Ethylene-based polymer foamed particles

Abstract

An ethylene-based polymer foamed particle having a foamed core layer composed of an ethylene-based polymer (I), and a coating layer coating the foamed core layer, wherein a gel fraction of the ethylene-based polymer foamed particle determined by a hot xylene extraction method is 40 mass% or more and 70 mass% or less, the coating layer contains an ethylene-based polymer (II), one or more resins (III) selected from the group consisting of a polypropylene-based resin and a polystyrene-based resin, and a radical scavenger, a mass proportion of the resin (III) in the coating layer in 100 mass% of the ethylene-based polymer (II) and the resin (III) is 1 mass% or more and 60 mass% or less, and a blending amount of the radical scavenger in the coating layer is 10 mass ppm or more and 10,000 mass ppm or less.

Description

Technical Field

[0001] This invention relates to ethylene-based polymer foam particles. Background Technology

[0002] Foamed particles composed of ethylene-based polymers possess high flexibility, making them suitable for applications such as packaging materials. Among ethylene-based polymers, for example, foamed particle molded bodies formed by in-mold molding of foamed particles containing olefin-based thermoplastic elastomers can achieve lightweighting while maintaining the excellent properties of olefin-based thermoplastic elastomers, such as flexibility and resilience. Therefore, they hold promise for further expansion of applications in sporting goods, automotive components, and building materials.

[0003] Patent Document 1 discloses a foamed particle with the aim of providing olefin-based thermoplastic elastomer foamed particles with excellent in-mold formability. The olefin-based thermoplastic elastomer constituting the foamed particle is a block copolymer having polyethylene blocks and ethylene / α-olefin copolymer blocks, and the hardness of the block copolymer on a type A hardness tester is within a specific range.

[0004] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2018-70735 Summary of the Invention The technical problem that the invention aims to solve However, when the foamed particles described in Patent Document 1 are molded after long-term storage, the in-mold bonding may sometimes decrease depending on the storage conditions.

[0005] Therefore, the technical problem of the present invention is to provide ethylene-based polymer foam particles that still exhibit excellent fusion properties during in-mold forming even after long-term storage.

[0006] Solution to the above technical problems The inventors have discovered that the above-mentioned technical problems can be solved by adopting the following configuration, thereby completing the present invention.

[0007] That is, the present invention is as follows.

[0008] <1> An ethylene-based polymer foamed particle has a foamed core layer composed of an ethylene-based polymer (I) and a coating layer covering the foamed core layer, wherein the gel fraction of the ethylene-based polymer foamed particle, as determined by hot xylene extraction, is 40% by mass or more and 70% by mass or less, the coating layer comprises an ethylene-based polymer (II), one or more resins (III) selected from the group consisting of polypropylene resins and polystyrene resins, and a free radical scavenger, wherein the resin (III) in the coating layer has a mass ratio of 1% by mass or more and 60% by mass or less in a total of 100% by mass of the ethylene-based polymer (II) and the resin (III), and the amount of the free radical scavenger in the coating layer is 10 ppm by mass or more and 10,000 ppm by mass or less.

[0009] <2> according to <1> The ethylene-based polymer foamed particles, wherein the free radical scavenger is one or more additives selected from the group consisting of antioxidants and weathering agents.

[0010] <3> according to <2> The ethylene-based polymer foamed particles, wherein the antioxidant is one or more compounds selected from the group consisting of phenolic antioxidants, phosphorus antioxidants and thioether antioxidants.

[0011] <4> according to <2> The ethylene-based polymer foamed particles, wherein the weather-resistant agent is a hindered amine light stabilizer.

[0012] <5> according to <1> ~ <4> The ethylene-based polymer foam particles according to any one of the following methods, wherein the ethylene-based polymer (I) is a thermoplastic elastomer.

[0013] <6> according to <5> The ethylene-based polymer foam particles, wherein the thermoplastic elastomer is a block copolymer with polyethylene blocks as hard segments.

[0014] <7> according to <1> ~ <6> The ethylene-based polymer foaming particles according to any one of the following methods, wherein the resin (III) is a polypropylene resin.

[0015] <8> according to <7> The ethylene-based polymer foamed particles, wherein the difference between the melting point Tmp of the polypropylene resin and the melting point Tme of the ethylene-based polymer (I) [Tmp-Tme] is greater than 5°C and less than 35°C.

[0016] <9> An ethylene-based polymer foamed particle, wherein the gel fraction of the ethylene-based polymer foamed particle, as determined by hot xylene extraction, is 40% by mass or more and 70% by mass or less, and the degree of oxidation of the ethylene-based polymer foamed particle, as determined by the following determination conditions (1), is 3.5 or less.

[0017] Measurement conditions (1): The foamed particles were divided into Group A, which was aged for 1 day in the dark at 23°C, and Group B, which was aged for 7 days in the dark at 80°C. The ATR of the foamed particles in Group A and Group B was measured and calculated using the following formula (1).

[0018] Oxidation degree = Oxidation degree of group B (A) 1730cm-1 / A 1460cm-1 ) / Oxidation degree of group A (A) 1730cm-1 / A 1460cm-1 Equation (1) [In equation (1), A represents absorbance, and the subscript indicates wavenumber.] Invention Effects According to the present invention, it is possible to provide vinyl polymer foam particles that exhibit excellent fusion properties during in-mold forming even after long-term storage. Detailed Implementation

[0019] [Ethylene-based polymer foamed particles] The ethylene polymer foamed particles of the present invention (hereinafter also simply referred to as ethylene polymer foamed particles or foamed particles) are ethylene polymer foamed particles having a foamed core layer composed of an ethylene polymer (I) and a coating layer covering the foamed core layer, wherein the gel fraction of the ethylene polymer foamed particles, as determined by hot xylene extraction, is 40% by mass or more and 70% by mass or less, the coating layer comprises an ethylene polymer (II), one or more resins (III) selected from the group consisting of polypropylene resins and polystyrene resins, and a free radical scavenger, wherein the resin (III) in the coating layer has a mass ratio of 1% by mass or more and 60% by mass or less in a total of 100% by mass of the ethylene polymer (II) and the resin (III), and the amount of the free radical scavenger in the coating layer is 10 ppm by mass or more and 10,000 ppm by mass or less.

[0020] The vinyl polymer foamed particles of the present invention have a multilayer structure having a foamed core layer composed of a vinyl polymer (I) and a coating layer covering the foamed core layer. The coating layer comprises a vinyl polymer (II), one or more resins (III) selected from the group consisting of polypropylene resins and polystyrene resins, and a free radical scavenger. Therefore, for example, even under long-term storage under harsh conditions such as exposure to external air, the reduction in the fusion properties of the foamed particles during in-mold molding can be suppressed, resulting in the formation of foamed particle molded bodies with excellent flexural strength.

[0021] In addition to the ethylene polymer (II), the coating layer also contains a specified amount of one or more resins (III) selected from the group consisting of polypropylene resins and polystyrene resins, as well as a free radical scavenger, thereby suppressing the decrease in the fusibility of the foamed particles. The reason for this is not yet clear, but it is speculated as follows.

[0022] The coating layer of the ethylene-based polymer foam particles of the present invention is formed by ethylene copolymer (II) and polypropylene resin and / or polystyrene resin, creating a morphology of an island structure with ethylene copolymer (II) as the ocean phase (continuous phase) and polypropylene resin and / or polystyrene resin as the island phase (dispersed phase). Additionally, "A and / or B" indicates A and B, or both A and B. If the foam particles are stored exposed to sunlight or external air, they are prone to generating free radicals due to light or heat. Because polypropylene resin and polystyrene resin have tertiary carbons with weak carbon-hydrogen bonding in their molecular backbone, they are more easily oxidized than ethylene polymer (II). Therefore, tertiary free radicals with a generation advantage are easily generated in polypropylene resin and polystyrene resin, and free radical scavengers capture and deactivate these tertiary free radicals. After the free radical scavenger is consumed, tertiary free radicals are further generated in the polypropylene resin and / or polystyrene resin, followed by secondary free radicals in the ethylene polymer (II). Here, because the secondary radicals generated in the vinyl polymer (II) are more likely to induce hydrogen abstraction reactions from polypropylene and / or polystyrene resins than from crosslinking among themselves, tertiary radicals are newly generated through hydrogen abstraction reactions of secondary radicals from polypropylene and / or polystyrene resins. As a result of following this cycle, the continued generation of secondary radicals in the vinyl polymer (II) can be delayed even when the foamed particles are stored under harsh conditions. Therefore, it is believed that the surface oxidation of the foamed particles is suppressed, and the decrease in the molecular weight of the polymer constituting the surface of the foamed particles can be suppressed. As a result, even under long-term storage under harsh conditions, it is believed that the decrease in the meltability of the foamed particles during in-mold molding can be suppressed.

[0023] On the other hand, when the coating layer does not contain polypropylene resin and / or polystyrene resin and free radical scavenger, but is composed of ethylene polymer (II), the continuous generation of secondary free radicals in ethylene polymer (II) cannot be delayed. Therefore, it is believed that under long-term storage of foamed particles, oxidation or cross-linking on the surface of the foamed particles will progress, resulting in a decrease in the fusibility of the foamed particles during in-mold molding.

[0024] <Foamed Core Layer> The foamed core layer (hereinafter also referred to as the foamed core layer or core layer) of the ethylene polymer foamed particles is composed of ethylene polymer (I) and is in a foamed state.

[0025] (Ethylene polymers (I)) The ethylene-based polymer (I) is a polymer containing 50% by mass or more of ethylene, preferably 60% by mass or more of ethylene, and more preferably 70% by mass or more of ethylene. Examples of ethylene-based polymers (I) include polyethylene resins, thermoplastic elastomers, or mixtures thereof. Among them, the ethylene-based polymer (I) preferably contains a thermoplastic elastomer, and more preferably a thermoplastic elastomer.

[0026] Examples of polyethylene-based resins include very low-density polyethylene (PE-VLD), low-density polyethylene (PE-LD), linear low-density polyethylene (PE-LLD), medium-density polyethylene (PE-MD), high-density polyethylene (PE-HD), ethylene-vinyl acetate copolymer (EVA), ethylene-methyl methacrylate copolymer (EMMA), and ethylene-ethyl acrylate copolymer (EEAK). Polyethylene-based resins can be used alone or in combination of two or more types. Low-density polyethylene is preferably characterized by a long-chain branched structure and a density of 910 kg / m³. 3 Above and below 930 kg / m 3 Polyethylene-based resins. Linear low-density polyethylene is preferably a copolymer of ethylene and an α-olefin with 4 to 8 carbon atoms, with essentially linear molecular chains and a density of 910 kg / m³. 3 Above and below 930 kg / m 3 Polyethylene-based resins. High-density polyethylene is preferably an ethylene homopolymer or a copolymer of ethylene and an α-olefin with 4 to 8 carbon atoms, and has a density of 930 kg / m³. 3 The above are polyethylene-based resins.

