JP7759491B2 - Hollow particles and their manufacturing method - Google Patents

Description

The present invention relates to hollow particles having a hollow shell inside and a method for producing the same.

For example, in the field of information and communications equipment, there is a demand for lower dielectric constants and dielectric loss tangents in electronic components (typically resin components) such as circuit boards to accommodate communications in high frequency bands. To achieve this, it has been proposed, for example, to incorporate air, which has a low relative dielectric constant, into the components. Specifically, as disclosed in Patent Documents 1 and 2, it has been proposed to introduce air using hollow particles.

International Publication No. 2021/171858 International Publication No. 2021/171859

In recent years, the increasing speed and capacity of information and communication devices has created a demand for hollow particles with further improved dielectric properties.

The present invention has been made to solve the above problems, and one of its objectives is to provide hollow particles with further improved dielectric properties.

(1) The hollow particles disclosed herein have a hollow shell. The shell is silica containing aluminum. The amount of aluminum present is greater in the first region than in the second region. The first region is located on the inner side of the shell in the thickness direction of the shell. The second region is located on the outer side of the first region in the thickness direction of the shell.

(2) The hollow particles disclosed herein are the hollow particles described in (1) above, in which the aluminum content is less than 1% of the components constituting the shell.

(3) In the hollow particles disclosed herein, the amount of aluminum present decreases from the inside to the outside of the shell in the hollow particles described in (1) or (2) above.

(4) The hollow particles of the present disclosure are the hollow particles according to any one of (1) to (3) above, wherein the epoxy resin oil absorption is less than 0.6 g/ ^m2 .

(5) The hollow particles disclosed herein are those described in any one of (1) to (4) above, wherein the shell further contains sodium. The sodium content of the shell is 3000 ppm or less.

(6) The hollow particles of the present disclosure are hollow particles described in any one of (1) to (5) above, wherein the silica is amorphous silica.

(7) The hollow particles disclosed herein are hollow particles described in any one of (1) to (6) above, wherein the shell has a hollowness of 30% or more and 95% or less.

(8) The hollow particles of the present disclosure are hollow particles described in any one of (1) to (7) above, wherein the thickness of the shell is 25 nm or more and 500 nm or less.

(9) The method for producing hollow particles disclosed herein is the method for producing hollow particles described in any one of (1) to (8) above. The method includes coating core particles with a shell-forming material to obtain core-shell particles, removing the core particles from the core-shell particles to obtain hollow particle precursors, and coating the hollow particle precursors with a shell-forming material.

(10) The method for producing hollow particles according to the present disclosure is the method for producing hollow particles described in (9) above, wherein the core particles contain an alunite-type compound represented by the following general formula (I):
M _a [Al _1-x M' _x ] ₃ (SO ₄ ^2- ) _y (OH) _z・mH ₂ O...(I)
In formula (I), M is at least one cation selected from the group consisting of Na ⁺ , K ⁺ , _NH4 ^+, and _H3O ⁺ . In formula (I), M' is at least one cation selected from the group consisting of Cu2 ⁺ , Zn2 ⁺ , Ni2 ⁺ , Sn4 ⁺ , Zr4 ^+, and Ti4 ⁺ . In formula (I), a satisfies 0.8≦a≦1.35. In formula (I), m satisfies 0≦m≦5. In formula (I), x satisfies 0≦x≦0.4. In formula (I), y satisfies 1.7≦y≦2.5. In formula (I), z satisfies 4≦z≦7.

Embodiments of the present invention can achieve excellent dielectric properties.

FIG. 2 is a schematic diagram illustrating a major axis and a minor axis. FIG. 1 is a cross-sectional view schematically illustrating a hollow particle according to one embodiment of the present invention. 1 is a graph showing an outline of an example of a change in the amount of Al present in the thickness direction of the shell. 10 is a graph showing an outline of another example of the change in the amount of Al present in the thickness direction of the shell. 10 is a graph outlining yet another example of the change in the amount of Al present in the thickness direction of the shell. 1 is a schematic cross-sectional view of a laminate according to one embodiment of the present invention. 1 is a TEM observation photograph of hollow particles of Example 1. 1 is a TEM observation photograph of hollow particles of Example 2. 1 is a TEM observation photograph of hollow particles of Example 3. 1 is a cross-sectional SEM photograph of a resin molded body of Example 1.

The following describes embodiments of the present invention, but the present invention is not limited to these embodiments.

(Definition of terms)
The definitions of terms used in this specification are as follows.
1. Particle Longitudinal Diameter The particle long diameter is a value measured using a scanning electron microscope (SEM). The particle long diameter is the average long diameter of randomly selected primary particles, as shown by L in Figure 1, for example. Note that primary particles are the smallest particles observed using SEM, and are distinguished from agglomerated secondary particles.
2. Particle Minor Diameter The particle minor diameter is a value measured by SEM observation. The particle minor diameter is the average minor diameter of randomly selected primary particles, as shown by T in Figure 1, for example.
3. Aspect Ratio The aspect ratio is a value calculated by dividing the minor axis of the particle by the major axis of the particle.

A. Hollow Particles In one embodiment of the present invention, hollow particles have a shell with a hollow interior. In other words, hollow particles have a shell and a space surrounded by this shell. The shell contains silica. The silica is typically amorphous silica. The silica content of the shell is, for example, 95% by weight or more, preferably 97% by weight or more, and more preferably 98% by weight or more.

The shell of the hollow particles contains aluminum (Al). By including Al, hollow particles with excellent strength (breaking strength) can be obtained. The Al content is preferably 0.1% or more, and more preferably 0.2% or more. Meanwhile, hollow particles containing Al tend to have a high dielectric constant and dielectric dissipation factor. Therefore, the Al content is preferably less than 1%, more preferably 0.9% or less, even more preferably 0.7% or less, and particularly preferably 0.5% or less. At such a content, the hollow particles can have extremely excellent dielectric properties. The Al content can be determined, for example, by composition analysis using inductively coupled plasma atomic emission spectrometry (ICP-AES).

The hollow particles are formed with a higher amount of Al present on the inner side of the shell thickness than on the outer side. This configuration allows for hollow particles with excellent strength (fracture strength), hollowness, and dielectric properties. Specifically, the Al-rich regions of the hollow particles are more likely to soften during firing. That is, hollow particles can be manufactured at lower firing temperatures. Furthermore, the Al-rich regions of the hollow particles can cause the hollow particle precursor that forms the shell to shrink more on the inside than on the outside during firing. The Al-rich regions of the hollow particles can increase the fracture strength. Furthermore, hollow particles maintain their particle diameter by suppressing softening and shrinkage on the outside of the shell, while expanding the internal space through softening and shrinkage on the inside, contributing to an improvement in the hollowness. In other words, the difference in the distribution of Al present on the outside and inside of the shell allows for improved shell strength and hollowness. The excellent strength and low dielectric properties of the hollow particles can effectively prevent the hollow particles from breaking, for example, when preparing a resin composition containing the resin and hollow particles. As a result, the hollow state of the particles is maintained, which can greatly contribute to improving the dielectric properties. Furthermore, if the shell thickness of the hollow particles is thin, Al tends to be uniformly distributed. For example, if Al is uniformly distributed in the hollow particles, the inside and outside of the shell tend to shrink equally, resulting in a smaller particle diameter.

Figure 2 is a cross-sectional view schematically illustrating a hollow particle according to one embodiment of the present invention. Note that hatching has been omitted from the cross section of the hollow particle in Figure 2 for clarity. The hollow particle 2 has a shell 4 and a space (hollow portion) 6 surrounded by the shell 4. The amount of Al present in the first portion 4a located on the inner side of the shell 4 in the thickness direction is greater than the amount of Al present in the second portion 4b located on the outer side of the first portion 4a in the thickness direction. For example, as shown in Figure 2, when the first portion 4a is located on or near the inner surface of the shell 4 and the second portion 4b is located on or near the outer surface of the shell 4, the amount of Al present in the first portion 4a is preferably at least two times, and more preferably at least three times, the amount of Al present in the second portion 4b. The distribution of Al abundance can be determined, for example, by compositional analysis using energy dispersive X-ray spectroscopy (TEM-EDS) with a transmission electron microscope (TEM). In one embodiment, the amount of Al present can be evaluated by the molar ratio of Al to Si.

