Method for producing expanded polypropylene-based resin particles and expanded polypropylene-based resin particles

Abstract

The present invention provides: expanded polypropylene-based resin particles including a biomass-derived polypropylene-based resin component and having satisfactory in-mold moldability; and a method for producing the expanded particles. This method for producing expanded polypropylene-based resin particles is for producing expanded polypropylene-based resin particles by expanding polypropylene-based resin particles. The polypropylene-based resin particles each comprises a core layer containing a polypropylene-based resin as a base resin and a resin layer covering the core layer and containing a polypropylene-based resin as a base resin. The resin layer includes a biomass-derived polypropylene-based resin having a biomass-derived monomer component in the molecular chain. The content X of the biomass-derived polypropylene-based resin in the resin layer is 50 mass% or higher. The content X (mass%) of the biomass-derived polypropylene-based resin in the resin layer is not lower than the content Y (mass%) of a biomass-derived polypropylene-based resin in the core layer.

Description

Method for producing expanded polypropylene resin beads and expanded polypropylene resin beads

[0001] The present invention relates to a method for producing expanded polypropylene resin beads and expanded polypropylene resin beads.

[0002] Expanded polyolefin resin bead molded articles, which are obtained by molding expanded polyolefin resin beads in a mold, are used in a wide range of fields, such as transport containers for foods and the like, packaging or cushioning materials for electric and electronic parts, precision parts, and vehicle components, building materials such as thermal insulation materials for homes, and shock absorbing materials for vehicle components, etc.

[0003] On the other hand, in recent years, there has been a demand for environmentally friendly products that can reduce the burden on the environment, even for products using polyolefin resins.As one method for producing such environmentally friendly products, the use of polyolefin resins made from natural materials such as plants (plant-derived polyolefin resins) as starting materials has been considered, instead of polyolefin resins made from fossil fuel-derived raw materials.

[0004] For example, Patent Document 1 discloses expanded polyethylene resin particles containing a plant-derived polyethylene resin with a plant content of 80% or more and a plant content of 1% or more, with the aim of providing expanded polyethylene resin particles that can contribute to solving environmental problems and the depletion of fossil fuel resources.

[0005] JP 2013-60514 A

[0006] Regarding expanded polypropylene resin particles produced using a polypropylene resin as a polyolefin resin, it is desirable to use a polypropylene resin made from natural materials such as plants as a starting material (biomass-derived polypropylene resin) instead of a polypropylene resin made from a fossil fuel-derived raw material.

[0007] However, when foamed beads containing biomass-derived polypropylene-based resin as a raw material are molded in a mold, the foamed beads have poor moldability in a mold compared to when foamed beads using only fossil fuel-derived polypropylene-based resin are molded in a mold, and there is a problem that it is difficult to obtain good polypropylene-based resin foamed bead moldings.

[0008] The present invention has been made in consideration of the above-mentioned problems, and an object of the present invention is to provide expanded polypropylene-based resin beads containing a biomass-derived polypropylene-based resin component, which have good in-mold moldability, and a method for producing the same.

[0009] The present inventors have found that the above-mentioned problems can be solved by producing expanded particles by expanding resin particles having a core layer and a resin layer, the resin layer of the resin particles containing a biomass-derived polypropylene resin containing a biomass-derived monomer component in the molecular chain, the content of the biomass-derived polypropylene resin in the resin layer being equal to or greater than a specific value, and the content of the biomass-derived polypropylene resin in the resin layer being equal to or greater than the content of the biomass-derived polypropylene resin in the core layer, and have completed the present invention. <1> A method for producing expanded polypropylene-based resin beads by expanding polypropylene-based resin particles to produce expanded polypropylene-based resin beads, wherein the polypropylene-based resin particles have a core layer containing a polypropylene-based resin as a base resin and a resin layer coating the core layer and also containing a polypropylene-based resin as a base resin, the resin layer containing a biomass-derived polypropylene-based resin having a biomass-derived monomer component in its molecular chain, the content X of the biomass-derived polypropylene-based resin in the resin layer being 50% by mass or more, and the content X (% by mass) of the biomass-derived polypropylene-based resin in the resin layer being equal to or greater than the content Y (% by mass) of the biomass-derived polypropylene-based resin in the core layer. <2> A method for producing expanded polypropylene-based resin beads according to <1>, wherein the polypropylene-based resin constituting the resin layer has a flexural modulus of 800 MPa or more and 1600 MPa or less. <3> A method for producing expanded polypropylene-based resin beads according to <1> or <2>, wherein the mass proportion of the resin layer in the polypropylene-based resin particles is 0.1% by mass or more and 20% by mass or less. <4> The method for producing expanded polypropylene-based resin beads according to any one of <1> to <3>, wherein the ratio of the flexural modulus of the polypropylene-based resin constituting the resin layer to the flexural modulus of the polypropylene-based resin constituting the core layer is 0.7 or more and 1.3 or less.<5> The method for producing expanded polypropylene-based resin beads according to any one of <1> to <4>, wherein the polypropylene-based resin constituting the core layer is a mixed resin of the biomass-derived polypropylene-based resin and a fossil fuel-derived polypropylene-based resin, and the mass ratio of the biomass-derived polypropylene-based resin to the fossil fuel-derived polypropylene-based resin in the mixed resin (biomass-derived polypropylene-based resin:fossil fuel-derived polypropylene-based resin) is 3:97 to 97:3. <6> The method for producing expanded polypropylene-based resin beads according to any one of <1> to <5>, wherein the resin layer has a biomass content of 10% or more as measured by ASTM D 6866-21. <7> The method for producing expanded polypropylene-based resin beads according to any one of <1> to <6>, wherein the expanded polypropylene-based resin beads have a biomass degree of 5% or more as measured by ASTM D 6866-21, and the biomass degree of the resin layer as measured by ASTM D 6866-21 is equal to or greater than the biomass degree of the expanded polypropylene-based resin beads. <8> Expanded polypropylene-based resin beads, comprising: an expanded core layer having a polypropylene-based resin as a base resin; and a resin layer coating the expanded core layer and also having a polypropylene-based resin as a base resin, the resin layer comprising a biomass-derived polypropylene resin containing a biomass-derived monomer component in its molecular chain, the expanded polypropylene-based resin beads having a biomass degree of 5% or more as measured by ASTM D 6866-21, and the biomass degree of the resin layer as measured by ASTM D 6866-21 is equal to or greater than the biomass degree of the expanded polypropylene-based resin beads.

[0010] According to the present invention, it is possible to provide expanded polypropylene resin beads containing a biomass-derived polypropylene resin component, which have good moldability in a mold, and a method for producing the same.

[0011] FIG. 2 is a diagram illustrating a DSC curve of expanded polypropylene resin beads at the time of the first heating.

[0012] [Method for producing expanded polypropylene-based resin beads] The method for producing expanded polypropylene-based resin beads of the present invention (hereinafter simply referred to as the method for producing expanded beads of the present invention or the method for producing the present invention) is a method for producing expanded polypropylene-based resin beads by expanding polypropylene-based resin beads, wherein the polypropylene-based resin particles have a core layer whose base resin is a polypropylene-based resin and a resin layer whose base resin is a polypropylene-based resin coating the core layer, the resin layer containing a biomass-derived polypropylene-based resin containing a biomass-derived monomer component in its molecular chain, the content X of the biomass-derived polypropylene-based resin in the resin layer being 50% by mass or more, and the content X (% by mass) of the biomass-derived polypropylene-based resin in the resin layer being equal to or greater than the content Y (% by mass) of the biomass-derived polypropylene-based resin in the core layer. In this specification, polypropylene-based resin refers to a polymer having a content of propylene-derived structural units of 50% by mass or more. In addition, in this specification, biomass means "renewable organic resources derived from living organisms, excluding fossil resources," as described in the "Biomass Nippon Comprehensive Strategy" approved by the Cabinet on March 31, 2006. Furthermore, in this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits.

[0013] In the method for producing expanded beads of the present invention, expanded polypropylene-based resin beads are obtained by expanding polypropylene-based resin beads having the core layer and the resin layer coating the core layer. When expanding the polypropylene-based resin beads, resin beads containing a blowing agent are expanded to obtain the expanded beads. The method for producing expanded beads preferably includes at least the following steps (A) to (C): step (A): preparing polypropylene-based resin beads having a core layer made of a polypropylene-based resin as a base resin and a resin layer coating the core layer, the resin layer also made of a polypropylene-based resin as a base resin; step (B): impregnating the polypropylene-based resin particles with a blowing agent; and step (C): expanding the polypropylene-based resin particles impregnated with the blowing agent to obtain expanded beads having an expanded core layer.

[0014] <Polypropylene-based resin particles> The polypropylene-based resin particles have a core layer made of a polypropylene-based resin as a base resin and a resin layer coated on the core layer, the core layer also made of a polypropylene-based resin as a base resin. In this specification, the core layer made of a polypropylene-based resin as a base resin means that the core layer is composed of a resin mainly made of a polypropylene-based resin. Specifically, the content of the polypropylene-based resin in the core layer is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more. Furthermore, the resin layer made of a polypropylene-based resin as a base resin means that the resin layer is composed of a resin mainly made of a polypropylene-based resin. Specifically, the content of the polypropylene-based resin in the resin layer is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more.

[0015] (Core layer) The core layer is made of a polypropylene-based resin as a base resin. The polypropylene-based resin constituting the core layer may be a biomass-derived polypropylene-based resin described later, a fossil fuel-derived polypropylene-based resin described later, or a mixture thereof. However, as described later, from the viewpoint of increasing the biomass content of the expanded beads as a whole, the polypropylene-based resin constituting the core layer preferably contains a biomass-derived polypropylene-based resin.

[0016] <Biomass-derived polypropylene resin> The biomass-derived polypropylene resin refers to a resin containing a biomass-derived monomer component (a component derived from a biomass-derived monomer) in the molecular chain. The biomass-derived polypropylene resin may be a polypropylene resin polymerized using only a biomass-derived monomer, or may be a polypropylene resin polymerized using a biomass-derived monomer and a fossil fuel-derived monomer.

