Elastomer composition containing hollow particles, and method for producing same

Abstract

Provided are: an elastomer composition containing hollow particles, wherein high dimensional stability is maintained during molding processes and the properties or functions imparted by the hollow particles are not easily compromised; and a production method for producing said elastomer composition through a kneading process in which the hollow particles are not easily collapsed, the void residual ratio is stably maintained, and the properties or functions imparted by the hollow particles are not easily compromised. This elastomer composition containing hollow particles comprises a base elastomer and hollow particles each having a shell containing a specific resin, has a storage modulus G' at 60°C of 2.5 MPa or less as obtained by dynamic viscoelasticity measurement, and has a void residual ratio of at least 80% as measured with a sheet-like molded body created by using the composition according to a predetermined test method. In addition, in the production method of the present disclosure, a raw material mixture, which comprises a base elastomer and hollow particles each having a shell containing a specific resin and has a storage modulus G' at 60°C of 2.5 MPa or less as obtained by dynamic viscoelasticity measurement, is kneaded at a temperature at which the storage modulus G' is 2.5 MPa or less, immediately after pre-kneading using a closed kneader at a temperature at which the storage modulus G' is 2.5 MPa or less, or after further preheating, at a temperature at which the storage modulus G' is 2.5 MPa or less, following the pre-heating.

Description

Hollow particle-containing elastomer composition and method for producing the same

[0001] The present disclosure relates to an elastomer composition containing hollow particles and a method for producing the same.

[0002] Elastomer products, focusing on the rubber-like elasticity or flexibility of elastomer materials, typified by rubber, are used in a wide range of fields for various applications such as shock absorbers, fluid-blocking gaskets, and tubing. Generally, an elastomer composition, in which a base elastomer is mixed with components required for the application, is kneaded in a molten state, and the base elastomer is crosslinked while being molded by a method such as extrusion molding or compression molding, to obtain various forms of elastomer products, such as parts, coatings, and filler chip materials. A known method for reducing the weight of elastomer products is to mix a foaming agent into the base elastomer, foam it by heating during the molding process, and produce a foamed elastomer molded product. Patent Document 1 describes an object of providing a rubber composition for vulcanization molding that has excellent dimensional stability, good surface properties, and is capable of producing effectively lightweight rubber products, and describes, as a means for achieving this object, the use of a rubber composition for vulcanization molding in which a base rubber having a specific Mooney viscosity at 100°C is mixed with hollow particles that are composed of an outer shell made of a thermoplastic resin and a foaming agent encapsulated in the shell and vaporizes when heated, and that have an expansion capacity rate of 20 to 80%.

[0003] However, in the method of producing a foamed elastomer molded body by mixing a blowing agent into a base elastomer, it is difficult to control the size of the pores formed by foaming, resulting in poor dimensional stability of the resulting elastomer product. While one of the technical objectives of the method of Patent Document 1 is to obtain a foamed elastomer molded body with excellent dimensional stability, further improvements in dimensional stability are desired. As a method for introducing a large number of micropores into a molded body to impart properties or functions such as weight reduction, thermal insulation, and opacity, it is known to use a molding material containing hollow particles in the base resin (Patent Documents 2 and 3). In molding materials containing hollow particles in the base resin, the pores are the cavities of the hollow particles contained in the molding material, so there is no need to control the size of the pores formed by foaming.

[0004] Patent No. 6116787 International Publication No. 2020 / 261926 International Publication No. 2021 / 112110

[0005] Molding materials containing hollow particles in a base elastomer are required to resist crushing of the hollow particles during molding, maintain high dimensional stability, and minimize loss of the properties or functions imparted by the hollow particles. Furthermore, to produce molding materials containing hollow particles in a base elastomer, the hollow particles are required to resist crushing when kneading a raw material mixture containing the base elastomer and hollow particles. In particular, when hollow particles are mixed into a base elastomer, a high shear force, such as roll kneading, is used as the finishing kneading process. This generates a higher shear force than when hollow particles are mixed into a base resin other than an elastomer and kneaded, making the hollow particles more susceptible to crushing during kneading.

[0006] The present disclosure has been made in consideration of the above-mentioned problems, and aims to provide a hollow particle-containing elastomer composition in which the hollow particles are resistant to crushing during the molding process, maintain high dimensional stability, and are not likely to lose the properties or functions imparted by the hollow particles. Another aim of the present disclosure is to provide a method for producing a hollow particle-containing elastomer composition in which the hollow particles are resistant to crushing during the process of kneading a raw material mixture containing a base elastomer and hollow particles, the residual void ratio after kneading is stable, and the properties or functions imparted by the hollow particles are not lost by kneading.

[0007] The present disclosure provides a hollow particle-containing elastomer composition comprising at least a base elastomer and hollow particles, the hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, the shell containing a polymer containing 50 parts by mass or more of crosslinkable monomer units per 100 parts by mass of total monomer units as the resin, the composition having a storage modulus G' at 60°C as determined by dynamic viscoelasticity measurement of 2.5 MPa or less, the composition being used to prepare a sheet-shaped molded product according to the method for measuring the residual void ratio of a hollow particle-containing elastomer molded product described below, and the measured residual void ratio being 80% or more. [Method for measuring the residual void ratio of a hollow particle-containing elastomer molded product] A sheet-shaped hollow particle-containing elastomer molded product is produced by press-molding the hollow particle-containing elastomer composition in a hot press at 120°C under a pressure of 1 MPa or less. The specific gravity of the resulting elastomer molded product is measured, and the residual void ratio of the hollow particles in the elastomer molded product is calculated according to the following formula (D): Void remaining rate (%) = {(c-a) / (c-b)} x 100 Formula (D) a: specific gravity of the sheet-like molded body after pressing, b: specific gravity of the molded body assuming that the voids are maintained (calculated value), c: specific gravity of the molded body assuming that all hollow particles are crushed (calculated value).

[0008] The present disclosure also provides a method for producing a hollow particle-containing elastomer composition comprising at least a base elastomer and hollow particles, the method comprising: preparing a raw material mixture having at least a base elastomer, a shell containing a resin, and hollow particles having a hollow portion surrounded by the shell, the shell containing, as the resin, a polymer containing 50 parts by mass or more of crosslinkable monomer units per 100 parts by mass of all monomer units; the raw material mixture having a storage modulus G' of 2.5 MPa or less at 60°C as determined by dynamic viscoelasticity measurement performed after homogenization of blended components; pre-kneading the raw material mixture using an internal kneader at a temperature at which the storage modulus G' of 2.5 MPa or less as determined by dynamic viscoelasticity measurement performed after the homogenization; The raw material mixture is pre-kneaded, and immediately thereafter, or after pre-heating at a temperature at which the storage modulus G' obtained by dynamic viscoelasticity measurement performed after the homogenization treatment is 2.5 MPa or less, and then kneaded at a temperature at which the storage modulus G' obtained by dynamic viscoelasticity measurement performed after the homogenization treatment is 2.5 MPa or less.

[0009] The hollow particle-containing elastomer composition of the present disclosure contains hollow particles whose strength is enhanced by a shell made of a resin containing a polymer with a high content of crosslinkable monomer units and whose strength does not decrease even in high-temperature environments due to the crosslinked structure, so the hollow particles are resistant to crushing during the molding process, maintain high dimensional stability, and are less likely to lose the properties or functions imparted by the hollow particles. Therefore, the hollow particle-containing elastomer composition of the present disclosure can produce hollow particle-containing molded articles with high dimensional stability and excellent properties or functions.

[0010] Furthermore, the method for producing a hollow particle-containing elastomer composition disclosed herein uses hollow particles that have increased strength due to a shell made of a resin containing a polymer with a high content of crosslinkable monomer units and whose strength does not decrease even in high-temperature environments due to the crosslinked structure, and further, by adjusting the storage modulus G' at 60°C of a raw material mixture containing a base elastomer and hollow particles to 2.5 MPa or less and performing pre-mixing and finish-mixing at a temperature at which the storage modulus G' of the raw material mixture is 2.5 MPa or less, loads such as internal pressure and shear force exerted on the hollow particles within the raw material mixture during kneading can be kept low. Therefore, according to the production method disclosed herein, the hollow particles are less likely to be crushed during the kneading step, the residual void ratio is stable, and the properties or functions imparted by the hollow particles are less likely to be impaired, resulting in a hollow particle-containing elastomer composition with excellent properties or functions.

[0011] 1 is a diagram illustrating an example of a method for producing hollow particles used in the present disclosure. FIG. 2 is a schematic diagram illustrating an embodiment of a suspension in a suspension step.

[0012] When a hollow particle-containing elastomer composition is molded, the hollow particles may be crushed by the application of pressure and heat during molding. Furthermore, when a raw material mixture containing a base elastomer and hollow particles is kneaded to produce a hollow particle-containing elastomer composition, the hollow particles may be crushed. In particular, when hollow particles are mixed into a base elastomer, higher shear forces are generated than when hollow particles are mixed into a base resin other than an elastomer, making the hollow particles more likely to be crushed during kneading. In particular, when hollow particles with a high porosity are used, the hollow particles are even more likely to be crushed because their shell thickness is generally thin or their particle size is large.

[0013] The researchers of the present disclosure have found that when the storage modulus G' at 60°C of an elastomer composition obtained by blending hollow particles having a shell made of a resin containing a polymer containing a certain amount or more of crosslinkable monomer units into a base elastomer is 2.5 MPa or less, the hollow particles are less likely to be crushed when molded using the elastomer composition, thereby maintaining the porosity of the hollow particles present inside the resulting elastomer molded product, and preventing the loss of the properties or functions imparted by the hollow particles. The researchers of the present disclosure have also found that when the storage modulus G' at 60°C of a raw material mixture obtained by blending hollow particles having a shell made of a resin containing a polymer containing a certain amount or more of crosslinkable monomer units into a base elastomer and then homogenizing the raw material mixture is 2.5 MPa or less, the porosity of the hollow particles present inside the elastomer composition obtained by kneading the raw material mixture can be maintained, and preventing the loss of the properties or functions imparted by the hollow particles. The present disclosure has been made based on the above findings.

[0014] The hollow particle-containing elastomer composition disclosed herein comprises at least a base elastomer and hollow particles, the hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, the shell being a polymer containing 50 parts by mass or more of crosslinkable monomer units per 100 parts by mass of total monomer units as the resin, the composition having a storage modulus G' at 60°C measured by dynamic viscoelasticity measurement of 2.5 MPa or less, the composition being characterized in that a sheet-shaped molded product is produced using the composition according to the method for measuring the void residual rate of a hollow particle-containing elastomer molded product described below, and the measured void residual rate is 80% or more. [Method for measuring the void residual rate of a hollow particle-containing elastomer molded product] A sheet-shaped hollow particle-containing elastomer molded product is produced by press-molding the hollow particle-containing elastomer composition using a hot press at 120°C under a pressure of 1 MPa or less. The specific gravity of the resulting elastomer molded product is measured, and the void residual rate of the hollow particles in the elastomer molded product is calculated according to the following formula (D): Void remaining rate (%) = {(c-a) / (c-b)} x 100 Formula (D) a: specific gravity of the sheet-like molded body after pressing, b: specific gravity of the molded body assuming that the voids are maintained (calculated value), c: specific gravity of the molded body assuming that all hollow particles are crushed (calculated value).

[0015] The method for producing a hollow particle-containing elastomer composition according to the present disclosure is a method for producing the hollow particle-containing elastomer composition, the method comprising: preparing a raw material mixture comprising at least a base elastomer, a shell containing a resin, and hollow portions surrounded by the shell, the shell containing hollow particles containing a polymer containing 50 parts by mass or more of crosslinkable monomer units per 100 parts by mass of all monomer units as the resin; the raw material mixture having a storage modulus G' at 60°C of 2.5 MPa or less as determined by dynamic viscoelasticity measurement performed after homogenization of blended components; pre-kneading the raw material mixture using an internal kneader at a temperature at which the storage modulus G' as determined by the dynamic viscoelasticity measurement performed after the homogenization is 2.5 MPa or less; and kneading the raw material mixture immediately after pre-kneading, or after pre-heating the raw material mixture at a temperature at which the storage modulus G' as determined by the dynamic viscoelasticity measurement performed after the homogenization is 2.5 MPa or less.

[0016] The hollow particle-containing elastomer composition, its manufacturing method, and its manufacturing method for an elastomer molded article of the present disclosure are described below. In this disclosure, the term "to" in a numerical range means that the numerical values ​​before and after it are included as the lower and upper limits. Also, in this disclosure, "(meth)acrylate" refers to acrylate and methacrylate, "(meth)acrylic" refers to acrylic and methacrylic, and "(meth)acryloyl" refers to acryloyl and methacryloyl.

[0017] In addition, in this disclosure, a polymerizable monomer is a compound having a functional group capable of addition polymerization (sometimes simply referred to as a polymerizable functional group in this disclosure). In this disclosure, a compound having an ethylenically unsaturated bond as a functional group capable of addition polymerization is generally used as the polymerizable monomer. In this disclosure, a polymerizable monomer having only one polymerizable functional group is referred to as a non-crosslinkable monomer, and a polymerizable monomer having two or more polymerizable functional groups is referred to as a crosslinkable monomer. A crosslinkable monomer is a polymerizable monomer that forms a crosslinked bond in a resin by a polymerization reaction.

[0018] [Hollow Particle-Containing Elastomer Composition] The hollow particle-containing elastomer composition of the present disclosure is a molding material for producing elastomer molded articles such as elastomer parts, elastomer parts integrally molded with parts made of other materials, coatings, filler chip materials, etc., by melt molding methods such as extrusion molding and compression molding. By incorporating hollow particles into the hollow particle-containing elastomer composition of the present disclosure, it is possible to impart various properties to the elastomer molded article, such as weight reduction, heat insulation, low dielectric constant, light reflection / scattering, and the retention of functional components such as antibacterial agents. Therefore, examples of applications of elastomer molded articles produced using the elastomer composition include light-reflecting materials, heat insulating materials, sound insulating materials, and low-dielectric materials used in various fields such as automobiles, electricity, electronics, construction, aviation, and space, overcoat materials or undercoat materials that require heat insulating properties, shock absorbing properties (cushioning properties), light reflecting properties, antibacterial properties, etc., shock absorbing materials (cushioning materials) for footwear such as sports shoes and sandals, home appliance parts, bicycle parts, stationery, tools, hollow particle-containing filaments for 3D printers, and buoyancy materials made from syntactic foam.

[0019] The hollow particle-containing elastomer composition of the present disclosure does not contain a blowing agent for making the elastomer molded body porous, and uses hollow particles that have already been molded into a hollow shape. Therefore, dimensional variations in the elastomer molded body due to foaming by the blowing agent do not occur.

[0020] Furthermore, the hollow particle-containing elastomer composition of the present disclosure contains hollow particles whose strength is enhanced by a shell made of a resin containing a polymer with a high content of crosslinkable monomer units and whose strength does not decrease even in high-temperature environments due to the crosslinked structure, so the hollow particles are resistant to crushing during the molding process, maintain high dimensional stability, and are less likely to lose the properties or functions imparted by the hollow particles. Therefore, the hollow particle-containing elastomer composition of the present disclosure can produce hollow particle-containing molded articles with high dimensional stability and excellent properties or functions.

[0021] [Storage Modulus] The hollow particle-containing elastomer composition of the present disclosure is characterized in that the storage modulus at 60°C, as determined by dynamic viscoelasticity measurement, is 2.5 MPa or less, preferably 1.7 MPa or less, from the viewpoint of reducing crushing of hollow particles within the hollow particle-containing elastomer composition during kneading. Furthermore, the lower limit of the storage modulus G' is not particularly limited, but is preferably 0.5 MPa or more, more preferably 0.8 MPa or more, in order to maintain the hardness of a molded article obtained from the elastomer composition.

[0022] The storage modulus of the hollow particle-containing elastomer composition at 60°C can be determined from the temperature dependence curve of the storage modulus obtained by measuring the dynamic viscoelasticity of the hollow particle-containing elastomer composition. In the present disclosure, a general method applied to measuring the dynamic viscoelasticity of a resin can be appropriately implemented, for example, by the following method. [Method for measuring dynamic viscoelasticity] Dynamic viscoelasticity is measured using a measuring device such as a HAAKE MARK III (manufactured by Thermo Fisher Scientific) or a rotating plate rheometer (manufactured by TA Instruments) using a parallel plate or crosshatch plate under the following conditions. <Measurement Conditions> Frequency: 1 Hz Geometry: Parallel plate or crosshatch plate (20 mmφ) Sample set: A test piece (2 to 4 mm thick) is fused to a 20 mmφ plate set at 170°C, then placed in a dynamic viscoelasticity measuring device, and the storage modulus G' (unit: MPa) from 150°C to room temperature is measured while the temperature is lowered from 170°C to room temperature (e.g., 25°C) at a rate of 5°C / min. Temperature lowering rate: 5°C / min Temperature range: 150°C to room temperature Strain: 0.001 Gap: 1.5 mm The test piece can be prepared, for example, by preparing a 2 mm thick sheet using a press at 160°C from the hollow particle-containing elastomer composition of the present disclosure and punching it into a 20 mmφ shape using a punching machine.

[0023] [Method for Adjusting Storage Modulus] The storage modulus G' of a hollow particle-containing elastomer composition, as determined by dynamic viscoelasticity measurement at 60°C, can be adjusted by varying one or more of the following factors: the amount of plasticizer added, the amount of hollow particles added, the particle diameter of the hollow particles, the surface composition of the hollow particles, and the type and amount of organic or inorganic fine particles other than hollow particles (e.g., carbon, silica, etc.). Of the above factors, the amount of plasticizer added and the amount of hollow particles added can be particularly significant factors that affect the storage modulus G'. Increasing the amount of plasticizer added can decrease the storage modulus G' of the hollow particle-containing elastomer composition, while decreasing the amount of plasticizer added can increase the storage modulus G' of the hollow particle-containing elastomer composition. The amount of plasticizer added is typically increased or decreased within a range of 35 to 100 parts by mass, preferably 45 to 90 parts by mass, per 100 parts by mass of the base elastomer, in order to adjust the storage modulus G' of the hollow particle-containing elastomer composition, as determined by dynamic viscoelasticity measurement, to 2.5 MPa or less. Furthermore, increasing the amount of hollow particles added can increase the storage modulus G' of the hollow particle-containing elastomer composition, while decreasing the amount of hollow particles added can decrease the storage modulus G' of the hollow particle-containing elastomer composition. The amount of hollow particles added is typically increased or decreased within a range of 5 to 80 parts by mass, per 100 parts by mass of the base elastomer, in order to adjust the storage modulus G' of the hollow particle-containing elastomer composition, as determined by dynamic viscoelasticity measurement, to 2.5 MPa or less, while taking into consideration the balance with the contribution of the hollow particles to their intended purpose, such as weight reduction, thermal insulation, and cushioning.

[0024] [Base Elastomer] The base material can be an elastomer, i.e., a polymer having rubber-like elasticity. The elastomer is not particularly limited, but examples include rubber and thermoplastic elastomers. Examples of rubber that can be used include natural rubber, isoprene rubber, butadiene rubber (BR), styrene butadiene rubber (SBR), chloroprene rubber, acrylonitrile butadiene rubber, ethylene-α-olefin copolymer rubber, ethylene-α-olefin-non-conjugated diene copolymer rubber such as ethylene-propylene-diene terpolymer (EPDM), halogenated ethylene-α-olefin-non-conjugated diene copolymer rubber, sulfonated ethylene-α-olefin-non-conjugated diene copolymer rubber, maleated ethylene-α-olefin-non-conjugated diene copolymer rubber, butyl rubber, isobutylene isoprene rubber, nitrile rubber, hydrogenated nitrile rubber, urethane rubber, silicone rubber, chlorosulfonated polyethylene rubber, chlorinated polyethylene rubber, acrylic rubber, epichlorohydrin rubber, fluororubber, polysulfide rubber, and propylene oxide rubber. Furthermore, thermoplastic elastomers generally exhibit rubber-like elasticity at room temperature (25°C) and have the property of being plasticized and moldable at high temperatures. Examples of thermoplastic elastomers that can be used include thermoplastic elastic polymers that have traditionally been used as molding resins, such as urethane-based elastomers, styrene-based elastomers, olefin-based elastomers, amide-based elastomers, and ester-based elastomers. These base elastomers can be used alone or in combination of two or more.The base elastomer preferably contains at least one selected from ethylene-α-olefin-non-conjugated diene copolymer rubber, butadiene rubber, styrene-butadiene rubber, natural rubber, isoprene rubber, nitrile rubber, hydrogenated nitrile rubber, butyl rubber, fluororubber, silicone rubber, acrylonitrile-butadiene rubber, chloroprene rubber, acrylic rubber, chlorosulfonated polyethylene rubber, chlorinated polyethylene rubber, urethane rubber, isobutylene-isoprene rubber, polysulfide rubber, propylene oxide rubber, and epichlorohydrin rubber, more preferably contains at least one selected from ethylene-α-olefin-non-conjugated diene copolymer rubber, butadiene rubber, styrene-butadiene rubber, natural rubber, isoprene rubber, and acrylic rubber, more preferably contains at least one selected from ethylene-α-olefin-non-conjugated diene copolymer rubber, butadiene rubber, and styrene-butadiene rubber, and even more preferably contains ethylene-α-olefin-non-conjugated diene copolymer rubber.

[0025] The ethylene-α-olefin-non-conjugated diene copolymer rubber is a random copolymer of ethylene, an α-olefin, and a non-conjugated diene. Examples of α-olefins include propylene, 1-butene, 1-pentene, 3-methyl-1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 1-undecene, and 1-dodecene. Among these, propylene, 1-hexene, and 1-octene are preferred, and propylene is particularly preferred. These α-olefins can be used alone or in combination of two or more. The molar ratio of ethylene to α-olefin (ethylene / α-olefin) is not particularly limited, but is preferably 40 / 60 to 95 / 5, more preferably 50 / 50 to 85 / 15, and even more preferably 60 / 40 to 80 / 20. Examples of non-conjugated dienes include 1,4-hexadiene, 3-methyl-1,4-hexadiene, 1,7-octadiene, 1,9-decadiene, 5-ethylidene-2-norbornene, 5-isopropylidene-2-norbornene, 5-isobutenyl-2-norbornene, cyclopentadiene, dicyclopentadiene, and norbornadiene. Among these, 5-ethylidene-2-norbornene and dicyclopentadiene are preferred. These non-conjugated dienes can be used alone or in combination of two or more.

