WO2026004743A1 - Polypropylene resin foam particles and polypropylene resin foam particle molded body - Google Patents

Abstract

The base resin constituting the foam particles includes a polypropylene resin A containing a biomass-derived monomer component and a fossil fuel-derived polypropylene resin B. Resin B is a propylene random copolymer containing 3-10 mass% of a comonomer component derived from one or more monomers selected from the group consisting of ethylene and butene. The flexural modulus M_B of resin B is 900 MPa or less. The mass ratio of resin A and resin B in the base resin is resin A:resin B=3:97-90:10 (where the total of resin A and resin B is 100 mass%.).

Description

Polypropylene resin expanded beads and polypropylene resin expanded bead molded body

The present invention relates to expanded polypropylene resin beads and expanded polypropylene resin bead molded articles.

Polypropylene resin foam bead moldings have excellent strength and cushioning properties, and are used in a variety of applications, including packaging materials, automotive components, and building materials.

Polypropylene resin foam bead molded articles are manufactured, for example, by a method known as in-mold molding, in which a mold is filled with polypropylene resin foam beads and then heated by supplying a heating medium such as steam into the mold. In this in-mold molding method, when a heating medium is supplied into the mold, the foam beads undergo secondary expansion and their surfaces melt. This causes the foam beads in the mold to fuse together, resulting in a molded article with a shape that corresponds to the shape of the mold cavity. Immediately after molding, the molded article is prone to expanding due to secondary expansion, so it is cooled in the mold with water or the like for a predetermined period of time before being released from the mold.

In recent years, there has been growing awareness of environmental impacts such as rising atmospheric carbon dioxide concentrations and the consumption of fossil fuel resources. To reduce this environmental impact, efforts are being made to produce expanded resin beads using raw materials derived from biomass. For example, in the case of expanded polyethylene resin beads, a technology has been proposed that uses plant-derived polyethylene resins with a high plant content (see, for example, Patent Document 1).

JP 2013-60514 A

On the other hand, with regard to expanded polypropylene resin beads, no technology has been proposed to date for expanded beads using biomass-derived polypropylene resin. Therefore, there is a need to propose technology for expanded polypropylene resin beads and polypropylene resin expanded bead molded articles that use biomass-derived raw materials and can contribute to reducing the environmental burden.

However, expanded beads containing biomass-derived polypropylene resin as a raw material can have poor moldability in molds, and depending on the molding conditions, molded articles may not be obtained. Furthermore, even when molded articles can be obtained, the range of molding pressures at which they can be obtained is narrow, and improvements in these areas have been strongly desired.

The present invention was made in light of this background, and aims to provide expanded polypropylene resin beads that contain biomass-derived polypropylene resin, contribute to reducing environmental impact, have excellent in-mold moldability, and can produce good expanded polypropylene resin bead moldings over a wide range of molding pressures, as well as expanded polypropylene resin bead moldings made from these expanded polypropylene resin beads.

One aspect of the present invention is the expanded polypropylene resin particles according to the following [1] to [7].

[1] Polypropylene-based resin expanded particles,
the base resin constituting the expanded beads comprises a polypropylene-based resin A containing a biomass-derived monomer component and a fossil fuel-derived polypropylene-based resin B;
the polypropylene-based resin B is a propylene-based random copolymer containing 3% by mass or more and 10% by mass or less of a comonomer component derived from one or more monomers selected from the group consisting of ethylene and butene,
The flexural modulus _M of the polypropylene-based resin B is 900 MPa or less,
The expanded polypropylene resin particles have a mass ratio of the polypropylene resin A to the polypropylene resin B in the base resin of 3:97 to 90:10 (wherein the total of the polypropylene resin A and the polypropylene resin B is 100 mass%).

[2] The expanded polypropylene resin beads according to [1], wherein the expanded beads have a biomass ratio of 1% or more and 50% or less as measured by ASTM D6866-21.
[3] The polypropylene resin A has a melting point Tm _A of 155°C or more and 170°C or less, and the polypropylene resin B has a melting point Tm _B of 130°C or more and 145°C or less. [1] Expanded polypropylene resin particles according to [2].

[4] The expanded polypropylene resin particles according to any one of [1] to [3], wherein the ratio M _B /M _A of the flexural modulus M _B of the polypropylene resin B to the flexural modulus M _A of the polypropylene resin A is 0.60 or less.
[5] The expanded polypropylene resin particles according to any one of [1] to [4], wherein the ratio MFR _A /MFR B of the melt mass-flow rate MFR _A of the polypropylene resin A at a temperature of 230° _C and a load of 2.16 kg to the melt mass-flow rate MFR _B of the polypropylene resin B at a temperature of 230°C and a load of 2.16 kg is 0.2 or more and 0.6 or less.

[6] The expanded polypropylene resin particles according to any one of [1] to [5], wherein the melt mass-flow rate MFR _A of the polypropylene resin A at a temperature of 230°C and a load of 2.16 kg is 5 g/10 min or less.
[7] The expanded polypropylene resin beads according to any one of [1] to [6], wherein the expanded beads have a bulk density of 10 kg/m ³ or more and 200 kg/m ³ or less.

Another aspect of the present invention is a polypropylene resin expanded bead molded article according to the following items [8] and [9].
[8] A polypropylene resin expanded bead molding obtained by molding the polypropylene resin expanded bead according to any one of [1] to [7] in a mold.
[9] The expanded polypropylene resin bead molding according to [8], wherein the expanded bead molding has a size that allows a sphere having a diameter of 55 mm to be cut out.

According to the above aspect, it is possible to provide expanded polypropylene resin beads that contain biomass-derived polypropylene resin, contribute to reducing environmental impact, have excellent in-mold moldability, and can produce excellent expanded polypropylene resin bead molded articles over a wide range of molding pressures.

FIG. 1 is an explanatory diagram showing a method for calculating the heat of fusion of the high-temperature peak.

(Polypropylene resin foam particles)
The expanded polypropylene resin beads (hereinafter also referred to as "expanded beads") are composed of a base resin containing a polypropylene resin A containing a biomass-derived monomer component in the molecular chain and a polypropylene resin B composed of a fossil fuel-derived monomer component. In this specification, "monomer component" refers to a structural unit constituting a polymer chain. The polypropylene resin B is the specific propylene random copolymer and has a flexural modulus M _B within the specific range. The mass ratio of the polypropylene resin A to the polypropylene resin B in the base resin is 3:97 to 90:10, where the total of the polypropylene resin A and the polypropylene resin B is taken as 100% by mass.

Expanded beads having this configuration contain a biomass-derived polypropylene resin, which can contribute to reducing environmental impact. Furthermore, because the expanded beads are made from a base resin containing the polypropylene resin A and the polypropylene resin B in the specified mass ratio, they have excellent moldability within a mold. Therefore, the expanded beads can broaden the range of molding pressures at which a good expanded polypropylene resin bead molded article (hereinafter also referred to as "expanded bead molded article" or "molded article") can be obtained.

After extensive research, the inventors discovered that expanded beads containing a biomass-derived polypropylene resin as a raw material have the problem of significantly narrowing the molding pressure range at which satisfactory molded bodies can be obtained when producing bodies with shapes that are relatively difficult to mold, such as thick bodies or bodies with thick and thin portions. In contrast, the expanded beads of the present invention easily avoid this problem by using a fossil fuel-derived polypropylene resin B having the specific configuration described above in addition to a biomass-derived polypropylene resin A, thereby easily widening the molding pressure range at which satisfactory molded bodies can be obtained, even when producing thick bodies or bodies with thick and thin portions. Furthermore, because the expanded beads of the present invention contain a biomass-derived polypropylene resin A, they can also contribute to reducing environmental impact. The detailed configuration of the expanded beads is described below.

[Base resin]
The base resin constituting the expanded beads contains a polypropylene-based resin A and a polypropylene-based resin B. In this specification, the polypropylene-based resin refers to a propylene homopolymer and a propylene-based copolymer having a propylene component (i.e., a monomer component derived from propylene) content of 50 mass% or more.

Examples of propylene homopolymers include isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene. Examples of propylene-based copolymers include copolymers of propylene with ethylene and/or α-olefins having 4 to 8 carbon atoms, such as propylene-ethylene copolymer, propylene-butene copolymer, and propylene-ethylene-butene copolymer, as well as propylene-acrylic acid copolymer and propylene-maleic anhydride copolymer. The copolymerization mode in propylene-based copolymers is not particularly limited. For example, propylene-based copolymers may be block copolymers, random copolymers, or graft copolymers.

Furthermore, the polypropylene-based resin may be crosslinked or non-crosslinked. From the perspective of contributing to reducing the environmental impact, it is preferable that the polypropylene-based resin be non-crosslinked. The detailed structures of polypropylene-based resin A and polypropylene-based resin B will be described later.

Mass ratio of polypropylene resin A to polypropylene resin B The mass ratio of polypropylene resin A to polypropylene resin B in the base resin is polypropylene resin A:polypropylene resin B = 3:97 to 90:10. That is, the proportion of polypropylene resin A relative to 100% by mass of the total of polypropylene resin A and polypropylene resin B is 3% by mass or more and 90% by mass or less, and the proportion of polypropylene resin B is 10% by mass or more and 97% by mass or less. By setting the mass ratio of polypropylene resin A to polypropylene resin B in the base resin within the above-mentioned specific range, the expanded beads can improve in-mold moldability and contribute to reducing environmental impact.

If the content of polypropylene-based resin A in the base resin is too low, the proportion of biomass-derived monomer components in the base resin will be low, which may make it difficult to contribute to reducing environmental impact. From the perspective of increasing the proportion of biomass-derived monomer components in the base resin and further enhancing the effect of reducing environmental impact, the content of polypropylene-based resin A in the base resin is preferably 5% by mass or more, more preferably 7% by mass or more, even more preferably 10% by mass or more, particularly preferably 15% by mass or more, and most preferably 20% by mass or more, relative to 100% by mass of the total content of polypropylene-based resin A and polypropylene-based resin B.

