CN115873363A - Molded body and impact absorbing material - Google Patents

Abstract

A molded article having a small compression set while maintaining the compression properties of a polystyrene resin expanded pellet molded article, and an impact absorbing material comprising the molded article are provided. A molded body (1) is obtained by in-mold molding a mixture of hollow particles (2) having an outer shell layer (21) and a hollow section (22) surrounded by the outer shell layer (21) and expanded particles (3) comprising a styrene resin as a base resin. The base resin of the outer shell layer (21) in the hollow particles (2) is a composite resin containing a (meth) acrylate component and a styrene component. Hollow particles in the cross-section of the shaped body (1)(2) The average value of the ratio S1/S2 of the total S1 of the areas of the expanded particles (3) to the total S2 of the areas of the expanded particles is 0.1 to 3, and the coefficient of variation of the ratio S1/S2 is 20% or less. 10% deformation compression stress sigma of the molded article (1) measured at 23 DEG C ₁₀ And 50% deformation compressive stress sigma ₅₀ Ratio of σ ₁₀ /σ ₅₀ Is 0.70 to 1.0 inclusive.

Description

Molded body and impact absorbing material

Technical Field

The present invention relates to a molded body and an impact absorbing material.

Background

A polystyrene resin expanded bead molded body obtained by in-mold molding polystyrene resin expanded beads is lightweight and has excellent compression properties and the like, and therefore, is used in various fields such as automobile materials, building materials, and material distribution materials. For example, patent document 1 describes the following examples: the polystyrene resin foamed particle molded article is used as an impact absorbing material for automobiles such as a floor spacer and a lower limb protection pad (tibia pad) by taking advantage of the lightweight property and compression property of the polystyrene resin foamed particle molded article.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open No. 2003-127796

Disclosure of Invention

Problems to be solved by the invention

On the other hand, polystyrene resin expanded bead molded bodies generally have a property of having a large compression set. Therefore, depending on the shape of the polystyrene resin expanded particle molded article, there is a possibility that the molded article is deformed by applying a compressive load or the like, and the molded article is hard to return to its original shape. In particular, when a polystyrene resin expanded particle molded product is used as an impact absorbing material, there is a possibility that a desired energy absorbing performance cannot be exhibited if the shape of the molded product is deformed.

The present invention has been made in view of the above-mentioned circumstances, and an object thereof is to provide a molded article having a small compression set while maintaining the compression properties of a polystyrene resin expanded bead molded article, and an impact absorbing material comprising the molded article.

Means for solving the problems

One embodiment of the present invention is a molded body according to the following [1] to [6 ].

[1] A molded article obtained by in-mold molding a mixed particle of a hollow particle having an outer shell layer and a hollow portion surrounded by the outer shell layer and an expanded particle comprising a styrene resin as a base resin,

the base resin of the outer shell layer in the hollow particle is a composite resin containing a (meth) acrylate component and a styrene component,

the average value of the ratio S1/S2 of the total S1 of the areas of the hollow particles to the total S2 of the areas of the expanded particles in the cross section of the molded body is 0.1 to 3, and the coefficient of variation of the ratio S1/S2 is 20% or less,

a 10% deformation compressive stress σ of the molded article measured at 23 DEG C ₁₀ And 50% deformation compressive stress sigma ₅₀ Ratio of σ ₁₀ /σ ₅₀ Is 0.70 to 1.0 inclusive.

[2]According to [1]The molded body, wherein the average value of the area of each of the hollow particles in the cross section of the molded body is 3mm ² Above and 60mm ² And a ratio of an average value of an area of each of the hollow particles to an average value of an area of each of the expanded particles is 0.7 or more and 1.4 or less.

[3]According to [1]]Or [2 ]]The molded article, wherein the molded article has a density of 20kg/m ³ Above and 100kg/m ³ The following.

[4] The molded body according to any one of [1] to [3], wherein the compression set of the molded body is 15% or less.

[5] The molded article according to any one of [1] to [4], wherein the hollow particles and the expanded particles contain a bromine-based flame retardant, and when a flammability test based on FMVSS No.302 is performed, (1) the molded article exhibits self-extinguishing properties, or (2) the molded article has a burning rate of 102 mm/min or less.

[6] The molded article according to [5], wherein the amount of the brominated flame retardant blended in the molded article is 0.5 mass% or more and 10 mass% or less.

Another embodiment of the present invention is an impact absorbing material according to the following [7 ].

[7] An impact absorbing material comprising the molded body according to any one of [1] to [6 ].

Effects of the invention

The molded body is formed by in-mold molding a mixture of hollow particles and expanded particles. The average value and the coefficient of variation of the ratio S1/S2 of the total S1 of the areas of the hollow particles to the total S2 of the areas of the expanded particles in the cross section of the molded body are within the specific ranges. In the molded article having such a configuration, the physical properties derived from the expanded particles and the physical properties derived from the hollow particles can be exhibited in a well-balanced manner, and the compression set can be reduced.

In addition, the molded article can exhibit good compression properties by expanded beads using a styrene resin as a base resin in the initial stage of compression. Further, the hollow particles are easily deformed as the compression progresses, whereby the compression load required for deformation of the molded body can be reduced. As a result, the 10% deformation compressive stress σ can be reduced ₁₀ And 50% deformation compressive stress sigma ₅₀ Ratio of σ ₁₀ /σ ₅₀ With the above-described specific range, the energy absorption efficiency can be improved in a wide range of strain amount.

In addition, the impact absorbing material is composed of the molded body, and therefore has a low compression set and a high energy absorption efficiency both in the case where the amount of strain is small and in the case where the amount of strain is large.

As a result, according to the above aspect, a molded article having a small compression set can be provided while maintaining the compressive physical properties of the expanded polystyrene resin pellet molded article. In addition to the above properties, the molded body is excellent in energy absorption efficiency in a wide range of strain amount, and thus is suitable as an impact absorbing material.

Drawings

FIG. 1 is an example of a photograph of a cut surface of a molded article.

Fig. 2 is an example of an electron micrograph (magnification: 50 magnifications) of the outer shell portions of the hollow particles in the compact.

Fig. 3 is an explanatory diagram illustrating a method of calculating the energy absorption efficiency.

Description of the reference numerals

1: a shaped body; 2: hollow particles; 21: an outer shell layer; 22: a hollow part; 3: and (3) foaming particles.

Detailed Description

(formed body)

The molded body is formed by in-mold molding a mixture of hollow particles and expanded particles. The hollow particles have an outer shell layer made of a composite resin containing a (meth) acrylate component and a styrene component as a base resin, and a hollow portion surrounded by the outer shell layer. The outer shell layer constitutes an outer shell of the hollow particle and has a resin film made of a base resin. The outer shell layer may also be composed of a single resin film. In addition, minute air bubbles partitioned by the resin film may also exist in the outer shell layer.

The expanded beads have a structure in which a styrene resin is used as a base resin and a plurality of cells are formed relatively uniformly throughout the entire bead. Further, the more detailed structure of the hollow particles and the expanded particles will be described later.

[ ratio of expanded particles to hollow particles ]

The average value of the ratio S1/S2 of the sum S1 of the areas of the hollow particles to the sum S2 of the areas of the expanded particles in the cross section of the molded body is 0.1 to 3, and the coefficient of variation of the ratio S1/S2 is 20% or less. By setting the average value and the coefficient of variation of the ratio S1/S2 in the molded body to the specific ranges, the properties derived from the expanded particles and the properties derived from the hollow particles can be exhibited in a well-balanced manner.

That is, when a compressive load is applied to the molded article, physical properties derived from the expanded beads are exhibited at the initial stage of compression, and the compressive load required for deformation of the molded article can be appropriately increased. As a result, the energy absorption efficiency in the case where the strain amount is relatively small can be improved.

Further, since the molded article has a low compression set, it is easily restored to the shape before compression when the compression load is released from a state in which the compression progresses and the strain amount increases to a certain extent. Further, the molded body is excellent in energy absorption efficiency even when the strain amount is relatively large.

The physical properties of the molded article are considered to be derived from the hollow particles. That is, hollow particles are more deformable than expanded particles, while having higher recovery properties than expanded particles. Therefore, it is considered that the hollow particles are deformed more largely than the expanded particles in a state where the molded body is compressed to some extent. Further, it is considered that when the compression load is released from this state, the shape of the deformed hollow particles is easily restored to the shape before compression, and thus the shape of the entire molded body is also easily restored.

Further, even when the compression progresses and the strain amount becomes further large, it is considered that the compression load required for deforming the molded body can be reduced by deforming the hollow particles more largely than the expanded particles. As described above, it is considered that the molded body can reduce the compression set by the physical properties derived from the hollow particles, and can improve the energy absorption efficiency when the strain amount is relatively large.

Therefore, by exhibiting the physical properties derived from the expanded particles and the physical properties derived from the hollow particles in a well-balanced manner, it is possible to exhibit good compression physical properties and reduce the compression set of the molded article. Further, the energy absorption efficiency of the molded body can be improved in a wide range of strain amount.

If the average value of the ratio S1/S2 is too low, the ratio of the hollow particles contained in the molded article becomes too low, and therefore, there is a concern that the compression set of the molded article may increase. In this case, when the strain amount is relatively large, a compressive load required for deformation of the molded article becomes large, which may cause a reduction in compressive physical properties and a reduction in energy absorption efficiency. The above problem can be easily avoided by setting the average value of the ratio S1/S2 to 0.1 or more, preferably 0.2 or more, and more preferably 0.3 or more.

On the other hand, if the average value of the ratio S1/S2 is too high, the proportion of the hollow particles contained in the molded body becomes too high, and therefore, there is a possibility that the compressive load required for deformation of the molded body becomes low when the strain amount is relatively small. As a result, there is a possibility that the compression physical properties at the initial stage of compression are reduced and the energy absorption efficiency is reduced. In this case, there is also a possibility that the flame retardancy of the molded article may be deteriorated. The above-mentioned problem can be easily avoided by setting the average value of the ratio S1/S2 to 3 or less, preferably 2 or less, more preferably 1 or less, further preferably 0.8 or less, and particularly preferably 0.6 or less.

When the coefficient of variation of the ratio is too high, the dispersion of the hollow particles in the molded body is large, and a relatively large proportion of hollow particles and a relatively small proportion of hollow particles are likely to be formed. Further, since the distribution of the hollow particles varies, there is a possibility that local physical properties in the molded body tend to vary greatly. As a result, for example, in the initial stage of compression, the compression load required for deformation of the molded article is reduced, which may lead to a reduction in the compression physical properties and a reduction in the energy absorption efficiency. This problem can be easily avoided by setting the coefficient of variation of the ratio S1/S2 to 20% or less, preferably 15% or less, more preferably 12% or less, and still more preferably 10% or less.

The value of the ratio S1/S2 between the total S1 of the areas of the hollow particles and the total S2 of the areas of the expanded particles, the average value thereof, and the coefficient of variation are specifically calculated as follows. In calculating the value of the ratio S1/S2, first, the molded body is randomly cut to expose the cut surface, and a photograph of the cut surface of the molded body 1 as shown in fig. 1 is obtained using a scanner or the like. Next, a square measurement region having a vertical direction of 50mm and a horizontal direction of 50mm is set on the photograph of the cut surface using image processing software or the like, and the total S1 of the areas occupied by the hollow particles 2 in the measurement region and the total S2 of the areas of the expanded particles 3 are calculated. In addition, in the cut surface of the molded body 1, for example, as shown in fig. 2, since the boundaries 11 between the particles (i.e., the outer surfaces of the respective particles before the in-mold molding) clearly appear, the portions surrounded by the boundaries 11 between the particles may be the areas of the respective particles.