An example of the distribution of Al abundance in the thickness direction is one in which the Al abundance continuously decreases from the inside to the outside of the shell. In this case, the manner in which the Al abundance changes is not particularly limited. For example, the Al abundance may decrease linearly with distance from the inner surface of the shell, as shown in Figure 3A. Alternatively, it may decrease exponentially with distance from the inner surface of the shell, as shown in Figure 3B. Specifically, the Al abundance of hollow particles can be controlled by controlling the addition of the Al raw material during the primary silica treatment of the core particles and the secondary silica treatment of the hollow particle precursor. Hollow particles with a linear decrease in Al abundance tend to have a higher hollow ratio than those with uniform distribution of Al in the shell. Hollow particles with an exponential decrease in Al abundance tend to have a higher hollow ratio than those with a linear decrease.

Another example of the distribution of Al abundance in the thickness direction is one in which the Al abundance gradually decreases from the inside to the outside of the shell. In other words, the hollow particles have a discontinuous decrease in Al abundance from the inside to the outside of the shell. For example, as shown in Figure 3C, a hollow particle may have a first region 41 with a constant Al abundance from the inner surface of the shell to the outside, and a second region 42 outside the first region 41 with a lower Al abundance than the first region 41. Note that Figure 3C illustrates the first region 41 and the second region 42 as a straight line for convenience, but a constant Al abundance means, for example, that the variation in the Al abundance is 20% or less. While Figure 3C shows an example in which the Al abundance decreases in two stages, three or more stages may also be used. A stepwise decrease in the Al abundance of hollow particles can enhance the effects of improving strength and hollowness.

The shell of the hollow particles may contain sodium (Na). If the Na content of the hollow particles is too high, it becomes difficult to increase the firing temperature during the hollow particle manufacturing process, and the particles tend to shrink. The Na content is preferably 3000 ppm or less, more preferably 2000 ppm or less, and even more preferably 1500 ppm or less. On the other hand, the Na content is preferably 100 ppm or more. The Na content can be determined, for example, by composition analysis using atomic absorption spectrometry (AAS).

In hollow particles, it is preferable that the amount of Na be greater on the inside of the shell in the thickness direction than on the outside of the shell in the thickness direction. It is thought that when some of the Si(4+) is replaced by Al(3+), Na easily enters the hollow particles to fill the electron vacancy. Therefore, it is thought that hollow particles tend to have a high concentration of Na in areas where Al is abundant.

In one embodiment, the primary particles of the hollow particles preferably satisfy 1≦D _SL ≦1.5. By satisfying 1≦D _SL ≦1.5, variations among particles can be suppressed and dielectric properties can be made uniform. The primary particles of the hollow particles more preferably satisfy 1≦D _SL ≦1.4, and even more preferably satisfy 1≦D _SL ≦1.3. Here, D _SL is D _75L /D _25L , and D _25L and D _75L represent the values of the 25th and 75th largest diameters, respectively, when 100 randomly selected primary particles are measured for their major axes and arranged in order of size from smallest to largest in observation using a scanning electron microscope.

The primary particles of the hollow particles preferably satisfy 1 ≦ D _ST ≦ 1.5, more preferably 1 ≦ D _ST ≦ 1.4, and even more preferably 1 ≦ D _ST ≦ 1.3, where D _ST is D _75T /D _25T , and D _25T and D _75T represent the 25th and 75th minor axis values, respectively, when 100 randomly selected primary particles are measured and arranged in order of size from smallest to largest in observation with a scanning electron microscope.

The aspect ratio of the hollow particles is preferably less than 2, more preferably 1.9 or less. On the other hand, the aspect ratio of the hollow particles is 1 or more, preferably greater than 1, more preferably 1.1 or more.

The hollow particles may have any suitable shape. Examples of the shape of the hollow particles include ellipsoids, spheres, agglomerates, scales, plates, membranes, cylinders, prisms, flat shapes, go stone shapes, and rice grain shapes. Spherical and go stone shapes are preferred. By adopting such shapes, the above _DSL and _DST can be satisfactorily satisfied.

The major axis of the hollow particles is preferably 0.5 μm or more, and more preferably 1 μm or more. This is because hollow particles with a major axis of 0.5 μm or more can fully satisfy the hollow ratio described below. On the other hand, the major axis of the hollow particles is preferably 10 μm or less, and more preferably 5 μm or less. This is because hollow particles with a major axis of 10 μm or less can significantly contribute to the miniaturization or thinning of components in which they are used.

The minor axis of the hollow particles is preferably 0.25 μm or more, and more preferably 0.5 μm or more. This is because hollow particles with a minor axis of 0.25 μm or more can fully satisfy the hollow ratio described below. On the other hand, the minor axis of the hollow particles is preferably 10 μm or less, and more preferably 5 μm or less. This is because hollow particles with a minor axis of 10 μm or less can significantly contribute to the miniaturization or thinning of components in which they are used.

The shell thickness of the hollow particles is preferably 25 nm or more, more preferably 50 nm or more, and even more preferably 75 nm or more. Such a thickness can effectively prevent the hollow particles from breaking, for example, when preparing a resin composition. On the other hand, the shell thickness of the hollow particles is preferably 500 nm or less, more preferably 350 nm or less, and even more preferably 250 nm or less. Such a thickness can fully satisfy the hollow ratio described below, significantly contributing to improved dielectric properties and weight reduction. Shell thickness can be measured by TEM observation. For example, it can be determined by measuring the shell thickness of randomly selected hollow particles and calculating the average of the measured shell thicknesses. The shell thickness of hollow particles may affect the distribution of Al abundance.

The hollow percentage of the hollow particles is preferably 30% or more, more preferably 40% or more, even more preferably 45% or more, and particularly preferably 50% or more. Such a hollow percentage can significantly contribute to, for example, improved dielectric properties and weight reduction. On the other hand, the hollow percentage of the hollow particles is preferably 95% or less, more preferably 90% or less, even more preferably 85% or less, and particularly preferably 80% or less. Such a hollow percentage can effectively prevent breakage of the hollow particles, for example, when preparing a resin composition. The hollow percentage can be calculated from the major and minor diameters and the shell thickness.

The BET specific surface area of the hollow particles is preferably 30 m ² /g or less, more preferably 20 m ² /g or less, and even more preferably 10 m ² /g or less. On the other hand, the BET specific surface area of the hollow particles is, for example, 0.5 m ² /g or more, and may be 1 m ² /g or more.

The cumulative pore volume of pores in hollow particles with diameters of 1 nm to 100 nm is preferably 0.1 cc/g or less, more preferably 0.08 cc/g or less, and even more preferably 0.06 cc/g or less. Such a pore volume can effectively prevent resin from penetrating into hollow particles in resin compositions, significantly contributing to improved dielectric properties. Meanwhile, the cumulative pore volume of pores in hollow particles with diameters of 1 nm to 100 nm is, for example, 0.01 cc/g or more.

The cumulative pore volume of pores in hollow particles with diameters of 1 to 10 nm is preferably 0.025 cc/g or less, more preferably 0.020 cc/g or less, and even more preferably 0.010 cc/g or less. Such a pore volume can effectively prevent resin from penetrating into hollow particles in resin compositions, significantly contributing to improved dielectric properties. Meanwhile, the cumulative pore volume of pores in hollow particles with diameters of 1 to 10 nm is, for example, 0.001 cc/g or more.

The density of the hollow particles is preferably 0.90 g/cc or less, more preferably 0.80 g/cc or less. On the other hand, the density of the hollow particles is preferably 0.55 g/cc or more, more preferably 0.60 g/cc or more. With such a density, the hollow particles can maintain the strength of the shell and the hollowness.