[0026] Examples of butadiene rubber (BR) include low cis BR, high cis BR, and high trans BR. Furthermore, modified BR into which a nitrogen-containing functional group, a silicon-containing functional group, an oxygen-containing functional group, or the like has been introduced may also be used as the butadiene rubber. Examples of styrene-butadiene rubber (SBR) include solution-polymerized SBR and emulsion-polymerized SBR. Furthermore, acid-modified SBR may also be used as the styrene-butadiene rubber. Commercially available acid-modified SBRs include products under the trade name Nipol LX206 (manufactured by Nippon Zeon Co., Ltd.), Nipol LX209 (manufactured by the same company), BM-430B (manufactured by the same company), and BM-451B (manufactured by the same company).

[0027] When at least one rubber selected from ethylene-α-olefin-non-conjugated diene copolymer rubber, butadiene rubber, and styrene-butadiene rubber is used as at least a portion of the base elastomer, the mass proportion of the rubber selected from ethylene-α-olefin-non-conjugated diene copolymer rubber, butadiene rubber, and styrene-butadiene rubber in the entire base elastomer is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more. The mass proportion of the rubber selected from ethylene-α-olefin-non-conjugated diene copolymer rubber, butadiene rubber, and styrene-butadiene rubber in the entire base elastomer is preferably 100% by mass. The mass proportion of the ethylene-α-olefin-non-conjugated diene copolymer rubber in the entire base elastomer is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more. The mass proportion of the ethylene-α-olefin-non-conjugated diene copolymer rubber in the entire base elastomer is preferably 10 ...100% by mass or more.

[0028] The iodine value of the base elastomer is not particularly limited, but is preferably 5 to 50 g / 100 g, more preferably 10 to 40 g / 100 g, and even more preferably 15 to 30 g / 100 g. When the iodine value of the base elastomer is within the above range, an elastomer composition with high crosslinking efficiency is obtained, and an elastomer composition that can provide vulcanized elastomer products with excellent compression set resistance and environmental degradation resistance is obtained.

[0029] When the base elastomer contains an elastomer having styrene monomer units, such as styrene-butadiene rubber or a styrene-based thermoplastic elastomer, the rigidity of the elastomer composition of the present disclosure can be adjusted by changing the content of styrene monomer units in the base elastomer. However, if the content of styrene monomer units in the base elastomer is too high, the rigidity of the elastomer composition and its raw material mixture during melt-kneading also increases, making it difficult to sufficiently reduce the storage modulus G' during melt-kneading. Therefore, the content of styrene monomer units in the base elastomer is preferably 0% by mass or more and 60% by mass or less, and more preferably 0% by mass or more and 50% by mass or less, of the total mass of the base elastomer.

[0030] From the viewpoint of improving the dimensional stability and surface properties of a molded article produced from the elastomer composition of the present disclosure, the Mooney viscosity (ML(1+4)100°C) of the base elastomer measured in accordance with JIS K6300 is preferably 20 or more and 75 or less, more preferably 20 or more and 60 or less, and even more preferably 20 or more and 55 or less. In the above expression "ML(1+4)100°C" relating to the Mooney viscosity, "M" stands for Mooney unit, "L" stands for an L-shaped rotor shape, "1+4" stands for a preheating time of 1 minute and a rotor rotation time of 4 minutes, and "100°C" stands for a measurement temperature of 100°C.

[0031] [Hollow Particles] The hollow particles used in the present disclosure are particles having a shell (outer shell) containing a resin and a hollow portion surrounded by the shell. In the present disclosure, the hollow portion is a hollow space clearly distinguishable from the shell of the hollow particle formed from a resin material. The shell of the hollow particle may have a porous structure, but in that case, the hollow portion has a size that is clearly distinguishable from the numerous microscopic spaces uniformly dispersed within the porous structure. From the viewpoint of pressure resistance, etc., the hollow portion of the hollow particle preferably has a solid shell. For example, the hollow portion of the hollow particle can be confirmed by SEM observation of the particle cross section or by TEM observation of the particle itself. Furthermore, the hollow portion of the hollow particle of the present disclosure may be filled with a gas such as air, may be in a vacuum or reduced pressure state, or may contain a solvent.

[0032] The hollow particles used in the present disclosure have increased strength due to a shell made of a resin containing a polymer with a high content of crosslinkable monomer units, and their strength does not decrease even in high-temperature environments due to the crosslinked structure, making them highly pressure-resistant and resistant to crushing when mixed with other materials and during molding after mixing.When added to a molded body, they impart a variety of effects, such as weight reduction, insulation, soundproofing, vibration damping, and light scattering.Another use is their excellent effectiveness as an encapsulating material that can encapsulate useful ingredients such as fragrances, medicines, pesticides, and ink components into the hollow interior by means of immersion treatment, reduced pressure or pressure immersion treatment, etc., making them suitable for use as additives for molded bodies.Furthermore, the hollow particles used in the present disclosure are resistant to crushing and resistant to a decrease in porosity even when subjected to processes that apply loads such as external pressure and shear force, such as kneading and injection molding, making them particularly suitable for use as additives for molded bodies obtained through processes that apply loads.

[0033] The hollow particles used in the present disclosure preferably have a porosity of 50% or more, more preferably 60% or more, and even more preferably 65% ​​or more. Having a porosity equal to or greater than the above-mentioned lower limit results in excellent properties such as light weight, heat resistance, heat insulation, and dielectric properties. The upper limit of the porosity of the hollow particles is not particularly limited, but is preferably 90% or less, more preferably 85% or less, and even more preferably 80% or less, in order to prevent a decrease in the pressure resistance of the hollow particles.

[0034] The porosity of the hollow particles is the apparent density D １ and true density D ０ The apparent density of the hollow particles D １ The measurement method is as follows: First, a volume of 100 cm ３ About 30 cm ３ The volumetric flask is filled with hollow particles, and the mass of the filled hollow particles is accurately weighed. Next, a volumetric flask with a capacity of 100 cm is filled with the hollow particles while being careful not to introduce air bubbles. ３ The volumetric flask is filled with isopropanol exactly up to the mark. The mass of the isopropanol added to the volumetric flask is accurately weighed, and the apparent density D of the hollow particles is calculated based on the following formula (I): １ (g / cm ３ ) is calculated using the formula (I): Apparent density D １ Apparent density D = [Mass of hollow particles] / (100 - [Mass of isopropanol] ÷ [Specific gravity of isopropanol at measurement temperature]) １ corresponds to the specific gravity of the entire hollow particle when the hollow portion is considered to be a part of the hollow particle.

[0035] True density D of hollow particles ０ The measurement method is as follows: After crushing the hollow particles in advance, ３ Approximately 10 g of crushed pieces of hollow particles are filled into a measuring flask, and the mass of the crushed pieces is accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol is added to the measuring flask, and the mass of the isopropanol is accurately weighed. The true density D of the hollow particles is calculated based on the following formula (II): ０ (g / cm ３ ) is calculated using the formula (II): True density D ０ = [mass of crushed hollow particle pieces] / (100 - [mass of isopropanol] ÷ [specific gravity of isopropanol at measurement temperature]) True density D ０ As is clear from the above measurement method, the true density D ０ In calculating the particle diameter, the hollow portion is not considered to be part of the hollow particle.

[0036] The porosity (%) of the hollow particles is calculated by multiplying the apparent density D １ and true density D ０ The porosity (%) is calculated by the following formula (III): １ / True density D ０ ) x 100

[0037] The crush resistance of hollow particles can be expressed by the void residual rate measured according to the following press test method. [Press test method] A mixture of polypropylene resin and hollow particles, with a mass ratio of polypropylene resin to hollow particles of 90:10, is melted and mixed at 200°C, placed in a heat press mold, and further heated at 200°C for 15 minutes, stirred, and then placed in a heat press set at 80°C. A cylinder heated to 80°C is placed in the mold. When the surface temperature of the mold reaches 140°C, a pressure of 15 MPa is applied. The mixture is then removed from the mold and pressed at a pressure of 1 MPa or less using a heat press set at 200°C to form a sheet. The specific gravity of the resulting sheet-like molded product is measured, and the void residual rate of the hollow particles is calculated according to the following formula (D): Void residual rate (%) = {(c-a) / (c-b)} x 100 Formula (D) The symbols in formula (D) have the following meanings. a: specific gravity of the sheet-shaped molded product after press molding, b: specific gravity (calculated value) of the molded product assuming that voids are maintained, c: specific gravity (calculated value) of the molded product assuming that all hollow particles are crushed. The specific gravity of the molded product after press molding was measured by the underwater displacement method in accordance with JIS K 7112. "Molded product assuming that voids are maintained" means a molded product assuming that the hollow particles mixed with the polypropylene resin are not crushed even in the molded product after the heat pressing process and that the porosity before mixing is maintained. The specific gravity b of the molded product assuming that voids are maintained was calculated by the following formula (E): b=1 / {(P Ａ / P Ｇ ) + (R Ａ / R Ｇ )} Formula (E) In the above formula for calculating b, P Ａ is the amount of hollow particles added, P Ｇ is the specific gravity of the hollow particles, R Ａ is the amount of base elastomer added, R Ｇand R represent the specific gravity of the base elastomer. The specific gravity c of the molded article, assuming that all hollow particles are crushed, was calculated by the following formula (F): c = [R Ｇ ×R Ａ +{D ０ ×P Ａ × (1-P Ｖ / 100)}] / {R Ａ +P Ａ × (1-P Ｖ / 100)} Formula (F) In the above calculation formula for finding c, R Ａ is the amount of base elastomer added, R Ｇ is the specific gravity of the base elastomer, D ０ is the true density of the hollow particle, P Ａ is the amount of hollow particles added, P Ｖ and represent the porosity (%) of the hollow particles.

[0038] The polypropylene resin used in the press test may have an MFR (melt flow rate) of 10 to 30 g / min, preferably 15 to 25 g / min, at 230° C. Commercially available polypropylene resins include, for example, Novatec PP, Grade MA1B (MFR of 21 g / min at 230° C.), manufactured by Japan Polypropylene Corporation.

[0039] The higher the void residual rate, the less likely the hollow particles are to be crushed during the process of producing the hollow particle-containing elastomer composition and during the process of producing an elastomer molded article using the hollow particle-containing elastomer composition, the more likely the effects imparted by the hollow particles are maintained without reduction, and the higher the dimensional stability during molding. When no hollow particle crushing occurs, the void residual rate is 100%. The hollow particles used in the present disclosure can achieve a void residual rate of 80% or more, and even 100%, according to the above test method.

[0040] The volume average particle diameter of the hollow particles used in the present disclosure has a lower limit of preferably 5.0 μm or more, more preferably 6.0 μm or more, and even more preferably 7.0 μm or more, and an upper limit of preferably 40.0 μm or less, more preferably 30.0 μm or less, and even more preferably 20.0 μm or less. When the volume average particle diameter of the hollow particles is equal to or greater than the above lower limit, it is easy to achieve both high porosity and excellent pressure resistance, and the tendency for hollow particles to aggregate with each other is reduced, thereby enabling excellent dispersibility to be exhibited. When the volume average particle diameter of the hollow particles is equal to or less than the above upper limit, it is easy to improve the uniformity of the shell, making it easy to obtain hollow particles with excellent pressure resistance. The particle diameter of the hollow particles disclosed in the present disclosure can be adjusted, for example, by the content of the dispersion stabilizer relative to the total mass of the polymerizable monomer and the hydrophobic solvent.

[0041] The shell thickness of the hollow particles used in the present disclosure is not particularly limited, but from the viewpoint of improving pressure resistance, it is preferably 0.30 μm or more, more preferably 0.40 μm or more, even more preferably 0.50 μm or more, and even more preferably 0.60 μm or more, and from the viewpoint of increasing porosity, it is preferably 3.00 μm or less, more preferably 2.00 μm or less, and even more preferably 1.50 μm or less. Note that in the present disclosure, the shell thickness of the hollow particles is a value calculated by calculating the inner diameter r of the hollow particles according to the following formula (1) using the volume average particle diameter R and porosity of the hollow particles, and then using the inner diameter r and volume average particle diameter R according to the following formula (2). Formula (1): 4 / 3π×(R / 2) ３ ×(porosity / 100)=4 / 3π×(r / 2) ３ Shell thickness=(R−r) / 2 Formula (2): Note that the porosity in the above formula (1) is a numerical value expressed as a percentage.

[0042] The particle size distribution (volume average particle size (Dv) / number average particle size (Dn)) of the hollow particles used in the present disclosure may be, for example, 1.1 or more and 2.5 or less. When the particle size distribution is 2.5 or less, particles with little variation in compressive strength characteristics and heat resistance can be obtained. Furthermore, when manufacturing a sheet-shaped molded product containing the hollow particles of the present disclosure, a product with a uniform thickness can be manufactured. The volume average particle size (Dv) and number average particle size (Dn) of the hollow particles can be determined, for example, by measuring the particle size of the hollow particles using a particle size distribution measuring device, calculating the number average and volume average, respectively, and using the obtained values ​​as the number average particle size (Dn) and volume average particle size (Dv) of the particles. The particle size distribution is the value obtained by dividing the volume average particle size by the number average particle size.

[0043] The shape of the hollow particles used in the present disclosure is not particularly limited as long as a hollow portion is formed inside, and examples include spherical, oval, and amorphous shapes. Among these, spherical shapes are preferred from the viewpoints of ease of production and pressure resistance. The hollow particles used in the present disclosure may have one or more hollow portions. However, in order to maintain a good balance between high porosity and mechanical strength, those having only one or two hollow portions are preferred, and those having only one hollow portion are preferred. The percentage of hollow particles used in the present disclosure having only one hollow portion is preferably 90% or more, more preferably 95% or more, and even more preferably more than 95%. Furthermore, the shells of hollow particles, and the partition walls separating adjacent hollow portions when the hollow particles have two or more hollow portions, may be porous, but are preferably solid in order to improve pressure resistance.

[0044] The hollow particles used in the present disclosure may have an average circularity of 0.950 to 0.995. One example of the shape of the hollow particles of the present disclosure is a bag made of a thin film and inflated with gas, the cross-sectional view of which is shown as hollow particle 10 in (5) of Figure 1. In this example, a single thin film is provided on the outside, and the interior is filled with gas. The particle shape can be confirmed, for example, by SEM or TEM.

[0045] Furthermore, the hollow particles of the present disclosure preferably have a thermal decomposition onset temperature of 150 to 400°C, more preferably 200 to 350°C. Hollow particles having a thermal decomposition onset temperature within the above range have excellent heat resistance. The thermal decomposition onset temperature of hollow particles means the temperature at which a 5% mass loss occurs, and can be measured using a TG-DTA device in an air atmosphere under conditions of an air flow rate of 230 mL / min and a temperature rise rate of 10°C / min.

[0046] The content of the hollow particles in the elastomer composition is not particularly limited, but is usually 5 to 80 parts by mass per 100 parts by mass of the base elastomer.

[0047] An example of a method for producing hollow particles used in the present disclosure will be described in detail below.

[0048] The hollow particles can be obtained by a production method including the steps of: preparing a mixed solution containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; suspending the mixed solution to prepare a suspension in which droplets of a monomer composition containing the polymerizable monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction to prepare a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a resin, and encapsulating the hydrophobic solvent in the hollow portion.

[0049] The above-mentioned production method is based on the following basic technique: a mixture containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium is suspended, whereby the polymerizable monomer and the hydrophobic solvent undergo phase separation to prepare a suspension in which droplets having a distribution structure in which the polymerizable monomer is unevenly distributed on the surface and the hydrophobic solvent is unevenly distributed in the center are dispersed in the aqueous medium; the suspension is then subjected to a polymerization reaction to harden the surfaces of the droplets and form hollow particles having hollows filled with the hydrophobic solvent. In the above-mentioned basic technique, by adjusting the composition of the polymerizable monomer and the type of hydrophobic solvent, etc., sufficient phase separation between the polymerizable monomer and the hydrophobic solvent occurs in the droplets of the monomer composition dispersed in the suspension; and when the suspension is subjected to a polymerization reaction, it is presumed that the polymerization reaction of the polymerizable monomer proceeds uniformly, resulting in the formation of shells with excellent uniformity in composition, thickness, etc.

[0050] The method for producing hollow particles includes a step of preparing a mixed solution, a step of preparing a suspension, and a step of subjecting the suspension to a polymerization reaction, and may further include other steps. Furthermore, as far as technically possible, two or more of the above steps and other additional steps may be performed simultaneously as a single step, or the order of the steps may be reversed. For example, the preparation of the mixed solution and the suspension may be performed simultaneously in a single process, such as by adding the materials for preparing the mixed solution and suspending them at the same time.

[0051] A preferred example of a method for producing hollow particles includes the following steps. (1) Mixed liquid preparation step: A step of preparing a mixed liquid containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium. (2) Suspension step: A step of suspending the mixed liquid to prepare a suspension in which droplets of a monomer composition containing a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator are dispersed in an aqueous medium. (3) Polymerization step: A step of subjecting the suspension to a polymerization reaction to prepare a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a resin, and encapsulating a hydrophobic solvent in the hollow portion. (4) Solid-liquid separation step: A step of obtaining precursor particles encapsulating a hydrophobic solvent in the hollow portion by solid-liquid separation of the precursor composition. (5) Solvent removal step: A step of removing the hydrophobic solvent encapsulated in the precursor particles obtained by the solid-liquid separation step to obtain hollow particles. In the present disclosure, hollow particles whose hollow portions are filled with a hydrophobic solvent may be referred to as "precursor particles" as they are considered to be intermediates of hollow particles whose hollow portions are filled with a gas. In this disclosure, "precursor composition" means a composition that includes precursor particles.

[0052] FIG. 1 is a schematic diagram illustrating an example of the manufacturing method of the present disclosure. (1) to (5) in FIG. 1 correspond to the above-described steps (1) to (5). The white arrows between the diagrams indicate the order of each step. Note that FIG. 1 is merely a schematic diagram for explanatory purposes, and the manufacturing method of the present disclosure is not limited to that shown in the diagram. Furthermore, the structure, dimensions, and shape of the materials used in the manufacturing method of the present disclosure are not limited to those of the various materials shown in these diagrams. (1) in FIG. 1 is a cross-sectional schematic diagram illustrating one embodiment of a mixed solution in the mixed solution preparation step. As shown in this figure, the mixed solution contains an aqueous medium 1 and a low-polarity material 2 dispersed in the aqueous medium 1. Here, the low-polarity material 2 refers to a material that has low polarity and is difficult to mix with the aqueous medium 1. In the present disclosure, the low-polarity material 2 contains a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator. (2) in FIG. 1 is a cross-sectional schematic diagram illustrating one embodiment of a suspension in the suspension step. The suspension includes an aqueous medium 1 and droplets 8 of a monomer composition dispersed in the aqueous medium 1. The droplets 8 of the monomer composition contain a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator, but the distribution within the droplets is non-uniform. The droplets 8 of the monomer composition are phase-separated into a hydrophobic solvent 4a and a material other than the hydrophobic solvent, including the polymerizable monomer, a material 4b, with the hydrophobic solvent 4a concentrated in the center and the material other than the hydrophobic solvent 4b concentrated on the surface, and a dispersion stabilizer (not shown) attached to the surface. (3) in FIG. 1 is a cross-sectional schematic diagram illustrating one embodiment of a precursor composition obtained by a polymerization process, including precursor particles encapsulating a hydrophobic solvent in hollow portions. The precursor composition includes an aqueous medium 1 and precursor particles 9 encapsulating a hydrophobic solvent 4a in hollow portions, dispersed in the aqueous medium 1. The shell 6 forming the outer surface of the precursor particle 9 is formed by polymerization of the polymerizable monomer in the droplet 8 of the monomer composition, and contains a polymer of the polymerizable monomer as a resin. (4) in FIG. 1 is a cross-sectional schematic diagram showing one embodiment of the precursor particle after the solid-liquid separation step. (4) in FIG. 1 shows a state in which the aqueous medium 1 has been removed from the state shown in (3) in FIG. 1. (5) in FIG. 1 is a cross-sectional schematic diagram showing one embodiment of the hollow particle after the solvent removal step. (5) in FIG. 1 shows a state in which the hydrophobic solvent 4a has been removed from the state shown in (4) in FIG.By removing the hydrophobic solvent from the precursor particles, hollow particles 10 are obtained, each having a gas-filled hollow portion 7 inside a shell 6. The above five steps and other steps will be described in order below.

[0053] (1) Mixed Liquid Preparation Step This step is a step of preparing a mixed liquid containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium. The mixed liquid may further contain other materials as long as the effects of the present disclosure are not impaired. The materials of the mixed liquid will be described in the following order: (A) polymerizable monomer, (B) hydrophobic solvent, (C) polymerization initiator, (D) dispersion stabilizer, and (E) aqueous medium.

[0054] (A) Polymerizable Monomer As the polymerizable monomer, a known polymerizable monomer conventionally used for producing hollow particles can be used, and although there are no particular limitations, at least a part of the polymerizable monomer contains a crosslinkable monomer. When the polymerizable monomer contains a crosslinkable monomer, the crosslink density of the shell can be increased, which makes it easier to form a shell with excellent strength, makes hollow particles more likely to become spherical, and makes it easier to form hollow portions within the particles that are clearly distinguishable from the shell.

[0055] Furthermore, from the viewpoint of facilitating a stable polymerization reaction, the polymerizable monomer is preferably a polymerizable monomer whose polymerizable functional group is a (meth)acryloyl group or a vinyl group, and more preferably an acrylic monomer containing a (meth)acryloyl group as the polymerizable functional group. In the present disclosure, "stable polymerization reaction" means that the reactivity of the polymerization reaction is good and the polymerization reaction proceeds uniformly. Furthermore, it is particularly preferable for the polymerizable monomer to contain an acrylic monomer and a hydrocarbon monomer, since this not only facilitates stability of the polymerization reaction but also improves the pressure resistance of the hollow particles. It is presumed that copolymerization of an acrylic monomer and a hydrocarbon monomer increases the reactivity of the hydrocarbon monomer, thereby improving the overall reactivity of the polymerizable monomers, thereby facilitating a stable polymerization reaction. Furthermore, when the polymerizable monomer contains an acrylic monomer and a hydrocarbon monomer, the compatibility with the hydrophobic solvent is appropriate, and therefore, when the suspension is subjected to a polymerization reaction, the polymerization reaction of the polymerizable monomer tends to proceed uniformly, and the formed shell tends to have excellent uniformity in composition, thickness, etc., which is presumably why the pressure resistance of the hollow particles is improved. As the hydrocarbon monomer, one whose polymerizable functional group is a vinyl group is preferred because it tends to stabilize the polymerization reaction. In this disclosure, a polymerizable monomer having a (meth)acryloyl group as the polymerizable functional group is referred to as an acrylic monomer, a crosslinkable monomer having a (meth)acryloyl group as the polymerizable functional group is referred to as a crosslinkable acrylic monomer, and a non-crosslinkable monomer having a (meth)acryloyl group as the polymerizable functional group is referred to as a non-crosslinkable acrylic monomer. In the crosslinkable acrylic monomer, at least one polymerizable functional group may be a (meth)acryloyl group, but it is preferable that all polymerizable functional groups are (meth)acryloyl groups. In addition, in the present disclosure, a polymerizable monomer consisting of carbon and hydrogen is referred to as a hydrocarbon monomer, a crosslinkable monomer consisting of carbon and hydrogen is referred to as a crosslinkable hydrocarbon monomer, and a non-crosslinkable monomer consisting of carbon and hydrogen is referred to as a non-crosslinkable hydrocarbon monomer.

