Production method for polypropylene resin foamed particles and polypropylene resin foamed particles

Abstract

Provided is a production method for polypropylene resin foamed particles (expanded beads) that involves foaming resin particles that contain a polypropylene resin as a base material resin to obtain foamed particles. The biomass percentage of the resin particles is at least 1%, the resin particles are 0.1–8 mass% carbon black, and the melt flow rate of the resin particles as measured at a temperature of 230°C and a load of 2.16 kg is no more than 20 g / 10 min.

Description

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

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

[0002] BACKGROUND ART Expanded polypropylene resin bead molded articles are excellent in strength and shock-absorbing properties and are therefore used in a variety of applications, such as packaging materials, automobile 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 an increase in the concentration of carbon dioxide in the atmosphere 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 relating to expanded polyethylene resin beads containing a plant-derived polyethylene resin with a high plant content has been proposed (e.g., Patent Document 1).

[0005] JP 2013-060514 A

[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 resin as a raw material may have poor moldability in a mold. Furthermore, even when a foamed bead molding can be obtained by molding the expanded beads in a mold, the water cooling time may be long, leaving room for improvement. Therefore, a need exists for a method for producing expanded polypropylene resin beads 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.

[0009] As a result of extensive research, the present inventors have found that a method for producing expanded beads by expanding specific resin particles can solve the above-mentioned problems. 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-based resin beads, which comprises expanding resin particles having a polypropylene-based 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 under a load of 2.16 kg. [2] The method for producing expanded polypropylene-based resin beads 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 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] The method for producing expanded polypropylene-based resin beads 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 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 method for producing expanded polypropylene resin beads according to any one of [1] to [3], wherein the bulk density of the expanded beads is 10 kg / m ３ More than 200kg / m ３[6] A method for producing expanded polypropylene resin beads according to any one of [1] to [4], which is as follows: [6] A method for producing expanded polypropylene resin beads according to any one of [1] to [5], which comprises: a dispersing step of dispersing the resin particles in an aqueous medium in a sealed container, a blowing agent adding step of adding a physical blowing 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. [7] Expanded beads having a polypropylene-based resin as a base resin, the expanded beads having a biomass content of 1% or more as measured by ASTM D6866-21, containing 0.1% to 8% by mass of carbon black, a closed cell rate of 85% or more, an average cell diameter D of 50 μm to 250 μm, and a 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 of 0.7 to 1.3. [8] The expanded polypropylene-based resin beads according to [7], wherein the average cell diameter Dc of the cells located at the center of the expanded beads is 220 μm or less.

[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.

[0011] [Method for Producing Expanded Polypropylene Resin Beads] The method for producing expanded polypropylene resin beads of the present invention involves expanding 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 by 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. Note that, in this specification, "expanded polypropylene resin beads" are also referred to simply as "expanded beads," "resin particles having a polypropylene resin as a base resin" are also referred to simply as "polypropylene resin particles" or "resin particles," and "polypropylene resin expanded bead molded body" are also simply referred to as "expanded bead molded body" or "molded body." Note that, in this specification, "base resin of resin particles" refers to the polymer component that constitutes the resin particles.

[0012] <Polypropylene-Based Resin Particles> The polypropylene-based resin particles used in the method for producing expanded polypropylene-based resin beads of the present invention have a biomass content of 1% or more as measured by 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. By expanding such resin particles, the resulting expanded beads have good moldability in a mold and a reduced water cooling time during molding, resulting in excellent productivity for molded articles. Furthermore, since the expanded beads contain a predetermined amount of biomass-derived components, they can also contribute to reducing environmental impact.

[0013] (Polypropylene-Based Resin) In this specification, the term "polypropylene-based resin" refers to a propylene homopolymer or a polypropylene-based 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 propylene-ethylene copolymer, propylene-butene copolymer, and propylene-ethylene-butene copolymer, as well as propylene-acrylic acid copolymer and propylene-maleic anhydride copolymer. These copolymers may be block copolymers, random copolymers, or graft copolymers. The above-described polymers may be crosslinked, but are preferably non-crosslinked in order to further contribute to reducing environmental impact.

[0014] (Biomass Degree) The polypropylene resin particles have a biomass degree (biobased carbon content) of 1% or more as measured by ASTM D6866-21. That is, the biomass degree of the resin particles as measured by ASTM D6866-21 is 1% or more. When the biomass degree of the resin particles is within this range, the use of fossil resources during the production of expanded bead moldings can be suppressed, and the amount of carbon dioxide emitted over the life cycle of the expanded bead moldings can also be reduced. From this perspective, the biomass degree of the resin particles as measured by ASTM D6866-21 is preferably 1.5% or more, more preferably 1.8% or more, and even more preferably 2% or more. From the perspective of further reducing the environmental impact, the biomass degree of the resin particles is even more preferably 5% or more, and particularly preferably 10% or more. Although there is no upper limit to the biomass degree of the resin particles, the biomass degree of the resin particles measured according to ASTM D6866-21 may be 100% or less, but 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 resin particles measured according to ASTM D6866-21 is preferably 80% or less, more preferably 50% or less, even more preferably 30% or less, still more preferably 20% or less, and even more preferably 10% or less. Note that the upper and lower limits of the biomass degree of the resin particles can be arbitrarily combined to determine a preferred range. Therefore, the biomass content of the resin particles is, for example, 1% or more and 100% or less, 1.5% or more and 80% or less, 2% or more and 50% or less, 2% or more and 30% or less, 1% or more and 20% or less, 1% or more and 10% or more and 5% or more and 20% or less, or 10% or more and 20% or less.

