hollow particles

Abstract

Provided are hollow particles with a reduced coefficient of thermal expansion. These hollow particles each comprise a shell containing a resin and a hollow section surrounded by the shell, have a porosity of 50% or more, and a volume average particle diameter of 1.0 μm or greater. The shell contains, as the resin, a polymer that contains 70-100 parts by mass of crosslinkable monomeric units per 100 parts by mass of the total monomeric units. The coefficient of thermal expansion of the hollow particles is 10×10-5 / °C or less at 80-200°C. When the hollow particles are subjected to an immersion test, in which 0.1 mg of the hollow particles are added to 4 mL of acetone, such mixture is shook for 10 minutes under at a shaking speed of 100 rpm, and the mixture is left to stand for 48 hours, the amount of the hollow particles that settle in the acetone is less than 10 mass%.

Description

[Technical Field]

[0001] The present disclosure relates to hollow particles. [Background technology]

[0002] Hollow particles (hollow resin particles) are particles with a cavity inside them, and compared with solid particles whose interiors are substantially filled with resin, they scatter light well and have low light transmittance, and therefore are widely used as organic pigments or hiding agents with excellent optical properties such as opacity and whiteness in applications such as water-based paints and paper coating compositions. In recent years, they have also been used as weight-reducing agents and heat-insulating agents for resins and paints used in various fields such as automobiles, electricity, electronics, and construction.

[0003] In electronic material applications, for example, in electronic circuit boards, hollow particles are sometimes contained in insulating resin layers to suppress the occurrence of crosstalk and an increase in transmission loss. Crosstalk and transmission loss in electronic circuit boards can be suppressed by reducing the relative dielectric constant and dielectric loss tangent of the insulating resin layer. Since hollow particles have a hollow interior, attempts have been made to lower the dielectric constant and dielectric loss tangent of insulating resin layers by adding hollow particles.

[0004] Furthermore, insulating resin layers used in electronic materials are also required to have adhesion and dimensional stability when used as laminated or bonded materials. In electronic materials, an insulating resin layer may be laminated on a metal plate, such as a copper-clad laminate. To improve the adhesion and dimensional stability of the insulating resin layer to the metal plate, it is effective to make the coefficient of thermal expansion (CTE) of the insulating resin layer closer to that of the metal plate. Conventionally, methods for reducing the CTE of the insulating resin layer have included adding inorganic fillers such as silica to the insulating resin layer. However, increasing the content of inorganic filler in the insulating resin layer leads to a deterioration in physical properties such as increased susceptibility to cracking, and also poses the problem of being unable to sufficiently reduce the relative dielectric constant and dielectric loss tangent. On the other hand, hollow particles typically have a lower CTE than insulating resins, so it is expected that incorporating hollow particles into an insulating resin layer will not only lower the dielectric constant and dielectric loss tangent, but also reduce the CTE.

[0005] Patent Documents 1 and 2 describe a technique of adding hollow particles to a resin composition used in electronic materials to achieve a low dielectric constant, a low dielectric loss tangent, and the like. Patent Document 1 discloses hollow particles obtained by suspending a solution obtained by mixing divinylbenzene, an initiator, and hexadecane in an aqueous solution in which polyvinyl alcohol has been dissolved, and then performing suspension polymerization. Patent Document 1 also discloses that a copper-clad laminate using a resin composition containing hollow particles has a reduced coefficient of thermal expansion. Patent Document 2 discloses hollow particles obtained by adding an oil component, which is a mixture of an epoxy skeleton component and an organic solvent, to an aqueous solution containing a curing agent and a water-soluble emulsifier, emulsifying the oil, and then subjecting the resulting mixture to interfacial polymerization. Patent Document 2 also discloses that when the ratio of the average linear expansion coefficient α2 at a temperature 10 to 50°C higher than the glass transition temperature to the average linear expansion coefficient α1 at a temperature 10 to 50°C lower than the glass transition temperature is 3 or less, the occurrence of wrinkles and warping near the glass transition temperature is suppressed. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2004 / 067638 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-48615 Summary of the Invention [Problem to be solved by the invention]

[0007] However, hollow particles used in electronic materials and the like are required to have a further reduced thermal expansion coefficient. An object of the present disclosure is to provide hollow particles with a reduced coefficient of thermal expansion. [Means for solving the problem]

[0008] The present inventors focused on the permeability of the shell of hollow particles to polar solvents and found that hollow particles with a shell that is less permeable to acetone by adjusting the composition and formation method of the shell have a low thermal expansion coefficient.

[0009] The present disclosure provides a hollow particle having a shell containing a resin and a hollow portion surrounded by the shell, The porosity is 50% or more, The volume average particle size is 1.0 μm or more, the shell contains, as the resin, a polymer containing 70 to 100 parts by mass of crosslinkable monomer units in 100 parts by mass of all monomer units, The thermal expansion coefficient at 80 to 200°C is 10.0 x 10 -5 / °C or less, The present invention provides hollow particles, which, in an immersion test in which 0.1 mg of hollow particles are added to 4 mL of acetone in an environment of 25°C, shaken at a shaking speed of 100 rpm for 10 minutes, and then allowed to stand for 48 hours, precipitate in acetone at less than 10 mass%.

[0010] The hollow particles of the present disclosure have a thermal expansion coefficient of 6.0 × 10 at 25 to 80 ° C. -5 / °C or less.

[0011] The hollow particles of the present disclosure can have a relative dielectric constant of 1.6 or less at a frequency of 1 GHz.

[0012] The hollow particles of the present disclosure can have a dielectric loss tangent of 0.010 or less at a frequency of 1 GHz.

[0013] In the hollow particle according to the present disclosure, the polymer contained in the shell may contain, as the crosslinkable monomer unit, a bifunctional crosslinkable monomer unit derived from a bifunctional crosslinkable monomer, and the content of the bifunctional crosslinkable monomer unit may be 70 to 100 parts by mass relative to 100 parts by mass of all monomer units of the polymer.

[0014] In the hollow particle according to the present disclosure, the polymer contained in the shell may contain, as the crosslinkable monomer unit, a tri- or higher functional crosslinkable monomer unit derived from a tri- or higher functional crosslinkable monomer, and the content of the tri- or higher functional crosslinkable monomer unit may be 5 to 50 parts by mass relative to 100 parts by mass of all monomer units of the polymer.

[0015] In the hollow particle of the present disclosure, the polymer contained in the shell further contains a hydrophilic non-crosslinkable monomer unit derived from a hydrophilic non-crosslinkable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C, The content of the hydrophilic non-crosslinkable monomer units may be 2 to 15 parts by mass, and the content of the crosslinkable monomer units may be 70 to 98 parts by mass, based on 100 parts by mass of all monomer units of the polymer.

[0016] In the hollow particles of the present disclosure, the crosslinkable monomer unit may contain a crosslinkable monomer derived from a (meth)acrylic crosslinkable monomer having a (meth)acryloyl group as a polymerizable functional group. [Effects of the Invention]

[0017] According to the present disclosure as described above, hollow particles having a reduced thermal expansion coefficient can be provided. [Brief explanation of the drawings]

[0018] [Figure 1] 1A to 1C are diagrams illustrating an example of a method for producing hollow particles according to the present disclosure. [Figure 2] FIG. 2 is a schematic diagram illustrating an embodiment of a suspension in a suspension step. DETAILED DESCRIPTION OF THE INVENTION

[0019] In the present disclosure, the use of "to" in a numerical range means that the numerical values ​​before and after it are included as the lower limit and upper limit. In the present disclosure, (meth)acrylate refers to each of acrylate and methacrylate, (meth)acrylic refers to each of acrylic and methacrylic, and (meth)acryloyl refers to each of acryloyl and methacryloyl. In the present disclosure, a polymerizable monomer is a compound having a functional group capable of addition polymerization (sometimes simply referred to as a polymerizable functional group in the present disclosure). In the present disclosure, a compound having an ethylenically unsaturated bond as the functional group capable of addition polymerization is generally used as the polymerizable monomer. Polymerizable monomers are classified into non-crosslinkable monomers and crosslinkable monomers. Non-crosslinkable monomers have only one polymerizable functional group, while crosslinkable monomers have two or more polymerizable functional groups and form crosslinks in the resin through a polymerization reaction. In addition, in the present disclosure, a polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C is referred to as a hydrophilic monomer, and a polymerizable monomer having a solubility of less than 0.3 g / L in distilled water at 20°C is referred to as a non-hydrophilic monomer.

[0020] 1.Hollow particles The hollow particles of the present disclosure are hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, The porosity is 50% or more, The volume average particle size is 1.0 μm or more, the shell contains, as the resin, a polymer containing 70 to 100 parts by mass of crosslinkable monomer units in 100 parts by mass of all monomer units, The thermal expansion coefficient at 80 to 200°C is 10.0 x 10 -5 / °C or less, In a hollow particle immersion test, 0.1 mg of hollow particles are added to 4 mL of acetone in a 25°C environment, shaken at 100 rpm for 10 minutes, and then allowed to stand for 48 hours. In this test, the amount of hollow particles that precipitates in the acetone is less than 10% by mass.

[0021] The hollow particles of the present disclosure are particles having a shell (outer shell) containing a resin and a hollow portion surrounded by the shell. In the present disclosure, the hollow portion is a hollow space that is clearly distinguishable from the shell of the hollow particle formed from a resin material. The shell of the hollow particle may have a porous structure, but in that case, the hollow portion has a size that allows it to be clearly distinguished from the numerous minute spaces uniformly dispersed within the porous structure. The hollow portion of the hollow particles can be confirmed by, for example, SEM observation of the cross section of the particles or TEM observation of the particles as they are. From the viewpoint of achieving a low dielectric constant and a low dielectric loss tangent, the hollow portion of the hollow particle according to the present disclosure is preferably filled with a gas such as air or nitrogen or is in a reduced pressure state close to vacuum.

[0022] Because hollow particles have a hollow portion inside the particle, various compositions and molded articles containing the hollow particles can be provided with performance such as weight reduction, heat insulation, and low dielectric constant. Because the hollow particles of the present disclosure have a reduced coefficient of thermal expansion (CTE), various compositions and molded articles containing the hollow particles of the present disclosure can have a further reduced CTE. The hollow particles of the present disclosure contain polymers in the shells containing 70 to 100 parts by mass of crosslinkable monomer units per 100 parts by mass of total monomer units, which is presumably why the crosslinkable monomer units account for a high proportion in the shell and why a dense covalent bond network is spread throughout the shell. Furthermore, the hollow particles of the present disclosure have a dense shell structure that is resistant to acetone penetration, which is presumably why the crosslinked structure in the shell is even denser. The hollow particles of the present disclosure have a low CTE, which is presumably because the porosity is 50% or more, the proportion of hollow portions in the particles is sufficiently large, i.e., the proportion of shells in the particles is sufficiently small, and the shell has a dense structure, which results in excellent heat resistance and resistance to thermal dimensional changes. The hollow particles of the present disclosure are particles with reduced dielectric constant and dielectric loss tangent. The hollow particles of the present disclosure have a porosity of 50% or more, a sufficiently large proportion of hollow spaces within the particle, and a dense shell structure, which significantly restricts molecular motion and makes dielectric relaxation less likely to occur. Therefore, it is presumed that they exhibit excellent dielectric properties. Furthermore, the hollow particles of the present disclosure are less likely to deform or collapse due to the excellent shell strength, and an increase in the dielectric constant caused by deformation or collapse of the hollow particles is suppressed, which is thought to enable them to maintain a low dielectric constant.

[0023] The hollow particles of the present disclosure are particles with a reduced coefficient of thermal expansion (CTE). The hollow particles of the present disclosure have a thermal expansion coefficient of 10.0 × 10 at 80 to 200 °C. -5 / °C or less, and in accordance with a preferred embodiment, -5 / ℃ or less, and furthermore, -5 The lower limit of the thermal expansion coefficient of the hollow particles of the present disclosure at 80 to 200°C is not particularly limited, but is usually 5.0 × 10 -5 / ℃ or more. In addition, the hollow particles of the present disclosure have a thermal expansion coefficient of 6.0 × 10 at 25 to 80 ° C. -5 / °C or less, and in a more preferred embodiment, 5.9 × 10 -5 / ℃ or less, and further 5.8×10 -5 The lower limit of the thermal expansion coefficient of the hollow particles of the present disclosure at 25 to 80°C is not particularly limited, but is usually 1.0 × 10 -5 / ℃ or more. In the present disclosure, the thermal expansion coefficient α of the hollow particles p is the thermal expansion coefficient α of a molded plate of a resin composition consisting of a matrix resin and hollow particles. c , the thermal expansion coefficient of the matrix resin alone α r、 The volume ratio SG of the matrix resin in the molded plate r , and the volume ratio W of hollow particles in the molded plate p It can be calculated from the following formula (E). α p =(αc- SG r ×α r ) / W p Formula (E) The thermal expansion coefficient is measured within a predetermined temperature range in accordance with JIS K7197:2012. The matrix resin may be, for example, an epoxy resin, and may contain additives for curing the resin, such as a curing agent and a curing catalyst.

