WO2025009411A1 - Nitrile rubber composition and crosslinked molded body - Google Patents

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

This disclosure relates to a nitrile rubber composition and a cross-linked molded article.

Traditionally, nitrile rubber (acrylonitrile-butadiene copolymer rubber) has been used in a wide range of rubber parts, such as hoses, tubes, seals, belts, hoses, and diaphragms, 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 also meeting the demand for lighter rubber parts.

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

International Publication No. 2019/058840

The object of the present invention is to provide a nitrile rubber composition that can give a lightweight cross-linked molded article that has excellent resistance to compression set and oil resistance and has appropriate hardness.

The inventors conducted extensive research to find a solution to the above problems, and discovered that the above problems could be solved by combining hollow particles having an iodine value of 2 g/100 g or more and 100 g/100 g or less with nitrile rubber having an iodine value of 2 g/100 g or more and 500 g/100 g or less, thereby completing the present invention.

In other words, according to the present disclosure, the following nitrile rubber composition and cross-linked molded article are provided.

[1] A nitrile rubber composition comprising a nitrile rubber and a hollow particle having a shell containing a resin and a hollow portion surrounded by the shell,
The iodine value of the nitrile rubber is 2 g/100 g or more and 500 g/100 g or less,
The iodine value of the hollow particles is 2 g/100 g or more and 100 g/100 g or less,
The iodine value is a value measured in accordance with JIS K 0070.
[2] The nitrile rubber composition according to [1], further comprising a liquid polymer having a reactive site.
[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 with respect to 100 parts by mass of the nitrile rubber.
[4] The nitrile rubber composition according to any one of [1] to [3], wherein the nitrile rubber has a Mooney viscosity ML(1+4,100°C) of 10 to 200.
[5] The nitrile rubber composition according to any one of [1] to [4], wherein the nitrile rubber contains an α,β-ethylenically unsaturated nitrile monomer unit and a conjugated diene monomer unit.
[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 with respect to 100 parts by mass of the nitrile rubber.
[7] The nitrile rubber composition according to any one of [1] to [6], wherein the apparent density of the hollow particles is 0.1 g/cm ³ or more and 0.8 g/cm ³ or less.
[8] The nitrile rubber composition according to any one of [1] to [7], wherein the volume average particle diameter of the hollow particles is 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 content of α,β-ethylenically unsaturated nitrile monomer units in the nitrile rubber is 10 to 60 mass%.
[10] The nitrile rubber composition according to any one of [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 number average molecular weight (Mw) of the liquid polymer is 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 and molding the nitrile rubber composition according to any one of [1] to [12].

The present invention provides a nitrile rubber composition that has excellent compression set resistance and oil resistance, and can give a lightweight cross-linked molded product with appropriate hardness.

FIG. 2 is a schematic diagram showing an example of a method for producing hollow particles used in the present disclosure.

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. In the present disclosure, the iodine value measured in accordance with JIS K 0070 may be simply referred to as the iodine value.

The iodine value of the hollow particles can be used as an index 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, at least during crosslinking, 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. Therefore, in the crosslinked molded product obtained from the nitrile rubber composition of the present disclosure, the outer surface of the hollow particles and the nitrile rubber are appropriately crosslinked, so that the adhesion of the interface between the hollow particles and the nitrile rubber is excellent, and the interface between the hollow particles and the nitrile rubber is not easily peeled off. In addition, the hollow particles used in the present disclosure have a high restoring force when an external force is applied, and are not easily plastically deformed. 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 believed that the interaction between such hollow particles and the nitrile rubber results in extremely excellent compression set resistance.

In addition, in the crosslinked molded article obtained from the nitrile rubber composition of the present disclosure, the hollow particles are difficult to crush and the voids are easily maintained, so that the hollow particles have an excellent weight reduction effect. It is presumed that the reason why the hollow particles are difficult to crush in the obtained 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 of the nitrile rubber. It is considered that the three-dimensional crosslinked structure improves the Young's modulus of the obtained crosslinked molded article, and makes it possible to reduce the compression set and improve the abrasion resistance. Furthermore, it is considered that the three-dimensional crosslinked structure provides a synergistic effect between the oil resistance of the nitrile rubber and the oil resistance of the hollow particles, and as a result, a crosslinked molded article with very excellent oil resistance is obtained.

The iodine value of the hollow particles used in this 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 more suitably form cross-linking bonds between the hollow particles and the nitrile rubber while favorably dispersing the hollow particles in the nitrile rubber, so that the effect of this disclosure becomes even more remarkable.

In order to further reduce the compression set of the resulting crosslinked molded article, the iodine value of the hollow particles used in this 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.

The hollow particles used in this disclosure are particles that have 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.

The reactive unsaturated bond may be, for example, a reactive unsaturated bond 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, or 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.

In the hollow particles used in this 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 in the surface treatment, but is preferably a reactive unsaturated bond contained in a crosslinkable monomer unit. In other words, it is preferable that at least one polymerizable functional group possessed by 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 this disclosure will be described later.

The resin contained in the shell of the hollow particles used in this disclosure is typically a polymer of a polymerizable monomer used in the manufacturing method of hollow particles described below. The shell of the hollow particles may further contain a surface treatment agent or additives different from the resin, as long as the object of this disclosure is not impaired. In the hollow particles used in this 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.

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 is surface-treated with a coupling agent. If the shell of the hollow particles used in the present disclosure is surface-treated, this is preferable in that the mechanical properties such as tensile strength, tear strength, and abrasion resistance are improved in the crosslinked molded article containing the hollow particles used in the present disclosure.

In the hollow particles used in this disclosure, the hollow portion is a hollow space that is clearly distinguishable from the shell. The shell of the hollow particles may have a porous structure, but in that case, the hollow portion has a size that is clearly distinguishable from the numerous tiny spaces uniformly dispersed within the porous structure. From the standpoint of mechanical strength, etc., it is preferable that the hollow particles used in this disclosure have a solid shell. In addition, from the standpoint of weight reduction effect, it is preferable that the hollow portions of the hollow particles used in this disclosure are filled with a gas such as air.

The hollow particles used in this 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 this 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%. In addition, the shell of the hollow particles used in this 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 terms of further improving the effect of reducing compression set.

The shape of the hollow particles used in this disclosure may be, for example, spherical, ellipsoidal, or amorphous, but from the standpoint of dispersibility and pressure resistance of the hollow particles, a spherical shape is preferable.

One example of the shape of the hollow particles used in this disclosure is a bag made of a thin film and inflated with gas, the cross-sectional view of which is shown as hollow particle 10 in (5) of Figure 1. In this example, a 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 as is with a TEM. The shape of the hollow particle can be confirmed, for example, by observing the hollow particle with an SEM or TEM.

The hollow particles used in the present disclosure may contain a small amount of impurities such as particles with low circularity caused by cracking or deformation, but from the viewpoint of further enhancing the effects of the present disclosure, the proportion of particles with a circularity of 0.85 or less out of 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.

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 this disclosure. By reducing the proportion of irregularly shaped particles contained in the hollow particles, the weight reduction effect of the hollow particles can be further improved. In addition, irregularly shaped particles are more likely to aggregate when dispersed in nitrile rubber than spherical particles, and have poor dispersibility. Therefore, by reducing the proportion of irregularly shaped particles contained in the 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.

Circularity is defined as the diameter of a circle having the same area as the projected image of a particle (equivalent diameter of circle area) divided by the diameter of a circle having the same perimeter as the projected image of a particle (equivalent diameter of oval). If a particle is a perfect sphere, the circularity is 1, and the more complex the surface shape of the particle, the smaller the circularity. The hollow particles used in this disclosure may have an average circularity of 0.950 to 0.995.

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

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 calculated from the apparent density _D1 and true density _D0 of the hollow particles.

The method for measuring the apparent density _D1 of the hollow particles is as follows. First, fill a 100 ^cm3 volumetric flask with about 30 ^cm3 of hollow particles, and accurately weigh the mass of the filled hollow particles. Next, fill the volumetric flask filled with the hollow particles with isopropanol precisely up to the mark, taking care not to introduce air bubbles. The mass of isopropanol added to the volumetric flask is accurately weighed, and the apparent density _D1 (g/ ^cm3 ) of the hollow particles is calculated based on the following formula (I).
Formula (I)
Apparent density D ₁ =[mass of hollow particles]/(100-[mass of isopropanol]/[specific gravity of isopropanol at measurement temperature])
The apparent density _D1 corresponds to the specific gravity of the entire hollow particle when the hollow portion is considered to be a part of the hollow particle.

