Nitrile rubber composition and crosslinked molded body

Abstract

The purpose is to provide a nitrile rubber composition with which it is possible to provide a lightweight crosslinked molded body excellent in compression set resistance and oil resistance and having moderate hardness. Provided is a nitrile rubber composition containing a nitrile rubber, a shell containing a resin, and hollow particles having a hollow part surrounded by the shell, wherein the nitrile rubber has an iodine value of 2-500 g / 100 g, the hollow particles have an iodine value of 2-100 g / 100 g, and the iodine value is a value measured according to JIS K0070.

Description

Nitrile rubber composition and crosslinked molded product

[0001] The present disclosure relates to a nitrile rubber composition and a crosslinked molded article.

[0002] Nitrile rubber (acrylonitrile-butadiene copolymer rubber) has traditionally been used in a wide range of rubber parts such as hoses, tubes, seals, belts, hoses, diaphragms, etc., taking advantage of its oil resistance, mechanical properties, chemical resistance, etc. There is a demand for a nitrile rubber composition that can improve the properties of nitrile rubber while meeting the demand for lighter rubber parts.

[0003] For example, Patent Document 1 describes a sealing member for mechanical seals which is a peroxide vulcanization molded product of hydrogenated nitrile rubber having a bound acrylonitrile content of less than 20% and an iodine value of 20 mg / 100 g or less.

[0004] International Publication No. 2019 / 058840

[0005] An object of the present invention is to provide a nitrile rubber composition which is excellent in compression set resistance and oil resistance, and which can give a lightweight crosslinked molded article having a suitable hardness.

[0006] The present inventors have conducted extensive research to solve the above-mentioned problems and have found that the above-mentioned problems can be solved by using a combination of hollow particles having an iodine value of 2 g / 100 g or more and 100 g / 100 g or less and nitrile rubber having an iodine value of 2 g / 100 g or more and 500 g / 100 g or less, and have thus completed the present invention.

[0007] That is, according to the present disclosure, the following nitrile rubber composition and crosslinked molded article are provided.

[0008] [1] A nitrile rubber composition containing a nitrile rubber and hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, wherein the iodine value of the nitrile rubber is 2 g / 100 g or more and 500 g / 100 g or less, and the iodine value of the hollow particles is 2 g / 100 g or more and 100 g / 100 g or less, the iodine value being a value measured in accordance with JIS K 0070. [2] The nitrile rubber composition according to [1], further containing a liquid polymer having reaction active sites. [3] The nitrile rubber composition according to [2], wherein the content of the liquid polymer is 1 part by mass or more and 20 parts by mass or less per 100 parts by mass of the nitrile rubber. [4] The nitrile rubber composition according to any one of [1] to [3], wherein the Mooney viscosity ML(1+4,100°C) of the nitrile rubber is 10 to 200. [5] The nitrile rubber composition according to any one of [1] to [4], wherein the nitrile rubber contains α,β-ethylenically unsaturated nitrile monomer units and conjugated diene monomer units. [6] The nitrile rubber composition according to any one of [1] to [5], wherein the content of the hollow particles is 1 part by mass or more and 100 parts by mass or less per 100 parts by mass of the nitrile rubber. [7] The apparent density of the hollow particles is 0.1 g / cm ３ 0.8g / cm or more ３ The nitrile rubber composition according to any one of [1] to [6], wherein the hollow particles have a volume average particle size of 0.1 μm or more and 100 μm or less. [9] The nitrile rubber composition according to any one of [1] to [8], wherein the nitrile rubber has a content of α,β-ethylenically unsaturated nitrile monomer units of 10 to 60 mass%.

[10] The nitrile rubber composition according to [2] to [9], wherein the liquid polymer is a liquid polymer having an ethylenic double bond.

[11] The nitrile rubber composition according to any one of [2] to

[10] , wherein the liquid polymer has a number average molecular weight (Mw) of 500 to 100,000.

[12] The nitrile rubber composition according to any one of [1] to

[11] , further comprising a crosslinking agent.

[13] A crosslinked molded article obtained by crosslinking the nitrile rubber composition according to any one of [1] to

[12] .

[0009] According to the present invention, it is possible to provide a nitrile rubber composition which is excellent in compression set resistance and oil resistance, and which can give a lightweight crosslinked molded article having an appropriate hardness.

[0010] FIG. 1 is a schematic diagram showing an example of a method for producing hollow particles used in the present disclosure.

[0011] 1. Hollow Particles The hollow particles used in the present disclosure are hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, and are characterized in that the iodine value measured in accordance with JIS K 0070 is 2 g / 100 g or more and 100 g / 100 g or less. Note that in the present disclosure, the iodine value measured in accordance with JIS K 0070 may sometimes be simply referred to as the iodine value.

[0012] The iodine value of hollow particles can be used as an indicator of the amount of reactive unsaturated bonds present on the outer surface of the hollow particles. The hollow particles used in the present disclosure have an iodine value of 2 g / 100 g or more and 100 g / 100 g or less, and therefore have an appropriate amount of reactive unsaturated bonds on the outer surface. When such hollow particles are mixed and kneaded with a nitrile rubber having an iodine value of 2 g / 100 g or more and 500 g / 100 g or less and crosslinked to obtain a crosslinked molded product, the reactive unsaturated bonds present on the outer surface of the hollow particles react with the reactive unsaturated bonds of the nitrile rubber to form covalent bonds, at least during the crosslinking process. Therefore, in the crosslinked molded product obtained from the nitrile rubber composition of the present disclosure, the outer surfaces of the hollow particles are appropriately crosslinked with the nitrile rubber, resulting in excellent adhesion at the interface between the hollow particles and the nitrile rubber, and the interface between the hollow particles and the nitrile rubber is less likely to peel. Furthermore, the hollow particles used in the present disclosure have a high restoring force when an external force is applied and are less likely to undergo plastic deformation. Therefore, when an external force is applied to a crosslinked molded article obtained from the nitrile rubber composition of the present disclosure, it is presumed that the nitrile rubber follows the restoration of the hollow particles, and it is considered that such an interaction between the hollow particles and the nitrile rubber realizes extremely excellent compression set resistance.

[0013] Furthermore, in the crosslinked molded article obtained from the nitrile rubber composition of the present disclosure, the hollow particles are resistant to crushing and easily maintain voids, resulting in an excellent weight-reducing effect due to the hollow particles. It is believed that the reason the hollow particles are resistant to crushing in the resulting crosslinked molded article is that a three-dimensional crosslinked structure is formed near the surface of the hollow particles after the reactive unsaturated bonds on the surface of the hollow particles react with the reactive unsaturated bonds in the nitrile rubber. It is believed that this three-dimensional crosslinked structure improves the Young's modulus of the resulting crosslinked molded article, making it possible to reduce compression set and improve abrasion resistance. Furthermore, it is believed that this three-dimensional crosslinked structure provides a synergistic effect between the oil resistance of the nitrile rubber and the oil resistance of the hollow particles, resulting in a crosslinked molded article with excellent oil resistance.

[0014] The iodine value of the hollow particles used in the present disclosure is not particularly limited as long as it is 2 g / 100 g or more and 100 g / 100 g or less, but is preferably 5 g / 100 g or more, more preferably 10 g / 100 g or more, and is preferably 95 g / 100 g or less, more preferably 90 g / 100 g or less. When the iodine value of the hollow particles is within the above range, it becomes possible to suitably form crosslinks between the hollow particles and the nitrile rubber while suitably dispersing the hollow particles in the nitrile rubber, thereby making the effects of the present disclosure more pronounced.

[0015] From the viewpoint of further reducing the compression set of the obtained crosslinked molded article, the iodine value of the hollow particles used in the present disclosure is preferably 20 g / 100 g or more, more preferably 25 g / 100 g or more, and even more preferably 30 g / 100 g or more.

[0016] 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, and have reactive unsaturated bonds on the outer surface of the shell.

[0017] Examples of the reactive unsaturated bond include reactive unsaturated bonds contained in a vinyl group, a (meth)acryloyl group, an allyl group, a butenyl group, a maleimide group, a nadimide group, a propargyl group, an ethynyl group, and the like. Among these, an ethylenically unsaturated bond is preferred, an ethylenically unsaturated bond contained in at least one group selected from the group consisting of a vinyl group, a (meth)acryloyl group, and an allyl group is more preferred, and an ethylenically unsaturated bond contained in at least one group selected from a vinyl group and a (meth)acryloyl group is even more preferred.

[0018] In the hollow particles used in the present disclosure, the reactive unsaturated bond on the outer surface of the shell may be a reactive unsaturated bond contained in a crosslinkable monomer unit or a reactive unsaturated bond contained in a coupling agent used for surface treatment, but is preferably a reactive unsaturated bond contained in a crosslinkable monomer unit. That is, it is preferable that at least one polymerizable functional group contained in the crosslinkable monomer is present unreacted on the surface of the shell. The crosslinkable monomer, coupling agent, etc. used in the production of the hollow particles used in the present disclosure will be described later.

[0019] The resin contained in the shell of the hollow particles used in the present disclosure is typically a polymer of a polymerizable monomer used in the hollow particle manufacturing method described below. The shell of the hollow particles may further contain a surface treatment agent or additive other than the resin, as long as the object of the present disclosure is not impaired. In the hollow particles used in the present disclosure, the content of the resin contained in the shell is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and even more preferably 98% by mass or more, and the shell may be made of resin.

[0020] The shell of the hollow particles used in the present disclosure may be a resin layer containing a polymer of a polymerizable monomer, the outer surface of which may be surface-treated with a coupling agent. Surface-treated shells of the hollow particles used in the present disclosure are preferred in that the crosslinked molded article containing the hollow particles used in the present disclosure has improved mechanical properties such as tensile strength, tear strength, and abrasion resistance.

[0021] In the hollow particles used in the present disclosure, the hollow portion is a hollow space that is clearly distinguishable from the shell. 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 minute spaces uniformly dispersed within the porous structure. From the viewpoint of mechanical strength, etc., the hollow particles used in the present disclosure preferably have a solid shell. Furthermore, from the viewpoint of weight reduction, the hollow portion of the hollow particles used in the present disclosure is preferably filled with a gas such as air.

[0022] The hollow particles used in the present disclosure may have one or more hollow portions, but from the viewpoint of maintaining 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 hollow particles used in the present disclosure preferably have a number ratio of particles having only one hollow portion of 90% or more, more preferably 95% or more, and even more preferably more than 95%.Furthermore, the shells of the hollow particles used in the present disclosure, 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 further improve the effect of reducing compression set.

[0023] The shape of the hollow particles used in the present disclosure may be, for example, spherical, oval, or irregular, but from the viewpoint of dispersibility and pressure resistance of the hollow particles, a spherical shape is preferred.

[0024] An example of the shape of the hollow particles used in the present disclosure is a bag made of a thin film and inflated with gas, and its cross-sectional view is shown as hollow particle 10 in (5) in Figure 1. In this example, a thin film is provided on the outside, and the inside is filled with gas. The hollow portion of the hollow particle can be confirmed, for example, by observing the cross-section of the particle with an SEM or by observing the particle directly with a TEM. The shape of the hollow particle can be confirmed, for example, by observing the hollow particle with an SEM or TEM.

[0025] The hollow particles used in the present disclosure may contain, as impurities, a small amount of particles with a low circularity due to particle cracking, deformation, or the like. However, from the viewpoint of further enhancing the effects of the present disclosure, the proportion of particles with a circularity of 0.85 or less in 100% by mass of hollow particles is preferably less than 15% by mass, more preferably less than 10% by mass, and even more preferably less than 8% by mass.

[0026] Particles with a circularity of 0.85 or less are typically particles that have deformations such as dents or cracks, and may be referred to as "irregularly shaped particles" in the present disclosure. By reducing the proportion of irregularly shaped particles contained in hollow particles, the weight-reducing effect of the hollow particles can be further improved. Furthermore, irregularly shaped particles are more likely to aggregate when dispersed in nitrile rubber than spherical particles, and are therefore less dispersible. Therefore, by reducing the proportion of irregularly shaped particles contained in hollow particles, the dispersibility of the hollow particles can be improved, and as a result, the effects of the present disclosure become even more pronounced.

[0027] The circularity is defined as the value obtained by dividing the diameter of a circle having the same area as the projected image of a particle (equivalent circular area diameter) by the diameter of a circle having the same perimeter as the projected image of the particle (equivalent circumferential diameter). When a particle is a perfect sphere, the circularity is 1, and the more complex the particle's surface shape, the smaller the circularity value. The hollow particles used in the present disclosure may have an average circularity of 0.950 to 0.995.

[0028] In this disclosure, circularity is measured using a flow-type particle image analyzer with an image resolution of 0.185 μm / pixel. A preferred example of a flow-type particle image analyzer is the "IF-3200" manufactured by Jasco International Co., Ltd. A measurement sample is prepared, for example, by dispersing 0.10 to 0.12 g of hollow particles in an aqueous solution of linear alkylbenzenesulfonate (0.3% concentration) in an ultrasonic cleaner for 5 minutes. The average circularity is the average value of the circularity of 1,000 to 3,000 arbitrarily selected particles.

[0029] The porosity of the hollow particles used in the present disclosure is not particularly limited, but from the viewpoint of making the effects of the present disclosure more pronounced, it is preferably 60% or more, more preferably 65% ​​or more, and even more preferably 70% or more. The upper limit of the porosity of the hollow particles is not particularly limited, but from the viewpoint of suppressing a decrease in the strength of the hollow particles and making them less likely to be crushed, it is preferably 90% or less, more preferably 85% or less, and even more preferably 80% or less. The porosity of the hollow particles is determined by the apparent density D １ and true density D ０ It is calculated from

[0030] Apparent density D of hollow particles 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, the volumetric flask filled with the hollow particles is accurately filled with isopropanol up to the marked line, taking care not to introduce air bubbles. The mass of 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 by 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.