[0027] Examples of thermoplastic elastomers include block copolymers with polyethylene blocks as hard segments, such as block copolymers of polyethylene blocks and ethylene / α-olefin copolymers, and mixtures composed of polypropylene resins and ethylene rubbers. From the viewpoint of the softness and resilience of foamed particle molded bodies (hereinafter also simply referred to as foamed particle molded bodies or molded bodies) formed by molding foamed particles of ethylene polymers, thermoplastic elastomers are preferably block copolymers with polyethylene blocks as hard segments, and more preferably block copolymers with polyethylene blocks as hard segments and ethylene / α-olefin copolymers as soft segments.

[0028] In mixtures composed of polypropylene-based resins and ethylene-based rubbers, examples of polypropylene-based resins include propylene homopolymers and copolymers of propylene with ethylene or α-olefins having 4 to 8 carbon atoms. On the other hand, examples of ethylene-based rubbers include copolymers of ethylene with α-olefins having 3 to 8 carbon atoms, and copolymers of ethylene with non-conjugated dienes such as 5-vinyl-2-norbornene, 5-ethylidene-2-norbornene, 5-methylene-2-norbornene, and dicyclopentadiene.

[0029] In block copolymers where polyethylene blocks are hard segments and ethylene / α-olefin copolymer blocks are soft segments, examples of polyethylene blocks include ethylene homopolymers and copolymers of ethylene with α-olefins having 3 to 8 carbon atoms. On the other hand, the ethylene / α-olefin copolymer blocks are preferably blocks of copolymers of ethylene with α-olefins having 3 to 20 carbon atoms. Examples of α-olefins copolymerized with ethylene include propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 3-methyl-1-butene, and 4-methyl-1-pentene.

[0030] As a thermoplastic elastomer, commercially available products can be used, such as those manufactured by Dow Chemical Company under the trade names "INFUSE" and "ENGAGE", Mitsubishi Chemical Company under the trade name "TREXPRENE", Mitsui Chemicals Corporation under the trade names "Milastomer" and "TAFMER", and Sumitomo Chemical Company under the trade name "ESPOLEX".

[0031] Melting Point Tech From the viewpoint of heat resistance of foamed particles, the melting point Tme of the ethylene polymer (I) is preferably 110°C or higher, more preferably 115°C or higher, and from the viewpoint of improving the fusibility of foamed particles, it is preferably 140°C or lower, more preferably 135°C or lower, even more preferably 130°C or lower, and even more preferably 125°C or lower.

[0032] The melting point Tme of the vinyl polymer (I) refers to the melting peak temperature determined by differential scanning calorimetry (DSC) based on the heat flux as described in JIS K 7121:2012. Specifically, a test piece made of vinyl polymer (I) was prepared and its condition was adjusted according to "(2) the condition of determining the melting temperature after certain heat treatment". The heating and cooling rates during condition adjustment were both 10 °C / min. A DSC curve was obtained by heating the condition-adjusted test piece from 30 °C to 200 °C at a heating rate of 10 °C / min. Then, the vertex temperature of the melting peak that appeared in the DSC curve was taken as the melting point of the vinyl polymer (I). In addition, in the case of multiple melting peaks in the DSC curve, the vertex temperature of the melting peak with the largest area was taken as the melting point.

[0033] Crystallization Temperature (Tce) The crystallization temperature Tce of the ethylene polymer (I) is preferably 90–120°C. When the crystallization temperature of the ethylene polymer (I) is within the above range, the in-mold formability of the foamed particles is more excellent. From the above point of view, the crystallization temperature of the ethylene polymer (I) is more preferably 100–115°C.

[0034] The Tce of ethylene-based polymers (I) was determined using a differential scanning calorimeter based on JIS K 7121:2012. Furthermore, in cases where multiple crystallization peaks appeared in the DSC curve, the peak temperature of the crystallization peak with the highest peak height was taken as the crystallization temperature.

[0035] Melt Flow Rate (MFR) From the viewpoint of obtaining foamed particles with better foaming and formability, the melt flow rate (MFR) of the ethylene polymer (I) is preferably 2 g / 10 min or more, more preferably 3 g / 10 min or more, even more preferably 4 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.

[0036] The MFR of ethylene polymers (I) was determined according to JIS K 7210-1:2014 at a temperature of 190°C and a load of 2.16 kg.

[0037] Flexural Modulus From the viewpoint of flexibility, the flexural modulus of the ethylene polymer (I) is preferably 10 MPa or more, more preferably 15 MPa or more, even more preferably 20 MPa or more, and preferably 300 MPa or less, more preferably 200 MPa or less, even more preferably 100 MPa or less, and even more preferably 50 MPa or less.

[0038] The flexural modulus of the vinyl polymer (I) was determined according to JIS K 7171:2016.

[0039] The foamed core layer may contain other polymers, such as resins or elastomers, besides the ethylene-based polymer (I) described above, without hindering the intended effects of the present invention. Examples of other polymers include thermoplastic resins such as polyolefin resins other than polyethylene resins and polystyrene resins, and thermoplastic elastomers other than the thermoplastic elastomers described above. However, the content of these other polymers in the foamed core layer relative to 100 parts by weight of the ethylene-based polymer (I) is preferably 30 parts by weight or less, more preferably 20 parts by weight or less, further preferably 10 parts by weight or less, even more preferably 5 parts by weight or less, even more preferably 1 part by weight or less, and most preferably 0 parts by weight.

[0040] Additives may be incorporated into the foamed core layer to a extent that does not impair the intended effects of the present invention. Examples of additives include bubble regulators, flame retardants, flame retardant aids, bubble nucleating agents, plasticizers, antistatic agents, antioxidants, weathering agents, ultraviolet absorbers, light stabilizers, conductive fillers, antibacterial agents, and lubricants. Examples of bubble regulators include inorganic powders such as talc, mica, zinc borate, calcium carbonate, silica, titanium dioxide, gypsum, zeolite, borax, aluminum hydroxide, and carbon; and organic powders such as phosphoric acid nucleating agents, phenolic nucleating agents, amine nucleating agents, and polyvinyl fluoride resin powder. The content of the above-mentioned additives in the foamed core layer is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less.

[0041] The foamed core layer may contain the free radical scavenger described later, but from the viewpoint of suppressing the crosslinking barrier of the foamed particles, the amount of free radical scavenger in the foamed core layer is preferably less than the amount of free radical scavenger in the coating layer. Furthermore, from the same viewpoint, the amount of free radical scavenger in the foamed core layer is preferably 100 ppm by mass or less (including 0 ppm by mass), more preferably 10 ppm by mass or less (including 0 ppm by mass), further preferably less than 10 ppm by mass (including 0 ppm by mass), particularly preferably 3 ppm by mass or less (including 0 ppm by mass), and most preferably 0 ppm by mass.

[0042] <Covering layer> The coating layer of the vinyl polymer foam particles (hereinafter also referred to as the coating layer) is a layer that covers the foam core layer composed of vinyl polymer (I), including vinyl polymer (II), one or more resins selected from the group consisting of polypropylene resins and polystyrene resins (III), and a free radical scavenger.

[0043] The coating layer can cover the entire surface of the foamed core layer or only a portion thereof. For example, the coating layer can be disposed on the side circumferential surface of the cylindrical foam particles to cover the foamed core layer. Furthermore, the coating layer can be in a foamed or non-foamed state, but is preferably substantially non-foamed. "Substantially non-foamed" includes a state where the coating layer is not foamed, contains no air bubbles, or has lost its air bubbles after foaming, meaning that there is almost no air bubble structure. The thickness of the coating layer is, for example, 0.5 μm or more and 100 μm or less. Additionally, an intermediate layer can be further disposed between the foamed core layer and the coating layer.

[0044] (Ethylene polymers (II)) The ethylene polymer (II) is a polymer containing 50% by mass or more of ethylene, preferably a polymer containing 60% by mass or more of ethylene, and more preferably a polymer containing 70% by mass or more of ethylene. The substances exemplified as the ethylene polymer (I) described above can be used as the ethylene polymer (II).

[0045] (Resin (III)) The resin (III) is one or more resins selected from the group consisting of polypropylene resins and polystyrene resins, preferably polypropylene resins.

[0046] Polypropylene Resins Examples of polypropylene-based resins include propylene homopolymers, propylene copolymers, or mixtures thereof. Examples of propylene homopolymers include isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene. Propylene copolymers are copolymers in which the content of structural units derived from propylene exceeds 50% by mass. Examples of propylene copolymers include propylene-ethylene copolymers, propylene-butene copolymers, propylene-ethylene-butene copolymers, copolymers of propylene with ethylene and / or α-olefins having 4 to 8 carbon atoms, propylene-acrylic acid copolymers, and propylene-maleic anhydride copolymers. Preferably, the polypropylene-based resin contains one or more copolymers selected from the group consisting of propylene-ethylene copolymers and propylene-ethylene-butene copolymers. Propylene copolymers can be atactic copolymers, block copolymers, or graft copolymers.

[0047] Polystyrene-based resins Polystyrene-based resins refer to thermoplastic resins in which the content of structural units derived from styrene exceeds 50% by mass. Examples of polystyrene-based resins include polystyrene (general purpose polystyrene: GPPS), styrene-methacrylic acid copolymer, styrene-methacrylic acid-methyl methacrylate copolymer, styrene-acrylic acid copolymer, styrene-maleic anhydride copolymer, styrene-methyl methacrylate copolymer, styrene-butyl acrylate copolymer, styrene-acrylonitrile copolymer, high-impact polystyrene (HIPS), and mixtures thereof.

[0048] -mass ratio- The resin (III) in the coating layer has a mass percentage of 1% or more out of a total of 100% by mass of the vinyl polymer (II) and resin (III). If the mass percentage of resin (III) is too low, depending on the storage conditions, it may be impossible to suppress the oxidation and surface crosslinking of the foamed particles during long-term storage, posing a risk of reduced meltability during in-mold molding. From the viewpoint of more reliably suppressing the reduction of meltability of the foamed particles during in-mold molding after long-term storage, and from the viewpoint of further shortening the cooling time during molding, the mass percentage of resin (III) in the coating layer out of a total of 100% by mass of the vinyl polymer (II) and resin (III) is preferably 2% or more, more preferably 5% or more, and even more preferably 8% or more. On the other hand, if the mass percentage of resin (III) is too high, depending on the storage conditions, there is a risk of reduced meltability during in-mold molding. In addition, the surface smoothness of the resulting foamed particle molded body may be significantly reduced. Furthermore, the soft texture characteristic of the vinyl polymer (II) may be compromised. From the viewpoint of easily avoiding these problems, the mass percentage of resin (III) in the coating layer in a total of 100% by mass of ethylene polymer (II) and resin (III) is 60% by mass or less, preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass or less.

