Polypropylene resin foam particles

Abstract

These polypropylene resin foam particles each have a foam core layer composed of a polypropylene resin, and a coating layer covering the foam core layer. The coating layer is composed of linear low-density polyethylene. The mass ratio of the coating layer to the foam core layer is 0.005-0.05. The polypropylene resin constituting the foam core layer satisfies a prescribed condition (i) or (ii). Said foam particles can remarkably reduce the steam pressure at the time of in-mold molding, can shorten the curing time, and furthermore makes it possible to obtain a foam particle molded article that has a desired shape and is excellent in appearance even when the curing time is shortened.

Description

Polypropylene resin foam particles

[0001] The present invention relates to expanded polypropylene resin particles.

[0002] Polypropylene resin expanded bead molded articles are used in various applications because they are lightweight and have excellent cushioning properties, rigidity, etc. Examples of methods for producing polypropylene resin expanded bead molded articles include an in-mold molding method in which expanded polypropylene resin beads are filled into a mold and heated with steam to secondary expand the expanded beads and melt their surfaces to fuse them together, thereby forming them into a desired shape, and then cooled in the mold with water, air, etc., and then released from the mold.

[0003] If the expanded bead molding is stored at room temperature after in-mold molding, the steam that flows into the cells of the expanded bead molding during in-mold molding and condenses in the cells, creating a negative pressure inside the cells and causing volumetric shrinkage of the expanded bead molding, which can result in significant deformation of the molding. Therefore, after releasing the expanded bead molding, a curing step is required in which the expanded bead molding is left to stand for a predetermined time in a high-temperature atmosphere adjusted to a temperature of about 60°C to 80°C to restore its shape. This curing step takes a long time, for example, about 12 hours.

[0004] Therefore, methods for shortening the curing time have been investigated to improve productivity. For example, Patent Document 1 discloses an expanded bead molding that does not require special equipment or processes, is easy to dry, and reduces dimensional shrinkage, eliminating the need for a high-temperature curing process, by filling a mold with expanded beads consisting of an expanded core layer made of a crystalline thermoplastic resin and an unexpanded coating layer, and introducing steam or hot air into the mold to heat and fuse the expanded beads, the expanded beads being obtained in such a way that fused portions are formed where the coating layers are in contact with each other and voids are formed between the expanded beads. Furthermore, Patent Document 2 discloses expanded polypropylene resin beads whose base resin is a propylene random copolymer or random block copolymer having a melt flow index, Z-average molecular weight, melting point, and endothermic energy of an endothermic peak appearing on the high-temperature side of a DSC curve within specific ranges, as expanded beads that can shorten the curing time and have improved secondary foaming properties, fusion properties, and cracking resistance.

[0005] JP 2003-039565 A JP 2000-129028 A

[0006] The technique described in Patent Document 1 can omit the curing step, but the resulting expanded bead molding has a significantly poor appearance due to the formation of numerous voids between the expanded beads in the molding, making it unusable for some applications. The technique described in Patent Document 2 can shorten the curing step, but there is room for improvement in its effectiveness. Furthermore, the technique described in Patent Document 2 requires a high steam pressure for molding, which places a heavy load on the equipment. Therefore, there is a need for a method that solves the above problems, particularly a method that reduces the steam pressure and shortens the curing step.

[0007] Therefore, an object of the present invention is to provide expanded beads that can significantly reduce the steam pressure during molding in a mold, shorten the curing time, and further, can produce expanded bead moldings that have a desired shape and excellent appearance even when the curing time is shortened.

[0008] The present inventors have found that the above-mentioned problems can be solved by adopting the following configuration, and have thus completed the present invention. That is, the present invention is as follows: <1> Expanded polypropylene-based resin beads having an expanded core layer made of a polypropylene-based resin and a coating layer coating the expanded core layer, wherein the coating layer is made of linear low-density polyethylene, the mass ratio of the coating layer to the expanded core layer is 0.005 or more and 0.05 or less, and the polypropylene-based resin constituting the expanded core layer satisfies the following (i) or (ii): (i) The polypropylene-based resin constituting the expanded core layer is an ethylene-butene-propylene copolymer i, and the total amount Ccmt of the ethylene component and the butene component in the copolymer i is 0.005 or more and 0.05 or less. （ｉ） (ii) the polypropylene-based resin constituting the foamed core layer is one or more copolymers ii selected from an ethylene-propylene copolymer and a butene-propylene copolymer, and the total amount Ccmt of the ethylene component and the butene component in the copolymer ii is 3.5% by mass or more; （ｉｉ）<2> The expanded polypropylene-based resin beads according to <1>, wherein the linear low-density polyethylene constituting the coating layer is a resin polymerized using a Ziegler-Natta polymerization catalyst. <3> The expanded polypropylene-based resin beads according to <1> or <2>, wherein the polypropylene-based resin constituting the foamed core layer has a flexural modulus of 400 MPa or more and less than 800 MPa. <4> The expanded polypropylene-based resin beads according to any one of <1> to <3>, wherein the coating layer has an average thickness (Tt) of 0.1 μm or more and 5 μm or less. <5> The expanded polypropylene-based resin beads according to any one of <1> to <4>, wherein the polypropylene-based resin constituting the foamed core layer is an ethylene-butene-propylene copolymer i. <6> The expanded polypropylene-based resin beads according to any one of <1> to <5>, wherein the polypropylene-based resin constituting the foamed core layer is a resin polymerized using a Ziegler-Natta polymerization catalyst. <7> The expanded polypropylene resin beads according to any one of <1> to <6>, wherein the linear low-density polyethylene constituting the coating layer has a flexural modulus of 120 MPa or more and 600 MPa or less. <8> The expanded polypropylene resin beads according to any one of <1> to <7>, wherein the expanded polypropylene resin beads have a bulk ratio of 5 times or more and 45 times or less.

[0009] The steam pressure during molding in a mold can be significantly reduced, and the curing time can be shortened. Furthermore, expanded beads can be provided that can produce expanded bead moldings having a desired shape and excellent appearance even when the curing time is shortened.

[0010] The expanded polypropylene-based resin beads of the present invention (hereinafter simply referred to as expanded polypropylene-based resin beads or expanded beads) are expanded polypropylene-based resin beads having an expanded core layer made of a polypropylene-based resin and a coating layer coating the expanded core layer, wherein the coating layer is made of linear low-density polyethylene, the mass ratio of the coating layer to the expanded core layer is 0.005 or more and 0.05 or less, and the polypropylene-based resin constituting the expanded core layer satisfies the following (i) or (ii): (i) the polypropylene-based resin constituting the expanded core layer is an ethylene-butene-propylene copolymer i, and the total amount Ccmt of the ethylene component and the butene component in the copolymer i is 0.005 or more and 0.05 or less. （ｉ） (ii) the polypropylene-based resin constituting the foamed core layer is one or more copolymers ii selected from an ethylene-propylene copolymer and a butene-propylene copolymer, and the total amount Ccmt of the ethylene component and the butene component in the copolymer ii is 3.5% by mass or more; （ｉｉ） is 4.0 mass % or more.

[0011] The expanded polypropylene resin beads of the present invention, having the above-described structure, can significantly reduce the steam pressure during in-mold molding and shorten the curing time. Furthermore, even with the shortened curing time, expanded bead moldings having the desired shape and excellent appearance can be obtained. In this specification, "shortening the time required for the curing step (curing time)" refers to, for example, shortening the curing time during the curing step, which typically requires leaving a molded product after release from a mold in a high-temperature environment of about 60°C to 80°C for about 12 hours to stabilize the shape of the molded product. The expanded beads of the present invention can stabilize the shape of a molded product, for example, by leaving a molded product after release from a mold in a high-temperature environment of about 60°C to 80°C for about 2 hours. Obtaining a molded product by shortening the curing time during in-mold molding of expanded beads, as described above, is also referred to as short-term curing molding. In this specification, the term "A to B" representing a numerical range is synonymous with "A or greater and B or less," and is used to include the values ​​A and B, which are the endpoints of the numerical range.

[0012] [Expanded Core Layer] The expanded polypropylene resin beads of the present invention have an expanded core layer composed of a polypropylene resin. The expanded core layer of the expanded polypropylene resin beads (hereinafter also simply referred to as the expanded core layer) is composed of a polypropylene resin and is in an expanded state. The polypropylene resin refers to a homopolymer of a propylene monomer and a propylene copolymer containing 50% by mass or more of structural units derived from propylene.

[0013] <Polypropylene Resin> In the expanded polypropylene resin beads of the present invention, the polypropylene resin constituting the expanded core layer satisfies the following (i) or (ii). That is, the polypropylene resin constituting the expanded core layer is either a polypropylene resin satisfying the following (i), a polypropylene resin satisfying the following (ii), or a mixed resin of a polypropylene resin satisfying the following (i) and a polypropylene resin satisfying the following (ii). (i) The polypropylene resin constituting the expanded core layer is an ethylene-butene-propylene copolymer i, and the total amount Ccmt of the ethylene component and the butene component in the copolymer i is （ｉ） (ii) the polypropylene-based resin constituting the foamed core layer is one or more copolymers ii selected from an ethylene-propylene copolymer and a butene-propylene copolymer, and the total amount Ccmt of the ethylene component and the butene component in the copolymer ii is 3.5% by mass or more; （ｉｉ） is 4.0% by mass or more. The term "total amount of ethylene components and butene components in the copolymer" refers to the "total (% by mass) of the content (% by mass) of ethylene-derived components and the content (% by mass) of butene-derived components relative to the total mass of the copolymer." However, when a copolymer contains only one of an ethylene-derived component and a butene-derived component, the total amount of ethylene components and butene components in the copolymer refers to the content (% by mass) of the one component. Thus, the foamed core layer is composed of a polypropylene-based resin with a high ratio of monomers other than propylene, which enhances the secondary foaming properties of the foamed core layer and provides excellent low-pressure moldability. Furthermore, because the amount of steam required for in-mold molding is reduced, the resulting molded article tends to maintain stable dimensions even after short curing periods.