[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 measured by measuring the radiocarbon ratio specified in ASTM D6866-21 using the resin particles as a measurement sample. 14It should be noted that, 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, the biomass degree can also be determined by calculation using these values.

[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 in-mold molding of the resulting expanded beads is shortened, improving the productivity of molded articles. In addition, the resulting molded articles have a luxurious appearance.

[0017] The reason why the inclusion of a predetermined amount of carbon black in the resin particles shortens the water-cooling time during in-mold molding is unclear, but it is thought to be as follows. Conventionally, expanded beads containing a biomass-derived polypropylene resin as a raw material sometimes required a long water-cooling time during in-mold molding, even when a foamed bead molding was obtained. This is thought to be because bubbles formed in the center of the resin beads during expansion coalesce, resulting in the resulting expanded beads having coarse bubbles in the center. In contrast, in the present invention, the inclusion of carbon black in resin particles containing a biomass-derived polypropylene resin is thought to moderately thicken the resin and inhibit the coalescence of central bubbles during expansion. 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, thereby shortening the water-cooling time during in-mold molding. It is thought that the effect of carbon black is particularly pronounced when the resin particles contain a biomass-derived polypropylene resin and have a specific melt flow rate, as described below.

[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 excessively high molding pressure or failure 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 of 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, based on the total amount of resin particles. From the viewpoint of in-mold moldability of the resulting expanded beads, the carbon black content of the resin particles is preferably 7% by mass or less, more preferably 6% by mass or less, even more preferably 4.5% by mass or less, and even more preferably 3% by mass or less, based on the total amount of resin particles. The upper and lower limits of the carbon black content of the resin particles can be arbitrarily combined to determine a preferred range. Therefore, the carbon black content of the resin particles is, for example, 0.5% by mass or more to 8% by mass or less, 1.5% by mass or more to 7% by mass or less, 2% by mass or more to 7% by mass or less, 0.5% by mass or more to 6% by mass or less, 1% by mass or more to 6% by mass or less, 1.5% by mass or more to 4.5% by mass or less, or 2% by mass or more to 3% by mass or less.

[0020] Carbon black is a material typically used as a black colorant. There are no particular limitations on the manufacturing method or composition of the carbon black. For example, channel black, roller black, furnace black, thermal black, acetylene black, ketjen black, etc. can be used. These carbon blacks may also be subjected to hydrophilization, hydrophobization, oxidation, reduction, acidification, basification, organification, etc. 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 50 mL / 100 g, more preferably 80 mL / 100 g. The upper and lower limits of the dibutyl phthalate (DBP) oil absorption of the carbon black can be arbitrarily combined to determine a preferred range. Therefore, the dibutyl phthalate (DBP) oil absorption of the carbon black is, for example, 50 mL / 100 g or more but less than 150 mL / 100 g, 50 mL / 100 g or more but 130 mL / 100 g or less, or 80 mL / 100 g or more but 110 mL / 100 g or less. 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, more preferably 150m ２ / g or less, more preferably 100m ２ The lower limit is preferably 30 m / g. ２ / g, more preferably 50m ２ The upper and lower limits of the BET specific surface area of ​​the carbon black can be arbitrarily combined to determine a preferred range. Therefore, the BET specific surface area of ​​the carbon black is, for example, 30 m ２ / g or more 200m ２ / g or less, 30m ２ / g or more 150m２ / g or less, 50m ２ / g or more 100m ２ The BET specific surface area of ​​carbon black is a value measured by the BET method in accordance with ASTM D3037.

[0023] The carbon black can be selected from those described above. Since the carbon black can be dispersed more uniformly in the polypropylene resin, it is preferable to blend the 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, 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 resin particles have a melt flow rate of 20 g / 10 min or less, measured at 230°C under a load of 2.16 kg. That is, the melt flow rate of the polypropylene resin particles is 20 g / 10 min or less, measured at 230°C under a load of 2.16 kg. If the melt flow rate of the polypropylene resin particles exceeds 20 g / 10 min, the thickening effect of the carbon black described above becomes insufficient, resulting in a longer water cooling time during in-mold molding of the resulting expanded beads, 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-based 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 even more preferably 9 g / 10 min or less. Furthermore, from the viewpoint of further enhancing the expandability of the resulting expanded beads, the melt flow rate of the polypropylene-based 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 even 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. Therefore, 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 melt mass flow rate 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) From the viewpoint of achieving an excellent balance between the moldability of the expanded beads and the strength of the molded product, 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. 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 more and 122° C. or less, more preferably 105° C. or more and 120° C. or less, even more preferably 108° C. or more and 118° C. or less, and particularly preferably 110° C. or more and 115° C. or less. The crystallization temperature of the resin particles can be determined by the same method as the crystallization temperature of the biomass-derived polypropylene resin described later, and more specifically, it 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 including a biomass-derived polypropylene-based resin in the base resin of the resin particles, the biomass content of the resulting expanded beads can be increased, contributing to a reduction in environmental impact. Furthermore, the base resin of the resin particles is preferably a mixed resin of a biomass-derived polypropylene-based resin and a fossil fuel-derived polypropylene-based resin. By including a biomass-derived polypropylene-based resin in addition to a fossil fuel-derived polypropylene-based resin in the base resin, the resulting expanded beads have better moldability. Furthermore, the viscoelasticity of the polypropylene-based resin constituting the resin particles 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 a shorter water cooling time during molding.