[0049] Furthermore, the mass percentage of resin (III) in the coating layer, out of a total of 100% by mass of the ethylene polymer (II) and resin (III), is preferably greater than the mass percentage of resin (III) in the foamed core layer, out of a total of 100% by mass of the ethylene polymer (I) and resin (III), more preferably more than 1% by mass, even more preferably more than 2% by mass, and particularly preferably more than 5% by mass. In this case, the intended effects of the present invention can be achieved more reliably without compromising the softness, resilience, and other properties of the foamed particles.

[0050] MFR From the viewpoint of obtaining foamed particles with better foaming and formability, the MFR of resin (III) is preferably 2 g / 10 min or more, more preferably 4 g / 10 min or more, even more preferably 6 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.

[0051] The MFR of resin (III) was determined according to JIS K 7210-1:2014. Specifically, when the resin is a polypropylene resin, the value was determined at a temperature of 230°C and a load of 2.16 kg; when the resin is a polystyrene resin, the value was determined at a temperature of 200°C and a load of 5 kg.

[0052] -Melting Point (Tmp)- When resin (III) is a polypropylene-based resin, from the viewpoint of improving the heat resistance of the foamed particles, the melting point Tmp of the polypropylene-based resin is preferably 120°C or higher, more preferably 125°C or higher, even more preferably 130°C or higher, and even more preferably 135°C or higher. On the other hand, from the viewpoint of further improving the fusibility of the foamed particles, it is preferably 160°C or lower, more preferably 155°C or lower, even more preferably 150°C or lower, and even more preferably 145°C or lower.

[0053] The melting point Tmp of polypropylene resin refers to the melting peak temperature determined by differential scanning calorimetry (DSC) based on heat flux as described in JIS K 7121:2012. Specifically, test pieces made of polypropylene resin are prepared, and the test pieces are conditioned according to "(2) the condition of measuring the melting temperature after certain heat treatment". The heating rate and cooling rate during conditioning are both 10°C / min. The DSC curve is obtained by heating the conditioned test piece from 30°C to 200°C at a heating rate of 10°C / min. Then, the vertex temperature of the melting peak that appears in the DSC curve is taken as the melting point of the polypropylene resin. In addition, when multiple melting peaks appear in the DSC curve, the vertex temperature of the melting peak with the largest area is taken as the melting point.

[0054] -Poor (Tmp-Tme)- When resin (III) is a polypropylene resin, the difference between the melting point Tmp of the polypropylene resin and the melting point Tme of the ethylene polymer (I) (Tmp-Tme) is preferably 5°C or more, more preferably 10°C or more, further preferably 15°C or more, even more preferably 18°C ​​or more, and preferably 35°C or less, more preferably 30°C or less, even more preferably 28°C or less, and even more preferably 25°C or less. Conventionally, to improve the fusibility of foamed particles, foamed particles with a multilayer structure having a coating layer composed of a polymer with a melting point lower than that of the polymer constituting the foamed core layer are typically used. However, in this invention, although the coating layer contains a polypropylene resin and this polypropylene resin has a melting point Tmp higher than the melting point Tme of the ethylene polymer (I) constituting the foamed core layer, the fusibility of the foamed particles remains excellent. This is believed to be because a predetermined amount of free radical scavenger is incorporated into the coating layer, resulting in a moderately lower degree of crosslinking compared to the foamed core layer.

[0055] (Free radical scavenger) Free radical scavengers are additives that capture free radicals and inhibit free radical chain reactions; they are also called free radical capture agents. The type of free radical scavenger is not limited as long as it is an additive that exhibits the aforementioned free radical capturing effect. However, from the viewpoint of further improving the effect of the present invention, it is preferable to select one or more additives from the group consisting of antioxidants and weathering agents. From the same viewpoint, it is preferable to use antioxidants and weathering agents in combination.

[0056] Antioxidants used as free radical scavengers can be known antioxidants, preferably one or more compounds selected from the group consisting of phenolic antioxidants, phosphorus antioxidants and thioether antioxidants.

[0057] Examples of phenolic antioxidants include pentaerythritol tetra[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (trade name "Irganox 1010" manufactured by BASF) and n-octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (trade name "Irganox 1010" manufactured by BASF). 1076”), 2,6-di-tert-butyl-p-cresol, 2,6-diphenyl-4-octadecyloxyphenol, distearate (3,5-di-tert-butyl-4-hydroxybenzyl)phosphonate, 1,6-hexamethylenebis[(3,5-di-tert-butyl-4-hydroxyphenyl)propionamide], 4,4'-thiobis(6-tert-butyl-m-cresol), 2,2'-methylenebis(4-methyl-6-tert-butylphenol), 2,2'-methylenebis(4-ethyl-6-tert-butylphenol), 4,4'-butylidenebis(6-tert-butyl-m-cresol) phenol), 2,2'-ethylidene bis(4,6-di-tert-butylphenol), 2,2'-ethylidene bis(4-sec-butyl-6-tert-butylphenol), 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,3,5-tris(2,6-dimethyl-3-hydroxy-4-tert-butylbenzyl)isocyanurate, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)isocyanurate, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-2,4,6-trimethylbenzene, 2- tert-butyl-4-methyl-6-(2-acryloyloxy-3-tert-butyl-5-methylbenzyl)phenol, stearyl[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], tetra[methyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]methane, diethylene thioglycol bis[(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 1,6-hexamethylenebis[(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], bis[3,3-bis(4-hydroxy-3-tert-butylphenyl)butyric acid]ethyl Diol esters, bis[2-tert-butyl-4-methyl-6-(2-hydroxy-3-tert-butyl-5-methylbenzyl)phenyl] terephthalate, 1,3,5-tris[(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxyethyl] isocyanurate, 3,9-bis[1,1-dimethyl-2-{(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy}ethyl]-2,4,8,10-tetraoxaspiro[5,5]undecane, triethylene glycol bis[(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate], etc. These can be used alone or in combination of two or more.

[0058] Examples of phosphorus-based antioxidants include tris(2,4-di-tert-butylphenyl) phosphite (manufactured by BASF under the trade name "Irgafos168"), trinonylphenyl phosphite, tris[2-tert-butyl-4-(3-tert-butyl-4-hydroxy-5-methylphenylthio)-5-methylphenyl] phosphite, tridecyl phosphite, octyl diphenyl phosphite, di(decyl) monophenyl phosphite, and di(tridecyl) Pentaerythritol diphosphite, bis(nonylphenyl)pentaerythritol diphosphite, bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite, bis(2,6-di-tert-butyl-4-methylphenyl)pentaerythritol diphosphite, bis(2,4,6-tri-tert-butylphenyl)pentaerythritol diphosphite, bis(2,4-dicumylphenyl)pentaerythritol diphosphite, tetra(tetrazyl)isopropylidene diphenol ... Alkyl)-4,4'-n-butylidene bis(2-tert-butyl-5-methylphenol) diphosphite, hexa(tetrazyl)-1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane triphosphite, tetra(2,4-di-tert-butylphenyl)biphenyl diphosphite, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 2,2'-methylenebis(4,6-tert-butylphenyl)-2-ethyl Hexyl phosphite, 2,2'-methylene bis(4,6-tert-butylphenyl)-octadecyl phosphite, 2,2'-ethylidene bis(4,6-di-tert-butylphenyl)fluorophosphite, tris(2-[(2,4,8,10-tetra-tert-butyldibenzo[d,f][1,3,2]dioxaphosphahepten-6-yl)oxy]ethyl)amine, phosphites of 2-ethyl-2-butylpropanediol and 2,4,6-tri-tert-butylphenol, etc. These can be used alone or in combination of two or more.

[0059] Examples of thioether antioxidants include dialkyl thiodipropionates such as dilaurate thiodipropionate, dimyristyl thiodipropionate, and distearate thiodipropionate; and pentaerythritol tetra(β-alkyl mercaptopropionates). These can be used alone or in combination of two or more.

[0060] Weather stabilizers that serve as free radical scavengers can be known weather stabilizers, preferably containing hindered amine light stabilizers, and more preferably hindered amine light stabilizers.

[0061] As hindered amine light stabilizers, for example, those manufactured by BASF under the trade names "Tinuvin" and "Flamestab", those manufactured by Solvay under the trade name "Cyasorb", those manufactured by Everlight Chemical under the trade name "EVERSORB", those manufactured by ADEKA Co., Ltd. under the trade name "Adekastab LA series", those manufactured by Nippon Emulsifier Co., Ltd. under the trade name "Newcol", and those manufactured by Seiko Chemical Co., Ltd. under the trade name "Nonflex" are used.Specifically, examples of hindered amine-based light stabilizers include dimethyl succinate-1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylpiperidine condensate (manufactured by BASF under the trade name "Tinuvin 622"), bis(1-octyloxy-2,2,6,6-tetramethyl-4-piperidinyl) sebacate (manufactured by BASF under the trade name "Tinuvin 123"), bis(1,2,2,6,6-pentamethyl-4-piperidinyl) sebacate (manufactured by BASF under the trade name "Tinuvin 765"), and 2-butyl-2-[(3,5-di-tert-butyl- [4-Hydroxyphenyl]methyl] bis(1,2,2,6,6-pentamethylpiperidin-4-yl) ester (manufactured by BASF under the trade name "Tinuvin 144", etc.), bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (manufactured by BASF under the trade name "Tinuvin 770DF"), N,N'-ethane-1,2-dimethylbis(1,3-propanediamine), cyclohexane, peroxides of 4-butylamino-2,2,6,6-tetramethylpiperidine, and reaction products of 2,4,6-trichloro-1,3,5-triazine (manufactured by BASF under the trade name "Flamestab"). NOR116, etc.), poly{[6-morpholino-1,3,5-triazine-2,4-diyl][(1,2,2,6,6-pentamethyl-4-piperidinyl)imino]hexamethylene[(1,2,2,6,6-pentamethyl-4-piperidinyl)imino]} (trade name "Cyasorb UV-3529" manufactured by Solvay, etc.), tetra(1,2,2,6,6-pentamethylpiperidin-4-yl)butane-1,2,3,4-tetracarboxylate (trade name "Adekastab LA-52" manufactured by ADEKA, etc.), tetra(2,2,6,6-tetramethyl-4-piperidinyl)=1,2,3,4-butanetetracarboxylate (trade name "Adekastab" manufactured by ADEKA, etc.). Examples of such products include LA-57, bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (trade name "Adekastab LA-77Y" manufactured by ADEKA Co., Ltd.), 1,2,2,6,6-pentamethyl-4-piperidinyl methacrylate (trade name "Newcol LS-3410" manufactured by Nippon Emulsifier Co., Ltd.), and 4,4'-bis(α,α-dimethylbenzyl)diphenylamine (trade name "NonflexDCD" manufactured by Seiko Chemical Co., Ltd.). These can be used alone or in combination of two or more.