[0056] The crosslinkable monomer used in the production method of the present disclosure is preferably a crosslinkable acrylic monomer or a crosslinkable hydrocarbon monomer. Examples of the crosslinkable acrylic monomer include bifunctional crosslinkable acrylic monomers such as allyl(meth)acrylate, vinyl(meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, and 2-hydroxy-3-(meth)acrylpropyl(meth)acrylate; and trifunctional or higher crosslinkable acrylic monomers such as trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol poly(meth)acrylate, and ethoxylated versions of these. Examples of the crosslinkable hydrocarbon monomer include bifunctional crosslinkable hydrocarbon monomers such as divinylbenzene, divinyldiphenyl, and divinylnaphthalene. Further examples of the crosslinkable monomer include crosslinkable allylic monomers such as diallyl phthalate. These crosslinkable monomers can be used alone or in combination of two or more.

[0057] In order to improve the strength of the shell and obtain hollow particles having excellent pressure resistance, the crosslinkable monomer preferably contains a trifunctional or higher crosslinkable monomer having three or more polymerizable functional groups. As the trifunctional or higher crosslinkable monomer, the above-mentioned trifunctional or higher crosslinkable acrylic monomers are preferred, and among them, pentaerythritol tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, and dipentaerythritol poly(meth)acrylate are preferred, and pentaerythritol tetra(meth)acrylate and trimethylolpropane tri(meth)acrylate are more preferred.

[0058] From the viewpoint of improving the shell strength and obtaining hollow particles with excellent pressure resistance, it is more preferable that the crosslinkable monomer contains a bifunctional crosslinkable monomer having only two polymerizable functional groups and a trifunctional or higher crosslinkable monomer having three or more polymerizable functional groups. The bifunctional crosslinkable monomer is preferably at least one selected from the group consisting of the above-mentioned bifunctional crosslinkable acrylic monomers and the above-mentioned bifunctional crosslinkable hydrocarbon monomers. Among the bifunctional crosslinkable acrylic monomers, ethylene glycol di(meth)acrylate and pentaerythritol di(meth)acrylate are preferred, with ethylene glycol di(meth)acrylate being more preferred. Among the bifunctional crosslinkable hydrocarbon monomers, divinylbenzene is preferred.

[0059] The content of the crosslinkable monomer is 50 parts by mass or more, more preferably 60 parts by mass or more, even more preferably 70 parts by mass or more, and even more preferably 80 parts by mass or more, per 100 parts by mass of the polymerizable monomer, in order to improve the pressure resistance of the hollow particles. When the content of the crosslinkable monomer is equal to or greater than the above-mentioned lower limit, hollow portions are more likely to be formed within the particles, the particles are more likely to become spherical, and the crosslinking density of the shell can be increased, thereby improving the solvent resistance, strength, heat resistance, etc. of the hollow particles. On the other hand, the polymerizable monomer may contain a non-crosslinkable monomer, provided that the effects of the present disclosure are not impaired. In this case, the content of the crosslinkable monomer may be, for example, 95 parts by mass or less, or 90 parts by mass or less, per 100 parts by mass of the polymerizable monomer. The content of the crosslinkable monomer refers to the total content of the bifunctional crosslinkable monomer and the trifunctional or higher crosslinkable monomer.

[0060] When the crosslinkable monomer contains a trifunctional or higher functional crosslinkable monomer, in order to improve the pressure resistance of the hollow particles, the content of the trifunctional or higher functional crosslinkable monomer is, relative to 100 parts by mass of the polymerizable monomer, preferably 10 parts by mass or more as a lower limit, more preferably 20 parts by mass or more, and preferably 50 parts by mass or less as an upper limit, more preferably 40 parts by mass or less, and even more preferably 30 parts by mass or less.

[0061] When the crosslinkable monomer contains a bifunctional crosslinkable monomer and a trifunctional or higher functional crosslinkable monomer, in order to improve the pressure resistance of the hollow particles, the content of the trifunctional crosslinkable monomer relative to 100 parts by mass of the total mass of the bifunctional crosslinkable monomer and the trifunctional or higher functional crosslinkable monomer is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 20 parts by mass or more, and the upper limit is preferably 50 parts by mass or less, more preferably 40 parts by mass or less.

[0062] The polymerizable monomer may contain a non-crosslinkable monomer within a range that does not impair the effects of the present disclosure. Examples of the non-crosslinkable monomer include non-crosslinkable acrylic monomers such as (meth)acrylic acid alkyl esters such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and lauryl (meth)acrylate, glycidyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, (meth)acrylic acid, (meth)acrylamide, N-methylol (meth)acrylamide, and N-butoxymethyl (meth)acrylamide; styrene, vinyl toluene, α- Examples of suitable non-crosslinkable hydrocarbon monomers include aromatic vinyl monomers such as methylstyrene, p-methylstyrene, ethylvinylbenzene, ethylvinylbiphenyl, and ethylvinylnaphthalene; monoolefin monomers such as ethylene, propylene, and butylene; and diene monomers such as butadiene and isoprene; vinyl carboxylate ester monomers such as vinyl acetate; halogenated aromatic vinyl monomers such as halogenated styrene; halogenated vinyl monomers such as vinyl chloride; halogenated vinylidene monomers such as vinylidene chloride; and vinylpyridine monomers. These non-crosslinkable monomers can be used alone or in combination of two or more. Among the non-crosslinkable monomers, (meth)acrylic acid alkyl esters and aromatic vinyl monomers are preferred, with aromatic vinyl monomers being more preferred, in terms of facilitating a stable polymerization reaction and preventing a decrease in the pressure resistance of the hollow particles. Among the (meth)acrylic acid alkyl esters, butyl acrylate and methyl methacrylate are preferred. Among the aromatic vinyl monomers, ethylvinylbenzene is preferred.

[0063] In order to improve the stability of the polymerization reaction, the content of the acrylic monomer is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, per 100 parts by mass of the polymerizable monomer.

[0064] Furthermore, from the viewpoint of improving the stability of the polymerization reaction and the pressure resistance of the hollow particles, the content of the acrylic monomer and the hydrocarbon monomer per 100 parts by mass of the polymerizable monomer is preferably 80 parts by mass or more, more preferably 90 parts by mass or more, even more preferably 98 parts by mass or more, and still more preferably 99 parts by mass or more. When the polymerizable monomer contains an acrylic monomer and a hydrocarbon monomer, from the viewpoint of improving the pressure resistance of the hollow particles, the content of the hydrocarbon monomer per 100 parts by mass of the total of the acrylic monomer and the hydrocarbon monomer is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, and even more preferably 30 parts by mass or more, and the upper limit is preferably 90 parts by mass or less, more preferably 80 parts by mass or less.

[0065] The content of the polymerizable monomer in the mixed liquid is not particularly limited, but from the viewpoint of the balance between the porosity, particle size, and mechanical strength of the hollow particles, the lower limit is preferably 30% by mass or more, more preferably 40% by mass or more, and the upper limit is preferably 60% by mass or less, more preferably 50% by mass or less, based on 100% by mass of the total mass of the components in the mixed liquid excluding the aqueous medium. Furthermore, from the viewpoint of the mechanical strength of the hollow particles, the content of the polymerizable monomer in the mixed liquid based on 100% by mass of the total mass of the solids excluding the hydrophobic solvent, among the materials that form the oil phase, is preferably 95% by mass or more, more preferably 97% by mass or more. In the present disclosure, the solids refer to all components excluding the solvent, and liquid polymerizable monomers and the like are considered to be included in the solids.

[0066] (B) Hydrophobic Solvent The hydrophobic solvent used in the manufacturing method of the present disclosure is a non-polymerizable, poorly water-soluble organic solvent. The hydrophobic solvent acts as a spacer material that forms hollow spaces inside the particles. In the suspension process described below, a suspension is obtained in which droplets of a monomer composition containing a hydrophobic solvent are dispersed in an aqueous medium. In the suspension process, phase separation occurs within the droplets of the monomer composition, and the hydrophobic solvent, which has low polarity, tends to collect inside the droplets of the monomer composition. Ultimately, the droplets of the monomer composition contain the hydrophobic solvent inside, and other materials other than the hydrophobic solvent are distributed around the periphery according to their respective polarities. Then, in the polymerization process described below, an aqueous dispersion containing precursor particles encapsulating the hydrophobic solvent is obtained. In other words, as the hydrophobic solvent collects inside the particles, hollow spaces filled with the hydrophobic solvent are formed inside the resulting precursor particles.

[0067] The hydrophobic solvent can be appropriately selected from known hydrophobic solvents and is not particularly limited. Examples include esters such as ethyl acetate and butyl acetate; ether esters such as propylene glycol monomethyl ether acetate and propylene glycol monoethyl ether acetate; and hydrocarbon solvents, of which hydrocarbon solvents are preferred. Examples of hydrocarbon solvents include aliphatic hydrocarbons including chain hydrocarbon solvents such as pentane, hexane, heptane, octane, 2-methylbutane, and 2-methylpentane, and cyclic hydrocarbon solvents such as cyclohexane, methylcyclohexane, and cycloheptane; and aromatic hydrocarbons such as benzene, toluene, and xylene. These hydrophobic solvents can be used alone or in combination of two or more.

[0068] In the suspension step, because phase separation between the polymerizable monomer and the hydrophobic solvent is likely to occur within the droplets of the monomer composition, it is preferable to select an organic solvent that has a lower solubility in water than the crosslinkable monomer contained in the polymerizable monomer as the hydrophobic solvent. Furthermore, when the polymerizable monomer contains an acrylic monomer and a hydrocarbon monomer, it is preferable to use a chain hydrocarbon solvent as the hydrophobic solvent. Among the chain hydrocarbon solvents, chain hydrocarbon solvents having 5 to 8 carbon atoms are preferred, and at least one selected from the group consisting of pentane, hexane, heptane, and octane is more preferred. On the other hand, when the polymerizable monomer contains an acrylic monomer but no hydrocarbon monomer, it is preferable to use a hydrocarbon solvent having 4 to 7 carbon atoms as the hydrophobic solvent, and it is more preferable to use a hydrocarbon solvent having 5 to 7 carbon atoms. Here, the hydrocarbon solvent may be either an aromatic hydrocarbon or an aliphatic hydrocarbon, but among these, an aliphatic hydrocarbon is preferred, a cyclic hydrocarbon solvent is more preferred, and at least one selected from the group consisting of cyclohexane, cycloheptane, and methylcyclohexane is even more preferred. The use of a combination of the above-mentioned polymerizable monomer and hydrophobic solvent is preferred because it is easy to improve the pressure resistance of the hollow particles. In particular, the use of a combination of a polymerizable monomer containing an acrylic monomer and a hydrocarbon monomer with the above-mentioned preferred hydrophobic solvent is preferred because it improves the uniformity of the shell and thereby improves the pressure resistance of the hollow particles.

[0069] Furthermore, although not particularly limited, the boiling point of the hydrophobic solvent is preferably 130° C. or lower, more preferably 100° C. or lower, from the viewpoint of ease of removal in the solvent removal step described below, and is preferably 50° C. or higher, more preferably 60° C. or higher, from the viewpoint of ease of inclusion in the precursor particles. When the hydrophobic solvent is a mixed solvent containing multiple types of hydrophobic solvents and has multiple boiling points, it is preferable that the boiling point of the solvent with the highest boiling point among the solvents contained in the mixed solvent is not higher than the above-mentioned upper limit, and it is preferable that the boiling point of the solvent with the lowest boiling point among the solvents contained in the mixed solvent is not lower than the above-mentioned lower limit.

[0070] Furthermore, the hydrophobic solvent used in the production method of the present disclosure preferably has a dielectric constant of 2.0 or less at 20°C. The dielectric constant is one of the indicators of the polarity of a compound. When the dielectric constant of the hydrophobic solvent is sufficiently small, such as 2.0 or less, phase separation proceeds rapidly in the droplets of the monomer composition, and hollow spaces are likely to be formed. Examples of hydrophobic solvents having a dielectric constant of 2.0 or less at 20°C are as follows. The values ​​in parentheses indicate the dielectric constant: pentane (1.8), hexane (1.9), heptane (1.9), octane (1.9), and cyclohexane (2.0). Regarding the dielectric constant at 20°C, reference can be made to values ​​described in known literature (e.g., "Chemical Handbook: Basics," 4th Revised Edition, edited by the Chemical Society of Japan, Maruzen Co., Ltd., published September 30, 1993, pages II-498 to II-503) and other technical information. The method for measuring the relative dielectric constant at 20°C includes, for example, a relative dielectric constant test carried out in accordance with JIS C 2101:1999, 23, at a measurement temperature of 20°C.

[0071] The porosity of the hollow particles can be adjusted by changing the amount of hydrophobic solvent in the mixed solution. In the polymerization step described below, the polymerization reaction proceeds with the hydrophobic solvent encapsulated in oil droplets containing the polymerizable monomer, etc., and therefore the higher the hydrophobic solvent content, the higher the porosity of the resulting hollow particles tends to be. In the present disclosure, the content of the hydrophobic solvent in the mixed solution is preferably 50 to 500 parts by mass per 100 parts by mass of the polymerizable monomer, as this facilitates control of the particle size of the hollow particles, increases the porosity while maintaining the strength of the hollow particles, and reduces the amount of residual hydrophobic solvent within the particles. The content of the hydrophobic solvent in the mixed solution is more preferably 70 to 300 parts by mass, and even more preferably 90 to 200 parts by mass per 100 parts by mass of the polymerizable monomer.

[0072] (C) Polymerization Initiator In the production method of the present disclosure, the mixed liquid preferably contains an oil-soluble polymerization initiator as a polymerization initiator. Methods for polymerizing droplets of the monomer composition after suspending the mixed liquid include emulsion polymerization using a water-soluble polymerization initiator and suspension polymerization using an oil-soluble polymerization initiator. Suspension polymerization can be carried out using an oil-soluble polymerization initiator. The oil-soluble polymerization initiator is not particularly limited as long as it is lipophilic and has a solubility in water of 0.2% by mass or less. Examples of the oil-soluble polymerization initiator include organic peroxides such as benzoyl peroxide, lauroyl peroxide, t-butyl peroxide-2-ethylhexanoate, t-butyl peroxydiethyl acetate, and t-butyl peroxypivalate; and azo compounds such as 2,2'-azobis(2,4-dimethylvaleronitrile), azobisisobutyronitrile, and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile).

[0073] The content of the polymerization initiator relative to 100 parts by mass of the polymerizable monomer in the mixed solution is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 7 parts by mass, and even more preferably 1 to 5 parts by mass. When the content of the polymerization initiator is equal to or greater than the above-mentioned lower limit, the polymerization reaction can proceed sufficiently, while when the content is equal to or less than the above-mentioned upper limit, there is little risk of the oil-soluble polymerization initiator remaining after completion of the polymerization reaction, and there is also little risk of an unexpected side reaction proceeding.

[0074] (D) Dispersion Stabilizer The dispersion stabilizer is an agent that disperses droplets of the monomer composition in an aqueous medium during the suspension process. In the present disclosure, an inorganic dispersion stabilizer is preferably used as the dispersion stabilizer because it facilitates control of the particle size of the droplets in the suspension, narrows the particle size distribution of the resulting hollow particles, and prevents the shell from becoming too thin, thereby suppressing a decrease in the strength of the hollow particles. Examples of inorganic dispersion stabilizers include inorganic compounds such as sulfates such as barium sulfate and calcium sulfate; carbonates such as barium carbonate, calcium carbonate, and magnesium carbonate; phosphates such as calcium phosphate; metal oxides such as aluminum oxide and titanium oxide; and metal hydroxides such as aluminum hydroxide, magnesium hydroxide, calcium hydroxide, barium hydroxide, and ferric hydroxide. These inorganic dispersion stabilizers can be used alone or in combination of two or more. Among the inorganic dispersion stabilizers, the above-mentioned poorly water-soluble metal salts such as sulfates, carbonates, phosphates, and metal hydroxides are preferred, metal hydroxides are more preferred, and magnesium hydroxide is particularly preferred. In the present disclosure, poor water solubility preferably means a solubility of 0.5 g or less in 100 g of water.

[0075] In the present disclosure, it is particularly preferable to use a poorly water-soluble inorganic dispersion stabilizer dispersed in an aqueous medium in the form of colloidal particles, i.e., in the form of a colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles. This not only narrows the particle size distribution of the droplets of the monomer composition, but also makes it easy to reduce the amount of inorganic dispersion stabilizer remaining in the resulting hollow particles by washing. A colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles can be prepared, for example, by reacting at least one selected from alkali metal hydroxides and alkaline earth metal hydroxides with a water-soluble polyvalent metal salt (excluding alkaline earth metal hydroxides) in an aqueous medium. Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Examples of alkaline earth metal hydroxides include barium hydroxide and calcium hydroxide. The water-soluble polyvalent metal salt may be any water-soluble polyvalent metal salt other than the above-mentioned alkaline earth metal hydroxides. Examples include magnesium metal salts such as magnesium chloride, magnesium phosphate, and magnesium sulfate; calcium metal salts such as calcium chloride, calcium nitrate, calcium acetate, and calcium sulfate; aluminum metal salts such as aluminum chloride and aluminum sulfate; barium salts such as barium chloride, barium nitrate, and barium acetate; and zinc salts such as zinc chloride, zinc nitrate, and zinc acetate. Among these, magnesium metal salts, calcium metal salts, and aluminum metal salts are preferred, magnesium metal salts are more preferred, and magnesium chloride is particularly preferred. The water-soluble polyvalent metal salts can be used alone or in combination of two or more. The method for reacting at least one selected from the above-mentioned alkali metal hydroxides and alkaline earth metal hydroxides with the above-mentioned water-soluble polyvalent metal salt in an aqueous medium is not particularly limited, and examples include a method of mixing an aqueous solution of at least one selected from the alkali metal hydroxides and alkaline earth metal hydroxides with an aqueous solution of the water-soluble polyvalent metal salt.

[0076] The content of the dispersion stabilizer is not particularly limited, but is preferably 0.5 to 15 parts by mass, more preferably 1 to 10 parts by mass, per 100 parts by mass of the total mass of the polymerizable monomer and the hydrophobic solvent. By having the content of the dispersion stabilizer at or above the lower limit, droplets of the monomer composition can be sufficiently dispersed so as not to coalesce in the suspension. On the other hand, by having the content of the dispersion stabilizer at or below the upper limit, an increase in the viscosity of the suspension during granulation can be prevented, and the problem of the suspension clogging in the granulator can be avoided. Furthermore, the content of the dispersion stabilizer is preferably 0.5 to 15 parts by mass, more preferably 0.5 to 10 parts by mass, per 100 parts by mass of the aqueous medium.

[0077] (E) Aqueous Medium In the present disclosure, the term "aqueous medium" refers to a medium selected from the group consisting of water, a hydrophilic solvent, and a mixture of water and a hydrophilic solvent. When using a mixture of water and a hydrophilic solvent, it is important that the polarity of the entire mixture is not too low in order to form droplets of the monomer composition. In this case, for example, the mass ratio of water to hydrophilic solvent (water:hydrophilic solvent) may be 99:1 to 50:50. The hydrophilic solvent in the present disclosure is not particularly limited as long as it is sufficiently miscible with water and does not cause phase separation. Examples of hydrophilic solvents include alcohols such as methanol and ethanol; tetrahydrofuran (THF); dimethyl sulfoxide (DMSO); and the like.

[0078] The content of the aqueous medium is not particularly limited, but from the viewpoint of setting the particle size and porosity of the hollow particles within the preferred ranges described below, the lower limit is preferably 200 parts by mass or more, more preferably 400 parts by mass or more, and even more preferably 600 parts by mass or more, relative to 100 parts by mass of the polymerizable monomer contained in the mixed liquid, and the upper limit is preferably 1,000 parts by mass or less, and more preferably 800 parts by mass or less.

[0079] The mixed liquid may further contain other materials different from the above-described materials (A) to (E) as long as the effects of the present disclosure are not impaired.

[0080] A mixed liquid is obtained by mixing the above-mentioned materials and other materials as needed, and stirring appropriately. In this mixed liquid, an oil phase containing lipophilic materials such as (A) the polymerizable monomer, (B) the hydrophobic solvent, and (C) the polymerization initiator is dispersed in an aqueous phase containing (D) the dispersion stabilizer and (E) the aqueous medium, with particles of about several mm in size. The dispersion state of these materials in the mixed liquid can be observed with the naked eye, depending on the type of material. In the mixed liquid preparation step, the mixed liquid may be obtained by simply mixing the above-mentioned materials and other materials as needed, and stirring appropriately. However, in terms of making the shell more uniform, it is preferable to prepare a mixed liquid by separately preparing an oil phase containing the polymerizable monomer, the hydrophobic solvent, and the polymerization initiator, and an aqueous phase containing the dispersion stabilizer and the aqueous medium, and then mixing them. In the present disclosure, a colloidal dispersion in which a poorly water-soluble inorganic dispersion stabilizer is dispersed in the form of colloidal particles in an aqueous medium is preferably used as the aqueous phase. By preparing the oil phase and the water phase separately in advance and then mixing them, hollow particles having a uniform shell composition can be produced, and the particle size of the hollow particles can be easily controlled.