[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 the molecular chain, which is formed by polymerizing a biomass-derived monomer. The biomass-derived polypropylene resin may be composed only 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 decomposing 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, waste liquor from palm oil production (palm oil mill effluent), oils and fats contained in microalgae, etc., from the viewpoint 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 decomposing the 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 product numbers "HP456J," "HP640J," and "EP348U."

[0033] The biomass degree (biobased carbon content) of the biomass-derived polypropylene resin measured by ASTM D6866-21 is preferably 1% or more and 100% or less. From the viewpoint of contribution to reducing environmental burden, the biomass degree of the biomass-derived polypropylene resin is more preferably 5% or more, even more preferably 10% or more, even more preferably 20% or more, even 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 molded articles, the biomass degree of the biomass-derived polypropylene resin is more preferably 80% or less, even 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 obtain a preferred biomass degree. Therefore, the biomass content of the biomass-derived polypropylene resin is, for example, preferably 1% or more and 100% or less, more preferably 5% or more and 80% or less, even more preferably 10% or more and 60% or less, still more preferably 20% or more and 50% or less, and particularly preferably 30% or more and 45% or less.

[0034] From the viewpoints of moldability of the expanded beads, productivity of the molded body, and strength of the molded body, 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 168°C or lower, and particularly preferably 160°C or higher and 165°C or lower. The melting point of the biomass-derived polypropylene resin can be determined based on JIS K 7121:2012. In this case, the condition of the test specimen is adjusted as follows: "(2) Measuring the melting temperature after a certain heat treatment." More specifically, a pelletized polypropylene resin is used as a test piece, and according to heat flux differential scanning calorimetry as described in JIS K 7121:2012, the test piece is 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 apex temperature of the melting peak determined by the DSC curve obtained when the test piece is heated from 23°C to 200°C at a heating rate of 10°C / min is taken as the melting point of the polypropylene resin. When two or more melting peaks appear in the DSC curve, the apex temperature of the melting peak with the largest area is taken as the melting point. In this case, the melting peak with the largest area can be determined by distinguishing each melting peak at the temperature of the valley of the DSC curve located between the apex temperatures of the melting peaks and comparing the areas (heat of fusion) of each melting peak. The valley temperature of the DSC curve can be determined by referring to the differential DSC curve (DDSC) and judging from the temperature at which the value on the vertical axis of the differential curve becomes 0. Examples of measuring devices 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 employed was "(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 appear, the temperature at the apex of the crystallization peak with the largest area was used 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 or more and 10 g / 10 min or less, more preferably 1 g / 10 min or more and 8 g / 10 min or less, even more preferably 1.5 g / 10 min or more and 5 g / 10 min or less, and even more preferably 2 g / 10 min or more and 5 g / 10 min or less. 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] From the viewpoint of expanding the range of molding pressures at which the expanded beads can be molded and from the viewpoint of obtaining an expanded bead molded article having excellent rigidity, 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. The flexural modulus of the biomass-derived polypropylene resin can be determined by heat-pressing the biomass-derived polypropylene resin to prepare a sheet-like test piece having a predetermined dimension, and measuring the flexural modulus of this test piece based on JIS K 7171:2016.

[0039] (Fossil fuel-derived polypropylene resin) A fossil fuel-derived polypropylene resin refers to a polypropylene resin obtained by polymerizing substantially only fossil fuel-derived monomers. Specifically, the fossil fuel-derived polypropylene resin preferably contains only fossil fuel-derived monomer components in the molecular chain. Furthermore, the biomass content (biobased carbon content) of the fossil fuel-derived polypropylene resin measured by 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 stably providing expanded beads that exhibit good in-mold moldability and from which expanded bead moldings with good surface properties can be obtained, 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] From the viewpoint of achieving an excellent balance between the moldability of the expanded beads and the strength of the molded product, 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. The melting point of the fossil fuel-derived polypropylene resin can be determined by the same method as the melting point of the biomass-derived polypropylene resin described above, and more specifically, it can be measured by the method described in the Examples.

[0043] From the viewpoint of obtaining expanded beads having an excellent moldable range, the crystallization temperature of the fossil fuel-derived polypropylene resin is preferably 90° C. or higher and 115° C. or lower, more preferably 92° C. or higher and 112° C. or lower, even more preferably 95° C. or higher and 110° C. or lower, and particularly preferably 100° C. or higher and 108° C. or lower. The crystallization temperature of the fossil fuel-derived polypropylene resin can be determined by the same method as that for the crystallization temperature of the biomass-derived polypropylene resin, and more specifically, it 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 resulting expanded beads can more reliably shorten the water-cooling time during molding. From the viewpoint of obtaining expanded beads that can more reliably shorten the water-cooling time while ensuring good in-mold moldability, 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] From the viewpoint of achieving an excellent balance between the moldability of the expanded beads and the strength of the molded product, 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. The flexural modulus of the fossil fuel-derived polypropylene resin can be determined by the same method as the flexural modulus of 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-based resin, preferably a mixed resin of a biomass-derived polypropylene-based resin and a fossil fuel-derived polypropylene-based resin, but may also contain other polymers other than polypropylene-based resins. The term "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-based resins, such as polyethylene-based resins, polystyrene-based resins, polyamide-based resins, and polyester-based resins, as well as thermoplastic elastomers such as olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers. Two or more of these other polymers may be contained. The blending ratio of the other polymer 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, even more preferably 1% by mass or less, and particularly preferably 0% by mass, based on 100% by mass of the base resin. It is particularly preferred that the base resin contains only polypropylene-based resins as polymers.