[0031] True density D of hollow particles ０ The measurement method is as follows: After crushing the hollow particles in advance, a 100 cm 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) ０= [mass of crushed pieces of hollow particles] / (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.

[0032] 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

[0033] The apparent density of the hollow particles used in the present disclosure is not particularly limited, but from the viewpoint of making the effects of the present disclosure more pronounced, it is preferably 0.8 g / cm ３ or less, more preferably 0.6 g / cm ３ More preferably, 0.4 g / cm ３ The lower limit of the apparent density of the hollow particles is not particularly limited, but is preferably 0.1 g / cm3 in order to prevent a decrease in the strength of the hollow particles and make them less likely to be crushed. ３ More preferably, 0.2 g / cm ３ That's all.

[0034] The volume average particle size of the hollow particles used in the present disclosure is not particularly limited, but the lower limit is preferably 0.1 μm or more, more preferably 1 μm or more, and even more preferably 3 μm or more, and the upper limit is preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 50 μm or less. When the volume average particle size of the hollow particles used in the present disclosure is within the above range, it becomes possible to suitably disperse the hollow particles in the nitrile rubber and more suitably form crosslinks between the hollow particles and the nitrile rubber, thereby making the effects of the present disclosure more pronounced.

[0035] The particle size distribution of the hollow particles (volume average particle size (Dv) / number average particle size (Dp)) is not particularly limited, but is preferably 1.05 or more and 1.30 or less, and more preferably 1.10 or more and 1.30 or less.

[0036] The volume average particle diameter (Dv) and number average particle diameter (Dp) of hollow particles can be determined by, for example, measuring the particle diameter of hollow particles using a particle size distribution measuring device based on the Coulter counter method, calculating the number average and volume average, respectively, and using the obtained values ​​as the number average particle diameter (Dp) and volume average particle diameter (Dv) of the particles. The particle size distribution is the value obtained by dividing the volume average particle diameter by the number average particle diameter. The Coulter counter method is a method for measuring particle diameter using an electrical resistance method known as the Coulter principle.

[0037] The residual volatile component content of the hollow particles used in the present disclosure is not particularly limited, but is preferably less than 100 ppm, more preferably less than 50 ppm, and even more preferably less than 30 ppm. Residual volatile components on the outer surface of hollow particles may inhibit the cross-linking reaction between the hollow particles and the nitrile rubber. When the residual volatile component content of hollow particles is within the above range, the reaction is less likely to be inhibited, and cross-linking between the hollow particles and the nitrile rubber can be more effectively formed, thereby further enhancing the effects of the present disclosure. The residual volatile components are typically organic compounds with a molecular weight of 500 or less, and specific examples include residual polymerizable monomers, residual hydrophobic solvents, and decomposition products of polymerization initiators. The residual volatile component content is the ratio of the mass of the residual volatile components contained in the hollow particles to the mass of the hollow particles. The lower limit of the residual volatile component content of the hollow particles used in the present disclosure is not particularly limited, but from the viewpoint of ease of production, it may be, for example, 1 ppm or more, 2 ppm or more, or 3 ppm or more. The residual volatile component content of the hollow particles can be measured by purge and trap / gas chromatography (P&T / GC). Specifically, the measurement method described in the Examples below can be used.

[0038] The hollow particles used in the present disclosure preferably have a total content of surfactants and water-soluble polymer stabilizers (hereinafter simply referred to as "surfactants, etc.") present on the outer surface of the particles of 500 ppm or less, more preferably 200 ppm or less, even more preferably 100 ppm or less, and even more preferably 50 ppm or less. The water-soluble polymer compounds may be either organic or inorganic. Surfactants, etc. present on the outer surface of hollow particles may inhibit the cross-linking reaction between the hollow particles and the nitrile rubber. When the content of surfactants, etc., present on the outer surface of hollow particles is below the upper limit, the reaction is less likely to be inhibited, and cross-linking between the hollow particles and the nitrile rubber can be more suitably formed, thereby making the effects of the present disclosure even more pronounced. By using only an inorganic dispersion stabilizer as a dispersion stabilizer in the hollow particle production process described below, the content of surfactants, etc. present on the surface of hollow particles can be kept below the measurement limit.

[0039] In the present disclosure, the content of surfactants, etc. present on the outer surface of hollow particles refers to the ratio of the mass of the surfactants, etc. present on the outer surface of hollow particles to the mass of the hollow particles. The surfactants, etc. present on the outer surface of hollow particles can be extracted, for example, by ultrasonically treating the hollow particles in water. The type and mass of the surfactants, etc. extracted into water can be identified from the peak positions and peak intensities in a 1H-NMR spectrum. In this method, the measurement limit for the amount of surfactants, etc. present on the surface of hollow particles is usually 0.05 ppm.

[0040] The thermal decomposition initiation temperature of the hollow particles used in the present disclosure is not particularly limited, but from the viewpoint of heat resistance, it is preferably 345°C or higher, more preferably 350°C or higher. The upper limit of the thermal decomposition initiation temperature of the hollow particles is not particularly limited, but may be, for example, 400°C or lower. In the present disclosure, the thermal decomposition initiation temperature of the hollow particles is the temperature at which a 5% mass loss occurs, and can be measured using a TG-DTA device under conditions of a nitrogen atmosphere, a nitrogen flow rate of 230 mL / min, and a heating rate of 10°C / min.

[0041] The hollow particles used in the present disclosure can impart various properties to the crosslinked molded article, such as heat insulation and retention of functional components such as antibacterial agents, in addition to the effects of reducing the weight of the crosslinked molded article, reducing compression set, and improving oil resistance. Applications of the nitrile rubber composition of the present disclosure and its crosslinked molded article will be described in detail in "2. Nitrile rubber composition" below.

[0042] The hollow particles used in the present disclosure are produced, for example, by a production method based on suspension polymerization, which will be described later.

[0043] An example of a method for producing hollow particles used in the present disclosure includes the following steps: preparing a mixture containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; suspending the mixture 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; subjecting the suspension to a polymerization reaction to prepare a precursor composition in which precursor particles having a hollow portion surrounded by a shell containing a resin and filled with the hydrophobic solvent are dispersed in the aqueous medium; and removing the hydrophobic solvent from the precursor particles. In the present disclosure, hollow particles filled with the hydrophobic solvent may be referred to as "precursor particles" as they are considered to be intermediates of hollow particles filled with gas. In the present disclosure, "precursor composition" refers to a composition containing precursor particles.

[0044] In the above-described manufacturing method, 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, resulting in the preparation of a suspension in which droplets of a monomer composition are dispersed in the aqueous medium, with the distribution structure being such that the polymerizable monomer is unevenly distributed on the surface and the hydrophobic solvent is unevenly distributed in the center. When this suspension is subjected to a polymerization reaction, a polymer begins to precipitate on the surface of the droplets of the monomer composition. As the polymerization reaction progresses, the droplet surfaces harden, forming shells, resulting in hollow particles with hollow spaces filled with the hydrophobic solvent. By adding a crosslinkable monomer to the mixture, reactive unsaturated bonds can be introduced into the outer surface of the shell.

[0045] The above-mentioned manufacturing method includes a step of preparing a mixed solution, a step of preparing a suspension, a step of subjecting the suspension to a polymerization reaction, and a step of removing the hydrophobic solvent from the precursor particles, 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 may be reversed. For example, the preparation of the mixed solution and the suspension may be performed simultaneously in a single step, such as by simultaneously adding the materials for preparing the mixed solution and suspending them.

[0046] A preferred example of the method for producing hollow particles includes the following steps: (1) mixed solution preparation step: preparing a mixed solution containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; (2) suspending step: suspending the mixed solution 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) polymerizing step: subjecting the suspension to a polymerization reaction to prepare a precursor composition in which precursor particles having a hollow portion surrounded by a shell containing a resin and filled with the hydrophobic solvent are dispersed in the aqueous medium; (4) solid-liquid separation step: performing solid-liquid separation of the precursor composition to obtain a solid content containing precursor particles; and (5) solvent removal step: removing the hydrophobic solvent from the precursor particles obtained in the solid-liquid separation step to obtain hollow particles.

[0047] FIG. 1 is a schematic diagram showing an example of the manufacturing method of the present disclosure. (1) to (5) in FIG. 1 correspond to the above-mentioned steps (1) to (5). The white arrows between the figures indicate the order of the steps. 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 figure. Furthermore, the structure, dimensions, and shape of the materials used in the manufacturing method of the present disclosure are not limited to the structure, dimensions, and shape of the various materials in these figures.

[0048] 1(1) is a cross-sectional schematic diagram showing 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.

[0049] 1(2) is a cross-sectional schematic diagram showing one embodiment of a suspension in a suspending 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 other than the hydrophobic solvent, a material other than the hydrophobic solvent, a material other than the hydrophobic solvent, a material other than the hydrophobic solvent, a material other than the hydrophobic solvent, a material other than the hydrophobic solvent, and a dispersion stabilizer (not shown) attached to the surface.

[0050] 1 (3) is a cross-sectional schematic diagram showing one embodiment of a precursor composition containing precursor particles encapsulating a hydrophobic solvent in their hollow spaces, obtained by a polymerization step. The precursor composition contains an aqueous medium 1 and precursor particles 9 encapsulating a hydrophobic solvent 4a in their hollow spaces, dispersed in the aqueous medium 1. A shell 6 forming the outer surface of the precursor particles 9 is formed by polymerization of a polymerizable monomer in droplets 8 of the monomer composition, and contains a polymer of the polymerizable monomer as a resin.

[0051] Fig. 1(4) is a cross-sectional schematic diagram showing one embodiment of precursor particles after the solid-liquid separation step, in which the aqueous medium 1 has been removed from the state shown in Fig. 1(3).

[0052] Fig. 1 (5) is a cross-sectional schematic diagram showing one embodiment of hollow particles after the solvent removal step. Fig. 1 (5) shows the state after removing the hydrophobic solvent 4a from the state shown in Fig. 1 (4). By removing the hydrophobic solvent from the precursor particles, hollow particles 10 having gas-filled hollow portions 7 inside the shells 6 are obtained.

[0053] The above five steps and other steps will be described below in order.

[0054] (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 purpose of the present disclosure is 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.

[0055] (A) Polymerizable Monomer In the present 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 the present disclosure). In the present disclosure, a compound having an ethylenically unsaturated bond as a functional group capable of addition polymerization is generally used as the polymerizable monomer. As the polymerizable functional group, a radically polymerizable group is preferred, and from the viewpoint of excellent reactivity, at least one selected from the group consisting of a (meth)acryloyl group, a vinyl group, and an allyl group is preferred, and at least one selected from a (meth)acryloyl group and a vinyl group is more preferred.

[0056] In addition, 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. The crosslinkable monomer can form a crosslinked bond in the polymer by a polymerization reaction. The crosslinkable monomer becomes a crosslinkable monomer unit in the shell, and the non-crosslinkable monomer becomes a non-crosslinkable monomer unit in the shell.

[0057] 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. Furthermore, a polymerizable monomer having a (meth)acryloyl group as a polymerizable functional group is referred to as an acrylic monomer, a crosslinkable monomer having a (meth)acryloyl group as a polymerizable functional group is referred to as a crosslinkable acrylic monomer, and a non-crosslinkable monomer having a (meth)acryloyl group as a polymerizable functional group is referred to as a non-crosslinkable acrylic monomer. In a crosslinkable acrylic monomer, at least one polymerizable functional group may be a (meth)acryloyl group, but it is preferred that all polymerizable functional groups are (meth)acryloyl groups. In the present disclosure, (meth)acrylate refers to each of acrylate and methacrylate, (meth)acrylic refers to each of acrylic and methacrylic, and (meth)acryloyl refers to each of acryloyl and methacryloyl.

[0058] The polymerizable monomer may be any known polymerizable monomer conventionally used in the preparation of hollow particles, and is not particularly limited, but preferably contains at least a crosslinkable monomer so that the iodine value of the hollow particles falls within the above range. It is presumed that the iodine value of the hollow particles falls within the above range because a portion of the reactive unsaturated bonds of the crosslinkable monomer remain unreacted on the outer surface of the shell.

[0059] Furthermore, when a crosslinkable monomer is included as the polymerizable monomer, the crosslink density of the polymer precipitated on the droplet surface increases when the suspension is subjected to a polymerization reaction, and the precipitates are also crosslinked to each other, thereby increasing the crosslink density of the shell. Therefore, a shell with excellent strength is easily formed, and the hollow particles are easily spherical, and hollow portions clearly distinguishable from the shell are easily formed within the particles. The stronger the shell, the less likely the hollow particles are to undergo plastic deformation. Therefore, by improving the shell strength, the effects of the present disclosure can be further enhanced.