The method for measuring the true density _D0 of hollow particles is as follows. After crushing the hollow particles in advance, about 10 g of crushed pieces of the hollow particles are filled into a measuring flask with a capacity of 100 ^cm3 , and the mass of the crushed pieces filled is accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol is added to the measuring flask, the mass of isopropanol is accurately weighed, and the true density _D0 (g/ ^cm3 ) of the hollow particles is calculated based on the following formula (II).
Formula (II)
True density D ₀ =[mass of crushed pieces of hollow particles]/(100-[mass of isopropanol]/[specific gravity of isopropanol at measurement temperature])
The true density _D0 corresponds to the specific gravity of only the shell portion of the hollow particle. As is clear from the above measurement method, the hollow portion is not considered to be part of the hollow particle when calculating the true density _D0 .

The porosity (%) of the hollow particles is calculated from the apparent density _D1 and the true density _D0 of the hollow particles by the following formula (III).
Formula (III)
Porosity (%)=100−(apparent density D ₁ /true density D ₀ )×100

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/cm3 or ^less , more preferably 0.6 g/ ^cm3 or less, and even more preferably 0.4 g/cm3 or ^less . The lower limit of the apparent density 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 0.1 g/ ^cm3 or more, more preferably 0.2 g/ ^cm3 or more.

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 more suitably form cross-linking bonds between the hollow particles and the nitrile rubber while favorably dispersing the hollow particles in the nitrile rubber, so that the effect of the present disclosure becomes even more remarkable.

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 to 1.30, and more preferably 1.10 to 1.30.

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

The residual volatile component content of the hollow particles used in this 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. The residual volatile components on the outer surface of the hollow particles may inhibit the cross-linking reaction between the hollow particles and the nitrile rubber. If the residual volatile component content of the hollow particles is within the above range, the above reaction inhibition is less likely to occur, and the cross-linking between the hollow particles and the nitrile rubber can be more suitably formed, so that the effect of this disclosure becomes more pronounced. The residual volatile components are usually 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 this 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 a purge and trap/gas chromatography (P&T/GC) method. Specifically, the measurement method in the examples described later can be adopted.

The hollow particles used in this disclosure 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 compound may be either organic or inorganic. The surfactants, etc. present on the outer surface of the hollow particles may inhibit the cross-linking reaction between the hollow particles and the nitrile rubber. If the content is below the upper limit value, the reaction inhibition is less likely to occur, and cross-linking between the hollow particles and the nitrile rubber can be more suitably formed, so that the effect of this disclosure becomes more pronounced. In the manufacturing process of the hollow particles described below, by using only inorganic dispersion stabilizers as dispersion stabilizers, the content of surfactants, etc. present on the hollow particle surface can be made less than the measurement limit value.

In this disclosure, the content of surfactants, etc. present on the outer surface of hollow particles refers to the ratio of the mass of 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 ultrasonicating the hollow particles in water. The type and mass of the surfactants, etc. extracted into the water can be identified from the peak position and peak intensity of the 1H-NMR spectrum. In this method, the measurement limit for the amount of surfactants, etc. present on the hollow particle surface is usually 0.05 ppm.

The thermal decomposition onset temperature of the hollow particles used in this disclosure is not particularly limited, but is preferably 345°C or higher, more preferably 350°C or higher, from the viewpoint of heat resistance. The upper limit of the thermal decomposition onset temperature of the hollow particles is not particularly limited, but may be, for example, 400°C or lower. In this disclosure, the thermal decomposition onset temperature of the hollow particles is the temperature at which a 5% mass reduction 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.

The hollow particles used in this disclosure can impart various properties to the crosslinked molded article, such as reducing the weight of the crosslinked molded article, reducing compression set, and improving oil resistance, as well as insulating the crosslinked molded article and retaining functional components such as antibacterial agents. Applications of the nitrile rubber composition and crosslinked molded article of this disclosure will be described in detail in "2. Nitrile rubber composition" below.

The hollow particles used in this disclosure are manufactured, for example, by the manufacturing method described below, which is based on suspension polymerization.

As an embodiment of the method for producing hollow particles used in the present disclosure, for example,
A step of preparing a mixed liquid containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium;
a step of suspending the mixed liquid 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;
a step of 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 the hollow portion filled with the hydrophobic solvent are dispersed in the aqueous medium;
removing the hydrophobic solvent from the precursor particles;
In the present disclosure, a hollow particle having a hollow portion filled with a hydrophobic solvent may be referred to as a "precursor particle" since the hollow particle having a hollow portion filled with a gas is considered to be an intermediate of the hollow particle having a hollow portion filled with a gas. In the present disclosure, a "precursor composition" refers to a composition containing the precursor particle.

In the above 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 are phase-separated, and a suspension is prepared in which droplets of the monomer composition having a distribution structure in which the polymerizable monomer is unevenly distributed on the surface side and the hydrophobic solvent is unevenly distributed in the center are dispersed in the aqueous medium. When this suspension is subjected to a polymerization reaction, a polymer begins to precipitate on the surface of the droplets of the monomer composition, and as the polymerization reaction progresses, the surface of the droplets hardens and a shell is formed, resulting in hollow particles with a hollow portion filled with the hydrophobic solvent. By including a crosslinkable monomer in the mixture, reactive unsaturated bonds can be introduced into the outer surface of the shell.

The above manufacturing method includes a step of preparing a mixed liquid, 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 one step, or the order may be changed. For example, the preparation of the mixed liquid and the suspension may be performed simultaneously in one step, such as adding the materials for preparing the mixed liquid and suspending them at the same time.

A preferred example of the method for producing hollow particles includes the following steps.
(1) Mixture preparation step: A step of preparing a mixture containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium;
(2) Suspension step: A step of preparing 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 by suspending the mixed liquid;
(3) A polymerization step: a step of 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 the hollow portion filled with the hydrophobic solvent are dispersed in the aqueous medium;
(4) a solid-liquid separation step: a step of obtaining a solid fraction containing precursor particles by performing solid-liquid separation of the precursor composition; and (5) a solvent removal step: a step of removing the hydrophobic solvent from the precursor particles obtained by the solid-liquid separation step, thereby obtaining hollow particles.

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 each diagram indicate the order of each step. Note that FIG. 1 is merely a schematic diagram for explanatory purposes, and the manufacturing method of the present disclosure is not limited to that shown in the diagram. Furthermore, the structure, dimensions, and shape of the materials used in the manufacturing method of the present disclosure are not limited to the structure, dimensions, and shape of the various materials in these diagrams.

(1) in FIG. 1 is a schematic cross-sectional view showing one embodiment of the mixed liquid in the mixed liquid preparation process. As shown in this figure, the mixed liquid 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 this disclosure, the low-polarity material 2 contains a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator.

(2) of FIG. 1 is a schematic cross-sectional view showing one embodiment of the suspension in the suspension process. 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 include 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 have a structure in which the hydrophobic solvent 4a and the material other than the hydrophobic solvent including the polymerizable monomer 4b are phase-separated, the hydrophobic solvent 4a is unevenly distributed in the center, the material other than the hydrophobic solvent 4b is unevenly distributed on the surface side, and a dispersion stabilizer (not shown) is attached to the surface.

(3) in FIG. 1 is a schematic cross-sectional view showing one embodiment of a precursor composition containing precursor particles having a hydrophobic solvent encapsulated in their hollow portions, obtained by a polymerization process. The precursor composition contains an aqueous medium 1 and precursor particles 9 that are dispersed in the aqueous medium 1 and encapsulate a hydrophobic solvent 4a in their hollow portions. The shell 6 that forms 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.

(4) in FIG. 1 is a schematic cross-sectional view showing one embodiment of precursor particles after the solid-liquid separation process. (4) in FIG. 1 shows the state after removing the aqueous medium 1 from the state shown in (3) in FIG. 1 above.

(5) in FIG. 1 is a schematic cross-sectional view showing one embodiment of hollow particles after the solvent removal step. (5) in FIG. 1 shows the state in which the hydrophobic solvent 4a has been removed from the state shown in (4) in FIG. 1 above. By removing the hydrophobic solvent from the precursor particles, hollow particles 10 having hollow portions 7 filled with gas inside the shells 6 are obtained.

The above five steps and other steps are explained below in order.

(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 within a range that does not impair the purpose of the present disclosure. The materials of the mixed liquid will be described in the order of (A) polymerizable monomer, (B) hydrophobic solvent, (C) polymerization initiator, (D) dispersion stabilizer, and (E) aqueous medium.

(A) Polymerizable Monomer In the present disclosure, the 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 radical 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.

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 crosslink 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.

