WO2025205814A1 - Polypropylene resin foam particles, polypropylene resin foam molded body, and method for manufacturing polypropylene resin foam particles - Google Patents

Abstract

The present invention addresses the problem of providing polypropylene resin foam particles or the like capable of providing a foam molded body in a short molding cycle. Provided are polypropylene resin foamed particles obtained by foaming resin particles containing a polypropylene-based random copolymer (A), and a propylene homopolymer (B1) and/or a polypropylene-based block copolymer (B2), wherein a mixture of (B1) and (B2), and (A), each have a specific melting point and MFR.

Description

Polypropylene-based resin expanded beads, polypropylene-based resin expanded molded article, and method for producing polypropylene-based resin expanded beads

The present invention relates to expanded polypropylene resin beads, expanded polypropylene resin molded articles, and a method for producing expanded polypropylene resin beads.

Polypropylene resin foam molded articles are used for a variety of purposes, including cushioning packaging materials, returnable boxes, and automotive components (see, for example, Patent Documents 1 and 2).

JP 2011-137172 A International Publication No. WO2023/190441

However, the above-mentioned conventional techniques are not sufficient from the perspective of the molding cycle when producing foamed molded articles using foamed beads, and there is room for further improvement.

One embodiment of the present invention has been developed in consideration of the above-mentioned problems, and its purpose is to provide new polypropylene-based resin foam beads that can produce polypropylene-based resin foam molded articles in a short molding cycle, and polypropylene-based resin foam molded articles obtained by molding these polypropylene-based resin foam beads.

The inventors conducted extensive research to solve the above problems, and as a result, they have completed the present invention.

That is, the expanded polypropylene resin particles according to one embodiment of the present invention are expanded polypropylene resin particles obtained by expanding polypropylene resin particles,
the polypropylene-based resin particles contain a polypropylene-based random copolymer (A) having a melting point of 140.0°C or higher and lower than 155.0°C, and a propylene homopolymer (B1) and/or a polypropylene-based block copolymer (B2),
the melting point of the mixture of the propylene homopolymer (B1) and the polypropylene-based block copolymer (B2) is 155.0°C or higher and lower than 165.0°C,
The ratio (MFR B /MFR _A ) of the melt flow rate (MFR B ) of the mixture of the propylene homopolymer (B1) and the _{polypropylene} block copolymer (B2) at 230°C and a load of 2160 g to the melt flow rate (MFR _A ) of the polypropylene random copolymer ( _A ) at 230°C and a load of 2160 g is 2.0 to 30.0.

A method for producing expanded polypropylene-based resin beads according to one embodiment of the present invention includes an extrusion step of melt-kneading a mixed resin containing a polypropylene-based random copolymer (A) having a melting point of 140.0°C or higher but lower than 155.0°C, and a propylene homopolymer (B1) and/or a polypropylene block copolymer (B2) to obtain polypropylene-based resin beads;
and an expansion step of expanding the polypropylene-based resin particles obtained in the extrusion step,
the melting point of the mixture of the propylene homopolymer (B1) and the polypropylene-based block copolymer (B2) is 155.0°C or higher and lower than 165.0°C,
The ratio (MFR B /MFR _A ) of the melt flow rate (MFR B ) of the mixture of the propylene homopolymer (B1) and the _{polypropylene} block copolymer (B2) at 230°C and a load of 2160 g to the melt flow rate (MFR _A ) of the polypropylene random copolymer ( _A ) at 230°C and a load of 2160 g is 2.0 to 30.0.

One embodiment of the present invention has the effect of providing novel expanded polypropylene resin beads that can produce expanded polypropylene resin molded articles in a short molding cycle, as well as a novel method for producing the expanded polypropylene resin beads.

One embodiment of the present invention is described below, but the present invention is not limited to this. The present invention is not limited to the configurations described below, and various modifications are possible within the scope of the claims. Furthermore, embodiments or examples obtained by combining the technical means disclosed in different embodiments or examples are also included in the technical scope of the present invention. Furthermore, new technical features can be created by combining the technical means disclosed in each embodiment. All academic literature and patent documents described in this specification are incorporated herein by reference. Furthermore, unless otherwise specified in this specification, the term "A to B" representing a numerical range means "greater than or equal to A (including A and greater than A) and less than or equal to B (including B and less than B)."

In this specification, a "structural unit derived from an X monomer" contained in a polymer, copolymer, or resin may be referred to as an "X unit."

Unless otherwise specified in this specification, a copolymer containing ^X1 units, ^X2 units, ..., and ^Xn units (n is an integer of 2 or more) as structural units is also referred to as an " ^X1 / ^X2 /.../ ^Xn copolymer." Unless otherwise specified, the polymerization mode of ^{the X1} / ^X2 /.../ ^Xn copolymer is not particularly limited, and it may be a random copolymer, an alternating copolymer, a block copolymer, or a graft copolymer.

In this specification, "polypropylene-based resin particles" may be referred to as "resin particles," "expanded polypropylene-based resin particles" may be referred to as "expanded particles," "expanded polypropylene-based resin particles according to one embodiment of the present invention" may be referred to as "the present expanded particles," "expanded polypropylene-based resin molded body" may be referred to as "expanded molded body," and "expanded polypropylene-based resin molded body according to one embodiment of the present invention" may be referred to as "the present expanded molded body." In this specification, "method for producing expanded polypropylene-based resin particles" may be referred to as "production method," and "method for producing expanded polypropylene-based resin particles according to one embodiment of the present invention" may be referred to as "the present production method."

1. Technical concept of one embodiment of the present invention
As a raw material for polypropylene-based resin foam moldings, polypropylene-based random copolymers can generally be used. On the other hand, polypropylene-based block copolymers can also be used as a raw material for polypropylene-based resin foam moldings. For example, when the impact resistance of polypropylene-based resin foam moldings is to be increased or recycled resins are used to reduce environmental impact.

As a recycled polypropylene resin, polypropylene block copolymers are in much greater circulation than polypropylene random copolymers.

Propylene homopolymer is also sometimes used as a raw material for polypropylene-based resin foam molded articles. For example, this is done to improve the dimensional stability of the polypropylene-based resin foam molded article.

With the above-mentioned motivations in mind, the present inventors have conducted extensive research into the following: To provide expanded polypropylene resin particles, which are the raw material for polypropylene resin foam moldings, by using a combination of a polypropylene random copolymer and a propylene homopolymer and/or a polypropylene block copolymer.

In the course of intensive research, the inventors independently discovered the following new finding: When foamed articles are obtained by foaming resin particles obtained by using a combination of a polypropylene random copolymer and a propylene homopolymer and/or a polypropylene block copolymer, the molding cycle may be lengthened.

The inventors therefore conducted further intensive research with the following objective: to provide expanded beads that can provide expanded molded articles in a short molding cycle, even when a polypropylene random copolymer is used in combination with a propylene homopolymer and/or a polypropylene block copolymer.

As a result, the present inventors independently discovered the following novel finding, which led to the completion of the present invention: the finding that, surprisingly, foamed molded articles can be provided in a short molding cycle when the following expanded beads are used, or when the expanded beads are obtained by the following production method:
Expanded polypropylene resin particles obtained by expanding polypropylene resin particles,
the polypropylene-based resin particles contain a polypropylene-based random copolymer (A) having a melting point of 140.0°C or higher and lower than 155.0°C, and a propylene homopolymer (B1) and/or a polypropylene-based block copolymer (B2),
the melting point of the mixture of the propylene homopolymer (B1) and the polypropylene-based block copolymer (B2) is 155.0°C or higher and lower than 165.0°C,
expanded polypropylene resin beads, in which the ratio (MFR B /MFR A ) of the melt flow rate (MFR _B ) of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) at 230°C and under a load of 2160 g to the melt flow rate (MFR _A ) of the polypropylene random copolymer ( _A ) at 230°C and under _a load of 2160 g is 2.0 to 30.0;
an extrusion step of melt-kneading a mixed resin containing a polypropylene random copolymer (A) having a melting point of 140.0°C or higher and lower than 155.0°C, and a propylene homopolymer (B1) and/or a polypropylene block copolymer (B2) to obtain polypropylene resin particles;
and an expansion step of expanding the polypropylene-based resin particles obtained in the extrusion step,
the melting point of the mixture of the propylene homopolymer (B1) and the polypropylene-based block copolymer (B2) is 155.0°C or higher and lower than 165.0°C,
The method for producing expanded polypropylene resin beads comprises the step of: (a) providing a mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) with a melt flow rate (MFR _B ) at 230°C under a load of 2160 g relative to the melt flow rate (MFR _A ) of the polypropylene random copolymer (A) at 230°C under a load of 2160 g; and (b) providing a melt flow rate (MFR _B /MFR _A ) of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) at 230°C under a load of 2160 g, the ratio being 2.0 to 30.0.

2. Polypropylene resin foam particles
The expanded polypropylene resin beads according to one embodiment of the present invention are obtained by expanding polypropylene resin particles, and the polypropylene resin particles contain a polypropylene random copolymer (A) having a melting point of 140.0°C or more and less than 155.0°C, and a propylene homopolymer (B1) and/or a polypropylene block copolymer (B2), wherein the melting point of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) is 155.0°C or more and less than 165.0°C, and the ratio (MFR B /MFR A ) of the melt flow rate (MFR _B ) of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) at 230°C and a load of 2160g to the melt flow rate (MFR _A ) of the polypropylene random copolymer ( _A ) at 230°C _and a load of 2160g is 2.0 to 30.0. In this specification, the configuration in which "the melting point of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) is 155.0°C or higher and lower than 165.0°C" may be referred to as "Configuration A," and the configuration in which "the ratio (MFR B /MFR _A ) of the melt flow rate at 230°C and a load of 2160g of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) at 230°C and a load of 2160g to the melt flow rate (MFR _A ) of the polypropylene random copolymer ( _A ) at 230°C and _a load of 2160g" is sometimes referred to as "Configuration B."

Because the present expanded beads have the above-mentioned configuration (particularly configuration A and configuration B), they have the advantage of being able to provide expanded molded articles in a short molding cycle. Furthermore, because the present expanded beads have the above-mentioned configuration, they also have the advantage of being able to provide expanded molded articles with excellent surface aesthetics. The molding cycle and methods for measuring and evaluating the surface aesthetics of expanded molded articles will be explained in detail in the examples below.

<Ingredients>
(Polypropylene resin particles)
(Resin component)
The polypropylene random copolymer (A), propylene homopolymer (B1), and polypropylene block copolymer (B2) can also be considered resin components of the resin particles and expanded beads. In other words, the resin particles and the expanded beads contain at least the polypropylene random copolymer (A) and the propylene homopolymer (B1) and/or the polypropylene block copolymer (B2) as resin components. In this specification, the "components that substantially constitute the resin particles" and the "components that substantially constitute the expanded beads" are both also referred to as "base resin." In other words, the resin particles contain a base resin, which contains the polypropylene random copolymer (A) and the propylene homopolymer (B1) and/or the polypropylene block copolymer (B2). In other words, the expanded beads contain a base resin, which contains the polypropylene random copolymer (A) and the propylene homopolymer (B1) and/or the polypropylene block copolymer (B2).

The polypropylene random copolymer (A), the propylene homopolymer (B1), and the polypropylene block copolymer (B2) are all polypropylene resins.

In this specification, the term "polypropylene-based resin" refers to a resin that contains the highest amount of propylene units among all the structural units that make up the resin. For example, a polypropylene-based resin contains 50 mol% or more of propylene units out of 100 mol% of all structural units.

(Polypropylene-based random copolymer (A))
The polypropylene random copolymer (A) has, in addition to propylene units, at least one structural unit derived from a monomer other than a propylene monomer.

In this specification, the "structural units other than propylene units" contained in the polypropylene resin may also be referred to as "comonomer units." In other words, the polypropylene random copolymer (A) contains at least propylene units and comonomer units. In the polypropylene random copolymer (A), the bonding order of the propylene units and comonomer units is random.

Comonomers include α-olefins having 2 or 4 to 12 carbon atoms, such as ethylene, 1-butene, isobutene, 1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 3,4-dimethyl-1-butene, 1-heptene, 3-methyl-1-hexene, 1-octene, and 1-decene.

The heating temperature of the expanded beads when they are molded (e.g., in-mold foam molding) is sometimes referred to as the "molding temperature." From the perspective of being able to lower the molding temperature, it is preferable that the polypropylene random copolymer (A) contains ethylene units as comonomer units. For example, it is preferable that the polypropylene random copolymer (A) is a propylene/ethylene random copolymer containing propylene units and ethylene units.

The polypropylene random copolymer (A) is not limited to a propylene/ethylene random copolymer. Examples of polypropylene random copolymers (A) other than a propylene/ethylene random copolymer include a propylene/1-butene random copolymer, a propylene/ethylene/1-butene random copolymer, a propylene/chlorinated vinyl random copolymer, and a propylene/maleic anhydride random copolymer. Note that the aforementioned 1-butene is synonymous with butene-1.