If the content of polypropylene-based resin A in the base resin is too high, it may result in a decrease in the in-mold moldability of the expanded beads, narrowing the range of molding pressures at which good molded articles can be formed. In some cases, it may even be impossible to mold good molded articles. From the perspective of further improving the in-mold moldability of the expanded beads and broadening the range of molding pressures at which good molded articles can be formed, the content of polypropylene-based resin A in the base resin is preferably 70% by mass or less, more preferably 60% by mass or less, even more preferably 50% by mass or less, particularly preferably 45% by mass or less, and most preferably 40% by mass or less, relative to 100% by mass of the total content of polypropylene-based resin A and polypropylene-based resin B.

When determining the preferred range of the mass ratio of polypropylene-based resin A to polypropylene-based resin B in the base resin, the upper and lower limits of the polypropylene-based resin A content described above can be combined in any way. For example, the preferred range of the mass ratio of polypropylene-based resin A to polypropylene-based resin B in the base resin may be polypropylene-based resin A:polypropylene-based resin B = 5:95 to 70:30, 7:93 to 60:40, 10:90 to 50:50, 15:85 to 45:55, or 20:80 to 40:60.

Other Polymers The base resin may contain, as necessary, other polymers different from these polypropylene-based resins, in addition to the polypropylene-based resin A and the polypropylene-based resin B. Examples of polymers other than the polypropylene-based resin A and the polypropylene-based resin B that may be contained in the base resin include polypropylene-based resins that do not fall under either the polypropylene-based resin A or the polypropylene-based resin B, thermoplastic resins other than polypropylene-based resins, such as polyethylene-based resins, polystyrene-based resins, polyamide-based resins, and polyester-based resins, and elastomers such as olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers.

The base resin may contain one or more types of polymers other than polypropylene-based resin A and polypropylene-based resin B. These polymers may be contained in polypropylene-based resin A or polypropylene-based resin B. Furthermore, during the manufacturing process of the expanded beads, polymers other than polypropylene-based resin A and polypropylene-based resin B may be added to the base resin as needed.

The content of polymers other than polypropylene resin A and polypropylene resin B in the base resin is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, even more preferably 3% by mass or less, and particularly preferably 1% by mass or less, and most preferably 0% by mass, i.e., the base resin contains only polypropylene resin A and polypropylene resin B as polymers, relative to 100% by mass of the base resin.

Additives The base resin may contain additives such as colorants, antioxidants, antistatic agents, surfactants, light stabilizers, UV absorbers, and flame retardants, as needed. The base resin may contain one or more of these additives. These additives may be contained in either the polypropylene-based resin A or the polypropylene-based resin B. Furthermore, these additives may be added to the base resin as needed during the production process of the expanded beads.

The amount of additives in the base resin is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, relative to 100% by mass of the base resin.

[Polypropylene Resin A]
Polypropylene-based resin A contains a biomass-derived monomer component in its molecular chain. In this specification, the biomass-derived monomer component refers to a structural unit derived from a monomer produced from a biomass raw material. The biomass-derived monomer component is incorporated into the molecular chain of polypropylene-based resin A by polymerization of a monomer produced from a biomass raw material (hereinafter also referred to as a "biomass-derived monomer"). In this specification, a polypropylene-based resin containing a biomass-derived monomer component may also be referred to as a biomass-derived polypropylene-based resin.

Polypropylene-based resin A may be composed solely of biomass-derived monomer components, or may be composed of biomass-derived monomer components and fossil fuel-derived monomer components (i.e., structural units derived from monomers produced from fossil fuels). The monomer produced from biomass raw materials may be, for example, propylene, or a comonomer copolymerizable with propylene, such as ethylene or an α-olefin having 4 to 8 carbon atoms.

In this specification, biomass refers to renewable, biologically derived organic resources (excluding organic resources derived from fossil fuels such as petroleum and coal). More specifically, biomass includes, for example, agricultural and livestock products, forestry products, and algae. Furthermore, biomass raw materials refer to substances produced from biomass that serve as raw materials for monomers. Examples of biomass raw materials that can be used include monosaccharides, polysaccharides, vegetable oils and animal fats obtained from biomass. The method for producing monomers from biomass raw materials is not particularly limited and can take various forms. Examples of methods for producing biomass-derived monomers include a method of dehydrating alcohol produced from biomass raw materials and a method of decomposing naphtha produced from biomass raw materials.

From the perspective of non-competition with food and contribution to a recycling-oriented society through the use of by-products and waste biomass, it is preferable to use bionaphtha produced from waste cooking oil, black liquor, tall oil, palm oil mill effluent, and oils contained in microalgae as the biomass raw material. From a similar perspective, it is preferable to use biomass-derived propylene obtained by decomposing bionaphtha as the biomass-derived monomer.

As polypropylene-based resin A, a propylene homopolymer containing a biomass-derived monomer component or a propylene-based copolymer containing a biomass-derived monomer component can be used. Furthermore, the propylene-based copolymer used as polypropylene-based resin A may be a random copolymer, a block copolymer, or a graft copolymer. The base resin may contain one of these polypropylene-based resins as polypropylene-based resin A, or two or more polypropylene-based resins.

From the perspective of further improving the moldability of the expanded beads in the mold, polypropylene-based resin A is preferably a propylene homopolymer or a propylene-ethylene block copolymer, and more preferably a propylene homopolymer. By using such polypropylene-based resin A, the range of molding pressures over which good molded articles can be obtained can be broadened.

As the biomass-derived polypropylene resin A, commercially available biomass-derived polypropylene resins can be used. Specific examples include Lyondellbasell's product numbers "HP456J," "HP640J," and "EP348U."

The biomass content (biobased carbon content) of polypropylene-based resin A, measured according to ASTM D6866-21, is preferably 1% or more. By producing expanded beads using polypropylene-based resin A with a biomass content of 1% or more, the proportion of biomass-derived monomer components contained in the expanded beads can be increased, further enhancing the effect of reducing environmental impact. From this perspective, the biomass content of polypropylene-based resin A is more preferably 10% or more, even more preferably 20% or more, and particularly preferably 30% or more. The upper limit of the biomass content of polypropylene-based resin A is, by definition, 100%. More specific methods for measuring the biomass content are described in detail in the Examples.

From the perspective of further improving the moldability of the expanded beads in the mold, the biomass content of polypropylene resin A is preferably 80% or less, more preferably 50% or less, and even more preferably 45% or less.

When determining the preferred range of the biomass degree of polypropylene-based resin A, the upper and lower limits of the biomass degree of polypropylene-based resin A described above can be combined in any way. For example, the preferred range of the biomass degree of polypropylene-based resin A may be 1% or more and 100% or less, 10% or more and 80% or less, 20% or more and 50% or less, or 30% or more and 45% or less.

The melting point Tm _A of polypropylene resin A is preferably in the range of 155°C or higher and 170°C or lower, more preferably in the range of 158°C or higher and 168°C or lower, and even more preferably in the range of 160°C or higher and 165°C or lower. In this case, the moldability of the expanded beads in the mold and the rigidity of the resulting molded article can be improved in a well-balanced manner. From the same viewpoint, it is preferable that the melting point Tm _A of polypropylene resin A is in the range of 155°C or higher and 170°C or lower, and that the melting point Tm _B of polypropylene resin B described below is in the range of 130°C or higher and 145°C or lower.

The melting point Tm _A of polypropylene-based resin A is determined in accordance with JIS K7121-1987. Specifically, a test specimen made of polypropylene-based resin A is first prepared, and the test specimen is conditioned in accordance with "(2) Measuring the melting temperature after a certain heat treatment" in "3. Conditioning of test specimen" in JIS K7121-1987. The heating rate and cooling rate during conditioning are both 10°C/min. The conditioned test specimen is heated from 30°C to 200°C at a heating rate of 10°C/min to obtain a DSC curve, and the apex temperature of the melting peak appearing on the DSC curve is taken as the melting point Tm _A of polypropylene-based resin A. The flow rate of nitrogen gas in the measurement environment is 30 mL per minute. If multiple melting peaks appear on the DSC curve, the apex temperature of the melting peak with the largest area is taken as the melting point Tm _A of polypropylene-based resin A.

From the viewpoint of further shortening the water cooling time during molding of the expanded beads, the crystallization temperature Tc _A of the polypropylene resin A is preferably 110°C or higher and 130°C or lower, more preferably 113°C or higher and 128°C or lower, and even more preferably 115°C or higher and 123°C or lower.

The crystallization temperature Tc _A of polypropylene resin A means the temperature at the apex of the crystallization peak determined by heat flux differential scanning calorimetry based on JIS K7121: 2012. When two or more crystallization peaks appear, the temperature at the apex of the crystallization peak with the largest area is taken as the crystallization temperature Tc _A.

The melt mass-flow rate MFR _A of polypropylene-based resin A at a temperature of 230°C and a load of 2.16 kg is preferably 0.5 g/10 min or more and 10 g/10 min or less. In this case, the in-mold moldability of the expanded beads can be further improved, and the range of molding pressures under which a good molded article can be obtained can be broadened. From this viewpoint, the melt mass-flow rate MFR _A of polypropylene-based resin A is more preferably 1 g/10 min or more and 8 g/10 min or less, and even more preferably 2 g/10 min or more and 5 g/10 min or less. The melt mass-flow rate MFR _A of polypropylene-based resin A described above is a value measured in accordance with JIS K7210-1:2014 under conditions of a test temperature of 230°C and a load of 2.16 kg.

The heat of fusion _ΔH of the polypropylene resin A is preferably 75 J/g or more and 130 J/g or less, more preferably 85 J/g or more and 120 J/g or less, and even more preferably 95 J/g or more and 110 J/g or less, which further improves the moldability of the expanded beads in the mold and broadens the range of molding pressures under which a good molded article can be formed.

The heat of fusion ΔH _A of polypropylene-based resin A can be determined based on a DSC curve obtained by performing differential scanning calorimetry (DSC) in accordance with JIS K7122-1987. Specifically, first, polypropylene-based resin A is used as a test piece, and the test piece is conditioned based on "(2) Measuring the melting temperature after a certain heat treatment" in "3. Conditioning of test piece" in JIS K7122-1987. The heating rate and cooling rate in the conditioning are both 10°C/min, and the temperature range is 23°C to 200°C. The conditioned test piece is then heated again from 23°C to 200°C at a rate of 10°C/min to obtain a DSC curve (DSC curve during second heating). The flow rate of nitrogen gas in the measurement environment is 30 mL per minute. On this DSC curve, a straight line is drawn connecting the point corresponding to 80°C and the high-temperature end point of the melting peak with the highest apex temperature. The heat of fusion ΔH _A of the polypropylene resin A can be calculated based on the area of the region surrounded by the straight line determined in this manner and the melting peak of the DSC curve.