[0062] The amount of free radical scavenger in the coating layer is 10 ppm by mass or more. If the amount of free radical scavenger in the coating layer is too small, depending on the storage conditions, it may not be able to suppress the oxidation and surface cross-linking of the foamed particles during long-term storage, posing a risk of reduced fusion properties during in-mold forming. From the viewpoint of more reliably suppressing the reduction of fusion properties of the foamed particles during in-mold forming after long-term storage, the amount of free radical scavenger in the coating layer is preferably 50 ppm by mass or more, more preferably 100 ppm by mass or more, further preferably 300 ppm by mass or more, and particularly preferably 500 ppm by mass or more. In addition, the amount of free radical scavenger in the coating layer is 10,000 ppm by mass or less. If the amount of free radical scavenger in the coating layer is too large, it may cause deviations in the in-mold formability of the foamed particles, resulting in deviations in the physical properties of various parts of the resulting molded article. From the viewpoint of easily avoiding such problems, the amount of free radical scavenger in the coating layer is preferably 8000 ppm by mass or less, more preferably 5000 ppm by mass or less, even more preferably 3000 ppm by mass or less, and even more preferably 2000 ppm by mass or less. Furthermore, when the foamed core layer also contains a free radical scavenger, it is preferable that the amount of free radical scavenger in the coating layer is greater than the amount of free radical scavenger in the foamed core layer.

[0063] As long as it does not impair the effects of the present invention, other polymers besides ethylene polymer (II) and resin (III) may be appropriately added to the coating layer. Examples of such other polymers include thermoplastic resins and thermoplastic elastomers. The content of such other polymers in the coating layer is preferably 30 parts by weight or less, more preferably 20 parts by weight or less, further preferably 10 parts by weight or less, even more preferably 5 parts by weight or less, even more preferably 1 part by weight or less, and most preferably 0 parts by weight, relative to 100 parts by weight of the total of ethylene polymer (II) and resin (III).

[0064] Additives may be appropriately added to the coating layer as long as they do not impair the effects of the present invention. Examples of additives added to the coating layer include substances exemplified as additives for the foamed core layer.

[0065] Properties of Vinyl Polymer Foamed Particles (apparent density) From the perspective of obtaining foamed particle molded articles with superior properties such as lightweight, softness, resilience, and recovery, the apparent density of ethylene-based polymer foamed particles is preferably 10 kg / m³. 3 The above, more preferably 30 kg / m 3 The above is further optimized to be 50 kg / m 3 The above is further optimized to 70 kg / m 3The above, and preferably 300 kg / m 3 The following is more preferably 200 kg / m 3 The following is a further preferred value: 150 kg / m 3 The following is a further preferred value: 100 kg / m 3 the following.

[0066] The apparent density of ethylene-based polymer foam particles was calculated as follows. First, the foam particle group was placed for 2 days at a relative humidity of 50%, 23°C, and 1 atm. Next, a graduated cylinder containing water at 23°C was prepared, and an arbitrary amount of foam particle group (mass W1 of the foam particle group) was submerged into the water in the graduated cylinder using tools such as a wire mesh. Then, considering the volume of the tools such as the wire mesh, the volume V1 [L] of the foam particle group was measured based on the rise in water level. The mass W1 [g] of the foam particle group placed in the graduated cylinder was divided by the volume V1 [L] (W1 / V1), and the units were converted to [kg / m³]. 3 ], thereby determining the apparent density of the bubbling particles.

[0067] (Average bubble diameter) From the viewpoint of further improving the formability of the foamed particles, the average bubble diameter of the ethylene polymer foamed particles is preferably 40 μm or more, more preferably 50 μm or more, and even more preferably 60 μm or more. From the viewpoint of further improving the surface smoothness of the obtained foamed particle molded body, it is preferably 150 μm or less, more preferably 120 μm or less, even more preferably 100 μm or less, and even more preferably 80 μm or less.

[0068] The average bubble diameter of vinyl polymer foam particles was determined as follows: More than 20 foam particles were randomly selected from the foam particle group. Each foam particle was divided into two halves, passing through its center, and magnified photographs of the entire cross-section were taken using a microscope such as a scanning electron microscope. In each cross-sectional photograph, four line segments were drawn at equal angles (45°) from the outermost surface of the foam particle through the center to the outermost surface on the opposite side. The number of bubbles intersecting each line segment was counted, and the total length of the four line segments was divided by the total number of bubbles intersecting the line segments to calculate the bubble diameter of each foam particle. The arithmetic mean of these values ​​was taken as the average bubble diameter of the foam particles.

[0069] (Xylene insoluble content (gel fraction)) From the viewpoint of obtaining molded articles with superior resilience, the xylene-insoluble content of the vinyl polymer foam particles, as determined by thermal xylene extraction, is 40% by mass or more, preferably 42% by mass or more, and from the viewpoint of obtaining molded articles with superior softness, it is 70% by mass or less, preferably 65% ​​by mass or less, more preferably 60% by mass or less, and even more preferably 55% by mass or less. The xylene-insoluble content determined by thermal xylene extraction is one of the indicators of the degree of crosslinking of the foam particles, and it increases with the increase of the degree of crosslinking. The xylene-insoluble content determined by thermal xylene extraction is also called the gel fraction.

[0070] The xylene insoluble content determined by the hot xylene extraction method is calculated as follows: Weigh about 1g of sample (let the mass of the weighed sample be G1[g]), boil it in 100g of xylene for 6 hours, filter it quickly with a 100-mesh metal wire mesh, and then dry the boiled xylene insoluble residue on the metal wire mesh in a vacuum dryer at 80℃ for 8 hours. Then weigh the mass of the boiled xylene insoluble residue (let the mass of the weighed boiled xylene insoluble residue be G2[g]), and calculate it by the following formula (2).

[0071] Xylene insoluble content (mass%) = [G2 / G1] × 100 ··· Equation (2) The technical problem of reduced meltability during in-mold molding when storing vinyl polymer foam particles as described above for extended periods is particularly pronounced in foam particles with a gel fraction within the aforementioned range. While the cause is not yet clear, it is believed to be due to factors such as crosslinking agent residues promoting deterioration. On the other hand, the vinyl polymer foam particles of the present invention, by having the above-described structure, can maintain good meltability even when the gel fraction is within the aforementioned range after long-term storage.

[0072] (Degree of oxidation) The oxidation degree of the vinyl polymer foam particles, as measured under the following testing conditions (1), is 3.5 or less. If the oxidation degree of the vinyl polymer foam particles, as measured under the following testing conditions (1), is too high, the melt-bonding properties of the foam particles during in-mold molding may decrease depending on the storage conditions when the foam particles are stored for a long period of time. Furthermore, in this case, the bending stress of the resulting foam particle molded body may decrease. From the viewpoint of more reliably suppressing the decrease in melt-bonding properties of the foam particles during in-mold molding after long-term storage, the oxidation degree of the vinyl polymer foam particles, as measured under the following testing conditions (1), is preferably 3.2 or less, more preferably 3.0 or less, further preferably 2.5 or less, and particularly preferably 2.0 or less. The lower limit of the oxidation degree of the foam particles is not particularly limited, for example, it is 0.3 or more, 0.5 or more, or 1.0 or more.

[0073] The degree of oxidation of ethylene-based polymer foam particles was determined under the following conditions (1).

[0074] Measurement conditions (1): The foamed particles were divided into Group A, which was placed in the dark at 23°C for 1 day, and Group B, which was placed in the dark at 80°C for 7 days. The ATR of the foamed particles after being placed in Group A and Group B was measured and calculated by the following formula (1).

[0075] Oxidation degree = Oxidation degree of group B (A) 1730cm-1 / A 1460cm-1 ) / Oxidation degree of group A (A) 1730cm-1 / A 1460cm-1 Equation (1) In equation (1), A represents absorbance, and the subscript indicates the wavenumber. Furthermore, the wavenumber is typically 5 cm⁻¹. -1 The error is left or right.

[0076] The details of the ATR measurement in measurement condition (1) are as follows. First, as the measuring device, for example, the product "FT / IR-4600typeA (ATR PRO450-S type)" manufactured by Nippon Spectroscopy Corporation is used, and the foamed particles are introduced at a rate of 340 kg / cm². 2 The pressure applied to the prism was used to seal it, and the infrared spectrum was measured to obtain the infrared absorption spectrum (without ATR correction). Next, the 1460 cm⁻¹ value obtained from the infrared absorption spectrum was measured. -1 Absorbance A at point 1460cm-1 and 1730cm -1 Absorbance A at point 1730cm-1 Calculate the absorbance ratio (A). 1730cm-1 / A 1460cm-1 The same measurements were performed on five foamed particles from both group A and group B, and the average value of these five values ​​was taken as the absorbance ratio of the foamed particles in group A and group B respectively (A). 1730cm-1 / A 1460cm-1 The detailed conditions for ATR determination are shown below.

[0077] ATR Measurement Conditions Prism: Zinc selenide Infrared light incident angle: 45° Total number of times: 64 Resolution: 4cm -1 Detector: TGS (Triglycine sulfate) Wavenumber measurement range: 600–4000 cm⁻¹ -1 Number of reflections: 1 In addition, the 1460 cm⁻¹ obtained from the infrared absorption spectrum -1 Absorbance A at point 1460cm-1 The 1460 cm⁻¹ peak mainly originates from the antisymmetric stretching vibration of methylene (-CH₂-) groups contained in ethylene-based polymers, polypropylene-based resins, and / or polystyrene-based resins. -1 The height of the nearby peak. Additionally, the 1730 cm⁻¹ peak obtained from the infrared absorption spectrum. -1 Absorbance A at point 1730cm-1 The presence of carbonyl groups (-C(=O)-) at 1730 cm⁻¹ is mainly due to the stretching vibrations of carbonyl groups (-C(=O)-) generated from the oxidation of ethylene-based polymers, polypropylene-based resins, and / or polystyrene-based resins. -1 The height of the nearby peaks.

[0078] The degree of oxidation of the foamed particles measured under the above-described conditions (1) is an indicator of how easily the foamed particles are oxidized. It can be said that the lower the degree of oxidation, the easier it is for the foamed particles to resist oxidation even after long-term storage. Such foamed particles, for example, as described above, can be obtained by having foamed particles with a foamed core layer composed of an ethylene polymer (I) and a coating layer covering the foamed core layer, and by having the coating layer contain an ethylene polymer (II), a specified amount of one or more resins (III) selected from the group consisting of polypropylene resins and polystyrene resins, and a specified amount of a free radical scavenger.

[0079] [Manufacturing method of vinyl polymer foam particles] The following illustrates an example of a method for manufacturing ethylene-based polymer foamed particles according to the present invention. Specifically, it is a method for manufacturing ethylene-based polymer foamed particles having a foamed core layer composed of an ethylene-based polymer (I) and a coating layer covering the foamed core layer, wherein the coating layer comprises an ethylene-based polymer (II), one or more resins (III) selected from the group consisting of polypropylene resins and polystyrene resins (preferably polypropylene resins), and a free radical scavenger, wherein the resin (III) in the coating layer has a mass ratio of 1% or more and 60% or less in a total of 100% by mass of the ethylene-based polymer (II) and the resin (III), and the amount of the free radical scavenger in the coating layer is 10 ppm or more and 10,000 ppm or less by mass, preferably including the following steps (A) to (C).