In addition, in this 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. Also, 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, it is sufficient that at least one polymerizable functional group is a (meth)acryloyl group, but it is preferable that all polymerizable functional groups are (meth)acryloyl groups. In this 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.

The polymerizable monomer may be any known polymerizable monomer that has been conventionally used to prepare hollow particles, and is not particularly limited, but it is preferable to include at least a crosslinkable monomer in order to set the iodine value of the hollow particles within the above range. It is presumed that the iodine value of the hollow particles falls within the above range as a result of a portion of the reactive unsaturated bonds of the crosslinkable monomer remaining unreacted on the outer surface of the shell.

Furthermore, when a crosslinkable monomer is included as the polymerizable monomer, the crosslink density of the polymer precipitated on the surface of the droplets increases when the suspension is subjected to a polymerization reaction, and the precipitates are also crosslinked with each other, so the crosslink density of the shell can be increased. As a result, a shell with excellent strength is easily formed, the hollow particles are easily spherical, and hollow portions that are 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, so by improving the strength of the shell, the effects of the present disclosure can be further enhanced.

Examples of crosslinkable monomers include aromatic divinyl monomers such as divinylbenzene, divinylbiphenyl, and divinylnaphthalene; straight-chain or branched diolefins such as butadiene, isoprene, 2,3-dimethylbutadiene, pentadiene, and hexadiene; diene monomers such as alicyclic diolefins such as dicyclopentadiene, cyclopentadiene, and ethylidenetetracyclododecene; crosslinkable hydrocarbon monomers such as crosslinkable macromers such as polybutadiene, polyisoprene, styrene and butadiene block copolymers (SBS), and styrene and 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 crosslinkable acrylic monomers include bifunctional 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, and crosslinkable acrylic monomers such as their ethoxylated derivatives; crosslinkable allylic monomers such as diallyl phthalate; crosslinkable macromers such as polyphenylene ether modified with vinyl at both ends and polyphenylene ether modified with (meth)acrylic at both ends. These crosslinkable monomers can be used alone or in combination of two or more.

In the present disclosure, in order to set the iodine value of the hollow particles within the above range and to improve the strength of the shell, it is preferable that the crosslinkable monomer contains at least one selected from a crosslinkable acrylic monomer and a crosslinkable hydrocarbon monomer, it is more preferable to contain at least a crosslinkable acrylic monomer, and it is particularly preferable to contain a combination of a crosslinkable acrylic monomer and a crosslinkable hydrocarbon monomer. Among crosslinkable monomers, crosslinkable acrylic monomers are preferable from the viewpoint of improving the strength of the shell, while crosslinkable hydrocarbon monomers are preferable since they facilitate the introduction of reactive unsaturated bonds into the outer surface of the shell.

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

In the present disclosure, in order to set the 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 be composed of a crosslinkable monomer, but may further contain a non-crosslinkable monomer 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.

In order to set the iodine value of the hollow particles within the above range and to improve 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.

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 the 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 the 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, if there is a demand to reduce the hardness (surface hardness) of the obtained crosslinked molded product, the content of the crosslinkable hydrocarbon monomer in 100% by mass of the 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 the 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.

In addition, when a bifunctional crosslinkable acrylic monomer and a trifunctional or higher crosslinkable acrylic monomer are combined, in order to set the iodine value of the hollow particles within the above range and to improve the strength of the shell, the content of the bifunctional crosslinkable acrylic monomer relative to the total 100% by mass of the bifunctional crosslinkable acrylic monomer and the trifunctional or higher crosslinkable acrylic monomer (the content of the bifunctional crosslinkable acrylic monomer unit relative to the total 100% by mass of the bifunctional crosslinkable acrylic monomer unit and the trifunctional or higher crosslinkable acrylic monomer unit in the polymer constituting the shell) is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more, and the upper limit is preferably 70% by mass or less, more preferably 60% by mass or less, and even more preferably 50% by mass or less.

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.

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 crosslinkable acrylic monomer and crosslinkable hydrocarbon monomer in 100 parts by mass of crosslinkable monomer (total content of crosslinkable acrylic monomer units and crosslinkable hydrocarbon monomer units in 100 parts by mass of 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 even more preferably 99 parts by mass or more.

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; methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, and t-butyl. Examples of the 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 polystyrene modified at the terminals with (meth)acrylic acid and polymethyl methacrylate modified at the terminals with (meth)acrylic acid. These non-crosslinkable monomers can be used alone or in combination of two or more.

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, in order to keep the iodine value of the hollow particles within the above range and to prevent 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.

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

In addition, 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 is the material that forms the oil phase in the mixed liquid, is preferably 96% by mass or more, and more preferably 97% by mass or more. In this disclosure, the solids are all components excluding the solvent, and liquid polymerizable monomers, etc. are considered to be included in the solids.

(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.

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 the monomer composition containing the hydrophobic solvent are dispersed in an aqueous medium. In the suspension process, phase separation occurs in 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 containing 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.

The hydrophobic solvent can be appropriately selected from known hydrophobic solvents, and is not particularly limited. Examples of the hydrophobic solvent 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 the hydrocarbon solvent 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.

In the suspension step, since phase separation between the polymerizable monomer and the hydrophobic solvent is likely to occur within the droplets of the monomer composition, it is preferable to select an organic solvent that has a lower solubility in water than the crosslinkable monomer contained in the polymerizable monomer as the hydrophobic solvent. Furthermore, when the polymerizable monomer contains 30% by mass or more of a hydrocarbon monomer, the hydrophobic solvent is preferably a hydrocarbon-based solvent, more preferably a chain hydrocarbon-based solvent, even more preferably a chain hydrocarbon-based 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.

Although not particularly limited, the boiling point of the hydrophobic solvent is preferably 130°C or lower, more preferably 100°C or lower, in order to facilitate removal in the solvent removal step described below, and is preferably 50°C or higher, more preferably 60°C or higher, in order to facilitate 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 equal to or lower than the upper limit value, 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 equal to or higher than the lower limit value.

Furthermore, it is preferable that the hydrophobic solvent has a relative dielectric constant of 2.5 or less at 20°C. The relative dielectric constant is one of the indicators that shows the polarity of a compound. When the relative dielectric constant of the hydrophobic solvent is sufficiently small, such as 2.5 or less, it is considered that phase separation proceeds quickly in the droplets of the monomer composition, and hollow portions are easily formed. Examples of hydrophobic solvents having a relative dielectric constant of 2.5 or less at 20°C are as follows. The values in parentheses are the values of the relative dielectric constant. Pentane (1.8), hexane (1.9), heptane (1.9), octane (1.9), cyclohexane (2.0). For the relative dielectric constant at 20°C, reference can be made to the values described in known literature (for example, "Chemical Handbook Basics" edited by the Chemical Society of Japan, Revised 4th Edition, Maruzen Co., Ltd., published on September 30, 1993, pages II-498 to II-50 3) and other technical information. An example of a method for measuring the relative dielectric constant at 20°C is a relative dielectric constant test that conforms to JIS C 2101:1999, paragraph 23, and is performed at a measurement temperature of 20°C.

By changing the amount of hydrophobic solvent in the mixture, the porosity of the hollow particles can be adjusted. In the suspension process described below, the polymerization reaction proceeds with the oil droplets containing the polymerizable monomer and the like encapsulating the hydrophobic solvent, so the higher the content of the hydrophobic solvent, the higher the porosity of the resulting hollow particles tends to be. In the present disclosure, the content of the hydrophobic solvent in the mixture is preferably 100 parts by mass or more and 650 parts by mass or less relative to 100 parts by mass of the polymerizable monomer, because this makes it easier to control the particle size of the hollow particles, makes it easier to increase the porosity while maintaining the strength of the hollow particles, and makes it easier to reduce the amount of residual hydrophobic solvent in the particles. The content of the hydrophobic solvent in the mixture is more preferably 120 parts by mass or more and 500 parts by mass or less, and even more preferably 140 parts by mass or more and 300 parts by mass or less relative to 100 parts by mass of the polymerizable monomer.

(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.

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-butyl peroxydiethyl acetate, and t-butyl peroxypivalate; and azo compounds such as 2,2'-azobis(2,4-dimethylvaleronitrile), azobisisobutyronitrile, and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile). 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, relative to 100 parts by mass of the polymerizable monomer in the mixed liquid. If the content of the polymerization initiator is equal to or greater than the lower limit, the polymerization reaction can proceed sufficiently, and if the content is equal to or less than the upper limit, there is little risk of the polymerization initiator remaining after completion of the polymerization reaction, and there is also little risk of an unexpected side reaction proceeding.

(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.

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.