[0014] The polypropylene resin constituting the foamed core layer is preferably a resin polymerized using a Ziegler-Natta polymerization catalyst. When the polypropylene resin constituting the foamed core layer is a resin polymerized using a Ziegler-Natta polymerization catalyst, it is particularly likely to exhibit stable formability during short-term curing. This resin is thought to have a broader molecular weight distribution and a relatively high content of low-molecular-weight components compared to resins polymerized using, for example, metallocene polymerization catalysts. The low-molecular-weight components melt early during molding in the mold, improving fusion properties and facilitating dimensional stability even during short-term curing.

[0015] (Ethylene-butene-propylene copolymer i) The polypropylene resin is preferably a polypropylene resin that satisfies the above (i) out of the above (i) and (ii). That is, the polypropylene resin preferably contains ethylene-butene-propylene copolymer i as a main component. Specifically, the mass proportion of ethylene-butene-propylene copolymer i in the polypropylene resin constituting the foamed core layer is preferably 60 mass% or more, more preferably 80 mass% or more. When the polypropylene resin is ethylene-butene-propylene copolymer i, the adhesion with the linear low-density polyethylene constituting the coating layer is improved, and the effects of the present invention can be more stably achieved. When the polypropylene resin constituting the foamed core layer is ethylene-butene-propylene copolymer i, the total amount Ccmt of the ethylene component and the butene component in copolymer i is （ｉ） The total amount Ccmt of the ethylene component and the butene component in the copolymer i is 3.5 mass% or more. （ｉ）is 3.5% by mass or more, preferably 4.0% by mass or more, and more preferably 4.5% by mass or more. It is also preferably 8.0% by mass or less, more preferably 7.0% by mass or less, and even more preferably 6.0% by mass or less. It is preferably 3.5 to 8.0% by mass, more preferably 4.0 to 8.0% by mass, even more preferably 4.0 to 7.0% by mass, and even more preferably 4.5 to 6.0% by mass. When the total amount of the ethylene component and the butene component in the copolymer i is within the above range, the secondary foaming property of the foamed core layer is enhanced, resulting in excellent low-pressure moldability. Furthermore, short-time curing moldability is also excellent. The mass ratio of the ethylene component to the butene component in the copolymer i (ethylene / butene) is preferably 50 / 50 to 98 / 2, more preferably 60 / 40 to 95 / 5, even more preferably 70 / 30 to 92 / 8, and even more preferably 80 / 20 to 90 / 10. When the mass ratio of the ethylene component to the butene component is within the above range, the adhesion to the linear low-density polyethylene that constitutes the coating layer becomes better.

[0016] (ii) One or more copolymers selected from ethylene-propylene copolymers and butene-propylene copolymers) When the polypropylene resin constituting the foamed core layer is one or more copolymers ii selected from ethylene-propylene copolymers and butene-propylene copolymers, the total amount Ccmt of the ethylene component and the butene component in the copolymer ii （ｉｉ） The polypropylene resin constituting the foamed core layer is preferably an ethylene-propylene copolymer. The copolymer ii may also be a mixed resin of an ethylene-propylene copolymer and a butene-propylene copolymer. The total amount Ccmt of the ethylene component and the butene component in the copolymer ii is （ｉｉ）is 4.0% by mass or more, preferably 4.2% by mass or more, and more preferably 4.5% by mass or more. It is also preferably 8.0% by mass or less, more preferably 7.0% by mass or less, and even more preferably 6.0% by mass or less. It is preferably 4.0 to 8.0% by mass, more preferably 4.0 to 7.0% by mass, even more preferably 4.2 to 6.0% by mass, and even more preferably 4.5 to 6.0% by mass. When the total amount of the ethylene component and the butene component in the copolymer ii is within the above range, the secondary foaming property of the foamed core layer is enhanced, resulting in excellent low-pressure moldability. Furthermore, short-time curing moldability is also improved.

[0017] The one or more copolymers ii selected from the group consisting of ethylene-butene-propylene copolymer i, ethylene-propylene copolymer, and butene-propylene copolymer constituting the foamed core layer may be random copolymers or block copolymers, and are preferably random copolymers from the viewpoint of more reliably improving the moldability of the foamed beads.

[0018] The content of each monomer component in the copolymer means the content of the constituent units derived from each monomer in the copolymer. The content of these monomer components can be determined by a known method using IR spectrum measurement.

[0019] (Bending Modulus Mc) From the viewpoint of improving moldability at low pressure and shortening curing time, the bending modulus Mc of the polypropylene-based resin constituting the foamed core layer is preferably 400 MPa or more, more preferably 500 MPa or more, even more preferably 550 MPa or more, particularly preferably 600 MPa or more, and preferably less than 800 MPa, more preferably 750 MPa or less, even more preferably 700 MPa or less, and particularly preferably 650 MPa or less. That is, the bending modulus Mc of the polypropylene-based resin constituting the foamed core layer is preferably 400 MPa or more and less than 800 MPa, more preferably 500 MPa or more and 750 MPa or less, even more preferably 550 MPa or more and 700 MPa or less, and particularly preferably 600 MPa or more and 650 MPa or less. By setting the bending modulus Mc of the polypropylene-based resin constituting the foamed core layer within the above range, excellent low-pressure moldability can be achieved, and dimensions can be stabilized by a short curing process. The flexural modulus Mc of the polypropylene-based resin is determined based on JIS K 7171:2016.

[0020] In the past, when foamed beads made of a polypropylene-based resin having a flexural modulus of less than 800 MPa were molded in a mold, the resistance to shrinkage and deformation after release from the mold was extremely low, and therefore, when the curing time was set short, the molded product tended to shrink or deform significantly. On the other hand, with the foamed beads described above, even when foamed beads made of a polypropylene-based resin having a flexural modulus of less than 800 MPa are used, the time required for the curing step can be shortened. This is because, as will be described later, the foamed beads have a foamed core layer made of a polypropylene-based resin of a predetermined composition, and also have a coating layer containing predetermined components and having a predetermined mass ratio relative to the foamed core layer.

[0021] (Melting Point Tmc) From the viewpoints of low-pressure moldability, short-time curing moldability, and the mechanical properties of the molded body, the melting point Tmc of the polypropylene resin constituting the foamed core layer is preferably 125°C or higher, more preferably 128°C or higher, even more preferably 130°C or higher, and preferably 160°C or lower, more preferably 150°C or lower, even more preferably 145°C or lower, and still more preferably 140°C or lower. That is, the melting point Tmc of the polypropylene resin constituting the foamed core layer is preferably 125 to 160°C, more preferably 128 to 150°C, even more preferably 130 to 145°C, and still more preferably 130 to 140°C. The melting point of polypropylene resins is determined based on JIS K 7121:1987, using "(2) Measurement of melting temperature after a certain heat treatment" as the conditioning of the test specimen. The conditioned test specimen is heated from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve, and the melting point is determined as the apex temperature of the melting peak associated with the melting of the resin on the DSC curve. Conditioning is performed under conditions of a heating rate and cooling rate of 10°C / min. The flow rate of nitrogen gas in the measurement environment is 30 mL per minute. If multiple melting peaks appear on the DSC curve, the apex temperature of the melting peak with the largest area is taken as the melting point.

[0022] (Melt Flow Rate (MFR)) From the viewpoint of improving foamability and improving low-pressure moldability and short-time curing moldability, the MFR of the polypropylene-based resin constituting the foam core layer is preferably 2 g / 10 min or more, more preferably 4 g / 10 min or more, even more preferably 5 g / 10 min or more, and preferably 15 g / 10 min or less, more preferably 12 g / 10 min or less, and even more preferably 10 g / 10 min or less. That is, the MFR of the polypropylene-based resin constituting the foam core layer is preferably 2 to 15 g / 10 min, more preferably 4 to 12 g / 10 min, and even more preferably 5 to 10 g / 10 min. The MFR of the polypropylene-based resin is measured in accordance with JIS K 7210-1:2014 at a temperature of 230°C and a load of 2.16 kg.

[0023] The foamed core layer may contain other polymers to the extent that the intended effects of the present invention are not impaired. Examples of other polymers include polypropylene-based resins that do not satisfy either (i) or (ii) above, other types of thermoplastic resins other than polypropylene-based resins, and thermoplastic elastomers. However, the content of the other polymers in the foamed core layer is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, and even more preferably 1% by mass or less, or 0% by mass. In other words, it is preferable that the foamed core layer contains only polypropylene-based resins that satisfy either (i) or (ii) above as polymers.

[0024] The foam core layer may contain additives as appropriate, provided that the additives do not impair the effects of the present invention. Examples of additives that can be added to the foam core layer include cell regulators, lubricants, flame retardants, flame retardant assistants, colorants, bubble nucleating agents, plasticizers, antistatic agents, antioxidants, ultraviolet inhibitors, light stabilizers, conductive fillers, and antibacterial agents.