When calculating the average value and the coefficient of variation of the ratio S1/S2, the molded body is first cut at random to produce test pieces with a plurality of cut surfaces exposed. Next, a photograph of each cut surface of the test piece is taken by using a scanner or the like, and a measurement area is randomly set on the photograph of each cut surface. Then, in each measurement region, the value of the ratio S1/S2 of the sum S1 of the areas of the hollow particles to the sum S2 of the areas of the expanded particles is calculated by the above-described method.

By arithmetically averaging the values of the ratios S1/S2 thus obtained, an average value of the ratios S1/S2 can be obtained. Further, the coefficient of variation of the ratio S1/S2 can be calculated by calculating the unbiased standard deviation of the ratio S1/S2 based on the values of the plurality of ratios S1/S2 and dividing the unbiased standard deviation of the ratio S1/S2 by the average value of the ratio S1/S2.

The unbiased standard deviation σ of the ratio S1/S2 is specifically represented by the following formula (1).

[ number 1]

Wherein n in the above formula (1) is the total number of measurement regions set on the cut surface, S _i Is the value of the ratio S1/S2 in the ith measurement area, S _ave Is the average of the ratio S1/S2.

When calculating the average value and the coefficient of variation of the ratio S1/S2, the total number of measurement regions set on the cut surface is 50 or more. The larger the number of measurement regions, the easier it is to obtain a mean value and a standard deviation with higher accuracy.

[ area of each hollow particle and ratio of area of each hollow particle to area of each expanded particle ]

Preferably, the average value of the area of each of the above hollow particles in the cross section of the shaped body is 3mm ² Above and 60mm ² And, the average value of the area of each of the above hollow particles andthe ratio of the average area of the foamed particles is 0.7 or more and 1.4 or less. In this way, by setting the average value of the area per hollow particle within the above-described specific range and further setting the average value of the area per expanded particle to a value relatively close to the average value of the area per hollow particle, it is possible to further reduce the variation in the distribution of the hollow particles in the molded body and to more uniformly distribute the hollow particles and the expanded particles in the molded body. As a result, the physical properties derived from the hollow particles and the physical properties derived from the expanded particles can be more reliably exhibited, good compression physical properties can be exhibited, and the compression set can be more easily reduced. Further, the energy absorption efficiency of the formed body can be more easily improved in a wide range of strain amount.

From the same viewpoint, the average value of the area of each hollow particle is more preferably 5mm ² Above and 40mm ² Hereinafter, more preferably 8mm ² Above and 30mm ² The following. The ratio of the average value of the area of each hollow particle to the average value of the area of each expanded particle is preferably 0.8 or more, more preferably 0.9 or more, and still more preferably 1.0 or more.

The method of calculating the average value of the area of each hollow particle described above is specifically described below. First, the molded body is cut into a desired surface to expose the cut surface, and a photograph of the cut surface is taken with a scanner or the like. Next, a measurement region including the hollow particles is set on the photograph of the cut surface using image processing software or the like, and the total S1 of the areas occupied by the hollow particles in the measurement region and the number of the hollow particles are calculated. Then, the total S1 of the areas occupied by the hollow particles in the measurement region is divided by the number of hollow particles to obtain a value as an average value of the areas per hollow particle.

The method of calculating the average value of the area per expanded particle is the same as the method of calculating the average value of the area per hollow particle described above, except that the total S2 of the areas of the expanded particles and the number of the expanded particles are used instead of the total S1 of the areas of the hollow particles and the number of the hollow particles in the measurement region. By dividing the average value of the area per hollow particle thus obtained by the average value of the area per expanded particle, the ratio of the average value of the area per hollow particle to the average value of the area per expanded particle can be obtained.

In calculating the average value of the area of each hollow particle or expanded particle, the number of hollow particles or expanded particles present in the measurement region is, for example, preferably 100 or more. When the number of hollow particles or expanded particles present in the measurement region is small, a plurality of measurement regions may be set at random on the photograph of the cut surface, and the areas and the numbers of hollow particles or expanded particles in the measurement regions may be added.

[ Density ]

The density of the molded article is preferably 20kg/m ³ Above and 100kg/m ³ The following. The molded body having the density in the above-described specific range is lightweight, has good mechanical strength, and can more reliably bring the compression load required for deformation of the molded body within a desired range in the initial stage of compression. As a result, the energy absorption efficiency of the molded body in the case where the strain amount is small can be more reliably improved. From the viewpoint of further improving the effect, the density of the molded article is more preferably 25kg/m ³ Above, more preferably 30kg/m ³ As described above. Further, the density of the molded article is more preferably 80kg/m ³ Hereinafter, more preferably 60kg/m ³ The amount of the surfactant is preferably 50kg/m or less ³ The following. In this case, the molded body can be more easily reduced in weight while the above-described operational effects are obtained.

[ compressive stress ]

Compression stress σ of 10% deformation of the shaped body at 23 ℃ ₁₀ (unit: kPa) and 50% deformation compressive stress [ sigma ] at 23 DEG C ₅₀ (unit: kPa) ratio σ ₁₀ /σ ₅₀ Is 0.70 to 1.0 inclusive. The energy absorption characteristics of the formed body can be evaluated based on the magnitude of the compressive stress under various amounts of strain. That is, the horizontal axis represents the amount of strain applied to the molded body, and the vertical axis represents the strain amountIn a stress-strain curve in which the axis is a compressive stress corresponding to the amount of strain, the strain amount epsilon is measured from the start of compression _a The closer the shape of the stress-strain curve to a rectangular shape, the certain strain amount epsilon _a The higher the energy absorption efficiency of the formed body under (b). In other words, it can be said that the strain amount ε _a Compressive stress σ of _a And less than strain amount epsilon _a Of strain epsilon _b Compressive stress σ of _b The smaller the difference, the closer the shape of the stress-strain curve is to a rectangle, and the amount of strain ε _a The higher the energy absorption efficiency of the formed body below.

The molded article can reduce the dispersion of the hollow particles and the distribution of the expanded particles in the molded article by in-mold molding the mixed particles of the hollow particles and the expanded particles, and can make the 10% deformation compressive stress σ at 23 ℃ ₁₀ (unit: kPa) and 50% deformation compressive stress [ sigma ] at 23 DEG C ₅₀ (unit: kPa) ratio σ ₁₀ /σ ₅₀ Within the above specified range. Further, a molded body having such characteristics has good compression physical properties and excellent energy absorption characteristics over a wide range of strain amounts.

From the same viewpoint, the molded article has a 10% strain compression stress σ at 23 ℃ ₁₀ (unit: kPa) and 75% deformation compressive stress [ sigma ] at 23 DEG C ₇₅ (unit: kPa) ratio σ ₁₀ /σ ₇₅ Preferably 0.30 or more and 1.0 or less, more preferably 0.38 or more and 0.70 or less, and still more preferably 0.40 or more and 0.60 or less.

In addition, 10% deformation compressive stress σ ₁₀ The ratio of the density of the molded article to the density of the molded article is preferably 8.0 kPa/(kg/m) ³ ) More preferably 8.5 kPa/(kg/m) ³ ) As described above. In this case, the compression physical properties of the molded body can be further improved, and the energy absorption characteristics of the molded body can be further improved in a wide range of strain amount. In addition, 10% deformation compressive stress σ ₁₀ The upper limit of the ratio of the density of the molded article to the density of the molded article is not particularly limited, and may be, for example, 15 kPa/(kg/m) ³ ) It may be 12 kPa/(kg/m) ³ )。

[ compression set ]

The compression set of the molded article is preferably 15% or less, more preferably 12% or less, and still more preferably 10% or less. A molded body having such characteristics is less likely to undergo permanent deformation even when a compressive load is applied thereto, and is easily restored to a shape before compression. Therefore, even when a compression load is applied to a molded article having a compression set within the above-specified range, the shape of the molded article is not easily deformed, and excellent energy absorption characteristics can be maintained for a longer period of time. Further, the compression set of the formed body was based on JIS K6767:1999, the method defined in the specification.

[ thickness of outer shell layer of hollow particle ]

The average value of the thicknesses of the outer shell layers of the hollow particles in the molded body is preferably 50 μm or more and 400 μm or less. In this case, the physical properties derived from the hollow particles can be more reliably exhibited, and the compression set of the molded article can be more easily reduced. In this case, an increase in the compressive load required for deformation of the molded body due to an increase in the amount of strain can be suppressed, and the energy absorption efficiency in the case where the amount of strain is large can be further improved.

The method of calculating the average value of the thicknesses of the outer shells of the hollow particles in the molded body is specifically as follows. First, the molded body is cut into a desired surface to expose the cut surface, and a cross-sectional photograph is taken using a scanner or the like. Next, a measurement region including hollow particles is set on the photograph of the cut surface using image processing software or the like, and the area of the hollow particles and the area of the hollow portion are calculated for each hollow particle existing in the measurement region.

Next, based on the area of the hollow particles and the area of the hollow portion, the equivalent circle diameter of the hollow particles (i.e., the diameter of a circle equal to the area of the hollow particles) and the equivalent circle diameter of the hollow portion (i.e., the diameter of a circle equal to the area of the hollow portion) are calculated. Then, 1/2 of the value obtained by subtracting the equivalent circle diameter of the hollow portion from the equivalent circle diameter of the hollow particle was taken as the thickness of the outer shell layer of each hollow particle.

After the above operation is performed on the plurality of hollow particles, the thicknesses of the shell layers of the obtained plurality of hollow particles are arithmetically averaged, whereby an average value of the thicknesses of the shell layers of the hollow particles can be obtained. The number of hollow particles used for calculating the average value of the thicknesses of the outer shells of the hollow particles is, for example, preferably 100 or more. When the number of hollow particles present in the measurement region is small, a plurality of measurement regions may be set at random on the photograph of the cut surface, and the hollow particles in the measurement regions may be used. When there are a plurality of cut surfaces, the measurement area may be set for each of the plurality of cut surfaces.

[ Total thickness of resin films in outer shells of hollow particles ]

The average value of the total thickness of the resin films in the outer shell layers of the hollow particles in the molded body is preferably 40 μm or more and 200 μm or less. In this case, the physical properties derived from the hollow particles can be more reliably expressed, and the compression set of the molded article can be more easily reduced. In this case, an increase in the compressive load required for deformation of the molded body due to an increase in the amount of strain can be suppressed, and the energy absorption efficiency in the case where the amount of strain is large can be further improved.

The method of calculating the average value of the total thickness of the resin films in the outer shell layer of the hollow particles is specifically as follows. First, the molded body is cut at an appropriate position to expose the cut surface. Using a scanning electron microscope, 30 or more hollow particles to be observed were randomly selected from among the hollow particles appearing on the cut surface. By observing these hollow particles at an appropriate magnification (for example, 200 times magnification), a magnified photograph of the outer shell layer of each hollow particle is taken.