The polypropylene random copolymer (A) may be a combination of a propylene/ethylene random copolymer and one or more polypropylene random copolymers other than a propylene/ethylene random copolymer.

The melting point of the polypropylene random copolymer (A) is 140.0°C or higher but lower than 155.0°C, preferably 140.0°C or higher but lower than 153.0°C, more preferably 140.0°C or higher but lower than 150.0°C, and even more preferably 142.0°C or higher but lower than 149.0°C. When the polypropylene random copolymer (A) has a melting point of (i) 140.0°C or higher, the foamed molded article obtained from the expanded beads has excellent heat resistance, and (ii) when it is lower than 155.0°C, it is easier to increase the expansion ratio of the expanded beads during production.

In this specification, the melting points of the polypropylene random copolymer (A), as well as the propylene homopolymer (B1) and polypropylene block copolymer (B2) described below, are values determined by measurement using a differential scanning calorimeter (hereinafter referred to as the "DSC method"). Specific operating procedures are as described in the examples described below. As a differential scanning calorimeter, for example, a DSC7020 model manufactured by Seiko Instruments Inc. can be used.

The melt flow rate (MFR _A ) of the polypropylene random copolymer (A) at 230°C and a load of 2160 g is not particularly limited as long as it satisfies the requirement B. The MFR _A of the polypropylene random copolymer (A) at 230°C and a load of 2160 g is preferably 3.0 g/10 min to 30.0 g/10 min, more preferably 3.0 g/10 min to 25.0 g/10 min, more preferably 4.0 g/10 min to 20.0 g/10 min, and even more preferably 5.0 g/10 min to 18.0 g/10 min. When the MFR _A of the polypropylene random copolymer (A) at 230°C and a load of 2160 g is within the above range, it is advantageous in that expanded beads having a relatively high expansion ratio are easily obtained. Furthermore, in this case, there are also advantages in that the surface beauty of the expanded molded article obtained from the expanded beads is excellent and the shrinkage rate of the expanded molded article is low.

In this specification, the MFR at 230°C and a load of 2,160 g of the polypropylene random copolymer (A), the propylene homopolymer (B1), the polypropylene block copolymer (B2), the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2), and the polypropylene resin particles described below is a value determined using a melt mass-flow rate measuring device as specified in JIS-K7210. Specific operating procedures are as described in the examples described below.

The polypropylene random copolymer (A) may be a newly produced one (non-recycled polypropylene random copolymer) produced by a known method, or a recycled polypropylene random copolymer derived from recycled materials. In other words, recycled materials containing recycled polypropylene random copolymers may be used as the polypropylene random copolymer (A). The polypropylene random copolymer (A) may also contain recycled polypropylene random copolymers. When only recycled materials are used as the polypropylene random copolymer (A), the content of polypropylene random copolymer (A) in the expanded beads is the product obtained by multiplying the amount of recycled material used (blended amount) by the content ratio of recycled polypropylene random copolymer in the recycled material. The polypropylene random copolymer (A) may also be a mixture of recycled polypropylene random copolymer and non-recycled polypropylene random copolymer.

The proportion of polypropylene random copolymer derived from recycled materials (i.e., recycled polypropylene random copolymer) in 100% by weight of polypropylene random copolymer (A) may be 50% or more, 70% or more, 80% or more, or even 100%. The polypropylene random copolymer (A) may be composed solely of recycled polypropylene random copolymer. The higher the proportion of polypropylene random copolymer derived from recycled materials (i.e., recycled polypropylene random copolymer) in the polypropylene random copolymer (A), the greater the advantage of being able to reduce the environmental impact.

As used herein, the term "recycled material" refers to (a) resin compositions (or pellets) that have been reconstituted by any means (e.g., crushing, shredding, melting, or a combination thereof) from resin products that were once used and/or discarded after being in the form of resin products (e.g., foam particles; foam molded products; films; food trays; packaging containers such as bags and bottles; medical containers such as IV drip packs and syringes; clothing cases; miscellaneous items such as clear files; home appliances; automobile parts; fishing gear such as fishing nets, ropes, and floats); and (b) resin compositions that have been reconstituted by any means (e.g., crushing, shredding, melting, or a combination thereof) from waste products generated during the manufacturing process of resin products. Resin products are often collected by their intended use and/or by their raw materials. Therefore, recycled materials may primarily contain resins of the same or nearly the same composition (e.g., polypropylene-based resins, polyethylene-based resins, etc.). However, when collecting resin products, the resin product to be collected may be mixed with other resin products for different purposes and/or resin products made from different raw materials. Therefore, in addition to the resin that is the main component, recycled materials may also contain resins with compositions other than the resin in question.

In this specification, the resin contained in recycled materials may be referred to as "recycled resin." For example, recycled materials obtained by (a) converting a resin product obtained primarily from polypropylene-based resin into a resin composition by any means, and/or (b) converting waste generated during the manufacturing process of a resin product using polypropylene-based resin as a primary raw material into a resin composition by any means, contain recycled polypropylene-based resin as the recycled resin. Recycled materials containing recycled polypropylene-based resin as the primary resin may contain recycled polyethylene-based resin as the recycled resin in an amount less than the amount of recycled polypropylene-based resin.

Recycled materials may contain additives used in the manufacturing process of resin products (e.g., foam nucleating agents (e.g., talc, calcium carbonate, silica, kaolin, barium sulfate, calcium hydroxide, aluminum hydroxide, aluminum oxide, titanium oxide, zinc borate, etc.) and colorants, as well as various other additives described in the "Additives" section below).

In this specification, the origin of the recycled material is not particularly limited. The recycled material may be derived from foamed materials such as foam particles and foam molded products. The recycled material may also be derived from non-foamed materials (for example, films; food trays; packaging containers such as bags and bottle containers; medical containers such as intravenous packs and syringes; clothing cases; miscellaneous items such as clear files; home appliances; automobile parts; fishing gear such as fishing nets, ropes, and floats, etc.).

In this specification, resins that have never been made into resin products may also be referred to as "non-recycled resins." For example, polypropylene-based resins that have never been made into resin products may also be referred to as "non-recycled polypropylene-based resins."

The following describes a case where resin particles contain multiple polypropylene random copolymers with different compositions, physical properties, and/or origins (whether recycled or not, etc.). In this case, the mixture of all polypropylene random copolymers contained in the resin particles is considered to be the "polypropylene random copolymer (A)." Furthermore, the physical properties (e.g., melting point and MFR _A , etc.) of the mixture of all polypropylene random copolymers contained in the resin particles are considered to be the physical properties (e.g., melting point and MFR _A , etc.) of the polypropylene random copolymer (A). For example, if the resin particles contain only two polypropylene random copolymers, a polypropylene random copolymer with a melting point of 139°C and a polypropylene random copolymer with a melting point of 150°C, the melting point of the mixture consisting of the entire amount of the polypropylene random copolymer with a melting point of 139°C and the entire amount of the polypropylene random copolymer with a melting point of 150°C contained in the resin particles is considered to be the melting point of the polypropylene random copolymer (A).

(Propylene homopolymer (B1))
The melting point of the propylene homopolymer (B1) is not particularly limited as long as it satisfies the structure A. The melting point of the propylene homopolymer (B1) is preferably 155.0°C or higher and lower than 165.0°C, more preferably 155.0°C or higher and 163.0°C or lower, more preferably 155.0°C or higher and 160.0°C or lower, and even more preferably 155.0°C or higher and 158.0°C or lower. When the melting point of the propylene homopolymer (B1) is (i) 155.0°C or higher, there is an advantage that a foamed molded article having excellent heat resistance can be obtained, and (ii) when it is lower than 165.0°C, there is an advantage that the foaming temperature and molding temperature can be lowered.

The melt flow rate (MFR) of the propylene homopolymer (B1) at 230°C and a load of 2160 g is not particularly limited as long as it satisfies the requirement B. The MFR of the propylene homopolymer (B1) at 230°C and a load of 2160 g is preferably 6 g/10 min to 350 g/10 min, more preferably 10 g/10 min to 300 g/10 min, even more preferably 15 g/10 min to 280 g/10 min, and particularly preferably 20 g/10 min to 250 g/10 min. Providing the MFR of the propylene homopolymer (B1) at 230°C and a load of 2160 g within the above range has the advantage of easily producing expanded beads with a relatively high expansion ratio. Furthermore, in this case, the expanded molded articles obtained from the expanded beads have the advantages of excellent surface appearance and a low shrinkage rate. The upper limit of the MFR of the propylene homopolymer (B1) at 230°C and a load of 2160 g may be 200 g/10 min or less, 150 g/10 min or less, or 100 g/min or less.

When the resin particles contain a polypropylene random copolymer (A) and a propylene homopolymer (B1), the content of the propylene homopolymer (B1) in the resin particles is preferably less than 95 parts by weight, more preferably 10 parts by weight or more and 50 parts by weight or less, even more preferably 10 parts by weight or more and 40 parts by weight or less, and particularly preferably 10 parts by weight or more and 30 parts by weight or less, per 100 parts by weight of the total of the polypropylene random copolymer (A) and the propylene homopolymer (B1). This configuration has the advantage of allowing for lower foaming temperatures and molding temperatures.

When the resin particles contain propylene homopolymer (B1), the ratio of the content of polypropylene random copolymer (A) to the content of propylene homopolymer (B1) in the resin particles (content (parts by weight) of polypropylene random copolymer (A):content (parts by weight) of propylene homopolymer (B1)) is preferably 90:10 to 50:50, more preferably 85:15 to 50:50, even more preferably 80:20 to 60:40, and particularly preferably 75:25 to 70:30. This configuration has the advantages of (i) being able to lower the foaming temperature and molding temperature, and (ii) the resulting foamed molded article having excellent heat resistance.

The propylene homopolymer (B1) may be a newly produced propylene homopolymer (non-recycled propylene homopolymer) produced by a known method, or a recycled propylene homopolymer derived from a recycled material. In other words, a recycled material containing a recycled propylene homopolymer may be used as the propylene homopolymer (B1). The propylene homopolymer (B1) may also contain a recycled propylene homopolymer. When a recycled material is used, the content of propylene homopolymer (B1) in the expanded beads is the product obtained by multiplying the amount (blended amount) of recycled material used by the content ratio of propylene homopolymer (B1) in the recycled material. The propylene homopolymer (B1) may be a mixture of recycled propylene homopolymer and non-recycled propylene homopolymer.

From the perspective of reducing the environmental impact, the proportion of propylene homopolymer derived from recycled materials (i.e., recycled propylene homopolymer) in 100% by weight of propylene homopolymer (B1) is preferably 50% or more, more preferably 70% or more, even more preferably 80% or more, and particularly preferably 100%. In other words, it is particularly preferable that propylene homopolymer (B1) is composed solely of recycled propylene homopolymer.

Propylene homopolymer (B1) may contain structural units derived from isoprene and structural units derived from conjugated dienes, but preferably does not contain any. In 100% by weight of propylene homopolymer (B1), the total content of structural units derived from isoprene and structural units derived from conjugated dienes is preferably 20% by weight or less, more preferably 10% by weight or less, even more preferably 5% by weight or less, and most preferably 0% by weight.

The following describes a case where the resin particles contain multiple types of propylene homopolymers that differ in composition, physical properties, and/or origin (whether recycled or not, etc.). In this case, the mixture of all propylene homopolymers contained in the resin particles is considered to be "propylene homopolymer (B1)." Furthermore, the physical properties (e.g., melting point, MFR, etc.) of the mixture of all propylene homopolymers contained in the resin particles are considered to be the physical properties (e.g., melting point, MFR, etc.) of the propylene homopolymer (B1).

(Polypropylene-based block copolymer (B2))
The polypropylene block copolymer (B2) has, in addition to propylene units, one or more structural units derived from a monomer other than propylene monomer. In other words, the polypropylene block copolymer (B2) contains at least propylene units and comonomer units. More specifically, the polypropylene block copolymer (B2) contains a propylene block composed of propylene units and a comonomer block composed of comonomer units. Specific examples of the comonomer units are the same as those described above in the section (Polypropylene Random Copolymer (A)), and therefore, the description therein is incorporated by reference and will not be repeated here.

From the viewpoint of being able to lower the molding temperature, the polypropylene-based block copolymer (B2) preferably contains ethylene units as comonomer units. For example, the polypropylene-based block copolymer (B2) is preferably a propylene/ethylene block copolymer containing a propylene block composed of propylene units and an ethylene block composed of ethylene units.

The polypropylene block copolymer (B2) is not limited to a propylene/ethylene block copolymer. Examples of polypropylene block copolymers (B2) other than a propylene/ethylene block copolymer include a propylene/1-butene block copolymer, a propylene/ethylene/1-butene block copolymer, a propylene/chlorinated vinyl block copolymer, and a propylene/maleic anhydride block copolymer.

