JP7624341B2 - Impact absorbing materials, resin particles and hollow particles - Google Patents

Description

The present invention relates to impact absorbing materials, resin particles, and hollow particles.

Impact absorbing materials such as floor spacers, tibia pads, and bumper cores are installed inside automobiles. These types of impact absorbing materials are required to have characteristics such as being resistant to deformation when compressive loads are repeatedly applied, having excellent energy absorption efficiency, and being lightweight. Conventionally, polystyrene resin foam bead moldings made by molding polystyrene resin foam beads in a mold have been used as impact absorbing materials (for example, Patent Document 1).

Patent Document 2 also describes that a molded body made of hollow spherical thermoplastic foam particles with a particle size of 0.5 to 20 mm and an average particle skin thickness of 3 μm or more can be advantageously used as a bumper core material.

JP 2003-127796 A Japanese Patent Application Publication No. 2-28225

However, the polystyrene resin foamed bead moldings described in Patent Document 1 have a large compression set. Therefore, depending on the application and shape of the polystyrene resin foamed bead molding, there is a risk that the moldings will gradually deform when a compressive load is repeatedly applied. In addition, when a polystyrene resin foamed bead molding is compressed, the compressive load required to deform the molding is likely to increase as the amount of strain increases, and it has been desired to further improve the energy absorption efficiency when the amount of strain increases.

On the other hand, the molded article obtained by molding the foam particles of Patent Document 2 in a mold is easily deformed even by a small compressive load because the particle skin is prone to buckling when a compressive load is applied. Therefore, the molded article obtained by molding the foam particles of Patent Document 2 in a mold has the problem that it has low energy absorption efficiency even when the amount of strain is small.

The present invention was made in light of this background, and aims to provide an impact absorbing material that has a small compression set and excellent energy absorption efficiency over a wide range of strain amounts, as well as resin particles and hollow particles for producing this impact absorbing material.

One aspect of the present invention is an impact absorbing material comprising a molded body obtained by molding hollow particles having an outer shell layer and a hollow portion surrounded by the outer shell layer,
the shell layer has a resin film having a base resin that is a composite resin containing a (meth)acrylic acid ester component derived from a (meth)acrylic acid ester monomer and a styrene-based component derived from a styrene-based monomer,
The apparent density ρ of the impact absorbing material is 20 kg/m ³ or more and 100 kg/m ³ or less,
The average particle diameter D of the hollow particles in the impact absorbing material is 2 mm or more and 9 mm or less,
The impact absorbing material has a fused portion in which the outer shell layers of the hollow particles are fused to each other, and the average value t _r of the total thickness of the resin film in the fused portion is 80 μm or more and 400 μm or less,
The average thickness t _t of the fused portion is more than 100 μm and 700 μm or less,
The impact absorbing material has a ratio t _r /t _t of an average value t r [μm] of the total thickness of the resin film in the fused portion to an _{average thickness t t [} μm] of the fused portion , which is 0.3 or more and 0.9 or less .

The impact absorbing material is composed of a molded body obtained by molding the specific hollow particles in a mold. The apparent density ρ of the impact absorbing material, the average particle diameter D of the hollow particles, and the average value t _r of the total thickness of the resin film at the fusion portion are each within the specific range. By having such a configuration, the impact absorbing material can reduce the compression set. Furthermore, the impact absorbing material can appropriately increase the compression load required for deformation in the initial stage of compression. This can improve the energy absorption efficiency of the impact absorbing material when the amount of strain is small.

Furthermore, the impact absorbing material can reduce the difference between the compressive load required for deformation when the strain amount is small and the compressive load required for deformation when the strain amount is large. As a result, the energy absorption efficiency can be improved even when the strain amount is large.

As a result, the above-mentioned embodiment can provide an impact absorbing material that has a small compression set and excellent energy absorption efficiency over a wide range of strain amounts.

FIG. 1 is a cross-sectional view of an impact absorbing material according to a first embodiment. FIG. 2 is an electron microscope photograph of the fusion-bonded portion of the impact absorbing material of Example 1, taken at a magnification of 200 times. FIG. 3 is an electron microscope photograph (magnification: 30 times) of the hollow particle in Example 1 in a state where it is divided into two. FIG. 4 is an electron microscope photograph (magnification: 1000 times) showing an enlarged view of the outer shell layer in FIG. FIG. 5 is an electron microscope photograph (magnification: 30 times) of an expanded bead in a reference example in a state where the expanded bead was divided into two. FIG. 6 is an explanatory diagram showing a method for calculating the energy absorption efficiency.

(shock absorbing material)
The impact absorbing material is composed of a molded body formed by molding hollow particles having an outer shell layer and a hollow portion surrounded by the outer shell layer in a mold. The hollow particles have an outer shell layer and a hollow portion surrounded by the outer shell layer. The outer shell layer constitutes the outer shell of the hollow particles and has a resin film made of a base resin. The outer shell layer may be composed of a single resin film. In addition, minute air bubbles partitioned by the resin film may be present in the outer shell layer. The structure of the hollow particles will be described in more detail later.

The shock absorbing material is used, for example, in automotive materials, construction materials, logistics materials, cushioning materials, bedding, etc. The shock absorbing material is suitable for automotive materials, for example, floor spacers, tibia pads, door pads, bumpers, etc. In particular, shock absorbing materials for automotive materials must adequately absorb the large impact that occurs during a collision, etc., and reduce the impact acceleration experienced by occupants. The shock absorbing material of the present invention has excellent energy absorption efficiency over a wide range of strain amounts, and therefore has excellent suitability as floor spacers, tibia pads, door pads, and bumpers for vehicles, and is particularly suitable as floor spacers and tibia pads.

The impact absorbing material is composed of hollow particles, but may contain particles other than hollow particles as long as the above-mentioned action and effect are not impaired. Examples of particles other than hollow particles include foamed particles and particles that do not have a hollow structure. When particles other than hollow particles are contained in the impact absorbing material, the ratio of particles other than hollow particles in the impact absorbing material is preferably 40% or less, more preferably 30% or less, even more preferably 20% or less, and particularly preferably 10% or less. By increasing the ratio of hollow particles in the impact absorbing material, the compression set can be easily reduced. In addition, by increasing the ratio of hollow particles in the impact absorbing material, the rate of increase in compression load accompanying an increase in the amount of strain can be reduced. As a result, the energy absorption efficiency of the impact absorbing material can be improved even when the amount of strain is large.

The ratio of particles other than hollow particles in the impact absorbing material is the ratio of the number of particles other than hollow particles to the total number of particles in any cut surface of the impact absorbing material, expressed as a percentage (unit: %). The calculation method for the ratio of particles other than hollow particles in the impact absorbing material is specifically as follows. First, randomly select three or more positions from which to take slices from the skin surface of the impact absorbing material, that is, from the part excluding the vicinity of the surface that was in contact with the mold during in-mold molding, and cut out slices with a thickness of about 2 mm at the selected positions. A scanner or the like is used to obtain a photograph of the surface of each slice, that is, the cross section of the impact absorbing material. A measurement area is set on each photograph obtained in this way so that it contains 100 or more particles. The ratio of the total number of particles other than hollow particles to the total number of particles in these measurement areas is taken as the ratio of particles other than hollow particles in the impact absorbing material.

[Apparent density ρ]
The apparent density ρ of the impact absorbing material is 20 kg/m ³ or more and 100 kg/m ³ or less. By setting the apparent density ρ of the impact absorbing material to the specific range, the compression load required for deformation of the impact absorbing material in the initial stage of compression can be appropriately increased. As a result, the energy absorption efficiency of the impact absorbing material when the strain amount is small can be improved. From the viewpoint of further enhancing this effect, the apparent density ρ of the impact absorbing material is preferably 25 kg/m ³ or more, more preferably 30 kg/m ³ or more. In addition, the apparent density ρ of the impact absorbing material is preferably 80 kg/m ³ or less, more preferably 60 kg/m ³ or less, and even more preferably 50 kg/m ³ or less. In this case, the impact absorbing material can be more easily reduced in weight.

If the apparent density ρ of the impact absorbing material is too low, the mechanical properties such as rigidity may fall below the range required for the impact absorbing material. If the apparent density ρ of the impact absorbing material is too high, the mass of the impact absorbing material may become excessively large. In this case, the rate of increase in the compressive load required to deform the impact absorbing material tends to increase as the amount of strain increases. This may result in a decrease in energy absorption efficiency when the amount of strain is large.

[Average particle diameter D of hollow particles]
The average particle diameter D of the hollow particles in the impact absorbing material is 2.0 mm or more and 9.0 mm or less. By setting the average particle diameter D of the hollow particles within the above-mentioned specific range, the total thickness of the resin film in the fusion part can be easily increased. And, by increasing the total thickness of the resin film, the increase rate of the compression load associated with an increase in the amount of strain can be reduced. As a result, the difference between the compression load required for deformation when the amount of strain is small and the compression load required for deformation when the amount of strain is large can be reduced, and the energy absorption efficiency of the impact absorbing material can be improved in a wide range of the amount of strain. From the viewpoint of further enhancing such an effect, the average particle diameter D of the hollow particles is preferably 3.0 mm or more, more preferably 3.5 mm or more. Moreover, the average particle diameter D of the hollow particles is more preferably 8.0 mm or less, and even more preferably 6.0 mm or less. In this case, the filling property of the hollow particles into the molding die during molding in the mold can be further improved, and the variation in the physical properties of the obtained impact absorbing material can be further reduced.

The method for measuring the average particle diameter D of the hollow particles described above is as follows. First, the impact absorbing material is cut along the thickness direction, etc., to expose the cross section of the impact absorbing material. A cross-sectional photograph of the impact absorbing material thus exposed is taken, and in the cross-sectional photograph obtained, five or more lines are drawn randomly from one edge of the cross-sectional photograph along the thickness direction, etc., of the molded body to the opposite edge. Next, the length of the lines is measured, and the number of particles that intersect with the lines is counted. The total length of the lines is then divided by the total number of particles that intersect with the lines to obtain the average particle diameter D of the hollow particles.

[Other particles]
The impact absorbing material may contain particles other than the hollow particles as long as the above-mentioned action and effect is not impaired. Examples of other particles include expanded particles having a cell structure in which a large number of bubbles are formed relatively uniformly throughout the particle.

[Fusing part]
The impact absorbing material has a fused portion formed by fusing the outer shell layers of the hollow particles together. The fused portion is composed of the outer shell layer of the hollow particle and the outer shell layer of the hollow particle adjacent to the hollow particle. The fused portion is usually in the shape of a flat plate.