Next, on the outer shell layer of each hollow particle, a line segment extending in the thickness direction of the outer shell layer is drawn from the outer surface of the outer shell layer (i.e., the surface welded to the other particles) to the inner surface of the outer shell layer (i.e., the surface facing the hollow portion). This operation is performed at ten or more positions randomly selected from within each magnified photograph, the length of a line segment extending in the thickness direction of the outer shell layer is measured, and the length of a portion of each line segment that overlaps with the bubble is calculated. Next, the length of the portion overlapping the bubble is subtracted from the length of each line segment, thereby calculating the total value of the thicknesses of the resin films of the outer shells on the line segments. The total thickness of the resin films of the outer shell layers in the respective hollow particles is calculated by arithmetically averaging the total value of the thicknesses of the resin films in the respective line segments.

Then, the total thickness of the resin films is calculated for 30 or more hollow particles, and then the average value of the total thickness of the resin films in the outer shell layers of the hollow particles can be obtained by arithmetically averaging the total thickness of the resin films.

[ flame retardancy ]

When the hollow particles and the expanded particles in the molded article contain a bromine-based flame retardant and a flammability test is performed using FMVSS (Federal Motor Vehicle Safety Standards) No.302, it is preferable that (1) the molded article exhibits self-extinguishing properties or (2) the molded article has a burning rate of 102 mm/min or less. The phrase "the molded article exhibits self-extinguishing properties" in the condition (1) means that any one of the conditions (1 a) to (1 c) is satisfied, where the condition (1 a): the molded body does not catch fire; condition (1 b): the combustion of the molded body is completed before reaching a position where the measurement of the combustion time is started; and condition (1 c): the combustion of the molded body is completed within 60 seconds after reaching the position at which the measurement of the combustion start time is started, and the combustion distance from the position at which the measurement of the combustion start time is started to the position at which the combustion is completed is within 51 mm. The molded body satisfying the above condition (1) or condition (2) has excellent flame retardancy in accordance with FMVSS No.302 standard, and therefore can be suitably used as an impact absorbing material for vehicles.

From the viewpoint of more reliably imparting excellent flame retardancy to the molded article, the amount of the bromine-based flame retardant incorporated in the molded article is preferably 0.5 mass% or more and 10 mass% or less. From the same viewpoint, it is more preferable that the amount of the bromine-based flame retardant blended in the hollow particles in the molded article is 0.05% by mass or more and 15% by mass or less, and the amount of the bromine-based flame retardant blended in the expanded particles is 0.05% by mass or more and 15% by mass or less.

When the molded article contains the brominated flame retardant, the content of the organic physical blowing agent in the molded article is preferably 4% by mass or less (including 0% by mass), and more preferably 3% by mass or less (including 0% by mass), from the viewpoint of more reliably improving the flame retardancy. Examples of the organic foaming agent in the molded article include hydrocarbons having 3 to 6 carbon atoms. The organic physical foaming agent in the molded article is derived from, for example, an organic physical foaming agent used in the process of producing expanded particles or hollow particles. Such an organic foaming agent generally remains in the pellets after the in-mold molding of the mixed pellets, and remains in the molded article.

[ use ]

As described above, the molded article has good compression physical properties and low compression set, and has excellent energy absorption characteristics over a wide range of strain amounts. Therefore, the molded article is suitable as an impact absorbing material. The impact absorbing material composed of the molded body can be used for, for example, automobile materials, building materials, material distribution materials, cushioning materials, bedding, and the like. In particular, a shock absorbing material used in a vehicle such as an automobile needs to appropriately absorb a large shock generated at the time of a collision or the like and reduce the shock acceleration to be generated to an occupant. The molded article of the present invention is excellent in energy absorption efficiency in a wide range of strain amount, and therefore, has excellent suitability as a vehicle impact absorbing material such as a floor spacer, a lower limb protection pad, a door pad, a bumper and the like for a vehicle, and is particularly suitable as a floor spacer and a lower limb protection pad.

(method for producing molded article)

The molded body is produced, for example, as follows. First, a mold having a cavity corresponding to the shape of a desired molded body is filled with mixed particles of the hollow particles and the expanded particles. When the mixed particles are filled into the mold, the hollow particles and the expanded particles may be mixed in advance, and then the mixed particles may be filled into the mold. Alternatively, the hollow particles and the expanded particles may be simultaneously supplied into the mold, and the hollow particles and the expanded particles may be filled into the mold while being mixed.

Preferably, the bulk density of the hollow particles filled in the forming mold is 20kg/m ³ Above and 100kg/m ³ Hereinafter, the ratio of the bulk density of the hollow particles to the bulk density of the expanded particles is 0.7 or more and 1.4 or less. In this way, by setting the bulk density of the hollow particles within the above-specified range and further setting the bulk density of the expanded particles to a value relatively close to the bulk density of the hollow particles, the hollow particles and the expanded particles can be more uniformly mixed, and the hollow particles and the expanded particles can be more uniformly distributed in the molding die. As a result, the molded article after in-mold molding can more easily exhibit both the physical properties derived from the hollow particles and the physical properties derived from the expanded particles, and can exhibit good compression physical properties, and the compression set can more easily be reduced. Further, the energy absorption efficiency of the molded body can be more easily improved in a wider range of strain amount.

From the same viewpoint, it is preferable that the average particle diameter of the hollow particles filled in the mold is 2mm or more and 9mm or less, and the ratio of the average particle diameter of the hollow particles to the average particle diameter of the expanded particles is 0.7 or more and 1.4 or less. Further, the average particle diameter of the hollow particles is a value of cumulative 63% diameter (i.e., d 63) calculated based on the particle size distribution on a volume basis of the hollow particles. The particle size distribution on a volume basis of the hollow particles can be obtained by using a particle size distribution measuring apparatus (for example, "Millitrack JPA" manufactured by Nikkiso K.K.) or the like. Likewise, the average particle diameter of the expanded particles is a value of cumulative 63% diameter (i.e., d 63) calculated based on the particle size distribution on a volume basis of the expanded particles. The particle size distribution on a volume basis of the expanded particles can be obtained by using a particle size distribution measuring apparatus (for example, "Millitrack JPA" manufactured by Nikkiso K.K.) or the like.

After the mixed particles are filled in the mold, the mixed particles in the mold are heated with a heating medium such as steam. The hollow particles and the expanded particles in the molding die are heated in the cavity to expand and are fused to each other. By the above operation, a molded body composed of mixed particles of hollow particles and expanded particles having a shape corresponding to the shape of the cavity is obtained. The molded article thus obtained can be used as it is as an impact absorbing material. Further, the molded body may be subjected to cutting or the like as necessary, and the molded body molded into a desired shape may be used as the impact absorbing material.

(hollow particle)

The hollow particles used for producing the molded body 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 foaming resin particles containing a physical foaming agent described later. The hollow particles of the present invention have a structure different from that of expanded particles having a bubble structure in which a plurality of bubbles are formed relatively uniformly throughout the entirety of the particles. The hollow particles before in-mold molding are preferably spherical.

The hollow portion of the hollow particle is located closer to the center portion of the hollow particle than the outer shell layer. The hollow portion, that is, the region surrounded by the outer shell layer may be substantially hollow. Here, "substantially hollow" means a state in which the structure of the shell layer is significantly different from that of the hollow portion when the cross section of the hollow particle is observed at a magnification of 20 to 1000 times using a transmission electron microscope or the like after the hollow particle is cut in a cross section passing through the center portion thereof.

For example, the hollow portion may be a single space separated from the outside of the hollow particle by the outer shell layer. In addition, a space wall which is made of a base resin and divides the hollow portion into a plurality of spaces may be present in the hollow portion. In this case, the hollow portion may have a hollow space wall formed therein, and several to several tens of spaces defined by the hollow space wall of the hollow portion and the inner surface of the envelope layer. In terms of easily expressing desired physical properties, the hollow portion is preferably divided into 50 or less spaces, more preferably 30 or less spaces, still more preferably 10 or less spaces, and particularly preferably 5 or less spaces in a cross section obtained by dividing the hollow particles into two.

If the region surrounded by the outer shell layer of the hollow particles is substantially hollow, a molded article having desired physical properties can be obtained by in-mold molding, and a molded article having a small compression set and excellent energy absorption efficiency in a wide range of strain amount can be produced. The hollow portion of the hollow particle is preferably a single space separated from the outside of the hollow particle by the outer shell layer. In addition, with respect to the structure and hollow portion of the hollow particles in the molded article after the in-mold molding, the description of the structure and hollow portion of the hollow particles used in the production of the molded article is referred to as appropriate.

[ average particle diameter ]

The average particle diameter of the hollow particles before in-mold molding is preferably 2mm or more and 9mm or less. By performing in-mold molding of the hollow particles having the average particle diameter in the above-described specific range, a molded body having desired characteristics can be obtained more easily. The average particle diameter of the hollow particles can be measured by the method described above.

[ bulk Density ]

The bulk density of the hollow particles before in-mold molding is preferably 20kg/m ³ Above and 100kg/m ³ The following. The hollow particles having the hollow structure and having the bulk density in the specific range described above are lightweight and excellent in both rigidity and recovery. Therefore, by performing in-mold molding of the hollow particles having the bulk density together with expanded particles, a molded body which is lightweight, has a small compression set, and is excellent in energy absorption efficiency in a wide range of strain amount can be easily obtained. From the viewpoint of improving the rigidity and recovery of the hollow particles, the bulk density of the hollow particles before in-mold molding is more preferably 22kg/m ³ Above, more preferably 25kg/m ³ As described above.

Further, from the viewpoint of further improving the lightweight property of the hollow particles, the bulk density of the hollow particles before in-mold molding is more preferably 80kg/m ³ Hereinafter, more preferably 60kg/m ³ The amount of the surfactant is preferably 50kg/m or less ³ The following.

As described aboveThe bulk density of the hollow particles is a value calculated by the following method. First, an arbitrary amount of hollow particles is prepared as a measurement target. The state of the measurement object was adjusted by leaving the measurement object under the conditions of a relative humidity of 50%, a temperature of 23 ℃ and 1atm for 10 days. After the measurement object is naturally accumulated in the measuring cylinder, the bottom surface of the measuring cylinder is lightly impacted to stabilize the filling height of the measurement object in the measuring cylinder. Then, the volume (unit: L) of the measurement object indicated by the scale of the measuring cylinder is read. The mass (unit: g) of the measurement object placed in the measuring cylinder was divided by the bulk volume, and the unit was converted. By the above operation, the bulk density (unit: kg/m) of the hollow particles can be obtained ³ )。

[ outer Shell layer ]

The outer shell layer of the hollow particle has a resin film composed of a base resin and a plurality of bubbles partitioned by the resin film. The outer shell layer may have, for example, a multilayer structure including a solid lower layer portion facing the hollow portion, a solid upper layer portion exposed to the outermost surface of the hollow particles (i.e., the outer surface of the outer shell layer), and a foamed layer portion located between the lower layer portion and the upper layer portion and including a plurality of bubbles. The solid state means a state in which the resin has substantially no bubbles.

The average value of the total thickness of the resin films in the hollow particles before in-mold molding is preferably 30 μm or more, more preferably 35 μm or more, and still more preferably 40 μm or more. By performing in-mold molding on such hollow particles, a molded body having a suitably high compression load required for deformation in the initial stage of compression and a high energy absorption efficiency in the case where the strain amount is large can be obtained more easily. The average value of the total thickness of the resin films in the hollow particles is preferably 200 μm or less, more preferably 150 μm or less, and still more preferably 120 μm or less. In this case, a molded body having a low density and excellent energy absorption efficiency can be obtained more easily.