[0081] (2) Suspension Step The suspension step is a step of preparing a suspension in which droplets of a monomer composition containing a hydrophobic solvent are dispersed in an aqueous medium by suspending the mixed liquid described above. The suspension method for forming droplets of the monomer composition is not particularly limited, and any known suspension method can be used. Examples of dispersers used in preparing the suspension include horizontal or vertical in-line dispersers such as Milder (product name) manufactured by Pacific Machinery Works, Ltd., Cavitron (product name) manufactured by Eurotec Co., Ltd., and in-line dispersers manufactured by IKA (e.g., DISPAX-REACTOR (registered trademark) DRS (product name)); and emulsifying dispersers such as the Homomixer MARK II series manufactured by Primix Corporation.

[0082] In the suspension prepared in the suspending step, droplets of the monomer composition containing the lipophilic material and having a particle size of approximately 5 to 40 μm are uniformly dispersed in the aqueous medium. Such droplets of the monomer composition are difficult to observe with the naked eye and can be observed using known observation equipment such as an optical microscope. During the suspending step, phase separation occurs in the droplets of the monomer composition, which makes it easier for the hydrophobic solvent, which has low polarity, to collect inside the droplets. As a result, the resulting droplets contain the hydrophobic solvent in their interiors and materials other than the hydrophobic solvent distributed around their peripheries.

[0083] FIG. 2 is a schematic diagram illustrating one embodiment of a suspension in the suspension step. The droplets 8 of the monomer composition in FIG. 2 are shown as a cross-section. Note that FIG. 2 is merely a schematic diagram, and the suspension in the present disclosure is not necessarily limited to that shown in FIG. 2. A portion of FIG. 2 corresponds to (2) in FIG. 1 described above. FIG. 2 illustrates droplets 8 of the monomer composition and polymerizable monomers 4c dispersed in the aqueous medium 1, dispersed in the aqueous medium 1. The droplets 8 are formed by surrounding an oil-soluble monomer composition 4 with a dispersion stabilizer 3. The monomer composition contains an oil-soluble polymerization initiator 5, as well as a polymerizable monomer and a hydrophobic solvent (none of which are shown). The droplets 8 are small oil droplets containing the monomer composition 4, and the oil-soluble polymerization initiator 5 generates polymerization-initiating radicals within the small oil droplets. Therefore, precursor particles of the desired particle size can be produced without excessive oil droplet growth. In the suspension polymerization method using such an oil-soluble polymerization initiator, there is no opportunity for the polymerization initiator to come into contact with the polymerizable monomer 4c dispersed in the aqueous medium 1. Therefore, by using an oil-soluble polymerization initiator, it is possible to prevent the by-production of excess resin particles such as dense solid particles having a relatively small particle size in addition to the desired resin particles having hollow portions.

[0084] (3) Polymerization Step This step is a step of preparing a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a resin and encapsulating a hydrophobic solvent in the hollow portion by subjecting the suspension obtained in the above-mentioned suspension step to a polymerization reaction. The precursor particles are formed by polymerization of a polymerizable monomer contained in droplets of the monomer composition, and the shell of the precursor particles contains a polymer of the polymerizable monomer as a resin.

[0085] The polymerization method is not particularly limited, and for example, a batch system, a semi-continuous system, a continuous system, etc. can be employed. The polymerization temperature is preferably 40 to 90°C, more preferably 50 to 80°C. The polymerization reaction time is preferably 1 to 48 hours, more preferably 4 to 36 hours. In the production method of the present disclosure, in the polymerization step, further polymerizable monomers may be added during the polymerization reaction of the polymerizable monomers in the suspension. By performing the polymerization reaction in two stages in this manner, the pressure resistance of the hollow particles may be improved. In the polymerization step, the shell portion of the droplets of the monomer composition containing the hydrophobic solvent therein is polymerized, and as described above, hollow portions filled with the hydrophobic solvent are formed inside the resulting precursor particles.

[0086] (4) Solid-Liquid Separation Step This step is a step of obtaining a solid content containing precursor particles by solid-liquid separation of the precursor composition containing precursor particles obtained by the above-mentioned polymerization step.

[0087] The method for solid-liquid separation of the precursor composition is not particularly limited, and known methods can be used. Examples of solid-liquid separation methods include centrifugation, filtration, and static separation. Among these, centrifugation or filtration can be used, and centrifugation may be used from the viewpoint of ease of operation. After the solid-liquid separation step, an optional step such as a pre-drying step may be performed before performing the solvent removal step described below. Examples of the pre-drying step include a step of pre-drying the solid content obtained after the solid-liquid separation step using a drying device such as a dryer or a drying appliance such as a hand dryer.

[0088] (5) Solvent Removal Step This step is a step of removing the hydrophobic solvent contained in the precursor particles obtained by the solid-liquid separation step. By removing the hydrophobic solvent contained in the precursor particles in air, the hydrophobic solvent inside the precursor particles is replaced with air, and hollow particles filled with gas are obtained.

[0089] In this process, "in the air" strictly refers to an environment in which no liquid is present outside the precursor particles, or an environment in which only a trace amount of liquid is present outside the precursor particles, so that the removal of the hydrophobic solvent is not affected. "In the air" can also be referred to as a state in which the precursor particles are not present in a slurry, or a state in which the precursor particles are present in a dry powder. In other words, in this process, it is important to remove the hydrophobic solvent in an environment in which the precursor particles are in direct contact with the external gas.

[0090] The method for removing the hydrophobic solvent from the precursor particles in air is not particularly limited, and known methods can be used. Examples of such methods include vacuum drying, heat drying, flash drying, or a combination of these methods. In particular, when heat drying is used, the heating temperature must be equal to or higher than the boiling point of the hydrophobic solvent and equal to or lower than the maximum temperature at which the shell structure of the precursor particles does not collapse. Depending on the shell composition and the type of hydrophobic solvent in the precursor particles, the heating temperature may be, for example, 50 to 200°C, 70 to 200°C, or 100 to 200°C. The drying operation in air replaces the hydrophobic solvent inside the precursor particles with the external gas, resulting in hollow particles whose hollow portions are filled with gas.

[0091] The drying atmosphere is not particularly limited and can be appropriately selected depending on the application of the hollow particles. Examples of the drying atmosphere include air, oxygen, nitrogen, argon, etc. Hollow particles with a temporary vacuum inside can also be obtained by filling the inside of the hollow particles with a gas and then drying under reduced pressure.

[0092] Alternatively, the hydrophobic solvent encapsulated in the precursor particles may be removed from the slurry containing the precursor particles and an aqueous medium without solid-liquid separation of the slurry-like precursor composition obtained in the polymerization step. In this method, the hydrophobic solvent encapsulated in the precursor particles can be removed by bubbling an inert gas through the precursor composition at a temperature equal to or higher than the boiling point of the hydrophobic solvent minus 35°C. When the hydrophobic solvent is a mixed solvent containing multiple hydrophobic solvents and has multiple boiling points, the boiling point of the hydrophobic solvent in the solvent removal step refers to the boiling point of the solvent with the highest boiling point among the solvents contained in the mixed solvent, i.e., the highest boiling point among the multiple boiling points. The temperature at which the inert gas is bubbled through the precursor composition is preferably equal to or higher than the boiling point of the hydrophobic solvent minus 30°C, more preferably equal to or higher than the boiling point of the hydrophobic solvent, in order to reduce the amount of hydrophobic solvent remaining in the hollow particles. The bubbling temperature is typically equal to or higher than the polymerization temperature in the polymerization step. Although not particularly limited, the temperature during bubbling may be 50°C or higher and 100°C or lower. The inert gas used for bubbling is not particularly limited, but examples thereof include nitrogen and argon. The bubbling conditions are appropriately adjusted depending on the type and amount of hydrophobic solvent so as to remove the hydrophobic solvent contained in the precursor particles. Although not particularly limited, for example, the inert gas may be bubbled at a rate of 1 to 3 L / min for 1 to 10 hours. This method produces a slurry of hollow particles containing an aqueous medium. The hollow particles obtained by solid-liquid separation of this slurry are dried, and the aqueous medium contained in the hollow particles is removed, thereby producing hollow particles whose hollow portions are filled with gas.

[0093] Comparing a method of obtaining hollow particles having hollow spaces filled with gas by performing solid-liquid separation on a slurry-like precursor composition and then removing the hydrophobic solvent from the precursor particles in an atmosphere of air, with a method of obtaining hollow particles having hollow spaces filled with gas by removing the hydrophobic solvent encapsulated in precursor particles in a slurry containing the precursor particles and an aqueous medium, performing solid-liquid separation, and then removing the aqueous medium from the hollow particles in an atmosphere of air, the former method has the advantage that the hollow particles are less likely to be crushed in the step of removing the hydrophobic solvent, while the latter method has the advantage that the amount of residual hydrophobic solvent is reduced by performing bubbling with an inert gas. Alternatively, as a method for removing the hydrophobic organic solvent contained in the precursor particles after the polymerization step and before the solid-liquid separation step without performing solid-liquid separation on the slurry precursor composition obtained in the polymerization step, for example, a method for evaporating and distilling off the hydrophobic organic solvent contained in the precursor particles from the precursor composition under a predetermined pressure (high pressure, normal pressure, or reduced pressure); or a method for introducing an inert gas such as nitrogen, argon, or helium, or water vapor, into the precursor composition under a predetermined pressure (high pressure, normal pressure, or reduced pressure), and then evaporating and distilling off the hydrophobic organic solvent may be used.

[0094] (6) Others Steps other than the above steps (1) to (5) may include, for example, the following (6-a) washing step and the following (6-b) hollow portion re-substitution step. (6-a) Washing Step The washing step is a step of adding an acid or alkali to wash the precursor composition containing the precursor particles before the solvent removal step in order to remove any dispersion stabilizer remaining in the precursor composition. When the dispersion stabilizer used is an acid-soluble inorganic dispersion stabilizer, it is preferable to add an acid to the precursor composition containing the precursor particles to perform washing. On the other hand, when the dispersion stabilizer used is an alkali-soluble inorganic compound, it is preferable to add an alkali to the precursor composition containing the precursor particles to perform washing. Furthermore, when an acid-soluble inorganic dispersion stabilizer is used as the dispersion stabilizer, it is preferable to add an acid to the precursor composition containing the precursor particles to adjust the pH to preferably 6.5 or less, more preferably 6 or less. The acid to be added may be an inorganic acid such as sulfuric acid, hydrochloric acid, or nitric acid, or an organic acid such as formic acid or acetic acid. However, sulfuric acid is particularly preferred because it has a high efficiency in removing the dispersion stabilizer and places a small burden on the production equipment.

[0095] (6-b) Hollow Portion Re-Substitution Process The hollow portion re-substitution process is a process of substituting the gas or liquid inside the hollow particles with another gas or liquid. This substitution can change the environment inside the hollow particles, selectively confine molecules inside the hollow particles, or modify the chemical structure inside the hollow particles to suit the application.

[0096] The hollow particles used in the present disclosure contain a polymer of the above-described polymerizable monomer as the main component of the shell, and the polymer forms the skeleton of the shell of the hollow particle. In the hollow particles of the present disclosure, the polymer contained in the shell contains a crosslinkable monomer unit in order to improve pressure resistance. The content of the crosslinkable monomer unit is 50 parts by mass or more, preferably 60 parts by mass or more, more preferably 70 parts by mass or more, and even more preferably 80 parts by mass or more, based on 100 parts by mass of all monomer units of the polymer. On the other hand, the polymer may contain a non-crosslinkable monomer unit, provided that the effects of the present disclosure are not impaired. In this case, the content of the crosslinkable monomer unit may be, for example, 95 parts by mass or less, or 90 parts by mass or less, based on 100 parts by mass of all monomer units of the polymer.

[0097] In order to improve pressure resistance, the hollow particles used in the present disclosure preferably contain a trifunctional or higher crosslinkable monomer unit in the shell of the polymer. The content of the trifunctional or higher crosslinkable monomer unit in 100 parts by mass of all monomer units of the polymer is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, as a lower limit, and preferably 50 parts by mass or less, more preferably 40 parts by mass or less, and even more preferably 30 parts by mass or less, as an upper limit.

[0098] In order to improve pressure resistance, the hollow particles used in the present disclosure preferably contain a polymer contained in the shell that contains a bifunctional crosslinkable monomer unit and a trifunctional or higher functional crosslinkable monomer unit. When the polymer contains a bifunctional crosslinkable monomer unit and a trifunctional or higher functional crosslinkable monomer unit, the content of the trifunctional or higher functional crosslinkable monomer unit is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 20 parts by mass or more, per 100 parts by mass of the bifunctional crosslinkable monomer unit and the trifunctional or higher functional crosslinkable monomer unit combined, and is preferably 50 parts by mass or less, more preferably 40 parts by mass or less. In the present disclosure, a crosslinkable monomer unit derived from a bifunctional crosslinkable monomer may be referred to as a "bifunctional crosslinkable monomer unit," and a crosslinkable monomer unit derived from a trifunctional or higher functional crosslinkable monomer may be referred to as a "trifunctional or higher functional crosslinkable monomer unit."

[0099] In addition, in the hollow particles used in the present disclosure, the polymer contained in the shell preferably contains an acrylic monomer unit, since this facilitates improving the uniformity of the shell. The content of the acrylic monomer unit is not particularly limited, but is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, relative to 100% by mass of all monomer units.

[0100] Furthermore, in the hollow particles used in the present disclosure, the polymer contained in the shell preferably contains acrylic monomer units and hydrocarbon monomer units, from the viewpoint of easily improving the uniformity of the shell and improving pressure resistance. The total content of the acrylic monomer units and hydrocarbon monomer units in 100 parts by mass of all monomer units in the polymer is preferably 80 parts by mass or more, more preferably 90 parts by mass or more, even more preferably 98 parts by mass or more, and even more preferably 99 parts by mass or more. When the polymer contained in the shell contains acrylic monomer units and hydrocarbon monomer units, from the viewpoint of improving the pressure resistance of the hollow particles, the content of the hydrocarbon monomer units in 100 parts by mass of the total of the acrylic monomer units and the hydrocarbon monomer units is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, and even more preferably 30 parts by mass or more, as a lower limit, and preferably 90 parts by mass or less, more preferably 80 parts by mass or less, as an upper limit.

[0101] In the hollow particles used in the present disclosure, the content of the polymer synthesized from the polymerizable monomer is preferably 96% by mass or more, more preferably 97% by mass or more, based on 100% by mass of the total solids content of the shell. By setting the content of the polymer at or above the lower limit, a decrease in the pressure resistance of the hollow particles can be suppressed. That is, to suppress a decrease in the pressure resistance of the hollow particles, the content of components other than the polymer is preferably 4% by mass or less, more preferably 3% by mass or less, based on 100% by mass of the total solids content of the shell. Examples of components other than the polymer contained in the hollow particles used in the present disclosure include unreacted polymerizable monomers, polymers other than the polymers of the polymerizable monomers, decomposition products of polymerization initiators, and low-molecular-weight compounds contained as impurities in the raw materials for the polymerizable monomers. Low-boiling components (e.g., boiling points of 200°C or less) are typically removed during the production process of hollow particles, while high-boiling components (e.g., boiling points of 250°C or more) may remain unreacted.

[0102] [Plasticizer] A plasticizer may be added to the hollow particle-containing elastomer composition of the present disclosure as needed to adjust the storage modulus G' of the hollow particle-containing elastomer composition measured under predetermined conditions to a certain value or less. The plasticizer may be one generally used as a plasticizer in applications such as automotive materials, general plastics, and rubber products, or one that imparts flexibility. Examples of suitable plasticizers include process oil, lubricating oil, paraffin, liquid paraffin, petroleum asphalt, and petrolatum; coal tar softeners such as coal tar and coal tar pitch; fatty oil softeners such as castor oil, linseed oil, rapeseed oil, and coconut oil; tall oil; waxes such as beeswax, carnauba wax, and lanolin; fatty acids and fatty acid salts such as ricinoleic acid, palmitic acid, barium stearate, calcium stearate, and zinc laurate; synthetic polymers such as petroleum resin, atactic polypropylene, and coumarone-indene resin; ester-based plasticizers such as dioctyl phthalate, dioctyl adipate, and dioctyl sebacate; carbonate ester-based plasticizers such as diisododecyl carbonate; and microcrystalline wax, sub(factice), liquid polybutadiene, modified liquid polybutadiene, liquid thiokol, and hydrocarbon synthetic lubricating oils. These plasticizers can be used alone or in combination.

[0103] To prevent bleeding of the plasticizer from molded articles produced using the hollow particle-containing elastomer composition of the present disclosure, the composition preferably contains a plasticizer selected from the group consisting of polymers having reactive sites that bond with the base elastomer and having a weight-average molecular weight of 1,000 to 100,000 (hereinafter sometimes referred to as a "reactive site-containing plasticizer"). The reactive site-containing plasticizer not only functions as a plasticizer during melt-kneading of the elastomer composition and its raw material mixture, but also bonds with the base elastomer molecules during melt-kneading of the elastomer composition and its raw material mixture or during production of a molded article using the elastomer composition. The reactive site-containing plasticizer itself integrates with the matrix of the elastomer composition and its raw material mixture, thereby preventing bleeding of the plasticizer. Bleeding of the plasticizer is particularly likely when a large amount of plasticizer is used. Therefore, when it is desired to add a large amount of plasticizer in order to sufficiently lower the storage modulus G' of the elastomer composition and its raw material mixture during melt-kneading, it is preferable to use the above-mentioned reactive site-containing plasticizer.

[0104] The polymer backbone, which forms the main part of the molecular structure of the reactive site-containing plasticizer, may have any chemical structure as long as it has adequate compatibility, softening or fluidity, and can function as a plasticizer when melt-kneaded with the elastomer composition and its raw material mixture. Examples include backbones having hydrocarbon polymer structures that may contain heteroatoms such as oxygen, nitrogen, and silicon in the main chain or side chain. The reactive site that bonds with the base elastomer refers to a chemical structure that has the function of forming a chemical, physical, or physicochemical bond with the reactive site present on the base elastomer. When the reactive site-containing plasticizer has two or more reactive sites that bond with the base elastomer in one molecule, the reactive site-containing plasticizer forms a crosslinked structure between two base elastomer molecules via the plasticizer, resulting in a high degree of integration between the plasticizer and the matrix, which is highly effective in preventing plasticizer bleeding. When a plasticizer containing reactive sites has two or more reactive sites per molecule and these reactive sites can also bond to hollow particles, the plasticizer forms crosslinked structures not only between two base elastomer molecules but also between the base elastomer molecule and the hollow particle, and between two hollow particles, thereby further preventing plasticizer bleeding. If the number of reactive sites per molecule of the plasticizer containing reactive sites is too large, the matrix in the elastomer composition and its raw material mixture will become excessively networked, resulting in poor melt flowability. Therefore, from the viewpoint of maintaining the storage modulus G' of the hollow particle-containing elastomer composition and its raw material mixture measured under specified conditions below a certain value, the number of reactive sites per molecule of the plasticizer containing reactive sites is preferably 2 to 10,000.

[0105] From the viewpoint of imparting sufficient plasticity, the reactive site-containing plasticizer is preferably liquid at at least one point within the range of room temperature (20° C.±15° C.), more preferably liquid at at least one point within the range of 10° C. to 30° C., and even more preferably liquid at at least one point within the range of 20° C. to 25° C. From the same viewpoint of imparting sufficient plasticity, the reactive site-containing plasticizer preferably has a glass transition temperature of −10° C. or lower, more preferably −120° C. to −20° C.

[0106] When the base elastomer has an ethylenic double bond, such as butadiene rubber or styrene-butadiene rubber, the ethylenic double bond on the base elastomer can serve as a reactive site, and the ethylenic double bond can be used as a reactive site for bonding to the base elastomer. Furthermore, when the shell of the hollow particle is synthesized from a monomer or crosslinkable monomer having an ethylenic double bond and unreacted ethylenic double bonds remain on the shell, the ethylenic double bond on the shell can serve as a reactive site, and the ethylenic double bond can be used as a reactive site for bonding to the hollow particle. Therefore, when an elastomer selected from the group consisting of butadiene rubber and styrene-butadiene rubber is used as the base elastomer, a polymer having an ethylenic double bond can be used as the reactive site-containing plasticizer.

[0107] As the reactive site-containing plasticizer selected from the group consisting of polymers having an ethylenic double bond and a weight average molecular weight of 1,000 or more and 100,000 or less, a diene rubber that is liquid at room temperature is preferably used. Examples of diene rubbers that are liquid at room temperature include unmodified liquid polybutadiene rubber; modified liquid polybutadiene rubber such as acrylate-modified liquid polybutadiene; liquid styrene-butadiene rubber; unmodified liquid polyisoprene rubber; and modified liquid polyisoprene rubber such as hydroxyl group-terminated liquid polyisoprene rubber. Of these, unmodified liquid polybutadiene rubber and modified liquid polybutadiene rubber are preferred.

[0108] Furthermore, polyolefins having terminal double bonds are also preferably used as the reactive site-containing plasticizer selected from the group consisting of polymers having ethylenic double bonds and a weight average molecular weight of 1,000 to 100,000. Examples of polyolefins having terminal double bonds include polypropylenes having terminal double bonds.

[0109] The content of the plasticizer in the elastomer composition is not particularly limited, but is typically 35 to 100 parts by mass, preferably 45 to 90 parts by mass, per 100 parts by mass of the base elastomer. When the above-mentioned reactive site-containing plasticizer is used, as long as the content is within an appropriate range, the effect of imparting plasticity to the composition becomes dominant over the increase in elasticity due to the formation of linking structures, thereby providing sufficient plasticizing functionality while preventing bleeding. However, if the content of the reactive site-containing plasticizer is too low, the increase in elasticity of the composition due to the formation of linking structures becomes dominant, thereby inhibiting the plasticizing effect. Furthermore, if the content of the reactive site-containing plasticizer is excessive, the plasticizing effect becomes greater, but the formation of linking structures becomes saturated, making the free plasticizer more likely to bleed. For the above reasons, when a plasticizer having reactive sites is used alone, the content of the plasticizer having reactive sites is preferably 65 to 90 parts by mass, more preferably 70 to 85 parts by mass, and even more preferably 70 to 80 parts by mass, relative to 100 parts by mass of the base elastomer. When a plasticizer having no reactive sites and a plasticizer having reactive sites are used in combination, the proportion of the plasticizer having reactive sites relative to the total amount of plasticizers is preferably 5 to 35% by mass, and even more preferably 10 to 30% by mass.

