Fiber-reinforced molded article and method for manufacturing a fiber-reinforced molded article

Abstract

To provide a lightweight fiber-reinforced molded body. [Solution] A method for producing a fiber-reinforced molded article, comprising the steps of: preparing a mixture containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium; preparing a suspension by suspending the mixture; and subjecting the suspension to a polymerization reaction, wherein the mixture contains a crosslinkable monomer as the first polymerizable monomer, the content of the crosslinkable monomer in 100 parts by mass of the first polymerizable monomer is 80 parts by mass or more, and when the polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more, a second polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C is added and subjected to a further polymerization reaction to produce hollow particles; preparing a resin composition containing the obtained hollow particles, a matrix resin, and a solvent; and impregnating reinforcing fibers with the resin composition.

Description

[Technical Field]

[0001] This disclosure relates to a fiber-reinforced molded article containing a matrix resin, reinforcing fibers, and hollow particles, and to a method for manufacturing the fiber-reinforced molded article. [Background technology]

[0002] Fiber-reinforced plastics (FRP), which are made by compounding reinforcing fibers with resin to improve its strength, are lightweight and have excellent mechanical properties such as strength and elastic modulus, so they are widely used in various fields such as automobiles, aircraft, ships, buildings, electrical and electronics.

[0003] Fiber-reinforced plastics can be obtained, for example, by impregnating reinforcing fibers with a matrix resin. However, when lightweighting agents such as foaming agents or inorganic balloons are added to further reduce the weight of fiber-reinforced plastics, problems arise such as the resin composition used to impregnate the reinforcing fibers becoming highly viscous, making impregnation of the matrix resin difficult, and the lightweighting agents not being able to disperse uniformly.

[0004] On the other hand, Patent Document 1 discloses a fiber-reinforced plastic containing hollow fine particles, which are obtained by polymerizing a reactive monomer containing 20% ​​by weight or more of a crosslinkable monomer, and which have at least one outer wall layer with a thickness of 20 nm or more on the outermost layer, a hollowness of 20 to 95 volume%, and an average particle diameter of 0.1 to 100 μm. In the examples of Patent Document 1, a reactive monomer containing 30 to 75% by weight of a crosslinkable monomer is used in the synthesis of the hollow fine particles. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2009-242477 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Patent Document 1 states that when the above-mentioned specific hollow fine particles are incorporated into fiber-reinforced plastic without being affected by the raw material monomers or organic solvents of the matrix resin, high strength and low specific gravity can be achieved. However, the hollow microparticles described in Patent Document 1 have insufficient solvent resistance to polar solvents. Therefore, if the resin composition used to impregnate the reinforcing fibers contains the hollow microparticles described in Patent Document 1 and a polar solvent as a diluent, the polar solvent penetrates into the hollow microparticles, making them prone to deformation or crushing. This prevents the maintenance of internal voids within the particles, and thus sufficient weight reduction cannot be achieved.

[0007] The object of this disclosure is to provide a lightweight fiber-reinforced molded article and a method for manufacturing the fiber-reinforced molded article. [Means for solving the problem]

[0008] The inventors focused on the permeability of the shell of hollow particles to polar solvents and found that by adjusting the composition and formation method of the shell, hollow particles with a shell that is resistant to acetone penetration can be suitably used as a lightweighting material for fiber-reinforced molded articles because they are less likely to be crushed in a solvent-containing resin composition and during the molding process of the resin composition.

[0009] This disclosure relates to a fiber-reinforced molded article comprising a matrix resin, reinforcing fibers, and hollow particles, The hollow particle is, It comprises a shell containing resin and a hollow portion surrounded by the shell, The shell contains a polymer as the resin, in which 80 parts by mass or more of crosslinkable monomer units are present in 100 parts by mass of total monomer units. In a hollow particle immersion test conducted at 25°C, 0.1 mg of hollow particles are added to 4 mL of acetone, shaken at a shaking speed of 100 rpm for 10 minutes, and then allowed to stand for 48 hours. The present invention provides a fiber-reinforced molded article in which less than 5% by mass of hollow particles precipitate in the acetone.

[0010] In the fiber reinforced molded body of the present disclosure, the polymer contained in the shell of the hollow particles further contains a hydrophilic non-crosslinkable monomer unit derived from a hydrophilic non-crosslinkable monomer having a solubility in distilled water at 20°C of 0.3 g / L or more, and in 100 parts by mass of all monomer units of the polymer, the content of the hydrophilic non-crosslinkable monomer unit is 2 to 20 parts by mass, and the content of the crosslinkable monomer unit is preferably 80 to 98 parts by mass.

[0011] In the fiber reinforced molded body of the present disclosure, the polymer contained in the shell of the hollow particles contains, as the crosslinkable monomer unit, a crosslinkable monomer unit having three or more functional groups derived from a crosslinkable monomer having three or more functional groups, and in 100 parts by mass of all monomer units of the polymer, the content of the crosslinkable monomer unit having three or more functional groups is preferably 5 to 50 parts by mass.

[0012] The present disclosure is a method for manufacturing a fiber reinforced molded body containing a matrix resin, reinforcing fibers, and hollow particles, a step of preparing a mixed solution containing a first polymerizable monomer, a hydrocarbon-based solvent, a dispersion stabilizer, and an aqueous medium, a step of preparing a suspension in which droplets of a monomer composition containing the first polymerizable monomer and the hydrocarbon-based solvent are dispersed in the aqueous medium by suspending the mixed solution, including a step of subjecting the suspension to a polymerization reaction, the mixed solution contains a crosslinkable monomer as the first polymerizable monomer, and the content of the crosslinkable monomer in 100 parts by mass of the first polymerizable monomer is 80 parts by mass or more, a step of manufacturing hollow particles by a manufacturing method in which, in the step of subjecting the suspension to a polymerization reaction, when the polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more, a second polymerizable monomer having a solubility in distilled water at 20°C of 0.3 g / L or more is added and further subjected to a polymerization reaction, a step of preparing a resin composition containing the hollow particles, the matrix resin, and a solvent obtained by the above step, and a step of impregnating the resin composition into reinforcing fibers, to provide a method for manufacturing a fiber reinforced molded body.

[0013] In the method for producing a fiber reinforced molded body of the present disclosure, in the step of subjecting the suspension in the step of producing the hollow particles to a polymerization reaction, the addition amount of the second polymerizable monomer is preferably 3 to 15 parts by mass with respect to 100 parts by mass of the first polymerizable monomer.

[0014] In the method for producing a fiber reinforced molded body of the present disclosure, in the step of preparing the mixed solution in the step of producing the hollow particles, the first polymerizable monomer contains a crosslinkable monomer having a functionality of 3 or more as the crosslinkable monomer, and the content of the crosslinkable monomer having a functionality of 3 or more in 100 parts by mass of the first polymerizable monomer is preferably 5 to 50 parts by mass.

[0015] In the method for producing a fiber reinforced molded body of the present disclosure, in the step of preparing the mixed solution in the step of producing the hollow particles, the dispersion stabilizer is preferably an inorganic dispersion stabilizer. In the method for producing a fiber reinforced molded body of the present disclosure, the inorganic dispersion stabilizer is more preferably a poorly water-soluble metal salt.

Advantages of the Invention

[0016] According to the present disclosure as described above, it is possible to provide a lightweight fiber reinforced molded body and a method for producing the fiber reinforced molded body.

Brief Description of the Drawings

[0017] [Figure 1] It is a diagram for explaining an example of a method for producing hollow particles used in the fiber reinforced molded body of the present disclosure. [Figure 2] It is a schematic diagram showing an embodiment of the suspension in the suspension step.

Modes for Carrying Out the Invention

[0018] In the present disclosure, “~” in a numerical range means including the numerical values described before and after it as the lower limit value and the upper limit value. Furthermore, in this disclosure, (meth)acrylate refers to acrylate and methacrylate, respectively; (meth)acrylic refers to acrylic and methacrylic, respectively; and (meth)acryloyl refers to acryloyl and methacryloyl, respectively. Furthermore, in this disclosure, a polymerizable monomer is a compound having a functional group capable of addition polymerization (which may be simply referred to as a polymerizable functional group in this disclosure). In this disclosure, a compound having an ethylenically unsaturated bond as the functional group capable of addition polymerization is generally used as the polymerizable monomer. Polymerizable monomers include non-crosslinkable monomers and crosslinkable monomers. Non-crosslinkable monomers are polymerizable monomers that have only one polymerizable functional group, while crosslinkable monomers are polymerizable monomers that have two or more polymerizable functional groups and form crosslinked bonds in the resin through polymerization reactions. Furthermore, in this disclosure, polymerizable monomers having a solubility of 0.3 g / L or more in distilled water at 20°C are referred to as hydrophilic monomers, and polymerizable monomers having a solubility of less than 0.3 g / L in distilled water at 20°C are referred to as non-hydrophilic monomers. Furthermore, in this disclosure, hardening means solidification with or without a chemical reaction.

[0019] I. Fiber-reinforced molded body The fiber-reinforced molded article of this disclosure is a fiber-reinforced molded article comprising a matrix resin, reinforcing fibers and hollow particles, The hollow particle is, It comprises a shell containing resin and a hollow portion surrounded by the shell, The shell contains a polymer as the resin, in which 80 parts by mass or more of crosslinkable monomer units are present in 100 parts by mass of total monomer units. In an immersion test of hollow particles, in which 0.1 mg of hollow particles are added to 4 mL of acetone at a temperature of 25°C, shaken at a shaking speed of 100 rpm for 10 minutes, and then allowed to stand for 48 hours, the amount of hollow particles that precipitate in the acetone is less than 5% by mass.

[0020] The fiber-reinforced molded articles of this disclosure are obtained, for example, by impregnating reinforcing fibers with a resin composition containing a matrix resin, hollow particles, and a solvent, as in the manufacturing method of this disclosure described later, and then molding them. The fiber-reinforced molded articles of this disclosure may be intermediate products such as prepregs in which the resin composition is in a semi-cured state, or they may be finished products obtained by curing the resin composition.

[0021] In conventional fiber-reinforced molded articles obtained by impregnating reinforcing fibers with a resin composition containing hollow particles, the solvent and resin in the resin composition (varnish) can penetrate into the interior of the hollow particles, making them prone to crushing. As a result, the hollow particles may not be able to maintain their internal voids, and the resulting molded article may not be sufficiently lightweight. In contrast, the fiber-reinforced molded article of this disclosure is a fiber-reinforced molded article that achieves further weight reduction compared to conventional fiber-reinforced molded articles containing hollow particles, because the contained hollow particles are less likely to be crushed in the varnish and during the process of impregnating the reinforcing fibers with the varnish, drying, and hardening, and the voids inside the particles are maintained even within the molded article. The hollow particles, matrix resin, and reinforcing fibers contained in the fiber-reinforced molded article of this disclosure are described below.

[0022] I-1.Hollow particles The hollow particles contained in the fiber-reinforced molded article of this disclosure are particles comprising a resin-containing shell (outer shell) and a hollow portion surrounded by the shell. In this disclosure, the hollow portion is a cavity-like space clearly distinguishable from the shell of the hollow particle formed from the resin material. The shell of the hollow particle may have a porous structure, in which case the hollow portion is of a size clearly distinguishable from a multitude of minute spaces uniformly dispersed within the porous structure. The hollow portion of a hollow particle can be confirmed, for example, by SEM observation of the particle cross-section, or by TEM observation of the particle itself. From the viewpoint of weight reduction, it is preferable that the hollow portion of the hollow particles in this disclosure is filled with air or a gas such as nitrogen, or is in a reduced-pressure state close to a vacuum.

[0023] The hollow particles used in the fiber-reinforced molded articles of this disclosure contain a polymer in the shell that has 80 parts by mass or more of crosslinkable monomer units per 100 parts by mass of total monomer units. Therefore, it is presumed that the proportion of crosslinkable monomer units in the shell is high, and that a dense covalent network is spread throughout the shell. Furthermore, since less than 5% by mass of the hollow particles used in the fiber-reinforced molded articles of this disclosure precipitate in acetone in the above immersion test, it is presumed that the shell has a dense structure that is resistant to acetone penetration, and that the crosslinking structure within the shell is even denser. The hollow particles used in the fiber-reinforced molded articles of this disclosure have a denser structure than conventional hollow particles that have a shell containing many crosslinkable monomer units, resulting in improved solvent resistance and strength. As a result, solvents do not easily penetrate into the inside of the particles, and because they also have superior strength, they are less likely to collapse even when added to varnish, and the varnish is impregnated into the reinforcing fibers, dried, and hardened, and it is believed that the voids inside the particles are maintained in the fiber-reinforced molded article. In the following, the hollow particles used in the fiber-reinforced molded articles of this disclosure may be referred to as the hollow particles of this disclosure or simply as hollow particles.

[0024] The hollow particles of this disclosure preferably have a porosity of 60% or more, more preferably 65% ​​or more, and even more preferably 70% or more. A porosity above the above lower limit results in superior effects such as weight reduction due to the hollow particles. While there is no particular upper limit to the porosity of the hollow particles of this disclosure, from the viewpoint of suppressing a decrease in the strength of the hollow particles and making them less susceptible to crushing, it is preferably 90% or less, more preferably 85% or less, and even more preferably 80% or less.

[0025] The porosity of the hollow particles in this disclosure is calculated from the apparent density D1 and true density D0 of the hollow particles. The method for measuring the apparent density D1 of hollow particles is as follows. First, a volume of 100 cm³ is used. 3 Approximately 30 cm in a volumetric flask 3Fill the volumetric flask with hollow particles and accurately weigh the mass of the filled hollow particles. Next, carefully fill the volumetric flask filled with hollow particles to the mark with isopropanol, taking care not to introduce air bubbles. Accurately weigh the mass of isopropanol added to the volumetric flask and calculate the apparent density D1 (g / cm³) of the hollow particles based on the following formula (I). 3 Calculate ). Equation (I) Apparent density D1 = [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, assuming that the hollow portion is considered part of the hollow particle.

[0026] The method for measuring the true density D0 of hollow particles is as follows: After pre-pulverizing the hollow particles, a volume of 100 cm³ is used. 3 Fill a volumetric flask with approximately 10 g of crushed hollow particles and accurately weigh the mass of the crushed particles. Then, add isopropanol to the volumetric flask in the same manner as the apparent density measurement described above, accurately weigh the mass of isopropanol, and calculate the true density D0 (g / cm³) of the hollow particles based on the following formula (II). 3 Calculate ). Formula (II) True density D0 = [Mass of hollow particle fragments] / (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 measurement method described above, the hollow portion is not considered part of the hollow particle when calculating the true density D0.