Average total thickness of the resin film t _r
The average value _tr of the total thickness of the resin film at the fusion portion is 80 μm or more and 400 μm or less. By setting the average value _tr of the total thickness of the resin film to 80 μm or more, the compression load required for deformation of the impact absorbing material in the initial stage of compression can be moderately increased. From the viewpoint of more reliably expressing such an effect, the average value _tr of the total thickness of the resin film is preferably 85 μm or more, more preferably 90 μm or more. If the average value _tr of the total thickness of the resin film is excessively small, there is a risk of a decrease in the energy absorption efficiency when the strain amount is large. In addition, if the average value _tr of the total thickness of the resin film is excessively small, in addition to the decrease in the energy absorption efficiency when the strain amount is large, there is also a risk of a decrease in the energy absorption efficiency when the strain amount is small.

In addition, by setting the average value t _r of the total thickness of the resin film to 400 μm or less, preferably 300 μm or less, more preferably 250 μm or less, and even more preferably 200 μm or less, it is possible to further improve the energy absorption efficiency of the impact absorbing material while ensuring a low apparent density ρ. If the average value t _r of the total thickness of the resin film is excessively large, the apparent density ρ of the impact absorbing material becomes high, which may lead to an increase in the mass of the impact absorbing material.

The method for calculating the average value t _r of the total thickness of the resin film is as follows. First, the impact absorbing material is cut at an appropriate position to expose the cut surface. Using a scanning electron microscope, 30 or more fusion parts to be observed are randomly selected from the multiple fusion parts that appear on the cut surface. These fusion parts are observed at an appropriate magnification (for example, 200 times magnification) to obtain enlarged photographs of each fusion part.

Next, on the fused portion in each enlarged photograph, a line extending in the thickness direction of the fused portion is drawn from one surface of the fused portion to the other surface.

The above operation is carried out at 10 or more positions randomly selected from each enlarged photograph, the length of the line segment extending in the thickness direction of the fusion portion is measured, and the length of the portion of each line segment overlapping with the air bubble is calculated. Next, the length of the portion overlapping with the air bubble is subtracted from the length of each line segment, and the total thickness of the resin film at each fusion portion is calculated by arithmetic averaging these values. Then, the total thickness of the resin film described above is calculated for the 30 or more fusion portions observed, and the arithmetic average value of the total thickness of these resin films is set as the average value _tr of the total thickness of the resin film at the fusion portion.

Average thickness of the fused portion _t
The average thickness t _t of the fused portion is preferably more than 100 μm and not more than 700 μm. By setting the average value t _r of the total thickness of the resin film in the fused portion within the specific range and setting the average thickness t _t of the fused portion within the specific range, the rate of increase in the compression load accompanying an increase in the amount of strain can be more reliably reduced. As a result, the energy absorption efficiency of the impact absorbing material when the amount of strain is large can be more reliably improved. From the viewpoint of further enhancing this effect, the average thickness t _t of the fused portion is more preferably 120 μm or more, and even more preferably 130 μm or more.

The average thickness t _t of the fused portion is more preferably 500 μm or less, even more preferably 400 μm or less, and particularly preferably 300 μm or more. In this case, even if the shape of the impact absorbing material is complicated, it is possible to more easily obtain an impact absorbing material having excellent appearance and good physical properties.

The method for calculating the average thickness t _t of the fused portion is as follows: First, similarly to the method for calculating the average total thickness t _t of the resin film described above, 30 or more fused portions randomly selected from the cut surface of the impact absorbing material are observed with a scanning electron microscope, and enlarged photographs of each fused portion are obtained.

Next, on the fused portion in each enlarged photograph, a line extending in the thickness direction of the fused portion is drawn from one surface of the fused portion to the other surface.

The above operation is performed at 10 or more randomly selected positions in each enlarged photograph, and the length of the line segment extending in the thickness direction of the fused portion is measured. The arithmetic average value of the lengths of these line segments is taken as the thickness of the fused portion shown in the enlarged photograph. This operation is performed for each of the 30 or more fused portions observed, and the arithmetic average value of the thicknesses of the fused portions is taken as the average thickness t _t of the fused portion.

The ratio t _r /t _t of the average total thickness t _r (unit: μm) of the resin film in the fusion part to the average thickness t _t (unit: μm) of the fusion part is preferably 0.3 or more and 1 or less. By setting the average total thickness t _r of the resin film in the fusion part within the specific range and setting the value of t _r /t _t within the specific range, the rate of increase in the compression load associated with an increase in the amount of strain can be more reliably reduced. As a result, the energy absorption efficiency of the impact absorbing material when the amount of strain is large can be more reliably improved. From the viewpoint of stably expressing such an effect, the ratio t _r /t _t is more preferably 0.4 or more, and even more preferably 0.5 or more.

In addition, the ratio t _r /t _{t of the average value t r of} the total thickness of the resin film in the fusion part to the average thickness t _t of the fusion part is more preferably 0.9 or less, and even more preferably 0.8 or less. In this case, the rate of increase in the compression load with an increase in the amount of strain can be further reduced, and the energy absorption efficiency of the impact absorbing material can be further improved when the amount of strain is large. Note that the ratio t _r /t _t of the average value t _r of the total thickness of the resin film in the fusion part to the average thickness t _t of the fusion part is 0.9 or less, which means that air bubbles are contained in the fusion part, and it is thought that the fusion part has a structure that contains air bubbles in an appropriate amount, which makes it easier to make the rate of increase in the compression load relative to the increase in the amount of strain gentle.

The ratio t _r /ρ (unit: μm/(kg/m ³ )) of the average total thickness t _r of the resin film at the fused portion to the apparent density ρ (unit: kg/m ³ ) of the impact absorbing material is preferably 2.0 to 8.0. In this case, the mass of the impact absorbing material can be easily reduced and the energy absorption efficiency of the impact absorbing material when the amount of strain is large can be improved.

[Compressive stress]
The energy absorption characteristics of an impact absorbing material can be evaluated based on the magnitude of the compressive stress at various strain amounts. That is, the energy absorption efficiency of an impact absorbing material at a certain strain amount ε _a is higher as the shape of the stress-strain curve from the start of compression to the strain amount ε a becomes closer to a rectangle in a stress-strain curve with the horizontal axis representing the strain amount applied to the impact absorbing material and the vertical axis representing the compressive stress corresponding to the strain amount. More specifically, the smaller the difference between the compressive stress σ _a at the strain amount ε _{a and} the compressive stress σ _b at a strain amount ε _b smaller than the strain amount ε _a , the closer the shape of the stress-strain curve becomes to a rectangle, and it can be said that the energy absorption efficiency of the impact absorbing material at the strain amount ε _a is high.

From this viewpoint, it is preferable that the ratio σ ₇₅ /σ ₅₀ of the 75% deformation compressive stress σ _{75 (unit: kPa) at 23° C. to the 50% deformation compressive stress σ 50 (} unit: kPa) at 23° C. of the impact absorbing material is 1.0 or more and less than 2.0. In this case, the energy absorption efficiency when the amount of strain is large can be improved, and even when a large impact is applied to the impact absorbing material, it is possible to absorb the energy of the impact while protecting the protected object.

It is also preferable that the ratio _σ10 / _σ75 of the compressive stress at 10% deformation _σ10 (unit: kPa) at 23° C. to the compressive stress at ₇₅ % deformation σ75 (unit: kPa) at 23° C. of the impact absorbing material is 0.30 or more and 1.0 or less. In this case, the energy absorption efficiency of the impact absorbing material can be improved over a wider range of strain amounts.

From the same viewpoint, it is preferable that the ratio σ ₂₅ /σ ₅₀ of the compressive stress at 25% deformation σ ₂₅ (unit: kPa) at 23° C. to the compressive stress at 50% deformation σ ₅₀ (unit: kPa) at 23° C. of the impact absorbing material is 0.50 or more and 1.0 or less.

[Compression set]
The compression set of the impact absorbing material is preferably 10% or less, more preferably 8% or less, and even more preferably 6% or less. Impact absorbing materials having such characteristics are less likely to accumulate permanent set even when compressive loads are repeatedly applied, and can easily restore to their pre-compression shape. Therefore, impact absorbing materials with compression set within the above-mentioned specific range can maintain excellent energy absorption properties for a longer period of time even when compressive loads are repeatedly applied.

(Method of manufacturing impact absorbing material)
The impact absorbing material is manufactured, for example, as follows. First, the hollow particles are filled into a mold having a cavity corresponding to the shape of the desired impact absorbing material. At this time, the particle group filled into the mold may contain particles other than hollow particles in addition to hollow particles. When the particle group filled into the mold contains particles other than hollow particles, the ratio of hollow particles in the particle group is preferably 50% or more, more preferably 60% or more, even more preferably 80% or more, and particularly preferably 90% or more. Next, the hollow particles in the mold are heated by a heating medium such as steam. The hollow particles expand and fuse to each other by being heated in the cavity. At this time, the outer shell layer of the hollow particle and the outer shell layer of the hollow particle next to the hollow particle are fused to each other to form a fused part. As a result, a molded body having a shape corresponding to the shape of the cavity and composed of hollow particles is obtained. The molded body obtained in this way may be used as an impact absorbing material as it is. If necessary, the molded article may be subjected to cutting or other processing to be molded into a desired shape, and the molded article may be used as an impact absorbing material.

(Hollow particles)
The hollow particles used in the manufacture of the impact absorbing material have an outer shell layer and a hollow portion surrounded by the outer shell layer. The hollow particles of the present invention are hollow particles obtained by expanding resin particles containing an organic physical foaming agent described below. The hollow particles of the present invention have a structure different from that of expanded particles having a cell structure in which a large number of bubbles are formed relatively uniformly throughout the particle. It is preferable that the hollow particles before molding in a mold are spherical.

The hollow part of the hollow particle is located closer to the center of the hollow particle than the outer shell layer. The hollow part, i.e., the area surrounded by the outer shell layer, may be substantially hollow. Here, "substantially hollow" refers to a state in which, when a hollow particle is cut at a cross section passing through its center and the cut surface is observed at a magnification of 20 to 1000 times using a transmission electron microscope or the like, the structure of the outer shell layer and the structure of the hollow part are clearly different.

For example, the hollow portion may be a single space separated from the outside of the hollow particle by the shell layer. The hollow portion may also have a space wall that is made of a base resin and divides the hollow portion into multiple spaces. In this case, the hollow portion may have several to several tens of spaces divided by the space wall of the hollow portion, or by the space wall of the hollow portion and the inner surface of the shell layer. From the viewpoint of easily expressing the desired physical properties, it is preferable that the hollow portion is divided into 50 or less spaces, more preferably 30 or less spaces, even more preferably 10 or less spaces, and particularly preferably 5 or less spaces, in a cross section when the hollow particle is divided in half.

