Method for producing expanded polypropylene resin particles, and expanded polypropylene resin particles

Abstract

To provide a polypropylene-based resin foamed particle containing a biomass-derived polypropylene-based resin component, which has good in-mold moldability while suppressing the occurrence of coalescence of resin particles during foaming, and a method for producing the same.SOLUTION: A method for producing expanded polypropylene resin particles by expanding polypropylene resin particles having a core layer containing a polypropylene resin as a base resin and a resin layer covering the core layer and containing a polypropylene resin as a base resin, wherein the polypropylene resin constituting the core layer contains a biomass-derived polypropylene resin containing a biomass-derived monomer component in a molecular chain, the content of the biomass-derived polypropylene resin in the core layer is 50 mass% or more, and the flexural modulus of the polypropylene resin constituting the resin layer is 800 MPa or more.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Expanded polyolefin resin bead moldings, which are obtained by molding expanded polyolefin resin beads in a mold, are used in a wide range of fields, including transport containers for food and other products, packaging or cushioning materials for electrical and electronic parts, precision parts, and vehicle components, building materials such as insulation for homes, and shock absorbing materials for vehicle components.

[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. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-60514 Summary of the Invention [Problem to be solved by the invention]

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

[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 it is difficult to obtain good polypropylene-based resin foamed bead moldings.

[0008] In order to improve the moldability of expanded beads containing biomass-derived polypropylene resin, it has been considered to produce expanded beads with a multilayer structure by expanding resin beads having a core layer containing biomass-derived polypropylene resin and a resin layer covering the core layer. However, in this case, the resin particles tend to coalesce during expansion, making it difficult to obtain the expanded beads themselves.

[0009] 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 while suppressing the occurrence of adhesion between resin particles during expansion, and a method for producing the same. [Means for solving the problem]

[0010] The present inventors have found that the above-mentioned problems can be solved by producing expanded beads by expanding resin particles having a core layer and a resin layer, wherein the polypropylene resin constituting the core layer contains a biomass-derived polypropylene resin containing a biomass-derived monomer component in its molecular chain, the content of the biomass-derived polypropylene resin in the core layer is a specific value or more, and the polypropylene resin constituting the resin layer has a flexural modulus of a specific value or more, and have thus completed the present invention. That is, the present invention is as follows. <1> A method for producing expanded polypropylene-based resin beads, comprising expanding polypropylene-based resin particles having a core layer whose base resin is a polypropylene-based resin and a resin layer coating the core layer and also having a polypropylene-based resin as the base resin, wherein the polypropylene-based resin constituting the core layer comprises a biomass-derived polypropylene-based resin containing a biomass-derived monomer component in its molecular chain, the content of the biomass-derived polypropylene-based resin in the core layer is 50% by mass or more, and the polypropylene-based resin constituting the resin layer has a flexural modulus of 800 MPa or more. <2> the content of the biomass-derived polypropylene-based resin in the resin layer is lower than the content of the biomass-derived polypropylene-based resin in the core layer; <1> 1. A method for producing expanded polypropylene resin particles according to claim 1. <3> The mass ratio of the resin layer in the polypropylene-based resin particles is 0.5 mass% or more and 20 mass% or less. <1> or <2> 1. A method for producing expanded polypropylene resin particles according to claim 1. <4> the polypropylene-based resin constituting the core layer is a mixed resin of the biomass-derived polypropylene-based resin and the 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 50:50 to 97:3; <1> ~ <3> 1. A method for producing the expanded polypropylene resin particles according to any one of the above. <5> 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 0.5 or more and 1.0 or less; <1> ~ <4> 1. A method for producing the expanded polypropylene resin particles according to any one of the above. <6> The expanded polypropylene-based resin particles 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 polypropylene-based resin constituting the expanded core layer comprises a biomass-derived polypropylene-based resin containing a biomass-derived monomer component in its molecular chain, the content of the biomass-derived polypropylene-based resin in the expanded core layer being 50% by mass or more, and the polypropylene-based resin constituting the resin layer has a flexural modulus of 800 MPa or more. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide expanded polypropylene-based resin beads containing a biomass-derived polypropylene-based resin component, which have good in-mold moldability while suppressing the occurrence of coalescence between resin particles during expansion, and a method for producing the same. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 2 is a diagram illustrating a DSC curve of expanded polypropylene resin beads at the time of the first heating. DETAILED DESCRIPTION OF THE INVENTION

[0013] [Method of manufacturing polypropylene resin foam 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 production method of the present invention) is a method for producing expanded polypropylene-based resin beads (hereinafter simply referred to as expanded beads) by expanding polypropylene-based resin beads having a core layer whose base resin is a polypropylene-based resin and a resin layer coating the core layer, the resin also having a base resin of a polypropylene-based resin. The polypropylene-based resin constituting the core layer comprises a biomass-derived polypropylene-based resin containing a biomass-derived monomer component in its molecular chain, the content of the biomass-derived polypropylene-based resin in the core layer is 50% by mass or more, and the polypropylene-based resin constituting the resin layer has a flexural modulus of 800 MPa or more. In this specification, polypropylene-based resin refers to a polymer containing 50% by mass or more of structural units derived from propylene. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0014] In the method for producing expanded beads of the present invention, expanded polypropylene resin beads are obtained by expanding polypropylene resin beads having the core layer and the resin layer covering the core layer. When expanding the polypropylene resin beads, resin beads containing a blowing agent are expanded to obtain expanded beads. The method for producing expanded beads preferably includes at least the following steps (A) to (C). Step (A): preparing polypropylene-based resin particles having a core layer containing a polypropylene-based resin as a base resin and a resin layer coating the core layer and containing a polypropylene-based resin as a base resin; Step (B): A step of impregnating polypropylene-based resin particles with a foaming agent; and Step (C): A step of expanding polypropylene resin particles impregnated with a foaming agent to obtain expanded particles having a foamed core layer.