[0027] The porosity (%) of a hollow particle is calculated using the following formula (III), given the apparent density D1 and true density D0 of the hollow particle. Formula (III) Porosity (%) = 100 - (Apparent density D1 / True density D0) × 100

[0028] The hollow particles in this disclosure preferably have a volume-average particle size of 1.0 to 80 μm. When the volume-average particle size of the hollow particles is above the lower limit, the aggregation of the hollow particles is reduced, resulting in excellent dispersibility. On the other hand, when the volume-average particle size of the hollow particles is below the upper limit, the decrease in the strength of the hollow particles is suppressed, and the hollow particles can easily penetrate between the reinforcing fibers, resulting in uniform dispersion in the molded article. The lower limit of the volume-average particle size of the hollow particles in this disclosure is more preferably 3.0 μm or more, even more preferably 5.0 μm or more, and even more preferably 7.0 μm or more. The upper limit of the volume-average particle size of the hollow particles in this disclosure is more preferably 30.0 μm or less, even more preferably 20.0 μm or less, and even more preferably 10.0 μm or less.

[0029] The particle size distribution (volume-average particle size (Dv) / number-average particle size (Dn)) of the hollow particles may be, for example, between 1.05 and 2.5. A particle size distribution of 2.5 or less allows for particles with less variation in properties such as pressure resistance and heat resistance between particles. Furthermore, a particle size distribution of 2.5 or less enables the production of products with uniform thickness, for example, when manufacturing sheet-like molded articles. The volume-average particle size (Dv) and number-average particle size (Dn) of hollow particles can be determined, for example, by measuring the particle size of the hollow particles using a laser diffraction particle size distribution analyzer, calculating the number average and volume average, respectively, and using the obtained values ​​as the number-average particle size (Dn) and volume-average particle size (Dv) of those particles. The particle size distribution is the value obtained by dividing the volume-average particle size by the number-average particle size.

[0030] The shape of the hollow particles in this disclosure is not particularly limited as long as a hollow portion is formed inside, and examples include spherical, ellipsoidal, and amorphous shapes. Among these, the spherical shape is preferred because it is easy to manufacture and the hollow particles have excellent strength and pressure resistance. Hollow particles may have one or more hollow sections. Furthermore, the shell of the hollow particle, and the partition walls separating adjacent hollow sections if there are two or more, may be porous. In order to maintain a good balance between the high porosity of the hollow particle and its mechanical strength, it is preferable that the particle has only one hollow section inside. The hollow particles may have an average circularity of 0.950 to 0.995. One example of the shape of a hollow particle is a bag consisting of a thin film and inflated with gas, the cross-section of which is shown as hollow particle 100 in Figure 1(5) below. In this example, a thin film is provided on the outside, and the inside is filled with gas. The particle shape can be confirmed, for example, by SEM or TEM. Furthermore, the internal shape of the particle can be confirmed by SEM or TEM after the particle has been sliced ​​crosswise using a known method.

[0031] The hollow particles of this disclosure contain a polymer as the resin in the shell, which contains 80 parts by mass or more of crosslinkable monomer units per 100 parts by mass of total monomer units. The polymer forms the framework of the shell of the hollow particles, and by containing crosslinkable monomer units in the above proportion, the shell of the hollow particles of this disclosure has a densely interwoven covalent network. In the above polymer, the content of crosslinkable monomer units in 100 parts by mass of total monomer units is preferably 85 parts by mass or more, more preferably 90 parts by mass or more, in order to improve the strength and solvent resistance of the hollow particles and to improve the effects such as weight reduction by the hollow particles. The upper limit of the content of the above crosslinkable monomer units is not particularly limited and may be 100 parts by mass or less, but it is preferably 98 parts by mass or less, more preferably 97 parts by mass or less, in order to sufficiently contain the hydrophilic non-crosslinkable monomer units described later. In this disclosure, the crosslinkable monomer unit is a monomer unit derived from a crosslinkable monomer, and in the polymer described above, if the content of the crosslinkable monomer unit is less than 100 parts by mass, the monomer units other than the crosslinkable monomer unit are non-crosslinkable monomer units derived from a non-crosslinkable monomer. The polymer described above is typically a polymer of a first polymerizable monomer and a second polymerizable monomer obtained by a first polymerization reaction and a second polymerization reaction in the method for producing hollow particles of the present disclosure described later. That is, in the hollow particles of the present disclosure, the crosslinkable monomer units and non-crosslinkable monomer units contained in the polymer described above are usually derived from the first polymerizable monomer and the second polymerizable monomer described later. The specific details of the crosslinkable and non-crosslinkable monomers used in the synthesis of the above polymer are as described in the method for producing hollow particles in this disclosure, which will be discussed later.

[0032] The above polymer contains at least one crosslinkable monomer unit selected from a difunctional crosslinkable monomer derived from a difunctional crosslinkable monomer, and a trifunctional or higher crosslinkable monomer unit derived from a trifunctional or higher crosslinkable monomer. In particular, it is preferable to include at least a difunctional crosslinkable monomer unit in order to improve the strength and solvent resistance of the hollow particles and to improve the effects such as weight reduction by the hollow particles, and it is even more preferable to include a combination of a difunctional crosslinkable monomer unit and a trifunctional or higher crosslinkable monomer unit in order to further improve the strength of the hollow particles. In this disclosure, crosslinkable monomer units derived from a bifunctional crosslinkable monomer may be referred to as "bifunctional crosslinkable monomer units," and crosslinkable monomer units derived from a trifunctional or more crosslinkable monomer may be referred to as "trifunctional or more crosslinkable monomer units."

[0033] When the above polymer contains bifunctional crosslinkable monomer units, the content of bifunctional crosslinkable monomer units in 100 parts by mass of the total monomer units of the above polymer is not particularly limited, but the lower limit is preferably 65 parts by mass or more, more preferably 70 parts by mass or more, and even more preferably 75 parts by mass or more, from the standpoint of improving the strength and solvent resistance of the hollow particles and improving the effects such as weight reduction by the hollow particles. On the other hand, the upper limit may be 100 parts by mass or less, but from the standpoint of sufficiently containing trifunctional or more crosslinkable monomer units or hydrophilic non-crosslinkable monomer units described later, it is preferably 98 parts by mass or less, more preferably 95 parts by mass or less, and even more preferably 90 parts by mass or less.

[0034] When the above polymer contains three or more crosslinkable monomer units, the content of three or more crosslinkable monomer units in 100 parts by mass of the total monomer units of the above polymer is not particularly limited, but the lower limit is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 15 parts by mass or more, from the standpoint of improving the strength and solvent resistance of the hollow particles and improving the effects such as weight reduction by the hollow particles. On the other hand, the upper limit is preferably 50 parts by mass or less, more preferably 40 parts by mass or less, and even more preferably 30 parts by mass or less, and even more preferably 25 parts by mass or less, from the standpoint of sufficiently containing two or more crosslinkable monomer units or hydrophilic non-crosslinkable monomer units described later.

[0035] Furthermore, the crosslinkable monomer units contained in the above polymer may also include crosslinkable monomer units derived from (meth)acrylic crosslinkable monomers having a (meth)acryloyl group as a polymerizable functional group. This makes it possible to produce hollow particles with excellent strength and heat resistance. When the polymer contains crosslinkable monomer units derived from a (meth)acrylic crosslinkable monomer, the content of the crosslinkable monomer units derived from the (meth)acrylic crosslinkable monomer is preferably 50 parts by mass or more, more preferably 70 parts by mass or more, and even more preferably 90 parts by mass or more, per 100 parts by mass of crosslinkable monomer units, and the crosslinkable monomer units may consist of (meth)acrylic crosslinkable monomer units. The specific details of the (meth)acrylic crosslinkable monomer will be described later in the method for producing hollow particles in this disclosure.

[0036] Furthermore, the polymer preferably further contains non-crosslinkable monomer units, more preferably hydrophilic non-crosslinkable monomer units having a solubility of 0.3 g / L or more in distilled water at 20°C, and particularly preferably contains hydrophilic non-crosslinkable monomer units derived from a second polymerizable monomer described later. By including a combination of crosslinkable and non-crosslinkable monomer units in the polymer, the mechanical properties of the hollow particle shell are improved. In particular, by including hydrophilic non-crosslinkable monomer units as non-crosslinkable monomer units, the shell tends to form a dense structure, which improves the strength and solvent resistance of the hollow particles, thereby improving the effects of weight reduction and other benefits of the hollow particles. In the above polymer, the content of non-crosslinkable monomer units in 100 parts by mass of total monomer units is 0 to 20 parts by mass. From the viewpoint of improving the strength of hollow particles, the lower limit is preferably 2 parts by mass or more, more preferably 3 parts by mass or more, and even more preferably 4 parts by mass or more, and the upper limit is preferably 15 parts by mass or less, more preferably 12 parts by mass or less, and even more preferably 10 parts by mass or less. In the above polymer, the content of hydrophilic non-crosslinked monomer units in 100 parts by mass of total monomer units is preferably 2 parts by mass or more, more preferably 3 parts by mass or more, and even more preferably 4 parts by mass or more, from the standpoint of improving the strength and solvent resistance of hollow particles and improving the effects such as weight reduction by hollow particles, as a lower limit, and preferably 20 parts by mass or less, more preferably 15 parts by mass or less, even more preferably 12 parts by mass or less, and even more preferably 10 parts by mass or less as an upper limit.

[0037] In the hollow particles of this disclosure, the content of the polymer is preferably 90% by mass or more, more preferably 95% by mass or more, of 100% by mass of the total solids content of the shell. By setting the content of the polymer to above the lower limit, the strength of the hollow particles can be improved.

[0038] In the hollow particles disclosed herein, in an immersion test in which 0.1 mg of hollow particles is added to 4 mL of acetone at 25°C, shaken at a shaking speed of 100 rpm for 10 minutes, and then allowed to stand for 48 hours, less than 5% by mass of hollow particles precipitates in the acetone. The percentage of hollow particles that precipitate in the acetone in the above immersion test serves as an indicator of the density of the hollow particle shell, and the lower the percentage of hollow particles that precipitate in the acetone in the above immersion test, the denser the hollow particle shell is presumed to be. In order to ensure that the amount of hollow particles that precipitate in acetone in the above immersion test is less than 5% by mass, for example, as in the method for producing hollow particles in the present disclosure described later, hollow particles can be produced by adding a second polymerizable monomer, which is a hydrophilic monomer, when the polymerization conversion rate of a first polymerizable monomer containing a specific amount or more of crosslinkable monomers reaches 93% by mass or more in the step of subjecting the suspension to a polymerization reaction, and further subjecting it to a polymerization reaction, such that the content ratio of crosslinkable monomer units in the polymer that forms the shell is 80% by mass or more.

[0039] Furthermore, in SEM observation, it is preferable that the hollow particles in this disclosure have 5 or fewer hollow particles with interconnecting holes or shell defects out of 100 hollow particles. Generally, hollow particles can be classified into two types: those in which the shell does not have connecting holes between the hollow portion and the external space of the particle, and those in which the shell has one or more connecting holes, allowing the hollow portion to connect to the outside of the particle through these holes. Depending on the size of the hollow particle, the diameter of the connecting holes is usually around 10 to 500 nm. While connecting holes can sometimes impart beneficial functions to hollow particles, they can also reduce the strength of the hollow particle and make it more prone to collapse because they represent areas where the shell is missing. Furthermore, hollow particles may have extremely large crack-like shell defects relative to their particle size. Depending on the size of the hollow particle, cracks with a length of 1 μm or more generally significantly degrade the strength of the hollow particle and are therefore recognized as shell defects. In the immersion test of hollow particles described above, if the amount of hollow particles that precipitate in acetone is less than 5% by mass, it can be considered that there are 5 or fewer hollow particles with interconnecting pores or shell defects out of 100 hollow particles. However, even if the shell does not have interconnecting pores or shell defects, the amount of hollow particles that precipitate in the immersion test described above may be 5% by mass or more. Therefore, in the immersion test of hollow particles described above, if the amount of precipitated hollow particles is less than 5% by mass, it is considered to indicate that the shell has very few interconnecting pores and shell defects, and that the shell has a dense cross-linking structure.

[0040] The content of hollow particles in the fiber-reinforced molded articles of this disclosure is not particularly limited, but on a mass basis, the lower limit is preferably 5% by mass or more, more preferably 10% by mass or more, and the upper limit is preferably 50% by mass or less, more preferably 40% by mass or less, even more preferably 30% by mass or less, and even more preferably 20% by mass or less. Furthermore, the content of hollow particles in the fiber-reinforced molded article of this disclosure is preferably 15% by volume or more, more preferably 25% by volume or more, and even more preferably 30% by volume or more, as a lower limit on a volume basis, and preferably 60% by volume or less, more preferably 50% by volume or less, and even more preferably 40% by volume or less as an upper limit. By setting the content of hollow particles above the lower limit, the effect of weight reduction of the fiber-reinforced molded article can be improved, and by setting it below the upper limit, the deterioration of the physical properties of the fiber-reinforced molded article can be suppressed.

[0041] To determine the mass-based content of hollow particles in a fiber-reinforced molded body, first, the resin content (mass%) in the fiber-reinforced molded body is calculated from the mass of the fiber-reinforced molded body and the mass of the reinforcing fibers in the fiber-reinforced molded body using the following formula (1). Next, the mass-based content of hollow particles (mass%) can be calculated from the resin content (mass%) and the content of hollow particles in the solid content of the resin composition impregnated into the reinforcing fibers using the following formula (2). Here, the resin content refers to the total of materials other than reinforcing fibers that make up the fiber-reinforced molded product, and is typically the sum of the matrix resin and hollow particles in the fiber-reinforced molded product. Furthermore, the fiber-reinforced molded article used when determining the content of hollow particles by mass only needs to have the solvent removed. An intermediate product such as a prepreg can be used to determine the content of hollow particles in the finished product obtained by curing the prepreg. Formula (1) Resin content (mass%) = (mass of fiber-reinforced molded body - mass of reinforcing fibers) ÷ mass of fiber-reinforced molded body × 100 Formula (2) Hollow particle content (mass%) = Resin content (mass%) × Hollow particle content in the solid content of the resin composition impregnated into the reinforcing fiber (mass%) ÷ 100

[0042] The volume-based content of hollow particles in a fiber-reinforced molded article can be calculated, for example, by determining the volume of the hollow particles, matrix resin, and reinforcing fibers constituting the fiber-reinforced molded article using the following formulas (3), (4), and (5), and then using the obtained values ​​to calculate the content using the following formula (6). Formula (3) Volume of hollow particles = Hollow particle content (mass%) ÷ Specific gravity of hollow particles (g / cm³) 3 ) Formula (4) Matrix resin volume = Matrix resin content (mass%) ÷ Specific gravity of the matrix resin after curing (g / cm³) 3 ) Formula (5) Volume of reinforcing fibers = Reinforcing fiber content (mass%) ÷ Specific gravity of reinforcing fibers (g / cm³) 3 ) Formula (6) Hollow particle content (volume %) = Volume of hollow particles ÷ (Volume of hollow particles + Volume of matrix resin + Volume of reinforcing fibers) × 100 The "hollow particle content (mass %)" used in formula (3) above is the value obtained by formula (2) above. The "matrix resin content (mass%)" used in formula (4) above can be calculated using the following formula (7). Formula (7) Content of matrix resin (mass%) = Content of resin component (mass%) × Content of matrix resin in solid content of resin composition impregnated in reinforcing fiber (mass%) ÷ 100 Here, the "content of resin component (mass%)" is the value obtained by the above formula (1). Also, the "specific gravity of matrix resin after curing (g / cm 3 )" for the cured product of the composition obtained by removing hollow resin particles from the resin composition impregnated in the reinforcing fiber can be the value of the true density obtained by the same method as for the hollow particles. The "content of reinforcing fiber (mass%)" used in the above formula (5) can be calculated by the following formula (8). Formula (8) Content of reinforcing fiber (mass%) = Mass of reinforcing fiber ÷ Mass of fiber reinforced molded body × 100

[0043] I-2. Matrix Resin The matrix resin used in the present disclosure can be one generally used as the matrix resin of the fiber reinforced molded body, and a thermoplastic resin, a thermosetting resin, or a room temperature curable resin may be used. The fiber reinforced molded body of the present disclosure can be produced by a wet method such as, for example, a method of immersing a resin composition containing a matrix resin, hollow particles and a solvent in a reinforcing fiber and then removing the solvent. When the matrix resin contains a thermoplastic resin, for example, a method of heating a resin composition containing a matrix resin and hollow particles to reduce its viscosity and impregnating it into a reinforcing fiber, or a method of forming a resin composition containing a matrix resin and hollow particles into a sheet shape and laminating it from both sides or one side of the reinforcing fiber and heating and pressing to impregnate the resin composition into the reinforcing fiber. The fiber reinforced molded body of the present disclosure can also be produced by a hot melt method. From the viewpoint that the weight reduction effect by the hollow particles used in the present disclosure is likely to be exhibited, it is preferable to use a matrix resin applicable to the above wet method, and for example, it is preferable to contain a thermosetting resin.

