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

Abstract

To provide a method for producing polypropylene-based resin foamed particles excellent in productivity of a foamed particle molded article because of good in-mold moldability and shortened water cooling time during in-mold molding, and to provide the polypropylene-based resin foamed particles. The present invention also provides a method for producing expanded polypropylene resin particles and expanded polypropylene resin particles, each of which can contribute to reduction in environmental load.SOLUTION: Wherein the polymer particles have a biomass ratio of not less than 1%, the polymer particles contain carbon black in an amount of not less than 0.1% by mass and not more than 8% by mass, and the polymer particles have a melt flow rate of not more than 16kg / 10 minutes as measured under conditions of a temperature of 230 °C and a load of 20g.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Polypropylene resin foam beads are excellent in strength and shock-absorbing properties and are therefore used in a variety of applications, such as packaging materials, automotive components, and building components.

[0003] Polypropylene resin foamed bead molded articles are produced, for example, by a method called in-mold molding, in which expanded polypropylene resin beads are filled into a mold and then heated by supplying a heating medium such as steam into the mold. In the in-mold molding method, when a heating medium is supplied into the mold, the expanded beads undergo secondary expansion and their surfaces melt. This causes the expanded beads in the mold to fuse together, resulting in a molded article having a shape corresponding to the shape of the mold cavity. Since the molded article is prone to expansion due to secondary expansion immediately after molding, it is cooled in the mold with water or the like for a predetermined time and then released from the mold.

[0004] In recent years, there has been growing awareness of environmental impacts such as the increase in atmospheric carbon dioxide concentration and the depletion of fossil fuel resources, and in order to reduce the environmental impact, the production of expanded resin beads using raw materials derived from biomass has been considered. For example, in the case of expanded polyethylene resin beads, a technology has been proposed for expanded polyethylene resin beads containing a plant-derived polyethylene resin with a high plant content (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-060514 Summary of the Invention [Problem to be solved by the invention]

[0006] On the other hand, with regard to expanded polypropylene resin bead moldings, no technology has been proposed so far relating to expanded beads using biomass-derived polypropylene resins, including plant-derived polypropylene resins. Therefore, there is a need for technology proposals relating to expanded polypropylene resin beads and expanded polypropylene resin bead moldings that use biomass-derived raw materials and can address the above-mentioned reduction in environmental load.

[0007] However, expanded beads containing biomass-derived polypropylene resins as raw materials may have poor moldability in molds, and even when expanded beads are molded in molds to obtain expanded bead molded articles, the water cooling time may be long, leaving room for improvement in these respects. Therefore, there has been a demand for expanded polypropylene resin beads and a method for producing the same that can solve these problems.

[0008] The present invention has been made in consideration of such demands, and an object of the present invention is to provide a method for producing expanded polypropylene-based resin beads, which have good in-mold moldability and a short water cooling time during in-mold molding, thereby providing excellent productivity for expanded bead moldings, and a method for producing expanded polypropylene-based resin beads. Another object of the present invention is to provide a method for producing expanded polypropylene-based resin beads, which can also contribute to reducing the environmental load. [Means for solving the problem]

[0009] As a result of extensive research, the present inventors have found that the above-mentioned problems can be solved by a method for producing expanded beads by expanding specific resin particles. That is, one aspect of the present invention is the method for producing expanded beads described in the following [1] to [6], and the expanded beads described in [7] and [8]. [1] A method for producing expanded polypropylene resin beads, comprising expanding polypropylene resin particles having a polypropylene resin as a base resin to obtain expanded beads, wherein the resin particles have a biomass content of 1% or more as measured according to ASTM D6866-21, contain 0.1% to 8% by mass of carbon black, and have a melt flow rate of 20 g / 10 min or less as measured at a temperature of 230°C and a load of 2.16 kg. [2] The method for producing expanded polypropylene resin particles according to [1], wherein the base resin of the resin particles is a mixed resin of a biomass-derived polypropylene resin and a fossil fuel-derived polypropylene resin, and the melt flow rate of the fossil fuel-derived polypropylene resin measured at a temperature of 230°C and a load of 2.16 kg is 35 g / 10 min or less. [3] The method for producing expanded polypropylene resin particles according to [1] or [2], wherein the base resin of the resin particles is a mixed resin of a biomass-derived polypropylene resin and a fossil fuel-derived polypropylene resin, and the mass ratio of the biomass-derived polypropylene resin to the fossil fuel-derived polypropylene resin in the mixed resin (biomass-derived polypropylene resin:fossil fuel-derived polypropylene resin) is 1:99 to 90:10. [4] The method for producing expanded polypropylene resin beads according to any one of [1] to [3], wherein the expanded beads have a crystalline structure in which a main endothermic peak having the largest peak area and a high-temperature peak, which is an endothermic peak that appears on the high-temperature side of the main endothermic peak, appear on a DSC curve obtained when the expanded beads are heated from 23°C to 230°C at a heating rate of 10°C / min, and the heat of fusion of the high-temperature peak is 10 J / g or more and 35 J / g or less. [5] The bulk density of the expanded particles is 10 kg / m 3 More than 200kg / m 3 The method for producing expanded polypropylene resin particles according to any one of [1] to [4] below. [6] A method for producing expanded polypropylene resin beads according to any one of [1] to [5], comprising: a dispersing step of dispersing the resin particles in an aqueous medium in a sealed container; a foaming agent adding step of adding a physical foaming agent into the sealed container; and an expansion step of impregnating the resin particles with the physical foaming agent in the sealed container, and then releasing the resin particles together with the aqueous medium from the sealed container into an atmosphere having a lower pressure than the pressure inside the sealed container, thereby expanding the resin particles to produce expanded beads. [7] Expanded polypropylene resin beads using a polypropylene resin as a base resin, wherein the expanded beads have a biomass content of 1% or more as measured by ASTM D6866-21, contain 0.1% to 8% by mass of carbon black, have a closed cell rate of 85% or more, have an average cell diameter D of 50 μm to 250 μm, and have a ratio Dc / D of the average cell diameter Dc of the bubbles located at the center of the expanded beads to the average cell diameter D of the expanded beads, of 0.7 to 1.3. [8] Expanded polypropylene resin beads according to [7], wherein the average cell diameter Dc of the bubbles located at the center of the expanded beads is 220 μm or less. [Effects of the Invention]

[0010] According to the present invention, a method for producing expanded polypropylene-based resin beads and expanded polypropylene-based resin beads can be provided, which have good in-mold moldability and a reduced water cooling time during in-mold molding, resulting in excellent productivity for expanded bead moldings.Furthermore, a method for producing expanded polypropylene-based resin beads and expanded polypropylene-based resin beads can be provided, which can also contribute to reducing environmental load. DETAILED DESCRIPTION OF THE INVENTION

[0011] [Method of manufacturing polypropylene resin foam beads] The method for producing expanded polypropylene-based resin beads of the present invention is a method for producing expanded polypropylene-based resin beads, which comprises expanding polypropylene-based resin particles having a polypropylene-based resin as a base resin to obtain expanded beads, wherein the resin particles have a biomass degree of 1% or more as measured according to ASTM D6866-21, contain 0.1% by mass or more and 8% by mass or less of carbon black, and have a melt flow rate of 20 g / 10 min or less as measured under conditions of a temperature of 230°C and a load of 2.16 kg. In this specification, "expanded polypropylene resin beads" will also be referred to simply as "expanded beads," "polypropylene resin particles having polypropylene resin as the base resin" will also be referred to simply as "expanded polypropylene resin beads" or "resin beads," and "expanded polypropylene resin bead molded body" will also be referred to simply as "expanded bead molded body" or "molded body."

[0012] <Polypropylene resin particles> The polypropylene resin particles used in the method for producing expanded polypropylene resin beads of the present invention have a biomass content of 1% or more as measured by ASTM D6866-21, contain 0.1% by mass or more and 8% by mass or less of carbon black, and have a melt flow rate of 20 g / 10 min or less as measured under conditions of a temperature of 230°C and a load of 2.16 kg. The expanded resin particles obtained by expanding such resin particles have good moldability and a shorter water cooling time during molding, resulting in excellent productivity of molded articles. Furthermore, since the expanded resin particles contain a predetermined amount of biomass-derived components, they can also contribute to reducing environmental impact.

[0013] (Polypropylene resin) In this specification, the term "polypropylene resin" refers to a propylene homopolymer or a polypropylene copolymer containing more than 50% by mass of structural units derived from propylene. Examples of the propylene homopolymer include isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene. Examples of the polypropylene copolymer include copolymers of propylene with ethylene or an α-olefin having 4 to 8 carbon atoms, such as a propylene-ethylene copolymer, a propylene-butene copolymer, and a propylene-ethylene-butene copolymer, as well as propylene-acrylic acid copolymers and propylene-maleic anhydride copolymers. These copolymers may be block copolymers, random copolymers, or graft copolymers. The above-mentioned polymer may be crosslinked, but is preferably non-crosslinked from the viewpoint of further contributing to reducing the environmental load.

[0014] (Biomass ratio) The polypropylene-based resin particles have a biomass content (biobased carbon content) of 1% or more as measured by ASTM D6866-21. That is, the biomass content of the resin particles as measured by ASTM D6866-21 is 1% or more. When the biomass degree of the resin beads is within the above range, the use of fossil resources can be suppressed during the production of the expanded bead molding, and the amount of carbon dioxide emitted during the life cycle of the expanded bead molding can also be reduced. From this viewpoint, the biomass degree of the resin particles measured by ASTM D6866-21 is more preferably 1.5% or more, more preferably 1.8% or more, and even more preferably 2% or more. From the viewpoint of further reducing the environmental load, the biomass degree of the resin particles is even more preferably 5% or more, and particularly preferably 10% or more. Furthermore, there is no upper limit to the biomass degree of the resin particles, and the biomass degree of the resin particles measured according to ASTM D6866 may be 100% or less. However, from the viewpoint of further improving moldability in a mold and further shortening the water cooling time during mold-in-a-mold molding, the biomass degree of the resin particles measured according to ASTM D6866-21 is preferably 80% or less, more preferably 50% or less, even more preferably 30% or less, and particularly preferably 20% or less, and from the viewpoint of further improving moldability in a mold, even more preferably 10% or less. The upper and lower limits of the biomass degree of the resin particles can be arbitrarily combined to determine a preferred range, such as 1% to 100%, 1% to 80%, 1.5% to 50%, 2% to 30%, 1% to 20%, or 5% to 20%.

[0015] The biomass ratio of the resin particles is measured according to ASTM D6866-21 and means the proportion of naturally occurring components contained in the resin particles. The biomass ratio is also measured by the radiocarbon standard specified in ASTM D6866-21 using the resin particles as a measurement sample. 14 It can be determined by carrying out a C measurement. When the biomass degree of the biomass-derived polypropylene resin used to produce the resin particles and the content of the biomass-derived polypropylene resin in the resin particles are known, these values ​​can be used to calculate the biomass degree.

[0016] (carbon black) The polypropylene resin particles contain 0.1% by mass or more and 8% by mass or less of carbon black. By including a predetermined amount of carbon black in the resin particles, the water cooling time during molding of the resulting expanded beads can be shortened, improving the productivity of the molded articles and providing the resulting molded articles with a luxurious appearance.