[0017] Examples of components derived from biomass-derived monomers (biomass-derived monomer components) contained in the molecular chain of biomass-derived polypropylene resins include components derived from propylene, ethylene, or α-olefins having 4 to 8 carbon atoms produced from biomass raw materials. The main component of the biomass-derived monomer component is preferably a component derived from propylene. In this specification, the term "biomass-derived monomer component" refers to a structural unit derived from a monomer in a polymer obtained by polymerizing a monomer such as propylene. The method for producing the biomass-derived monomer is not particularly limited, and the biomass-derived monomer can be obtained by conventionally known methods. For example, the biomass-derived monomer can be obtained by dehydrating alcohol derived from biomass raw materials or by decomposing naphtha derived from biomass raw materials. From the perspective of relatively high supply stability to the market, the biomass-derived monomer is preferably a monomer obtained from bionaphtha. In the examples of the present invention, a biomass-derived polypropylene resin obtained by polymerizing a propylene-containing monomer obtained by decomposing bionaphtha is used. Examples of biomass raw materials used in producing biomass-derived monomers include monosaccharides, polysaccharides, vegetable oils and animal fats obtained from agricultural and livestock products, forestry products, algae, etc. From the viewpoint of low competition with food and the potential for contributing to a recycling-oriented society by utilizing by-products and waste biomass, it is preferable to use biomass-derived monomers produced from at least one biomass raw material selected from waste cooking oil, black liquor, tall oil, palm oil production waste liquor (palm oil mill effluent), oils and fats contained in microalgae, etc.

[0018] Examples of biomass-derived polypropylene-based resins include propylene homopolymers and polypropylene copolymers containing 50 mol% or more of structural units derived from propylene. Examples of polypropylene-based copolymers include copolymers of propylene with ethylene and / or α-olefins having 4 or more carbon atoms, such as ethylene-propylene copolymers, propylene-butene copolymers, and ethylene-propylene-butene copolymers, as well as propylene-acrylic acid copolymers and propylene-maleic anhydride copolymers. These copolymers may be block copolymers, random copolymers, or graft copolymers. Furthermore, the biomass-derived polypropylene-based resin may be a mixture of two or more biomass-derived polypropylene-based resins. Among these, from the viewpoint of relatively stable supply to the market, it is preferable to use one or more selected from the group consisting of propylene homopolymers and ethylene-propylene copolymers, and it is more preferable to use propylene homopolymers.

[0019] From the viewpoint of increasing the mechanical strength of a polypropylene resin expanded bead molded article (hereinafter also referred to simply as expanded bead molded article or molded article) obtained by in-mold molding of the expanded beads obtained by the production method of the present invention while also increasing the in-mold moldability of the expanded beads, the flexural modulus of the biomass-derived polypropylene resin is preferably 800 MPa or more, more preferably 1000 MPa or more, even more preferably 1200 MPa or more, still more preferably 1400 MPa or more, and preferably 2000 MPa or less, more preferably 1800 MPa or less, even more preferably 1700 MPa or less, and still more preferably 1600 MPa or less. Therefore, the flexural modulus of the biomass-derived polypropylene resin is preferably 800 MPa or more and 2000 MPa or less, more preferably 1000 MPa or more and 1800 MPa or less, even more preferably 1200 MPa or more and 1700 MPa or less, and still more preferably 1400 MPa or more and 1600 MPa or less. The flexural modulus of the biomass-derived polypropylene resin can be determined based on JIS K 7171:2016.

[0020] The melting point of the biomass-derived polypropylene resin is preferably 130°C or higher, more preferably 135°C or higher, from the viewpoint of improving the mechanical properties of the resulting molded article. On the other hand, from the viewpoint of easily improving the in-mold moldability of the expanded beads under conditions of low molding pressure, the melting point of the biomass-derived polypropylene resin is preferably 165°C or lower. Therefore, the melting point of the biomass-derived polypropylene resin is preferably 130°C or higher and 165°C or lower, more preferably 135°C or higher and 165°C or lower. The melting point of the biomass-derived polypropylene resin is measured in accordance with JIS K 7121:2012 using the biomass-derived polypropylene resin as a test piece.

[0021] The heat of fusion of the biomass-derived polypropylene resin is preferably 60 J / g or more and 120 J / g or less, or 70 J / g or more and 110 J / g or less, from the viewpoint of improving the mechanical properties of the resulting molded article and facilitating the production of expanded beads with good in-mold moldability. The heat of fusion of the biomass-derived polypropylene resin can be determined from a DSC curve obtained by performing differential scanning calorimetry (DSC) in accordance with JIS K 7122:2012 on a test piece of the biomass-derived polypropylene resin.

[0022] The melt flow rate of the biomass-derived polypropylene resin, measured at a temperature of 230°C and a load of 2.16 kg, is preferably 1 g / 10 min or more and 8 g / 10 min or less, more preferably 2 g / 10 min or more and 5 g / 10 min or less, from the viewpoint of easily improving the in-mold moldability of the expanded beads. The melt flow rate of the biomass-derived polypropylene resin is a value measured at a temperature of 230°C and a load of 2.16 kg. More specifically, it can be measured in accordance with JIS K 7210-1:2014 by the method described in the examples.

[0023] When a mixture of two or more biomass-derived polypropylene resins is used as the biomass-derived polypropylene resin, the various physical properties such as flexural modulus and heat of fusion measured for this mixture are used as the various physical properties such as flexural modulus and heat of fusion of the biomass-derived polypropylene resin.

[0024] <<Fossil Fuel-Derived Polypropylene Resin>> Fossil fuel-derived polypropylene resin refers to a polypropylene resin polymerized substantially exclusively using fossil fuel-derived monomers. Examples of fossil fuel-derived polypropylene resins include conventionally known polypropylene resins, with propylene homopolymers and polypropylene copolymers being preferred. Examples of polypropylene copolymers include copolymers of propylene with ethylene and / or α-olefins having 4 or more carbon atoms, such as ethylene-propylene copolymers, propylene-butene copolymers, and ethylene-propylene-butene copolymers, as well as propylene-acrylic acid copolymers and propylene-maleic anhydride copolymers. These copolymers may be block copolymers, random copolymers, or graft copolymers. Furthermore, the fossil fuel-derived polypropylene resin may be a mixture of two or more fossil fuel-derived polypropylene resins. When the core layer contains a fossil fuel-derived polypropylene resin, the biomass-derived polypropylene resin and the fossil fuel-derived polypropylene resin may be the same or different polypropylene resins. When the core layer contains a fossil fuel-derived polypropylene-based resin, from the viewpoint of easily improving the in-mold moldability of the expanded beads, the fossil fuel-derived polypropylene-based resin is preferably one or more polypropylene-based resins selected from the group consisting of propylene homopolymer, ethylene-propylene copolymer, propylene-butene copolymer, and ethylene-propylene-butene copolymer.

[0025] From the viewpoint of easily improving the in-mold moldability of the expanded beads while containing the biomass-derived polypropylene resin, the flexural modulus of the fossil fuel-derived polypropylene resin is preferably 600 MPa or more, more preferably 700 MPa or more, even more preferably 800 MPa or more, and preferably 1600 MPa or less, more preferably 1500 MPa or less. Therefore, the flexural modulus of the fossil fuel-derived polypropylene resin is preferably 600 MPa or more and 1600 MPa or less, more preferably 700 MPa or more and 1500 MPa or less, even more preferably 800 MPa or more and 1500 MPa or less. Furthermore, in the polypropylene resin constituting the core layer, the ratio of the flexural modulus of the fossil fuel-derived polypropylene resin to the flexural modulus of the biomass-derived polypropylene resin is preferably 0.7 or more and 1.3 or less, more preferably 0.8 or more and 1.2 or less. The flexural modulus of the fossil fuel-derived polypropylene resin can be determined based on JIS K 7171:2016, similarly to the flexural modulus of the biomass-derived polypropylene resin.

[0026] From the viewpoint of easily improving the in-mold moldability of the expanded beads while containing the biomass-derived polypropylene resin, the melting point of the fossil fuel-derived polypropylene resin is preferably 130° C. or higher, more preferably 135° C. or higher, and preferably 165° C. or lower. Therefore, the melting point of the fossil fuel-derived polypropylene resin is preferably 130° C. or higher and 165° C. or lower, more preferably 135° C. or higher and 165° C. or lower. The melting point of the fossil fuel-derived polypropylene resin is measured in accordance with JIS K 7121:2012, in the same manner as for the biomass-derived polypropylene resin, except that the fossil fuel-derived polypropylene resin is used as the test piece.

[0027] The heat of fusion of the fossil fuel-derived polypropylene resin is preferably 60 J / g or more and 120 J / g or less, more preferably 70 J / g or more and 110 J / g or less, from the viewpoint of easily improving the in-mold moldability of the expanded beads while containing the biomass-derived polypropylene resin. Furthermore, in the polypropylene resin constituting the core layer, the ratio of the heat of fusion of the fossil fuel-derived polypropylene resin to the heat of fusion of the biomass-derived polypropylene resin is preferably 0.7 or more and 1.3 or less, more preferably 0.8 or more and 1.2 or less. The heat of fusion of the fossil fuel-derived polypropylene resin can be determined from a DSC curve obtained by differential scanning calorimetry (DSC) in accordance with JIS K 7122:2012, in the same manner as for the biomass-derived polypropylene resin, except that the fossil fuel-derived polypropylene resin is used as the test piece.

[0028] The melt flow rate of the fossil fuel-derived polypropylene resin, measured at a temperature of 230°C and a load of 2.16 kg, is preferably 1 g / 10 min to 15 g / 10 min, more preferably 3 g / 10 min to 12 g / 10 min, from the viewpoint of easily improving the in-mold moldability of the expanded beads. Furthermore, in the polypropylene resin constituting the core layer, the ratio of the melt flow rate of the fossil fuel-derived polypropylene resin to the melt flow rate of the biomass-derived polypropylene resin is preferably 0.2 to 5, more preferably 0.3 to 4. The melt flow rate of the fossil fuel-derived polypropylene resin is a value measured at a temperature of 230°C and a load of 2.16 kg. More specifically, like the melt flow rate of the biomass-derived polypropylene resin, it can be measured in accordance with JIS K 7210-1:2014 by the method described in the Examples.