[0060] Examples of the crosslinkable monomer include aromatic divinyl monomers such as divinylbenzene, divinylbiphenyl, and divinylnaphthalene; diene monomers such as linear or branched diolefins such as butadiene, isoprene, 2,3-dimethylbutadiene, pentadiene, and hexadiene; and alicyclic diolefins such as dicyclopentadiene, cyclopentadiene, and ethylidenetetracyclododecene; crosslinkable hydrocarbon monomers such as crosslinkable macromers such as polybutadiene, polyisoprene, styrene-butadiene block copolymers (SBS), and styrene-isoprene block copolymers (SIS); allyl (meth)acrylate, vinyl (meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, and the like. Examples of suitable crosslinkable acrylic monomers include difunctional crosslinkable acrylic monomers such as 1,3-bis(methacryloyloxy)-2-hydroxypropane, 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, and dipentaerythritol poly(meth)acrylate, as well as crosslinkable acrylic monomers such as ethoxylated versions of these monomers; crosslinkable allylic monomers such as diallyl phthalate; and crosslinkable macromers such as polyphenylene ether vinyl-modified at both ends and polyphenylene ether (meth)acrylic-modified at both ends. These crosslinkable monomers can be used alone or in combination of two or more.

[0061] In the present disclosure, from the viewpoint of setting the iodine value of the hollow particles within the above range and from the viewpoint of improving the strength of the shell, it is preferable to include at least one crosslinkable monomer selected from a crosslinkable acrylic monomer and a crosslinkable hydrocarbon monomer, more preferably to include at least a crosslinkable acrylic monomer, and particularly preferably to include a combination of a crosslinkable acrylic monomer and a crosslinkable hydrocarbon monomer. Among crosslinkable monomers, crosslinkable acrylic monomers are preferred from the viewpoint of improving the strength of the shell, while crosslinkable hydrocarbon monomers are preferred from the viewpoint of easily introducing reactive unsaturated bonds into the outer surface of the shell.

[0062] Among the crosslinkable hydrocarbon monomers, aromatic divinyl monomers are preferred, with divinylbenzene being particularly preferred. As the crosslinkable acrylic monomer, both the bifunctional crosslinkable acrylic monomers and trifunctional or higher crosslinkable acrylic monomers are preferred. From the viewpoint of improving the strength of the shell, it is preferred to contain at least a bifunctional crosslinkable acrylic monomer, and it is more preferred to contain a combination of a bifunctional crosslinkable acrylic monomer and a trifunctional or higher crosslinkable acrylic monomer. Among the bifunctional crosslinkable acrylic monomers, at least one selected from the group consisting of ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, and pentaerythritol di(meth)acrylate is preferred, with ethylene glycol di(meth)acrylate being more preferred, and ethylene glycol dimethacrylate being even more preferred. Among them, the trifunctional or higher crosslinkable acrylic monomer is preferably at least one selected from the group consisting of trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, and pentaerythritol tetra(meth)acrylate, more preferably trimethylolpropane tri(meth)acrylate, and even more preferably trimethylolpropane trimethacrylate.

[0063] In the present disclosure, in order to achieve an iodine value of the hollow particles within the above range and to improve the strength of the shell, the content of the crosslinkable monomer in 100% by mass of the polymerizable monomer is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 90% by mass or more, and even more preferably 95% by mass or more. In the present disclosure, the polymerizable monomer may consist of a crosslinkable monomer, but may also contain a non-crosslinkable monomer, which will be described later as a polymerizable monomer. Therefore, the content of the crosslinkable monomer in 100% by mass of the polymerizable monomer may be, for example, 99% by mass or less, 98% by mass or less, or 97% by mass or less. The content of each monomer in 100% by mass of the polymerizable monomer corresponds to the content of each monomer unit in 100% by mass of all monomer units of the polymer constituting the shell.

[0064] From the viewpoint of setting the iodine value of the hollow particles within the above range and improving the strength of the shell, the content of the crosslinkable hydrocarbon monomer in 100% by mass of the polymerizable monomer is not particularly limited, and the lower limit is preferably 5% by mass or more, more preferably 10% by mass or more, and the upper limit may be 100% by mass or less. From the same viewpoint, the content of the crosslinkable acrylic monomer in 100% by mass of the polymerizable monomer is not particularly limited, and the lower limit may be 0% by mass or more, and the upper limit is preferably 95% by mass or less, more preferably 90% by mass or less.

[0065] From the viewpoint of obtaining an extremely excellent effect of reducing compression set, the content of the crosslinkable hydrocarbon monomer in 100% by mass of polymerizable monomer is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more. From the same viewpoint, the content of the crosslinkable acrylic monomer in 100% by mass of polymerizable monomer is preferably 80% by mass or less, more preferably 70% by mass or less, and even more preferably 60% by mass or less. On the other hand, when there is a demand for reducing the hardness (surface hardness) of the obtained crosslinked molded article, the content of the crosslinkable hydrocarbon monomer in 100% by mass of polymerizable monomer is preferably 80% by mass or less, more preferably 60% by mass or less, and even more preferably 55% by mass or less. From the same viewpoint, the content of the crosslinkable acrylic monomer in 100% by mass of polymerizable monomer is preferably 20% by mass or more, more preferably 40% by mass or more, and even more preferably 45% by mass or more.

[0066] Furthermore, when a bifunctional crosslinkable acrylic monomer and a tri- or higher functional crosslinkable acrylic monomer are contained in combination, from the viewpoint of setting the iodine value of the hollow particles within the above range and improving the strength of the shell, the content of the bifunctional crosslinkable acrylic monomer relative to the total 100 mass% of the bifunctional crosslinkable acrylic monomer and the tri- or higher functional crosslinkable acrylic monomer (the content of the bifunctional crosslinkable acrylic monomer units relative to the total 100 mass% of the bifunctional crosslinkable acrylic monomer units and the tri- or higher functional crosslinkable acrylic monomer units in the polymer constituting the shell) is preferably 20 mass% or more, more preferably 30 mass% or more, and even more preferably 40 mass% or more as the lower limit, and is preferably 70 mass% or less, more preferably 60 mass% or less, and even more preferably 50 mass% or less as the upper limit.

[0067] In order to set the iodine value of the hollow particles within the above range and to improve the strength of the shell, when the crosslinkable monomer contains a crosslinkable acrylic monomer and a crosslinkable hydrocarbon monomer, the content of the crosslinkable acrylic monomer relative to 100 parts by mass of the total of the crosslinkable acrylic monomer and the crosslinkable hydrocarbon monomer (the content of the crosslinkable acrylic monomer units relative to 100 parts by mass of the total of the crosslinkable acrylic monomer units and the crosslinkable hydrocarbon monomer units in the polymer constituting the shell) 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 the lower limit, and is preferably 95 parts by mass or less, more preferably 90 parts by mass or less as the upper limit.

[0068] In order to set the iodine value of the hollow particles within the above range and to improve the strength of the shell, the total content of the crosslinkable acrylic monomer and the crosslinkable hydrocarbon monomer per 100 parts by mass of the crosslinkable monomer (the total content of the crosslinkable acrylic monomer units and the crosslinkable hydrocarbon monomer units per 100 parts by mass of the crosslinkable monomer units in the polymer constituting the shell) is preferably 80 parts by mass or more, more preferably 90 parts by mass or more, even more preferably 95 parts by mass or more, and still more preferably 99 parts by mass or more.

[0069] In the present disclosure, the polymerizable monomer may further include a non-crosslinkable monomer. Examples of the non-crosslinkable monomer include aromatic monovinyl monomers such as styrene, vinyltoluene, α-methylstyrene, p-methylstyrene, ethylvinylbenzene, ethylvinylbiphenyl, and ethylvinylnaphthalene, linear or branched monoolefins such as ethylene, propylene, and butylene, and alicyclic monoolefins such as vinylcyclohexane, norbornene, tricyclododecene, and 1,4-methano-1,4,4a,9a-tetrahydrofluorene, non-crosslinkable hydrocarbon monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, and t-butyl Examples of suitable non-crosslinkable acrylic monomers include aminoethyl (meth)acrylate, glycidyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, (meth)acrylic acid, (meth)acrylamide, N-methylol (meth)acrylamide, and N-butoxymethyl (meth)acrylamide; 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; vinylpyridine monomers; and non-crosslinkable macromers such as (meth)acrylic-terminated polystyrene and (meth)acrylic-terminated polymethyl methacrylate. These non-crosslinkable monomers can be used alone or in combination of two or more.

[0070] The content of the non-crosslinkable monomer in 100% by mass of the polymerizable monomer is preferably 50% by mass or less, more preferably 30% by mass or less, even more preferably 10% by mass or less, and more preferably 5% by mass or less, from the viewpoint of setting the iodine value of the hollow particles within the above range and of suppressing a decrease in shell strength. The lower limit of the content of the non-crosslinkable monomer in 100% by mass of the polymerizable monomer is not particularly limited, and may be, for example, 1% by mass or more, 2% by mass or more, or 3% by mass or more.

[0071] The content of the polymerizable monomer in the mixed solution is not particularly limited, but from the viewpoint of the balance between the porosity, particle size, and mechanical strength of the hollow particles, it is preferably 15 to 50 mass %, more preferably 20 to 40 mass %, relative to 100 mass % of the total mass of the components in the mixed solution excluding the aqueous medium.

[0072] In order to prevent a decrease in the strength of the resulting hollow particles, the content of the polymerizable monomer relative to the total mass of the solids excluding the hydrophobic solvent, which constitutes the oil phase in the mixed liquid, is preferably 96% by mass or more, and more preferably 97% by mass or more. In the present disclosure, the solids include all components excluding the solvent, and the liquid polymerizable monomer and the like are considered to be included in the solids.

[0073] (B) Hydrophobic Solvent The hydrophobic solvent used in the production method of the present disclosure is a non-polymerizable and poorly water-soluble organic solvent.

[0074] 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. 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.

[0075] 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. Among these, hydrocarbon solvents are preferred, and hydrocarbon solvents having 5 to 8 carbon atoms are more preferred. Examples of hydrocarbon solvents include aliphatic hydrocarbons including chain hydrocarbon solvents such as pentane, hexane, heptane, octane, 2-methylbutane, 2-methylpentane, and paraffin solvents, 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.

[0076] In the suspension step, since phase separation between the polymerizable monomer and the hydrophobic solvent is likely to occur in 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 a hydrocarbon monomer in a proportion of 30% by mass or more, the hydrophobic solvent is preferably a hydrocarbon solvent, more preferably a chain hydrocarbon solvent, even more preferably a chain hydrocarbon solvent having 5 to 8 carbon atoms, and even more preferably at least one selected from the group consisting of pentane, hexane, heptane, and octane.

[0077] 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.

[0078] Furthermore, the hydrophobic solvent preferably has a dielectric constant of 2.5 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 2.5 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 2.5 or less at 20°C are as follows. The values ​​in parentheses are the dielectric constant values: 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," edited by the Chemical Society of Japan, Revised 4th Edition, Maruzen Co., Ltd., published September 30, 1993, pages II-498 to II-50 3) 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.

[0079] The porosity of the hollow particles can be adjusted by changing the amount of hydrophobic solvent in the mixed solution. In the suspension process 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 100 to 650 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 120 to 500 parts by mass, and even more preferably 140 to 300 parts by mass per 100 parts by mass of the polymerizable monomer.

[0080] (C) Polymerization Initiator In the production method of the present disclosure, the mixed liquid preferably contains an oil-soluble polymerization initiator as the polymerization initiator.

[0081] 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, and examples thereof include organic peroxides such as benzoyl peroxide, lauroyl peroxide, t-butyl peroxide-2-ethylhexanoate, t-butylperoxydiethyl acetate, and t-butylperoxypivalate; and azo compounds such as 2,2'-azobis(2,4-dimethylvaleronitrile), azobisisobutyronitrile, and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile). The content of the polymerization initiator 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, per 100 parts by mass of the polymerizable monomer in the mixed solution. When the content of the polymerization initiator is equal to or greater than the above lower limit, the polymerization reaction can proceed sufficiently, and when the content is equal to or less than the above upper limit, there is little risk that the polymerization initiator will remain after completion of the polymerization reaction, and there is also little risk that an unexpected side reaction will proceed.

[0082] (D) Dispersion stabilizer The dispersion stabilizer is an agent that disperses droplets of the monomer composition in an aqueous medium in the suspension step. Examples of the dispersion stabilizer include inorganic dispersion stabilizers, organic or inorganic water-soluble polymer stabilizers, and surfactants.

[0083] In the present disclosure, it is preferable to use an inorganic dispersion stabilizer as the dispersion stabilizer because it is easy to control the particle size of the droplets in the suspension, the dispersion stabilizer can be easily removed by a washing process, and the shell is prevented from becoming too thin, thereby preventing a decrease in the strength of the hollow particles. Examples of inorganic dispersion stabilizers include 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; metal hydroxides such as aluminum hydroxide, magnesium hydroxide, calcium hydroxide, barium hydroxide, and ferric hydroxide; and inorganic compounds such as silicon dioxide. These inorganic dispersion stabilizers can be used alone or in combination of two or more.

[0084] Among the inorganic dispersion stabilizers, a poorly water-soluble inorganic dispersion stabilizer can be preferably used. Here, poorly water-soluble means that the solubility in water at 25° C. is preferably less than 1 g / L. Among the poorly water-soluble inorganic dispersion stabilizers, metal hydroxides are preferred, and magnesium hydroxide is more preferred.

[0085] In the present disclosure, it is particularly preferred to use the poorly water-soluble inorganic dispersion stabilizer in a state in which it is dispersed in the form of colloidal particles in an aqueous medium, i.e., in the form of a colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles, which allows the inorganic dispersion stabilizer to be easily removed by the washing step described below.

[0086] 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.

[0087] Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, potassium hydroxide, etc. Examples of alkaline earth metal hydroxides include barium hydroxide, calcium hydroxide, etc.