The flexural modulus _M of the polypropylene resin A is preferably 1000 MPa or more and 2000 MPa or less, more preferably 1200 MPa or more and 1800 MPa or less, and even more preferably 1300 MPa or more and 1700 MPa or less, in which case the moldability of the expanded beads in the mold can be further improved.

The flexural modulus M _A of the polypropylene resin A can be determined based on JIS K7171:2008.

When two or more types of polypropylene resin A are used in producing the expanded beads, a test piece made of a molten mixture of polypropylene resin A is prepared by melt-mixing the multiple types of polypropylene resin A in the same ratio as the compounding ratio in the expanded bead production process, and the biomass ratio, melting point, crystallization temperature, melt mass-flow rate, heat of fusion, and flexural modulus measured using this test piece by the above-mentioned methods are defined as the biomass ratio, melting point Tm _A , crystallization temperature Tc _A , melt mass-flow rate MFR _A , heat of fusion ΔH _A , and flexural modulus M _A of the polypropylene resin A, respectively.

・Polypropylene resin B
The base resin of the expanded beads contains a fossil fuel-derived polypropylene resin B. In this specification, a fossil fuel-derived polypropylene resin refers to a polypropylene resin obtained by polymerizing fossil fuel-derived monomers and containing only fossil fuel-derived monomer components in its molecular chain. In other words, the polypropylene resin B is composed only of fossil fuel-derived monomer components. Therefore, the biomass content (biobased carbon content) of the polypropylene resin B measured in accordance with ASTM D6866-21 is 0%.

In this specification, a fossil fuel-derived monomer component refers to a structural unit derived from a monomer produced from a fossil fuel. The monomer produced from a fossil fuel (hereinafter also referred to as a "fossil fuel-derived monomer") may be, for example, propylene, or a comonomer copolymerizable with propylene, such as ethylene or an α-olefin having 4 to 8 carbon atoms. The method for producing a monomer from a fossil fuel is not particularly limited and can take various forms. One example of a method for producing a fossil fuel-derived monomer is the thermal cracking and fractional distillation of naphtha obtained during petroleum refining.

The polypropylene-based resin B is a propylene-based random copolymer containing 3% to 10% by mass of a comonomer component (hereinafter also referred to as a comonomer-derived structural unit) derived from one or more monomers selected from the group consisting of ethylene and butene. The flexural modulus _M of the polypropylene-based resin _B is 900 MPa or less. The base resin may contain one or more propylene-based copolymers having a flexural modulus M within the specific range and containing the specific comonomer component as the polypropylene-based resin B. The expanded beads can contribute to reducing environmental impact and improving moldability in a mold by using the specific polypropylene-based resin B together with the biomass-derived polypropylene-based resin A. Furthermore, the expanded beads can be molded into good molded articles over a wide range of molding pressures, even when molding thick articles or articles having both thick and thin portions in a mold.

The flexural modulus _M of polypropylene-based resin B is 900 MPa or less. If the flexural modulus M of polypropylene-based resin _B is too high, the range of molding pressures at which a good molded product can be formed may be excessively narrow, particularly when attempting to obtain a thick molded product or a molded product having thick and thin portions by in-mold molding. By setting the flexural modulus _M of polypropylene-based resin B to 900 MPa or less, the above-mentioned problems can be easily avoided and the range of molding pressures at which a good molded product can be formed can be widened. From this viewpoint, the flexural modulus _M of polypropylene-based resin B is preferably less than 900 MPa, and more preferably 880 MPa or less.

On the other hand, from the viewpoint of improving the recovery of the molded body, further expanding the upper limit of the range of in-mold molding pressure at which a good molded body can be molded, and improving the physical properties of the molded body, the flexural modulus _M of the polypropylene-based resin B is preferably 500 MPa or more, more preferably 600 MPa or more, even more preferably 610 MPa or more, particularly preferably 650 MPa or more, and most preferably 700 MPa or more.

In determining a preferred range of the flexural modulus _M of the polypropylene-based resin B, any combination of the above-mentioned upper and lower limits of the flexural modulus _M of the polypropylene-based resin B can be used. For example, the preferred range of the flexural modulus _M of the polypropylene-based resin B may be 500 MPa or more and 900 MPa or less, 600 MPa or more and 900 MPa or less, 610 MPa or more and 900 MPa or less, 650 MPa or more and less than 900 MPa, or 700 MPa or more and 880 MPa or less.

From the viewpoint of broadening the range of molding pressures at which good molded articles can be formed, the ratio M _B /M A of the flexural modulus M _B of polypropylene resin B to the flexural modulus M _A of polypropylene resin _A is preferably 0.60 or less, more preferably 0.30 or more and 0.60 or less, even more preferably 0.35 or more but less than 0.60, particularly preferably 0.40 or more but less than 0.60, and most preferably 0.45 or more and 0.58 or less. From the same viewpoint, the difference M _A -M _B between the flexural modulus M _A of polypropylene resin A and the flexural modulus M _B of polypropylene resin B is preferably 580 MPa or more and 1000 MPa or less, more preferably 600 MPa or more and 900 MPa or less, and even more preferably 620 MPa or more and 890 MPa or less.

Furthermore, polypropylene-based resin B is a propylene-based random copolymer containing 3% by mass or more and 10% by mass or less of the specific comonomer component. When polypropylene-based resin B contains both ethylene and butene, the total amount is 3% by mass or more and 10% by mass or less. If polypropylene-based resin B does not contain the comonomer component, or if the content of the comonomer component is too low, secondary foaming properties may be insufficient, resulting in excessively poor in-mold moldability and making it impossible to obtain a satisfactory molded product. Even if a molded product can be obtained, the molding pressure may be excessively high or the range of possible molding pressures may be narrow. These problems can be easily avoided by setting the content of the comonomer component in polypropylene-based resin B to 3% by mass or more. To more reliably avoid the above-mentioned problems, polypropylene-based resin B preferably contains 3.2% by mass or more of the comonomer component, more preferably 3.3% by mass or more, and even more preferably 3.4% by mass or more.

On the other hand, by setting the content of the comonomer component in polypropylene-based resin B to 10% by mass or less, the upper limit of the range of molding pressures at which a good molded article can be produced can be expanded, and the physical properties of the molded article can be improved. To more reliably achieve this effect, polypropylene-based resin B preferably contains 8% by mass or less of the comonomer component, more preferably 5% by mass or less, even more preferably 4.5% by mass or less, and particularly preferably 4% by mass or less.

When determining the preferred range of the content of the comonomer component in polypropylene-based resin B, the above-mentioned upper and lower limits of the content of the comonomer component in polypropylene-based resin B can be combined in any manner. For example, the preferred range of the content of the structural unit derived from the specific comonomer in polypropylene-based resin B may be 3% by mass or more and 8% by mass or less, 3.2% by mass or more and 5% by mass or less, 3.3% by mass or more and 4.5% by mass or less, or 3.4% by mass or more and 4% by mass or less.

The content of the comonomer component in polypropylene-based resin B can be determined, for example, based on IR spectroscopy. The method for measuring the content of the comonomer component in polypropylene-based resin B will be explained in detail in the Examples.

The melting point Tm _B of the polypropylene resin B is preferably 130° C. or higher and 145° C. or lower, more preferably 133° C. or higher and 142° C. or lower, and even more preferably 135° C. or higher and 140° C. or lower. In this case, the lower limit of the molding pressure at which a molded article can be formed can be further lowered, and the range of molding pressure at which a good molded article can be formed can be more reliably expanded.

The crystallization temperature Tc _B of the polypropylene resin B is preferably 85° C. or higher and 110° C. or lower, more preferably 90° C. or higher and 105° C. or lower, even more preferably 92° C. or higher and 104° C. or lower, and particularly preferably 95° C. or higher and 102° C. or lower. In this case, the thermal processability of the expanded beads during heating for molding and the rate of crystallization and solidification during cooling can be appropriately adjusted, and the range of molding pressures under which a good molded article can be molded can be more reliably expanded.

From the same viewpoint, the difference [Tc _A - Tc _B ] between the crystallization temperature Tc _A of polypropylene resin A and the crystallization temperature Tc _B of polypropylene resin B is preferably 15°C or more and 35°C or less, more preferably 16°C or more and 28°C or less, even more preferably 16°C or more and 25°C or less, and particularly preferably 18°C or more and 23°C or less.

The melt mass-flow rate MFR _B of polypropylene resin B at a temperature of 230°C and a load of 2.16 kg is preferably 1 g/10 min or more and 20 g/10 min or less, more preferably 3 g/10 min or more and 15 g/10 min or less, even more preferably 4 g/10 min or more and 12 g/10 min or less, and particularly preferably 5 g/10 min or more and 10 g/10 min or less. In this case, the moldability of the expanded beads in the mold can be further improved, and the range of molding pressures under which a good molded article can be formed can be wider.

From the same viewpoint, the ratio MFR _A /MFR B of the melt mass-flow rate MFR _A of the polypropylene resin A at a temperature of 230°C and a load of 2.16 kg to the melt mass-flow rate MFR _B of the polypropylene resin _B at a temperature of 230°C and a load of 2.16 kg is preferably 0.2 or more and 0.6 or less, more preferably 0.3 or more and 0.5 or less.

The heat of fusion ΔH of polypropylene resin _B is preferably 30 J/g or more and 100 J/g or less, more preferably 40 J/g or more and 90 J/g or less, even more preferably 50 J/g or more and 80 J/g or less, and particularly preferably 60 J/g or more and 75 J/g or less. In this case, the moldability of the expanded beads in the mold can be further improved, and the range of molding pressures under which a good molded article can be formed can be wider.