[0044] Examples of thermoplastic resins that can be used as matrix resins include polyolefins such as polypropylene and polyethylene; polyamides such as PA6, PA66, and PA12; polyimides, polyamideimides, polyetherimides, polyetherketones, polyvinyl chloride, polystyrene, poly(meth)acrylate, polycarbonate, polyvinylidene fluoride, acrylonitrile-butadiene-styrene copolymer (ABS), acrylonitrile-styrene copolymer (AS), polyphenylene ether, polyphenylene sulfide, polyester, polytetrafluoroethylene, and thermoplastic elastomers. These thermoplastic resins can be used individually or in combination of two or more.

[0045] Examples of thermosetting resins that can be used as matrix resins include phenolic resins, melamine resins, urea resins, unsaturated polyester resins, epoxy resins, polyurethane resins, silicon resins, alkyd resins, thermosetting polyphenylene ether resins, thermosetting polyimide resins, benzoxazine resins, urea resins, allyl resins, aniline resins, maleimide resins, bismaleimidotriazine resins, liquid crystalline polyester resins, vinyl ester resins, unsaturated polyester resins, cyanate ester resins, and polyetherimide resins. These thermosetting resins can be used individually or in combination of two or more. Examples of room-temperature curing resins that can be used as matrix resins include epoxy resins and urethane resins. These room-temperature curing resins can be used individually or in combination of two or more types.

[0046] The matrix resin may, if necessary, contain additives for curing the resin, such as curing agents, curing catalysts, or curing accelerators. The specific contents of these resin curing additives are as described in the method for manufacturing fiber-reinforced molded articles, which will be described later.

[0047] The content of the matrix resin in the fiber-reinforced molded article of this disclosure is not particularly limited, but from the viewpoint of mechanical properties, the content of the matrix resin per 100 parts by mass of the total of the matrix resin and hollow particles in the fiber-reinforced molded article is preferably 50 parts by mass or more, more preferably 70 parts by mass or more, and from the viewpoint of weight reduction, it is preferably 95 parts by mass or less, more preferably 90 parts by mass or less, and even more preferably 85 parts by mass or less. In this disclosure, the content of the matrix resin also includes the content of additives for curing the resin, such as curing agents, curing catalysts, or curing accelerators.

[0048] I-3. Reinforced Fibers The reinforcing fibers used in this disclosure can be those commonly used in fiber-reinforced molded articles, and are not particularly limited. Examples include organic or inorganic reinforcing fibers such as carbon fibers, glass fibers, aramid fibers, alumina fibers, boron fibers, tyranno fibers, and SiC fibers. Among these, carbon fibers are preferred due to their excellent mechanical properties, heat oxidation resistance, and dimensional stability.

[0049] The form of the reinforcing fiber is not particularly limited, and for example, reinforcing fibers processed into woven fabrics, nonwoven fabrics, mats, etc., can be used. Furthermore, the reinforcing fiber may be long fiber or short fiber.

[0050] The content of reinforcing fibers in the fiber-reinforced molded article of this disclosure is not particularly limited, but is preferably 20 to 80% by mass of 100% by mass of the fiber-reinforced molded article of this disclosure. When the reinforcing fiber content is within the above range, a fiber-reinforced molded article with excellent fatigue strength and impact properties can be obtained.

[0051] Furthermore, the fiber-reinforced molded articles of this disclosure may, to the extent that they do not impair the effects of this disclosure, further contain additives such as inorganic fillers, antioxidants, heat stabilizers, ultraviolet absorbers, light stabilizers, lubricants, flame retardants, mold release agents, antistatic agents, and colorants, as needed, in addition to the hollow particles, matrix resin, and reinforcing fibers described above.

[0052] I-4. Physical properties of fiber-reinforced molded articles The fiber-reinforced molded articles of this disclosure can be made to have physical properties suitable for their intended use by appropriately selecting the type and amount of each component described above.

[0053] The tensile modulus of the fiber-reinforced molded article of this disclosure is not particularly limited, but is preferably 13 GPa or more, more preferably 14 GPa or more, as a lower limit, and preferably 20 GPa or less, more preferably 18 GPa or less, as an upper limit. The tensile strength of the fiber-reinforced molded articles of this disclosure is not particularly limited, but is preferably 350 MPa or more, more preferably 400 MPa or more, as a lower limit, and preferably 500 MPa or less, more preferably 450 MPa or less, as an upper limit. In this disclosure, the tensile modulus and tensile strength of the fiber-reinforced molded article are measured in accordance with JIS K 7165:2008.

[0054] The shape of the fiber-reinforced molded article of this disclosure is not particularly limited and can be any shape that can be molded, such as a sheet, film, plate, tube, and any other three-dimensional shape.

[0055] I-5. Applications of Fiber-Reinforced Molded Products The applications of the fiber-reinforced molded articles of this disclosure are not particularly limited, but they are suitably used in various fields such as automobiles, bicycles, aerospace, railway vehicles, ships, buildings, electrical and electronic equipment, and sporting goods, where mechanical properties and weight reduction are required.

[0056] II. Method for Manufacturing Fiber-Reinforced Molded Articles The present disclosure is a method for producing a fiber-reinforced molded article comprising a matrix resin, reinforcing fibers, and hollow particles, A step of preparing a mixture containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium, The steps include: preparing a suspension in which droplets of the monomer composition containing the first polymerizable monomer and the hydrocarbon solvent are dispersed in the aqueous medium by suspending the aforementioned mixture; The step includes subjecting the suspension to a polymerization reaction, The aforementioned mixture contains a crosslinkable monomer as the first polymerizable monomer, and the content of the crosslinkable monomer in 100 parts by mass of the first polymerizable monomer is 80 parts by mass or more. A manufacturing method for producing hollow particles, comprising the step of subjecting the suspension to a polymerization reaction, wherein when the polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more, a second polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C is added and the suspension is subjected to a further polymerization reaction, A step of preparing a resin composition containing hollow particles, matrix resin, and solvent obtained in the above step, The method is characterized by comprising the step of impregnating the reinforcing fibers with the resin composition.

[0057] The method for manufacturing a fiber-reinforced molded article according to this disclosure includes the steps of manufacturing hollow particles, preparing a resin composition, and impregnating reinforcing fibers with the resin composition, and may also include other steps. Furthermore, the manufacturing method according to this disclosure may perform two or more of the steps simultaneously as a single step, or in a different order, as long as it is technically possible.

[0058] II-1. Process for manufacturing hollow particles The hollow particles used in this disclosure are A step of preparing a mixture containing a first polymerizable monomer including a crosslinkable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium, The steps include: preparing a suspension in which droplets of the monomer composition containing the first polymerizable monomer and the hydrocarbon solvent are dispersed in the aqueous medium by suspending the aforementioned mixture; The step includes subjecting the suspension to a polymerization reaction, The aforementioned mixture contains a crosslinkable monomer as the first polymerizable monomer, and the content of the crosslinkable monomer in 100 parts by mass of the first polymerizable monomer is 80 parts by mass or more. These hollow particles can be obtained by a method for producing hollow particles, in which, in the step of subjecting the suspension to a polymerization reaction, when the polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more, a second polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C is added and the polymerization reaction is further carried out.

[0059] The above method for producing hollow particles follows a basic technique in which a mixture containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium is suspended, causing the first polymerizable monomer and the hydrocarbon solvent to undergo phase separation, resulting in a suspension in which droplets having a distribution structure in which the first polymerizable monomer is concentrated on the surface and the hydrocarbon solvent is concentrated in the center are dispersed in the aqueous medium, and this suspension is subjected to a polymerization reaction to harden the surface of the droplets and form hollow particles having a hollow portion filled with the hydrocarbon solvent. In this basic technology, when the polymerization conversion rate of the first polymerizable monomer containing a crosslinkable monomer reaches 93% by mass or more in the step of subjecting the suspension to polymerization, a second polymerizable monomer, which is a hydrophilic monomer whose solubility in distilled water at 20°C is 93% by mass or more, is added and subjected to further polymerization, thereby enabling the production of hollow particles in which less than 5% by mass of hollow particles precipitate in acetone in the above-mentioned immersion test of hollow particles. When a crosslinkable monomer is used as the polymerizable monomer for forming the shell of the hollow particles, unreacted polymerizable functional groups tend to remain in the shell. The more unreacted polymerizable functional groups that remain, the coarser the crosslinked structure of the shell becomes. Therefore, in hollow particles obtained by conventional manufacturing methods, it is thought that 5% or more by mass of hollow particles precipitate in acetone in the above-mentioned immersion test of hollow particles due to the remaining unreacted polymerizable functional groups. In the above method for producing hollow particles, a suspension in which droplets of a monomer composition containing a first polymerizable monomer containing a large amount of crosslinkable monomers are dispersed in an aqueous medium is subjected to a polymerization reaction. The first polymerization reaction is carried out until the polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more. Then, a second polymerizable monomer, which is a hydrophilic monomer, is added and a second polymerization reaction is carried out further. It is believed that this can improve the overall reaction rate of the polymerizable monomer, including the first and second polymerizable monomers. In this disclosure, particles having a shell containing a polymer of the first polymerizable monomer and a hollow portion filled with a hydrocarbon solvent, obtained by the first polymerization reaction described above, may be referred to as first precursor particles, and compositions containing these first precursor particles may be referred to as first precursor compositions. Furthermore, particles having a shell containing polymers of the first polymerizable monomer and the second polymerizable monomer and a hollow portion filled with a hydrocarbon solvent, obtained by the second polymerization reaction described above, may be considered intermediates of hollow particles with a hollow portion filled with gas, and may be referred to as second precursor particles, and compositions containing these second precursor particles may be referred to as second precursor compositions. In the above method for producing hollow particles, the second polymerizable monomer has a solubility in distilled water at 20°C that is greater than or equal to the specified value, so when added to the first precursor composition, it is easily incorporated into the shell of the first precursor particles. The second polymerizable monomer, being a hydrophilic monomer, has affinity for both the first polymerizable monomer and the aqueous medium. Therefore, when added to the first precursor composition, it is thought to be incorporated into the shell formed by the first polymerizable monomer, thereby promoting the thermal motion of the shell. During the second polymerization reaction, the polymerization reaction proceeds with the second polymerizable monomer incorporated into the shell formed by the first polymerizable monomer, while the thermal motion of the shell is promoted. As a result, the reaction rate is high, and the polymerization reaction of the second polymerizable monomer incorporated into the shell and the polymerizable functional groups of the first polymerizable monomer that remained unreacted proceeds sufficiently, resulting in a dense crosslinking structure. It is therefore estimated that a shell is formed that is resistant to acetone penetration.

[0060] The above method for producing hollow particles includes the steps of preparing a mixture, preparing a suspension, and subjecting the suspension to a polymerization reaction, and may also include other steps. Furthermore, to the extent that it is technically possible, two or more of the above steps and other additional steps may be performed simultaneously as a single step, or in any order. For example, the preparation of the mixture and the suspension may be performed simultaneously in a single process, such as adding the materials for preparing the mixture while simultaneously performing the suspension.

[0061] A preferred example of a method for producing the above-mentioned hollow particles is a manufacturing method that includes the following steps. (1) Mixed liquid preparation process A process for preparing a mixture containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium. (2) Suspension process The process involves suspending the aforementioned mixture to prepare a suspension in which droplets of a monomer composition containing a first polymerizable monomer and a hydrocarbon solvent are dispersed in an aqueous medium. (3) Polymerization process (3-1) First polymerization step A first polymerization reaction is performed in which the suspension is subjected to a polymerization reaction until the polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more, thereby preparing a first precursor composition comprising a shell containing a polymer of the first polymerizable monomer and first precursor particles having a hollow portion filled with a hydrocarbon solvent. (3-2) Second polymerization process A second precursor composition is prepared by adding a second polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C to the first precursor composition and carrying out a second polymerization reaction, thereby preparing a second precursor composition containing a shell containing polymers of the first polymerizable monomer and the second polymerizable monomer, and second precursor particles having a hollow portion filled with a hydrocarbon solvent. (4) Solid-liquid separation process A step of obtaining second precursor particles containing a hydrocarbon solvent in a hollow portion by performing solid-liquid separation of the second precursor composition, and (5) Solvent removal process A step of removing the hydrocarbon solvent contained within the second precursor particles obtained by the solid-liquid separation step to obtain hollow particles.