[0088] The water-soluble polyvalent metal salt may be any water-soluble polyvalent metal salt other than the compounds corresponding to the alkaline earth metal hydroxides, and examples thereof include magnesium metal salts such as magnesium chloride, magnesium phosphate, magnesium sulfate, etc.; calcium metal salts such as calcium chloride, calcium nitrate, calcium acetate, calcium sulfate, etc.; aluminum metal salts such as aluminum chloride, aluminum sulfate, etc.; barium salts such as barium chloride, barium nitrate, barium acetate, etc.; zinc salts such as zinc chloride, zinc nitrate, zinc acetate, etc. 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.

[0089] The method for reacting at least one selected from the alkali metal hydroxides and alkaline earth metal hydroxides with the water-soluble polyvalent metal salt in an aqueous medium is not particularly limited, but for example, an aqueous solution of at least one selected from the alkali metal hydroxides and alkaline earth metal hydroxides may be mixed with an aqueous solution of the water-soluble polyvalent metal salt.

[0090] Furthermore, colloidal silica can also be used as the colloidal dispersion liquid containing poorly water-soluble inorganic dispersion stabilizer colloidal particles.

[0091] Examples of organic water-soluble polymer stabilizers include polyvinyl alcohol, polycarboxylic acids (such as polyacrylic acid), celluloses (such as hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose, and ethyl cellulose), polyvinylpyrrolidone, polyacrylimide, polyethylene oxide, and poly(hydroxystearic acid-g-methyl methacrylate-co-methacrylic acid) copolymers. Examples of inorganic water-soluble polymer compounds include sodium tripolyphosphate. Surfactants are compounds that have both hydrophilic and hydrophobic groups in one molecule, and include known ionic surfactants such as anionic surfactants, cationic surfactants, and amphoteric surfactants, as well as nonionic surfactants. Water-soluble polymer stabilizers and surfactants typically have a solubility of 1 g / L or more in water at 25°C.

[0092] 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 combined weight of the polymerizable monomer and the hydrophobic solvent. By ensuring that the content of the dispersion stabilizer is 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 ensuring that the content of the dispersion stabilizer is at or below the upper limit, an increase in the viscosity of the suspension during granulation can be prevented, thereby avoiding problems such as clogging of the suspension in the granulator. 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.

[0093] In the present disclosure, from the viewpoint of suppressing a decrease in the reactivity between the hollow particles and the nitrile rubber, the smaller the residual amount of dispersion stabilizer, the more preferable, and it is most preferable that the dispersion stabilizer is not contained, and it is particularly preferable that the dispersion stabilizer is not contained. By using only an inorganic dispersion stabilizer as the dispersion stabilizer, hollow particles can be obtained in which both the water-soluble polymer compound and the surfactant are below the detection limit.

[0094] (E) Aqueous Medium In the present disclosure, the 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.

[0095] When a mixture of water and a hydrophilic solvent is used, it is important that the polarity of the mixture as a whole is not too low, from the viewpoint of forming droplets of the monomer composition. In this case, for example, the mass ratio of water to the hydrophilic solvent (water:hydrophilic solvent) may be 99:1 to 50:50.

[0096] 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 the hydrophilic solvent include alcohols such as methanol and ethanol, tetrahydrofuran (THF), and dimethyl sulfoxide (DMSO).

[0097] 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.

[0098] The mixed liquid may further contain other materials different from the above-described materials (A) to (E) as long as the object of the present disclosure is not impaired.

[0099] A mixed liquid is obtained by mixing the above-mentioned materials and other materials as necessary and stirring them 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 particle sizes of about several mm. The dispersion state of these materials in the mixed liquid can be observed with the naked eye, depending on the type of material.

[0100] In the mixed solution preparation step, the mixed solution may be obtained by simply mixing the above-mentioned materials and other materials as necessary and appropriately stirring, etc. However, in order to make the shell more uniform, it is preferable to prepare the mixed solution in advance separately from an oil phase containing a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator, and an aqueous phase containing a dispersion stabilizer and an aqueous medium, and then mixing these. 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 can be preferably used as the aqueous phase.

[0101] 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.

[0102] (2) Suspension Step The suspension step is a step of suspending the above-mentioned mixed liquid to prepare a suspension in which droplets of the monomer composition containing a hydrophobic solvent are dispersed in an aqueous medium.

[0103] The suspension method for forming droplets of the monomer composition is not particularly limited, and any known suspension method can be employed. Examples of the disperser used in preparing the suspension include horizontal or vertical in-line dispersers such as Milder manufactured by Pacific Machinery Works, Ltd., Cavitron manufactured by Eurotec Co., Ltd., and in-line dispersers manufactured by IKA (e.g., DISPAX-REACTOR (registered trademark) DRS); and emulsifying dispersers such as the Homomixer MARK II series manufactured by Primix Corporation.

[0104] In the dispersion for preparing the suspension, the rotation speed of the disperser is preferably 100 rpm or more, more preferably 200 rpm or more, and even more preferably 300 rpm or more, from the viewpoint of forming hollow portions and setting the volume average particle size of the hollow particles within the above-mentioned preferred range, while from the viewpoint of reducing the proportion of irregularly shaped particles, it is preferably 30,000 rpm or less, more preferably 10,000 rpm or less, and even more preferably 5,000 rpm or less.

[0105] In the suspension prepared in the suspending step, droplets of the monomer composition containing the lipophilic material and having a particle size of approximately 0.1 to 100 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 the low-polarity hydrophobic solvent more likely to collect inside the droplets. As a result, the resulting droplets contain the hydrophobic solvent inside and materials other than the hydrophobic solvent distributed around the periphery. The droplets of the monomer composition dispersed in the aqueous medium are composed of the oil-soluble monomer composition surrounded by a dispersion stabilizer. The droplets of the monomer composition contain an oil-soluble polymerization initiator, a polymerizable monomer, and a hydrophobic solvent.

[0106] The droplets of the monomer composition are minute oil droplets, and the oil-soluble polymerization initiator generates polymerization initiation radicals inside the minute oil droplets. Therefore, precursor particles of the desired particle size can be produced without excessive growth of the minute oil droplets. 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 dispersed in the aqueous medium. Therefore, by using an oil-soluble polymerization initiator, it is possible to suppress the by-production of excess resin particles, such as dense particles with a relatively small particle size, in addition to the desired resin particles having hollow portions.

[0107] (3) Polymerization Step This step is a step of subjecting the suspension obtained in the above-mentioned suspension step to a polymerization reaction to prepare a precursor composition in which precursor particles having a hollow portion surrounded by a shell containing a resin and filled with a hydrophobic solvent are dispersed in an aqueous medium. 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.

[0108] The polymerization method is not particularly limited, and for example, a batch method, a semi-continuous method, a continuous method, etc. can be used. 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 1 to 36 hours.

[0109] In the polymerization process, the shell portion of the droplets of the monomer composition containing the hydrophobic solvent therein is polymerized, and as described above, a hollow portion filled with the hydrophobic solvent is formed inside the resulting precursor particles.

[0110] (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.

[0111] The method for solid-liquid separation of the precursor composition is not particularly limited, and known methods can be used. Examples of the solid-liquid separation method include centrifugation, filtration, and static separation. Among them, filtration is preferred because it is easy to operate and has a high efficiency of removing the dispersion stabilizer.

[0112] After the solid-liquid separation step, an optional step such as a pre-drying step may be carried out before the solvent removal step described below is carried out. 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.

[0113] (5) Solvent Removal Step This step is a step for removing the hydrophobic solvent contained in the precursor particles.

[0114] For example, after the solid-liquid separation process described above, the hydrophobic solvent contained in the precursor particles can be removed in the air, replacing the hydrophobic solvent inside the precursor particles with air, resulting in hollow particles filled with gas. 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, such that the removal of the hydrophobic solvent is not affected. "In the air" can also refer to 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.

[0115] 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. Therefore, 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.

[0116] By drying in air, the hydrophobic solvent inside the precursor particles is replaced with the external gas, resulting in hollow particles with the hollows filled with gas. 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. Alternatively, hollow particles with a temporary vacuum inside can be obtained by first filling the interior of the hollow particles with gas and then drying under reduced pressure.

[0117] 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 precursor composition obtained in the polymerization step. In this method, the hydrophobic solvent encapsulated in the precursor particles can be removed, for example, 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. Here, 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 is 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.

[0118] The temperature at which the inert gas is bubbled through the precursor composition is preferably at least 30°C below the boiling point of the hydrophobic solvent, more preferably at least 20°C below that, in order to reduce the amount of hydrophobic solvent remaining in the hollow particles. The bubbling temperature is typically at least the polymerization temperature in the polymerization step. Although not particularly limited, the bubbling temperature may be 50°C or higher and 100°C or lower. The inert gas to be bubbled 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, the inert gas may be bubbled at a rate of 1 to 3 L / min for 1 to 10 hours.

[0119] In this method, a slurry of hollow particles containing an inert gas is obtained. The slurry is subjected to solid-liquid separation to obtain hollow particles, which are then dried to remove the aqueous medium remaining in the hollow particles, thereby obtaining hollow particles whose hollow spaces are filled with gas.

[0120] 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 in 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 remaining in 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 remaining hydrophobic solvent is reduced by performing bubbling with an inert gas.

[0121] Alternatively, as a method for removing the hydrophobic solvent encapsulated in the precursor particles after the polymerization step and before the solid-liquid separation step without subjecting the slurry precursor composition obtained in the polymerization step to solid-liquid separation, for example, a method for evaporating and distilling off the hydrophobic solvent encapsulated 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 evaporating and distilling off the hydrophobic solvent may be used.

[0122] (6) Others As steps other than the above steps (1) to (5), for example, the following steps (6-a) surface treatment step, (6-b) sieving step, (6-c) washing step, and (6-d) particle interior substitution step may be added.

[0123] (6-a) Surface Treatment Step The method for producing hollow particles used in the present disclosure may include a surface treatment step of treating the outer surface of the shell with a coupling agent after the above-mentioned polymerization step.

[0124] Coupling agents have functional groups capable of bonding with organic substances and functional groups capable of bonding with inorganic substances in one molecule, thereby increasing the affinity between organic and inorganic materials. Coupling agents preferably have functional groups in their molecular structure that are capable of crosslinking with nitrile rubber, as described below. The functional group capable of crosslinking with nitrile rubber is appropriately selected depending on the type of nitrile rubber and is not particularly limited. Examples of such functional groups include hydroxyl groups, carboxyl groups, carbonyl groups, amino groups, mercapto groups, halogen groups, vinyl groups, methacryloyl groups, acryloyl groups, siloxyl groups, peroxide groups, and epoxy groups. Of these functional groups, carboxyl groups, carbonyl groups, and epoxy groups are preferred, with epoxy groups being particularly preferred. Examples of coupling agents include silane coupling agents, titanium coupling agents, and aluminum coupling agents.

[0125] Examples of the silane coupling agent include alkoxysilanes having a vinyl group, such as vinyltriethoxysilane and vinyltris(β-methoxyethoxy)silane; alkoxysilanes having a methacryloyl group or an acryloyl group, such as γ-acryloxypropyltrimethoxysilane and γ-methacryloxypropyltrimethoxysilane; and alkoxysilanes having an epoxy group, such as γ-glycidoxypropyltrimethoxysilane, β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane and γ-glycidoxypropylmethyldiethoxysilane. Examples of silanes include alkoxysilanes having an amino group, such as γ-aminopropyltriethoxysilane, N-β-(aminoethyl)γ-aminopropyltrimethoxysilane, and N-β-(aminoethyl)γ-aminopropylmethyldimethoxysilane; alkoxysilanes having a mercapto group, such as γ-mercaptopropyltrimethoxysilane; alkoxysilanes having a halogen group, such as γ-chloropropyltrimethoxysilane; silanes having a vinyl group and a halogen group, such as vinyltrichlorosilane; and methyltriacetoxysilane.

[0126] Examples of titanium coupling agents include isopropyl triisostearoyl titanate, isopropyl tridodecylbenzenesulfonyl titanate, isopropyl tris(dioctyl pyrophosphate) titanate, tetraisopropyl bis(dioctyl phosphite) titanate, tetraoctyl bis(ditridecyl phosphite) titanate, tetra(2,2-diallyloxymethyl-1-butyl)bis(ditridecyl)phosphite titanate, bis(dioctyl pyrophosphate), bis(dioctyl pyrophosphate)ethylene titanate, isopropyl trioctanoyl titanate, isopropyl dimethacryl isostearoyl titanate, isopropyl isostearoyl diacryl titanate, isopropyl tri(dioctylphosphate)titanate, isopropyl tri(N-amidoethylaminoethyl)titanate, dicumylphenyloxyacetate titanate, diisostearoyl ethylene titanate, and the like.

[0127] Examples of aluminum coupling agents include acetoalkoxyaluminum diisopropylate.

[0128] Among these, silane coupling agents are preferred, and particularly preferred are alkoxysilanes having an epoxy group, such as γ-glycidoxypropyltrimethoxysilane, β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, etc. The coupling agent is used as is or dissolved in a solvent.

[0129] (6-b) Sieving Step (Foreign Matter Removal Step) The method for producing hollow particles used in the present disclosure preferably includes a sieving step after the solvent removal step. By performing the sieving step, coarse powder and aggregates can be removed, and foreign matter can be easily removed.

[0130] The sieving method may be any known method and is not particularly limited. For example, sieving may be performed using a metal mesh such as a stainless steel mesh or a resin mesh such as a nylon mesh. More specifically, the sieved hollow particles can be obtained by vibrating the mesh on which the hollow particles are placed and obtaining the hollow particles that have passed through the mesh. The mesh opening used in the sieving step is appropriately selected depending on the size of the hollow particles. It is preferable that the mesh opening be such that the proportion of particles with a circularity of 0.85 or less in the obtained hollow particles is less than 15 mass%.