The average thickness of the outer shell layer in the hollow particles before in-mold forming is preferably more than 50 μm and 350 μm or less. The ratio of the average value of the total thickness of the resin films in the hollow particles before the in-mold molding to the average thickness of the outer shell layer is preferably 0.3 or more, and more preferably 0.5 or more. By configuring the outer shell layer to have a structure determined based on the above average thickness and the ratio of the average value of the total thicknesses of the resin films to the average thickness of the outer shell layer, the durability of the outer shell layer against a load can be improved. This can improve the resilience of the hollow particle against repeated loading.

The average thickness of the outer shell layer is a value calculated by the following method. First, the hollow pellet before the in-mold forming is divided into approximately halved portions, and the cut surface of the outer shell layer is exposed. Three or more observation positions were randomly set in one of the regions obtained by approximately quartering the cut surface of the outer shell layer using a scanning electron microscope. By observing the observation positions at an appropriate magnification (for example, 1000 magnifications), magnified photographs of the cut surfaces of the shell layer at the respective observation positions are obtained.

On the outer shell layer in the obtained magnified photograph, a line segment extending in the thickness direction of the outer shell layer (i.e., the radial direction of the hollow particles) is drawn from the outer surface to the inner surface of the outer shell layer. The above operation is performed at ten or more positions randomly selected from within each magnified photograph, and the length of line segments at positions of thirty or more positions in total is measured. The arithmetic average of the lengths of the line segments thus obtained was taken as the thickness of the outer shell of each hollow particle. Then, the thickness of the outer shell layer was calculated for five or more hollow particles selected at random, and the arithmetic average of the thicknesses of the outer shell layers was defined as the average thickness of the outer shell layer.

In the measurement of the thickness of the outer shell layer, there may be a case where large bubbles having a length of 100 μm or more in the thickness direction of the outer shell layer exist in the vicinity of the inner surface of the outer shell layer or the like. Such large bubbles are treated as hollow portions. Specifically, when the large bubbles 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 point where the bubble is reached may be the thickness of the outer shell layer.

In calculating the average value of the total thickness of the resin films of the outer shell layer in the hollow particles before in-mold molding, three or more observation positions are randomly set for one region in which the cut surface of the outer shell layer is roughly divided into four equal parts, and magnified photographs of the cut surface of the outer shell layer at each observation position are obtained, in the same manner as the method of calculating the average thickness of the outer shell layer. Next, a line segment extending in the thickness direction of the shell layer is drawn from the outer surface to the inner surface of the shell layer in each enlarged photograph.

The above operation is performed at ten or more positions randomly selected from within each magnified photograph, the length of a line segment extending in the thickness direction of the outer shell layer is measured, and the length of a portion of each line segment that overlaps with the bubble is calculated. Next, the length of the portion overlapping the bubble is subtracted from the length of each line segment, and the arithmetic mean is performed on the values, thereby calculating the total thickness of the resin film of each hollow particle. Then, the total thickness of the resin films is calculated for five or more randomly selected hollow particles, and the arithmetic average of the total thicknesses of the resin films is defined as the average of the total thicknesses of the resin films.

[ base resin ]

The base resin of the resin film in the hollow particles is a composite resin containing a (meth) acrylate component derived from a (meth) acrylate monomer and a styrene component derived from a styrene monomer (sometimes referred to as "styrene monomer component"). As described later, the composite resin is preferably a composite resin obtained by impregnating and polymerizing a (meth) acrylate monomer and a styrene monomer in a styrene resin (styrene resin core particle). The total of the proportion of the (meth) acrylate component and the proportion of the styrene component in the composite resin is preferably 80% by mass or more, more preferably 90% by mass or more, and still more preferably 95% by mass or more.

Examples of the (meth) acrylate component contained in the composite resin include components derived from an acrylate monomer such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate, and components derived from a methacrylate monomer such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, and 2-ethylhexyl methacrylate. The composite resin may contain one component selected from the (meth) acrylate components described above, or may contain two or more components.

In the composite resin, a component derived from methyl methacrylate is preferably contained as the (meth) acrylate component. In this case, the content ratio of the component derived from methyl methacrylate in the (meth) acrylate component is preferably 50% by mass or more, more preferably 60% by mass or more, still 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 desired.

Examples of the styrene component contained in the composite resin include components derived from styrene monomers such as styrene, α -methylstyrene, o-methylstyrene, m-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, styrenesulfonic acid, and sodium styrenesulfonate, and components derived from copolymers of styrene monomers such as rubber-modified polystyrene, acrylonitrile-butadiene-styrene copolymer, acrylonitrile-styrene copolymer, and acrylonitrile-ethylene propylene rubber-styrene copolymer, with other monomers or polymers. The composite resin may contain one component selected from the styrene-based components described above, or may contain two or more components.

In the composite resin, a styrene-based component is preferably contained as the styrene-based component. In this case, the content of the styrene-derived component in the styrene-based component is preferably 50% by mass or more, more preferably 60% by mass or more, still 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 desired.

The composite resin may contain other components than the (meth) acrylate component and the styrene component. Examples of the component include components derived from monomers having a carbon-carbon double bond such as a hydroxyl group-containing vinyl compound such as hydroxyethyl acrylate and a nitrile group-containing vinyl compound such as acrylonitrile.

The amount of the other components other than the (meth) acrylate component and the styrene component in the composite resin is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and still more preferably 5 parts by mass or less, based on 100 parts by mass of the total of the (meth) acrylate component and the styrene component. In this case, the structure and physical properties of the hollow particles can be more reliably made to be desired.

The composite resin can be obtained, for example, by impregnating a polymerized (meth) acrylate monomer and a styrenic monomer into a styrenic resin core particle. As the base resin of the styrene-based resin core particle, a styrene-based resin composed of the above styrene-based component can be used. More specifically, as the base resin of the styrene-based resin core particle, a homopolymer of a styrene-based monomer, a copolymer of a styrene-based monomer and another monomer or polymer, or the like can be used. The base resin of the styrenic resin core particle is preferably polystyrene.

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

In the case of producing a composite resin by impregnating a core particle made of a base resin containing a (meth) acrylate component and/or a styrene component with a polymerized (meth) acrylate monomer and/or a styrene component, the mass ratio of the (meth) acrylate component to the styrene 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 subjected to the impregnation polymerization, or the like.

[ flame retardant ]

The hollow particles may contain a bromine-based flame retardant. In this case, the flame retardancy of the molded article can be further improved. The amount of the bromine-based flame retardant to be blended in the hollow particles is preferably 0.05 mass% or more and 15 mass% or less from the viewpoint of improving the fused state between the particles and further improving the flame retardancy of the molded article. As the bromine-based flame retardant, for example, a brominated bisphenol-based flame retardant, a brominated styrene-butadiene-based copolymer, or the like can be used. Examples of the brominated bisphenol flame retardant include a bromide having a bisphenol a skeleton, a bromide having a bisphenol F skeleton, and a bromide having a bisphenol S skeleton. More specifically, as the brominated bisphenol flame retardant, a brominated bisphenol A flame retardant such as 2, 2-bis (4- (2, 3-dibromo-2-methylpropyloxy) -3, 5-dibromophenyl) propane or 2, 2-bis (4- (2, 3-dibromopropyloxy) -3, 5-dibromophenyl) propane, or a brominated bisphenol S flame retardant such as bis [3, 5-dibromo-4- (2, 3-dibromopropyloxy) phenyl ] sulfone can be used. Further, as the brominated styrene-butadiene copolymer, a brominated styrene-butadiene block copolymer can be cited. The bromine-based flame retardant may be used alone, or two or more kinds of bromine-based flame retardants may be used in combination.

Further, the method of adding the flame retardant to the hollow particles is not particularly limited. For example, a hollow particle containing a flame retardant can be obtained by impregnating a core particle made of a styrene resin as a base resin with a polymerized (meth) acrylate monomer and a styrene monomer in the presence of the flame retardant to obtain a composite resin particle containing the flame retardant, and then impregnating the composite resin particle with a blowing agent and then foaming the composite resin particle.

[ additives ]

The hollow particles may contain additives such as a bubble control agent, a catalyst neutralizer, a lubricant, a crystal nucleating agent, and an antistatic agent in a range not to impair the effects of the above-described actions. The content of the additive in the hollow particles is, for example, preferably 20% by mass or less, more preferably 15% by mass or less, further preferably 10% by mass or less, and particularly preferably 5% by mass or less, relative to the total mass of the hollow particles.

(method for producing hollow particles)

As a method for producing the hollow particles, for example, the following method can be employed: after resin particles containing the composite resin as a base resin and an organic foaming agent are prepared, the resin particles are foamed.

As a method for producing the resin particles, for example, a method of impregnating a styrene-based resin core particle (hereinafter, appropriately referred to as "core particle") with a (meth) acrylate monomer and a styrene-based monomer, polymerizing the (meth) acrylate monomer and the styrene-based monomer, and impregnating an organic physical blowing agent can be employed.

More specifically, the method for producing resin particles may have:

a dispersion step of dispersing the core particles in an aqueous medium;

a modification step of impregnating the core particles dispersed in the aqueous medium with a (meth) acrylate monomer and a styrene monomer to polymerize the (meth) acrylate monomer and the styrene monomer to obtain resin particles; and

and an impregnation step of impregnating the organic physical blowing agent into the core particles or the resin particles in the impregnation step at least once before the modification step, during the modification step, and after the modification step is completed. The dispersing step, the modifying step and the impregnating step may be continuously performed in a single closed vessel, or may be performed using different vessels. Hereinafter, each step will be described in detail.

[ Dispersion step ]

In the dispersion step, a suspension is prepared by dispersing the core particles in an aqueous medium. As the aqueous medium, for example, deionized water or the like can be used. In the aqueous medium, a suspending agent, a surfactant, and the like may be added as necessary in addition to the core particles.

The base resin of the core particle used in the dispersion step is preferably the above-mentioned styrene-based resin, more preferably a styrene-based resin containing not less than 50% by mass of a component derived from styrene, and still more preferably polystyrene. In the core particle, other resins than a styrene-based resin such as an acrylic resin as a polymer of a (meth) acrylate monomer may be contained within a range not impairing the above-described action and effect. The core particle may contain additives such as a bubble control agent, a pigment, a slip agent, an antistatic agent, and a flame retardant.

The method for producing the core particle is not particularly limited. For example, a method of producing the core particles may be a method using a granulating apparatus equipped with an extruder. In this case, a wire cutting method, an underwater cutting method, a thermal cutting method, or the like can be employed. The core particle can be produced by, for example, 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 particle is preferably 0.6mm or more and 2.0mm or less, and more preferably 0.7mm or more and 1.5mm or less. In this case, the polymerization stability in the modification step is improved, and the weight average molecular weight of the composite resin is easily increased. Further, by performing the modification step and the impregnation step using the core particles having an average particle diameter within the above-described specific range, the structure of the finally obtained hollow particles can be formed into a desired structure more reliably.

The average particle diameter of the core particle is a cumulative 63% diameter (i.e., d 63) calculated from the particle size distribution based on the volume of the core particle, and can be measured using a particle size distribution measuring apparatus (for example, "millirack JPA" manufactured by japan electronics corporation).

