Thermally-expandable microcapsule

Abstract

Provided is a thermally-expandable microcapsule that does not require a special molding machine even if engineering plastic or the like is used, exhibits excellent heat resistance in a high temperature range, exhibits high foaming performance even in a high temperature range, and is also excellent in moldability. The present invention is a thermally-expandable microcapsule in which a volatile expansion agent is encapsulated as a core agent in a shell containing a polymer, wherein a difference between the Tg% of a foaming start temperature Ts and the Tg% of Ts + 20°C in measurement of TG / DTA is 3% or less.

Description

Thermally Expandable Microcapsules

[0001] The present invention relates to a thermally expandable microcapsule.

[0002] In recent years, in the automotive field, there has been a trend toward replacing metal parts of automobiles with lightweight foam molded articles in line with the trend toward weight reduction and electrification. For this reason, the practical application of foam molded articles has been actively investigated.

[0003] For example, a method for producing a foamed molded article involves foaming a resin material using a foaming agent, and the foaming agent is generally a thermally expandable microcapsule or a chemical foaming agent. A widely known example of such a thermally expandable microcapsule is one in which a volatile blowing agent that becomes gaseous at a temperature below the softening point of the thermoplastic polymer is encapsulated in a shell containing a thermoplastic polymer.

[0004] Patent Document 1 discloses heat-expandable microspheres comprising a thermoplastic resin shell and a blowing agent encapsulated therein that vaporizes upon heating, the thermoplastic resin containing a nitrile monomer (A) essentially containing methacrylonitrile, a carboxyl group-containing monomer (B), and a monomer (C) having a group reactive with a carboxyl group. Patent Document 2 discloses heat-expandable microspheres comprising a thermoplastic resin shell and a blowing agent encapsulated therein that vaporizes upon heating, the thermoplastic resin containing a nitrile monomer (A) essentially containing acrylonitrile and methacrylonitrile, a carboxyl group-containing monomer (B), and a monomer (C) having one polymerizable double bond.

[0005] Furthermore, as a method for foaming highly heat-resistant engineering plastics and super engineering plastics (hereinafter referred to as engineering plastics, etc.), a method is adopted in which a molten resin such as an engineering plastic is melted by shear mixing with a high-pressure supercritical fluid, and then foam injection molding is performed.

[0006] International Publication No. WO 2016 / 190178 International Publication No. WO 2016 / 084612

[0007] However, foam injection molding using a supercritical fluid requires a special molding machine, and it is difficult to introduce the manufacturing equipment. On the other hand, when the thermally expandable microcapsules of Patent Documents 1 and 2 are used for foam molding of engineering plastics, etc., it is difficult to maintain the volatile expansion agent encapsulated in the microcapsules due to thermal expansion in a high temperature range, resulting in problems with foaming performance.

[0008] The present invention aims to provide a thermally expandable microcapsule that does not require a special molding machine even when using engineering plastics, etc., has excellent heat resistance at high temperatures, has high foaming performance even at high temperatures, and also has excellent moldability.

[0009] Disclosure (1) is a thermally expandable microcapsule having a polymer-containing shell and a volatile blowing agent encapsulated as a core agent, wherein, in TG / DTA measurement, the difference between the Tg% at the foaming initiation temperature Ts and the Tg% at Ts + 20°C is 3% or less. Disclosure (2) is a thermally expandable microcapsule of Disclosure (1), wherein the maximum foaming temperature Tmax is 240°C or higher. Disclosure (3) is a thermally expandable microcapsule of Disclosure (1) or (2), wherein the foaming initiation temperature Ts is 185°C or higher. Disclosure (4) is a thermally expandable microcapsule in any combination with any of Disclosures (1) to (3), wherein the polymer contains 45% by weight to 65% by weight of structural units derived from a nitrile monomer (I) and 35% by weight to 55% by weight of structural units derived from a carboxyl group-containing monomer (II), and the nitrile monomer (I) includes acrylonitrile. The present disclosure (5) is a thermally expandable microcapsule according to the present disclosure (4), wherein the nitrile monomer (I) contains methacrylonitrile and acrylonitrile, the weight ratio of the structural units derived from the acrylonitrile to the structural units derived from the methacrylonitrile (acrylonitrile / methacrylonitrile) is 1.05 or more, and the weight ratio of the structural units derived from the methacrylonitrile to the structural units derived from the carboxyl group-containing monomer (II) (methacrylonitrile / carboxyl group-containing monomer (II)) is 0.8 or less. The present disclosure (6) is a thermally expandable microcapsule in any combination with any of the present disclosures (1) to (5), wherein the degree of gelation measured in N,N-dimethylformamide (DMF) solvent is 30% by weight or less. The present disclosure (7) is a thermally expandable microcapsule in any combination with any of the present disclosures (1) to (6), wherein the polymer does not contain structural units derived from a polymerizable monomer having two or more double bonds in the molecule. The present disclosure (8) is a thermally expandable microcapsule in any combination with any of the present disclosures (1) to (7), in which the content of the metal cation salt in the shell is 0.5% by weight or more and 10% by weight or less. The present invention will be described in detail below.