[0017] The reason why the water cooling time during molding in a mold can be shortened when the resin particles contain a predetermined amount of carbon black is not clear, but is thought to be as follows. Conventionally, expanded beads containing biomass-derived polypropylene resin as a raw material have sometimes required a long water-cooling time during in-mold molding, even when foamed bead moldings can be obtained. This is thought to be because bubbles formed in the center of the resin beads during expansion of the resin beads coalesce, resulting in the resulting expanded beads having coarse bubbles in the center. On the other hand, in the present invention, the incorporation of carbon black into resin beads containing biomass-derived polypropylene resin is thought to moderately thicken the resin and inhibit the coalescence of central bubbles during expansion of the resin beads. As a result, the resulting expanded beads inhibit the formation of coarse bubbles in the center and have relatively small and uniform cell diameters overall, which is thought to shorten the water-cooling time during in-mold molding. It is believed that such an effect of carbon black is significantly exhibited when the resin particles contain a biomass-derived polypropylene resin and have a specific melt flow rate, which will be described later.

[0018] The polypropylene-based resin particles contain 0.1% by mass or more and 8% by mass or less of carbon black. That is, the carbon black content in the resin particles is 0.1% by mass or more and 8% by mass or less, based on the total amount of resin particles. If the carbon black content in the resin particles is less than 0.1% by mass, the resulting expanded beads may require a long water cooling time during in-mold molding, resulting in poor productivity. In addition, the resulting molded article may not be able to have a luxurious appearance. On the other hand, if the carbon black content in the resin particles exceeds 8% by mass, the resulting expanded beads may have poor in-mold moldability, resulting in an excessively high molding pressure or making it impossible to obtain a good molded article.

[0019] From the viewpoint of further shortening the water cooling time during in-mold molding, the carbon black content in the resin particles is preferably 0.5% by mass or more, more preferably 1% by mass or more, even more preferably 1.5% by mass or more, and even more preferably 2% by mass or more. From the viewpoint of the in-mold moldability of the resulting expanded beads, the carbon black content in the resin particles is preferably 7% by mass or less, more preferably 6% by mass or less, more preferably 4.5% by mass or less, and even more preferably 3% by mass or less. The carbon black content in the resin particles means the proportion (% by mass) of carbon black relative to the total amount of resin particles. The upper and lower limits of the carbon black content in the resin particles can be arbitrarily combined to determine a preferred range. Therefore, the content of carbon black in the resin particles is, for example, 0.5% by mass or more and 8% by mass or less, 1.5% by mass or more and 7% by mass or less, 2% by mass or more and 7% by mass or less, 0.5% by mass or more and 6% by mass or less, 1% by mass or more and 6% by mass or less, 1.5% by mass or more and 4.5% by mass or less, and 2% by mass or more and 3% by mass or less.

[0020] Carbon black is a material that is commonly used as a black colorant. The manufacturing method and composition of the carbon black are not particularly limited. For example, channel black, roller black, furnace black, thermal black, acetylene black, ketjen black, etc. can be used. In addition, these carbon blacks may be hydrophilized, hydrophobized, oxidized, reduced, acidified, basicized, or organically modified. From the viewpoint of excellent dispersibility in polypropylene-based resins, furnace black is preferred as the carbon black.

[0021] The dibutyl phthalate (DBP) oil absorption of the carbon black is preferably less than 150 mL / 100 g, more preferably 130 mL / 100 g or less, and even more preferably 110 mL / 100 g or less. The lower limit is preferably 80 mL / 100 g. The DBP oil absorption is a value measured in accordance with ASTM D2414-79.

[0022] In order to more appropriately exert the thickening effect of the resin, the BET specific surface area of ​​the carbon black is preferably 200 m 2 / g or less, and more preferably 150m 2 / g or less, more preferably 100m 2 The lower limit is preferably 50 m / g or less. 2 The BET specific surface area of ​​carbon black is a value measured by the BET method in accordance with ASTM D3037.

[0023] Carbon black can be selected from those described above. Since carbon black can be dispersed more uniformly in the polypropylene resin, it is preferable to blend carbon black into the polypropylene resin as a masterbatch. The concentration of carbon black in the masterbatch is preferably 10% by mass or more and 80% by mass or less, and more preferably 30% by mass or more and 60% by mass or less. The base resin of the masterbatch may be a resin different from the polypropylene resin, but is preferably a polypropylene resin.

[0024] (Melt Flow Rate) The polypropylene-based resin particles have a melt flow rate of 20 g / 10 min or less when measured at 230°C under a load of 2.16 kg. That is, the melt flow rate of the polypropylene-based resin particles is 20 g / 10 min or less when measured at 230°C under a load of 2.16 kg. If the melt flow rate of the polypropylene-based resin particles exceeds 20 g / 10 min, the thickening effect of the carbon black described above becomes insufficient. As a result, the water cooling time during in-mold molding of the resulting expanded beads becomes longer, which may impair productivity. Furthermore, the in-mold moldability of the expanded beads may be reduced.

[0025] From the above viewpoints, the melt flow rate of the polypropylene resin particles measured under conditions of a temperature of 230°C and a load of 2.16 kg is preferably 18 g / 10 min or less, more preferably 15 g / 10 min or less, even more preferably 10 g / 10 min or less, and still more preferably 9 g / 10 min or less. Also, the melt flow rate of the polypropylene resin particles is preferably 3 g / 10 min or more, more preferably 5 g / 10 min or more, even more preferably 6 g / 10 min or more, and still more preferably 7 g / 10 min or more. The upper and lower limits of the melt flow rate of the resin particles can be arbitrarily combined to determine a preferred range, and the melt flow rate of the resin particles is, for example, 3 g / 10 min to 20 g / 10 min, 5 g / 10 min to 18 g / 10 min, 6 g / 10 min to 15 g / 10 min, 7 g / 10 min to 10 g / 10 min, or 7 g / 10 min to 9 g / 10 min. The melt flow rate of the polypropylene resin particles is a value measured under conditions of a temperature of 230°C and a load of 2.16 kg. More specifically, it can be measured in accordance with JIS K 7210-1:2014 by the method described in the examples.

[0026] (Melting Point) The melting point of the resin particles is preferably 135°C or higher and 160°C or lower, more preferably 140°C or higher and 155°C or lower, and even more preferably 142°C or higher and 152°C or lower, from the viewpoint of achieving an excellent balance between the moldability of the expanded beads and the strength of the molded body. The melting point of the resin particles can be determined by the same method as that for the melting point of the biomass-derived polypropylene resin described below, and more specifically, can be measured by the method described in the Examples.

[0027] (crystallization temperature) From the viewpoint of further shortening the water cooling time during molding of the expanded beads, the crystallization temperature of the resin particles is preferably 102°C or higher and 120°C or lower, more preferably 105°C or higher and 118°C or lower, and even more preferably 110°C or higher and 115°C or lower. The crystallization temperature of the resin particles can be determined by the same method as that for the crystallization temperature of the biomass-derived polypropylene resin described below, and more specifically, can be measured by the method described in the Examples.

[0028] <Base resin and mixed resin> The base resin of the resin particles contains a biomass-derived polypropylene-based resin. By containing a biomass-derived polypropylene-based resin as the base resin of the resin particles, the biomass content of the resulting expanded beads can be increased, which can also contribute to reducing the environmental load. The base resin of the resin particles is preferably a mixed resin of a biomass-derived polypropylene resin and a fossil fuel-derived polypropylene resin. By including a fossil fuel-derived polypropylene resin in addition to a biomass-derived polypropylene resin in the base resin, the resulting expanded beads have better moldability. Furthermore, the viscoelasticity of the polypropylene resin constituting the resin beads is appropriately adjusted, making it easier to obtain the effects of the carbon black, and the resulting expanded beads can be more reliably molded with less water cooling time. In this specification, the term "base resin of resin particles" refers to the polymer component that constitutes the resin particles.

[0029] (Biomass-derived polypropylene resin) In the present invention, biomass refers to renewable organic resources derived from living organisms (excluding organic resources derived from fossil fuels such as petroleum and coal). Biomass-derived polypropylene resin refers to a polypropylene resin containing a biomass-derived monomer component in its molecular chain, which is formed by polymerizing a biomass-derived monomer. The biomass-derived polypropylene resin may be composed solely of biomass-derived monomer components, or may be composed of biomass-derived monomer components and fossil fuel-derived monomer components.

[0030] In the present invention, the biomass-derived monomer refers to propylene, ethylene, or an α-olefin having from 4 to 8 carbon atoms produced from a biomass raw material. Furthermore, in the present invention, the biomass-derived monomer component refers to a structural unit in a polymer produced by addition polymerization of the monomers propylene, ethylene, or an α-olefin having from 4 to 8 carbon atoms. The method for producing the biomass-derived monomer is not particularly limited, and the biomass-derived monomer can be obtained by a conventionally known method, such as dehydrating alcohol derived from a biomass raw material or cracking naphtha derived from a biomass raw material. Examples of biomass raw materials used in the production of the biomass-derived monomer include monosaccharides, polysaccharides, vegetable oils and animal oils obtained from agricultural and livestock products, forestry products, algae, etc. In the present invention, the biomass raw material used in producing the biomass-derived monomer is preferably bionaphtha produced from waste cooking oil, black liquor, tall oil, palm oil mill effluent, oils and fats contained in microalgae, etc., from the viewpoints of non-competition with food and contribution to a recycling-oriented society through the use of by-products and waste biomass, and in the present invention, propylene obtained by cracking such bionaphtha is preferably used.

[0031] The biomass-derived polypropylene-based resin is composed of the above-mentioned polypropylene-based resin. The biomass-derived polypropylene-based resin may contain two or more types of biomass-derived polypropylene-based resins. From the viewpoint of expanding the steam pressure range in which an expanded bead molding can be produced and providing expanded beads that more reliably shorten the water cooling time during molding, the biomass-derived polypropylene-based resin is preferably a propylene homopolymer or a propylene-ethylene block copolymer, and more preferably a propylene homopolymer.

[0032] The biomass-derived polypropylene resin may be a commercially available polypropylene resin derived from biomass. Specific examples include products manufactured by Lyondellbasell under the product numbers "HP456J," "HP640J," and "EP348U."

[0033] The biomass-derived polypropylene resin preferably has a biomass degree (biobased carbon content) measured according to ASTM D6866-21 of 1% or more and 100% or less. From the viewpoint of contribution to reducing environmental impact, the biomass degree of the biomass-derived polypropylene resin is preferably 5% or more, more preferably 10% or more, even more preferably 20% or more, still more preferably 25% or more, and particularly preferably 30% or more. On the other hand, from the viewpoint of moldability of the expanded beads and productivity of the molded articles, the biomass degree of the biomass-derived polypropylene resin is preferably 80% or less, more preferably 60% or less, even more preferably 50% or less, and particularly preferably 45% or less. The above upper and lower limits can be arbitrarily combined to determine a preferred biomass degree. Therefore, the biomass degree of the biomass-derived polypropylene resin is, for example, preferably 5% or more and 80% or less, more preferably 10% or more and 60% or less, even more preferably 20% or more and 50% or less, and even more preferably 30% or more and 45% or less.

[0034] The melting point of the biomass-derived polypropylene resin is preferably 150°C or higher and 170°C or lower, more preferably 155°C or higher and 170°C or lower, even more preferably 158°C or higher and 170°C or lower, and particularly preferably 160°C or higher and 170°C or lower, from the viewpoints of moldability of the expanded beads, productivity of the molded body, and strength of the molded body. The melting point of biomass-derived polypropylene resins can be determined based on JIS K 7121: 2012. In this case, the test specimen is conditioned as follows: (2) Measuring the melting temperature after a certain heat treatment. More specifically, a pelletized polypropylene resin was used as a test specimen, and the DSC curve obtained by the heat flux differential scanning calorimetry method described in JIS K 7121:2012 was used. The test specimen was heated from 23°C to 200°C at a heating rate of 10°C / min, cooled to 23°C at a cooling rate of 10°C / min, and then heated again from 23°C to 200°C at a heating rate of 10°C / min. The melting point of the polypropylene resin was determined as the apex temperature of the melting peak. If two or more melting peaks appear in the DSC curve, the melting point was determined as the apex temperature of the melting peak with the largest area. The melting peak with the largest area can be determined by distinguishing each melting peak at the temperature between the valleys of the DSC curve located between the peak temperatures of the melting peaks and comparing the areas (heats of fusion) of each melting peak. The valley temperature of the DSC curve can be determined by referring to the DSC differential curve (DDSC) and the temperature at which the value on the vertical axis of the differential curve becomes 0. Examples of the measuring device include a heat flux differential scanning calorimeter (manufactured by SII NanoTechnology Inc., model number: DSC7020).