[0080] Process (A): Dispersion process, in which multilayer polymer particles having a core layer composed of ethylene polymer (I) in a non-foamed state and a coating layer comprising ethylene polymer (II), one or more resins selected from the group consisting of polypropylene resins and polystyrene resins (III) and a free radical scavenger and covering the core layer are dispersed in a dispersion medium in a closed container. Process (B): Foaming agent impregnation process, which impregnates the multilayer polymer particles with a foaming agent; and Process (C): Foaming process, in which foamable multilayer polymer particles impregnated with foaming agent are released from a sealed container together with a dispersion medium into an atmosphere at a lower pressure than that inside the sealed container, so that at least the core layer foams to form a foamed core layer, thereby manufacturing foamed particles.

[0081] The order of process (A) and process (B) is not limited, and they can be performed simultaneously. However, from the point of view of efficient production, it is preferable to perform process (B) after process (A).

[0082] <Methods for Manufacturing Multilayer Polymer Particles> The manufacturing method of multilayer polymer particles can, for example, use an extruder having a core-forming extruder, a coating-forming extruder, and a multilayer strand-forming die disposed on the outlet side of these extruders. A core-forming melt-knead is prepared by supplying an ethylene-based polymer (I) constituting the core layer and, if necessary, additives to the core-forming extruder. A coating-forming melt-knead is prepared by supplying an ethylene-based polymer (II) constituting the coating layer, one or more resins (III) selected from the group consisting of polypropylene-based resins and polystyrene-based resins, a free radical scavenger, and, if necessary, additives to the coating-forming extruder. The core-forming melt-knead and the coating-forming melt-knead are then fed into the multilayer strand-forming die and combined to form a composite having a core-sheath structure, consisting of a non-foamed core layer and a non-foamed coating layer covering the core layer. Then, the composite is extruded into strands through a small orifice of a die attached to the front end of an extruder. After cooling in water, it is cut into specified sizes using a granulator (strand cutting method). This yields multilayer polymer particles with a non-foamed core layer and a coating layer covering the core layer. In addition to the methods described above, other methods for cutting the extruded composite include underwater cutting (extruding the composite into water) and thermal cutting (extruding the composite into air and immediately cutting it).

[0083] (Particle size of multilayer polymer particles) The particle size of the multilayer polymer particles is preferably 0.1 to 5.0 mm, more preferably 0.5 to 3.0 mm.

[0084] (Mass of multilayer polymer particles) The average mass of the multilayer polymer particles is preferably adjusted to 0.1–20 mg, more preferably 0.2–10 mg, even more preferably 0.3–5 mg, and even more preferably 0.4–3 mg.

[0085] (Mass ratio of core layer to cladding layer) The mass ratio of the foamed core layer to the coating layer of the foamed particles can be adjusted by the mass ratio of the core layer to the coating layer in the non-foamed state of the multilayer polymer particles. From this point of view, the preferred mass ratio of the core layer to the coating layer of the multilayer polymer particles is core layer:coating layer = 99:1 to 80:20, more preferably 97:3 to 85:15, and even more preferably 95:5 to 90:10.

[0086] (Aspect ratio L / D of multilayer polymer particles) From the viewpoint of obtaining foamed particles with excellent filling properties, the aspect ratio L / D of the multilayer polymer particles is preferably 0.5 to 5.0, more preferably 1.0 to 4.0. The aspect ratio of the multilayer polymer particles is obtained by measuring the maximum axial length (L) of the multilayer polymer particles (i.e., the extrusion direction) and the maximum cross-sectional diameter (D) of the polymer particles in a direction orthogonal to the length direction of the maximum length using vernier calipers, calculating the ratio (L / D), and then taking the arithmetic mean of these values.

[0087] (additive) As needed, additives such as bubble regulators, flame retardants, flame retardant auxiliaries, bubble nucleating agents, plasticizers, antistatic agents, antioxidants, weathering agents, ultraviolet absorbers, light stabilizers, conductive fillers, and antibacterial agents can be added to the multilayer polymer particles having a non-foamed core layer and a coating layer. When adding additives, they can be added in step (A). Examples of bubble regulators include inorganic powders such as talc, mica, zinc borate, calcium carbonate, silica, titanium dioxide, gypsum, zeolite, borax, aluminum hydroxide, and carbon; and organic powders such as phosphoric acid nucleating agents, phenolic nucleating agents, amine nucleating agents, and polyvinyl fluoride resin powder. When adding bubble regulators, the content of the bubble regulator in the multilayer polymer particles is preferably 0.01 to 1 part by mass relative to 100 parts by mass of the multilayer polymer particles.

[0088] Furthermore, the particle size, aspect ratio, and average mass of the multilayer polymer particles can be adjusted in the above-mentioned wire cutting method by appropriately changing the polymer ejection speed, traction speed, and cutting machine speed.

[0089] <Process (A)> In process (A), for example, the multilayer polymer particles can be dispersed in a dispersion medium using a mixer, such as in a sealable container that can withstand heating and pressurization, such as an autoclave.

[0090] There are no particular limitations on the dispersion medium as long as it does not dissolve the multilayer polymer particles. Examples include water, ethylene glycol, glycerol, methanol, ethanol, and other alcohols, with water being the preferred choice.

[0091] In step (A), to prevent the multilayer polymer particles from fusing together, it is preferable to further add a dispersant to the dispersion medium. Examples of dispersants include organic dispersants such as polyvinyl alcohol, polyvinylpyrrolidone, and methylcellulose; and insoluble inorganic salts such as alumina, zinc oxide, kaolin, mica, magnesium phosphate, and tricalcium phosphate. These can be used alone or in combination of two or more. From the perspective of ease of processing, insoluble inorganic salts are preferred, and kaolin is more preferred. When adding a dispersant, it is preferable to add approximately 0.001 to 5 parts by mass relative to 100 parts by mass of multilayer polymer particles.

[0092] Surfactants may be further added to the dispersion medium. Examples of surfactants include sodium dodecylbenzenesulfonate, sodium alkyl sulfonate, sodium oleate, sodium lauryl sulfate, sodium polyoxyethylene alkyl ether phosphate, sodium polyoxyethylene alkyl ether sulfate, and other anionic and nonionic surfactants commonly used in suspension polymerization. When a surfactant is added, it is preferable to add approximately 0.001 to 1 part by weight per 100 parts by weight of the multilayer polymer particles.

[0093] A crosslinking process to crosslink the multilayer polymer particles can be performed after step (A) and before step (B) described later. However, the crosslinking process can also be performed simultaneously with step (A). In the crosslinking process, for example, in a container such as the aforementioned autoclave, the crosslinking agent is dispersed together with the multilayer polymer particles in a dispersion medium.

[0094] There are no particular limitations on crosslinking agents; any substance capable of crosslinking ethylene-based polymers can be used. Examples of crosslinking agents include bis(1-methyl-1-phenylethyl) peroxide (10-hour half-life temperature: 116°C), 2,5-tert-butyl peroxide benzoate (10-hour half-life temperature: 104°C), 1,1-bis(tert-butylperoxide)cyclohexane (10-hour half-life temperature: 91°C), 1,1-di(tert-hexylperoxide)cyclohexane (10-hour half-life temperature: 87°C), and tert-butylperoxide-2-ethylhexyl monocarbonate (10-hour half-life temperature: 99°C). These can be used alone or in combination of two or more.

[0095] The amount of crosslinking agent incorporated relative to 100 parts by weight of multilayer polymer particles is preferably 0.1 parts by weight or more, more preferably 0.2 parts by weight or more, and preferably 5 parts by weight or less, more preferably 2.5 parts by weight or less. If the amount of crosslinking agent incorporated is within the above range, the crosslinking efficiency is improved, and crosslinked polymer particles with the desired xylene insoluble content can be obtained. The crosslinked polymer particles have excellent foaming properties and can impart sufficient strength to the ethylene polymer (I) to withstand foaming.

[0096] In the crosslinking process, for example, the temperature is heated to above the temperature at which the ethylene polymer (I) constituting the core layer softens and the crosslinking agent substantially decomposes (crosslinking temperature), and held for a specified time (crosslinking holding time) to crosslink the multilayer polymer particles containing the ethylene polymer (I), thereby obtaining crosslinked multilayer polymer particles.

[0097] The crosslinking temperature is preferably above the melting point of the vinyl polymer (I) constituting the multilayer polymer particles and below +80°C, specifically, preferably 100°C to 170°C. The holding time at the crosslinking temperature (crosslinking holding time) is preferably 1 minute or more, more preferably 20 minutes or more, and preferably 100 minutes or less, more preferably 60 minutes or less.

[0098] <Process (B)> In process (B), for example, the ethylene polymer (I) constituting the core layer is heated to a temperature above which softens, and a foaming agent is impregnated to obtain foamable multilayer polymer particles.

[0099] There are no particular limitations on the foaming agent as long as it can foam multilayer polymer particles. Examples of foaming agents include inorganic physical foaming agents such as air, nitrogen, carbon dioxide, argon, helium, oxygen, and neon; aliphatic hydrocarbons such as propane, n-butane, isobutane, n-pentane, isopentane, and n-hexane; alicyclic hydrocarbons such as cyclohexane and cyclopentane; halogenated hydrocarbons such as chlorofluoromethane, trifluoromethane, 1,1-difluoroethane, 1,1,1,2-tetrafluoroethane, chloromethane, chloroethane, and dichloromethane; and dialkyl ethers such as dimethyl ether, diethyl ether, and methyl ethyl ether. Among these, inorganic physical foaming agents that do not deplete the ozone layer and are inexpensive are preferred, nitrogen, air, and carbon dioxide are more preferred, and carbon dioxide is particularly preferred. These can be used alone or in combination of two or more.

[0100] The amount of foaming agent added is determined by considering the desired apparent density of the foamed particles, the type of ethylene polymer (I), and the type of foaming agent. Generally, relative to 100 parts by mass of multilayer polymer particles, the organic physical foaming agent is preferably 5 to 50 parts by mass, and the inorganic physical foaming agent is preferably 0.1 to 30 parts by mass, more preferably 0.5 to 15 parts by mass.

[0101] The heating temperature in step (B) is preferably above the melting point of the ethylene polymer (I) and below +80°C, specifically, preferably 100°C to 230°C. The holding time at this 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.

[0102] <Process (C)> In step (C), for example, foamable multilayer polymer particles impregnated with a foaming agent and heated in step (B) are released from a sealed container into an atmosphere at a lower pressure than that inside the sealed container, causing at least the core layer to foam and become a foamed core layer, thereby enabling the manufacture of foamed particles.