[0015] <Polypropylene 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 covering the core layer and also made of a polypropylene-based resin as a base resin. In this specification, a core layer having a polypropylene-based resin as a base resin means that the core layer is composed of a resin containing a polypropylene-based resin as a main component. 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, a resin layer having a polypropylene-based resin as a base resin means that the resin layer is composed of a resin containing a polypropylene-based resin as a main component. 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.

[0016] (Core layer) The core layer contains a biomass-derived polypropylene resin that uses a polypropylene resin as a base resin and contains a biomass-derived monomer component (a component derived from a biomass-derived monomer) in its molecular chain. The content of the biomass-derived polypropylene resin in the core layer is 50% by mass or more. By including a specific amount or more of the biomass-derived polypropylene resin in the core layer, expanded beads with reduced environmental impact can be obtained. In this specification, biomass means "renewable, biologically derived organic resources excluding fossil resources," as described in the "Biomass Nippon Comprehensive Strategy" approved by the Cabinet on March 31, 2006. The biomass-derived polypropylene-based resin may be a polypropylene-based resin polymerized using only biomass-derived monomers, or may be a polypropylene-based resin polymerized using biomass-derived monomers and fossil fuel-derived monomers. Furthermore, the polypropylene-based resin constituting the core layer may be composed essentially of biomass-derived polypropylene-based resin alone, or may be composed of biomass-derived polypropylene-based resin and a fossil fuel-derived polypropylene-based resin described below.

[0017] Examples of components derived from biomass-derived monomers (biomass-derived monomer components) contained in the molecular chain of the biomass-derived polypropylene resin 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 a biomass-derived monomer is not particularly limited, and the biomass-derived monomer can be obtained by a conventionally known method. For example, the biomass-derived monomer can be obtained by a method of dehydrating alcohol derived from a biomass raw material or a method of decomposing naphtha derived from a biomass raw material. From the viewpoint 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 is used, which is obtained by polymerizing a propylene-containing monomer obtained by decomposing bionaphtha. 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 through the use of 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 mill effluent, oils and fats contained in microalgae, etc.

[0018] Examples of biomass-derived polypropylene-based resins that can be used include propylene homopolymers and polypropylene copolymers containing 50 mol% or more of propylene-derived structural units. 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 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] 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 is 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, from the viewpoint of increasing the mechanical strength of the expanded polypropylene resin expanded bead molded article (hereinafter simply referred to 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 improving the in-mold moldability of the expanded beads. The flexural modulus of the biomass-derived polypropylene resin can be determined in accordance with 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, the melting point of the biomass-derived polypropylene resin is preferably 165° C. or lower, from the viewpoint of easily improving the in-mold moldability of the expanded beads under low molding pressure conditions. The melting point of the biomass-derived polypropylene resin is measured based on 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 body while making it easier to obtain 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 using a biomass-derived polypropylene resin as a test piece.

[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 moldability of the expanded beads in a mold. The melt flow rate of the biomass-derived polypropylene resin is a value measured under conditions of 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] The polypropylene resin constituting the core layer may contain a fossil fuel-derived polypropylene resin in addition to the biomass-derived polypropylene resin. In this specification, the fossil fuel-derived polypropylene resin means a polypropylene resin polymerized using substantially only fossil fuel-derived monomers. Examples of fossil fuel-derived polypropylene-based resins include conventionally known polypropylene-based resins, and preferred examples include propylene homopolymers and polypropylene-based copolymers. 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 fossil fuel-derived polypropylene-based resin may be a mixture of two or more fossil fuel-derived polypropylene-based resins. When the core layer contains a fossil fuel-derived polypropylene-based resin, the biomass-derived polypropylene-based resin and the fossil fuel-derived polypropylene-based resin may be the same type of polypropylene-based resin or different types of polypropylene-based 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, it is preferable that the fossil fuel-derived polypropylene-based resin is 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] The flexural modulus of the fossil fuel-derived polypropylene resin is preferably 600 MPa or more, more preferably 700 MPa or more, and even more preferably 800 MPa or more, from the viewpoint of easily improving the in-mold moldability of the expanded beads while containing the biomass-derived polypropylene resin, and is preferably 1600 MPa or less, more preferably 1500 MPa or less. Furthermore, in the polypropylene-based resin constituting the core layer, the ratio of the flexural modulus of the biomass-derived polypropylene-based resin to the flexural modulus of the fossil fuel-derived polypropylene-based 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] 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, from the viewpoint of easily improving the moldability of the expanded beads while containing the biomass-derived polypropylene resin. The melting point of the fossil fuel-derived polypropylene resin is measured based on 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, or 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-based resin constituting the core layer, the ratio of the heat of fusion of the biomass-derived polypropylene-based resin to the heat of fusion of the fossil fuel-derived polypropylene-based 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 performing 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 under conditions of a temperature of 230°C and a load of 2.16 kg, is preferably 1 g / 10 min or more and 15 g / 10 min or less, more preferably 3 g / 10 min or more and 12 g / 10 min or less, from the viewpoint of easily improving the in-mold moldability of the expanded beads. In addition, in the polypropylene-based resin constituting the core layer, the ratio of the melt flow rate of the biomass-derived polypropylene-based resin to the melt flow rate of the fossil fuel-derived polypropylene-based resin is preferably 0.2 or more and 5 or less, more preferably 0.3 or more and 4 or less. The melt flow rate of the fossil fuel-derived polypropylene resin is a value measured under conditions of 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 further reducing the environmental load, the content of the biomass-derived polypropylene resin in the core layer is 50% by mass or more, preferably 55% by mass or more, and the upper limit is not particularly limited, but is 100% by mass. 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, from the viewpoint of easily improving the moldability of the expanded beads in a mold while containing the biomass-derived polypropylene resin, 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 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.