[0035] From the viewpoint of further shortening the water cooling time during molding of the expanded beads, the crystallization temperature of the biomass-derived polypropylene resin is preferably 110°C or higher and 130°C or lower, more preferably 113°C or higher and 128°C or lower, and even more preferably 115°C or higher and 123°C or lower.

[0036] The crystallization temperature of biomass-derived polypropylene resins refers to the temperature at the apex of the crystallization peak determined by heat flux differential scanning calorimetry in accordance with JIS K 7121:2012. The specimen conditioning method used was "(2) Measurement of melting temperature after a certain heat treatment," with a cooling rate of 10°C per minute. If two or more crystallization peaks appear, the temperature at the apex of the crystallization peak with the largest area is taken as the crystallization temperature.

[0037] The melt flow rate of the biomass-derived polypropylene resin, measured in accordance with JIS K 7210-1:2014 at 230°C under a load of 2.16 kg, is preferably 0.5 g / 10 min to 10 g / 10 min, more preferably 1 g / 10 min to 8 g / 10 min, even more preferably 1.5 g / 10 min to 5 g / 10 min, and still more preferably 2 g / 10 min to 5 g / 10 min. When the melt flow rate is within the above range, the resulting expanded beads have better moldability in a mold. Furthermore, the resulting expanded bead molding exhibits sufficient rigidity.

[0038] The flexural modulus of the biomass-derived polypropylene resin is preferably 1000 MPa or more and 1800 MPa or less, more preferably 1200 MPa or more and 1800 MPa or less, even more preferably 1300 MPa or more and 1700 MPa or less, and still more preferably 1400 MPa or more and 1600 MPa or less, from the viewpoint of expanding the moldable range of the expanded beads and obtaining expanded bead molded articles with excellent rigidity. The flexural modulus of the biomass-derived polypropylene resin can be determined by heat-pressing the biomass-derived polypropylene resin to prepare a sheet-shaped test piece having a predetermined dimension, and measuring the flexural modulus of this test piece based on JIS K 7171:2016.

[0039] (Polypropylene resin derived from fossil fuels) Fossil fuel-derived polypropylene resin refers to a polypropylene resin obtained by polymerizing substantially only fossil fuel-derived monomers. Fossil fuel-derived polypropylene resins preferably contain only fossil fuel-derived monomer components in their molecular chains. Furthermore, the biomass content (biobased carbon content) of fossil fuel-derived polypropylene resins measured according to ASTM D6866-21 is 0%.

[0040] In the present invention, the fossil fuel-derived monomer refers to propylene, ethylene, or an α-olefin having 4 to 8 carbon atoms produced from a fossil fuel. The method for producing the fossil fuel-derived monomer is not particularly limited, and the fossil fuel-derived monomer can be produced by a known method practiced in the petrochemical industry. For example, the fossil fuel-derived monomer can be obtained by thermal cracking and fractional distillation of naphtha obtained in the process of petroleum refining.

[0041] The fossil fuel-derived polypropylene resin is composed of the above-mentioned polypropylene resin. The fossil fuel-derived polypropylene resin may be a mixture of two or more fossil fuel-derived polypropylene resins as long as they are derived from a fossil fuel such as a petroleum fuel. From the viewpoint of consistently providing expanded beads that exhibit good in-mold moldability and can give expanded bead moldings with good surface properties, the fossil fuel-derived polypropylene resin is preferably at least one selected from the group consisting of a propylene-ethylene random copolymer, a propylene-butene random copolymer, and a propylene-ethylene-butene random copolymer, and more preferably a propylene-ethylene random copolymer.

[0042] The melting point of the fossil fuel-derived polypropylene resin is preferably 130°C or higher and 155°C or lower, more preferably 135°C or higher and 150°C or lower, and even more preferably 138°C or higher and 145°C or lower, from the viewpoint of achieving an excellent balance between the moldability of the expanded beads and the strength of the molded body. The melting point of the fossil fuel-derived polypropylene resin can be determined by the same method as that for the biomass-derived polypropylene resin described above, and more specifically, can be measured by the method described in the Examples.

[0043] The crystallization temperature of the fossil fuel-derived polypropylene resin is preferably 90°C or higher and 110°C or lower, more preferably 92°C or higher and 109°C or lower, and even more preferably 94°C or higher and 108°C or lower, from the viewpoint of obtaining expanded beads with an excellent molding range. The crystallization temperature of the fossil fuel-derived polypropylene resin can be determined by the same method as that for the biomass-derived polypropylene resin, and more specifically, can be measured by the method described in the Examples.

[0044] The melt flow rate of the fossil fuel-derived polypropylene resin, measured in accordance with JIS K 7210-1:2014 at 230°C under a load of 2.16 kg, is preferably 35 g / 10 min or less. When the melt flow rate of the fossil fuel-derived polypropylene resin is within the above range, the viscoelasticity of the polypropylene resin constituting the resin particles is appropriately adjusted, making it easier to obtain the effects of the carbon black, and the water cooling time during molding of the resulting expanded particles is more reliably shortened. From the viewpoint of obtaining expanded beads that can more reliably shorten the water cooling time while ensuring good moldability in a mold, the melt flow rate of the fossil fuel-derived polypropylene resin is preferably 1 g / 10 min or more and 35 g / 10 min or less, more preferably 3 g / 10 min or more and 20 g / 10 min or less, even more preferably 5 g / 10 min or more and 15 g / 10 min or less, and particularly preferably 6 g / 10 min or more and 12 g / 10 min or less.

[0045] The flexural modulus of the fossil fuel-derived polypropylene resin is preferably 700 MPa or more and 1200 MPa or less, more preferably 800 MPa or more and 1100 MPa or less, and even more preferably 900 MPa or more and 1000 MPa or less, from the viewpoint of achieving an excellent balance between the moldability of the expanded beads and the strength of the molded body. The flexural modulus of the fossil fuel-derived polypropylene resin can be determined by the same method as that for the biomass-derived polypropylene resin described above, and more specifically, can be measured by the method described in the Examples.

[0046] (Composition and properties of base resin and mixed resin) The base resin of the resin particles contains a biomass-derived polypropylene resin, preferably a mixed resin of a biomass-derived polypropylene resin and a fossil fuel-derived polypropylene resin, but may contain a polymer other than the polypropylene resin. The polymer generally refers to a polymer having a weight-average molecular weight of 10,000 or more. Examples of other polymers include thermoplastic resins other than polypropylene resins, such as polyethylene resins, polystyrene resins, polyamide resins, and polyester resins, and thermoplastic elastomers, such as olefin thermoplastic elastomers and styrene thermoplastic elastomers. Two or more of these other polymers may be contained. The blending ratio of other polymers in the base resin is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 3% by mass or less, still more preferably 1% by mass or less, and particularly preferably 0% by mass, based on 100% by mass of the base resin. It is even more preferable that the base resin contains only polypropylene-based resin as the polymer.

[0047] The expanded beads may also contain additives. Examples of additives include one or more functional additives such as colorants, antioxidants, antistatic agents, surfactants, heat stabilizers, light stabilizers, UV absorbers, and flame retardants. The blending ratio of the additives in the expanded beads is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 5 parts by mass or less, per 100 parts by mass of the base resin.

[0048] When the base resin of the resin particles is a mixed resin of a biomass-derived polypropylene resin and a fossil fuel-derived polypropylene resin, the mass ratio of the biomass-derived polypropylene resin to the fossil fuel-derived polypropylene resin in the mixed resin (biomass-derived polypropylene resin:fossil fuel-derived polypropylene resin) is preferably 1:99 to 90:10, more preferably 2:98 to 75:25, even more preferably 3:97 to 60:40, and still more preferably 4:96 to 50:50 (where the total of the biomass-derived polypropylene resin and the fossil fuel-derived polypropylene resin is 100). By using a mass ratio of the biomass-derived polypropylene resin to the fossil fuel-derived polypropylene resin within the above range, the moldability of the resulting expanded beads is improved, the water cooling time during molding is shortened, and the productivity of molded articles is excellent, which also contributes to reducing the environmental load.

[0049] The polypropylene-based resin contained in the biomass-derived polypropylene-based resin constituting the base resin and the polypropylene-based resin contained in the fossil fuel-derived polypropylene-based resin may be the same type of polypropylene-based resin or different polypropylene-based resins. For example, the biomass-derived polypropylene-based resin and the fossil fuel-derived polypropylene-based resin may both be propylene homopolymers, or both may be polypropylene-based copolymers, or one may be a propylene homopolymer and the other a polypropylene-based copolymer.

[0050] A combination in which the biomass-derived polypropylene-based resin is a propylene homopolymer or a propylene-ethylene block copolymer and the fossil fuel-derived polypropylene-based resin is a polypropylene-based copolymer is preferred, and a combination in which the biomass-derived polypropylene-based resin is a propylene homopolymer and the fossil fuel-derived polypropylene-based resin is a polypropylene-based copolymer is more preferred.

[0051] The fossil fuel-derived polypropylene-based resin preferably contains a polypropylene-based copolymer, more preferably one or more polypropylene-based copolymers selected from the group consisting of a propylene-ethylene random copolymer, a propylene-butene random copolymer, and a propylene-ethylene-butene random copolymer. Blending a fossil fuel-derived polypropylene-based resin containing the polypropylene-based copolymer with a biomass-derived polypropylene-based resin further improves the moldability of the resulting expanded beads. From this perspective, when the fossil fuel-derived polypropylene-based resin is a propylene-ethylene random copolymer, the ethylene content of the propylene-ethylene random copolymer is preferably 1% by mass or more and 5% by mass or less, more preferably 1.5% by mass or more and 4% by mass or less, and even more preferably 2.0% by mass or more and 3.5% by mass or less.

[0052] When the base resin of the resin particles is a mixed resin of a biomass-derived polypropylene resin and a fossil fuel-derived polypropylene resin, the ratio of the melt flow rate (MFR) of the biomass-derived polypropylene resin measured at 230°C under a load of 2.16 kg to the melt flow rate of the fossil fuel-derived polypropylene resin measured at 230°C under a load of 2.16 kg (MFR of biomass-derived polypropylene resin / MFR of fossil fuel-derived polypropylene resin) is preferably 0.2 or more and 0.8 or less, more preferably 0.3 or more and 0.7 or less. By having this ratio within this range, the moldability of the resulting expanded beads and the strength of the molded article are well balanced. Furthermore, since the viscoelasticity of the entire resin particles is appropriately adjusted, it is believed that the effects of the carbon black described above are more easily realized.

[0053] The polypropylene-based resin particles used in the method for producing expanded beads of the present invention can be obtained by feeding, for example, a biomass-derived polypropylene-based resin, a fossil fuel-derived polypropylene-based resin, and an optional cell regulator into an extruder, heating and kneading the mixture to form a resin melt, and then extruding the resin melt from the extruder and pelletizing it by a strand cut method, a hot cut method, an underwater cut method, or the like.