[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 even more preferably 4:96 to 50:50 (where the total of the biomass-derived polypropylene resin and the fossil fuel-derived polypropylene resin is taken as 100). Having a mass ratio of the biomass-derived polypropylene resin to the fossil fuel-derived polypropylene resin within the above range improves the moldability of the resulting expanded beads, shortens the water cooling time during molding, and improves the productivity of molded articles. This also contributes to reducing environmental impact.

[0049] The polypropylene resin contained in the biomass-derived polypropylene resin constituting the base resin and the polypropylene resin contained in the fossil fuel-derived polypropylene resin may be the same type of polypropylene resin or different polypropylene resins. For example, the biomass-derived polypropylene resin and the fossil fuel-derived polypropylene resin may both be propylene homopolymers, or both may be polypropylene copolymers, or one may be a propylene homopolymer and the other a polypropylene 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, and more preferably contains 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 resin particles as a whole 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 external shape of the resin particles is cylindrical, 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 take-up speed of the strands, the cutter speed when cutting the strands, and the like.

[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] As the cell regulator, for example, inorganic powders or organic powders can be used. Examples of inorganic powders include metal borate salts such as zinc borate and magnesium borate, and examples of organic powders include fluororesin powders such as polytetrafluoroethylene (PTFE). From the viewpoint of stably obtaining expanded beads having a desired bulk density and little variation in cell diameter, the amount of cell regulator in the resin particles 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. Furthermore, from the viewpoint of easily adjusting the average cell diameter of the expanded beads to a desired range, it is preferable to use metal borate salts, and more preferably zinc borate, as the cell regulator. Furthermore, when zinc borate is used, its number-based arithmetic mean particle diameter is preferably 0.5 μm or more and 10 μm or less, more preferably 1 μm or more and 8 μm or less. 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 the particle shape to be spherical, and then arithmetically averaging 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, even more preferably 90% by mass or more, and may be 100% by mass. Examples of dispersion mediums 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 so that the resin particles heated in the container do not fuse 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 of these may be used alone, or two or more may be used 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 such physical blowing agents 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. Alternatively, inorganic and organic physical blowing agents may be used in combination. From the viewpoint of facilitating the production of desired expanded beads, 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, and the like. 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 maintained at that temperature for about 5 to 30 minutes, after which they are released from the sealed container into an atmosphere of a pressure lower than the pressure inside the sealed container (e.g., atmospheric pressure) to expand the resin beads.

[0067] Expanded beads having a crystalline structure in which a main endothermic peak and a high-temperature peak appear in the first DSC curve described below 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 (the melting point of the resin constituting the resin particles -15°C) and (the 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 (holding step). Next, the resin particles that have undergone this holding step are expanded to obtain expanded beads that exhibit the above-mentioned melting peak. In producing expanded beads, resin particles that have undergone the holding step may be prepared in advance and then expanded to obtain expanded beads. Alternatively, the resin particles may be subjected to the holding step as part of the dispersing step or the blowing agent impregnation step, and the resin particles that have undergone this 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 under a 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 having a crystalline structure in which the above-mentioned main endothermic peak and high-temperature peak appear.

[0068] In the manufacturing method, when resin particles are expanded, the resin particles may be expanded in one stage as described above, or in two or more stages. When expanding resin particles in two stages, the resin particles are first expanded by a direct expansion method or the like in the first expansion stage 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. Expanding resin particles in multiple stages in this manner makes it easy to obtain expanded beads with a higher expansion ratio (i.e., a lower bulk density).

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

[0070] (Heat of fusion of 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 by a DSC curve obtained by performing differential scanning calorimetry (DSC) according to JIS K 7122:2012 on a test piece of expanded beads. The heat of fusion of the high-temperature peak can also be determined from the same DSC curve. Specifically, the crystalline structure can be confirmed if a DSC curve obtained by heating the expanded beads from 23°C to 230°C at a heating rate of 10°C / min (the DSC curve in the first heating) 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. The main endothermic peak with the largest peak area is an intrinsic melting peak that appears due to the melting of crystals normally present in the base resin. 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) (DSC curve in the second heating), only a melting peak due to melting of crystals normally possessed by the base resin appears. This makes it possible to confirm which peak in the DSC curve in 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. The heat of fusion of the high-temperature peak of the expanded beads is 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 in accordance with JIS K 7122:2012 using the expanded beads as a test piece. 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, 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 in the dispersion step and / or the blowing agent impregnation step.

[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 or less, more preferably 15 kg / m ３ More than 150kg / m ３ More preferably 20 kg / m or less ３ More than 100kg / m ３ More preferably, 20 kg / m or less ３ More than 80kg / m ３ 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 of 23°C, 50% relative humidity, and 1 atm. The expanded beads having a mass W (g) thus obtained 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 mass 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 as kg / m ３ The bulk density of the expanded particles (kg / m ３ ) can be obtained.