[0031] 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 such 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, as well as elastomers, such as olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers. When the core layer contains such other polymers, the content of such 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.

[0032] It is preferable that a bubble adjusting agent is added to the core layer. As the bubble adjusting agent, 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 the desired bulk density and little variation in bubble diameter, the amount of bubble control agent added in 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. From the viewpoint of easily adjusting the average cell diameter of the expanded beads to a desired range, it is preferable to use a metal borate, and more preferably zinc borate, as the cell adjusting agent. 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.

[0033] <Biomass degree Bc> The biomass degree (biobased carbon content) of the core layer, as measured by ASTM D 6866-21, is preferably 1% or more. From the viewpoint of further reducing the environmental load, the biomass degree Bc of the core layer is 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.

[0034] <Flexural modulus Fc> 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 is 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, from the viewpoint of increasing the mechanical strength of the resulting molded article and improving the in-mold moldability of the expanded beads. 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, or 70 J / g or more and 110 J / g or less, from the viewpoint of improving the mechanical properties of the obtained molded body while improving in-mold moldability. The heat of fusion of the polypropylene-based resin can be determined from a DSC curve obtained by performing differential scanning calorimetry (DSC) on a test piece of the polypropylene-based resin in accordance with JIS K 7122: 2012. Specifically, it can be measured by the method described in the examples.

[0036] 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.

[0037] (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 covering 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.

[0038] <Flexural modulus Fr> The flexural modulus Fr of the polypropylene resin constituting the resin layer is 800 MPa or more from the viewpoint of suppressing adhesion of resin particles to each other during expansion of the resin particles and improving moldability of the resulting expanded beads in a mold. Expanded polypropylene resin beads containing a biomass-derived polypropylene resin component have a problem in that they have poor moldability in a mold compared to expanded beads made solely from a fossil fuel-derived polypropylene resin, making it difficult to obtain good expanded polypropylene resin bead moldings. The present inventors have investigated the possibility of obtaining expanded beads with good in-mold moldability by expanding multilayer polypropylene-based resin beads having a core layer containing a biomass-derived polypropylene-based resin and a resin layer covering the core layer, thereby improving the fusion between the expanded beads during in-mold molding. However, it was found that in this case, the resin beads tend to coalesce during expansion, making it difficult to obtain the expanded beads themselves. In the method for producing expanded polypropylene resin beads of the present invention, expanded beads having a core layer containing a biomass-derived polypropylene resin as a main component and a resin layer covering the core layer are produced. By forming the resin layer from a polypropylene resin having a specific flexural modulus, it is possible to produce expanded beads with good in-mold moldability while suppressing the occurrence of adhesion between resin particles during expansion. The reason for this is unclear, but is presumed to be as follows. When expanding multilayered polypropylene resin beads, a polypropylene resin with a low flexural modulus that tends to elongate when softened by heating is usually used as the resin layer. On the other hand, a high content of biomass-derived polypropylene resin in the core layer tends to result in a relatively high temperature during expansion. In this case, if the flexural modulus of the polypropylene resin forming the resin layer is low, the resin layer is likely to soften and deform excessively, which is thought to increase the tendency for adhesion between resin particles to occur. On the other hand, by forming the resin layer from a polypropylene-based resin having a specific flexural modulus, as in the manufacturing method of the present invention, excessive softening and deformation of the resin layer is suppressed during expansion of the resin particles, and adhesion between the resin particles is suppressed.As a result, it is thought that it is possible to obtain good polypropylene-based resin expanded particles having a resin layer in which adhesion between the expanded particles is also suppressed. From the viewpoint of stably suppressing adhesion of resin particles during expansion of the resin particles and from the viewpoint of easily improving moldability of the expanded beads in a mold, the flexural modulus Fr is preferably 900 MPa or more, more preferably 950 MPa or more. Furthermore, from the viewpoint of easily improving moldability of the expanded beads in a mold, the flexural modulus Fr is preferably 1600 MPa or less, more preferably 1500 MPa or less, and even more preferably 1400 MPa or less. The flexural modulus Fr of the polypropylene resin constituting the resin layer can be determined based on JIS K 7171:2016.

[0039] <Flexural modulus ratio Fr / Fc> 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, and is preferably 1.0 or less, more preferably 0.9 or less, from the viewpoint of being able to stably suppress adhesion of resin particles during expansion of the resin particles while stably obtaining expanded beads with good in-mold moldability.

[0040] 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 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more, from the viewpoint of more stably suppressing adhesion of the resin particles to each other during expansion while more stably improving the in-mold moldability of the expanded beads by the resin layer formed from a specific polypropylene-based resin, and is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 12% by mass or less. The mass ratio of the resin layer in the polypropylene-based resin particles generally corresponds to the mass ratio of the resin layer in the expanded polypropylene-based resin beads produced using the resin particles.