[0054] The average mass per resin particle is preferably adjusted to 0.1 to 20 mg, more preferably 0.2 to 10 mg, even more preferably 0.3 to 5 mg, and even more preferably 0.4 to 2 mg. The external shape of the resin particles is not particularly limited as long as the intended object of the present invention can be achieved, but is preferably cylindrical. However, the resin particles may have a cylindrical external shape with through holes so that expanded beads having through holes can be produced. When the resin particles have a cylindrical outer shape, the particle diameter (length in the extrusion direction) of the resin particles is preferably 0.1 to 3.0 mm, more preferably 0.3 to 1.5 mm. The ratio (length / diameter ratio) of the length of the resin particles in the extrusion direction to the length of the resin particles in the direction perpendicular to the extrusion direction (diameter of the resin particles) is preferably 0.5 to 5.0, more preferably 1.0 to 3.0.

[0055] When pelletizing by the strand cutting method, the particle size, length / diameter ratio, and average mass of the resin particles can be adjusted by appropriately changing the extrusion speed when extruding the resin melt, the strand take-up speed, and the cutter speed when cutting the strand.

[0056] The resin particles may contain additives as appropriate within the range that does not impair the effects of the present invention. Examples of additives include antioxidants, ultraviolet absorbers, antistatic agents, flame retardants, pigments, dyes, and cell regulators. These additives can be incorporated into the expanded beads by adding them to the resin particles during the process of producing the resin particles.

[0057] The cell regulator may be, for example, an inorganic powder or an organic powder. Examples of inorganic powders include metal borates such as zinc borate and magnesium borate, and examples of organic powders include fluororesin powders such as polytetrafluoroethylene (PTFE). From the viewpoint of stably obtaining expanded beads having a desired bulk density and little variation in cell diameter, the amount of the cell regulator in the resin beads is preferably 50 ppm by mass or more and 5000 ppm by mass or less, more preferably 100 ppm by mass or more and 2000 ppm by mass or less, and even more preferably 150 ppm by mass or more and 1500 ppm by mass or less. From the viewpoint of easily adjusting the average cell diameter of the expanded beads to a desired range, it is preferable to use a metal borate, more preferably zinc borate, as the cell adjusting agent. When zinc borate is used, the arithmetic mean particle diameter based on the number is preferably 0.5 μm to 10 μm, more preferably 1 μm to 8 μm. The number-based arithmetic mean particle diameter of zinc borate can be determined by converting the volume-based particle size distribution measured by laser diffraction scattering method into a number-based particle size distribution by assuming that the particles have a spherical shape, and then calculating the arithmetic mean of the particle diameters based on this number-based particle size distribution. Note that the particle diameter refers to the diameter of a hypothetical sphere having the same volume as the particle.

[0058] <Production of expanded polypropylene resin beads> The method for producing the expanded polypropylene resin beads of the present invention may be any method as long as it is a method for obtaining expanded beads by expanding the polypropylene resin beads, but the following methods are preferred.

[0059] A preferred method for producing expanded polypropylene resin beads of the present invention is a method for producing expanded polypropylene resin beads, comprising the steps of: a dispersing step of dispersing the resin beads in an aqueous medium in a sealed container; a foaming agent adding step of adding a physical foaming agent to the sealed container; and an expansion step of impregnating the resin beads with the physical foaming agent in the sealed container, and then releasing the resin beads together with the aqueous medium from the sealed container into an atmosphere under a pressure lower than that inside the sealed container to expand the resin beads to produce expanded beads. Such an expansion method is also called a direct foaming method.

[0060] In the dispersion step, an aqueous dispersion medium is preferably used as a dispersion medium for dispersing the resin particles obtained as described above in a sealed container. The aqueous dispersion medium is a dispersion medium containing water as a main component. The proportion of water in the aqueous dispersion medium is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, and may be 100% by mass. Examples of dispersion media other than water in the aqueous dispersion medium include ethylene glycol, glycerin, methanol, and ethanol.

[0061] In this method, it is preferable to add a dispersant to the dispersion medium to prevent the resin particles heated in the container from fusing together within the container. Any dispersant can be used as long as it prevents the resin particles from fusing together within the container, but inorganic dispersants are preferably used. Examples of inorganic dispersants include natural or synthetic clay minerals such as kaolin, mica, and clay, as well as aluminum oxide, titanium oxide, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, and iron oxide. One or more of these may be used alone or in combination. Of these, natural or synthetic clay minerals are preferred. The amount of the dispersant added is preferably 0.001 to 5 parts by mass per 100 parts by mass of the resin particles.

[0062] When a dispersant is used, it is preferable to use an inorganic salt such as aluminum sulfate or an anionic surfactant such as sodium dodecylbenzenesulfonate, sodium alkylsulfonate, or sodium oleate as a dispersing aid in combination. The dispersing aid is preferably added in an amount of about 0.001 to 1 part by mass per 100 parts by mass of the resin particles.

[0063] In the blowing agent addition step, a physical blowing agent is preferably used as the blowing agent for expanding the resin particles. Examples of the physical blowing agent include inorganic and organic physical blowing agents. Examples of inorganic physical blowing agents include carbon dioxide, air, nitrogen, helium, and argon. Examples of organic physical blowing agents include aliphatic hydrocarbons such as propane, n-butane, isobutane, n-pentane, isopentane, and hexane; cyclic aliphatic hydrocarbons such as cyclopentane and cyclohexane; and halogenated hydrocarbons such as ethyl chloride, 2,3,3,3-tetrafluoropropene, trans-1,3,3,3-tetrafluoropropene, and trans-1-chloro-3,3,3-trifluoropropene. The physical blowing agents may be used alone or in combination. In addition, inorganic and organic physical blowing agents may be used in combination. From the viewpoint of facilitating the production of desired expanded particles, the blowing agent used in this production method is preferably an inorganic physical blowing agent, and more preferably carbon dioxide.

[0064] The amount of foaming agent to be added is determined taking into consideration the desired bulk density of the expanded beads, the type of foaming agent, etc. For example, when a physical foaming agent is used, the amount of physical foaming agent to be added per 100 parts by mass of resin particles is preferably 0.1 to 30 parts by mass, more preferably 0.5 to 15 parts by mass.

[0065] In the expansion step, a preferred method for impregnating the resin particles with the blowing agent is to disperse the resin particles in an aqueous dispersion medium in a sealed container, inject the blowing agent under pressure into the sealed container, and then heat and pressurize the sealed container to impregnate the resin particles with the blowing agent. In this case, the resin particles are impregnated with the blowing agent in the aqueous dispersion medium in the sealed container.

[0066] In the expansion step, the pressure (internal pressure) inside the sealed container during expansion is preferably 0.5 MPa (G) or more, more preferably 0.6 MPa (G) or more, even more preferably 0.8 MPa (G) or more, still more preferably 1.0 MPa (G) or more, and even more preferably 1.5 MPa (G) or more. The upper limit is preferably 3.0 MPa (G) or less, more preferably 2.5 MPa (G) or less. Within the above range, the desired expanded beads can be safely produced without risk of damage or explosion of the sealed container. Preferably, the temperature is raised to 110 to 210°C, more preferably 130 to 170°C, and the resin beads containing the blowing agent are then preferably expanded by releasing them from the sealed container into an atmosphere with a pressure lower than the pressure inside the sealed container (e.g., atmospheric pressure).

[0067] Further, expanded beads having a crystalline structure in which a main endothermic peak and a high-temperature peak appear in the first DSC curve can be produced, for example, as follows. First, resin particles dispersed in a dispersion medium in a sealed container are heated to a temperature between (melting point of the resin constituting the resin particles -15°C) and (melting point of the resin constituting the resin particles +10°C) and maintained at this temperature for a sufficient time, preferably about 10 to 60 minutes (maintenance step). Next, the resin particles that have undergone this maintenance step are expanded to obtain expanded particles that exhibit the above-mentioned melting peak. In the production of expanded beads, resin particles that have undergone the holding step may be prepared in advance, and the resin particles that have undergone the holding step may be expanded to obtain expanded beads. Alternatively, for example, the resin particles may be subjected to the holding step as part of the dispersion step or the blowing agent impregnation step, and the resin particles that have undergone the holding step may be expanded to obtain expanded beads. From the viewpoint of increasing the productivity of expanded beads, it is preferable to carry out the above-mentioned holding step by heating resin particles dispersed in a dispersion medium in a sealed container in the presence of a blowing agent, and then release the contents of the sealed container from the sealed container into an atmosphere of pressure lower than the pressure inside the sealed container, thereby expanding the resin particles that have undergone the above-mentioned holding step, thereby obtaining expanded beads that exhibit a melting peak described below.

[0068] In the manufacturing method, when expanding resin particles, the resin particles may be expanded in one stage as described above, or the resin particles may be expanded in two or more stages. When expanding resin particles in two stages, first, in the first expansion stage, the resin particles are expanded by a direct expansion method or the like to obtain first-stage expanded particles. In the second expansion stage, for example, the first-stage expanded particles may be pressurized with air or the like to increase the pressure (internal pressure) within the cells of the first-stage expanded particles, and then the first-stage expanded particles may be heated with steam or the like to further expand them. By expanding resin particles in multiple stages in this manner, expanded beads with a higher expansion ratio (i.e., a lower bulk density) can be easily obtained.

[0069] <Polypropylene-based resin expanded particles produced by the above-mentioned production method> The expanded polypropylene resin particles produced by the method for producing expanded polypropylene resin particles of the present invention are preferably the expanded polypropylene resin particles described in the section [Expanded Polypropylene Resin Particles] below, and the same applies to more preferred expanded polypropylene resin particles.

[0070] (heat of fusion at high temperature peak) The expanded polypropylene resin beads preferably have a crystalline structure in which a DSC curve obtained when the expanded beads are heated from 23°C to 230°C at a heating rate of 10°C / min shows a main endothermic peak with the largest peak area and a high-temperature peak which is an endothermic peak that appears on the high-temperature side of the main endothermic peak.

[0071] The crystalline structure can be confirmed from a DSC curve obtained by performing differential scanning calorimetry (DSC) in accordance with JIS K 7122:2012 using the expanded beads as a test piece, and the heat of fusion of the high-temperature peak can also be determined from the same DSC curve. Specifically, if the DSC curve obtained by heating the expanded beads from 23°C to 230°C at a heating rate of 10°C / min (DSC curve in the first heating) shows a main endothermic peak with the largest peak area and a high-temperature peak that is an endothermic peak that appears on the high-temperature side of the main endothermic peak, it can be confirmed that the expanded beads have the above-mentioned crystalline structure. The main endothermic peak with the largest peak area is a characteristic melting peak that appears due to the melting of crystals that the base resin normally has. The high-temperature peak is presumed to indicate the presence of secondary crystals in the resin. In addition, in the DSC curve obtained by heating the expanded beads from 23°C to 230°C at a heating rate of 10°C / min (first heating), cooling from 230°C to 23°C at a cooling rate of 10°C / min, and then heating again from 23°C to 230°C at a heating rate of 10°C / min (second heating), only a melting peak due to the melting of crystals normally present in the base resin appears. This makes it possible to identify which peak in the DSC curve from the first heating is the main endothermic peak and which peak is the high-temperature peak.

[0072] From the viewpoint of moldability of the resulting expanded beads, the heat of fusion of the high-temperature peak of the expanded beads is preferably 10 J / g or more and 35 J / g or less, more preferably 10 J / g or more and 30 J / g or less, even more preferably 10 J / g or more and 25 J / g or less, and particularly preferably 12 J / g or more and 20 J / g or less. As described above, the heat of fusion of the high-temperature peak can be determined by heat flux differential scanning calorimetry using the expanded beads as a test piece in accordance with JIS K 7122: 2012. More specifically, it can be measured by the method described in the examples.