[0103] Specifically, while maintaining the pressure inside the sealed container above the vapor pressure of the foaming agent, the end of the sealed container below the water surface is opened. The foaming multilayer polymer particles impregnated with the foaming agent, along with the dispersion medium, are released from the sealed container into an atmosphere at a lower pressure than the pressure inside the sealed container (usually atmospheric pressure). This causes at least the core layer of the foaming multilayer polymer particles to foam, forming a foamed core layer. This allows the production of foamed particles with a multilayer structure having a foamed core layer and a covering layer encapsulating the foamed core layer. Alternatively, the foaming multilayer polymer particles from step (B) can be cooled and removed, and then heated using a heating medium such as warm air or steam to cause them to foam, thereby producing foamed particles.

[0104] In process (C), the foaming temperature is preferably between 110°C and 170°C. Furthermore, the pressure inside the sealed container is preferably 0.5 MPa (G) or higher and 5 MPa (G) or lower. Note that "0.5 MPa (G)" refers to a gauge pressure of 0.5 MPa.

[0105] The above-mentioned processes (A) to (C) are preferably performed as a series of processes in a single sealed container, but they can also be performed as separate processes, for example, by taking out multilayer polymer particles in each process and putting them back into the sealed container for the next process.

[0106] Furthermore, in the above-mentioned process, it is preferable to perform a holding process, that is, when heating in a sealed container, holding the product at a temperature above (the melting point of the ethylene polymer (I) constituting the core layer -20°C) and below (the melting end temperature of the ethylene polymer (I) constituting the core layer) for a sufficient time, preferably about 10 to 60 minutes. Then, it is preferable to perform a second holding process, that is, adjusting the temperature to between (the melting point of the ethylene polymer (I) -15°C) and (the melting end temperature of the ethylene polymer (I) +10°C), and holding the product at this temperature for a sufficient time, preferably about 10 to 60 minutes, as needed. Then, by releasing the foaming multilayer polymer particles containing the foaming agent from the sealed container under low pressure for foaming, foamed particles with a crystalline structure exhibiting a high-temperature peak can also be obtained.

[0107] The foamed particles obtained as described above can be pressurized with inorganic gases such as air to increase the internal pressure, and then heated with steam or the like to make them foam (two-stage foaming). They can also be made into foamed particles with a higher foaming ratio (lower apparent density).

[0108] The vinyl polymer foam particles of the present invention are foam particles capable of forming foam particle molded bodies that satisfy a specific relationship between foam particle molded bodies formed by in-mold molding after undergoing an accelerated test of curing (standing) at 80°C in a dark environment for 7 days and foam particle molded bodies formed by in-mold molding without undergoing the accelerated test. Specifically, the vinyl polymer foam particles of the present invention are foam particles capable of forming foam particle molded bodies in the following way: the ratio of the maximum flexural strength of the foam particle molded body formed by in-mold molding of the foam particle that has undergone the accelerated test to the maximum flexural strength of the foam particle molded body formed by in-mold molding of the foam particle that has not undergone the accelerated test is 0.75 or more. As will be explained later, if the ratio of the maximum flexural strength is 0.75 or more, it is determined that even after undergoing the accelerated test, the reduction in the weldability of the foam particle during in-mold molding is suppressed. Furthermore, the above-mentioned accelerated test is a condition set as an example of a foam particle storage method under harsh conditions. In addition, the maximum flexural strength of the foamed particle molded body can be determined according to JIS K 7221-2:2006, specifically by the method described in the examples.

[0109] <Ethylene-based polymer foamed particle molding> (apparent density) The apparent density of the foamed particle molded body is preferably 20–300 kg / m³. 3 More preferably, it is 30–150 kg / m 3 A further preferred value is 35–120 kg / m³. 3 A further preferred value is 40–100 kg / m³. 3 When the apparent density of the foamed particle molded body is within the above range, it achieves an excellent balance between lightweight and mechanical properties.

[0110] The apparent density of the foamed particle molded body was calculated as follows. The foamed particle molded body was placed for 2 days at a relative humidity of 50%, a temperature of 23°C, and a pressure of 1 atm. Next, a container filled with water at 23°C was prepared, and an arbitrary amount of foamed particle molded body (mass W[g]) was submerged in the water using a tool such as a wire mesh. Then, considering the volume of the tool such as the wire mesh, the volume V[L] of the foamed particle molded body was measured based on the rise in water level. The mass W[g] of the foamed particle molded body placed in the container was divided by the volume V[L] (W / V), and the units were converted to [kg / m³]. 3 ], thereby determining the apparent density of the foamed particle-shaped body.

[0111] (Independent bubble rate) The percentage of independent bubbles in the foamed particle molded article is preferably 60% or more, more preferably 65% ​​or more, and even more preferably 70% or more. In this case, it is easier to make the molded article exhibit the desired physical properties.

[0112] The independent bubble ratio of the foamed particle molded body was determined according to ASTM D2856-70, step C. Specifically, first, a specimen measuring 15mm in length, 15mm in width, and 15mm in height was cut from the center of the molded body, and the geometric volume Va of the specimen (unit: cm³) was calculated. 3 The true volume Vx of the sample is calculated as the product of its longitudinal dimension (cm), transverse dimension (cm), and height dimension (cm). Next, following step C as described in ASTM-D2856-70, an air comparator hydrometer (specifically, the Beckman Model 1000 Air Comparison Pycnometer manufactured by Tokyo Scientific Co., Ltd.) is used to determine the true volume Vx of the sample. Furthermore, the true volume Vx obtained by the air comparator hydrometer is the sum of the volume of the polymer constituting the sample and the total volume of the individual air bubbles within the sample (cm³). 3 ).

[0113] The independent bubble rate of the sample (in %) is calculated using the sample mass W (in g) and the density ρ of the polymer constituting the foam particles (in g / cm³). 3 The geometric volume Va and the true volume Vx of the sample obtained by the above method are expressed by the following formula (3).

[0114] Independent bubble rate = (Vx - W / ρ) × 100 / (Va - W / ρ) ··· Equation (3) Perform the above operation on 5 samples and calculate the independent bubble rate of each sample. Then, take the arithmetic mean of the independent bubble rates of these 5 samples as the independent bubble rate of the molded body.

[0115] <Manufacturing Method of Vinyl Polymer Foamed Particles> A method for manufacturing ethylene-based polymer foamed particle molded articles can be implemented by filling ethylene-based polymer foamed particles into a molding die and heating them using a heating medium such as steam. Specifically, after filling the foamed particles into the molding die, a heating medium such as steam is introduced into the molding die, thereby heating the foamed particles to cause them to foam and fuse together, thus obtaining a foamed particle molded article with a shaped molded space. Furthermore, it is preferable to use a pressure molding method (for example, Japanese Patent Publication No. 51-22951) in which the foamed particles are pre-pressurized with a pressurized gas such as air to increase the pressure inside the bubbles of the foamed particles, and the pressure inside the foamed particles is adjusted to a pressure 0.01 to 0.3 MPa higher than atmospheric pressure. Then, the foamed particles are filled into the molding die under atmospheric pressure or reduced pressure, and then a heating medium such as steam is supplied into the mold to heat and fuse the foamed particles. In addition, molding can also be performed using the following compression filling molding method (Japanese Patent Publication No. 4-46217): Foamed particles pressurized to above atmospheric pressure are filled into a molding die pressurized to above atmospheric pressure, and then a heating medium such as steam is supplied to the mold cavity for heating, causing the foamed particles to fuse together. Alternatively, molding can be performed using an atmospheric pressure filling molding method (Japanese Patent Publication No. 6-49795) or a combination of the above methods (Japanese Patent Publication No. 6-22919). In the atmospheric pressure filling molding method, foamed particles with high secondary foaming power obtained under special conditions are filled into the mold cavity of the molding die under atmospheric pressure or reduced pressure, and then a heating medium such as steam is supplied for heating, causing the foamed particles to fuse together.

[0116] Example The present invention will now be described in detail with reference to the embodiments, but the present invention is not limited to these embodiments.

[0117] The raw materials, obtained foamed particles, and foamed particle molded articles used in the examples and comparative examples were measured or evaluated as follows. Furthermore, unless otherwise stated, the physical property measurements of the foamed particles were performed using foamed particles that had undergone state conditioning by standing for 24 hours at 50% relative humidity, 23°C, and 1 atm. Additionally, unless otherwise stated, the physical property measurements and evaluations of the foamed particle molded articles were performed using molded articles that had undergone state conditioning by standing for 12 hours at 50% relative humidity, 60°C, and 1 atm after demolding.

[0118] [Measurement] <Ingredients> (Melting point) The melting point of the raw materials was determined based on differential scanning calorimetry (DSC) as described in JIS K 7121:2012. Specifically, test pieces composed of each raw material were prepared and the test pieces were conditioned according to "(2) the condition of determining the melting temperature after certain heat treatment". The heating and cooling rates during conditioning were both 10 °C / min. The DSC curve was obtained by heating the conditioned test piece from 30 °C to 200 °C at a heating rate of 10 °C / min, and the vertex temperature of the melting peak accompanying the melting of the raw material on the DSC curve was determined as the melting point. In addition, when multiple melting peaks appeared in the DSC curve, the vertex temperature of the melting peak with the largest area was taken as the melting point. The measuring apparatus used was a differential scanning calorimeter (manufactured by SII NanoTechnology Inc., model: DSC7020).

[0119] (Crystallization temperature) The crystallization temperature of the raw material was determined according to JIS K 7121:2012. It was determined by taking the peak temperature of the exothermic peak accompanying crystallization in the DSC curve obtained when the raw material, heated from 30°C to 200°C at a heating rate of 10°C / min to melt, was cooled back to 30°C at a cooling rate of 10°C / min. Furthermore, in cases where multiple exothermic peaks appeared in the DSC curve, the peak temperature of the crystallization peak with the largest area was taken as the crystallization temperature. The measurement was performed using a differential scanning calorimetry (DSC7020, manufactured by SII NanoTechnology Inc.)

[0120] (MFR) The MFR of the raw materials was determined according to JIS K 7210-1:2014, at a temperature of 190℃ or 230℃ and a load of 2.16 kg. The MFR of polypropylene resins was determined at a temperature of 230℃, and the MFR of polyethylene resins was determined at a temperature of 190℃.

[0121] (Flexural modulus) The raw material was hot-pressed at 180°C to form a 4mm thick sheet, from which test pieces measuring 80mm long × 10mm wide × 4mm thick were cut. The flexural modulus of elasticity of the test piece, calculated according to JIS K 7171:2016, was used as the flexural modulus of elasticity of the raw material. Furthermore, the radius of the indenter R1 and the radius of the support R2 were both 5mm, the distance between the support points was 64mm, and the testing speed was 2mm / minute.