If the region surrounded by the shell layer of the hollow particles is substantially hollow, a molded product having the desired physical properties can be obtained by molding in a mold, and an impact absorbing material having a small compression set and excellent energy absorption efficiency over a wide range of strain amounts can be manufactured. It is preferable that the hollow portion of the hollow particle is a single space separated from the outside of the hollow particle by the shell layer. In addition, for the structure and hollow portion of the hollow particle in the molded product after molding in a mold, the explanation of the structure and hollow portion of the hollow particle used in manufacturing the molded product can be appropriately referred to.

[Average particle size]
The average particle size of the hollow particles before molding in a mold is preferably 2.0 mm or more and 9.0 mm or less. By molding hollow particles having an average particle size in the specific range in a mold, the average particle size of the hollow particles in the impact absorbing material can be easily adjusted to be within the specific range.

The average particle size of the hollow particles before molding in the mold is the cumulative 63% diameter (i.e., d63) calculated based on the particle size distribution of the hollow particles on a volume basis. The particle size distribution of the hollow particles on a volume basis can be obtained using a particle size distribution measuring device (for example, "Militrack JPA" manufactured by Nikkiso Co., Ltd.).

[Bulk density]
The bulk density of the hollow particles before molding in the mold is preferably 20 kg/m ³ or more and 100 kg/m ³ or less. The hollow particles having the hollow structure described above and the bulk density in the specific range are lightweight and excellent in both rigidity and restorability. Therefore, by molding hollow particles having such a bulk density in the mold, it is possible to easily obtain an impact absorbing material that is lightweight, has a small compression set, and has excellent energy absorption efficiency in a wide range of strain. From the viewpoint of increasing the rigidity and restorability of the hollow particles, the bulk density of the hollow particles before molding in the mold is more preferably 22 kg/m ³ or more, and even more preferably 25 kg/m ³ or more.

From the viewpoint of further improving the light weight of the hollow particles, the bulk density of the hollow particles before molding in a mold is more preferably 80 kg/m3 ^{or less} , further preferably 60 kg/m3 or ^less , and particularly preferably 50 kg/ ^m3 or less.

The bulk density of the hollow particles described above is a value calculated by the following method. First, an arbitrary amount of hollow particles is prepared as the measurement object. The measurement object is left for 10 days under conditions of 50% relative humidity, 23°C temperature, and 1 atm to adjust the state of the measurement object. After the measurement object is allowed to naturally pile up in the measuring cylinder, a light impact is applied to the bottom of the measuring cylinder to stabilize the filling height of the measurement object in the measuring cylinder. Then, the bulk volume (unit: L) of the measurement object indicated by the graduations on the measuring cylinder is read. The mass (unit: g) of the measurement object placed in the measuring cylinder is divided by the bulk volume, and the units are converted. From the above, the bulk density (unit: kg/m ³ ) of the hollow particles can be obtained.

[Outer shell layer]
The shell layer of the hollow particle has a resin film made of a base resin and a plurality of bubbles partitioned by the resin film. The shell layer may have a multi-layer structure including, for example, a solid lower layer facing the hollow portion, a solid upper layer exposed on the outermost surface of the hollow particle (i.e., the outer surface of the shell layer), and a foam layer having a plurality of bubbles located between the lower layer and the upper layer. Note that solid means a state in which there are substantially no bubbles in the resin.

The average total thickness of the resin film in the hollow particles before molding in a mold is preferably 30 μm or more, more preferably 35 μm or more, and even more preferably 40 μm or more. By molding such hollow particles in a mold, a molded body can be more easily obtained in which the compression load required for deformation in the initial stage of compression is moderately high and the energy absorption efficiency is high when the amount of strain is large. In addition, the average total thickness of the resin film in the hollow particles is preferably 200 μm or less, more preferably 150 μm or less, and even more preferably 120 μm. In this case, a molded body with a low apparent density and excellent energy absorption efficiency can be more easily obtained.

The average thickness of the shell layer of the hollow particles before molding is preferably more than 50 μm and not more than 350 μm. In addition, the ratio of the average value of the total thickness of the resin film to the average thickness of the shell layer of the hollow particles before molding is preferably 0.3 or more, and more preferably 0.5 or more. By making the structure of the shell layer a structure specified by the aforementioned average thickness and the ratio of the average value of the total thickness of the resin film to the average thickness of the shell layer, the durability of the shell layer against load can be improved. This can increase the restorability of the hollow particles against repeated load.

From the viewpoint of further improving the restorability of the hollow particles, the average thickness of the outer shell layer in the hollow particles before molding in the mold is more preferably 60 μm or more, and even more preferably 70 μm or more. From the same viewpoint, the ratio of the average value of the total thickness of the resin film to the average thickness of the outer shell layer in the hollow particles before molding in the mold is more preferably 0.50 or more, and even more preferably 0.55 or more.

The average thickness of the outer shell layer mentioned above is a value calculated by the following method. First, the hollow particles before molding in a mold are divided into roughly two equal parts to expose the cut surface of the outer shell layer. Using a scanning electron microscope, three or more observation positions are set randomly within one of the roughly four equal parts of the cut surface of the outer shell layer. These observation positions are observed at an appropriate magnification (e.g., 1000x magnification) to obtain enlarged photographs of the cut surface of the outer shell layer at each observation position.

On the outer shell layer in the enlarged photograph obtained, a line segment extending in the thickness direction of the outer shell layer (i.e., the radial direction of the hollow particle) is drawn from the outer surface to the inner surface of the outer shell layer. The above operation is performed at 10 or more positions randomly selected from each enlarged photograph, and the length of the line segment is measured at a total of 30 or more positions. The arithmetic mean value of the lengths of the line segments thus obtained is regarded as the thickness of the outer shell layer of each hollow particle. Then, the aforementioned outer shell layer thickness is calculated for 5 or more randomly selected hollow particles, and the arithmetic mean value of these outer shell layer thicknesses is regarded as the average thickness of the outer shell layer.

When calculating the average total thickness of the resin film of the outer shell layer of hollow particles before molding in a mold, similar to the method for calculating the average thickness of the outer shell layer, three or more observation positions are set randomly for one of the roughly four equal regions of the cut surface of the outer shell layer, and enlarged photographs of the cut surface of the outer shell layer at each observation position are obtained. Next, on the outer shell layer in each enlarged photograph, a line segment extending in the thickness direction of the outer shell layer is drawn from the outer surface to the inner surface of the outer shell layer.

The above operations are carried out at 10 or more randomly selected positions within each enlarged photograph, the length of the line segments extending in the thickness direction of the outer shell layer is measured, and the length of each line segment that overlaps with an air bubble is calculated. Next, the length of the portion that overlaps with an air bubble is subtracted from the length of each line segment, and these values are arithmetically averaged to calculate the total thickness of the resin film of each hollow particle. The aforementioned total thickness of the resin film is then calculated for 5 or more randomly selected hollow particles, and the arithmetic average of these total resin film thicknesses is regarded as the average total resin film thickness.

[Base resin]
The base resin of the resin film in the hollow particle is a composite resin containing a (meth)acrylic acid ester component derived from a (meth)acrylic acid ester monomer and a styrene-based component derived from a styrene-based monomer. As described later, the composite resin is preferably a composite resin obtained by impregnating and polymerizing a core particle having a styrene-based resin as a base resin with a (meth)acrylic acid ester monomer and a styrene-based monomer.

The (meth)acrylic acid ester components contained in the composite resin include components derived from acrylic acid ester monomers such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate, and components derived from methacrylic acid ester monomers such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, and 2-ethylhexyl methacrylate. The composite resin may contain one component selected from these (meth)acrylic acid ester components, or two or more components.

It is preferable that the composite resin contains a component derived from methyl methacrylate as the (meth)acrylic acid ester component. In this case, the content of the component derived from methyl methacrylate in the (meth)acrylic acid ester component is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more. In this case, the structure and physical properties of the hollow particles can be more reliably made to be the desired structure and physical properties.

Examples of styrene-based components contained in the composite resin include components derived from styrene-based monomers such as styrene, α-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, p-ethylstyrene, 2,4-dimethylstyrene, p-methoxystyrene, p-n-butylstyrene, p-t-butylstyrene, o-chlorostyrene, m-chlorostyrene, p-chlorostyrene, 2,4,6-tribromostyrene, divinylbenzene, styrene sulfonic acid, and sodium styrene sulfonate, components derived from homopolymers of styrene-based monomers such as polystyrene, and components derived from copolymers of styrene-based monomers and other monomers or polymers such as rubber-modified polystyrene, acrylonitrile-butadiene-styrene copolymer, acrylonitrile-styrene copolymer, and acrylonitrile-ethylene propylene rubber-styrene copolymer. The composite resin may contain one or more of these styrene-based components.

It is preferable that the composite resin contains a component derived from styrene as the styrene-based component. In this case, the content of the component derived from styrene in the styrene-based component is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more. In this case, the structure and physical properties of the hollow particles can be more reliably made to be the desired structure and physical properties.

The composite resin may contain components other than the (meth)acrylic acid ester component and the styrene-based component. Examples of such components include components derived from monomers having a carbon-carbon double bond, such as vinyl compounds containing hydroxyl groups, vinyl compounds containing nitrile groups, organic acid vinyl compounds, olefin compounds, diene compounds, vinyl halides, vinylidene halides, and maleimide compounds.

Examples of vinyl compounds containing a hydroxyl group include hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, and hydroxypropyl methacrylate. Examples of vinyl compounds containing a nitrile group include acrylonitrile and methacrylonitrile.

Examples of organic acid vinyl compounds include vinyl acetate and vinyl propionate. Examples of olefin compounds include ethylene, propylene, 1-butene, and 2-butene. Examples of diene compounds include butadiene, isoprene, and chloroprene.

Examples of vinyl halide compounds include vinyl chloride and vinyl bromide. Examples of vinylidene halide compounds include vinylidene chloride. Examples of maleimide compounds include N-phenylmaleimide and N-methylmaleimide.

The composite resin may contain one component selected from the components derived from monomers having these carbon-carbon double bonds, or may contain two or more components.

The amount of components other than the (meth)acrylic acid ester component and the styrene-based component is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 5 parts by mass or less, per 100 parts by mass of the total of the (meth)acrylic acid ester component and the styrene-based component. In this case, the structure and physical properties of the hollow particles can be more reliably achieved as desired.