Polypropylene-based block copolymers (B2) include substances that are considered to be polypropylene-based block copolymers in the technical field of polypropylene-based resins. For example, a propylene/ethylene block copolymer contains a homopolypropylene matrix and a polyethylene layer covered with an ethylene/propylene elastic copolymer as a domain, and is sometimes referred to as an impact copolymer.

The polypropylene-based block copolymer (B2) may be a combination of a propylene/ethylene block copolymer and one or more polypropylene-based block copolymers (B2) other than a propylene/ethylene block copolymer.

The melting point of the polypropylene block copolymer (B2) is not particularly limited as long as it satisfies the structure A. The melting point of the polypropylene block copolymer (B2) is preferably 155.0°C or higher but lower than 165.0°C, more preferably 155.0°C or higher but lower than 164.0°C, more preferably 156.0°C or higher but lower than 163.0°C, more preferably 156.0°C or higher but lower than 162.0°C, and even more preferably 156.0°C or higher but lower than 161.0°C. When the melting point of the polypropylene block copolymer (B2) is (i) 155.0°C or higher, it has the advantage of being able to produce a foamed molded article with excellent heat resistance, and (ii) when it is lower than 165.0°C, it has the advantage of excellent moldability.

The melt flow rate (MFR) of the polypropylene block copolymer (B2) at 230°C and a load of 2160 g is not particularly limited as long as it satisfies the requirement B. The MFR of the polypropylene block copolymer (B2) at 230°C and a load of 2160 g is preferably 6 g/10 min to 350 g/10 min, more preferably 10 g/10 min to 300 g/10 min, more preferably 15 g/10 min to 280 g/10 min, and even more preferably 20 g/10 min to 250 g/10 min. When the MFR of the polypropylene block copolymer (B2) at 230°C and a load of 2160 g falls within the above range, it has the advantage of easily obtaining expanded beads with a relatively high expansion ratio. Furthermore, in this case, it also has the advantage that the expanded molded articles obtained from the expanded beads have excellent surface appearance and a low shrinkage rate.

When the resin particles contain a polypropylene-based block copolymer (B2), the ratio of the content of the polypropylene-based random copolymer (A) to the content of the polypropylene-based block copolymer (B2) in the resin particles (content (parts by weight) of the polypropylene-based random copolymer (A):content (parts by weight) of the polypropylene-based block copolymer (B2)) is preferably 90:10 to 40:60, more preferably 85:15 to 55:45, even more preferably 80:20 to 60:40, and particularly preferably 75:25 to 65:35. This configuration has the advantages of (i) being able to lower the foaming temperature and molding temperature, and (ii) the resulting foamed molded article having excellent heat resistance.

The polypropylene-based block copolymer (B2) may be a newly produced polypropylene-based block copolymer (non-recycled polypropylene-based block copolymer) produced by a known method, or a recycled polypropylene-based block copolymer derived from recycled materials. In other words, a recycled material containing a recycled polypropylene-based block copolymer may be used as the polypropylene-based block copolymer (B2). The polypropylene-based block copolymer (B2) may also contain a recycled polypropylene-based block copolymer. When a recycled material is used, the content of the polypropylene-based block copolymer (B2) in the expanded beads is the product obtained by multiplying the amount (blended amount) of recycled material used by the content ratio of the polypropylene-based block copolymer (B2) in the recycled material. The polypropylene-based block copolymer (B2) may be a mixture of a recycled polypropylene-based block copolymer and a non-recycled polypropylene-based block copolymer. Preferably, the polypropylene-based block copolymer (B2) contains a recycled polypropylene-based block copolymer derived from recycled materials. As mentioned above, the distribution volume of polypropylene-based block copolymers as recycled polypropylene-based resins is greater than the distribution volume of polypropylene-based random copolymers. Therefore, recycled polypropylene-based block copolymer resins are relatively easy to obtain.

From the viewpoint of reducing the environmental impact, the proportion of recycled polypropylene block copolymers derived from recycled materials (i.e., recycled polypropylene block copolymers) in 100% by weight of polypropylene block copolymer (B2) is preferably 50% or more, more preferably 70% or more, even more preferably 80% or more, and particularly preferably 100%. In other words, it is particularly preferable that polypropylene block copolymer (B2) is composed solely of recycled polypropylene block copolymers.

The polypropylene block copolymer (B2) may contain structural units derived from isoprene and structural units derived from conjugated dienes, but preferably does not contain any. In 100% by weight of the polypropylene block copolymer (B2), the total content of structural units derived from isoprene and structural units derived from conjugated dienes is preferably 20% by weight or less, more preferably 10% by weight or less, even more preferably 5% by weight or less, and most preferably 0% by weight.

This section explains the case where resin particles contain multiple types of polypropylene-based block copolymers that differ in composition, physical properties, and/or origin (whether recycled or not, etc.). In this case, the mixture consisting of all the polypropylene-based block copolymers contained in the resin particles is considered to be the "polypropylene-based block copolymer (B2)." Furthermore, the physical properties (e.g., melting point, MFR, etc.) of the mixture consisting of all the polypropylene-based block copolymers contained in the resin particles are considered to be the physical properties (e.g., melting point, MFR, etc.) of the polypropylene-based block copolymer (B2).

It is preferable that at least one of the polypropylene random copolymer (A), propylene homopolymer (B1), and polypropylene block copolymer (B2) contains a recycled resin derived from recycled materials. Using recycled resin not only reduces environmental pollution, but also significantly reduces the amount of plastic waste generated and the amount of plastic used in manufacturing. Therefore, embodiments that use recycled resin as all or part of the resin have the advantage of contributing to the achievement of Sustainable Development Goals (SDGs), such as Goal 12, "Ensure sustainable consumption and production patterns," and Goal 14, "Conserve and sustainably use the oceans and marine resources for sustainable development."

(Mixture of Propylene Homopolymer (B1) and Polypropylene Block Copolymer (B2))
The melting point of the mixture of propylene homopolymer (B1) and polypropylene-based block copolymer (B2) is 155°C or higher but lower than 165°C, more preferably 155°C or higher but lower than 164°C, more preferably 155°C or higher but lower than 163°C, more preferably 155°C or higher but lower than 162°C, more preferably 155°C or higher but lower than 161°C, more preferably 155°C or higher but lower than 160°C, and even more preferably 155°C or higher but lower than 158°C. This configuration has the advantage that the expanded beads can provide a foamed molded article in a short molding cycle. Furthermore, when the melting point of the mixture of propylene homopolymer (B1) and polypropylene-based block copolymer (B2) is (i) 155°C or higher, it has the advantage that a foamed molded article with excellent heat resistance can be obtained, and (ii) when it is lower than 165°C, it has the advantage that the foaming temperature and molding temperature can be lowered and the molding processability is excellent.

The melt flow rate (MFR _B ) of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) at 230°C under a load of 2160 g is not particularly limited, as long as it satisfies the structure B. The MFR _B of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) at 230°C under a load of 2160 g is preferably 6 g/10 min to 350 g/10 min, more preferably 10 g/10 min to 300 g/10 min, still more preferably 15 g/10 min to 280 g/10 min, and particularly preferably 20 g/10 min to 250 g/10 min. When the MFR _B of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) at 230°C under a load of 2160 g is within the above range, there is an advantage that expanded beads having a relatively high expansion ratio are easily obtained. Furthermore, in this case, there are also advantages that the surface of the foamed molded article obtained from the present expanded beads is excellent and the shrinkage rate of the foamed molded article is small.

In one embodiment of the present invention, the resin particles may (i) contain a propylene homopolymer (B1) and not contain a polypropylene-based block copolymer (B2), (ii) contain a polypropylene-based block copolymer (B2) and not contain a propylene homopolymer (B1), or (iii) contain both a propylene homopolymer (B1) and a polypropylene-based block copolymer (B2). Therefore, the "melting point and MFR _B of a mixture of propylene homopolymer (B1) and polypropylene-based block copolymer (B2)" respectively mean (i) the melting point and MFR of the propylene homopolymer (B1) when the resin particles contain propylene homopolymer (B1) but not polypropylene-based block copolymer (B2), (ii) the melting point and MFR of the polypropylene-based block copolymer (B2) when the resin particles contain polypropylene-based block copolymer (B2) but not propylene homopolymer (B1), and (iii) the melting point and MFR of the mixture of propylene homopolymer (B1) and polypropylene-based block copolymer (B2) when the resin particles contain both propylene homopolymer (B1) and polypropylene-based block copolymer (B2).

In the resin particles, out of a total of 100 parts by weight of polypropylene random copolymer (A), propylene homopolymer (B1), and polypropylene block copolymer (B2), it is preferable that (a) the content of polypropylene random copolymer (A) is 70 to 95 parts by weight and the total content of propylene homopolymer (B1) and polypropylene block copolymer (B2) is 5 to 30 parts by weight; it is more preferable that (b) the content of polypropylene random copolymer (A) is 70 to 90 parts by weight and the total content of propylene homopolymer (B1) and polypropylene block copolymer (B2) is 10 to 30 parts by weight; and it is even more preferable that (c) the content of polypropylene random copolymer (A) is 70 to 80 parts by weight and the total content of propylene homopolymer (B1) and polypropylene block copolymer (B2) is 20 to 30 parts by weight. When the contents of the polypropylene random copolymer (A), propylene homopolymer (B1), and polypropylene block copolymer (B2) in the resin particles are within the above-mentioned ranges, the expanded beads have the advantage of providing superior productivity for producing polypropylene resin foamed molded articles. The term "total content of propylene homopolymer (B1) and polypropylene block copolymer (B2)" refers to (i) the content of propylene homopolymer (B1) when the resin particles contain propylene homopolymer (B1) but not polypropylene block copolymer (B2); (ii) the content of polypropylene block copolymer (B2) when the resin particles contain polypropylene block copolymer (B2) but not propylene homopolymer (B1); and (iii) the total content of propylene homopolymer (B1) and polypropylene block copolymer (B2) when the resin particles contain both propylene homopolymer (B1) and polypropylene block copolymer (B2).

(MFR _B /MFR _A )
The ratio (MFR B /MFR _A ) of the melt flow rate (MFR B ) of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) at 230°C and a load of 2160 g to the melt flow rate (MFR _A ) of the polypropylene random copolymer ( _A ) at 230°C _and a load of 2160 g is 2.0 to 30.0, preferably 4.0 to 30.0, and more preferably 5.0 to 30.0. According to this configuration, the expanded beads have the advantage that a foamed molded article can be provided in a short molding cycle. Furthermore, according to this configuration, there is the advantage that a foamed molded article with excellent surface beauty can be provided.

The resin particles may further contain resins other than the polypropylene random copolymer (A), propylene homopolymer (B1), and polypropylene block copolymer (B2) (sometimes referred to as "other resins, etc.") as resin components, provided that the effects of one embodiment of the present invention are not impaired. Examples of such other resins include: (a) polypropylene resins other than polypropylene random copolymers, propylene homopolymers, and polypropylene block copolymers having a melting point of less than 140.0°C or 155.0°C or higher; (b) ethylene resins such as high-density polyethylene, medium-density polyethylene, low-density polyethylene, linear low-density polyethylene, linear very-low-density polyethylene, ethylene/vinyl acetate copolymer, ethylene/acrylic acid copolymer, and ethylene/methacrylic acid copolymer; (c) styrene resins such as polystyrene, styrene/maleic anhydride copolymer, and styrene/ethylene copolymer; (d) polyolefin waxes such as propylene-α-olefin wax; and (e) olefin rubbers such as ethylene/propylene rubber, ethylene/butene rubber, ethylene/hexene rubber, and ethylene/octene rubber. The content of other resins in the resin particles is preferably 10 parts by weight or less, and more preferably 5 parts by weight or less, per 100 parts by weight of the total of the polypropylene random copolymer (A), the propylene homopolymer (B1), and the polypropylene block copolymer (B2).

(Additives)
In addition to the resin components described above, the resin particles may further contain optional additives, such as colorants, water-absorbing substances (e.g., (i) polyols such as glycerin and diglycerin, (ii) polyethers such as polyethylene glycol and polyethylene oxide, and (iii) metal borates such as borax and zinc borate), foam nucleating agents (e.g., inorganic substances such as talc, calcium carbonate, silica, kaolin, barium sulfate, calcium hydroxide, aluminum hydroxide, aluminum oxide, and titanium oxide), antistatic agents (e.g., glycerin monostearate, glycerin monodistearate), and flame retardants (e.g., hydroxybenzoates). Examples of suitable additives include hindered amine flame retardants, bromine flame retardants, phosphate ester flame retardants, and melamine flame retardants, antioxidants (e.g., hindered phenol antioxidants), heat stabilizers (e.g., phosphorus-based heat stabilizers and sulfur-based heat stabilizers), light stabilizers (e.g., benzotriazole UV absorbers, triazine UV absorbers, HALS and/or hindered amine light stabilizers), nucleating agents, conductive agents (e.g., carbon, carbon nanotubes, and metal fillers), lubricants, acid scavengers, antiblocking agents, metal chelating agents (e.g., IRGANOX® MD1024 and ADK STAB® CDA-1), lubricants, antibacterial agents, organic peroxides, and MFR adjusters. The resin particles may also contain an additive for recycled materials containing any one or a combination of two or more of these additives. A "metal chelating agent" is sometimes referred to as a "metal deactivator." Such additives may be added directly to the mixed resin or polypropylene resin composition described below in the production of polypropylene resin particles.