[0073] The heat of fusion of the high-temperature peak can be adjusted, for example, by controlling the rate of temperature rise in the pressure vessel during the dispersion step and / or the blowing agent impregnation step, or by controlling the holding temperature and holding time when the temperature in the pressure vessel is held at a predetermined temperature for a predetermined time.

[0074] (bulk density) The bulk density of the expanded beads is preferably 10 kg / m from the viewpoint of achieving an excellent balance between the light weight and mechanical strength of the resulting molded article. 3 More than 200kg / m 3 More preferably, it is 15 kg / m or less. 3 More than 150kg / m 3 More preferably, it is 20 kg / m or less. 3 More than 100kg / m 3 and even more preferably 20 kg / m or less. 3 More than 80kg / m 3 and even more preferably 25 kg / m or less. 3 More than 60kg / m 3 The following is the result.

[0075] The bulk density of expanded beads is measured by the following method. First, the expanded beads to be measured are left to stand for 24 hours or more in an environment with a temperature of 23°C, a relative humidity of 50%, and 1 atm. The expanded beads having a weight W (g) obtained in this way are filled into a measuring cylinder, and the bottom of the measuring cylinder is lightly tapped on a horizontal surface several times to stabilize the filling height of the expanded beads in the measuring cylinder. The bulk volume V (L) of the expanded beads indicated on the measuring cylinder scale is read, and the weight W of the expanded beads is divided by the bulk volume V of the expanded beads (W / V). The value obtained in this way is expressed in kg / m 3 The bulk density of the expanded particles (kg / m 3 ) can be obtained.

[0076] [Polypropylene resin foam particles] The expanded polypropylene resin beads of the present invention are polypropylene resin beads that use a polypropylene resin as a base resin, and the expanded beads have a biomass degree of 1% or more as measured by ASTM D6866-21, contain 0.1% by mass or more and 8% by mass or less of carbon black, have a closed cell rate of 85% or more, have an average cell diameter D of 50 μm or more and 250 μm or less, and have a ratio Dc / D of the average cell diameter Dc of the bubbles located at the center of the expanded beads to the average cell diameter D of the expanded beads of 0.7 or more and 1.3 or less. The expanded polypropylene resin beads of the present invention are preferably those produced by the above-mentioned method for producing expanded polypropylene resin beads, and the method for producing the expanded polypropylene resin beads of the present invention is preferably the above-mentioned method for producing expanded polypropylene resin beads, and the same is true for more preferred methods. The expanded beads described in the section [Expanded Polypropylene Resin Beads] of the present invention are preferably expanded beads produced by the above-mentioned [Method for producing expanded polypropylene resin beads], and the same is true for more preferred expanded beads.

[0077] (Polypropylene resin) The expanded polypropylene resin particles of the present invention have a polypropylene resin as a base resin. The polypropylene resin is as described above. The base resin is preferably a mixed resin of a biomass-derived polypropylene resin and a fossil fuel-derived polypropylene resin, and the properties of each of these polypropylene resins are also as described above.

[0078] (Biomass ratio) The biobased carbon content of the expanded polypropylene resin particles is 1% or more as measured by ASTM D6866-21. When the biomass degree of the expanded beads is within the above range, the use of fossil resources can be suppressed in the production of the expanded bead molding, and the amount of carbon dioxide emitted in the life cycle of the expanded bead molding can also be reduced. From the above viewpoint, the biomass degree of the expanded beads measured according to ASTM D6866-21 is 1% or more, preferably 1.5% or more, more preferably 1.8% or more, and even more preferably 2% or more. From the viewpoint of further reducing the environmental load, the biomass degree of the expanded beads is even more preferably 5% or more, and particularly preferably 10% or more. There is no upper limit to the biomass degree of the expanded beads, but the biomass degree of the expanded beads measured according to ASTM D6866 may be 100% or less. From the viewpoint of further improving in-mold moldability and further shortening the water cooling time during in-mold molding, the biomass degree of the expanded beads measured according to ASTM D6866-21 is preferably 80% or less, more preferably 50% or less, even more preferably 30% or less, and particularly preferably 20% or less. From the viewpoint of further improving in-mold moldability in particular, it is even more preferably 10% or less. The upper and lower limits of the biomass degree of the expanded beads can be arbitrarily combined to determine a preferred range, such as 1% to 100%, 1% to 80%, 1.5% to 50%, 2% to 30%, 1% to 20%, or 5% to 20%. The biomass ratio of the expanded beads is measured according to ASTM D6866-21 and means the proportion of naturally occurring components contained in the expanded beads. The biomass ratio is also measured by radiocarbon spectroscopy as defined in ASTM D6866-21 using the expanded beads as a measurement sample. 14 It can be determined by carrying out a C measurement. When the biomass degree of the biomass-derived polypropylene resin used to produce the expanded beads and the content of the biomass-derived polypropylene resin in the expanded beads are known, these values ​​can be used to calculate the biomass degree.

[0079] (carbon black) The expanded polypropylene resin particles contain 0.1 to 8 parts by mass of carbon black relative to 100 parts by mass of the base resin, as described above in detail. By including a predetermined amount of carbon black in the expanded beads, the water cooling time during molding can be shortened, improving the productivity of molded articles and providing molded articles with a luxurious appearance. The reason why the foamed beads have improved moldability and the water cooling time during molding is shortened when the foamed beads contain a predetermined amount of carbon black is not clear, but is thought to be as described above.

[0080] The expanded polypropylene resin beads contain 0.1% by mass or more and 8% by mass or less of carbon black. If the carbon black content in the expanded beads is less than 0.1% by mass, the expanded beads may require a long water cooling time during in-mold molding, resulting in poor productivity. Furthermore, the resulting molded article may not have a luxurious appearance. On the other hand, if the carbon black content in the expanded beads exceeds 8% by mass, the in-mold moldability of the expanded beads may decrease, resulting in an excessively high molding pressure and making it impossible to obtain a good molded article.

[0081] From the viewpoint of further shortening the water cooling time during in-mold molding, the carbon black content in the expanded beads is preferably 0.5% by mass or more, more preferably 1% by mass or more, even more preferably 1.5% by mass or more, and even more preferably 2% by mass or more. From the viewpoint of in-mold moldability of the expanded beads, the carbon black content in the expanded beads is preferably 7% by mass or less, more preferably 6% by mass or less, more preferably 4.5% by mass or less, and even more preferably 3% by mass or less. The upper and lower limits of the carbon black content in the expanded beads can be arbitrarily combined to determine a preferred range. Therefore, the carbon black content in the expanded beads is, for example, 0.5% by mass to 8% by mass or less, 1.5% by mass to 7% by mass or less, 2% by mass to 7% by mass or less, 0.5% by mass to 6% by mass or less, 1% by mass to 6% by mass or less, 1.5% by mass to 4.5% by mass or less, and 2% by mass to 3% by mass or less.

[0082] (closed cell ratio) The closed cell content of the expanded beads is 85% or more, preferably 90% or more, more preferably 92% or more, and even more preferably 93% or more, from the viewpoint of widening the moldable steam pressure range during molding of the expanded beads in a mold and obtaining expanded beads with an excellent moldability. The upper limit of the closed cell content is not particularly limited, but is generally 98%.

[0083] The closed cell ratio of expanded beads is measured as follows: First, a bulk volume of approximately 20 cm 3The expanded beads are immersed in water to measure the apparent volume Va of the expanded beads. After measuring the apparent volume Va, the expanded beads are thoroughly dried, and the volume (the sum of the volume of the resin constituting the expanded beads and the total volume of the cells in the closed cell portion within the expanded beads) (true volume Vx) of the expanded beads is measured according to procedure C described in ASTM D2856-94. An air comparison hydrometer is used to measure this true volume Vx. An example of the air comparison hydrometer is the Model 1000 Air Comparison Hydrometer manufactured by Tokyo Science Co., Ltd. The closed cell ratio is then calculated using the following formula (1): Using different measurement samples, the closed cell ratio is measured five times using the same procedure as above, and the arithmetic mean value of the values ​​obtained in each measurement is calculated, which is the closed cell ratio of the expanded beads. Closed cell ratio (%)=(Vx-W / ρ)×100 / (Va-W / ρ) (1) Vx: The true volume (cm ) of the expanded particle group measured by the above method 3 ) Va: Apparent volume (cm) of the foam particles measured from the rise in water level when the foam particles are submerged in water in a measuring cylinder. 3 ) W: Weight of foam particles (g) ρ: Density of the resin that makes up the foamed particles (g / cm 3 )

[0084] (Average bubble diameter D) The expanded beads have an average cell diameter D of 50 μm or more and 250 μm or less. If the average cell diameter D of the expanded beads is too small, the moldability of the expanded beads may be impaired. On the other hand, if the average cell diameter D of the expanded beads is too large, the water cooling time during in-mold molding may be long. From this viewpoint, the average cell diameter of the expanded beads is preferably 60 μm or more and 220 μm or less, more preferably 80 μm or more and 200 μm or less, even more preferably 90 μm or more and 180 μm or less, and even more preferably 100 μm or more and 160 μm or less.

[0085] The average cell diameter D of expanded beads is determined as follows. First, a photograph of a cross section of an expanded bead is taken, dividing it into two equal parts. A straight line is drawn on the photograph so as to roughly bisect the area of ​​the expanded bead cross section. The length L of the line segment from one edge of the expanded bead to the opposite edge is divided by the number N of all the cells in contact with this line segment (L / N), and this value is taken as the average cell diameter of one expanded bead. This procedure is performed on 10 or more expanded beads, and the arithmetic mean value is taken as the average cell diameter D of the expanded beads.

[0086] The ratio Dc / D of the average cell diameter Dc of the cells located at the center of the expanded beads to the average cell diameter D of the expanded beads (the average cell diameter Dc of the cells located at the center of the expanded beads is described below) is 0.7 or more and 1.3 or less. This indicates that the average cell diameter of the expanded beads and the average cell diameter of the cells located at the center of the expanded beads are relatively close in value, meaning that a relatively uniform cell structure is formed throughout the expanded beads. Therefore, the expanded beads of the present invention have excellent moldability and exhibit the effect of shortening the water cooling time during mold molding. Furthermore, even when the expanded beads contain a relatively large amount of carbon black within the above range, an increase in molding pressure is likely to be suppressed. From this viewpoint, the ratio Dc / D is preferably 0.8 or more and 1.2 or less.

[0087] (Average diameter of bubbles located in the center Dc) The average cell diameter Dc of the cells located at the center of the expanded beads is preferably 220 μm or less, more preferably 200 μm or less, even more preferably 180 μm or less, and still more preferably 170 μm or less, particularly from the viewpoint of further shortening the water cooling time during molding and improving the productivity of molded articles. The lower limit of the average cell diameter Dc of the cells located at the center of the expanded beads is preferably 50 μm or more, more preferably 80 μm or more, and even more preferably 100 μm or more, from the viewpoint of moldability of the expanded beads.

[0088] The average cell diameter Dc of the bubbles located at the center of an expanded bead is determined as follows. First, the expanded bead is cut into approximately two equal halves passing through the center of the expanded bead, and one of the cut surfaces is photographed using a scanning electron microscope so that the entire cut surface is captured. The shortest line is drawn from the center (centroid) of the image of the approximately circular cut surface of the expanded bead to the cell membrane that separates each nearby bubble for each bubble. Then, from the bubbles located near the center, 10 bubbles are selected in order of the shortest shortest line from the center to the cell membrane. A rectangle is drawn circumscribing each of the selected 10 bubbles, and the vertical and horizontal lengths of each rectangle are measured. The average of these lengths is taken as the cell diameter of the bubbles located at the center of the expanded bead. The same procedure is performed on 10 randomly selected expanded beads. The arithmetic mean of the cell diameters at the centers of the 10 expanded beads is taken as the average cell diameter Dc of the bubbles located at the center of the expanded bead.