[0041] The polypropylene-based resin constituting the resin layer is a polypropylene-based resin having the specific flexural modulus described above. Examples of such polypropylene-based resins include the biomass-derived polypropylene-based resins described for the core layer and fossil fuel-derived polypropylene-based resins. Furthermore, polypropylene-based copolymers are preferably used as the polypropylene-based resin constituting the resin layer, and more preferably, the polypropylene-based resin is one or more polypropylene-based resins selected from the group consisting of ethylene-propylene copolymers, propylene-butene copolymers, and ethylene-propylene-butene copolymers. For the biomass-derived polypropylene resin and fossil fuel-derived polypropylene resin in the resin layer, the explanations for the biomass-derived polypropylene resin and fossil fuel-derived polypropylene resin in the core layer can be referred to as appropriate.

[0042] From the viewpoint of easily improving the moldability of the expanded beads while containing biomass-derived polypropylene resin as a whole, the content CR of biomass-derived polypropylene resin in the resin layer is BPP However, the content of biomass-derived polypropylene resin in the core layer CC BPP It is preferable that the content of the biomass-derived polypropylene resin in the core layer is less than CC BPP and the content of biomass-derived polypropylene resin in the resin layer, CR BPP Difference with (CC BPP -CR BPP ) is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, still more preferably 40% by mass or more, and even more preferably 50% by mass or more. The content of biomass-derived polypropylene resin in the resin layer is CR BPP is the content (mass%) of biomass-derived polypropylene resin in the resin layer when the resin layer is taken as 100% by mass. CC is the content (mass%) of biomass-derived polypropylene resin in the core layer. BPP is the content (mass%) of biomass-derived polypropylene resin in the core layer when the core layer is taken as 100 mass%.

[0043] <Biomass ratio> From the viewpoint of easily improving the in-mold moldability of the expanded beads, the biomass degree Br of the resin layer measured according to ASTM D 6866-21 is preferably lower than the biomass degree Bc of the core layer measured according to ASTM D 6866-21. The difference (Bc - Br) between the biomass degree Bc of the core layer and the biomass degree Br of the resin layer is preferably 10% or more, more preferably 15% or more, and even more preferably 20% or more. On the other hand, the upper limit of the difference (Bc - Br) between the biomass degree Bc of the core layer and the biomass degree Br of the resin layer may be 100%, 90%, 80%, or 60%. The polypropylene resin constituting the resin layer may be a biomass-derived polypropylene resin or a fossil fuel-derived polypropylene resin. From the viewpoint of stably improving in-mold moldability, the polypropylene resin constituting the resin layer preferably contains a fossil fuel-derived polypropylene resin. The proportion of the fossil fuel-derived polypropylene resin in the polypropylene resin constituting the resin layer is preferably 50% by mass, more preferably 60% by mass or more, even more preferably 70% by mass or more, still more preferably 80% by mass or more, and even more preferably 90% by mass or more.

[0044] <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, or 75 J / g or more and 95 J / g or less, from the viewpoint of more stably suppressing adhesion of resin particles to each other during foaming while easily improving the moldability of the expanded beads in the mold. In addition, the ratio (ΔHr / ΔHc) of the heat of fusion ΔHr of the polypropylene resin constituting the resin layer to the heat of fusion ΔHc of the polypropylene 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) on a test piece of the polypropylene-based resin in accordance with JIS K 7122: 2012. Specifically, it can be measured by the method described in the examples.

[0045] 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.

[0046] 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.

[0047] 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 resin layer within the scope that allows the intended object of the invention to be achieved. From the viewpoint of stably improving the moldability of the expanded beads in the mold, it is preferable that the resin layer does not substantially contain a bubble control agent, or that the resin layer contains a bubble control agent and the content of the bubble control agent in the resin layer is less than the content of the bubble control agent in the core layer.

[0048] <Step (A): Preparation of Polypropylene Resin Particles> In step (A), resin particles having a core layer and a resin layer coating the core layer can be prepared by granulating resin particles using an extrusion device having, for example, an extruder for forming a core layer, an extruder for forming a resin layer (coating layer), and a co-extrusion die such as a die for forming a multilayer strand connected at the outlet side of these extruders. A polypropylene-based resin, which is the base resin of the core layer, and additives added as needed are supplied to the extruder for forming the core layer and melt-kneaded to form a resin melt for forming the core layer. A polypropylene-based resin, which is the base resin of the resin layer, is supplied to the extruder for forming the resin layer (coating 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 multilayer 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.

[0049] (mass of resin particles) The average mass of the polypropylene resin particles is preferably adjusted to be 0.1 to 20 mg, more preferably 0.2 to 10 mg, even more preferably 0.3 to 5 mg, and even more preferably 0.4 to 2 mg.

[0050] (shape of resin particles) The external shape of the polypropylene 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 resin particles have a cylindrical outer shape, 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. 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 0.6 to 3.0.

[0051] <Step (B): Preparation of Polypropylene Resin Particles Containing a Foaming Agent> In the step (B), the polypropylene resin constituting the core layer is impregnated with a foaming agent, thereby obtaining polypropylene resin particles containing a foaming 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. At the same time, a blowing agent is added to the sealed container and maintained under a predetermined temperature and pressure atmosphere, thereby allowing the polypropylene-based resin particles to be impregnated with the blowing agent.

[0052] 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.

[0053] 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, more preferably kaolin, from the viewpoint 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.

[0054] 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.