Examples of the suspending agent used in the dispersion step include inorganic suspending agents formed from fine particles of inorganic substances such as tricalcium phosphate, hydroxyapatite, magnesium pyrophosphate, magnesium phosphate, aluminum hydroxide, iron 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 hydroxypropylmethyl cellulose. The above-mentioned 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.

Examples of the surfactant include anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants. The surfactant is preferably an anionic surfactant, more preferably an alkali metal salt of an alkylsulfonic acid having 8 to 20 carbon atoms, and still more preferably sodium alkylsulfonate having 8 to 20 carbon atoms. By using the surfactant, when the (meth) acrylate monomer and the styrene-based monomer are added in the subsequent modification step, the state in which the core particles, the monomers, and the like are suspended in the aqueous medium can be maintained more easily.

An electrolyte composed of an inorganic salt such as lithium chloride, potassium chloride, sodium sulfate, sodium nitrate, sodium carbonate, or sodium hydrogen carbonate may be added to the suspension as needed.

[ modification step ]

In the modification step, a (meth) acrylate monomer and a styrene monomer are added to the suspension, and the (meth) acrylate monomer and the styrene monomer are impregnated into the core particles and polymerized. Thereby, composite resin particles are obtained which have a composite resin comprising a styrene component and a (meth) acrylate component as a base resin.

The proportion of styrene in the styrene-based monomer added in the modification step is preferably 50 mass% or more, more preferably 60 mass% or more, still more preferably 80 mass% or more, and particularly preferably 90 mass% or more. In this case, the structure and physical properties of the hollow particles can be more reliably made to be desired. From the same viewpoint, the proportion of methyl methacrylate in the (meth) acrylate monomer added in the modification step is preferably 50% by mass or more, more preferably 60% by mass or more, still more preferably 80% by mass or more, and particularly preferably 90% by mass or more.

The total amount of the (meth) acrylate monomer and the styrene monomer added in the modification step is 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, with respect to 100 parts by mass of the core particle, for example. The ratio of the amount of the (meth) acrylate monomer to the amount of the styrene monomer may be appropriately set according to the desired composition of the composite resin. From the viewpoint of easily obtaining good hollow particles, the ratio of the amount of the (meth) acrylate monomer to the amount of the styrene-based monomer is preferably (meth) acrylate monomer to styrene-based monomer = 50: 50 to 85: 15, and more preferably (meth) acrylate monomer to styrene-based monomer = 60: 40 to 75: 25 in terms of mass ratio.

In the modification step, a polymerization initiator is used to polymerize the (meth) acrylate monomer and the styrene monomer. The polymerization initiator is not particularly limited as long as it is a polymerization initiator suitable for suspension polymerization of a styrene-based monomer. As the polymerization initiator, for example, a polymerization initiator soluble in a vinyl monomer and having a 10-hour half-life temperature of 50 ℃ or more and 120 ℃ or less can be used. Examples of the polymerization initiator include cumyl hydroperoxide, dicumyl peroxide, t-butyl peroxy-2-ethylhexanoate, t-butyl peroxybenzoate, benzoyl peroxide, t-butyl peroxyisopropyl carbonate, t-amyl peroxy-2-ethylhexyl carbonate, hexyl peroxy-2-ethylhexyl carbonate, organic peroxides such as lauroyl peroxide, and azo compounds such as azobisisobutyronitrile. The polymerization initiators may be used alone or in combination of two or more.

The polymerization initiator may be added to the aqueous medium in a state of being dissolved in a solvent or a monomer, and impregnated into the core particle together with the (meth) acrylate monomer and the styrene monomer. In this case, as the solvent, for example, aromatic hydrocarbons such as ethylbenzene and toluene, and aliphatic hydrocarbons such as heptane and octane can be used. The amount of the polymerization initiator added is preferably 0.01 to 3 parts by mass based on 100 parts by mass of the total of the (meth) acrylate monomer and the styrene monomer added in the modification step.

In the modification step, a bubble regulator, a plasticizer, an oil-soluble polymerization inhibitor, a flame retardant, a dye, and the like may be added as necessary. The bubble control agent is added to the aqueous medium in a state of being dissolved or dispersed in the monomer and/or the solvent, for example. Examples of the cell regulator include fatty acid monoamide, fatty acid bisamide, talc, silica, polyethylene wax, methylene bisstearic acid, methyl methacrylate copolymer, and silicone. Examples of the fatty acid monoamide include oleic acid amide and stearic acid amide. Examples of the fatty acid bisamide include ethylene bisstearamide.

The heating temperature in the modification step, the time required for adding the (meth) acrylate monomer and the styrene monomer may be appropriately set depending on the chemical structure of the styrene resin of the core particle, the chemical structures of the (meth) acrylate monomer and the styrene monomer, the characteristics of the polymerization initiator, the desired degree of polymerization of the composite resin, and the like.

The composite resin particle obtained after the modification step preferably has a structure in which a large amount of a (meth) acrylate component is contained in a surface layer portion of the resin particle, while a large amount of a styrene component is contained in the interior of the resin particle. In the foaming step, by foaming the resin particles in which the (meth) acrylate component and the styrene component are distributed as described above, the foaming inside the resin particles is promoted, the progress of the foaming in the surface layer portion of the resin particles is suppressed, and the yield of the hollow particles having the outer shell layer and the hollow portion is further improved.

[ immersion step ]

In the impregnation step, the organic foaming agent is impregnated into the core particles or the composite resin particles at a timing of at least one or more of before the modification step, during the modification step, and after the modification step is completed, thereby obtaining the resin particles. That is, the impregnation step may be performed on the core particles before the (meth) acrylate monomer and the styrene monomer are impregnated, or may be performed on the composite resin particles during the polymerization of the (meth) acrylate monomer and the styrene monomer in the modification step, or on the composite resin particles after the polymerization of the (meth) acrylate monomer and the styrene monomer is completed. In addition, the organic physical blowing agent may be impregnated into the core particles or the composite resin particles at two or more times among the above timings. The impregnation step is preferably performed at least before the modification step, from the viewpoint of sufficiently impregnating the organic foaming agent into the composite resin particles and easily forming the structure of the hollow particles into a desired structure. From the same viewpoint, the impregnation step is more preferably performed before the modification step, and is performed at least at one timing of the middle of the modification step and after the end of the modification step. When the impregnation step is performed a plurality of times, the organic physical blowing agents used in the respective impregnation steps may be the same or different from each other. As the organic foaming agent, for example, a known organic foaming agent used for expandable styrene resin particles, such as a hydrocarbon having 3 to 6 carbon atoms, can be used.

When the core particles or the composite resin particles are impregnated with the organic physical blowing agent, for example, the organic physical blowing agent may be supplied into a closed container containing the particles to increase the pressure in the closed container. By maintaining this state, the organic physical blowing agent can be impregnated into the core particles and the composite resin particles.

The inside of the closed vessel may be heated as necessary while the organic foaming agent is immersed in the core particles or the composite resin particles. By heating the inside of the closed container, impregnation of the organic foaming agent into the core particles or the composite resin particles can be further promoted. The temperature and time for immersing the organic foaming agent may be appropriately set according to the timing of the immersing step. For example, in the impregnation step performed before the modification step, the organic physical blowing agent is preferably impregnated into the core particles by holding the temperature of approximately 40 ℃ to 90 ℃ for 0.5 hours to 3 hours. In the impregnation step performed during the modification step and/or after the completion of the modification step, the organic physical blowing agent is preferably impregnated into the core particles or the composite resin particles by holding the temperature of approximately 80 ℃ to 120 ℃ for 3 hours to 5 hours.

The amount of the organic physical blowing agent added 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 still 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.

By performing the impregnation step as described above, resin pellets containing an organic foaming agent can be obtained. After the impregnation step, the obtained resin particles may be dehydrated and dried as necessary. The method of dehydration drying is not particularly limited, and for example, a method of dehydrating and drying resin particles by blowing hot air to the resin particles using an air flow dryer can be employed. By dehydrating and drying the resin particles, the amount of water contained in the resin particles can be reduced, and the remaining organic foaming agent contained in the surface layer portion of the resin particles can be easily dispersed while maintaining the foamability of the resin particles. Thus, even in the case of hollow particles having a low bulk density, a good outer shell layer can be easily formed on the hollow particles. When a pneumatic dryer is used, for example, the hot air temperature may be appropriately set within a range of 30 ℃ to 60 ℃ and the drying time may be appropriately set within a range of 0.5 hours to 4 hours.

The resin particles have an average particle diameter of 1.0mm to 5.0 mm. By making the resin particles have an average particle diameter in the above-described specific range, the structure of the finally obtained hollow particles can be formed into a desired structure more reliably. From the viewpoint of further improving the yield of the hollow particles having the above-described specific structure, the average particle diameter of the resin particles is preferably 1.1mm or more and 4.0mm or less, and more preferably 1.2mm or more and 3.0mm or less.

The average particle diameter of the resin particles is a cumulative 63% diameter (i.e., d 63) calculated from the particle size distribution based on the volume of the resin particles, and can be measured using a particle size distribution measuring apparatus (for example, "Millitrack JPA" manufactured by Nikkiso Co., ltd.).

Examples of the surface coating agent used in the coating step include zinc stearate, triglycerol stearate, monoglycerol stearate, and hydrogenated castor oil. Further, as the surface coating agent in the coating step, an antistatic agent or the like may be used. The amount of the surface coating agent added is preferably 0.01 part by mass or more and 2 parts by mass or less with respect to 100 parts by mass of the resin particles.

The hollow particles can be obtained by foaming resin particles containing an organic physical foaming agent. As a method of foaming the resin particles, for example, a method of heating the resin particles with a heating medium can be employed. Specifically, the resin pellets can be foamed by introducing a heating medium such as steam into a prefoamer to which the resin pellets containing the organic physical foaming agent are supplied.

When the resin particles containing the organic physical foaming agent are foamed, the resin particles may be foamed in one stage or may be foamed in a plurality of stages. In the latter case, for example, the resin particles may be foamed to produce primary hollow particles having a bulk density greater than a target bulk density, and the primary hollow particles may be further foamed to produce hollow particles having a desired bulk density.

(foaming granule)

The expanded beads used for producing the molded article have a structure in which a styrene resin is used as a base resin and a large number of cells are formed relatively uniformly throughout the entire bead. The styrene-based resin used in the expanded particles is the same as that used in the above-described styrene-based resin core particles. The base resin of the expanded beads is preferably a styrene-based resin containing 50 mass% or more of a component derived from styrene.

The styrene resin constituting the expanded beads preferably contains not less than 80 mass%, more preferably not less than 90 mass%, and still more preferably not less than 95 mass% of a styrene-derived component, from the viewpoint of stably obtaining a molded article which is light in weight and has good mechanical properties such as compressive properties. The styrene resin constituting the expanded beads is particularly preferably polystyrene.

The expandable beads are obtained by expanding expandable styrene resin beads containing an organic physical foaming agent, using a styrene resin as a base resin. Examples of the organic foaming agent used for the expandable styrene resin particles include hydrocarbons having 3 to 6 carbon atoms.