[0010] In one embodiment of the present invention, the thermally expandable microcapsules have a difference of 3% or less between the Tg% at the foaming initiation temperature Ts and the Tg% at Ts + 20°C in TG / DTA measurements. By satisfying this relationship, the thermally expandable microcapsules can be obtained with excellent heat resistance at high temperatures, reduced silver streaking during foaming, and excellent moldability. Furthermore, by using such thermally expandable microcapsules, molded articles can be obtained without the use of special molding machines, even when engineering plastics are used as the base resin. The difference (Tg%(Ts + 20°C) - Tg%(Ts)) is preferably 3% or less, more preferably 2% or less. The smaller the difference (Tg%(Ts + 20°C) - Tg%(Ts)), the better, for example, 0% or more. From the viewpoint of foaming ability, 0.5% or more is preferred, and 1.0% or more is more preferred. Furthermore, the Tg%(Ts + 20°C) is preferably 0% or more, 5% or less is preferred, and 3% or less is more preferred. The Tg% refers to the rate of change in sample weight and can be measured using a thermogravimetric / differential thermal analyzer (TG / DTA). Specifically, for example, 20 μg of sample is placed in an aluminum container with a diameter of 5 mm and a depth of 2 mm, heated from 40° C. to 350° C. at a heating rate of 5° C. / min, and the weight change of the sample is measured based on the weight at 40° C. (weight change of 0%). The Tg% at the foaming initiation temperature Ts obtained from the TMA measurement results and the Tg% at the foaming initiation temperature Ts + 20° C. are measured, and the difference therebetween can be calculated. The difference between the Tg% at Ts and the Tg% at Ts + 20° C. is considered to correspond to a change in state, such as the weight of gas outflowing from the foaming initiation temperature to a high-temperature range.

[0011] The difference (Tg %(Ts+20°C)-Tg %(Ts)) can be adjusted by adjusting the types and ratios of monomer components in the polymer, the weight average molecular weight (Mw) of the polymer, the polymerization conditions, the content of metal cation salt in the shell, and the like. In particular, the difference can be achieved by optimizing the weight ratio of structural units derived from acrylonitrile to structural units derived from methacrylonitrile (acrylonitrile / methacrylonitrile), the weight ratio of structural units derived from methacrylonitrile to structural units derived from carboxyl group-containing monomer (II) (hereinafter also referred to as "structural units having a carboxyl group") (methacrylonitrile / carboxyl group-containing monomer (II)), and the ratio of other monomers.

[0012] The preferred lower limit of the true density of the thermally expandable microcapsules of the present invention is 1.08 g / cm ３ The preferred upper limit is 1.50 g / cm ３ By setting the density within the above range, the uniformity of the thermally expandable microcapsules can be improved. A more preferable lower limit is 1.10 g / cm ３ , and a more preferable upper limit is 1.40 g / cm ３ The true density can be measured using a true density meter (Ultrapyc 5000, manufactured by Anton Parr, etc.) under conditions of a pressure of 12 psi, a temperature of 25° C., and a gas type of helium.

[0013] The thermally expandable microcapsules according to one embodiment of the present invention preferably have a maximum foaming temperature (Tmax) of 240°C or higher. A temperature of 240°C or higher increases heat resistance, preventing the thermally expandable microcapsules from bursting or shrinking when a composition containing the thermally expandable microcapsules is molded at high temperatures. Furthermore, aggregation of the thermally expandable microcapsules during molding can be suppressed, resulting in a good appearance. The Tmax is more preferably 245°C or higher, more preferably 290°C or lower, and even more preferably 270°C or lower. In this specification, the maximum foaming temperature refers to the temperature at which the diameter of the thermally expandable microcapsules reaches its maximum (maximum displacement) when the diameter is measured while the thermally expandable microcapsules are heated from room temperature.

[0014] The foaming initiation temperature (Ts) is preferably 185°C or higher. By setting the temperature at 185°C or higher, it is possible to obtain thermally expandable microcapsules with excellent heat resistance. The Ts is more preferably 190°C or higher, and is preferably 290°C or lower, and more preferably 250°C or lower. The Ts can be adjusted by the type of core agent, etc.

[0015] The thermally expandable microcapsules according to one embodiment of the present invention preferably have a gelation degree of 30% by weight or less as measured in N,N-dimethylformamide (DMF) solvent. By achieving this range, a high expansion ratio can be achieved even at high temperatures. The gelation degree can be measured by a method conforming to ASTM D2765.

[0016] The preferred lower limit of the volume average particle diameter of the thermally expandable microcapsules according to one embodiment of the present invention is 1 μm, and the preferred upper limit is 100 μm. If the diameter is less than 1 μm, the cells in the resulting molded article may be too small, resulting in an insufficient expansion ratio. If the diameter exceeds 100 μm, the cells in the resulting molded article may be too large, resulting in problems with appearance. A more preferred lower limit is 3 μm, and a more preferred upper limit is 50 μm. The volume average particle diameter of the thermally expandable microcapsules can be measured using a laser diffraction / scattering particle size distribution analyzer or the like.

[0017] The shell constituting the thermally expandable microcapsule according to one embodiment of the present invention contains a polymer. The polymer is preferably a polymer obtained by polymerizing a monomer composition containing a nitrile monomer (I) and a carboxyl group-containing monomer (II). That is, the polymer preferably has a structural unit derived from the nitrile monomer (I) and a structural unit derived from the carboxyl group-containing monomer (II).