[0076] [Polypropylene-Based Resin Expanded Beads] The polypropylene-based resin expanded beads of the present invention are expanded beads using a polypropylene-based resin as a base resin, and have a biomass content of 1% or more as measured by ASTM D6866-21. The expanded beads contain 0.1% to 8% by mass of carbon black. The closed cell content of the expanded beads is 85% or more. The average cell diameter D of the expanded beads is 50 μm to 250 μm. 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 to 1.3. The polypropylene-based resin expanded beads of the present invention are preferably produced by the above-mentioned method for producing expanded polypropylene-based resin beads. The method for producing the expanded polypropylene-based resin beads of the present invention is preferably the above-mentioned method for producing expanded polypropylene-based resin beads, and the same applies to more preferred methods. The expanded beads described in the section [Polypropylene-Based Resin Expanded Beads] of the present invention are preferably expanded beads produced by the above-mentioned [Method for Producing Expanded Polypropylene-Based Resin Beads], and the same applies to more preferred expanded beads.

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

[0078] (Biomass Degree) The biomass degree (biobased carbon content) of the expanded polypropylene resin beads, as measured by ASTM D6866-21, is 1% or more. Having the biomass degree of the expanded beads within this range reduces the use of fossil resources during the production of expanded bead moldings and also reduces the amount of carbon dioxide emitted over the life cycle of the expanded bead moldings. From this perspective, the biomass degree of the expanded beads, as measured by 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 perspective of further reducing the environmental impact, the biomass degree of the expanded beads is even more preferably 5% or more, and particularly preferably 10% or more. Although there is no upper limit to the biomass degree of the expanded beads, the biomass degree of the expanded beads measured according to ASTM D6866-21 must be 100% or less, and from the viewpoints 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 expanded beads measured according to ASTM D6866-21 is preferably 80% or less, more preferably 50% or less, even more preferably 30% or less, particularly preferably 20% or less, and particularly 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 expanded beads can be arbitrarily combined to determine a preferred range. Therefore, the biomass ratio of the expanded beads is, for example, 1% to 100%, 1.5% to 80%, 2% to 50%, 2% to 30%, 1% to 20%, 1% to 10%, 5% to 20%, or 10% 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 measured by the radiocarbon standard specified in ASTM D6866-21 using the expanded beads as a measurement sample. 14It should be noted that, 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, the biomass degree can also be determined by calculation using these values.

[0079] (Carbon Black) The expanded polypropylene resin beads contain 0.1 parts by mass or more and 8 parts by mass or less of carbon black per 100 parts by mass of the base resin. Details of carbon black are as described above. When the expanded beads contain a predetermined amount of carbon black, the water cooling time during in-mold molding is shortened, improving the productivity of molded articles. In addition, the obtained molded articles have a luxurious appearance. The reason why the moldability of the expanded beads is improved and the water cooling time during molding is shortened when the expanded beads contain a predetermined amount of carbon black is not clear, but it 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 be reduced, resulting in 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, even 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 or more to 8% by mass or less, 1.5% by mass or more to 7% by mass or less, 2% by mass or more to 7% by mass or less, 0.5% by mass or more to 6% by mass or less, 1% by mass or more to 6% by mass or less, 1.5% by mass or more to 4.5% by mass or less, or 2% by mass or more to 3% by mass or less.

[0082] (Closed Cell Ratio) The closed cell ratio 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 ratio is not particularly limited, but is generally 98%.

[0083] The closed cell ratio of the expanded beads is measured as follows: First, a bulk volume of about 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 of the expanded beads (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) 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 according to 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 determined, which is the closed cell ratio of the expanded beads. Closed cell ratio (%) = (Vx - W / ρ) x 100 / (Va - W / ρ) (1) Vx: true volume (cm) of the expanded particle group measured by the above method ３ ) Va: Apparent volume (cm) of the expanded particles measured from the rise in the water level when the expanded particles are submerged in water in a measuring cylinder ３ ) W: Mass of the expanded particle group (g) ρ: Density of the resin constituting the expanded particle (g / cm ３ )

[0084] (Average Cell Diameter D) The average cell diameter D of the expanded beads is 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 still 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 the expanded bead into two equal parts. A straight line is drawn on the photograph so as to roughly bisect the cross section of the expanded bead. 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 touching the 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 easily suppressed. From this viewpoint, the ratio Dc / D is preferably 0.8 or more and 1.2 or less, more preferably 0.9 or more and 1.1 or less.

[0087] (Average Cell Diameter Dc of Centrally Located Bubbles) The average cell diameter Dc of the centrally located bubbles of the expanded beads is preferably 220 μm or less, more preferably 200 μm or less, even more preferably 180 μm or less, and even 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 centrally located bubbles 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. The upper and lower limits of the average cell diameter Dc of the centrally located bubbles of the expanded beads can be arbitrarily combined to determine a preferred range. Therefore, the average cell diameter Dc of the centrally located bubbles of the expanded beads is, for example, 50 μm or more and 220 μm or less, 50 μm or more and 200 μm or less, 80 μm or more and 180 μm or less, or 100 μm or more and 170 μm or less.

[0088] The average cell diameter Dc of the cells 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 photographed image of the approximately circular cut surface of the expanded bead to the cell membrane that separates each cell located nearby. Then, from the cells located near the center, 10 cells 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 cells, and the vertical and horizontal lengths of each rectangle are measured. The average of these lengths is taken as the cell diameter of the cells 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 cells located at the center of the expanded beads.