[0062] Figure 1 is a schematic diagram illustrating an example of the above manufacturing method. (1) to (5) in Figure 1 correspond to each of the above steps (1) to (5). The white arrows between the figures indicate the order of each step. Note that Figure 1 is merely a schematic diagram for illustrative purposes, and the above manufacturing method is not limited to that shown in the figure. Furthermore, the structure, dimensions, and shape of the materials used in the manufacturing method of this disclosure are not limited to the structure, dimensions, and shape of the various materials shown in these figures. Figure 1(1) is a schematic cross-sectional view showing one embodiment of the mixture in the mixture preparation process. As shown in this figure, the mixture includes an aqueous medium 1 and a low-polarity material 2 dispersed in the aqueous medium 1. Here, the low-polarity material 2 means a material that has low polarity and does not easily mix with the aqueous medium 1. In this disclosure, the low-polarity material 2 includes a first polymerizable monomer and a hydrocarbon solvent. Figure 1(2) is a schematic cross-sectional view showing one embodiment of a suspension in the suspension process. The suspension comprises an aqueous medium 1 and droplets 10 of a monomer composition dispersed in the aqueous medium 1. The droplets 10 of the monomer composition contain a first polymerizable monomer and a hydrocarbon solvent, but the distribution within the droplets is not uniform. The droplets 10 of the monomer composition have a structure in which the hydrocarbon solvent 4a and a material 4b other than the hydrocarbon solvent containing the first polymerizable monomer are phase-separated, with the hydrocarbon solvent 4a unevenly distributed in the center and the material 4b other than the hydrocarbon solvent unevenly distributed on the surface side, and a dispersion stabilizer (not shown) attached to the surface. Figure 1(3) is a schematic cross-sectional view showing one embodiment of a composition (second precursor composition) that includes hollow particles (second precursor particles) containing a hydrocarbon solvent in their hollow portions, obtained by a polymerization process. The composition includes an aqueous medium 1 and hollow particles (second precursor particles) 20 dispersed in the aqueous medium 1, each containing a hydrocarbon solvent 4a in its hollow portion. The shell 6 forming the outer surface of the second precursor particles 20 is formed by the polymerization of the first polymerizable monomer in a droplet 10 of the monomer composition, and the polymerization of a second polymerizable monomer added later. Figure 1(4) is a schematic cross-sectional view showing one embodiment of hollow particles (second precursor particles) containing a hydrocarbon solvent in the hollow portion after the solid-liquid separation process. Figure 1(4) shows the state after the aqueous medium 1 has been removed from the state shown in Figure 1(3). Figure 1(5) is a schematic cross-sectional view showing one embodiment of hollow particles after the solvent removal process. Figure 1(5) shows the state after the hydrocarbon solvent 4a has been removed from the state shown in Figure 1(4). By removing the hydrocarbon solvent from the hollow particles (second precursor particles) that contain the hydrocarbon solvent in the hollow portion, hollow particles 100 are obtained that have a gas-filled hollow portion 8 inside the shell 6. The following describes the five processes mentioned above, as well as other processes, in order.

[0063] (1) Mixed liquid preparation process This process involves preparing a mixture containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium. The mixture preferably contains an oil-soluble polymerization initiator as a polymerization initiator. Furthermore, the mixture may contain other materials, such as suspension stabilizers, to the extent that they do not impair the effects of the present disclosure. The materials of the mixture will be described in the following order: (A) the first polymerizable monomer, (B) the oil-soluble polymerization initiator, (C) the hydrocarbon solvent, (D) the dispersion stabilizer, and (E) the aqueous medium.

[0064] (A) First polymerizable monomer The first polymerizable monomer comprises at least a crosslinkable monomer and may further comprise non-crosslinkable monomers to the extent that it does not impair the effects of the present disclosure. As the first polymerizable monomer, a (meth)acrylic polymerizable monomer having a (meth)acryloyl group as the polymerizable functional group is preferably used because the polymerization reaction is easily stabilized and hollow particles with high heat resistance can be obtained.

[0065] [Cross-linkable monomers] Because crosslinkable monomers have multiple polymerizable functional groups, they can be linked together, thereby increasing the crosslinking density of the shell. Examples of crosslinkable monomers include divinylbenzene, divinyldiphenyl, divinylnaphthalene, diallylphthalate, diallylamine, allyl(meth)acrylate, vinyl(meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, tricyclodecanedimethanol di(meth)acrylate, 2-hydroxy-3-(meth)acrylpropyl(meth)acrylate, etc. Examples include difunctional crosslinkable monomers having two polymerizable functional groups; trifunctional or more crosslinkable monomers having three or more polymerizable functional groups, such as trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol poly(meth)acrylate, and their ethoxylated derivatives. These crosslinkable monomers can be used individually or in combination of two or more. Among these crosslinkable monomers, hydrophilic crosslinkable monomers with a solubility of 0.3 g / L or more in distilled water at 20°C include, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, allyl methacrylate, vinyl methacrylate, 2-hydroxy-3-methacrylatelpropyl acrylate, diallylamine, and the like. The crosslinkable monomer contained in the first polymerizable monomer may be a hydrophilic crosslinkable monomer with a solubility of 0.3 g / L or more in distilled water at 20°C, or a non-hydrophilic crosslinkable monomer with a solubility of less than 0.3 g / L, and is not particularly limited.

[0066] The first polymerizable monomer includes at least one crosslinkable monomer selected from a bifunctional crosslinkable monomer and a trifunctional or more crosslinkable monomer. In particular, it is more preferable to include a combination of a bifunctional crosslinkable monomer and a trifunctional or more crosslinkable monomer in order to improve the strength and solvent resistance of the hollow particles and to enhance the effects such as weight reduction by the hollow particles. When the first polymerizable monomer includes a trifunctional or more crosslinkable monomer, it is advantageous in that a denser covalent network can be spread in the shell, but there is a tendency for unreacted polymerizable functional groups to remain after the first polymerization reaction. In the above manufacturing method, even when the first polymerizable monomer includes a trifunctional or more crosslinkable monomer, by adding a hydrophilic monomer as the second polymerizable monomer and carrying out a second polymerization reaction, the polymerization reaction of the unreacted polymerizable functional groups remaining after the first polymerization reaction can proceed more easily. Therefore, by including a trifunctional or crosslinkable monomer in the first polymerizable monomer, the crosslinked structure of the shell is made denser, improving the strength and solvent resistance of the hollow particles, thereby enhancing the effects of weight reduction and other benefits of the hollow particles.

[0067] Furthermore, as the crosslinkable monomer included in the first polymerizable monomer, a (meth)acrylic crosslinkable monomer having a (meth)acryloyl group as a polymerizable functional group is preferred because the polymerization reaction is easily stabilized and hollow particles with excellent strength and heat resistance can be obtained. In other words, as the bifunctional crosslinkable monomer used in the first polymerizable monomer, bifunctional (meth)acrylic crosslinkable monomers such as allyl (meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, and pentaerythritol di(meth)acrylate are preferred, and among these, ethylene glycol di(meth)acrylate and pentaerythritol di(meth)acrylate are more preferred. The crosslinkable monomer with three or more functions used in the first polymerizable monomer is preferably a (meth)acrylic crosslinkable monomer with three or more functions, such as trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol poly(meth)acrylate, and ethoxylated derivatives thereof. However, pentaerythritol tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, and dipentaerythritol poly(meth)acrylate are more preferred, and trimethylolpropane tri(meth)acrylate and pentaerythritol tetra(meth)acrylate are even more preferred. In this disclosure, the (meth)acrylic crosslinkable monomer may be any crosslinkable monomer having at least one (meth)acryloyl group as a polymerizable functional group, but it is preferable that all polymerizable functional groups are (meth)acryloyl groups.

[0068] When the first polymerizable monomer contains a (meth)acrylic crosslinkable monomer, the content of the (meth)acrylic crosslinkable monomer is preferably 50 parts by mass or more, more preferably 70 parts by mass or more, and even more preferably 90 parts by mass or more, in 100 parts by mass of the crosslinkable monomer contained in the first polymerizable monomer, and the crosslinkable monomer contained in the first polymerizable monomer may consist of a (meth)acrylic crosslinkable monomer.

[0069] In 100 parts by mass of the first polymerizable monomer, the content of the crosslinkable monomer is 80 parts by mass or more, preferably 85 to 100 parts by mass, more preferably 90 to 100 parts by mass, and even more preferably 95 to 100 parts by mass. By having a crosslinkable monomer content above the above lower limit, the polymer contained in the formed shell tends to be a polymer containing 80 to 100 parts by mass of crosslinkable monomer units per 100 parts by mass of total monomer units. Furthermore, because the content of crosslinkable monomer units in the shell of the hollow particles is sufficiently high, a dense covalent network is formed in the shell, resulting in improved strength and solvent resistance of the hollow particles, and thereby improving effects such as weight reduction by the hollow particles.

[0070] When the first polymerizable monomer contains a bifunctional crosslinkable monomer as a crosslinkable monomer, the content of the bifunctional crosslinkable monomer in 100 parts by mass of the first polymerizable monomer is not particularly limited, but the lower limit is 70 parts by mass or more, more preferably 80 parts by mass or more, in order to improve the strength and solvent resistance of the hollow particles and to improve the effects such as weight reduction by the hollow particles, while the upper limit may be 100 parts by mass or less, but in order to sufficiently contain trifunctional or more crosslinkable monomer units, it is preferably 95 parts by mass or less, more preferably 90 parts by mass or less.

[0071] When the first polymerizable monomer contains a crosslinkable monomer with three or more functions as a crosslinkable monomer, the content of the crosslinkable monomer with three or more functions in 100 parts by mass of the first polymerizable monomer is not particularly limited, but the lower limit is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 15 parts by mass or more, from the standpoint of improving the strength and solvent resistance of the hollow particles and improving the effects such as weight reduction by the hollow particles, the upper limit is preferably 50 parts by mass or less, more preferably 40 parts by mass or less, and even more preferably 30 parts by mass or less, and even more preferably 25 parts by mass or less, from the standpoint of sufficiently containing a crosslinkable monomer with two or more functions.

[0072] [Non-crosslinkable monomers] The first polymerizable monomer may further contain a non-crosslinkable monomer. As non-crosslinkable monomers, monovinyl monomers are preferably used. A monovinyl monomer is a compound having one polymerizable vinyl functional group. Examples of monovinyl monomers include (meth)acrylate alkyl esters having alkyl groups with 6 or more carbon atoms, such as 2-ethylhexyl (meth)acrylate and lauryl (meth)acrylate; aromatic vinyl monomers such as styrene, vinyltoluene, α-methylstyrene, p-methylstyrene, and halogenated styrene; monoolefin monomers such as ethylene, propylene, and butylene; diene monomers such as butadiene and isoprene; vinyl carboxylate monomers such as vinyl acetate; halogenated vinyl monomers such as vinyl chloride; and halogenated vinyl monomers such as vinylidene chloride. Examples include non-hydrophilic, non-crosslinkable monomers such as vinylidene genate monomers and vinylpyridine monomers, and hydrophilic, non-crosslinkable monomers such as alkyl (meth)acrylates having C1-C5 alkyl groups such as methyl (meth)acrylate, ethyl (meth)acrylate, and butyl (meth)acrylate; (meth)acrylamides and their derivatives such as (meth)acrylamide, N-methylol(meth)acrylamide, and N-butoxymethyl(meth)acrylamide; nitrile (meth)acrylates and their derivatives; and non-crosslinkable monomers containing polar groups. Examples of non-crosslinkable monomers containing polar groups include, for example, non-crosslinkable monomers containing polar groups selected from carboxyl groups, hydroxyl groups, sulfonic acid groups, amino groups, polyoxyethylene groups, and epoxy groups. More specifically, examples include carboxyl group-containing monomers such as ethylenically unsaturated carboxylic acid monomers like (meth)acrylic acid, crotonic acid, cinnamic acid, itaconic acid, fumaric acid, maleic acid, and butentricarboxylic acid; hydroxyl group-containing monomers such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate; sulfonic acid group-containing monomers such as styrene sulfonic acid; amino group-containing monomers such as dimethylaminoethyl (meth)acrylate and diethylaminoethyl (meth)acrylate; polyoxyethylene group-containing monomers such as methoxypolyethylene glycol (meth)acrylate; and epoxy group-containing monomers such as glycidyl (meth)acrylate, allyl glycidyl ether, and 4-hydroxybutyl acrylate glycidyl ether. These non-crosslinkable monomers can be used individually or in combination of two or more types. As the non-crosslinkable monomer used as the first polymerizable monomer, hydrophilic non-crosslinkable monomers are preferred in terms of improving the strength of the hollow particles, alkyl (meth)acrylates having an alkyl group with 1 to 5 carbon atoms are more preferred, and alkyl (meth)acrylates having an alkyl group with 1 to 4 carbon atoms are even more preferred.

[0073] In the first polymerizable monomer, polymerizable monomers other than the crosslinkable monomer are non-crosslinkable monomers. The content of non-crosslinkable monomers in the first polymerizable monomer is preferably 0 to 15 parts by mass per 100 parts by mass of the first polymerizable monomer. In order to suppress the decrease in reactivity of the first polymerizable monomer, improve the strength and solvent resistance of the hollow particles, and enhance the effects such as weight reduction by the hollow particles, the content of non-crosslinkable monomers per 100 parts by mass of the first polymerizable monomer is more preferably 10 parts by mass or less, and even more preferably 5 parts by mass or less. It is particularly preferable that the first polymerizable monomer does not contain non-crosslinkable monomers.

[0074] The content of the first polymerizable monomer in the mixture is not particularly limited, but from the viewpoint of balancing the porosity, particle size, and mechanical strength of the hollow particles, it is usually 15 to 55% by mass, more preferably 25 to 40% by mass, based on 100% by mass of the total mass of the components in the mixture excluding the aqueous medium.

[0075] Furthermore, from the viewpoint of improving the mechanical strength of the hollow particles, the content of the first polymerizable monomer relative to 100% by mass of the total solid content of the material that becomes the oil phase in the mixed liquid, excluding the hydrocarbon solvent, is preferably 90% by mass or more, more preferably 95% by mass or more.

[0076] (B) Oil-soluble polymerization initiator In this disclosure, it is preferable that the mixture contains an oil-soluble polymerization initiator as a polymerization initiator. As a method for polymerizing droplets of monomer composition after suspending the mixture, there is an emulsion polymerization method using a water-soluble polymerization initiator and a suspension polymerization method using an oil-soluble polymerization initiator, and suspension polymerization can be performed by using an oil-soluble polymerization initiator. The oil-soluble polymerization initiator is not particularly limited as long as it is lipophilic and has a solubility in water of 0.2% by mass or less. Examples of oil-soluble polymerization initiators include benzoyl peroxide, lauroyl peroxide, t-butyl peroxide-2-ethylhexanoate, 2,2'-azobis(2,4-dimethylvaleronitrile), and azobisisobutyronitrile. When the total mass of the first polymerizable monomer in the mixture is 100 parts by mass, the content of the oil-soluble 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. By having an oil-soluble polymerization initiator content of 0.1 to 10 parts by mass, the polymerization reaction proceeds sufficiently, and there is little risk of residual oil-soluble polymerization initiator after the polymerization reaction is completed, and the risk of unexpected side reactions is also small.

[0077] (C) Hydrocarbon solvents In this disclosure, a hydrocarbon solvent is used as a non-polymerizable and poorly water-soluble organic solvent. The hydrocarbon solvent acts as a spacer material that forms hollow spaces inside the particles. In the suspension step described later, a suspension is obtained in which droplets of the monomer composition containing the hydrocarbon solvent are dispersed in an aqueous medium. In the suspension step, phase separation occurs within the droplets of the monomer composition, resulting in the less polar hydrocarbon solvent tending to accumulate inside the droplets. Finally, in the droplets of the monomer composition, the hydrocarbon solvent is distributed inside, and other materials other than the hydrocarbon solvent are distributed around its periphery according to their respective polarities. Then, in the polymerization process described later, an aqueous dispersion containing hollow particles encapsulating a hydrocarbon solvent is obtained. That is, as the hydrocarbon solvent accumulates inside the particles, a hollow space filled with the hydrocarbon solvent is formed inside the resulting precursor particles.