Colorants include chromatic pigments and carbon black. Chromatic pigments include (i) blue pigments such as copper phthalocyanine blue, ultramarine, cobalt blue, and Prussian blue; (ii) red pigments such as perylene red, quinacridone red, and cadmium red; (iii) yellow pigments such as condensed azo yellow, cadmium yellow, and barium chromate; (iv) green pigments that combine blue and yellow pigments; (v) orange pigments that combine red and yellow pigments; and (vi) purple pigments such as cobalt violet and pigments that combine blue and red pigments. The resin particles may contain carbon black.

<Physical properties>
The physical properties of the resin particles and the present expanded particles will be described below.

(MFR of polypropylene-based resin particles)
The polypropylene resin particles preferably have an MFR at 230°C under a load of 2,160 g of 7.0 g/10 min to 30.0 g/10 min, more preferably 10.0 g/10 min to 28.0 g/10 min, and even more preferably 13.0 g/10 min to 26.0 g/10 min or more. This configuration offers the advantage that the expanded beads can provide a foamed molded article in a short molding cycle. This configuration also offers the advantage that a foamed molded article having excellent internal fusion properties and/or surface beauty can be provided.

(DSC ratio of expanded polypropylene resin particles)
The expanded beads preferably have at least two melting peaks in a DSC curve obtained by a differential scanning calorimetry (differential scanning calorimetry) described below. Of the melting peaks, the heat of fusion determined from the higher-temperature melting peak is referred to as the "high-temperature heat of fusion (Q _h )," and the heat of fusion determined from the lower-temperature melting peak is referred to as the "low-temperature heat of fusion (Q _l )." When there are three or more melting peaks, the heat of fusion determined from the highest-temperature melting peak is referred to as the "high-temperature heat of fusion (Q _h )," and the heat of fusion determined from the remaining melting peaks is referred to as the "low-temperature heat of fusion (Q _l )." The ratio of the high-temperature heat of fusion to the total heat of fusion, or more specifically, the "ratio of the high-temperature heat of fusion to 100% of the total heat of fusion," is also referred to as the "DSC ratio." The DSC ratio can be calculated using the following formula:
DSC ratio (%)=Q _h /(Q _l +Q _h )×100.

The ratio of the heat of fusion on the higher temperature side to the total heat of fusion (100%) of the expanded beads, i.e., the DSC ratio, is not particularly limited. The DSC ratio of the expanded beads is preferably 30.0% to 50.0%, more preferably 30.0% to 40.0%, and even more preferably 30.0% to 35.0%. When the DSC ratio of the expanded beads is 30.0% or more, the expanded beads have the advantage of being able to provide a foamed molded article with sufficient strength. On the other hand, when the DSC ratio of the expanded beads is 50.0% or less, the expanded beads have the advantage of being able to be molded at a relatively low temperature (molding temperature) to provide a foamed molded article. The method for measuring the DSC ratio of the expanded beads will be explained in detail in the Examples below.

The DSC ratio of the expanded beads also serves as a guide to the amount of high-melting-point crystals contained in the expanded beads. In other words, a DSC ratio of 30.0% to 50.0% indicates that the expanded beads contain a relatively large amount of high-melting-point crystals. The DSC ratio of the expanded beads also plays a significant role in the viscoelasticity of the resin beads and the expanded beads when they are foamed and expanded. In other words, when the DSC ratio of the expanded beads is 30.0% to 50.0%, the resin beads and the expanded beads can exhibit excellent foaming and expansion properties, respectively, when they are foamed and molded. As a result, the expanded beads have the advantage of being able to produce foamed molded articles with excellent internal fusion at low molding pressures and excellent mechanical strength, such as compressive strength.

In the present expanded beads, methods for controlling the DSC ratio within a predetermined range include adjusting the conditions during production of the present expanded beads (particularly, the foaming temperature, foaming pressure, holding time, and the temperature of the region (space) from which the dispersion is released, etc.). Because of the ease of adjustment, adjusting the foaming temperature, foaming pressure, and/or holding time is preferred as a method for controlling the DSC ratio within a predetermined range.

For example, increasing the foaming temperature tends to decrease the DSC ratio, and conversely, decreasing the foaming temperature tends to increase the DSC ratio. This is because the amount of unmelted crystals changes depending on the foaming temperature. Also, increasing the foaming pressure tends to decrease the DSC ratio, and conversely, decreasing the foaming pressure tends to increase the DSC ratio. This is because the degree of plasticization changes depending on the foaming pressure, which in turn changes the amount of unmelted crystals. Also, the longer the holding time, the greater the DSC ratio tends to become. This is because the amount of growth of unmelted crystals changes depending on the holding time.

(Expansion ratio of expanded polypropylene resin beads)
The expansion ratio of the expanded beads is preferably 10 to 50, more preferably 18 to 40, and even more preferably 18 to 30. If the expansion ratio of the expanded beads is (i) 10 or more, a lightweight expanded molded article can be obtained efficiently, and (ii) if it is 50 or less, there is no risk of the strength of the resulting expanded molded article being insufficient. The method for calculating the expansion ratio of the expanded beads will be explained in detail in the Examples below.

(Average cell diameter of expanded polypropylene resin particles)
The average cell diameter of the expanded beads is not particularly limited. The average cell diameter of the expanded beads is preferably 80 μm to 500 μm, more preferably 85 μm to 400 μm, even more preferably 90 μm to 300 μm, and particularly preferably 95 μm to 250 μm. When the average cell diameter of the expanded beads is (i) 80 μm or more, the expanded beads can provide a polypropylene resin foam molded article having excellent compressive strength, and (ii) when the average cell diameter is 500 μm or less, there is no risk of the molding cycle becoming longer, which has the advantage of improving productivity. The method for measuring the average cell diameter of the expanded beads will be described in detail in the Examples below.

(molding pressure)
The present expanded beads have the advantage that they can be used to obtain a foamed molded article having an excellent fusion rate (e.g., a fusion rate of 80% or more) at a low molding pressure. In other words, the present expanded beads have the advantage that a foamed molded article having an excellent fusion rate can be provided at a molding pressure equivalent to that of foamed beads obtained using only a polypropylene-based random copolymer as the base resin.

In this specification, the lowest molding pressure that can provide a foamed molded article with an excellent fusion rate (e.g., a fusion rate of 80% or more) when producing a foamed molded article using expanded beads is also referred to as the "minimum molding pressure." The present expanded beads also have the advantage of a low minimum molding pressure. In other words, the minimum molding pressure of the present expanded beads can be equivalent to that of expanded beads obtained using only a polypropylene-based random copolymer as the base resin.

The molding pressure for the expanded beads is not particularly limited, but is preferably 0.34 MPa (gauge pressure) or less, more preferably 0.32 MPa (gauge pressure) or less, and even more preferably 0.30 MPa (gauge pressure) or less. The lower limit of the molding pressure is not particularly limited, but may be, for example, 0.15 MPa (gauge pressure) or more. When the minimum molding pressure is within the above-mentioned range, there is the advantage that a foamed molded article can be provided with little economic burden.

3. Method for producing expanded polypropylene resin beads
A method for producing expanded polypropylene-based resin beads according to one embodiment of the present invention comprises: an extrusion step of melt-kneading a mixed resin containing a polypropylene-based random copolymer (A) having a melting point of 140.0°C or more and less than 155.0°C, and a propylene homopolymer (B1) and/or a polypropylene block copolymer (B2) to obtain polypropylene-based resin beads; and an expansion step of expanding the polypropylene-based resin beads obtained in the extrusion step, wherein the melting point of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) is 155.0°C or more and less than 165.0°C, and the ratio of the melt flow rate (MFR _{B ) of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) at 230°C and a load of 2160 g to the melt flow rate (MFR A )} of the polypropylene random copolymer (A) at 230°C and a load of 2160 g (MFR _B /MFR _A ) is 2.0 to 30.0.

Because this manufacturing method includes the above-described configuration, it has the advantage of being able to provide expanded beads that can provide expanded molded articles in a short molding cycle. Furthermore, because this manufacturing method includes the above-described configuration, it also has the advantage of being able to provide expanded beads that can provide expanded molded articles with excellent internal fusion properties and/or surface beauty. Furthermore, because this manufacturing method includes the above-described configuration, it is possible to provide expanded beads that can provide expanded molded articles with excellent internal fusion properties and/or surface beauty using a smaller amount of steam than conventional methods, despite using a propylene homopolymer and/or a polypropylene-based block copolymer. In other words, because this manufacturing method includes the above-described configuration, it also has the advantage of low energy consumption (energy saving). As described above, this manufacturing method also has Configuration A and Configuration B.

(Extrusion process)
In the extrusion step, a mixed resin containing the polypropylene random copolymer (A) and the propylene homopolymer (B1) and/or the polypropylene block copolymer (B2) is melt-kneaded.

The specific embodiment of the extrusion process is not particularly limited. For example, a method for obtaining resin particles can be a method using an extruder. Specifically, resin particles can be produced by any of the following methods (1) to (5): (1) blending a polypropylene random copolymer (A), a propylene homopolymer (B1), and/or a polypropylene block copolymer (B2), and, if necessary, one or more selected from the group consisting of other resins and additives, to produce a mixed resin; (2) feeding the mixed resin into an extruder and melt-kneading the mixed resin to prepare a polypropylene resin composition; (3) extruding the polypropylene resin composition through a die equipped in the extruder; (4) solidifying the extruded polypropylene resin composition by cooling it, for example, by passing it through water; (5) then shredding the solidified polypropylene resin composition with a cutter into desired shapes, such as cylindrical, elliptical, spherical, cubic, rectangular, hollow cylinder, or polygonal prism. Alternatively, in (3), the melt-mixed polypropylene resin composition may be extruded directly into water through a die provided in the extruder, and immediately after extrusion, the polypropylene resin composition may be cut into particles, cooled, and solidified. By melt-mixing the mixed resin in this manner, more uniform resin particles can be obtained. The extrusion step may also include a peroxide treatment step in which the mixed resin is treated with a peroxide.

The weight per particle of the resin particles obtained in this manner is preferably 0.2 mg/particle to 10.0 mg/particle, and more preferably 0.5 mg/particle to 6.0 mg/particle. When the weight per particle of the resin particles is (a) 0.2 mg/particle or more, the handleability of the resin particles tends to be improved, and the shrinkage rate of the foamed molded article obtained by molding the resulting foamed beads tends to be small. When the weight per particle of the resin particles is (b) 10.0 mg/particle or less, the mold filling ability in the in-mold foam molding process tends to be improved.

(Peroxide treatment step)
This production method preferably includes a peroxide treatment step in which a propylene homopolymer (B1') and/or a polypropylene-based block copolymer (B2') is treated with a peroxide to obtain the propylene homopolymer (B1) and/or the polypropylene-based block copolymer (B2). For example, by melt-kneading an organic peroxide with (i) a propylene homopolymer (B1'), (ii) a polypropylene-based block copolymer (B2'), or (iii) a mixture of the propylene homopolymer (B1') and the polypropylene-based block copolymer (B2') in an extruder, these resins can be treated with the peroxide, and the resulting polymers can be used as (i) a propylene homopolymer (B1), (ii) a polypropylene-based block copolymer (B2), or (iii) a mixture of the propylene homopolymer (B1) and the polypropylene-based block copolymer (B2), respectively. In other words, in this production method, the propylene homopolymer (B1) and/or the polypropylene-based block copolymer (B2) may be treated with a peroxide. Even if the MFR at 230°C and a load of 2160 g of a mixture of propylene homopolymer and polypropylene-based block copolymer prepared as raw materials is low and does not satisfy the structure B according to one embodiment of the present invention, the MFR of the mixture can be increased by treating the propylene homopolymer and/or polypropylene-based block copolymer with peroxide, thereby making the mixture satisfy the structure B. A propylene homopolymer and a polypropylene-based block copolymer that satisfy the structure B can be respectively referred to as the propylene homopolymer (B1) and the polypropylene-based block copolymer (B2) according to one embodiment of the present invention. In other words, the propylene homopolymer (B1') and the polypropylene-based block copolymer (B2') respectively refer to a propylene homopolymer and a polypropylene-based block copolymer that do not satisfy the structure B. The specific embodiment of the peroxide treatment step is not particularly limited as long as the structure B is satisfied.