The methods for measuring the biomass degree, flexural modulus M _B , melting point Tm _B , crystallization temperature Tc _B , melt mass-flow rate MFR _B and heat of fusion ΔH _B of polypropylene-based resin B are the same as the methods for measuring the biomass degree, flexural modulus M _A , melting point Tm _A , crystallization temperature Tc _A , melt mass-flow rate MFR _A and heat of fusion ΔH _A of polypropylene-based resin A, respectively, except that polypropylene-based resin B is used instead of polypropylene-based resin A.

Furthermore, when two or more types of polypropylene-based resin B are used in the production of expanded beads, a test piece made of a molten mixture of polypropylene-based resin B is prepared by melt-mixing the multiple types of polypropylene-based resin B in the same ratio as the compounding ratio in the production process of the expanded beads, and the biomass degree, melting point, crystallization temperature, melt mass-flow rate, heat of fusion, and flexural modulus measured using this test piece by the above-mentioned methods are defined as the biomass degree, melting point Tm _B , crystallization temperature Tc _B , melt mass-flow rate MFR _B , heat of fusion ΔH _B , and flexural modulus M _B of polypropylene-based resin B, respectively.

[Closed cell ratio of expanded beads]
The closed cell ratio of the expanded beads is preferably 80% or more, more preferably 85% or more, even more preferably 90% or more, and particularly preferably 92% or more. In this case, the moldability of the expanded beads in a mold can be further improved, and the physical properties of the molded article obtained by molding the expanded beads in a mold can be more easily improved. The upper limit of the closed cell ratio of the expanded beads is not particularly limited, but may be, for example, 99%.

The closed cell ratio of expanded beads is a value measured using an air comparison hydrometer in accordance with Procedure C of ASTM-D2856-70. The specific method for measuring the closed cell ratio of expanded beads is as follows. After conditioning, expanded beads with a bulk volume of approximately 20 ^cm3 are used as a measurement sample. The measurement sample is submerged in a graduated cylinder containing ethanol, and the apparent volume Va of the measurement sample is measured from the rise in the liquid level. After measuring the apparent volume Va, the measurement sample is thoroughly dried, and the true volume Vx of the measurement sample is measured using an air comparison hydrometer ("Beckman Model 1000 Air Comparison Pycnometer" manufactured by Tokyo Science Co., Ltd.) in accordance with Procedure C described in ASTM-D2856-70. These volume values Va and Vx are then used to calculate the closed cell ratio (unit: %) of the measurement sample according to the following formula (1): The above operation is repeated five times using different measurement samples, and the arithmetic mean value (N=5) of the closed cell ratios of the five measurement samples is taken as the closed cell ratio (unit: %) of the expanded beads.
Closed cell ratio = (Vx-W/ρ) x 100/(Va-W/ρ)...(1)

However, the meanings of the symbols in the above formula (1) are as follows:
Vx: the true volume of the measurement sample measured by the above method, i.e., the sum of the volume of the resin constituting the expanded beads and the total volume of the closed cells in the expanded beads (unit: cm ³ ).
Va: Apparent volume of the measurement sample measured from the rise in the liquid level when the measurement sample is submerged in a measuring cylinder containing ethanol (unit: cm ³ )
W: Mass of the measurement sample (unit: g)
ρ: density of the resin constituting the expanded beads (unit: g/cm ³ )

[Biomass content of expanded beads]
The biomass degree of the expanded beads, as measured by ASTM D6866-21, is preferably 1% or more and 50% or less, more preferably 2% or more and 40% or less, even more preferably 3% or more and 35% or less, and particularly preferably 5% or more and 30% or less. By setting the biomass degree of the expanded beads within the above-mentioned specific range, the effect of reducing the environmental load can be further enhanced, and the effect of improving the in-mold moldability of the expanded beads can be more reliably obtained.

The biomass degree of the expanded beads can be determined by measuring the concentration of radioactive carbon ¹⁴ C as specified in ASTM D6866-21 using the expanded beads as a measurement sample. When the biomass degree of the biomass-derived polypropylene resin A used to produce the expanded beads and the content of the biomass-derived polypropylene resin A in the expanded beads are known, the biomass degree of the expanded beads can also be calculated using these values.

[Bulk density of expanded particles]
The expanded beads preferably have a bulk density of 10 kg/m or ^more and 200 kg/m ^{or less} , more preferably 15 kg/m or ^more and 150 kg/m ^or less, even more preferably 20 kg/m or ^more and 120 kg/m ^{or less} , particularly preferably 25 kg/m or ^more and 100 kg/m ^{or less} , and most preferably ³⁰ kg/m or more and 80 kg/m or ^less . By setting the bulk density of the expanded beads within the above-mentioned specific range, a molded article that is lightweight and has good physical properties can be more easily obtained.

The bulk density of expanded beads is calculated as follows. First, the expanded beads are left to stand for 24 hours or more in an environment of 50% relative humidity, 23°C temperature, and 1 atm atmospheric pressure to condition the expanded beads. The conditioned expanded beads are filled into a measuring cylinder, and the bottom of the measuring cylinder is lightly tapped against the floor several times to stabilize the filling height of the expanded beads in the measuring cylinder. The bulk volume (unit: L) of the expanded beads is read from the measuring cylinder scale. The mass (unit: g) of the expanded beads in the measuring cylinder is then divided by the aforementioned bulk volume, and the units are converted to obtain the bulk density (unit: kg/ ^m3 ) of the expanded beads.

[Fusing layer]
The expanded beads have a foam layer composed of a base resin containing the polypropylene-based resin A and the polypropylene-based resin B. The expanded beads may have a single-layer structure consisting of only the foam layer, or a multilayer structure comprising the foam layer and a fusion layer covering the foam layer. The fusion layer is provided to enhance fusion between the expanded beads during in-mold molding. The fusion layer may be present on the entire surface of the expanded beads or on a portion of the surface. The fusion layer may be in a foamed or non-foamed state, but is preferably in a substantially non-foamed state. The aforementioned "non-foamed state" includes a state in which the fusion layer is not foamed and does not contain bubbles, and a state in which the bubbles have disappeared after foaming, meaning that there is almost no bubble structure in the fusion layer. When the expanded beads have a foam layer and a fusion layer, the foam layer covered by the fusion layer is sometimes referred to as the "foamed core layer."

The method for producing expanded beads having a fusion layer is not particularly limited, and examples include a method of foaming resin beads having an unfoamed core layer and a fusion layer covering the core layer, or a method of foaming an unfoamed core layer to obtain an expanded core layer and then attaching a fusion layer to the surface of the expanded core layer. When foaming resin beads having a fusion layer on their surface to obtain expanded beads, it is preferable to use an extrusion device capable of co-extrusion to co-extrude the resin melt for forming the core layer and the resin melt for forming the fusion layer when producing the resin beads, thereby laminating the fusion layer on the surface of the core layer.

If the expanded beads have a fusion layer, the proportion of the fusion layer in the expanded beads is preferably approximately 0.5% by mass or more and 20% by mass or less, and more preferably 1% by mass or more and 12% by mass or less.

Examples of the base resin that makes up the fusion layer include crystalline polyolefin resins with a melting point lower than that of the base resin that makes up the foam core layer, and amorphous polyolefin resins with a softening point lower than that of the base resin that makes up the foam core layer. The base resin that makes up the fusion layer is preferably a polypropylene resin and/or a polyethylene resin.

If the adhesive layer is made of a crystalline polyolefin resin having a melting point, the melting point of the crystalline polyolefin resin is preferably 110°C or higher and 150°C or lower, more preferably 120°C or higher and 145°C or lower, and even more preferably 125°C or higher and 142°C or lower.

Furthermore, the difference between the melting point of the base resin constituting the foamed core layer and the melting point of the crystalline polyolefin resin constituting the fusion layer (i.e., the melting point of the base resin constituting the foamed core layer minus the melting point of the crystalline polyolefin resin constituting the fusion layer) is preferably approximately 1°C or higher and 40°C or lower, more preferably 2°C or higher and 35°C or lower, and even more preferably 5°C or higher and 30°C or lower. In this case, the in-mold moldability of the foamed beads can be improved even when the molding pressure is relatively low.

[Melting point of expanded particles]
The melting point of the expanded beads is preferably 135°C or higher and 165°C or lower, more preferably 140°C or higher and 162°C or lower, even more preferably 145°C or higher and 160°C or lower, and particularly preferably 150°C or higher and 158°C or lower. In this case, the in-mold moldability of the expanded beads can be more easily improved, and the compression properties of the resulting molded article can be further improved. The method for measuring the melting point of the expanded beads is the same as the method for measuring the melting point Tm _A of the polypropylene-based resin A described above, except that expanded beads or the base resin constituting the expanded beads are used instead of the polypropylene-based resin A.

[High-temperature peak, heat of fusion at the high-temperature peak, and total heat of fusion of the expanded particles]
The expanded beads preferably have a crystalline structure in which a DSC curve obtained when heated from 23°C to 200°C at a heating rate of 10°C/min exhibits a melting peak due to the melting of crystals inherent to the resin component contained in the expanded beads and one or more melting peaks located at higher temperatures than the melting peak. Expanded beads with such a crystalline structure exhibit excellent moldability in a mold. Furthermore, by molding such expanded beads in a mold, molded articles with excellent compression properties can be more easily obtained. Hereinafter, the melting peak due to the melting of crystals inherent to the resin component appearing in the DSC curve is referred to as the "resin-specific peak," and the melting peak appearing at a higher temperature than the resin-specific peak is referred to as the "high-temperature peak." The resin-specific peak appears due to the melting of crystals normally possessed by the resin component contained in the expanded beads. On the other hand, the high-temperature peak is presumed to appear due to the melting of secondary crystals formed in the resin component during the manufacturing process of the expanded beads. In other words, when a high-temperature peak appears in a DSC curve, it is presumed that secondary crystals are formed in the resin component.

Whether or not the expanded beads have the aforementioned crystalline structure can be determined based on the DSC curve obtained by performing differential scanning calorimetry (DSC) under the conditions described above in accordance with JIS K7122-1987. The flow rate of nitrogen gas in the measurement environment is 30 mL per minute. When performing DSC, 1 to 3 mg of expanded beads can be used as the sample.