[0024] Furthermore, the hollow particles of the present disclosure are particles with reduced relative dielectric constant and dielectric loss tangent. The hollow particles of the present disclosure may have a dielectric constant at a frequency of 1 GHz of 1.6 or less, and in a more preferred embodiment, 1.5 or less. The lower limit of the dielectric constant of the hollow particles of the present disclosure is not particularly limited, but is usually 1.0 or more. Furthermore, the hollow particles of the present disclosure can have a dielectric loss tangent at a frequency of 1 GHz of 0.010 or less, and in a more preferred embodiment, 0.009 or less. The lower limit of the dielectric loss tangent of the hollow particles of the present disclosure is not particularly limited, but is usually 0.001 or more. In the present disclosure, the relative permittivity and dielectric loss tangent of hollow particles are measured using a perturbation type measuring device at a measurement frequency of 1 GHz.

[0025] The hollow particles of the present disclosure have a porosity of 50% or more, preferably 60% or more. When the porosity is equal to or greater than the above-mentioned lower limit, the hollow particles have low CTE, relative dielectric constant, and dielectric loss tangent, and are also lightweight, heat resistance, and heat insulation properties. The upper limit of the porosity of the hollow particles of the present disclosure is not particularly limited, but is preferably 90% or less, more preferably 85% or less, and even more preferably 80% or less, in order to prevent a decrease in the strength of the hollow particles and make them less likely to be crushed.

[0026] The porosity of the hollow particles of the present 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, 330cm into a measuring flask 3 The volumetric flask is filled with hollow particles, and the mass of the filled hollow particles is accurately weighed. Next, the volumetric flask filled with hollow particles is accurately filled with isopropanol up to the marked line, taking care not to introduce air bubbles. The mass of isopropanol added to the volumetric flask is accurately weighed, and the apparent density D1 (g / cm) of the hollow particles is calculated based on the following formula (I): 3 ) is calculated. Formula (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 when the hollow portion is considered to be a part of the hollow particle.

[0027] The true density D0 of hollow particles is measured as follows: After crushing the hollow particles, 3 Approximately 10 g of crushed pieces of hollow particles are filled into the measuring flask, and the mass of the crushed pieces is accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol is added to the measuring flask, and the mass of the isopropanol is accurately weighed. The true density D0 (g / cm) of the hollow particles is calculated based on the following formula (II): 3 ) is calculated. Formula (II) True density D0 = [mass of crushed 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 above measurement method, the hollow portion is not considered to be part of the hollow particle when calculating the true density D0.

[0028] The porosity (%) of the hollow particles is calculated from the apparent density D1 and true density D0 of the hollow particles by the following formula (III). Formula (III) Porosity (%) = 100 - (apparent density D1 / true density D0) x 100

[0029] The hollow particles of the present disclosure have a volume average particle size of 1.0 μm or more. When the volume average particle size of the hollow particles is equal to or greater than the lower limit, the tendency for the hollow particles to aggregate with each other is reduced, thereby enabling the hollow particles to exhibit excellent dispersibility. Even when the hollow particles are contained in an insulating resin layer of an electronic circuit board, wiring defects are unlikely to occur, and the hollow particles are therefore suitable for use as a material for the electronic circuit board. The lower limit of the volume average particle diameter of the hollow particles of the present disclosure is preferably 3.0 μm or more, more preferably 5.0 μm or more, and even more preferably 7.0 μm or more. The upper limit of the volume average particle diameter of the hollow particles of the present disclosure is not particularly limited, but is preferably 30.0 μm or less, more preferably 20.0 μm or less, and even more preferably 15.0 μm or less, from the viewpoints of improving the strength of the hollow particles, facilitating uniformity of the shell thickness, and facilitating use as a material for electronic circuit boards. In order to set the volume average particle size of the hollow particles within the above-mentioned preferred range, for example, it is preferable to use the above-mentioned preferred dispersion stabilizer and further use the above-mentioned preferred hydrophobic solvent in the mixed liquid preparation step.

[0030] The particle size distribution of the hollow particles (volume average particle size (Dv) / number average particle size (Dn)) may be, for example, 1.1 or more and 2.5 or less. When the particle size distribution is 2.5 or less, particles with little variation in compressive strength characteristics and heat resistance among particles can be obtained. Furthermore, when producing a sheet-like molded product, for example, a product with a uniform thickness can be produced. The volume average particle size (Dv) and number average particle size (Dn) of the hollow particles can be determined by, for example, measuring the particle size of the hollow particles using a laser diffraction particle size distribution analyzer, calculating the number average and volume average, and using the resulting values ​​as the number average particle size (Dn) and volume average particle size (Dv) of the particles. The particle size distribution is calculated by dividing the volume average particle size by the number average particle size.

[0031] The shell thickness of the hollow particles of the present disclosure has a lower limit of preferably 0.1 μm or more, more preferably 0.3 μm or more, and even more preferably 0.5 μm or more, from the viewpoint of improving the strength of the hollow particles, and an upper limit of preferably 6 μm or less, more preferably 4 μm or less, even more preferably 2 μm or less, and even more preferably 1 μm or less, from the viewpoint of increasing the porosity. Having a shell thickness of the hollow particles equal to or greater than the above-mentioned lower limit improves the strength of the shell. On the other hand, the hollow particles of the present disclosure have a dense shell structure that is resistant to acetone penetration, so even if the shell thickness is thin and equal to or less than the above-mentioned upper limit, the hollow particles have excellent strength and are resistant to crushing. The shell thickness of the hollow particles can be calculated by calculating the inner diameter r of the hollow particles using the volume average particle diameter R and porosity of the hollow particles according to the following formula (1), and then using the inner diameter r and the volume average particle diameter R to calculate the shell thickness of the hollow particles according to the following formula (2). The porosity in the following formula (1) is a numerical value expressed as a percentage. 4 / 3π×(R / 2) 3 ×Porosity=4 / 3π×(r / 2) 3 Formula (1) Shell thickness = (Rr) / 2 Equation (2) The difference between the shell thickness calculated in this manner and the average thickness measured at 20 points on the shell is usually within ±10% of these average values, so the shell thickness calculated as above can be considered to be the shell thickness of the hollow particles. The thickness at each point of the shell of a hollow particle used to calculate the average thickness at 20 points on the shell can be measured, for example, by breaking the hollow particle and observing the shell fragments obtained with an SEM.

[0032] The shape of the hollow particles of the present disclosure is not particularly limited as long as a hollow portion is formed inside, and examples thereof include spherical, oval spherical, irregular, etc. Among these, spherical is preferred from the viewpoint of ease of production. The hollow particles may have one or more hollow portions. The shell of the hollow particles, and when the hollow particles have two or more hollow portions, the partition walls separating adjacent hollow portions, may be porous. The interior of the particle preferably has only one hollow portion in order to maintain a good balance between the high porosity of the hollow particles and the mechanical strength of the hollow particles. The hollow particles may have an average circularity of 0.950 to 0.995. An example of the shape of a hollow particle is a bag made of a thin membrane and inflated with gas, and its cross section is shown in hollow particle 100 in Fig. 1 (5) described below. In this example, a thin membrane is provided on the outside, and the inside is filled with gas. The particle shape can be confirmed by, for example, SEM or TEM. Furthermore, the internal shape of a particle can be confirmed by SEM or TEM after cutting the particle into cross sections by a known method.

[0033] The hollow particles of the present disclosure contain, as the resin in the shell, a polymer containing 70 to 100 parts by mass of crosslinkable monomer units per 100 parts by mass of all monomer units. The polymer forms the skeleton of the shell of the hollow particles, and by containing the crosslinkable monomer units in the above ratio, the shell of the hollow particles of the present disclosure has a dense network of covalent bonds. In the above polymer, the content of the crosslinkable monomer units in 100 parts by mass of all monomer units is preferably 75 parts by mass or more, more preferably 85 parts by mass or more, from the viewpoint of reducing the CTE, relative dielectric constant, and dielectric dissipation factor of the hollow particles and improving the strength of the hollow particles, while, when a hydrophilic non-crosslinkable monomer is added as the second polymerizable monomer, the content is preferably 98 parts by mass or less, more preferably 97 parts by mass or less, from the viewpoint of adding a sufficient amount of the second polymerizable monomer. In the present disclosure, a crosslinkable monomer unit is a monomer unit derived from a crosslinkable monomer, and when the content of the crosslinkable monomer unit in the above polymer 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 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, which will be 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 are usually derived from the first polymerizable monomer and the second polymerizable monomer, which will be described later. Specific details of the crosslinkable monomer and non-crosslinkable monomer used in the synthesis of the polymer are as described in the method for producing hollow particles of the present disclosure, which will be described later.

[0034] The polymer contains, as a crosslinkable monomer unit, at least one selected from a bifunctional crosslinkable monomer unit derived from a bifunctional crosslinkable monomer and a trifunctional or higher crosslinkable monomer unit derived from a trifunctional or higher crosslinkable monomer. Among these, from the viewpoint of lowering the CTE and improving the strength of the hollow particles, it is preferable to contain at least a bifunctional crosslinkable monomer unit, and from the viewpoint of further improving the strength of the hollow particles, it is more preferable to contain a combination of a bifunctional crosslinkable monomer unit and a trifunctional or higher crosslinkable monomer unit. In the present disclosure, a crosslinkable monomer unit derived from a difunctional crosslinkable monomer may be referred to as a "difunctional crosslinkable monomer unit," and a crosslinkable monomer unit derived from a trifunctional or higher crosslinkable monomer may be referred to as a "trifunctional or higher crosslinkable monomer unit."

[0035] When the polymer contains a bifunctional crosslinkable monomer unit, the content of the bifunctional crosslinkable monomer unit in 100 parts by mass of all monomer units of the polymer is not particularly limited, but the lower limit is preferably 70 parts by mass or more, more preferably 75 parts by mass or more, from the viewpoint of lowering the CTE and improving the strength of the hollow particles. On the other hand, the upper limit may be 100 parts by mass or less, but 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, from the viewpoint of sufficiently containing trifunctional or higher functional crosslinkable monomer units or hydrophilic non-crosslinkable monomer units described later.

[0036] When the polymer contains trifunctional or higher crosslinkable monomer units, the content of the trifunctional or higher crosslinkable monomer units in 100 parts by mass of all monomer units of the polymer is not particularly limited, but from the viewpoint of lowering the CTE and improving the strength of the hollow particles, 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. On the other hand, the upper limit is preferably 50 parts by mass or less, more preferably 40 parts by mass or less, and from the viewpoint of sufficiently containing bifunctional or higher crosslinkable monomer units or hydrophilic non-crosslinkable monomer units described later, the upper limit is more preferably 30 parts by mass or less, and even more preferably 25 parts by mass or less.

[0037] The crosslinkable monomer units contained in the polymer may also contain crosslinkable monomer units derived from a (meth)acrylic crosslinkable monomer having a (meth)acryloyl group as a polymerizable functional group, which allows the hollow particles of the present disclosure to have excellent strength and heat resistance, and further allows the resin composition containing the hollow particles of the present disclosure to have improved adhesion. 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 the crosslinkable monomer units, and the crosslinkable monomer units may be composed of (meth)acrylic crosslinkable monomer units. Specific details of the (meth)acrylic crosslinkable monomer are as described later in the method for producing hollow particles of the present disclosure.

[0038] The polymer preferably further contains a non-crosslinkable monomer unit, more preferably a hydrophilic non-crosslinkable monomer unit having a solubility of 0.3 g / L or more in distilled water at 20° C., and particularly preferably a hydrophilic non-crosslinkable monomer unit derived from a second polymerizable monomer described below. When the polymer contains a combination of a crosslinkable monomer unit and a non-crosslinkable monomer unit, the mechanical properties of the shell of the hollow particle are improved. In particular, when the polymer contains a hydrophilic non-crosslinkable monomer unit as a non-crosslinkable monomer unit, the shell tends to have a dense structure, which tends to reduce the CTE, dielectric constant, and dielectric loss tangent of the hollow particle and also tends to improve the strength of the hollow particle. In the above polymer, the content of the non-crosslinkable monomer units in 100 parts by mass of all monomer units is 0 to 30 parts by mass, and from the viewpoint of improving the strength of the hollow particles, it is preferably 2 to 25 parts by mass, more preferably 4 to 15 parts by mass. In the above polymer, the content of the hydrophilic non-crosslinkable monomer units in 100 parts by mass of all monomer units is preferably 2 to 15 parts by mass, more preferably 3 to 13 parts by mass, and even more preferably 4 to 10 parts by mass, from the viewpoint of improving the strength of the hollow particles and reducing the CTE, relative dielectric constant, and dielectric loss tangent of the hollow particles.

[0039] In the hollow particles of the present disclosure, the content of the polymer is preferably 90% by mass or more, more preferably 95% by mass or more, based on 100% by mass of the total solid content of the shell. By making the content of the polymer equal to or greater than the lower limit, the strength of the hollow particles can be improved.

[0040] The shell of the hollow particle of the present disclosure may further contain a polar component, such as an organic acid or a metal salt thereof, or a polar resin. Specific details of the polar component will be described later in the method for producing hollow particles of the present disclosure. The presence of a polar component in the shell of the hollow particle and the content thereof can be confirmed by, for example, pyrolysis gas chromatography.