[0018] The nitrile monomer (I) is not particularly limited, and examples thereof include acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, α-ethoxyacrylonitrile, fumaronitrile, and mixtures thereof. Among these, acrylonitrile is preferred, and acrylonitrile and methacrylonitrile are particularly preferred. These may be used alone or in combination of two or more.

[0019] The content of the structural unit derived from the nitrile monomer (I) in the polymer is preferably 45% by weight or more and 65% by weight or less. By making it 45% by weight or more, the expansion ratio can be improved. By making it 65% by weight or less, the heat resistance can be improved. The content is more preferably 48% by weight or more and more preferably 60% by weight or less.

[0020] In the constituent units derived from the nitrile monomer (I), the weight ratio of the constituent units derived from acrylonitrile to the constituent units derived from methacrylonitrile (acrylonitrile / methacrylonitrile) is preferably 1.05 or more. By making it 1.05 or more, it becomes easier to make the difference between the Tg% at the foaming initiation temperature Ts and the Tg% at Ts + 20°C in the obtained thermally expandable microcapsules 3% or less. Therefore, it is possible to obtain more excellent heat resistance and moldability. The weight ratio is more preferably 1.10 or more, preferably 2.00 or less, and more preferably 1.50 or less.

[0021] The content of the acrylonitrile-derived structural units in the polymer is preferably 23% by weight or more, more preferably 26% by weight or more, and preferably 43% by weight or less, more preferably 37% by weight or less. The content of the methacrylonitrile-derived structural units in the polymer is preferably 15% by weight or more, more preferably 20% by weight or more, and preferably 31% by weight or less, more preferably 28% by weight or less.

[0022] The carboxyl group-containing monomer (II) may be, for example, a radically polymerizable unsaturated carboxylic acid monomer having a carboxyl group and 3 to 8 carbon atoms. Specific examples include unsaturated monocarboxylic acids, unsaturated dicarboxylic acids, their anhydrides, and monoesters of unsaturated dicarboxylic acids. These may be used alone or in combination of two or more. Examples of the unsaturated carboxylic acids include unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid, ethacrylic acid, crotonic acid, and cinnamic acid, and unsaturated dicarboxylic acids such as maleic acid, itaconic acid, fumaric acid, citraconic acid, and chloromaleic acid. Examples of the monoesters of unsaturated dicarboxylic acids include monomethyl maleate, monoethyl maleate, monobutyl maleate, monomethyl fumarate, monoethyl fumarate, monomethyl itaconate, monoethyl itaconate, and monobutyl itaconate. Among these, acrylic acid, methacrylic acid, maleic acid, maleic anhydride, and itaconic acid are preferred, with methacrylic acid being more preferred.

[0023] The content of the structural unit having a carboxyl group in the polymer is preferably 35% by weight or more, and preferably 55% by weight or less. By making it 35% by weight or more, the heat resistance of the obtained thermally expandable microcapsules can be improved. Furthermore, by making it 55% by weight or less, the powder fluidity of the obtained thermally expandable microcapsules can be improved. The content is more preferably 40% by weight or more, and more preferably 53% by weight or less.

[0024] In the polymer, the weight ratio of the methacrylonitrile-derived structural unit to the carboxyl group-containing structural unit (methacrylonitrile / carboxyl group-containing monomer (II)) is preferably 0.8 or less. By setting it to 0.8 or less, the difference between the Tg% at the foaming initiation temperature Ts and the Tg% at Ts + 20°C in the obtained thermally expandable microcapsules can be easily set to 3% or less. Therefore, the heat resistance and moldability can be improved. The weight ratio is preferably 0 or more, more preferably 0.35 or more, and more preferably 0.75 or less.

[0025] The polymer may contain a structural unit derived from a polymerizable monomer having two or more double bonds in the molecule, but preferably does not contain such a structural unit. When the polymer does not contain such a structural unit, it can exhibit a high expansion ratio even when molded in a high temperature range.

[0026] Examples of the polymerizable monomer include monomers having two or more radically polymerizable double bonds, and specific examples include divinylbenzene, di(meth)acrylate, and tri- or higher functional (meth)acrylates. Examples of the di(meth)acrylate include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, and 1,4-butanediol di(meth)acrylate. Other examples include 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, glycerin di(meth)acrylate, trimethylolpropane di(meth)acrylate, and dimethylol-tricyclodecane di(meth)acrylate. Furthermore, di(meth)acrylate of polyethylene glycol having a weight average molecular weight of 200 to 600 may also be used. Examples of the trifunctional (meth)acrylate include trimethylolpropane tri(meth)acrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, triallyl formal tri(meth)acrylate, etc. Examples of the tetrafunctional or higher (meth)acrylate include pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, etc.

[0027] The polymer may contain a structural unit derived from a monomer other than the structural unit derived from the nitrile monomer (I) and the structural unit having a carboxyl group. Examples of the other monomer include (meth)acrylic acid esters, as well as vinyl monomers such as vinyl chloride, vinylidene chloride, vinyl acetate, and styrene. These may be used alone or in combination of two or more. Among these, (meth)acrylic acid esters are preferred, and alkyl methacrylates such as methyl methacrylate, ethyl methacrylate, and n-butyl methacrylate, or methacrylic acid esters containing an alicyclic ring, an aromatic ring, or a heterocyclic ring, such as cyclohexyl methacrylate, benzyl methacrylate, and isobornyl methacrylate, are particularly preferred.