[0025] [Coating Layer] The coating layer of the expanded polypropylene resin beads (hereinafter simply referred to as the coating layer) is composed of linear low-density polyethylene and coats the expanded core layer composed of a polypropylene resin. If the expanded beads do not have the coating layer or if the resin constituting the coating layer is, for example, a polypropylene resin, the amount of steam required for heat fusion increases excessively, and the low-pressure moldability and short-term sintering moldability that are the advantages of the present invention are not fully achieved. From the viewpoint of reliably achieving the intended effects of the present invention, the content of linear low-density polyethylene in the coating layer is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, even more preferably 80% by mass or more, and even more preferably 90% by mass or more. The proportion of the total surface area of ​​the expanded beads that is occupied by the coating layer (the coverage rate of the expanded core layer by the coating layer) is preferably 50% by mass or more, more preferably 60% by mass or more, from the viewpoint of fully obtaining the effects of the coating layer. Examples of expanded beads having a coating layer include multilayer expanded beads having a cylindrical foamed core layer in an expanded state and a coating layer covering the lateral surface of the foamed core layer. The coating layer is preferably in a non-expanded state from the viewpoint of improving low-pressure moldability and short-time curing moldability. Here, the non-expanded state includes not only a state in which no bubbles are present in the coating layer, but also a substantially non-expanded state in which only a small number of very small bubbles are present. Furthermore, the state in which no bubbles are present in the coating layer also includes a state in which bubbles that have formed once have broken and disappeared.

[0026] <Linear Low-Density Polyethylene (PE-LLD)> The coating layer is composed of linear low-density polyethylene (PE-LLD). Linear low-density polyethylene is a linear copolymer of ethylene and an α-olefin. The α-olefin constituting the copolymer typically has 4 to 10 carbon atoms. By using linear low-density polyethylene for the coating layer, it is possible to improve low-pressure moldability and short-curing formability. This is thought to be due to the linear low-density polyethylene's ability to improve the fusion between the foamed particles. Another reason is thought to be that the melting point of linear low-density polyethylene is within a suitable range and that it has suitable crystallinity. If the resin constituting the coating layer were a polyethylene-based resin other than linear low-density polyethylene, such as high-density polyethylene (PE-HD), there is a risk that low-pressure moldability would be impaired or that dimensional change would not be suppressed if the curing time was short. Furthermore, when the resin constituting the coating layer is low-density polyethylene (PE-LD), there is a risk that dimensional changes cannot be suppressed if the curing time is short.

[0027] (Heat of Fusion) From the viewpoint of short-term curing and formability, the heat of fusion of the PE-LLD constituting the coating layer is preferably 95 J / g or more and 130 J / g or less, more preferably 98 J / g or more and 120 J / g or less, and even more preferably 100 J / g or more and 110 J / g or less. The heat of fusion of the PE-LLD is determined based on JIS K 7122:1987, using "(2) Measurement of heat of fusion after a certain heat treatment" as the conditioning of the test specimen. The conditioned test specimen is heated from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve, and the heat of fusion is determined from the area of ​​the melting peak associated with resin melting on the DSC curve. Conditioning is performed under conditions of a heating rate and a cooling rate of 10°C / min. The flow rate of nitrogen gas in the measurement environment is 30 mL per minute. When a plurality of melting peaks appear on the DSC curve, the heat of fusion calculated from the area of ​​the largest melting peak is regarded as the heat of fusion of PE-LLD.

[0028] (Crystallization Temperature) From the viewpoint of short-term cure formability, the crystallization temperature of the PE-LLD constituting the coating layer is preferably 100°C or higher and 115°C or lower, more preferably 100°C or higher and 110°C or lower, and even more preferably 102°C or higher and 108°C or lower. The crystallization temperature of PE-LLD can be determined based on JIS K 7121:1987. Specifically, a test piece is heated from 23°C to 200°C at a heating rate of 10°C / min, held at 200°C for 10 minutes, and then cooled to 30°C at a cooling rate of 10°C / min to obtain a DSC curve. The crystallization temperature is determined as the apex temperature of the exothermic peak associated with resin crystallization on the DSC curve. The flow rate of nitrogen gas in the measurement environment is 30 mL / min. Note that if multiple exothermic peaks appear on the DSC curve, the apex temperature of the exothermic peak with the largest area is taken as the crystallization temperature.

[0029] (Density) The density of the linear low-density polyethylene (PE-LLD) constituting the coating layer is preferably 0.900 g / cm from the viewpoint of low-pressure moldability and short-time curing moldability. ３ More preferably, 0.905 g / cm ３ More preferably, it is 0.910 g / cm ３ and preferably 0.940 g / cm ３ or less, more preferably 0.930 g / cm ３ More preferably, it is 0.920 g / cm or less. ３ That is, the density of the linear low-density polyethylene (PE-LLD) constituting the coating layer is preferably 0.900 g / cm or less. ３ 0.940g / cm or more ３ or less, more preferably 0.905 g / cm ３ 0.930g / cm or more ３ More preferably, it is 0.910 g / cm or less. ３ 0.920g / cm or more ３ The density of PE-LLD is measured, for example, in accordance with Method B (pycnometer method) described in JIS K 7112:1999.

[0030] (Melting point Tms) From the viewpoints of low-pressure moldability and short-time hardening moldability, the melting point Tms of the PE-LLD constituting the coating layer is preferably 105°C or higher, more preferably 110°C or higher, and even more preferably 115°C or higher. Furthermore, the melting point Tms of the PE-LLD constituting the coating layer is preferably 128°C or lower, more preferably 125°C or lower, and even more preferably 122°C or lower. From the above viewpoints, the melting point Tms of the PE-LLD constituting the coating layer is preferably 105 to 128°C, more preferably 110 to 125°C, and even more preferably 115 to 122°C. The melting point Tms of PE-LLD is determined based on JIS K 7121:1987, using the "(2) Measurement of melting temperature after a certain heat treatment" method for conditioning the test specimen. The conditioned test specimen is heated from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve, and the melting point Tms is determined as the apex temperature of the melting peak associated with the melting of the resin on the DSC curve. Conditioning is performed under conditions of a heating rate and cooling rate of 10°C / min. The flow rate of nitrogen gas in the measurement environment is 30 mL per minute. If multiple melting peaks appear on the DSC curve, the apex temperature of the melting peak with the largest area is taken as the melting point.

[0031] The melting point Tms (°C) of the linear low-density polyethylene constituting the coating layer is preferably lower than the melting point Tmc (°C) of the polypropylene-based resin constituting the foamed core layer. In other words, it is preferable that Tms < Tmc. In this case, the moldability of the expanded beads is improved, particularly at low pressures. From this viewpoint, Tmc - Tms is preferably 3°C or higher, more preferably 5°C or higher, and even more preferably 10°C or higher. On the other hand, from the viewpoint of suppressing peeling between the foamed core layer and the coating layer and mutual adhesion between the expanded beads during production, Tmc - Tms is preferably 35°C or lower, more preferably 25°C or lower, and even more preferably 20°C or lower. From the above viewpoint, Tmc - Tms is preferably 3 to 35°C, more preferably 5 to 25°C, and even more preferably 10 to 20°C.

[0032] The linear low-density polyethylene constituting the coating layer is preferably a resin polymerized using a Ziegler-Natta polymerization catalyst. When the linear low-density polyethylene constituting the coating layer is a resin polymerized using a Ziegler-Natta polymerization catalyst, formability is more likely to be exhibited stably, particularly during short-term curing. This resin is thought to have a broader molecular weight distribution and a relatively large amount of low-molecular-weight components compared to resins polymerized using, for example, metallocene polymerization catalysts. The low-molecular-weight components melt early during molding in the mold, improving fusion properties and facilitating dimensional stability even during short-term curing.

[0033] The expanded beads exhibit excellent adhesion between the foamed core layer and the coating layer composed of linear low-density polyethylene polymerized using a Ziegler-Natta polymerization catalyst. This is thought to be due to the fact that the foamed core layer is composed of a polypropylene-based resin satisfying the above-mentioned (i) or (ii) criteria, containing a high copolymer content, which reduces the likelihood of molecular chain elimination at the interface between the core layer and the coating layer during the process of laminating the coating layer to the core layer by coextrusion (described below), and also because the core layer contains a sufficient amount of components compatible with the ethylene component of the coating layer. The excellent adhesive layer between the foamed core layer and the coating layer prevents peeling from the interface between the foamed core layer and the coating layer when the expanded bead molding is folded, thereby consistently producing moldings with excellent fusion properties at molding pressures expected from the configuration of the foamed core layer and the coating layer of the expanded beads. Therefore, expanded beads having the above configuration can be molded more easily at low pressures and more consistently exhibit the effect of short-term curing and formability.

[0034] In the past, expanded beads having a foamed core layer made of a polypropylene resin and a coating layer made of a linear low-density polyethylene that coats the foamed core layer have preferably used a resin polymerized using a metallocene polymerization catalyst as the linear low-density polyethylene that constitutes the coating layer, from the viewpoint of excellent adhesion between the foamed core layer and the coating layer. On the other hand, the expanded beads have excellent adhesion between the foamed core layer and the coating layer even when a resin polymerized using a Ziegler-Natta polymerization catalyst is used as the linear low-density polyethylene that constitutes the coating layer. Therefore, as described above, the foamed beads are more likely to exhibit stable moldability, especially in short-term curing.

[0035] (Bending Modulus Ms) The bending modulus Ms of the PE-LLD constituting the coating layer is preferably 120 MPa or more. In this case, the sliding properties and toughness of the molded article can be further improved. From this viewpoint, the bending modulus Ms of the PE-LLD constituting the coating layer is more preferably 150 MPa or more, even more preferably 180 MPa or more, and particularly preferably 200 MPa or more. On the other hand, the bending modulus Ms of the PE-LLD constituting the coating layer is preferably 600 MPa or less. In this case, the moldability of the expanded beads and the toughness of the resulting molded article can be further improved. From this viewpoint, the bending modulus Ms of the PE-LLD constituting the coating layer is more preferably 500 MPa or less, even more preferably 400 MPa or less, particularly preferably 300 MPa or less, and most preferably 250 MPa or less. From the above viewpoints, the flexural modulus Ms of the PE-LLD constituting the coating layer is preferably 120 to 600 MPa, more preferably 150 to 500 MPa, even more preferably 180 to 400 MPa, still more preferably 200 to 300 MPa, and even more preferably 200 to 250 MPa. The flexural modulus Ms of the PE-LLD is determined based on JIS K 7171:2016.