[0122] <Foaming Particles> (apparent density) The apparent density of the foamed particles was calculated as follows. First, the foamed particle group was placed for 2 days at a relative humidity of 50%, 23°C, and 1 atm. Next, a graduated cylinder containing water at 23°C was prepared, and an arbitrary amount of the foamed particle group (mass W1 of the foamed particle group) was submerged into the water in the graduated cylinder using tools such as a wire mesh. Then, considering the volume of the tools such as the wire mesh, the volume V1 [L] of the foamed particle group was measured based on the rise in water level. The mass W1 [g] of the foamed particle group placed in the graduated cylinder was divided by the volume V1 [L] (W1 / V1), and the units were converted to [kg / m³]. 3 ], thereby determining the apparent density of the bubbling particles.

[0123] (Average bubble diameter) The average bubble diameter of the foamed particles was determined as follows: Twenty foamed particles were randomly selected from the group. Each particle was cut in half lengthwise through its center, and magnified photographs of the entire cross-section were taken using a scanning electron microscope. In each cross-sectional photograph, four line segments were drawn at equal angles (45°) from the outermost surface of the foamed particle through the center to the outermost surface on the opposite side. The number of bubbles intersecting each line segment was counted, and the total length of the four line segments was divided by the total number of bubbles intersecting the line segments to calculate the bubble diameter of each foamed particle. The arithmetic mean of these values ​​was taken as the average bubble diameter of the foamed particles.

[0124] (Xylene insoluble content (gel fraction)) The xylene insoluble content of foamed particles determined by hot xylene extraction is obtained as follows: Weigh about 1g of sample (let the mass of the weighed sample be G1[g]), boil it in 100g of xylene for 6 hours, filter it quickly with a 100-mesh metal wire mesh, and then dry the boiling xylene insoluble residue on the metal wire mesh in a vacuum dryer at 80℃ for 8 hours. Then weigh the mass of the boiling xylene insoluble residue (let the mass of the weighed boiling xylene insoluble residue be G2[g]), and obtain it by the following formula (2).

[0125] Xylene insoluble content (mass%) = [G2 / G1] × 100 ··· Equation (2) (Degree of oxidation) The degree of oxidation of the foamed particles was determined under the following conditions (1).

[0126] Measurement conditions (1): The foamed particles were divided into group A, which was left to stand in the dark at 23°C for 1 day, and group B, which was left to stand in the dark at 80°C for 7 days. The ATR of the foamed particles after standing was measured in groups A and B, and calculated using the following formula (1). The degree of oxidation of the foamed particles is an indicator of how easily the foamed particles are oxidized. It can be said that the lower the degree of oxidation, the easier it is for the foamed particles to be oxidized even after long-term storage. In this embodiment and comparative example, the foamed particles with an oxidation degree of 3.5 or less achieved the objective effect of the present invention.

[0127] Oxidation degree = Oxidation degree of group B (A) 1730cm-1 / A 1460cm-1 ) / Oxidation degree of group A (A) 1730cm-1 / A 1460cm-1 Equation (1) In equation (1), A represents absorbance, and the subscript indicates the wavenumber. Furthermore, the wavenumber is typically 5 cm⁻¹. -1 The error is approximately 100%. The detailed contents of the ATR measurement under measurement conditions (1) are as follows. First, as the measuring device, the product name "FT / IR-4600typeA (ATR PRO450-S type)" manufactured by Japan Spectrophotometry Co., Ltd. was used. The foamed particles were then subjected to a flow rate of 340 kg / cm². 2 The pressure applied to the prism was used to seal it, and the infrared spectrum was measured to obtain the infrared absorption spectrum (without ATR correction). Next, the 1460 cm⁻¹ value obtained from the infrared absorption spectrum was measured. -1 Absorbance A at point 1460cm-1 and 1730cm -1 Absorbance A at point 1730cm-1 Calculate the absorbance ratio (A). 1730cm-1 / A 1460cm-1 The same measurements were performed on five foamed particles from both group A and group B, and the average value of these five values ​​was taken as the absorbance ratio of the foamed particles in group A and group B respectively (A). 1730cm-1 / A 1460cm-1 The detailed conditions for ATR determination are shown below.

[0128] ATR Measurement Conditions Prism: Zinc selenide Infrared light incident angle: 45° Total number of times: 64 Resolution: 4cm -1 Detector: TGS (Triglycine sulfate) Wavenumber measurement range: 600–4000 cm⁻¹ -1 Number of reflections: 1 <Foamed Particle Molding Body> (apparent density) The apparent density of the foamed particle molded body was calculated as follows. The foamed particle molded body was placed for 2 days at a relative humidity of 50%, a temperature of 23°C, and a pressure of 1 atm. Next, a container filled with water at 23°C was prepared, and an arbitrary amount of foamed particle molded body (mass W[g]) was submerged in the water using a tool such as a wire mesh. Then, considering the volume of the tool such as the wire mesh, the volume V[L] of the foamed particle molded body was measured based on the rise in water level. The mass W[g] of the foamed particle molded body placed in the container was divided by the volume V[L] (W / V), and the units were converted to [kg / m³]. 3 ], thereby determining the apparent density of the foamed particle-shaped body.

[0129] (Cooldown time) During the in-mold forming of foamed particle molded bodies, the time from the end of the main heating to the point when the pressure (surface pressure) generated on the inner surface of the metal mold reaches 0.04 MPa (G) is calculated as the cooling time. Furthermore, the cooling time is an indicator of the forming cycle time; a shorter cooling time indicates a shorter and more efficient forming cycle.

[0130] (Independent bubble rate) The independent bubble ratio of the foamed particle molded body was determined using an air comparison hydrometer or similar method, following step C as described in ASTM-D2856-70. The determination was specifically performed using the method described above.

[0131] (Fusion rate (fusion property)) The fusion rate of the foamed particle molded body is evaluated by the material breakage rate. Specifically, the foamed particle molded body is bent until it breaks, and the fracture surface (a fracture surface containing more than 100 foamed particles) is observed. The number of broken foamed particles and the number of foamed particles detached at the interface are counted visually. The fusion rate is then calculated as the percentage of the number of broken foamed particles relative to the total number of broken foamed particles and the number of foamed particles detached at the interface.

[0132] [evaluate] <Foamed Particle Molding Body> (Restoration) The thickness of the foamed particle molded body was measured near the four corners (specifically, within 10 mm from the corners towards the center) and the thickness of the center (the part that is equally divided longitudinally and laterally) when viewed from above. Then, the ratio (in %) of the thickness of the thinnest part to the thickness of the thickest part was calculated. Cases with a ratio of 98% or higher were rated as "A", and cases with a ratio of less than 98% were rated as "B".

[0133] (Secondary foaming properties (surface properties)) Draw a 100mm×100mm rectangle in the center of the obtained foamed particle molded body. Draw lines from the corners of the rectangular area to the diagonal and count the number of gaps (gaps) with a size of 1mm×1mm or larger on the line. Evaluate the surface properties (secondary foaming) of the foamed particle molded body as follows.

[0134] A: The number of gaps is less than 3 B: The number of gaps is 3 or more In addition, the aforementioned fusion properties, resilience, and secondary foaming properties are used as indicators of the formability of foamed particles.

[0135] (Surface smoothness) The surface smoothness of the obtained foamed particle molded body was evaluated according to the following criteria. The better the interstices of the foamed particles on the surface of the molded body, the better the surface performance. Specifically, a 100mm × 100mm square was drawn in the center of the foamed particle molded body, and the number of depressions with a depth of 1mm or more in this area was measured.

[0136] A: Less than 2 B: 3-9 C: 10 or more (Ratio of maximum bending strength) Foamed particle molded bodies formed by in-mold molding of foamed particles that underwent an accelerated curing (standing) test at 80°C in the dark for 7 days, and foamed particle molded bodies formed by in-mold molding of foamed particles that did not undergo the accelerated test, were prepared, and their maximum flexural strength was measured. Then, the ratio of the maximum flexural strength of the foamed particle molded body formed by in-mold molding of the foamed particle that underwent the accelerated test to the maximum flexural strength of the foamed particle molded body formed by in-mold molding of the foamed particle that did not undergo the accelerated test was calculated. When the ratio of the maximum flexural strength is 0.75 or higher, it is determined that even after the accelerated test, the reduction in the weldability of the foamed particles during in-mold molding was suppressed.

[0137] The maximum flexural strength of the foamed particle molded body was determined according to JIS K 7221-2:2006, with the point of maximum flexural strength being measured. Specifically, test pieces measuring 120 mm in length, 25 mm in width, and 20 mm in thickness were cut from the foamed particle molded body without removing the surface skin layer. Using these test pieces, except that the descent speed of the pressure wedge was set to 10 mm / min, the distance between the support points was set to 100 mm, the radius of the support front end was set to 5 mm, and the radius of the pressure wedge front end was set to 5 mm, the maximum flexural strength was measured according to JIS K 7221-2:2006. The maximum flexural strength was measured by cutting five test pieces near the center of the foamed particle molded body along its length without removing the skin layer, and the arithmetic mean was taken. Furthermore, the skin layer of the foamed particle molded body refers to the surface of the molded body that is in contact with the inner surface of the mold during in-mold molding, and the skin layer refers to the portion extending from the surface of the molded body to a depth of 10 mm.

[0138] [raw material] Table 1 shows the raw materials used in the examples and comparative examples.

[0139] Table 1

[0140] Examples 1-5, Comparative Examples 3 and 5 <Fabrication of Multilayer Polymer Particles> A co-extruder was used to produce strands by employing an extruder for forming a core layer with an inner diameter of 65 mm and an extruder for forming a coating layer with an inner diameter of 30 mm, and by having a die on the outlet side capable of co-extruding multiple multilayer strands.

[0141] The ethylene polymer (I) shown in Table 2, talc (trade name "KHB-125B" manufactured by Hayashi Kasei Corporation) as a bubble regulator, and 0.1 parts by weight of erucamide as a lubricant are supplied to an extruder for core layer formation, and the mixture is melt-kneaded to obtain a polymer melt for the core layer. Simultaneously, the ethylene polymer (II) shown in Table 2, the resin (III) shown in Table 2, and the free radical scavenger shown in Table 2 are supplied to the extruder in the proportions shown in Table 2, and the mixture is melt-kneaded to obtain a polymer melt for the coating layer.

[0142] Next, the core layer polymer melt and the coating layer polymer melt are fed into a co-extrusion die. In the die, the core layer polymer melt is stacked so that it covers the core layer polymer melt. The mixture is extruded into a filament and water-cooled. It is then cut into approximately 2 mg particles using a granulator and dried to obtain multilayer polymer particles (particle size: 1.0 mm, aspect ratio L / D: 2.0).

[0143] At this point, the mass ratio of the amount of polymer melt sprayed for the core layer to the amount of polymer melt sprayed for the coating layer is set to core layer:coating layer = 95:5. In addition, the bubble regulator is supplied in the form of masterbatch, and only 0.1 parts by mass of talc is added to the core layer relative to 100 parts by mass of ethylene-based polymer (I), while no bubble regulator is added to the coating layer.