[0089] Expanded beads satisfying the above-mentioned Dc / D ratio and Dc ranges can be obtained by the expanded bead manufacturing method of the present invention. As described above, according to the manufacturing method of the present invention, coalescence of the central cells is suppressed during expansion of the resin beads, so that the average cell diameter of the centrally located cells is controlled to be small, and as a result, a relatively uniform cell structure is formed in the expanded beads obtained.

[0090] (Melt Flow Rate) From the viewpoints of moldability in a mold, water cooling time, and the like, the expanded beads preferably have a melt flow rate measured under conditions of a temperature of 230°C and a load of 2.16 kg of 3 g / 10 min or more and 20 g / 10 min or less, more preferably 5 g / 10 min or more and 18 g / 10 min or less, even more preferably 6 g / 10 min or more and 15 g / 10 min or less, and particularly preferably 7 g / 10 min or more and 10 g / 10 min or less. The melt flow rate of the expanded polypropylene resin particles is a value measured under conditions of a temperature of 230°C and a load of 2.16 kg.

[0091] (fusion layer) The expanded beads may have a single-layer structure consisting of only a foamed layer made of the polypropylene-based resin, or a multilayer structure comprising a foamed layer made of the polypropylene-based resin and a fusion layer covering the foamed core layer, which is provided to enhance fusion between the expanded beads during in-mold molding. The fusion layer may be present on the entire surface of the expanded beads or on a portion of the surface. The fusion layer may be in a foamed or non-foamed state, but is preferably in a substantially non-foamed state. The "non-foamed state" mentioned above includes a state in which the fusion layer is not foamed and does not contain bubbles, and a state in which the bubbles have disappeared after foaming, meaning that there is almost no bubble structure in the fusion layer. When the expanded beads have a foamed layer and a fusion layer, the foamed layer covered by the fusion layer is sometimes referred to as the "foamed core layer."

[0092] The method for producing expanded beads having a fusion layer is not particularly limited, and examples thereof include a method of foaming resin beads having a non-foamed core layer and a fusion layer covering the core layer, a method of foaming a non-foamed core layer to obtain an expanded core layer, and then attaching a fusion layer to the surface of the expanded core layer, etc. When foamed beads are obtained by foaming resin beads having a fusion layer on their surface, it is preferable to use a method in which, when producing the resin beads, a resin melt for forming the core layer and a resin melt for forming the fusion layer are co-extruded using an extrusion device capable of co-extrusion, thereby laminating the fusion layer on the surface of the core layer.

[0093] When the expanded beads have a fusion layer, the proportion of the fusion layer in the expanded beads is preferably approximately 0.5% by mass to 20% by mass, more preferably 1% by mass to 10% by mass.

[0094] Examples of the base resin constituting the fusion layer include a crystalline polyolefin resin having a melting point lower than that of the polypropylene resin constituting the foamed core layer, and an amorphous polyolefin resin having a softening point lower than that of the polypropylene resin constituting the foamed core layer. The base resin constituting the fusion layer is preferably a polypropylene resin and / or a polyethylene resin.

[0095] When the adhesive layer is made of a crystalline polyolefin resin having a melting point, the melting point of the crystalline polyolefin resin is preferably 110°C or higher and 155°C or lower, more preferably 120°C or higher and 150°C or lower, and even more preferably 125°C or higher and 145°C or lower.

[0096] The difference between the melting point of the polypropylene resin constituting the foamed core layer and the melting point of the crystalline polyolefin resin constituting the fusion layer is preferably approximately 1° C. to 40° C., more preferably 2° C. to 35° C., and even more preferably 5° C. to 30° C. In this case, the in-mold moldability of the expanded beads can be improved even when the molding pressure is relatively low.

[0097] <Polypropylene resin foam bead molding> The expanded beads of the present invention or the expanded beads obtained by the method for producing expanded beads of the present invention can be molded in a mold to obtain an expanded polypropylene resin bead molded article. That is, the polypropylene resin expanded bead molded article is obtained by molding the expanded beads in a mold.

[0098] The foamed bead molded article can be produced by filling a mold with the foamed beads and heat-molding them using a heating medium such as steam. Specifically, after filling the mold with the foamed beads, a heating medium such as steam is introduced into the mold to heat and expand (secondary foaming) the foamed beads and fuse them together, thereby obtaining a foamed bead molded article having the shape of the molding space. The in-mold molding of the present invention can also be carried out by a pressure molding method (e.g., JP-B-51-22951), in which the foamed beads are pre-pressurized with a pressurized gas such as air to increase the pressure within the cells of the foamed beads and adjust the pressure within the foamed beads to a pressure 0.01 to 0.3 MPa higher than atmospheric pressure, and then the foamed beads are filled into the mold under atmospheric or reduced pressure, and a heating medium such as steam is then supplied into the mold to heat-fuse the foamed beads (e.g., JP-B-51-22951). Alternatively, molding can be performed by a compression filling molding method (Japanese Patent Publication No. 4-46217), in which a mold pressurized to atmospheric pressure or higher by a compressed gas is filled with expanded beads pressurized to atmospheric pressure or higher, and then a heating medium such as steam is supplied into the cavity to heat and fuse the expanded beads. Alternatively, molding can be performed by a normal pressure filling molding method (Japanese Patent Publication No. 6-49795), in which expanded beads with high secondary expansion power obtained under special conditions are filled into the cavity of a mold under atmospheric or reduced pressure, and then a heating medium such as steam is supplied to heat and fuse the expanded beads, or a combination of the above methods (Japanese Patent Publication No. 6-22919).

[0099] From the viewpoint of achieving both light weight and mechanical properties, the density of the expanded bead molding is preferably 10 kg / m 3 More than 300kg / m 3 More preferably, it is 20 kg / m or less. 3 More than 200kg / m 3 More preferably, it is 25 kg / m or less. 3 More than 100kg / m 3 and even more preferably 30 kg / m or less. 3 More than 80kg / m 3 The following is the result. The density of the expanded bead molding can be determined by dividing the mass of the expanded bead molding by the volume calculated based on the dimensions of the expanded bead molding.

[0100] The polypropylene resin foam bead moldings are lightweight and have excellent mechanical properties, and therefore can be used as shock absorbers, heat insulating materials, various packaging materials, etc., for transporting food, packaging and cushioning materials for electric and electronic parts, vehicle components such as automobile bumpers, building components such as heat insulating materials for houses, miscellaneous goods, etc. [Example]

[0101] The present invention will now be described in more detail with reference to examples, but the present invention is not limited to these examples in any way.

[0102] [Measurement and Evaluation] The resins, expanded beads, and expanded bead molded articles used in the examples and comparative examples were subjected to the following measurements and evaluations. The expanded beads and expanded bead molded articles were evaluated after being left to condition for 2 days under conditions of 50% relative humidity, 23°C, and 1 atm.

[0103] <Biomass ratio> The biomass content (%) of the polypropylene resin used in the examples and comparative examples was determined as follows in accordance with ASTM D6866-21. Polypropylene resin was used as the resin for measurement, and carbon dioxide (CO2) was generated by burning the resin, and the carbon dioxide was purified in a vacuum line. The purified carbon dioxide was reduced with hydrogen using iron as a catalyst, thereby producing graphite (C). The graphite was then packed into a cathode with an inner diameter of 1 mm using a hand press, which was then fitted into a wheel and used in a tandem accelerator based on a NEC Corporation tandem accelerator. 14 The device was installed in a dedicated C-AMS (accelerator mass spectrometry) device. 14 The number of C's, 13 C concentration ( 13 C / 12 C), 14C concentration (14 C / 12 C) was measured. For the measurements, oxalic acid (HOxII) provided by the National Institute of Standards (NIST) was used as the standard sample. Measurements of this standard sample and a background sample were also carried out simultaneously. From the measurement results, the carbon content of the sample relative to the modern carbon content of the standard sample was 14 Calculate the C percentage, then compare it with the reference sample. 13 By correcting for the deviation in the carbon concentration, a corrected pMC (percent modern carbon) value was obtained. The biomass degree was calculated using the corrected pMC value. The atmospheric correction factor used was the value for 2019-2021 described in ASTM D6866-21 (100.0 pMC). To measure the biomass degree of the polypropylene-based resin of the present invention, the atmospheric correction factor described in ASTM D6866-21 for the year the polypropylene-based resin was manufactured was used to determine the biomass degree. In addition, the biomass content (%) of the resin particles and expanded beads was calculated from the biomass content of the biomass-derived polypropylene-based resin used to produce the resin particles and expanded beads and the content of the biomass-derived polypropylene-based resin in the resin particles and expanded beads.

[0104] <Melt flow rate (MFR) of polypropylene resin, melt flow rate (MFR) of resin particles, melt flow rate (MFR) of expanded particles> The melt flow rates (MFR) of the polypropylene resins used in the Examples and Comparative Examples, the melt flow rates (MFR) of the resin particles in the Examples and Comparative Examples, and the melt flow rates (MFR) of the expanded particles in the Examples and Comparative Examples were measured at a temperature of 230°C and a load of 2.16 kg in accordance with JIS K 7210-1: 2014. In this specification, the melt flow rate refers to the melt mass flow rate described in JIS K 7210-1: 2014.

[0105] <Melting point> The melting points of polypropylene resins were measured by heat flux differential scanning calorimetry in accordance with JIS K 7121:2012. A high-sensitivity differential scanning calorimeter, "EXSTAR DSC7020" (manufactured by SII NanoTechnology Inc.), was used as the measuring device. The specimen conditioning method employed was "(2) Measurement of melting temperature after a certain heat treatment." Approximately 5 mg of polypropylene resin was collected as a test specimen. The specimen was heated from 23 to 200 °C at a heating rate of 10 °C / min under a nitrogen inflow rate of 30 mL / min. It was then held at that temperature for 10 minutes, cooled to 23 °C at a cooling rate of 10 °C / min, and then heated again to 200 °C at a heating rate of 10 °C / min to obtain a DSC curve (DSC curve for the second heating). The apex temperature of the melting peak in the DSC curve was determined, and this value was taken as the melting point. If multiple melting peaks appear on a DSC curve, the apex temperature of the melting peak with the largest area is used as the melting point. The melting peak with the largest area can be determined by distinguishing each melting peak using the valley temperature of the DSC curve located between the peak temperatures of the melting peaks as a boundary and comparing the areas (heat of fusion) of each melting peak. The valley temperature of the DSC curve corresponds to the temperature at which the vertical axis of the differential DSC curve (DDSC) becomes 0, so it can also be determined from the DSC differential curve. The melting points of the resin particles in the examples and comparative examples were measured in the same manner as in measuring the melting point of polypropylene-based resins, except that about 5 mg of resin particles was used as a test piece.

[0106] <Crystallization temperature> Using a polypropylene resin as the measurement sample, the temperature at the apex of the crystallization peak was determined by heat flux differential scanning calorimetry according to JIS K 7121:2012. The specimen was conditioned as described in "(2) Measurement of melting temperature after a certain heat treatment," with a cooling rate of 10°C per minute. When two or more crystallization peaks appeared, the temperature at the apex of the crystallization peak with the largest area was taken as the crystallization temperature. The crystallization temperatures of the resin particles used in the examples and comparative examples were measured in the same manner as in measuring the crystallization temperature of polypropylene-based resins, except that about 5 mg of resin particles was used as a test piece.