The composite resin can be obtained, for example, by impregnating and polymerizing core particles having a styrene-based resin as the base resin with a (meth)acrylic acid ester monomer and a styrene-based monomer. Examples of styrene-based resins that can be used to prepare the composite resin include homopolymers of the above-mentioned styrene-based monomers and copolymers of styrene-based monomers with other monomers or polymers.

The weight average molecular weight of the composite resin is preferably 30×10 ⁴ or more and 50×10 ⁴ or less, and more preferably 35×10 ⁴ or more and 45×10 ⁴ or less. In this case, the rigidity and restorability of the hollow particles can be further improved. The weight average molecular weight of the composite resin is a polystyrene-equivalent molecular weight measured by gel permeation chromatography using polystyrene as a standard substance.

The mass ratio of the (meth)acrylic ester component and the styrene-based component contained in the composite resin is preferably (meth)acrylic ester component:styrene-based component = 70:30 to 30:70, and more preferably (meth)acrylic ester component:styrene-based component = 60:40 to 40:60. In this case, the yield of hollow particles having the specific structure can be further increased.

When a composite resin is produced by impregnating and polymerizing a core particle composed of a base resin containing a (meth)acrylic acid ester component and/or a styrene-based component with a (meth)acrylic acid ester monomer and/or a styrene-based monomer, the mass ratio of the (meth)acrylic acid ester component and the styrene-based component contained in the composite resin can be determined from the relationship between the mass of each component in the core particle and the mass of each monomer to be impregnated and polymerized.

[Additives]
The hollow particles may contain additives such as a bubble regulator, a catalyst neutralizer, a lubricant, a crystal nucleating agent, an antistatic agent, etc., within a range that does not impair the above-mentioned action and effect. The content of the additives in the hollow particles is, for example, preferably 20% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less, based on the total mass of the hollow particles.

(Resin particles)
The hollow particles are obtained by expanding resin particles having the following structure: the resin particles have a base resin that is a composite resin containing a (meth)acrylic acid ester component derived from a (meth)acrylic acid ester monomer and a styrene-based component derived from a styrene-based monomer, and contain an organic physical blowing agent;
The average particle size of the resin particles is 1 mm or more and 5 mm or less,
a mass ratio of the (meth)acrylic acid ester component to the styrene-based component contained in the composite resin is (meth)acrylic acid ester component:styrene-based component=70:30 to 30:70;
In an infrared absorption spectrum of the surface of the resin particle obtained by a total reflection measurement method, the value of the surface absorbance ratio S represented by the following formula (1) is 0.70 or more.
S=As _acryl /As _styrene ...(1)

In the above formula (1), As _styrene is the maximum absorbance in the wave number range of 650 cm ^-1 to 750 cm ^-1 , and As _acryl is the maximum absorbance in the wave number range of 1680 cm ^-1 to 1780 cm ^-1 .

The base resin in the resin particles is the same as the base resin in the hollow particles described above.

As the organic physical blowing agent, for example, a volatile organic compound having a boiling point of 90°C or less at atmospheric pressure, such as propane, normal butane, isobutane, normal pentane, isopentane, neopentane, hexane, cyclobutane, cyclopentane, dimethyl ether, diethyl ether, etc., can be used. These volatile organic compounds may be used alone, or two or more types of volatile organic compounds may be used in combination.

The content of the organic physical foaming agent in the resin particles is preferably 3% by mass or more and 8% by mass or less, and more preferably 4% by mass or more and 7% by mass or less. In this case, the yield of hollow particles having the desired structure can be further increased.

In the infrared absorption spectrum of the surface of the resin particle obtained by total reflection measurement, the value of the surface absorbance ratio S represented by the above formula (1) is 0.70 or more. The maximum absorbance value As _styrene in the above formula (1) is the intensity of a peak derived from the out-of-plane bending vibration of the benzene ring contained in the styrene-based component. In addition, the maximum absorbance value As _acryl in the above formula (1) is the intensity of a peak derived from the C=O stretching vibration of the carbonyl group contained in the (meth)acrylic acid ester component.

Therefore, the value of the surface absorbance ratio S represented by the above formula (1) is related to the ratio of the styrene-based component and the (meth)acrylic acid ester component present on the surface of the resin particle. The (meth)acrylic acid ester component is abundantly present on the surface of the resin particle having the surface absorbance ratio S value within the above specific range. By expanding such resin particles, hollow particles having the desired structure and characteristics can be easily obtained. From the viewpoint of more reliably obtaining hollow particles having a good hollow structure, the upper limit of the value of the surface absorbance ratio S represented by the above formula (1) is preferably about 3.0, more preferably 2.0, and even more preferably 1.5. If the value of the surface absorbance ratio S is too low, it may be difficult to obtain hollow particles having the desired outer shell layer structure when the resin particles are expanded.

The method of calculating the surface absorbance ratio S described above is specifically as follows. First, a diamond prism with an incident angle of 45 degrees is pressed against a resin particle with a pressure of 170 kg/ ^cm2 to obtain an uncorrected infrared absorption spectrum. The absorbance of the obtained infrared absorption spectrum is corrected over the entire spectrum so that the penetration depth of the infrared light at each wave number coincides with the penetration depth of the infrared light at a wave number of 1000 cm ^-1 . Based on the thus obtained corrected infrared absorption spectrum, the maximum absorbance values As _styrene and As _acryl are determined. Then, the value obtained by dividing the value of As _acryl by the value of As _styrene is taken as the surface absorbance ratio S.

In addition, it is preferable that the ratio S/I of the surface absorbance ratio S represented by the above formula (1) to the internal absorbance ratio I represented by the following formula (2) in an infrared absorption spectrum of the inside of a resin particle obtained by a total reflection measurement method is 0.3 or more and 3 or less.
I=Ai _acryl /Ai _styrene ...(2)

In the above formula (2), Ai _styrene is the maximum absorbance in the wave number range of 650 cm ^-1 to 750 cm ^-1 , and Ai _acryl is the maximum absorbance in the wave number range of 1680 cm ^-1 to 1780 cm ^-1 .

The maximum absorbance Ai _styrene in the formula (2) is the intensity of a peak derived from the out-of-plane bending vibration of a benzene ring contained in a styrene-based component, similar to the maximum absorbance As _styrene in the formula (1). The maximum absorbance Ai _acryl in the formula (2) is the intensity of a peak derived from the C=O stretching vibration of a carbonyl group contained in a (meth)acrylic acid ester component, similar to the maximum absorbance As _acryl in the formula (1).

Therefore, the value of the internal absorbance ratio I represented by the above formula (2) is related to the ratio of the styrene-based component and the (meth)acrylic acid ester component present inside the resin particle. And the ratio S/I of the surface absorbance ratio S to the internal absorbance ratio I is related to the ratio of the concentration of the styrene-based component inside the resin particle to the concentration of the styrene-based component on the surface of the resin particle.

Resin particles having a surface absorbance ratio S within the specific range and a ratio S/I of the surface absorbance ratio S to the internal absorbance ratio I within the specific range can further increase the yield of hollow particles with desired structure and properties. From the same viewpoint, the lower limit of the ratio S/I of the surface absorbance ratio S to the internal absorbance ratio I is preferably 0.4, more preferably 0.5, and even more preferably 0.6. The upper limit of the ratio S/I is preferably 2, and more preferably 1.

The method for calculating the internal absorbance ratio I described above is the same as the method for calculating the surface absorbance ratio S, except that the resin particle is divided into roughly two equal parts, and then a diamond prism is placed in contact with the split surface exposed by the division of one of the two hemispherical particles obtained by the division.

(Method of manufacturing hollow particles)
As a method for producing the hollow particles, for example, a method can be adopted in which resin particles containing an organic physical foaming agent and the composite resin as a base resin are prepared, and then the resin particles are foamed.

As a method for producing resin particles, for example, a method can be adopted in which styrene-based resin core particles (hereinafter referred to as "core particles") using a styrene-based resin as the base resin are impregnated with a (meth)acrylic acid ester monomer and a styrene-based monomer, the (meth)acrylic acid ester monomer and the styrene-based monomer are polymerized, and the particles are impregnated with a physical foaming agent.

More specifically, the method for producing resin particles includes a dispersing step of dispersing core particles in an aqueous medium,
a modification step of impregnating the core particles dispersed in the aqueous medium with a (meth)acrylic acid ester monomer and a styrene-based monomer, and polymerizing the (meth)acrylic acid ester monomer and the styrene-based monomer to obtain resin particles;
and an impregnation step of impregnating the core particles or the resin particles with an organic physical foaming agent at least once or more times among before, during, and after the modification step. The dispersion step, the modification step, and the impregnation step may be performed consecutively in a single sealed container, or may be performed using separate containers. Each step will be described in detail below.

[Dispersion process]
In the dispersion step, the core particles are dispersed in an aqueous medium to prepare a suspension. As the aqueous medium, for example, deionized water or the like can be used. In addition to the core particles, a suspending agent, a surfactant, or the like can be added to the aqueous medium as necessary.

The base resin of the core particles used in the dispersion process is preferably a styrene-based resin. As the styrene-based resin, a homopolymer of the above-mentioned styrene-based monomer, a copolymer of a styrene-based monomer and another monomer or polymer, etc. can be used. More specifically, as the styrene-based resin, for example, a homopolymer of a styrene-based monomer such as polystyrene, or a copolymer containing 50 mol% or more of a component derived from a styrene-based monomer can be used. In addition, the core particles may contain additives such as a bubble adjuster, a pigment, a slip agent, an antistatic agent, and a flame retardant, as long as the above-mentioned action and effect are not impaired.

The core particles may contain other resins than the styrene resin, such as an acrylic resin that is a polymer of a (meth)acrylic acid ester monomer. From the viewpoint of more reliably obtaining a desired structure for the hollow particles, the content of the resin other than the styrene resin in the core particles is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, and even more preferably 10 parts by mass or less, per 100 parts by mass of the styrene resin.

The method for producing the core particles is not particularly limited. For example, the strand cut method, the underwater cut method, the hot cut method, etc. can be used as the method for producing the core particles. In addition, the core particles may be produced, for example, by a suspension polymerization method in which a styrene-based monomer or the like is polymerized in an aqueous medium.

The average particle diameter of the core particles is preferably 0.6 mm or more and 2.0 mm or less, and more preferably 0.7 mm or more and 1.5 mm or less. In this case, the polymerization stability in the modification process is improved, and it is easy to increase the weight average molecular weight of the composite resin. In addition, by performing the modification process and the impregnation process using core particles whose average particle diameter is within the specific range, the average particle diameter of the resin particles and the surface absorbance ratio S of the resin particles can be easily adjusted to the specific range, and the structure of the hollow particles finally obtained can be more reliably made to the desired structure.