As the inorganic dispersion stabilizer, 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. As the poorly water-soluble inorganic dispersion stabilizer, a metal hydroxide is preferable, and magnesium hydroxide is more preferable.

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

A colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles can be prepared, for example, by reacting at least one selected from alkali metal hydroxides and alkaline earth metal hydroxides with a water-soluble polyvalent metal salt (excluding alkaline earth metal hydroxides) in an aqueous medium.

Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Examples of alkaline earth metal hydroxides include barium hydroxide and calcium hydroxide.

The water-soluble polyvalent metal salt may be any water-soluble polyvalent metal salt other than the above-mentioned alkaline earth metal hydroxides, 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.

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

In addition, colloidal silica can be used as a colloidal dispersion liquid containing poorly water-soluble inorganic dispersion stabilizer colloidal particles.

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

The content of the dispersion stabilizer is not particularly limited, but is preferably 0.5 to 15 parts by mass, and more preferably 1 to 10 parts by mass, per 100 parts by mass of the total mass of the polymerizable monomer and the hydrophobic solvent. By having the content of the dispersion stabilizer be equal to or greater than the lower limit, the droplets of the monomer composition can be sufficiently dispersed so as not to coalesce in the suspension. On the other hand, by having the content of the dispersion stabilizer be equal to or less than the upper limit, it is possible to prevent the viscosity of the suspension from increasing during granulation, and to avoid the problem of the suspension clogging the granulator. In addition, the content of the dispersion stabilizer is preferably 0.5 to 15 parts by mass, and more preferably 0.5 to 10 parts by mass, per 100 parts by mass of the aqueous medium.

In the present disclosure, in order to suppress 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 in which both the water-soluble polymer compound and the surfactant are below the detection limit can be obtained.

(E) Aqueous Medium In the present disclosure, an aqueous medium means a medium selected from the group consisting of water, a hydrophilic solvent, and a mixture of water and a hydrophilic solvent.

When using a mixture of water and a hydrophilic solvent, it is important that the polarity of the entire mixture is not too low in terms of forming droplets of the monomer composition. In this case, for example, the mass ratio of water to hydrophilic solvent (water:hydrophilic solvent) may be 99:1 to 50:50.

The hydrophilic solvent in this disclosure is not particularly limited as long as it is sufficiently miscible with water and does not cause phase separation. Examples of hydrophilic solvents include alcohols such as methanol and ethanol; tetrahydrofuran (THF); dimethyl sulfoxide (DMSO); etc.

The amount 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 1000 parts by mass or less, and more preferably 800 parts by mass or less.

The mixture may further contain other materials different from the above-mentioned materials (A) to (E) as long as the purpose of this disclosure is not impaired.

The above-mentioned materials and other materials as necessary are mixed and appropriately stirred to obtain a mixed liquid. In the mixed liquid, the oil phase containing the above-mentioned (A) polymerizable monomer, (B) hydrophobic solvent, and (C) lipophilic materials such as a polymerization initiator is dispersed with particle sizes of about several mm in the aqueous phase containing (D) a dispersion stabilizer and (E) an aqueous medium. The dispersion state of these materials in the mixed liquid can be observed with the naked eye depending on the type of material.

In the mixed solution preparation step, the mixed solution may be obtained by simply mixing the above-mentioned materials and other materials as necessary and stirring appropriately, but in terms of making the shell more uniform, it is preferable to prepare the mixed solution by separately preparing in advance 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 mixing these. In the present disclosure, a colloidal dispersion in which a poorly water-soluble inorganic dispersion stabilizer is dispersed in an aqueous medium in the form of colloidal particles can be preferably used as the aqueous phase.

In this way, by preparing the oil phase and the water phase separately and then mixing them, hollow particles with a uniform shell composition can be produced, and the particle size of the hollow particles can be easily controlled.

(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.

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

In dispersing to prepare a suspension, the rotation speed of the dispersing machine is preferably 100 rpm or more, more preferably 200 rpm or more, and even more preferably 300 rpm or more, in order to form hollow portions and to set the volume average particle size of the hollow particles within the above-mentioned preferred range, while it is preferably 30,000 rpm or less, more preferably 10,000 rpm or less, and even more preferably 5,000 rpm or less, in order to reduce the proportion of irregularly shaped particles.

In the suspension prepared in the suspension process, droplets of the monomer composition containing the lipophilic material and having a particle size of about 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. In the suspension process, phase separation occurs in the droplets of the monomer composition, so that the hydrophobic solvent with low polarity tends to collect inside the droplets. As a result, the droplets obtained have the hydrophobic solvent distributed 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 the dispersion stabilizer. The droplets of the monomer composition contain an oil-soluble polymerization initiator, a polymerizable monomer, and a hydrophobic solvent.

The droplets of the monomer composition are tiny oil droplets, and the oil-soluble polymerization initiator generates polymerization initiation radicals inside the tiny oil droplets. Therefore, precursor particles of the desired particle size can be produced without the tiny oil droplets growing too much. In suspension polymerization methods using such oil-soluble polymerization initiators, 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 solid particles with a relatively small particle size in addition to the desired resin particles with hollow portions.

(3) Polymerization step This step is a step of preparing 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 by subjecting the suspension obtained in the above-mentioned suspension step to a polymerization reaction. The precursor particles are formed by polymerization of a polymerizable monomer contained in droplets of the monomer composition, and the shell of the precursor particles contains a polymer of the polymerizable monomer as a resin.

There are no particular limitations on the polymerization method, and for example, a batch method, a semi-continuous method, or a continuous method can be used. The polymerization temperature is preferably 40 to 90°C, and more preferably 50 to 80°C. The polymerization reaction time is preferably 1 to 48 hours, and more preferably 1 to 36 hours.

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

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

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

After the solid-liquid separation step, an optional step such as a pre-drying step may be carried out before carrying out the solvent removal step described below. An example of a pre-drying step is a step in which the solid obtained after the solid-liquid separation step is pre-dried using a drying device such as a dryer or a drying tool such as a hand dryer.

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

For example, after the solid-liquid separation process described above, the hydrophobic solvent contained in the precursor particles is removed in the air, and the hydrophobic solvent inside the precursor particles is replaced with air, resulting in hollow particles filled with gas. In this process, "in the air" strictly speaking means an environment in which there is absolutely no liquid outside the precursor particles, and an environment in which there is only a very small amount of liquid outside the precursor particles that does not affect the removal of the hydrophobic solvent. "In the air" can also be expressed as a state in which the precursor particles are not present in a slurry, or a state in which the precursor particles are present in a dry powder. That is, 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.

The method for removing the hydrophobic solvent in the precursor particles in air is not particularly limited, and known methods can be used. Examples of such methods include vacuum drying, heat drying, air flow drying, or a combination of these methods. In particular, when using heat drying, 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 composition of the shell in the precursor particles and the type of hydrophobic solvent, the heating temperature may be, for example, 50 to 200°C, 70 to 200°C, or 100 to 200°C.

By drying in air, the hydrophobic solvent inside the precursor particles is replaced by the external gas, resulting in hollow particles with the hollow space filled with gas. The drying atmosphere is not particularly limited and can be selected appropriately depending on the application of the hollow particles. Possible drying atmospheres include, for example, air, oxygen, nitrogen, argon, etc. Also, hollow particles with a temporary vacuum inside can be obtained by filling the inside of the hollow particles with gas and then drying under reduced pressure.

As an alternative method, the hydrophobic solvent contained in the precursor particles may be removed in a slurry containing the precursor particles and an aqueous medium without performing solid-liquid separation of the slurry-like precursor composition obtained in the polymerization step. In this method, for example, the hydrophobic solvent contained in the precursor particles can be removed by bubbling an inert gas into 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 types of 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.

The temperature at which the inert gas is bubbled into the precursor composition is preferably at least 30°C below the boiling point of the hydrophobic solvent, and more preferably at least 20°C below that, in order to reduce the amount of hydrophobic solvent remaining in the hollow particles. The temperature at which the bubbling is performed is usually at least the polymerization temperature in the polymerization step. Although not particularly limited, the temperature at which the bubbling is performed may be 50°C or higher and 100°C or lower. Although not particularly limited, the inert gas to be bubbled may be, for example, nitrogen or argon. The bubbling conditions are appropriately adjusted depending on the type and amount of the hydrophobic solvent so that the hydrophobic solvent contained in the precursor particles can be removed. Although not particularly limited, for example, the inert gas may be bubbled at a rate of 1 to 3 L/min for 1 to 10 hours.