[0131] (6-c) Washing Step The washing step is a step of adding an acid or alkali to wash the precursor particles or hollow particles to remove any dispersion stabilizer remaining in the precursor particles or hollow particles. When the dispersion stabilizer used is an inorganic dispersion stabilizer soluble in acid, washing is preferably carried out by adding an acid to a slurry containing the precursor particles or hollow particles. On the other hand, when the dispersion stabilizer used is an inorganic dispersion stabilizer soluble in alkali, washing is preferably carried out by adding an alkali to a slurry containing the precursor particles or hollow particles.

[0132] Furthermore, when an acid-soluble inorganic dispersion stabilizer is used as the dispersion stabilizer, it is preferable to add an acid to a slurry containing precursor particles or hollow particles to adjust the pH of the slurry 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, but sulfuric acid is particularly preferable because it has a high efficiency in removing the dispersion stabilizer and places a small burden on the production equipment.

[0133] (6-d) Particle Interior Substitution Process The particle interior substitution process is a process in which the gas or liquid inside the hollow particles is replaced 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.

[0134] 2. Nitrile Rubber The nitrile rubber composition of the present disclosure contains, as a base material, a nitrile rubber having an iodine value measured in accordance with JIS K 0070 of 2 g / 100 g or more and 500 g / 100 g or less.

[0135] The nitrile rubber used in the present disclosure is not particularly limited, but examples thereof include copolymers obtained by copolymerizing an α,β-ethylenically unsaturated nitrile monomer and an optionally used conjugated diene monomer.

[0136] The α,β-ethylenically unsaturated nitrile monomer is not limited as long as it is an α,β-ethylenically unsaturated compound having a nitrile group, and examples thereof include acrylonitrile; α-halogenoacrylonitriles such as α-chloroacrylonitrile and α-bromoacrylonitrile; and α-alkylacrylonitriles such as methacrylonitrile; with acrylonitrile being preferred. The α,β-ethylenically unsaturated nitrile monomer may be used alone or in combination. The content of the α,β-ethylenically unsaturated nitrile monomer unit in the nitrile rubber used in the present disclosure is preferably 10 to 60% by mass, more preferably 20 to 50% by mass, and even more preferably 30 to 45% by mass.

[0137] As the conjugated diene monomer, conjugated diene monomers having 4 to 6 carbon atoms such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, and chloroprene are preferred, with 1,3-butadiene and isoprene being more preferred, and 1,3-butadiene being particularly preferred. These may be used alone or in combination. The content of conjugated diene monomer units (including saturated conjugated diene monomer units) in the nitrile rubber used in the present disclosure is preferably 40 to 90% by mass, more preferably 50 to 80% by mass, and even more preferably 55 to 70% by mass.

[0138] Furthermore, the nitrile rubber used in the present disclosure may be obtained by copolymerizing, in addition to the α,β-ethylenically unsaturated nitrile monomer and the conjugated diene monomer used as needed, other monomers copolymerizable therewith. Examples of such other monomers include carboxyl group-containing monomers, α,β-ethylenically unsaturated monocarboxylic acid ester monomers, ethylene, α-olefin monomers, aromatic vinyl monomers, fluorine-containing vinyl monomers, and copolymerizable antioxidants. These other copolymerizable monomers may be used alone or in combination.

[0139] The carboxyl group-containing monomer is not particularly limited as long as it is copolymerizable with the α,β-ethylenically unsaturated nitrile monomer and has one or more unsubstituted (free) carboxyl groups that are not esterified or the like. Examples of the carboxyl group-containing monomer include α,β-ethylenically unsaturated monocarboxylic acid monomers, α,β-ethylenically unsaturated polycarboxylic acid monomers, and α,β-ethylenically unsaturated dicarboxylic acid monoester monomers. The carboxyl group-containing monomer also includes monomers in which the carboxyl groups of these monomers form carboxylate salts. Furthermore, anhydrides of α,β-ethylenically unsaturated polycarboxylic acids can also be used as carboxyl group-containing monomers because they form carboxyl groups by cleavage of the acid anhydride group after copolymerization.

[0140] Examples of the α,β-ethylenically unsaturated monocarboxylic acid monomer include acrylic acid, methacrylic acid, ethylacrylic acid, crotonic acid, and cinnamic acid.

[0141] Examples of the α,β-ethylenically unsaturated polycarboxylic acid monomer include butenedioic acids such as fumaric acid and maleic acid, itaconic acid, citraconic acid, mesaconic acid, glutaconic acid, allylmalonic acid, and teraconic acid. Examples of the anhydrides of α,β-unsaturated polycarboxylic acids include maleic anhydride, itaconic anhydride, and citraconic anhydride.

[0142] Examples of the α,β-ethylenically unsaturated dicarboxylic acid monoester monomer include monoalkyl maleates such as monomethyl maleate, monoethyl maleate, monopropyl maleate, and mono-n-butyl maleate; monocycloalkyl maleates such as monocyclopentyl maleate, monocyclohexyl maleate, and monocycloheptyl maleate; monoalkylcycloalkyl maleates such as monomethylcyclopentyl maleate and monoethylcyclohexyl maleate; monoalkyl fumarate esters such as monomethyl fumarate, monoethyl fumarate, monopropyl fumarate, and mono-n-butyl fumarate; monocycloalkyl fumarate esters such as monocyclopentyl fumarate, monocyclohexyl fumarate, and monocycloheptyl fumarate; monoalkylcycloalkyl fumarate esters such as monomethylcyclopentyl fumarate and monoethylcyclohexyl fumarate; monoalkyl citraconic acid esters such as monomethyl citraconic acid, monoethyl citraconic acid, monopropyl citraconic acid, and mono-n-butyl citraconic acid; monocycloalkyl citraconic acid esters such as monocyclopentyl citraconic acid, monocyclohexyl citraconic acid, and monocycloheptyl citraconic acid; monoalkyl citraconic acid cycloalkyl esters such as monomethylcyclopentyl citraconic acid and monoethylcyclohexyl citraconic acid; monoalkyl itaconic acid esters such as monomethyl itaconic acid, monoethyl itaconic acid, monopropyl itaconic acid, and mono-n-butyl itaconic acid; monocycloalkyl itaconic acid esters such as monocyclopentyl itaconic acid, monocyclohexyl itaconic acid, and monocycloheptyl itaconic acid; and monoalkyl itaconic acid cycloalkyl esters such as monomethylcyclopentyl itaconic acid and monoethylcyclohexyl itaconate.

[0143] The α,β-ethylenically unsaturated monocarboxylic acid ester monomer is not particularly limited, but examples thereof include α,β-ethylenically unsaturated monocarboxylic acid alkyl ester monomers, α,β-ethylenically unsaturated monocarboxylic acid alkoxyalkyl ester monomers, α,β-ethylenically unsaturated monocarboxylic acid aminoalkyl ester monomers, α,β-ethylenically unsaturated monocarboxylic acid hydroxyalkyl ester monomers, and α,β-ethylenically unsaturated monocarboxylic acid fluoroalkyl ester monomers.

[0144] Specific examples of the α,β-ethylenically unsaturated monocarboxylic acid alkyl ester monomer include alkyl acrylate monomers such as methyl acrylate, ethyl acrylate, propyl acrylate, isobutyl acrylate, n-butyl acrylate, n-pentyl acrylate, 2-ethylhexyl acrylate, and n-dodecyl acrylate; cycloalkyl acrylate monomers such as cyclopentyl acrylate and cyclohexyl acrylate; alkyl acrylate cycloalkyl ester monomers such as methylcyclopentyl acrylate, ethylcyclopentyl acrylate, and methylcyclohexyl acrylate; and methacrylates such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, n-pentyl methacrylate, and n-octyl methacrylate. Examples thereof include acrylic acid alkyl ester monomers; methacrylic acid cycloalkyl ester monomers such as cyclopentyl methacrylate, cyclohexyl methacrylate, and cyclopentyl methacrylate; methacrylic acid alkyl cycloalkyl ester monomers such as methylcyclopentyl methacrylate, ethylcyclopentyl methacrylate, and methylcyclohexyl methacrylate; crotonate alkyl ester monomers such as propyl crotonate, n-butyl crotonate, and 2-ethylhexyl crotonate; crotonate cycloalkyl ester monomers such as cyclopentyl crotonate, cyclohexyl crotonate, and cyclooctyl crotonate; and crotonate alkyl cycloalkyl ester monomers such as methylcyclopentyl crotonate and methylcyclohexyl crotonate.

[0145] Specific examples of the α,β-ethylenically unsaturated monocarboxylic acid alkoxyalkyl ester monomer include acrylates such as methoxymethyl acrylate, methoxyethyl acrylate, methoxybutyl acrylate, ethoxymethyl acrylate, ethoxyethyl acrylate, ethoxypropyl acrylate, ethoxydodecyl acrylate, n-propoxyethyl acrylate, i-propoxyethyl acrylate, n-butoxyethyl acrylate, i-butoxyethyl acrylate, t-butoxyethyl acrylate, methoxypropyl acrylate, and methoxybutyl acrylate. Acid alkoxyalkyl ester monomers; methacrylic acid alkoxyalkyl ester monomers such as methoxymethyl methacrylate, methoxyethyl methacrylate, methoxybutyl methacrylate, ethoxymethyl methacrylate, ethoxyethyl methacrylate, ethoxypentyl methacrylate, n-propoxyethyl methacrylate, i-propoxyethyl methacrylate, n-butoxyethyl methacrylate, i-butoxyethyl methacrylate, t-butoxyethyl methacrylate, methoxypropyl methacrylate, and methoxybutyl methacrylate; and the like.

[0146] The α-olefin monomer preferably has 3 to 12 carbon atoms, and examples thereof include propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene.

[0147] Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, and vinylpyridine.

[0148] Examples of the fluorine-containing vinyl monomer include fluoroethyl vinyl ether, fluoropropyl vinyl ether, o-trifluoromethylstyrene, vinyl pentafluorobenzoate, difluoroethylene, and tetrafluoroethylene.

[0149] Examples of the copolymerizable antioxidant include N-(4-anilinophenyl)acrylamide, N-(4-anilinophenyl)methacrylamide, N-(4-anilinophenyl)cinnamamide, N-(4-anilinophenyl)crotonamide, N-phenyl-4-(3-vinylbenzyloxy)aniline, and N-phenyl-4-(4-vinylbenzyloxy)aniline.

[0150] The content of these other monomer units in the nitrile rubber used in the present disclosure is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less.

[0151] The iodine value of the nitrile rubber used in the present disclosure is not particularly limited as long as it is 2 g / 100 g or more and 500 g / 100 g or less, but from the viewpoint of being able to further enhance the effects of the present disclosure by more suitably forming cross-linking bonds between the hollow particles and the nitrile rubber while suppressing excessive reaction between the nitrile rubber and components other than the hollow particles and the nitrile rubber, the iodine value is preferably 4 g / 100 g or more, more preferably 8 g / 100 g or more, and even more preferably 10 g / 100 g or more, and is preferably 450 g / 100 g or less, and more preferably 400 g / 100 g or less.

[0152] From the viewpoint of further reducing the compression set of the obtained crosslinked molded article, the iodine value of the nitrile rubber is preferably 50 g / 100 g or more, more preferably 100 g / 100 g or more, even more preferably 200 g / 100 g or more, and particularly preferably 300 g / 100 g or more.

[0153] The Mooney viscosity (ML1+4, 100°C) of the nitrile rubber used in the present disclosure is preferably 10 to 200, more preferably 15 to 100, and even more preferably 20 to 80. When the Mooney viscosity of the nitrile rubber is within the above range, the processability of the nitrile rubber composition of the present disclosure can be improved, and the effects of the present disclosure can also be further enhanced.

[0154] The method for producing the raw rubber used in the present invention is not particularly limited, but it is preferable to produce the raw rubber by copolymerizing the above-mentioned monomers by emulsion polymerization using an emulsifier to prepare a latex of copolymer rubber, and then hydrogenating it as necessary. During emulsion polymerization, commonly used polymerization secondary materials such as an emulsifier, a polymerization initiator, and a molecular weight modifier can be used.

[0155] The emulsifier is not particularly limited, but examples thereof include nonionic emulsifiers such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenol ethers, polyoxyethylene alkyl esters, and polyoxyethylene sorbitan alkyl esters; anionic emulsifiers such as salts of fatty acids such as myristic acid, palmitic acid, stearic acid, oleic acid, and linolenic acid, alkylbenzenesulfonates such as sodium dodecylbenzenesulfonate, polycondensates of naphthalenesulfonates and formalin, higher alcohol sulfates, and alkyl sulfosuccinates; and copolymerizable emulsifiers such as sulfoesters of α,β-unsaturated carboxylic acids, sulfate esters of α,β-unsaturated carboxylic acids, and sulfoalkylaryl ethers. The amount of emulsifier added is preferably 0.1 to 10 parts by mass, and more preferably 0.5 to 5 parts by mass, per 100 parts by mass of the monomers used in the polymerization.