[0041] When the shell of the hollow particles of the present disclosure contains an organic acid or a metal salt thereof as a polar component, the total content of the organic acid or the metal salt thereof in the shell is preferably 0.0001 to 0.1 mass%, more preferably 0.001 to 0.01 mass%. On the other hand, when the shell of the hollow particles of the present disclosure contains a polar resin as a polar component, the content of the polar resin in the shell is preferably 0.1 to 10.0 mass %, more preferably 0.3 to 8.0 mass %. By including an organic acid or a metal salt thereof or a polar resin in the shell, it is possible to prevent the strength of the shell from decreasing and further prevent the hollow particles from collapsing.

[0042] In an immersion test of the hollow particles of the present disclosure, 0.1 mg of hollow particles are added to 4 mL of acetone at 25°C, shaken at 100 rpm for 10 minutes, and then allowed to stand for 48 hours. The percentage of hollow particles that precipitate in acetone in the immersion test is an indicator of the density of the hollow particle shell. It is estimated that the lower the percentage of hollow particles that precipitate in acetone in the immersion test, the denser the hollow particle shell. It is also estimated that the denser the hollow particle shell, the lower the CTE, dielectric constant, and dielectric loss tangent of the hollow particle. In the hollow particles of the present disclosure, the proportion of hollow particles that precipitate in acetone in the above immersion test may be less than 10% by mass, and more preferably less than 5% by mass. In order to reduce the percentage of hollow particles that precipitate in acetone in the above-mentioned immersion test to less than 10 mass % for hollow particles with a porosity of 50% or more, for example, hollow particles can be produced by a method similar to that described below in the present disclosure, in which, in a step of subjecting a suspension to a polymerization reaction, when the polymerization conversion rate of a first polymerizable monomer containing a specific amount or more of a crosslinkable monomer reaches 93 mass % or more, a second polymerizable monomer that is a hydrophilic monomer is added and the resulting mixture is subjected to a further polymerization reaction, and the content of crosslinkable monomer units in the polymer that forms the shell is 70 mass % or more.

[0043] Furthermore, in the hollow particles of the present disclosure, it is preferable that, when observed under an SEM, 5 or less of 100 hollow particles have interconnected pores or shell defects. Generally, hollow particles are classified into those in which the shell does not have any interconnecting holes that connect the hollow portion to the external space of the particle, and those in which the shell has one or more interconnecting holes through which the hollow portion communicates with the outside of the particle. Although the diameter of the interconnecting holes varies depending on the size of the hollow particle, it is usually about 10 to 500 nm. While the interconnecting holes can impart beneficial functions to hollow particles, they can also reduce the strength of the hollow particles and make them more susceptible to crushing because they are missing portions of the shell. Furthermore, hollow particles may have shell defects in the form of cracks that are extremely large compared to the particle size. Although it depends on the size of the hollow particle, cracks with a length of 1 μm or more generally significantly reduce the strength of the hollow particle and are therefore recognized as shell defects. In the above-mentioned hollow particle immersion test, if less than 10% by mass of hollow particles precipitate in acetone, it can be considered that 5 or fewer hollow particles have interconnected pores or shell defects out of 100 hollow particles. Even if the shell has no interconnected pores or shell defects, the above-mentioned hollow particle immersion test may result in 10% or more of the hollow particles precipitating. Therefore, in the above-mentioned hollow particle immersion test, if less than 10% by mass of hollow particles precipitate, this is considered to indicate that the shell has very few interconnected pores and shell defects and has a dense crosslinked structure.

[0044] 2. Manufacturing method of hollow particles The hollow particles of the present disclosure may be, for example: preparing a mixed liquid containing a first polymerizable monomer including a crosslinkable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium; a step of suspending the mixed liquid to prepare a suspension in which droplets of a monomer composition containing the first polymerizable monomer and the hydrocarbon solvent are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction, In the step of subjecting the suspension to a polymerization reaction, when the polymerization conversion rate of the first polymerizable monomer reaches 93 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 further subjected to a polymerization reaction.

[0045] The method for producing the hollow particles follows a basic technique in which a mixed liquid containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium is suspended, causing phase separation between the first polymerizable monomer and the hydrocarbon solvent, thereby preparing a suspension in which droplets having a distribution structure in which the first polymerizable monomer is unevenly distributed on the surface side and the hydrocarbon solvent is unevenly distributed in the center are dispersed in the aqueous medium, and this suspension is subjected to a polymerization reaction to harden the surfaces of the droplets, thereby forming hollow particles having hollow portions filled with the hydrocarbon solvent. In this basic technique, when the polymerization conversion rate of the first polymerizable monomer, including the crosslinkable monomer, reaches 93% by mass or more in the step of subjecting the suspension to a polymerization reaction, a second polymerizable monomer, which is a hydrophilic monomer having a solubility in distilled water at 20°C equal to or greater than the specific value, is added and the resulting mixture is further subjected to a polymerization reaction. This allows the production of hollow particles in which less than 10% by mass of the hollow particles precipitate in acetone in the above-mentioned hollow particle immersion test. When a crosslinkable monomer is used as the polymerizable monomer for forming the shell of the hollow particles, unreacted polymerizable functional groups are likely to remain in the shell. The more unreacted polymerizable functional groups remain, the coarser the crosslinked structure of the shell becomes. Therefore, it is believed that the remaining unreacted polymerizable functional groups in hollow particles obtained by conventional manufacturing methods cause 10% or more by mass of the hollow particles to precipitate in acetone in the above-mentioned hollow particle immersion test. In the above-mentioned method for producing hollow particles, a suspension in which droplets of a monomer composition containing a first polymerizable monomer containing a large amount of a crosslinkable monomer are dispersed in an aqueous medium is subjected to a polymerization reaction, and the first polymerization reaction is carried out until the polymerization conversion rate of the first polymerizable monomer reaches 93 mass% or more. Thereafter, a second polymerizable monomer, which is a hydrophilic monomer, is added and a second polymerization reaction is further carried out, which is thought to improve the reaction rate of the entire polymerizable monomers including the first polymerizable monomer and the second polymerizable monomer. In the present disclosure, particles obtained by the first polymerization reaction and having a shell containing a polymer of a first polymerizable monomer and a hollow portion filled with a hydrocarbon-based solvent may be referred to as first precursor particles, and a composition containing the first precursor particles may be referred to as a first precursor composition. Furthermore, particles obtained by the second polymerization reaction and having a shell containing a polymer of a first polymerizable monomer and a second polymerizable monomer and a hollow portion filled with a hydrocarbon-based solvent may be considered as an intermediate of hollow particles whose hollow portions are filled with gas and may be referred to as second precursor particles, and a composition containing the second precursor particles may be referred to as a second precursor composition. In the above-described method for producing hollow particles, the second polymerizable monomer has a solubility in distilled water at 20°C equal to or greater than the specific value, and thus is easily incorporated into the shell of the first precursor particle when added to the first precursor composition. The hydrophilic second polymerizable monomer has affinity for both the first polymerizable monomer and the aqueous medium. Therefore, when added to the first precursor composition, it is incorporated into the shell formed by the first polymerizable monomer, promoting the thermal motion of the shell. During the second polymerization reaction, the second polymerizable monomer is incorporated into the shell formed by the first polymerizable monomer, and the thermal motion of the shell is promoted while the polymerization reaction proceeds. This results in a high reaction rate, and the polymerization reaction of the second polymerizable monomer incorporated into the shell and the polymerizable functional groups of the remaining unreacted first polymerizable monomer proceeds sufficiently, resulting in a dense crosslinked structure and the formation of a shell that is resistant to acetone penetration.

[0046] The method for producing hollow particles includes a step of preparing a mixed solution, a step of preparing a suspension, and a step of subjecting the suspension to a polymerization reaction, and may further include other steps. Furthermore, as far as technically possible, two or more of the above steps and other additional steps may be performed simultaneously as a single step, or the order may be reversed. For example, the preparation of the mixed solution and the suspension may be performed simultaneously in a single process, such as by adding the materials for preparing the mixed solution and suspending them at the same time.

[0047] A preferred example of the method for producing the hollow particles includes the following steps. (1) Mixed liquid preparation process a step of preparing a mixed liquid containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium; (2) Suspension process a step of suspending the mixed liquid 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 step of performing a first polymerization reaction in which the suspension is subjected to a polymerization reaction until a polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more, thereby preparing a first precursor composition containing first precursor particles having a shell containing a polymer of the first polymerizable monomer and a hollow portion filled with a hydrocarbon-based solvent. (3-2) Second polymerization step a step of 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 second precursor particles having a shell containing a polymer of the first polymerizable monomer and the second polymerizable monomer and a hollow portion filled with a hydrocarbon-based solvent; (4) Solid-liquid separation process A step of obtaining second precursor particles having hollow portions containing a hydrocarbon solvent by solid-liquid separation of the second precursor composition; and (5) Solvent removal process a step of removing the hydrocarbon solvent contained in the second precursor particles obtained by the solid-liquid separation step to obtain hollow particles;

[0048] FIG. 1 is a schematic diagram showing an example of the above-mentioned manufacturing method. (1) to (5) in FIG. 1 correspond to the above-mentioned steps (1) to (5). The white arrows between the figures indicate the order of the steps. Note that FIG. 1 is merely a schematic diagram for explanation, and the above-mentioned manufacturing method is not limited to those shown in the figure. Furthermore, the structure, dimensions, and shape of the materials used in the manufacturing method of the present disclosure are not limited to the structure, dimensions, and shape of the various materials in these figures. 1(1) is a cross-sectional schematic diagram showing one embodiment of a mixed solution in the mixed solution preparation step. As shown in this figure, the mixed solution contains an aqueous medium 1 and a low-polarity material 2 dispersed in the aqueous medium 1. Here, the low-polarity material 2 refers to a material that has low polarity and is difficult to mix with the aqueous medium 1. In the present disclosure, the low-polarity material 2 contains a first polymerizable monomer and a hydrocarbon solvent. FIG. 1 (2) is a cross-sectional schematic diagram showing one embodiment of a suspension in a suspending step. The suspension includes an aqueous medium 1 and droplets 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 are phase-separated into a hydrocarbon solvent 4a and a material other than the hydrocarbon solvent, including the first polymerizable monomer, 4b, with the hydrocarbon solvent 4a unevenly distributed in the center and the material other than the hydrocarbon solvent 4b unevenly distributed on the surface, and a dispersion stabilizer (not shown) attached to the surface. 1(3) is a cross-sectional schematic diagram showing one embodiment of a composition (second precursor composition) containing hollow particles (second precursor particles) encapsulating a hydrocarbon solvent in their hollow spaces, obtained by a polymerization step. The composition contains an aqueous medium 1 and hollow particles (second precursor particles) 20 encapsulating a hydrocarbon solvent 4a in their hollow spaces, dispersed in the aqueous medium 1. A shell 6 forming the outer surface of the second precursor particle 20 is formed by polymerization of a first polymerizable monomer in droplets 10 of the monomer composition and polymerization of a second polymerizable monomer that is added later. Fig. 1(4) is a cross-sectional schematic diagram showing one embodiment of hollow particles (second precursor particles) containing a hydrocarbon solvent in the hollow portion after the solid-liquid separation step, which shows a state in which the aqueous medium 1 has been removed from the state shown in Fig. 1(3). Fig. 1 (5) is a cross-sectional schematic diagram showing one embodiment of hollow particles after the solvent removal step. Fig. 1 (5) shows a state in which the hydrocarbon solvent 4a has been removed from the state shown in Fig. 1 (4). By removing the hydrocarbon solvent from hollow particles (second precursor particles) containing the hydrocarbon solvent in their hollow portions, hollow particles 100 having gas-filled hollow portions 8 inside their shells 6 are obtained. The above five steps and other steps will be explained below in order.

[0049] (1) Mixed liquid preparation process This step is a step of preparing a mixed liquid containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium. The mixed liquid may further contain a polar component. The mixed liquid preferably contains an oil-soluble polymerization initiator as a polymerization initiator. The mixed liquid may further contain other materials such as a suspension stabilizer, as long as the effects of the present disclosure are not impaired. The materials of the mixed liquid will be explained in the following order: (A) first polymerizable monomer, (B) polar component, (C) oil-soluble polymerization initiator, (D) hydrocarbon solvent, (E) dispersion stabilizer, and (F) aqueous medium.

[0050] (A) First Polymerizable Monomer The first polymerizable monomer includes at least a crosslinkable monomer, and may further include a non-crosslinkable monomer within a range that 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 a polymerizable functional group can be preferably used, since the polymerization reaction is likely to be stable and hollow particles having high heat resistance can be obtained. On the other hand, from the viewpoint of reducing the relative dielectric constant and dielectric loss tangent of the hollow particles, hydrocarbon monomers consisting of carbon and hydrogen can also be preferably used.