The dielectric constant of the hollow particles at 25°C and 10 GHz is preferably 2.2 or less, and more preferably 2.0 or less. Meanwhile, the dielectric constant of the hollow particles at 25°C and 10 GHz is, for example, 1.0 or more. The dielectric dissipation factor of the hollow particles at 25°C and 10 GHz is preferably 0.005 or less, and more preferably 0.002 or less. Meanwhile, the dielectric dissipation factor of the hollow particles at 25°C and 10 GHz is, for example, 0.0001 or more.

The breaking strength of the hollow particles is preferably 10 MPa or more, more preferably 12 MPa or more, even more preferably 14 MPa or more, and particularly preferably 16 MPa or more. Such a breaking strength can effectively prevent the hollow particles from breaking, for example, when preparing a resin composition. As a result, the hollow state of the particles is maintained, which can greatly contribute to improving the dielectric properties. On the other hand, the breaking strength of the hollow particles is, for example, 800 MPa or less.

The breaking strength of hollow particles can be measured, for example, with a microcompression tester ("MCT-510" manufactured by Shimadzu Corporation) using a length measurement kit and a side observation kit. Specifically, the breaking strength can be measured by scattering a very small amount of hollow particles on the lower pressure plate and performing a breaking test on each particle under the following measurement conditions.
Test force: 0.980 mN
・Load speed: 0.0223mN/sec
・Upper pressure indenter: flat surface φ20μm

For example, 10 particles are measured, and the breaking strength Cs is calculated from the particle diameter d (mm) of each measured particle and the test force P (N) at the breaking point. The breaking strength Cs is calculated using the following formula from JIS R 1639-5 "Measurement methods for fine ceramic granule properties - Part 5: Single granule crushing strength."
Cs=2.48P/πd ²
The breaking strength of the hollow particles can be determined by calculating the average value of the breaking strength Cs (MPa) of 10 particles.

The epoxy resin oil absorption per 1 g of hollow particles is preferably 2.3 g or less, more preferably 2.0 g or less, and is preferably less than 0.6 g/ ^m² , more preferably 0.46 g/m² or ^less , and even more preferably 0.35 g/m² or ^less .

In one embodiment, the hollow particles are preferably surface-treated with any suitable surface treatment agent. Examples of the surface treatment agent include at least one selected from the group consisting of higher fatty acids, anionic surfactants, cationic surfactants, phosphate esters, coupling agents, esters of polyhydric alcohols and fatty acids, acrylic polymers, and silicone treatment agents.

Any appropriate method can be used to produce the hollow particles. According to one embodiment of the present invention, a method for producing hollow particles includes coating core particles with a shell-forming material to obtain core-shell particles, removing the core particles from the core-shell particles to obtain hollow particle precursors, and coating the hollow particle precursors with a shell-forming material. By coating the hollow particle precursors with a shell-forming material, for example, a strong shell can be formed while suppressing a decrease in hollowness. Specifically, the hollow particle precursors obtained by removing the core particles may have pores and may be brittle. By filling the pores of the hollow particle precursors with the shell-forming material, hollow particles with a strong shell can be obtained without shrinking the hollow particle precursors. The shrinkage of the hollow particle precursors is, for example, softening shrinkage.

The primary particles of the core particles preferably satisfy 1≦D _SL ≦1.5, more preferably 1≦D _SL ≦1.4, and even more preferably 1≦D _SL ≦1.3. The primary particles of the core particles preferably satisfy 1≦D _ST ≦1.5, more _{preferably 1≦D ST ≦1.4, and even more preferably 1≦D ST ≦1.3. D SL and D ST are as described} above.

The aspect ratio of the core particle is preferably less than 2, more preferably 1.9 or less. On the other hand, the aspect ratio of the core particle is 1 or greater, preferably greater than 1, more preferably 1.1 or greater. Examples of the shape of the core particle include ellipsoid, sphere, agglomerate, scale, plate, membrane, cylinder, prism, flat, go stone, and rice grain. Preferably, the core particle is spherical or go stone shaped.

The major axis of the core particle is preferably 0.5 μm or more, more preferably 1 μm or more. On the other hand, the major axis of the core particle is preferably 10 μm or less, more preferably 5 μm or less. On the other hand, the minor axis of the core particle is preferably 0.25 μm or more, more preferably 0.5 μm or more. On the other hand, the minor axis of the core particle is preferably 10 μm or less, more preferably 5 μm or less.

Any appropriate material can be used as the material for forming the core particle. For example, a material containing aluminum can be used to satisfy the above-mentioned distribution of Al abundance in the resulting hollow particle. In one embodiment, the core particle is preferably formed from an alunite-type compound represented by the following general formula (I):
M _a [Al _1-x M' _x ] ₃ (SO ₄ ^2- ) _y (OH) _z・mH ₂ O...(I)
In formula (I), M is at least one cation selected from the group consisting of Na ⁺ , K ⁺ , _NH4 ^+, and _H3O ⁺ . In formula (I), M' is at least one cation selected from the group consisting of Cu2 ⁺ , Zn2 ⁺ , Ni2 ⁺ , Sn4 ⁺ , Zr4 ^+, and Ti4 ⁺ . In formula (I), a satisfies 0.8≦a≦1.35. In formula (I), m satisfies 0≦m≦5. In formula (I), x satisfies 0≦x≦0.4. In formula (I), y satisfies 1.7≦y≦2.5. In formula (I), z satisfies 4≦z≦7.

Examples of the shell-forming material that can be used include sodium silicate (Na ₂ O.nSiO ₂ ) and alkoxysilanes such as tetraethoxysilane (Si(OCH ₂ CH ₃ ) ₄ ). In one embodiment, sodium silicate (water glass) is used. The presence of Na may, for example, facilitate softening during the firing process described below, making it easier to control the firing temperature. The shell-forming material may contain Al. For example, when core particles are coated multiple times using shell-forming materials, the Al concentration in the shell-forming materials used for each treatment can be adjusted to ensure a satisfactory distribution of Al abundance in the resulting hollow particles.

The amount of coating with the shell-forming material can be adjusted by any appropriate method. For example, the coating amount can be adjusted by controlling the pH value when coating with a shell-forming material containing sodium silicate. Specifically, sodium silicate can be stable in high pH ranges. For example, sodium silicate can be stable at a pH of 11 or higher. Therefore, lowering the pH value can condense sodium silicate molecules, allowing silica to efficiently precipitate on the core particles. Here, when the core particles contain the above-mentioned alunite-type compound, the aqueous slurry of the alunite-type compound itself can exhibit acidity. The acidity of the aqueous slurry of the alunite-type compound itself is, for example, pH 3 to 5. Because the aqueous slurry of the alunite-type compound itself is acidic, silica can be efficiently precipitated on the core particles without, for example, using a pH adjuster to lower the pH value. When core particles containing an alunite-type compound are coated multiple times with a shell-forming material containing sodium silicate, a pH adjuster may be used in one or more of the multiple coating times. In this case, an acidic solution such as hydrochloric acid or sulfuric acid can be used as the pH adjuster. By using an acidic solution, hollow particles with a low Na content can be obtained.

In addition, heating when coating with the shell-forming material can also promote coating with the shell-forming material. Specifically, this can promote shell precipitation and increase the shell formation rate. The heating temperature when coating with the shell-forming material is, for example, 80°C to 90°C. The amount of coating can also be adjusted by controlling the concentration and amount of the shell-forming material when coating. When coating core particles with a shell-forming material containing sodium silicate, the concentration of sodium silicate in the shell-forming material is, for example, 0.1 mol/L to 2 mol/L.