[0029] When a mixture of two or more fossil fuel-derived polypropylene resins is used as the fossil fuel-derived polypropylene resin, the various physical properties such as flexural modulus and heat of fusion measured for this mixture are used as the various physical properties such as flexural modulus and heat of fusion of the fossil fuel-derived polypropylene resin.

[0030] From the viewpoint of increasing the biomass content of the expanded beads as a whole, the polypropylene resin constituting the core layer preferably contains a biomass-derived polypropylene resin, and from this viewpoint, the content Y of the biomass-derived polypropylene resin in the core layer is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, and still more preferably 50% by mass or more.

[0031] From the viewpoint of easily improving the moldability of the expanded beads in a mold, the polypropylene resin constituting the core layer is preferably a mixed resin of a biomass-derived polypropylene resin and a fossil fuel-derived polypropylene resin. In this case, the mass ratio of the biomass-derived polypropylene resin to the fossil fuel-derived polypropylene resin in the mixed resin (biomass-derived polypropylene resin:fossil fuel-derived polypropylene resin) is preferably 3:97 to 97:3, more preferably 5:95 to 95:5, even more preferably 10:90 to 90:10, and still more preferably 20:80 to 80:20, from the viewpoint of easily improving the moldability of the expanded beads in a mold while containing the biomass-derived polypropylene resin. Furthermore, from the viewpoint of further increasing the content of the biomass-derived polypropylene resin while ensuring the moldability of the expanded beads in a mold, the ratio of the biomass-derived polypropylene resin to the fossil fuel-derived polypropylene resin is preferably 50:50 to 97:3, more preferably 50:50 to 95:5, even more preferably 50:50 to 90:10, and still more preferably 50:50 to 80:20.

[0032] The core layer may contain a polymer other than the polypropylene-based resin described above, as long as the intended effects of the present invention can be achieved. Examples of other polymers include thermoplastic resins other than polypropylene-based resins, such as polyethylene-based resins, polystyrene-based resins, polyamide-based resins, and polyester-based resins, and elastomers, such as olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers. When the core layer contains such other polymers, the content of the other polymers in the core layer is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, and even more preferably 10 parts by mass or less, per 100 parts by mass of the polypropylene-based resin.

[0033] It is preferable that a bubble adjuster be added to the core layer. As the bubble adjuster, for example, one or more selected from inorganic powders and organic powders can be used. Examples of inorganic powders include metal borate salts such as zinc borate and magnesium borate, and examples of organic powders include fluororesin powders such as polytetrafluoroethylene (PTFE). From the viewpoint of stably obtaining expanded beads having a desired bulk density and little variation in bubble diameter, the amount of bubble adjuster added to the core layer is preferably 0.005% by mass or more and 1% by mass or less, more preferably 0.01% by mass or more and 0.5% by mass or less, and even more preferably 0.02% by mass or more and 0.2% by mass or less. Furthermore, from the viewpoint of easily adjusting the average bubble diameter of the expanded beads to a desired range, it is preferable to use a metal borate salt, and more preferably zinc borate, as the bubble adjuster. In addition, additives such as flame retardants, flame retardant assistants, antistatic agents, antioxidants, ultraviolet absorbers, light stabilizers, conductive materials, and colorants may be added to the core layer within the scope that allows the intended object of the present invention to be achieved.

[0034] -Flexural Modulus Fc- From the viewpoint of improving the in-mold moldability of the expanded beads while increasing the mechanical strength of the resulting molded article, the flexural modulus Fc of the polypropylene resin constituting the core layer is preferably 800 MPa or more, more preferably 1000 MPa or more, even more preferably 1200 MPa or more, still more preferably 1400 MPa or more, and preferably 2000 MPa or less, more preferably 1800 MPa or less, even more preferably 1700 MPa or less, and still more preferably 1600 MPa or less. Therefore, the flexural modulus Fc of the polypropylene resin constituting the core layer is preferably 800 MPa or more and 2000 MPa or less, more preferably 1000 MPa or more and 1800 MPa or less, even more preferably 1200 MPa or more and 1700 MPa or less, and still more preferably 1400 MPa or more and 1600 MPa or less. The flexural modulus Fc of the polypropylene resin constituting the core layer can be determined in accordance with JIS K 7171:2016.

[0035] -Heat of fusion ΔHc- The heat of fusion ΔHc of the polypropylene resin constituting the core layer is preferably 60 J / g or more and 120 J / g or less, more preferably 70 J / g or more and 110 J / g or less, from the viewpoint of improving the mechanical properties of the resulting molded article and enhancing moldability in the mold. The heat of fusion of the polypropylene resin constituting the core layer can be determined from a DSC curve obtained by performing differential scanning calorimetry (DSC) in accordance with JIS K 7122:2012 using a polypropylene resin as a test piece. Specifically, it can be measured by the method described in the examples.

[0036] -Biomass Degree Bc- From the viewpoint of further reducing the environmental impact, the biomass degree (biobased carbon content) of the core layer, measured according to ASTM D 6866-21, is preferably 1% or more, more preferably 5% or more, even more preferably 10% or more, and even more preferably 15% or more. The upper limit of the biomass degree Bc of the core layer is not particularly limited, but may be 100%, 90%, 80%, or 60%. The biomass degree Bc of the core layer may be calculated by directly measuring the core layer, or may be calculated from the relationship between the biomass degree of the polypropylene-based resin or the like used to form the core layer or the foamed core layer and its blending amount in the core layer or the foamed core layer.

[0037] In addition, when multiple types of polypropylene-based resins are used to form the core layer, a measurement mixture is prepared by melt-kneading each resin using an extruder or the like at the blending ratio of each polypropylene-based resin when forming the core layer, and the various physical properties measured for the measurement mixture are used as the various physical properties of the polypropylene-based resin that constitutes the core layer.

[0038] (Resin Layer) The resin layer, which uses a polypropylene-based resin as the base resin, covers the core layer. Therefore, the resin layer can also be called a coating layer. The resin layer may cover a portion of the core layer, or may completely cover the entire outer surface of the core layer. The resin layer preferably covers 50% or more of the core layer, more preferably 70% or more, and even more preferably 80% or more. The resin layer preferably does not have a bubble structure and is in a non-foamed state.

[0039] The resin layer contains a biomass-derived polypropylene resin containing a biomass-derived monomer component in its molecular chain. The content X of the biomass-derived polypropylene resin in the resin layer is 50% by mass or more, and the content Y (% by mass) of the biomass-derived polypropylene resin in the core layer is equal to or greater than that. That is, the resin layer is primarily composed of a biomass-derived polypropylene resin. This also means that the content of the biomass-derived polypropylene resin in the resin layer is equal to or greater than the content of the biomass-derived polypropylene resin in the core layer. In expanded polypropylene resin beads having a single-layer (single-layer) structure containing a biomass-derived polypropylene resin as a resin component, the fusion strength between the expanded beads tends to decrease as the content of the biomass-derived polypropylene resin increases, resulting in decreased in-mold moldability. Therefore, in-mold molding is difficult, requiring in-mold molding at extremely high steam pressures to obtain an expanded bead molded article, and there is room for improvement in this regard. In the method for producing expanded polypropylene resin beads of the present invention, the resin layer covering the core layer contains a biomass-derived polypropylene resin as a main component, and the content of the biomass-derived polypropylene resin in the resin layer is equal to or greater than the content of the biomass-derived polypropylene resin in the core layer. This allows the production of expanded beads with good moldability while still containing the biomass-derived polypropylene resin compared to single-layer expanded beads with the same biomass content. While the reason for this is unclear, it is thought that the use of multilayer expanded beads with a resin layer increases the amount of resin in the surface layer of the expanded beads compared to single-layer expanded beads, possibly resulting in an increase in the resin components that can contribute to fusion even at relatively low steam pressures. It is also thought that the effect of including a relatively high amount of biomass-derived polypropylene resin in the resin layer on moldability is less than the effect of including a biomass-derived polypropylene resin in the core layer on moldability. Therefore, it is believed that by incorporating a relatively large amount of biomass-derived polypropylene resin in the resin layer, the content of biomass-derived polypropylene resin in the entire expanded beads can be increased while improving the moldability of the expanded beads in the mold.Furthermore, since the resin layer does not require the addition of a cell control agent, unlike the core layer, it is believed that the amount of cell control agent present on the surface layer of the expanded beads can be reduced, which is thought to reduce the effect of the cell control agent on the fusion properties of the expanded beads, and is also believed to be one factor in improving in-mold moldability.

[0040] The method for producing expanded beads of the present invention can also be said to have the following effects. In single-layer expanded beads, in order to obtain expanded beads with a higher biomass content, it is necessary to increase the content of biomass-derived polypropylene resin in the polypropylene resin constituting the expanded beads. However, this may result in a decrease in in-mold moldability of the expanded beads. On the other hand, if a multi-layer expanded bead having a resin layer and an expanded core layer is produced in which a fossil fuel-derived polypropylene resin, as has been conventionally used, is used as the resin layer to improve the in-mold moldability of the expanded beads, the biomass content of the expanded beads as a whole will decrease. In the expanded beads of the present invention, by using a multi-layer expanded bead having a predetermined amount or more of biomass-derived polypropylene resin in the resin layer and a content of biomass-derived polypropylene resin in the resin layer that is equal to or greater than the content of biomass-derived polypropylene resin in the foam core layer, even when expanding beads with a higher biomass content as a whole can be obtained, thereby maintaining a high biomass content and achieving good in-mold moldability.

[0041] From the viewpoint of increasing the biomass-derived polypropylene resin content of the expanded beads as a whole while ensuring good moldability of the expanded beads, the content X of the biomass-derived polypropylene resin in the resin layer is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and even more preferably 90% by mass or more. Furthermore, from the viewpoint of easily increasing the moldability of the expanded beads as a whole while containing the biomass-derived polypropylene resin, the difference (X-Y) between the content X of the biomass-derived polypropylene resin in the resin layer and the content Y of the biomass-derived polypropylene resin in the core layer is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, and preferably 90% by mass or less, more preferably 80% by mass or less. Therefore, the difference (X-Y) is preferably 10% by mass or more and 90% by mass or less, more preferably 20% by mass or more and 80% by mass or less, even more preferably 30% by mass or more and 80% by mass or less. The content X of the biomass-derived polypropylene resin in the resin layer is the content (mass%) of the biomass-derived polypropylene resin in the resin layer when the resin layer is taken as 100% by mass, and the content Y of the biomass-derived polypropylene resin in the core layer is the content (mass%) of the biomass-derived polypropylene resin in the core layer when the core layer is taken as 100% by mass.