[0156] The polymerization initiator is not particularly limited as long as it is a radical initiator, and examples thereof include inorganic peroxides such as potassium persulfate, sodium persulfate, ammonium persulfate, potassium perphosphate, and hydrogen peroxide; organic peroxides such as t-butyl peroxide, cumene hydroperoxide, p-menthane hydroperoxide, di-t-butyl peroxide, t-butylcumyl peroxide, acetyl peroxide, isobutyryl peroxide, octanoyl peroxide, dibenzoyl peroxide, 3,5,5-trimethylhexanoyl peroxide, t-butylperoxyisobutyrate, and diisopropylbenzene hydroperoxide; and azo compounds such as azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, azobiscyclohexanecarbonitrile, and methyl azobisisobutyrate. These polymerization initiators can be used alone or in combination of two or more. Inorganic or organic peroxides are preferred as the polymerization initiator. When a peroxide is used as a polymerization initiator, it can also be used as a redox polymerization initiator in combination with a reducing agent such as sodium bisulfite, ferrous sulfate, etc. The amount of the polymerization initiator added is preferably 0.01 to 2 parts by mass per 100 parts by mass of the monomers used in the polymerization.

[0157] The molecular weight modifier is not particularly limited, but examples thereof include mercaptans such as t-dodecyl mercaptan, n-dodecyl mercaptan, and octyl mercaptan; halogenated hydrocarbons such as carbon tetrachloride, methylene chloride, and methylene bromide; α-methylstyrene dimer; and sulfur-containing compounds such as tetraethylthiuram disulfide, dipentamethylenethiuram disulfide, and diisopropylxanthogen disulfide. These may be used alone or in combination of two or more. Of these, mercaptans are preferred, and t-dodecyl mercaptan is more preferred. The amount of molecular weight modifier added is preferably 0.1 to 5 parts by mass per 100 parts by mass of the monomers used in the polymerization.

[0158] Water is usually used as the medium for emulsion polymerization, and the amount of water is preferably 80 to 500 parts by mass, more preferably 80 to 300 parts by mass, per 100 parts by mass of the monomers used in the polymerization.

[0159] In emulsion polymerization, if necessary, it is possible to use additional polymerization auxiliary materials such as stabilizers, dispersants, pH adjusters, oxygen scavengers, particle size adjusters, etc. When these are used, the types and amounts used are not particularly limited.

[0160] Furthermore, the copolymer obtained may be subjected to hydrogenation (hydrogenation reaction) as needed. In this case, the hydrogenation method is not particularly limited, and any known method may be used.

[0161] 3. Nitrile Rubber Composition The nitrile rubber composition of the present disclosure is characterized by containing the hollow particles used in the present disclosure and the nitrile rubber used in the present disclosure, and is used as a molding material for producing a crosslinked molded article. The nitrile rubber composition of the present disclosure can provide a lightweight crosslinked molded article that is excellent in compression set resistance and oil resistance and has appropriate hardness.

[0162] [Hollow Particles] The hollow particles contained in the nitrile rubber composition of the present disclosure are the hollow particles used in the present disclosure described above. The content of the hollow particles contained in the nitrile rubber composition of the present disclosure is not particularly limited, but from the viewpoint of improving the processability of the nitrile rubber composition and making the effects of the present disclosure more pronounced, the lower limit is preferably 1 part by mass or more, more preferably 5 parts by mass or more, even more preferably 15 parts by mass or more, particularly preferably 25 parts by mass or more, and most preferably 40 parts by mass or more, relative to 100 parts by mass of the nitrile rubber, and the upper limit is preferably 200 parts by mass or less, more preferably 150 parts by mass or less, even more preferably 100 parts by mass or less, and particularly preferably 75 parts by mass or less.

[0163] [Nitrile rubber] The nitrile rubber contained in the nitrile rubber composition of the present disclosure is the nitrile rubber used in the present disclosure described above. The content of the nitrile rubber in the nitrile rubber composition of the present disclosure is not particularly limited, but the lower limit is preferably 30% by mass or more, more preferably 35% by mass or more, and even more preferably 40% by mass or more, and the upper limit is preferably 90% by mass or less, more preferably 85% by mass or less, and even more preferably 80% by mass or less.

[0164] [Crosslinking Agent] The nitrile rubber composition of the present disclosure preferably contains a crosslinking agent for crosslinking the nitrile rubber used in the present disclosure. Examples of crosslinking agents include organic peroxide crosslinking agents and sulfur crosslinking agents. From the viewpoint of more suitably forming crosslinks between the hollow particles and the nitrile rubber and within the nitrile rubber, organic peroxide crosslinking agents are preferred as the crosslinking agent. Examples of organic peroxide crosslinking agents include 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 crosslinking agents can be used alone or in combination of two or more. The content of the crosslinking agent is appropriately selected depending on the type thereof, 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 nitrile rubber used in the present disclosure.

[0165] [Liquid polymer having reactive active sites] The nitrile rubber composition of the present disclosure preferably contains a liquid polymer having reactive active sites. The liquid polymer having reactive active sites functions as a plasticizer during melt-kneading of the nitrile rubber composition and its raw material mixture. Furthermore, the liquid polymer having reactive active sites bonds with nitrile rubber molecules during melt-kneading of the nitrile rubber composition and its raw material mixture or during molding of the nitrile rubber composition, and the liquid polymer having reactive active sites itself can be integrated with the matrix of the nitrile rubber composition and its raw material mixture. By containing a liquid polymer having reactive active sites in the nitrile rubber composition of the present disclosure, it becomes possible to favorably disperse the hollow particles in the nitrile rubber and more favorably form crosslinks between the hollow particles and the nitrile rubber and crosslinks within the nitrile rubber, thereby making the effects of the present disclosure more pronounced.

[0166] The polymer backbone, which is the main part of the molecular structure of the liquid polymer having reactive active sites, preferably has appropriate compatibility, softening property or fluidity, and can function as a plasticizer when the nitrile rubber composition and its raw material mixture are melt-kneaded. For example, the polymer backbone may be a backbone having a hydrocarbon polymer structure which may contain heteroatoms such as oxygen, nitrogen, silicon, etc. in the main chain or side chain.

[0167] The reactive sites in a liquid polymer having reactive active sites refer to chemical structures that function to form chemical, physical, or physicochemical bonds with reactive active sites present on the nitrile rubber. When a liquid polymer having reactive active sites has two or more reactive active sites per molecule that bond with the nitrile rubber, the liquid polymer having reactive active sites forms a crosslinked structure between two nitrile rubber molecules via molecular chains derived from the liquid polymer, thereby making the effects of the present disclosure even more pronounced. When a liquid polymer having reactive active sites has two or more reactive active sites per molecule and the reactive active sites can also bond to hollow particles, the liquid polymer having reactive active sites not only forms a crosslinked structure between two nitrile rubber molecules, but also forms a crosslinked structure between the nitrile rubber molecule and the hollow particle, and between two hollow particles, thereby making the effects of the present disclosure even more pronounced. From the viewpoint of being able to appropriately introduce the above-mentioned crosslinked structure while suppressing excessive networking of the matrix, the number of reactive active sites per molecule of the liquid polymer having reactive active sites is preferably 2 to 10,000.

[0168] From the viewpoint of achieving an excellent effect of imparting plasticity, the liquid polymer having reactive sites is preferably liquid at at least one point within the range of 5° C. to 35° 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. Furthermore, from the viewpoint of achieving an excellent effect of imparting plasticity, the liquid polymer having reactive sites preferably has a glass transition temperature of −10° C. or lower, more preferably −120° C. to −20° C. The Mooney viscosity (1+4,100° C.) of the liquid polymer having reactive sites is usually 1 or lower, or is so low that the Mooney viscosity cannot be measured.

[0169] As the liquid polymer having reactive sites, for example, a liquid polymer having an ethylenic double bond can be used. When a liquid polymer having an ethylenic double bond is used, the ethylenic double bond in the liquid polymer can react with and bond to the ethylenic double bond present on the surface of the nitrile rubber or the hollow particles.

[0170] Examples of liquid polymers having ethylenic double bonds as reactive active sites include unmodified liquid diene rubbers such as liquid polybutadiene rubber, unmodified liquid polyisoprene rubber, and liquid styrene-butadiene rubber; modified liquid diene rubbers such as acrylate-modified liquid polybutadiene and hydroxyl-terminated liquid polyisoprene rubber; and polyolefins having terminal double bonds such as polypropylene with double bonds at both ends; and among these, unmodified liquid diene rubber and polyolefins having terminal double bonds are preferred, and liquid polybutadiene rubber is more preferred. The 1,2-vinyl structure ratio (the proportion of 1,2-vinyl bonds present) in the unmodified liquid diene rubber and modified liquid diene rubber is preferably 10 to 100%, more preferably 30 to 99%, even more preferably 50 to 98%, and particularly preferably 70 to 97%.

[0171] The number average molecular weight (Mw) of the liquid polymer having reactive active sites is not particularly limited, but is preferably 500 to 100,000, more preferably 800 to 30,000, and even more preferably 1,000 to 10,000. When the number average molecular weight (Mw) is within the above range, volatilization of the liquid polymer having reactive active sites and phase separation from the nitrile rubber are suppressed, and further, the effect of improving crosslinkability by the liquid polymer having reactive active sites is more suitably obtained, making the effects of the present disclosure more pronounced.

[0172] The content of the liquid polymer having reactive active sites in the nitrile rubber composition is not particularly limited, but from the viewpoint of making the effects of the present disclosure more pronounced, it is preferably 1 part by mass or more, more preferably 3 parts by mass or more, even more preferably 5 parts by mass or more, and also preferably 20 parts by mass or less, even more preferably 18 parts by mass or less, and particularly preferably 15 parts by mass or less, relative to 100 parts by mass of the nitrile rubber used in the present disclosure.

[0173] [Other Components] The nitrile rubber composition of the present disclosure may contain additives such as plasticizers, reinforcing agents, fillers, vulcanization accelerators, vulcanization accelerator assistants, softeners, processing aids, antioxidants, ultraviolet absorbers, foaming agents, foaming assistants, lubricants, pigments, colorants, dispersants, and flame retardants, as needed, within the scope of the present disclosure.

[0174] Furthermore, the reinforcing agent, filler, etc. contained in the nitrile rubber composition of the present disclosure may be surface-treated with the above-mentioned coupling agent that can be used for the hollow particles used in the present disclosure. If at least one selected from the reinforcing agent and filler is surface-treated with the above-mentioned coupling agent, it is preferable in that the mechanical properties such as tensile strength, tear strength, and abrasion resistance of the obtained crosslinked molded article are improved. It is more preferable that the hollow particles used in the present disclosure and at least one selected from the reinforcing agent and filler are surface-treated with the above-mentioned coupling agent.

[0175] As the plasticizer, those generally used as plasticizers in applications such as vehicle materials, general plastics, rubber products, etc., or those having properties that impart flexibility can be used. For example, 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; 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; microcrystalline wax, sub(factice), liquid thiokol, and hydrocarbon-based synthetic lubricating oils can be used. These plasticizers can be used alone or in combination of two or more. The content of the plasticizer in the nitrile rubber composition is not particularly limited, and is usually 0 to 100 parts by mass, preferably 1 to 90 parts by mass, per 100 parts by mass of the nitrile rubber used in the present disclosure.

[0176] The reinforcing agent has the effect of improving mechanical properties such as tensile strength, tear strength, and abrasion resistance. Specific examples of such reinforcing agents include carbon black such as SRF, GPF, FEF, HAF, ISAF, SAF, FT, and MT, carbon blacks surface-treated with silane coupling agents, finely powdered silica, and silica. These reinforcing agents can be used alone or in combination of two or more. The amount of reinforcing agent to be added is not particularly limited, and is usually less than 230 parts by mass per 100 parts by mass of the nitrile rubber used in the present disclosure.

[0177] 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 0 to 200 parts by mass per 100 parts by mass of the nitrile rubber used in the present disclosure.

[0178] 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; and N-t-butyl-2-benzothiazole. Examples of suitable vulcanization accelerators include sulfenamides such as thiazylsulfenamide; thiurams such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide, and tetramethylthiuram monosulfide; 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 of two or more. The amount of vulcanization accelerator to be used is typically 0 to 20 parts by mass, preferably 0 to 10 parts by mass, per 100 parts by mass of the nitrile rubber used in the present disclosure.

[0179] Specific examples of the vulcanization accelerator aid include metal oxides such as magnesium oxide and zinc oxide; and organic acids (salts) such as stearic acid, oleic acid, and zinc stearate, with zinc oxide and stearic acid being particularly preferred. These vulcanization accelerator aids can be used alone or in combination of two or more. The amount of vulcanization accelerator aid blended is usually in the range of 0 to 20 parts by mass per 100 parts by mass of the nitrile rubber used in the present disclosure.

[0180] 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 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 0 to 200 parts by mass per 100 parts by mass of the nitrile rubber used in the present disclosure.

[0181] 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.

[0182] Examples of the antioxidant include amine-based, hindered phenol-based, and sulfur-based antioxidants.

[0183] Examples of lubricants include hydrocarbon-based lubricants such as liquid paraffin, fatty acid-based lubricants such as stearic acid, fatty acid amide-based lubricants such as stearic acid amide, ester-based lubricants such as butyl stearate, and alcohol-based lubricants such as stearyl alcohol, as well as compounds or mixtures thereof, and metal soaps.

[0184] Examples of pigments include inorganic pigments such as titanium dioxide, zinc oxide, ultramarine, red iron oxide, lithopone, lead, cadmium, iron, cobalt, aluminum, hydrochlorides, and nitrates; 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.

[0185] The nitrile rubber composition of the present disclosure may contain a base elastomer other than the nitrile rubber used in the present disclosure, as long as the object of the present disclosure is not impaired. Examples of other base elastomers include rubbers such as natural rubber, isoprene rubber, butadiene rubber, styrene butadiene rubber, chloroprene rubber, hydrogenated acrylonitrile butadiene rubber (HNBR), ethylene-α-olefin copolymer rubber, ethylene-α-olefin-non-conjugated diene copolymer rubbers 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, urethane rubber, silicone rubber, chlorosulfonated polyethylene, acrylic rubber, epichlorohydrin rubber, fluororubber, polysulfide rubber, and propylene oxide rubber; thermoplastic elastomers such as urethane-based elastomers, styrene-based elastomers, olefin-based elastomers, amide-based elastomers, and ester-based elastomers; and the like. These base elastomers can be used alone or in combination of two or more.