In this method, a slurry of hollow particles containing an inert gas is obtained. The slurry is separated into solid and liquid to obtain hollow particles, which are then dried and the aqueous medium remaining in the hollow particles is removed to obtain hollow particles whose hollow portions are filled with gas.

Compared between a method of obtaining hollow particles whose hollow portions are filled with gas by performing solid-liquid separation of a slurry-like precursor composition and then removing the hydrophobic solvent in the precursor particles in air, and a method of obtaining hollow particles whose hollow portions are filled with gas by removing the hydrophobic solvent contained in the precursor particles in a slurry containing precursor particles and an aqueous medium, performing solid-liquid separation, and removing the aqueous medium remaining in the hollow particles in air, the former method has the advantage that the hollow particles are less likely to be crushed in the process of removing the hydrophobic solvent, and the latter method has the advantage that the amount of remaining hydrophobic solvent is reduced by performing bubbling with an inert gas.

　Other methods for removing the hydrophobic solvent contained in the precursor particles after the polymerization step and before the solid-liquid separation step without performing solid-liquid separation of the slurry-like precursor composition obtained in the polymerization step include, for example, a method of evaporating and distilling off the hydrophobic solvent contained in the precursor particles from the precursor composition under a predetermined pressure (high pressure, normal pressure, or reduced pressure); or a method of 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) to evaporate and distill off the solvent.

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

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

A coupling agent has a functional group capable of bonding with an organic substance and a functional group capable of bonding with an inorganic substance in one molecule, and can increase the affinity between the organic material and the inorganic material. As a coupling agent, it is preferable that the functional group in its molecular structure is a functional group capable of crosslinking with nitrile rubber, which will be described later. The functional group capable of crosslinking with nitrile rubber is appropriately selected depending on the type of nitrile rubber, and is not particularly limited, but examples thereof 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, and epoxy groups are particularly preferred. Examples of coupling agents include silane coupling agents, titanium coupling agents, and aluminum coupling agents.

Silane coupling agents include, for example, 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; alkoxysilanes having an epoxy group, such as γ-glycidoxypropyltrimethoxysilane, β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane and γ-glycidoxypropylmethyldiethoxysilane. Examples 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; methyltriacetoxysilane; and the like.

Titanium coupling agents include, for example, 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(dioctylpyrophosphate)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, etc.

Examples of aluminum coupling agents include acetoalkoxyaluminum diisopropylate.

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

(6-b) Sieving process (foreign matter removal process)
The method for producing hollow particles used in the present disclosure preferably includes a sieving step after the above-mentioned solvent removal step. By carrying out the sieving step, coarse powder and aggregates can be removed, and foreign matter can be easily removed.

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 mesh carrying the hollow particles is vibrated to obtain the hollow particles that have passed through the mesh, thereby obtaining the sieved hollow particles. The mesh openings used in the sieving step are appropriately selected according to the size of the hollow particles. It is preferable that the mesh openings are such that the proportion of particles with a circularity of 0.85 or less in the obtained hollow particles is less than 15 mass%.

(6-c) Washing step The washing step is a step of adding an acid or an alkali to wash the precursor particles or hollow particles to remove the dispersion stabilizer remaining in the precursor particles or hollow particles. When the dispersion stabilizer used is an inorganic dispersion stabilizer soluble in acid, it is preferable to add an acid to the slurry containing the precursor particles or hollow particles to perform washing, whereas when the dispersion stabilizer used is an inorganic dispersion stabilizer soluble in alkali, it is preferable to add an alkali to the slurry containing the precursor particles or hollow particles to perform washing.

When an inorganic dispersion stabilizer soluble in acid is used as the dispersion stabilizer, it is preferable to add an acid to the slurry containing the 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 of its high efficiency in removing the dispersion stabilizer and its small burden on the manufacturing equipment.

(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. By such substitution, it is possible to change the environment inside the hollow particles, selectively confine molecules inside the hollow particles, or modify the chemical structure inside the hollow particles according to the application.

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.

The nitrile rubber used in this disclosure is not particularly limited, but examples include copolymers obtained by copolymerizing an α,β-ethylenically unsaturated nitrile monomer and a conjugated diene monomer used as needed.

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-halogenoacrylonitrile such as α-chloroacrylonitrile and α-bromoacrylonitrile; and α-alkylacrylonitrile such as methacrylonitrile; with acrylonitrile being preferred. The α,β-ethylenically unsaturated nitrile monomer may be used alone or in combination with a plurality of types thereof. 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.

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 this disclosure is preferably 40 to 90% by mass, more preferably 50 to 80% by mass, and even more preferably 55 to 70% by mass.

The nitrile rubber used in this disclosure may be a copolymer of the α,β-ethylenically unsaturated nitrile monomer and the conjugated diene monomer used as necessary, as well as 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, copolymerizable antiaging agents, and the like. These other copolymerizable monomers may be used alone or in combination.

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 at least one unsubstituted (free) carboxyl group that is not esterified. 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 monomers also include monomers in which the carboxyl groups of these monomers form carboxylates. Furthermore, anhydrides of α,β-ethylenically unsaturated polycarboxylic acids can also be used as carboxyl group-containing monomers because they form carboxyl groups by cleaving the acid anhydride group after copolymerization.

α,β-ethylenically unsaturated monocarboxylic acid monomers include acrylic acid, methacrylic acid, ethylacrylic acid, crotonic acid, and cinnamic acid.

Examples of α,β-ethylenically unsaturated polycarboxylic acid monomers 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 anhydrides of α,β-unsaturated polycarboxylic acids include maleic anhydride, itaconic anhydride, and citraconic anhydride.

Examples of α,β-ethylenically unsaturated dicarboxylic acid monoester monomers 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; monoalkyl cycloalkyl 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; monoalkyl cycloalkyl fumarate esters such as monomethylcyclopentyl fumarate and monoethylcyclohexyl fumarate; Monoalkyl esters of citraconic acid such as monomethyl citraconic acid, monoethyl citraconic acid, monopropyl citraconic acid, and mono-n-butyl citraconic acid; monocycloalkyl esters of citraconic acid such as monocyclopentyl citraconic acid, monocyclohexyl citraconic acid, and monocycloheptyl citraconic acid; monoalkyl cycloalkyl esters of citraconic acid such as monomethylcyclopentyl citraconic acid and monoethylcyclohexyl citraconic acid; monoalkyl esters of itaconic acid such as monomethyl itaconic acid, monoethyl itaconic acid, monopropyl itaconic acid, and mono-n-butyl itaconic acid; monocycloalkyl esters of itaconic acid such as monocyclopentyl itaconic acid, monocyclohexyl itaconic acid, and monocycloheptyl itaconic acid; monoalkyl cycloalkyl esters of itaconic acid such as monomethylcyclopentyl itaconic acid and monoethylcyclohexyl itaconic acid; and the like.

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 monomer, α,β-ethylenically unsaturated monocarboxylic acid alkoxyalkyl ester monomer, α,β-ethylenically unsaturated monocarboxylic acid aminoalkyl ester monomer, α,β-ethylenically unsaturated monocarboxylic acid hydroxyalkyl ester monomer, and α,β-ethylenically unsaturated monocarboxylic acid fluoroalkyl ester monomer.

Specific examples of α,β-ethylenically unsaturated monocarboxylic acid alkyl ester monomers 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; alkyl acrylate cycloalkyl ester monomers such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, n-pentyl methacrylate, and n-octyl methacrylate. Examples of such monomers include alkyl methacrylate ester monomers; cycloalkyl methacrylate ester monomers such as cyclopentyl methacrylate, cyclohexyl methacrylate, and cyclopentyl methacrylate; alkyl cycloalkyl methacrylate ester monomers such as methylcyclopentyl methacrylate, ethylcyclopentyl methacrylate, and methylcyclohexyl methacrylate; alkyl crotonate ester monomers such as propyl crotonate, n-butyl crotonate, and 2-ethylhexyl crotonate; cycloalkyl crotonate ester monomers such as cyclopentyl crotonate, cyclohexyl crotonate, and cyclooctyl crotonate; alkyl cycloalkyl crotonate ester monomers such as methylcyclopentyl crotonate and methylcyclohexyl crotonate; and the like.

Specific examples of α,β-ethylenically unsaturated monocarboxylic acid alkoxyalkyl ester monomers 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.

The α-olefin monomer preferably has 3 to 12 carbon atoms, such as propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene.

Aromatic vinyl monomers include styrene, α-methylstyrene, vinylpyridine, etc.

Examples of fluorine-containing vinyl monomers include fluoroethyl vinyl ether, fluoropropyl vinyl ether, o-trifluoromethylstyrene, vinyl pentafluorobenzoate, difluoroethylene, and tetrafluoroethylene.