[0107] <Flexural modulus> The flexural modulus of the polypropylene-based resin was measured as follows. First, polypropylene resin was heat-pressed at 160°C to produce 4 mm sheets, and test pieces measuring 80 mm in length, 10 mm in width, and 4 mm in thickness were cut from these sheets. The flexural modulus of these test pieces was measured in accordance with JIS K 7171:2016. The radius R1 of the indenter and the radius R2 of the support table were both 5 mm, the distance between supports was 64 mm, and the test speed was 2 mm / min.

[0108] <Ethylene component content, butene component content> The content of monomer components (ethylene component, butene component) in polypropylene resins was determined by a known method based on IR spectroscopy. Specifically, it was determined by the method described in Polymer Analysis Handbook (edited by the Polymer Analysis Research Forum of the Japan Society for Analytical Chemistry, published January 1995, published by Kinokuniya Shoten, page numbers and item names: 615-616 "II.2.3 2.3.4 Propylene / ethylene copolymer", 618-619 "II.2.3 2.3.5 Propylene / butene copolymer"), that is, by a method of quantifying the absorbance of ethylene and butene corrected by a predetermined coefficient and the relationship between the thickness of a film-like test piece, etc.

[0109] More specifically, first, a polypropylene resin was hot-pressed at 180°C to form a film, and then a plurality of test pieces with different thicknesses were prepared. Next, the IR spectrum of each test piece was measured, and the 722 cm -1 and 733 cm -1 Absorbance (A 722 , A 733 ) and butene-derived 766 cm -1 Absorbance (A 766) was read. Next, for each test piece, the ethylene component content (unit: mass%) in the polypropylene-based resin was calculated using the following formulas (2) to (4). The ethylene component contents obtained for each test piece were calculated as an arithmetic average, which was taken as the ethylene component content (unit: mass%) in the polypropylene-based resin.

[0110] (K' 733 ) c =1 / 0.96{(K' 733 ) a -0.268(K' 722 ) a}···(2) (K' 722 ) c =1 / 0.96{(K' 722 ) a -0.150(K' 733 ) a}···(3) Ethylene content = 0.575 {(K' 722 ) c +(K' 733 ) c}···(4)

[0111] However, K' in formulas (2) to (4) a is the apparent extinction coefficient (K') at each wavenumber a =A / ρt) and K' c is the corrected extinction coefficient, A is the absorbance, and ρ is the density of the resin (unit: g / cm 3 ) and t is the thickness (unit: cm) of the film-like test piece. The above formulas (2) to (4) can be applied to random copolymers.

[0112] Furthermore, for each test piece, the butene component content (unit: mass%) in the polypropylene-based resin was calculated using the following formula (5): The arithmetic mean of the butene component contents obtained for each test piece was taken as the butene component content (unit: mass%) in the polypropylene-based resin. Butene content = 12.3 (A 766 / L)···(5) In the formula (5), A is the absorbance, and L is the thickness of the film-like test piece (unit: mm).

[0113] <Bulk density of expanded particles> A measuring cylinder was filled with expanded particles with a mass W (g), and the bottom of the measuring cylinder was lightly tapped several times on a horizontal surface to stabilize the filling height of the expanded particles in the measuring cylinder. The bulk volume V (L) of the expanded particles indicated on the measuring cylinder was read, and the mass W of the expanded particles was divided by the bulk volume V of the expanded particles (W / V). The value obtained in this way was expressed in kg / m 3 The value obtained by converting the unit to the bulk density of the expanded particles (kg / m 3 ) was decided.

[0114] <Closed cell ratio of expanded beads> Bulk volume approximately 20cm 3 The expanded beads were immersed in water to measure the apparent volume Va of the expanded beads. After measuring the apparent volume Va, the expanded beads were thoroughly dried, and the volume of the expanded beads (the sum of the volume of the resin constituting the expanded beads and the total volume of the closed cell portion within the expanded beads) (true volume Vx) was measured according to procedure C described in ASTM D2856-94. The true volume Vx was measured using an air comparison hydrometer, Model 1000, manufactured by Tokyo Science Co., Ltd. Next, the closed cell ratio was calculated using the following formula (1): Using different measurement samples, the closed cell ratio was measured five times using the same procedure as above, and the arithmetic mean value of the values ​​obtained in each measurement was calculated and used as the closed cell ratio of the expanded beads. Closed cell ratio (%)=(Vx-W / ρ)×100 / (Va-W / ρ) (1) Vx: The true volume (cm ) of the expanded particle group measured by the above method 3 ) Va: Apparent volume (cm) of the foam particles measured from the rise in water level when the foam particles are submerged in water in a measuring cylinder. 3 ) W: Weight of foam particles (g) ρ: Density of the resin that makes up the foamed particles (g / cm 3 )

[0115] <Average cell diameter D of expanded particles> The average cell diameter D of the expanded beads was measured as follows. Thirty expanded beads were randomly selected from the group of expanded beads. Each expanded bead was cut through the center and divided into two, and an enlarged photograph of one cross section was taken. In each cross-sectional photograph, four lines were drawn from the outermost surface of the expanded bead through the center to the outermost surface on the opposite side, so that the angles between any two adjacent lines were equal. The number of cells intersecting each line was counted, and the average cell diameter of each expanded bead was calculated by dividing the total length of the four line segments by the total number of cells intersecting the lines. The average cell diameter D of the expanded beads was calculated by arithmetically averaging these values.

[0116] <Average bubble diameter Dc of bubbles located at the center of foamed beads> Thirty expanded beads were randomly selected from the group of expanded beads. The expanded beads were cut into approximately two equal halves passing through the center of the expanded beads, and one of the cut surfaces was photographed using a scanning electron microscope so that the entire cut surface was captured. The shortest line from the center (centroid) of the photographed image of the approximately circular cut surface of the expanded beads to the cell membrane separating each nearby cell was drawn for each cell. Then, from the cells present near the center, 10 cells were selected in order of the shortest shortest line from the center to the cell membrane. A rectangle was drawn circumscribing each of the selected 10 cells, and the vertical and horizontal lengths of each rectangle were measured. The average of these was taken as the cell diameter. The average of the 30 cell diameters was taken as the cell diameter of the cell located at the center of the expanded beads. The same procedure was performed on 30 randomly selected expanded beads. The arithmetic mean of the cell diameters at the center of the 30 expanded beads was taken as the average cell diameter Dc of the cells located at the center of the expanded beads.

[0117] <Heat of fusion of the high-temperature peak of foamed beads> The heat of fusion of the high-temperature peak of the expanded beads was measured by heat flux differential scanning calorimetry in accordance with JIS K 7122:2012. Specifically, approximately 2 mg of expanded beads was collected as a test piece and heated from 23°C to 230°C at a heating rate of 10°C / min using a differential scanning calorimeter (EXSTAR DSC7020). A DSC curve (DSC curve from the first heating) with two or more melting peaks was obtained. The measurement was performed under conditions of a nitrogen inflow rate of 30 mL / min. In the obtained DSC curve, the main endothermic peak was designated A, and the high-temperature peak appearing above it was designated B. A straight line (α-β) was drawn connecting point α on the DSC curve corresponding to 80°C and point β on the DSC curve corresponding to the melting end temperature T of the test piece. The melting end temperature T is the high-temperature end point of high-temperature peak B, and refers to the intersection point of the high-temperature peak and the high-temperature-side baseline. Next, a straight line parallel to the vertical axis of the graph was drawn from point γ on the DSC curve corresponding to the valley between the main endothermic peak A and high-temperature peak B, and the point where this line intersected with the straight line (α-β) was designated δ. The area enclosed by the curve of the high-temperature peak B in the DSC curve, the line segment (δ-β), and the line segment (γ-δ) was determined, and the heat of fusion of each high-temperature peak was calculated from this area. The heat of fusion of the high-temperature peak was measured for three different test pieces, and the arithmetic mean value of the obtained values ​​was taken as the heat of fusion of the high-temperature peak of the expanded beads.

[0118] <Density of foamed bead molding> First, the mass of the expanded bead molding was measured and designated as W [g]. Next, based on the dimensions of the foamed bead molding, the volume V [cm 3 ] was measured. The mass W [g] of the foamed bead molding is divided by the volume V (W / V), and the unit is [kg / m 3 The density of the expanded bead molding was calculated by converting the calculated value into the value of the density of the expanded bead molding.

[0119] <Compressive stress at 50% strain of foamed bead moldings> Two test pieces (without skin) measuring 5 cm length x 5 cm width x 1.25 cm height were taken from the center of the molded articles obtained in the Examples and Comparative Examples. These test pieces were stacked to a height of 2.5 cm and compressed at a compression rate of 10 mm / min to measure the stress at 50% strain. The higher the stress, the better the strength (rigidity) of the expanded bead molded article. The test pieces were taken from molded articles molded at the lowest possible steam pressure.

[0120] [Polypropylene resin] The polypropylene resins used in the examples and comparative examples are shown in Table 1. The polypropylene resins A (PP-A1 and PP-A2 in Table 1) used in these examples are all propylene homopolymers containing biomass-derived monomer components. Specifically, PP-A1 is "HP456J" manufactured by Lyondellbasell, and PP-A2 is "HP640J" manufactured by Lyondellbasell.

[0121] [Table 1]

[0122] [Production of expanded polypropylene resin beads and expanded bead molded articles] Examples 1 to 4 and Comparative Examples 1 to 2 <Production of expanded polypropylene resin beads> A production apparatus was prepared, which was equipped with an extruder having an inner diameter of 50 mm and a strand-forming die attached to the downstream side of the extruder. The types of biomass-derived polypropylene resin and fossil fuel-derived polypropylene resin shown in Table 2, as well as carbon black and zinc borate as a cell regulator (0.1 part by mass per 100 parts by mass of the total amount of the resins) were fed to an extruder in the blending ratios shown in Table 2, melt-kneaded, and a resin melt was obtained. The resin melt was introduced into a strand-forming die and extruded into strands. The extruded strands were cooled with water and cut with a pelletizer to obtain polypropylene resin particles with an average mass of 1.0 mg per particle. The carbon black used was furnace black (DBP oil absorption 100 mL / 100 g, BET specific surface area 80 m 2 / g, average particle size 20 nm) was used.

[0123] 1 kg of the obtained polypropylene-based resin particles was supplied to a 5 L pressure vessel capable of being pressurized together with 3 L of water as an aqueous dispersion medium. Furthermore, 0.3 parts by mass of kaolin as an inorganic dispersant and 0.004 parts by mass (as an active ingredient) of a surfactant (trade name: NEOGEN, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) were added to the pressure vessel per 100 parts by mass of the resin particles. Next, the pressure vessel was heated with stirring at a rate of 2°C / min until the foaming temperature shown in Table 2 was reached, after which carbon dioxide was injected as a foaming agent into the pressure vessel, and the pressure was increased to the foaming pressure shown in Table 2, and the temperature and pressure were maintained for 15 minutes. This adjustment was made so that a high-temperature peak would appear in the DSC curve of the resulting expanded beads when measured by DSC. Thereafter, the contents of the pressure vessel (resin particles and water) were released under atmospheric pressure to obtain expanded polypropylene resin particles. The obtained expanded polypropylene resin particles were subjected to the above-mentioned measurements and evaluations, and the results are shown in Table 2.