[0110] [Vulcanizing / Crosslinking Agent] A vulcanizing / crosslinking agent may be added to the hollow particle-containing elastomer composition of the present disclosure, as needed, to crosslink the base elastomer. Examples of vulcanizing / crosslinking agents include sulfur, such as powdered sulfur, precipitated sulfur, colloidal sulfur, and insoluble sulfur; inorganic vulcanizing agents, such as sulfur chloride, selenium, and tellurium; sulfur-containing organic compounds, such as morpholine disulfide, alkylphenol disulfides, thiuram disulfides, and dithiocarbamates; and organic peroxides, such as 1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane, di-t-butylperoxide, dicumyl peroxide, t-butylcumyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and 1,3-bis-(t-butylperoxy-isopropyl)benzene. These vulcanizing / crosslinking agents may be used alone or in combination. The amount of vulcanization / crosslinking agent to be added is determined appropriately depending on the type of agent, but is usually in the range of 0.1 to 10 parts by mass, preferably 0.5 to 5 parts by mass, per 100 parts by mass of the base elastomer. If necessary, a vulcanization accelerator or a vulcanization accelerator assistant can also be used in combination.

[0111] [Other Components] The hollow particle-containing elastomer composition of the present disclosure may optionally contain conventionally known additives, such as reinforcing agents, fillers, vulcanization accelerators, softeners, processing aids, antioxidants, UV absorbers, blowing agents, foaming aids, lubricants, pigments, colorants, dispersants, and flame retardants, provided that the objectives of the present disclosure are not impaired. Reinforcing agents are effective in improving the mechanical properties of elastomers, such as tensile strength, tear strength, and abrasion resistance. Specific examples of such reinforcing agents include carbon blacks such as SRF, GPF, FEF, HAF, ISAF, SAF, FT, and MT, carbon blacks surface-treated with silane coupling agents, finely divided silica, and silica. These reinforcing agents can be used alone or in combination. The amount of reinforcing agent is not particularly limited, but is typically 230 parts by mass or less per 100 parts by mass of the base elastomer. Examples of fillers include inorganic fillers such as calcium carbonate, light calcium carbonate, heavy calcium carbonate, magnesium carbonate, talc, clay, glass beads, and glass balloons; and organic fillers such as high styrene resin, coumarone-indene resin, phenolic resin, lignin, modified melamine resin, and petroleum resin, with inorganic fillers being particularly preferred. These fillers can be used alone or in combination of two or more. The amount of filler to be added is not particularly limited, and is typically 30 to 200 parts by mass per 100 parts by mass of the base elastomer.

[0112] Specific examples of the vulcanization accelerator include aldehyde ammonias such as hexamethylenetetramine; guanidines such as diphenylguanidine, di(o-tolyl)guanidine, and o-tolyl-piguanide; thioureas such as thiocarbanilide, di(o-tolyl)thiourea, N,N'-diethylthiourea, and dilaurylthiourea; thiazoles such as mercaptobenzothiazole, dibenzothiazole disulfide, and N,N'-di(ethylthiocarbamoylthio)benzothiazole; Examples of suitable vulcanization accelerators include sulfenamides such as butyl-2-benzothiazylsulfenamide; thiurams such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, and tetrabutylthiuram disulfide; carbamates such as zinc dimethylthiocarbamate, sodium dimethyldithiocarbamate, copper dimethyldithiocarbamate, tellurium dimethylthiocarbamate, and iron dimethylthiocarbamate; and xanthates such as zinc butylthioxanthate. These vulcanization accelerators can be used alone or in combination. The amount of vulcanization accelerator added is typically 0.1 to 20 parts by mass, preferably 0.2 to 10 parts by mass, per 100 parts by mass of the base elastomer. Specific examples of vulcanization accelerator aids include metal oxides such as magnesium oxide and zinc white; and organic acids (salts) such as stearic acid, oleic acid, and zinc stearate. Zinc white and stearic acid are particularly preferred. These vulcanization accelerators can be used alone or in combination of two or more. The amount of the vulcanization accelerator added is usually in the range of 0.5 to 20 parts by mass per 100 parts by mass of the base elastomer.

[0113] Examples of softeners include petroleum-based softeners such as process oil, lubricating oil, paraffin, liquid paraffin, petroleum asphalt, and Vaseline; coal tar-based softeners such as coal tar and coal tar pitch; fatty oil-based softeners such as castor oil, linseed oil, rapeseed oil, and coconut oil; tall oil; sap; waxes such as beeswax, carnauba wax, and lanolin; fatty acids and fatty acid salts such as ricinoleic acid, palmitic acid, barium stearate, calcium stearate, and zinc laurate; synthetic polymers such as petroleum resin, atactic polypropylene, and coumarone-indene resin; ester-based plasticizers such as dioctyl phthalate, dioctyl adipate, and dioctyl sebacate; carbonate ester-based plasticizers such as diisododecyl carbonate; and other microcrystalline waxes, sap (factice), liquid polybutadiene, modified liquid polybutadiene, liquid thiokol, and hydrocarbon-based synthetic lubricating oils. Among these, petroleum-based softeners are preferred, and process oils are particularly preferred. The amount of the softener to be added is not particularly limited, and is usually 10 to 200 parts by mass per 100 parts by mass of the base elastomer.

[0114] Examples of processing aids include higher fatty acids such as ricinoleic acid, stearic acid, palmitic acid, and lauric acid; salts of higher fatty acids such as barium stearate, zinc stearate, and calcium stearate; and esters of higher fatty acids such as ricinoleic acid, stearic acid, palmitic acid, and lauric acid. Examples of antioxidants include amine-based, hindered phenol-based, and sulfur-based antioxidants.

[0115] Examples of lubricants include hydrocarbon compounds or mixtures such as liquid paraffin, fatty acid compounds such as stearic acid, fatty acid amide compounds such as stearic acid amide, ester compounds such as butyl stearate, and alcohol compounds such as stearyl alcohol, as well as metal soaps. Examples of pigments include inorganic pigments such as titanium dioxide, zinc oxide, ultramarine, red iron oxide, lithopone, lead, cadmium, iron, cobalt, aluminum, hydrochloride salts, and nitrate salts, and organic pigments such as azo pigments, phthalocyanine pigments, quinacridone pigments, quinacridonequinone pigments, dioxazine pigments, anthrapyrimidine pigments, anthanthrone pigments, indanthrone pigments, flavanthrone pigments, perylene pigments, perinone pigments, diketopyrrolopyrrole pigments, quinonaphthalone pigments, anthraquinone pigments, thioindigo pigments, benzimidazolone pigments, isoindoline pigments, and carbon black.

[0116] [Method for Producing Hollow Particle-Containing Elastomer Composition] The method for producing the hollow particle-containing elastomer composition of the present disclosure is not particularly limited. Generally, a raw material mixture containing a base elastomer, hollow particles, and, optionally, other components such as a plasticizer and a vulcanizing / crosslinking agent is pre-mixed at a temperature at which the base elastomer softens to homogenize the blended components. Then, a finish mix is ​​performed by applying a high shear force, such as roll mix, to further homogenize and refine the blended components, thereby obtaining a hollow particle-containing elastomer composition. To prevent crosslinking of the raw material mixture by the vulcanizing / crosslinking agent during the pre-mixing, a vulcanizing / crosslinking agent may be added in the finish mix after the pre-mixing. Furthermore, hollow particles may be added in the finish mix.

[0117] In the present disclosure, in order to reduce crushing of hollow particles during the kneading of a raw material mixture for a hollow particle-containing elastomer composition, it is preferable to prepare a raw material mixture including at least a base elastomer, a shell containing a resin, and hollow portions surrounded by the shell, wherein the shell contains hollow particles containing a polymer containing 50 parts by mass or more of crosslinkable monomer units per 100 parts by mass of all monomer units as the resin, and having a storage modulus G' at 60°C of 2.5 MPa or less as determined by dynamic viscoelasticity measurement performed after a homogenization treatment of blended components, pre-kneading the raw material mixture using an internal kneader at a temperature at which the storage modulus G' as determined by the dynamic viscoelasticity measurement performed after the homogenization treatment becomes 2.5 MPa or less, and kneading the raw material mixture immediately after pre-kneading or after pre-heating at a temperature at which the storage modulus G' as determined by the dynamic viscoelasticity measurement performed after the homogenization treatment becomes 2.5 MPa or less. In order to mix the components uniformly, the kneading step (finish kneading step) is preferably carried out by roll kneading.

[0118] In the above manufacturing method, "kneading at a kneading temperature of 60° C. or higher" means that the set temperature of the kneading device is 60° C. or higher. Similarly, "kneading at a temperature of A° C.," "kneading at a temperature of A° C.," "preheating at a temperature of A° C.," or "molding at a temperature of A° C." means that the temperature of the device, such as the kneading device, heating device, or molding device, is set to the corresponding value (A).

[0119] The above-described manufacturing method uses hollow particles whose strength is enhanced by a shell made of a resin containing a polymer with a high content of crosslinkable monomer units and whose strength does not decrease even in high-temperature environments due to the crosslinked structure. Furthermore, by adjusting the storage modulus G' at 60°C of the raw material mixture containing the base elastomer and hollow particles to 2.5 MPa or less and performing pre-mixing and final mix-kneading at a temperature at which the storage modulus G' of the raw material mixture is 2.5 MPa or less, the loads imposed on the hollow particles within the raw material mixture during final mix-kneading, such as internal pressure and shear stress, are kept low. Therefore, according to the manufacturing method disclosed herein, the hollow particles are resistant to crushing during the mix-kneading process, the residual void ratio is stable, and the properties or functions imparted by the hollow particles are not easily lost, resulting in a hollow particle-containing elastomer composition with excellent properties or functions. Furthermore, according to the above-described manufacturing method, the initial void ratio of the hollow particles present within the raw material mixture can be maintained even when the raw material mixture is mixed repeatedly, thereby preventing the effects of the hollow particles from being impaired. Therefore, the elastomer composition recovered from the molding machine can be reused as a raw material mixture before it is crosslinked (vulcanized).

[0120] To carry out the above method, a formulation is specified in advance that results in a storage modulus G' of 2.5 MPa or less at 60°C as determined by dynamic viscoelasticity measurement of the raw material mixture. The dynamic viscoelasticity measurement of the raw material mixture can be performed using the same method as that used to measure the dynamic viscoelasticity of the hollow particle-containing elastomer composition. However, since the raw material mixture is a composition before kneading and the blended components are not sufficiently uniform, performing dynamic viscoelasticity measurement on the raw material mixture as is will result in large variations in measured values ​​or even be impossible. Therefore, the dynamic viscoelasticity measurement of the raw material mixture must be performed after the blended components contained in the raw material mixture have been homogenized to a state in which stable measured values ​​can be obtained. The conditions for this homogenization are not particularly limited, but the homogenization may be performed, for example, under the following conditions: [Homogenization] A mixture containing at least the base elastomer and the hollow particles is homogenized by kneading it using an internal kneader at a kneading temperature of 100°C and a kneading member rotation speed of 50 rpm.

[0121] When dynamic viscoelasticity measurement is performed to identify a raw material mixture having a storage modulus of 2.5 MPa or less at 60° C., as long as the resistance to crushing of hollow particles during the production stage of the hollow particle-containing elastomer composition can be properly evaluated, a simplified measurement may be performed using a pseudo raw material mixture that does not contain some of the components contained in the hollow particle-containing elastomer composition, within a range in which the effect on the change in storage modulus at 60° C. is small. For example, the vulcanization / crosslinking agent for the hollow particle-containing elastomer composition usually has a small effect on the change in storage modulus at 60° C. within the range of the amount of vulcanization / crosslinking agent typically used, so that a pseudo mixture that does not contain the vulcanization / crosslinking agent can be used as a sample for dynamic viscoelasticity measurement.

[0122] Generally, in order to uniformly knead a composition containing a base elastomer, hollow particles, and, if necessary, other components such as a plasticizer, a vulcanizing / crosslinking agent, etc., it is desirable to apply a high shear force, such as with roll kneading, but the hollow particles are easily crushed by such high shear force. In the present disclosure, the specific raw material mixture is pre-kneaded using an internal kneader at a temperature at which the storage modulus G′, as measured by dynamic viscoelasticity measurement after the homogenization treatment, becomes 2.5 MPa or less, and then immediately, or after pre-kneading, the mixture is preheated at a temperature at which the storage modulus G′, as measured by dynamic viscoelasticity measurement after the homogenization treatment, becomes 2.5 MPa or less, and then finish-kneaded at a temperature at which the storage modulus G′, as measured by dynamic viscoelasticity measurement after the homogenization treatment, becomes 2.5 MPa or less. This prevents excessive load on the hollow particles even when kneading is performed using a high shear force, such as with roll kneading, and therefore makes it possible to produce a uniformly mixed elastomer composition while avoiding crushing of the hollow particles.

[0123] In the present disclosure, the term "closed kneader" used in the preliminary kneading and the homogenization treatment refers to a machine having a chamber in the center for containing the rubber material to be processed, two rotors (stirring members) installed in the chamber, and a mechanism for grinding and kneading the rubber placed between the two rotors. The rotor is a roll-shaped rotating shaft with blades attached to it to mix the entire material uniformly without leaving any material behind. The rubber material is mixed in the chamber while being subjected to the shear force of the rotors. Examples of the closed kneader include a kneader and a Banbury mixer. More specifically, examples of the kneader include the PLASTICODER LABOSTATION (manufactured by Brabender) and the MS-type pressure kneader (manufactured by Moriyama Corporation), and examples of the Banbury mixer include the MIXTRON BB MIXER (manufactured by Kobe Steel, Ltd.). Furthermore, as a kneader for finish kneading, for example, a twin mixing roll can be used, and more specifically, a product name: Mixing Roll DY6-15 (manufactured by Daihan Co., Ltd.) can be mentioned. The internal kneader used for pre-kneading and the internal kneader used for homogenization treatment do not need to be devices with exactly the same configuration. However, from the viewpoint of predicting as accurately as possible the viscoelastic behavior of the raw material mixture in the stage of pre-mixing the raw material mixture based on the viscoelastic properties of the raw material mixture measured in the stage of specifying the recipe for the raw material mixture, it is preferable to use an internal kneader with the same device configuration in the stage of pre-kneading and the stage of homogenization treatment.

[0124] As an example, mixing can be performed using the following method. First, using an internal mixer such as a kneader or Banbury mixer, which has a weaker shear force than roll mixers, the mixing temperature is set to 100°C. After the temperature of the internal mixer has stabilized, the base elastomer is added. Next, while rotating the rotor of the internal mixer at a rotation speed of 30 to 100 rpm, components such as hollow particles, plasticizer, vulcanization / crosslinking agent, etc. are added in any order to perform premixing. Next, the mixing temperature of the roll mixer is set to 60°C or higher. After the temperature of the roll mixer has stabilized, the premixed raw material mixture is immediately added to the roll mixer and roll mixed, thereby obtaining the hollow particle-containing elastomer composition of the present disclosure. By immediately adding the premixed raw material mixture from the internal mixer to the roll mixer, the temperature of the raw material mixture decreases during transfer from the premixing step to the final mixer, reducing its melt fluidity and preventing increased crushing of the hollow particles during the final mixer. Here, the "immediately" in "immediately charged into the roll mixer from the internal mixer" means that the time required to transfer the raw material mixture from the pre-mixing step to the finish-mixing step is short enough to keep the degree of increased hollow particle crushing during the finish-mixing to a practically negligible range. More specifically, the difference (unit: %) between the porosity of an elastomer molded product produced from the pre-mixed raw material mixture, measured according to the method for measuring the residual void ratio of a hollow particle-containing elastomer molded product described below, and the porosity of an elastomer molded product produced from the final hollow particle-containing elastomer composition, measured by the same method, is preferably 10% or less, more preferably 5% or less. Furthermore, the time required for the start of finish-mixing immediately after pre-mixing without preheating, expressed specifically in hours, is preferably within 10 minutes, more preferably within 5 minutes, of pre-mixing.

[0125] If, due to process considerations, the temperature of the raw material mixture drops below 60°C when it is removed from the preliminary kneading device and transferred to the finishing kneading device, it is preferable to preheat the raw material mixture in a heating device such as an oven to a preheating temperature of 60°C or higher for an appropriate period of time, for example, about one hour, before loading it into the finishing kneading device and starting kneading, in order to maintain the temperature at which the storage modulus G' of the raw material mixture is 2.5 MPa or less. In order to prevent the raw material mixture from being crosslinked by the vulcanizing / crosslinking agent during the preliminary kneading, a vulcanizing / crosslinking agent may be added during the roll kneading step after the preliminary kneading. Furthermore, hollow particles may be added during the roll kneading step. In both the preliminary kneading and the roll kneading, it is preferable to perform the kneading at a kneading temperature of 100°C or lower in order to prevent the raw material mixture from being crosslinked by the vulcanizing / crosslinking agent.

[0126] The hollow particle-containing elastomer composition obtained as described above can be made into a molding material product having any desired shape. For example, the hollow particle-containing elastomer composition in a molten state may be molded into a long sheet, a block, a filler, or the like, or may be subjected to secondary processing into a roll obtained by winding the long sheet or into strips obtained by cutting the long sheet to a predetermined length.

[0127] [Method for Producing Hollow Particle-Containing Elastomer Molded Articles] The method for producing a molded article using the hollow particle-containing elastomer composition of the present disclosure is not particularly limited, but preferably includes a step of kneading the hollow particle-containing elastomer composition at a molding temperature of 60°C or higher. Generally, when producing an elastomer molded article using a molding elastomer composition, the components are kneaded to a uniform molten state, and then any molding method such as extrusion molding, compression molding, or extrusion lamination is carried out. By molding the hollow particle-containing elastomer composition of the present disclosure at a molding temperature of 60°C or higher, the storage modulus G' of the hollow particle-containing elastomer composition during molding can be set to 2.5 MPa or less. Since the loads such as external pressure and shear force acting on the hollow particles are not excessive, an elastomer molded article can be produced while avoiding crushing of the hollow particles.

[0128] The hollow particle-containing elastomer molded article obtained in the present disclosure can have a void residual rate of 80% or more, preferably 90% or more, more preferably 95% or more, and even more preferably 100%, as measured according to the following test method. [Method for Measuring the Void Residual Rate of Hollow Particle-Containing Elastomer Molded Article] A hollow particle-containing elastomer composition is press-molded in a hot press at 120°C under a pressure of 1 MPa or less to produce a sheet-like hollow particle-containing elastomer molded article. The specific gravity of the resulting elastomer molded article is measured, and the void residual rate of the hollow particles in the elastomer molded article is calculated according to the following formula (D). Note that formula (D) is the same as formula (D) used to calculate the void residual rate in the above-mentioned press test method performed to evaluate the crushability of hollow particles. Void remaining rate (%) = {(c-a) / (c-b)} x 100 Formula (D) a: specific gravity of compact after pressing, b: specific gravity of compact assumed to maintain voids (calculated value), c: specific gravity of compact assumed to have all hollow particles crushed (calculated value).

[0129] The present disclosure will be described in more detail below with reference to examples and comparative examples, but the present disclosure is not limited to these examples. Note that parts and percentages are by mass unless otherwise specified.

[0130] [Preparation of Hollow Particles] [Production Example 1 (Hollow Particles A)] (1) Mixture Preparation Step First, the following materials were mixed to form an oil phase: Crosslinkable monomer: ethylene glycol dimethacrylate 25 parts Crosslinkable monomer: trimethylolpropane trimethacrylate 30 parts Crosslinkable monomer: divinylbenzene 45 parts Oil-soluble polymerization initiator: 2,2'-azobis(2,4-dimethylvaleronitrile) 3 parts Hydrophobic solvent: hexane 160 parts Meanwhile, in a stirring tank at room temperature, an aqueous solution of 12.1 parts of sodium hydroxide (alkali metal hydroxide) in 121 parts of ion-exchanged water was gradually added with stirring to an aqueous solution of 17.1 parts of magnesium chloride (a water-soluble polyvalent metal salt) in 494 parts of ion-exchanged water to prepare a magnesium hydroxide colloid (poorly water-soluble metal hydroxide colloid) dispersion (magnesium hydroxide 4 parts), which served as an aqueous phase. The resulting aqueous phase and oil phase were mixed to prepare a mixed liquid.

[0131] (2) Suspension step The mixture obtained in the mixture preparation step was stirred and suspended for 1 minute using a disperser (manufactured by Primix Corporation, product name: Homomixer) at a rotation speed of 4,000 rpm, to prepare a suspension in which droplets of the monomer composition encapsulating the hydrophobic solvent were dispersed in water.

[0132] (3) Polymerization step: The suspension obtained in the suspension step was stirred for 1 hour and 30 minutes under a nitrogen atmosphere at a temperature of 65°C to carry out a polymerization reaction, thereby obtaining a precursor composition containing precursor particles encapsulating a hydrophobic solvent.

[0133] (4) Washing step and solid-liquid separation step The precursor composition was washed with dilute sulfuric acid (25°C, 10 minutes) to adjust the pH to 5.5 or less. Next, after separating the water by filtration, 200 parts of ion-exchanged water was added to re-slurry the mixture. The water washing treatment (washing, filtration, dehydration) was repeated several times at room temperature (25°C), and the mixture was filtered to obtain a solid fraction. The obtained solid fraction was dried in a dryer at a temperature of 40°C to obtain precursor particles encapsulating the hydrophobic solvent.

[0134] (5) Solvent Removal Step The precursor particles obtained in the solid-liquid separation step were heat-treated in a vacuum dryer at 200°C in a nitrogen atmosphere for 12 hours to remove the hydrophobic solvent contained in the particles, thereby obtaining hollow particles A. From the results of observation with a scanning electron microscope and the porosity value, it was confirmed that the obtained hollow particles were spherical and had hollow portions.

[0135] [Production Example 2 (Hollow Particles B)] Hollow particles (hollow particles B) of Production Example 2 were obtained in the same procedure as Production Example 1, except that in the above "(1) Mixture Preparation Step" of Production Example 1, the formulation of the oil phase was changed as follows: Crosslinkable monomer: ethylene glycol dimethacrylate 70 parts Crosslinkable monomer: trimethylolpropane trimethacrylate 30 parts Oil-soluble polymerization initiator: 2,2'-azobis(2,4-dimethylvaleronitrile) 3 parts Hydrophobic solvent: cyclohexane 48 parts Hydrophobic solvent: toluene 112 parts It was confirmed from the results of observation with a scanning electron microscope and the porosity value that the obtained hollow particles were spherical and had hollow portions.

[0136] [Evaluation of Physical Properties of Hollow Particles] The hollow particles obtained in each production example were evaluated for the following physical properties.