[0089] Expanded beads satisfying the above-mentioned Dc / D ratio and Dc range 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 cells in the center portion during expansion of the resin beads is suppressed, and the average cell diameter of the cells located in the center portion 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 viewpoint of moldability in a mold, water cooling time, etc., the expanded beads have a melt flow rate measured at a temperature of 230°C under a load of 2.16 kg of preferably 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 beads is the melt mass flow rate value measured at a temperature of 230°C under a load of 2.16 kg.

[0091] (Fusing 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 fusing layer covering the foamed core layer, which is provided to enhance the fusing between the expanded beads during in-mold molding. The fusing layer may be present on the entire surface of the expanded beads or on a portion of the surface. The fusing 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 fusing 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 fusing layer. When the expanded beads have a foamed layer and a fusing layer, the foamed layer covered by the fusing 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] Furthermore, 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 (i.e., the value obtained by subtracting the melting point of the crystalline polyolefin resin constituting the fusion layer from the melting point of the polypropylene resin constituting the foamed core layer) is preferably approximately 1° C. or more and 40° C. or less, more preferably 2° C. or more and 35° C. or less, and even more preferably 5° C. or more and 30° C. or less. 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 Expanded Bead Molded Article> A polypropylene resin expanded bead molded article can be obtained by molding the expanded beads of the present invention or the expanded beads obtained by the method for producing expanded beads of the present invention in a mold. That is, the polypropylene resin expanded bead molded article is obtained by molding the expanded beads in a mold.

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

[0099] From the viewpoint of achieving both light weight and mechanical properties, the density of the foamed bead molding is preferably 10 kg / m 3 More than 300kg / m 3 or less, more preferably 20 kg / m 3 More than 200kg / m 3 More preferably 25 kg / m or less3 More than 100kg / m 3 More preferably, 30 kg / m or less 3 More than 80kg / m 3 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 expanded polypropylene resin 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 parts such as automobile bumpers, building parts such as heat insulating materials for houses, miscellaneous goods, etc.

[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] [Measurements and Evaluations] The resins, expanded beads, and expanded bead molded articles used in the Examples and Comparative Examples were measured and evaluated as follows: 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 Degree> The biomass degree (%) of the polypropylene-based resin used in the examples and comparative examples was determined as follows in accordance with ASTM D6866-21. A polypropylene-based resin was used as the resin to be measured, and carbon dioxide (CO ２ ) was generated and the carbon dioxide was purified in a vacuum line. The purified carbon dioxide was reduced with hydrogen using iron as a catalyst, thereby producing graphite (C). The graphite was then packed into a cathode with an inner diameter of 1 mm using a hand press, which was then fitted into a wheel and used in a tandem accelerator based on a NEC Corporation tandem accelerator. １４ The sample was then attached to a dedicated C-AMS (accelerator mass spectrometry) device. １４ The number of C's, １３ C concentration ( １３ C / １２ C), 14C concentration ( １４C / １２ C) was measured. In the measurement, oxalic acid (HOxII) provided by the National Institute of Standards (NIST) was used as the standard sample. Measurements of this standard sample and a background sample were also carried out at the same time. From the measurement results obtained, the ratio of sample carbon to modern carbon of the standard sample was １４ The C ratio was calculated, and then the １３ By correcting for the deviation in the C concentration, a corrected pMC (percent modern carbon) value was obtained. The biomass degree was calculated using the corrected pMC value. The atmospheric correction factor used was the value for 2019-2021 described in ASTM D6866-21 (100.0 pMC). To measure the biomass degree of the polypropylene-based resin according to the present invention, the atmospheric correction factor described in ASTM D6866-21 for the year the polypropylene-based resin was produced was used to determine the biomass degree. The biomass degree (%) of the resin particles and expanded beads was calculated from the biomass degree of the biomass-derived polypropylene-based resin used to produce the resin particles and expanded beads and the content of 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 beads> The melt flow rate (MFR) of the polypropylene resin used in the examples and comparative examples, the melt flow rate (MFR) of the resin particles in the examples and comparative examples, and the melt flow rate (MFR) of the expanded beads 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, melt flow rate refers to the melt mass flow rate described in JIS K 7210-1: 2014.

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

[0106] <Crystallization Temperature> Using a polypropylene resin as a measurement sample, the temperature at the apex of the crystallization peak obtained by heat flux differential scanning calorimetry was determined based on JIS K 7121:2012. The test specimen was conditioned using "(2) Measurement of melting temperature after a certain heat treatment," and the cooling rate was 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 the measurement of the crystallization temperature of the polypropylene resin, except that approximately 5 mg of resin particles were used as the test specimen.

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

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

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

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

[0112] Further, 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 component content = 12.3 (A 766 / L) (5) where A in formula (5) is absorbance, and L is the thickness (unit: mm) of the film-like test piece.

[0113] <Bulk Density of Expanded Beads> A measuring cylinder was filled with expanded beads having a mass W (g), and the measuring cylinder was lightly tapped on a horizontal surface several times with the bottom of the measuring cylinder to stabilize the packed height of the expanded beads in the measuring cylinder. The bulk volume V (L) of the expanded beads indicated on the scale of the measuring cylinder was read, and the mass W of the expanded beads was divided by the bulk volume V of the expanded beads (W / V). The value obtained in this way was expressed as kg / m ３ The value obtained by converting the unit to３ )