[0036] (Melt Flow Rate (MFR)) The melt flow rate (MFR) of the PE-LLD constituting the coating layer is preferably 0.5 to 5.0 g / 10 min, more preferably 1.0 to 3.5 g / 10 min, and even more preferably 1.5 to 2.5 g / 10 min, from the viewpoint of more easily producing expanded beads having excellent filling properties, for example, an aspect ratio of 0.8 to 1.3, and from the viewpoint of improving the coverage of the foamed core layer with the coating layer and more stably exhibiting the intended effects of the present application. The MFR of the PE-LLD is measured in accordance with JIS K 7210-1:2014 at a temperature of 190°C and a load of 2.16 kg.

[0037] The coating layer may contain a polyethylene resin other than linear low-density polyethylene, such as low-density polyethylene (PE-LD), high-density polyethylene (PE-HD), etc. However, from the viewpoint of further improving the effects of the linear low-density polyethylene, the proportion of linear low-density polyethylene in the resin constituting the coating layer is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 100% by mass, i.e., the resin constituting the coating layer contains only linear low-density polyethylene as a polymer.

[0038] The coating layer may contain other polymers or additives besides polyethylene-based resins, as long as the effects of the present invention are not impaired. Examples of such other polymers include thermoplastic resins such as polypropylene-based resins and polystyrene-based resins, and thermoplastic elastomers. The content of such other polymers in the coating layer is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less, based on the total mass of the coating layer. It is particularly preferred that the coating layer contains only polyethylene-based resins as polymers. Examples of additives include cell regulators, lubricants, flame retardants, flame retardant assistants, colorants, bubble nucleating agents, plasticizers, antistatic agents, antioxidants, ultraviolet inhibitors, light stabilizers, conductive fillers, and antibacterial agents.

[0039] [Characteristics of Expanded Polypropylene-Based Resin Beads] The expanded polypropylene-based resin beads of the present invention are polypropylene-based resin beads having an expanded core layer made of a polypropylene-based resin and a coating layer covering the expanded core layer, wherein the coating layer is made of linear low-density polyethylene, the mass ratio of the coating layer to the expanded core layer is 0.005 or more and 0.05 or less, and the polypropylene-based resin constituting the expanded core layer satisfies the above (i) or (ii). Preferably, the expanded beads further have the following characteristics:

[0040] <Mass Ratio of Foamed Core Layer to Coating Layer> The mass ratio of the coating layer to the foamed core layer [mass of coating layer / mass of foamed core layer] is 0.005 or more and 0.05 or less. If the mass ratio of the coating layer is too large, the molding pressure reduction effect is sufficient, but the formability during short-term curing is impaired. In particular, when attempting to produce a molded body with a low apparent density, a short curing time may not be sufficient to suppress dimensional change in the molded body. In other words, a long curing process is required. From the viewpoint of further suppressing significant shrinkage and deformation of the molded body after in-mold molding, the mass ratio of the coating layer to the foamed core layer is preferably 0.045 or less, more preferably 0.04 or less, and even more preferably 0.035 or less. On the other hand, if the mass ratio of the coating layer is too small, the molding pressure reduction effect of the coating layer may not be fully exerted, and significant shrinkage and deformation may not be suppressed when the curing time is shortened. From the viewpoint of suppressing shrinkage and deformation even when the curing time is shortened, the mass ratio of the coating layer to the foamed core layer is preferably 0.01 or more, more preferably 0.015 or more, and even more preferably 0.02 or more. The mass ratio of the coating layer to the foamed core layer is preferably 0.01 or more and 0.045 or less, more preferably 0.015 or more and 0.04 or less, and even more preferably 0.02 or more and 0.035 or less. The mass ratio of the foamed core layer to the coating layer in the expanded beads corresponds to the mass ratio of the core layer to the coating layer in the multilayer resin beads described below.

[0041] The reason why adjusting the mass ratio of the coating layer as described above makes it easier to suppress shrinkage and deformation of the molded body even when the curing time is shortened is believed to be as follows. To reduce the weight of the molded body, expanded beads with a low apparent density are typically used for in-mold molding. Furthermore, the coating layer of the expanded beads is typically composed of, for example, a substantially non-expanded resin. Therefore, when the mass ratio of the coating layer is high, it is necessary to further reduce the apparent density of the expanded core layer in order to reduce the apparent density of the entire expanded beads. As a result, it is believed that the resistance of the molded body to shrinkage and deformation after demolding is likely to be reduced. Furthermore, the coating layer is typically composed of linear low-density polyethylene, which typically has a lower melting point than the polypropylene-based resin that constitutes the foamed core layer. Therefore, when the mass ratio of the coating layer is high, it is believed that the expanded beads are more susceptible to the heat from steam during in-mold molding, making the entire expanded beads more likely to shrink. Based on the above, if the mass ratio of the coating layer is too high, it is believed that although the molding pressure of the expanded beads can be reduced, particularly when attempting to produce a molded body with a low apparent density, it becomes difficult to suppress significant shrinkage and deformation of the molded body during short-term curing. Therefore, the expanded beads of the present invention can achieve both low-pressure moldability and short-time curing moldability at a high level by forming an expanded core layer from a specific polypropylene-based resin and forming the expanded core layer and a coating layer composed of linear low-density polyethylene in a specific mass ratio.

[0042] From the viewpoint of more reliably improving formability during short-term curing, the average thickness (Tt) of the coating layer is preferably 5 μm or less, more preferably 4 μm or less, even more preferably 3.5 μm or less, and even more preferably 3 μm or less. Furthermore, from the viewpoint of fully exerting the molding pressure reducing effect of the coating layer and more reliably suppressing significant shrinkage and deformation when the curing time is shortened, the average thickness (Tt) of the coating layer is preferably 0.1 μm or more, more preferably 0.3 μm or more, even more preferably 0.5 μm or more, and even more preferably 1 μm or more. From the above viewpoints, the average thickness (Tt) of the coating layer is preferably 0.1 μm or more and 5 μm or less, more preferably 0.3 μm or more and 4 μm or less, even more preferably 0.5 μm or more and 3.5 μm or less, and even more preferably 1 μm or more and 3 μm or less.

[0043] In this specification, the average thickness (Tt) of the coating layer is a value calculated using the following formulas (1), (2), and (3). Formulas (1) to (3) are derived as the average value (Tt) of the coating layer of expanded beads obtained by expanding cylindrical multilayer resin beads having a coating layer into a similar shape. However, even when the expanded beads of the present invention are obtained from multilayer resin beads having a shape other than cylindrical, the average thickness (Tt) of the coating layer is calculated using the following formulas (1), (2), and (3). The physical properties of the expanded multilayer beads used to calculate the average thickness (Tt) of the coating layer of the expanded beads are values ​​determined from a group of expanded multilayer beads obtained by expanding a group of multilayer resin beads having the physical properties of the multilayer resin beads used to calculate the average thickness (Tt).

[0044] Pd={(4×W)÷(π×Ld×Db)} （１／３） ... (1) where the diameter of the cylindrical multi-layer foamed beads is Pd [cm], the mass of the multi-layer resin beads is W [g], and the apparent density of the multi-layer foamed beads is Db [g / cm ３ ], and the aspect ratio L / D of the resulting expanded beads is designated as Ld.

[0045] Cd = {Pd ２ −(4×R×W) ÷ (π×Pd×Ld×ρ) （１／２）... (2) where Cd [cm] is the diameter of only the core layer portion of the cylindrical multilayer foamed particle, R (dimensionless) is the mass ratio of the resin in the coating layer of the multilayer resin particle (mass of resin in coating layer (S) / (mass of resin in coating layer (S) + mass of resin in core layer (C))), and ρ [g / cm ３ ]

[0046] Tt [μm] = {(Pd-Cd)÷2}×10000...(3)

[0047] The method for calculating the apparent density Db of the expanded beads is as follows. First, the expanded beads are allowed to stand for 24 hours in an environment of 50% relative humidity, 23°C temperature, and 1 atm atmospheric pressure to adjust the state of the expanded beads. After measuring the mass (unit: g) of the expanded beads, the expanded beads are submerged in a measuring cylinder containing water at 23°C using a wire net or the like, and the volume (unit: L) of the expanded beads is determined by subtracting the volume of the wire net or the like from the rise in the liquid level. Then, the apparent density (unit: g / cm) of the expanded beads is calculated by converting the value obtained by dividing the mass of the expanded beads by the volume of the expanded beads. ３ ) can be calculated.