[0042] The mass proportion of the resin layer in the polypropylene-based resin particles (the mass proportion of the resin layer in the resin particles when the resin particles are 100% by mass) is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, even more preferably 1% by mass or more, still more preferably 2% by mass or more, and preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 12% by mass or less, from the viewpoint of increasing the content of biomass-derived polypropylene-based resin while more stably improving the in-mold moldability of the expanded beads. Therefore, the mass proportion of the resin layer in the polypropylene-based resin particles is preferably 0.1% by mass or more and 20% by mass or less, more preferably 0.5% by mass or more and 15% by mass or less, even more preferably 1% by mass or more and 12% by mass or less. The mass proportion of the resin layer in the polypropylene-based resin particles usually corresponds to the mass proportion of the resin layer in the expanded polypropylene-based resin beads produced using the resin particles.

[0043] <<Flexural Modulus Fr>> The flexural modulus Fr of the polypropylene resin constituting the resin layer is preferably 800 MPa or more, more preferably 900 MPa or more, even more preferably 950 MPa or more, from the viewpoint of increasing the content of biomass-derived polypropylene resin in the expanded beads as a whole while easily improving the in-mold moldability of the expanded beads. It is preferably 1700 MPa or less, more preferably 1600 MPa or less, and even more preferably 1550 MPa or less. Therefore, the flexural modulus Fr is preferably 800 MPa or more and 1700 MPa or less, more preferably 900 MPa or more and 1600 MPa or less, and even more preferably 950 MPa or more and 1550 MPa or less. The flexural modulus Fr of the polypropylene resin constituting the resin layer can be determined based on JIS K 7171:2016.

[0044] <Flexural Modulus Ratio Fr / Fc> From the viewpoint of being able to stably obtain expanded beads with good in-mold moldability while increasing the content of biomass-derived polypropylene resin in the expanded beads as a whole, the ratio Fr / Fc of the flexural modulus Fr of the polypropylene resin constituting the resin layer to the flexural modulus Fc of the polypropylene resin constituting the core layer is preferably 0.5 or more, more preferably 0.6 or more, even more preferably 0.7 or more, still more preferably 0.8 or more, and preferably 2 or less, more preferably 1.5 or less, even more preferably 1.3 or less, and still more preferably 1.2 or less. Thus, the ratio Fr / Fc is preferably 0.5 or more and 2 or less, more preferably 0.6 or more and 1.5 or less, even more preferably 0.7 or more and 1.3 or less, and still more preferably 0.8 or more and 1.2 or less.

[0045] The polypropylene-based resin constituting the resin layer can be the biomass-derived polypropylene-based resin described in the core layer as the main component. Furthermore, in addition to the biomass-derived polypropylene-based resin, the fossil fuel-derived polypropylene-based resin described in the core layer can also be used. The polypropylene-based resin constituting the resin layer is preferably one or more polypropylene-based resins selected from the group consisting of propylene homopolymer, ethylene-propylene copolymer, propylene-butene copolymer, and ethylene-propylene-butene copolymer. Regarding the biomass-derived polypropylene-based resin and fossil fuel-derived polypropylene-based resin in the resin layer, the descriptions of the biomass-derived polypropylene-based resin and fossil fuel-derived polypropylene-based resin in the core layer can be referenced as appropriate.

[0046] <Biomass Degree Br> The biomass degree (biobased carbon content) Br of the resin layer, measured according to ASTM D 6866-21, is preferably 10% or more, more preferably 20% or more, from the viewpoint of further reducing the environmental load. The upper limit of the biomass degree Br of the resin layer is not particularly limited, but may be 100%, 90%, 80%, or 60%. The biomass degree Br of the resin layer may be calculated by directly measuring the resin layer, or may be calculated from the relationship between the biomass degree of the polypropylene resin or the like used to form the resin layer and its blending amount in the resin layer. From the viewpoint of easily improving the in-mold moldability of the expanded beads while containing a biomass-derived polypropylene resin, the difference (Br - Bc) between the biomass degree Br of the resin layer and the biomass degree Bc of the core layer measured by ASTM D 6866-21 is preferably 5% or more, more preferably 10% or more, and preferably 100% or less, more preferably 90% or less, even more preferably 80% or less, and still more preferably 60% or less. Thus, the difference (Br - Bc) is preferably 5% or more and 100% or less, more preferably 10% or more and 90% or less, even more preferably 10% or more and 80% or less, and still more preferably 10% or more and 60% or less.

[0047] <<Heat of Fusion ΔHr>> The heat of fusion ΔHr of the polypropylene-based resin constituting the resin layer is preferably 70 J / g or more and 100 J / g or less, more preferably 75 J / g or more and 95 J / g or less, from the viewpoint of increasing the content of biomass-derived polypropylene-based resin in the entire expanded beads while easily improving the in-mold moldability of the expanded beads. Furthermore, the ratio (ΔHr / ΔHc) of the heat of fusion ΔHr of the polypropylene-based resin constituting the resin layer to the heat of fusion ΔHc of the polypropylene-based resin constituting the core layer is preferably 0.5 or more and 2 or less, more preferably 0.6 or more and 1 or less. The heat of fusion of the polypropylene-based resin constituting the resin layer can be determined from a DSC curve obtained by performing differential scanning calorimetry (DSC) in accordance with JIS K 7122:2012 on a polypropylene-based resin test piece. Specifically, it can be measured by the method described in the examples.

[0048] In addition, when multiple types of polypropylene-based resins are used to form the resin layer, a measurement mixture is prepared by melt-kneading each resin using an extruder or the like at the blending ratio of each polypropylene-based resin when forming the resin layer, and the various physical properties measured for the measurement mixture are used as the various physical properties of the polypropylene-based resin that constitutes the resin layer.

[0049] The resin layer may contain a polymer other than the polypropylene-based resin described above, as long as the intended effects of the present invention can be achieved. Examples of the other polymer include thermoplastic resins other than polypropylene-based resins, such as polyethylene-based resins, polystyrene-based resins, polyamide-based resins, and polyester-based resins, and elastomers such as olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers. When the resin layer contains the other polymer, the content of the other polymer in the resin layer is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, and even more preferably 10 parts by mass or less, per 100 parts by mass of the polypropylene-based resin.

[0050] Furthermore, the resin layer may contain additives such as flame retardants, flame retardant auxiliaries, antistatic agents, antioxidants, ultraviolet absorbers, light stabilizers, conductive materials, colorants, etc. From the viewpoint of stably improving the in-mold moldability of the expanded beads, it is preferable that the resin layer does not substantially contain a cell control agent, or that the resin layer contains a cell control agent and the content of the cell control agent in the resin layer is lower than the content of the cell control agent in the core layer.

[0051] <Step (A): Preparation of Polypropylene-Based Resin Particles> In step (A), for example, resin particles are granulated using an extrusion device having a core layer forming extruder, a resin layer (coating layer) forming extruder, and a co-extrusion die such as a multilayer strand forming die connected at the outlet side of these extruders, thereby preparing resin particles having a core layer and a resin layer coating the core layer. The core layer forming extruder is supplied with a polypropylene-based resin as the base resin of the core layer and additives added as needed, and melt-kneaded to form a resin melt for forming the core layer. The resin layer (coating layer) forming extruder is supplied with a polypropylene-based resin as the base resin of the resin layer, and melt-kneaded to form a resin melt for forming the resin layer (coating layer). The resin melt for forming the core layer and the resin melt for forming the resin layer (coating layer) are introduced into a co-extrusion die and merged to form a multilayered composite. The composite is extruded from the extrusion device and granulated to a predetermined mass, thereby obtaining multilayered polypropylene resin particles having a non-foamed core layer and a non-foamed coating layer that coats the core layer. The multilayered polypropylene resin particles can be granulated by a method such as a strand-cutting method in which the composite is extruded in the form of a strand from a small hole in a die attached to the downstream side of the extrusion device, cooled in water, and then cut, an underwater cutting method in which the composite is extruded into water and then cut, or a hot cutting method in which the composite is extruded into air and then cut immediately thereafter.

[0052] (Mass of Resin Particles) The average mass per polypropylene resin particle is preferably adjusted to 0.1 to 20 mg, more preferably 0.2 to 10 mg, even more preferably 0.3 to 5 mg, and still more preferably 0.4 to 2 mg.

[0053] (Shape of Resin Particles) The external shape of the polypropylene-based resin particles is not particularly limited as long as it is within a range that allows the intended object of the present invention to be achieved, but is preferably cylindrical. When the external shape of the resin particles is cylindrical, the particle diameter (length in the extrusion direction) of the resin particles is preferably 0.2 to 4 mm, more preferably 0.5 to 3 mm. Furthermore, the ratio (length / diameter ratio) of the length of the resin particles in the extrusion direction to the length of the resin particles in the direction perpendicular to the extrusion direction (diameter of the resin particles) is preferably 0.5 to 5.0, more preferably 1.0 to 3.0.

[0054] <Step (B): Preparation of polypropylene-based resin particles containing a blowing agent> In step (B), the polypropylene-based resin constituting the core layer is impregnated with a blowing agent to obtain polypropylene-based resin particles containing a blowing agent. In step (B), for example, a dispersion medium and polypropylene-based resin particles are placed in a sealed container such as an autoclave that can be sealed and can withstand heat and pressure, and the polypropylene-based resin particles are dispersed in the dispersion medium using a stirrer or the like, and a blowing agent is added to the sealed container and maintained under a predetermined temperature and pressure atmosphere, thereby impregnating the polypropylene-based resin particles with the blowing agent.

[0055] The dispersion medium is not particularly limited as long as it does not dissolve the polypropylene-based resin particles. For example, water, ethylene glycol, glycerin, and alcohols such as methanol and ethanol can be used, and among these, water is preferably used.