[0028] The content of the structural units derived from the other monomers in the polymer is preferably 0% by weight or more, more preferably 0.1% by weight or more, and is preferably 1.5% by weight or less, more preferably 1.3% by weight or less. By setting the content within the above range, the dispersibility of the composition using the thermally expandable microcapsules can be improved, and the thermal expandability can also be improved.

[0029] The weight-average molecular weight (Mw) of the polymer is preferably 200,000 or more, more preferably 250,000 or more, and preferably 500,000 or less, and more preferably 450,000 or less. By setting the weight-average molecular weight in the above range, it becomes easier to keep the difference in Tg% at the expansion initiation temperature Ts and Tg% at Ts + 20°C in the obtained thermally expandable microcapsules to 3% or less. The weight-average molecular weight can be measured, for example, by gel permeation chromatography using DMF as an eluent and a Shodex LF-804 two-arm column with a monodisperse polystyrene standard.

[0030] A polymerization initiator is added to the monomer composition to polymerize the monomers. Suitable examples of the polymerization initiator include dialkyl peroxides, diacyl peroxides, peroxyesters, peroxydicarbonates, and azo compounds. Specific examples include dialkyl peroxides such as methyl ethyl peroxide, di-t-butyl peroxide, and dicumyl peroxide; and diacyl peroxides such as isobutyl peroxide, benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, and 3,5,5-trimethylhexanoyl peroxide. Other examples include t-butyl peroxypivalate, t-hexyl peroxypivalate, t-butyl peroxyneodecanoate, t-hexyl peroxyneodecanoate, 1-cyclohexyl-1-methylethyl peroxyneodecanoate, and 1,1,3,3-tetramethylbutyl peroxyneodecanoate. Other examples include peroxyesters such as cumyl peroxy neodecanoate and (α,α-bis-neodecanoylperoxy)diisopropylbenzene; bis(4-t-butylcyclohexyl)peroxydicarbonate, di-n-propyl-oxydicarbonate, and diisopropyl peroxydicarbonate. Further examples include peroxydicarbonates such as di(2-ethylethylperoxy)dicarbonate, dimethoxybutyl peroxydicarbonate, and di(3-methyl-3-methoxybutylperoxy)dicarbonate. Additionally, other examples include azo compounds such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), and 1,1'-azobis(1-cyclohexanecarbonitrile).

[0031] The shell may contain a metal cation salt. When the shell contains a metal cation salt, if the copolymer constituting the shell contains carboxyl groups, the metal cations derived from the metal cation salt react with the carboxyl groups to ionically crosslink the copolymer, improving heat resistance and enabling the formation of thermally expandable microcapsules that do not burst or shrink for long periods of time at high temperatures. Furthermore, since the elastic modulus of the shell is less likely to decrease even at high temperatures, the thermally expandable microcapsules do not burst or shrink even when subjected to molding processes that apply strong shear forces, such as kneading, calendaring, extrusion, and injection molding. The aforementioned ionic crosslinking refers to the formation of crosslinks between free carboxyl groups present as side chains of the copolymer. The number of carboxyl groups arranged per monovalent metal cation varies depending on the metal type.

[0032] The metal cation is not particularly limited as long as it reacts with the carboxyl groups of the copolymer to ionically crosslink the copolymer, and examples thereof include ions of Li, Na, K, Zn, Mg, Ca, Ba, Sr, Mn, Al, Ti, Ru, Fe, Ni, Cu, Cs, Sn, Cr, and Pb. These may be used alone or in combination of two or more. Among these, Ca, Zn, and Al ions are preferred, with Zn ions being particularly preferred. When two or more of the metal cations are used, the combination is not particularly limited, but it is preferable to use an alkali metal ion in combination with a metal cation other than the alkali metal. The presence of the alkali metal ion activates functional groups such as carboxyl groups, thereby promoting the reaction between the metal cations other than the alkali metal and the carboxyl groups of the copolymer. Examples of the alkali metal include Na, K, and Li.

[0033] The content of the metal cation salt in the shell is 0% by weight or more, preferably 0.5% by weight or more, and preferably 10% by weight or less. By setting the content within this range, heat resistance can be further improved. The content is more preferably 0.8% by weight or more, and more preferably 8% by weight or less.

[0034] The shell constituting the thermally expandable microcapsule according to one embodiment of the present invention preferably further contains at least one inorganic compound selected from the group consisting of Si-based compounds and Mg-based compounds. By containing the inorganic compound, it is possible to suppress fusion of the thermally expandable microcapsules with each other in the resin during molding.

[0035] The Si-based compound and Mg-based compound preferably contain an oxide, hydroxide, carbonate, or hydrogencarbonate of silicon or magnesium. These Si-based compounds and Mg-based compounds may be used alone or in combination of two or more.

[0036] Examples of the Si-based compounds include colloidal silica, silicate sol, No. 3 water glass, sodium orthosilicate, sodium metasilicate, etc. Among these, colloidal silica is preferred. Examples of the Mg-based compounds include magnesium oxide, magnesium hydroxide, magnesium hydroxide, hydrotalcite, dihydrotalcite, magnesium carbonate, basic magnesium carbonate, magnesium calcium carbonate, magnesium phosphate, magnesium hydrogen phosphate, magnesium pyrophosphate, magnesium borate, etc. Among these, magnesium hydroxide is preferred.