[0051] [Crosslinking monomer] Since the crosslinkable monomer has a plurality of polymerizable functional groups, the monomers can be linked together, thereby increasing the crosslink density of the shell. Examples of the crosslinkable monomer include difunctional crosslinkable monomers having two polymerizable functional groups, such as divinylbenzene, divinyldiphenyl, divinylnaphthalene, diallyl phthalate, diallylamine, allyl(meth)acrylate, vinyl(meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, and 2-hydroxy-3-(meth)acrylpropyl(meth)acrylate; and trifunctional or higher crosslinkable 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 ethoxylated versions of these. These crosslinkable monomers can be used either alone or in combination of two or more. Among these crosslinkable monomers, examples of hydrophilic crosslinkable monomers having a solubility of 0.3 g / L or more in distilled water at 20°C include ethylene glycol dimethacrylate, diethylene glycol diacrylate, allyl methacrylate, vinyl methacrylate, 2-hydroxy-3-methacrylpropyl acrylate, and diallylamine. The crosslinkable monomer contained in the first polymerizable monomer is not particularly limited and may be a hydrophilic crosslinkable monomer having a solubility in distilled water at 20°C of 0.3 g / L or more, or a non-hydrophilic crosslinkable monomer having a solubility of less than 0.3 g / L.

[0052] The first polymerizable monomer contains at least one crosslinkable monomer selected from a bifunctional crosslinkable monomer and a trifunctional or higher functional crosslinkable monomer. In particular, from the viewpoint of reducing the CTE of the hollow particles, it is preferable to contain at least a bifunctional crosslinkable monomer. Furthermore, from the viewpoint of improving the strength of the hollow particles, it is more preferable to contain a combination of a bifunctional crosslinkable monomer and a trifunctional or higher functional crosslinkable monomer. When the first polymerizable monomer contains a trifunctional or higher functional crosslinkable monomer, it is advantageous in that a covalent bond network can be more densely spread throughout the shell, but unreacted polymerizable functional groups tend to remain after the first polymerization reaction. In the above-mentioned production method, even when the first polymerizable monomer contains a trifunctional or higher functional crosslinkable monomer, adding a hydrophilic monomer as the second polymerizable monomer and performing the second polymerization reaction facilitates the polymerization reaction of the unreacted polymerizable functional groups remaining after the first polymerization reaction. Therefore, when the first polymerizable monomer contains a trifunctional or higher crosslinkable monomer, the crosslinked structure of the shell becomes denser, thereby reducing the CTE, relative dielectric constant, and dielectric loss tangent of the hollow particles and improving the strength of the hollow particles.

[0053] Furthermore, the crosslinkable monomer contained in the first polymerizable monomer is preferably a (meth)acrylic crosslinkable monomer having a (meth)acryloyl group as a polymerizable functional group, because the polymerization reaction is likely to be stable, hollow particles having excellent strength and heat resistance can be obtained, and the adhesion of a resin composition containing the obtained hollow particles is improved. That is, as the bifunctional crosslinkable monomer used for 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 trifunctional or higher crosslinkable monomer used in the first polymerizable monomer is preferably a trifunctional or higher (meth)acrylic crosslinkable monomer 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, or an ethoxylated version of any of these. 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 the present disclosure, the (meth)acrylic crosslinkable monomer may be a crosslinkable monomer having at least one (meth)acryloyl group as a polymerizable functional group, but it is preferable that all of the polymerizable functional groups are (meth)acryloyl groups.

[0054] 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, per 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.

[0055] In addition, as the crosslinkable monomer contained in the first polymerizable monomer, hydrocarbon-based crosslinkable monomers such as divinylbenzene, divinylbiphenyl, and divinylnaphthalene are also preferred because they can easily reduce the relative dielectric constant and dielectric loss tangent of the hollow particles, and among these, divinylbenzene is more preferred.

[0056] The content of the crosslinkable monomer per 100 parts by mass of the first polymerizable monomer is preferably 75 to 100 parts by mass, more preferably 80 to 100 parts by mass, even more preferably 85 to 100 parts by mass, and even more preferably 90 to 100 parts by mass. When the content of the crosslinkable monomer is at or above the lower limit, the polymer contained in the formed shell is likely to be a polymer containing 70 to 100 parts by mass of crosslinkable monomer units per 100 parts by mass of all monomer units. Furthermore, because the content of the crosslinkable monomer units in the shell of the hollow particle is sufficiently high, a covalent bond network is densely spread throughout the shell, resulting in reduced CTE, dielectric constant, and dielectric loss tangent of the hollow particle and improved strength.

[0057] When the first polymerizable monomer contains a bifunctional crosslinkable monomer as the 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 75 parts by mass or more, from the viewpoint of reducing the CTE, relative dielectric constant, and dielectric dissipation factor of the hollow particles and improving the strength of the hollow particles. On the other hand, the upper limit may be 100 parts by mass or less, but is preferably 95 parts by mass or less, more preferably 90 parts by mass or less, from the viewpoint of sufficiently containing trifunctional or higher functional crosslinkable monomer units.

[0058] When the first polymerizable monomer contains a trifunctional or higher crosslinkable monomer as a crosslinkable monomer, the content of the trifunctional or higher crosslinkable monomer 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 viewpoint of reducing the CTE, dielectric constant, and dielectric dissipation factor of the hollow particles and improving the strength of the hollow particles, and the upper limit is preferably 50 parts by mass or less, more preferably 40 parts by mass or less, and from the viewpoint of sufficiently containing the bifunctional or higher crosslinkable monomer, the content is even more preferably 30 parts by mass or less, and even more preferably 25 parts by mass or less.

[0059] [Non-crosslinkable monomer] The first polymerizable monomer may further include a non-crosslinkable monomer. As the non-crosslinkable monomer, a monovinyl monomer is preferably used. A monovinyl monomer is a compound having one polymerizable vinyl functional group. Examples of the monovinyl monomer include (meth)acrylic acid alkyl esters having an alkyl group of 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 carboxylic acid ester monomers, such as vinyl acetate; halogenated vinyl monomers, such as vinyl chloride; and halogenated vinyl monomers, such as vinylidene chloride. Examples of the hydrophilic non-crosslinkable monomer include non-hydrophilic non-crosslinkable monomers such as vinylidene dichloride monomers and vinylpyridine monomers, and hydrophilic non-crosslinkable monomers such as (meth)acrylic acid alkyl esters having an alkyl group of 1 to 5 carbon atoms, such as methyl (meth)acrylate, ethyl (meth)acrylate, and butyl (meth)acrylate; (meth)acrylamides and derivatives thereof, such as (meth)acrylamide, N-methylol (meth)acrylamide, and N-butoxymethyl (meth)acrylamide; (meth)acrylic acid nitrile and derivatives thereof; and polar group-containing non-crosslinkable monomers. Preferred examples of the polar group-containing non-crosslinkable monomer include non-crosslinkable monomers containing a polar group selected from a carboxyl group, a hydroxyl group, a sulfonic acid group, an amino group, a polyoxyethylene group, and an epoxy group. More specifically, examples of the monomer include carboxyl group-containing monomers such as ethylenically unsaturated carboxylic acid monomers, such as (meth)acrylic acid, crotonic acid, cinnamic acid, itaconic acid, fumaric acid, maleic acid, and butenetricarboxylic 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 styrenesulfonic 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 either alone or in combination of two or more. As the non-crosslinkable monomer used as the first polymerizable monomer, from the viewpoint of improving the strength of the hollow particles, hydrophilic non-crosslinkable monomers are preferred, (meth)acrylic acid alkyl esters having an alkyl group with 1 to 5 carbon atoms are more preferred, (meth)acrylic acid alkyl esters having an alkyl group with 1 to 4 carbon atoms are more preferred, and methyl (meth)acrylate is even more preferred.

[0060] In the first polymerizable monomer, polymerizable monomers other than the crosslinkable monomer are non-crosslinkable monomers. The content of the non-crosslinkable monomer in the first polymerizable monomer is preferably 0 to 25 parts by mass per 100 parts by mass of the first polymerizable monomer. From the viewpoints of suppressing a decrease in the reactivity of the first polymerizable monomer, reducing the CTE, dielectric constant, and dielectric dissipation factor of the hollow particles, and improving the strength of the hollow particles, the content of the non-crosslinkable monomer in the first polymerizable monomer is more preferably 20 parts by mass or less, even more preferably 15 parts by mass or less, and even more preferably 10 parts by mass or less. It is particularly preferred that the first polymerizable monomer does not contain a non-crosslinkable monomer.

[0061] The content of the first polymerizable monomer in the mixed solution is not particularly limited, but from the viewpoint of the balance between the porosity, particle size, and mechanical strength of the hollow particles, it is usually 15 to 55 mass%, more preferably 25 to 40 mass%, relative to 100 mass% of the total mass of the components in the mixed solution excluding the aqueous medium.

[0062] Furthermore, from the viewpoint of improving the mechanical strength of the hollow particles, the content of the first polymerizable monomer relative to the total mass (100 mass%) of the solid content excluding the hydrocarbon solvent among the materials that form the oil phase in the mixed liquid is preferably 90 mass% or more, and more preferably 95 mass% or more.

[0063] (B) Polar component The mixed liquid may further contain a polar component. By including a polar component in the mixed liquid, hollow particles that are resistant to crushing even when the porosity is high can be obtained. As the polar component, an organic acid or a metal salt thereof, or a polar resin as described below can be used.

[0064] Examples of organic acids include rosin acid and higher fatty acids, such as higher fatty acids having 10 to 25 carbon atoms and not including a carbon atom in the carboxyl group. Examples of metals used in metal salts of organic acids include alkali metals such as Li, Na, and K, and alkaline earth metals such as Mg and Ca. Of these, alkali metals are preferred, and at least one selected from Li, Na, and K is more preferred.

[0065] When an organic acid or a metal salt thereof is used as the polar component, the total content of the organic acid or the metal salt thereof is preferably 0.0001 to 0.1 parts by mass, more preferably 0.001 to 0.01 parts by mass, and even more preferably 0.0015 to 0.006 parts by mass, per 100 parts by mass of the first polymerizable monomer and the hydrocarbon solvent combined. When the content is equal to or greater than the lower limit, the shell thickness of the hollow particles can be easily controlled, and the strength of the hollow particles can be improved. On the other hand, when the content is equal to or less than the upper limit, a decrease in the polymerizable monomer content can be suppressed, thereby suppressing a decrease in shell strength and further suppressing collapse of the hollow particles.

[0066] The polar resin refers to a polymer containing a repeating unit containing a heteroatom, and specific examples thereof include acrylic resins, polyester resins, and vinyl resins containing heteroatoms. The polar resin may be a homopolymer or copolymer of a heteroatom-containing monomer, or a copolymer of a heteroatom-containing monomer and a non-heteroatom-containing monomer. When the polar resin is a copolymer of a heteroatom-containing monomer and a non-heteroatom-containing monomer, the proportion of heteroatom-containing monomer units in 100% by mass of all repeating units constituting the copolymer is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more, in order to easily control the particle size of the hollow particles.

[0067] The number average molecular weight (Mn) of the polar resin is not particularly limited, but is preferably in the range of 3,000 to 20,000, more preferably 4,000 to 17,000, and even more preferably 6,000 to 15,000, in terms of polystyrene, as measured by gel permeation chromatography (GPC) using tetrahydrofuran. When the number average molecular weight (Mn) of the polar resin is equal to or greater than the lower limit, the solubility of the polar resin is improved and the particle size of the hollow particles can be easily controlled. When the number average molecular weight (Mn) is equal to or less than the upper limit, a decrease in shell strength can be suppressed.

[0068] When a polar resin is used as the polar component, the content of the polar resin is preferably 0.1 to 10.0 parts by mass, more preferably 0.3 to 8.0 parts by mass, and even more preferably 0.5 to 8.0 parts by mass, relative to 100 parts by mass of the first polymerizable monomer. When the content is equal to or greater than the lower limit, the shell thickness of the hollow particles can be easily controlled, and the strength of the hollow particles can be improved. On the other hand, when the content is equal to or less than the upper limit, a decrease in the polymerizable monomer content can be suppressed, thereby suppressing a decrease in shell strength and further suppressing collapse of the hollow particles.

[0069] (C) Oil-soluble polymerization initiator In the present disclosure, the mixed liquid preferably contains an oil-soluble polymerization initiator as a polymerization initiator. As a method for polymerizing droplets of the monomer composition after suspending the mixed liquid, there are an emulsion polymerization method using a water-soluble polymerization initiator and a suspension polymerization method using an oil-soluble polymerization initiator, and the 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 the oil-soluble polymerization initiator 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 mixed solution 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 the content of the oil-soluble polymerization initiator be 0.1 to 10 parts by mass, the polymerization reaction can be sufficiently progressed, and there is little risk of the oil-soluble polymerization initiator remaining after completion of the polymerization reaction, and there is also little risk of an unexpected side reaction proceeding.

[0070] (D) Hydrocarbon solvents In the present disclosure, a hydrocarbon-based solvent is used as a non-polymerizable, poorly water-soluble organic solvent. The hydrocarbon-based solvent acts as a spacer material that forms hollow spaces inside the particles. In the suspension process described below, a suspension is obtained in which droplets of a monomer composition containing a hydrocarbon-based solvent are dispersed in an aqueous medium. In the suspension process, phase separation occurs within the droplets of the monomer composition, and as a result, the hydrocarbon-based solvent, which has low polarity, tends to collect inside the droplets of the monomer composition. Ultimately, the droplets of the monomer composition contain the hydrocarbon-based solvent inside, and other materials other than the hydrocarbon-based solvent are distributed around the periphery according to their respective polarities. Then, in the polymerization step described below, an aqueous dispersion containing hollow particles encapsulating the hydrocarbon solvent is obtained. That is, the hydrocarbon solvent collects inside the particles, and hollow portions filled with the hydrocarbon solvent are formed inside the obtained precursor particles.

