wherein the base resin contains a thermoplastic elastomer.

6. A method for producing a foamed molded article, comprising the steps of: preparing the resin composition according to claim 4; and heating the resin composition to expand the thermally expandable microcapsules.

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