[0037] Other examples of the inorganic compounds that may be added include calcium phosphate, aluminum hydroxide, ferric hydroxide, barium sulfate, calcium sulfate, sodium sulfate, calcium oxalate, calcium carbonate, barium carbonate, etc. Furthermore, inorganic salts such as sodium chloride and sodium sulfate, alkali metal nitrite, stannous chloride, stannic chloride, potassium dichromate, etc. may also be added as needed.

[0038] The content of the inorganic compound is 0.01% by weight, preferably 7% by weight, of the total thermally expandable microcapsules. By making it 0.01% by weight or more, it is possible to suppress fusion of the thermally expandable microcapsules in the resin during molding. By making it 7% by weight or less, it is possible to further improve the resin dispersibility during molding. A more preferred lower limit is 0.3% by weight, and a more preferred upper limit is 5% by weight. The content of the inorganic compound can be calculated from the weight of the monomer composition and volatile expanding agent that form the thermally expandable microcapsules.

[0039] The shell may further contain, as necessary, a stabilizer, an ultraviolet absorber, an antioxidant, an antistatic agent, a flame retardant, a silane coupling agent, a coloring agent, and the like.

[0040] In one embodiment of the thermally expandable microcapsules of the present invention, a volatile expanding agent is encapsulated in the shell as a core agent. The volatile expanding agent is a substance that becomes gaseous at a temperature below the softening point of the polymer that constitutes the shell, and a low-boiling organic solvent is suitable. Examples of the volatile expanding agent include low-molecular-weight hydrocarbons such as ethane, ethylene, propane, propene, n-butane, isobutane, butene, isobutene, n-pentane, isopentane, neopentane, n-hexane, heptane, petroleum ether, isooctane, octane, decane, isododecane, dodecane, and hexanedecane. Also, CCl ３ F, CCl ２ F ２ , CClF ３ , CClF ２ -CClF ２Examples of suitable volatile expanding agents include chlorofluorocarbons such as tetramethylsilane, trimethylethylsilane, trimethylisopropylsilane, and trimethyl-n-propylsilane; and tetraalkylsilanes such as tetramethylsilane, trimethylethylsilane, trimethylisopropylsilane, and trimethyl-n-propylsilane. Among these, isobutane, n-butane, n-pentane, isopentane, n-hexane, isooctane, isododecane, and mixtures thereof are preferred. These volatile expanding agents may be used alone or in combination of two or more. Furthermore, a thermally decomposable compound that decomposes into a gaseous form upon heating may also be used as the volatile expanding agent. In particular, the core agent preferably contains 50% by weight or more, more preferably 70% by weight or more, and even more preferably 90% by weight or more of those having 8 or more carbon atoms.

[0041] In the thermally expandable microcapsules according to one embodiment of the present invention, among the above-mentioned volatile expanding agents, the use of a high-boiling hydrocarbon having 8 or more carbon atoms can improve the maximum expansion temperature, and the use of a low-boiling hydrocarbon having 5 or less carbon atoms can increase the expansion ratio and allow expansion to begin quickly. Furthermore, a thermally decomposable compound that decomposes into a gaseous form when heated may also be used as the volatile expanding agent.

[0042] The method for producing the thermally expandable microcapsules according to one embodiment of the present invention is not particularly limited, but they can be produced, for example, by carrying out the steps of preparing an aqueous dispersion medium, dispersing an oily mixture containing a monomer composition, a volatile expanding agent, a metal cation salt, etc. in the aqueous dispersion medium, and polymerizing the monomer composition. The monomer composition may contain the nitrile monomer (I), the carboxyl group-containing monomer (II), and other monomers.

[0043] When producing thermally expandable microcapsules, which is one embodiment of the present invention, a step of preparing an aqueous dispersion medium is first carried out. Specifically, for example, an aqueous dispersion medium containing silicon dioxide is prepared by adding water and a dispersion stabilizer containing silicon dioxide, and optionally a co-stabilizer, to a polymerization reaction vessel. Furthermore, alkali metal nitrite, stannous chloride, stannic chloride, potassium dichromate, etc. may also be added as necessary.

[0044] Examples of the silicon dioxide-containing dispersion stabilizer include colloidal silica. As the colloidal silica, alkaline colloidal silica having a colloidal solution (aqueous dispersion) with a pH of more than 7 may be used, or acidic colloidal silica having a pH of less than 7 may be used. Of these, alkaline colloidal silica is more preferred. Furthermore, the colloidal silica preferably contains 10 to 50% by weight of silicon dioxide as a solid content and is monodispersed.

[0045] Examples of dispersion stabilizers other than silicon dioxide include calcium phosphate, magnesium hydroxide, aluminum hydroxide, ferric hydroxide, barium sulfate, calcium sulfate, sodium sulfate, calcium oxalate, calcium carbonate, barium carbonate, and magnesium carbonate.

[0046] The amount of the dispersion stabilizer containing silicon dioxide to be added is determined appropriately depending on the particle size of the thermally expandable microcapsules, but the preferred lower limit is 2.5 parts by weight and the preferred upper limit is 7 parts by weight relative to 100 parts by weight of the oily mixture (oil phase).The more preferred lower limit is 3 parts by weight and the even more preferred upper limit is 5 parts by weight.The amount of the oil phase refers to the total amount of the monomer and the volatile expanding agent.