[0055] The blowing agent is not particularly limited as long as it can expand polypropylene-based 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.

[0056] The amount of foaming agent to be added is determined taking into consideration 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.

[0057] 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.

[0058] <Process (C)> In step (C), for example, the polypropylene resin particles containing the 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 resin particles containing the 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 the production of expanded polypropylene resin beads having a foamed core layer formed by the expansion of the core layer. Specifically, it is preferable to open one end of the sealed container below the water surface while maintaining the pressure inside the sealed container at a pressure equal to or higher than the vapor pressure of the blowing agent, and release the polypropylene resin particles containing the blowing agent together with the dispersion medium from the sealed container into an atmosphere of a pressure lower than the pressure inside the sealed container, usually atmospheric pressure, to expand the core layer of the polypropylene resin particles to form an expanded core layer. This allows the production of expanded polypropylene resin particles with a multilayer structure having an expanded core layer and a resin layer (coating layer) covering the expanded core layer.

[0059] 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° C. to 170° C. The pressure inside the sealed container is preferably 0.5 MPa (G) to 5 MPa (G). The pressures marked with (G) are gauge pressures, i.e., pressure values ​​based on atmospheric pressure.

[0060] 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-mentioned steps (A) to (C). However, 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.

[0061] In addition, the expanded polypropylene resin beads obtained as described above can be subjected to a pressure treatment using air or the like to increase the internal pressure within the cells of the expanded beads, and then the expanded beads can be heated using steam or the like to expand them (two-stage expansion), thereby obtaining expanded beads with an even higher expansion ratio (lower bulk density).

[0062] [Polypropylene resin foam particles] The expanded polypropylene-based resin beads of the present invention (hereinafter simply referred to as expanded beads of the present invention, 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 polypropylene-based resin constituting the expanded core layer comprises a biomass-derived polypropylene-based resin containing a biomass-derived monomer component in its molecular chain, the content of the biomass-derived polypropylene-based resin in the expanded core layer is 50% by mass or more, and the flexural modulus of the polypropylene-based resin constituting the resin layer is 800 MPa or more. The expanded polypropylene resin beads of the present invention can be preferably produced by the above-mentioned [Method for producing expanded polypropylene resin beads]. Therefore, for the explanation of the polypropylene resin etc. related to the expanded polypropylene resin beads of the present invention, the explanation of the polypropylene resin etc. in the corresponding [Method for producing expanded polypropylene resin beads] can be appropriately referred to.

[0063] <Biomass ratio> From the viewpoint of reducing the environmental impact, the biomass degree of the expanded polypropylene resin beads, as 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 of the expanded beads is not particularly limited, but may be 100%, 90%, 80%, or 60%. The biomass degree of the expanded beads may be measured directly on the expanded beads, or may be 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.

[0064] <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 Less than or equal to 100 kg / m 3 More preferably, 60 kg / m or less 3 More preferably, 50 kg / m or less 3 The following is the result. The bulk density of expanded particles can be calculated as follows: First, a measuring cylinder is filled with expanded particles 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 particles in the measuring cylinder. Next, the volume V1 ([L]) of the expanded particles indicated on the measuring cylinder's scale is read. The mass W1 of the expanded particles 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.

[0065] <High temperature peak> The expanded polypropylene resin beads preferably have a crystalline structure in which a DSC curve obtained when the expanded beads are heated from 23°C to 200°C at a heating rate of 10°C / min shows a melting peak due to the melting of crystals specific to the polypropylene resin (i.e., the resin-specific peak) and one or more melting peaks (i.e., high-temperature peaks) on the higher temperature side of the melting peak (i.e., high-temperature peaks). 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 specific to the polypropylene resin that constitutes the expanded beads, and is considered to be an endothermic peak that appears due to the endothermic heat that occurs when crystals typically melt in polypropylene resins. On the other hand, a 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 resin that makes up 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, in step (B) 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 time. Specifically, for example, in step (B), a first-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 melting end temperature of the propylene-based resin) for approximately 10 to 60 minutes. Thereafter, the temperature in the sealed container is adjusted to a temperature between (the melting point of the polypropylene-based resin -15°C) and lower than (the melting end temperature of the polypropylene-based resin). At this time, a second-stage holding step in which the temperature is maintained for an additional 10 to 60 minutes may be performed, if necessary. Then, step (C) is performed to produce expanded polypropylene-based resin beads having a high-temperature peak.

[0066] <Heat of fusion at high temperature peak> 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. Figure 1 shows an example of a DSC curve (DSC curve from the first heating cycle) obtained when polypropylene-based resin foam particles are heated from 23°C to 200°C at a heating rate of 10°C / min. This curve shows a resin-specific peak P1 and a high-temperature peak P2 higher than the resin-specific peak. When a high-temperature peak P2 appears as shown in Figure 1, its heat of fusion can be calculated as follows. First, the point on the DSC curve at 80°C is designated as α, and the point on the DSC curve corresponding to the melting end temperature T is designated as β. A line L1 is then drawn connecting these points. Next, a line L2 is drawn parallel to the vertical axis of the graph from point γ on the DSC curve, which corresponds to the valley between the resin-specific peak and the high-temperature peak. The point where this line L2 intersects with line L1 is designated as δ. The area (2) enclosed by the high-temperature peak curve, the line (δ-β), and the line L2 is taken as the area of ​​the high-temperature peak. The heat of fusion of the high-temperature peak can be calculated from this area.