Specifically, the DSC curve obtained when heating from 23°C to 200°C at a heating rate of 10°C/min (i.e., the first heating) as described above shows both the resin peak characteristic of the resin component contained in the expanded beads and a high-temperature peak. In contrast, the DSC curve obtained when the first heating is followed by cooling from 200°C to 23°C at a cooling rate of 10°C/min, and then heating again from 23°C to 200°C at a heating rate of 10°C/min (i.e., the second heating) shows only the resin peak characteristic of the resin component contained in the expanded beads. Therefore, by comparing the DSC curve obtained during the first heating with the DSC curve obtained during the second heating, it is possible to distinguish between the resin peak characteristic of the resin component and the high-temperature peak.

From the viewpoint of further improving the in-mold moldability of the expanded beads and the compression properties of the molded body, the apex temperature of the high-temperature peak of the expanded beads is preferably 155°C or higher and 178°C or lower, and more preferably 160°C or higher and 175°C or lower. From the same viewpoint, the heat of fusion of the high-temperature peak is preferably 10 J/g or higher and 50 J/g or lower, more preferably 12 J/g or higher and 40 J/g or lower, even more preferably 15 J/g or higher and 35 J/g or lower, and particularly preferably 20 J/g or higher and 30 J/g or lower.

Furthermore, the total heat of fusion of the expanded beads is preferably 60 J/g or more and 120 J/g or less, more preferably 70 J/g or more and 110 J/g or less, even more preferably 75 J/g or more and 105 J/g or less, and particularly preferably 80 J/g or more and 100 J/g or less. In this case, the moldability of the expanded beads in a mold can be more easily improved. Furthermore, by molding such expanded beads in a mold, a molded product with excellent compression properties can be more easily obtained.

From the perspective of further improving the in-mold moldability of the expanded beads and the compression properties of the molded body, the ratio of the heat of fusion of the high-temperature peak to the total heat of fusion of the expanded beads is preferably 0.15 or more and 0.35 or less, and more preferably 0.20 or more and 0.30 or less.

The methods for measuring the apex temperature of the high-temperature peak, the heat of fusion of the high-temperature peak, and the total heat of fusion are as follows. First, a DSC curve is obtained by performing differential scanning calorimetry using 1 to 3 mg of expanded beads as a sample, heating from 23°C to 200°C at a heating rate of 10°C/min. Figure 1 shows an example of a DSC curve. When the expanded beads have a high-temperature peak, the DSC curve will show a resin-specific peak ΔH1 and a high-temperature peak ΔH2, which has a peak higher than the peak of the resin-specific peak ΔH1, as shown in Figure 1. The temperature corresponding to the peak of this high-temperature peak ΔH2 is the apex temperature of the high-temperature peak.

Next, draw a line L1 connecting point α, which corresponds to 80°C on the DSC curve, and point β, which corresponds to the melting end temperature T of the expanded beads. Note that the melting end temperature T is the high-temperature end point of the high-temperature peak ΔH2, i.e., the intersection of the high-temperature peak ΔH2 and the baseline on the higher temperature side of the high-temperature peak ΔH2 on the DSC curve.

After drawing line L1, draw line L2, parallel to the vertical axis of the graph, through the maximum point γ that exists between the resin-specific peak ΔH1 and the high-temperature peak ΔH2. This line L2 separates the resin-specific peak ΔH1 from the high-temperature peak ΔH2. The heat of fusion of the high-temperature peak ΔH2 can be calculated based on the area of the part of the DSC curve that constitutes the high-temperature peak ΔH2 and the area surrounded by lines L1 and L2. The total heat of fusion of the expanded beads can be calculated based on the area of the part of the DSC curve that constitutes the resin-specific peak ΔH1, the part of the high-temperature peak ΔH2, and the area surrounded by line L1. In other words, the total heat of fusion of the expanded beads is the sum of the heat of fusion of the resin-specific peak ΔH1 and the high-temperature peak ΔH2.

(Method for producing expanded beads)
The method for producing the expanded beads is not particularly limited, and for example, a conventional method for producing expanded polypropylene resin beads can be used as the method for producing the expanded beads. More specifically, examples of the method for producing the expanded beads include a method of producing resin beads composed of the base resin and then expanding the resin beads using a foaming agent, and a method of extruding a molten base resin while supplying a foaming agent to foam it, and then cutting the extrudate to a desired size.

There are no particular limitations on the method for producing resin particles. For example, when producing resin particles by the strand cutting method, the polypropylene resin A, the polypropylene resin B, and any additives used as needed are supplied to an extruder, and the resin raw materials are heated and kneaded inside the extruder to obtain a molten mixture. This molten mixture is extruded in the form of strands through small holes in a die attached downstream of the extruder. Resin particles can be obtained by collecting and cutting this strand-like extrudate to the desired length.

When foaming resin particles, methods such as the "direct foaming method" can be used, in which resin particles containing a foaming agent dispersed in an aqueous medium in a container are released together with the aqueous medium into an atmosphere with a lower pressure than the pressure inside the container, or the "impregnation foaming method," in which resin particles are impregnated with a foaming agent in the gas phase and then heated to foam them.

In the direct foaming method, resin particles are first dispersed in an aqueous medium in a container. The aqueous medium can be, for example, water. If necessary, a dispersant or dispersion aid may be added to disperse the resin particles in the aqueous medium in the container.

As dispersants, inorganic fine particles such as aluminum oxide, tricalcium phosphate, magnesium pyrophosphate, zinc oxide, kaolin, and mica can be used. These inorganic fine particles may be used alone, or two or more types of inorganic fine particles may be used in combination. As dispersing aids, inorganic salts such as aluminum sulfate, and anionic surfactants such as sodium alkylbenzenesulfonate, sodium dodecylbenzenesulfonate, and sodium alkanesulfonate can be used. These dispersing aids may be used alone, or two or more types of dispersing aids may be used in combination.

Next, a blowing agent is supplied into the container, and the pressure inside the container is increased to impregnate the resin particles with the blowing agent. This produces resin particles containing the blowing agent. At this time, the resin particles in the container can be heated together with the aqueous medium to promote the impregnation of the resin particles with the blowing agent.

The blowing agent used in the foaming process can be, for example, inorganic physical blowing agents such as carbon dioxide, air, nitrogen, helium, or argon; or organic physical blowing agents such as hydrocarbons such as propane, n-butane, isobutane, n-pentane, isopentane, or hexane; or halogenated hydrocarbons such as ethyl chloride, 2,3,3,3-tetrafluoropropene, trans-1,3,3,3-tetrafluoropropene, or trans-1-chloro-3,3,3-trifluoropropene. From the perspective of environmental impact and ease of handling, carbon dioxide is preferably used as the blowing agent. The amount of blowing agent added is preferably 0.1 to 30 parts by weight, and more preferably 0.5 to 15 parts by weight, per 100 parts by weight of resin particles.

The pressure inside the container immediately before foaming is preferably 0.5 MPa (G) or higher in gauge pressure. On the other hand, the pressure inside the container is preferably 4.0 MPa (G) or lower in gauge pressure. Within the above range, expanded beads can be produced safely without risk of the container breaking or exploding.

After the resin particles have been impregnated with the blowing agent, the contents of the container are released into an atmosphere with a lower pressure than the container. This causes the resin particles to expand and form a cellular structure, which is then cooled by the outside air (i.e., the atmosphere), stabilizing the cellular structure and resulting in expanded particles.

In the above-described manufacturing method, a step of adjusting the crystalline structure of the resin component contained in the resin particles may be carried out between dispersing the resin particles in the aqueous medium and expanding the resin particles. By expanding the resin particles after adjusting the crystalline structure of the resin component, expanded beads with excellent in-mold moldability can be easily obtained.

The crystal structure of the resin component can be adjusted, for example, as follows. First, a holding step is performed in which the temperature of the resin particles is maintained within a temperature range of (the melting point of the base resin - 20°C) or more and (the temperature at which the base resin finishes melting) for a sufficient period of time, preferably 10 to 60 minutes. The temperature inside the pressure vessel is then adjusted to a temperature between (the melting point of the base resin - 15°C) or more and (the temperature at which the base resin finishes melting). If necessary, a two-stage holding step is then performed in which the temperature is maintained for an additional 10 to 60 minutes. By expanding the resin particles that have undergone this holding step, expanded particles with a crystal structure that exhibits the aforementioned high-temperature peak on the DSC curve can be easily obtained.

In the above-mentioned manufacturing method, expanded beads may be obtained by preparing resin particles that have undergone the holding step in advance, and then impregnating these resin particles with a blowing agent to cause expansion. From the perspective of increasing productivity of expanded beads, it is preferable to carry out the above-mentioned holding step by heating resin particles dispersed in an aqueous medium in a container in the presence of a blowing agent, and then releasing the contents of the sealed container from the container into an atmosphere with a pressure lower than the pressure inside the container to expand the resin particles, thereby obtaining expanded beads having a crystalline structure that exhibits the above-mentioned high-temperature peak.

The reason why heating and foaming under the conditions described above can improve the in-mold moldability and compression properties of the expanded beads is thought to be due to the formation of secondary crystals of polypropylene resin in the base resin that makes up the expanded beads. Whether or not secondary crystals of polypropylene resin have formed in the base resin can be determined by the presence or absence of a high-temperature peak in the DSC curve.

Furthermore, in the above-mentioned manufacturing method, when expanding the resin particles, the resin particles may be expanded in a single stage as described above, or the resin particles may be expanded in two or more stages. When expanding the resin particles in two stages, first, in the first expansion stage, the resin particles are expanded by a direct expansion method or the like to obtain first-stage expanded particles. In the second expansion stage, for example, the first-stage expanded particles may be pressurized with air or the like to increase the pressure (internal pressure) within the cells of the first-stage expanded particles, and then the first-stage expanded particles may be heated with steam or the like to further expand them. By expanding the resin particles in multiple stages in this way, expanded beads with a higher expansion ratio (i.e., a lower bulk density) can be easily obtained.

(Polypropylene resin foamed particle molding)
After the expanded beads are filled into a mold, a molded body can be obtained by in-mold molding using a heating medium such as steam. The base resin of the expanded beads constituting the molded body contains a polypropylene-based resin A containing a biomass-derived monomer component and a fossil-fuel-derived polypropylene-based resin B. Furthermore, the polypropylene-based resin B is composed of the specific random copolymer and has a flexural modulus M _B within the specific range. Therefore, by using the expanded beads, good molded bodies can be obtained over a wide range of molding pressures. Furthermore, molded bodies composed of the expanded beads can contribute to reducing environmental impact. The molded bodies, like conventional expanded bead molded bodies composed solely of fossil-fuel-derived polypropylene-based resins, can be suitably used for various applications such as packaging materials, automotive components, and building materials.