[0137] 1. Volume Average Particle Size of Hollow Particles The volume average particle size of hollow particles was measured using a particle size distribution analyzer (manufactured by Beckman Coulter, Inc., product name: Multisizer 4e). The measurement conditions were aperture diameter: 50 μm, dispersion medium: Isoton II (product name), concentration: 10%, and number of particles measured: 100,000. Specifically, 0.2 g of particle sample was placed in a beaker, and a surfactant aqueous solution (manufactured by Fujifilm Corporation, product name: Drywell) was added as a dispersant. 2 ml of dispersion medium was added to wet the particles, and then 10 ml of dispersion medium was added. The mixture was dispersed in an ultrasonic disperser for 1 minute, and then measured using the particle size distribution analyzer.

[0138] 2. Density and porosity of hollow particles 2-1. Measurement of apparent density of hollow particles First, a 100 cm ３ About 30 cm ３The hollow particles were packed into the measuring flask, and the mass of the packed hollow particles was accurately weighed. Next, the measuring flask packed with the hollow particles was accurately filled with isopropanol up to the marked line, taking care not to introduce air bubbles. The mass of the isopropanol added to the measuring flask was accurately weighed, and the apparent density D of the hollow particles was calculated based on the above formula (I). １ (g / cm ３ ) was calculated.

[0139] 2-2. Measurement of true density of hollow particles After crushing the hollow particles in advance, ３ Approximately 10 g of crushed pieces of hollow particles was filled into a measuring flask, and the mass of the crushed pieces was accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol was added to the measuring flask, and the mass of the isopropanol was accurately weighed. Based on the above formula (II), the true density D of the hollow particles was calculated. ０ (g / cm ３ ) was calculated.

[0140] 2-3. Calculation of porosity Apparent density D of hollow particles １ and true density D ０ The porosity of the hollow particles was calculated based on the above formula (III).

[0141] 3. Void Residual Ratio of Hollow Particles The void residual ratio of hollow particles was measured according to the following press test method. [Press Test Method] A mixture of polypropylene resin (manufactured by Japan Polypropylene Corporation, product name: Novatec PP, grade: MA1B) with a mass ratio of polypropylene resin:hollow particles of 90:10 and hollow particles was melted and mixed at 200°C, placed in a heat press mold, and further heated at 200°C for 15 minutes. The mixture was then stirred with a spatula and placed in a heat press set at 80°C. A cylinder heated to 80°C was placed in the mold. When the surface temperature of the mold reached 140°C, a pressure of 15 MPa was applied. The mixture was then removed from the mold and pressed at a pressure of 1 MPa or less using a heat press set at 200°C to form a sheet-like molded product with a thickness of 0.3 mm. The specific gravity of the resulting sheet-like molded product was measured, and the void residual ratio of hollow particles was calculated according to the following formula (D): Void remaining rate (%) = {(c-a) / (c-b)} x 100 Equation (D) The symbols in equation (D) have the following meanings: a: specific gravity of the sheet-shaped molded product after press molding, b: specific gravity of the molded product assuming that the voids are maintained (calculated value), c: specific gravity of the molded product assuming that all hollow particles are crushed (calculated value).

[0142] The amounts of materials added (parts by mass) and the results of physical property evaluation for the hollow particles obtained in each production example are shown in Table 1. The void remaining rate measured by the press test shown in Table 1 is the average value for 10 samples (n = 10).

[0143]

[0144] [Production of elastomer composition, production of elastomer molded body] [Example 1] 100 parts by mass of ethylene-propylene-diene terpolymer (EPDM) (Mooney viscosity at 100°C (JIS K6300), ML(1+4)100°C: 25, product name: Nordel IP 4725, manufacturer: Dow Chemical Company) was placed in a kneader (Plasticorder Labostation, manufactured by Brabender), and kneading was started at a kneading temperature of 100°C and a rotation speed of 50 rpm to produce hollow particles A obtained in Production Example 1. Preliminary mixing was carried out while adding 25 parts by mass of the mixture, 45 parts by mass of carbon (product name: Seest S, manufacturer: Tokai Carbon Co., Ltd.), 29 parts by mass of silica (product name: Nipsil VN-3, manufacturer: Tosoh Silica Corporation), and 75 parts of process oil as a plasticizer (hydrogenated high-viscosity oil: Diana Process Oil PW-300, manufacturer: Idemitsu Kosan Co., Ltd.) The mixture after preliminarily mixing was preheated in a 60°C oven for at least 1 hour before roll mixing, and the temperature of the mixture was maintained at 60°C. The mixture, preheated to 60°C, was placed in a two-roll mixing mixer (model: DY6-15, roll diameter: 6 inches, roll clearance: 0.5 mm, manufactured by Daihan Co., Ltd.) with the mixer temperature set to 80°C. Subsequently, 1.5 parts of sulfur as a vulcanizing agent and 2 parts of tetrathium monosulfide (reagent grade) as a vulcanization accelerator were added, while the mixer was roll-mixed (rotation speed: front roll 24 rpm / rear roll 21 rpm, mix time: 15 minutes). The molten mixture after roll-mixing was heated and dried at 80°C for 6 hours to obtain the hollow particle-containing elastomer composition of Example 1. The resulting hollow particle-containing elastomer composition was press-molded at a pressure of 1 MPa or less using a hot press at 120°C to form a 0.3 mm-thick sheet-like molded product (hollow particle-containing elastomer molded product).

[0145] [Example 2] A hollow particle-containing elastomer composition of Example 2 was obtained in the same manner as in Example 1, except that the preheating temperature was changed from 60°C to 80°C. The obtained hollow particle-containing elastomer composition was press-molded under the same molding conditions as in Example 1 to obtain a hollow particle-containing elastomer molded article of Example 2.

[0146] [Example 3] A hollow particle-containing elastomer composition of Example 3 was obtained in the same manner as in Example 1, except that 25 parts by mass of hollow particles B obtained in Production Example 2 was used instead of 25 parts by mass of hollow particles A obtained in Production Example 1. The obtained hollow particle-containing elastomer composition was press-molded under the same molding conditions as in Example 1 to obtain a hollow particle-containing elastomer molded article of Example 3.

[0147] [Example 4] The non-oil-extended styrene-butadiene rubber used in Example 4 means a styrene-butadiene rubber to which no oily component as a plasticizer has been added. 100 parts by mass of non-oil-extended styrene-butadiene rubber (Mooney viscosity at 100°C (JIS K6300), ML(1+4)100°C: 52.0, styrene unit content: 23.5% by mass, product name: Nipol (registered trademark) 1502, manufacturer: Nippon Zeon Co., Ltd.) was charged into a kneader (Plasticoda Labostation, manufactured by Brabender), and kneading was started at a kneading temperature of 100°C and a rotation speed of 50 rpm to produce hollow particles A obtained in Production Example 1. Preliminary mixing was carried out while adding 25 parts by mass of carbon (product name: Seest S, manufacturer: Tokai Carbon Co., Ltd.), 25 parts by mass of silica (product name: Nipsil VN-3, manufacturer: Tosoh Silica Corporation), 65 parts by mass of process oil (hydrogenated high-viscosity oil: Diana Process Oil PW-300, manufacturer: Idemitsu Kosan Co., Ltd.) as a plasticizer, and 10 parts by mass of liquid polybutadiene (weight average molecular weight: 2100, product name: Liquid Polybutadiene B2000, manufacturer: Nippon Soda Co., Ltd.) as a plasticizer having an ethylenic double bond. The mixture after preliminary mixing was preheated in a 60°C oven for at least 1 hour before roll mixing, and the temperature of the mixture was maintained at 60°C. The mixture, preheated to 60°C, was placed in a two-roll mixing mixer (model: DY6-15, roll diameter: 6 inches, roll clearance: 0.5 mm, manufactured by Daihan Co., Ltd.) with the mixer temperature set to 80°C. Subsequently, 1.5 parts of sulfur as a vulcanizing agent and 2 parts of tetrathium monosulfide (reagent grade) as a vulcanization accelerator were added, while the mixer was roll-mixed (rotation speed: front roll 24 rpm / rear roll 21 rpm, mix time: 15 minutes). The molten mixture after roll-mixing was heated and dried at 80°C for 6 hours to obtain the hollow particle-containing elastomer composition of Example 4. The obtained hollow particle-containing elastomer composition was press-molded in a hot press at 120°C under a pressure of 1 MPa or less to obtain a 0.3 mm-thick sheet-like molded product (hollow particle-containing elastomer molded product).

[0148] [Example 5] A hollow particle-containing elastomer composition of Example 5 was obtained in the same manner as in Example 4, except that the amount of carbon was changed from 25 parts by mass to 45 parts by mass, the amount of process oil as a plasticizer was changed from 65 parts by mass to 55 parts by mass, and the amount of liquid polybutadiene as a plasticizer having an ethylenic double bond was changed from 10 parts by mass to 20 parts by mass in Example 4. The obtained hollow particle-containing elastomer composition was press-molded under the same molding conditions as in Example 4, to obtain a hollow particle-containing elastomer molded article of Example 5.

[0149] [Example 6] The hollow particle-containing elastomer composition of Example 6 was obtained in the same manner as in Example 4, except that the amount of carbon was changed from 25 parts by mass to 45 parts by mass, the amount of process oil as a plasticizer was changed from 65 parts by mass to 0 parts by mass, and the amount of liquid polybutadiene as a plasticizer having an ethylenic double bond was changed from 10 parts by mass to 60 parts by mass. The obtained hollow particle-containing elastomer composition was press-molded under the same molding conditions as in Example 4, to obtain the hollow particle-containing elastomer molded article of Example 6.

[0150] Example 7 The hollow particle-containing elastomer composition of Example 7 was obtained in the same manner as in Example 4, except that the amount of carbon was changed from 25 parts by mass to 45 parts by mass, the amount of process oil as a plasticizer was changed from 65 parts by mass to 0 parts by mass, the amount of liquid polybutadiene as a plasticizer having an ethylenic double bond was changed from 10 parts by mass to 75 parts by mass, and the heating conditions involved changing the preheating temperature from 60° C. to 80° C. The obtained hollow particle-containing elastomer composition was press-molded under the same molding conditions as in Example 4, to obtain the hollow particle-containing elastomer molded article of Example 7.

[0151] [Example 8] The hollow particle-containing elastomer composition of Example 8 was obtained in the same manner as in Example 4, except that the amount of carbon was changed from 25 parts by mass to 45 parts by mass, the amount of process oil as a plasticizer was changed from 65 parts by mass to 0 parts by mass, and the amount of liquid polybutadiene as a plasticizer having an ethylenic double bond was changed from 10 parts by mass to 80 parts by mass in Example 4. The obtained hollow particle-containing elastomer composition was press-molded under the same molding conditions as in Example 4 to obtain the hollow particle-containing elastomer molded article of Example 8.

[0152] [Example 9] The oil-extended styrene-butadiene rubber used in Example 9 refers to styrene-butadiene rubber to which several mass % of an oily component as a plasticizer has been added. 100 mass parts of oil-extended styrene-butadiene rubber (Mooney viscosity at 100°C (JIS K6300), ML(1+4)100°C: 49.0, styrene unit content: 40.0 mass %, product name: Nipol (registered trademark) 1739, manufacturer: Nippon Zeon Co., Ltd.) were charged into a kneader (Plasticoda Labostation, manufactured by Brabender), and kneading was started at a kneading temperature of 100°C and a rotation speed of 50 rpm to produce hollow particles A obtained in Production Example 1. Preliminary mixing was carried out while adding 25 parts by mass of the mixture, 45 parts by mass of carbon (product name: Seest S, manufacturer: Tokai Carbon Co., Ltd.), 29 parts by mass of silica (product name: Nipsil VN-3, manufacturer: Tosoh Silica Corporation), and 60 parts by mass of liquid polybutadiene (weight average molecular weight: 2100, product name: Liquid Polybutadiene B2000, manufacturer: Nippon Soda Co., Ltd.) as a plasticizer having an ethylenic double bond. The mixture after preliminary mixing was preheated in a 60°C oven for at least 1 hour before roll mixing, and the temperature of the mixture was maintained at 60°C. The mixture, preheated to 60°C, was placed in a two-roll mixing mixer (model: DY6-15, roll diameter: 6 inches, roll clearance: 0.5 mm, manufactured by Daihan Co., Ltd.) with the mixer temperature set to 80°C. Subsequently, 1.5 parts of sulfur as a vulcanizing agent and 2 parts of tetrathium monosulfide (reagent grade) as a vulcanization accelerator were added, while the mixer was roll-mixed (rotation speed: front roll 24 rpm / rear roll 21 rpm, mix time: 15 minutes). The molten mixture after roll-mixing was heated and dried at 80°C for 6 hours to obtain the hollow particle-containing elastomer composition of Example 9. The resulting hollow particle-containing elastomer composition was press-molded at a pressure of 1 MPa or less using a hot press at 120°C to form a 0.3 mm-thick sheet-like molded product (hollow particle-containing elastomer molded product).

[0153] [Example 10] A hollow particle-containing elastomer composition of Example 10 was obtained in the same manner as in Example 9, except that the amount of liquid polybutadiene, which is a plasticizer having an ethylenic double bond, was changed from 60 parts by mass to 75 parts by mass, and further, regarding the heating conditions, the preheating temperature was changed from 60° C. to 80° C. The obtained hollow particle-containing elastomer composition was press-molded under the same molding conditions as in Example 9, to obtain a hollow particle-containing elastomer molded article of Example 10.

[0154] [Example 11] A hollow particle-containing elastomer composition of Example 11 was obtained in the same manner as in Example 9, except that the amount of liquid polybutadiene, a plasticizer having an ethylenic double bond, was changed from 60 parts by mass to 80 parts by mass in Example 9. The obtained hollow particle-containing elastomer composition was press-molded under the same molding conditions as in Example 9, to obtain a hollow particle-containing elastomer molded article of Example 11.

[0155] [Example 12] 100 parts by mass of butadiene rubber (Mooney viscosity at 100°C (JIS K6300), ML(1+4)100°C: 44.0, styrene unit content: 0 mass%, product name: Nipol (registered trademark) BR1220, manufacturer: Zeon Corporation) was placed in a kneader (Plasticorder Labostation, manufactured by Brabender), and kneading was started at a kneading temperature of 100°C and a rotation speed of 50 rpm to produce hollow particles A obtained in Production Example 1. Preliminary mixing was carried out while adding 25 parts by mass of the mixture, 45 parts by mass of carbon (product name: Seest S, manufacturer: Tokai Carbon Co., Ltd.), 29 parts by mass of silica (product name: Nipsil VN-3, manufacturer: Tosoh Silica Corporation), and 60 parts by mass of liquid polybutadiene (weight average molecular weight: 2100, product name: Liquid Polybutadiene B2000, manufacturer: Nippon Soda Co., Ltd.) as a plasticizer having an ethylenic double bond. The mixture after preliminary mixing was preheated in a 60°C oven for at least 1 hour before roll mixing, and the temperature of the mixture was maintained at 60°C. The mixture, preheated to 60°C, was placed in a two-roll mixing mixer (model: DY6-15, roll diameter: 6 inches, roll clearance: 0.5 mm, manufactured by Daihan Co., Ltd.) with the mixer temperature set to 80°C. Subsequently, 1.5 parts of sulfur as a vulcanizing agent and 2 parts of tetrathium monosulfide (reagent grade) as a vulcanization accelerator were added, while the mixer was roll-mixed (rotation speed: front roll 24 rpm / rear roll 21 rpm, mix time: 15 minutes). The molten mixture after roll-mixing was heated and dried at 80°C for 6 hours to obtain the hollow particle-containing elastomer composition of Example 12. The resulting hollow particle-containing elastomer composition was press-molded at a pressure of 1 MPa or less using a hot press at 120°C to form a 0.3 mm-thick sheet-like molded product (hollow particle-containing elastomer molded product).

[0156] Example 13 A hollow particle-containing elastomer composition of Example 13 was obtained in the same manner as in Example 12, except that the amount of liquid polybutadiene, which is a plasticizer having an ethylenic double bond, was changed from 60 parts by mass to 75 parts by mass, and further, regarding the heating conditions, the preheating temperature was changed from 60° C. to 80° C. The obtained hollow particle-containing elastomer composition was press-molded under the same molding conditions as in Example 12, and a hollow particle-containing elastomer molded article of Example 13 was obtained.

[0157] [Example 14] A hollow particle-containing elastomer composition of Example 14 was obtained in the same manner as in Example 12, except that the amount of liquid polybutadiene, a plasticizer having an ethylenic double bond, was changed from 60 parts by mass to 80 parts by mass in Example 12. The obtained hollow particle-containing elastomer composition was press-molded under the same molding conditions as in Example 12, to obtain a hollow particle-containing elastomer molded article of Example 14.

[0158] [Comparative Example 1] A hollow particle-containing elastomer composition of Comparative Example 1 was obtained in the same manner as in Example 1, except that preheating was not performed. The obtained hollow particle-containing elastomer composition was press-molded under the same molding conditions as in Example 1 to obtain a hollow particle-containing elastomer molded article of Comparative Example 1.

[0159] [Comparative Example 2] A hollow particle-containing elastomer composition of Comparative Example 2 was obtained in the same manner as in Example 1, except that preheating was not performed and roll kneading was also performed without heating. The obtained hollow particle-containing elastomer composition was press-molded under the same molding conditions as in Example 1 to obtain a hollow particle-containing elastomer molded article of Comparative Example 2.

[0160] [Comparative Example 3] A hollow particle-containing elastomer composition of Comparative Example 3 was obtained in the same manner as in Example 1, except that the amount of plasticizer in Example 1 was changed to 30 parts by mass. The obtained hollow particle-containing elastomer composition was press-molded under the same molding conditions as in Example 1 to obtain a hollow particle-containing elastomer molded article of Comparative Example 3.

[0161] [Evaluation of Physical Properties of Hollow Particle-Containing Elastomer Compositions and Elastomer Molded Products] 1. Measurement of Storage Modulus G' Dynamic viscoelasticity measurements were performed on the hollow particle-containing elastomer compositions obtained in each Example and Comparative Example, and on the raw material mixtures obtained in the pre-mixing stage in each Example and Comparative Example, using the following method. The raw material mixtures in the pre-mixing stage were pseudo-raw material mixtures containing no vulcanizing agent or vulcanization accelerator. Adding 1.5 parts of sulfur as a vulcanizing agent and 2 parts of tetrathium monosulfide as a vulcanization accelerator to the raw material mixtures in the pre-mixing stage resulted in the final raw material mixtures and hollow particle-containing elastomer compositions. From the measurement results, the storage modulus G' of the elastomer composition at 60°C, the storage modulus G' of the raw material mixture at 60°C, the storage modulus G' at the temperature at which the raw material mixture started to be roll-mixed (preheating temperature), and the storage modulus G' of the raw material mixture at the roll-mixing temperature were determined. [Method for measuring dynamic viscoelasticity] Dynamic viscoelasticity was measured using a HAAKE MARK III (manufactured by Thermo Fisher Scientific). Test specimens were prepared by using a hollow particle-containing elastomer composition and a raw material mixture to prepare a 2 mm thick sheet using a press at 160°C, and then punching out a 20 mm diameter specimen using a punching machine. The test specimens (2 mm thick) were fused using 20 mm diameter parallel plates set at 170°C, and then placed in a dynamic viscoelasticity measuring device. The storage modulus G' (unit: MPa) from 150°C to room temperature was measured while the temperature was lowered from 170°C to room temperature at a rate of 5°C / min. <Measurement conditions> Frequency: 1 Hz Geometry: Parallel plates (20 mm diameter) Strain: 0.001 Gap: 1.5 mm

[0162] 2. Measurement of Void Residual Ratio of Hollow Particle-Containing Elastomer Molded Article The specific gravity of the sheet-like elastomer molded article obtained in each Example and Comparative Example was measured, and the void residual ratio of hollow particles in the elastomer molded article was calculated according to the following formula (D): Void residual ratio (%) = {(c-a) / (c-b)} x 100 Formula (D) a: specific gravity of sheet-like molded article obtained by press molding, b: specific gravity of molded article assuming voids are maintained (calculated value), c: specific gravity of molded article assuming all hollow particles are crushed (calculated value).

[0163] Table 2 shows the amounts of materials added (parts by mass), the kneading conditions, and the results of physical property evaluation for the elastomer compositions and elastomer molded articles obtained in each Example and Comparative Example. The void remaining rate of the elastomer molded articles after press molding shown in Table 2 is the average value for 10 samples (n=10). The kneading conditions in each Example and Comparative Example are generally as follows. <Pre-kneading> Apparatus: Plasticoda Labostation (manufactured by Brabender) Mixer: W 250E Blade: Roller type Rotation speed: 50 rpm Kneading temperature: 100°C <Roll kneading conditions> Apparatus: Twin mixing roll DY6-15 (manufactured by Daihan Co., Ltd.) Roll diameter: 6 inches Rotation speed: Front roll 24 rpm / rear roll 21 rpm Clearance between rolls: 0.5 mm Set temperature: 25 to 80°C Kneading time: 15 minutes

[0164]

[0165]

[0166] [Discussion] In Examples 1 and 3, elastomer compositions were produced using a raw material mixture containing a base elastomer and hollow particles containing a polymer in which the resin forming the shell contained 50 parts by mass or more of crosslinkable monomer units per 100 parts by mass of all monomer units. The steps from pre-kneading to roll kneading were carried out at a pre-kneading temperature of 100°C, a pre-kneading rotation speed of 50 rpm, a preheating temperature of 60°C, and a roll kneading temperature of 80°C. As a result, in Example 1, the storage modulus of the elastomer composition at 60°C and the storage modulus of the raw material mixture after homogenization were 1.7 MPa. In Example 3, the storage modulus of the elastomer composition at 60°C and the storage modulus of the raw material mixture after homogenization were 1.6 MPa. Thus, in both Examples 1 and 3, the storage modulus of the elastomer composition at 60°C and the storage modulus of the raw material mixture after homogenization were 2.5 MPa or less. The recipe of the raw material mixture in the preliminary kneading stage differed from that of the finally obtained elastomer composition in that it did not contain a vulcanizing agent or a vulcanization accelerator, but the storage modulus at 60°C did not vary significantly due to this difference in composition.