The average particle size of the core particles is the cumulative 63% diameter (i.e., d63) calculated based on the particle size distribution of the core particles on a volume basis, and can be measured using a particle size distribution measuring device (for example, "Militrack JPA" manufactured by Nikkiso Co., Ltd.).

Examples of suspending agents used in the dispersion step include inorganic suspending agents consisting of inorganic fine particles such as tricalcium phosphate, hydroxyapatite, magnesium pyrophosphate, magnesium phosphate, aluminum hydroxide, ferric hydroxide, titanium hydroxide, magnesium hydroxide, barium phosphate, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, barium sulfate, talc, kaolin, and bentonite, and organic suspending agents such as polyvinylpyrrolidone, polyvinyl alcohol, ethyl cellulose, and hydroxypropylmethylcellulose. These suspending agents may be used alone or in combination of two or more. The suspending agent is preferably one or more of tricalcium phosphate, hydroxyapatite, and magnesium pyrophosphate.

The amount of the suspending agent added is preferably 0.05 parts by mass or more and 10 parts by mass or less, and more preferably 0.3 parts by mass or more and 5 parts by mass or less, in terms of solid content, per 100 parts by mass of the aqueous medium. By making the amount of the suspending agent added 0.05 parts by mass or more, more preferably 0.3 parts by mass or more, it becomes easier to maintain the state in which the core particles and monomers are suspended in the aqueous medium when the (meth)acrylic acid ester monomer and the styrene-based monomer are added in the subsequent modification process. In addition, by making the amount of the suspending agent added 10 parts by mass or less, more preferably 5 parts by mass or less, it is possible to narrow the particle size distribution of the hollow particles finally obtained.

Examples of surfactants that can be used include anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants. Examples of anionic surfactants include sodium alkylsulfonate, sodium alkylbenzenesulfonate, sodium lauryl sulfate, sodium α-olefinsulfonate, and sodium dodecylphenylether disulfonate. Examples of nonionic surfactants include polyoxyethylene dodecyl ether, polyoxyethylene nonylphenyl ether, and polyoxyethylene lauryl ether.

Examples of cationic surfactants include alkylamine salts such as coconut amine acetate and stearyl amine acetate, and quaternary ammonium salts such as lauryl trimethyl ammonium chloride and stearyl trimethyl ammonium chloride. Examples of amphoteric surfactants include alkyl betaines such as lauryl betaine and stearyl betaine, and alkyl amine oxides such as lauryl dimethyl amine oxide. These surfactants may be used alone or in combination of two or more.

The surfactant is preferably an anionic surfactant, more preferably an alkali metal alkylsulfonate having 8 to 20 carbon atoms, and even more preferably sodium alkylsulfonate having 8 to 20 carbon atoms. By using these surfactants, the core particles, monomers, etc. can be more easily maintained in a suspended state in the aqueous medium when the (meth)acrylic acid ester monomer and the styrene monomer are added in the subsequent modification process.

If necessary, electrolytes consisting of inorganic salts such as lithium chloride, potassium chloride, sodium chloride, sodium sulfate, sodium nitrate, sodium carbonate, and sodium bicarbonate can be added to the suspension.

[Modification process]
In the modification step, a (meth)acrylic acid ester monomer and a styrene-based monomer are added to the suspension, the core particles are impregnated with the (meth)acrylic acid ester monomer and the styrene-based monomer, and these monomers are polymerized to obtain composite resin particles having a composite resin containing a styrene-based component and a (meth)acrylic acid ester component as a base resin.

The total amount of the (meth)acrylic acid ester monomer and the styrene-based monomer added in the modification step is, for example, preferably 200 parts by mass or more and 700 parts by mass or less, and more preferably 250 parts by mass or more and 600 parts by mass or less, relative to 100 parts by mass of the core particles. The ratio of the amount of the (meth)acrylic acid ester monomer to the amount of the styrene-based monomer may be appropriately set according to the composition of the desired composite resin. From the viewpoint of making it easier to obtain good hollow particles, the ratio of the amount of the (meth)acrylic acid ester monomer to the amount of the styrene-based monomer is preferably (meth)acrylic acid ester monomer:styrene monomer = 50:50 to 85:15, and more preferably (meth)acrylic acid ester monomer:styrene monomer = 60:40 to 75:25, in terms of mass ratio.

In the modification step, a polymerization initiator is used to polymerize the (meth)acrylic acid ester monomer and the styrene-based monomer. The polymerization initiator is not particularly limited as long as it is applicable to the suspension polymerization of the styrene-based monomer. For example, a polymerization initiator that is soluble in a vinyl monomer and has a 10-hour half-life temperature of 50°C or more and 120°C or less can be used. Examples of such polymerization initiators include organic peroxides such as cumene hydroxyperoxide, dicumyl peroxide, t-butylperoxy-2-ethylhexanoate, t-butylperoxybenzoate, benzoyl peroxide, t-butylperoxyisopropyl carbonate, t-amylperoxy-2-ethylhexyl carbonate, hexylperoxy-2-ethylhexyl carbonate, and lauroyl peroxide, and azo compounds such as azobisisobutyronitrile. These polymerization initiators may be used alone or in combination of two or more.

The polymerization initiator may be added to an aqueous medium in a state dissolved in a solvent or monomer, for example, and impregnated into the core particles together with the (meth)acrylic acid ester monomer and the styrene-based monomer. In this case, the solvent may be, for example, an aromatic hydrocarbon such as ethylbenzene or toluene, or an aliphatic hydrocarbon such as heptane or octane. The amount of the polymerization initiator added is preferably 0.01 parts by mass or more and 3 parts by mass or less per 100 parts by mass of the total of the (meth)acrylic acid ester monomer and the styrene-based monomer added in the modification process.

In the modification process, a bubble regulator, a plasticizer, an oil-soluble polymerization inhibitor, a flame retardant, a dye, etc. can be added as necessary. The bubble regulator is added to the aqueous medium in a state where it is dissolved or dispersed in a monomer and/or a solvent. Examples of the bubble regulator include fatty acid monoamides, fatty acid bisamides, talc, silica, polyethylene wax, methylene bisstearic acid, methyl methacrylate copolymers, and silicones. Examples of fatty acid monoamides include oleic acid amide and stearic acid amide. Examples of fatty acid bisamides include ethylene bisstearic acid amide.

The plasticizer, oil-soluble polymerization inhibitor, flame retardant, and dye can be added to the aqueous medium in a state where they are dissolved or dispersed in, for example, a (meth)acrylic acid ester monomer and/or a styrene-based monomer. Examples of the plasticizer include fatty acid esters such as glycerin tristearate, glycerin trioctoate, glycerin trilaurate, sorbitan tristearate, sorbitan monostearate, and butyl stearate, acetylated monoglycerides such as glycerin diacetomonolaurate, oils and fats such as hardened beef tallow and hardened castor oil, and organic compounds such as cyclohexane and liquid paraffin. Examples of the oil-soluble polymerization inhibitor include para-t-butylcatechol, hydroquinone, and benzoquinone.

The heating temperature in the modification process and the time required for adding the (meth)acrylic acid ester monomer and the styrene-based monomer can be appropriately set depending on the chemical structure of the styrene-based resin of the core particles, the chemical structures of the (meth)acrylic acid ester monomer and the styrene-based monomer, the properties of the polymerization initiator, the desired degree of polymerization of the composite resin, etc.

The composite resin particles obtained after the modification process preferably have a structure in which the surface layer of the resin particles contains a large amount of (meth)acrylic ester components, while the interior of the resin particles contains a large amount of styrene-based components. In the foaming process, by foaming the resin particles in which the (meth)acrylic ester components and the styrene-based components are distributed as described above, foaming inside the resin particles is promoted and the progress of foaming on the surface layer of the resin particles is suppressed, thereby further increasing the yield of hollow particles having an outer shell layer and a hollow portion.

[Impregnation process]
In the impregnation step, the core particles or composite resin particles are impregnated with the organic physical foaming agent at least once among the timings before the modification step, during the modification step, and after the completion of the modification step, to obtain resin particles. That is, the impregnation step may be performed on the core particles before the (meth)acrylic acid ester monomer and the styrene-based monomer are impregnated, or on the composite resin particles in the middle of the polymerization of the (meth)acrylic acid ester monomer and the styrene-based monomer in the modification step, or on the composite resin particles after the polymerization of the (meth)acrylic acid ester monomer and the styrene-based monomer is completed. In addition, the core particles or composite resin particles may be impregnated with the organic physical foaming agent at two or more of these timings. From the viewpoint of sufficiently impregnating the organic physical foaming agent into the inside of the composite resin particles and facilitating the hollow particle structure to have a desired structure, it is preferable to perform the impregnation step at least before the modification step. From the same viewpoint, it is more preferable that the impregnation step is carried out before the modification step, and at least one of during the modification step and after the completion of the modification step. When the impregnation step is carried out multiple times, the organic physical foaming agent used in each impregnation step may be the same or different from each other.

To impregnate the core particles or composite resin particles with the organic physical foaming agent, for example, the organic physical foaming agent may be supplied into a sealed container in which these particles are housed, and the pressure inside the sealed container may be increased. By maintaining this state, the organic physical foaming agent can be impregnated into the core particles or composite resin particles.

While the core particles or composite resin particles are being impregnated with the organic physical foaming agent, the sealed container may be heated as necessary. Heating the sealed container can further promote the impregnation of the core particles or composite resin particles with the organic physical foaming agent. The temperature and time for impregnating the organic physical foaming agent may be appropriately set according to the timing of the impregnation process. For example, in the impregnation process performed before the modification process, it is preferable to impregnate the core particles with the organic physical foaming agent by maintaining a temperature of approximately 40°C to 90°C for 0.5 hours to 3 hours. In the impregnation process performed during the modification process and/or after the completion of the modification process, it is preferable to impregnate the core particles or composite resin particles with the organic physical foaming agent by maintaining a temperature of approximately 80°C to 120°C for 3 hours to 5 hours.

The amount of the organic physical foaming agent in the impregnation step is, for example, preferably 1 part by mass or more and 10 parts by mass or less, more preferably 3 parts by mass or more and 9 parts by mass or less, and even more preferably 5 parts by mass or more and 8 parts by mass or less, per 100 parts by mass of the composite resin particles.