[Density of Molded Product]
The density of the molded body is preferably 10 kg/m or ^more and 200 kg/m ^or less, more preferably ¹⁵ kg/m or more and 150 kg/m ^or less, even more preferably 20 kg/m ^or more and 120 kg/m ^{or less} , and particularly preferably 30 kg/m or ^more and 90 kg/m or ^less . In this case, the balance between the light weight and physical properties of the molded body is better. The density of the molded body is calculated by dividing the mass (unit: g) of the molded body by the volume (unit: L) calculated from the external dimensions of the molded body and converting the result into units.

Examples of the expanded beads are described below.

(Polypropylene Resin A)
Table 1 shows the properties of two types of biomass-derived polypropylene resin A used in the production of expanded beads. The polypropylene resins A used in this example (Table 1, PP-A1 and PP-A2) are both propylene homopolymers containing a biomass-derived monomer component. Specifically, PP-A1 is "HP456J" manufactured by Lyondellbasell, and PP-A2 is "HP640J" manufactured by Lyondellbasell. In Table 1, propylene homopolymers are referred to as "h-PP."

The physical properties of Polypropylene Resin A shown in Table 1 were measured as follows:

[Biomass ratio]
The biomass content of polypropylene resin A was measured in accordance with ASTM D6866-21. More specifically, polypropylene resin A was first combusted to generate combustion gas containing carbon dioxide (CO ₂ ). This combustion gas was introduced into a vacuum line, and the carbon dioxide (CO ₂ ) was purified in the vacuum line. Graphite (C) was produced by reducing this carbon dioxide using hydrogen. Iron was used as a catalyst for the reduction of carbon dioxide.

Next, the graphite, background sample, and standard sample were measured using an accelerator mass spectrometer (AMS). The number of ¹⁴ C atoms in each sample, the concentration of ¹³ C atoms relative to the number of ¹² C atoms ( ¹³ C/ ¹² C), and the concentration of ¹⁴ C atoms relative to the number of ¹² C atoms ( ¹⁴ C/ ¹² C) were calculated. The isotope ratios were measured using a dedicated ¹⁴ C-AMS instrument based on an NEC tandem accelerator. For the measurements, graphite was packed into a 1 mm inner diameter cathode using a hand press, which was then fitted into a wheel and installed in the dedicated ¹⁴ C-AMS instrument. Oxalic acid (HOx II), provided by the National Bureau of Standards (NIST), was used as the standard sample.

Based on these measurement results, δ ¹⁴ C, which represents the deviation in the concentration of ¹⁴ C atoms in graphite from the concentration of modern ¹⁴ C atoms obtained based on the measurement results of the standard sample, expressed in parts per thousand (‰), and δ ¹³ C, which represents the deviation in the concentration of ¹³ C atoms in graphite from the concentration of modern ¹³ C atoms obtained based on the measurement results of the standard sample, expressed in parts per thousand (‰), were calculated. To correct for the isotope effect due to ¹³ C, the δ ¹⁴ C obtained by the above-mentioned method was corrected using the δ ¹³ C value. The pMC (percent modern carbon) value was then obtained based on the corrected δ ¹⁴ C value (Δ ¹⁴ C). The biomass content was then calculated by applying atmospheric correction to this pMC value.

The atmospheric correction factor used for atmospheric correction was the atmospheric correction factor for 2019-2021 specified in ASTM D6866-21 (100.0 pMC). More specifically, the atmospheric correction factor specified in ASTM D6866 for the year the polypropylene resin was manufactured was used to determine the biomass content.

[Melting point Tm _A ]
The melting point Tm _A of polypropylene-based resin A was measured in accordance with JIS K7121-1987. Specifically, first, the condition of a test specimen made of polypropylene-based resin A was adjusted in accordance with "(2) Measurement of melting temperature after a certain heat treatment" in "3. Conditioning of test specimen" described in JIS K7121-1987. In the conditioning, the heating rate and cooling rate were 10°C/min. A DSC curve was obtained by heating the conditioned test specimen from 23°C to 200°C at a heating rate of 10°C/min. The flow rate of nitrogen gas in the measurement environment was 30 mL per minute. The apex temperature of the melting peak that appeared in the DSC curve was taken as the melting point Tm _A of polypropylene-based resin A. The measurement device used was a heat flux differential scanning calorimeter (manufactured by SII Nanotechnology Inc., model number: DSC7020). When a plurality of melting peaks appeared in the DSC curve, the apex temperature of the melting peak with the largest area was taken as the melting point Tm _A of the polypropylene resin A.

[Crystallization temperature Tc _A ]
The crystallization temperature Tc _A of polypropylene-based resin A was measured based on JIS K7121-1987. Specifically, using a heat flux differential scanning calorimeter (manufactured by SII Nanotechnology Inc., model number: DSC7020), a test piece made of polypropylene-based resin A was heated from 23°C to 200°C at a heating rate of 10°C/min, and then cooled from 200°C to 30°C at a cooling rate of 10°C/min to obtain a DSC curve. The peak temperature of the crystallization peak in this DSC curve was taken as the crystallization temperature. The flow rate of nitrogen gas under the measurement environment was 30 mL per minute. When multiple crystallization peaks appeared in the DSC curve, the apex temperature of the crystallization peak with the largest area was taken as the crystallization temperature Tc _A of polypropylene-based resin A.

[Heat of fusion ΔH _A ]
The heat of fusion ΔH _A of polypropylene resin A was determined from a DSC curve obtained by differential scanning calorimetry in accordance with JIS K7122-1987. Specifically, first, polypropylene resin A was used as a test specimen, and the condition of the test specimen was adjusted based on "(2) Measurement of melting temperature after a certain heat treatment" in "3. Conditioning of test specimen" in JIS K7122-1987. In the conditioning, the heating rate and cooling rate were 10°C/min. Thereafter, the conditioned test specimen was again heated from 23°C to 200°C at a rate of 10°C/min to obtain a DSC curve (DSC curve at the time of the second heating). The flow rate of nitrogen gas in the measurement environment was 30 mL per minute.

On the DSC curve thus obtained, a straight line was drawn connecting the point corresponding to 80°C on the DSC curve and the high-temperature end point of the melting peak with the highest apex temperature. Then, the heat of fusion ΔH of polypropylene resin _A was calculated based on the area enclosed by the straight line thus determined and the melting peak of the DSC curve.

[Melt mass-flow rate MFR _A ]
The melt mass-flow rate MFR _A of polypropylene resin A was measured in accordance with JIS K7210-1:2014 under conditions of a temperature of 230°C and a load of 2.16 kg.

[Flexural modulus M _A ]
Polypropylene resin A was heat-pressed at 180°C to prepare a 4 mm thick sheet, and a test piece measuring 80 mm long x 10 mm wide x 4 mm thick was cut out from this sheet. The flexural modulus of the test piece determined in accordance with JIS K7171:2008 was taken as the flexural modulus M _A of polypropylene resin A. The radius R1 of the indenter and the radius R2 of the support table were both 5 mm, the distance between supports was 64 mm, and the test speed was 2 mm/min.

(Polypropylene Resin B)
Table 2 shows the properties of the four types of fossil fuel-derived polypropylene resins B used in the production of the expanded beads. Of the four types of polypropylene resins B used in this example, PP-B1 and PP-B2 are propylene-ethylene random copolymers composed of fossil fuel-derived monomer components. Of the four types of polypropylene resins B, PP-B3 is a propylene-ethylene-butene random copolymer composed of fossil fuel-derived monomer components. Of the four types of polypropylene resins B, PP-B4 is a propylene homopolymer composed of fossil fuel-derived monomer components. In Table 2, propylene-ethylene random copolymer is referred to as "r-PP," propylene-ethylene-butene random copolymer is referred to as "ter-PP," and propylene homopolymer is referred to as "h-PP."

The methods for measuring the physical properties shown in Table 2 are the same as those for measuring the corresponding physical properties of Polypropylene Resin A described above. The "ethylene component content" and "butene component content" of Polypropylene Resin B in Table 2 were calculated as follows:

The ethylene and butene content in polypropylene resin B was determined by a known method using IR spectroscopy. Specifically, it was determined using the method described in the Polymer Analysis Handbook (edited by the Polymer Analysis Research Forum of the Japan Society for Analytical Chemistry, published January 1995, published by Kinokuniya Shoten, page numbers and section names: 615-616 "II.2.3 2.3.4 Propylene/ethylene copolymer", 618-619 "II.2.3 2.3.5 Propylene/butene copolymer"), which quantifies the absorbance of ethylene and butene based on the relationship between the absorbance corrected by a predetermined coefficient and the thickness of the film test piece, etc.

More specifically, polypropylene-based resin B was first heat-pressed at 180°C to form a film, and a plurality of test specimens with thicknesses ranging from 0.1 to 0.3 mm were prepared. Next, the IR spectrum of each test specimen was measured to determine the absorbances at 722 ^cm and 733 ^cm (A ₇₂₂ , A ₇₃₃ ) attributable to ethylene and the absorbance at 766 ^cm (A ₇₆₆ ) attributable to butene. The ethylene content (unit: mass%) in polypropylene-based resin B was then calculated for each test specimen using the following formulas (2) to (4). The arithmetic mean of the ethylene content values obtained for each test specimen was taken as the ethylene content (unit: mass%) in polypropylene-based resin B.

(K' ₇₃₃ ) _c = 1/0.96 {(K _{' 733} ) _a -0.268 (K' ₇₂₂ ) _a }...(2)
(K' ₇₂₂ ) _c = 1/0.96 {(K _{' 722} ) _a -0.150 (K' ₇₃₃ ) _a }...(3)
Ethylene content = 0.575 {(K _{' 722} ) _c + (K' ₇₃₃ ) _c } (4)

where _K'a in equations (2) to (4) is the apparent absorption coefficient at each wave number ( _K'a = A/ρt), _K'c is the corrected absorption coefficient, A is the absorbance, ρ is the density of the resin (unit: g/ ^cm3 ), and t is the thickness of the film-like test piece (unit: cm). Note that the above equations (2) to (4) can be applied to random copolymers.