[0056] In order to more stably prevent the polypropylene resin particles from adhering to each other, it is preferable to further add a dispersant to the dispersion medium. Examples of dispersants include organic dispersants such as polyvinyl alcohol, polyvinylpyrrolidone, and methyl cellulose; and sparingly soluble inorganic salts such as aluminum oxide, zinc oxide, kaolin, mica, magnesium phosphate, and tricalcium phosphate. These can be used alone or in combination of two or more. Among these, the dispersant is preferably a sparingly soluble inorganic salt, and more preferably kaolin, from the perspective of ease of handling. When a dispersant is added, it is preferable to add the dispersant in an amount of about 0.001 to 5 parts by mass per 100 parts by mass of the polypropylene resin particles.

[0057] A surfactant can also be added to the dispersion medium. Examples of surfactants include sodium dodecylbenzenesulfonate, sodium alkylsulfonate, sodium oleate, sodium lauryl sulfate, polyoxyethylene alkyl ether sodium phosphate, polyoxyethylene alkyl ether sodium sulfate, and other anionic surfactants and nonionic surfactants commonly used in suspension polymerization. When a surfactant is added, it is preferable to add the surfactant in an amount of about 0.001 to 1 part by mass per 100 parts by mass of the polypropylene resin particles.

[0058] The blowing agent is not particularly limited as long as it can expand the polypropylene resin particles. Examples of the blowing agent include inorganic physical blowing agents such as air, nitrogen, carbon dioxide, argon, helium, oxygen, and neon; aliphatic hydrocarbons such as propane, normal butane, isobutane, normal pentane, isopentane, and normal hexane; alicyclic hydrocarbons such as cyclohexane and cyclopentane; halogenated hydrocarbons such as ethyl chloride, 2,3,3,3-tetrafluoropropene, trans-1,3,3,3-tetrafluoropropene, and trans-1-chloro-3,3,3-trifluoropropene; and organic physical blowing agents such as dialkyl ethers such as dimethyl ether, diethyl ether, and methyl ethyl ether. Among these, from the viewpoints of environmental friendliness and economic efficiency, inorganic physical blowing agents are preferred, more preferably at least one selected from the group consisting of nitrogen, air, and carbon dioxide, and even more preferably carbon dioxide. These blowing agents can be used alone or in combination of two or more.

[0059] The amount of foaming agent to be added is determined in consideration of the desired bulk density of the expanded polypropylene resin particles, the type of polypropylene resin, the type of foaming agent, etc. For example, when an inorganic physical foaming agent is used, the amount to be added is preferably 0.1 to 30 parts by mass, more preferably 0.5 to 15 parts by mass, per 100 parts by mass of the polypropylene resin particles.

[0060] The heating temperature in step (B) is preferably 100 to 180° C., more preferably 130 to 175° C. The time for maintaining the heating temperature is preferably 1 to 100 minutes, more preferably 10 to 60 minutes.

[0061] <Step (C)> In step (C), for example, the polypropylene-based resin particles containing a blowing agent obtained in step (B) are expanded to obtain expanded beads having a foamed core layer. In step (C), it is preferable to expand the core layer by releasing the polypropylene-based resin particles containing a blowing agent together with a dispersion medium from a sealed container into an atmosphere with a pressure lower than that inside the sealed container. This allows for the production of expanded polypropylene-based resin beads having a foamed core layer formed by the expansion of the core layer. Specifically, while maintaining the pressure inside the sealed container at a pressure equal to or higher than the vapor pressure of the blowing agent, one end of the sealed container below the water surface is opened, and the polypropylene-based resin particles containing a blowing agent together with the dispersion medium are released from the sealed container into an atmosphere with a pressure lower than that inside the sealed container, usually atmospheric pressure, to expand the core layer of the polypropylene-based resin particles and form an expanded core layer. This allows for the production of expanded polypropylene-based resin beads having a multilayer structure having an expanded core layer and a resin layer (coating layer) covering the expanded core layer.

[0062] When polypropylene resin particles are released from a sealed container into an atmosphere with a lower pressure than the pressure inside the sealed container to cause foaming, the temperature during foaming is usually preferably 110 to 170° C. The pressure inside the sealed container is preferably 0.5 MPa (G) to 5 MPa (G). The pressures indicated with (G) are gauge pressures, i.e., pressure values ​​based on atmospheric pressure.

[0063] The above steps (B) and (C) may be carried out as separate steps, but from the viewpoint of increasing the productivity of expanded beads, it is preferable to carry out them as a series of steps using a single sealed container. The expanded beads can be preferably produced by a method including the above steps (A) to (C), but the expanded beads can also be produced, for example, by impregnating resin particles with a blowing agent in a sealed container, removing the resin particles containing the blowing agent from the sealed container, and heating the resin particles containing the blowing agent with a heating medium such as steam to expand them.

[0064] In addition, expanded polypropylene resin beads obtained as described above can be pressurized with air or the like to increase the internal pressure within the cells of the expanded beads, and then the expanded beads can be heated with steam or the like to expand (two-stage expansion), thereby obtaining expanded beads with an even higher expansion ratio (lower bulk density).

[0065] [Polypropylene-Based Resin Expanded Beads] The polypropylene-based resin expanded beads of the present invention (hereinafter also simply referred to as expanded polypropylene-based resin beads or expanded beads) have an expanded core layer whose base resin is a polypropylene-based resin, and a resin layer whose base resin is a polypropylene-based resin and covers the expanded core layer, wherein the resin layer contains a biomass-derived polypropylene-based resin that contains a biomass-derived monomer component in its molecular chain, and the biomass degree of the expanded polypropylene-based resin beads measured according to ASTM D 6866-21 is 5% or more, and the biomass degree of the resin layer measured according to ASTM D 6866-21 is equal to or greater than the biomass degree of the expanded polypropylene-based resin beads.

[0066] <Biomass Degree Be> From the viewpoint of reducing environmental impact, the biomass degree Be of the expanded polypropylene resin beads, as measured according to ASTM D 6866-21, is preferably 5% or more, more preferably 10% or more, and even more preferably 12% or more. The upper limit of the biomass degree Be of the expanded beads is not particularly limited, but may be 100%, 90%, 80%, or 60%. The biomass degree Be of the expanded beads may be measured directly on the expanded beads, or calculated from the relationship between the biomass degree of the resin beads or the polypropylene resin used to produce the expanded beads and their blending amounts. The difference (Br - Be) between the biomass degree Br of the resin layer and the biomass degree Be of the expanded polypropylene resin beads is 0% or more, preferably 5% or more, more preferably 10% or more, and preferably 100% or less, more preferably 90% or less, even more preferably 80% or less, and even more preferably 60% or less. Therefore, the difference (Br-Be) is preferably 0% or more and 90% or less, more preferably 5% or more and 80% or less, and even more preferably 10% or more and 60% or less.

[0067] <Bulk Density> The bulk density of the expanded polypropylene resin beads is preferably 10 kg / m from the viewpoint of increasing the mechanical strength of the expanded bead molding. ３ More preferably, 15 kg / m ３ More preferably, 20 kg / m ３ In addition, from the viewpoint of increasing the lightness of the expanded bead molding, the expanded polypropylene resin beads preferably have a strength of 200 kg / m ３ or less, more preferably 100 kg / m ３ More preferably 60 kg / m or less ３ More preferably, 50 kg / m or less ３ Therefore, the bulk density of the expanded polypropylene resin particles is preferably 10 kg / m or less. ３ Above 200 kg / m ３ More preferably, it is 15 kg / m or less. ３ Above 100 kg / m ３ More preferably, it is 20 kg / m or less. ３ Above 60 kg / m ３ and even more preferably 20 kg / m or less. ３ Above 50 kg / m ３ The bulk density of expanded beads is determined as follows. First, a measuring cylinder is filled with expanded beads having a mass W1 [g], and the bottom of the measuring cylinder is lightly tapped on the floor several times to stabilize the filling height of the expanded beads in the measuring cylinder. Next, the volume V1 [L] of the expanded beads indicated on the measuring cylinder scale is read. The mass W1 of the expanded beads is divided by the volume V1 (W1 / V1) and the unit is expressed as [kg / m ３ The bulk density of the expanded beads can be determined by converting the value of the bulk density into the value of the particle diameter.

[0068] <High-Temperature Peak> The expanded polypropylene resin beads preferably have a crystalline structure in which a melting peak due to the melting of crystals inherent to the polypropylene resin (i.e., the resin-specific peak) and one or more melting peaks (i.e., high-temperature peaks) appear on the higher temperature side of the DSC curve obtained when the expanded beads are heated from 23°C to 200°C at a heating rate of 10°C / min. The DSC curve is obtained by performing differential scanning calorimetry (DSC) in accordance with JIS K 7121:1987 using 1 to 3 mg of expanded beads as a test sample. The resin-specific peak is a melting peak due to the melting of crystals inherent to the polypropylene resin constituting the expanded beads, and is considered to be an endothermic peak that appears due to the endothermic heat that occurs when crystals typically possessed by polypropylene resins melt. On the other hand, the melting peak on the higher temperature side of the resin-specific peak (i.e., the high-temperature peak) is a melting peak that appears on the higher temperature side of the resin-specific peak on the DSC curve. The appearance of this high-temperature peak suggests the presence of secondary crystals in the resin. In addition, in the DSC curve obtained by heating the expanded beads from 23°C to 200°C at a heating rate of 10°C / min (i.e., the first heating), cooling from 200°C to 23°C at a cooling rate of 10°C / min, and then heating again from 23°C to 200°C at a heating rate of 10°C / min (i.e., the second heating), it is preferable that only a melting peak due to the melting of crystals specific to the polypropylene-based resin constituting the expanded beads appears. In this way, the resin-specific peak and the high-temperature peak can be distinguished. The high-temperature peak can be adjusted, for example, by controlling the rate of temperature rise in the sealed container or by maintaining the temperature in the sealed container at a predetermined temperature for a predetermined period of time in the above-mentioned step (B). Specifically, for example, in step (B), a one-stage holding step is performed in which the temperature in the sealed container is maintained at a temperature equal to or higher than (the melting point of the polypropylene-based resin - 20°C) and lower than (the end temperature of melting of the propylene-based resin) for approximately 10 to 60 minutes. Thereafter, the temperature inside the sealed container is adjusted to a temperature between (the melting point of the polypropylene-based resin - 15°C) and below (the melting end temperature of the polypropylene-based resin). At this time, if necessary, a second-stage holding step may be performed in which the temperature is held for an additional 10 to 60 minutes. Thereafter, by carrying out step (C), expanded polypropylene-based resin beads having a high-temperature peak can be produced.