In this production method, (i) when propylene homopolymer (B1) is used but polypropylene-based block copolymer (B2) is not used, only the propylene homopolymer (B1') needs to be treated with peroxide in the peroxide treatment step; (ii) when polypropylene-based block copolymer (B2) is used but propylene homopolymer (B1) is not used, only the polypropylene-based block copolymer (B2') needs to be treated with peroxide in the peroxide treatment step; (iii) when both propylene homopolymer (B1) and polypropylene-based block copolymer (B2) are used, in the peroxide treatment step, (iii-1) only the propylene homopolymer (B1') may be treated with peroxide, (iii-2) only the polypropylene-based block copolymer (B2') may be treated with peroxide, or (iii-3) both the propylene homopolymer (B1') and the polypropylene-based block copolymer (B2') may be treated with peroxide.

The propylene homopolymer (B1') and the polypropylene block copolymer (B2') may have the same configuration as the propylene homopolymer (B1) and the polypropylene block copolymer (B2), respectively, except that the MFR of the blend of the propylene homopolymer (B1') and the polypropylene block copolymer (B2') does not satisfy configuration B. In other words, the descriptions in the above sections (Propylene homopolymer (B1)) and (Polypropylene block copolymer (B2)) can be used, as appropriate, for the respective configurations of the propylene homopolymer (B1') and the polypropylene block copolymer (B2').

In the peroxide treatment step, the amount of peroxide used is preferably 0.01 to 1.00 parts by weight, more preferably 0.05 to 0.50 parts by weight, and even more preferably 0.10 to 0.40 parts by weight, per 100 parts by weight of the total of the propylene homopolymer (B1') and the polypropylene block copolymer (B2').

Usable peroxides include, for example, 1,1-bis(t-butylperoxy)3,3,5-trimethylcyclohexane, t-butylperoxylaurate, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, t-butylperoxybenzoate, dicumyl peroxide, 1,3-bis(t-butylperoxyisopropyl)benzene, t-butylperoxyisopropyl monocarbonate, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 1,1-di(t-butylperoxy)cyclohexane, n-butyl-4,4-di(t-butylperoxy)valerate, α,α'-di(t-butylperoxy)diisopropylbenzene, and 2,5-dimethyl-2,5-bis(t-butylperoxy)hexyne-3.

In this manufacturing method, in addition to the peroxide treatment step described above, the extrusion step may further include a step of treating the mixed resin with peroxide.

(Foaming process)
The foaming step in this production method is not particularly limited, but may include, for example, an embodiment having the following steps: (i) a dispersing step of dispersing polypropylene-based resin particles, an aqueous dispersion medium, and a foaming agent in a container to obtain a dispersion, (ii) a heating step of heating the dispersion to a temperature equal to or higher than the softening temperature of the polypropylene-based resin particles, (iii) a pressurizing step of increasing the pressure in the container, and (iv) a releasing step of opening one end of the container and releasing the dispersion in the container into a region with a pressure lower than the pressure in the container. Hereinafter, an example of the foaming step will be described using an example in which the foaming step includes the dispersing step, the heating step, the pressurizing step, and the releasing step.

(Dispersion process)
The vessel used in the dispersion step is not particularly limited. Examples of the vessel include a pressure vessel and an autoclave-type pressure vessel (pressure autoclave). The vessel may be equipped with a stirrer inside.

The aqueous dispersion medium is not particularly limited as long as it contains water. From the viewpoint of enabling stable production of expanded beads, it is preferable to use pure water such as RO water (water purified by reverse osmosis membrane method), distilled water, deionized water (water purified by ion exchange resin), or ultrapure water as the aqueous dispersion medium.

Foaming agents include (a) (a-1) inorganic gases such as nitrogen, carbon dioxide, and air (a mixture of oxygen, nitrogen, and carbon dioxide), and (a-2) inorganic blowing agents such as water; and (b) organic blowing agents. From the perspective of reducing environmental impact and reducing the risk of combustion, inorganic blowing agents are preferred, inorganic gases and/or water are more preferred, and carbon dioxide and/or water are even more preferred. The water used as the aqueous dispersion medium can also be used as the blowing agent. In this case, both the aqueous dispersion medium and the blowing agent in the dispersion step can be water.

In this manufacturing method, it is preferable to use a dispersant (e.g., inorganic dispersants such as tribasic calcium phosphate, tribasic magnesium phosphate, basic magnesium carbonate, calcium carbonate, barium sulfate, kaolin, talc, and clay) and a dispersing aid (e.g., sodium dodecylbenzenesulfonate, sodium alkanesulfonate, sodium alkylsulfonate, sodium alkyldiphenyletherdisulfonate, and sodium α-olefinsulfonate). This configuration reduces adhesion of resin particles (sometimes referred to as blocking) and increases the stability of the dispersion in the container. This has the advantage of enabling stable production of expanded beads.

(Heating process)
In this specification, the "softening temperature of the polypropylene-based resin particles" means the melting point of the resin with the highest melting point among the non-recycled polypropylene-based resin contained in the base resin that constitutes the polypropylene-based resin particles and the resin components contained in the recycled material - 10.0°C.

In this specification, "a temperature equal to or higher than the softening temperature of the polypropylene-based resin particles" may be referred to as "foaming temperature." In other words, the heating step is a step of heating the temperature of the dispersion to the foaming temperature. The foaming temperature in the heating step is not particularly limited as long as it is equal to or higher than the softening temperature of the polypropylene-based resin particles. In the heating step, the temperature to which the dispersion is heated, i.e., the upper limit of the foaming temperature, is not particularly limited. The foaming temperature is preferably equal to or lower than the softening temperature of the polypropylene-based resin particles + 20.0°C, more preferably equal to or lower than the softening temperature of the polypropylene-based resin particles + 15.0°C, and even more preferably equal to or lower than the softening temperature of the polypropylene-based resin particles + 10.0°C. This configuration has the advantage of eliminating the risk of the polypropylene-based resin particles adhering to each other within the container.

(Pressure application process)
In this specification, the pressure (constant pressure) after being pressurized in the pressurizing step may be referred to as the “foaming pressure.” In other words, in the pressurizing step, the pressure inside the container is increased to the foaming pressure.

In the pressurizing step, the pressure inside the container, i.e., the foaming pressure, is not particularly limited. The foaming pressure in the pressurizing step is preferably (i) 1.0 MPa (gauge pressure) to 10.0 MPa (gauge pressure), more preferably (ii) 1.5 MPa (gauge pressure) to 5.0 MPa (gauge pressure), and even more preferably (iii) 1.5 MPa (gauge pressure) to 3.5 MPa (gauge pressure). When the foaming pressure is 1.0 MPa (gauge pressure) or higher, foamed particles with a suitable density can be obtained.

The heating step and the pressurizing step may be performed in any order, or may be performed simultaneously.

(holding process)
The present manufacturing method may further include a holding step, after the heating step and the pressurizing step and before the releasing step, of holding the temperature inside the container at the foaming temperature and the pressure inside the container at (or near) the foaming pressure.

In the holding step, the time (holding time) for which the temperature of the dispersion is held at the foaming temperature and the pressure inside the container is held at (or near) the foaming pressure is not particularly limited. The holding time is preferably 10 to 60 minutes, more preferably 12 to 55 minutes, and even more preferably 15 to 50 minutes. A holding time of 10 minutes or longer has the advantage that a sufficient amount of unmelted crystals (crystals of the base resin) are present, thereby reducing shrinkage of the resulting expanded beads and/or an increase in the open cell ratio. On the other hand, a holding time of 60 minutes or less has the advantage that an excessive amount of unmelted crystals are not present, allowing the expanded beads to be molded at a lower molding temperature.

(Release process)
The releasing step can be carried out after the heating step and the pressurizing step (a) when the holding step is not carried out, or after the holding step (b) when the holding step is carried out. The releasing step can expand the resin particles, resulting in expanded particles.

In the release process, "a region under pressure lower than the pressure inside the container" refers to "a region under pressure lower than the pressure inside the container" or "a space under pressure lower than the pressure inside the container," and can also be referred to as "an atmosphere under pressure lower than the pressure inside the container." A region under pressure lower than the pressure inside the container can also be referred to as a region under pressure lower than the foaming pressure, and may be, for example, a region under atmospheric pressure. A low-pressure region is, for example, a gas phase. Furthermore, in order to improve foaming properties, the low-pressure region (space) may be filled with saturated water vapor.

In the release process, when the dispersion is released into a region with a lower pressure than the pressure inside the container, the dispersion can be released through an orifice with a diameter of 1 mm to 5 mm in order to adjust the flow rate of the dispersion and reduce variation in the expansion ratio of the resulting expanded beads.

As mentioned above, the process of producing expanded beads from resin beads is called the "first-stage expansion process," and the resulting expanded beads are called "first-stage expanded beads." In order to obtain expanded beads with a high expansion ratio, the first-stage expanded beads obtained in the first-stage expansion process may be expanded again. The process of increasing the expansion ratio of the first-stage expanded beads is called the "second-stage expansion process," and the polypropylene resin expanded beads obtained by the second-stage expansion process are called "second-stage expanded beads." The specific method for the second-stage expansion process is not particularly limited, and any known method can be used.

4. Polypropylene resin foam molded article
A polypropylene-based resin foam molded article according to one embodiment of the present invention is a foam molded article obtained by molding the polypropylene-based resin foam beads described in Section [2. Expanded Polypropylene Resin Beads]. The polypropylene-based resin foam molded article according to one embodiment of the present invention may be a foam molded article obtained by molding the expanded polypropylene resin beads obtained by the manufacturing method described in Section [3. Manufacturing Method for Expanded Polypropylene Resin Beads]. It can also be said that the polypropylene-based resin foam molded article according to one embodiment of the present invention comprises the expanded polypropylene resin beads described in Section [2. Expanded Polypropylene Resin Beads] or the expanded polypropylene resin beads obtained by the manufacturing method described in Section [3. Manufacturing Method for Expanded Polypropylene Resin Beads].

In this specification, the "polypropylene resin foam molded article according to one embodiment of the present invention" may be referred to as the "present foam molded article."

Because of the above-mentioned structure, this foamed molded article has the advantage of excellent internal fusion and/or surface beauty.

(internal fusion)
In this specification, the internal fusion property of the present foamed molded article is evaluated by the number of all foamed beads present and the number of foamed beads broken at locations other than particle interfaces on the fracture surface of the foamed molded article. The greater the number of foamed beads broken at locations other than particle interfaces among all foamed beads present, in other words, the higher the internal fusion rate, the better the internal fusion property.

The internal fusion rate of this foamed molded article is preferably 60% or more, more preferably 70% or more, even more preferably 80% or more, and particularly preferably 90% or more.

(Surface beauty)
In this specification, the surface aesthetics of the present foamed molded article are evaluated based on the degree of gaps between the foam particles (hereinafter sometimes referred to as "intergranular gaps") on the surface of the foamed molded article and the wrinkles on the surface of the foamed molded article. The smaller the size and number of intergranular gaps on the surface of the foamed molded article, the better the surface aesthetics of the foamed molded article. Furthermore, the fewer wrinkles on the surface of the foamed molded article, the better the surface aesthetics of the foamed molded article.

(Molding cycle)
In this specification, the length of the molding cycle of the foamed molded article is evaluated by the time (seconds) until the cooling (water cooling) of the molded article is completed. The completion of cooling (water cooling) of the molded article was defined as the time when the mold was opened and demolded when the surface pressure measured by a pressure gauge attached to the surface of the Planck mold had dropped to 0.01 MPa (gauge pressure).

One embodiment of the present invention may have the following configuration.

[1] Expanded polypropylene resin particles obtained by expanding polypropylene resin particles,
the polypropylene-based resin particles contain a polypropylene-based random copolymer (A) having a melting point of 140.0°C or higher and lower than 155.0°C, and a propylene homopolymer (B1) and/or a polypropylene-based block copolymer (B2),
the melting point of the mixture of the propylene homopolymer (B1) and the polypropylene-based block copolymer (B2) is 155.0°C or higher and lower than 165.0°C,
The expanded polypropylene resin particles have a ratio (MFR B /MFR _A ) of the melt flow rate (MFR B ) of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) at 230°C and a load of 2160 g to the melt flow rate (MFR _A ) of the polypropylene random copolymer ( _A ) at 230°C and a load of 2160 g, _of 2.0 to 30.0.

[2] The expanded polypropylene resin particles described in [1], wherein, in the polypropylene resin particles, the content of the polypropylene random copolymer (A) is 70 to 95 parts by weight, and the total content of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) is 5 to 30 parts by weight, based on a total of 100 parts by weight of the polypropylene random copolymer (A), the propylene homopolymer (B1), and the polypropylene block copolymer (B2).

[3] Expanded polypropylene resin particles according to [1] or [2], which have two or more melting peaks in a DSC curve obtained by measurement using a differential scanning calorimeter.

[4] Expanded polypropylene resin particles according to any one of [1] to [3], having a DSC ratio of 30.0% to 50.0%.

