A method for producing expanded beads and a method for producing a foamed bead molded article.

Abstract

To provide: a production method of foamed particles having excellent in-mold moldability; and the foamed particles; as well as a foamed particle molded body formed by in-mold molding of the foamed particles.SOLUTION: In a production method of foamed particles, in which a melting point of the polypropylene-based resin is 140°C or higher, difference between the melting point of the polypropylene-based resin TmPP and a glass transition temperature of the polystyrene-based resin TgPS, TmPP-TgPS, is 35-60°C, a melt flow rate of the polypropylene-based resin is 4-10 g / 10 min, difference between the melt flow rate of the polypropylene-based resin MFRPP and a melt flow rate of the polystyrene-based resin MFRPS, MFRPP-MFRPS, is (-3)-3 g / 10 min, a mass ratio of the polypropylene-based resin to the polystyrene-based resin is 60:40-90:10, and MFRRP of the resin particles is 5-10 g / 10 min.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing expanded beads and a method for producing a foamed bead molded article. [Background technology]

[0002] Polypropylene foamed bead molded articles obtained by molding expanded polypropylene resin beads in a mold have excellent chemical resistance, impact resistance, compression strain recovery, etc., and are therefore used in a variety of applications, including as shock absorbers, heat insulating materials, and various packaging materials, as well as for food transport containers, packaging or cushioning materials for electrical and electronic components, vehicle components such as automobile bumpers, building components such as residential insulation, and miscellaneous goods.

[0003] The polypropylene resin foamed bead molded article is produced, for example, by an in-mold molding method in which expanded polypropylene resin beads are filled into a mold and heated with a heating medium such as steam to cause secondary expansion of the expanded beads and to fuse the expanded beads together, thereby molding the expanded beads into a desired shape.

[0004] In order to improve the moldability of expanded polypropylene resin beads in a mold, for example, Patent Document 1 discloses expanded beads obtained by expanding a polypropylene resin composition comprising a polypropylene resin, a polystyrene resin, and a compatibilizer. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 6-345914 Summary of the Invention [Problem to be solved by the invention]

[0006] When producing an expanded bead molding by in-mold molding of expanded polypropylene resin beads, in order to improve the in-mold moldability of the expanded beads, a pretreatment such as pressurizing the expanded beads with air or the like to increase the pressure inside the cells of the expanded beads may be performed. On the other hand, from the viewpoint of improving the productivity of expanded bead moldings, it is desirable that the expanded beads have good in-mold moldability even without such pretreatment. However, the expanded beads obtained by expanding the composition described in Patent Document 1 do not have sufficiently improved in-mold moldability, and in particular, when the bulk density of the expanded beads is reduced, it is difficult to obtain expanded beads having good in-mold moldability even without pretreatment.

[0007] The present invention has been made in view of the above background, and aims to provide a method for producing expanded beads that have excellent in-mold moldability even without subjecting the expanded beads to pressure treatment using air or the like, and a method for producing an expanded bead molding by in-mold molding the expanded beads produced by the method. [Means for solving the problem]