[0071] The type of hydrocarbon solvent is not particularly limited, and examples of the hydrocarbon solvent include saturated hydrocarbon solvents such as butane, pentane, normal 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 below, the polymerization reaction proceeds in a state in which the hydrocarbon solvent is encapsulated in oil droplets containing the crosslinkable monomer, etc., and therefore the porosity of the resulting hollow particles tends to increase as the hydrocarbon solvent content increases. The hydrocarbon solvent preferably contains 50% by mass or more of saturated hydrocarbon solvents in a total amount of 100% by mass of hydrocarbon solvents. This allows sufficient phase separation to occur within the droplets of the polymerizable monomer liquid, making it easier to obtain hollow particles having only one hollow portion and suppressing the generation of porous particles. The ratio of saturated hydrocarbon solvent is preferably 60% by mass or more, more preferably 80% by mass or more, in order to further suppress the generation of porous particles and to make the hollow portions of each hollow particle more uniform. Furthermore, the hydrocarbon solvent is preferably a hydrocarbon solvent having 4 to 7 carbon atoms. A hydrocarbon compound having 4 to 7 carbon atoms is easily encapsulated in the first precursor particles during the polymerization step, and can be easily removed from the second precursor particles during the solvent removal step. Among these, a hydrocarbon solvent having 5 or 6 carbon atoms is particularly preferred. Although not particularly limited, the hydrocarbon solvent preferably has a boiling point of 130° C. or less, more preferably 100° C. or less, because it is easily removed in the solvent removal step described below. Furthermore, the hydrocarbon solvent preferably has a boiling point of 50° C. or more, more preferably 60° C. or more, because it is easily encapsulated in the first precursor particles.

[0072] Furthermore, the hydrocarbon solvent preferably has a relative dielectric constant of 3 or less at 20°C. The relative dielectric constant is one of the indicators that indicates the polarity of a compound. When the relative dielectric constant of the hydrocarbon solvent is sufficiently small, 3 or less, it is thought that phase separation proceeds quickly in the droplets of the monomer composition, making it easy to form hollow spaces. Examples of solvents with a relative dielectric constant of 3 or less at 20°C are as follows. The value in parentheses is the relative dielectric constant. Heptane (1.9), normal hexane (1.9), cyclohexane (2.0), benzene (2.3), toluene (2.4). For the relative permittivity at 20°C, reference can be made to values ​​described in known literature (for example, "Chemical Handbook Basics" edited by the Chemical Society of Japan, 4th revised edition, Maruzen Co., Ltd., published September 30, 1993, pp. II-498 to II-503) and other technical information. Examples of methods for measuring the relative permittivity at 20°C include a relative permittivity test conducted in accordance with JISC 2101:1999, 23, at a measurement temperature of 20°C.

[0073] In the present disclosure, the content of the hydrocarbon solvent in the mixed solution is preferably 50 to 500 parts by mass relative to 100 parts by mass of the total mass of the first polymerizable monomer, because this makes it easier to control the particle size of the hollow particles, to increase the porosity while maintaining the strength of the hollow particles, and to reduce the amount of residual hydrocarbon solvent in the particles. The content of the hydrocarbon solvent in the mixed solution is more preferably 60 to 400 parts by mass, even more preferably 70 to 300 parts by mass, and even more preferably 80 to 200 parts by mass relative to 100 parts by mass of the total mass of the first polymerizable monomer.

[0074] (E) Dispersion stabilizer The dispersion stabilizer is an agent for dispersing droplets of the monomer composition in an aqueous medium in the suspension step. In the present disclosure, it is preferable to use an inorganic dispersion stabilizer as the dispersion stabilizer, because it is easy to control the particle size of the droplets in the suspension, it is possible to narrow the particle size distribution of the obtained hollow particles, and it is possible to prevent the shell from becoming too thin, thereby preventing a decrease in the strength of the hollow particles. Examples of inorganic dispersion stabilizers include inorganic compounds such as sulfates such as barium sulfate and calcium sulfate, carbonates such as barium carbonate, calcium carbonate and magnesium carbonate, phosphates such as calcium phosphate, metal oxides such as aluminum oxide and titanium oxide, and metal hydroxides such as aluminum hydroxide, magnesium hydroxide, calcium hydroxide, barium hydroxide and ferric hydroxide. These inorganic dispersion stabilizers can be used alone or in combination of two or more. Among the inorganic dispersion stabilizers, the above-mentioned poorly water-soluble 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 the present disclosure, the poorly water-soluble inorganic metal salt is preferably an inorganic metal salt having a solubility of 0.5 g or less in 100 g of water. In the present disclosure, it is particularly preferred to use the poorly water-soluble inorganic dispersion stabilizer in a state in which it is dispersed in the form of colloidal particles in an aqueous medium, i.e., in the state of a colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles, which not only makes it possible to narrow the particle size distribution of the droplets of the monomer composition but also makes it possible to easily reduce the amount of inorganic dispersion stabilizer remaining in the obtained hollow particles by washing. A colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles can be prepared, for example, by reacting at least one selected from alkali metal hydroxides and alkaline earth metal hydroxides with a water-soluble polyvalent metal salt (excluding alkaline earth metal hydroxides) in an aqueous medium. Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, potassium hydroxide, etc. Examples of alkaline earth metal hydroxides include barium hydroxide, calcium hydroxide, etc. The water-soluble polyvalent metal salt may be any water-soluble polyvalent metal salt other than the compounds corresponding to the alkaline earth metal hydroxides, for example, magnesium metal salts such as magnesium chloride, magnesium phosphate, magnesium sulfate, etc.; calcium metal salts such as calcium chloride, calcium nitrate, calcium acetate, calcium sulfate, etc.; aluminum metal salts such as aluminum chloride, aluminum sulfate, etc.; barium salts such as barium chloride, barium nitrate, barium acetate, etc.; zinc salts such as zinc chloride, zinc nitrate, zinc acetate, etc. Among these, magnesium metal salts, calcium metal salts, and aluminum metal salts are preferred, magnesium metal salts are more preferred, and magnesium chloride is particularly preferred. The water-soluble polyvalent metal salts can be used alone or in combination of two or more. The method for reacting at least one selected from the alkali metal hydroxides and alkaline earth metal hydroxides with the water-soluble polyvalent metal salt in an aqueous medium is not particularly limited, but examples include a method of mixing an aqueous solution of at least one selected from the alkali metal hydroxides and alkaline earth metal hydroxides with an aqueous solution of the water-soluble polyvalent metal salt.

[0075] The content of the dispersion stabilizer is not particularly limited, but is preferably 0.5 to 10 parts by mass, more preferably 1.0 to 8.0 parts by mass, relative to 100 parts by mass of the total mass of the first polymerizable monomer and the hydrocarbon solvent. When the content of the dispersion stabilizer is equal to or greater than the lower limit, droplets of the monomer composition can be sufficiently dispersed so as not to coalesce in the suspension. On the other hand, when the content of the dispersion stabilizer is equal to or less than the upper limit, an increase in the viscosity of the suspension during granulation can be prevented, and the problem of the suspension clogging in the granulator can be avoided. The content of the dispersion stabilizer is usually 2 parts by mass or more and 15 parts by mass or less, and preferably 3 parts by mass or more and 8 parts by mass or less, relative to 100 parts by mass of the aqueous medium.

[0076] (F)Aqueous medium In the present disclosure, the 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 the present disclosure is not particularly limited as long as it is sufficiently miscible with water and does not cause phase separation. Examples of hydrophilic solvents include alcohols such as methanol and ethanol, tetrahydrofuran (THF), 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 polarity of the entire mixture is not too low in order to form droplets of the monomer composition. In this case, for example, the mixing ratio (mass ratio) of water to hydrophilic solvent may be set to 99:1 to 50:50, etc.

[0077] A mixed liquid is obtained by mixing the above-mentioned materials and other materials as needed and stirring them appropriately. In this mixed liquid, an oil phase containing the above-mentioned (A) first polymerizable monomer, (B) polar component, (C) oil-soluble polymerization initiator, and (D) lipophilic material such as a hydrocarbon solvent is dispersed in an aqueous phase containing (E) dispersion stabilizer and (F) aqueous medium with particle sizes of about several mm. The dispersion state of these materials in the mixed liquid can be observed with the naked eye, depending on the type of material. In the mixed solution preparation step, the mixed solution may be obtained by simply mixing the above-mentioned materials and other materials as necessary and appropriately stirring, etc. However, in order to make the shell more uniform, it is preferable to prepare the mixed solution in advance separately from an oil phase containing the first polymerizable monomer, a polar component, and a hydrocarbon solvent, and an aqueous phase containing the dispersion stabilizer and an aqueous medium, and then mixing them. In the present disclosure, a colloidal dispersion in which a poorly water-soluble inorganic dispersion stabilizer is dispersed in the form of colloidal particles in an aqueous medium can be preferably used as the aqueous phase. In this way, by preparing the oil phase and the water phase separately in advance and then mixing them, hollow particles having a uniform shell composition can be produced.

[0078] (2) Suspension process The suspending step is a step of preparing a suspension in which droplets of the monomer composition containing a hydrocarbon solvent are dispersed in an aqueous medium by suspending the mixed liquid described above. The method of suspension for forming droplets of the monomer composition is not particularly limited, and may be carried out using, for example, an apparatus capable of strong stirring, such as an (in-line type) emulsifying disperser (horizontal in-line type dispersers such as Milder, manufactured by Pacific Machinery Works, and Cavitron, manufactured by Eurotech Co., Ltd.; vertical in-line type dispersers such as DRS 2000 / 5, manufactured by IKA), or a high-speed emulsifying disperser (TK Homomixer MARK II, manufactured by Primix Corporation). In the suspension prepared in the suspending step, droplets of the monomer composition containing the lipophilic material and having a particle size of about 1 to 60 μm are uniformly dispersed in the aqueous medium. Such droplets of the monomer composition are difficult to observe with the naked eye, but can be observed using a known observation device such as an optical microscope. In the suspension step, phase separation occurs in the droplets of the monomer composition, and the hydrocarbon solvent with low polarity tends to collect inside the droplets. As a result, the obtained droplets have the hydrocarbon solvent distributed inside and materials other than the hydrocarbon solvent distributed around the periphery.

[0079] Fig. 2 is a schematic diagram showing one embodiment of the suspension in the suspension step. A droplet 10 of the monomer composition in Fig. 2 is intended to show a schematic cross section thereof. Note that Fig. 2 is merely a schematic diagram, and the suspension in the present disclosure is not necessarily limited to that shown in Fig. 2. A part of Fig. 2 corresponds to (2) in Fig. 1 described above. 2 shows droplets 10 of the monomer composition and a first polymerizable monomer 4c dispersed in the aqueous medium 1, which are dispersed in the aqueous medium 1. The droplets 10 are formed by surrounding an oil-soluble monomer composition 4 with a dispersion stabilizer 3. The monomer composition contains an oil-soluble polymerization initiator 5, as well as a first polymerizable monomer and a hydrocarbon solvent (none of which are shown). The droplets 10 are minute oil droplets containing the monomer composition 4, and the oil-soluble polymerization initiator 5 generates polymerization-initiating radicals inside the minute oil droplets. Therefore, precursor particles of the desired particle size can be produced without causing the minute oil droplets to grow too large. In the suspension polymerization method using such an oil-soluble polymerization initiator, there is no opportunity for the polymerization initiator to come into contact with the polymerizable monomer 4c dispersed in the aqueous medium 1. Therefore, by using an oil-soluble polymerization initiator, it is possible to prevent the by-production of excess resin particles such as dense particles having a relatively small particle size in addition to the desired resin particles having hollow portions.

[0080] (3) Polymerization process (3-1) First polymerization step In the above production method, the polymerization step is carried out in two stages. In the first polymerization step, the suspension is subjected to a first polymerization reaction until the polymerization conversion rate of the first polymerizable monomer reaches 93 mass% or more, thereby preparing a first precursor composition containing first precursor particles having a shell containing a polymer of the first polymerizable monomer and a hollow portion filled with a hydrocarbon solvent. During the first polymerization reaction, the droplets of the monomer composition are subjected to the polymerization reaction while encapsulating the hydrocarbon solvent, which allows the polymerization reaction to proceed while maintaining the shape. Therefore, during the first polymerization reaction, the size and porosity of the resulting hollow particles can be easily adjusted by adjusting the amount of hydrocarbon solvent, the amount of polar component, the type of dispersion stabilizer, and the like. Furthermore, since the 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, making it difficult for the hydrocarbon solvent to blend with the shell. This leads to sufficient phase separation, resulting in only one hollow portion.

[0081] In the first polymerization reaction, the polymerization method is not particularly limited, and for example, a batch method, a semi-continuous method, a continuous method, etc. can be used. In the first polymerization reaction, the polymerization temperature is preferably 40 to 80°C, and more preferably 50 to 70°C. In the first polymerization reaction, the temperature rise rate when the temperature is raised to the polymerization temperature is preferably 10°C / h to 60°C / h, and more preferably 15°C / h to 55°C / h. The reaction time for the first polymerization reaction is preferably 0.5 to 5 hours, and more preferably 1 to 3 hours.