As described above, by carrying out the impregnation process, resin particles containing an organic physical foaming agent can be obtained. After carrying out the impregnation process, the obtained resin particles may be dehydrated and dried as necessary. The method of dehydration and drying is not particularly limited, but for example, a method of dehydrating and drying the resin particles by blowing hot air onto the resin particles using an airflow dryer can be adopted. By dehydrating and drying the resin particles, the amount of water contained in the resin particles can be reduced, and the excess organic physical foaming agent contained in the surface layer of the resin particles can be easily dispersed while maintaining the expandability of the resin particles. This makes it easier to form a good outer shell layer on the hollow particles, even if the hollow particles have a low apparent density. When using an airflow dryer, for example, the hot air temperature can be appropriately set within a range of 30°C to 60°C, and the drying time can be appropriately set within a range of 0.5 hours to 4 hours.

The average particle diameter of the resin particles is 1.0 mm or more and 5.0 mm or less. By having the resin particles have an average particle diameter in the specific range and a surface absorbance ratio S in the specific range, the structure of the hollow particles finally obtained can be more reliably made to be the desired structure. From the viewpoint of further increasing the yield of hollow particles having the specific structure, the average particle diameter of the resin particles is preferably 1.1 mm or more and 4.0 mm or less, and more preferably 1.2 mm or more and 3.0 mm or less.

The average particle diameter of the resin particles is the cumulative 63% diameter (i.e., d63) calculated based on the particle size distribution of the resin particles on a volume basis, and can be measured using a particle size distribution measuring device (for example, "Militrack JPA" manufactured by Nikkiso Co., Ltd.).

From the viewpoint of facilitating the formation of a good outer shell layer on hollow particles even in the case of hollow particles with a low apparent density, the moisture content of the resin particles is preferably 0.01% by mass or more and 1% by mass or less, more preferably 0.03% by mass or more and 0.5% by mass or less, and even more preferably 0.05% by mass or more and 0.4% by mass or less. In addition, after the impregnation process, a coating process may be carried out, if necessary, in which the surfaces of the resin particles are coated with a surface coating agent.

Surface coating agents used in the coating process include, for example, zinc stearate, stearate triglyceride, stearate monoglyceride, and castor oil. Antistatic agents can also be used as surface coating agents in the coating process. The amount of surface coating agent added is preferably 0.01 to 2 parts by mass per 100 parts by mass of resin particles.

Hollow particles can be obtained by expanding resin particles containing an organic physical blowing agent. For example, a method of heating the resin particles with a heating medium can be used to expand the resin particles. Specifically, the resin particles can be expanded by introducing a heating medium such as steam into a pre-expander to which resin particles containing an organic physical blowing agent have been supplied.

When expanding resin particles containing an organic physical blowing agent, the resin particles may be expanded in one stage, or in multiple stages. In the latter case, for example, the resin particles may be expanded to produce first-stage hollow particles with an apparent density greater than the desired apparent density, and the first-stage hollow particles may then be further expanded to produce hollow particles with the desired apparent density.

Specific aspects of the impact absorbing material and its manufacturing method will be described below. Note that the specific aspects of the impact absorbing material according to the present invention are not limited to the aspects of Examples 1 to 4 described below, and the configuration can be changed as appropriate within the scope of the spirit of the present invention.

Example 1
The impact absorbing material 1 of this example is produced by molding hollow particles 2 having an outer shell layer 21 and a hollow portion 22 surrounded by the outer shell layer 21 in a mold, as shown in Figure 1. The outer shell layer 21 of the hollow particle 2 has a resin film 211 whose base resin is a composite resin containing a (meth)acrylic acid ester component and a styrene-based component. In addition, as shown in Figures 1 and 2, the impact absorbing material 1 has a fusion portion 11 formed by fusing the outer shell layers 21 of the hollow particles 2 to each other. The apparent density of the impact absorbing material 1, the average particle diameter of the hollow particles 2 in the impact absorbing material 1, and the average value of the total thickness of the resin film 211 in the fusion portion 11 are as shown in Table 1.

In producing the impact absorbing material 1 of this example, first, core particles containing a styrene-based resin were produced by suspension polymerization, and then these core particles were used to produce resin particles containing an organic physical foaming agent. Hollow particles were obtained by foaming these resin particles. The hollow particles thus obtained were molded in a mold to obtain the impact absorbing material 1. The method for producing the impact absorbing material 1 is described in detail below.

[Preparation of core particles]
First, 765 g of deionized water, 0.84 g of a suspending agent, and 3.2 g of a surfactant were placed in a 3 L autoclave equipped with a stirrer. The suspending agent used was tribasic calcium phosphate (manufactured by Taihei Chemical Industry Co., Ltd.), and the surfactant used was sodium dodecylbenzenesulfonate (manufactured by Tokyo Chemical Industry Co., Ltd.).

Next, while stirring the contents of the autoclave, a polymerization initiator, a plasticizer, and 848 g of styrene were added to the autoclave. As the polymerization initiator, 2.2 g of t-butylperoxy 2-ethylhexanoate ("Perbutyl (registered trademark) O" manufactured by NOF Corporation) and 0.44 g of t-butylperoxy 2-ethylhexyl monocarbonate ("Perbutyl (registered trademark) E" manufactured by NOF Corporation) were used in combination. As the plasticizer, 12.7 g of cyclohexane and 6.2 g of hardened beef tallow were used in combination. The plasticizer was dissolved in styrene and added to the autoclave.

After the contents of the autoclave were stirred at room temperature for 30 minutes, the temperature inside the autoclave was increased to 90°C over 1.5 hours. After the temperature inside the autoclave reached 90°C, the autoclave was further heated, and the temperature inside the autoclave was increased to 100°C over 5.5 hours. After the temperature inside the autoclave reached 100°C, the autoclave was further heated, and the temperature inside the autoclave was increased to 110°C over 1.5 hours. This temperature was maintained for 2 hours, and then the autoclave was cooled to 30°C over 4 hours. Through the above operations, core particles with polystyrene as the base resin were produced.

After cooling was completed, the core particles were removed from the autoclave, and tricalcium phosphate adhering to the surface of the core particles was removed using nitric acid. The core particles were dehydrated and washed using a centrifuge, and then the water adhering to the surface of the core particles was removed using an airflow dryer. The average particle size of the core particles obtained as described above was as shown in Table 1. The average particle size of the core particles is the cumulative 63% diameter (i.e., d63) value in the volume-based particle size distribution obtained using a particle size distribution measuring device (Militrack JPA, manufactured by Nikkiso Co., Ltd.).

[Preparation of resin particles containing organic physical foaming agents]
965 g of deionized water and 6.0 g of sodium pyrophosphate were charged into a 3 L autoclave equipped with a stirrer. Then, 12.9 g of powdered magnesium nitrate hexahydrate was charged into the autoclave, and the contents of the autoclave were stirred at 40° C. for 30 minutes. As a result, the contents of the autoclave were converted into a slurry of magnesium pyrophosphate as a suspending agent.

Next, the surfactant, 200 g of core particles, and a polymerization initiator were placed in the autoclave. As the surfactant, a 1% by mass aqueous solution of sodium lauryl sulfonate was used, and the amount of surfactant added was 0.04 g of sodium lauryl sulfonate. As the polymerization initiator, 2.4 g of benzoyl peroxide (NOF Corp.'s "Niper (registered trademark) BW") was used.

After replacing the air in the autoclave with nitrogen, 18 g of an organic physical blowing agent was added to the autoclave over a period of 10 minutes. The organic physical blowing agent used was a mixture of about 70% by mass of normal butane and about 30% by mass of isobutane.

After the addition of the organic physical blowing agent, the temperature inside the autoclave was kept at 40°C for 1 hour. The temperature inside the autoclave was then raised to 80°C over 1 hour. While maintaining this temperature, a mixture of 200 g of styrene, 400 g of methyl methacrylate, and a polymerization initiator was added to the autoclave over 6 hours while stirring the inside of the autoclave at a stirring speed of 450 rpm. 1.2 g of t-butyl peroxybenzoate ("Perbutyl Z" manufactured by NOF Corporation) was used as the polymerization initiator.

30 minutes after the addition of styrene and methyl methacrylate, 80 g of pentane as an organic physical foaming agent was added to the autoclave over 30 minutes. Also, 30 minutes after the addition of styrene and methyl methacrylate, the temperature inside the autoclave was raised to 110°C over 1.5 hours and maintained at this temperature for 4 hours. The autoclave was then cooled to 35°C over approximately 6 hours. Through the above operations, a composite resin containing a component derived from styrene and a component derived from methyl methacrylate was used as the base resin, and resin particles containing an organic physical foaming agent were produced.

After cooling was completed, the resin particles were removed from the autoclave, and tricalcium phosphate adhering to the surface of the resin particles was removed using nitric acid. The resin particles were dehydrated and washed using a centrifuge, and then the moisture adhering to the surface of the resin particles was removed using an airflow dryer. The average particle size of the resin particles obtained as above, the weight average molecular weight of the composite resin, the moisture content in the resin particles, and the content of the organic physical foaming agent in the resin particles are as shown in Table 1. The average particle size of the resin particles is the cumulative 63% diameter (i.e., d63) value in the volume-based particle size distribution obtained using a particle size distribution measuring device (Militrack JPA, manufactured by Nikkiso Co., Ltd.).

In this example, the surfaces of the resin particles obtained as described above were coated with a surface coating agent. Specifically, a mixture of 0.11 parts by mass of zinc stearate, 0.053 parts by mass of glycerin monostearate, 0.004 parts by mass of talc, and 0.065 parts by mass of an antistatic agent ("Resistat (registered trademark) PE132" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) was added to 100 parts by mass of the resin particles, and the surfaces of the resin particles were coated with the surface coating agent containing these.

After the surfaces of the resin particles were coated with a surface coating agent, the resin particles were further dried using an airflow dryer by heating them at 40°C for 1 hour to remove excess moisture and the organic physical foaming agent.

[Preparation of hollow particles]
The resin particles obtained by the above-mentioned method were put into a normal pressure batch expansion machine with a volume of 30 L, and steam was supplied into the expansion machine. The temperature of the steam was 120° C., and the heating time was 90 seconds. In this way, the resin particles were expanded, and hollow particles having the bulk density shown in Table 1 were obtained.