The expandable styrene resin particles can be produced by a conventionally known method such as suspension polymerization. As for the method for foaming the expandable styrene resin particles, conventionally known methods can be used. For example, expandable styrene resin particles can be expanded by supplying a heating medium such as steam to the expandable styrene resin particles and heating the expandable styrene resin particles. More specifically, the expandable styrene resin beads can be expanded by heating the expandable styrene resin beads with steam or the like using, for example, a cylindrical expander with a stirring device.

[ flame retardant ]

The bromine-based flame retardant may be contained in the expanded particles. In this case, the flame retardancy of the molded article can be further improved. The amount of the brominated flame retardant to be incorporated in the expanded beads is preferably 0.05 mass% or more and 15 mass% or less from the viewpoint of improving the fusibility between the beads and further improving the flame retardancy of the molded article. Examples of the bromine-based flame retardant used in the expanded particles include brominated bisphenol-based flame retardants exemplified as bromine-based flame retardants used in the hollow particles, and brominated styrene-butadiene-based copolymers. The bromine-based flame retardant may be used alone, or two or more kinds of bromine-based flame retardants may be used in combination.

Further, the method of adding the flame retardant to the expanded beads is not particularly limited. For example, expanded beads containing a flame retardant can be obtained by obtaining styrene-based resin beads obtained by suspension polymerization of a styrene-based monomer in the presence of a flame retardant, impregnating the resin beads with a blowing agent to obtain expandable beads, and then expanding the expandable beads.

[ other additives ]

In addition, additives such as a cell regulator, a pigment, a slip agent, and an antistatic agent may be contained in the expanded beads within a range not to impair the above-described effects.

[ average particle diameter ]

The average particle diameter of the expanded beads before in-mold molding is not particularly limited, but is preferably 2mm or more and 9mm or less. From the viewpoint of more uniformly mixing the hollow particles and the expanded particles, the ratio of the average particle diameter of the hollow particles to the average particle diameter of the expanded particles is preferably 0.7 or more and 1.4 or less. The average particle diameter of the expanded particles can be measured by the method described above.

[ bulk Density ]

The bulk density of the expanded beads before in-mold molding is not particularly limited, but is preferably 20kg/m ³ Above and 100kg/m ³ The following. From the viewpoint of more uniformly mixing the hollow particles and the expanded particles, it is preferable that the ratio of the bulk density of the hollow particles to the bulk density of the expanded particles is 0.7 or more and 1.4 or less.

The method for measuring the bulk density of the expanded particles is the same as the method for measuring the bulk density of the hollow particles described above, except that the expanded particles are used instead of the hollow particles.

[ average bubble diameter ]

The average cell diameter of the expanded beads is preferably 15 μm or more and 150 μm or less, more preferably 20 μm or more and 120 μm or less, and still more preferably 30 μm or more and 90 μm or less. Such expanded particles have excellent in-mold moldability, and therefore, a molded article having good mechanical strength can be stably obtained.

The average cell diameter of the expanded beads was measured in the following manner. First, the expanded beads were divided into two parts so as to pass through the center of the expanded beads, and a magnified photograph of the cut surface was taken with a scanning electron microscope. Next, a straight line is drawn from the surface of the expanded beads to the surface on the opposite side through the vicinity of the center on the photograph, and the number of cells intersecting the straight line is counted. The length of the straight line (i.e., the actual length of the cut surface of the expanded beads) is divided by the number of cells to obtain the cell diameter (unit: μm) of each expanded bead. The arithmetic mean of the cell diameters of the expanded beads obtained by similarly performing this operation on ten expanded beads was defined as the average cell diameter (unit: μm) of the expanded beads.

[ examples ] A

Specific embodiments of the molded article and the method for producing the same will be described. First, the hollow particles and expanded particles used for producing the molded article will be described.

(hollow particle)

The hollow particles used for producing the molded article have an outer shell layer made of a base resin composed of a composite resin containing a (meth) acrylate component and a styrene component, and a hollow portion surrounded by the outer shell layer. The average particle diameter and the bulk density of the hollow particles, the average value of the total thickness of the resin films of the outer shell layer, and the flame retardant contained in the hollow particles are shown in table 1. In addition, the hollow particles A3 shown in table 1 are not blended with a flame retardant, and therefore, the column of the flame retardant is indicated by the symbol "-".

In the production of hollow particles, first, styrene-based resin core particles are produced by suspension polymerization, and then resin particles containing an organic physical blowing agent are produced using the core particles. By foaming the resin particles, hollow particles can be obtained. Hereinafter, the method for producing the hollow particles will be described in detail.

[ preparation of core particles ]

First, 765g of deionized water, 0.84g of a suspension and 3.2g of a surfactant were put into a 3-L autoclave equipped with a stirrer. In addition, tricalcium phosphate (manufactured by taiping chemical industry co., ltd.) was used as a suspension agent, and sodium dodecylbenzenesulfonate (manufactured by tokyo chemical industry co., ltd.) was used as a surfactant.

Then, 848g of styrene, a polymerization initiator, and a plasticizer were charged into the autoclave while stirring the contents of the autoclave. As the polymerization initiator, 2.2g of t-butyl peroxy-2-ethylhexanoate ("PERBUTYL (registered trademark) O" manufactured by Nichikoku K.K.) and 0.44g of t-butyl peroxy-2-ethylhexyl monocarbonate ("PERBUTYL (registered trademark) E" manufactured by Nichikoku K.K.) were used in combination. Further, 12.7g of cyclohexane and 6.2g of hydrogenated tallow were used in combination as a plasticizer. The plasticizer was put into the autoclave in a state of being dissolved in styrene.

After stirring the contents of the autoclave at room temperature for 30 minutes, the temperature in the autoclave was raised to 90 ℃ over a period of 1 and a half hours. After the temperature in the autoclave reached 90 ℃, the inside of the autoclave was further heated for 5 and a half hours to raise the temperature in the autoclave to 100 ℃. After the temperature in the autoclave reached 100 ℃, the inside of the autoclave was further heated to raise the temperature in the autoclave to 110 ℃ for 1 hour and a half. After maintaining the temperature for 2 hours, the autoclave was cooled to 30 ℃ over 4 hours. Through the above operations, core particles using polystyrene as a base resin were produced.

After the completion of the cooling, the core particles in the autoclave were taken out, and tricalcium phosphate attached to the surfaces of the core particles was removed by nitric acid. After dehydration and washing of the core particles were performed using a centrifugal separator, moisture adhering to the surfaces of the core particles was removed using an air flow dryer.

[ production of resin particles containing organic physical blowing agent ]

965g of deionized water and 6.0g of sodium pyrophosphate were charged into a 3L autoclave equipped with a stirrer. Thereafter, 12.9g of powdery magnesium nitrate hexahydrate was charged into the autoclave, and the contents of the autoclave were stirred at 40 ℃ for 30 minutes. In this way, the contents of the autoclave were made into a slurry of magnesium pyrophosphate as a suspension agent.

Next, a surfactant, 200g of core particles, and a polymerization initiator were charged into the autoclave. Further, as the surfactant, a1 mass% aqueous solution of sodium lauryl sulfate was used, and the amount of the surfactant added was 0.04g in terms of sodium lauryl sulfate. Further, as a polymerization initiator, 2.4g of benzoyl peroxide ("NYPER (registered trademark) BW" manufactured by Nichigan was used).

After the autoclave was purged with nitrogen, 18g of an organic foaming agent was added to the autoclave over 10 minutes. As the organic blowing agent, a mixture of about 70 mass% of n-butane and about 30 mass% of isobutane was used.

After the addition of the organic physical blowing agent, the temperature in the autoclave was maintained at 40 ℃ for 1 hour. Then, the temperature in the autoclave was raised to 80 ℃ over 1 hour. While the autoclave was stirred at a stirring speed of 450rpm while maintaining the temperature, a mixture of 200g of styrene, 400g of methyl methacrylate and a polymerization initiator was added to the autoclave over 6 hours. Further, as the polymerization initiator, 1.2g of t-butyl peroxybenzoate (1.2 g of "PERBUTYL Z" manufactured by Nichikoku K.K.) was used. In addition, in the preparation of the hollow particles A1, A2, A4 and A5 shown in table 1, 80g of a flame retardant was previously mixed with the above-mentioned mixture of styrene, methyl methacrylate and a polymerization initiator, and the mixture was added to an autoclave. 2, 2-bis (4 ' - (2 ",3 ″ -dibromo-2 ″ -methylpropyloxy-3 ',5' -dibromophenyl) propane (" SR-130 "manufactured by first Industrial pharmaceutical Co., ltd.) was used as the flame retardant.

After 30 minutes of addition of styrene and methyl methacrylate, 80g of pentane was added as an organic physical blowing agent to the autoclave over 30 minutes. After 1 hour of addition of pentane, the temperature in the autoclave was raised to 120 ℃ over 1.5 hours, and the temperature was maintained for 4 hours. Thereafter, the autoclave was cooled to 35 ℃ over about 6 hours. By the above-described operation, resin pellets containing an organic physical foaming agent and a composite resin containing a styrene-derived component and a methyl methacrylate-derived component as a base resin were produced.

After completion of the cooling, the resin particles in the autoclave were taken out, and tricalcium phosphate adhering to the surfaces of the resin particles was removed by nitric acid. After dehydration and washing of the resin pellets by a centrifugal separator, moisture adhering to the surfaces of the resin pellets was removed by an air dryer.

In this example, the surface of the resin particles obtained as described above was coated with a surface coating agent. Specifically, a mixture of 0.11 parts by mass of zinc stearate, 0.053 parts by mass of glycerol monostearate, 0.004 parts by mass of talc, and 0.065 parts by mass of an antistatic agent ("resist (registered trademark) PE132" manufactured by first industrial pharmaceutical co., ltd.) was added to 100 parts by mass of the resin particles, thereby coating the surfaces of the resin particles with a surface coating agent containing them.

After the surface of the resin particles was coated with the surface coating agent, the resin particles were further subjected to a drying treatment in which the resin particles were heated at a temperature of 40 ℃ for 1 hour by using a pneumatic dryer to remove excess moisture and organic foaming agents.

[ production of hollow particles ]

The resin pellets obtained by the above method were charged into an atmospheric pressure batch foaming machine having a capacity of 30L, and steam was supplied into the foaming machine. The steam temperature is 105 to 125 ℃ and the heating time is 50 to 150 seconds. The resin particles were foamed to obtain hollow particles A1 to A5 having average values of the average particle diameter, the bulk density, and the total thickness of the resin film of the outer shell layer shown in table 1.

In addition, the average particle diameter shown in table 1 is a cumulative 63% diameter calculated based on the particle size distribution of the hollow particles on a volume basis. The average values of the bulk density and the total thickness of the resin film of the outer shell layer are values measured by the above-described methods.

(foaming granule)

The expanded beads used for producing the molded article have a structure in which a styrene resin is used as a base resin and a large number of cells are formed relatively uniformly throughout the entire bead. The average particle diameter and bulk density of the expanded particles and the flame retardant contained in the expanded particles are shown in table 1.

When the expandable beads are produced, expandable styrene resin beads containing a styrene resin as a base resin and an organic foaming agent are first produced by suspension polymerization. The expandable styrene resin particles are expanded to obtain expanded particles. Hereinafter, a method for producing expanded beads will be described in detail.