[0067] <Total heat of fusion> The total heat of fusion of the expanded polypropylene resin beads is preferably 60 J / g or more, more preferably 70 J / g or more, and even more preferably 80 J / g or more, from the viewpoint of increasing the mechanical strength of the resulting expanded bead molding while stably obtaining expanded beads with good in-mold moldability, and is preferably 120 J / g or less, and more preferably 110 J / g or less. The total heat of fusion of expanded polypropylene resin beads can be determined from a DSC curve obtained by performing differential scanning calorimetry (DSC) according to JIS K 7122:2012 on the expanded beads as a test specimen. Specifically, the test specimen was first conditioned using "(2) Measurement of melting temperature after a certain heat treatment." The test specimen was heated from 23 to 200°C at a heating rate of 10°C / min. After reaching 200°C, the temperature was lowered from 200 to 23°C at a rate of 10°C / min. After heating again, the DSC curve (DSC curve during the second heating) was obtained. The point at 80°C on the obtained DSC curve during the second heating was designated as α, and the point on the DSC curve corresponding to the end of melting 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) is measured, and the total heat of fusion of the expanded beads can be calculated from this area.

[0068] [Polypropylene resin foam bead molding] Expanded polypropylene resin beads can be molded in a mold to obtain an expanded polypropylene resin bead molded article.

[0069] 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 then a heating medium such as steam is supplied into the mold to heat-fuse the expanded beads (e.g., JP-B 51-22951). 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 is filled with expanded beads, 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 mold cavity 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).

[0070] <density> The density of the expanded polypropylene resin bead molding is preferably 10 kg / m from the viewpoint of easily obtaining a molding 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 Less than or equal to 100 kg / m 3 More preferably, 80 kg / m or less 3 More preferably, 60 kg / m or less 3The 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.

[0071] <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. [Example]

[0072] 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.

[0073] The resin 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.

[0074] [Measurement method] <Resin raw materials> (Biomass ratio) The biomass content of the biomass-derived polypropylene resin used as the resin raw material was measured as follows based on ASTM D6866-21. Carbon dioxide (CO2) was generated by burning the raw materials, 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 the C-AMS dedicated device. 14 The number of C's, 13 C concentration ( 13 C / 12 C). 14 C concentration ( 14 C / 12 C) was measured. For the measurements, 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 simultaneously. From the measurement results, the carbon obtained from the measurement sample relative to the modern carbon of the standard sample was 14 Calculate the C percentage, then calculate the C percentage from the standard sample. 13 Corrected pMC (percent modern carbon) values ​​were obtained by correcting for deviations in C concentration. 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 of the present invention, the atmospheric correction factor described in ASTM D6866 for the year the polypropylene-based resin was manufactured was used to determine the biomass degree.

[0075] (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 produce a 4 mm thick sheet, and a standard test piece measuring 80 mm in length, 10 mm in width, and 4 mm in thickness 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.

[0076] (heat of fusion) The heat of fusion of the polypropylene resin as the resin raw material was determined as follows by heat flux differential scanning calorimetry based on JIS K 7122:2012. The measurement device used was a high-sensitivity differential scanning calorimeter, "EXSTAR DSC7020" (Hitachi High-Tech Science Corporation). The specimen conditioning method employed was "(2) Measurement of melting temperature after a certain heat treatment." Approximately 2 mg of polypropylene resin was collected as a specimen. The 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. It was then 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 for 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.

[0077] (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. If multiple melting peaks appear on a DSC curve, the apex temperature of the melting peak with the largest area is used as the melting point. The melting peak with the largest area can be determined by distinguishing each melting peak using the valley temperature of the DSC curve located between the peak temperatures of the melting peaks as a boundary and comparing the areas (heat of fusion) of each melting peak. The valley temperature of the DSC curve corresponds to the temperature at which the vertical axis of the differential DSC curve (DDSC) becomes 0, so it can also be determined from the DSC differential curve.

[0078] (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 at a temperature of 230°C and a load of 2.16 kg.

[0079] (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.

[0080] <Polypropylene resin foam particles> (heat of fusion at high temperature peak) The heat of fusion of the high-temperature peak of the expanded polypropylene resin beads was determined as follows by heat flux differential scanning calorimetry in accordance with JIS K 7122:2012. 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" (Hitachi High-Tech Science Corporation), obtaining a DSC curve (DSC curve from the first heating) with two or more melting peaks. The measurement was performed under conditions of a nitrogen inflow rate of 30 mL / min. In the obtained DSC curve, the peak specific to the polypropylene resin constituting the expanded beads was designated P1, and the high-temperature peak appearing above that was designated P2. A straight line (α-β) was drawn connecting point α corresponding to 80°C on the DSC curve and point β on the DSC curve corresponding to the melting end temperature of the test piece. The melting end temperature is the high-temperature end point of the high-temperature peak P2, and refers to the intersection point between the high-temperature peak and the high-temperature-side baseline. Next, a straight line parallel to the vertical axis of the graph was drawn from point γ on the DSC curve corresponding to the valley between the intrinsic peak P1 and the high-temperature peak P2, and the point where this line intersected was designated δ. The area enclosed by the curve of the high-temperature peak P2 on the DSC curve, 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 above-mentioned high-temperature peak was measured for three different test pieces, and the arithmetic mean value of the obtained values ​​was taken as the heat of fusion ΔH2 of the high-temperature peak of the expanded beads.

[0081] (Total heat of fusion) The total heat of fusion of the expanded polypropylene resin particles was measured by heat flux differential scanning calorimetry in accordance with JIS K 7122:2012 in the same manner as in the heat of fusion of the polypropylene resin described above, except that expanded polypropylene resin particles were used as the test specimen.