[0069] <Heat of fusion of high-temperature peak ΔH2> The heat of fusion of the high-temperature peak of the expanded polypropylene resin beads is preferably 10 to 50 J / g, more preferably 15 to 40 J / g, and even more preferably 20 to 35 J / g, from the viewpoint of improving the in-mold moldability of the expanded beads and obtaining an expanded bead molding having an excellent balance between cushioning properties and rigidity. In the DSC curve (DSC curve at the first heating) obtained when the expanded polypropylene resin beads are heated from 23°C to 200°C at a heating rate of 10°C / min, the resin-specific peak P １ and a high-temperature peak P ２ An example of a DSC curve in which a high-temperature peak P appears is shown in FIG. ２ appears, its heat of fusion can be determined as follows. First, the point on the DSC curve at a temperature of 80°C is designated as α, and the point on the DSC curve corresponding to the melting end temperature T is designated as β, and a line L1 is drawn connecting these. Next, a line L2 parallel to the vertical axis of the graph is drawn from point γ on the DSC curve corresponding to the valley between the resin's intrinsic peak and the high-temperature peak, and the point where it intersects with line L1 is designated as δ. The area (2) enclosed by the curve of the high-temperature peak portion of the DSC curve, the line segment (δ-β), and the line segment L2 is defined as the area of ​​the high-temperature peak, and the heat of fusion ΔH2 of the high-temperature peak can be determined from this area.

[0070] <Total Heat of Fusion ΔH> From the viewpoint of stably obtaining expanded beads with good in-mold moldability while increasing the mechanical strength of the resulting expanded bead molding, the total heat of fusion ΔH of the expanded polypropylene resin beads is preferably 60 J / g or more, more preferably 70 J / g or more, even more preferably 80 J / g or more, and preferably 120 J / g or less, more preferably 110 J / g or less. Therefore, the total heat of fusion of the expanded polypropylene resin beads is preferably 60 J / g or more and 120 J / g or less, more preferably 70 J / g or more and 110 J / g or less, even more preferably 80 J / g or more and 110 J / g or less. The total heat of fusion ΔH of the expanded polypropylene resin beads can be determined from a DSC curve obtained by performing differential scanning calorimetry (DSC) in accordance with JIS K 7122:2012 using the expanded beads as a test piece. Specifically, the test specimen was first conditioned using "(2) Measurement of melting temperature after a certain heat treatment," in which the test specimen was heated from 23°C to 200°C at a heating rate of 10°C / min. After reaching 200°C, the test specimen was cooled from 200°C to 23°C at a rate of 10°C / min, and then heated a second time from 23°C to 200°C at a rate of 10°C / min to obtain a DSC curve (DSC curve during second heating). The point at a temperature of 80°C on the obtained DSC curve during second heating is designated as α, and the point on the DSC curve corresponding to the melting end temperature is designated as β. The area enclosed by the DSC curve in the section between points α and β and the line segment (α-β) was measured, and the total heat of fusion of the expanded beads could be calculated from this area.

[0071] <Ratio (ΔH2 / ΔH)> When the expanded polypropylene resin beads have a high-temperature peak, the ratio (ΔH2 / ΔH) of the heat of fusion ΔH2 of the high-temperature peak of the expanded polypropylene resin beads to the total heat of fusion ΔH of the expanded polypropylene resin beads is preferably 0.1 or more and 0.5 or less, more preferably 0.2 or more and 0.4 or less, from the viewpoint of improving the in-mold moldability of the expanded beads while making it easier to obtain an expanded bead molding having excellent mechanical strength.

[0072] [Polypropylene Resin Expanded Bead Molded Article] A polypropylene resin expanded bead molded article can be obtained by molding the polypropylene resin expanded beads in a mold.

[0073] The in-mold molding method can be performed by filling a mold with expanded beads and heat-molding them using a heating medium such as steam. Specifically, after filling the mold with the expanded beads, a heating medium such as steam is introduced into the mold to heat and expand the expanded beads and fuse them together, thereby obtaining an expanded bead molded article having the shape of the molding space. The in-mold molding method of the present invention can be performed by a pressure molding method (e.g., JP-B 51-22951) in which the expanded beads are pre-pressurized with a pressurized gas such as air to increase the pressure within the cells of the expanded beads and adjust the pressure within the expanded beads to a pressure 0.01 to 0.3 MPa higher than atmospheric pressure, and then the expanded beads are filled into the mold under atmospheric pressure or reduced pressure, and a heating medium such as steam is then supplied into the mold to heat-fuse the expanded beads. Alternatively, molding can be performed by a compression filling molding method (Japanese Patent Publication No. 4-46217) in which a mold pressurized to atmospheric pressure or higher by a compressed gas is filled with expanded beads pressurized to atmospheric pressure or higher, and then a heating medium such as steam is supplied into the cavity to heat and fuse the expanded beads. Alternatively, molding can be performed by a normal pressure filling molding method (Japanese Patent Publication No. 6-49795) in which expanded beads with high secondary expansion power obtained under special conditions are filled into the cavity of a mold under atmospheric or reduced pressure, and then a heating medium such as steam is supplied to heat and fuse the expanded beads, or a combination of the above methods (Japanese Patent Publication No. 6-22919).

[0074] <Density> The density of the expanded polypropylene resin bead molded product is preferably 10 kg / m from the viewpoint of easily obtaining a molded product having a good balance between mechanical strength and light weight. ３ More preferably, 15 kg / m ３ More preferably, 20 kg / m ３ and preferably 200 kg / m ３ or less, more preferably 100 kg / m ３ More preferably, 80 kg / m or less３ More preferably, 60 kg / m or less ３ Therefore, the density of the expanded polypropylene resin bead molding is preferably 10 kg / m or less. ３ More than 200kg / m ３ More preferably, it is 15 kg / m or less. ３ More than 100kg / m ３ More preferably, it is 20 kg / m or less. ３ More than 80kg / m ３ More preferably, it is 20 kg / m or less. ３ More than 60kg / m ３ The density of the expanded polypropylene resin bead molding is calculated by dividing the mass of the expanded bead molding by the volume determined from the outer dimensions of the molding, and converting the result into units.

[0075] <Compressive stress at 50% strain> The compressive stress at 50% strain of the expanded polypropylene resin bead molding is preferably 100 kPa or more, more preferably 200 kPa or more, from the viewpoint of obtaining a molding that is resistant to excessive deformation due to stress. On the other hand, the upper limit is not particularly limited, but is preferably 1 MPa, more preferably 800 kPa. The compressive stress at 50% strain of the expanded polypropylene resin bead molding is measured based on the method specified in JIS K 6767:1999.

[0076] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0077] The raw materials, expanded beads, and expanded bead molded articles of the Examples and Comparative Examples were measured and evaluated as follows. The physical properties of the expanded beads were measured using expanded beads that had been conditioned by standing at 50% RH, 23°C, and 1 atm for 24 hours. The physical properties of the expanded bead molded articles were measured and evaluated using expanded bead molded articles that had been conditioned by standing at 50% RH, 80°C, and 1 atm for 12 hours after demolding.

[0078] [Measurement Method] <Resin Raw Material> (Biomass Degree) The biomass degree of a biomass-derived polypropylene resin used as a resin raw material was measured as follows based on ASTM D6866-21. By burning the raw material, carbon dioxide (CO ２ ) was generated and the carbon dioxide was purified in a vacuum line. The purified carbon dioxide was reduced with hydrogen using iron as a catalyst, thereby producing graphite (C). The graphite was then packed into a cathode with an inner diameter of 1 mm using a hand press, which was then fitted into a wheel and used in a tandem accelerator based on NEC Corporation. １４ The measurement was carried out using a dedicated C-AMS device. １４ The number of C's, １３ C concentration ( １３ C / １２ C), １４ C concentration ( １４ C / １２ C) was measured. In the measurement, oxalic acid (HOxII) provided by the National Institute of Standards (NIST) was used as the standard sample. Measurements of this standard sample and a background sample were also carried out at the same time. From the measurement results obtained, the carbon content of the measured sample relative to the modern carbon content of the standard sample was １４ Calculate the C ratio, then calculate the C ratio from the standard sample. １３ By correcting for the deviation in the C concentration, a corrected pMC (percent modern carbon) value was obtained. The biomass degree was calculated using the corrected pMC value. The atmospheric correction factor used was the value for 2019-2021 described in ASTM D6866-21 (100.0 pMC). To measure the biomass degree of the polypropylene-based resin according to the present invention, the atmospheric correction factor described in ASTM D6866 for the year the polypropylene-based resin was produced was used to determine the biomass degree.

[0079] (Flexural modulus) The flexural modulus of the polypropylene-based resin used as the resin raw material was measured in accordance with JIS K 7171:2016. First, the polypropylene-based resin was heat-pressed at 230 ° C to prepare a 4 mm thick sheet, and a length of 80 mm x width of 10 mm x thickness of 4 mm (standard test piece) was cut out from the sheet. Using this test piece, a bending test was performed with the indenter radius R1 and the support table radius R2 both set to 5 mm, the support distance set to 64 mm, and the test speed set to 2 mm / min. The flexural modulus of the polypropylene-based resin was measured from the results of the bending test.

[0080] (Density) The density of the polypropylene resin used as the resin raw material was measured in accordance with Method B (pycnometer method) described in JIS K 7112:1999.

[0081] (Heat of fusion) The heat of fusion of the polypropylene-based resin used as the resin raw material was determined by heat flux differential scanning calorimetry according to JIS K 7122:2012 as follows. A high-sensitivity differential scanning calorimeter "EXSTAR DSC7020" (manufactured by Hitachi High-Tech Science Corporation) was used as the measuring device. The test specimen was conditioned using "(2) Measurement of melting temperature after a certain heat treatment." Approximately 2 mg of polypropylene-based resin was collected as a test specimen. The test specimen was heated from 23°C to 200°C at a heating rate of 10°C / min under conditions of a nitrogen inflow rate of 30 mL / min, then held at that temperature for 10 minutes, cooled to 23°C at a cooling rate of 10°C / min, and then heated again to 200°C at a heating rate of 10°C / min to obtain a DSC curve (DSC curve at the second heating). The point at 80°C on the DSC curve obtained during the second heating was designated as α, and the point on the DSC curve corresponding to the melting end temperature was designated as β. The area of ​​the portion enclosed by the DSC curve in the section between points α and β and the line segment (α-β) was measured, and the heat of fusion of the polypropylene-based resin was calculated from this area.