[5] Expanded polypropylene resin particles according to any one of [1] to [4], wherein at least one of the polypropylene random copolymer (A), the propylene homopolymer (B1), and the polypropylene block copolymer (B2) contains a recycled resin.

[6] Expanded polypropylene resin particles according to any one of [1] to [5], wherein the melt flow rate (MFR) of the polypropylene resin particles at 230°C under a load of 2160 g is 7.0 g/10 min to 30.0 g/10 min.

[7] Expanded polypropylene resin particles according to any one of [1] to [6], wherein the polypropylene random copolymer (A) is at least one selected from the group consisting of propylene/ethylene random copolymer, propylene/1-butene random copolymer, propylene/ethylene/1-butene random copolymer, propylene/chlorinated vinyl random copolymer, and propylene/maleic anhydride random copolymer.

[8] The expanded polypropylene resin particles according to any one of [1] to [7], wherein the polypropylene random copolymer (A) has a melt flow rate (MFR _A ) at 230°C under a load of 2160 g of 3.0 g/10 min to 30.0 g/10 min.

[9] Expanded polypropylene resin particles according to any one of [1] to [8], wherein the polypropylene random copolymer (A) includes a recycled polypropylene random copolymer.

[10] The expanded polypropylene resin particles described in [9], wherein the proportion of the recycled polypropylene random copolymer in 100% by weight of the polypropylene random copolymer (A) is 50% or more.

[11] Expanded polypropylene resin particles according to any one of [1] to [10], wherein the melting point of the propylene homopolymer (B1) is 155.0°C or higher and lower than 165.0°C.

[12] Expanded polypropylene resin particles according to any one of [1] to [11], wherein the melt flow rate of the propylene homopolymer (B1) at 230°C under a load of 2160 g is 6 g/10 min to 350 g/10 min.

[13] Expanded polypropylene resin particles according to any one of [1] to [12], wherein when the polypropylene resin particles contain the propylene homopolymer (B1), the ratio of the content of the polypropylene random copolymer (A) to the content of the propylene homopolymer (B1) in the polypropylene resin particles (content (parts by weight) of the polypropylene random copolymer (A):content (parts by weight) of the propylene homopolymer (B1)) is 90:10 to 50:50.

[14] Expanded polypropylene resin particles according to any one of [1] to [13], wherein the total content of structural units derived from isoprene and structural units derived from conjugated dienes is 20% by weight or less, based on 100% by weight of the propylene homopolymer (B1).

[15] Expanded polypropylene resin particles according to any one of [1] to [14], wherein the polypropylene block copolymer (B2) is at least one selected from the group consisting of propylene/ethylene block copolymers, propylene/1-butene block copolymers, propylene/ethylene/1-butene block copolymers, propylene/chlorinated vinyl block copolymers, and propylene/maleic anhydride block copolymers.

[16] Expanded polypropylene resin particles according to any one of [1] to [15], wherein the melting point of the polypropylene block copolymer (B2) is 155.0°C or higher but lower than 165.0°C.

[17] Expanded polypropylene resin particles according to any one of [1] to [16], wherein the polypropylene block copolymer (B2) has a melt flow rate of 6 g/10 min to 350 g/10 min at 230°C under a load of 2160 g.

[18] Expanded polypropylene resin particles according to any one of [1] to [17], wherein when the polypropylene resin particles contain the polypropylene block copolymer (B2), the ratio of the content of the polypropylene random copolymer (A) in the polypropylene resin particles to the content of the polypropylene block copolymer (B2) (content (parts by weight) of the polypropylene random copolymer (A):content (parts by weight) of the polypropylene block copolymer (B2)) is 90:10 to 40:60.

[19] Expanded polypropylene resin particles according to any one of [1] to [18], wherein the polypropylene block copolymer (B2) includes a recycled polypropylene block copolymer.

[20] Expanded polypropylene resin particles according to any one of [1] to [19], wherein the proportion of the recycled polypropylene block copolymer in 100% by weight of the polypropylene block copolymer (B2) is 50% or more.

[21] Expanded polypropylene resin particles according to any one of [1] to [20], wherein the total content of structural units derived from isoprene and structural units derived from conjugated dienes is 20% by weight or less, based on 100% by weight of the polypropylene block copolymer (B2).

[22] The expanded polypropylene resin particles according to any one of [1] to [21], wherein the melt flow rate (MFR _B ) of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) at 230°C under a load of 2160 g is 6 g/10 min to 350 g/10 min.

[23] Expanded polypropylene resin particles according to any one of [1] to [22], wherein the polypropylene resin particles contain carbon black.

[24] Expanded polypropylene resin particles according to any one of [1] to [23], having an expansion ratio of 10 to 50 times.

[25] Expanded polypropylene resin particles according to any one of [1] to [24], wherein the average cell diameter is 80 μm to 500 μm.

[26] A polypropylene resin foam molded article obtained by molding the polypropylene resin foam beads described in any one of [1] to [25].

[27] The polypropylene resin foam molded article according to [26], having an internal fusion rate of 60% or more.

[28] An extrusion step of melt-kneading a mixed resin containing a polypropylene-based random copolymer (A) having a melting point of 140.0°C or more and less than 155.0°C, and a propylene homopolymer (B1) and/or a polypropylene-based block copolymer (B2) to obtain polypropylene-based resin particles;
and an expansion step of expanding the polypropylene-based resin particles obtained in the extrusion step,
the melting point of the mixture of the propylene homopolymer (B1) and the polypropylene-based block copolymer (B2) is 155.0°C or higher and lower than 165.0°C,
The method for producing expanded polypropylene resin beads comprises the step of: (a) providing a mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) with a melt flow rate (MFR _B ) at 230°C under a load of 2160 g relative to the melt flow rate (MFR _A ) of the polypropylene random copolymer (A) at 230°C under a load of 2160 g; and (b) providing a melt flow rate (MFR _B /MFR _A ) of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) at 230°C under a load of 2160 g, the ratio being 2.0 to 30.0.

[29] A method for producing expanded polypropylene resin beads according to [28], comprising a peroxide treatment step of treating a propylene homopolymer (B1') and/or a polypropylene block copolymer (B2') with a peroxide to obtain the propylene homopolymer (B1) and/or the polypropylene block copolymer (B2).

[30] The method for producing expanded polypropylene resin beads according to [28] or [29], wherein the amount of peroxide used in the peroxide treatment step is 0.01 to 1.00 parts by weight per 100 parts by weight of the total of the propylene homopolymer (B1') and the polypropylene block copolymer (B2').

[31] The method for producing expanded polypropylene resin particles described in any one of [28] to [30], wherein the melt flow rate (MFR) of the polypropylene resin particles at 230°C and a load of 2160 g is 7.0 g/10 min to 30.0 g/10 min.

[32] The method for producing expanded polypropylene resin beads according to any one of [28] to [31], wherein in the extrusion step, the amount of the polypropylene random copolymer (A) used is 70 to 95 parts by weight, and the total amount of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) used is 5 to 30 parts by weight, based on a total of 100 parts by weight of the polypropylene random copolymer (A), the propylene homopolymer (B1), and the polypropylene block copolymer (B2).

[33] The method for producing expanded polypropylene resin beads described in any one of [28] to [32], wherein the expanded polypropylene resin beads have two or more melting peaks in a DSC curve obtained by measurement using a differential scanning calorimeter.

[34] The method for producing expanded polypropylene resin beads described in any one of [28] to [33], wherein the DSC ratio of the expanded polypropylene resin beads is 30.0% to 50.0%.

[35] The method for producing expanded polypropylene resin beads described in any one of [28] to [34], wherein at least one of the polypropylene random copolymer (A), the propylene homopolymer (B1), and the polypropylene block copolymer (B2) contains a recycled resin.

[36] The method for producing expanded polypropylene resin beads according to any one of [28] to [35], wherein the polypropylene random copolymer (A) is at least one selected from the group consisting of propylene/ethylene random copolymers, propylene/1-butene random copolymers, propylene/ethylene/1-butene random copolymers, propylene/chlorinated vinyl random copolymers, and propylene/maleic anhydride random copolymers.

[37] The method for producing expanded polypropylene resin beads according to any one of [28] to [36], wherein the polypropylene random copolymer (A) has a melt flow rate (MFR _A ) of 3.0 g/10 min to 30.0 g/10 min at 230°C and a load of 2160 g.

[38] The method for producing expanded polypropylene resin beads described in any one of [28] to [37], wherein the polypropylene random copolymer (A) includes a recycled polypropylene random copolymer.

[39] The method for producing expanded polypropylene resin beads according to [38], wherein the proportion of the recycled polypropylene random copolymer in 100% by weight of the polypropylene random copolymer (A) is 50% or more.

[40] The method for producing expanded polypropylene resin beads described in any one of [28] to [39], wherein the melting point of the propylene homopolymer (B1) is 155.0°C or higher and lower than 165.0°C.

[41] The method for producing expanded polypropylene resin beads according to any one of [28] to [40], wherein the melt flow rate of the propylene homopolymer (B1) at 230°C and a load of 2160 g is 6 g/10 min to 350 g/10 min.

[42] The method for producing expanded polypropylene resin beads according to any one of [28] to [41], wherein, when the propylene homopolymer (B1) is used in the extrusion step, the ratio of the amount of the polypropylene random copolymer (A) used in the extrusion step to the amount of the propylene homopolymer (B1) used (amount of the polypropylene random copolymer (A) used (parts by weight): amount of the propylene homopolymer (B1) used (parts by weight)) is 90:10 to 50:50.

[43] The method for producing expanded polypropylene resin beads according to any one of [28] to [42], wherein the total content of structural units derived from isoprene and structural units derived from conjugated dienes is 20% by weight or less, based on 100% by weight of the propylene homopolymer (B1).

[44] The method for producing expanded polypropylene resin beads according to any one of [28] to [43], wherein the polypropylene block copolymer (B2) is at least one selected from the group consisting of propylene/ethylene block copolymers, propylene/1-butene block copolymers, propylene/ethylene/1-butene block copolymers, propylene/chlorinated vinyl block copolymers, and propylene/maleic anhydride block copolymers.

[45] The method for producing expanded polypropylene resin beads described in any one of [28] to [44], wherein the melting point of the polypropylene block copolymer (B2) is 155.0°C or higher but lower than 165.0°C.

[46] The method for producing expanded polypropylene resin beads according to any one of [28] to [45], wherein the polypropylene block copolymer (B2) has a melt flow rate of 6 g/10 min to 350 g/10 min at 230°C under a load of 2160 g.

[47] The method for producing expanded polypropylene resin beads according to any one of [28] to [46], wherein, when the polypropylene block copolymer (B2) is used in the extrusion step, the ratio of the amount of the polypropylene random copolymer (A) used in the extrusion step to the amount of the polypropylene block copolymer (B2) used (amount of the polypropylene random copolymer (A) used (parts by weight):amount of the polypropylene block copolymer (B2) used (parts by weight)) is 90:10 to 40:60.

[48] The method for producing expanded polypropylene resin beads described in any one of [28] to [47], wherein the polypropylene block copolymer (B2) includes a recycled polypropylene block copolymer.

[49] The method for producing expanded polypropylene resin beads according to any one of [28] to [48], wherein the proportion of the recycled polypropylene block copolymer in 100% by weight of the polypropylene block copolymer (B2) is 50% or more.

[50] The method for producing expanded polypropylene resin beads according to any one of [28] to [49], wherein the total content of structural units derived from isoprene and structural units derived from conjugated dienes is 20% by weight or less in 100% by weight of the polypropylene block copolymer (B2).

[51] The method for producing expanded polypropylene resin beads according to any one of [28] to [50], wherein the melt flow rate (MFR _B ) of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) at 230°C under a load of 2160 g is 6 g/10 min to 350 g/10 min.

[52] The method for producing expanded polypropylene resin beads described in any one of [28] to [51], wherein the mixed resin contains carbon black.

[53] The method for producing expanded polypropylene resin beads described in any one of [28] to [52], wherein the expansion ratio of the expanded polypropylene resin beads is 10 to 50 times.

[54] The method for producing expanded polypropylene resin beads described in any one of [28] to [53], wherein the expanded polypropylene resin beads have an average cell diameter of 80 μm to 500 μm.

[55] A method for producing a polypropylene resin foam molded article, comprising a step of molding expanded polypropylene resin beads produced by the method for producing expanded polypropylene resin beads described in any one of [28] to [54].

[56] The method for producing a polypropylene resin foam molded article according to [55], wherein the internal fusion rate of the polypropylene resin foam molded article is 60% or more.

Below, one embodiment of the present invention will be explained in more detail using examples and comparative examples. The present invention is not limited to these examples.