[0047] Examples of the auxiliary stabilizer include a condensation product of diethanolamine and an aliphatic dicarboxylic acid, a condensation product of urea and formaldehyde, etc. Further examples include polyvinylpyrrolidone, polyethylene oxide, polyethyleneimine, tetramethylammonium hydroxide, gelatin, methylcellulose, polyvinyl alcohol, dioctyl sulfosuccinate, sorbitan ester, various emulsifiers, etc.

[0048] In addition to the co-stabilizer, a condensation product or a water-soluble nitrogen compound may be added. As the condensation product, a condensation product of diethanolamine and an aliphatic dicarboxylic acid is preferred, and a condensation product of diethanolamine and adipic acid or a condensation product of diethanolamine and itaconic acid is particularly preferred.

[0049] Examples of the water-soluble nitrogen compounds include polyvinylpyrrolidone, polyethyleneimine, polyoxyethylene alkylamine, polydialkylaminoalkyl(meth)acrylates such as polydimethylaminoethyl methacrylate and polydimethylaminoethyl acrylate, polydialkylaminoalkyl(meth)acrylamides such as polydimethylaminopropyl acrylamide and polydimethylaminopropyl methacrylamide, polyacrylamide, polycationic acrylamide, polyamine sulfone, and polyallylamine. Among these, polyvinylpyrrolidone is preferably used.

[0050] Next, in the method for producing thermally expandable microcapsules, a step of dispersing an oily mixture containing a monomer composition and a volatile expanding agent in an aqueous dispersion medium is carried out. Specifically, a step of dispersing an oily mixture containing a monomer composition and a volatile expanding agent in an aqueous dispersion medium is carried out. In this step, the monomer composition and the volatile expanding agent may be added separately to the aqueous dispersion medium to prepare the oily mixture in the aqueous dispersion medium, but typically the two are mixed together to form an oily mixture before adding it to the aqueous dispersion medium. In this case, the oily mixture and the aqueous dispersion medium may be prepared in separate containers, and the oily mixture may be dispersed in the aqueous dispersion medium by mixing them while stirring in the separate containers, and then added to the polymerization reaction vessel. In this step, an inorganic compound is present at the interface between the oil droplets of the oily mixture and the aqueous dispersion medium, resulting in the inorganic compound being present on the surface of the resulting thermally expandable microcapsules. A polymerization initiator is used to polymerize the monomers. The polymerization initiator may be added to the oily mixture in advance, or may be added after the aqueous dispersion medium and the oily mixture are stirred and mixed in a polymerization reaction vessel.

[0051] Examples of a method for emulsifying and dispersing the oily mixture in an aqueous dispersion medium to a predetermined particle size include a method of stirring with a homomixer (for example, manufactured by Tokushu Kika Kogyo Co., Ltd.) or a method of passing the mixture through a static dispersion device such as a line mixer or an element-type static disperser. The aqueous dispersion medium and the polymerizable mixture may be supplied separately to the static dispersion device, or a dispersion liquid that has been mixed and stirred in advance may be supplied.

[0052] The thermally expandable microcapsules according to one embodiment of the present invention can be produced by subjecting the dispersion obtained through the above-described steps to a step of polymerizing the monomer by heating, a step of washing, and a step of drying. In the polymerization step, the polymerization temperature is preferably 50°C or higher and 60°C or lower. The polymerization time is preferably 4 hours or higher and 24 hours or lower. The polymerization pressure is preferably 0.1 MPa or higher and 1 MPa or lower.

[0053] Masterbatch pellets can be obtained by mixing the thermally expandable microcapsules of the present invention with a resin (base resin). Alternatively, a foamable resin composition can be obtained by adding a matrix resin, such as a thermoplastic resin, to the thermally expandable microcapsules of the present invention. An ink containing the thermally expandable microcapsules and resin can also be used as a foamable ink. Compositions containing the thermally expandable microcapsules and resin of the present invention are preferably used for adhesives, rubber chips, foam chips, flooring materials, rock consolidation materials, paints, coating materials, reinforcing fibers, composite materials, electronic components, molding materials, and the like. The molding materials are preferably used for molding by injection molding, extrusion molding, blow molding, rotational molding, vacuum molding, inflation molding, calendar molding, slush molding, dip molding, foam molding, fused deposition modeling, inkjet molding, stereolithography, laser sintering, and the like.

[0054] The resin used for the base resin is not particularly limited, and thermoplastic resins and curable resins commonly used in conventional foam molding can be used. Specific examples of the thermoplastic resin include polyolefins such as low-density polyethylene (LDPE) and polypropylene (PP), polyvinyl acetate, ethylene-vinyl acetate copolymer (EVA), vinyl chloride, polystyrene, thermoplastic elastomers, and ethylene-methyl methacrylate copolymer (EMMA). Among these, LDPE, EVA, EMMA, and thermoplastic elastomers are preferred due to their low melting points and ease of processability. These may be used alone or in combination of two or more. Examples of the curable resin include epoxy resins, (meth)acrylic resins, urethane resins, phenolic resins, cyanate resins, isocyanate resins, maleimide resins, benzoxazine resins, silicone resins, fluororesins, polyimide resins, and phenoxy resins. Of these, epoxy resins are preferred as the curable resin. These curable resins may be used alone or in combination of two or more.

[0055] The content of the thermally expandable microcapsules in the masterbatch pellets is not particularly limited, but a preferred lower limit is 10 parts by weight and a preferred upper limit is 90 parts by weight per 100 parts by weight of the thermoplastic resin.