[0082] In the above production 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 the present disclosure, the polymerization conversion rate is calculated from 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 using the following formula (A). In the present disclosure, the solid content refers to all components excluding the solvent, and liquid polymerizable monomers and the like are considered to be 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 first polymerizable monomer / mass of solid content of first precursor particles)×100 Formula (A)

[0083] (3-2) Second polymerization step 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 to carry out a second polymerization reaction, thereby preparing a second precursor composition including second precursor particles having a shell including a polymer of the first polymerizable monomer and the second polymerizable monomer, and a hollow portion filled with a hydrocarbon solvent. In the second polymerization reaction, the polymerization reaction proceeds in a state in which the second polymerizable monomer is incorporated into the shell of the first precursor particle. Since the thermal motion of the shell of the first precursor particle is promoted by the incorporation of the second polymerizable monomer, it is presumed that in the second polymerization reaction, the polymerization reaction of the polymerizable functional groups of the first polymerizable monomer remaining unreacted in the shell and the second polymerizable monomer proceeds, resulting in the formation of a dense crosslinked structure.

[0084] The second polymerizable monomer is not particularly limited as long as it has a solubility of 0.3 g / L or more in distilled water at 20°C. However, from the viewpoint of reducing the CTE, dielectric constant, and dielectric dissipation factor of the hollow particles and improving the strength of the hollow particles, a non-crosslinkable monomer, i.e., a hydrophilic non-crosslinkable monomer, having a solubility of 0.3 g / L or more in distilled water at 20°C is preferred. Examples of hydrophilic non-crosslinkable monomers used as the second polymerizable monomer include those similar to the hydrophilic non-crosslinkable monomers usable as the first polymerizable monomer. Among these, from the viewpoint of reducing the CTE, dielectric constant, and dielectric dissipation factor of the hollow particles and improving the strength of the hollow particles, at least one selected from the group consisting of (meth)acrylic acid alkyl esters having an alkyl group of 1 to 5 carbon atoms, (meth)acrylic acid nitrile and its derivatives, and polar group-containing non-crosslinkable monomers is preferred. The number of carbon atoms in the alkyl group of the (meth)acrylic acid alkyl esters is preferably 1 to 4, more preferably 1 to 3. As the (meth)acrylic acid alkyl esters, methyl (meth)acrylate is particularly preferred. Among the 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 serving as the polar group-containing non-crosslinkable monomers, glycidyl (meth)acrylate is preferred, and among the hydroxyl group-containing monomers, 2-hydroxyethyl methacrylate is preferred.

[0085] Furthermore, a hydrophilic crosslinkable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C can also be used as the second polymerizable monomer. Examples of hydrophilic crosslinkable monomers used as the second polymerizable monomer include those similar to the hydrophilic crosslinkable monomers usable as the first polymerizable monomer. Among the hydrophilic crosslinkable monomers used as the second polymerizable monomer, hydrophilic crosslinkable monomers containing hydroxyl groups or amino groups are preferred. Examples of hydrophilic crosslinkable monomers containing hydroxyl groups include 2-hydroxy-3-methacrylpropyl acrylate, and examples of hydrophilic crosslinkable monomers containing amino groups include diallylamine. The second polymerizable monomer preferably has a solubility in distilled water at 20° C. of 2 g / L or more, more preferably 10 g / L or more, and even more preferably 15 g / L or more, so that the second polymerizable monomer is easily incorporated into the shell of the first precursor particle to promote thermal motion and improve the strength of the hollow particle. 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.

[0086] Furthermore, since the second polymerizable monomer is easily incorporated into the shell of the first precursor particle to promote thermal motion, the CTE, relative dielectric constant, and dielectric loss tangent of the hollow particle are reduced, and the strength of the hollow particle is easily improved, the molecular weight of the second polymerizable monomer is preferably 200 or less, more preferably 100 or less. The lower limit of the molecular weight of the second polymerizable monomer is not particularly limited, and is usually 50 or more.

[0087] The amount of the second polymerizable monomer added is preferably 3 to 15 parts by mass, more preferably 4 to 10 parts by mass, per 100 parts by mass of the first polymerizable monomer. When the amount of the second polymerizable monomer added is equal to or greater than the lower limit, the effect of accelerating the polymerization reaction by adding the second polymerizable monomer is improved, and the crosslinked structure of the hollow particle shell becomes denser, thereby reducing the CTE, dielectric constant, and dielectric dissipation factor of the hollow particle and improving the strength of the hollow particle. On the other hand, when the amount of the second polymerizable monomer added is equal to or less than the upper limit, a decrease in the content of the first polymerizable monomer relative to the total polymerizable monomer used to form the shell can be suppressed. Because the first polymerizable monomer contains a large amount of crosslinkable monomer, suppressing a decrease in the content of the first polymerizable monomer can result in hollow particles with excellent strength that contain a large amount of crosslinked structures formed by the crosslinkable monomer.

[0088] In the second polymerization reaction carried out after the addition of the second polymerizable monomer, the polymerization method is not particularly limited, and for example, the same polymerization method as that used in the first polymerization reaction can be adopted. 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.

[0089] According to the above production method, the amount of unreacted polymerizable monomer remaining after the second polymerization reaction can be reduced to preferably 750 ppm or less, more preferably 500 ppm or less, and even more preferably 300 ppm or less. In the present disclosure, the amount of unreacted polymerizable monomer remaining after the second polymerization reaction is the ratio of the mass of the unreacted polymerizable monomer to the mass of the solid content of the hollow particles obtained by the second polymerization reaction. The mass of the unreacted polymerizable monomer can be measured using gas chromatography (GC).

[0090] (4) Solid-liquid separation process This step is a step of obtaining a solid content containing second precursor particles by solid-liquid separation of a second precursor composition containing hollow particles (second precursor particles) encapsulating a hydrocarbon solvent, which is obtained by the above-mentioned polymerization step.

[0091] The method for solid-liquid separation of the second precursor composition is not particularly limited, and any known method can be used. Examples of the solid-liquid separation method include centrifugation, filtration, and static separation. Among these, centrifugation or filtration can be used, and centrifugation may be used from the viewpoint of ease of operation. After the solid-liquid separation step, an optional step such as a pre-drying step may be carried out before the solvent removal step described below is carried out. Examples of the pre-drying step include a step of pre-drying the solid content obtained after the solid-liquid separation step using a drying device such as a dryer or a drying appliance such as a hand dryer.

[0092] (5) Solvent removal process This step is a step of removing the hydrocarbon solvent contained in the hollow particles (second precursor particles) obtained in the solid-liquid separation step. By removing the hydrocarbon solvent contained in the second precursor particles in the air, the hydrocarbon solvent inside the second precursor particles is replaced with air, and hollow particles filled with gas are obtained.

[0093] In this process, "in the air" strictly refers to an environment in which no liquid is present outside the second precursor particles, or an environment in which only a trace amount of liquid is present outside the second precursor particles, to the extent that it does not affect the removal of the hydrocarbon solvent. "In the air" can also be referred to as a state in which the second precursor particles are not present in a slurry, or a state in which the second precursor particles are present in a 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.

[0094] The method for removing the hydrocarbon solvent from the second precursor particles in air is not particularly limited, and any known method can be used, such as vacuum drying, heat drying, flash drying, or a combination of these methods. In particular, when a heat drying method is used, the heating temperature must be equal to or higher than the boiling point of the hydrocarbon solvent and equal to or lower than the maximum temperature at which the shell structure of the second precursor particles does not collapse. Therefore, depending on the shell composition of the second precursor particles and the type of hydrocarbon solvent, the heating temperature may be, for example, 50 to 200°C, 70 to 200°C, or 100 to 200°C. By the drying operation in air, the hydrocarbon solvent inside the second precursor particles is replaced by the external gas, resulting in hollow particles whose hollow portions are filled with gas.

[0095] The drying atmosphere is not particularly limited and can be appropriately selected depending on the application of the hollow particles. Examples of the drying atmosphere include air, oxygen, nitrogen, argon, etc. Hollow particles with a temporary vacuum inside can also be obtained by filling the inside of the hollow particles with a gas and then drying under reduced pressure.

[0096] As an alternative method, the hydrocarbon-based solvent may be removed by replacing the hydrocarbon-based solvent contained in the second precursor particles with the aqueous medium of the slurry containing the second precursor particles in a slurry containing the second precursor particles and an aqueous medium, without subjecting the slurry-like second precursor composition obtained in the polymerization step to solid-liquid separation. In this method, the hydrocarbon solvent contained in the second precursor particles can be removed by bubbling an inert gas through the second precursor composition at a temperature equal to or higher than the boiling point of the hydrocarbon solvent minus 35°C. Here, when 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 is the boiling point of the solvent with the highest boiling point among the solvents contained in the mixed solvent, i.e., the highest boiling point among the multiple boiling points. The temperature at which the inert gas is bubbled into the second precursor composition is preferably at least 30°C below the boiling point of the hydrocarbon solvent, more preferably at least 20°C below that, in order to reduce the amount of hydrocarbon solvent remaining in the hollow particles. The bubbling temperature is usually set to a temperature equal to or higher than the polymerization temperature in the polymerization step. While not particularly limited, the bubbling temperature may be set to 50°C or higher and 100°C or lower. The inert gas to be bubbled is not particularly limited, but examples thereof include nitrogen and argon. The bubbling conditions are appropriately adjusted depending on the type and amount of the hydrocarbon solvent so as to remove the hydrocarbon solvent contained in the second precursor particles, and are not particularly limited, but may be, for example, bubbling an inert gas at a rate of 1 to 3 L / min for 1 to 10 hours. In this method, an aqueous slurry is obtained in which the aqueous medium is encapsulated in the second precursor particles, and the slurry is subjected to solid-liquid separation to obtain hollow particles, which are then dried to remove the aqueous medium from the hollow particles, thereby obtaining hollow particles whose hollow spaces are filled with gas.

[0097] Comparing a method of obtaining hollow particles having hollow portions filled with gas by performing solid-liquid separation on a slurry-like second precursor composition and then removing the hydrocarbon-based solvent from the second precursor particles in air, with a method of obtaining hollow particles having hollow portions filled with gas by replacing the hydrocarbon-based solvent contained in second precursor particles in a slurry containing the second precursor particles and an aqueous medium with the aqueous medium of the slurry, performing solid-liquid separation, and removing the aqueous medium from the second precursor particles in air, the former method has the advantage that the hollow particles are less likely to be crushed in the step of removing the hydrocarbon-based solvent, and the latter method has the advantage that the amount of residual hydrocarbon-based solvent is reduced by performing bubbling with an inert gas. When the hydrocarbon solvent contained in the second precursor particles is replaced with water, the resulting hollow resin particles will collapse unless the particles are filled with water in the same volume as the hydrocarbon solvent that has escaped from the particles. One way to prevent this is to adjust the pH of the slurry to 7 or higher, swell the particle shells with alkali, and then remove the hydrocarbon solvent. In this case, the particle shells acquire flexibility, allowing the hydrocarbon solvent inside the particles to be replaced with water more quickly. Alternatively, as a method for removing the hydrophobic organic solvent contained in the precursor particles after the polymerization step and before the solid-liquid separation step without performing solid-liquid separation on the slurry precursor composition obtained in the polymerization step, for example, a method of evaporating and distilling off the hydrophobic organic solvent contained in the precursor particles from the precursor composition under a predetermined pressure (high pressure, normal pressure, or reduced pressure); or a method of introducing an inert gas such as nitrogen, argon, or helium, or water vapor, into the precursor composition under a predetermined pressure (high pressure, normal pressure, or reduced pressure) and evaporating and distilling off the hydrophobic organic solvent may be used.

[0098] (6) Other As steps other than the above steps (1) to (5), for example, the following step (6-a) of cleaning and the following step (6-b) of replacing the hollow portion again may be added. (6-a) Cleaning process The washing step is a step of adding an acid or alkali to wash the second precursor composition containing the second precursor particles before the solid-liquid separation step in order to remove any dispersion stabilizer remaining in the second precursor composition. When the dispersion stabilizer used is an inorganic dispersion stabilizer soluble in acid, it is preferable to add an acid to the second precursor composition containing the second precursor particles to wash the second precursor composition. On the other hand, when the dispersion stabilizer used is an inorganic compound soluble in alkali, it is preferable to add an alkali to the second precursor composition containing the second precursor particles to wash the second precursor composition. 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 to adjust the pH to preferably 6.5 or less, more preferably 6 or less. The acid to be added may be an inorganic acid such as sulfuric acid, hydrochloric acid, or nitric acid, or an organic acid such as formic acid or acetic acid, but sulfuric acid is particularly preferable because it has a high efficiency in removing the dispersion stabilizer and places a small burden on the production equipment.

[0099] (6-b) Re-replacement process of hollow part The hollow re-substitution process is a process in which the gas or liquid inside the hollow particles is replaced with another gas or liquid. This substitution can change the environment inside the hollow particles, selectively confine molecules inside the hollow particles, or modify the chemical structure inside the hollow particles to suit the application.