〔material〕
<Base resin>
(Polypropylene-based random copolymer (A))
A-1, A-2 [non-recycled resin; manufactured by Prime Polymer Co., Ltd., propylene/ethylene random copolymer]
A-3 [non-recycled resin; manufactured by Prime Polymer Co., Ltd., propylene/ethylene/1-butene random copolymer]
A-4 [recycled material: a foamed molded article containing A-3 as a base resin was pulverized, and the pulverized material was melt-kneaded to obtain a resin composition in the form of pellets. The obtained resin composition in the form of pellets was used as the recycled material.]
(Propylene homopolymer (B1))
B1-1 [non-recycled resin; manufactured by Prime Polymer, propylene homopolymer]
B1-2, B1-3 [non-recycled resin; resin obtained by treating B1-1 with peroxide] B1-4 [non-recycled resin; propylene homopolymer manufactured by Prime Polymer Co., Ltd.]
B1-5 [non-recycled resin; resin obtained by treating B1-4 with peroxide]
(Polypropylene-based block copolymer (B2))
B2-1 [non-recycled resin; manufactured by Prime Polymer Co., Ltd., propylene/ethylene block copolymer]
B2-2 [non-recycled resin; resin obtained by treating B2-1 with peroxide]
B2-3 [non-recycled resin; manufactured by Prime Polymer, propylene/ethylene block copolymer]
B2-4, B2-5 [non-recycled resin; resin obtained by treating B2-3 with peroxide] B2-6 [non-recycled resin; propylene/ethylene block copolymer manufactured by Prime Polymer Co., Ltd.]
B2-7 and B2-8 [non-recycled resins; resins obtained by treating B2-6 with peroxide]; B2-9 [recycled material; automobile parts containing a propylene/ethylene block copolymer as a base resin were pulverized, and the pulverized material was melt-kneaded to obtain a resin composition in the form of pellets. The obtained resin composition in the form of pellets was used as the recycled material]
B2-10 to B2-12 [recycled material; material obtained by treating B2-9 with peroxide]
B2-13 [Recycled material: A foamed sheet containing a propylene/ethylene block copolymer as a base resin was pulverized, and the pulverized material was melt-kneaded to obtain a resin composition in the form of pellets. The obtained resin composition in the form of pellets was used as the recycled material.]
B2-14, B2-15 [recycled material; material obtained by treating B2-13 with peroxide]
(peroxide)
Peroxide [NOF, Perbutyl I]
Peroxide [NOF Perhexa 25B-40]
(MFR adjustment of polypropylene resin and recycled material (peroxide treatment process))
B1-2 and B1-3 were obtained by dry-blending B1-1 with the peroxides shown in Table 1 using a blender, melt-kneading the mixture at a resin temperature of 220°C using a twin-screw extruder (TEM26-SX, manufactured by Toshiba Machine Co., Ltd.), and cutting the extruded strands (approximately 5 mg/grain). Similarly, B2-2 was obtained by treating B2-1, B1-5 by treating B1-4, B2-4 and B2-5 by treating B2-3, B2-7 and B2-8 by treating B2-6, B2-10 to B2-12 by treating B2-9, and B2-14 and B2-15 by treating B2-13 with the peroxides shown in Table 1. The amount of peroxide (parts by weight) in Table 1 is the amount of peroxide used per 100 parts by weight of non-recycled resin or recycled material.

Table 1 shows the composition of the non-recycled resin used, the composition of the recycled polypropylene resin contained in the recycled material, and the peroxide treatment. The melting point, MFR, number-average molecular weight (Mn), weight-average molecular weight (Mw), and z-average molecular weight (Mz) of the non-recycled resin and recycled material used were measured using the methods described below. The carbon black content and ash content of the recycled material used were measured using the methods described below. The results are shown in Table 2. In Table 2, resins marked with "-" in the "Carbon Black (wt%)" column indicate that they contain no carbon black (0 wt%). Similarly, recycled materials marked with "-" in the "Ash Content (wt%)" column indicate that they contain no ash (0.00 wt% or more and 0.05 wt% or less). In Table 2, the content of recycled polypropylene resin contained in the recycled material is calculated by subtracting the carbon black content and ash content listed in Table 2 from 100 wt% of the recycled material.

<Resin particle additive>
Talc [Hayashi Kasei Co., Ltd., Talc Powder PK-S]
Glycerin [Lion Corporation, refined glycerin D]
Zinc borate [Tomita Pharmaceutical Co., Ltd., Zinc Borate 2335]
Carbon black masterbatch [a mixture obtained by mixing 40 parts by weight of carbon black with 60 parts by weight of a polypropylene resin (MFR at 230°C = 7.5 g/10 min)].

[Measurement method]
The evaluation methods used in the examples and comparative examples are described below.

<Melt point measurement of polypropylene resin and recycled materials>
The melting points of the polypropylene resin and the recycled material were measured using a differential scanning calorimeter (DSC7020, manufactured by Hitachi High-Tech Science Corporation). The specific measurement method was as follows: (1) 5 to 6 mg of the sample to be measured was heated from 40°C to 220°C at a heating rate of 10°C/min to melt it; (2) Then, the temperature was lowered from 220°C to 40°C at a heating rate of 10°C/min to crystallize it; (3) The temperature was further increased from 40°C to 220°C at a heating rate of 10°C/min. The temperature of the peak (melting peak) of the DSC curve obtained during the second heating (i.e., during (3)) was taken as the melting point of the polypropylene resin and the recycled material. In addition, when there are multiple peaks (melting peaks) in the DSC curves of the polypropylene-based resin and the recycled material obtained by the second temperature increase using the above-mentioned method, the temperature of the peak (melting peak) with the largest heat of fusion was taken as the melting point of the polypropylene-based resin and the recycled material.

<MFR Measurement of Polypropylene Resin and Recycled Materials>
The MFR of the polypropylene resin and the recycled material was measured using a melt mass-flow rate measuring device according to JIS K7210 under the following conditions: orifice diameter: 2.0959±0.005 mm, orifice length: 8.000±0.025 mm, load: 2160 g, and temperature: 230±0.2°C.

<GPC Measurement of Polypropylene Resin and Recycled Materials>
The number average molecular weight (Mn), weight average molecular weight (Mw) and z average molecular weight (Mz) of the polypropylene resin and the recycled material were measured as polystyrene equivalent values by gel permeation chromatography (GPC). The GPC measurement conditions for measuring the molecular weights of the polypropylene resin and the recycled material were as follows:

20 mg of the polypropylene resin and recycled material to be measured were completely dissolved in 20 mL of o-dichlorobenzene at 145°C, and the resulting solution was hot filtered through a sintered filter with a pore size of 1.0 µm, and the filtrate was used as an analytical sample.
Measurement equipment: HLC-8321GPC/HT type high temperature gel permeation chromatograph (manufactured by Tosoh Corporation) Analysis equipment: Data processing software Empower 3 (manufactured by Waters Japan)
Columns: 2 TSKgel GMH6-HT, 2 TSKgel GMH6-HTL (inner diameter 7.5 mm x length 300 mm, manufactured by Tosoh Corporation)
Mobile phase: o-dichlorobenzene (containing 0.025% BHT)
Column temperature: 140°C
Detector: differential refractometer Flow rate: 1.0 mL/min Injection amount: 1.0 mL/min

The recycled materials used in the examples and comparative examples did not contain any resins other than the polypropylene resins listed in the "Composition of recycled polypropylene resins contained in recycled materials" column; in addition to polypropylene resins, they contained only carbon black and ash. Carbon black and ash do not affect the melting point, MFR, number average molecular weight (Mn), weight average molecular weight (Mw), and z-average molecular weight (Mz). Therefore, the melting point, MFR, number average molecular weight (Mn), weight average molecular weight (Mw), and z-average molecular weight (Mz) of the recycled materials can be considered to be the melting point, MFR, number average molecular weight (Mn), weight average molecular weight (Mw), and z-average molecular weight (Mz), respectively, of the polypropylene resins listed in the "Composition of recycled polypropylene resins contained in recycled materials" column.

<Quantitative determination of carbon black in recycled materials>
The carbon black content in the recycled material was measured using a thermogravimetric and differential thermal analyzer (STA200RV, Hitachi High-Tech Science Corp.) according to the following procedure (1) to (3): (1) 6 to 8 mg of the sample to be measured was weighed into a Pt measuring vessel; (2) The sample was heated to 600°C at 10°C/min in a nitrogen atmosphere, then cooled to 400°C at 10°C/min, and then placed in a simulated air (oxygen:nitrogen = 21%:79%) atmosphere and heated to 800°C at 10°C/min; (3) The weight ratio (wt%) of carbon black was calculated from the difference between the weight loss rate (wt%) at 400°C and the weight loss rate (wt%) at 800°C in the TG weight loss curve obtained in step (2) above.

<Quantitative determination of ash content in recycled materials>
The ash content of the recycled material was calculated from the weight of the recycled material and the weight of the residue after burning the recycled material. The specific operating procedures were as follows: (1) The recycled material was heated at 150°C for 1 hour to completely remove moisture from the recycled material; (2) 1 g to 2 g of the recycled material was placed in a crucible and held at 300°C in an electric furnace for 30 minutes or more, and then burned at 750°C for 1 hour or more; (3) The crucible was removed from the electric furnace and cooled in a desiccator at 23°C for 1 hour; (4) The ash content of the recycled material was calculated using the following formula:
W1: Crucible weight (g)
W2: Weight of the crucible + recycled material before combustion (g)
W3: Weight of the crucible and recycled materials after combustion (g)
Ash content in recycled material (wt%) = {(W3 - W1) * 100}/(W2 - W1).

<Measurement of MFR of Polypropylene Resin Particles>
The method for measuring the MFR of polypropylene-based resin particles is the same as the method described in the section <Measurement of MFR of Polypropylene-based Resin and Recycled Material> above, except that the sample is polypropylene-based resin particles. Therefore, the description in that section is incorporated herein by reference, and further description will be omitted here.

<Measurement of DSC ratio of expanded polypropylene resin beads>
The DSC ratio was measured using a differential scanning calorimeter (DSC7020, manufactured by Hitachi High-Tech Science Corporation). Specifically, 5 to 6 mg of expanded polypropylene resin beads were heated from 40°C to 220°C at a heating rate of 10°C/min. The DSC curve obtained during the first heating cycle had a melting peak area on the low temperature side as _Ql and a melting peak area on the high temperature side as _Qh , and the DSC ratio was calculated using the following formula:
DSC ratio (%)=Q _h /(Q _l +Q _h )×100.

Strictly speaking, the measurement was carried out as follows. A line segment was drawn connecting the maximum point between the low-temperature melting peak and the high-temperature melting peak and the point on the curve at a temperature of 100°C, and this line segment was designated as line segment _L. The area enclosed by the low-temperature melting peak and line segment _L was designated as _Ql . A line segment was also drawn from the maximum point between the low-temperature melting peak and the high-temperature melting peak to the melting end baseline, and this line segment was designated as line segment _H. The area enclosed by the high-temperature melting peak and line segment _H was designated as _Qh . The temperatures at the top of the high-temperature melting peak and the low-temperature melting peak were also read.

<Measurement of Expansion Ratio of Expanded Polypropylene Resin Beads>
The expansion ratio of the expanded beads was measured as follows (1) to (4): (1) Approximately 3 g to 10 g of expanded polypropylene resin beads were taken, dried at 60°C for 6 hours, and then conditioned indoors at 23°C and 50% humidity, and the weight _w1 (g) was measured; (2) The expanded beads used for weight measurement were submerged in a measuring cylinder containing ethanol, and the volume v ( ^cm3 ) was measured by measuring the rise in the water level in the measuring cylinder (submersion method); (3) The true specific gravity _ρb of the expanded polypropylene resin beads was calculated as _w1 /v; (4) The density _ρr of the expanded polypropylene resin beads before expansion was divided by the true specific gravity _ρb of the expanded polypropylene resin beads, and the resulting value was used as the expansion ratio of the expanded polypropylene resin beads. In the examples and comparative examples shown below, the density _ρr of the expanded polypropylene resin beads before expansion was 0.9 g/ ^cm3 .

<Average cell diameter of expanded polypropylene resin particles>
The average cell diameter of expanded polypropylene resin beads was measured by the following methods (1) to (5): (1) Using a razor (high-quality stainless steel double-edged blade manufactured by Feather Corporation), the expanded beads were cut so as to pass through the center of the expanded beads, while being careful not to destroy the cell membranes of the expanded beads; (2) The cut surface of the expanded beads was observed using a microscope (manufactured by Hirox Co., Ltd., RH-2000), and an image of the observed surface was taken; (3) A line segment corresponding to a length of 2000 μm was drawn on the image obtained at any part of the expanded beads except for the surface layer; (4) The number n of cells through which the line segment passed was measured, and the cell diameter was calculated from the formula (cell diameter = 2000/n (μm)); (5) The same procedure was performed on 10 expanded beads, and the arithmetic mean value of the calculated cell diameters was taken as the average cell diameter of the expanded polypropylene resin beads.