[0186] The content of the other base elastomer is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably substantially 0 parts by mass per 100 parts by mass of the nitrile rubber used in the present disclosure.

[0187] A crosslinked molded article obtained by melt molding or the like using the nitrile rubber composition of the present disclosure as a molding material may be, for example, a rubber member, a rubber member integrally molded with a part made of another material, a coating, or a filler chip material.

[0188] Crosslinked molded articles obtained by crosslinking the nitrile rubber composition of the present disclosure are lightweight molded articles that have excellent compression set resistance and oil resistance and moderate hardness. Examples of uses for crosslinked molded articles produced using the nitrile rubber composition of the present disclosure include various rubber components used in various fields such as automobiles, electrical appliances, electronics, construction, aviation, and space. More specific examples include automotive parts such as hoses, seals, anti-vibration rubber, and weather strips; construction materials such as waterproof sheets and seals; electrical rubber parts such as high-voltage cables and connectors; and industrial products such as heat-resistant conveyor belts, chemical-resistant rolls, and heat-resistant hoses. Sealing materials are particularly required to have low compression set because their sealing properties deteriorate upon plastic deformation. Crosslinked molded articles produced using the nitrile rubber composition of the present disclosure have reduced compression set, making them particularly suitable for use in applications requiring low compression set, such as sealing materials.

[0189] Furthermore, examples of applications of the crosslinked molded article produced using the nitrile rubber composition of the present disclosure include overcoat materials or undercoat materials that require heat insulation, shock-absorbing properties (cushioning properties), etc., shock-absorbing materials (cushioning materials) for footwear such as sports shoes and sandals, home appliance parts, bicycle parts, stationery, tools, and 3D printer filaments.

[0190] [Method for producing nitrile rubber composition] A general method can be used as a method for producing the nitrile rubber composition of the present disclosure, and is not particularly limited. Examples of the method for producing the nitrile rubber composition of the present disclosure include a method in which, while kneading the nitrile rubber used in the present disclosure, the hollow particles used in the present disclosure and other components such as a crosslinking agent added as needed are added to the nitrile rubber, and the mixture is further kneaded. Alternatively, a method can be used in which a raw material mixture containing the nitrile rubber used in the present disclosure, the hollow particles used in the present disclosure, and other components such as a crosslinking agent added as needed is prepared, and then the raw material mixture is kneaded.

[0191] The kneading of the nitrile rubber or raw material mixture used in the present disclosure is carried out at a temperature at which the nitrile rubber softens. Furthermore, kneaders that can be used for the kneading include, for example, known kneaders such as single-screw kneaders, twin-screw kneaders, kneaders, Banbury mixers, pressure kneaders, and roll kneaders. Among these, kneading that applies high shear force, such as roll kneading, is preferred. A kneader that can be used for roll kneading is, for example, a twin mixing roll, and more specifically, a mixing roll DY6-15 (manufactured by Daihan Co., Ltd.). Furthermore, in the production of the nitrile rubber composition of the present disclosure, the compounding components may be homogenized by pre-kneading at a temperature at which the nitrile rubber softens, and then finish kneading that applies high shear force, such as roll kneading, may be carried out. This allows for the production of a nitrile rubber composition in which the compounding components are further homogenized and refined. Furthermore, when crosslinking and molding the nitrile rubber composition, the nitrile rubber composition before crosslinking recovered from the molding device can be reused as the raw material mixture.

[0192] The conditions for kneading the nitrile rubber or raw material mixture used in the present disclosure are not particularly limited, but when a radical generator is used as a crosslinking agent, the kneading temperature is preferably equal to or lower than the 10-hour half-life temperature of the crosslinking agent (radical generator). The kneading temperature here refers to the set temperature of the kneading device. The kneading time is preferably within 1 hour. By adopting such kneading conditions, the crosslinking reaction of the nitrile rubber during kneading can be prevented, scorching can be prevented, and the compounding components can be mixed uniformly.

[0193] As an example, kneading can be performed by the following method: Using a roll kneader, the kneading temperature is set to 70 to 90°C, and after the temperature of the kneader has stabilized, the nitrile rubber used in the present disclosure is added, and then, while rotating the rotor of the kneader at a rotation speed of 10 to 35 rpm, components such as hollow particles and a crosslinking agent are added in any order, and after all materials have been added, the mixture is kneaded for 10 to 30 minutes. This produces the nitrile rubber composition of the present disclosure.

[0194] 4. Crosslinked Molded Article The crosslinked molded article of the present disclosure is a molded article obtained by crosslinking and molding the above-described nitrile rubber composition of the present disclosure.

[0195] By crosslinking and molding the nitrile rubber composition of the present disclosure, the crosslinking reaction of the nitrile rubber used in the present disclosure progresses, and the reactive unsaturated bonds of the hollow particles react with the reactive unsaturated bonds of the nitrile rubber used in the present disclosure to form crosslinks, thereby obtaining the crosslinked molded article of the present disclosure. The method for crosslinking and molding the nitrile rubber composition can be appropriately selected from known methods depending on the type and content of the hollow particles and other additives, the shape of the desired molded article, etc. Known methods for crosslinking and molding the nitrile rubber composition include, for example, melt molding methods such as extrusion molding, compression molding, extrusion lamination, injection molding, press molding, and blow molding.

[0196] The temperature when crosslinking and molding the nitrile rubber composition of the present disclosure is not particularly limited, but from the viewpoint of sufficiently progressing the crosslinking reaction of the nitrile rubber used in the present disclosure and the reaction between the hollow particles and the nitrile rubber used in the present disclosure, when a radical generator is used as a crosslinking agent, it is preferably equal to or higher than the 10-hour half-life temperature of the crosslinking agent. The upper limit of the temperature when crosslinking and molding the nitrile rubber composition of the present disclosure is not particularly limited, but when a radical generator is used as a crosslinking agent, it is preferably equal to or lower than the 1-minute half-life temperature. Furthermore, the conditions for crosslinking and molding the nitrile rubber composition of the present disclosure are not particularly limited, but for example, the pressure can be 1 MPa to 20 MPa and the heating and pressing time can be 1 minute to 180 minutes.

[0197] The form of the crosslinked molded article of the present disclosure is not particularly limited, and for example, the nitrile rubber composition in a molten state may be molded into a long sheet, a block, a filler, or the like, or the long sheet may be wound up into a roll, or the long sheet may be cut to a predetermined length and subjected to secondary processing into a strip, or the like.

[0198] The specific gravity of the crosslinked molded article of the present disclosure is not particularly limited, but is preferably 0.9 or less, more preferably 0.8 or less, and even more preferably 0.7 or less. The lower limit of the specific gravity of the nitrile rubber composition is not particularly limited, but is usually 0.3 or more, preferably 0.4 or more. The specific gravity of the crosslinked molded article can be measured by the underwater displacement method in accordance with JIS K 7112:1999.

[0199] The hardness (Duro-A) of the crosslinked molded article of the present disclosure is not particularly limited. From the viewpoint that the crosslinked molded article of the present disclosure can be suitably used for applications such as sealing members, the hardness (Duro-A) of the crosslinked molded article of the present disclosure is preferably 60 or more, more preferably 65 or more, even more preferably 70 or more, particularly preferably 75 or more, and is preferably 90 or less, more preferably 85 or less, and even more preferably 82 or less. The hardness (Duro-A) of the crosslinked molded article can be measured using a Type A durometer in accordance with JIS K 6253-3.

[0200] The crosslinked molded article of the present disclosure preferably has a compression set of less than 60%, more preferably less than 40%, and particularly preferably less than 35%, as measured in accordance with the room temperature test of JIS K 6262:2013. The lower limit of the compression set of the crosslinked molded article of the present disclosure is not particularly limited, but is usually 10% or more. Furthermore, the nitrile rubber composition of the present disclosure used as a molding material for the crosslinked molded article of the present disclosure usually has a compression set of 60% or more.

[0201] 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.

[0202] [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 acrylic monomer: ethylene glycol dimethacrylate 25 parts Crosslinkable acrylic monomer: trimethylolpropane trimethacrylate 30 parts Crosslinkable hydrocarbon monomer: divinylbenzene 45 parts Oil-soluble polymerization initiator: 2,2'-azobis(2,4-dimethylvaleronitrile) 3 parts Hydrophobic solvent: hexane 160 parts

[0203] Meanwhile, in a stirring tank, at room temperature, an aqueous solution prepared by dissolving 17.1 parts of magnesium chloride (a water-soluble polyvalent metal salt) in 494 parts of ion-exchanged water was gradually added with stirring to prepare an aqueous solution prepared by dissolving 12.1 parts of sodium hydroxide (an alkali metal hydroxide) in 121 parts of ion-exchanged water, thereby preparing a magnesium hydroxide colloid (a poorly water-soluble metal hydroxide colloid) dispersion (4 parts of magnesium hydroxide), which served as the aqueous phase. The obtained aqueous phase and oil phase were mixed to prepare a mixed liquid.

[0204] (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.

[0205] (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.

[0206] (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.

[0207] (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.

[0208] (6) Sieving Step The hollow particles obtained in the solvent removal step were sieved using a nylon mesh with an opening of 100 μm, and the hollow particles that passed through the mesh and fell to the bottom were collected to remove coarse particles, thereby obtaining hollow particles A.

[0209] [Production Example 2 (Hollow Particles B)] Hollow particles of Production Example 2 (hollow particles B) were obtained in the same manner as in Production Example 1, except that the amount of polymerizable monomer added to the oil phase in the above "(1) Mixture preparation step" was changed according to Table 1.

[0210] [Production Example 3 (Hollow Particles C)] Hollow particles of Production Example 3 (hollow particles C) were obtained in the same procedure as in Production Example 1, except that the rotation speed of the disperser in the above "(2) suspension step" was changed to 400 rpm.

[0211] [Production Example 4 (Hollow Particles D)] Hollow particles of Production Example 4 (hollow particles D) were obtained in the same procedure as in Production Example 1, except that the amount of polymerizable monomer added to the oil phase in the above "(1) Mixture preparation step" was changed according to Table 1.

[0212] [Production Example 5 (Hollow Particles E)] Hollow particles of Production Example 5 (hollow particles E) were obtained in the same manner as in Production Example 1, except that in the above-mentioned "(1) mixed solution preparation step", the polymerizable monomer added to the oil phase was changed to 100 parts of divinylbenzene alone.

[0213] [Comparative Production Example 1 (Dense Solid Particles A)] Dense solid particles (dense solid particles A) of Comparative Production Example 2 were obtained in the same manner as in Production Example 1, except that in the above-mentioned "(2) Suspension Step", the rotation speed of the disperser was changed to 10 rpm.

[0214] [Evaluation of Particle Properties] The hollow particles obtained in Production Examples 1 to 4 and Comparative Production Example 1, and the solid particles obtained in Comparative Production Example 2 were evaluated for the following physical properties.

[0215] 1. Iodine Value Measurement The iodine value of the particles was measured in accordance with JIS K 0070. The specific measurement method is as follows: 0.7 to 2 g of particles (sample) and 10 mL of chloroform were added to a 300 mL iodine flask, and 25 mL of Wiess's solution was added as a reaction solution. The flask was gently stirred and sealed, and then allowed to stand in the dark at 25°C for 30 minutes. Next, 20 mL of 100 g / L potassium iodide solution and 100 mL of purified water were added and stirred. Using a burette, titration was performed with a titrant (0.1 mol / L sodium thiosulfate solution). When the solution turned pale yellow, an indicator (1% starch solution) was added. The titration was continued, and the point at which the blue color disappeared was the endpoint. Separately from this main test, a blank test was performed using a solution without added particles, and the iodine value of the particles was calculated using the following formula. The iodine value is the amount of halogen that bonds when 100 g of a sample is reacted with a halogen, converted into grams of iodine. Iodine value (g / 100 g) = {(V0 - V1) x f x 1.269} / S, where S is the mass of the sample (g), V1 is the amount of titrant in this test (mL), V0 is the amount of titrant in the blank test (mL), and f is the factor of the titrant.

[0216] 2. Particle size and particle size distribution Particle size was measured using a particle size distribution analyzer (product name: Multisizer 4e, manufactured by Beckman Coulter, Inc.) using the Coulter counter method, and the number average and volume average were calculated to determine the number average particle size (Dp) and volume average particle size (Dv). The particle size distribution (Dv / Dp) was calculated by dividing the volume average particle size by the number average particle size. 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 (product name: Drywell, manufactured by Fujifilm Corporation) was added as a dispersant. 2 ml of dispersion medium was added to the beaker 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.