[0100] 3. Uses of hollow particles The hollow particles of the present disclosure have excellent strength and are therefore resistant to crushing during kneading with other materials and during molding after kneading, and when added to a molded body, they are effective as a lightweight material, heat insulating material, soundproofing material, vibration damping material, etc., and are therefore suitable as additives for molded bodies, and are particularly suitable for use as additives for resin molded bodies. The hollow particles of the present disclosure can also be contained as a filler in molded bodies formed using a thermoplastic or thermosetting resin, and in molded bodies formed using a thermoplastic or thermosetting resin and a material further containing organic or inorganic fibers, for example. Examples of uses of the hollow particles of the present disclosure include additives used in light reflectors, heat insulating materials, sound insulating materials, and low-dielectric materials used in various fields such as automobiles, electrical appliances, electronics, architecture, aviation, and spacecraft, as well as in food containers, footwear such as sports shoes and sandals, home appliance parts, bicycle parts, stationery, tools, and 3D printer filaments. In particular, the hollow particles of the present disclosure have a low CTE, dielectric constant, and dielectric loss tangent, making them suitable for use as additives in resin compositions used in electrical or electronic applications. For example, the hollow particles of the present disclosure are suitable for use as materials for electronic circuit boards. Specifically, by incorporating the hollow particles of the present disclosure into an insulating resin layer of an electronic circuit board, the CTE, dielectric constant, and dielectric loss tangent of the insulating resin layer can be reduced. The hollow particles according to the present disclosure are also suitable for use as additives in semiconductor materials such as interlayer insulating materials, dry film resists, solder resists, bonding wires, magnet wires, semiconductor encapsulants, epoxy encapsulants, mold underfills, underfills, die bond pastes, buffer coating materials, copper-clad laminates, flexible substrates, high-frequency device modules, antenna modules, and automotive radars. Among these, the hollow particles are particularly suitable as additives in semiconductor materials such as interlayer insulating materials, solder resists, magnet wires, epoxy encapsulants, underfills, buffer coating materials, copper-clad laminates, flexible substrates, high-frequency device modules, antenna modules, and automotive radars. Furthermore, the hollow particles of the present disclosure have high porosity, are not easily crushed, and have excellent heat resistance, so they meet the heat insulation and shock-absorbing properties (cushioning properties) required for undercoating materials and also meet the heat resistance required for thermal paper applications.The hollow particles of the present disclosure are also useful as plastic pigments that are excellent in gloss, hiding power, etc. Furthermore, the hollow particles of the present disclosure can be used for various purposes depending on the components contained therein, since useful components such as fragrances, medicines, agricultural chemicals, and ink components can be encapsulated therein by means of immersion treatment, reduced pressure or pressure immersion treatment, or the like.

[0101] 4.Resin composition The resin composition of the present disclosure is characterized by containing the hollow particles of the present disclosure and a matrix resin. The resin composition of the present disclosure is typically obtained by mixing or kneading the hollow particles of the present disclosure, a matrix resin, and additives added as needed, and may be in the form of pellets, for example. In a resin composition containing the hollow particles of the present disclosure, the hollow particles of the present disclosure are resistant to crushing during mixing or kneading and subsequent molding, and therefore the hollow particles exhibit the effects of weight reduction, low CTE, low dielectric constant, etc.

[0102] The matrix resin used in the resin composition of the present disclosure is not particularly limited, but is preferably a thermoplastic resin or a thermosetting resin. The thermoplastic resin may be any known one, and is not particularly limited. Examples of the thermoplastic resin include polyolefins such as polypropylene and polyethylene; polyamides such as PA6, PA66, and PA12; polyimide, polyamideimide, polyetherimide, polyetherketoneketone, 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 either alone or in combination of two or more. The thermosetting resin may be any known resin, and is not particularly limited. Examples of the thermosetting resin include phenolic resins, melamine resins, urea resins, unsaturated polyester resins, epoxy resins, polyurethane resins, silicon resins, alkyd resins, thermosetting modified polyphenylene ether resins, thermosetting polyimide resins, benzoxazine resins, urea resins, allyl resins, aniline resins, maleimide resins, bismaleimide triazine resins, liquid crystalline polyester resins, vinyl ester resins, unsaturated polyester resins, cyanate ester resins, and polyetherimide resins. These thermosetting resins can be used either alone or in combination of two or more. The thermosetting resin is preferably used together with additives for curing the resin, such as a curing agent and a curing catalyst. Known curing agents and curing catalysts can be used and may be appropriately selected depending on the type of resin. Examples of such curing agents and curing catalysts include amines, acid anhydrides, imidazoles, thiols, phenols, naphthols, benzoxazines, cyanate esters, and carbodiimides. Furthermore, when the resin composition of the present disclosure contains a thermosetting resin, it may further contain a solvent or the like for dissolving or dispersing each component, as necessary.

[0103] When the resin composition of the present disclosure is used as a material for electronic circuit boards, the matrix resin is preferably an insulating resin. The insulating resin is not particularly limited, and examples thereof include epoxy resins, thermosetting modified polyphenylene ether resins, thermosetting polyimide resins, silicon resins, benzoxazine resins, melamine resins, urea resins, allyl resins, phenol resins, unsaturated polyester resins, polyurethane resins, and aniline resins. Among these, epoxy resins, thermosetting polyimide resins, modified polyphenylene ether resins, silicon resins, benzoxazine resins, and melamine resins are preferred. These insulating resins may be used alone or in combination of two or more.

[0104] The content of the matrix resin in the resin composition of the present disclosure, based on 100% by total mass, is not particularly limited, but is preferably 50 to 95% by mass. When the content of the matrix resin is equal to or greater than the lower limit, the resin composition can be molded into a molded article with excellent moldability, and the resulting molded article has excellent mechanical strength. On the other hand, when the content of the matrix resin is equal to or less than the upper limit, the hollow particles of the present disclosure can be sufficiently incorporated, thereby imparting to the resin composition the properties of the hollow particles of the present disclosure, such as weight reduction, low CTE, and low dielectric constant. In the present disclosure, when the matrix resin is a thermosetting resin, the content of the matrix resin also includes the content of additives for curing the resin, such as a curing agent and a curing catalyst.

[0105] The content of the hollow particles of the present disclosure in a total mass of 100% by mass of the resin composition of the present disclosure is not particularly limited, but is preferably 5 to 50% by mass. When the content of the hollow particles is equal to or greater than the lower limit, the hollow particles of the present disclosure can impart performance such as weight reduction, low CTE, and low dielectric constant to the resin composition. On the other hand, when the content of the hollow particles is equal to or less than the upper limit, the matrix resin can be sufficiently contained, thereby improving moldability and mechanical strength.

[0106] In addition to the hollow particles and matrix resin of the present disclosure, the resin composition of the present disclosure may further contain additives such as an ultraviolet absorber, a colorant, a heat stabilizer, and a filler, as well as a solvent, as necessary, within a range that does not impair the effects of the present disclosure. The resin composition of the present disclosure may further contain organic or inorganic fibers such as carbon fibers, glass fibers, aramid fibers, and polyethylene fibers.

[0107] The resin composition of the present disclosure can be obtained, for example, by mixing the hollow particles of the present disclosure, a matrix resin, and, if necessary, additives, a solvent, and the like. When the matrix resin in the resin composition of the present disclosure is a thermoplastic resin, the hollow particles of the present disclosure and any additives added as needed may be added to a molten thermoplastic resin and mixed by melt kneading. The resin composition of the present disclosure obtained in this manner may be a liquid resin composition, or may be a resin molded product obtained by molding the liquid resin composition by a known method.

[0108] The resin composition of the present disclosure can be used as a molded article. By containing the hollow particles of the present disclosure, the molded article of the resin composition of the present disclosure can effectively exhibit the effects of the hollow particles of the present disclosure, such as weight reduction, low CTE, and low dielectric constant.

[0109] When the resin composition of the present disclosure contains a thermoplastic resin as a matrix resin and is a liquid resin composition obtained by melting the resin, a molded article can be obtained by molding the liquid resin composition into a desired shape using a known molding method such as extrusion molding, injection molding, press molding, or compression molding.

[0110] When the resin composition of the present disclosure is a liquid resin composition in which hollow particles or the like are contained in a liquid matrix resin before the curing reaction, or a liquid resin composition in which each component is dissolved or dispersed in a solvent, the liquid resin composition can be applied to a support, and dried and cured as necessary to obtain a molded body. Examples of materials for the support include resins such as polyethylene terephthalate and polyethylene naphthalate; and metals such as copper, aluminum, nickel, chromium, gold, and silver. The liquid resin composition can be applied by any known method, such as dip coating, roll coating, curtain coating, die coating, slit coating, and gravure coating. When the liquid resin composition contains a solvent, it is preferable to dry the resin composition after the application. The drying temperature is preferably a temperature at which the matrix resin does not harden, and is usually 20°C to 200°C, preferably 30°C to 150°C. The drying time is usually 30 seconds to 1 hour, preferably 1 minute to 30 minutes. The curing reaction of the resin composition is carried out by a method appropriate for the type of matrix resin, and is not particularly limited. When a matrix resin that cures upon heating is included, the heating temperature for the curing reaction is adjusted appropriately depending on the type of resin, and is not particularly limited, but is usually 30°C or higher and 400°C or lower, preferably 70°C or higher and 300°C or lower, and more preferably 100°C or higher and 200°C or lower. The curing time is 5 minutes to 5 hours, and preferably 30 minutes to 3 hours. The heating method is not particularly limited, and may be carried out using, for example, an electric oven. The matrix resin contained in the liquid resin composition obtained by dissolving or dispersing each component in a solvent may be a thermosetting resin or a thermoplastic resin.

[0111] The shape of the molded body is not particularly limited, and can be any shape that can be molded using the resin composition of the present disclosure, such as a sheet, a film, a plate, a tube, or any other three-dimensional shape. Furthermore, when the molded body contains fibers, the fibers in the molded body may be in the form of a nonwoven fabric. Furthermore, when the molded body contains fibers, the molded body may be a molded body of a resin composition in which hollow particles of the present disclosure are added to a fiber-reinforced plastic containing the resin and fibers as described above.

[0112] Examples of applications of the resin composition of the present disclosure include light reflecting materials, heat insulating materials, sound insulating materials, and low dielectric materials used in various fields such as automobiles, electricity, electronics, architecture, aviation, and space, as well as food containers, footwear such as sports shoes and sandals, home appliance parts, bicycle parts, stationery, tools, etc. The resin composition of the present disclosure has a low CTE, a low dielectric constant, and a low dielectric loss tangent, and is therefore particularly suitable for use in insulating resin layers used in the electrical or electronic fields. [Example]

[0113] The present disclosure will be described in more detail below with reference to examples and comparative examples, but the present disclosure is not limited to these examples. Note that parts and percentages are by mass unless otherwise specified.

[0114] [Example 1] (1) Mixed liquid preparation process First, the following materials were mixed to form an 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 Rosin acid 0.007 parts 120 parts cyclohexane Next, in a stirring tank, an aqueous solution of 12.1 parts of sodium hydroxide (alkali metal hydroxide) in 121 parts of ion-exchanged water was gradually added under stirring to an aqueous solution of 17.1 parts of magnesium chloride (a water-soluble polyvalent metal salt) in 494 parts of ion-exchanged water at room temperature to prepare a magnesium hydroxide colloid (poorly water-soluble metal hydroxide colloid) dispersion (4 parts magnesium hydroxide) as the aqueous phase. The resulting aqueous phase and oil phase were mixed to prepare a mixed liquid.

[0115] (2) Suspension process The mixture obtained in the mixture preparation step was stirred and suspended for 1 minute using a disperser (manufactured by Primix Corporation, trade name: Homomixer) at a rotation speed of 4,000 rpm, to prepare a suspension in which droplets of the monomer composition encapsulating cyclohexane were dispersed in water.

[0116] (3) Polymerization process The suspension obtained in the suspension step was heated in a nitrogen atmosphere from 40°C to 65°C over 30 minutes (heating rate: 50°C / hour), and stirred at 65°C for 1 hour and 30 minutes to carry out a first polymerization reaction, yielding a first precursor composition containing 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 a second polymerizable monomer was added to the stirring tank, and the mixture was stirred under a nitrogen atmosphere at 65°C for 2 hours and 30 minutes to carry out a second polymerization reaction. This second polymerization reaction yielded a second precursor composition containing second precursor particles encapsulating cyclohexane.

[0117] (4) Washing process and solid-liquid separation process The second precursor composition was washed with dilute sulfuric acid (25°C, 10 minutes) to adjust the pH to 5.5 or less. Next, after separating the water by filtration, 200 parts of ion-exchanged water was added to re-slurry the mixture. The water washing process (washing, filtration, dehydration) was repeated several times at room temperature (25°C), and the mixture was filtered to obtain a solid fraction. The obtained solid fraction was dried in a dryer at 40°C to obtain second precursor particles containing cyclohexane.

[0118] (5) Solvent removal process The second precursor particles obtained in the solid-liquid separation step were heat-treated in a vacuum dryer at 200°C for 6 hours to remove the hydrocarbon solvent contained in the particles, thereby obtaining the hollow particles of Example 1. From the results of observation with a scanning electron microscope and the porosity value, it was confirmed that the obtained hollow particles were spherical and had hollow portions.