[0082] (bulk density) The bulk density of the expanded polypropylene resin particles was determined as follows. First, a measuring cylinder was filled with expanded particles having a mass W1 [g], and the bottom of the measuring cylinder was lightly tapped on the floor several times to stabilize the filling height of the expanded particles in the measuring cylinder. Next, the volume V1 [L] of the expanded particles indicated on the measuring cylinder was read. The mass W1 [g] of the expanded particles 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.

[0083] (Biomass ratio) 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.

[0084] <Foamed bead molding> (Molded object density) The density of the expanded bead molding was determined as the arithmetic mean value of the densities of three test pieces, calculated by dividing the mass of the expanded bead molding by the volume calculated based on the molding dimensions. The density was measured using an expanded bead molding obtained by in-mold molding at the lowest molding pressure within the moldable range described below.

[0085] (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 specimen measuring 50 mm long, 50 mm wide, and 25 mm thick was cut out. This test specimen was compressed at a rate of 10 mm / min using an A&D RTF-1350 in accordance with JIS K 6767:1999. The load at 50% strain was determined, and this was divided by the pressure-receiving area of ​​the specimen to determine the compressive stress at 50% strain [kPa].

[0086] [Evaluation method] <Minimum molding pressure to obtain good polypropylene resin expanded bead molded products> In the <Preparation of Expanded Bead Molded Articles> 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), and the minimum molding pressure required to obtain expanded bead molded articles 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. (Fusing ability) The flat expanded bead molding 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 the fusion property was evaluated. (Surface unevenness) The surface unevenness was evaluated by rating the foamed bead moldings such that the shape of the molding die was sufficiently formed at the periphery and the unevenness caused by adjacent foamed beads was not noticeable, and rating the foamed bead moldings such that the shape of the molding die was not sufficiently formed and the unevenness caused by adjacent foamed beads was not noticeable. (shape) The thickness of a flat-plate-shaped expanded bead molding was measured near both ends in the longitudinal direction (the intersection of a position 10 mm inward from the end toward the center in the longitudinal direction of the molding and a position dividing the molding in half in the transverse direction of the molding) and at the center (the intersection of a position dividing the molding in half in the longitudinal direction and a position dividing the molding in half in the transverse direction). 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 considered pass (no excessive sink marks in the center of the molding), and a ratio of less than 95% was considered fail (sink marks in the center of the molding).

[0087] <Range of molding pressure that can be molded> In the evaluation of the "minimum molding pressure required to obtain a satisfactory expanded polypropylene resin bead molded article," expanded bead molded articles were prepared by varying the molding pressure (steam pressure) in increments of 0.01 MPa (G) from the minimum molding pressure required to obtain an expanded bead molded article that passed all the evaluations. Table 3 shows the molding pressure scores required to obtain an expanded bead molded article that passed all the evaluations of fusion, appearance, and recovery. Note that because the molding temperature is controlled by the molding pressure, the larger the number of molding pressures that are possible to mold and the wider the range from the lower limit to the upper limit, the wider the range of molding temperatures that can be molded. Furthermore, those that can be molded under low steam pressure conditions are preferred because they allow for a reduction in the amount of steam required for molding, resulting in excellent productivity.

[0088] [Resin raw material] Table 1 shows the polypropylene resins used to form the core layers in the examples and comparative examples. In Table 1, the biomass-derived polypropylene resin used to form C1 to C3 was "propylene homopolymer manufactured by Lyondellbasell, product name: CirculenRenew C14 HP456," with a biomass content of 42%, a melting point of 163°C, a melt flow rate of 3g / 10min, a flexural modulus of 1500MPa, and a heat of fusion of 102J / g. The fossil-fuel-derived polypropylene resin used to form C1 and C3 was "propylene homopolymer manufactured by Prime Polymer Corporation, product name: Prime Polypro J105G," with a biomass content of 0%, a melting point of 163°C, a melt flow rate of 10g / 10min, a flexural modulus of 1500MPa, and a heat of fusion of 104J / g. The biomass degree of C1 and C3 was calculated from the biomass degree of the biomass-derived polypropylene resin used to form C1 and C3 and the mass ratio of the biomass-derived polypropylene resin to the fossil fuel-derived polypropylene resin in C1 and C3.

[0089] [Table 1]

[0090] Table 2 shows the polypropylene resins used to form the resin layers in the examples and comparative examples. In Table 2, the polypropylene resins used to form S1 to S3 are all fossil fuel-derived polypropylene resins, and have a biomass content of 0. The polypropylene resins used to form S1 and S2 are ethylene-propylene random copolymers, and the polypropylene resin used to form S3 is an ethylene-propylene-butene random copolymer.

[0091] [Table 2]

[0092] Examples 1 to 4 <Preparation of resin particles> A manufacturing apparatus was prepared, which included 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 to the multilayer strand forming die. The manufacturing apparatus was also designed to be capable of laminating the resin melts for forming each layer within the die, as well as co-extrusion. The polypropylene resin shown in Table 3 was used to form the core layer, and zinc borate (0.1 part by mass per 100 parts by mass of the polypropylene resin used to form the core layer) was used as a cell control agent. The polypropylene resin shown in Table 3 was used to form the resin layer (coating layer). The resin melts shown in Table 3 were used to form the resin layer (coating layer). The melts were introduced into a multilayer strand die and merged within the die. Multilayer strands with a two-layer structure consisting of a core layer and a resin layer (coating layer), with the proportion of the resin layer (coating layer) shown in Table 3, were extruded. The extruded strands were water-cooled and cut using a pelletizer to obtain multilayered resin particles with an average mass of 1 mg per particle, a particle diameter (length in the extrusion direction) of 2.0 mm, and a length / diameter ratio of 2.0.