Examples of copolymerizable anti-aging agents 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.

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

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 further enhancing the effects of the present disclosure by more suitably forming cross-linking bonds between the hollow particles and the nitrile rubber while suppressing excessive reactions between the hollow particles, components other than the nitrile rubber, and the nitrile rubber, the iodine value is preferably 4 g/100 g or more, more preferably 8 g/100 g or more, even more preferably 10 g/100 g or more, and is preferably 450 g/100 g or less, more preferably 400 g/100 g or less.

In order to further reduce the compression set of the resulting 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.

The Mooney viscosity (ML1+4, 100°C) of the nitrile rubber used in this disclosure is preferably 10 to 200, more preferably 15 to 100, and even more preferably 20 to 80. By having the Mooney viscosity of the nitrile rubber in the above range, the processability of the nitrile rubber composition of this disclosure can be improved while further enhancing the effects of this disclosure.

The method for producing the raw rubber used in the present invention is not particularly limited, but it is preferable to produce it 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 auxiliary materials such as emulsifiers, polymerization initiators, and molecular weight regulators can be used.

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 linoleic acid, alkylbenzene sulfonates such as sodium dodecylbenzene sulfonate, polycondensates of naphthalene sulfonates 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, relative to 100 parts by mass of the monomers used in the polymerization.

The polymerization initiator is not particularly limited as long as it is a radical initiator, but 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. As the polymerization initiator, inorganic or organic peroxides are preferred. When a peroxide is used as a polymerization initiator, it can be used as a redox polymerization initiator in combination with a reducing agent such as sodium bisulfite or ferrous sulfate. The amount of polymerization initiator added is preferably 0.01 to 2 parts by mass per 100 parts by mass of the monomer used in the polymerization.

The molecular weight regulator 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 can 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 regulator added is preferably 0.1 to 5 parts by mass per 100 parts by mass of the monomer used in the polymerization.

Water is usually used as the medium for emulsion polymerization. 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 monomer used in the polymerization.

In emulsion polymerization, it is possible to use, as necessary, polymerization auxiliary materials such as stabilizers, dispersants, pH adjusters, oxygen scavengers, and particle size adjusters. When using these, there are no particular limitations on the type or amount used.

The resulting copolymer may be hydrogenated (hydrogenation reaction) as necessary. In this case, the hydrogenation method is not particularly limited, and any known method may be used.

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 a moderate hardness.

[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 effect of the present disclosure more prominent, 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, 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, relative to 100 parts by mass of the nitrile rubber.

[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 ratio of the nitrile rubber in the nitrile rubber composition of the present disclosure is not particularly limited, but the lower limit is preferably 30 mass% or more, more preferably 35 mass% or more, and even more preferably 40 mass% or more, and the upper limit is preferably 90 mass% or less, more preferably 85 mass% or less, and even more preferably 80 mass% or less.

[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 the crosslinking agent include organic peroxide crosslinking agents and sulfur crosslinking agents. From the viewpoint of more suitably forming crosslinking bonds between the hollow particles and the nitrile rubber and crosslinking bonds within the nitrile rubber, the crosslinking agent is preferably an organic peroxide crosslinking agent. Examples of the organic peroxide crosslinking agent include 1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane, di-t-butylperoxide, dicumyl peroxide (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, but is usually in the range of 0.1 to 10 parts by mass, preferably 0.5 to 5 parts by mass, relative to 100 parts by mass of the nitrile rubber used in the present disclosure.

[Liquid polymer having reactive sites]
The nitrile rubber composition of the present disclosure preferably contains a liquid polymer having a reactive active site. The liquid polymer having a reactive active site functions as a plasticizer during melt-kneading of the nitrile rubber composition and its raw material mixture. Furthermore, the liquid polymer having a reactive active site bonds with the nitrile rubber molecule 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 a reactive active site itself can be integrated with the matrix in the nitrile rubber composition and its raw material mixture. By containing a liquid polymer having a reactive active site in the nitrile rubber composition of the present disclosure, it becomes possible to more suitably form cross-linking between the hollow particles and the nitrile rubber and cross-linking within the nitrile rubber while favorably dispersing the hollow particles in the nitrile rubber, so that the effect of the present disclosure becomes more remarkable.

The polymer skeleton that forms the main part of the molecular structure of the liquid polymer having reactive active sites preferably has appropriate compatibility, softening 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 skeleton may be a skeleton having a hydrocarbon-based polymer structure that may contain heteroatoms such as oxygen, nitrogen, and silicon in the main chain or side chain.

The reactive active points in the liquid polymer having reactive active points refer to a chemical structure that has the function of forming a chemical, physical or physicochemical bond with the reactive active points present on the nitrile rubber. When the liquid polymer having reactive active points has two or more reactive active points in one molecule that bond with the nitrile rubber, the liquid polymer having reactive active points forms a cross-linked structure between two nitrile rubber molecules via a molecular chain derived from the liquid polymer, so that the effect of the present disclosure becomes even more remarkable. When the liquid polymer having reactive active points has two or more reactive active points in one molecule and the reactive active points can also bond to hollow particles, the liquid polymer having reactive active points not only forms a cross-linked structure between two nitrile rubber molecules, but also forms a cross-linked structure between the nitrile rubber molecule and the hollow particle, and between two hollow particles, so that the effect of the present disclosure becomes even more remarkable. From the viewpoint of being able to suitably introduce the above-mentioned cross-linked structure while suppressing excessive networking of the matrix, the number of reactive active points per molecule of the liquid polymer having reactive active points is preferably 2 to 10,000.

From the viewpoint of excellent plasticity imparting effect, it is preferable that the liquid polymer having reactive active sites is liquid at least at one point within the range of 5°C to 35°C, more preferably at least at one point within the range of 10°C to 30°C, and even more preferably at least at one point within the range of 20°C to 25°C. Also, from the viewpoint of excellent plasticity imparting effect, it is preferable that the glass transition temperature of the liquid polymer having reactive active sites is -10°C or lower, and more preferably -120°C to -20°C. The Mooney viscosity (1+4,100°C) of the liquid polymer having reactive active sites is usually 1 or lower, or the viscosity is so low that the Mooney viscosity cannot be measured.

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

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) in 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%.

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 with the nitrile rubber are suppressed, and further, the effect of improving crosslinking by the liquid polymer having reactive active sites is more suitably obtained, so that the effect of the present disclosure becomes more pronounced.

The amount 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 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.

[Other ingredients]
The nitrile rubber composition of the present disclosure may contain additives such as plasticizers, reinforcing agents, fillers, vulcanization accelerators, vulcanization acceleration assistants, softeners, processing assistants, antiaging agents, UV absorbers, foaming agents, foaming assistants, lubricants, pigments, colorants, dispersants, and flame retardants, as necessary, within the scope that does not impair the object of the present disclosure.

Furthermore, the reinforcing agent and filler 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, this is preferable in that the mechanical properties such as tensile strength, tear strength, and abrasion resistance of the obtained crosslinked molded product 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.

As plasticizers, those that are commonly used as plasticizers in applications such as automotive materials, general plastics, rubber products, etc., or those that have properties that impart flexibility can be used. For example, the following plasticizers can be used: 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 polymeric substances 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, sub (factice), liquid thiokol, and hydrocarbon-based synthetic lubricating oils. 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 this disclosure.

The reinforcing agent has the effect of enhancing 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 that have been surface-treated with a silane coupling agent, finely powdered silicic acid, and silica. These reinforcing agents can be used alone or in combination of two or more. The amount of reinforcing agent is not particularly limited, and is usually less than 230 parts by mass per 100 parts by mass of the nitrile rubber used in this disclosure.

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 mixed is not particularly limited, and is usually 0 to 200 parts by mass per 100 parts by mass of the nitrile rubber used in this disclosure.

Specific examples of vulcanization accelerators include aldehyde ammonias such as hexamethylenetetramine; guanidines such as diphenylguanidine, di(o-tolyl)guanidine, and o-tolyl-pyguanide; 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; N-t-butyl-2-benzothiazole; Examples of the vulcanization accelerator include sulfenamides such as zothiazylsulfenamide; 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 the vulcanization accelerator is usually 0 to 20 parts by mass, preferably 0 to 10 parts by mass, based on 100 parts by mass of the nitrile rubber used in the present disclosure.

Specific examples of vulcanization accelerators 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 accelerators can be used alone or in combination of two or more. The amount of vulcanization accelerator used is usually in the range of 0 to 20 parts by mass per 100 parts by mass of the nitrile rubber used in this disclosure.