[0167] Furthermore, when the elastomer compositions obtained in Examples 1 and 3 were press-molded to produce sheet-like molded articles, the void retention rate of the hollow particles present in the molded articles was 100%, confirming that the hollow particles were resistant to crushing during the molding process and that molded articles with a high void retention rate were obtained. In Example 1, the storage modulus at the temperature at which the raw material mixture started to be roll-kneaded (equivalent to the preheating temperature), i.e., 60°C, was 1.7 MPa, which was below 2.5 MPa. However, the storage modulus at the roll-kneading temperature of the raw material mixture, i.e., 80°C, was 1.2 MPa, which was even lower than the storage modulus at the start of roll-kneading. It is believed that this suppressed the load due to roll-kneading, making the hollow particles less likely to be crushed. In Example 3, the storage modulus of the raw material mixture at the start of roll kneading (equivalent to the preheating temperature), i.e., 60°C, was 1.6 MPa, which was not more than 2.5 MPa, but the storage modulus of the raw material mixture at the roll kneading temperature, i.e., 80°C, was 1.1 MPa, which was even lower than the storage modulus at the start of roll kneading. This is thought to have suppressed the load due to roll kneading, making the hollow particles less likely to be crushed.

[0168] In Example 2, the same raw material mixture as in Example 1 was used, and the steps from pre-mixing to roll mixing were carried out at a pre-mixing temperature of 100°C, a pre-mixing rotation speed of 50 rpm, a preheating temperature of 80°C, and a roll mixing temperature of 80°C to produce an elastomer composition. The raw material mixture and the resulting elastomer composition used in Example 2 were the same as those used in Example 1, and their storage moduli at 60°C were 2.5 MPa or less. Furthermore, when a sheet-shaped molded product was produced by press molding the elastomer composition obtained in Example 2, the void retention rate of the hollow particles present in the molded product was 100%, confirming that the hollow particles were resistant to crushing during the molding process and that molded products with a high void retention rate were obtained. Examples 1 and 2 used raw material mixtures with the same composition, but the roll mixing start temperatures (equivalent to the preheating temperature) of the raw material mixtures were different. That is, the roll mixing start temperature in Example 1 was 60°C, while the roll mixing start temperature in Example 2 was 80°C. Despite these differences in process temperature, the void remaining rate of the hollow particles in the elastomer molding was 100% in both Examples 1 and 2, and the results of the void remaining rate show no discernible difference in the resistance to hollow particle crushing. However, in Example 2, the temperature at the start of roll mixing of the raw material mixture and the roll mixing temperature of the raw material mixture were both 80°C, and the storage modulus at the process temperature of the raw material mixture, i.e., 80°C, was 1.2 MPa from the initial stage to the final stage of roll mixing, thereby suppressing the load due to roll mixing compared to Example 1. Therefore, it is believed that kneading conditions were achieved that made the hollow particles even more resistant to crushing than those in Example 1.

[0169] In Comparative Examples 1 and 2, the same raw material mixture as in Example 1 was used. The raw material mixture and the resulting elastomer composition used in Comparative Examples 1 and 2 had the same compositions as in Example 1, and therefore their storage moduli at 60°C were 2.5 MPa or less. However, in Comparative Example 1, preheating was not performed, and the temperature at the start of roll kneading was low, at room temperature (25°C). When a sheet-like molded product was produced by press-molding the elastomer composition obtained in Comparative Example 1, the residual void ratio of the hollow particles present in the molded product was 75%. In Comparative Example 1, a raw material mixture having the same composition as in Example 1 was used, but the storage modulus at the temperature at the start of roll kneading of the raw material mixture, i.e., 25°C, was 4.6 MPa, exceeding 2.5 MPa. Therefore, it is believed that the load due to roll kneading was greater in the initial stage of roll kneading than in Example 1, resulting in the crushing of many hollow particles.

[0170] In Comparative Example 2, the elastomer composition was left at room temperature without preheating, and the roll kneading step was also carried out at room temperature, so that the roll kneading was carried out at room temperature (25°C) from the initial stage to the final stage. When a sheet-like molded product was produced by press-molding the elastomer composition obtained in Comparative Example 2, the residual void ratio of the hollow particles present in the molded product was 14%. Comparative Example 2 used a raw material mixture having the same composition as Example 1 and Comparative Example 1, but the storage modulus of the raw material mixture at the process temperature, i.e., 25°C, from the initial stage to the final stage of roll kneading was 4.6 MPa, exceeding 2.5 MPa. Therefore, it is believed that the load due to roll kneading was greater than that of Example 1 and Comparative Example 1 from the initial stage to the final stage of roll kneading, resulting in the crushing of many hollow particles.

[0171] In Comparative Example 3, when the plasticizer content in the raw material mixture of Example 1 was reduced to 30 parts by mass, the storage modulus of the elastomer composition at 60°C and the storage modulus of the raw material mixture after homogenization were both 3.2 MPa, exceeding 2.5 MPa. Furthermore, when the resulting elastomer composition was press-molded to produce a sheet-like molded product, the residual void ratio of the hollow particles present in the molded product was 45%, making it impossible to maintain the initial porosity of the hollow particles. In Comparative Example 3, the storage modulus at the temperature at which the raw material mixture started to be roll-kneaded (equivalent to the preheating temperature), i.e., 60°C, was 3.2 MPa, and the storage modulus at the roll-kneading temperature of the raw material mixture, i.e., 80°C, was 2.2 MPa. The storage modulus at the process temperature of the raw material mixture, i.e., 60 to 80°C, was greater than 2.5 MPa throughout the entire process, from the initial to the final stages of roll-kneading. It is believed that this is why the load caused by roll kneading increased from the initial stage to the final stage of roll kneading compared to Example 1, causing many hollow particles to be crushed.

[0172] Examples 4 and 5 are examples in which the base elastomer in the raw material mixture of Example 1 was changed from EPDM to non-oil-extended styrene-butadiene rubber, and part of the plasticizer without reactive sites was replaced with a plasticizer containing reactive sites. The raw material mixture of Example 4 was the same as the raw material mixture of Example 1, except that the base elastomer was changed from EPDM to non-oil-extended styrene-butadiene rubber, the carbon content was changed from 45 parts by mass to 25 parts by mass, the plasticizer (process oil) content was reduced from 75 parts by mass to 65 parts by mass, and 10 parts by mass of a plasticizer containing reactive sites (liquid polybutadiene) was added, and the total amount of the plasticizer and plasticizer containing reactive sites was 75 parts by mass, the same as the plasticizer content in Example 1. The kneading conditions were the same as in Example 1, and the steps from pre-kneading to roll kneading were carried out at a pre-kneading temperature of 100°C, a pre-kneading rotation speed of 50 rpm, a preheating temperature of 60°C, and a roll kneading temperature of 80°C to produce an elastomer composition. As a result, the storage modulus at 60°C of the elastomer composition obtained in Example 4 and the storage modulus at 60°C of the raw material mixture after homogenization were 1.7 MPa, both of which were 2.5 MPa or less. The raw material mixture of Example 5 was the same as that of Example 1, except that the base elastomer was changed from EPDM to non-oil-extended styrene-butadiene rubber, the content of plasticizer (process oil) was reduced from 75 parts by mass to 55 parts by mass, 20 parts by mass of reactive site-containing plasticizer (liquid polybutadiene) was added, and the total amount of plasticizer and reactive site-containing plasticizer was 75 parts by mass, the same as the plasticizer content in Example 1. The kneading conditions were the same as in Example 1, and the steps from pre-kneading to roll kneading were carried out at a pre-kneading temperature of 100°C, a pre-kneading rotation speed of 50 rpm, a preheating temperature of 60°C, and a roll kneading temperature of 80°C to produce an elastomer composition. As a result, the storage modulus at 60°C of the elastomer composition obtained in Example 5 was 1.7 MPa, and the storage modulus at 60°C of the raw material mixture after the homogenization treatment was 1.6 MPa, both of which were 2.5 MPa or less. In Examples 4 and 5, a part of the plasticizer without reactive sites was replaced with a plasticizer containing reactive sites, and therefore, a crosslinked structure may have been formed between the molecules of the base elastomer via the plasticizer containing reactive sites.However, since the amount of the reactive site-containing plasticizer used was small, no significant difference was observed between the storage modulus at 60°C of the raw material mixture in the pre-kneading stage and the storage modulus at 60°C of the resulting elastomer composition.

[0173] Furthermore, when the elastomer compositions obtained in Examples 4 and 5 were press-molded to produce sheet-shaped molded articles, the void retention rate of the hollow particles present in the molded articles was 100%, confirming that the hollow particles were resistant to crushing during the molding process and that molded articles with a high void retention rate were obtained. In Example 4, the storage modulus at the temperature at which the raw material mixture started to be roll-kneaded (equivalent to the preheating temperature), i.e., 60°C, was 1.7 MPa, which was below 2.5 MPa. However, the storage modulus at the roll-kneading temperature of the raw material mixture, i.e., 80°C, was 1.1 MPa, which was even lower than the storage modulus at the start of roll-kneading. It is believed that this suppressed the load caused by roll-kneading, making the hollow particles less likely to be crushed. In Example 5, the storage modulus of the raw material mixture at the start of roll kneading (equivalent to the preheating temperature), i.e., at 60°C, was 1.7 MPa, which was not more than 2.5 MPa, but the storage modulus of the raw material mixture at the roll kneading temperature, i.e., at 80°C, was 1.2 MPa, which was even lower than the storage modulus at the start of roll kneading. This is thought to have suppressed the load due to roll kneading, making the hollow particles less likely to be crushed.

[0174] Examples 6 to 8 are examples in which the base elastomer in the raw material mixture of Example 1 was changed from EPDM to non-oil-extended styrene-butadiene rubber, and the plasticizer without reactive sites was changed to a plasticizer containing reactive sites. The raw material mixture of Example 6 was the same as that of Example 1, except that the base elastomer was changed from EPDM to non-oil-extended styrene-butadiene rubber, and 60 parts by mass of a plasticizer containing reactive sites (liquid polybutadiene) was used instead of 75 parts by mass of the plasticizer (process oil). The kneading conditions were the same as in Example 1, and the steps from pre-kneading to roll kneading were carried out at a pre-kneading temperature of 100°C, a pre-kneading rotation speed of 50 rpm, a preheating temperature of 60°C, and a roll kneading temperature of 80°C to produce the elastomer composition. As a result, the storage modulus at 60°C of the elastomer composition obtained in Example 6 was 2.1 MPa, and the storage modulus at 60°C of the raw material mixture after the homogenization treatment was 1.7 MPa, both of which were 2.5 MPa or less. In Example 6, it is believed that a crosslinked structure was formed between the molecules of the base elastomer via the reactive site-containing plasticizer, which resulted in the storage modulus at 60°C of the obtained elastomer composition being greater than the storage modulus at 60°C of the raw material mixture in the pre-kneading stage.

[0175] Furthermore, when the elastomer composition obtained in Example 6 was press-molded to produce a sheet-like molded article, the void retention rate of the hollow particles present in the molded article was 85%, confirming that the hollow particles were resistant to crushing during the molding process and that a molded article with a high void retention rate was obtained. In Example 6, the storage modulus at the temperature at which the raw material mixture started to be roll-kneaded (equivalent to the preheating temperature), i.e., 60°C, was 1.6 MPa, which was below 2.5 MPa. However, the storage modulus at the roll-kneading temperature of the raw material mixture, i.e., 80°C, was 1.2 MPa, which was even lower than the storage modulus at the start of roll-kneading. It is believed that this suppressed the load due to roll-kneading, making the hollow particles less likely to be crushed.

[0176] The raw material mixture of Example 7 was the same as that of Example 1, except that the base elastomer was changed from EPDM to non-oil-extended styrene-butadiene rubber, and 75 parts by mass of a reactive site-containing plasticizer (liquid polybutadiene) was used instead of 75 parts by mass of a plasticizer (process oil). The kneading conditions were the same as those of Example 1, and the steps from pre-kneading to roll kneading were carried out at a pre-kneading temperature of 100°C, a pre-kneading rotation speed of 50 rpm, a preheating temperature of 60°C, and a roll kneading temperature of 80°C to produce an elastomer composition. The storage modulus at 60°C of the elastomer composition obtained in Example 7 was 1.9 MPa, and the storage modulus at 60°C of the raw material mixture after homogenization was 1.7 MPa, both of which were below 2.5 MPa. In Example 7, the storage modulus of the elastomer composition at 60°C was greater than the storage modulus of the raw material mixture in the pre-mixing stage, but the difference in the increase in storage modulus was smaller than in Example 6. This is thought to be because the amount of reactive site-containing plasticizer used in Example 7 was greater than the amount of reactive site-containing plasticizer used in Example 6. More specifically, the effect of increasing the storage modulus due to the formation of crosslinked structures between the molecules of the base elastomer via the reactive site-containing plasticizer was counterbalanced by the effect of increasing plasticity due to the increased amount of reactive site-containing plasticizer used, which is thought to be why the difference in the increase in storage modulus of the elastomer composition in Example 7 was smaller than in Example 6.

[0177] Furthermore, when the elastomer composition obtained in Example 7 was press-molded to produce a sheet-shaped molded product, the void retention rate of the hollow particles present in the molded product was 90%, confirming that the hollow particles were resistant to crushing during the molding process and that a molded product with a high void retention rate was obtained. In Example 7, the storage modulus at the temperature at which the raw material mixture started to be roll-kneaded (equivalent to the preheating temperature), i.e., 60°C, was 1.6 MPa, which was below 2.5 MPa. However, the storage modulus at the roll-kneading temperature of the raw material mixture, i.e., 80°C, was 1.2 MPa, which was even lower than the storage modulus at the start of roll-kneading. This is thought to have suppressed the load caused by roll-kneading, making the hollow particles less likely to be crushed. The storage modulus of the raw material mixture in Example 7 at the temperature at which roll-kneading started (60°C) and at the roll-kneading temperature of the raw material mixture (80°C) were the same as in Example 6, but the void retention rate of the molded product in Example 7 was 90%, which was higher than the void retention rate of the molded product in Example 6 (85%). The reason for this is thought to be that in the elastomer composition of Example 7, the effect of increasing the storage modulus due to the formation of crosslinked structures between the molecules of the base elastomer via the reactive site-containing plasticizer was counteracted by the effect of increasing the plasticity due to the increased amount of reactive site-containing plasticizer used. In other words, due to the counteracting effects of increasing plasticity, the storage modulus of the raw material mixture rapidly decreased in the process of increasing the roll-kneading temperature of the raw material mixture from 60°C to 80°C, and therefore the load due to roll-kneading was suppressed more than in Example 6, and the hollow particles were less likely to be crushed.

[0178] The raw material mixture of Example 8 was the same as that of Example 1, except that the base elastomer was changed from EPDM to non-oil-extended styrene-butadiene rubber, and 80 parts by mass of a reactive site-containing plasticizer (liquid polybutadiene) was used instead of 75 parts by mass of a plasticizer (process oil). The kneading conditions were the same as those of Example 1, and the steps from pre-kneading to roll kneading were carried out at a pre-kneading temperature of 100°C, a pre-kneading rotation speed of 50 rpm, a preheating temperature of 60°C, and a roll kneading temperature of 80°C to produce an elastomer composition. The storage modulus at 60°C of the elastomer composition obtained in Example 8 was 1.6 MPa, and the storage modulus at 60°C of the raw material mixture after homogenization was 1.7 MPa, both of which were below 2.5 MPa. In Example 8, a larger amount of reactive site-containing plasticizer was used than in Example 7, but the storage modulus of the resulting elastomer composition at 60°C was as low as that of Examples 4 and 5, in which a smaller amount of reactive site-containing plasticizer was used. The reason for this is thought to be that the reaction in which a crosslinked structure is formed between the molecules of the base elastomer via the reactive site-containing plasticizer became saturated, and the effect of increasing the storage modulus reached its upper limit, whereas the content of the reactive site-containing plasticizer in a free state that was not bonded to the base elastomer increased, and the effect of increasing plasticity became dominant.

[0179] Furthermore, when the elastomer composition obtained in Example 8 was press-molded to produce a sheet-like molded article, the void retention rate of the hollow particles present in the molded article was 100%, confirming that the hollow particles were resistant to crushing during the molding process and that a molded article with a high void retention rate was obtained. In Example 8, the storage modulus at the temperature at which the raw material mixture started to be roll-kneaded (equivalent to the preheating temperature), i.e., 60°C, was 1.7 MPa, which was below 2.5 MPa. However, the storage modulus at the roll-kneading temperature of the raw material mixture, i.e., 80°C, was 1.1 MPa, which was even lower than the storage modulus at the start of roll-kneading. It is believed that this suppressed the load due to roll-kneading, making the hollow particles less likely to be crushed. The storage modulus of the raw material mixture in Example 8 at the start of roll kneading (60°C) and at the roll kneading temperature (80°C) were almost the same as those in Examples 6 and 7, but the void residual rate of the molded body in Example 8 was 100%, which was higher than the void residual rate of the molded bodies in Examples 6 and 7. This is thought to be because, in the elastomer composition of Example 8, the effect of increasing plasticity due to the increased amount of reactive site-containing plasticizer used was dominant over the effect of increasing the storage modulus due to the formation of crosslinked structures between the base elastomer molecules via the reactive site-containing plasticizer. In other words, because the effect of increasing plasticity became dominant, the storage modulus of the raw material mixture rapidly decreased as the roll kneading temperature of the raw material mixture increased from 60°C to 80°C. Therefore, the load due to roll kneading was suppressed more than in Examples 6 and 7, and the hollow particles were less likely to be crushed.

[0180] Examples 9 to 11 are examples in which the base elastomer in the raw material mixture of Example 1 was changed from EPDM to oil-extended styrene-butadiene rubber, and the plasticizer without reactive sites was changed to a plasticizer containing reactive sites. The raw material mixture of Example 9 was the same as that of Example 1, except that the base elastomer was changed from EPDM to oil-extended styrene-butadiene rubber, and 60 parts by mass of a plasticizer containing reactive sites (liquid polybutadiene) was used instead of 75 parts by mass of the plasticizer (process oil). The kneading conditions were the same as in Example 1, and the steps from pre-kneading to roll kneading were carried out at a pre-kneading temperature of 100°C, a pre-kneading rotation speed of 50 rpm, a preheating temperature of 60°C, and a roll kneading temperature of 80°C to produce the elastomer composition. As a result, the storage modulus at 60°C of the elastomer composition obtained in Example 9 was 2.2 MPa, and the storage modulus at 60°C of the raw material mixture after the homogenization treatment was 1.6 MPa, both of which were 2.5 MPa or less. In Example 9, it is believed that a crosslinked structure was formed between the molecules of the base elastomer via the reactive site-containing plasticizer, which resulted in the storage modulus at 60°C of the obtained elastomer composition being greater than the storage modulus at 60°C of the raw material mixture in the pre-kneading stage.

[0181] Furthermore, when a sheet-like molded article was produced by press-molding the elastomer composition obtained in Example 9, the void retention rate of the hollow particles present in the molded article was 87%, confirming that the hollow particles were less likely to be crushed during the molding process and that a molded article with a high void retention rate was obtained. In Example 9, the storage modulus at the temperature at which the raw material mixture started to be roll-kneaded (equivalent to the preheating temperature), i.e., 60°C, was 1.7 MPa, which was below 2.5 MPa. However, the storage modulus at the roll-kneading temperature of the raw material mixture, i.e., 80°C, was 1.2 MPa, which was even lower than the storage modulus at the start of roll-kneading. It is believed that this suppressed the load due to roll-kneading, making the hollow particles less likely to be crushed.

[0182] The raw material mixture of Example 10 was the same as that of Example 1, except that the base elastomer was changed from EPDM to oil-extended styrene-butadiene rubber, and 75 parts by mass of a reactive site-containing plasticizer (liquid polybutadiene) was used instead of 75 parts by mass of a plasticizer (process oil). The kneading conditions were the same as in Example 1, and the steps from pre-kneading to roll kneading were carried out at a pre-kneading temperature of 100°C, a pre-kneading rotation speed of 50 rpm, a preheating temperature of 60°C, and a roll kneading temperature of 80°C to produce an elastomer composition. The storage modulus at 60°C of the elastomer composition obtained in Example 10 was 1.9 MPa, and the storage modulus at 60°C of the raw material mixture after homogenization was 1.7 MPa, both of which were below 2.5 MPa. In Example 10, the storage modulus of the elastomer composition at 60°C was greater than the storage modulus of the raw material mixture in the pre-mixing stage, but the difference in the increase in storage modulus was smaller than in Example 9. This is thought to be because the amount of reactive site-containing plasticizer used in Example 10 was greater than the amount of reactive site-containing plasticizer used in Example 9. More specifically, the effect of increasing the storage modulus due to the formation of a crosslinked structure between the molecules of the base elastomer via the reactive site-containing plasticizer was countered by the effect of increasing plasticity due to the increased amount of reactive site-containing plasticizer used, which is thought to be the reason why the difference in the increase in storage modulus was smaller than in Example 9.

[0183] Furthermore, when a sheet-shaped molded article was produced by press-molding the elastomer composition obtained in Example 10, the void retention rate of the hollow particles present in the molded article was 90%, confirming that the hollow particles were resistant to crushing during the molding process and that a molded article with a high void retention rate was obtained. In Example 10, the storage modulus at the temperature at which the raw material mixture started to be roll-kneaded (equivalent to the preheating temperature), i.e., 60°C, was 1.6 MPa, which was below 2.5 MPa. However, the storage modulus at the roll-kneading temperature of the raw material mixture, i.e., 80°C, was 1.2 MPa, which was even lower than the storage modulus at the start of roll-kneading. It is believed that this suppressed the load due to roll-kneading, making the hollow particles less likely to be crushed. The storage modulus of the raw material mixture in Example 10 at the start of roll kneading (60°C) and at the roll kneading temperature (80°C) were almost the same as those in Example 9, but the void residual rate of the molded body in Example 10 was 90%, which was higher than the void residual rate (87%) of the molded body in Example 9. This is thought to be because the effect of increasing the storage modulus of the elastomer composition in Example 10 due to the formation of crosslinked structures between the base elastomer molecules via the reactive site-containing plasticizer was counterbalanced by the effect of increasing plasticity due to the increased amount of reactive site-containing plasticizer used. In other words, due to the counterbalanced effect of increasing plasticity, the storage modulus of the raw material mixture rapidly decreased as the roll kneading temperature of the raw material mixture increased from 60°C to 80°C. Therefore, the load due to roll kneading was suppressed more than in Example 9, and the hollow particles were less likely to be crushed.