[0078] The type of hydrocarbon solvent is not particularly limited. Examples of hydrocarbon solvents include saturated hydrocarbon solvents such as butane, pentane, n-hexane, cyclohexane, heptane, and octane; aromatic hydrocarbon solvents such as benzene, toluene, and xylene; and relatively volatile solvents such as carbon disulfide and carbon tetrachloride. The porosity of the hollow particles can be adjusted by changing the amount of hydrocarbon solvent in the mixture. In the suspension step described later, the polymerization reaction proceeds with oil droplets containing crosslinkable monomers, etc., encapsulating the hydrocarbon solvent. Therefore, the higher the hydrocarbon solvent content, the higher the porosity of the resulting hollow particles tends to be. Preferably, the hydrocarbon solvent contains 50% by mass or more of saturated hydrocarbon solvent in a total amount of 100% by mass of hydrocarbon solvents. This allows for sufficient phase separation within droplets of polymerizable monomer liquid, making it easier to obtain hollow particles with only one hollow portion and suppressing the formation of porous particles. The proportion of saturated hydrocarbon solvent is preferably 60% by mass or more, and more preferably 80% by mass or more, from the standpoint of further suppressing the formation of porous particles and ensuring that the hollow portions of each hollow particle are uniform. Furthermore, hydrocarbon solvents having 4 to 7 carbon atoms are preferred. Hydrocarbon compounds having 4 to 7 carbon atoms are easily encapsulated in the first precursor particles during the polymerization process and can be easily removed from the second precursor particles during the solvent removal process. Among these, hydrocarbon solvents having 5 or 6 carbon atoms are particularly preferred. Furthermore, although not particularly limited, hydrocarbon solvents are preferred to have a boiling point of 130°C or lower, and more preferably 100°C or lower, as they are easily removed in the solvent removal process described later. Also, hydrocarbon solvents are preferred to have a boiling point of 50°C or higher, and more preferably 60°C or higher, as they are easily encapsulated in the first precursor particles.

[0079] Furthermore, the hydrocarbon solvent preferably has a relative permittivity of 3 or less at 20°C. Relative permittivity is one indicator of the polarity of a compound. When the relative permittivity of the hydrocarbon solvent is sufficiently small, such as 3 or less, it is thought that phase separation proceeds rapidly in the droplets of the monomer composition, and hollow spaces are easily formed. Examples of solvents with a relative permittivity of 3 or less at 20°C are as follows. (The value in parentheses is the relative permittivity.) Heptane (1.9), n-hexane (1.9), cyclohexane (2.0), benzene (2.3), toluene (2.4). Regarding the relative permittivity at 20°C, values ​​can be found in publicly available literature (for example, "Basic Chemical Handbook," edited by the Chemical Society of Japan, 4th revised edition, Maruzen Co., Ltd., published September 30, 1993, pp. II-498 to II-503), as well as other technical information. Methods for measuring the relative permittivity at 20°C include, for example, relative permittivity tests conducted in accordance with JISC 2101:1999, item 23, with the measurement temperature set at 20°C.

[0080] In this disclosure, the content of the hydrocarbon solvent in the mixture is preferably 50 parts by mass or more and 500 parts by mass or less per 100 parts by mass of the total mass of the first polymerizable monomer, as this makes it easier to control the particle size of the hollow particles, easier to increase the porosity while maintaining the strength of the hollow particles, and easier to reduce the amount of residual hydrocarbon solvent in the particles. The content of the hydrocarbon solvent in the mixture is more preferably 60 parts by mass or more and 400 parts by mass or less, even more preferably 70 parts by mass or more and 300 parts by mass or less, and even more preferably 80 parts by mass or more and 200 parts by mass or less per 100 parts by mass of the total mass of the first polymerizable monomer.

[0081] (D) Dispersion stabilizer A dispersion stabilizer is an agent used in the suspension process to disperse droplets of a monomer composition in an aqueous medium. In this disclosure, it is preferable to use an inorganic dispersion stabilizer as the dispersion stabilizer because it allows for easy control of the particle size of the droplets in the suspension, narrows the particle size distribution of the resulting hollow particles, and suppresses a decrease in the strength of the hollow particles by preventing the shell from becoming too thin. 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; and metal hydroxides such as aluminum hydroxide, magnesium hydroxide, calcium hydroxide, barium hydroxide, and ferric hydroxide. These inorganic dispersion stabilizers can be used individually or in combination of two or more. Among the inorganic dispersion stabilizers mentioned above, poorly water-soluble inorganic metal salts such as sulfates, carbonates, phosphates, and metal hydroxides are preferred, metal hydroxides are more preferred, and magnesium hydroxide is particularly preferred. In this disclosure, the poorly water-soluble inorganic metal salt is preferably an inorganic metal salt whose solubility in 100 g of water is 0.5 g or less. In this disclosure, it is particularly preferable to use a poorly water-soluble inorganic dispersion stabilizer in the form of colloidal particles dispersed in an aqueous medium, that is, in the form of a colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles. This allows for a narrower particle size distribution of the monomer composition droplets, and also makes it easy to keep the amount of residual inorganic dispersion stabilizer in the resulting hollow particles low after washing. A colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloid 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 can be any water-soluble polyvalent metal salt other than the alkaline earth metal hydroxide compounds mentioned above. Examples include magnesium metal salts such as magnesium chloride, magnesium phosphate, and magnesium sulfate; calcium metal salts such as calcium chloride, calcium nitrate, calcium acetate, and calcium sulfate; aluminum metal salts such as aluminum chloride and aluminum sulfate; barium salts such as barium chloride, barium nitrate, and barium acetate; and zinc salts such as zinc chloride, zinc nitrate, and zinc acetate. Among these, magnesium metal salts, calcium metal salts, and aluminum metal salts are preferred, magnesium metal salts are more preferred, and magnesium chloride is particularly preferred. The method for reacting at least one selected from the alkali metal hydroxide and alkaline earth metal hydroxide mentioned above with the water-soluble polyvalent metal salt mentioned above in an aqueous medium is not particularly limited, but one method is to mix an aqueous solution of at least one selected from the alkali metal hydroxide and alkaline earth metal hydroxide with an aqueous solution of the water-soluble polyvalent metal salt.

[0082] The content of the dispersion stabilizer is not particularly limited, but is preferably 0.5 to 10 parts by mass, and more preferably 1.0 to 8.0 parts by mass, per 100 parts by mass of the total mass of the first polymerizable monomer and the hydrocarbon solvent. By having a dispersion stabilizer content above the lower limit, the monomer composition droplets can be sufficiently dispersed so that they do not coalesce in the suspension. On the other hand, by having a dispersion stabilizer content below 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. Furthermore, the content of the dispersion stabilizer is usually between 2 and 15 parts by mass, and preferably between 3 and 8 parts by mass, per 100 parts by mass of the aqueous medium.

[0083] (E)Aqueous medium In this disclosure, "aqueous medium" means a medium selected from the group consisting of water, a hydrophilic solvent, and a mixture of water and a hydrophilic solvent. The hydrophilic solvent in this disclosure is not particularly limited as long as it mixes well with water and does not undergo phase separation. Examples of hydrophilic solvents include alcohols such as methanol and ethanol; tetrahydrofuran (THF); and dimethyl sulfoxide (DMSO). Among aqueous media, water is preferred due to its high polarity. When using a mixture of water and a hydrophilic solvent, it is important that the overall polarity of the mixture does not become too low, from the viewpoint of forming droplets of the monomer composition. In this case, for example, the mixing ratio (mass ratio) of water and hydrophilic solvent may be water:hydrophilic solvent = 99:1 to 50:50.

[0084] A mixture is obtained by mixing the aforementioned materials and other materials as needed, and stirring as appropriate. In this mixture, the oil phase containing (A) the first polymerizable monomer, (B) an oil-soluble polymerization initiator, and (C) a lipophilic material such as a hydrocarbon solvent is dispersed in the aqueous phase containing (D) a dispersion stabilizer and (E) an aqueous medium, with particle sizes of several millimeters. Depending on the type of material, the dispersion state of these materials in the mixture can be observed with the naked eye. In the mixture preparation step, the mixture may be obtained by simply mixing the aforementioned materials and other materials as needed, and stirring as appropriate. However, in order to ensure a uniform shell, it is preferable to prepare the mixture by separately preparing an oil phase containing the first polymerizable monomer and a hydrocarbon solvent, and an aqueous phase containing a dispersion stabilizer and an aqueous medium, and then mixing these together. In this 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. By preparing the oil phase and aqueous phase separately in this manner and then mixing them, hollow particles with a uniform shell composition can be produced.

[0085] (2) Suspension process The suspension step is a process of preparing a suspension in which droplets of a monomer composition containing a hydrocarbon solvent are dispersed in an aqueous medium by suspending the above-mentioned mixture. The suspension method for forming droplets of monomer compositions is not particularly limited, but can be carried out using equipment capable of strong stirring, such as (in-line) emulsifiers / dispersers (horizontal in-line dispersers such as Taiheiyo Kiko Co., Ltd., product name: Milder, and Eurotech Co., Ltd., product name: Cavitron; vertical in-line dispersers such as IKA, product name: DRS 2000 / 5, etc.) or high-speed emulsifiers / dispersers (product name: TK Homomixer MARK II, etc., manufactured by Primix Co., Ltd.). In the suspension prepared in the suspension process, droplets of a monomer composition containing the above-mentioned lipophilic material and having a particle size of approximately 1 to 80 μm are uniformly dispersed in an aqueous medium. Such droplets of monomer composition are difficult to observe with the naked eye and can be observed using known observation equipment such as an optical microscope. During the suspension process, phase separation occurs within the monomer composition droplets, causing the less polar hydrocarbon solvent to accumulate inside the droplets. As a result, the resulting droplets will have the hydrocarbon solvent distributed inside and other materials distributed around their periphery.

[0086] Figure 2 is a schematic diagram showing one embodiment of a suspension in the suspension process. The droplet 10 of the monomer composition in Figure 2 is schematically shown as its cross-section. Note that Figure 2 is merely a schematic diagram, and the suspension in this disclosure is not necessarily limited to that shown in Figure 2. Part of Figure 2 corresponds to Figure 1(2) described above. Figure 2 shows how droplets 10 of the monomer composition and the first polymerizable monomer 4c dispersed in the aqueous medium 1 are dispersed in the aqueous medium 1. The droplets 10 are formed by a dispersion stabilizer 3 surrounding the oil-soluble monomer composition 4. The monomer composition contains an oil-soluble polymerization initiator 5, as well as a first polymerizable monomer and a hydrocarbon solvent (neither of which are shown). The droplet 10 is a micro-oil droplet containing the monomer composition 4, and the oil-soluble polymerization initiator 5 generates polymerization initiation radicals within the micro-oil droplet. Therefore, precursor particles of the desired particle size can be produced without over-growing the micro-oil droplets. In suspension polymerization using such an oil-soluble polymerization initiator, there is no opportunity for the polymerization initiator to come into contact with the polymerizable monomer 4c dispersed in the aqueous medium 1. Therefore, by using an oil-soluble polymerization initiator, it is possible to suppress the formation of extra resin particles, such as relatively small, dense particles, in addition to the desired hollow resin particles.

[0087] (3) Polymerization process (3-1) First polymerization step In the above manufacturing method, the polymerization process is carried out in two stages. In the first polymerization step, a first polymerization reaction is carried out in which the suspension is subjected to a polymerization reaction until the polymerization conversion rate of the first polymerizable monomer is 93% by mass or more, thereby preparing a first precursor composition containing a shell containing a polymer of the first polymerizable monomer and first precursor particles having a hollow portion filled with a hydrocarbon solvent. During the first polymerization reaction, droplets of the monomer composition are subjected to the polymerization reaction while encapsulating the hydrocarbon solvent, allowing the polymerization reaction to proceed easily while maintaining their shape. Therefore, the size and porosity of the resulting hollow particles can be easily adjusted by adjusting the amount of hydrocarbon solvent and the type of dispersion stabilizer during the first polymerization reaction. Furthermore, because the above-mentioned first polymerizable monomer and hydrocarbon solvent are used in combination, the polarity of the hydrocarbon solvent is low relative to the shell of the first precursor particle, and the hydrocarbon solvent does not readily accrete with the shell, so phase separation occurs sufficiently, and only one hollow portion tends to be formed.

[0088] In the first polymerization reaction, there are no particular limitations on the polymerization method; for example, batch, semi-continuous, or continuous methods can be used. In the first polymerization reaction, the polymerization temperature is preferably 40 to 80°C, and more preferably 50 to 70°C. Furthermore, in the first polymerization reaction, the heating rate when raising the temperature to the polymerization temperature is preferably 10°C / h to 60°C / h, and more preferably 15°C / h to 55°C / h. Furthermore, the reaction time for the first polymerization reaction is preferably 0.5 to 5 hours, and more preferably 1 to 3 hours.

[0089] In the above manufacturing method, the first polymerization reaction is carried out until the polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more, preferably 95% by mass or more, more preferably 97% by mass or more, and even more preferably 99% by mass or more. In this disclosure, the polymerization conversion rate is determined by formula (A) below, using the mass of the solid content of the first precursor particles obtained by the first polymerization reaction and the mass of the first polymerizable monomer remaining unreacted after the first polymerization reaction. In this disclosure, the solid content refers to all components excluding the solvent, and liquid polymerizable monomers, etc., are included in the solid content. The mass of the unreacted first polymerizable monomer can be measured using gas chromatography (GC). Polymerization conversion rate (mass%) = 100 - (mass of unreacted primary polymerizable monomer / mass of solid content of primary precursor particles) × 100 Equation (A)

[0090] (3-2) Second polymerization process In the second polymerization step, a second polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C is added to the first precursor composition obtained in the first polymerization step, and a second polymerization reaction is carried out to prepare a second precursor composition containing a shell containing polymers of the first polymerizable monomer and the second polymerizable monomer, and second precursor particles having a hollow portion filled with a hydrocarbon solvent. In the second polymerization reaction, the polymerization proceeds with the second polymerizable monomer incorporated into the shell of the first precursor particle. As the incorporation of the second polymerizable monomer into the shell of the first precursor particle promotes thermal motion, it is presumed that in the second polymerization reaction, the polymerization reaction of the polymerizable functional groups of the first polymerizable monomer that remained unreacted within the shell, and the polymerization of the second polymerizable monomer proceeds, forming a dense crosslinked structure.