[0119] [Examples 2 to 3 and 8] The hollow particles of Examples 2 to 3 and 8 were produced in the same manner as in Example 1, except that the material of the second polymerizable monomer added in the above "(3) polymerization step" was changed to that shown in Table 1.

[0120] [Example 4] The hollow particles of Example 4 were produced in the same manner as in Example 1, except that in the above "(1) mixed solution preparation step," the material and amount of the first polymerizable monomer were changed as shown in Table 1.

[0121] [Examples 5 to 6] The hollow particles of Examples 5 and 6 were produced in the same manner as in Example 1, except that the amount of the second polymerizable monomer added in the above "(3) polymerization step" was set as shown in Table 1.

[0122] [Example 7] The hollow particles of Example 7 were produced in the same manner as in Example 1, except that in the above "(1) mixed solution preparation step," the material and amount of the first polymerizable monomer and the material and amount of the hydrocarbon solvent were changed as shown in Table 1.

[0123] [Comparative Example 1] Hollow particles of Comparative Example 1 were produced in the same manner as in Example 1, except that in the above "(3) polymerization step", the second polymerizable monomer was not added and the second polymerization reaction was not carried out.

[0124] Comparative Example 2 Hollow particles of Comparative Example 2 were produced in the same manner as in Example 1, except that in the above "(3) polymerization step" in Example 1, 5 parts of styrene (solubility in distilled water at 20°C: 0.2 g / L) was added as the second polymerizable monomer instead of 5 parts of methyl acrylate.

[0125] Comparative Example 3 The hollow particles of Comparative Example 3 were produced in the same manner as in Example 1, except that the amount of hydrocarbon solvent in the above "(1) mixed solution preparation step" was changed to the amount shown in Table 1.

[0126] Comparative Example 4 The hollow particles of Comparative Example 4 were produced in the same manner as in Example 1, except that in the above "(1) mixed solution preparation step", the material and amount of the first polymerizable monomer were as shown in Table 1, and in the above "(3) polymerization step", the second polymerizable monomer was not added, and the second polymerization reaction was not performed.

[0127] Comparative Example 5 The hollow particles of Comparative Example 5 were produced in the same manner as in Example 1, except that in the above "(1) mixed solution preparation step", the material and amount of the first polymerizable monomer and the material and amount of the hydrocarbon solvent were as shown in Table 1, and in the above "(3) polymerization step", the second polymerizable monomer was not added, and the second polymerization reaction was not performed.

[0128] [Table 1]

[0129] [Evaluation] 1. Polymerization conversion rate In the polymerization step of each example and each comparative example, 50 g of the first precursor composition generated in the first polymerization reaction was collected and subjected to pressure filtration to obtain the first precursor particles (including moisture and a hydrocarbon solvent) contained in the first precursor composition, and weighed accurately to the unit of 1 mg. To about 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 another 10 minutes. The obtained solution was allowed to stand to precipitate insoluble matters, 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 ( <0.25mm×30m> ) Column temperature: 80 °C Injection temperature: 200 °C FID detection side temperature: 200 °C

[0130] For the hollow particles obtained in each example and each comparative example, the content ratio (mass %) of each monomer unit in the polymer contained in the shell is shown in Table 2. In addition, the following measurements and evaluations were performed on the hollow particles obtained in each example and each comparative example. The results are shown in Table 2.

[0131] 2. Volume average particle diameter of hollow particles The particle diameter of the hollow particles was measured using a laser diffraction particle size distribution analyzer (Shimadzu Corporation, trade name: SALD-2000), and the volume average was calculated to obtain the volume average particle diameter.

[0132] 3. Density and porosity of hollow particles 3-1. Measurement of apparent density of hollow particles First, a capacity of 100cm 3 30cm into a measuring flask 3 The volumetric flask was filled with 10 ... 3 ) was calculated. Formula (I) Apparent density D1 = [Mass of hollow particles] / (100 - [Mass of isopropanol] ÷ [Specific gravity of isopropanol at measurement temperature])

[0133] 3-2. Measurement of true density of hollow particles After crushing the hollow particles in advance, 3 Approximately 10 g of crushed pieces of hollow particles was filled into a measuring flask, and the mass of the packed crushed pieces was accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol is added to the measuring flask, the mass of isopropanol is accurately weighed, and the true density D0 (g / cm) of the hollow particles is calculated based on the following formula (II). 3 ) was calculated. Formula (II) True density D0 = [mass of crushed hollow particle fragments] / (100 - [mass of isopropanol] ÷ [specific gravity of isopropanol at measurement temperature])

[0134] 3-3. 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 according to the following formula (III). Formula (III) Porosity (%) = 100 - (apparent density D1 / true density D0) x 100

[0135] 4. Shell thickness of hollow particles The inner diameter r of the hollow particles was calculated using the volume average particle diameter R and porosity of the hollow particles according to the following formula (1), and the shell thickness of the hollow particles was calculated using the inner diameter r and the volume average particle diameter R according to the following formula (2). 4 / 3π×(R / 2) 3 ×Porosity=4 / 3π×(r / 2) 3 Formula (1) Shell thickness = (Rr) / 2 Equation (2)

[0136] 5. Immersion test In an environment of 25°C, 0.1 mg of hollow particles were added to 4 mL of acetone, and the mixture was shaken for 10 minutes at a shaking speed of 100 rpm using a shaker. After leaving the mixture to stand for 48 hours, the percentage of hollow particles that precipitated was determined and evaluated according to the following evaluation criteria. The hollow particles that precipitated in the acetone were separated using a centrifuge, dried, and the mass of the hollow particles that precipitated in the acetone was measured. The percentage of the precipitated hollow particles was determined by calculating the ratio of the mass of the hollow particles that precipitated in the acetone to the total mass of the hollow particles immersed in the acetone. (Evaluation criteria for immersion test) ○: The precipitated hollow particles are less than 10% by mass ×: The precipitated hollow particles are 10% by mass or more

[0137] 6.Coefficient of thermal expansion 6-1. Preparation of measurement molded plate 50 parts of epoxy resin (Daicel, product number: EHPE3150CE), 24.9 parts of curing agent (DIC, product number: LF6161), 0.1 parts of 2-ethyl-4-methylimidazole (Nacalai Tesque, product number: 2E4MZ) as a curing catalyst, and 26 parts of methyl ethyl ketone (MEK) were mixed and stirred for 30 minutes. After stirring, 16.6 parts of hollow particles were added and stirred for an additional hour to produce a varnish. 15 g of the varnish was placed on copper foil and degassed in a vacuum dryer at 130 °C until no bubbles remained. It was then cured for 2 hours in a hot air circulating oven at 110 °C. After heating and pressurizing at 0.5 MPa in a 110 °C press, the temperature was increased to 205 °C (4 °C / min) and held for 1 hour to produce a molded plate. The molded plate was cut into a 20 mm x 40 mm x 0.5 mm shape to serve as a measurement sample.

[0138] 6-2. Measurement of the thermal expansion coefficient of hollow particles In accordance with JIS K 7197:2012, the thermal expansion coefficient of the produced molded plate was measured in a tensile mode in the range of 25 to 250°C using a TMA device (manufactured by Rigaku Corporation, model: TMA-8311). The thermal expansion coefficient α of the above molded plate in the temperature range of 80 to 200°C and the temperature range of 25 to 80°C c , the thermal expansion coefficient of epoxy resin alone α r , the volume ratio SG of the epoxy resin in the molded plate r , and the volume fraction W of hollow particles in the molded plate p From this, the thermal expansion coefficient of the hollow particles, α p was calculated using the following formula (E). α p =(α c- SG r ×α r ) / W p Formula (E) The coefficient of thermal expansion (CTE) values ​​are shown in Table 2. -5 is the value multiplied by

[0139] 7. Measurement of relative permittivity and dielectric loss tangent Using a perturbation type measuring device (manufactured by AET, model: ADMS01Nc), the relative permittivity and dielectric loss tangent of the hollow particles were measured at a frequency of 1 GHz and room temperature (25°C).

[0140] [Table 2]

[0141] [Consideration] As shown in Table 2 above, the hollow particles obtained in each comparative example had a higher CTE at 80 to 200°C and a higher CTE at 25 to 80°C, as well as a higher relative permittivity and dielectric loss tangent at a frequency of 1 GHz, compared to the examples using the same type of crosslinkable monomer. The hollow particles obtained in Comparative Examples 1, 2, 4, and 5 had 10 mass% or more of hollow particles precipitated in acetone in the above-mentioned immersion test, and it is presumed that the shell was not dense enough, and therefore the CTE, relative dielectric constant, and dielectric loss tangent were not sufficiently reduced. In Comparative Examples 1 and 5, it is presumed that the shell was not dense enough because the second polymerizable monomer was not added. In Comparative Example 2, it is presumed that the shell was not dense enough because styrene, which has a solubility of 0.2 g / L in distilled water at 20°C, was used as the second polymerizable monomer instead of a hydrophilic monomer, which has a solubility of 0.3 g / L or more in distilled water at 20°C. In Comparative Example 4, it is presumed that the density of the shell was insufficient because methyl methacrylate was added to the mixed solution in one step together with the first polymerizable monomer, rather than as the second polymerizable monomer, and the polymerization reaction was carried out in one step. On the other hand, it is presumed that the hollow particles obtained in Comparative Example 3 had a low porosity and an insufficient size of the hollow portion, and therefore the CTE, relative dielectric constant, and dielectric loss tangent were not sufficiently reduced.

[0142] In contrast, the hollow particles obtained in each example had low CTE at 80 to 200°C and low CTE at 25 to 80°C, and also low relative permittivity and dielectric loss tangent at a frequency of 1 GHz. The hollow particles obtained in Examples 1 to 8 had a porosity of 50% or more, a sufficiently large proportion of hollow portions in the particle, i.e., a sufficiently reduced proportion of shells in the particle, and the shells contained a polymer containing 70 to 100 parts by mass of crosslinkable monomer units per 100 parts by mass of all monomer units. Furthermore, the hollow particles had a dense structure in which less than 10% by mass of the particles precipitated in acetone in the above-mentioned immersion test. Therefore, it is presumed that the CTE, relative dielectric constant, and dielectric loss tangent were sufficiently reduced. [Explanation of symbols]

[0143] 1 Aqueous medium 2 Low polarity material 3. Dispersion stabilizer 4. Monomer composition 4a Hydrocarbon solvents 4b Materials other than hydrocarbon solvents 4c Polymerizable monomer dispersed in an aqueous medium 5. Oil-soluble polymerization initiator 6 shells 8 Hollow part 10 droplets 20 Hollow particles containing a hydrocarbon solvent in the hollow space (second precursor particles) 100 Hollow particles with the hollow space filled with gas

Claims

1. A hollow particle having a shell containing a resin and a hollow portion surrounded by the shell, The porosity is 50% or more, The volume average particle size is 1.0 μm or more, the shell contains, as the resin, a polymer containing 70 to 100 parts by mass of crosslinkable monomer units per 100 parts by mass of all monomer units; Thermal expansion coefficient at 80 to 200°C is 10.0 x 10 －５ / °C or less, In an immersion test of hollow particles, 0.1 mg of hollow particles are added to 4 mL of acetone in an environment of 25°C, shaken at a shaking speed of 100 rpm for 10 minutes, and then allowed to stand for 48 hours. In this test, the amount of hollow particles that precipitate in acetone is less than 10 mass%.

2. Thermal expansion coefficient at 25 to 80°C is 6.0 x 10 －５ The hollow particles according to claim 1, wherein the viscosity is 1 / °C or less.

3. 3. The hollow particle according to claim 1, which has a relative dielectric constant of 1.6 or less at a frequency of 1 GHz.

4. The hollow particle according to any one of claims 1 to 3, having a dielectric loss tangent of 0.010 or less at a frequency of 1 GHz.

5. 5. The hollow particle according to claim 1, wherein the polymer contained in the shell contains, as the crosslinkable monomer unit, a bifunctional crosslinkable monomer unit derived from a bifunctional crosslinkable monomer, and the content of the bifunctional crosslinkable monomer unit is 70 to 100 parts by mass relative to 100 parts by mass of all monomer units of the polymer.

6. 5. The hollow particle according to claim 1, wherein the polymer contained in the shell contains, as the crosslinkable monomer unit, a tri- or higher functional crosslinkable monomer unit derived from a tri- or higher functional crosslinkable monomer, and the content of the tri- or higher functional crosslinkable monomer unit is 5 to 50 parts by mass relative to 100 parts by mass of all monomer units of the polymer.

7. the polymer contained in the shell 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; 5. The hollow particle according to claim 1, wherein the content of the hydrophilic non-crosslinkable monomer unit is 2 to 15 parts by mass and the content of the crosslinkable monomer unit is 70 to 98 parts by mass, relative to 100 parts by mass of all monomer units of the polymer.

8. The hollow particle according to any one of claims 1 to 7, wherein the crosslinkable monomer unit comprises a crosslinkable monomer unit derived from a (meth)acrylic crosslinkable monomer having a (meth)acryloyl group as a polymerizable functional group.

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