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; sab; 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 polymer substances 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, sab (factice), liquid thiokol, and hydrocarbon-based synthetic lubricating oils. Of these, petroleum-based softeners are preferred, and process oil is particularly preferred. The amount of softener used is not particularly limited, but is usually 0 to 200 parts by mass per 100 parts by mass of the nitrile rubber used in this disclosure.

Processing aids include higher fatty acids such as ricinoleic acid, stearic acid, palmitic acid, and lauric acid; salts of higher fatty acids such as barium stearate, zinc stearate, and calcium stearate; and esters of higher fatty acids such as ricinoleic acid, stearic acid, palmitic acid, and lauric acid.

Examples of antioxidants include amine-based, hindered phenol-based, and sulfur-based antioxidants.

Lubricants include compounds or mixtures of hydrocarbons such as liquid paraffin, fatty acids such as stearic acid, fatty acid amides such as stearic acid amide, esters such as butyl stearate, alcohols such as stearyl alcohol, metal soaps, etc.

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.

The nitrile rubber composition of the present disclosure may contain base elastomers other than the nitrile rubber used in the present disclosure, provided that the objectives of the present disclosure are 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-propylene-diene terpolymer (EPDM) and other ethylene-α-olefin-non-conjugated diene copolymer rubbers, halogenated ethylene-α-olefin-non-conjugated diene copolymer rubbers, sulfonated ethylene-α-olefin-non-conjugated diene copolymer rubbers, maleated ethylene-α-olefin-non-conjugated diene copolymer rubbers, butyl rubber, isobutylene isoprene rubber, urethane rubber, silicone rubber, chlorosulfonated polyethylene, acrylic rubber, epichlorohydrin rubber, fluororubber, polysulfide rubber, and propylene oxide rubber; and thermoplastic elastomers such as urethane-based elastomers, styrene-based elastomers, olefin-based elastomers, amide-based elastomers, and ester-based elastomers. These base elastomers can be used alone or in combination of two or more.

The content of other base elastomers 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 this disclosure.

The crosslinked molded article obtained by using the nitrile rubber composition of the present disclosure as a molding material and subjecting it to a melt molding method or the like may be, for example, a rubber part, a rubber part integrally molded with a part made of another material, a coating, or a filler chip material.

The crosslinked molded article obtained by crosslinking the nitrile rubber composition of the present disclosure is a lightweight molded article that has excellent compression set resistance and oil resistance and moderate hardness. Applications of the crosslinked molded article produced using the nitrile rubber composition of the present disclosure include, for example, various rubber parts used in various fields such as automobiles, electricity, electronics, construction, aviation, and space. More specifically, examples include automobile 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 decrease when they are plastically deformed. The crosslinked molded article produced using the nitrile rubber composition of the present disclosure has reduced compression set, and is therefore particularly suitable for use in applications where low compression set is required, such as sealing materials.

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

[Method of producing nitrile rubber composition]
The method for producing the nitrile rubber composition of the present disclosure can be a general method and is not particularly limited. For example, the method for producing the nitrile rubber composition of the present disclosure can be a method in which the nitrile rubber used in the present disclosure is kneaded while 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 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.

The kneading of the nitrile rubber or the raw material mixture used in the present disclosure is carried out at a temperature at which the nitrile rubber softens. In addition, as the kneading machine used for the kneading, for example, a single-shaft kneader, a twin-shaft kneader, a kneader, a Banbury mixer, a pressure kneader, a roll kneader, or other known kneading machines can be used. Among them, it is preferable to perform kneading by applying a high shear force such as roll kneading. As the kneading machine used for roll kneading, for example, a two-roll mixing roll can be used, and more specifically, a mixing roll DY6-15 (manufactured by Daihan Co., Ltd.) can be mentioned. In addition, in the production of the nitrile rubber composition of the present disclosure, the compounding components are homogenized by preliminary kneading at a temperature at which the nitrile rubber softens, and then finishing kneading by applying a high shear force such as roll kneading can be performed. This makes it possible to obtain a nitrile rubber composition in which the compounding components are further homogenized and finely divided. In addition, when the nitrile rubber composition is crosslinked and molded, the nitrile rubber composition before crosslinking recovered from the molding device can be reused as the raw material mixture.

The conditions for kneading the nitrile rubber or raw material mixture used in this disclosure are not particularly limited, but when a radical generator is used as the crosslinking agent, the kneading temperature is preferably equal to or lower than the 10-hour half-life temperature of the crosslinking agent (radical generator). Note that the kneading temperature here refers to the set temperature of the kneading device. In addition, the kneading time is preferably within 1 hour. By using such kneading conditions, it is possible to prevent the crosslinking reaction of the nitrile rubber during kneading, prevent scorching, and mix the compounded components uniformly.

As an example, kneading can be performed in the following manner: Using a roll kneader, set the kneading temperature to 70-90°C, and once the kneader temperature has stabilized, add the nitrile rubber used in this disclosure, then add components such as hollow particles and crosslinking agent in any order while rotating the kneader rotor at a rotation speed of 10-35 rpm, and once all materials have been added, knead for 10-30 minutes. This produces the nitrile rubber composition of this disclosure.

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.

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, and 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.

The temperature when crosslinking and molding the nitrile rubber composition of the present disclosure is not particularly limited, but from the viewpoint of sufficiently proceeding with 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 the crosslinking agent, it is preferable that the temperature is 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 the crosslinking agent, it is preferable that the temperature is equal to or lower than the 1-minute half-life temperature. In addition, the conditions when 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 pressurizing time can be 1 minute to 180 minutes.

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 then secondary processed into a strip, or the like.

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, and 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.

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.

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. In addition, the nitrile rubber composition of the present disclosure used as the molding material for the crosslinked molded article of the present disclosure typically has a compression set of 60% or more.

The present disclosure will be explained 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 weight unless otherwise specified.

[Preparation of hollow particles]
[Manufacture example 1 (hollow particles A)]
(1) Mixture Preparation Step First, the following materials were mixed to prepare 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

Meanwhile, in a stirring tank, an aqueous solution of 12.1 parts of sodium hydroxide (alkali metal hydroxide) in 121 parts of ion-exchanged water was gradually added to an aqueous solution of 17.1 parts of magnesium chloride (a water-soluble polyvalent metal salt) in 494 parts of ion-exchanged water at room temperature while stirring to prepare a magnesium hydroxide colloid (poorly water-soluble metal hydroxide colloid) dispersion (4 parts of magnesium hydroxide) as the aqueous phase. The resulting aqueous phase and oil phase were mixed to prepare a mixed liquid.

(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.

(3) Polymerization Step The suspension obtained in the above 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.

(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 reslurry the mixture, and the water washing treatment (washing, filtration, dehydration) was repeated several times at room temperature (25°C), followed by filtration and separation to obtain a solid content. The obtained solid content was dried in a dryer at a temperature of 40°C to obtain precursor particles containing a hydrophobic solvent.

(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.

(6) Sieving Step The hollow particles obtained in the above-mentioned solvent removal step were sieved using a nylon mesh having an opening of 100 μm, and the hollow particles that passed through the mesh and fell to the bottom were collected, thereby removing coarse particles, and hollow particles A were obtained.

[Manufacture example 2 (hollow particles B)]
In the above "(1) Mixture preparation step", the amount of polymerizable monomer added to the oil phase was changed according to Table 1. As a result, hollow particles No. 2 (hollow particles B) were obtained.

[Manufacture example 3 (hollow particles C)]
The hollow particles of Production Example 3 (hollow particles C) were produced in the same manner as in Production Example 1, except that the rotation speed of the dispersing machine in the above "(2) Suspension step" was changed to 400 rpm. Got it.

[Manufacture example 4 (hollow particles D)]
In the above "(1) Mixture preparation step", the amount of polymerizable monomer added to the oil phase was changed according to Table 1. As a result, hollow particles No. 4 (hollow particles D) were obtained.

[Manufacture example 5 (hollow particles E)]
The same procedure as in Production Example 1 was repeated except that in the above "(1) Mixture preparation step", the polymerizable monomer added to the oil phase was changed to 100 parts of divinylbenzene. As a result, hollow particles No. 5 (hollow particles E) were obtained.

[Comparative production example 1 (dense particles A)]
The dense solid particles of Comparative Production Example 2 (Dense Solid Particles) were prepared in the same manner as in Production Example 1, except that in the above "(2) Suspension Step", the rotation speed of the dispersing machine was changed to 10 rpm. A) was obtained.

[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 subjected to the following physical property evaluations.