[0008] The present inventors have found that the above problems can be solved by employing the following configuration, and have thus completed the present invention. That is, the present invention is as follows. <1> The resin particles are made of a base resin that is a mixture of polypropylene resin and polystyrene resin, and are expanded to a bulk density of 10 kg / m 3 More than 200kg / m 3 A method for producing expanded beads, comprising the steps of: PP is 140°C or higher, and the melting point Tm PP and the glass transition temperature Tg of the polystyrene resin PS Difference from Tm PP -Tg PS The melt flow rate MFR of the polypropylene resin measured under the conditions of a temperature of 230°C and a load of 2.16 kg is 35°C or higher and 60°C or lower. PPis 4 g / 10 min or more and 10 g / 10 min or less, and the melt flow rate MFR PP and the melt flow rate MFR of the polystyrene resin measured under the conditions of a temperature of 230°C and a load of 2.16 kg. PS Difference between MFR PP -MFR PS a mass ratio of the polypropylene-based resin to the polystyrene-based resin of 60:40 to 90:10; and a melt flow rate MFR of the resin particles measured under conditions of a temperature of 230°C and a load of 2.16 kg. RP A method for producing expanded beads, wherein the viscosity is 5 g / 10 min or more and 10 g / 10 min or less. <2> The melting point Tm of the polypropylene resin PP and the glass transition temperature Tg of the polystyrene resin PS Difference from Tm PP -Tg PS is between 45°C and 60°C, <1> A method for producing the expanded beads described above. <3> The resin particles have a resin layer on the surface of the particles, the resin layer having a polypropylene-based resin (S) as a base resin, and the polypropylene-based resin (S) has a melting point Tm S and the glass transition temperature Tg of the polystyrene resin PS Difference from Tm S -Tg PS is between 15°C and 45°C, <1> or <2> A method for producing the expanded beads described above. <4> The resin particles have an average aspect ratio L / D of 1.4 or more and 5.0 or less. <1> ~ <3> 10. A method for producing expanded beads according to any one of the above. <5> The resin particles contain a brominated flame retardant. <1> ~ <4> 10. A method for producing expanded beads according to any one of the above. <6> The bulk density of the expanded particles is 15 kg / m 3 More than 35kg / m 3 Below is the <1> ~ <5> 10. A method for producing expanded beads according to any one of the above. <7> <1> ~ <6> 1. A method for producing a foamed bead molding, comprising molding the foamed beads produced by the method for producing foamed beads according to any one of the above items 1 to 5 in a mold. [Effects of the Invention]

[0009] According to the present invention, there are provided a method for producing expanded beads that have excellent in-mold moldability even without subjecting the expanded beads to pressure treatment using air or the like, and a method for producing an expanded bead molding by in-mold molding the expanded beads produced by the method. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 10 is a diagram for explaining how to determine the high-temperature peak of expanded beads. [Figure 2] 1 is a cross-sectional photograph of the resin particles of Example 1 taken along the extrusion direction. DETAILED DESCRIPTION OF THE INVENTION

[0011] [Method of manufacturing foam beads] The method for producing expanded beads of the present invention (hereinafter also referred to simply as the method for producing expanded beads or the method for producing the present invention) is to expand resin beads having a base resin that is a mixed resin obtained by kneading a polypropylene-based resin and a polystyrene-based resin, to produce expanded beads having a bulk density of 10 kg / m 3 More than 200kg / m 3 A method for producing expanded beads, comprising the steps of: PP is 140°C or higher, and the melting point Tm PP and the glass transition temperature Tg of the polystyrene resin PS Difference from Tm PP -Tg PS The melt flow rate MFR of the polypropylene resin measured under the conditions of a temperature of 230°C and a load of 2.16 kg is 35°C or higher and 60°C or lower. PP is 4 g / 10 min or more and 10 g / 10 min or less, and the melt flow rate MFR PP and the melt flow rate MFR of the polystyrene resin measured under the conditions of a temperature of 230°C and a load of 2.16 kg. PS Difference between MFR PP -MFR PSa mass ratio of the polypropylene-based resin to the polystyrene-based resin of 60:40 to 90:10; and a melt flow rate MFR of the resin particles measured under conditions of a temperature of 230°C and a load of 2.16 kg. RP is 5g / 10min or more and 10g / 10min or less.

[0012] In the method for producing expanded beads of the present invention, the melting point Tm of the polypropylene-based resin is PP is a specific value or more, and the melting point Tm of the polypropylene resin PP and the glass transition temperature Tg of polystyrene resin PS Difference from Tm PP -Tg PS is in a specific range, and the melt flow rate MFR of polypropylene resin measured under the conditions of a temperature of 230°C and a load of 2.16 kg PP is in a specific range, and the melt flow rate MFR of the polypropylene resin PP and the melt flow rate MFR of the polystyrene resin measured under the conditions of a temperature of 230°C and a load of 2.16 kg. PS Difference between MFR PP -MFR PS is in a specific range, the mass ratio of the polypropylene resin to the polystyrene resin is in a specific range, and the melt flow rate MFR of the resin particles measured under the conditions of a temperature of 230°C and a load of 2.16 kg is RP This makes it possible to obtain expanded beads having excellent moldability in a mold.