[0048] <Bulking ratio> The bulking ratio of the expanded polypropylene resin beads is preferably 50 times or less, more preferably 45 times or less, even more preferably 40 times or less, and particularly preferably 35 times or less. On the other hand, the bulking ratio of the expanded polypropylene resin beads is preferably 5 times or more, more preferably 10 times or more, even more preferably 15 times or more, and particularly preferably 20 times or more. From the above viewpoints, the bulking ratio of the expanded polypropylene resin beads is preferably 5 to 50 times, more preferably 5 to 45 times, even more preferably 10 to 45 times, still more preferably 15 to 40 times, and particularly preferably 20 to 35 times. By having the bulking ratio of the expanded polypropylene resin beads within the above range, the effects of the present invention can be fully exhibited, low-pressure moldability can be improved, and curing time can be shortened. The bulking ratio of the expanded beads can be determined as follows. First, the expanded beads are allowed to stand for 24 hours in an environment of 50% relative humidity, 23°C, and 1 atm pressure to adjust the state of the expanded beads. After conditioning, expanded beads were randomly taken out from the group of expanded beads and placed in a measuring cylinder with a volume of 1 L. A large number of expanded beads were placed up to the 1 L mark so as to form a natural pile. The mass W1 [g] of the placed expanded beads was divided by the placed volume V1 (1 [L]) (W1 / V1), and the bulk density [kg / m] of the expanded beads was calculated by converting the unit. ３ Then, the density [kg / m ] of the resin constituting the foamed core layer of the foamed beads is calculated. ３ ] is calculated by multiplying the bulk density [kg / m ３ ] can be calculated by dividing

[0049] <Average Cell Diameter> From the viewpoint of further improving the moldability of the expanded beads, the average cell diameter of the expanded polypropylene resin beads is preferably 80 μm or more, more preferably 100 μm or more, and even more preferably 150 μm or more. On the other hand, from the viewpoint of further improving the surface smoothness of the resulting expanded bead molding, the average cell diameter of the expanded polypropylene resin beads is preferably 300 μm or less, more preferably 250 μm or less, and even more preferably 220 μm or less. From the above viewpoints, the average cell diameter of the expanded polypropylene resin beads is preferably 80 to 300 μm, more preferably 100 to 250 μm, even more preferably 150 to 250 μm, and even more preferably 150 to 220 μm. The average cell diameter of the expanded beads is measured as follows: 20 or more expanded beads are randomly selected from a group of expanded beads. The expanded beads are cut through the center to divide them into two, and enlarged photographs of the entire cross sections of each are taken using a microscope such as a scanning electron microscope. In each cross-sectional photograph, four lines are drawn at equal angles (45°) from the outermost surface of the expanded beads through the center to the outermost surface on the opposite side. The number of cells intersecting each line is counted, and the total length of the four lines is divided by the total number of cells intersecting the lines to determine the cell diameter of each expanded bead. The arithmetic mean of these values ​​is used as the average cell diameter of the expanded beads.

[0050] <Closed Cell Ratio> The closed cell ratio of the expanded beads is preferably 85% or more, more preferably 90% or more, and even more preferably 95% or more. In this case, the moldability of the expanded beads and the rigidity of the resulting expanded bead molding can be further improved. The closed cell ratio of the expanded beads can be measured using an air comparison hydrometer in accordance with ASTM D2856-70.

[0051] <High-Temperature Peak> The expanded beads preferably have a crystalline structure in which, in a DSC curve obtained by heating the expanded beads from 23°C to 200°C at a heating rate of 10°C / min using heat flux differential scanning calorimetry (DSC), one or more endothermic peaks (hereinafter referred to as "high-temperature peaks") appear higher than the apex of the endothermic peak (hereinafter referred to as "intrinsic peak") inherent to the polypropylene-based resin constituting the core layer. In this case, the moldability of the expanded beads can be further improved, and the rigidity of the resulting expanded bead molded article can be further increased. From this perspective, the heat of fusion of the high-temperature peak is preferably 5 to 50 J / g, more preferably 8 to 40 J / g, and even more preferably 10 to 30 J / g.

[0052] <Other Properties> The expanded beads may have a columnar shape, such as a cylindrical shape, but are not particularly limited thereto. The aspect ratio L / D of the expanded polypropylene resin beads is preferably 0.8 to 1.4, in order to improve the packing properties of the expanded beads and the rigidity of the molded article. The aspect ratio L / D is more preferably 0.9 to 1.3, and even more preferably greater than 0.9 and less than 1.3. The aspect ratio L / D of the expanded beads is determined by measuring the maximum axial length (L) of 100 randomly selected expanded beads and the maximum cross-sectional diameter (D) of the cross-section of the beads in a direction perpendicular to the longitudinal direction of the maximum length using a vernier caliper, calculating the ratio (L / D), and arithmetically averaging the values. The expanded beads may also have a cylindrical shape with through holes passing through them in the axial direction. From the viewpoint of further improving the appearance of the resulting molded article, it is preferable that the expanded beads have no through holes.

[0053] [Method for producing expanded polypropylene-based resin beads] The expanded polypropylene-based resin beads of the present invention are polypropylene-based resin expanded beads having an expanded core layer made of a polypropylene-based resin and a coating layer coating the expanded core layer, wherein the coating layer is made of linear low-density polyethylene, the mass ratio of the coating layer to the expanded core layer is 0.005 or more and 0.05 or less, and the polypropylene-based resin constituting the expanded core layer satisfies the above (i) or (ii). There are no limitations on the production method, but the expanded polypropylene-based resin beads are preferably produced by the method shown below.

[0054] A preferred method for producing expanded polypropylene resin beads of the present invention includes the following steps (A) to (C): step (A): a dispersion step in which multilayer resin beads having an unfoamed core layer composed of a polypropylene resin and a coating layer composed of a linear low-density polyethylene coating the core layer are dispersed in a dispersion medium in a sealed container; step (B): a foaming agent impregnation step in which the multilayer resin beads are impregnated with a foaming agent; and step (C): an expansion step in which the expandable multilayer resin beads impregnated with the foaming agent are released from the sealed container together with the dispersion medium into an atmosphere having a pressure lower than that inside the sealed container, thereby foaming at least the core layer into an expanded core layer, thereby producing expanded beads. Steps (A) and (B) can be performed in any order, or simultaneously. However, from the viewpoint of efficient production, it is preferable to perform step (B) after step (A).

[0055] <Multilayer Resin Particles and Method for Producing Multilayer Resin Particles> The multilayer resin particles used in step (A) are multilayer resin particles having a non-foamed core layer made of a polypropylene-based resin and a coating layer made of linear low-density polyethylene and coating the core layer. The multilayer resin particles used in step (A) are preferably multilayer resin particles having a non-foamed cylindrical core layer made of a polypropylene-based resin and a coating layer made of linear low-density polyethylene and laminated on the lateral surface of the core layer by coextrusion to coat the core layer.

[0056] The method for producing multilayer resin particles can use, for example, an extruder having a core layer extruder, a coating layer extruder, and a co-extrusion die for forming a multilayer strand installed on the outlet side of these extruders. The polypropylene resin constituting the core layer and optional additives are supplied to the core layer extruder and melt-kneaded to form a core layer-forming melt-kneaded mixture, while the coating layer extruder is supplied with a linear low-density polyethylene constituting the coating layer and optional additives and melt-kneaded to form a coating layer-forming melt-kneaded mixture. The core layer-forming melt-kneaded mixture and the coating layer-forming melt-kneaded mixture are introduced into the co-extrusion die for forming a multilayer strand and merged to form a composite having a core-sheath structure consisting of a non-foamed core layer and a non-foamed coating layer covering the lateral surface of the core layer. The composite is then extruded in the form of a strand from a small hole in a die attached to the tip of the extruder, cooled in water, and cut to a predetermined mass using a pelletizer (strand cutting method), thereby obtaining multilayer resin particles having a non-foamed core layer and a coating layer coating the core layer. In addition to the above-mentioned method, methods for cutting the extruded composite include an underwater cutting method in which the composite is extruded into water and then cut, and a hot cutting method in which the composite is extruded into air and then cut immediately thereafter.

[0057] The particle diameter of the multilayer resin particles is preferably 0.1 to 5.0 mm, and more preferably 0.5 to 3.0 mm.

[0058] The mass of the multilayer resin particles 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 even more preferably 0.4 to 2 mg. The mass of the multilayer resin particles is determined by dividing the total mass of 100 randomly selected multilayer resin particles by 100.

[0059] The mass ratio of the coating layer to the expanded core layer of the expanded beads can be adjusted by the mass ratio of the coating layer to the unexpanded core layer of the multilayer resin beads. From this viewpoint, the mass ratio of the coating layer to the core layer of the multilayer resin beads is 0.005 or more and 0.05 or less, preferably 0.01 or more and 0.045 or less, more preferably 0.015 or more and 0.04 or less, and even more preferably 0.02 or more and 0.035 or less.

[0060] Additives such as cell control agents, flame retardants, flame retardant auxiliaries, cell nucleating agents, plasticizers, antistatic agents, antioxidants, UV inhibitors, light stabilizers, conductive fillers, and antibacterial agents can be added to the multilayer resin particles having a non-foamed core layer and a coating layer, as needed. When additives are added, they can be added in step (A). Examples of cell control agents include inorganic powders such as talc, mica, zinc borate, calcium carbonate, silica, titanium oxide, gypsum, zeolite, borax, aluminum hydroxide, and carbon; and organic powders such as phosphoric acid-based nucleating agents, phenol-based nucleating agents, amine-based nucleating agents, and polyethylene fluoride-based resin powder. When a cell control agent is added, the content of the cell control agent is preferably 0.01 to 1 part by mass per 100 parts by mass of the polymer constituting the core layer of the multilayer resin particles.

[0061] The particle size, average mass, and mass ratio of the coating layer to the core layer of the multilayer resin particles can be adjusted by appropriately changing the resin discharge speed, take-up speed, cutter speed, etc. in the strand cutting method.

[0062] <Step (A)> In step (A), the multilayered resin particles can be dispersed in a dispersion medium using, for example, a stirrer in a sealable container that can withstand heat and pressure, such as an autoclave.

[0063] The dispersion medium is not particularly limited as long as it does not dissolve the multilayer resin particles, and examples thereof include water, ethylene glycol, glycerin, and alcohols such as methanol and ethanol, with water being preferred.