1. Measurement of iodine value The iodine value of the particles was measured according to 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 solution was added as a reaction solution, stirred gently, sealed, and left to stand in a dark place 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. Titration was performed using a titrant (0.1 mol/L sodium thiosulfate solution) using a burette, and when the solution turned light yellow, an indicator (1% starch solution) was added, and the titration was continued until the point at which the blue color disappeared was the end point. Apart from this main test, a blank test was performed on a solution to which no particles were added, and the iodine value of the particles was calculated using the following formula. The iodine value is the amount of halogen bonded when 100 g of a sample is reacted with a halogen, and the amount is converted into grams of iodine.
Iodine value (g/100g)={(V0-V1)×f×1.269}/S
S: mass of sample (g)
V1: Volume of titrant in this test (mL)
V0: Volume of titration solution in blank test (mL)
f: titrant factor

2. Particle size and particle size distribution The particle size of the particles was measured using a particle size distribution measuring instrument (product name: Multisizer 4e, manufactured by Beckman Coulter, Inc.) by the Coulter counter method, and the number average and volume average were calculated, respectively, to obtain the number average particle size (Dp) and the volume average particle size (Dv). In addition, the particle size distribution (Dv/Dp) was obtained 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 measured particles: 100,000. Specifically, 0.2 g of a particle sample was placed in a beaker, and a surfactant aqueous solution (product name: Drywell, manufactured by Fuji Film Co., Ltd.) was added thereto as a dispersant. 2 ml of dispersion medium was further added thereto to wet the particles, and then 10 ml of dispersion medium was added, and the particles were dispersed for 1 minute using an ultrasonic disperser, and then the measurement was performed using the above particle size distribution measuring instrument.

3. Porosity 3-1. Measurement of apparent density of hollow particles First, about 30 ^cm3 of hollow particles was filled into a measuring flask with a capacity of 100 ^cm3 , and the mass of the filled hollow particles was precisely weighed. Next, the measuring flask filled with the hollow particles was precisely filled with isopropanol up to the marked line, taking care not to introduce air bubbles. The mass of isopropanol added to the measuring flask was precisely weighed, and the apparent density _D1 (g/ ^cm3 ) of the hollow particles was calculated based on the following formula (I).
Formula (I)
Apparent density D ₁ =[mass of hollow particles]/(100-[mass of isopropanol]/[specific gravity of isopropanol at measurement temperature])

3-2. Measurement of true density of hollow particles After crushing the hollow particles in advance, about 10 g of crushed pieces of the hollow particles were filled into a measuring flask with a capacity of 100 ^cm3 , and the mass of the crushed pieces was accurately weighed. Then, isopropanol was added to the measuring flask in the same manner as in the measurement of the apparent density described above, and the mass of isopropanol was accurately weighed, and the true density D ₀ (g/cm ³ ) of the hollow particles was calculated based on the following formula (II).
Formula (II)
True density D ₀ =[mass of crushed pieces of hollow particles]/(100-[mass of isopropanol]/[specific gravity of isopropanol at measurement temperature])

3-3. Calculation of porosity The porosity of the hollow particles was calculated from the apparent density _D1 and true density _D0 of the hollow particles according to the following formula (III).
Formula (III)
Porosity (%)=100−(apparent density D ₁ /true density D ₀ )×100

[Production of nitrile rubber composition and production of crosslinked molded article]
[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 two-roll mixing 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 rotor of the kneader 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) (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, and then 2 parts of 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane (product name "Perhexa 25B", manufactured by NOF Corporation) were added as a crosslinking agent to obtain a mixture. The obtained mixture was kneaded for 15 minutes to obtain the nitrile rubber composition of Example 1.

The obtained nitrile rubber composition was press molded for 15 minutes at 10 MPa pressure using a hot press at 160°C to obtain a cylindrical cross-linked molded product with a diameter of 29±0.5 mm and a height of 12.5±0.5 mm. The obtained cross-linked molded product can be used as a test piece for the compression set measurement (JIS K 6262) described below.

[Examples 2 to 3]
In Example 1, instead of liquid polybutadiene 1 (PB1), liquid polybutadiene 2 (PB2) (liquid polymer having reactive active 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 active 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 having a crosslinked molded article number of 3,200, a 1,2-vinyl structure ratio of 90% or more, a trans-1,4 structure ratio of 10% or less, a viscosity at 45°C of 210 Poise, and a specific gravity of 0.87 was used.

[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.

[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 the liquid polybutadiene 1 in Example 1.

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

[Examples 10 to 11]
In Example 1, except that the amount of hollow particles A obtained in Production Example 1 was changed according to Table 2, the same procedure as in Example 1 was carried out to obtain nitrile rubber compositions and crosslinked molded articles of Examples 10 and 11.

[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), 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), and hydrogenated nitrile rubber 4 (HNBR5) (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) were 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) were used.

[Comparative Example 1]
In Example 1, the hollow particles A obtained in Production Example 1 were replaced with the dense solid particles A obtained in Comparative Production Example 1, and the same procedure was followed as in Example 1 to obtain a nitrile rubber composition and a crosslinked molded product of Comparative Example 1.

[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.

[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 in Example 1, low-density polyethylene (LDPE) (product name: NUC-8008, manufactured by NUC Corporation, iodine value: lower limit of measurement or less) was used as the base elastomer instead of nitrile rubber 1 (NBR1).

[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 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) was used instead of nitrile rubber 1 (NBR1) in Example 1.

[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.

[Evaluation of physical properties of crosslinked molded product]
1. Hardness (Duro-A)
The hardness (Duro-A) of the crosslinked molded article was measured using a type A durometer in accordance with JIS K 6253-3.

2. Specific Gravity and Weight Reduction Rate Using a sample cut into a 1 cm square and 2 mm thick piece 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 material elastomer used in the crosslinked molded article according to the following formula (1).
Formula (1):
Weight reduction rate (%)={(specific gravity of base elastomer−specific gravity of crosslinked molded body)/specific gravity of base elastomer}×100
The specific gravity of the base elastomer was as follows:
Specific gravity of NBR1, HNBR2-4: 1.0
Specific gravity of LDPE: 0.918
Specific gravity of EPDM: 0.9

3. Oil resistance
The crosslinked molded body was punched out to obtain a test piece having a size of 30 mm x 20 mm x 2 mm. 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 perform a fuel oil immersion test. From the result, 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 and less than 150% C: ΔV is 150% or more

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

More specifically, the compression set was measured using the following procedure. First, the thickness of the center of the test piece was measured at standard temperature. The test piece was placed on a compression plate (smooth stainless steel plate) and a spacer (9.3 mm thick) was placed on the outside of the test piece, after which the compression plate was 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 center of the test piece was measured. The compression set (%) was calculated from the thickness of the test piece and the thickness of the spacer before and after compression using the following formula (2).

CS: Compression set (%)
t0: original thickness of the test piece (mm)
t1: Spacer thickness (mm)
t2: Thickness (mm) of the test piece after 30 minutes from the compression device

As is clear from Tables 2 and 3, a nitrile rubber composition containing hollow particles with an iodine value of 2 g/100 g or more and 100 g/100 g or less and a nitrile rubber with an iodine value of 2 g/100 g or more and 500 g/100 g or less was capable of producing a lightweight crosslinked molded article with excellent compression set resistance and oil resistance and moderate hardness (Examples 1 to 14).

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

Claims (13)

A nitrile rubber composition comprising a nitrile rubber and a hollow particle having a shell containing a resin and a hollow portion surrounded by the shell,
The iodine value of the nitrile rubber is 2 g/100 g or more and 500 g/100 g or less,
The iodine value of the hollow particles is 2 g/100 g or more and 100 g/100 g or less,
The iodine value is a value measured in accordance with JIS K 0070. The nitrile rubber composition according to claim 1, further comprising a liquid polymer having reactive active sites. The nitrile rubber composition according to claim 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. The nitrile rubber composition according to any one of claims 1 to 3, wherein the Mooney viscosity ML(1+4,100°C) of the nitrile rubber is 10 to 200. The 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. The 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. The nitrile rubber composition according to any one of claims 1 to 6, wherein the hollow particles have an apparent density of 0.1 g/ ^cm3 or more and 0.8 g/ ^cm3 or less. The nitrile rubber composition according to any one of claims 1 to 7, wherein the volume average particle size of the hollow particles is 0.1 μm or more and 100 μm or less. The 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%. The nitrile rubber composition according to claim 2 or 3, wherein the liquid polymer is a liquid polymer having an ethylenic double bond. The 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. The nitrile rubber composition according to any one of claims 1 to 11, further comprising a crosslinking agent. A cross-linked molded article obtained by cross-linking and molding the nitrile rubber composition according to any one of claims 1 to 12.

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