[0013] The method for producing expanded beads of the present invention preferably includes at least the following steps (A) to (C). Step (A): A step of kneading a polypropylene-based resin and a polystyrene-based resin to obtain resin particles having a mixed resin of a polypropylene-based resin and a polystyrene-based resin as a base resin; Step (B): A step of impregnating resin particles with a foaming agent to obtain expandable resin particles; and Step (C): A step of expanding the expandable resin particles to obtain expanded particles.

[0014] (Polypropylene resin) The polypropylene resin refers to a polymer containing 50% by mass or more of structural units derived from propylene. Examples of the polypropylene resin include a propylene homopolymer, a propylene copolymer, or a mixture thereof, and the propylene copolymer is preferably used. Examples of the propylene copolymer include a copolymer of propylene and at least one selected from the group consisting of ethylene and an α-olefin having 4 to 20 carbon atoms, such as 1-butene, 1-pentene, 1-hexene, 1-octene, and 4-methyl-1-butene. The copolymer is preferably a copolymer of propylene and ethylene and / or a copolymer of propylene, ethylene, and butene, and more preferably a copolymer of propylene and ethylene.

[0015] <Melting point Tm PP ≫ Melting point Tm of polypropylene resin PP The melting point Tm of polypropylene resin is 140°C or higher. PP If the melting point Tm of the polypropylene resin is too low, when the resulting expanded beads are molded in a mold, the expanded beads molded article immediately after molding is likely to shrink and deform, which may result in a decrease in moldability of the expanded beads. PP When the melting point Tm of the polypropylene resin is equal to or greater than the above value, excessive deformation of the expanded bead molding immediately after molding can be suppressed, and expanded beads having excellent moldability in the mold can be obtained, and a molding having a small dimensional change rate relative to the mold can be easily obtained. PP is preferably 142°C or higher, more preferably 145°C or higher, even more preferably 148°C or higher, and particularly preferably 150°C or higher. In addition, the melting point Tm of polypropylene resin PP The temperature is preferably 165°C or lower, more preferably 160°C or lower, and even more preferably 155°C or lower, from the viewpoint of easily improving the moldability of the expanded beads in a mold under low molding pressure conditions. Melting point Tm of polypropylene resin PPThe DSC curve (DSC curve at the second heating) is obtained by heating the test piece from 30°C to 200°C at a heating rate of 10°C / min. After reaching 200°C, the temperature is lowered from 200°C to 30°C at a rate of 10°C / min. Then, the test piece is heated again from 30°C to 200°C at a rate of 10°C / min. The DSC curve at the second heating is obtained by heating the test piece for a second time from 30°C to 200°C at a rate of 10°C / min. The apex temperature of the melting peak accompanying the melting of the resin on the DSC curve at the second heating is the melting point Tm of the polypropylene resin. PP If multiple melting peaks appear on the DSC curve, the apex temperature of the melting peak with the largest area is taken as the melting point Tm of the polypropylene resin. PP The melting peak with the largest area can be determined by comparing the areas (heat of fusion) of each melting peak, distinguishing the melting peaks using the temperature of the valley of the DSC curve located between the peak temperatures of each melting peak. The valley temperature of the DSC curve can be determined by looking at the temperature at which the value on the vertical axis of the DSC differential curve (DDSC) becomes 0.