[0091] The second polymerizable monomer is not particularly limited as long as it is a polymerizable monomer with a solubility of 0.3 g / L or more in distilled water at 20°C. However, among these, non-crosslinked monomers with a solubility of 0.3 g / L or more in distilled water at 20°C, i.e., hydrophilic non-crosslinked monomers, are preferred in order to improve the strength and solvent resistance of the hollow particles and to enhance the effects of weight reduction by the hollow particles. Examples of hydrophilic non-crosslinked monomers used as the second polymerizable monomer include those similar to the hydrophilic non-crosslinked monomers that can be used as the first polymerizable monomer. In particular, from the standpoint of improving the strength and solvent resistance of hollow particles and enhancing the effects of weight reduction by hollow particles, at least one selected from the group consisting of alkyl (meth)acrylates having an alkyl group having 1 to 5 carbon atoms, nitrile (meth)acrylates and their derivatives, and non-crosslinkable monomers containing polar groups is preferred, alkyl (meth)acrylates having an alkyl group having 1 to 5 carbon atoms are more preferred, and alkyl (meth)acrylates having an alkyl group having 1 to 4 carbon atoms are even more preferred. The alkyl acrylates and nitrile acrylates mentioned above are preferred, respectively. When the polymerizable functional group is an acryloyl group instead of a methacryloyl group, the reactivity is excellent, which improves effects such as weight reduction by hollow particles, and also reduces the likelihood of unreacted second polymerizable monomers remaining. In this disclosure, the acrylic crosslinkable monomer may be any crosslinkable monomer having at least one acryloyl group as a polymerizable functional group and no methacryloyl group, but it is preferable that all polymerizable functional groups are acryloyl groups. Furthermore, the number of carbon atoms in the alkyl group of the above (meth)acrylate alkyl esters is preferably 1 to 4 from the viewpoint of improving effects such as weight reduction due to hollow particles, more preferably 1 to 3 from the viewpoint of preventing the residue of unreacted second polymerizable monomers, and even more preferably the alkyl group is a methyl group. Among the above polar group-containing non-crosslinkable monomers, epoxy group-containing monomers, hydroxyl group-containing monomers, and amino group-containing monomers are preferred. Among the epoxy group-containing monomers among the above polar group-containing non-crosslinkable monomers, glycidyl (meth)acrylate is preferred, and among the hydroxyl group-containing monomers, 2-hydroxyethyl methacrylate is preferred.

[0092] Furthermore, as the second polymerizable monomer, a hydrophilic crosslinkable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C can also be used. Examples of hydrophilic crosslinkable monomers used as the second polymerizable monomer include those similar to those usable as the first polymerizable monomer. Among the hydrophilic crosslinkable monomers used as the second polymerizable monomer, hydrophilic crosslinkable monomers containing a hydroxyl group or an amino group are preferred. As a hydrophilic crosslinkable monomer containing a hydroxyl group, for example, 2-hydroxy-3-methacrylate is preferably used, and as a hydrophilic crosslinkable monomer containing an amino group, for example, diallylamine is preferably used. Furthermore, the second polymerizable monomer is preferably 2 g / L or more, more preferably 10 g / L or more, and even more preferably 15 g / L or more, in distilled water at 20°C, because the second polymerizable monomer is easily incorporated into the shell of the first precursor particles, promoting thermal motion and improving the strength of the hollow particles. The upper limit of the solubility of the second polymerizable monomer in distilled water at 20°C is not particularly limited, but is usually 80 g / L or less.

[0093] Furthermore, the molecular weight of the second polymerizable monomer is preferably 200 or less, more preferably 100 or less, as this facilitates the incorporation of the second polymerizable monomer into the shell of the first precursor particles, thereby promoting thermal motion, improving the strength and solvent resistance of the hollow particles, and enhancing the effects of weight reduction and other benefits of the hollow particles. The lower limit of the molecular weight of the second polymerizable monomer is not particularly limited, but is usually 50 or more.

[0094] The amount of the second polymerizable monomer added is preferably 3 to 15 parts by mass, and more preferably 4 to 10 parts by mass, per 100 parts by mass of the first polymerizable monomer. If the amount of the second polymerizable monomer added is above the lower limit, the effect of promoting the polymerization reaction by adding the second polymerizable monomer is improved, and the crosslinked structure of the hollow particle shell becomes denser, thereby improving the strength and solvent resistance of the hollow particle, and consequently improving the effects of weight reduction and other benefits of the hollow particle. On the other hand, if the amount of the second polymerizable monomer added is below the upper limit, it is possible to suppress the decrease in the content ratio of the first polymerizable monomer to the total polymerizable monomer used to form the shell. Since the first polymerizable monomer contains a large amount of crosslinkable monomers, by suppressing the decrease in the content ratio of the first polymerizable monomer, it is possible to obtain hollow particles with excellent strength that contain a large amount of crosslinked structure formed by crosslinkable monomers.

[0095] In the second polymerization reaction, which is carried out after adding the second polymerizable monomer, there are no particular limitations on the polymerization method; for example, the same polymerization method used in the first polymerization reaction can be employed. In the second polymerization reaction, the polymerization temperature is preferably 40 to 80°C, and more preferably 50 to 70°C. The reaction time for the second polymerization reaction is preferably 1 to 6 hours, and more preferably 2 to 4 hours.

[0096] According to the above manufacturing method, the amount of unreacted polymerizable monomers remaining after the second polymerization reaction can be preferably 750 ppm or less, more preferably 500 ppm or less, and even more preferably 300 ppm or less. If the amount of unreacted polymerizable monomers remaining after the second polymerization reaction is below the above upper limit, it is suggested that the reaction rate of polymerizable monomers is high. When the reaction rate of polymerizable monomers is high, the cross-linking structure in the shell tends to become denser, which in turn tends to improve the solvent resistance and strength of the hollow particles. In this disclosure, the amount of unreacted polymerizable monomers remaining after the second polymerization reaction is the ratio of the mass of unreacted polymerizable monomers to the mass of solids of the hollow particles obtained by the second polymerization reaction. The mass of unreacted polymerizable monomers can be measured using gas chromatography (GC).

[0097] (4) Solid-liquid separation process This process involves obtaining a solid component containing the second precursor particles by performing solid-liquid separation of the second precursor composition, which contains hollow particles (second precursor particles) encapsulating a hydrocarbon solvent, obtained by the polymerization process described above.

[0098] The method for solid-liquid separation of the second precursor composition is not particularly limited, and known methods can be used. Examples of solid-liquid separation methods include centrifugation, filtration, and static separation. Among these, centrifugation or filtration can be used, and centrifugation may be used from the viewpoint of ease of operation. After the solid-liquid separation process, any optional steps such as a pre-drying process may be performed before carrying out the solvent removal process described later. An example of a pre-drying process is to pre-dry the solid obtained after the solid-liquid separation process using a drying device such as a dryer or a drying apparatus such as a hand dryer.

[0099] (5) Solvent removal process This step involves removing the hydrocarbon solvent contained within the hollow particles (second precursor particles) obtained in the solid-liquid separation step. By removing the hydrocarbon solvent encapsulated within the second precursor particle in the air, the hydrocarbon solvent inside the second precursor particle is replaced with air, resulting in a hollow particle filled with gas.

[0100] In this process, "in the air" strictly refers to an environment where there is absolutely no liquid outside the second precursor particles, or an environment where there is only a very small amount of liquid outside the second precursor particles that does not affect the removal of the hydrocarbon solvent. "In the air" can also be rephrased as a state in which the second precursor particles are not in the slurry, or a state in which the second precursor particles are in the dry powder. In other words, in this process, it is important to remove the hydrocarbon solvent in an environment in which the second precursor particles are in direct contact with the external gas.

[0101] The method for removing the hydrocarbon solvent from the second precursor particles in the air is not particularly limited, and known methods can be employed. Examples of such methods include vacuum drying, heat drying, airflow drying, or a combination of these methods. In particular, when using the heat drying method, the heating temperature must be above the boiling point of the hydrocarbon solvent and below the maximum temperature at which the shell structure of the second precursor particles does not collapse. Therefore, depending on the shell composition in the second precursor particles and the type of hydrocarbon solvent, the heating temperature may be, for example, 50-200°C, 70-200°C, or 100-200°C. Through a drying operation in air, the hydrocarbon solvent inside the second precursor particle is replaced by the external gas, resulting in hollow particles in which the hollow portion is filled with gas.

[0102] The drying atmosphere is not particularly limited and can include, for example, air, oxygen, nitrogen, or argon. Furthermore, hollow particles with a temporarily vacuumed interior can be obtained by first filling the inside of the hollow particles with gas and then drying them under reduced pressure.

[0103] Alternatively, instead of performing solid-liquid separation on the slurry-like second precursor composition obtained in the polymerization step, the hydrocarbon solvent may be removed by replacing the hydrocarbon solvent encapsulated in the second precursor particles with the aqueous medium of the slurry in a slurry containing the second precursor particles and an aqueous medium. In this method, the hydrocarbon solvent encapsulated in the second precursor particles can be removed by bubbling an inert gas through the second precursor composition at a temperature at or above the boiling point of the hydrocarbon solvent minus 35°C. Here, if the hydrocarbon solvent is a mixed solvent containing multiple types of hydrocarbon solvents and has multiple boiling points, the boiling point of the hydrocarbon solvent in the solvent removal step shall be 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 bubbling through the second precursor composition is preferably at or above the boiling point of the hydrocarbon solvent minus 30°C, and more preferably at or above the boiling point of the hydrocarbon solvent minus 20°C, in order to reduce the amount of residual hydrocarbon solvent in the hollow particles. The bubbling temperature is usually at or above the polymerization temperature in the polymerization step. Although not particularly limited, the bubbling temperature may be between 50°C and 100°C. The inert gas used for bubbling is not particularly limited, but examples include nitrogen and argon. The bubbling conditions are adjusted as appropriate to remove the hydrocarbon solvent encapsulated in the second precursor particles, depending on the type and amount of hydrocarbon solvent, and are not particularly limited. For example, an inert gas may be bubbled at a rate of 1 to 3 L / min for 1 to 10 hours. In this method, an aqueous slurry is obtained in which a water-based medium is encapsulated within the second precursor particles. By separating this slurry into solid and liquid phases and drying the resulting hollow particles, the water-based medium within the hollow particles is removed, thereby obtaining hollow particles in which the hollow portion is occupied by gas.

[0104] Comparing a method in which a slurry-like second precursor composition is subjected to solid-liquid separation and then the hydrocarbon solvent in the second precursor particles is removed in the air to obtain hollow particles with a hollow portion filled with gas, with a method in which the hydrocarbon solvent contained in the second precursor particles is replaced with the aqueous medium of the slurry in a slurry containing the second precursor particles and an aqueous medium, then solid-liquid separation is performed and the aqueous medium in the second precursor particles is removed in the air to obtain hollow particles with a hollow portion filled with gas, the former method has the advantage that the hollow particles are less likely to be crushed in the step of removing the hydrocarbon solvent, while the latter method has the advantage that the residue of hydrocarbon solvent is reduced by performing bubbling with an inert gas. Furthermore, when replacing the hydrocarbon solvent encapsulated in the second precursor particle with water, there is a problem that the resulting hollow resin particles will collapse unless an equal volume of water is added to the particle to replace the hydrocarbon solvent removed from it. To prevent this, for example, the pH of the slurry can be set to 7 or higher, and the hydrocarbon solvent can be removed after the particle shell has been alkaline-swelled. In this case, the particle shell gains flexibility, allowing the replacement of the hydrocarbon solvent with water inside the particle to proceed rapidly.

[0105] In addition, after the polymerization step and before the solid-liquid separation step, a method may be used to remove the hydrocarbon solvent contained within the second precursor particles without performing solid-liquid separation of the slurry-like second precursor composition obtained in the polymerization step. For example, this could involve evaporating the hydrocarbon solvent contained within the second precursor particles from the second precursor composition under a predetermined pressure (high pressure, atmospheric pressure, or reduced pressure); or introducing an inert gas such as nitrogen, argon, or helium, or water vapor, into the second precursor composition under a predetermined pressure (high pressure, atmospheric pressure, or reduced pressure) and then evaporating the solvent.

[0106] (6) Others In addition to the above steps (1) to (5), for example, the following steps may be added: (6-a) the cleaning step and (6-b) the re-replacement step of the hollow portion. (6-a) Washing process The washing step is a step performed before the solid-liquid separation step in order to remove any remaining dispersion stabilizer in the second precursor composition containing the second precursor particles by adding an acid or alkali. If the dispersion stabilizer used is an inorganic dispersion stabilizer soluble in acid, it is preferable to wash the second precursor composition containing the second precursor particles by adding an acid. On the other hand, if the dispersion stabilizer used is an inorganic compound soluble in alkali, it is preferable to wash the second precursor composition containing the second precursor particles by adding an alkali. Furthermore, when an acid-soluble inorganic dispersion stabilizer is used as the dispersion stabilizer, it is preferable to add an acid to the second precursor composition containing the second precursor particles and adjust the pH to preferably 6.5 or lower, more preferably 6 or lower. As the added acid, inorganic acids such as sulfuric acid, hydrochloric acid, and nitric acid, and organic acids such as formic acid and acetic acid can be used, but sulfuric acid is particularly preferred because it has a high efficiency in removing the dispersion stabilizer and places little burden on the manufacturing equipment.

[0107] (6-b) Replacement process of the hollow section The hollow portion re-substitution process is a process of replacing the gas or liquid inside a hollow particle with another gas or liquid. Such substitution can change the environment inside the hollow particle, selectively confine molecules inside the hollow particle, or modify the chemical structure inside the hollow particle to suit the application.

[0108] II-2. Steps for preparing the resin composition The method for producing a fiber-reinforced molded article according to the present disclosure includes a step of preparing a resin composition containing hollow particles, a matrix resin, and a solvent obtained in the above step. In this step, for example, the resin composition is prepared by mixing the hollow particles, the matrix resin, the solvent, and additives added as needed.

[0109] The content of hollow particles in the resin composition is not particularly limited, but from the viewpoint of reducing the weight of the fiber-reinforced molded article, it is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more, of the total solid content of the resin composition, and from the viewpoint of suppressing a decrease in the physical properties of the fiber-reinforced molded article, it is preferably 50% by mass or less, and more preferably 30% by mass or less.

[0110] The matrix resin used in this process is the matrix resin used in the fiber-reinforced molded article of the present disclosure described above, but before curing. The matrix resin used in this process preferably includes, for example, an epoxy resin. Examples of epoxy resins include bixylenol-type epoxy resin, bisphenol A-type epoxy resin, bisphenol F-type epoxy resin, bisphenol S-type epoxy resin, bisphenol AF-type epoxy resin, dicyclopentadiene-type epoxy resin, trisphenol-type epoxy resin, naphthol novolac-type epoxy resin, phenol novolac-type epoxy resin, tert-butyl-catechol-type epoxy resin, naphthalene-type epoxy resin, naphthol-type epoxy resin, anthracene-type epoxy resin, glycidylamine-type epoxy resin, and glycidyl ester-type epoxy resin. Examples include cresol novolac type epoxy resins, phenol aralkyl type epoxy resins, biphenyl type epoxy resins, linear aliphatic epoxy resins, epoxy resins having a butadiene structure, alicyclic epoxy resins, heterocyclic epoxy resins, spiroring-containing epoxy resins, cyclohexane type epoxy resins, cyclohexanedimethanol type epoxy resins, naphthylene ether type epoxy resins, trimethylol type epoxy resins, tetraphenylethane type epoxy resins, isocyanurate type epoxy resins, phenolphthalein type epoxy resins, and the like. These epoxy resins can be used individually or in combination of two or more types. The matrix resin used in this process may contain additives for curing the resin, such as curing agents, curing catalysts, or curing accelerators. The additives for curing the resin contained in the matrix resin can be selected from known ones depending on the type of resin and are not particularly limited. Examples include amines, acid anhydrides, imidazoles, thiols, phenols, naphthols, benzoxazines, cyanate esters, and carbodiimides. Examples of curing agents used in combination with epoxy resins include amine-based curing agents, amide-based curing agents, acid anhydride-based curing agents, phenol-based curing agents, active ester-based curing agents, carboxyl group-containing curing agents, and thiol-based curing agents. Furthermore, examples of curing catalysts used in combination with epoxy resins include phosphorus compounds, tertiary amine compounds, imidazole compounds, and organometallic salts. These additives for curing the resins can be used individually or in combination of two or more types. The viscosity of the matrix resin used in this process is not particularly limited; the viscosity of the entire resin composition can be adjusted by controlling the type and amount of solvent to ensure excellent impregnation into the reinforcing fibers.