[0064] In step (A), it is preferable to further add a dispersant to the dispersion medium to prevent fusion between the multilayer resin particles. 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, sparingly soluble inorganic salts are preferred, and kaolin is more preferred, due to ease of handling. When a dispersant is added, the amount of dispersant added is preferably 0.001 to 5 parts by mass per 100 parts by mass of the multilayer resin particles.

[0065] A surfactant may also be added to the dispersion medium as a dispersion aid. 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, the amount of the surfactant added is preferably 0.001 to 1 part by mass per 100 parts by mass of the multilayer resin particles.

[0066] <Step (B)> In step (B), for example, the core layer is heated to a temperature equal to or higher than the softening temperature of the polypropylene-based resin constituting the core layer, and the foaming agent is impregnated therein, thereby obtaining expandable multilayer resin particles.

[0067] The blowing agent is not particularly limited as long as it can expand the multilayer 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 chlorofluoromethane, trifluoromethane, 1,1-difluoroethane, 1,1,1,2-tetrafluoroethane, methyl chloride, ethyl chloride, and methylene chloride; and dialkyl ethers such as dimethyl ether, diethyl ether, and methyl ethyl ether. Among these, inorganic physical blowing agents that do not deplete the ozone layer and are inexpensive are preferred, with nitrogen, air, and carbon dioxide being more preferred, and carbon dioxide being even more preferred. These blowing agents may be used alone or in combination of two or more.

[0068] The amount of foaming agent to be added is determined taking into consideration the desired bulk density of the expanded beads, the type of polypropylene resin, the type of foaming agent, and the like. In the case of an organic physical foaming agent, the amount of foaming agent to be added is preferably 5 to 50 parts by mass relative to 100 parts by mass of the multilayer resin particles. In the case of an inorganic physical foaming agent, the amount of foaming agent to be added is preferably 0.1 to 30 parts by mass, and more preferably 0.5 to 15 parts by mass, relative to 100 parts by mass of the multilayer resin particles.

[0069] The heating temperature in step (B) is preferably equal to or higher than the melting point of the polypropylene resin and equal to or lower than the melting point + 80°C, specifically, 100°C to 230°C. The time for which the resin is held at the heating temperature is preferably 1 minute or more, more preferably 20 minutes or more, and preferably 100 minutes or less, more preferably 60 minutes or less. That is, the time for which the resin is held at the heating temperature is preferably 1 to 100 minutes, more preferably 20 to 60 minutes.

[0070] <Step (C)> In step (C), for example, the expandable multilayer resin particles impregnated with a blowing agent in step (B) and heated are released from a sealed container into an atmosphere with a pressure lower than that of the sealed container, thereby expanding at least the core layer into a foamed core layer, thereby producing expanded beads. Specifically, while maintaining the pressure in 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 expandable multilayer resin particles impregnated with a blowing agent are released from the sealed container together with a dispersion medium into an atmosphere with a pressure lower than that of the sealed container, usually atmospheric pressure, thereby expanding at least the core layer of the expandable multilayer resin particles into a foamed core layer, thereby producing expanded beads with a multilayer structure having a foamed core layer and a coating layer covering the foamed core layer. Alternatively, the expandable multilayer resin particles that have undergone step (B) can be cooled and removed, and then heated with a heating medium such as hot air or steam to expand the expandable multilayer resin particles, thereby producing expanded beads.

[0071] In step (C), the temperature during foaming is preferably 110°C to 170°C. The pressure inside the sealed container is preferably 0.5 MPa (G) or more and 5 MPa (G) or less. Note that "0.5 MPa (G)" means a gauge pressure of 0.5 MPa.

[0072] The above steps (A) to (C) are preferably carried out as a series of steps in a single sealed container. However, they may also be carried out as separate steps, such as by removing the multilayer resin particles or the like after each step and then putting them back into the sealed container to carry out the next step.

[0073] Furthermore, in the above process, it is preferable to carry out a first-stage holding step in which, during heating in the sealed container, the temperature is held at a temperature equal to or higher than (the melting point of the polypropylene-based resin constituting the core layer -20°C) and lower than (the melting end temperature of the polypropylene-based resin constituting the core layer) for a sufficient time, preferably about 10 to 60 minutes. Furthermore, it is preferable to subsequently carry out a second-stage holding step in which the temperature is adjusted to a range from (the melting point of the polypropylene-based resin -15°C) to (the melting end temperature of the polypropylene-based resin +10°C) and, if necessary, held at that temperature for an additional sufficient time, preferably 10 to 60 minutes. The expandable resin particles containing a blowing agent are then released from the sealed container under low pressure and expanded to obtain expanded particles having a crystalline structure that exhibits the above-mentioned high-temperature peak.

[0074] The expanded beads obtained as described above can be pressurized with an inorganic gas such as air to increase the internal pressure in the cells, and then heated with steam or the like to expand them (second-stage expansion) to produce expanded beads with a higher expansion ratio (lower apparent density).

[0075] <Production of Expanded Bead Molded Articles> Expanded bead molded articles can be obtained by in-mold molding of expanded beads. By using the expanded polypropylene resin beads of the present invention, the steam pressure during in-mold molding can be significantly reduced, and even if the curing time is shortened, an expanded bead molded article having the desired shape and excellent appearance can be obtained. In-mold molding of expanded beads can be carried out by filling the expanded beads into a molding die and heat-molding using a heating medium such as steam. Specifically, after filling the molding die with the expanded beads, a heating medium such as steam is introduced into the molding die to heat the expanded beads to cause secondary expansion, and the surfaces are melted and fused together to obtain an expanded bead molded article having the shape of the molding space. Alternatively, it is preferable to use a pressure molding method (e.g., JP-B-51-22951) in which the expanded beads are first 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 cells to 0.01 to 0.3 MPa higher than atmospheric pressure, and then the expanded beads are filled into a mold under atmospheric or reduced pressure, and a heating medium such as steam is supplied into the mold to heat and fuse the expanded beads. Alternatively, molding can also be performed using a compression filling molding method (JP-B-4-46217) in which expanded beads pressurized to atmospheric pressure or higher are filled into a mold pressurized to the pressure or higher with a compressed gas, and then a heating medium such as steam is supplied into the cavity to heat and fuse the expanded beads. Alternatively, foamed beads having a high secondary expansion power obtained under special conditions may be filled into a cavity of a mold under atmospheric pressure or reduced pressure, and then a heating medium such as steam is supplied to heat and fuse the foamed beads, as in a normal pressure filling molding method (Japanese Patent Publication No. 6-49795), or a combination of the above methods (Japanese Patent Publication No. 6-22919).

[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 resins and expanded beads used in the examples and comparative examples were subjected to the following measurements and evaluations. The expanded beads were conditioned by being left to stand at 50% relative humidity, 23°C, and 1 atm for 24 hours before the measurements of their physical properties.

[0078] [Measurement and Evaluation] <Raw Material Resin> (Monomer Component Content of Polypropylene Resin) The monomer component content of polypropylene resins (specifically, ethylene-propylene copolymer, ethylene-propylene-butene copolymer) was determined by a known method of determining by IR spectroscopy. Specifically, the content was determined by the method described in Polymer Analysis Handbook (edited by the Polymer Analysis Research Forum of the Japan Society for Analytical Chemistry, published in January 1995, published by Kinokuniya Shoten, pages 615-616, "II.2.3 2.3.4 Propylene / ethylene copolymer," and pages 618-619, "II.2.3 2.3.5 Propylene / butene copolymer"), that is, by a method of quantifying the absorbance of the ethylene component and the butene component based on the relationship between the value corrected by a predetermined coefficient and the thickness of a film-like test piece.

[0079] More specifically, first, a polypropylene-based resin was hot-pressed in an environment of 180°C to form a film, and a plurality of test pieces with different thicknesses were prepared. Next, the IR spectrum of each test piece was measured, and the 722 cm －１ and 733 cm －１ The absorbance (A ７２２ , A ７３３ ) and butene-derived 766 cm －１ The absorbance (A ７６６ ) was read. Next, for each test piece, the ethylene component content (unit: mass%) in the polypropylene-based resin was calculated using the following formulas (4) to (6). The ethylene component contents obtained for each test piece were arithmetically averaged to obtain the ethylene component content (unit: mass%) in the polypropylene-based resin.

[0080] (K' ７３３ ) ｃ =1 / 0.96 {(K' ７３３ ) ａ -0.268 (K' ７２２ ) ａ}...(4) (K' ７２２ ) ｃ =1 / 0.96 {(K' ７２２ ) ａ -0.150 (K' ７３３ )ａ} (5) Ethylene component content = 0.575 {(K' ７２２ ) ｃ +(K' ７３３ ) ｃ} ... (6)

[0081] However, K' in formulas (4) to (6) ａ is the apparent extinction coefficient (K') at each wave number ａ = A / ρt), and K' ｃ is the corrected extinction coefficient, A is the absorbance, and ρ is the density of the resin (unit: g / cm ３ ) and t is the thickness of the film-like test piece (unit: cm). The above formulas (4) to (6) can be applied to random copolymers.

[0082] Further, for each test piece, the butene component content (unit: mass%) in the polypropylene-based resin was calculated using the following formula (7). The arithmetic mean of the butene component contents obtained for each test piece was taken as the butene component content (unit: mass%) in the polypropylene-based resin. Butene component content = 12.3 (A ７６６ / L) (7) where A in formula (7) is absorbance, and L is the thickness (unit: mm) of the film-like test piece.