The core particles are typically removed by dissolving them in an acidic solution. Examples of the acidic solution include hydrochloric acid, sulfuric acid, and nitric acid. The dissolution temperature is, for example, 30°C to 90°C, and preferably 50°C to 80°C. Such temperatures allow for efficient dissolution of the core particles while suppressing problems such as the shell becoming more fragile. In one embodiment, sulfuric acid is used as the acidic solution, for example, from the perspective of reusing the substance obtained by reaction with the core particles. Examples of substances obtained by reaction with the core particles include salts. The sulfuric acid concentration is, for example, 0.1 mol/L to 3.5 mol/L, and preferably 0.5 mol/L or higher. When dissolving core particles containing alunite-type compounds, the Al content can be favorably achieved by adjusting, for example, the acidic solution, the dissolution temperature, and the dissolution time.

The method for producing hollow particles may include calcining the core-shell particles before removing the core particles. When the core particles contain the alunite-type compound, calcination is preferably performed before removing the core particles. Calcination is preferably performed, for example, in an air atmosphere. This is because the alunite-type compound can be acid-resistant, and calcination changes the alunite-type compound, making the calcined core particles more soluble in acidic solutions. Specifically, for core particles containing an alunite-type compound, portions with low agglomeration density are more soluble in acidic solutions, but portions with high agglomeration density are less soluble in acidic solutions, with the amount of dissolution in acidic solutions remaining at, for example, about 30% by weight. Calcination produces aluminum oxide (Al ₂ O ₃ ), which is more soluble in acidic solutions, from the alunite-type compound, thereby improving the solubility of the core particles in acidic solutions.

The firing temperature for the core-shell particles is, for example, 300°C to 900°C, preferably 300°C to 650°C. Such a firing temperature can suppress crystallization of the shell and produce the aluminum oxide. The firing time is, for example, 0.5 hours to 20 hours. Firing may be performed continuously or in multiple stages at different temperatures. When firing in multiple stages, the firing time is the total firing time for each stage. In one embodiment, the firing temperature for the core-shell particles may be adjusted to satisfy the distribution of Al abundance in the resulting hollow particles.

The shell-forming material used to coat the hollow particle precursor is preferably selected to correspond to the shell-forming material used to coat the core particles. Specifically, when sodium silicate is used as the shell-forming material used to coat the core particles, sodium silicate is preferably used as the shell-forming material used to coat the hollow particle precursor.

When coating hollow particle precursors with a shell-forming material containing sodium silicate, it is preferable to adjust the coating amount by controlling the pH value. Specifically, sodium silicate can be stable in a high pH range. A high pH range is, for example, a pH of 11 or higher. For sodium silicate, it is preferable to lower the pH value using a pH adjuster, for example, to condense the sodium silicate molecules and efficiently precipitate silica on the hollow particle precursor. Examples of pH adjusters that can be used include acidic solutions such as hydrochloric acid and sulfuric acid.

The hollow particle manufacturing method may include calcining a hollow particle precursor coated with a shell-forming material. Specifically, the calcination of the hollow particle precursor may be performed after coating the hollow particle precursor with the shell-forming material. Calcining the hollow particle precursor can produce hollow particles that significantly contribute to improving dielectric properties. Specifically, the hydrophobicity of the hollow particle precursor surface can be improved. More specifically, silanol groups on the hollow particle precursor surface can be converted to siloxanes. Furthermore, hollow particles that are easy to incorporate or disperse into resins when preparing resin compositions can be obtained. Specifically, appropriate shrinkage through calcination can prevent fusion between particles while filling the pores on the hollow particle precursor surface, resulting in hollow particles with a smooth surface. Obtaining hollow particles with a smooth surface can, for example, favorably achieve the above-mentioned BET specific surface area and pore volume. In one embodiment, the calcination temperature of the hollow particle precursor may be adjusted to satisfy the above-mentioned distribution of Al abundance in the resulting hollow particles.

The calcination temperature for the hollow particle precursor is, for example, 300°C to 1300°C, preferably 700°C to 1300°C, and more preferably 900°C to 1300°C. Such a calcination temperature can effectively achieve the hydrophobicity described above. Furthermore, calcination at such a temperature can achieve a high hollowness ratio. Specifically, calcining the hollow particle precursor in a state in which the pores are filled with a coating of shell-forming material can effectively achieve hydrophobicity while suppressing shrinkage of the hollow particle precursor. The calcination time for the hollow particle precursor is, for example, 0.1 to 10 hours. Calcination may be performed continuously or in multiple stages at different temperatures. For example, calcination may be performed at a temperature T1 below 950°C, followed by calcination at a temperature T2 of 950°C or higher. In this case, the difference between temperatures T1 and T2 (T2 - T1) is preferably 150°C or higher, more preferably 250°C or higher. When firing is performed in multiple stages, the firing time is the total of the firing times for each stage.

The method for producing the hollow particles may include subjecting the hollow particle precursor to an acid treatment before calcining the hollow particle precursor. The acid treatment is preferably carried out using an acidic solution, typically hydrochloric acid, sulfuric acid, or the like. When sodium silicate is used as the shell-forming material, the hollow particle precursor may contain Na. By removing Na through the acid treatment, the calcination temperature of the hollow particle precursor can be set high, resulting in hollow particles that can significantly improve dielectric properties.

In the method for producing hollow particles, the step of obtaining the core-shell particles preferably includes a drying process. Specifically, when the core particles are coated with the shell-forming material multiple times, it is preferable to dry the core particles coated with the shell-forming material and then coat them again with the shell-forming material. The drying temperature in the step of obtaining core-shell particles is, for example, 90°C to 120°C. The drying time in the step of obtaining core-shell particles is, for example, 3 hours to 24 hours. By including such a drying process, the above oil absorption can be achieved satisfactorily, for example, regardless of whether a pH adjuster is used when coating with the shell-forming material.

In one embodiment of the present invention, the hollow particles are used as a functional agent for a resin material. Below, we will explain a resin composition containing the hollow particles.

B. Resin Composition In one embodiment of the present invention, the resin composition includes a resin and the hollow particles. In the resin composition, the hollow particles can be well maintained in their hollow state. Furthermore, in a resin molded product molded using the resin composition, the hollow particles can be well maintained in their hollow state.

Any appropriate resin may be selected as the resin, depending on, for example, the intended use of the resulting resin composition. For example, the resin may be a thermoplastic resin or a thermosetting resin. Specific examples of resins include epoxy resins, polyimide resins, polyamide resins, polyamideimide resins, polyether ether ketone resins, polyester resins, polyhydroxy polyether resins, polyolefin resins, fluororesins, liquid crystal polymers, and modified polyimides. These may be used alone or in combination of two or more.

The content of the hollow particles in the resin composition is preferably 0.1% by weight or more, and more preferably 0.5% by weight or more. On the other hand, the content is preferably 90% by weight or less, and more preferably 85% by weight or less.

The resin composition preferably contains 0.5 parts by weight or more of hollow particles per 100 parts by weight of resin, and more preferably 1 part by weight or more. On the other hand, the resin composition preferably contains 300 parts by weight or less of hollow particles per 100 parts by weight of resin, and more preferably 200 parts by weight or less.

The volume ratio of hollow particles in the resin composition is preferably 0.1% or more, and more preferably 0.5% or more. On the other hand, the volume ratio of hollow particles in the resin composition is preferably 70% or less, and more preferably 60% or less. This is because, for example, this can result in excellent processability when producing the resin composition.

The hollow ratio of the hollow particles in the resin composition is preferably 30% or more, more preferably 40% or more, even more preferably 45% or more, and particularly preferably 50% or more.

The resin composition may contain optional components. Examples of optional components include curing agents, stress reducing agents, colorants, adhesion improvers, release agents, flow control agents, defoaming agents, solvents, and fillers. These may be used alone or in combination of two or more. In one embodiment, the resin composition contains a curing agent. The content of the curing agent is, for example, 1 to 150 parts by weight per 100 parts by weight of the resin.

Any appropriate method can be used to prepare the resin composition. Specifically, the resin composition is obtained by dispersing the hollow particles in the resin using any appropriate dispersion method. Examples of dispersion methods include dispersion using various agitators such as a homomixer, disperser, or ball mill, dispersion using a planetary mixer, dispersion using shear force using a three-roll mill, and dispersion using ultrasonic treatment.