[Preparation of impact absorbing material]
A molded product molding machine (Taho Precision Co., Ltd. "DSM-0705VS") was used to mold the hollow particles in the mold. First, the hollow particles obtained as described above were aged for one day in a constant temperature room at a temperature of 23 ° C. Next, the hollow particles were filled into a mold having a rectangular molding cavity of 300 mm x 75 mm x 25 mm equipped in the molding machine. Steam with a gauge pressure of 0.07 MPa (G) was introduced into this mold to heat the hollow particles, thereby performing in-mold molding. The obtained molded body was dried for one day in a drying chamber at a temperature of 40 ° C., and then further aged for one day in a constant temperature room at a temperature of 23 ° C. In this way, an impact absorbing material consisting of a molded body with an apparent density of 25 kg / m ³ was obtained. In addition, when the ratio of particles other than the hollow particles in the impact absorbing material was measured by the above-mentioned method, in this example, no particles other than the hollow particles were contained in the impact absorbing material.

Example 2
The manufacturing method of the impact absorbing material of Example 2 is the same as that of Example 1, except that after the surfaces of the resin particles containing an organic physical foaming agent are coated with a surface coating agent, drying with an air flow dryer is not performed before foaming, and the foaming conditions are changed so that the bulk density of the hollow particles becomes the value shown in Table 1.

Example 3
The manufacturing method of the impact absorbing material of Example 3 was the same as that of Example 1, except that the foaming conditions were changed so that the bulk density of the hollow particles would be the value shown in Table 1.

(Example 4 and Comparative Examples 1 to 2)
The manufacturing methods of the impact absorbing material of Example 4 and Comparative Example 1-2 are similar to the manufacturing method of the impact absorbing material of Example 1, except that the polymerization conditions were changed so that the average particle diameters of the core particles and the resin particles had the values shown in Table 1, that after the surfaces of the resin particles containing an organic physical foaming agent were coated with a surface coating agent, drying with an airflow dryer was not performed before foaming, and that the foaming conditions were changed so that the bulk density of the hollow particles had the values shown in Table 1.

(Reference example)
The impact absorbing material of the Reference Example is produced by molding expanded polystyrene resin particles in a mold. The expanded polystyrene resin particles used in producing the impact absorbing material of the Reference Example are produced by expanding polystyrene resin particles having an average particle diameter shown in Table 1. The bulk density of the expanded polystyrene resin particles used in producing the impact absorbing material of the Reference Example is as shown in Table 1. The impact absorbing material of the Reference Example is produced by molding such expanded polystyrene resin particles in a mold under the same conditions as in Example 1.

Next, we will explain how to evaluate the properties of the resin particles, hollow particles, and impact absorbing materials obtained as described above.

[Surface absorbance ratio S and internal absorbance ratio I of resin particles]
Using a Fourier transform infrared spectrophotometer ("FT/IR-460plus" manufactured by JASCO Corporation) and a total reflection absorption measurement device ("ATR PRO450-S" manufactured by JASCO Corporation), the infrared absorption spectrum of the surface and the infrared absorption spectrum of the internal of the resin particles were obtained by the method described above. Then, based on these infrared absorption spectra, the surface absorbance ratio S, the internal absorbance ratio I, and the ratio S/I of the surface absorbance ratio S to the internal absorbance ratio I were calculated. These values for the resin particles of the Examples, Comparative Examples, and Reference Examples were as shown in Table 1.

[Weight average molecular weight of base resin]
A chromatogram of the hollow particles was obtained by gel permeation chromatography (GPC) using polystyrene as a standard substance, and the weight average molecular weight calculated from the obtained chromatogram was regarded as the weight average molecular weight of the base resin.

Specifically, the chromatogram was obtained using HLC-8320GPC EcoSEC manufactured by Tosoh Corporation. After preparing a sample solution with a concentration of 0.1 wt% by dissolving resin particles as a measurement sample in tetrahydrofuran (THF), one TSKguard column SuperH-H and two TSK-GEL SuperHM-H columns were connected in series, and the measurement sample was separated by differences in molecular size using gel permeation chromatography (GPC) under the following separation conditions: eluent: tetrahydrofuran (THF), THF flow rate: 0.6 ml/min, to obtain a chromatogram. Then, the retention time in the chromatogram was converted to molecular weight using a calibration curve created using standard polystyrene, and a differential molecular weight distribution curve was obtained. The weight average molecular weight of the base resin was calculated from this differential molecular weight distribution curve. The "Weight average molecular weight of base resin" column in Table 1 shows the weight average molecular weight of the composite resin in the examples and comparative examples, and the weight average molecular weight of the styrene-based resin in the reference example.

[Water content in resin particles]
About 0.3 g of the resin particles was heated to a temperature of 160° C. using a thermal moisture vaporizer to vaporize the moisture in the resin particles. The vaporized moisture was introduced into a Karl Fischer moisture analyzer ("AQ-6" manufactured by Hiranuma Sangyo Co., Ltd.) connected to the thermal moisture vaporizer to measure the moisture content in the resin particles.

[Content of organic physical foaming agent in resin particles]
Approximately 1 g of resin particles accurately weighed to four decimal places were dried for 4 hours in a hot air dryer set at a temperature of 120° C. The dried resin particles were cooled to room temperature, and then the mass of the dried resin particles was accurately measured to four decimal places. The total volatile content was calculated from the change in mass before and after heating according to the following formula.
Total volatile content (unit: mass%) = {mass before heating (unit: g) - mass after heating (unit: g)} ÷ mass before heating (unit: g) × 100

The content of organic physical foaming agent in the resin particles was calculated by subtracting the moisture content (unit: mass%) from the total volatile content.

[Structure of hollow particles]
After dividing the hollow particle into roughly two equal parts, the cut surface of the hollow particle was observed using an electron microscope. As an example, the cut surface of the hollow particle 2 in Example 1 is shown in Figures 3 and 4. As shown in Figures 3 and 4, the hollow particle 2 in Example 1 has an outer shell layer 21 provided with a resin film 211 and a hollow portion 22 surrounded by the outer shell layer 21.

The hollow portion 22 of the hollow particle 2 is a single space separated from the external space of the hollow particle 2 by the outer shell layer 21, as shown in FIG. 3, for example. Although the hollow portion 22 of the hollow particle 2 shown in FIG. 3 is composed of a single space, the hollow portion 22 may be partitioned into several to several tens of spaces by space walls. As shown in FIG. 4, a plurality of air bubbles 212 partitioned by a resin film 211 are formed in the outer shell layer 21 of the hollow particle 2.

The hollow particles in Examples 2 to 4 and Comparative Examples 1 and 2 also have an outer shell layer and a hollow portion surrounded by the outer shell layer, similar to the hollow particles in Example 1.

On the other hand, as shown in FIG. 5, in the expanded beads 8 of the reference example, air bubbles are formed relatively uniformly throughout the entire interior of the beads, and no hollow portions exist.

[Bulk density of hollow particles and expanded particles]
1.5 g of hollow particles or expanded particles were left for 10 days under conditions of 50% relative humidity, 23°C, and 1 atm to adjust the condition. A measuring cylinder with a volume of 200 mL was prepared, and the particles after the adjustment were placed in the measuring cylinder and allowed to accumulate naturally. Then, a light impact was applied to the bottom of the measuring cylinder to stabilize the filling height of the particles in the measuring cylinder. Then, the bulk volume (unit: L) of the particles indicated by the graduations on the measuring cylinder was read. The mass of the particles placed in the measuring cylinder was divided by the bulk volume and converted into units to calculate the bulk density (unit: kg/m ³ ) of the hollow particles or expanded particles. The bulk density of the hollow particles or expanded particles in the examples and comparative examples was as shown in Table 1.

[Average thickness of outer shell layer of hollow particles before molding]
The method for calculating the average thickness of the outer shell layer of the hollow particles before molding is as described above. The average thicknesses of the outer shell layers of the hollow particles of the Examples and Comparative Examples are as shown in Table 1.

When measuring the thickness of the outer shell layer, there may be giant bubbles near the inner surface of the outer shell layer, the bubbles having a length of 100 μm or more in the thickness direction of the outer shell layer. In this example, such bubbles were treated as hollow portions. Specifically, when the giant bubbles described above are present near the inner surface of the outer shell layer, the length of the line segment from the outer surface of the hollow particle to the bubble described above was taken as the thickness of the outer shell layer.

In addition, the expanded beads of the reference example have uniform bubbles formed across the entire cut surface, so the thickness of the outer shell layer cannot be measured. For this reason, the symbol "-" is entered in the "Average thickness of outer shell layer" column for the reference example. The thickness of the resin film on the outermost surface of the expanded beads of the reference example is approximately 2 μm.

[Average value of total thickness of resin film in outer shell layer of hollow particles before molding in mold]
The method for calculating the average value of the total thickness of the resin film in the outer shell layer of the hollow particles before molding in the mold is as described above. The average values of the total thickness of the resin film in the hollow particles of the Examples and Comparative Examples are as shown in Table 1. In addition, the "Resin film/outer shell layer" column in Table 1 lists the value obtained by dividing the average value of the total thickness of the resin film by the average thickness of the outer shell layer.

[Apparent density ρ of impact absorbing material]
The apparent density ρ (unit: kg/m ³ ) of the impact absorbing material was determined by dividing the mass (unit: kg) of the impact absorbing material by the volume (unit: m ³ ) calculated based on the external dimensions. The apparent densities ρ of the impact absorbing materials of the Examples, Comparative Examples and Reference Examples are as shown in Table 2.

[Average particle diameter D of hollow particles or foam particles in impact absorbing material]
The impact absorbing material was cut in the center of its length along the thickness direction of the impact absorbing material to expose the cut surface. Next, a photograph of the cut surface of the molded body was taken, and five lines along the thickness direction of the impact absorbing material were drawn randomly on the obtained photograph from one edge to the opposite edge of the photograph. Next, the number of particles intersecting with the lines was counted. Then, the total length of each line was divided by the total number of particles intersecting with each line to calculate the average particle diameter D (unit: mm) of the hollow particles or expanded particles in the impact absorbing material. The average particle diameter D of the hollow particles in the impact absorbing materials of the examples and comparative examples, and the average particle diameter D of the expanded particles in the impact absorbing material of the reference example were as shown in Table 2. Note that the impact absorbing materials of the examples and comparative examples did not contain any particles other than the hollow particles.

[Average value t _r of total thickness of resin film in fused portion]
The impact absorbing material was cut in the center of its longitudinal direction to expose the cut surface. Using a scanning electron microscope, 30 fusion parts to be observed were randomly selected from the multiple fusion parts that appeared on the cut surface. Each fusion part to be observed was observed at a magnification of 200 times to obtain an enlarged photograph of each fusion part. Next, a line segment extending in the thickness direction of the fusion part was drawn on the fusion part in each enlarged photograph from one surface of the fusion part to the other surface of the fusion part.