[0016] <Melt flow rate MFR PP ≫ MFR of polypropylene resin measured at a temperature of 230°C and a load of 2.16 kg PP MFR is 4g / 10min or more. PP If the MFR is too low, the secondary foaming property of the expanded beads tends to decrease, and the in-mold moldability of the expanded beads tends to decrease. PP is preferably 5 g / 10 min or more, more preferably 6 g / 10 min or more. In addition, the MFR of polypropylene resin PP The MFR is 10g / 10min or less. ppIf the flow rate is too high, the expanded bead molding tends to shrink excessively after molding in the mold, and the moldability in the mold tends to decrease. From the viewpoint of easily and stably suppressing excessive shrinkage immediately after molding of the expanded bead molding, the flow rate is preferably 9 g / 10 min or less, more preferably 8 g / 10 min or less. MFR of polypropylene resin PP is measured in accordance with JIS K 7210-1:2014 at a temperature of 230°C and a load of 2.16 kg.

[0017] ≪Flexural modulus F PP ≫ Flexural modulus F of polypropylene resin PP is preferably 800 MPa or more, more preferably 1000 MPa or more, still more preferably 1100 MPa or more, particularly preferably 1200 MPa or more, from the viewpoint of further suppressing shrinkage of the expanded bead molding after in-mold molding and improving the in-mold moldability of the expanded beads. PP is preferably 1600 MPa or less, more preferably 1500 MPa or less, and even more preferably 1400 MPa or less. Flexural modulus F of polypropylene resin PP can be calculated based on JIS K 7171:2016.

[0018] <Total heat of fusion> The total heat of fusion of the polypropylene resin is preferably 60 J / g or more, more preferably 70 J / g or more, and even more preferably 80 J / g or more from the viewpoint of improving the mechanical properties of the resulting molded article. Also, the total heat of fusion of the polypropylene resin is preferably 120 J / g or less, more preferably 110 J / g or less, and even more preferably 100 J / g or less from the viewpoint of improving the in-mold moldability of the expanded beads. The total heat of fusion of a polypropylene-based resin can be determined from a DSC curve obtained by subjecting the polypropylene-based resin to differential scanning calorimetry (DSC) in accordance with JIS K 7122:2012. Specifically, a DSC curve of the polypropylene-based resin at the second heating is first obtained in the same manner as in the melting point measurement described above. The point at 80°C on the obtained DSC curve at the second heating is designated as α, and the point on the DSC curve corresponding to the melting end temperature is designated as β. The area enclosed by the DSC curve in the section between points α and β and the line segment (α-β) is measured, and this is defined as the total heat of fusion of the polypropylene-based resin.

[0019] (Polystyrene resin) The polystyrene resin refers to a thermoplastic resin containing 50% by mass or more of structural units derived from styrene. Examples of polystyrene resins include polystyrene (general-purpose polystyrene: GPPS), styrene-methacrylic acid copolymer, styrene-methacrylic acid-methyl methacrylate copolymer, styrene-acrylic acid copolymer, styrene-maleic anhydride copolymer, styrene-methyl methacrylate copolymer, styrene-butyl acrylate copolymer, styrene-acrylonitrile copolymer, high impact polystyrene (HIPS), and mixtures thereof. Among these, polystyrene is preferably used. Furthermore, as the polystyrene-based resin, recycled raw materials derived from polystyrene-based resin products, obtained by recycling polystyrene-based resin products, etc., can also be used. In the present invention, even when a polystyrene-based resin containing recycled raw materials derived from polystyrene-based resin products is used, expanded beads having excellent in-mold moldability can be stably obtained. Examples of recycled raw materials derived from polystyrene-based resin products that can be used include recycled raw materials from expanded polystyrene-based resin bead moldings used in fish boxes, etc., and recycled raw materials from extruded polystyrene-based resin foams such as foam sheets used in food containers, etc., and their scraps. The recycled raw materials are produced, for example, by feeding crushed foam or the like into an extruder, melt-kneading the foam to form a molten resin, and then extruding the molten resin from the extruder and pelletizing it into a predetermined shape.