The above resin composition is typically formed into a resin molded article in a desired shape. For example, it is formed into a resin molded article in a desired shape using a mold. When forming the resin molded article, the resin composition may be subjected to any appropriate treatment. For example, the resin composition may be subjected to a curing treatment.

In one embodiment of the present invention, the resin composition is used as a resin layer included in a laminate. Below, we will explain a laminate having a resin layer formed from the resin composition.

C. Laminate FIG. 4 is a schematic cross-sectional view of a laminate according to one embodiment of the present invention. The laminate 10 includes a resin layer 11 and a metal foil 12. The resin layer 11 is formed from the resin composition. Specifically, the resin layer 11 includes the resin and the hollow particles. Although not shown, the laminate 10 may include other layers. Examples of such layers include a substrate. The substrate may be laminated on one side of the resin layer 11. Specifically, the substrate may be disposed on the side of the resin layer 11 where the metal foil 12 is not disposed. A resin film is typically used as the substrate. Unlike the illustrated example, the laminate 10 may have a configuration in which the metal foil 12 is disposed on the surface of a laminate structure including multiple resin layers 11. The laminate 10 may also have a configuration in which the metal foil 12 is disposed on each side of a laminate structure including multiple resin layers 11. The laminate 10 may also have a configuration in which multiple combinations of resin layers 11 and metal foils 12 are stacked. The laminate 10 is typically used as a wired circuit board. The metal foil 12 is formed into a predetermined shape so as to form, for example, a predetermined circuit wiring.

The thickness of the resin layer is, for example, 5 μm or more, preferably 10 μm or more. On the other hand, the thickness of the resin layer is, for example, 100 μm or less, preferably 50 μm or less, and more preferably 25 μm or less. Such a thickness can adequately accommodate, for example, the recent trend toward miniaturization of electronic components.

Any suitable metal can be used to form the metal foil. Examples include copper, aluminum, nickel, chromium, and gold. These can be used alone or in combination of two or more. The thickness of the metal foil is, for example, 2 μm to 35 μm.

Any appropriate method can be used to prepare the laminate. For example, the resin composition is applied to the substrate to form a coating layer, and the metal foil is then laminated on the coating layer to obtain a laminate. In another specific example, the resin composition is applied to the metal foil to form a coating layer to obtain a laminate. Typically, the coating layer is cured by applying energy such as heating or light irradiation at any appropriate time. When applying the resin composition, it may be dissolved in any appropriate solvent.

The present invention will be specifically described below using examples, but the present invention is not limited to these examples. The measurement methods for each property are as follows unless otherwise specified. Furthermore, "%" and "ppm" are by weight unless otherwise specified.
1. Particle Longitudinal Diameter The long diameter of the particles was calculated by FE-SEM observation. Specifically, the long diameter of 100 primary particles randomly selected from the SEM photograph of the particles was measured, and the arithmetic mean (average long diameter) of the obtained measurements was calculated. The magnification of the SEM observation was 10,000 times.
2. Minor diameter of particles The minor diameter of particles was calculated by FE-SEM observation. Specifically, the minor diameter of 100 primary particles randomly selected from the SEM photograph of the particles was measured, and the arithmetic mean (average minor diameter) of the obtained measurements was calculated. The magnification of the SEM observation was 10,000 times.
3. _DSL and _DST
DSL and _DST were calculated by FE-SEM observation. Specifically, the major axis of each of 100 primary particles randomly selected from the SEM photograph of the particles was measured, and the 75th value ( _D75L ) was divided by the 25th value ( _D25L ) to calculate _DSL . Furthermore, the minor axis of each of 100 primary particles randomly selected from the SEM photograph of the particles was measured, and the 75th value ( _D75T ) was divided by the 25th value ( _D25T ) to calculate _DST .
4. Aspect Ratio The aspect ratio was calculated by FE-SEM observation. Specifically, the aspect ratio was calculated by dividing the average major axis of the particles by the average minor axis of the particles.

[Example 1]
(Preparation of hollow silica particles)
800 g of elliptical alunite particle powder (NaAl ₃ (SO ₄ ) ₂ (OH) ₆ , major axis: 1.98 μm, D _SL : 1.13, minor axis: 1.35 μm, D _ST : 1.18, aspect ratio: 1.47) was suspended in 5 L of ion-exchanged water to obtain an alunite particle slurry.

The resulting alunite particle slurry was then heated to 90°C with stirring, and 816 ml of 0.55 mol/ _L No. 3 water glass ( _{Na2O.2.97SiO2} , Fujifilm Wako Pure Chemical Industries, Ltd.) was added thereto over 4 hours. The resulting slurry was aged for 1 hour, then dehydrated and washed with water to obtain a cake of first core-shell particle precursor. The cake of first core-shell particle precursor was then dried at 105°C for 1 day to obtain a powder of core-shell particles.

Next, the powder of the first core-shell particle precursor was suspended in 5 L of ion-exchanged water and heated to 90°C while stirring. 136 ml of 0.55 mol/L No. 3 water glass was then added over 10 minutes. Subsequently, 680 ml of 0.55 mol/L No. 3 water glass and 757 ml of 0.50 mol/L sulfuric acid were added simultaneously. The No. 3 water glass was added over 50 minutes, and the sulfuric acid over 60 minutes. The resulting slurry was aged for 30 minutes, then dehydrated and washed with water to obtain a cake of the second core-shell particle precursor.

The resulting cake of second core-shell particle precursor was suspended in 5 L of ion-exchanged water and heated to 90°C while stirring. 136 ml of 0.55 mol/L No. 3 water glass was then added over 10 minutes. Then, 680 ml of 0.55 mol/L No. 3 water glass and 757 ml of 0.50 mol/L sulfuric acid were added simultaneously. The No. 3 water glass was added over 50 minutes, and the sulfuric acid over 60 minutes. The resulting slurry was aged for 30 minutes, then dehydrated and washed with water, and then dried at 105°C for one day to obtain a powder of core-shell particles.

The resulting powder of core-shell particles was then fired at 500° C. for 3 hours and then at 550° C. for 1 hour. It is believed that the firing caused the alunite particles to change as follows.
_NaAl3 ( _SO4 ) ₂ (OH) ₆ → _NaAl ( _SO4 ) ₂ + _Al2O3 + _3H2O

Next, 6 L of ion-exchanged water was added to 737 g of the calcined core-shell particles and resuspended under stirring at room temperature. 3.95 L of 2 mol/L sulfuric acid was added to this, heated to 90°C, and allowed to react for 5 hours to dissolve the core particles, yielding a slurry of the first hollow silica precursor. The first hollow silica precursor is also a hollow particle precursor. The resulting slurry of the first hollow silica precursor was dehydrated and washed with water to yield a cake of the first hollow silica precursor.

Next, the resulting first hollow silica precursor cake (88 g of powder) was suspended in 4 L of ion-exchanged water and heated to 90°C with stirring. 40 ml of 0.55 mol/L No. 3 water glass was added over 10 minutes, followed by the simultaneous addition of 200 ml of 0.55 mol/L No. 3 water glass and 210 ml of 0.5 mol/L sulfuric acid. The No. 3 water glass was added over 50 minutes, and the sulfuric acid was added over 60 minutes. The resulting slurry was aged for 30 minutes, then dehydrated and washed with water to obtain a second hollow silica precursor cake.
The resulting second hollow silica precursor cake was then suspended in 4 L of ion-exchanged water and heated to 90°C with stirring. 40 ml of 0.55 mol/L No. 3 water glass was added over 10 minutes, followed by the simultaneous addition of 200 ml of 0.55 mol/L No. 3 water glass and 210 ml of 0.50 mol/L sulfuric acid. The No. 3 water glass was added over 50 minutes, and the sulfuric acid was added over 60 minutes. The resulting slurry was aged for 30 minutes, then dehydrated and washed with water to obtain a third hollow silica precursor cake. The resulting third hollow silica precursor cake was then suspended in 4 L of ion-exchanged water and heated to 90°C with stirring. 40 ml of 0.55 mol/L No. 3 water glass was added over 10 minutes, followed by the simultaneous addition of 200 ml of 0.55 mol/L No. 3 water glass and 210 ml of 0.50 mol/L sulfuric acid. The No. 3 water glass was added over 50 minutes, and the sulfuric acid was added over 60 minutes. The slurry thus obtained was aged for 30 minutes, and then dehydrated and washed with water to obtain a cake of the fourth hollow silica precursor.