[ production of Expandable styrene resin particles ]

First, 16kg of deionized water, 14.4g of a suspension agent, 24.4g of a surfactant and 24.4g of an electrolyte were put into a 50L autoclave equipped with a stirrer. Further, tricalcium phosphate (manufactured by taiping chemical industries, ltd.) was used as a suspension agent. As the surfactant, 0.6g of sodium alpha-olefin sulfonate and 0.2g of disodium alkylbiphenyldisulfonate were used in combination. As the electrolyte, sodium acetate was used.

Next, a polymerization initiator, a flame retardant aid, a plasticizer, a cell regulator, and a polymerization initiator were mixed with 16kg of styrene to prepare a mixture. The mixture was charged into the autoclave while the contents of the autoclave were stirred. Further, as the polymerization initiator, 43.2g of t-butyl peroxy-2-ethylhexanoate ("PERBUTYL (registered trademark) O" manufactured by Nichigan corporation) and 25.6g of t-butyl peroxy-2-ethylhexyl monocarbonate ("PERBUTYL (registered trademark) E" manufactured by Nichigan corporation) were used in combination. As the flame retardant, a brominated styrene-butadiene block copolymer ("Emerald Innovation (registered trademark) 3000" manufactured by Chemtura Japan corporation) was used in a mass ratio to styrene shown in table 1. As the flame-retardant auxiliary, 51.2g of dicumyl peroxide ("PERCUMYL (registered trademark) D" manufactured by Nichigan corporation) was used. As the plasticizer, 12.8g of liquid paraffin ("MORESCO WHITE P-60" manufactured by MORESCO Co., ltd.) was used. As the cell controlling agent, 3.2g of polyethylene wax powder ("polyethylene wax 1000" manufactured by Toyo ADL Co., ltd.) was used. As the polymerization inhibitor, 0.32g of 4-tert-butylcatechol ("DIC-TBC" manufactured by DIC corporation) was used. In table 1, the flame retardant is abbreviated as "E3000".

After the inside of the autoclave was replaced with nitrogen, the temperature in the autoclave was raised to 90 ℃ for 1 hour and a half. After the temperature in the autoclave reached 90 ℃, the inside of the autoclave was further heated to raise the temperature in the autoclave to 100 ℃ for 6 and a half hours. In the temperature raising step, the injection of the organic physical foaming agent into the autoclave was started after 5 hours and 30 minutes from the time when the temperature in the autoclave reached 90 ℃, and the injection of the organic physical foaming agent was completed after 30 minutes from the start of the injection. As organic blowing agents, 320g of pentane (mixture of n-pentane 80% and isopentane 20%) and 880g of butane (mixture of n-butane 70% and isobutane 30%) were used.

After the temperature in the autoclave reached 100 ℃, the autoclave was further heated, and the temperature in the autoclave was raised to 120 ℃ over 2 hours. The temperature was maintained for 5 hours. After that, the autoclave was cooled to 30 ℃ over about 6 hours. Through the above-described operations, expandable styrene resin particles containing a styrene resin as a base resin and an organic physical foaming agent are produced.

[ production of expanded beads ]

The expandable styrene resin beads obtained by the above method were charged into an atmospheric pressure batch foaming machine having a volume of 30L, and steam was supplied into the foaming machine. The steam temperature is 105 to 125 ℃ and the heating time is 50 to 150 seconds. The resin particles were foamed, and foamed particles B1 to B6 having the average particle diameter and bulk density shown in table 1 were obtained.

The average particle diameter shown in table 1 is a cumulative 63% diameter calculated based on the particle size distribution of the expanded particles on a volume basis. The bulk density is a value measured by the above method.

(examples 1 to 6)

As shown in fig. 1 and 2, the molded bodies 1 according to examples 1 to 6 were produced by in-mold molding a mixed particle of hollow particles 2 and expanded particles 3, which has an outer shell layer 21 and a hollow portion 22 surrounded by the outer shell layer 21. That is, the molded bodies 1 of examples 1 to 6 contain the hollow particles 2 and the expanded particles 3. The method for producing the molded articles of examples 1 to 6 is specifically as follows.

[ production of molded article ]

The mixed particles of the hollow particles and the expanded particles were molded in a mold using a molding machine ("PEONY-AD/0907" manufactured by Chimaphila industries, ltd.). First, the hollow particles and expanded particles obtained as described above were aged in a thermostatic chamber at a temperature of 23 ℃ for 1 day. The cured hollow particles and expanded particles were mixed in the combination and mass ratio shown in table 2 to prepare mixed particles. The mixed pellets were charged into a molding die having a rectangular parallelepiped cavity of 300mm × 300mm × 100mm provided in a molding machine.

After the mixed pellets were charged, steam of 0.07MPa (G) in gauge pressure was introduced into the mold, and the hollow pellets and expanded pellets were heated to perform in-mold molding. The obtained molded body was dried in a drying chamber at a temperature of 40 ℃ for 1 day, and then further cured in a thermostatic chamber at a temperature of 23 ℃ for 1 day. The molded bodies of examples 1 to 6 were obtained in the above manner.

Comparative example 1

As shown in table 3, the molded body of comparative example 1 did not contain expanded particles, and was substantially composed of hollow particles. The method for producing the molded article of comparative example 1 was the same as the method for producing the molded article of examples 1 to 6, except that only the hollow particles were filled in the molding machine.

Comparative example 2

As shown in table 3, the molded body of comparative example 2 was composed of expanded particles without containing hollow particles. The method for producing the molded article of comparative example 2 was the same as the method for producing the molded article of examples 1 to 6, except that only the expanded beads were filled in the molding machine.

Comparative examples 3 and 4

The molded bodies of comparative examples 3 and 4 had the same structures as the molded bodies of examples 1 to 6, except that the combination of the hollow particles and the expanded particles was changed as shown in table 3. The method for producing the molded bodies of comparative examples 3 and 4 was the same as the method for producing the molded bodies of examples 1 to 6, except that the combination of the hollow particles and the expanded particles was changed as shown in table 3.

Next, a method for evaluating each characteristic of the molded article obtained as described above will be described below.

[ Density of molded article ]

The mass (unit: kg) of the molded body was divided by the volume (unit: m) calculated based on the outer dimensions ³ ) The obtained value was used as the density of the molded article (unit: kg/m ³ ). Practice ofThe densities of the molded bodies of the examples and comparative examples are shown in tables 2 and 3.

[ foaming agent amount ]

About 1g of a sample was collected from the molded body and heated for 4 hours using a hot air dryer set at 120 ℃ to evaporate the water and organic foaming agent in the sample. The total volatile content (unit: mass%) in the sample was determined as a value representing the ratio of the mass loss by heating to the mass before heating in percentage. In addition to the measurement of the total volatile matter content, a sample of about 0.3g was collected from the molded article, and the water content (unit: mass%) in the sample was measured using a Karl Fischer water meter.

Then, the value obtained by subtracting the water content (unit: mass%) from the total volatile content (unit: mass%) in the sample calculated as described above was defined as the amount (unit: mass%) of the organic foaming agent remaining in the molded article. That is, the amount of the foaming agent in the molded article can be calculated by the following formula.

Amount of foaming agent (% by mass) = total volatile component (% by mass) -amount of water (% by mass)

[ average value and coefficient of variation of area ratio of hollow particles to expanded particles ]

The above-described molded body having a rectangular parallelepiped shape with a longitudinal direction of 300mm, a lateral direction of 300mm, and a thickness of 100mm was cut along a plane orthogonal to the thickness direction at a position of a depth of 20mm from the surface of the molded body in the thickness direction, and a cut piece in which the cut surface was exposed on both surfaces in the thickness direction was prepared. The size of the cut piece is specifically 300mm in the longitudinal direction, 300mm in the transverse direction, and 60mm in thickness.

Using a scanner, photographs of each cut surface in the slice were acquired. Next, using image processing software ("WinROOF" manufactured by mitsubishi corporation), 15 measurement regions each having a vertical direction of 50mm and a horizontal direction of 50mm were randomly set on the photograph of each cut surface, and the total of the 15 measurement regions was set to 30. Then, the area ratio S1/S2 of the total S1 of the areas of the hollow particles, the total S2 of the areas of the expanded particles, and the total S1 of the areas of the hollow particles to the total S2 of the areas of the expanded particles in each measurement region is calculated. The total of the areas S1 of the hollow particles and the total of the areas S2 of the expanded particles are not included in the area of the particles intersecting the boundary line of the measurement region.

Next, the cut pieces were cut into a lattice shape when viewed from any cut surface, and 20 small cut pieces were prepared. A total of 31 small slices (specifically, a plane orthogonal to a plane having a length of 300mm and a width of 300mm of the molded body before cutting) were taken using a scanner so that the cut planes were not overlapped. Then, using image processing software ("WinROOF" manufactured by mitsubishi co., ltd.), measurement regions having a vertical direction of 50mm and a horizontal direction of 50mm were randomly set on the photograph of each cut surface, and the total S1 of the areas of the hollow particles and the total S2 of the areas of the expanded particles in each measurement region were measured. Further, based on the above values, the area ratio S1/S2 of the total S1 of the areas of the hollow particles to the total S2 of the areas of the expanded particles is calculated. The total S1 of the areas of the hollow particles and the total S2 of the areas of the expanded particles are not included in the areas of the particles intersecting the boundary line of the measurement region.

A value obtained by arithmetically averaging the area ratio S1/S2 of the sum S1 of the areas of the hollow particles and the sum S2 of the areas of the expanded particles in the measurement region at the total 61 obtained above is taken as an average value of the area ratio S1/S2 of the sum S1 of the areas of the hollow particles and the sum S2 of the areas of the expanded particles in the cross section of the molded body. Further, a value obtained by dividing the unbiased standard deviation (see the above expression (1)) of the ratio S1/S2 of the total S1 of the areas of the hollow particles to the total S2 of the areas of the expanded particles in the measurement region at 61 by the average value of the area ratios S1/S2 is used as the coefficient of variation of the area ratio S1/S2 of the total S1 of the areas of the hollow particles to the total S2 of the areas of the expanded particles in the cross section of the molded body. The average values and the coefficients of variation of the area ratios S1/S2 of the molded bodies of examples and comparative examples are shown in tables 2 and 3.

[ average value of area per hollow particle and average value of area per expanded particle ]

Using the photographs of the cut surfaces obtained in the calculation of the average value of the area ratios of the hollow particles and the expanded particles, the total S1 of the areas of the hollow particles, the number of hollow particles, the total S2 of the areas of the expanded particles, and the number of expanded particles in the measurement region were calculated by the above-described method. Using the above values, the average value of the area of each hollow particle and the average value of the area of each expanded particle were calculated. The average value (a) of the area of each hollow particle, the average value (B) of the area of each expanded particle, and the ratio (a)/(B) of the average value (a) of the area of each hollow particle to the average value (B) of the area of each expanded particle in the molded bodies of examples and comparative examples are shown in tables 2 and 3. In addition, the number of hollow particles and the number of expanded particles are not counted for the particles intersecting the boundary line of the measurement region.

[ average value of thickness of outer shell layer, average value of total thickness of resin film in outer shell layer ]

The average value of the thicknesses of the outer shell layers of the hollow particles and the average value of the total thickness of the resin films in the outer shell layers were calculated by the above-described method. The average value of the thicknesses of the outer shell layers of the hollow particles and the average value of the total thickness of the resin films in the outer shell layers present in the molded bodies of examples and comparative examples are shown in tables 2 and 3.