[0144] In addition, pentaerythritol tetra[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (trade name "Irganox 1010" manufactured by BASF) was used as a phenolic antioxidant; tris(2,4-di-tert-butylphenyl) phosphite (trade name "Irgafos168" manufactured by BASF) was used as a phosphorus-based antioxidant; and dimethyl succinate-1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylpiperidine condensate (trade name "Tinuvin 622" manufactured by BASF) was used as a weathering agent. These antioxidants and weathering agents function as free radical scavengers.

[0145] <Making of Foamed Particles> 1 kg of the multilayer polymer particles and 3 L of water as a dispersion medium were added to a sealed container. Then, 3 g of kaolin as a dispersant, 0.4 g of surfactant (Neogen, a trade name manufactured by Daiichi Kogyo Pharmaceutical Co., Ltd.), and 8 g of Perccumyl D, a trade name manufactured by Nippon Yu Co., Ltd., were added as a crosslinking agent. Carbon dioxide was added to the sealed container as a foaming agent at a pressure of 2.3 MPa (carbon dioxide pressure). The container was heated to 160.0°C (foaming temperature) with stirring and maintained at this temperature for 30 minutes. The contents of the container were then released to atmospheric pressure to obtain foamed particles. The coating layer in the foamed particles was in a non-foamed state.

[0146] <Fabrication of Foamed Particle Moldings> After undergoing a pretreatment and pressurization process, the obtained foamed particles are filled into the cavity of a forming mold with a flat mold cavity measuring 250mm x 250mm x 20mm. Specifically, the pretreatment and pressurization process involves placing the foamed particles into a pressure-resistant container and pressurizing the container with air, causing the air to permeate the bubbles within the foamed particles, thereby increasing the internal pressure of the bubbles. Furthermore, the pressure (i.e., internal pressure) within the bubbles of the foamed particles removed from the pressure-resistant container is 0.12 MPa (G). Additionally, the cracking amount (specifically, the ratio of the mold opening amount to the thickness dimension) when filling the forming mold with foamed particles is set to 20% (i.e., 4mm). After filling, the forming mold is closed in the thickness direction, mechanically compressing the foamed particles. Next, steam is supplied from both sides of the mold for 5 seconds for preheating (venting process). Then, single-sided heating is performed from one side of the mold until the steam pressure reaches 0.08 MPa (G) lower than the molding pressure recorded in Table 2. Then, single-sided heating is performed from the other side of the mold until the steam pressure reaches 0.04 MPa (G) lower than the molding pressure recorded in Table 2. Afterward, heating continues until the molding pressure recorded in Table 2 is reached (main heating). This causes the foamed particles to foam a second time while simultaneously melting the surface, allowing the foamed particles to fuse together. After heating, the pressure is released, and the molded body is water-cooled until the surface pressure generated by the foaming force of the molded body becomes 0.04 MPa (G). Then, the mold is opened and the molded body is removed. The resulting molded body is cured in an oven at 60°C for 12 hours to obtain the foamed particle molded body.

[0147] The internal pressure of the foamed particles after the pretreatment and pressurization process is determined by the following method. Specifically, the mass Q (in g) of the foamed particle group before filling into the molding die in the state of increased internal pressure and the mass U (in g) of the foamed particle group after 48 hours from the pretreatment and pressurization process are measured. Using these values, the bubble internal pressure P (in MPa (G) of the foamed particles after the pretreatment and pressurization process is calculated based on the following formula (4).

[0148] P={(QU)÷M}×R×T÷V···Formula (4) Furthermore, in the above formula (4), M is the molecular weight of air, R is the gas constant, T is the absolute temperature of the foamed particle group (unit: K), and V is the volume (unit: L) after subtracting the volume of resin occupied by the foamed particle group from the apparent volume of the foamed particle group. In this example, M = 28.8 g / mol, R = 0.0083 MPa·L / (K·mol), and T = 296 K were used.

[0149] Comparative Example 1 Except that polypropylene resin and free radical scavenger were not incorporated into the coating layer, foamed particles and foamed particle molded articles were obtained in the same manner as in Example 1.

[0150] Comparative Example 2 In the above-mentioned <Preparation of Multilayer Polymer Particles>, an extruder with an inner diameter of 50 mm and a manufacturing apparatus for forming strands attached to the downstream side of the extruder were prepared. The ethylene-based polymer shown in Table 2, the polypropylene-based resin shown in Table 2, and BASF's trade name "Irganox 1010" as a free radical scavenger were fed into the extruder in the blending ratios shown in Table 2, respectively, and were melt-kneaded and extruded to obtain single-layer polymer particles. Otherwise, foamed particles and foamed particle molded bodies were obtained in the same manner as in Example 1.

[0151] Comparative Example 4 Except that no free radical scavenger was incorporated into the coating layer, foamed particles and foamed particle molded articles were obtained in the same manner as in Example 1.

[0152] Comparative Example 6 Except that polypropylene resin was not incorporated into the coating layer, but instead linear low-density polyethylene was incorporated, foamed particles and foamed particle molded articles were obtained in a manner substantially similar to those in Example 1.

[0153] Table 2

[0154] As can be seen from Table 2, the foamed particles of the embodiments have a low degree of oxidation, and oxidation is easily suppressed even during long-term storage. As a result, the reduction in fusion during in-mold forming is easily suppressed, and the reduction in maximum flexural strength after a heating test assuming long-term storage is also suppressed. In addition, the foamed particles of the embodiments have a shorter cooling time compared to the foamed particles of the comparative examples, and the molding cycle tends to be shortened.

[0155] On the other hand, Comparative Example 1, which does not contain polypropylene-based resin, polystyrene-based resin, or free radical scavenger in its coating layer, exhibits a high degree of oxidation. Furthermore, the maximum flexural strength of the foamed particle molded body formed after accelerated foaming was reduced. This is believed to be because the generated free radicals were not deactivated, leading to rapid oxidation of the foamed particle surface.

[0156] Comparative Example 2, which is a non-multilayer structure, showed poor recoverability of the foamed particle molded article. This is believed to be because cross-linking is hindered by free radical scavengers during the foamed particle manufacturing process, resulting in a lower gel fraction of the foamed particles and consequently poor recoverability of the molded article.

[0157] Comparative Example 3, which does not contain polypropylene or polystyrene resins in its coating layer, also exhibited a high degree of oxidation. Furthermore, the maximum flexural strength of the foamed particle molded from the foamed particles after accelerated molding was reduced. This is believed to be because the oxidation-retarding effect provided by the polypropylene and / or polystyrene resins could not be obtained, thus causing oxidation on the surface of the foamed particles to progress rapidly.

[0158] Comparative Example 4, which lacks a free radical scavenger in its coating layer, also exhibits a high degree of oxidation. Furthermore, the maximum flexural strength of the foamed particle molded body formed after accelerated testing decreased. This is believed to be due to the absence of free radical scavenging and deactivation in the initial stages of the oxidation reaction caused by the free radical scavenger, leading to rapid oxidation of the foamed particle surface.

[0159] Comparative Example 5, which has a higher mass proportion of polypropylene resin in the coating layer, also has a higher degree of oxidation. The decrease in maximum flexural strength after accelerated testing was not suppressed, and the flexural test evaluation was also poor. This is believed to be because the polypropylene resin, which is more prone to oxidation and degradation than ethylene-based polymers, forms a marine phase (continuous phase). Furthermore, the molded body formed by in-mold molding the foamed particles of Comparative Example 5 has significantly poorer surface smoothness. Moreover, the characteristic softness of ethylene-based polymers is compromised.

[0160] Comparative Example 6, which uses linear low-density polyethylene (PE-LLD) instead of polypropylene-based resin and / or polystyrene-based resin in the coating layer, also exhibited a high degree of oxidation. Furthermore, the maximum flexural strength of the foamed particle molded body formed after accelerated testing was reduced. This is believed to be because the linear low-density polyethylene (specifically, an ethylene-hexene copolymer polymerized via a metallocene-based catalyst) is compatible with the ethylene-based polymer (II), and the oxidation-retarding effect provided by the polypropylene-based resin and / or polystyrene-based resin cannot be obtained, thus causing oxidation on the surface of the foamed particles to progress rapidly.

[0161] Industrial applicability The ethylene-based polymer foam particles of the present invention can suppress the decrease in the fusion properties of the foam particles even when stored for a long time under harsh conditions such as exposure to external air, resulting in the formation of foam particle molded articles with excellent flexural strength.

Claims

1. A type of ethylene-based polymer foamed particle, comprising a foamed core layer composed of an ethylene-based polymer (I) and a covering layer covering the foamed core layer, wherein, The gel fraction of the ethylene-based polymer foam particles, determined by hot xylene extraction, was 40% by mass or more and 70% by mass or less. The coating layer comprises an ethylene polymer (II), one or more resins selected from the group consisting of polypropylene resins and polystyrene resins (III), and a free radical scavenger. The resin (III) in the coating layer has a mass ratio of more than 1% and less than 60% of the total 100% mass of the ethylene polymer (II) and the resin (III). The amount of the free radical scavenger in the coating layer is more than 10 ppm by mass and less than 10,000 ppm by mass.

2. The ethylene-based polymer foam particles according to claim 1, wherein, The free radical scavenger is one or more additives selected from the group consisting of antioxidants and weathering agents.

3. The ethylene-based polymer foam particles according to claim 2, wherein, The antioxidant is one or more compounds selected from the group consisting of phenolic antioxidants, phosphorus antioxidants and thioether antioxidants.

4. The ethylene-based polymer foam particles according to claim 2, wherein, The weathering agent is a hindered amine light stabilizer.

5. The ethylene-based polymer foam particles according to any one of claims 1 to 4, wherein, The ethylene-based polymer (I) is a thermoplastic elastomer.

6. The ethylene-based polymer foam particles according to claim 5, wherein, The thermoplastic elastomer is a block copolymer with polyethylene blocks as hard segments.

7. The ethylene-based polymer foam particles according to any one of claims 1 to 4, wherein, The resin (III) is a polypropylene resin.

8. The ethylene-based polymer foam particles according to claim 7, wherein, The difference between the melting point Tmp of the polypropylene resin and the melting point Tme of the ethylene polymer (I), [Tmp-Tme], is greater than 5°C and less than 35°C.

9. A type of ethylene-based polymer foaming particle, wherein, The gel fraction of the ethylene-based polymer foam particles, determined by hot xylene extraction, was ≥40% by mass and ≤70% by mass. The oxidation degree of the ethylene-based polymer foam particles, measured under the following conditions (1), is below 3.

5. Measurement conditions (1): The foamed particles were divided into group A, which was placed in the dark at 23°C for 1 day, and group B, which was placed in the dark at 80°C for 7 days. The ATR of the foamed particles in group A and group B was measured and calculated using the following formula (1). Oxidation degree = Oxidation degree of group B (A) 1730cm-1 / A 1460cm-1 ) / Oxidation degree of group A (A) 1730cm-1 / A 1460cm-1 Equation (1) In equation (1), A represents absorbance and the subscript indicates wavenumber.

Images