Next, 2 L of ion-exchanged water and 1,083 ml of 2 mol/L sulfuric acid were added to the resulting cake of fourth hollow silica precursor (282 g of powder) and left at 90°C for 1.5 hours to obtain a slurry of fifth hollow silica precursor. The cake was then dehydrated and washed with water, and dried at 105°C for 1 day to obtain a powder of fifth hollow silica precursor.

The resulting fifth hollow silica precursor powder was then calcined in an electric furnace at 800°C for 1 hour and at 1100°C for 1 hour to obtain hollow silica particles. Specifically, the fifth hollow silica precursor powder was calcined in an air atmosphere to obtain hollow particles.

(Preparation of Resin Composition)
95.2 phr of bisphenol F epoxy resin ("JER806" manufactured by Mitsubishi Chemical Corporation) and 40 phr of the obtained hollow silica particles were mixed for 3 minutes under atmospheric pressure at a rotation speed of 1000 rpm and a revolution speed of 2000 rpm using a planetary centrifugal mixer ("ARV-310P" manufactured by Thinky Corporation) to obtain a mixture. Hereinafter, the hollow silica particles may be referred to as a "filler."
The resulting mixture and 4.8 phr of an imidazole-based epoxy resin curing agent ("2E4MZ" manufactured by Shikoku Chemicals Co., Ltd.) were mixed for 3 minutes using a planetary centrifugal mixer under conditions of a rotation of 1000 rpm, a revolution of 2000 rpm, and a pressure of 0.7 kPa to obtain a resin composition.

(Production of resin molded body)
The obtained resin composition was poured into a 1.0 mm thick Teflon mold and press-molded at 80°C for 1 hour. After cooling, the molded product was removed from the mold and placed in a dryer at 150°C for 4 hours to further heat-cure. The molded product was then cooled to obtain an evaluation sample.

[Example 2]
(Preparation of hollow silica particles)
200 g of elliptical alunite particle powder (NaAl ₃ (SO ₄ ) ₂ (OH) ₆ , major axis: 1.98 μm, D _SL : 1.13, minor axis: 1.35 μm, D _ST : 1.18, aspect ratio: 1.47) was suspended in 1.4 L of ion-exchanged water to obtain an alunite particle slurry.

The resulting alunite particle slurry was then heated to 90°C with stirring, and 202 ml of 0.56 mol/ _L No. 3 water glass ( _{Na2O.2.97SiO2} , Fujifilm Wako Pure Chemical Industries, Ltd.) was added thereto over 4 hours. The resulting slurry was aged for 1 hour, then dehydrated and washed with water to obtain a cake of first core-shell particle precursor. The cake of first core-shell particle precursor was then dried at 105°C for 1 day to obtain a powder of core-shell particles.

Next, the powder of the first core-shell particle precursor was suspended in 1.4 L of ion-exchanged water and heated to 90°C while stirring. 34 ml of 0.56 mol/L No. 3 water glass was then added over 10 minutes. Subsequently, 168 ml of 0.56 mol/L No. 3 water glass and 188 ml of 0.51 mol/L sulfuric acid were added simultaneously. The No. 3 water glass was added over 50 minutes, and the sulfuric acid over 60 minutes. The resulting slurry was aged for 30 minutes, then dehydrated and washed with water to obtain a cake of the second core-shell particle precursor.

The resulting cake of second core-shell particle precursor was suspended in 1.4 L of ion-exchanged water and heated to 90°C while stirring. 34 ml of 0.56 mol/L No. 3 water glass was then added over 10 minutes. Then, 168 ml of 0.56 mol/L No. 3 water glass and 188 ml of 0.51 mol/L sulfuric acid were added simultaneously. The No. 3 water glass was added over 50 minutes, and the sulfuric acid over 60 minutes. The resulting slurry was aged for 30 minutes, then dehydrated and washed with water, and then dried at 105°C for one day to obtain a powder of core-shell particles.

The resulting powder of core-shell particles was then fired at 500° C. for 3 hours and then at 550° C. for 1 hour. It is believed that the firing caused the alunite particles to change as follows.
_NaAl3 ( _SO4 ) ₂ (OH) ₆ → _NaAl ( _SO4 ) ₂ + _Al2O3 + _3H2O

Next, 1305 ml of ion-exchanged water was added to 177 g of the calcined core-shell particles and resuspended under stirring at room temperature. 555 ml of 3.35 mol/L sulfuric acid was added to this, heated to 90°C, and allowed to react for 5 hours to dissolve the core particles, yielding a slurry of the first hollow silica precursor. The resulting slurry of the first hollow silica precursor was dehydrated and washed with water to yield a cake of the first hollow silica precursor.

Next, the resulting first hollow silica precursor cake (18.8 g of powder) was suspended in 1.36 L of ion-exchanged water and heated to 90°C with stirring. To this was added 20 ml of 0.56 mol/L No. 3 water glass over 10 minutes, followed by simultaneous addition of 100 ml of 0.56 mol/L No. 3 water glass and 105 ml of 0.51 mol/L sulfuric acid. The No. 3 water glass was added over 50 minutes, and the sulfuric acid was added over 60 minutes. The resulting slurry was aged for 30 minutes, then dehydrated and washed with water to obtain a second hollow silica precursor cake.
The resulting second hollow silica precursor cake was then suspended in 1.36 L of ion-exchanged water and heated to 90°C with stirring. 20 ml of 0.56 mol/L No. 3 water glass was added over 10 minutes, followed by the simultaneous addition of 100 ml of 0.56 mol/L No. 3 water glass and 105 ml of 0.51 mol/L sulfuric acid. The No. 3 water glass was added over 50 minutes, and the sulfuric acid was added over 60 minutes. The resulting slurry was aged for 30 minutes, then dehydrated and washed with water to obtain a third hollow silica precursor cake. The resulting third hollow silica precursor cake was then suspended in 1.36 L of ion-exchanged water and heated to 90°C with stirring. 20 ml of 0.56 mol/L No. 3 water glass was added over 10 minutes, followed by the simultaneous addition of 100 ml of 0.56 mol/L No. 3 water glass and 105 ml of 0.51 mol/L sulfuric acid. The No. 3 water glass was added over 50 minutes, and the sulfuric acid was added over 60 minutes. The resulting slurry was aged for 30 minutes, then dehydrated and washed with water to obtain a fourth hollow silica precursor cake.

Next, 1.36 L of ion-exchanged water and 146 ml of 3.35 mol/L sulfuric acid were added to the resulting cake of fourth hollow silica precursor (52.8 g of powder) and left at 90°C for 1.5 hours to obtain a slurry of fifth hollow silica precursor. The cake was then dehydrated and washed with water, and dried at 105°C for 1 day to obtain a powder of fifth hollow silica precursor.

The resulting fifth hollow silica precursor powder was then calcined in an electric furnace (in an air atmosphere) at 800°C for 1 hour and at 1100°C for 1 hour to obtain hollow silica particles.

(Production of resin molded body)
A molded body (evaluation sample) was obtained in the same manner as in Example 1, except that the above hollow silica particles were used.

[Example 3]
(Preparation of hollow silica particles)
1,500 g of elliptical alunite particle powder (NaAl ₃ (SO ₄ ) ₂ (OH) ₆ , major axis: 1.98 μm, D _SL : 1.13, minor axis: 1.35 μm, D _ST : 1.18, aspect ratio: 1.47) was suspended in 7.5 L of ion-exchanged water to obtain an alunite particle slurry.