[0111] The matrix resin content in the resin composition is not particularly limited, but from the viewpoint of impregnation of the resin composition and mechanical properties of the fiber-reinforced molded article, it is preferably 50% by mass or more, more preferably 70% by mass or more, of the total solid content of the resin composition, and from the viewpoint of weight reduction of the fiber-reinforced molded article, it is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less.

[0112] The solvent used in this process can be any known solvent usable in resin compositions, and is not particularly limited; it can be appropriately selected depending on the type and viscosity of the matrix resin. Examples of suitable solvents include aromatic hydrocarbons such as benzene, toluene, and xylene; ketones such as dioxane, acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; and ethers such as ethylene glycol monomethyl ether and propylene glycol monomethyl ether. These solvents can be used individually or in combination of two or more. In particular, the solvent preferably contains at least one selected from ketones and ethers, and more preferably contains ketones, as this allows for easier demonstration of the effects of weight reduction and other benefits due to the hollow particles of the present disclosure.

[0113] The solvent content in the resin composition is adjusted as appropriate to achieve a viscosity that facilitates impregnation of the reinforcing fibers, and is not particularly limited. However, from the perspective of easily exhibiting the effects of weight reduction and other benefits due to the hollow particles of the present disclosure, the solvent content is preferably 10 to 50% by mass, more preferably 20 to 40% by mass, of the total mass of the resin composition. The viscosity of the resin composition is not particularly limited, as long as it is sufficient to impregnate the reinforcing fibers.

[0114] The resin composition may further contain additives such as UV absorbers, colorants, defoamers, thickeners, heat stabilizers, leveling agents, lubricants, antistatic agents, and fillers, as needed, to the extent that they do not impair the effects of the present disclosure.

[0115] In this process, the mixing of each component may be carried out at a temperature at which the solvent does not volatilize, for example, at a temperature above room temperature but below the boiling point of the solvent. Furthermore, if the resin composition includes a thermosetting resin as the matrix resin, the process may be carried out in a temperature environment below the curing temperature of the thermosetting resin, and although not particularly limited, it is usually carried out in a temperature environment of 240°C or lower. When the resin composition includes a thermoplastic resin as the matrix resin, the mixing of each component may be performed by melt kneading, which involves melting the thermoplastic resin by heating. The temperature during melt kneading should be any temperature that can melt the thermoplastic resin used, and is not particularly limited, but it is preferable to be 250°C or lower in order to suppress the crushing of hollow particles.

[0116] II-3. Process of impregnating reinforcing fibers with resin composition The method for manufacturing a fiber-reinforced molded article according to this disclosure includes a step of impregnating reinforcing fibers with the resin composition obtained in the above step. The reinforcing fibers impregnated with the resin composition obtained in this step are generally called prepregs. The reinforcing fibers used in this process are the same as the reinforcing fibers used in the fiber-reinforced molded articles of the present disclosure described above. The method for impregnating the reinforcing fibers with the resin composition is not particularly limited and can include, for example, a wet method conventionally used in the manufacture of prepregs, such as a method in which the resin composition is immersed in the reinforcing fibers and then the solvent is removed. The method for removing the solvent from the resin composition impregnated into the reinforcing fibers is not particularly limited and can include, for example, natural drying, heat drying, or a combination thereof. The heat drying temperature is appropriately selected according to the type of solvent and matrix resin, so as to be able to remove the solvent while leaving the resin composition uncured or semi-cured, and is not particularly limited, but is usually between 40 and 250°C. A general drying device such as an oven can be used for heat drying. The drying time is not particularly limited, but is usually between 1 minute and 1 hour.

[0117] II-4. Other Processes The method for producing a fiber-reinforced molded article according to this disclosure may further include a step of curing a resin composition impregnated with reinforcing fibers. For example, the process of curing the resin composition impregnated into the reinforcing fibers can involve heating and pressurizing the reinforcing fibers impregnated with the resin composition using methods such as press molding, autoclave molding, sheet winding molding, bagging molding, wrapping tape molding, internal pressure molding, or sheet wrap molding. The curing of the resin composition may be carried out by laminating multiple layers of reinforcing fibers impregnated with the resin composition, or by laminating a support or the like. Examples of support materials include resins such as polyethylene terephthalate and polyethylene naphthalate, and metals such as copper, stainless steel, aluminum, nickel, chromium, gold, and silver. The heating and pressing conditions for curing the resin composition are adjusted as appropriate depending on the composition of the resin composition, the content of each component, the shape of each component, etc., and are not particularly limited, but for example, it may be carried out at a temperature of 23 to 250°C for 5 minutes to 24 hours. [Examples]

[0118] The present disclosure will be further described below with reference to examples and comparative examples, but the present disclosure is not limited to these examples. Parts and percentages are by mass unless otherwise specified.

[0119] [Manufacturing Example 1] (1) Mixed liquid preparation process First, the following materials were mixed to form the oil phase. First polymerizable monomer: 80 parts ethylene glycol dimethacrylate and 20 parts pentaerythritol tetraacrylate 2,2'-Azobis(2,4-dimethylvaleronitrile) (oil-soluble polymerization initiator, manufactured by Wako Pure Chemical Industries, Ltd., trade name: V-65) 3 parts Cyclohexane 125 parts Next, in a stirred tank, under room temperature conditions, an aqueous solution prepared by dissolving 17.1 parts of magnesium chloride (water-soluble polyvalent metal salt) in 494 parts of deionized water was gradually added under stirring to an aqueous solution prepared by dissolving 12.1 parts of sodium hydroxide (alkali metal hydroxide) in 121 parts of deionized water to prepare a magnesium hydroxide colloid (poorly water-soluble metal hydroxide colloid) dispersion (4 parts magnesium hydroxide), which was then used as the aqueous phase. A mixture was prepared by mixing the obtained aqueous phase and oil phase.

[0120] (2) Suspension process The mixture obtained in the above mixture preparation step was suspended by stirring it for 1 minute at a rotation speed of 4,000 rpm using a disperser (Primix Corporation, product name: Homomixer) to prepare a suspension in which droplets of the monomer composition containing cyclohexane were dispersed in water.

[0121] (3) Polymerization process The suspension obtained in the above suspension step was heated in a nitrogen atmosphere from 40°C to 65°C over 30 minutes (heating rate: 50°C / hour), and stirred for 1 hour and 30 minutes under the temperature of 65°C to carry out the first polymerization reaction, obtaining a first precursor composition containing the first precursor particles. The polymerization conversion rate at the end of the first polymerization reaction was 99.2% by mass. Subsequently, 5 parts of methyl acrylate as the second polymerizable monomer were added to the stirring tank, and the second polymerization reaction was carried out by stirring for 2 hours and 30 minutes under the nitrogen atmosphere and temperature of 65°C. This second polymerization reaction obtained a second precursor composition containing the second precursor particles encapsulating cyclohexane.

[0122] (4) Washing process and solid-liquid separation process The second precursor composition described above was washed with dilute sulfuric acid (25°C, 10 minutes) to reduce the pH to 5.5 or less. Next, after separating the water by filtration, 200 parts of freshly deionized water were added to re-form a slurry, and the water washing treatment (washing, filtration, dewatering) was repeated several times at room temperature (25°C), and the solid was separated by filtration. The obtained solid was dried in a dryer at a temperature of 40°C to obtain second precursor particles containing cyclohexane.

[0123] (5) Solvent removal process The second precursor particles obtained in the solid-liquid separation process described above were heat-treated in a vacuum dryer at 200°C for 6 hours to remove the hydrocarbon solvent contained within the particles, thereby obtaining the hollow particles of Production Example 1. Based on observations using a scanning electron microscope and porosity values, it was confirmed that the obtained hollow particles were spherical and contained hollow portions.

[0124] [Manufacturing Example 2] In Production Example 1, the hollow particles of Production Example 2 were produced using the same procedure as in Production Example 1, except that the amount of cyclohexane added in the "(1) Mixture Preparation Step" was changed to 160 parts.

[0125] [Manufacturing Examples 3-8] In Production Example 1, the hollow particles of Production Examples 3 to 8 were produced using the same procedure as in Production Example 1, except that the type or amount of the second polymerizable monomer added in the "(3) polymerization step" was as shown in Table 1.

[0126] [Comparative Manufacturing Example 1] In Production Example 1, the hollow particles of Comparative Production Example 1 were produced using the same procedure as in Production Example 1, except that the second polymerizable monomer was not added and the second polymerization reaction was not carried out in the "(3) polymerization step" described above.

[0127] [Comparative Manufacturing Example 2] In Production Example 1, the hollow particles of Comparative Production Example 2 were produced using the same procedure as in Production Example 1, except that in the "(3) Polymerization Step" described above, the reaction time of the first polymerization reaction was changed from 1 hour and 30 minutes to 30 minutes, and when the sum of the polymerization conversion rates of the first polymerizable monomers, ethylene glycol dimethacrylate and pentaerythritol tetraacrylate, reached 91.0% by mass, the second polymerizable monomer was added and the second polymerization reaction was carried out.

[0128] [Comparative Manufacturing Example 3] In Manufacturing Example 1, the hollow particles of Comparative Manufacturing Example 3 were produced using the same procedure as in Manufacturing Example 1, except that the materials and quantities of the first polymerizable monomer were as shown in Table 1 in the "(1) Mixture Preparation Step" described above.

[0129] [Comparative Production Example 4] In Production Example 1, in the above “(3) Polymerization step”, as the second polymerizable monomer, except that 5 parts of styrene (solubility in distilled water at 20°C is 0.2 g / L) was added instead of 5 parts of methyl acrylate, hollow particles of Comparative Production Example 4 were produced in the same procedure as in Production Example 1.

[0130]

Table 1

[0131] [Evaluation] 1. Polymerization conversion rate In the polymerization step of each production example and each comparative production example, 50 g of the first precursor composition generated in the first polymerization reaction was collected and pressure-filtered to obtain the first precursor particles (containing moisture and a hydrocarbon solvent) contained in the first precursor composition, and weighed accurately to the unit of 1 mg. To approximately 3 g of the accurately weighed first precursor particles, 27 g of ethyl acetate was added and stirred for 15 minutes, and then 13 g of methanol was added and stirred for a further 10 minutes. The obtained solution was allowed to stand to precipitate the insoluble matter, and the supernatant of this solution was collected as a measurement sample. 2 μL of the measurement sample was injected into a gas chromatograph, and the amount of the polymerizable monomer in the measurement sample was quantified by gas chromatography (GC) under the following conditions, and this was taken as the mass of the unreacted first polymerizable monomer. Further, the first precursor particles obtained by pressure filtration were dried at 200°C for 2 hours to remove moisture and the hydrocarbon solvent, and the mass of the solid content of the first precursor particles was determined. Then, the polymerization conversion rate was calculated by the following formula (A). Polymerization conversion rate (mass %) = 100 - (mass of unreacted first polymerizable monomer / mass of solid content of first precursor particles) × 100 Formula (A) <GC conditions> Column: TC-WAX (0.25 mm × 30 m) Column temperature: 80°C Injection temperature: 200°C FID detection side temperature: 200°C

[0132] 2. Residual monomer amount Precisely weigh 3 g of hollow particles to the nearest 1 mg, add 27 g of ethyl acetate, and stir for 15 minutes. Then add 13 g of methanol and stir for an additional 10 minutes. Leave the resulting solution to stand to precipitate the insoluble matter, and collect the supernatant of this solution as a measurement sample. Inject 2 μl of the measurement sample into a gas chromatograph and quantify the amount of unreacted polymerizable monomer in the measurement sample by gas chromatography (GC) under the following conditions. Calculate the content ratio of the unreacted polymerizable monomer contained in the hollow particles and use it as the residual monomer amount. <GC Conditions> Column: TC-WAX (0.25 mm × 30 m) Column temperature: 80 °C Injection temperature: 200 °C FID detector temperature: 200 °C

[0133] For the hollow particles obtained in each production example and each comparative production example, Table 2 shows the content ratio (mass %) of each monomer unit in the polymer contained in the shell. In addition, the following measurements and evaluations were performed on the hollow particles obtained in each production example and each comparative production example. The results are shown in Table 2.

[0134] 3. Volume-average particle diameter Using a laser diffraction particle size distribution analyzer (manufactured by Shimadzu Corporation, product name: SALD-2000), measure the particle size of the hollow particles, calculate the volume average, and use it as the volume-average particle diameter.

[0135] 4. Density and porosity 4-1. Measurement of apparent density First, fill a 100 cm 3 volumetric flask with approximately 30 cm 3 of hollow particles, and accurately weigh the mass of the filled hollow particles. Next, while taking care not to let air bubbles enter, precisely fill the volumetric flask filled with hollow particles with isopropanol up to the calibration mark. Accurately weigh the mass of the isopropanol added to the volumetric flask, and calculate the apparent density D1 (g / cm 3 ) of the hollow particles based on the following formula (I). Formula (I) Apparent density D1 = [Mass of hollow particles] / (100 - [Mass of isopropanol] ÷ [Specific gravity of isopropanol at measurement temperature])

[0136] 4-2. Measurement of true density After crushing the hollow particles in advance, a volume of 100 cm³ 3 Approximately 10 g of hollow particle fragments were packed into a volumetric flask, and the mass of the packed fragments was accurately weighed. Next, as with the measurement of apparent density above, isopropanol is added to a volumetric 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). 3 ) was calculated. Formula (II) True density D0 = [Mass of hollow particle fragments] / (100 - [Mass of isopropanol] ÷ [Specific gravity of isopropanol at measurement temperature])

[0137] 4-3. Calculation of void ratio The porosity of the hollow particles was calculated from the apparent density D1 and true density D0 of the hollow particles based on the following equation (III). Formula (III) Porosity (%) = 100 - (Apparent density D1 / True density D0) × 100

[0138] 5. Immersion Test Under conditions of 25°C, 0.1 mg of hollow particles were added to 4 mL of acetone, shaken for 10 minutes at a shaking speed of 100 rpm using a shaker, and then allowed to stand for 48 hours. The proportion of precipitated hollow particles was determined and evaluated according to the evaluation criteria below. The precipitated hollow particles in the acetone were separated using a centrifuge, dried, and their mass was measured. The proportion of precipitated hollow particles was determined by calculating the ratio of the mass of the precipitated hollow particles to the total mass of the hollow particles immersed in the acetone. (Evaluation criteria for immersion tests) ○: Less than 5% by mass of precipitated hollow particles ×: Precipitated hollow particles make up 5% or more by mass.