[0056] The method for producing the masterbatch pellets is not particularly limited, but examples include pre-kneading raw materials such as a base resin and various additives using a co-rotating twin-screw extruder or the like. Next, the mixture is heated to a predetermined temperature, a blowing agent such as thermally expandable microcapsules is added, and the resulting mixture is further kneaded. The resulting mixture is then cut into pellets of the desired size using a pelletizer to form the masterbatch. Alternatively, a pellet-shaped masterbatch may be produced by kneading raw materials such as the base resin and thermally expandable microcapsules using a batch kneader and then granulating them using a granulator. The kneader is not particularly limited as long as it can knead the materials without destroying the thermally expandable microcapsules, and examples include a pressure kneader and a Banbury mixer.

[0057] Furthermore, foamed molded articles can be obtained using the thermally expandable microcapsules and expandable masterbatch. In particular, the thermally expandable microcapsules can be suitably used in applications requiring post-processing at high temperatures, allowing for the production of foamed sheets with high-quality appearance, such as uneven surfaces. Specifically, foamed molded articles can be obtained by kneading the thermally expandable microcapsules or expandable masterbatch containing the thermally expandable microcapsules with a matrix resin and molding the mixture. According to the present invention, durability can be improved during thermal expansion, and the expansion ratio and heat resistance can also be improved.

[0058] The molding method for the foamed molded article is not particularly limited, and examples thereof include kneading molding, calendar molding, extrusion molding, injection molding, etc. In the case of injection molding, the process is not particularly limited, and examples thereof include the short shot method in which a resin material is partially placed in a mold and foamed, and the core back method in which the mold is fully filled with the resin material and then opened to the desired foaming point.

[0059] According to the present invention, it is possible to provide thermally expandable microcapsules that do not require special molding machines even when using engineering plastics, etc., have excellent heat resistance at high temperatures, have high foaming performance even at high temperatures, and also have excellent moldability.

[0060] The following examples will further illustrate the present invention, but the present invention is not limited to these examples.

[0061] Examples 1 to 16, Comparative Examples 1 to 14 Preparation of Thermally Expandable Microcapsules An aqueous dispersion medium was prepared by adding 8 L of water, 5 parts by weight of colloidal silica as a dispersant, and 0.3 parts by weight of polyvinylpyrrolidone to a polymerization reaction vessel. An oily mixture containing the monomers and metal cation salts in the amounts shown in Tables 1 and 2 was then added to the aqueous dispersion medium and suspended to prepare a dispersion. The resulting dispersion was stirred and mixed using a homogenizer and placed in a nitrogen-purged pressure polymerization vessel. The reaction was carried out at the polymerization pressure shown in Tables 1 and 2, at the polymerization temperature and for the polymerization time shown in Tables 1 and 2, to obtain a reaction product. The resulting reaction product was repeatedly filtered and washed with water, and then dried to obtain thermally expandable microcapsules.

[0062] (Preparation of Masterbatch Pellets) 100 parts by weight of a propylene-based elastomer and 10 parts by weight of a fatty acid ester lubricant were kneaded in a Banbury mixer. When the temperature reached approximately 100°C, 100 parts by weight of the obtained thermally expandable microcapsules were added to the propylene-based elastomer. The mixture was further kneaded for 30 seconds, extruded, and simultaneously pelletized to obtain masterbatch pellets. The propylene-based elastomer used had a melt index of 3.7 g / 10 min, a melting point of 75°C, a propylene content of 91% by weight, and an ethylene content of 9% by weight.

[0063] (Preparation of foamed molded article) Masterbatch pellets and 100 parts by weight of PA66 resin (Zytel 103HSL NC010 (Celanese)) were mixed to obtain mixed pellets containing 6 wt% masterbatch pellets. The obtained mixed pellets were fed into the hopper of a screw-type injection molding machine equipped with an accumulator, melt-kneaded, and injection-molded using a core-back method with a core-back amount of 2 mm to obtain a plate-shaped molded article. The molding conditions were a cylinder temperature of 260°C, an injection speed of 100 mm / s, a cooling time of 20 s, a back pressure of 5 MPa, a dwell pressure of 0 MPa, and a mold temperature of 80°C.

[0064] (Evaluation Method) The obtained thermally expandable microcapsules were evaluated by the following methods. The results are shown in Tables 1 and 2.

[0065] (1) Measurement of Volume Average Particle Diameter The volume average particle diameter of the obtained thermally expandable microcapsules was measured using a particle size distribution diameter measuring instrument (LA-910, manufactured by HORIBA Co., Ltd.).

[0066] (2) Measurement of Foaming Initiation Temperature and Maximum Foaming Temperature The foaming initiation temperature (Ts) and maximum foaming temperature (Tmax) were measured using a thermomechanical analyzer (TMA) (TMA2940, manufactured by TA Instruments). Specifically, 25 μg of a sample was placed in an aluminum container with a diameter of 7 mm and a depth of 1 mm, and heated from 80° C. to 250° C. at a temperature increase rate of 5° C. / min under a force of 0.1 N applied from above. The vertical displacement of the measuring probe was measured, and the temperature at which the displacement began to increase was defined as the foaming initiation temperature, the maximum value of the displacement as the maximum displacement, and the temperature at the maximum displacement as the maximum foaming temperature.