The resulting alunite particle slurry was then heated to 90°C with stirring, and 1507 ml of 0.54 mol/L No. 3 water glass (Na ₂ O·3.14SiO ₂ , manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added thereto over 4 hours. The resulting slurry was aged for 1 hour, then dehydrated and washed with water to obtain a cake of first core-shell particle precursors.

The resulting cake of first core-shell particle precursor was then suspended in 7.5 L of ion-exchanged water and heated to 90°C with stirring. 1507 ml of 0.54 mol/L No. 3 water glass was added over 2 hours. The resulting slurry was aged for 1 hour, left to stand with stirring for 15 hours, then dehydrated and washed with water to obtain a cake of second core-shell particle precursor. The resulting cake of second core-shell particle precursor was then suspended in 7.5 L of ion-exchanged water and heated to 90°C with stirring. 1507 ml of 0.54 mol/L No. 3 water glass was added over 2 hours. The resulting slurry was aged for 1 hour, left to stand with stirring for 15 hours, then dehydrated and washed with water, and then dried at 100°C for 1 day to obtain a powder of core-shell particles.

The resulting powder of core-shell particles was then fired at 500° C. for 3 hours and then at 550° C. for 1 hour. It is believed that the firing caused the alunite particles to change as follows.
_NaAl3 ( _SO4 ) ₂ (OH) ₆ → _NaAl ( _SO4 ) ₂ + _Al2O3 + _3H2O

Next, 6 L of ion-exchanged water was added to 864 g of the calcined core-shell particles and resuspended under stirring at room temperature. 7.8 L of 1 mol/L sulfuric acid was added to this, heated to 90°C, and allowed to react for 5 hours to dissolve the core particles, yielding a slurry of the first hollow silica precursor. The resulting slurry of the first hollow silica precursor was dehydrated and washed with water to yield a cake of the first hollow silica precursor.

Next, the resulting first hollow silica precursor cake (130 g of powder) was suspended in 860 ml of ion-exchanged water and heated to 90°C while stirring. 33 ml of 0.54 mol/L No. 3 water glass was added over 10 minutes, followed by the simultaneous addition of 163 ml of 0.54 mol/L No. 3 water glass and 186 ml of 0.50 mol/L sulfuric acid. The No. 3 water glass was added over 50 minutes, and the sulfuric acid over 60 minutes. The resulting slurry was aged for 30 minutes, then dehydrated and washed with water to obtain a hollow silica precursor cake. This same procedure was repeated two more times to obtain a second hollow silica precursor cake.

Next, 2 L of ion-exchanged water and 400 ml of 2.87 mol/L sulfuric acid were added to the resulting cake of second hollow silica precursor (284 g of powder) and left at 90°C for 1.5 hours to obtain a slurry of third hollow silica precursor. The cake was then dehydrated and washed with water, and dried at 100°C for 1 day to obtain a powder of third hollow silica precursor.

The resulting third hollow silica precursor powder was then calcined in an electric furnace (in an air atmosphere) at 800°C for 1 hour and at 1100°C for 1 hour to obtain hollow silica particles.

(Production of resin molded body)
A molded body (evaluation sample) was obtained in the same manner as in Example 1, except that the hollow silica particles were used and the blending amount of the hollow silica particles was 25 phr. An attempt was made to blend the hollow silica particles in the same amount as in Example 1, but it was not possible to blend any more.

[Comparative Example 1]
A molded body (evaluation sample) was obtained in the same manner as in Example 1, except that hollow silica particles were not blended.

<XRD measurement>
The hollow silica particles of Examples 1, 2 and 3 were analyzed by X-ray diffraction ("EMPYRIAN" manufactured by PANalytical) and found to be amorphous silica.

<TEM-EDS Measurement>
For the hollow silica particles of Examples 1, 2, and 3, images were captured using a transmission electron microscope ("JEM-2100PLUS" manufactured by JEOL Ltd.) and an attached energy dispersive X-ray analyzer (JED2300 series manufactured by JEOL Ltd.) under conditions of an acceleration voltage of 200 kV and a magnification of 200,000 times, and composition analysis was performed at three points in the thickness direction in point analysis mode.
The observation results for the hollow silica particles of Example 1 are shown in Figure 5A, those for the hollow silica particles of Example 2 are shown in Figure 5B, and those for the hollow silica particles of Example 3 are shown in Figure 5C. The atomic ratios of oxygen (O), aluminum (Al), and silicon (Si) calculated from the peak intensities are also shown. In all cases, a tendency for the amount of Al to decrease from the inside to the outside of the shell was confirmed.

The hollow silica particles of Examples 1, 2 and 3 were also evaluated as follows. The evaluation results are summarized in Table 1.
1. Al Content The Al content was measured by preparing an Al content measurement sample from hollow silica particles through pretreatment and analyzing it using inductively coupled plasma atomic emission spectroscopy (ICP-AES). Specifically, 250 mg of hollow silica particles were weighed into a PMP resin beaker. The hollow silica particles were then soaked in ultrapure water, and 3 ml of nitric acid and 5 ml of hydrofluoric acid were added and heated to dissolve. The mixture was then concentrated to 5 ml, and a 1% boric acid solution was added to mask the remaining hydrofluoric acid. The mixture was then inactivated so as not to damage the analytical equipment, and heated for 10 minutes. After cooling, the mixture was diluted to 50 ml with ultrapure water to prepare an Al content measurement sample, which was then subjected to analysis.
From the obtained results, the Al content was calculated.
2. Na Content The Na content was measured by preparing a sample for measuring the Na content from hollow silica particles through pretreatment and analyzing it by atomic absorption spectrometry (AAS). Specifically, 250 mg of hollow silica particles were weighed into a PMP resin beaker. The hollow silica particles were then soaked in ultrapure water, and 3 ml of nitric acid and 5 ml of hydrofluoric acid were added and heated to dissolve. The solution was then concentrated to 5 ml, and a 1% boric acid solution was added to mask the remaining hydrofluoric acid. The solution was then inactivated without damaging the analytical equipment and heated for 10 minutes. After cooling, the solution was diluted to 50 ml with ultrapure water to prepare a sample for measuring the Na content, which was then subjected to analysis.
From the obtained results, the Na content was calculated.
3. Shell Thickness of Hollow Particles The shell thickness of the particles was calculated by TEM observation. Specifically, the shell thickness of 10 primary particles randomly selected from the TEM photograph of the particles was measured, and the arithmetic mean (average thickness) of the obtained measurements was calculated. The magnifications for TEM observation were 10,000x and 100,000x.
4. Hollow ratio The hollow ratio was calculated from the major and minor diameters and the shell thickness. Specifically, the primary particle diameter (major and minor diameters) and the shell thickness were used to approximate the particle shape with the volume of a cylinder, and the hollow ratio was calculated using the following formula:
Hollowness = hollow volume of hollow particle ÷ volume of hollow particle × 100
Volume of hollow particle = π × radius ² × height = π × (major axis ÷ 2) ² × minor axis Volume of hollow space of hollow particle = π × ((major axis - shell thickness × 2) ÷ 2) ² × (minor axis - shell thickness × 2)
5. BET Specific Surface Area The specific surface area was measured using a "BELsorp-mini" manufactured by Microtrac BEL Co., Ltd. Specifically, the specific surface area was measured by a constant volume gas adsorption method using nitrogen gas, and the specific surface area was determined by analysis using the BET multipoint method.
6. Pore Volume The pore volume was measured using a "BELsorp-max" manufactured by Microtrac-Bell Co., Ltd. Specifically, the measurement was carried out by a constant volume gas adsorption method using nitrogen gas, and the pore volume (the cumulative pore volume of pores with diameters of 1 nm to 100 nm and the cumulative pore volume of pores with diameters of 1 nm to 10 nm) was determined by analysis using the BJH method.
7. Density The density was determined by the nitrogen gas substitution method. Specifically, a gas substitution density measurement device (Micromeritics, dry automatic density meter Accupic II 1340) was used, and 0.15 to 0.2 g of sample was placed in a ¹ cm3 cell and measured using nitrogen gas. The measurement was repeated five or more times, and the average value was calculated.
8. Dielectric constant and dielectric loss tangent The dielectric constant and dielectric loss tangent were measured by a cavity resonance method using a powder dielectric constant measuring device. The measurement was performed under the conditions of a temperature of 25°C, a humidity of 44% RH, and a frequency of 10 GHz. The density required for calculating the dielectric properties was determined by the nitrogen gas substitution method.
9. Oil Absorption Epoxy resin ("JER819" manufactured by Mitsubishi Chemical Corporation) was added dropwise to 1 g of hollow silica particles to be measured and mixed, and the amount of epoxy resin added when the particles turned into a paste was used to calculate the oil absorption using the following conversion formula. Specifically, the epoxy resin was repeatedly added dropwise to the hollow silica particles and mixed, and the point at which the hollow silica particles turned into a paste was designated the end point. The epoxy resin was added dropwise near the end point while being blended in. The amount of epoxy resin per 1 g of the sample was then calculated using the following formula, and the resulting oil absorption per 1 g was divided by the specific surface area of the sample to calculate the oil absorption per unit surface area ( ^m2 ).
Oil absorption per 1 g = epoxy resin titration amount (g) / mass of powder to be measured (g)
Oil absorption per unit surface area (m ² )=oil absorption per 1 g (g/g)/specific surface area of powder to be measured (m ² /g)