The above operation was performed at 10 positions randomly selected from each enlarged photograph, the length of the line segment extending in the thickness direction of the fusion portion was measured, and the length of the portion of each line segment overlapping with the air bubble was calculated. Next, the length of the portion overlapping with the air bubble was subtracted from the length of each line segment, and the total thickness of the resin film at each fusion portion was calculated by arithmetically averaging these values. Then, the total thickness of the resin film described above was calculated for 30 randomly selected fusion portions, and the arithmetic average value of the total thickness of these resin films was obtained, which was taken as the average value t _r (unit: μm) of the total thickness of the resin film at the fusion portion. The average value t _r of the total thickness of the resin film at the fusion portion of the impact absorbing material of the embodiment and the comparative example was as shown in Table 2.

[Average thickness of fused portion t _t ]
The average thickness t t (unit: μm) of the fused portion exposed on the cut surface was calculated by arithmetically averaging the lengths of the line segments extending in the thickness direction of the fused portion measured in the measurement of the average total thickness _{t r} of the resin film in the fused portion described above. The average thickness t _t of the fused portion in the impact absorbing materials of the examples and comparative examples was as shown in Table 2.

[Compression characteristics]
A rectangular parallelepiped test piece measuring 50 mm in length, 50 mm in width, and 25 mm in thickness was taken from the center of the impact absorbing material. A compression test was performed on the test piece based on the method specified in JIS K7220:2006, and a stress-strain curve was obtained. The compression test was performed in a laboratory at 23°C.

The 10% deformation compressive stress σ ₁₀ (unit: kPa), 25% deformation compressive stress σ ₂₅ (unit: kPa), 50% deformation compressive stress σ ₅₀ (unit: kPa) and 75% deformation compressive stress σ ₇₅ (unit: kPa) in the Examples, Comparative Examples and Reference Examples calculated based on the stress-strain curves were as shown in Table 2. The "σ ₂₅ /σ ₅₀ " column in Table 2 shows the ratio of the 25% deformation compressive stress σ ₂₅ to the 50% deformation compressive stress σ ₅₀ expressed as a percentage, the "σ ₇₅ /σ ₅₀ " column shows the ratio of the 75% deformation compressive stress σ ₇₅ to the 50% deformation compressive stress σ ₅₀ expressed as a percentage, and the "σ ₁₀ /σ ₇₅ " column shows the ratio of the 10% deformation compressive stress σ ₁₀ to the 75% deformation compressive stress σ ₇₅ expressed as a percentage.

[Energy absorption characteristics]
The energy absorption efficiency (unit: %) at a certain strain amount ε ₁ (unit: %) is a value calculated by the following method based on the stress-strain curve obtained by the above-mentioned method. First, as shown in FIG. 6, the area S 1 of the region on the stress-strain curve surrounded by the portion L1 from the point P ₀ where the strain amount is 0% to the point P ₁ where the strain amount is ε ₁ , the horizontal axis, and the straight line L2 that passes through the point P ₁ and is parallel to the vertical axis is calculated. Separately, the area S ₀ of the rectangular region on the stress-strain curve surrounded by the straight line L3 that passes through the point P ₁ and is parallel to the horizontal axis, the straight line L2 that passes through the point P ₁ and is parallel to the vertical axis, the vertical axis, and the horizontal axis is calculated. Then, the ratio S ₁ /S ₀ of the area S ₁ to _the area S ₀ expressed as a percentage is defined as the energy absorption efficiency (unit: %) at the strain amount ε ₁ .

Table 2 shows the energy absorption efficiency at a strain of 25%, at a strain of 50%, and at a strain of 75% for the examples, comparative examples, and reference examples.

In addition, the area enclosed by the horizontal axis and a straight line parallel to the vertical axis that passes through the portion of the stress-strain curve from 0% strain to 50% strain measured in the compression test and the point at 50% strain on the stress-strain curve was calculated, and this value was taken as the energy absorption amount up to 50% strain. The energy absorption amount per volume was then calculated by dividing this energy absorption amount by the volume of the impact absorbing material used in the test. The energy absorption amount per mass was also calculated by dividing the energy absorption amount per volume by the apparent density of the impact absorbing material used in the test. These values were as shown in Table 2.

The aforementioned energy absorption amount per mass is preferably 5 J/g or more, and more preferably 6 J/g. In this case, the weight of the impact absorbing material component can be made lighter while maintaining good energy absorption performance. The aforementioned energy absorption amount per mass is preferably 10 J/g or less, and more preferably 8 J/g or less. In this case, the impact absorbing material does not become too hard, and can absorb impact appropriately, and when used as an automobile material, it becomes easier to ensure the safety of passengers in the event of a collision.

[Compression set]
A rectangular parallelepiped test piece measuring 50 mm in length, 50 mm in width, and 25 mm in thickness was taken from the center of the impact absorbing material. A compression test of the test piece was carried out based on the method specified in JIS K7220:2006, and after a 25% strain was applied to the test piece, the compression load was removed. After the load was completely removed, the magnitude of the strain remaining in the test piece was taken as the compression set (unit: %). The values of the compression set in the examples, comparative examples, and reference examples were as shown in Table 2.

As shown in Tables 1 and 2, the impact absorbing materials of Examples 1 to 4 are composed of hollow particles with the specific composite resin as the base resin. The apparent density ρ, the average particle diameter D of the hollow particles, and the average value _tr of the total thickness of the resin film at the fusion portion of these impact absorbing materials are all within the specific ranges. Therefore, as shown in Table 2, these impact absorbing materials have a smaller compression set than the polystyrene resin foamed bead molding (Reference Example).

In addition, because the impact absorbing materials of Examples 1 to 4 have the above-mentioned specific structure, excessive buckling of the hollow particles that make up the molded body due to an increase in compressive load is suppressed. Therefore, even when the amount of strain applied to the impact absorbing material is large, the increase in the load required for deformation is gradual. Therefore, the impact absorbing materials of Examples 1 to 4 have superior energy absorption efficiency when the amount of strain is large compared to the impact absorbing material of the reference example.

The average value t _r of the total thickness of the resin film of the fused portion in the impact absorbing material of Comparative Example 1 is smaller than the specific range. Therefore, the impact absorbing material of Comparative Example 1 is more easily deformed than the impact absorbing materials of Examples 1 to 4, and has inferior energy absorption efficiency both when the strain amount is large and when the strain amount is small.

The average value t _r of the total thickness of the resin film of the fusion portion in the impact absorbing material of Comparative Example 2 is larger than that of Comparative Example 1, but smaller than the specific range. Therefore, in the impact absorbing material of Comparative Example 2, the hollow particles constituting the molded body are likely to buckle with an increase in the compressive load, and when the amount of strain applied to the impact absorbing material increases, the ratio of increase in the load required for deformation relative to the amount of strain increases. As a result, the impact absorbing material of Comparative Example 2 is inferior in energy absorption efficiency when the amount of strain is large compared to the impact absorbing materials of Examples 1 to 4.

Reference Signs List 1: Impact absorbing material 2: Hollow particle 21: Outer shell layer 22: Hollow portion

Claims (8)

An impact absorbing material comprising a molded body obtained by molding hollow particles having an outer shell layer and a hollow portion surrounded by the outer shell layer,
the shell layer has a resin film having a base resin that is a composite resin containing a (meth)acrylic acid ester component derived from a (meth)acrylic acid ester monomer and a styrene-based component derived from a styrene-based monomer,
The apparent density ρ of the impact absorbing material is 20 kg/m ³ or more and 100 kg/m ³ or less,
The average particle diameter D of the hollow particles in the impact absorbing material is 2 mm or more and 9 mm or less,
The impact absorbing material has a fused portion in which the outer shell layers of the hollow particles are fused to each other, and the average value t _r of the total thickness of the resin film in the fused portion is 80 μm or more and 400 μm or less,
The average thickness t _t of the fused portion is more than 100 μm and 700 μm or less,
An impact absorbing material, wherein a ratio t _r /t _t of an average value t _r [μm] of a total thickness of the resin film in the fused portion to an average thickness t _t [μm] of the fused portion is 0.3 or more and 0.9 or less . The impact absorbing material according to claim 1, wherein the mass ratio of the (meth)acrylic ester component and the styrene-based component contained in the composite resin is (meth)acrylic ester component:styrene-based component = 70:30 to 30:70. 3. The impact absorbing material according to claim 1, wherein the ratio t _r /ρ [μm/(kg/m ³ )] of the average total thickness t _r [μm] of the resin film in the fused portion to the apparent density ρ [kg/m ³ ] of the impact absorbing material is 2.0 or more and 8.0 or less. The impact absorbing material according to any one of claims 1 to 3, wherein the ratio σ ₇₅ /σ ₅₀ of the 50% deformation compressive stress σ ₅₀ [kPa] at 23 ° C. to the 75% deformation compressive stress σ ₇₅ [kPa] at 23 ° C. of the impact absorbing material is 1.0 or more and less than 2.0. The impact absorbing material according to any one of claims 1 to 4, wherein the ratio σ ₁₀ /σ ₇₅ of the compressive stress at 10% deformation σ ₁₀ [kPa] at 23°C to the compressive stress at 75% deformation σ ₇₅ [kPa] at 23°C of the impact absorbing material is 0.30 or more and 1.0 or less. The impact absorbing material according to any one of claims 1 to 5 , wherein the impact absorbing material has a compression set of 10% or less. Resin particles used for producing the hollow particles to be used for forming the molded body constituting the impact absorbing material according to any one of claims 1 to 6,
A composite resin containing a (meth)acrylic acid ester component derived from a (meth)acrylic acid ester monomer and a styrene-based component derived from a styrene-based monomer is used as a base resin, and the resin particles contain an organic physical blowing agent,
The average particle size of the resin particles is 1 mm or more and 5 mm or less,
a mass ratio of the (meth)acrylic acid ester component to the styrene-based component contained in the composite resin is (meth)acrylic acid ester component:styrene-based component=70:30 to 30:70;
Resin particles, in which a surface absorbance ratio S represented by the following formula (1) in an infrared absorption spectrum of the surface of the resin particles obtained by a total reflection measurement method is 0.70 or more:
S=As _acryl /As _styrene ...(1)
(In the above formula (1), As _styrene is the maximum absorbance in the wave number range of 650 cm ^-1 or more and 750 cm ^-1 or less, and As _acryl is the maximum absorbance in the wave number range of 1680 cm ^-1 or more and 1780 cm ^-1 or less.) A hollow particle obtained by expanding the resin particle according to claim 7 ,
Hollow particles having a bulk density of 20 kg/m3 or ^more and 100 kg/m3 ^{or less} .

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