[0114] <Closed cell ratio of expanded particles> Bulk volume: approx. 20 cm 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 cells in the closed cell portion within the expanded beads) (true volume Vx) was measured according to procedure C described in ASTM D2856-94. An air comparison hydrometer, Model 1000, manufactured by Tokyo Science Co., Ltd. was used to measure this true volume Vx. The closed cell ratio was then calculated according to 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) x 100 / (Va - W / ρ) (1) Vx: true volume (cm) of the expanded beads measured by the above method. ３ ) Va: Apparent volume (cm) of the expanded particles measured from the rise in the water level when the expanded particles are submerged in water in a measuring cylinder ３ ) W: Mass of the expanded particle group (g) ρ: Density of the resin constituting the expanded particle (g / cm ３ )

[0115] <Average Cell Diameter D of Expanded Beads> The average cell diameter D of expanded beads was measured as follows. Thirty expanded beads were randomly selected from a group of expanded beads. Each expanded bead was cut through the center to divide it into two, and an enlarged photograph of one cross section was taken. In each cross-sectional photograph, four line segments 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 line segments were equiangular. The number of cells intersecting each line segment was counted, and the average cell diameter of each expanded bead was determined by dividing the total length of the four line segments by the total number of cells intersecting the lines. These values ​​were arithmetically averaged to determine the average cell diameter D of the expanded beads.

[0116] <Average Cell Diameter Dc of Bubbles Located at the Center of Expanded Beads> Thirty expanded beads were randomly selected from a group of expanded beads. Each expanded bead was cut into approximately two equal halves passing through the center of the expanded bead, and one of the cut surfaces was photographed using a scanning electron microscope so that the entire cut surface was captured. For each of the photographed images of the approximately circular cut surfaces of the expanded beads, the shortest line was drawn from the center (centroid) of the image to the cell membrane separating each of the nearby cells. Then, from the cells located near the center, 10 cells were selected in order of the shortest shortest line from the center to the cell membrane. A rectangle circumscribing each of the selected 10 cells was drawn, and the vertical and horizontal lengths of each rectangle were measured, and the average was taken as the cell diameter. The average of the 10 cell diameters thus calculated was taken as the cell diameter of the cell located at the center of the expanded bead. The same procedure was performed on 30 randomly selected expanded beads. The arithmetic mean value of the cell diameters at the center of 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 High-Temperature Peak of Expanded 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 specimen 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 for 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. Regarding 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 corresponding to 80°C on the DSC curve and point β on the DSC curve corresponding to the end-of-melting temperature T of the test specimen. The melting end temperature T is the high-temperature end point of high-temperature peak B, and refers to the intersection of the high-temperature peak and the high-temperature baseline. Next, a line parallel to the vertical axis of the graph was drawn from point γ on the DSC curve, which corresponds to the valley between the main endothermic peak A and high-temperature peak B, and the point at which this line intersected with the line (α-β) was designated as δ. The area enclosed by the curve of the high-temperature peak B portion of the DSC curve, the line segment (δ-β), and the line segment (γ-δ) 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 of the obtained values ​​was designated as the heat of fusion of the high-temperature peak of the expanded beads.

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

[0119] <Compression stress of expanded bead moldings at 50% strain> 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 moldings obtained in the Examples and Comparative Examples, and the 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 moldings. The test pieces were taken from moldings 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 a biomass-derived monomer component. Specifically, PP-A1 is "HP456J" manufactured by Lyondellbasell, and PP-A2 is "HP640J" manufactured by Lyondellbasell.

[0121]

[0122] [Production of Expanded Polypropylene Resin Beads and Expanded Bead Moldings] Examples 1 to 4 and Comparative Examples 1 and 2 <Production of Expanded Polypropylene Resin Beads> A production apparatus was prepared, equipped with an extruder with an inner diameter of 50 mm and a strand-forming die attached downstream of the extruder. The 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 into the extruder in the blending ratios shown in Table 2, and melt-kneaded to obtain a resin melt. The resin melt was introduced into the strand-forming die to extrude strands. The extruded strands were water-cooled and cut with a pelletizer to obtain polypropylene resin particles with an average mass of 1.0 mg per particle. Furnace black (DBP oil absorption: 100 mL / 100 g, BET specific surface area: 80 m) was used as the carbon black. ２ / g, average particle size 20 nm) was used.

[0123] One kilogram of the resulting polypropylene-based resin particles was placed in a 5-liter pressurized pressure vessel along with 3 liters 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 of a surfactant (trade name: NEOGEN, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., sodium dodecylbenzenesulfonate) (as an active ingredient) were added to the pressure vessel per 100 parts by mass of the resin particles. The pressure vessel was then heated at a rate of 2°C / min with stirring to the foaming temperature shown in Table 2. Carbon dioxide was then injected as a foaming agent into the pressure vessel, and the pressure was increased to the foaming pressure shown in Table 2. The temperature and pressure were maintained for 15 minutes. This resulted in a DSC curve of the resulting expanded beads being adjusted so that a high-temperature peak appeared. The contents of the pressure vessel (resin particles and water) were then released under atmospheric pressure to obtain expanded polypropylene-based resin beads. The resulting expanded polypropylene-based resin beads were subjected to the above-described measurements and evaluations. The results are shown in Table 2.