[0124] <Production of foamed bead molded body> The expanded polypropylene resin beads were filled into a mold having a molding cavity capable of forming a rectangular expanded bead molding having dimensions of 250 mm length x 200 mm width x 20 mm height, and heated by the following heating method: A metal mold was used as the molding mold. The heating method involved preheating (exhaust step) by supplying steam to the mold for 5 seconds with the drain valves on both sides open. The drain valves were then closed, and steam was supplied from one side of the mold until a pressure 0.08 MPa (G) lower than the molding pressure during main heating was reached, resulting in a first one-way heating. Next, steam was supplied from the other side of the mold until a pressure 0.04 MPa (G) lower than the molding pressure during main heating was reached, resulting in a second one-way heating. Steam was then supplied from both sides of the mold until the molding pressure shown in Table 2 was reached, resulting in main heating. After the main heating was completed, the pressure was released and water cooling was promptly initiated until the surface pressure due to the expansion force of the expanded bead molding (specifically, the value of a surface pressure gauge attached to the inner surface of the mold) reached 0.04 MPa (G). After water cooling was completed, the expanded bead molding was removed from the mold. The water cooling time refers to the time (seconds) required from the start of water cooling to the end of water cooling. The shorter the water cooling time, the shorter the molding cycle, which can be considered to result in superior productivity of expanded bead moldings. After demolding, the molded article was left to stand for 12 hours in an atmosphere at 80° C. for aging. The expanded bead molded article thus obtained was subjected to the above-mentioned measurements and evaluations. The results are shown in Table 2. The amount of cracking during molding (specifically, the ratio of the mold opening amount to the internal height dimension) was 20% (i.e., 4 mm). Furthermore, when filling the mold with the foamed beads, no pre-pressurization was performed, in which the foamed beads are impregnated with air to increase the internal pressure within the bubbles of the foamed beads (internal particle pressure).

[0125] The molding pressures shown in Table 2 were determined by leaving the expanded bead moldings at a relative humidity of 50%, 23°C, and 1 atm for 24 hours after curing, and then evaluating the fusion, secondary expansion, and recovery properties described below. The molding pressures shown in Table 2 were the lowest among those that could produce expanded bead moldings that passed all the evaluations. The lower the molding pressure, the better the moldability of the expanded beads.

[0126] (Fusing ability) Test pieces (100 mm long x 100 mm wide x thickness: the thickness of the foamed bead molding) were cut from the center of the foamed bead molding. A 5 mm incision was made in each test piece in the thickness direction with a utility knife, and the test piece was broken at the incision. The number of foamed beads (n) present on the fractured surface of the foamed bead molding and the number of foamed beads that had broken (b) were then measured. The number of foamed beads that had broken (b) relative to the number of foamed beads present on the fractured surface (n) was expressed as a percentage to obtain the fusion rate (%), which was evaluated as follows: Pass: The fusion rate was 80% or more. Failed: The fusion rate was less than 80%.

[0127] (Secondary foaming) The surface of the expanded bead molding was visually observed, and the secondary expandability was evaluated as follows. Pass: The gaps between the foam particles on the surface of the foamed bead molding are fully filled. Failed: The gaps between the foam particles on the surface of the foamed bead molding are clearly not filled.

[0128] (Recoverability) The thickness of the expanded bead molding was measured at positions 10 mm away from each of the four corners of the expanded bead molding in a plan view in the thickness direction, toward the center of the plane. The largest of these thicknesses was taken as the thickness of the corners of the expanded bead molding. Separately, the thickness of the expanded bead molding was measured at a central position in both the vertical and horizontal directions in a plan view in the thickness direction, and this value was taken as the thickness of the central part of the expanded bead molding. The ratio (%) of the thickness of the central part to the thickness of the corners of the expanded bead molding was calculated and evaluated as follows. Pass: The ratio was 90% or higher. Fail: The ratio was less than 90%.

[0129] Reference example 1 <Production of expanded polypropylene resin beads> In Comparative Example 1, polypropylene-based resin expanded particles (expanded particles in which the base resin is composed only of fossil fuel-derived polypropylene-based resin and does not contain carbon black) were obtained in the same manner as in Comparative Example 1, except that no biomass-derived polypropylene-based resin was blended, only fossil fuel-derived polypropylene-based resin was used, the base resin was entirely fossil fuel-derived polypropylene-based resin, and the foaming temperature was set to 151.8°C. The average cell diameter D of the obtained expanded beads was 120 μm, the average cell diameter Dc of the bubbles located at the center of the expanded beads was 160 μm, and the ratio Dc / D of the average cell diameter Dc of the bubbles located at the center of the expanded beads to the average cell diameter D of the expanded beads was 1.3. An expanded bead molding was carried out in a mold in the same manner as in the above Examples and Comparative Examples using the expanded beads of Reference Example 1. At this time, the molding pressure was 0.22 MPa (G) and the water cooling time was 36 seconds.

[0130] Reference example 2 <Production of expanded polypropylene resin beads> In Example 1, polypropylene-based resin expanded particles (expanded particles whose base resin is composed only of fossil fuel-derived polypropylene-based resin and contains a predetermined amount of carbon black) were obtained in the same manner as in Example 1, except that no biomass-derived polypropylene-based resin was blended, only fossil fuel-derived polypropylene-based resin was used, the base resin was entirely fossil fuel-derived polypropylene-based resin, and the foaming temperature was set to 152.0°C. The average cell diameter D of the obtained expanded beads was 115 μm, the average cell diameter Dc of the bubbles located at the center of the expanded beads was 155 μm, and the ratio Dc / D of the average cell diameter Dc of the bubbles located at the center of the expanded beads to the average cell diameter D of the expanded beads was 1.3. An expanded bead molding was performed in a mold using the expanded beads of Reference Example 2 in the same manner as in the above Examples and Comparative Examples, with the molding pressure being 0.24 MPa (G) and the water cooling time being 33 seconds.

[0131] [Table 2]

[0132] The results shown in Table 2 show that the expanded beads of Examples 1 to 4 have good moldability and require a short water cooling time during molding. As described above, the expanded polypropylene resin beads obtained by the production method of the present invention have good moldability and excellent productivity for molded articles. Furthermore, since a biomass-derived polypropylene resin is used, it can also contribute to reducing environmental impact. Furthermore, since the expanded polypropylene resin beads contain carbon black and have a black color, they have a luxurious appearance and excellent design properties.

[0133] The expanded beads of Comparative Example 1 did not contain carbon black. Therefore, the average cell diameter Dc of the cells located at the center of the obtained expanded beads was larger than that of the expanded beads of Examples, and the ratio Dc / D of the average cell diameter Dc of the cells located at the center of the expanded beads to the average cell diameter D of the expanded beads was excessively large. As a result, the water cooling time during molding was long, and the productivity of the expanded bead molding was poor.

[0134] The expanded beads of Comparative Example 2 had an excessively large MFR of resin particles. Therefore, the average cell diameter Dc of the cells located at the center of the expanded beads obtained was larger than that of the expanded beads of Examples, and the ratio Dc / D of the average cell diameter Dc of the cells located at the center of the expanded beads to the average cell diameter D of the expanded beads was excessively large. As a result, the water cooling time during molding was long, and the productivity of the expanded bead moldings was poor.

[0135] The expanded beads of Reference Examples 1 and 2 are examples of expanded beads made of a fossil fuel-derived polypropylene resin without blending any biomass-derived polypropylene resin as the polypropylene resin. The difference between the expanded beads of Reference Example 1 and Reference Example 2 is that the expanded beads of Reference Example 1 do not contain carbon black, whereas the expanded beads of Reference Example 2 contain a predetermined amount of carbon black. The MFR of the resin particles in both examples is roughly equivalent to that of the Examples. The expanded beads of Reference Example 2 contained a predetermined amount of carbon black, but the average cell diameter Dc of the cells located at the center of the expanded beads was roughly equivalent to that of the expanded beads of Reference Example 1, and the ratio of the average cell diameter Dc of the cells located at the center of the expanded beads to the average cell diameter D of the expanded beads was also roughly equivalent. As a result, there was almost no effect of shortening the water cooling time during molding. This indicates that the effect of suppressing the coalescence (coarsening) of centrally located bubbles and the effect of shortening the water cooling time due to the incorporation of carbon black, as discovered in the present invention, are unique to cases where the polypropylene-based resin contains a biomass-derived polypropylene-based resin (specifically, cases where the biomass content of the resin particles or expanded particles is a predetermined level or higher). Furthermore, the foamed beads exhibited a suppressed increase in molding pressure due to the incorporation of carbon black, compared to foamed beads derived from non-biomass raw materials (that is, derived from fossil fuels).

Claims

1. A method for producing expanded polypropylene resin beads by expanding polypropylene resin beads having a polypropylene resin as a base resin to obtain expanded beads, comprising: The resin particles have a biomass content of 1% or more as measured by ASTM D6866-21, the resin particles contain 0.1% by mass or more and 8% by mass or less of carbon black, The method for producing expanded polypropylene resin beads, wherein the resin beads have a melt flow rate of 20 g / 10 min or less as measured under conditions of a temperature of 230° C. and a load of 2.16 kg.

2. 2. The method for producing expanded polypropylene resin beads according to claim 1, wherein the base resin of the resin particles is a mixed resin of a biomass-derived polypropylene resin and a fossil fuel-derived polypropylene resin, and the fossil fuel-derived polypropylene resin has a melt flow rate of 35 g / 10 min or less as measured at a temperature of 230°C and a load of 2.16 kg.

3. 3. The method for producing expanded polypropylene resin beads according to claim 1 or 2, wherein a base resin of the resin particles is a mixed resin of a biomass-derived polypropylene resin and a fossil fuel-derived polypropylene resin, and a mass ratio of the biomass-derived polypropylene resin to the fossil fuel-derived polypropylene resin in the mixed resin (biomass-derived polypropylene resin:fossil fuel-derived polypropylene resin) is 1:99 to 90:

10.

4. the expanded beads have a crystalline structure in which a DSC curve obtained when the expanded beads are heated from 23°C to 230°C at a heating rate of 10°C / min shows a main endothermic peak having the largest peak area and a high-temperature peak which is an endothermic peak appearing on the high-temperature side of the main endothermic peak, 3. The method for producing expanded polypropylene resin beads according to claim 1, wherein the heat of fusion of the high-temperature peak is 10 J / g or more and 35 J / g or less.

5. The bulk density of the expanded beads is 10 kg / m ３ More than 200kg / m ３ The method for producing expanded polypropylene resin beads according to claim 1 or 2, wherein the method is as follows:

6. a dispersing step of dispersing the resin particles in an aqueous medium in a sealed container; a foaming agent adding step of adding a physical foaming agent into the sealed container; and an expansion step of impregnating the resin particles with the physical blowing agent in the sealed container, and then releasing the resin particles together with the aqueous medium from the sealed container into an atmosphere having a pressure lower than the pressure inside the sealed container, thereby expanding the resin particles to produce expanded beads. The method for producing the expanded polypropylene resin beads according to claim 1 or 2.

7. A polypropylene-based resin expanded particle having a polypropylene-based resin as a base resin, The expanded beads have a biomass content of 1% or more as measured by ASTM D6866-21; the expanded beads contain 0.1% by mass or more and 8% by mass or less of carbon black, The foamed beads have a closed cell rate of 85% or more, The average cell diameter D of the expanded beads is 50 μm or more and 250 μm or less, Expanded polypropylene resin beads, wherein the ratio Dc / D of the average cell diameter Dc of the cells located at the center of the expanded beads to the average cell diameter D of the expanded beads is 0.7 or more and 1.3 or less.

8. The expanded polypropylene resin beads according to claim 7, wherein the average cell diameter Dc of the cells located at the center of the expanded beads is 220 μm or less.

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