[ compression characteristics ]

A rectangular parallelepiped test piece having a vertical dimension of 50mm, a horizontal dimension of 50mm and a thickness of 25mm was taken from the central portion of the molded body. Based on JIS K7220: the compression test of the test piece was performed by the method specified in 2006, and a stress-strain curve was obtained. Furthermore, the compression test was carried out in a laboratory at 23 ℃.

Compressive stress σ at 10% deformation in examples and comparative examples calculated based on stress-strain curve 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) are shown in tables 2 and 3. In addition, "σ" in tables 2 and 3 ₁₀ Density column shows 10% deformation pressureCompressive stress sigma ₁₀ Value obtained by dividing by the density of the compact, at "σ ₁₀ /σ ₅₀ "column shows the 10% deformation compressive stress σ in percent ₁₀ And 50% deformation compressive stress sigma ₅₀ The value of the ratio is at ₁₀ /σ ₇₅ "the compressive stress at 10% deformation σ in percent is shown in the column ₁₀ And 75% deformation compressive stress sigma ₇₅ The value of the ratio of the two.

Further, based on the stress-strain curve, values of the amount of strain (C) at a compressive stress of 300kPa, the amount of strain (D) at a compressive stress of 600kPa, and the differences (D) - (C) between the amount of strain (D) at a compressive stress of 600kPa and the amount of strain (C) at a compressive stress of 300kPa were calculated. The values of the molded articles of examples and comparative examples are shown in tables 2 and 3.

[ energy absorption Properties ]

An example of a stress-strain curve measured by a compression test is shown in fig. 3. Dependent variable epsilon _a Energy absorption amount (unit: J/g) of (A) and (B) from 0% strain amount to ε strain amount in a stress-strain amount curve _a The portion L1 up to and through the amount of strain ε on the stress-strain curve _a A straight line L2 parallel to the vertical axis and an area S surrounded by the horizontal axis _a Are equal.

The energy absorption amount per unit mass was calculated by calculating the energy absorption amount at the strain amount of 50% based on the stress-strain curves of the molded bodies of examples and comparative examples, and dividing the energy absorption amount by the mass of the molded body used in the test. The energy absorption amount per unit mass of the molded bodies of examples and comparative examples are shown in tables 2 and 3.

The energy absorption amount per unit mass is preferably 4J/g or more, more preferably 5J/g or more, and still more preferably 6J/g or more. In this case, the weight of the impact absorbing material can be further reduced while ensuring good energy absorption performance. The energy absorption amount per unit mass is preferably 10J/g or less, and more preferably 8J/g or less. In this case, the molded body does not become excessively hard, can appropriately absorb an impact, and when used as an impact absorbing material for an automobile, safety of an occupant at the time of collision is easily ensured.

[ compression set ]

Three rectangular parallelepiped test pieces having a vertical direction of 50mm, a horizontal direction of 50mm, and a thickness of 25mm were sampled from the center in the thickness direction of the molded body so as not to include a skin surface, i.e., a surface that comes into contact with an inner surface of the molding die during in-mold molding. Based on JIS K6767: the test piece was subjected to a compression test by the method defined in 1999. Specifically, each test piece was compressed in a state of being deformed by 25% in the thickness direction of the test piece under an environment of a temperature of 23 ℃ and a relative humidity of 50%, and left for 22 hours in this state. Thereafter, each test piece was released from the compressed state, and the thickness of each test piece was measured 24 hours after the compression was completed. The compression set (unit:%) of each test piece was determined by dividing the amount of change in the thickness of the test piece before and after the test by the thickness of the test piece before the test, and the arithmetic average of these values was taken as the compression set (unit:%). The values of compression set in examples and comparative examples are shown in tables 2 and 3.

[ flame retardancy ]

Flammability tests were performed by a method according to the regulations of FMVSS (Federal Motor Vehicle Safety Standards: U.S. Federal Motor Vehicle Safety Standards) No. 302. Specifically, first, a molded article having a rectangular parallelepiped shape with a longitudinal direction of 585mm, a transverse direction of 485mm, and a thickness of 100mm was prepared by in-mold molding of a mixed particle of hollow particles and expanded particles using a molding machine ("VS-1300" manufactured by DAISEN K.K.). The molded article was left at a temperature of 40 ℃ for 3 days, and further left at room temperature for 1 day to cure the molded article. Thereafter, the molded article was left at 40 ℃ for 3 days, and further left at room temperature for 1 day, whereby the molded article was cured. Thereafter, five flat test pieces of 356mm in the longitudinal direction, 102mm in the transverse direction and 13mm in the thickness were cut out from the molded body.

Next, one end of the test piece in the longitudinal direction was mounted on a mounting device of a flammability tester (MVSS-2 manufactured by shiga tester) according to FMVSS No.302, and the test piece was held horizontally. The flame of the burner was allowed to contact the other end of the test piece in the longitudinal direction, i.e., below the end not held by the fixture, for 15 seconds, and then the flame of the burner was separated from the test piece. The height of the flame of the burner was 38mm, and the distance from the tip of the burner to the lower surface of the test piece was 19mm.

Then, measurement of the combustion time was started at a point of time when the flame generated from the test piece reached a position 38mm away from the open end of the test piece (i.e., the end portion of the test piece in the longitudinal direction which was not held by the mounting fixture). In the case where the flame generated from the test piece reaches a position at a distance of 38mm from the fixed end of the test piece (i.e., the end held by a jig or the like in the longitudinal direction of the test piece), the measurement of the combustion time is completed at the point of time when the flame reaches the position. In addition, in the case where the combustion of the test piece was completed before reaching the position of 38mm in distance from the fixed end of the test piece, the measurement of the combustion time was completed at the time point when the combustion was completed. When the combustion of the test piece was completed before the test piece reached the position 38mm away from the fixed end of the test piece, the combustion time of the test piece was 0 second.

The above test was carried out using five test pieces, and the flame retardancy was evaluated based on the test results. The symbols shown in the column "flame retardancy" in tables 2 and 3 have the following meanings.

A: the test piece was found to satisfy either of the conditions (1) and (2) that all of the five test pieces exhibited self-extinguishing properties, or (2) that the combustion rate was 102 mm/min or less although there was a test piece that did not exhibit self-extinguishing properties.

B: for any of the test pieces, the flame generated from the test piece did not propagate on the surface of the test piece, and the burning rate exceeded 102 mm/min.

C: for more than one test piece, the flame generated from the test piece propagates on the surface of the test piece.

The burning rate used for the above evaluation is a value obtained by arithmetically averaging the burning rates of the test pieces in the combustibility test performed on five test pieces. When the combustion rate for evaluation was calculated, the test piece showing self-extinguishing property in the flammability test was treated so that the combustion rate of the test piece was 0 mm/min. The combustion rate of the test piece which did not exhibit self-extinguishing property was calculated by the following formula (I).

B＝60×D/T(I)

In the formula (I), the symbol B indicates the combustion speed (unit: mm/min), the symbol D indicates the distance traveled by the flame (unit: mm), and the symbol T indicates the time required for the flame to travel Dmm (unit: sec).

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As shown in tables 1 and 2, the molded bodies of examples 1 to 6 were composed of mixed particles of hollow particles and expanded particles. In the molded body, an average value of a ratio S1/S2 of a total S1 of areas of the hollow particles to a total S2 of areas of the expanded particles in a cross section of the molded body, a coefficient of variation of the ratio S1/S2, and a 10% deformation compressive stress σ of the molded body ₁₀ And 50% deformation compressive stress sigma ₅₀ Ratio of σ ₁₀ /σ ₅₀ Each within the above-specified ranges. Therefore, the molded article exhibits good compression properties and has low permanent set. Further, the molded body has a wide range of strain amount with a compressive stress of 300 to 600kPa, and has excellent energy absorption efficiency in a wide range of strain amount.

Since the molded article of comparative example 1 was composed of hollow particles, the compression stress at 10% deformation was higher than those of the molded articles of examples 1 to 6σ ₁₀ Low, the energy absorption efficiency in the initial stage of compression is poor. In the molded article of comparative example 1, the flame generated from the test piece in the flammability test propagated on the surface, and the test piece was burned as a whole.

Since the molded article of comparative example 2 was composed of expanded particles, the compression set was larger than those of the molded articles of examples 1 to 6. In addition, in the molded article of comparative example 2, when the amount of strain is larger than the molded articles of examples 1 to 6, the compressive load required for deformation is higher, and the energy absorption efficiency in the case where the amount of strain is larger is inferior.

In the molded bodies of comparative examples 3 and 4, when the hollow particles and the expanded particles are mixed, it is difficult to uniformly mix them, for example, because the difference in the bulk density between the hollow particles and the expanded particles used for producing the molded bodies is large. Therefore, in the molded body of the comparative example, the distribution of the hollow particles varies, and the coefficient of variation S1/S2 of the ratio S1 of the total S1 of the areas of the hollow particles to the total S2 of the areas of the expanded particles is larger than the specific range. Further, as a result of variation in the distribution of the hollow particles in the molded article, the 10% distortion compressive stress σ was generated ₁₀ And leads to a reduction in energy absorption efficiency at the early stage of compression. In the molded bodies of comparative examples 3 and 4, the flame generated from the test piece in the flammability test propagated on the surface, and the test piece was burned as a whole.

While specific embodiments of the molded article according to the present invention have been described above with reference to examples 1 to 6, the specific embodiments of the molded article and the impact absorbing material according to the present invention are not limited to the embodiments of examples 1 to 6, and the configuration may be appropriately modified within a range not to impair the gist of the present invention.

Claims (7)

1. A molded article obtained by in-mold molding a mixture of hollow particles having an outer shell layer and a hollow portion surrounded by the outer shell layer and expanded particles comprising a styrene resin as a base resin,

the base resin of the outer shell layer in the hollow particle is a composite resin containing a (meth) acrylate component and a styrene component,

an average value of a ratio S1/S2 of a total S1 of areas of the hollow particles to a total S2 of areas of the expanded particles in a cross section of the molded body is 0.1 or more and 3 or less, and a coefficient of variation of the ratio S1/S2 is 20% or less,

a 10% deformation compressive stress σ of the molded article measured at 23 DEG C ₁₀ And 50% deformation compressive stress sigma ₅₀ Ratio of σ ₁₀ /σ ₅₀ Is 0.70 to 1.0 inclusive.

2. The shaped body as claimed in claim 1, wherein the average of the area of each of the hollow particles in the cross-section of the shaped body is 3mm ² Above and 60mm ² And a ratio of an average value of an area of each of the hollow particles to an average value of an area of each of the expanded particles is 0.7 or more and 1.4 or less.

3. The shaped body as claimed in claim 1 or 2, wherein the shaped body has a density of 20kg/m ³ Above and 100kg/m ³ The following.

4. The shaped body as claimed in any of claims 1 to 3, wherein the shaped body has a compression set of 15% or less.

5. The molded body according to any one of claims 1 to 4, wherein the hollow particles and the expanded particles contain a bromine-based flame retardant, and when a flammability test of FMVSS No.302 is performed, (1) the molded body exhibits self-extinguishing properties, or (2) the molded body has a burning rate of 102 mm/min or less.

6. The molded body according to claim 5, wherein the amount of the bromine-based flame retardant blended in the molded body is 0.5 mass% or more and 10 mass% or less.

7. An impact absorbing material comprising the formed body according to any one of claims 1 to 6.

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