[0083] (Flexural Modulus) The flexural modulus of the resin was measured by the following method. First, the resin was heat-pressed at 230°C to produce a 4 mm sheet, and a test piece measuring 80 mm in length, 10 mm in width, and 4 mm in thickness was cut out from this sheet. The flexural modulus of this test piece was determined in accordance with JIS K 7171:2016. The radius R1 of the indenter and the radius R2 of the support table were both 5 mm, the distance between the supports was 64 mm, and the test speed was 2 mm / min.

[0084] (MFR) The MFR of a resin was measured at a temperature of 190°C or 230°C under a load of 2.16 kg in accordance with JIS K 7210-1: 2014. When measuring the MFR of a polypropylene-based resin, a temperature of 230°C was used, and when measuring the MFR of a linear low-density polyethylene, a temperature of 190°C was used.

[0085] (Melting Point (Tm)) The melting point of the resin was measured in accordance with JIS K 7121:1987. Specifically, "(2) Measurement of melting temperature after a certain heat treatment" was adopted, and the test specimen was conditioned at a heating rate and cooling rate of 10°C / min. The conditioned test specimen was heated from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve, and the melting point was determined as the apex temperature of the melting peak associated with the melting of the resin on the DSC curve. The conditioning was performed at a heating rate and cooling rate of 10°C / min. The flow rate of nitrogen gas in the measurement environment was 30 mL per minute. When multiple melting peaks appear on the DSC curve, the apex temperature of the melting peak with the largest area was taken as the melting point. The measurement device used was a heat flux differential scanning calorimeter (manufactured by SII Nanotechnology Inc., model number: DSC7020).

[0086] (Density) The density of the resin was measured in accordance with Method B (pycnometer method) described in JIS K 7112: 1999. The density of the polypropylene resin used in the present invention was 900 kg / m ３ is.

[0087] (Heat of fusion) The heat of fusion of linear low-density polyethylene was measured based on JIS K 7122:1987, using "(2) Measurement of heat of fusion after a certain heat treatment" as the conditioning of the test specimen. The conditioned test specimen was heated from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve, and the heat of fusion was determined from the area of ​​the melting peak associated with the melting of the resin on the DSC curve. The conditioning was performed under conditions of a heating rate and cooling rate of 10°C / min. The flow rate of nitrogen gas in the measurement environment was 30 mL per minute. When multiple melting peaks appear on the DSC curve, the heat of fusion calculated from the area of ​​the largest melting peak was used as the heat of fusion of the linear low-density polyethylene. The measurement device used was a heat flux differential scanning calorimeter (manufactured by SII Nanotechnology Inc., model number: DSC7020).

[0088] (Crystallization Temperature) The crystallization temperature of linear low-density polyethylene was determined based on JIS K 7121:1987. Specifically, a test piece was heated from 23°C to 200°C at a heating rate of 10°C / min, held at 200°C for 10 minutes, and then cooled to 30°C at a cooling rate of 10°C / min to obtain a DSC curve. The apex temperature of the exothermic peak associated with resin crystallization on the DSC curve was taken as the crystallization temperature. The flow rate of nitrogen gas in the measurement environment was 30 mL per minute. When multiple exothermic peaks appear on the DSC curve, the apex temperature of the exothermic peak with the largest area was taken as the crystallization temperature. The measurement device used was a heat flux differential scanning calorimeter (manufactured by SII Nanotechnology Inc., model number: DSC7020).

[0089] <Expanded beads> (Bulk ratio) The bulk ratio of the expanded beads was determined as follows: Expanded beads were randomly taken out from the expanded bead group and placed in a measuring cylinder with a volume of 1 L. A large number of expanded beads were placed up to the 1 L mark so as to form a natural pile. The mass W1 [g] of the placed expanded beads was divided by the placed volume V1 (1 [L]) to obtain the bulk density [kg / m ３ The density [kg / m ] of the polypropylene resin constituting the foamed core layer of the foamed beads was then determined. ３ ] is calculated by multiplying the bulk density [kg / m ３ ] was calculated by dividing by

[0090] (Heat of Fusion of High-Temperature Peak) The heat of fusion of the high-temperature peak of the expanded beads was determined as follows. Approximately 3 mg of expanded beads was sampled and heated from 23°C to 200°C at a heating rate of 10°C / min using a differential scanning calorimeter (TA Instruments DSC Q1000). A DSC curve was obtained, showing an endothermic peak (resin-specific peak) due to the melting inherent to the polypropylene resin and one or more melting peaks (high-temperature peaks) on the higher temperature side of the peak. In the following explanation, the resin-specific peak is designated as A, and the high-temperature peak appearing on the higher temperature side is designated as B. 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 T of the expanded beads. The melting end temperature T refers to the intersection of the DSC curve on the higher temperature side of high-temperature peak B with the high-temperature-side baseline. Next, a 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 resin-specific peak A and the high-temperature peak B, and the point where this line intersected with the line (α-β) was designated as δ. The area of ​​high-temperature peak B was the area enclosed by the curve of the high-temperature peak B portion of the DSC curve, the line segment (δ-β), and the line segment (γ-δ), and this was designated as the heat of fusion of the high-temperature peak.

[0091] (Closed Cell Ratio) The closed cell ratio of the expanded beads was measured using an air comparison type hydrometer based on ASTM D2856-70. Specifically, first, the bulk volume of the expanded beads after conditioning, that is, the value of the mark when naturally deposited in a measuring cylinder, was measured. ３After collecting a measurement sample so that the apparent volume of the measurement sample satisfies the above equation, the apparent volume of the measurement sample was measured. Specifically, the apparent volume of the measurement sample corresponds to the amount of rise in the liquid level when the measurement sample is submerged in a measuring cylinder containing ethanol at 23°C. The measurement sample whose apparent volume has been measured was thoroughly dried, and the true volume of the measurement sample was measured using an air comparison hydrometer 930 manufactured by Toshiba Beckman Corporation in accordance with Procedure C described in ASTM-D2856-70. These volume values ​​were then used to calculate the closed cell ratio of the measurement sample according to the following formula (8). The above operation was performed five times using different measurement samples, and the arithmetic mean value of the closed cell ratios obtained from these five measurements was taken as the closed cell ratio of the expanded beads. Closed cell ratio (%) = (Vx - W / ρ) x 100 / (Va - W / ρ) (8) where Vx (unit: cm) in the above formula (8) is the closed cell ratio of the expanded beads. ３ ) is the true volume of the expanded beads (i.e., the sum of the volume of the resin constituting the expanded beads and the total volume of the closed cells in the expanded beads), and Va (unit: cm ３ ) is the apparent volume of the expanded beads (i.e., the volume measured from the rise in the liquid level when the expanded beads are submerged in a measuring cylinder containing ethanol), W (unit: g) is the mass of the measurement sample, and ρ (unit: g / cm ３ ) is the density of the polypropylene resin constituting the foamed core layer.

[0092] (Average Cell Diameter) The average cell diameter of expanded beads was measured as follows. Twenty expanded beads were randomly selected from a group of expanded beads. Each expanded bead was cut through its center to divide it into two halves, and an enlarged photograph of the entire cross section was taken using a scanning electron microscope. In each cross-sectional photograph, four lines were drawn at equal angles (45°) from the outermost surface of the expanded bead through the center to the outermost surface on the opposite side. The number of cells intersecting each line was counted, and the total length of the four line segments was divided by the total number of cells intersecting the lines to determine the cell diameter of each expanded bead. The arithmetic mean of these values ​​was determined as the average cell diameter of the expanded beads.

[0093] (Aspect Ratio (L / D)) The aspect ratio L / D of the expanded beads was determined by measuring the maximum axial length (L) and the maximum cross-sectional diameter (D) of the cross section of the particle in a direction perpendicular to the longitudinal direction of the maximum length with a vernier caliper for 100 randomly selected expanded beads, calculating the ratio (L / D), and arithmetically averaging the values.

[0094] (Average Thickness of Coating Layer) The average thickness of the coating layer of the expanded beads was obtained by the above-mentioned method. Specifically, the values ​​calculated by the above-mentioned formulas (1) to (3) were shown in Table 2 as the average thickness of the coating layer of the expanded beads.

[0095] (Adhesion) The adhesion between the foamed core layer and the coating layer of the expanded beads was evaluated by the following method. 1,000 expanded beads obtained in the Examples and Comparative Examples were randomly selected and visually observed. Specifically, the expanded beads were observed to see whether or not there were any areas where the coating layer had peeled off from the foamed core layer. The observation results were evaluated according to the following criteria. Expanded beads with no peeling of the coating layer have excellent adhesion to the coating layer and are preferred. (Evaluation criteria) A: Fewer than 5 expanded beads had areas where the coating layer had peeled off from the foamed core layer B: 5 or more expanded beads had areas where the coating layer had peeled off from the foamed core layer

[0096] (Lower limit molding pressure) After drying the expanded beads at 23°C for 24 hours, the expanded beads were pressurized to apply internal pressure to the cells of the expanded beads to the internal particle pressure listed in Table 2. The expanded beads were filled into a flat mold measuring 250mm long x 200mm wide x 50mm thick, with the cracking amount adjusted to 10% (i.e., 5mm). The mold was then clamped and subjected to an exhaust process in which steam was supplied from both sides of the mold for 5 seconds to preheat. One-sided heating was then performed by supplying steam from one side of the mold until a pressure 0.04 MPa (G) lower than the predetermined molding pressure was reached. Next, one-sided heating was performed by supplying steam from the other side of the mold until a pressure 0.02 MPa (G) lower than the molding pressure was reached, and heating was then performed until the predetermined molding pressure was reached (i.e., main heating). After heating was completed, the pressure was released, and the molded product was water-cooled until the surface pressure due to the foaming force reached 0.04 MPa (G), and then released from the mold. The molded body after demolding was cured in an 80°C atmosphere for 12 hours, and then allowed to stand in a room temperature atmosphere until the temperature of the molded body reached room temperature (23°C), resulting in a molded body. The obtained molded body was evaluated for three criteria, namely, fusion property, surface property, and recovery property, according to the criteria described below. A molded body that passed all of the evaluations was deemed to be a passing product. In this specification, a molded body that passed both surface property and recovery property was deemed to have a good appearance. The molding and evaluation of the above molded body were performed by changing the molding pressure between 0.10 and 0.28 MPa (G) in increments of 0.02 MPa, and the lowest molding pressure at which a passing product could be obtained was designated as the lower limit molding pressure. The lower the lower limit molding pressure, the better the moldability of the expanded beads, and the less steam required for molding.