[0082] (Melting Point) The melting point of the polypropylene resin used as the resin raw material was measured by heat flux differential scanning calorimetry in accordance with JIS K 7121:2012. A high-sensitivity differential scanning calorimeter "EXSTAR DSC7020" (manufactured by Hitachi High-Tech Science Corporation) was used as the measuring device. The test specimen was conditioned using "(2) Measurement of melting temperature after a certain heat treatment." Approximately 2 mg of polypropylene resin was collected as a test specimen. The test specimen was heated from 23°C to 200°C at a heating rate of 10°C / min under conditions of a nitrogen inflow rate of 30 mL / min, then held at that temperature for 10 minutes, cooled to 23°C at a cooling rate of 10°C / min, and then heated again to 200°C at a heating rate of 10°C / min to obtain a DSC curve (DSC curve at the second heating). The apex temperature of the melting peak in the DSC curve was determined, and this value was taken as the melting point. In addition, when a plurality of melting peaks appear in a DSC curve, the apex temperature of the melting peak with the largest area is adopted as the melting point. In this case, the melting peaks are distinguished using the temperature of the valley of the DSC curve located between the apex temperatures of the melting peaks as a boundary, and the areas (heat of fusion) of the melting peaks are compared, thereby determining the melting peak with the largest area. The valley temperature of the DSC curve corresponds to the temperature at which the value on the vertical axis of the differential curve of the DSC curve (DDSC) becomes 0, so it can also be determined from the differential curve of the DSC curve.

[0083] (Melt Flow Rate) The melt flow rate of the polypropylene resin used as the resin raw material was measured in accordance with JIS K 7210-1:2014 under conditions of a temperature of 230°C and a load of 2.16 kg.

[0084] <Polypropylene-based resin expanded beads> (Heat of fusion of high-temperature peak) The heat of fusion of the high-temperature peak of the polypropylene-based resin expanded beads was determined by heat flux differential scanning calorimetry in accordance with JIS K 7122:2012 as follows. Specifically, approximately 2 mg of the expanded beads was collected as a test piece and heated from 23°C to 200°C at a heating rate of 10°C / min using a differential scanning calorimeter "EXSTAR DSC7020" (manufactured by Hitachi High-Tech Science Corporation) to obtain a DSC curve (DSC curve in the first heating) having two or more melting peaks. The measurement was performed under conditions of a nitrogen inflow rate of 30 mL / min. With respect to the obtained DSC curve, the characteristic peak of the polypropylene-based resin constituting the expanded beads was determined as P １ The high-temperature peak that appears on the higher temperature side is P ２ A straight line (α-β) was drawn connecting point α on the DSC curve corresponding to 80°C and point β on the DSC curve corresponding to the melting end temperature T of the test piece. ２ The intrinsic peak P is the end point on the high temperature side of the peak P and is the intersection point between the high temperature peak and the high temperature base line. １ and high-temperature peak P ２ A straight line parallel to the vertical axis of the graph was drawn from point γ on the DSC curve, which corresponds to the valley between the points γ and δ, and the point where the straight line intersects with the line was taken as δ. ２ The area surrounded by the curve of the part, the line segment (δ-β), and the line segment (γ-δ) was determined, and the heat of fusion of each high-temperature peak was calculated from this area. The heat of fusion of the high-temperature peak was measured for three different test pieces, and the arithmetic mean value of the obtained values ​​was defined as the heat of fusion ΔH2 of the high-temperature peak of the expanded beads.

[0085] (Total Heat of Fusion) The total heat of fusion of the expanded polypropylene-based resin beads was measured by heat flux differential scanning calorimetry in accordance with JIS K 7122:2012 in the same manner as in the above-described heat of fusion of the polypropylene-based resin, except that expanded polypropylene-based resin beads were used as test pieces.

[0086] (Bulk Density) The bulk density of the expanded polypropylene resin beads was determined as follows. First, a measuring cylinder was filled with expanded beads having a mass W1 [g], and the bottom of the measuring cylinder was lightly tapped against the floor several times to stabilize the filling height of the expanded beads in the measuring cylinder. Next, the volume V1 [L] of the expanded beads indicated on the measuring cylinder was read. The mass W1 [g] of the expanded beads was divided by the volume V1 [L] (W1 / V1) and the unit was expressed as [kg / m ３ The bulk density of the expanded beads was calculated by converting the value into the value of the particle diameter.

[0087] (Biomass Degree) The biomass degree of the expanded polypropylene resin beads was calculated from the relationship between the biomass degree of the polypropylene resin used to produce the resin beads and the blending amount thereof.

[0088] <Expanded Bead Molded Article> (Molded Article Density) The density of the expanded bead molded article was determined as the arithmetic mean value of the densities of three test pieces calculated by dividing the mass of the expanded bead molded article by the volume calculated based on the molded article dimensions. The density was measured using an expanded bead molded article obtained by in-mold molding at the lowest molding pressure within the moldable range described below.

[0089] (Compressive Stress at 50% Strain) The compressive stress at 50% strain of an expanded bead molding was determined as follows using an expanded bead molding obtained by in-mold molding at the lowest molding pressure within the moldable range described below. The skin layer was removed from the expanded bead molding, and a rectangular parallelepiped test piece measuring 50 mm long, 50 mm wide, and 25 mm thick was cut out. This test piece was compressed at a rate of 10 mm / min using an "RTF-1350" manufactured by A&D Co., Ltd. in accordance with JIS K 6767:1999 to determine the load at 50% strain. This load was then divided by the pressure-receiving area of ​​the test piece to determine the compressive stress [kPa] at 50% strain.

[0090] [Evaluation Method] <Minimum Molding Pressure Required to Obtain a Good Expanded Polypropylene Resin Bead Molded Article> In the <Preparation of Expanded Bead Molded Article> described below, expanded bead molded articles were prepared by increasing the molding pressure (steam pressure) in increments of 0.01 MPa (G) within the range of 0.40 to 0.50 MPa (G). The minimum molding pressure required to obtain an expanded bead molded article that passed all of the following evaluations of weldability, surface irregularities, and shape (sink marks (dents) in the center of the molded article) was determined. (Weldability) A flat expanded bead molded article was bent and broken, and the number of expanded beads present on the fracture surface (C1) and the number of broken expanded beads (C2) were determined. The ratio of the number of broken expanded beads to the total number of expanded beads (C2 / C1 × 100) was calculated as the material failure rate. A material failure rate of 80% or more was considered acceptable, and weldability was evaluated. (Surface Unevenness) The surface unevenness was evaluated as follows: if the shape of the molding die was sufficiently formed at the periphery of the expanded bead molded article, and the unevenness caused by adjacent expanded beads was not noticeable, the molded article was evaluated as pass; if the shape of the molding die was not sufficiently formed, and the unevenness caused by adjacent expanded beads was noticeable, the molded article was evaluated as fail. (Shape) For a flat expanded bead molded article, the thickness near both longitudinal ends (the intersection of a position 10 mm inward from the end toward the center in the longitudinal direction of the molded article and a position dividing the molded article in half in the transverse direction) and the thickness at the center (the intersection of a position dividing the molded article in half in the longitudinal direction and a position dividing the molded article in half in the transverse direction) were measured. Next, the ratio (%) of the thickness at the center to the thickness of the thickest point near both ends was calculated. A ratio of 95% or more was evaluated as pass (no excessive sink marks occurred in the center of the molded article), and a ratio of less than 95% was evaluated as fail (sink marks occurred in the center of the molded article).

[0091] <Moldable molding pressure range> For the expanded beads obtained in the examples, in the evaluation of the above-mentioned <Minimum molding pressure at which a good polypropylene resin expanded bead molded article can be obtained>, the molding pressure (steam pressure) was increased in increments of 0.01 MPa (G) from the minimum molding pressure at which a foamed bead molded article that passed all the evaluations could be obtained, and the molding pressure at which a foamed bead molded article that passed all the evaluations of fusion, appearance, and recovery was obtained was calculated. As a result, the molding pressure scores for Examples 1 and 4 were 4 points, for Examples 2, 3, and 5, 5 points, and for Example 6, 6 points. Note that, since the molding temperature is controlled by the molding pressure, the higher the molding pressure score and the wider the range from the lower limit to the upper limit, the wider the range of molding heating temperature that can be molded. In addition, those that can be molded even under low steam pressure conditions are preferable because the amount of steam required for molding can be reduced and productivity is excellent.

[0092] [Raw Materials] The polypropylene resins used to form the core layers in the Examples and Comparative Examples are shown in Table 1. In Table 1, the biomass-derived polypropylene resin used to form C1 to C4 was "propylene homopolymer manufactured by Lyondellbasell, product name: CirculenRenew C14 HP456," which had a biomass content of 42%, a melting point of 163°C, a melt flow rate of 3 g / 10 min, a flexural modulus of 1500 MPa, and a heat of fusion of 102 J / g. The fossil fuel-derived polypropylene resin used to form C2 to C4 was a "propylene homopolymer manufactured by Prime Polymer Co., Ltd., product name: Prime Polypro J105G," which had a biomass content of 0%, a melting point of 163°C, a melt flow rate of 10 g / 10 min, a flexural modulus of 1500 MPa, and a heat of fusion of 104 J / g. The biomass contents of C2 to C4 were calculated from the biomass contents of the biomass-derived polypropylene resins used to form C2 to C4 and the mass ratios of the biomass-derived polypropylene resin and the fossil fuel-derived polypropylene resin in C2 to C4, respectively.

[0093]

[0094] The polypropylene-based resins used to form the resin layers in the examples and comparative examples are shown in Table 2. In Table 2, the polypropylene-based resin used to form S1 is the same as the polypropylene-based resin C1 used to form the core layer, "propylene homopolymer manufactured by Lyondellbasell, product name: CirculenRenew C14 HP456." The polypropylene-based resin used to form S2 is "ethylene-propylene copolymer manufactured by Lyondellbasell, product name: CirculenRenew C14 EP448T."