[0184] The raw material mixture of Example 11 was prepared by replacing the base elastomer in the raw material mixture of Example 1 with an oil-extended styrene-butadiene rubber instead of EPDM, and replacing 75 parts by weight of the plasticizer (process oil) with 80 parts by weight of a reactive site-containing plasticizer (liquid polybutadiene). The kneading conditions were the same as in Example 1, with the steps from pre-kneading to roll kneading being carried out at a pre-kneading temperature of 100°C, a pre-kneading rotation speed of 50 rpm, a preheating temperature of 60°C, and a roll kneading temperature of 80°C to produce an elastomer composition. The storage modulus at 60°C of the elastomer composition obtained in Example 11 was 1.6 MPa, and the storage modulus at 60°C of the raw material mixture after homogenization was 1.7 MPa, both of which were below 2.5 MPa. Despite the use of a larger amount of reactive site-containing plasticizer in Example 11 than in Example 10, the resulting elastomer composition had a lower storage modulus at 60°C than Example 10. The reason for this is thought to be that the reaction of forming a crosslinked structure between the molecules of the base elastomer via the plasticizer containing reactive sites reaches saturation, and the effect of increasing the storage modulus reaches its upper limit, while the content of the free plasticizer containing reactive sites that is not bonded to the base elastomer increases, and the effect of increasing plasticity becomes dominant.

[0185] Furthermore, when the elastomer composition obtained in Example 11 was press-molded to produce a sheet-like molded article, the void retention rate of the hollow particles present in the molded article was 100%, confirming that the hollow particles were resistant to crushing during the molding process and that a molded article with a high void retention rate was obtained. In Example 11, the storage modulus at the temperature at which the raw material mixture started to be roll-kneaded (equivalent to the preheating temperature), i.e., 60°C, was 1.7 MPa, which was below 2.5 MPa. However, the storage modulus at the roll-kneading temperature of the raw material mixture, i.e., 80°C, was 1.2 MPa, which was even lower than the storage modulus at the start of roll-kneading. It is believed that this suppressed the load due to roll-kneading, making the hollow particles less likely to be crushed. The storage modulus of the raw material mixture in Example 11 at the start of roll kneading (60°C) and at the roll kneading temperature (80°C) were almost the same as those in Examples 9 and 10. However, the void residual rate of the molded body in Example 11 was 100%, which was higher than the void residual rates of the molded bodies in Examples 9 and 10. This is thought to be because, in the elastomer composition of Example 11, the effect of increasing plasticity due to the increased amount of reactive site-containing plasticizer used was dominant over the effect of increasing the storage modulus due to the formation of crosslinked structures between the base elastomer molecules via the reactive site-containing plasticizer. In other words, because the effect of increasing plasticity became dominant, the storage modulus of the raw material mixture rapidly decreased as the roll kneading temperature of the raw material mixture increased from 60°C to 80°C. Therefore, the load due to roll kneading was suppressed compared to Examples 9 and 10, and the hollow particles were less likely to be crushed.

[0186] Examples 12 to 14 are examples in which the base elastomer in the raw material mixture of Example 1 was changed from EPDM to butadiene rubber, and the plasticizer without reactive sites was changed to a reactive site-containing plasticizer. The raw material mixture of Example 12 was the same as the raw material mixture of Example 1, except that the base elastomer was changed from EPDM to butadiene rubber, and 60 parts by mass of a reactive site-containing plasticizer (liquid polybutadiene) was used instead of 75 parts by mass of the plasticizer (process oil). The kneading conditions were the same as in Example 1, and the steps from pre-kneading to roll kneading were carried out at a pre-kneading temperature of 100°C, a pre-kneading rotation speed of 50 rpm, a preheating temperature of 60°C, and a roll kneading temperature of 80°C to produce the elastomer composition. The storage modulus at 60°C of the elastomer composition obtained in Example 12 was 2.5 MPa, and the storage modulus at 60°C of the raw material mixture after the homogenization treatment was 1.7 MPa, both of which were below 2.5 MPa. In Example 12, a crosslinked structure was formed between the molecules of the base elastomer via the reactive site-containing plasticizer, which is thought to have resulted in a larger storage modulus at 60°C of the resulting elastomer composition than the storage modulus at 60°C of the raw material mixture in the pre-mixing stage.

[0187] Furthermore, when the elastomer composition obtained in Example 12 was press-molded to produce a sheet-like molded article, the void retention rate of the hollow particles present in the molded article was 85%, confirming that the hollow particles were resistant to crushing during the molding process and that a molded article with a high void retention rate was obtained. In Example 12, the storage modulus at the temperature at which the raw material mixture started to be roll-kneaded (equivalent to the preheating temperature), i.e., 60°C, was 1.6 MPa, which was below 2.5 MPa. However, the storage modulus at the roll-kneading temperature of the raw material mixture, i.e., 80°C, was 1.2 MPa, which was even lower than the storage modulus at the start of roll-kneading. It is believed that this suppressed the load due to roll-kneading, making the hollow particles less likely to be crushed.

[0188] The raw material mixture of Example 13 was the same as that of Example 1, except that the base elastomer was changed from EPDM to butadiene rubber, and 75 parts by mass of a reactive site-containing plasticizer (liquid polybutadiene) was used instead of 75 parts by mass of a plasticizer (process oil). The kneading conditions were the same as those of Example 1, and the steps from pre-kneading to roll kneading were carried out at a pre-kneading temperature of 100°C, a pre-kneading rotation speed of 50 rpm, a preheating temperature of 60°C, and a roll kneading temperature of 80°C to produce an elastomer composition. The storage modulus at 60°C of the elastomer composition obtained in Example 13 was 2.3 MPa, and the storage modulus at 60°C of the raw material mixture after homogenization was 1.7 MPa, both of which were 2.5 MPa or less. In Example 13, the storage modulus of the elastomer composition at 60°C was greater than the storage modulus of the raw material mixture in the pre-kneading stage, but the difference in the increase in storage modulus was smaller than that in Example 12. This is thought to be because the amount of reactive site-containing plasticizer used in Example 13 was greater than that in Example 12. More specifically, the effect of increasing the storage modulus due to the formation of crosslinked structures between the molecules of the base elastomer via the reactive site-containing plasticizer was countered by the effect of increasing plasticity due to the increased amount of reactive site-containing plasticizer used, which is thought to be the reason why the difference in the increase in storage modulus was smaller than that in Example 12.

[0189] Furthermore, when the elastomer composition obtained in Example 13 was press-molded to produce a sheet-like molded article, the void retention rate of the hollow particles present in the molded article was 90%, confirming that the hollow particles were resistant to crushing during the molding process and that a molded article with a high void retention rate was obtained. In Example 13, the storage modulus at the temperature at which the raw material mixture started to be roll-kneaded (equivalent to the preheating temperature), i.e., 60°C, was 1.7 MPa, which was below 2.5 MPa. However, the storage modulus at the roll-kneading temperature of the raw material mixture, i.e., 80°C, was 1.2 MPa, which was even lower than the storage modulus at the start of roll-kneading. It is believed that this suppressed the load due to roll-kneading, making the hollow particles less likely to be crushed. The storage modulus of the raw material mixture in Example 13 at the start of roll kneading (60°C) and at the roll kneading temperature (80°C) were almost the same as those in Example 12, but the void remaining rate of the molded body in Example 13 was 90%, which was higher than the void remaining rate (85%) of the molded body in Example 12. This is thought to be because the effect of increasing the storage modulus of the elastomer composition in Example 13 due to the formation of crosslinked structures between the base elastomer molecules via the reactive site-containing plasticizer was counterbalanced by the effect of increasing plasticity due to the increased amount of reactive site-containing plasticizer used. In other words, due to the counterbalanced effect of increasing plasticity, the storage modulus of the raw material mixture rapidly decreased as the roll kneading temperature of the raw material mixture increased from 60°C to 80°C. This suggests that the load due to roll kneading was more suppressed than in Example 12, and the hollow particles were less likely to be crushed.

[0190] The raw material mixture of Example 14 was prepared by replacing the base elastomer in the raw material mixture of Example 1 with butadiene rubber instead of EPDM, and replacing 75 parts by mass of the plasticizer (process oil) with 80 parts by mass of a reactive site-containing plasticizer (liquid polybutadiene). The kneading conditions were the same as in Example 1, with the steps from pre-kneading to roll kneading being carried out at a pre-kneading temperature of 100°C, a pre-kneading rotation speed of 50 rpm, a preheating temperature of 60°C, and a roll kneading temperature of 80°C to produce an elastomer composition. The storage modulus at 60°C of the elastomer composition obtained in Example 14 was 2.2 MPa, and the storage modulus at 60°C of the raw material mixture after homogenization was 1.6 MPa, both of which were below 2.5 MPa. Despite the use of a larger amount of reactive site-containing plasticizer in Example 14 than in Example 13, the storage modulus at 60°C of the resulting elastomer composition was lower than that of Example 13. The reason for this is thought to be that the reaction of forming a crosslinked structure between the molecules of the base elastomer via the plasticizer containing reactive sites reaches saturation, and the effect of increasing the storage modulus reaches its upper limit, while the content of the free plasticizer containing reactive sites that is not bonded to the base elastomer increases, and the effect of increasing plasticity becomes dominant.

[0191] Furthermore, when the elastomer composition obtained in Example 14 was press-molded to produce a sheet-like molded article, the void retention rate of the hollow particles present in the molded article was 100%, confirming that the hollow particles were resistant to crushing during the molding process and that a molded article with a high void retention rate was obtained. In Example 14, the storage modulus at the temperature at which the raw material mixture started to be roll-kneaded (equivalent to the preheating temperature), i.e., 60°C, was 1.7 MPa, which was below 2.5 MPa. However, the storage modulus at the roll-kneading temperature of the raw material mixture, i.e., 80°C, was 1.2 MPa, which was even lower than the storage modulus at the start of roll-kneading. It is believed that this suppressed the load due to roll-kneading, making the hollow particles less likely to be crushed. The storage modulus of the raw material mixture in Example 14 at the start of roll kneading (60°C) and at the roll kneading temperature (80°C) were almost the same as those in Examples 12 and 13. However, the void residual rate of the molded body in Example 14 was 100%, which was higher than the void residual rates of the molded bodies in Examples 12 and 13. This is thought to be because, in the elastomer composition of Example 14, the effect of increasing plasticity due to the increased amount of reactive site-containing plasticizer used was dominant over the effect of increasing the storage modulus due to the formation of crosslinked structures between the base elastomer molecules via the reactive site-containing plasticizer. In other words, because the effect of increasing plasticity became dominant, the storage modulus of the raw material mixture rapidly decreased as the roll kneading temperature of the raw material mixture increased from 60°C to 80°C. Therefore, the load due to roll kneading was suppressed compared to Examples 12 and 13, and the hollow particles were less likely to be crushed.

[0192] REFERENCE SIGNS LIST 1 aqueous medium 2 low-polarity material 3 dispersion stabilizer 4 monomer composition 4a hydrophobic solvent 4b material other than hydrophobic solvent 4c polymerizable monomer dispersed in aqueous medium 5 oil-soluble polymerization initiator 6 shell 7 hollow portion 8 droplet 9 precursor particle 10 hollow particle with hollow portion filled with gas

Claims

1. It comprises at least a base elastomer and hollow particles, The hollow particle comprises a shell containing a resin and a hollow portion surrounded by the shell, wherein the shell is a hollow particle containing a polymer in which 50 parts by mass or more of crosslinkable monomer units are present in 100 parts by mass of total monomer units. A composition having a storage modulus G' at 60°C of 2.5 MPa or less, obtained by dynamic viscoelasticity measurement, A hollow particle-containing elastomer composition in which a sheet-like molded article is prepared using the composition according to the method for measuring the void retention rate of a hollow particle-containing elastomer molded article described below, and the measured void retention rate is 80% or more. [Method for measuring the void retention rate of elastomer molded articles containing hollow particles] A sheet-like molded article of hollow particle-containing elastomer is produced by press-molding a hollow particle-containing elastomer composition at a pressure of 1 MPa or less using a hot press at 120°C. The specific gravity of the obtained elastomer molded body is measured, and the void retention rate of hollow particles within the elastomer molded body is calculated according to the following formula (D). Vacancy remaining rate (%) = {(ca) / (c-b)}×100 Formula (D) a: Specific gravity of the sheet-like molded body after pressing, b: Specific gravity of the molded body assuming that voids are maintained (calculated value) c: Specific gravity of the molded body assuming all hollow particles are crushed (calculated value)

2. The hollow particle-containing elastomer composition according to claim 1, wherein the void retention rate of the hollow particles, as measured according to the following press test method, is 80% or more. [Press Test Method] A mixture of polypropylene resin and hollow particles, with a mass ratio of polypropylene resin to hollow particles of 90:10, is melted and mixed at 200°C. The mixture is then placed in a mold for a hot press and heated further at 200°C for 15 minutes, after which it is stirred. Next, it is placed in a hot press set to 80°C, a cylinder heated to 80°C is placed in the mold, and when the surface temperature of the mold reaches 140°C, it is pressurized at 15 MPa. After that, the mixture is removed from the mold and molded into a sheet by pressurizing it at a pressure of 1 MPa or less using a hot press set to 200°C. The specific gravity of the obtained sheet-like molded body is measured, and the void retention rate of the hollow particles is calculated according to the following formula (D). Vacancy remaining rate (%) = {(ca) / (c-b)}×100 Formula (D) The symbols in equation (D) have the following meanings: a: Specific gravity of the sheet-like molded body after press molding, b: Specific gravity of the molded body assuming that voids are maintained (calculated value) c: Specific gravity of the molded body assuming all hollow particles are crushed (calculated value)

3. The hollow particle-containing elastomer composition according to claim 1, wherein the porosity of the hollow particles is 50% or more.

4. The hollow particle-containing elastomer composition according to claim 1, wherein the volume-average particle size of the hollow particles is 5.0 μm or more and 40.0 μm or less.

5. The hollow particle-containing elastomer composition according to claim 1, wherein the polymer contains, as the crosslinkable monomer units, three- or more crosslinkable monomer units derived from a three- or more crosslinkable monomer, and the content of the three- or more crosslinkable monomer units is 10 parts by mass or more and 50 parts by mass or less in 100 parts by mass of the total monomer units of the polymer.

6. The hollow particle-containing elastomer composition according to claim 1, wherein the polymer contains, as the crosslinkable monomer units, a difunctional crosslinkable monomer unit derived from a difunctional crosslinkable monomer and a trifunctional or more crosslinkable monomer unit derived from a trifunctional or more crosslinkable monomer.

7. The hollow particle-containing elastomer composition according to claim 6, wherein, in a total of 100 parts by mass of the bifunctional crosslinkable monomer units and the trifunctional or more crosslinkable monomer units, the content of the trifunctional or more crosslinkable monomer units is 5 parts by mass or more and 40 parts by mass or less.

8. The hollow particle-containing elastomer composition according to claim 1, wherein the base elastomer is at least one selected from the group consisting of ethylene-α-olefin-non-conjugated diene copolymer rubber, butadiene rubber, styrene-butadiene rubber, natural rubber, isoprene rubber, nitrile rubber, hydrogenated nitrile rubber, butyl rubber, fluororubber, silicone rubber, acrylonitrile butadiene rubber, chloroprene rubber, acrylic rubber, chlorosulfonated polyethylene rubber, chlorinated polyethylene rubber, urethane rubber, isobutylene isoprene rubber, polysulfide rubber, propylene oxide rubber, and epichlorohydrin rubber.

9. The hollow particle-containing elastomer composition according to claim 8, wherein at least a portion of the base elastomer is at least one selected from the group consisting of ethylene-α-olefin-nonconjugated diene copolymer rubber, butadiene rubber, and styrene-butadiene rubber.

10. The hollow particle-containing elastomer composition according to claim 1, comprising 35 to 100 parts by mass of a plasticizer per 100 parts by mass of a base elastomer.

11. The hollow particle-containing elastomer composition according to claim 10, wherein at least a portion of the base elastomer is at least one selected from the group consisting of butadiene rubber and styrene-butadiene rubber, and at least a portion of the plasticizer is a polymer having reactive active sites that bind to the base elastomer and having a weight-average molecular weight of 1,000 or more and 100,000 or less.

12. The hollow particle-containing elastomer composition according to claim 1, wherein the Mooney viscosity (ML(1+4)100℃) of the base elastomer, as measured in accordance with JIS K6300, is 20 or more and 75 or less.

13. The hollow particle-containing elastomer composition according to claim 1, wherein the content of styrene monomer units contained in the base elastomer is 0% by mass or more and 60% by mass or less.

14. A method for producing a hollow particle-containing elastomer composition comprising at least a base elastomer and hollow particles, A raw material mixture is prepared comprising at least a base elastomer, a shell containing a resin, and a hollow portion surrounded by the shell, wherein the shell contains hollow particles containing a polymer in which 50 parts by mass or more of crosslinkable monomer units are included in 100 parts by mass of total monomer units, and the storage modulus G' at 60°C, obtained by dynamic viscoelasticity measurement performed after homogenization treatment of the compounding components, is 2.5 MPa or less. The raw material mixture is pre-kneaded using a closed kneader at a temperature at which the storage modulus G' obtained by dynamic viscoelasticity measurement performed after the homogenization treatment is 2.5 MPa or less. A method for producing a hollow particle-containing elastomer composition, comprising pre-kneading the raw material mixture, then pre-heating it to a temperature at which the storage modulus G' obtained by dynamic viscoelasticity measurement performed after the homogenization treatment is 2.5 MPa or less, and then kneading it to a temperature at which the storage modulus G' obtained by dynamic viscoelasticity measurement performed after the homogenization treatment is 2.5 MPa or less.

15. The method for producing a hollow particle-containing elastomer composition according to claim 14, wherein in the preliminary kneading, the raw material mixture is pre-kneaded using a closed-type kneader at a kneading temperature of 100°C or higher.

16. A method for producing a hollow particle-containing elastomer composition according to claim 14, wherein, in the case where preheating is not performed, kneading of the raw material mixture is started within 10 minutes after pre-kneading.

17. A method for producing a hollow particle-containing elastomer composition according to claim 14, wherein the preheating temperature is 60°C or higher in the preheating step.

18. A method for producing a hollow particle-containing elastomer composition according to claim 14, wherein the kneading temperature is 60°C or higher during the kneading process.

19. A method for producing a hollow particle-containing elastomer composition according to claim 14, wherein the kneading step is performed by roll kneading.

20. A method for producing a hollow particle-containing elastomer composition according to claim 14, wherein the void retention rate of the hollow particles, as measured according to the following press test method, is 80% or more. [Press Test Method] A mixture of polypropylene resin and hollow particles, with a mass ratio of polypropylene resin to hollow particles of 90:10, is melted and mixed at 200°C. The mixture is then placed in a mold for a hot press and heated further at 200°C for 15 minutes, after which it is stirred. Next, it is placed in a hot press set to 80°C, a cylinder heated to 80°C is placed in the mold, and when the surface temperature of the mold reaches 140°C, it is pressurized at 15 MPa. After that, the mixture is removed from the mold and molded into a sheet by pressurizing it at a pressure of 1 MPa or less using a hot press set to 200°C. The specific gravity of the obtained sheet-like molded body is measured, and the void retention rate of the hollow particles is calculated according to the following formula (D). Vacancy remaining rate (%) = {(ca) / (c-b)}×100 Formula (D) The symbols in equation (D) have the following meanings: a: Specific gravity of the sheet-like molded body after press molding, b: Specific gravity of the molded body assuming that voids are maintained (calculated value) c: Specific gravity of the molded body assuming all hollow particles are crushed (calculated value)

21. A method for producing the hollow particle-containing elastomer composition according to claim 14, wherein the porosity of the hollow particles is 50% or more.

22. A method for producing a hollow particle-containing elastomer composition according to claim 14, wherein the volume-average particle size of the hollow particles is 5.0 μm or more and 40.0 μm or less.

23. A method for producing a hollow particle-containing elastomer composition according to claim 14, wherein the polymer contains, as the crosslinkable monomer units, three- or more crosslinkable monomer units derived from a three- or more crosslinkable monomer, and the content of the three- or more crosslinkable monomer units is 10 parts by mass or more and 50 parts by mass or less in 100 parts by mass of the total monomer units of the polymer.

24. A method for producing a hollow particle-containing elastomer composition according to claim 14, wherein the polymer contains, as the crosslinkable monomer units, a difunctional crosslinkable monomer unit derived from a difunctional crosslinkable monomer and a trifunctional or more crosslinkable monomer unit derived from a trifunctional or more crosslinkable monomer.

25. A method for producing a hollow particle-containing elastomer composition according to claim 24, wherein, in a total of 100 parts by mass of the bifunctional crosslinkable monomer units and the trifunctional or more crosslinkable monomer units, the content of the trifunctional or more crosslinkable monomer units is 5 parts by mass or more and 40 parts by mass or less.

26. A method for producing a hollow particle-containing elastomer composition according to claim 14, wherein the base elastomer is at least one selected from the group consisting of ethylene-α-olefin-non-conjugated diene copolymer rubber, butadiene rubber, styrene-butadiene rubber, natural rubber, isoprene rubber, nitrile rubber, hydrogenated nitrile rubber, butyl rubber, fluororubber, silicone rubber, acrylonitrile butadiene rubber, chloroprene rubber, acrylic rubber, chlorosulfonated polyethylene rubber, chlorinated polyethylene rubber, urethane rubber, isobutylene isoprene rubber, polysulfide rubber, propylene oxide rubber, and epichlorohydrin rubber.

27. A method for producing a hollow particle-containing elastomer composition according to claim 26, wherein at least a portion of the base elastomer is at least one selected from the group consisting of ethylene-α-olefin-nonconjugated diene copolymer rubber, butadiene rubber, and styrene-butadiene rubber.

28. The method for producing a hollow particle-containing elastomer composition according to claim 14, wherein the raw material mixture contains 35 to 100 parts by mass of a plasticizer per 100 parts by mass of a base elastomer.

29. A method for producing a hollow particle-containing elastomer composition according to claim 28, wherein at least a portion of the base elastomer is at least one selected from the group consisting of butadiene rubber and styrene-butadiene rubber, and at least a portion of the plasticizer is a polymer having reactive active sites that bind to the base elastomer and having a weight-average molecular weight of 1,000 or more and 100,000 or less.

30. A method for producing a hollow particle-containing elastomer composition according to claim 14, wherein the Mooney viscosity (ML(1+4)100℃) of the base elastomer, as measured in accordance with JIS K6300, is 20 or more and 75 or less.

31. A method for producing a hollow particle-containing elastomer composition according to claim 14, wherein the content ratio of styrene monomer units contained in the base elastomer is 0% by mass or more and 60% by mass or less.