[0139] [Table 2]

[0140] [Consideration] As shown in Table 2 above, in the comparative manufacturing examples 1 to 4, the amount of hollow particles that precipitated in acetone in the immersion test was 5% by mass or more. In comparative manufacturing example 1, it is presumed that the polymerization reaction was carried out in a single step, resulting in unreacted polymerizable functional groups remaining in the shell and a rough cross-linking structure of the shell, which allowed acetone to easily penetrate the resulting hollow particles. In comparative production example 2, the second polymerizable monomer was added before the polymerization conversion rate of the first polymerizable monomer reached 93% by mass. It is presumed that the timing of adding the second polymerizable monomer was too early, resulting in unreacted polymerizable functional groups remaining in the shell and a rough cross-linking structure of the shell, which made the resulting hollow particles more susceptible to acetone penetration. In comparative production example 3, the content of crosslinkable monomers in the first polymerizable monomer was low, and furthermore, a large amount of unreacted non-crosslinkable monomers remained. As a result, the crosslinking structure of the shell was rough, and it is presumed that the resulting hollow particles were easily permeated with acetone. In comparative production example 4, instead of using a hydrophilic monomer with a solubility of 0.3 g / L or higher in distilled water at 20°C as the second polymerizable monomer, styrene with a solubility of 0.2 g / L in distilled water at 20°C was used. As a result, the second polymerizable monomer was not easily incorporated into the shell, leaving unreacted polymerizable functional groups in the shell. This resulted in a rough cross-linking structure of the shell, which is presumed to have made the resulting hollow particles more susceptible to acetone penetration.

[0141] In contrast, the hollow particles obtained in Production Examples 1 to 8 contained a polymer in the shell that included 80 parts by mass or more of crosslinkable monomer units per 100 parts by mass of total monomer units, and the amount of hollow particles that precipitated in acetone in the immersion test was less than 5% by mass. In Production Examples 1 to 8, the first polymerizable monomer contained in the mixture contained a sufficient amount of crosslinkable monomers, and in the polymerization process, when the polymerization conversion rate of the first polymerizable monomer reached 93% by mass or more, a second polymerizable monomer with a solubility of 0.3 g / L or more in distilled water at 20°C was added and subjected to further polymerization. Therefore, although a large amount of crosslinkable monomers were used, almost no unreacted polymerizable functional groups remained in the shell, and the crosslinked structure of the shell became dense, so it is presumed that hollow particles that are difficult for acetone to penetrate were obtained.

[0142] [Examples 1-10, Comparative Examples 1-4] (1) Preparation of resin composition (varnish) 50 parts epoxy resin (Daicel Corporation, product code: EHPE3150CE), 24.9 parts curing agent (DIC Corporation, product code: LF6161, 65% solids MEK solution), 0.1 parts 2-ethyl-4-methylimidazole (Nacalai Tesque Corporation, 2E4MZ) as a curing catalyst, and the amount of methyl ethyl ketone (MEK) shown in Table 3 were added and stirred at room temperature for 30 minutes. After stirring, the hollow particles obtained in the above production examples 1 to 8 or comparative production examples 1 to 4 were added in the amount shown in Table 3, and the mixture was stirred for a further 1 hour to prepare a resin composition (varnish) with a solids content of approximately 70%.

[0143] (2) Preparation of prepregs Carbon fiber cut to 300mm x 200mm (Mitsubishi Chemical Corporation, product name: TR3110MS, thickness: 200μm, basis weight: 200g / m²) 2 25-30 ml of the above resin composition was applied to the carbon fiber. The carbon fiber coated with the above resin composition was suspended to allow excess resin composition to fall off by its own weight, and was allowed to air dry naturally. The carbon fiber was then dried in a hot air circulating oven at 140°C for 30 minutes to obtain a prepreg.

[0144] (3) Fabrication of fiber-reinforced molded articles A PET film (manufactured by Nippa Co., Ltd., product name: SFL, thickness: 50 μm) was placed on top of a SUS plate, and a laminate of two prepregs obtained above was placed on top of that. Another PET film and a SUS plate were then placed on top of that laminate in this order and the laminate was placed into a vacuum press. After heating and pressurizing in the vacuum press at 120°C and 0.5 MPa for 20 minutes, the temperature was further increased to 205°C (4°C / min) and held for 1 hour, after which it was removed from the vacuum press to produce a plate-shaped molded body.

[0145] [Comparative Example 5] 50 parts epoxy resin (manufactured by Daicel Corporation, product code: EHPE3150CE), 24.9 parts hardener (manufactured by DIC Corporation, product code: LF6161, 65% solids MEK solution), 0.1 parts 2-ethyl-4-methylimidazole (manufactured by Nacalai Tesque Corporation, 2E4MZ) as a curing catalyst, and 20 parts methyl ethyl ketone (MEK) were added and stirred at room temperature for 30 minutes to prepare a resin composition (varnish). Using the obtained varnish, molded articles were prepared using the same procedure as in Examples 1-10 and Comparative Examples 1-4.

[0146] [evaluation] 6. Specific gravity of fiber-reinforced molded articles The specific gravity of the obtained fiber-reinforced molded articles was measured by the water displacement method in accordance with JIS K7112.

[0147] 7. Mass-based content of hollow particles and reinforcing fibers in fiber-reinforced molded articles The prepregs obtained as intermediates in each example and comparative example were cut to 150 mm x 150 mm to be used as prepregs for measurement. The mass of the prepregs for measurement was measured, and the mass of carbon fibers in the prepregs for measurement was measured against the basis weight of the carbon fibers (200 g / m²). 2The following calculations were performed for the prepreg used for measurement: The resin content (mass%) in the prepreg was calculated from the mass of the prepreg and the mass of the reinforcing fibers in the prepreg using formula (1) above. The hollow particle content (mass%) on a mass basis was calculated from the resin content (mass%) and the hollow particle content (mass%) in the solid content of the resin composition impregnated into the reinforcing fibers using formula (2) above. In formula (1) above, the mass of the prepreg used for measurement was used as the "mass of the fiber-reinforced molded product". Furthermore, the reinforcing fiber content (mass %) was calculated as the ratio of the mass of reinforcing fibers in the prepreg to the mass of the prepreg used for measurement. The mass-based content of hollow particles and reinforcing fibers was calculated for six prepregs cut from the prepregs obtained in each example and comparative example. The mass-based content of hollow particles and reinforcing fibers in the fiber-reinforced molded articles obtained in each example and comparative example was the average value obtained from the six prepregs.

[0148] 8. Volume-based content of hollow particles in fiber-reinforced molded articles Using the content of hollow particles (mass%), matrix resin (mass%), and reinforcing fibers (mass%) in the measurement prepreg used in "7. Mass-based content of hollow particles and reinforcing fibers in fiber-reinforced molded articles" above, the volumes of hollow particles, matrix resin, and reinforcing fibers were calculated using equations (3), (4), and (5) above, and the volume-based content of hollow particles (volume%) was calculated using equation (6) above. Furthermore, the true density of the cured product obtained by heating and pressurizing the resin composition prepared in Comparative Example 5 under the same conditions as in Example 1 was measured, and the specific gravity (g / cm³) of the cured matrix resin used in formula (4) above was determined. 3 )

[0149] 9. Tensile modulus and tensile strength Test specimens were prepared by cutting the fiber-reinforced molded bodies obtained in each example and comparative example perpendicular to the winding direction of the cloth into measurement samples, and then cutting them into 130 mm x 15 mm strips so that the winding direction of the cloth was in the short axis direction. Tensile tests were performed on these test specimens under the following conditions in accordance with JIS K 7165:2008 to determine the tensile modulus and tensile strength. <Conditions for tensile testing> Test machine: Shimadzu Corporation, model AG-5kNI Load cell: 5t Chuck: Wedge type Tensile speed: 1 mm / min Chuck spacing: 60mm Temperature: 25℃ Humidity: 50%RH The tensile modulus and tensile strength were measured on five test pieces cut from the sample used for measurement. The tensile modulus and tensile strength of the fiber-reinforced molded articles obtained in each example and comparative example were calculated as the average of three measurements from the five test pieces, excluding the maximum and minimum values.

[0150] [Table 3]

[0151] [Consideration] As shown in Table 3 above, the fiber-reinforced molded articles of Comparative Examples 1 to 4, obtained using varnish containing 20% ​​by mass of the hollow particles of Comparative Manufacturing Examples 1 to 4, had a specific gravity of 1.36 to 1.45 g / cm³. 3 Comparative Example 5, which does not contain hollow particles, is a fiber-reinforced molded body (specific gravity: 1.48 g / cm³). 3 Compared to that, the specific gravity is 0.03~0.12 g / cm³. 3 It had only decreased by a small amount. In contrast, the fiber-reinforced molded article of Example 1, obtained using a varnish containing 10% by mass of the hollow particles of Production Example 1, has a lower hollow particle content compared to Comparative Examples 1-4, but its specific gravity is 1.30 g / cm³. 3 Therefore, its specific gravity is 0.18 g / cm³ compared to the fiber-reinforced molded article of Comparative Example 5, which does not contain hollow particles. 3 It had also decreased. The fiber-reinforced molded article of Example 2, obtained using a varnish containing 20% ​​by mass of the hollow particles of Production Example 1, has a specific gravity of 1.18 g / cm³. 3 Therefore, its specific gravity is 0.30 g / cm³ compared to the fiber-reinforced molded article of Comparative Example 5, which does not contain hollow particles. 3 It had also decreased. The fiber-reinforced molded article of Example 3, obtained using a varnish containing 25% by mass of the hollow particles of Production Example 1, has a specific gravity of 1.11 g / cm³. 3 Therefore, its specific gravity is 0.37 g / cm³ compared to the fiber-reinforced molded article of Comparative Example 5, which does not contain hollow particles. 3 It had also decreased. The fiber-reinforced molded articles of Examples 4-10, obtained using a varnish containing 20% ​​by mass of the hollow particles from Production Examples 2-8, had a specific gravity of 1.13-1.20 g / cm³. 3 Therefore, compared to the fiber-reinforced molded article of Comparative Example 5, which does not contain hollow particles, its specific gravity is 0.28 to 0.35 g / cm³. 3 It had also decreased. As shown in Table 2 above, the hollow particles in Comparative Manufacturing Examples 1-4 had a sedimentation rate of 5% or more by mass in the acetone during the immersion test, suggesting that they were easily crushed due to the rough cross-linking structure of the shells. Therefore, it is presumed that in the fiber-reinforced molded articles of Comparative Examples 1-4 containing the hollow particles of Comparative Manufacturing Examples 1-4, the hollow particles were crushed during the manufacturing process, and the voids were not maintained, resulting in insufficient weight reduction.

[0152] In contrast, the hollow particles in Production Examples 1-8, as shown in Table 2 above, contain a large amount of crosslinkable monomer units in their shells. Less than 5% by mass of hollow particles precipitated in acetone during the immersion test, and the crosslinking structure of the shells was dense, suggesting that they were resistant to crushing. Therefore, in the fiber-reinforced molded articles of Examples 1-10 containing the hollow particles of Production Examples 1-8, the hollow particles were resistant to crushing during the manufacturing process, and the voids were maintained, resulting in significant weight reduction. Furthermore, comparing Examples 1 to 3, the fiber-reinforced molded body was lighter as the content of hollow particles increased. Comparing Examples 2 and 4, it was found that the higher the porosity of the hollow particles, the lighter the fiber-reinforced molded body. Comparing Examples 2, 5, and 6, it was found that the higher the content of crosslinkable monomer units in the shell of the hollow particles, the lighter the fiber-reinforced molded article. This is presumed to be because a higher content of crosslinkable monomer units in the shell resulted in a denser crosslinking structure of the shell, improving the solvent resistance and strength of the hollow particles. Comparing Examples 2, 7-10, the specific gravity of the fiber-reinforced molded articles differed depending on the type of second polymerizable monomer used to produce the hollow particles. When alkyl acrylates having alkyl groups with 1 to 4 carbon atoms were used as the second polymerizable monomer, the specific gravity tended to decrease. Furthermore, the fiber-reinforced molded articles obtained in Examples 1 to 10 had tensile modulus and tensile strength within a range that is practical for use as fiber-reinforced molded articles. [Explanation of symbols]

[0153] 1 Aqueous medium 2 Low polarity material 3. Dispersion stabilizer 4. Monomer composition 4a Hydrocarbon solvents 4b Materials other than hydrocarbon solvents 4c Polymerizable monomers dispersed in an aqueous medium 5. Oil-soluble polymerization initiators 6 Shells 8 Hollow part 10 droplets 20 Hollow particles containing a hydrocarbon solvent in their hollow space (second precursor particles) 100 Hollow particles with a hollow space filled with gas

Claims

1. A method for producing a fiber-reinforced molded article containing a matrix resin, reinforcing fibers, and hollow particles, A step of preparing a mixture containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium, The steps include: preparing a suspension in which droplets of the monomer composition containing the first polymerizable monomer and the hydrocarbon solvent are dispersed in the aqueous medium by suspending the aforementioned mixture; The step includes subjecting the suspension to a polymerization reaction, The aforementioned mixture contains a crosslinkable monomer as the first polymerizable monomer, and the content of the crosslinkable monomer in 100 parts by mass of the first polymerizable monomer is 80 parts by mass or more. A manufacturing method for producing hollow particles, comprising the step of subjecting the suspension to a polymerization reaction, wherein when the polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more, a second polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C is added and the suspension is subjected to a further polymerization reaction, A step of preparing a resin composition containing hollow particles, matrix resin, and solvent obtained in the above step, A method for producing a fiber-reinforced molded article, comprising the step of impregnating reinforcing fibers with the resin composition.

2. The method for producing a fiber-reinforced molded article according to claim 1, wherein in the step of subjecting the suspension of the step for producing the hollow particles to a polymerization reaction, the amount of the second polymerizable monomer added is 3 to 15 parts by mass per 100 parts by mass of the first polymerizable monomer.

3. The method for producing a fiber-reinforced molded article according to claim 1 or 2, wherein in the step of preparing the mixed solution in the step of producing the hollow particles, the first polymerizable monomer includes a crosslinkable monomer with three or more functions as the crosslinkable monomer, and the content of the crosslinkable monomer with three or more functions in 100 parts by mass of the first polymerizable monomer is 5 to 50 parts by mass.

4. A method for producing a fiber-reinforced molded article according to any one of claims 1 to 3, wherein in the step of preparing the mixed liquid in the step of producing the hollow particles, the dispersion stabilizer is an inorganic dispersion stabilizer.

5. The method for producing a fiber-reinforced molded article according to claim 4, wherein the inorganic dispersion stabilizer is a poorly water-soluble metal salt.

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