Furthermore, for each test piece, the butene component content (unit: mass%) in the polypropylene-based resin was calculated using the following formula (5): The arithmetic mean of the butene component contents obtained for each test piece was taken as the butene component content (unit: mass%) in the polypropylene-based resin.
Butene component content=12.3(A ₇₆₆ /L) (5)
In the formula (5), A is the absorbance, and L is the thickness (unit: mm) of the film-like test piece.

Next, we will explain the structure and manufacturing method of the expanded beads in this example.

Example 1
PP-A1 as polypropylene resin A, PP-B1 as polypropylene resin B, and a cell control agent were fed into an extruder, and a molten mixture of these resins was formed in the extruder. The mass ratios of PP-A1 and PP-B1 were as shown in Table 3. Zinc borate was used as the cell control agent. The amount of zinc borate added was 0.1% by mass relative to 100% by mass of the total of PP-A1 and PP-B1. In Tables 3 to 5, polypropylene resin A is abbreviated as "PP(A)" and polypropylene resin B is abbreviated as "PP(B)."

The molten mixture was then extruded in the form of strands through small holes in a die located downstream of the extruder. The strand-like extrudate was collected, cooled, and then cut to an appropriate length using a pelletizer to obtain resin particles. The average mass of each resin particle was approximately 1.0 mg.

The resin particles obtained in this manner were expanded using the direct foaming method. Specifically, 1 kg of resin particles was first placed in a 5 L container along with 3 L of water as an aqueous medium. Next, 0.3 parts by mass of dispersant and 0.004 parts by mass of dispersion aid were added to the container per 100 parts by mass of resin particles, and the resin particles were dispersed in the aqueous medium. Kaolin was used as the dispersant. Sodium dodecylbenzenesulfonate was also used as the dispersion aid. The amount of dispersion aid added mentioned above is the amount of active ingredient in the dispersion aid.

Then, while stirring the contents of the container, the container was heated at a temperature increase rate of 2°C/min, and the temperature inside the container was raised to the foaming temperature shown in Table 3. After the temperature inside the container reached the foaming temperature, carbon dioxide was supplied into the container as a foaming agent, and the pressure inside the container was raised to the foaming pressure shown in Table 3. By maintaining this temperature and pressure for 15 minutes, the resin particles were impregnated with the foaming agent and the crystalline structure of the base resin that makes up the resin particles was adjusted. The container was then opened, and the contents were released into an atmospheric pressure atmosphere, causing the resin particles to foam. In this way, the expanded beads of Example 1 were obtained.

(Examples 2 and 3)
The expanded beads of Examples 2 and 3 have the same structure as the expanded beads of Example 1, except that the mass ratio between PP-A1 and PP-B1 was changed as shown in Table 3. The manufacturing method of the expanded beads of Examples 2 and 3 is generally the same as the manufacturing method of the expanded beads of Example 1, except that the mass ratio between PP-A1 and PP-B1 was changed as shown in Table 3.

(Examples 4 and 5)
The expanded beads of Examples 4 and 5 have the same structure as the expanded beads of Example 1, except that PP-B2 or PP-B3 was used instead of PP-B1 as the polypropylene-based resin B. The manufacturing method of the expanded beads of Examples 4 and 5 is generally the same as the manufacturing method of the expanded beads of Example 1, except that the polypropylene-based resin B was changed as described above.

Example 6
The expanded beads of Example 6 have the same structure as the expanded beads of Example 1, except that PP-A2 was used instead of PP-A1 as the polypropylene-based resin A. The manufacturing method of the expanded beads of Example 6 is generally the same as the manufacturing method of the expanded beads of Example 1, except that the polypropylene-based resin A was changed as described above.

(Comparative Example 1)
The expanded beads of Comparative Example 1 have the same structure as the expanded beads of Example 1, except that they do not contain polypropylene-based resin B and are composed of polypropylene-based resin A, as shown in Table 5. The manufacturing method of the expanded beads of Comparative Example 1 is generally the same as the manufacturing method of the expanded beads of Example 1, except that polypropylene-based resin B is not blended.

(Comparative Example 2 and Reference Example 1)
As shown in Table 5, the expanded beads of Comparative Example 2 and Reference Example 1 have the same structure as the expanded beads of Example 1, except that PP-B4 was used instead of PP-B1 as the polypropylene-based resin B. The manufacturing methods of the expanded beads of Comparative Example 2 and Reference Example 1 are generally the same as the manufacturing method of the expanded beads of Example 1, except that the polypropylene-based resin B was changed as described above.

Tables 3 to 5 show the physical properties of the expanded beads of Examples 1 to 6, Comparative Examples 1 and 2, and Reference Example 1, as well as the molded articles obtained using these expanded beads. The methods for measuring and evaluating the physical properties shown in Tables 3 to 5 are as follows.

(Biomass content of expanded beads)
The biomass degree of the expanded beads was calculated from the biomass degree of the polypropylene resin A used to produce the expanded beads and the content of the polypropylene resin A in the expanded beads.

(Bulk density of expanded particles)
The expanded beads were left to stand for one day in an environment of 50% relative humidity, 23°C, and 1 atm atmospheric pressure to condition the expanded beads. The condition-conditioned expanded beads were filled into a measuring cylinder, and the bottom of the measuring cylinder was lightly tapped against the floor several times to stabilize the filling height of the expanded beads in the measuring cylinder. Next, the bulk volume (unit: L) of the expanded beads was read from the scale on the measuring cylinder. The mass (unit: g) of the expanded beads in the measuring cylinder was then divided by the aforementioned bulk volume, and the value was converted into units to calculate the bulk density (unit: kg/ ^m3 ) of the expanded beads.

(Closed cell ratio of expanded beads)
The closed cell content of the expanded beads was measured using an air comparison hydrometer according to ASTM-D2856-70 Procedure C. Specifically, the expanded beads were first left to stand for one day under an environment of 50% relative humidity, 23°C temperature, and 1 atm atmospheric pressure to condition the expanded beads. After conditioning, the expanded beads with a bulk volume of approximately 20 cm ³ were used as a measurement sample. The measurement sample was submerged in a measuring cylinder containing ethanol, and the apparent volume Va of the measurement sample was measured from the rise in the liquid level. After measuring the apparent volume Va, the measurement sample was thoroughly dried, and the true volume Vx of the measurement sample was measured using an air comparison hydrometer ("Beckman Model 1000 Air Comparison Pycnometer" manufactured by Tokyo Science Co., Ltd.) according to Procedure C described in ASTM-D2856-70. Using these volume values Va and Vx, the closed cell ratio (unit: %) of the measurement sample was calculated based on the following formula (1): The above operation was repeated five times using different measurement samples, and the arithmetic mean value (N=5) of the closed cell ratios of the five measurement samples was taken as the closed cell ratio (unit: %) of the expanded beads.
Closed cell ratio = (Vx-W/ρ) x 100/(Va-W/ρ)...(1)

However, the meanings of the symbols in the above formula (1) are as follows:
Vx: the true volume of the measurement sample measured by the above method, i.e., the sum of the volume of the resin constituting the expanded beads and the total volume of the closed cells in the expanded beads (unit: cm ³ ).
Va: Apparent volume of the measurement sample measured from the rise in the liquid level when the measurement sample is submerged in a measuring cylinder containing ethanol (unit: cm ³ )
W: Mass of the measurement sample (unit: g)
ρ: density of the resin constituting the expanded beads (unit: g/cm ³ )

(Melting point of expanded particles)
The method for measuring the melting point of the expanded beads is the same as the method for measuring the melting point Tm _A of the polypropylene resin A described above, except that expanded beads are used instead of the polypropylene resin A.

(Top temperature of high-temperature peak, heat of fusion of high-temperature peak, and total heat of fusion of expanded particles)
A DSC curve was obtained by differential scanning calorimetry using approximately 3 mg of expanded beads as a sample, heated from 23°C to 200°C at a heating rate of 10°C/min. The flow rate of nitrogen gas in the measurement environment was 30 mL/min. Based on this DSC curve, the apex temperature of the high-temperature peak ΔH2 was determined.

Next, a line L1 (see Figure 1) was drawn connecting point α, which corresponds to 80°C on the DSC curve, and point β, which corresponds to the melting end temperature T of the expanded beads. Furthermore, a line L2 was drawn parallel to the vertical axis of the graph, passing through the maximum point γ, which exists between the resin-specific peak ΔH1 and the high-temperature peak ΔH2, to separate the resin-specific peak ΔH1 and the high-temperature peak ΔH2.

The heat of fusion of the sample's high-temperature peak ΔH2 was then calculated based on the area of the portion of the DSC curve that is surrounded by the line L1 and the line L2 and the portion that constitutes the high-temperature peak ΔH2. The total heat of fusion of the sample was also calculated based on the area of the portion of the DSC curve that is surrounded by the line L1 and the portion that constitutes the resin-specific peak ΔH1 and the portion that constitutes the high-temperature peak ΔH2.

The above procedure was performed three times using different samples, and the arithmetic mean value of the heat of fusion of the high-temperature peak ΔH2 obtained in each measurement was used as the heat of fusion of the high-temperature peak ΔH2 of the expanded beads. In addition, the arithmetic mean value of the total heat of fusion obtained in each measurement was used as the total heat of fusion of the expanded beads.

(Moldable range)
In evaluating the moldable range, molded bodies were produced by in-mold molding while changing the molding pressure during main heating from 0.26 MPa (G) to 0.45 MPa (G) in increments of 0.01 MPa, and the lower limit molding pressure and moldable range were determined based on the surface properties, fusion properties, and recovery properties of the obtained molded bodies.

The manufacturing method for molded articles using the expanded beads of Examples 1-6 and Comparative Examples 1-2 is as follows. First, thoroughly dried expanded beads were filled into a mold by the cracking filling method. In this example, a mold was used with a cavity capable of molding a flat-plate-shaped molded article measuring 250 mm in length, 200 mm in width, and 60 mm in thickness. In the cracking filling method, the expanded beads were filled into the mold with a cracking gap of 12 mm in the thickness direction of the molded article (i.e., a 20% cracking amount), and then the mold was completely closed to mechanically compress the expanded beads in the mold.