<Internal fusion properties of polypropylene resin foam molded body>
The internal fusion property of the polypropylene resin foam molded article was evaluated by determining the internal fusion rate of the molded article.The measurement method of the internal fusion rate of the polypropylene resin foam molded article was as follows (1) to (4): (1) a cutter was used to make a 5 mm long cut on any one surface of the foam molded article in a direction perpendicular to the surface; (2) the foam molded article was then manually broken along the cut; (3) the area of the obtained fracture surface excluding the cut was visually observed, and the total number of foamed beads present in the area and the number of foamed beads broken at areas other than the particle interface in the area (i.e., the foamed beads broken themselves) were counted; (4) the internal fusion rate was calculated according to the following formula:
Internal fusion rate (%)=(number of expanded particles broken at other than particle interfaces in a region/total number of expanded particles present in a region)×100.
The values shown in the "Internal fusion property" column in Tables 3 to 5 are the internal fusion rates (%).

<Surface aesthetics of polypropylene resin foam molded products>
The resulting polypropylene resin foam molded article was visually observed on a surface measuring 350 mm long x 450 mm wide, and the surface aesthetics was judged according to the following criteria: Intergranular gaps (gaps between the polypropylene resin foam particles), which are one of the evaluation indices for surface aesthetics, were judged by visually counting the number of gaps present within a 50 mm square on the surface of the central part of the molded article.
4 (beautiful surface appearance): No wrinkles and 0 to 1 gap between grains.
3 (good surface appearance): no wrinkles and 2-3 gaps between grains.
2 (Surface appearance is acceptable): Wrinkles are observed or there are 4 to 5 gaps between the grains.
1 (surface appearance unacceptable): Wrinkles are observed or there are 6 or more gaps between grains.

<Molding cycle for polypropylene resin foam molded body>
The molding cycle in the method for producing a polypropylene-based resin foam molded article was measured from the start of molding to the end of molding, when the molded article was released from the mold. The start of molding was the point at which the polypropylene-based resin foam beads began to be filled into the mold. A mold capable of forming a molding space measuring 370 mm long, 320 mm wide, and 50 mm thick was used. Next, with the drain valve of the drain line of the molding machine open, steam at 0.10 MPa (gauge pressure) was pumped into the mold for 3 seconds to heat the foamed beads and expel air from the mold (preheating step). Next, steam was flowed from the fixed mold side to the movable mold side for 6 seconds (one-way heating step), and then steam was flowed from the movable mold side to the fixed mold side for 3 seconds (reverse one-way heating step) to expel air and heat the beads. Next, with the drain valve of the drain line of the molding machine closed, steam at 0.30 MPa (gauge pressure) was pumped into the mold for 9 seconds to heat the foamed beads (double-sided heating step), fusing them to form a foam molded article. The foam molded article in the mold was then water-cooled. Next, the mold was opened when the surface pressure measured by a pressure gauge attached to the surface of the Planck mold had dropped to 0.01 MPa (gauge pressure), and molding was completed when the mold was released. The evaluation criteria for the molding cycle (productivity) were as follows:
3 (Excellent): Molding cycle is within 180 seconds.
2 (poor): Molding cycle is longer than 180 seconds and less than 210 seconds.
1 (very poor): Molding cycle longer than 210 seconds.

The methods for producing polypropylene-based resin particles, expanded polypropylene-based resin particles, and expanded polypropylene-based resin molded articles in the examples and comparative examples are described below.

Example 1
[Preparation of Polypropylene Resin Particles]
75 parts by weight of A-3 as the polypropylene random copolymer (A), 25 parts by weight of B1-2 as the propylene homopolymer (B1), 0.2 parts by weight of talc, and 0.2 parts by weight of glycerin were weighed and dry-blended using a blender to obtain a mixed resin. The obtained mixed resin was melt-kneaded at a resin temperature of 220 ° C. using a twin-screw extruder (Toshiba Machine Co., Ltd., TEM26-SX), and the extruded strand was water-cooled in a 2 m long water tank and then cut to produce polypropylene resin particles (1.2 mg/particle) (extrusion step).

[Preparation of expanded polypropylene resin beads (expansion process)]
In a 10 L pressure-resistant autoclave, 100 parts by weight (2.4 kg) of the polypropylene-based resin particles obtained as described above, 200 parts by weight of water, 0.3 parts by weight of kaolin (manufactured by BASF, ASP170) as a poorly water-soluble inorganic compound, and 0.06 parts by weight of sodium dodecylbenzenesulfonate (manufactured by Kao Corporation, Neopelex G-15) as a surfactant were charged, and then 5 parts by weight of carbon dioxide was added as a blowing agent under stirring (dispersion step). The contents of the autoclave were heated to a foaming temperature of 160 ° C. (heating step), and after holding for 10 minutes, additional carbon dioxide was injected to increase the internal pressure of the autoclave to a foaming pressure of 2.80 MPa (gauge pressure) (pressurization step). After holding at the foaming temperature and foaming pressure for 20 minutes (holding step), the valve at the bottom of the autoclave was opened, and the contents were released to atmospheric pressure through a 3.6 mm diameter orifice to obtain polypropylene-based resin foamed particles with an expansion ratio of 18 times (release step). During this release, the pressure in the container was maintained by injecting carbon dioxide to prevent a decrease in the pressure inside the container. The expanded polypropylene resin beads had a DSC curve with two peaks, a high-temperature melting peak at 169°C and a low-temperature melting peak at 150°C, a DSC ratio of 30.1%, and an average cell diameter of 280 µm.

[Preparation of polypropylene resin foam molded article]
The obtained polypropylene-based resin foamed beads were dried at 80°C. The dried polypropylene-based resin foamed beads were placed in a pressure-resistant container and impregnated with pressurized air, adjusting the internal pressure of the polypropylene-based resin foamed beads to 0.20 MPa (absolute pressure). Next, the polypropylene-based resin foamed beads to which internal pressure had been applied were filled into a mold measuring 370 mm in length, 320 mm in width, and 50 mm in thickness. The mold chamber was then heated with steam at 0.30 MPa (gauge pressure) (molding pressure) to fuse the foamed beads together. After water-cooling the inside of the mold and the surface of the molded body, the molded body was released from the mold to obtain a polypropylene-based resin foamed molded body. The obtained foamed molded body was left to stand at 23°C for 2 hours and then aged at 75°C for 16 hours.

(Examples 2 to 20, Comparative Examples 1 to 8)
In [Preparation of Polypropylene-Based Resin Particles], polypropylene-based resin particles were prepared by the same procedure as in Example 1, except that the formulation of the base resin was changed as shown in Tables 3 to 5. For example, in Example 12, polypropylene-based resin particles were prepared by the same procedure as in Example 1, except for the following: 75 parts by weight of A-3 as the polypropylene-based random copolymer (A), 25 parts by weight of B2-12- as the propylene homopolymer (B1), 0.2 parts by weight of talc, 0.2 parts by weight of glycerin, and 7.5 parts by weight of the carbon black masterbatch were weighed out and dry-blended using a blender to obtain a mixed resin. For example, in Example 13, polypropylene-based resin particles were prepared in the same manner as in Example 1, except for the following: 75 parts by weight of A-3 as the polypropylene-based random copolymer (A), 25 parts by weight of B2-12- as the propylene homopolymer (B1), 0.05 parts by weight of zinc borate, and 7.5 parts by weight of a carbon black masterbatch were weighed and dry-blended using a blender to obtain a mixed resin. Next, polypropylene-based resin expanded beads and polypropylene-based resin foam molded articles were prepared in the same manner as in Example 1, except that the foaming conditions in [Preparation of Expanded Polypropylene-Based Resin Beads] were changed as shown in Tables 3 to 5. The formulation conditions of the base resin, the foaming conditions, and the evaluation results of the obtained expanded polypropylene-based resin beads and expanded polypropylene-based resin foam molded articles are shown in Tables 3 to 5.

In Examples 1 to 20, it was found that when polypropylene resin particles containing a polypropylene random copolymer (A) and a propylene homopolymer (B1) and/or a polypropylene block copolymer (B2) and having an MFR _B /MFR _A of 2.0 to 30.0 are used, even when recycled polypropylene random copolymer (A), or propylene homopolymer (B1) and/or polypropylene block copolymer (B2) is used as in Examples 8 to 20, foamed molded articles can be obtained in a short molding cycle and foamed molded articles with excellent surface beauty can be obtained.

In Comparative Examples 1 to 8, it was found that when either configuration A or configuration B was not met, the molding cycle was poor and there was room for improvement in surface aesthetics.

According to one embodiment of the present invention, novel polypropylene-based resin particles can be provided that can produce polypropylene-based resin foam molded articles in a short molding cycle. Therefore, one embodiment of the present invention can be used in a variety of applications, including automotive interior components, core materials for automotive bumpers, heat insulation materials, cushioning packaging materials, and returnable boxes.

Claims (13)

Expanded polypropylene resin particles obtained by expanding polypropylene resin particles,
the polypropylene-based resin particles contain a polypropylene-based random copolymer (A) having a melting point of 140.0°C or higher and lower than 155.0°C, and a propylene homopolymer (B1) and/or a polypropylene-based block copolymer (B2),
the melting point of the mixture of the propylene homopolymer (B1) and the polypropylene-based block copolymer (B2) is 155.0°C or higher and lower than 165.0°C,
The expanded polypropylene resin particles have a ratio (MFR B /MFR _A ) of the melt flow rate (MFR B ) of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) at 230°C and a load of 2160 g to the melt flow rate (MFR _A ) of the polypropylene random copolymer ( _A ) at 230°C and a load of 2160 g, _of 2.0 to 30.0. The expanded polypropylene resin particles according to claim 1, wherein, in the polypropylene resin particles, the content of the polypropylene random copolymer (A) is 70 to 95 parts by weight, and the total content of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) is 5 to 30 parts by weight, based on a total of 100 parts by weight of the polypropylene random copolymer (A), the propylene homopolymer (B1), and the polypropylene block copolymer (B2). The expanded polypropylene resin particles according to claim 1, which have two or more melting peaks in a DSC curve obtained by measurement using a differential scanning calorimeter. The expanded polypropylene resin particles according to claim 1, having a DSC ratio of 30.0% to 50.0%. The expanded polypropylene resin particles according to claim 1, wherein at least one of the polypropylene random copolymer (A), the propylene homopolymer (B1), and the polypropylene block copolymer (B2) contains a recycled resin. The expanded polypropylene resin particles according to claim 1, wherein the melt flow rate (MFR) of the polypropylene resin particles at 230°C and a load of 2160 g is 7.0 g/10 min to 30.0 g/10 min. the propylene homopolymer (B1) has a melt flow rate of 6 g/10 min to 350 g/10 min at 230°C under a load of 2160 g;
2. The expanded polypropylene resin particles according to claim 1, wherein the polypropylene block copolymer (B2) has a melt flow rate of 6 g/10 min to 350 g/10 min at 230° C. under a load of 2160 g. The expanded polypropylene resin particles according to claim 1, wherein the polypropylene resin particles contain carbon black. A polypropylene resin foam molded article obtained by molding the polypropylene resin foam beads described in any one of claims 1 to 8. an extrusion step of melt-kneading a mixed resin containing a polypropylene random copolymer (A) having a melting point of 140.0°C or higher and lower than 155.0°C, and a propylene homopolymer (B1) and/or a polypropylene block copolymer (B2) to obtain polypropylene resin particles;
and an expansion step of expanding the polypropylene-based resin particles obtained in the extrusion step,
the melting point of the mixture of the propylene homopolymer (B1) and the polypropylene-based block copolymer (B2) is 155.0°C or higher and lower than 165.0°C,
The method for producing expanded polypropylene resin beads comprises the step of: (a) providing a mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) with a melt flow rate (MFR _B ) at 230°C under a load of 2160 g relative to the melt flow rate (MFR _A ) of the polypropylene random copolymer (A) at 230°C under a load of 2160 g; and (b) providing a melt flow rate (MFR _B /MFR _A ) of the mixture of the propylene homopolymer (B1) and the polypropylene block copolymer (B2) at 230°C under a load of 2160 g, the ratio being 2.0 to 30.0. The method for producing expanded polypropylene resin beads according to claim 10, comprising a peroxide treatment step of treating a propylene homopolymer (B1') and/or a polypropylene block copolymer (B2') with a peroxide to obtain the propylene homopolymer (B1) and/or the polypropylene block copolymer (B2). The method for producing expanded polypropylene resin beads according to claim 10 or 11, wherein the melt flow rate (MFR) of the polypropylene resin beads at 230°C and a load of 2160 g is 7.0 g/10 min to 30.0 g/10 min. The method for producing expanded polypropylene resin beads according to claim 11, wherein the amount of peroxide used in the peroxide treatment step is 0.01 to 1.00 parts by weight per 100 parts by weight of the total of the propylene homopolymer (B1') and the polypropylene block copolymer (B2').