[0124] <Production of Expanded Bead Molded Articles> The expanded polypropylene resin beads were filled into a mold having a molding cavity capable of molding a rectangular parallelepiped expanded bead molded article measuring 250 mm long, 200 mm wide, and 20 mm high, and heated using the following heating method. A metal mold was used as the mold. The heating method was as follows: Steam was supplied to the mold for 5 seconds with drain valves on both sides open, to perform preheating (exhaust step). 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, to perform 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, to perform second one-way heating. Then, steam was supplied from both sides of the mold until the molding pressure shown in Table 2 was reached, to perform main heating. After the completion of this heating, the pressure was released and water cooling was promptly initiated. Water cooling was continued until the surface pressure due to the expansion force of the expanded bead molding (specifically, the value of a pressure gauge attached to the inner surface of the mold) reached 0.04 MPa (G). After water cooling, 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. A shorter water cooling time indicates a shorter molding cycle and therefore superior productivity of the expanded bead molding. After demolding, the molded product was left to cure in an 80°C atmosphere for 12 hours. The expanded bead molding thus obtained was subjected to the above-described measurements and evaluations. The results are shown in Table 2. The amount of cracking during molding (specifically, the ratio of the mold opening length to the internal height dimension) was 20% (i.e., 4 mm). Furthermore, no pre-pressurization was performed when filling the mold with the expanded beads, in which air is impregnated into the expanded beads to increase the internal pressure within the bubbles of the expanded beads (internal particle pressure).

[0125] The molding pressures shown in Table 2 were determined by allowing the expanded bead moldings to stand for 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm after curing, and then evaluating the fusion properties, secondary foaming properties, and recovery properties described below. The molding pressures shown in Table 2 were the lowest among those capable of molding expanded bead moldings that were "pass" in all evaluations. The lower the molding pressure, the better the moldability of the expanded beads can be determined to be.

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

[0127] (Secondary Expandability) The surface of the expanded bead molding was visually observed and the secondary expandability was evaluated as follows: Pass: The gaps between the expanded beads on the surface of the expanded bead molding were sufficiently filled. Fail: The gaps between the expanded beads on the surface of the expanded bead molding were clearly not filled.

[0128] (Recovery) The thickness of the expanded bead molding was measured at positions 10 mm away from each of the four corners in a plan view of the expanded bead molding 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 of the expanded bead molding 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 more. Fail: The ratio was less than 90%.

[0129] Reference Example 1 <Production of Expanded Polypropylene-Based Resin Beads> Expanded polypropylene-based resin beads (expanded beads whose base resin is composed solely 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, and all base resins were fossil fuel-derived polypropylene-based resins, and the foaming temperature was 151.8°C. The average cell diameter D of the resulting 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. Expanded bead moldings were obtained using the expanded beads of Reference Example 1 in the same manner as in the above Examples and Comparative Examples. The molding pressure was 0.22 MPa (G), and the water cooling time was 36 seconds.

[0130] Reference Example 2 <Production of Expanded Polypropylene-Based Resin Beads> Expanded polypropylene-based resin beads (expanded beads whose base resin is composed solely 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 the biomass-derived polypropylene-based resin was not blended, and all of the base resins were fossil fuel-derived polypropylene-based resins, and the foaming temperature was 152.0°C. The average cell diameter D of the resulting 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. Expanded bead moldings were obtained using the expanded beads of Reference Example 2 in a mold in the same manner as in the above Examples and Comparative Examples. The molding pressure was 0.24 MPa (G), and the water cooling time was 33 seconds.

[0131]

[0132] The results shown in Table 2 indicate that the expanded beads of Examples 1 to 4 have good moldability and a short water cooling time during molding. Thus, it can be seen that the expanded polypropylene resin beads obtained by the manufacturing 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 beads contain carbon black and are black in 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 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 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 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 incorporating a biomass-derived 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, while the expanded beads of Reference Example 2 incorporate a predetermined amount of carbon black. The MFR of the resin beads in both examples is generally equivalent to that of the Examples. The expanded beads of Reference Example 2 incorporate a predetermined amount of carbon black, but the average cell diameter Dc of the cells located at the center of the expanded beads is generally 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 is also generally equivalent. As a result, there was little effect in shortening the water cooling time during molding. This indicates that the effects of the present invention, which are found to suppress the coalescence (coarsening) of centrally located bubbles by the incorporation of carbon black and the effect of shortening the water cooling time, are specific to cases where the polypropylene resin contains a biomass-derived polypropylene resin (specifically, when the biomass content of the resin particles or expanded particles is a predetermined level or higher). Furthermore, the increase in molding pressure due to the incorporation of carbon black was suppressed in the expanded particles compared to cases where the expanded particles were derived from non-biomass raw materials (i.e., fossil fuel-derived).

Claims

1. A method for producing expanded polypropylene resin beads, which comprises expanding resin particles having a polypropylene resin as the base resin to obtain expanded beads, wherein the resin particles have a biomass content of 1% or more as measured by 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 under a load of 2.16 kg.

2. A method for producing expanded polypropylene resin particles 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 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. A method for producing expanded polypropylene resin particles according to claim 1 or 2, wherein the base resin of the resin particles is a mixed resin of biomass-derived polypropylene resin and 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. A method for producing expanded polypropylene resin beads according to claim 1 or 2, wherein the expanded beads have a crystalline structure in which 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, 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 beads according to claim 1 or 2, wherein the method is as follows:

6. A method for producing expanded polypropylene resin beads according to claim 1 or 2, 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 having a polypropylene-based resin as a base resin, wherein the expanded beads have a biomass ratio of 1% or more as measured by ASTM D6866-21, the expanded beads contain 0.1% to 8% by mass of carbon black, the expanded beads have a closed cell rate of 85% or more, the expanded beads have an average cell diameter D of 50 μm to 250 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 is 0.7 to 1.

3.

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.

Images