Next, steam was supplied into the mold to perform in-mold molding. For in-mold molding, steam was first supplied into the mold for 5 seconds with the mold's drain valve open to perform preheating. The drain valve was then closed, and steam was supplied from one side of the mold to perform a first one-sided heating until a pressure 0.08 MPa (G) lower than the molding pressure during main heating was reached. Next, steam was supplied from the other side of the mold to perform a second one-sided heating until a pressure 0.04 MPa (G) lower than the molding pressure during main heating was reached. After that, steam was supplied from both sides of the mold to perform main heating until the molding pressure during main heating was reached. After main heating was completed, the pressure inside the mold was released, and the molded body inside the mold was cooled with water until the surface pressure generated on the inner surface of the mold by the foaming force of the molded body reached 0.04 MPa (G).

The molded body was then removed from the mold and placed in an oven at 80°C for 12 hours for a curing process. After the curing process, the molded body was left to stand for 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm to condition the molded body. The molded body was large enough to cut out a sphere with a diameter of 55 mm.

The method for producing a molded product using the expanded beads of Reference Example 1 is the same as the method for producing a molded product using the expanded beads of Comparative Example 2, except that a mold was used that had a cavity capable of molding a flat plate-shaped molded product measuring 250 mm in length, 200 mm in width, and 20 mm in thickness.

The surface roughness, fusion, and recovery of the molded bodies obtained in this manner were evaluated. The range of molding pressures that met all criteria (i.e., molding pressures at which acceptable products could be obtained) according to the evaluation criteria described below was defined as the moldable range, and the lowest molding pressure within the moldable range was defined as the lower limit molding pressure. The "Water cooling time at lower limit molding pressure" column in Tables 3 to 5 lists the time required from the start of cooling until the surface pressure reached 0.04 MPa (G) when molding in a mold at the lower limit molding pressure. The "Number of moldable conditions" column in Tables 3 to 5 lists the number of molding pressures at which acceptable products could be obtained. The larger the number of moldable conditions, the wider the range of molding pressures at which acceptable products could be molded in a mold. The evaluation methods for surface roughness, fusion, and recovery in the moldable range were as follows. The above-mentioned acceptable products are sometimes referred to as "good molded bodies."

[Surface properties (secondary foaming properties)]
A 100 mm x 100 mm square was drawn in the center of one skin surface in the thickness direction of the molded article, i.e., the surface that had been in contact with the inner surface of the mold during in-mold molding. A diagonal line was then drawn from one corner of the square. The number of voids present on the diagonal line, i.e., gaps formed between the expanded beads, and gaps measuring 1 mm x 1 mm or more were counted. A sample with two or fewer voids was judged to pass, and a sample with three or more voids was judged to fail.

[Weldability]
A test piece measuring 100 mm long, 100 mm wide, and 20 mm thick was cut from the center of the molded body. A 5 mm deep incision was made on one surface of the test piece in the thickness direction, roughly dividing the test piece in half, and then the test piece was bent to fracture along the incision. One hundred or more randomly selected expanded beads exposed on the fracture surface were visually observed to determine whether they had fractured internally (i.e., expanded beads with material fracture) or at the interface between the expanded beads. The ratio of the number of expanded beads with internal fracture to the total number of observed expanded beads was calculated as a percentage (i.e., material fracture rate), and this value was taken as the fusion rate. A fusion rate of 80% or more was judged to be acceptable, and a fusion rate of less than 80% was judged to be unacceptable.

[Recovery]
In a plan view of the molded body from the thickness direction, the thickness of the corners of the molded body at four positions 10 mm inward from each vertex of the surface bounded by the 250 mm vertical side and the 200 mm horizontal side toward the center of the surface was measured, as well as the thickness of the molded body at the center of the surface. Next, the ratio (unit: %) of the thickness of the center to the thickness of the thickest corner among the four corners was calculated. If the thickness ratio obtained in this way was 90% or more, it was judged to be acceptable, and if it was less than 90%, it was judged to be unacceptable.

(Density of Molded Body)
A compact was obtained by molding in a mold at the minimum molding pressure in the above-described method for producing a compact. The mass (unit: g) of this compact was divided by the volume (unit: L) calculated from the outer dimensions of the compact, and the density (unit: kg/ ^m3 ) of the compact was calculated by converting the unit.

(Compressive stress at 50% strain)
In the above-described method for producing a molded body, a molded body was obtained by performing in-mold molding at the minimum molding pressure. In Examples 1 to 6 and Comparative Example 2, a rectangular parallelepiped test piece measuring 50 mm in length, 50 mm in width, and 25 mm in thickness was taken from the center of the molded body. In Reference Example 1, two rectangular parallelepiped pieces measuring 50 mm in length, 50 mm in width, and 12.5 mm in thickness were taken from the center of the molded body, and these pieces were stacked to prepare a test piece measuring 50 mm in length, 50 mm in width, and 25 mm in thickness. Then, a compression test was performed on the above test piece at a compression rate of 10 mm/min based on the method specified in JIS K7220:2006, and a stress-strain curve was obtained. The compression test was performed in a laboratory at 23 °C. The compressive stress at 50% strain in this stress-strain curve was defined as the compressive stress at 50% strain of the molded body.

As shown in Tables 3 and 4, the base resin constituting the expanded beads of Examples 1 to 6 contains biomass-derived polypropylene-based resin A and fossil fuel-derived polypropylene-based resin B in the specific mass ratios described above. Furthermore, polypropylene-based resin B is the specific random copolymer described above, and the flexural modulus _M of polypropylene-based resin B falls within the specific range described above. Therefore, the expanded beads of Examples 1 to 6 have excellent moldability within a mold, and can be molded into good molded articles over a wide range of molding pressures. Furthermore, because these expanded beads contain a biomass-derived monomer component in the base resin, they can contribute to reducing environmental impact.

In contrast, as shown in Table 5, the base resin constituting the expanded beads of Comparative Example 1 did not contain polypropylene-based resin B. As a result, the expanded beads of Comparative Example 1 had excessively low secondary expandability and poor in-mold moldability, and it was not possible to obtain a molded article with acceptable surface properties at any molding pressure.

The polypropylene resin B used in the expanded beads of Comparative Example 2 has a flexural modulus _M higher than the specific range. Therefore, the expanded beads of Comparative Example 2 have poor moldability in a mold, and when attempting to mold a thick molded article of 60 mm in a mold, the moldable range was narrowed.

Reference Example 1 is an example in which a thinner molded body was produced using the same expanded beads as in Comparative Example 2. As shown in Table 5, there were five possible moldable conditions in Reference Example 1, while there was only one possible moldable condition in Comparative Example 2. Therefore, a comparison of Comparative Example 2 and Reference Example 1 shows that when molding a thick molded body or a molded body having both thick and thin portions, the moldable range is likely to be excessively narrow compared to when molding a thin molded body. Even with expanded beads made from fossil fuel-derived polypropylene-based resin, the moldable range for a thick molded body may be narrower than the moldable range for a thin molded body, but it is not excessively narrow as described above. In other words, the above issue can be said to be unique to expanded beads containing biomass-derived polypropylene-based resin.

In contrast, as shown in Examples 1 to 6, by using expanded beads containing polypropylene resin A and polypropylene resin B in mass ratios within the above-mentioned specific range, it is clear that the moldable range can be greatly expanded, even when molding a thick molded product or a molded product having thick and thin parts, and that good molded products can be easily obtained.

The above describes the embodiments of the expanded polypropylene resin beads and expanded polypropylene resin bead molded articles according to the present invention based on the examples, but the specific embodiments of the expanded polypropylene resin beads and expanded polypropylene resin bead molded articles according to the present invention are not limited to those in the examples, and the configurations can be modified as appropriate within the scope of the spirit of the present invention.

Claims (9)

Polypropylene-based resin foam particles,
the base resin constituting the expanded beads comprises a polypropylene-based resin A containing a biomass-derived monomer component and a fossil fuel-derived polypropylene-based resin B;
the polypropylene-based resin B is a propylene-based random copolymer containing 3% by mass or more and 10% by mass or less of a comonomer component derived from one or more monomers selected from the group consisting of ethylene and butene,
The flexural modulus _M of the polypropylene-based resin B is 900 MPa or less,
The expanded polypropylene resin particles have a mass ratio of the polypropylene resin A to the polypropylene resin B in the base resin of 3:97 to 90:10 (wherein the total of the polypropylene resin A and the polypropylene resin B is 100 mass%). The expanded polypropylene resin beads according to claim 1, wherein the biomass content of the expanded beads is 1% or more and 50% or less, as measured by ASTM D6866-21. The expanded polypropylene resin particles according to claim 1 or 2, wherein the polypropylene resin A has a melting point Tm _A of 155°C or more and 170°C or less, and the polypropylene resin B has a melting point Tm _B of 130°C or more and 145°C or less. 3. The expanded polypropylene resin beads according to claim 1, wherein the ratio M _B /M _A of the flexural modulus M _B of the polypropylene resin B to the flexural modulus M _A of the polypropylene resin A is 0.60 or less. 3. The expanded polypropylene resin beads according to claim 1, wherein the ratio MFR _A /MFR B of the melt mass-flow rate MFR _A of the polypropylene resin _A at a temperature of 230°C and a load of 2.16 kg to the melt mass-flow rate MFR _B of the polypropylene resin B at a temperature of 230°C and a load of 2.16 kg is 0.2 or more and 0.6 or less. 3. The expanded polypropylene resin particles according to claim 1, wherein the polypropylene resin A has a melt mass-flow rate MFR _A of 5 g/10 min or less at a temperature of 230° C. and a load of 2.16 kg. 3. The expanded polypropylene resin beads according to claim 1, wherein the expanded beads have a bulk density of 10 kg/m ^<3> or more and 200 kg/m ^<3> or less. A polypropylene resin foamed bead molded article obtained by molding the polypropylene resin foamed beads according to claim 1 or 2 in a mold. The polypropylene resin foam bead molded article according to claim 8, wherein the molded article is large enough to cut out a sphere with a diameter of 55 mm.