[0020] <Glass transition temperature Tg PS ≫ Glass transition temperature Tg of polystyrene resin PS is the melting point Tm of the polypropylene resin described below. PP Difference from Tm PP -Tg PS is not particularly limited as long as it is within a specific range, but from the viewpoint of easily suppressing shrinkage of the expanded bead molding immediately after molding, it is preferably 80°C or higher, more preferably 90°C or higher, even more preferably 95°C or higher, and even more preferably 98°C or higher. PS From the viewpoint of easily improving the moldability of the expanded beads in a mold, the temperature is preferably 130°C or lower, more preferably 120°C or lower, even more preferably 110°C or lower, and still more preferably 105°C or lower. Glass transition temperature Tg of polystyrene resin PS can be determined as the midpoint glass transition temperature of the DSC curve obtained by heat flux differential scanning calorimetry according to JIS K 7121: 2012. In measuring the glass transition temperature, the test specimen is conditioned in accordance with "Measurement of the glass transition temperature after a certain heat treatment" described in "3. Conditioning of test specimen (3)" of JIS K 7121: 2012, and the glass transition temperature can be measured.

[0021] ≪Difference Tm PP -Tg PS ≫ Melting point Tm of polypropylene resin PP and the glass transition temperature Tg of polystyrene resin PS Difference from TmPP -Tg PS The difference Tm is 35°C or higher. PP -Tg PS If the difference Tm is too small, the secondary foaming property of the expanded beads tends to decrease, and the moldability of the expanded beads in a mold tends to decrease. The reason for this is not clear, but it is thought that the polystyrene resin follows the elongation of the polypropylene resin due to secondary foaming, making it difficult to deform. From the viewpoint of being able to stably obtain expanded beads that are excellent in secondary foaming property and moldability in a mold, and from the viewpoint of being able to easily obtain a molded product with a small dimensional change rate relative to the mold, it is PP -Tg PS is preferably 38°C or higher, more preferably 40°C or higher, even more preferably 42°C or higher, and particularly preferably 45°C or higher. PP -Tg PS The difference Tm is 60°C or less. PP -Tg PS If the difference Tm is too large, the expanded bead molding tends to shrink excessively immediately after molding, and the in-mold moldability of the expanded beads tends to decrease. The reason for this is unclear, but it is thought that the mixed resin is difficult to solidify when the expanded beads filled in the molding die are cooled after heating, and the time until the mixed resin solidifies becomes longer. From the viewpoint of suppressing excessive shrinkage immediately after molding of the expanded bead molding and stably obtaining expanded beads with excellent in-mold moldability, the difference Tm PP -Tg PS is preferably 58°C or less, more preferably 55°C or less.

[0022] <Melt flow rate MFR PS ≫ Melt flow rate (MFR) of polystyrene resin measured at 230°C and a load of 2.16 kg PS is the melt flow rate MFR of polypropylene resin, which will be described later. PP Difference between MFR PP -MFR PSis not particularly limited as long as it is within a specific range, but from the viewpoint of easily increasing the secondary expandability of the resulting expanded beads, it is preferably 3 g / 10 min or more, more preferably 4 g / 10 min or more, even more preferably 5 g / 10 min or more, and still more preferably 6 g / 10 min or more. PS is preferably 10 g / 10 min or less, more preferably 9 g / 10 min or less, and even more preferably 8 g / 10 min or less, from the viewpoint of easily suppressing shrinkage immediately after molding of an expanded bead molding when the resulting expanded bead molding is molded in a mold. MFR of polystyrene resin PS is measured in accordance with JIS K 7210-1:2014 at a temperature of 230°C and a load of 2.16 kg.