[0067] (3) Tg% Measurement The weight change rate (Tg%) of the thermally expandable microcapsules was measured using a thermogravimetric / differential thermal analyzer (TG / DTA) (TG / DTA6200 (manufactured by Hitachi, Ltd.)). Specifically, 20 μg of the sample was placed in an aluminum container with a diameter of 5 mm and a depth of 2 mm, heated from 40°C to 350°C at a heating rate of 5°C / min, and the weight change of the sample was measured. The difference between the Tg% at the foaming onset temperature Ts obtained from the TMA measurement results and the Tg% at the foaming onset temperature Ts + 20°C was calculated.

[0068] (4) Degree of gelation The thermally expandable microcapsules were immersed in DMF for 24 hours, and the degree of gelation in DMF was measured. Specifically, the solution was adjusted so that the sample was 3% by weight, and after leaving it to stand for 24 hours, centrifuged at 3000 rpm for 3 minutes to separate the solvent and the sample. After removing the supernatant solvent, the sample was dried in a vacuum dryer, and the degree of gelation was calculated using the following formula based on the dry weight of the gel and the weight of the sample. Degree of gelation = (dry weight of gel / weight of polymer contained in the sample) x 100

[0069] (5) Weight-average molecular weight (Mw) 5 ml of solvent (dimethylformamide with 0.05 M lithium bromide added) was added to 10 mg of the obtained thermally expandable microcapsules, and the mixture was gently stirred at room temperature. After visually checking for unwanted substances, the mixture was filtered using a 0.45 μm filter. The obtained measurement solution was subjected to molecular weight distribution measurement under the following measurement conditions, and the weight-average molecular weight (Mw) of the polymer constituting the shell was measured. (Measurement conditions) Apparatus: Gel permeation chromatograph GPC (manufactured by JASCO Corporation) Detector: Differential refractive index detector RI (JASCO Corporation RI-4030) Column: Two connected Shodex LF-804 columns Flow rate: 0.8 mL / min Column temperature: 40°C Injection volume: 0.200 mL Standard sample: Monodisperse polystyrene (manufactured by Tosoh Corporation)

[0070] (6) Specific Gravity The density of the obtained foamed molded article was measured by a method conforming to JIS K 7112 Method A (underwater displacement method). The specific gravity is an index representing the heat resistance, which is the object of the present invention. If the heat resistance is poor, the specific gravity becomes high because the microcapsule shrinks after expanding at high temperatures. Therefore, if a foamed molded article with a low specific gravity is obtained, it can be said that the thermally expandable microcapsule has excellent heat resistance.

[0071] (7) Surface Properties (Moldability) Using a color computer (CM-3600d, manufactured by Konica Minolta Japan Inc.), the L* value of the surface of the obtained foam molded article was measured, and the color difference was measured using the unfoamed state (black) as the standard, and evaluated according to the following criteria. For foam molded articles, when the color difference is small, the occurrence of silver streaks is small, molding can be performed under a wide range of molding conditions, and it can be said that the moldability is excellent. ◎: Color difference is 0 or more and 2 or less. ◯: Color difference is more than 2 and 6 or less. △: Color difference is more than 6.

[0072]

[0073]

[0074] According to the present invention, it is possible to provide thermally expandable microcapsules that do not require special molding machines even when using engineering plastics, etc., have excellent heat resistance at high temperatures, have high foaming performance even at high temperatures, and also have excellent moldability.

Claims

1. A thermally expandable microcapsule in which a volatile expanding agent is encapsulated as a core agent in a shell containing a polymer, and in which, in TG / DTA measurement, the difference between the Tg% at the foaming initiation temperature Ts and the Tg% at Ts + 20°C is 3% or less.

2. The thermally expandable microcapsule according to claim 1, wherein the maximum foaming temperature Tmax is 240°C or higher.

3. The thermally expandable microcapsule according to claim 1 or 2, wherein the foaming initiation temperature Ts is 185°C or higher.

4. A thermally expandable microcapsule as described in claim 1, 2 or 3, wherein the polymer contains 45% by weight or more and 65% by weight or less of structural units derived from a nitrile-based monomer (I) and 35% by weight or more and 55% by weight or less of structural units derived from a carboxyl group-containing monomer (II), and the nitrile-based monomer (I) includes acrylonitrile.

5. The thermally expandable microcapsule according to claim 4, wherein the nitrile monomer (I) contains methacrylonitrile and acrylonitrile, the weight ratio of the constituent units derived from the acrylonitrile to the constituent units derived from the methacrylonitrile (acrylonitrile / methacrylonitrile) is 1.05 or more, and the weight ratio of the constituent units derived from the methacrylonitrile to the constituent units derived from the carboxyl group-containing monomer (II) (methacrylonitrile / carboxyl group-containing monomer (II)) is 0.8 or less.

6. The thermally expandable microcapsule according to claim 1, 2, 3, 4 or 5, which has a gelation degree of 30% by weight or less as measured in N,N-dimethylformamide (DMF) solvent.

7. A thermally expandable microcapsule according to claim 1, 2, 3, 4, 5 or 6, wherein the polymer does not contain any structural unit derived from a polymerizable monomer having two or more double bonds in the molecule.

8. A thermally expandable microcapsule according to claim 1, 2, 3, 4, 5, 6 or 7, wherein the content of the metal cation salt in the shell is 0.5% by weight or more and 10% by weight or less.

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