[0093] <Preparation of expanded particles> 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, 0.9 parts by mass of kaolin as an inorganic dispersant and 0.6 parts by mass of a surfactant (sodium dodecylbenzenesulfonate, manufactured by Daiichi 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.

[0094] <Production of foamed bead molded body> First, the obtained expanded beads were placed in a pressure-resistant container, and the pressure container was pressurized with air to impregnate the expanded beads with air, applying an internal pressure (internal bubble pressure) of 0.10 MPa (G) to the expanded beads. Next, the expanded beads to which internal pressure had been applied were filled into a mold having a molding cavity capable of molding a flat-plate-shaped expanded bead molding product measuring 250 mm in length, 200 mm in width, and 40 mm in height, and heated by the following heating method. A metal mold was used as the mold. First, with the drain valves on both sides of the mold open, steam was supplied to the mold to perform preheating (exhaust process). Then, steam was supplied from one side of the mold to heat it, and then steam was supplied from the other side of the mold to heat it again. 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, and water cooling was continued until the surface pressure due to the expansion force of the expanded bead molding reached 0.04 MPa (G). After water cooling was completed, the expanded bead molding was removed from the mold, and an expanded bead molding with the density shown in Table 3 was obtained. The measurement and evaluation results of the obtained expanded bead molding are shown in Table 3.

[0095] Comparative Examples 1 and 2 <Preparation of resin particles> A manufacturing apparatus was prepared, equipped with an extruder with an inner diameter of 50 mm and a strand-forming die attached downstream of the extruder. 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 control agent 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.

[0096] <Preparation of expanded particles> 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, 0.9 parts by mass of kaolin as an inorganic dispersant and 0.6 parts by mass of a surfactant (sodium dodecylbenzenesulfonate, manufactured by Daiichi Kogyo Seiyaku Co., Ltd., product name "Neogen") (as an active ingredient) were added to the pressure vessel per 100 parts by mass of the resin particles. 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 with stirring and maintained at that temperature for 15 minutes. This adjusted the DSC curve of the resulting expanded beads so that a high-temperature peak would appear. 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.

[0097] <Production of foamed bead molded body> As in Example 1, internal pressure was applied to the resulting expanded beads, and the expanded beads were then filled into a mold to attempt in-mold molding. As a result, in Comparative Examples 1 and 2, the expanded beads were not sufficiently fused together, and a good expanded bead molding could not be obtained.

[0098] Comparative Example 3 Multilayered resin particles were obtained in the same manner as in Example 1, except that the polypropylene resin shown in Table 3 was used as the polypropylene resin for forming the resin layer (coating layer). Note that the mass ratio of the resin layer in the polypropylene resin particles in Comparative Example 3 was 5%, and the biomass ratio of the polypropylene resin particles was 24%. Using the obtained resin particles, an attempt was made to prepare expanded particles in the same manner as in Example 1. However, when the contents of the sealed container were released under atmospheric pressure, the resin particles had coalesced together and formed clumps inside the sealed container, making it impossible to release the resin particles properly, and expanded particles could not be produced.

[0099] [Table 3]

[0100] As can be seen from Table 3, the expanded beads of the present invention can be produced with good moldability while suppressing adhesion of resin particles during expansion. Specifically, the expanded beads of the present invention can be molded at low molding pressures and over a wide range of molding pressures. [Industrial Applicability]

[0101] 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 applications, 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 beads having a core layer made of a polypropylene-based resin as a base resin and a resin layer covering the core layer and also made of a polypropylene-based resin as a base resin, the method comprising: the polypropylene-based resin constituting the core layer contains a biomass-derived polypropylene-based resin containing a biomass-derived monomer component in its molecular chain, The content of the biomass-derived polypropylene resin in the core layer is 50% by mass or more, The method for producing expanded polypropylene resin beads, wherein the polypropylene resin constituting the resin layer has a flexural modulus of 800 MPa or more.

2. 2. The method for producing expanded polypropylene resin beads according to claim 1, wherein the content of the biomass-derived polypropylene resin in the resin layer is lower than the content of the biomass-derived polypropylene resin in the core layer.

3. The method for producing expanded polypropylene resin beads according to claim 1 or 2, wherein a mass ratio of the resin layer in the polypropylene resin beads is 0.5 mass % or more and 20 mass % or less.

4. 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; 3. The method for producing expanded polypropylene resin beads according to claim 1, wherein a 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 50:50 to 97:

3.

5. 3. The method for producing expanded polypropylene-based resin beads according to claim 1, 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 0.5 or more and 1.0 or less.

6. Polypropylene-based resin foam particles, The foamed core layer has a polypropylene-based resin as a base resin, and a resin layer that coats the foamed core layer and also has a polypropylene-based resin as a base resin, the polypropylene-based resin constituting the foamed core layer contains a biomass-derived polypropylene-based resin containing a biomass-derived monomer component in its molecular chain, The content of the biomass-derived polypropylene resin in the foamed core layer is 50% by mass or more, The expanded polypropylene resin particles have a flexural modulus of elasticity of the polypropylene resin constituting the resin layer of 800 MPa or more.

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