[0097] (Fusing property) The molded article was bent and broken, and the number C1 of expanded beads present on the fracture surface and the number C2 of broken expanded beads were determined, and the ratio of the number of broken expanded beads to the number of expanded beads present on the fracture surface (i.e., material failure rate) was calculated. The material failure rate was calculated using the formula (C2 / C1) x 100. The above measurement was carried out five times using different test pieces, and the material failure rate was calculated for each. An arithmetic mean value of the material failure rate of 80% or more was considered to be acceptable.

[0098] (Surface Properties) A ​​100 mm × 100 mm rectangle was drawn in the center of the molded article, and a diagonal line was drawn from one corner of the rectangle. The number of voids (gaps between expanded beads) larger than a square with sides of 2 mm and resulting from through-holes in the expanded beads that were formed so as to overlap the diagonal line were counted, and a total of less than 5 was deemed acceptable.

[0099] (Recovery) A molded body was obtained using a flat mold having a length of 250 mm, a width of 200 mm, and a thickness of 50 mm. The thicknesses of the four corners (specifically, 10 mm inward from the corners toward the center) and the center (the part dividing the body into two equal parts in both the length and width directions) were measured. Next, the ratio (unit: %) of the thickness of the thinnest part to the thickness of the thickest part among the measured parts was calculated, and a ratio of 98% or more was deemed to be acceptable.

[0100] (Formability after short-term curing) A molded body was obtained in the same manner as in the molding described above for (lower limit molding pressure), except that the molding pressure was the lower limit molding pressure shown in Table 2, and the molded body after demolding was cured for 2 hours in an atmosphere at 80°C. The obtained molded body was evaluated for (recovery) as described above. The better the recovery after 2 hours of curing, the more stable the dimensions will be even if the curing time is shortened, and the more preferable it is. (Evaluation criteria) A: Passed recovery after 2 hours of curing (the ratio of the thickness of the thinnest part of the molded body to the thickness of the thickest part is 98% or more) B: Failed recovery after 2 hours of curing (the ratio of the thickness of the thinnest part of the molded body to the thickness of the thickest part is less than 98%)

[0101] <Expanded Bead Molded Articles> The expanded bead molded articles of the Examples and Comparative Examples were subjected to the following measurements and evaluations. The density of the molded articles was measured using the molded articles obtained at the lower limit molding pressure in the molding process described above (lower limit molding pressure). The molded articles were conditioned by standing for 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm before use in the measurements.

[0102] (Density) The weight of the expanded bead molding divided by the volume calculated based on the dimensions is the molding density (kg / m) of the expanded bead molding. ３ )

[0103] [Raw Materials] The resins used in the Examples and Comparative Examples are shown in Table 1. In Tables 1 and 2, resin names "LL" 1 to 4 and structure "LLDPE" are linear low-density polyethylene (PE-LLD), and "PP" 1 to 5 are polypropylene-based resins. Structure "Et-Bt-PP" is an ethylene-butene-propylene copolymer, and "Et-PP" is an ethylene-propylene copolymer.

[0104]

[0105] Examples 1 to 3 and Comparative Examples 1 to 5 <Preparation of Multilayer Resin Particles> A polypropylene-based resin shown in Table 2 was melt-kneaded in an extruder at a maximum set temperature of 240°C to obtain a resin melt for the core layer. Simultaneously, a linear low-density polyethylene or polypropylene-based resin shown in Table 2 was melt-kneaded in an extruder at a maximum set temperature of 240°C to obtain a resin melt for the coating layer. The resin melt for the core layer and the resin melt for the coating layer were then fed into a co-extrusion die, and laminated by co-extrusion in the die so that the resin melt for the coating layer covered the lateral surface of the resin melt for the core layer. The resulting strands were extruded and water-cooled, and cut into pellets with a mass of approximately 1.0 mg using a pelletizer. These pellets were then dried to obtain cylindrical multilayer resin particles (particle diameter: 2.2 mm, aspect ratio L / D: 2.7). The amounts of the resin melt for the core layer and the resin melt for the coating layer were adjusted so that the mass ratio of the coating layer to the core layer [mass of coating layer / mass of core layer] was equal to the "mass ratio of coating layer" in Table 2. In producing the multilayer resin particles, zinc borate was supplied as a bubble nucleating agent to the extruder of the resin melt for the core layer. The bubble nucleating agent was supplied as a masterbatch, and 1000 ppm of zinc borate was contained as a bubble nucleating agent only in the polypropylene resin for the core layer, while no bubble nucleating agent was contained in the polymer for the coating layer.

[0106] <Preparation of Expanded Beads> 1 kg of the multilayer 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 a dispersant and 0.2 parts by mass of a surfactant (Neogen, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.; sodium alkylbenzene sulfonate) (as an active ingredient) were added to the sealed container relative to 100 parts by mass of the multilayer resin particles. Carbon dioxide as a blowing agent was added to the sealed container so that the pressure inside the container reached the pressure shown in Table 2. The mixture was heated with stirring to the expansion temperature shown in Table 2 and maintained at that temperature for 15 minutes. The contents of the container were then released to atmospheric pressure to obtain expanded beads (first-stage expanded beads). The coating layer of the expanded beads was unexpanded in all Examples and Comparative Examples. The expanded beads were then placed in a pressure-resistant container (specifically, a metal drum), and air was injected into the pressure-resistant container to increase the pressure inside the container, thereby impregnating the air bubbles and increasing the internal pressure inside the bubbles of the expanded beads. Next, steam was supplied to the expanded beads (first-stage expanded beads) removed from the pressure vessel so that the pressure inside the pressure vessel (i.e., drum pressure) became the pressure shown in Table 2, and the beads were heated under atmospheric pressure. The pressure inside the cells of the first-stage expanded beads removed from the pressure vessel (i.e., internal pressure) was the value shown in Table 2. In this way, the apparent density of the first-stage expanded beads was reduced, and expanded beads (second-stage expanded beads) were obtained. The evaluation results of the obtained expanded beads are shown in Table 2.

[0107]

[0108] The expanded beads of the Examples have a significantly low lower limit of molding pressure capable of producing a good molded article with excellent fusibility, surface properties, and recovery, and can be molded in a mold with a very low steam pressure. Furthermore, the expanded beads of the Examples exhibit excellent recovery, suppressing shrinkage and deformation even after a short curing time of 2 hours in a high-temperature atmosphere. From the above, the expanded polypropylene resin beads of the present invention can significantly reduce the steam pressure during molding in a mold, and can produce expanded bead molded articles with the desired shape and excellent appearance even with a shortened curing time. Furthermore, the expanded beads of the Examples exhibited excellent adhesion, even though the resin constituting the coating layer was linear low-density polyethylene polymerized using a Ziegler-Natta polymerization catalyst.

Claims

1. A polypropylene-based resin expanded particle having a foamed core layer made of a polypropylene-based resin and a coating layer coating the foamed core layer, the coating layer being made of linear low-density polyethylene, the mass ratio of the coating layer to the foamed core layer being 0.005 or more and 0.05 or less, and the polypropylene-based resin constituting the foamed core layer satisfies the following (i) or (ii): (i) the polypropylene-based resin constituting the foamed core layer is an ethylene-butene-propylene copolymer i, and the total amount Ccmt of the ethylene component and the butene component in the copolymer i is 0.005 or more and 0.05 or less. （ｉ） (ii) the polypropylene-based resin constituting the foamed core layer is one or more copolymers ii selected from an ethylene-propylene copolymer and a butene-propylene copolymer, and the total amount Ccmt of the ethylene component and the butene component in the copolymer ii is 3.5% by mass or more; （ｉｉ） is 4.0 mass % or more.

2. The polypropylene resin expanded particles according to claim 1, wherein the linear low-density polyethylene constituting the coating layer is a resin polymerized using a Ziegler-Natta polymerization catalyst.

3. The polypropylene-based resin expanded particles according to claim 1 or 2, wherein the flexural modulus of the polypropylene-based resin constituting the foamed core layer is 400 MPa or more and less than 800 MPa.

4. The expanded polypropylene resin particles according to claim 1 or 2, wherein the average thickness (Tt) of the coating layer is 0.1 μm or more and 5 μm or less.

5. The polypropylene resin expanded particles according to claim 1 or 2, wherein the polypropylene resin constituting the foamed core layer is an ethylene-butene-propylene copolymer i.

6. The polypropylene resin expanded particles according to claim 1 or 2, wherein the polypropylene resin constituting the foamed core layer is a resin polymerized using a Ziegler-Natta polymerization catalyst.

7. The polypropylene resin expanded particles according to claim 1 or 2, wherein the linear low-density polyethylene constituting the coating layer has a flexural modulus of 120 MPa or more and 600 MPa or less.

8. The expanded polypropylene resin beads according to claim 1 or 2, wherein the expanded beads have a bulk ratio of 5 to 45 times.

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