<Evaluation of resin molded product>
The resin molded bodies (evaluation samples) of the examples and comparative examples were subjected to density measurement, cross-sectional observation, and dielectric property measurement. The porosity of the resin molded body, the filler density in the resin molded body, and the filler hollowness in the resin molded body were also calculated. The evaluation results are summarized in Table 1. The results of observing the resin molded body of Example 1 at 2000x magnification are shown in Figure 6.

(Measurement of density)
The density of the resin molded body was measured using an electronic density meter ("SD120L" manufactured by Alpha Mirage Co., Ltd.) Specifically, the obtained resin molded body was cut into a size of 5 cm x 6 cm using an ultrasonic cutter, and the molded body sample obtained by cutting was subjected to measurement.

1. Porosity of Resin Molded Body The porosity of the resin molded body (proportion of the volume of air in the resin molded body) Va % was calculated from the results of the density measurement.
2. Filler density in resin molded body The filler density in the resin molded body was calculated using the following formula. The filler density in the resin molded body was calculated by subtracting the volume of the resin component from the volume of the filler-containing resin molded body to calculate the filler volume, and then dividing the amount of filler added by the calculated filler volume. The volume of the filler-containing resin molded body was measured using an electronic density meter ("SD120L" manufactured by Alpha Mirage Co., Ltd.). The volume of the resin component was calculated from the weight and density of a blank resin molded body separately prepared without containing filler.
Filler density in resin molded body = wf/(Vc-mr/ρr) = wf/(Vc-100/ρr)
wf: weight part of particles (filler) when the weight of resin is 100 parts by weight (phr) Vc: volume of resin molded body mr: weight of resin (g)
ρr: Density of resin (g/ml)
3. Filler Hollow Ratio in Resin Molded Product The filler hollow ratio Va/(Va+Vs) in the resin molded product was calculated from the void ratio Va% of the resin molded product. Here, Vs can be calculated using the following formula:
Vc: volume of resin molded body Vs: volume of particle shell Va: volume of air in resin molded body Vr: volume of resin mc: weight of resin molded body (g)
ρc: Density of resin molded body (g/ml)
ms: weight of particle (shell) (g)
ρs: particle shell density (2.28 (g/ml))
mr: resin weight (g)
ρr: Density of resin (g/ml)
wf: parts by weight (phr) of particles (filler) when the weight of the resin is 100 parts by weight (phr)

(Cross-section observation of resin molded body)
The obtained molded body sample was cut using a cross-section polisher ("IB-09010CP" manufactured by JEOL Ltd.), and the cross section was observed using an FE-SEM ("JSM-7600F" manufactured by JEOL Ltd., magnification: 2000x, 5000x, or 10000x).

(Measurement of dielectric constant and dielectric loss tangent)
The dielectric constant and dielectric loss tangent of the obtained molded sample were measured under the following conditions.
Measurement method: Compliant with IEC 62810 (cavity resonator perturbation method)
- Sample shape: length 55 mm or more, width 1.6 to 2.4 mm, height 0.7 to 1.0 mm
Test conditions: Frequency 10 GHz
Measurements: 2 times Conditioning: 23°C ± 1°C, 50% RH ± 5% RH, 24 hours
- Test room environment: 23°C ± 1°C, 50% RH ± 5% RH
Measurement equipment: PNA network analyzer N5222B (manufactured by Keysight Technologies, Inc.)
Cavity resonator: CP531 for 10 GHz (manufactured by Kanto Electronics Application Development Co., Ltd.)

In each example, as shown in Figure 6, cross-sectional observation of the resin molded body confirmed that no resin had penetrated the hollow regions, and destruction of hollow particles was suppressed. In Examples 1 and 2, which had low oil absorption, it was possible to incorporate more filler into the resin, and it was found that the dielectric properties of the resin composition (resin molded body) were superior.

The hollow particles of the present invention can be suitably used, typically, in electronic materials. They can also be used, for example, in thermal insulation materials, soundproofing materials, shock-absorbing materials, stress-absorbing materials, optical materials, and lightweight materials.

L Long axis T Short axis 2 Hollow particle 4 Shell 6 Space (hollow part)
10 Laminate 11 Resin layer 12 Metal foil

Claims (10)

A hollow particle having an internal hollow shell,
The shell is silica containing aluminum, the aluminum content is less than 1% of the components constituting the shell, and the amount of aluminum present in a first portion of the shell located on the inner side in the thickness direction of the shell is at least twice as much as that present in a second portion of the shell located near the surface on the outer side in the thickness direction of the shell than in the first portion. 2. The hollow particle of claim 1, wherein the amount of aluminum present decreases from the inside to the outside of the shell . 3. The hollow particle according to claim 2, wherein the amount of aluminum present decreases continuously from the inside to the outside of the shell, or the amount of aluminum present decreases stepwise from the inside to the outside of the shell . 2. The hollow particle according to claim 1, wherein the epoxy resin oil absorption is less than 0.6 g/ ^m2 . The hollow particle described in claim 1, wherein the shell further contains sodium, and the sodium content of the shell is 3000 ppm or less. The hollow particle described in claim 1, wherein the silica is amorphous silica. The hollow particle described in claim 1, wherein the shell has a hollowness ratio of 30% or more and 95% or less. The hollow particle described in claim 1, wherein the shell thickness is 25 nm or more and 500 nm or less. coating the core particles with a shell-forming material to obtain core-shell particles;
removing the core particles from the core-shell particles to obtain hollow particle precursors; and
coating the hollow particle precursor with a shell-forming material;
The method for producing hollow particles according to any one of claims 1 to 8, comprising: The method according to claim 9, wherein the core particles contain an alunite-type compound represented by the following general formula (I):
M _a [Al _1-x M' _x ] ₃ (SO ₄ ^2- ) _y (OH) _z・mH ₂ O...(I)
In formula (I), M is at least one cation selected from the group consisting of Na ⁺ , K ⁺ , NH ₄ ⁺ and H ₃ O ⁺ ; M′ is at least one cation selected from the group consisting of Cu ²⁺ , Zn ²⁺ , Ni ²⁺ , Sn ⁴⁺ , Zr ⁴⁺ and Ti ⁴⁺ ; a satisfies 0.8≦a≦1.35; m satisfies 0≦m≦5; x satisfies 0≦x≦0.4; y satisfies 1.7≦y≦2.5; and z satisfies 4≦z≦7.