[0217] 3. Porosity 3-1. Measurement of apparent density of hollow particles First, a volume of 100 cm ３ About 30 cm ３The volumetric flask was filled with hollow particles, and the mass of the filled hollow particles was accurately weighed. Next, the volumetric flask filled 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 volumetric flask was accurately weighed, and the apparent density D of the hollow particles was calculated based on the following formula (I): １ (g / cm ３ The apparent density D was calculated using the formula (I). １ = [Mass of hollow particles] / (100 - [Mass of isopropanol] / [Specific gravity of isopropanol at measurement temperature])

[0218] 3-2. 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. The true density D of the hollow particles was calculated based on the following formula (II): ０ (g / cm ３ ) was calculated. ０ = [Mass of crushed hollow particle fragments] / (100 - [Mass of isopropanol] / [Specific gravity of isopropanol at measurement temperature])

[0219] 3-3. 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 following formula (III): Formula (III) Porosity (%) = 100 - (apparent density D １ / True density D ０ ) x 100

[0220]

[0221] [Production of Nitrile Rubber Composition, and Production of Crosslinked Molded Product] [Example 1] As a base elastomer, 100 parts by mass of nitrile rubber 1 (NBR1) (acrylonitrile butadiene rubber, product name: Nipol (registered trademark) DN3350, manufactured by Zeon Corporation, iodine value: 380 g / 100 g, bound acrylonitrile content: 33.0%, Mooney viscosity ML (1+4,100°C): 50.0) was charged into a twin mixing roll kneader (model name: DY6-15, roll diameter: 6 inches, roll clearance: 0.5 mm, manufactured by Daihan Co., Ltd.) kept at a temperature of 80°C. The rotation speed of the kneader rotor was set to 10 to 35 rpm, and the base elastomer (NBR) was wound around the roll. Then, 50 parts by mass of the hollow particles A obtained in Production Example 1 and 10 parts by mass of liquid polybutadiene 1 (PB1) (a liquid polymer having reactive active sites, product name: NISSO-PB B-2000, manufactured by Nippon Soda Co., Ltd., number average molecular weight 2100, 1,2-vinyl structure ratio: 90% or more, trans-1,4 structure ratio: 10% or less, viscosity at 45°C: 62 poise, specific gravity: 0.86) were added to the kneader, followed by adding 2 parts of 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane (product name "Perhexa 25B", manufactured by NOF Corporation) as a crosslinking agent to obtain a mixture. The resulting mixture was kneaded for 15 minutes to obtain the nitrile rubber composition of Example 1.

[0222] The obtained nitrile rubber composition was press-molded for 15 minutes at a pressure of 10 MPa using a hot press at 160°C to obtain a cylindrical crosslinked molded article having a diameter of 29±0.5 mm and a height of 12.5±0.5 mm. The obtained crosslinked molded article can be used as a test piece for compression set measurement (JIS K 6262) described later.

[0223] [Examples 2 and 3] In Example 1, instead of liquid polybutadiene 1 (PB1), liquid polybutadiene 2 (PB2) (liquid polymer having reactive sites, product name: NISSO-PB B-1000, manufactured by Nippon Soda Co., Ltd., number average molecular weight 1200, 1,2-vinyl structure ratio: 85% or more, trans-1,4 structure ratio: 15% or less, viscosity at 45°C: 10 poise, specific gravity: 0.86) or liquid polybutadiene 3 (PB3) (liquid polymer having reactive sites, product name: NISSO-PB Nitrile rubber compositions and crosslinked molded articles of Examples 2 and 3 were obtained in the same manner as in Example 1, except that a nitrile rubber composition B-3000 manufactured by Nippon Soda Co., Ltd., number average molecular weight 3200, 1,2-vinyl structure ratio: 90% or more, trans-1,4 structure ratio: 10% or less, viscosity at 45°C: 210 poise, specific gravity: 0.87) was used.

[0224] Example 4 A nitrile rubber composition and a crosslinked molded article of Example 4 were obtained in the same manner as in Example 1, except that the liquid polybutadiene 1 was not used.

[0225] [Example 5] A nitrile rubber composition and a crosslinked molded article of Example 5 were obtained in the same manner as in Example 1, except that polypropylene (PP) having double bonds at both ends (liquid polymer having reactive active sites, product name: Polypropylene 2.0, manufactured by San-ei Kogyo Co., Ltd., number average molecular weight 10,000) was used instead of liquid polybutadiene 1 in Example 1.

[0226] [Examples 6 to 9] Nitrile rubber compositions and crosslinked molded articles of Examples 6 to 9 were obtained in the same manner as in Example 1, except that hollow particles B obtained in Production Example 2, hollow particles C obtained in Production Example 3, hollow particles D obtained in Production Example 4, or hollow particles E obtained in Production Example 5 were used in accordance with Table 2 instead of hollow particles A obtained in Production Example 1 in Example 1.

[0227] [Examples 10 to 11] Nitrile rubber compositions and crosslinked molded articles of Examples 10 to 11 were obtained in the same manner as in Example 1, except that the amount of hollow particles A obtained in Production Example 1 was changed according to Table 2.

[0228] [Examples 12 to 14] In Example 1, instead of nitrile rubber 1 (NBR1), hydrogenated nitrile rubber 2 (HNBR2) (hydrogenated acrylonitrile butadiene rubber, product name: Zetpol (registered trademark) 2030L, manufactured by Zeon Corporation, iodine value: 56.60 g / 100 g, bound acrylonitrile content: 36.2%, Mooney viscosity ML (1+4, 100°C): 57.5) or hydrogenated nitrile rubber 3 (HNBR3) (hydrogenated acrylonitrile butadiene rubber, product name: Zetpol (registered trademark) 2010L, manufactured by Zeon Corporation, iodine value: 56.60 g / 100 g, bound acrylonitrile content: 36.2%, Mooney viscosity ML (1+4, 100°C): 57.5) was used as the base elastomer according to Table 3. Nitrile rubber compositions and crosslinked molded articles of Examples 12 to 14 were obtained in the same manner as in Example 1, except that hydrogenated nitrile rubber 4 (HNBR4) (hydrogenated acrylonitrile butadiene rubber, product name: Zetpol (registered trademark) 2000L, manufactured by Zeon Corporation, iodine value: 4.00 g / 100 g, bound acrylonitrile amount: 36.2%, Mooney viscosity ML (1+4,100°C): 57.5) or hydrogenated nitrile rubber 4 (HNBR4) (hydrogenated acrylonitrile butadiene rubber, product name: Zetpol (registered trademark) 2000L, manufactured by Zeon Corporation, iodine value: 4.00 g / 100 g, bound acrylonitrile amount: 36%, Mooney viscosity ML (1+4,100°C): 65) was used.

[0229] [Comparative Example 1] A nitrile rubber composition and a crosslinked molded article of Comparative Example 1 were obtained in the same manner as in Example 1, except that the dense particles A obtained in Comparative Production Example 1 were used instead of the hollow particles A obtained in Production Example 1.

[0230] Comparative Example 2 A nitrile rubber composition and a crosslinked molded article of Comparative Example 2 were obtained in the same manner as in Example 1, except that the hollow particles A obtained in Production Example 1 were not used.

[0231] [Comparative Example 3] An elastomer composition and a molded article of Comparative Example 3 were obtained in the same manner as in Example 1, except that low-density polyethylene (LDPE) (product name: NUC-8008, manufactured by NUC Corporation, iodine value: not more than the lower limit of measurement) was used as the base elastomer instead of nitrile rubber 1 (NBR1) in Example 1.

[0232] Comparative Example 4 An elastomer composition and a crosslinked molded article of Comparative Example 4 were obtained in the same manner as in Example 1, except that nitrile rubber 1 (NBR1) in Example 1 was replaced with ethylene-propylene-diene terpolymer (EPDM) (product name: NORDEL 4725P, manufactured by The Dow Chemical Company, iodine value: 200 g / 100 g, Mooney viscosity ML (1+4, 100°C): 25).

[0233] Comparative Example 5 An elastomer composition and a crosslinked molded article of Comparative Example 5 were obtained in the same manner as in Comparative Example 4, except that the hollow particles A obtained in Production Example 1 were not used.

[0234] [Evaluation of Physical Properties of Crosslinked Molded Articles] 1. Hardness (Duro-A) The hardness (Duro-A) of the crosslinked molded articles was measured using a type A durometer in accordance with JIS K 6253-3.

[0235] 2. Specific Gravity and Weight Reduction Rate Using samples cut into 1 cm square and 2 mm thick pieces from the crosslinked molded article, the specific gravity of the crosslinked molded article was measured by the underwater displacement method in accordance with JIS K 7112:1999. The weight reduction rate (%) was calculated from the specific gravity of the crosslinked molded article measured above and the specific gravity of the base elastomer used in the crosslinked molded article using the following formula (1): Formula (1): Weight reduction rate (%) = {(specific gravity of base elastomer - specific gravity of crosslinked molded article) / specific gravity of base elastomer} x 100 The following values ​​were used for the specific gravity of the base elastomer: Specific gravity of NBR1, HNBR2-4: 1.0 Specific gravity of LDPE: 0.918 Specific gravity of EPDM: 0.9

[0236] 3. Oil Resistance The crosslinked molded article was punched out to a size of 30 mm x 20 mm x 2 mm to obtain a test piece. The obtained test piece was immersed in a fuel oil (Fuel-C) of toluene / isooctane = 50 / 50 (volume ratio) at 40°C for 168 hours to conduct a fuel oil immersion resistance test. From this, the volume swelling degree ΔV (unit: %) after immersion was calculated according to JIS K 6258. The lower the volume swelling degree ΔV after immersion, the better the oil resistance. A: ΔV is less than 80% B: ΔV is 80% or more but less than 150% C: ΔV is 150% or more

[0237] 4. Compression Set In accordance with the room temperature test of JIS K 6262:2013, a cylindrical cross-linked molded body having a diameter of 29±0.5 mm and a height of 12.5±0.5 mm was used as a test piece, and the compression set (%) of the cross-linked molded body was measured under the conditions of a standard temperature of 23±2°C, a test temperature of 30°C, a test time of 168 hours, and a compression ratio of the test piece of 25%, and evaluated based on the following evaluation criteria: SA: Compression set less than 35% A: Compression set 35% or more and less than 40% B: Compression set 40% or more and less than 60% C: Compression set 60% or more

[0238] More specifically, the compression set was measured by the following procedure. First, the thickness of the central part of the test piece was measured at standard temperature. The test piece was placed on a compression plate (a smooth stainless steel plate), and a spacer (9.3 mm thick) was sandwiched between the outside of the test piece. The compression plate was then compressed until it was in close contact with the spacer. The device compressing the test piece was kept in a thermostatic chamber at the test temperature for the test time. After the test time had elapsed, the device was removed, and the test piece was immediately released from the compressed state. After leaving it at standard temperature for 30 minutes, the thickness of the central part of the test piece was measured. The compression set (%) was calculated from the thickness of the test piece before and after compression and the thickness of the spacer using the following formula (2):

[0239]

[0240] CS: Compression set (%) t0: Original thickness of test piece (mm) t1: Spacer thickness (mm) t2: Thickness of test piece after removal from compression device 30 minutes later (mm)

[0241]

[0242]

[0243] As is clear from Tables 2 and 3, the nitrile rubber compositions containing hollow particles having an iodine value of 2 g / 100 g or more and 100 g / 100 g or less and nitrile rubber having an iodine value of 2 g / 100 g or more and 500 g / 100 g or less were capable of providing lightweight crosslinked molded articles that were excellent in compression set resistance and oil resistance and had appropriate hardness (Examples 1 to 14).

[0244] On the other hand, when solid particles were used instead of hollow particles, the weight-reducing effect of hollow particles was not obtained (Comparative Example 1), and when hollow particles were not used, not only was the weight-reducing effect of hollow particles not obtained, but the compression set resistance was also poor (Comparative Example 2).Furthermore, when LDPE or EPDM was used instead of nitrile rubber, the oil resistance was poor (Comparative Examples 3 to 5).

Claims

1. A nitrile rubber composition containing a nitrile rubber and hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, wherein the iodine value of the nitrile rubber is 2 g / 100 g or more and 500 g / 100 g or less, and the iodine value of the hollow particles is 2 g / 100 g or more and 100 g / 100 g or less, and the iodine value is a value measured in accordance with JIS K 0070.

2. The nitrile rubber composition according to claim 1, further comprising a liquid polymer having reactive sites.

3. A nitrile rubber composition according to claim 2, wherein the content of said liquid polymer is 1 part by mass or more and 20 parts by mass or less per 100 parts by mass of said nitrile rubber.

4. A nitrile rubber composition according to any one of claims 1 to 3, wherein the nitrile rubber has a Mooney viscosity ML(1+4, 100°C) of 10 to 200.

5. A nitrile rubber composition according to any one of claims 1 to 4, wherein the nitrile rubber contains α,β-ethylenically unsaturated nitrile monomer units and conjugated diene monomer units.

6. A nitrile rubber composition according to any one of claims 1 to 5, wherein the content of the hollow particles is 1 part by mass or more and 100 parts by mass or less per 100 parts by mass of the nitrile rubber.

7. The apparent density of the hollow particles is 0.1 g / cm ３ 0.8g / cm or more ３ The nitrile rubber composition according to any one of claims 1 to 6, wherein:

8. A nitrile rubber composition according to any one of claims 1 to 7, wherein the volume average particle diameter of the hollow particles is from 0.1 µm to 100 µm.

9. A nitrile rubber composition according to any one of claims 1 to 8, wherein the content of α,β-ethylenically unsaturated nitrile monomer units in the nitrile rubber is 10 to 60 mass %.

10. A nitrile rubber composition according to claim 2 or 3, wherein the liquid polymer is a liquid polymer having an ethylenic double bond.

11. A nitrile rubber composition according to any one of claims 2, 3 and 10, wherein the number average molecular weight (Mw) of the liquid polymer is 500 to 100,000.

12. The nitrile rubber composition according to any one of claims 1 to 11, further comprising a crosslinking agent.

13. A crosslinked molded article obtained by crosslinking and molding the nitrile rubber composition according to any one of claims 1 to 12.