[0095]

[0096] Examples 1 to 5 <Preparation of Resin Particles> A manufacturing apparatus was prepared, including a core layer forming extruder with an inner diameter of 50 mm, a multilayer strand forming die attached downstream of the core layer forming extruder, and a resin layer (coating layer) forming extruder with an inner diameter of 30 mm. The manufacturing apparatus was connected downstream of the resin layer (coating layer) forming extruder and the multilayer strand forming die. The manufacturing apparatus was also designed to enable lamination of the resin melts for forming each layer within the die and co-extrusion. The polypropylene resins shown in Table 3 and zinc borate (0.1 parts by mass per 100 parts by mass of the polypropylene resin for forming the core layer) as a cell regulator were fed to the core layer forming extruder and melt-kneaded. Meanwhile, the polypropylene resins shown in Table 3 were fed to the resin layer (coating layer) forming extruder and melt-kneaded. The resin melts for forming each layer obtained by melt-kneading were introduced into a multilayer strand-forming die and merged within the die to extrude multilayer strands having a two-layer structure consisting of a core layer and a resin layer (coating layer), with the proportion of the resin layer (coating layer) being the value shown in Table 3. The extruded strands were cooled with water and cut with a pelletizer to obtain multilayered resin particles having an average mass per particle of 1 mg, a particle diameter (length in the extrusion direction) of 2.0 mm, and a length / diameter ratio of 2.0.

[0097] <Preparation of Expanded Beads> 1 kg of the resulting resin particles was placed in a 5-L sealed container together with 3 L of water as a dispersion medium. Furthermore, 0.3 parts by mass of kaolin as an inorganic dispersant and 0.2 parts by mass of a surfactant (sodium dodecylbenzenesulfonate, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., product name "Neogen") (as an active ingredient) were added to the sealed container per 100 parts by mass of the resin particles. Next, carbon dioxide was injected into the sealed container as a blowing agent, and the pressure was increased to a gauge pressure of 2.0 MPa (G). The contents of the sealed container were then heated to 169°C at a heating rate of 2°C / min while stirring, and the temperature was maintained at 169°C for 15 minutes. This adjusted the endothermic curve of the resulting expanded beads to a high-temperature peak in DSC measurement. The contents of the sealed container (resin particles and water) were then released under atmospheric pressure to obtain expanded beads with the bulk densities shown in Table 3. The measurement results for the resulting expanded beads are shown in Table 3. The mass ratio of the core layer to the resin layer (coating layer) in the expanded beads was the same as the mass ratio of the core layer to the resin layer (coating layer) in the resin beads.

[0098] <Production of Expanded Bead Molded Article> First, the obtained expanded beads were placed in a pressure-resistant vessel, and the vessel was pressurized with air to impregnate the expanded beads, thereby applying an internal pressure (internal bubble pressure) of 0.20 MPa (G) to the expanded beads. Next, the expanded beads to which the internal pressure had been applied were filled into a mold having a molding cavity capable of molding a flat expanded bead molded article having dimensions of 250 mm length x 200 mm width x 40 mm height, and heated by the following heating method. A metal mold was used as the mold. First, steam was supplied to the mold with drain valves on both sides open to perform preheating (exhaust step). Then, steam was supplied from one side of the mold to heat it, and then steam was supplied from the other side to heat it. Next, steam was supplied from both sides of the mold at a predetermined molding steam pressure to heat it. After heating was completed, the pressure was released and water cooling was quickly initiated. Water cooling was continued until the surface pressure due to the expansion force of the expanded bead molded article reached 0.04 MPa (G). After water cooling, the expanded bead molding was removed from the mold to obtain an expanded bead molding having the density shown in Table 3. Table 3 shows the measurement and evaluation results of the obtained expanded bead molding.

[0099] Comparative Examples 1 and 2 <Preparation of Resin Particles> A manufacturing apparatus equipped with an extruder with an inner diameter of 50 mm and a strand-forming die attached downstream of the extruder was prepared. The polypropylene resin shown in Table 3 and zinc borate (0.1 parts by mass per 100 parts by mass of polypropylene resin) as a cell regulator were fed into the extruder and melt-kneaded to obtain a resin melt. The resin melt was introduced into the strand-forming die to extrude strands. The extruded strands were water-cooled and cut with a pelletizer to obtain single-layer resin particles with an average mass of 1 mg per particle, a particle diameter of 2.0 mm, and a length / diameter ratio of 2.0.

[0100] <Preparation of Expanded Beads> 1 kg of the resulting resin particles was placed in a 5-L pressurizable sealed container together with 3 L of water as an aqueous dispersion medium. Furthermore, 0.3 parts by mass of kaolin as an inorganic dispersant and 0.2 parts by mass of a surfactant (sodium dodecylbenzenesulfonate, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., product name "Neogen") (as an active ingredient) per 100 parts by mass of the resin particles were added to the pressure vessel. Next, carbon dioxide was injected as a blowing agent into the sealed container, and the pressure was increased to a gauge pressure of 2.0 MPa (G). The contents of the sealed container were then heated to 169°C at a heating rate of 2°C / min while stirring, and the temperature was maintained at 169°C for 15 minutes. This adjusted the DSC curve of the resulting expanded beads so that a high-temperature peak appeared in the DSC measurement. The contents of the sealed container (resin particles and water) were then released under atmospheric pressure to obtain expanded beads with the bulk densities shown in Table 3. The measurement results for the resulting expanded beads are shown in Table 3.

[0101] <Production of Expanded Bead Molded Article> After applying internal pressure to the obtained expanded beads, the expanded beads were filled into a mold and in-mold molding was attempted in the same manner as in Example 1. As a result, in Comparative Examples 1 and 2, when the molding pressure (steam pressure) was in the range of 0.40 to 0.50 MPa (G), the expanded beads were not sufficiently fused together, and a good expanded bead molded article could not be obtained.

[0102]

[0103] Comparing Example 1 and Comparative Example 1, both have the same biomass content throughout the expanded beads. Comparative Example 1, which has a single-layer structure and does not satisfy the requirements of the present invention, was unable to mold a good molded article. On the other hand, Example 1, which has a multilayer structure and does satisfy the requirements of the present invention, was able to mold a good molded article. Similarly, comparing Example 2 and Comparative Example 2, both have the same biomass content throughout the expanded beads. Comparative Example 2, which has a single-layer structure and does not satisfy the requirements of the present invention, was unable to mold a good molded article. On the other hand, Example 2, which has a multilayer structure and does satisfy the requirements of the present invention, was able to mold a good molded article. Furthermore, Examples 3 to 5 are examples in which the polypropylene resin constituting the core layer is a mixed resin of a biomass-derived polypropylene resin and a fossil fuel-derived polypropylene resin. In this case, too, expanded beads with good in-mold moldability were obtained while increasing the biomass content throughout the expanded beads.

[0104] As described above, the method for producing expanded beads of the present invention can provide expanded beads with good moldability in a mold. Specifically, the expanded beads of the present invention can be molded in a mold at a low molding pressure and over a wide range of molding pressures.

[0105] The expanded beads of the present invention can contribute to reducing environmental load and have good moldability in molds. Therefore, expanded bead moldings obtained by molding the expanded beads of the present invention in molds can be used in a wide range of fields, such as shock absorbers, heat insulating materials, various packaging materials, food transport containers, electrical and electronic parts, precision parts, packaging or cushioning materials for vehicle components, building materials such as residential insulation materials, and miscellaneous goods.

Claims

1. A method for producing expanded polypropylene-based resin beads, comprising expanding polypropylene-based resin particles to produce expanded polypropylene-based resin beads, wherein the polypropylene-based resin particles have a core layer whose base resin is a polypropylene-based resin, and a resin layer whose base resin is a polypropylene-based resin and covers the core layer, the resin layer containing a biomass-derived polypropylene-based resin that contains a biomass-derived monomer component in its molecular chain, the content X of the biomass-derived polypropylene-based resin in the resin layer being 50% by mass or more, and the content X (% by mass) of the biomass-derived polypropylene-based resin in the resin layer being equal to or greater than the content Y (% by mass) of the biomass-derived polypropylene-based resin in the core layer.

2. The method for producing expanded polypropylene resin beads according to claim 1, wherein the flexural modulus of the polypropylene resin constituting the resin layer is 800 MPa or more and 1600 MPa or less.

3. A method for producing expanded polypropylene resin particles according to claim 1 or 2, wherein the mass ratio of the resin layer in the polypropylene resin particles is 0.1 mass % or more and 20 mass % or less.

4. A method for producing expanded polypropylene resin beads according to any one of claims 1 to 3, wherein the ratio of the flexural modulus of the polypropylene resin constituting the resin layer to the flexural modulus of the polypropylene resin constituting the core layer is 0.7 or more and 1.3 or less.

5. A method for producing expanded polypropylene resin beads according to any one of claims 1 to 4, wherein the polypropylene resin constituting the core layer is a mixed resin of the biomass-derived polypropylene resin and the fossil fuel-derived polypropylene resin, and the mass ratio of the biomass-derived polypropylene resin to the fossil fuel-derived polypropylene resin in the mixed resin (biomass-derived polypropylene resin:fossil fuel-derived polypropylene resin) is 3:97 to 97:

3.

6. The method for producing expanded polypropylene resin beads according to any one of claims 1 to 5, wherein the biomass content of the resin layer measured according to ASTM D 6866-21 is 10% or more.

7. The method for producing expanded polypropylene resin beads according to any one of claims 1 to 6, wherein the expanded polypropylene resin beads have a biomass degree of 5% or more as measured by ASTM D 6866-21, and the resin layer has a biomass degree of equal to or greater than the biomass degree of the expanded polypropylene resin beads as measured by ASTM D 6866-21.

8. Expanded polypropylene resin beads, comprising: an expanded core layer whose base resin is a polypropylene resin; and a resin layer whose base resin is a polypropylene resin and covers the expanded core layer; the resin layer contains a biomass-derived polypropylene resin that contains a biomass-derived monomer component in its molecular chain; the expanded polypropylene resin beads have a biomass degree of 5% or more as measured according to ASTM D 6866-21; and the resin layer has a biomass degree of equal to or greater than that of the expanded polypropylene resin beads as measured according to ASTM D 6866-21.

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