[0023] <Difference MFR PP -MFR PS ≫ Melt flow rate (MFR) of polypropylene resin PP and the melt flow rate (MFR) of polystyrene resins PS Difference between MFR PP -MFR PS is between -3g / 10min and 3g / 10min. PP -MFR PS If the difference MFR is too large, when the polypropylene resin and the polystyrene resin are melt-kneaded to obtain resin particles, the two may not be mixed sufficiently, and when the resin particles are expanded, it may be difficult to obtain expanded beads with a high closed cell ratio. Furthermore, when the obtained expanded beads are molded in a mold, the fusion between the expanded beads may decrease and the expanded bead molding may easily shrink after molding in a mold, which may reduce the moldability of the expanded beads in a mold. From the viewpoint of being able to stably obtain expanded beads with excellent moldability in a mold, the difference MFR PP -MFR PS is preferably −2.0 g / 10 min or more and 2.0 g / 10 min or less, and more preferably −1.5 g / 10 min or more and 1.5 g / 10 min or less.

[0024] ≪Weight average molecular weight Mw≫ The weight-average molecular weight Mw of the polystyrene resin is preferably 180,000 or more, more preferably 200,000 or more, from the viewpoint of stably improving the moldability of the expanded beads in a mold. From the same viewpoint, the weight-average molecular weight Mw of the polystyrene resin is preferably 350,000 or less, more preferably 320,000 or less, even more preferably 300,000 or less, and still more preferably 280,000 or less. The average molecular weights (Mw, Mn) of the polystyrene resin in the present invention are polystyrene-equivalent average molecular weights (relative average molecular weights) measured by gel permeation chromatography (GPC) using polystyrene as the standard substance.

[0025] <Ratio of weight average molecular weight Mw to number average molecular weight Mn (Mw / Mn)> The ratio Mw / Mn of the weight-average molecular weight Mw of the polystyrene resin to the number-average molecular weight Mn of the polystyrene resin is preferably 2.0 or more, more preferably 2.2 or more, from the viewpoints of improving the dispersibility of the polystyrene resin in the polypropylene resin during resin particle production, enabling the production of resin particles in which the polypropylene resin and the polystyrene resin are well mixed, and stably improving the in-mold moldability of the expanded beads obtained by expanding the resin particles. Also, from the same viewpoint, the ratio Mw / Mn is preferably 4.0 or less, more preferably 3.5 or less, even more preferably 3.2 or less, and even more preferably 3.0 or less.

[0026] ≪Flexural modulus F PS ≫ Flexural modulus of polystyrene resin F PS is the difference in flexural modulus F PP -F PS is not particularly limited as long as it can be set within a specific range, but from the viewpoint of easily improving the moldability of the expanded beads in a mold, it is preferably 2500 MPa or more, more preferably 2800 MPa or more, and even more preferably 3000 MPa or more. PS is preferably 4000 MPa or less, more preferably 3700 MPa or less, and even more preferably 3500 MPa or less. The flexural modulus of the polystyrene resin can be determined based on JIS K 7171:2016.

[0027] <Difference in flexural modulus F PS -F PP ≫ Flexural modulus of polystyrene resin F PS and the flexural modulus F of polypropylene resin PP Difference from F PS -F PP is preferably 1600 MPa or more, more preferably 1700 MPa or more, and even more preferably 1800 MPa or more, from the viewpoint of easily suppressing excessive shrinkage of the expanded bead molding immediately after molding. PS -F PP is preferably 2400 MPa or less, more preferably 2100 MPa or less, and even more preferably 2000 MPa or less, from the viewpoint of easily increasing the secondary expandability of the expanded beads and easily obtaining a molded product with a small dimensional change rate relative to the mold.

[0028] <Step (A): Production of Resin Particles Using Mixed Resin as Base Resin> The mixed resin is obtained by kneading the polypropylene-based resin and the polystyrene-based resin, and can be obtained, for example, by melt-kneading the polypropylene-based resin and the polystyrene-based resin in an extruder. Furthermore, resin particles having a mixed resin as a base resin can be obtained, for example, by kneading a polypropylene-based resin and a polystyrene-based resin in an extruder to form a mixed resin melt of the polypropylene-based resin and the polystyrene-based resin, extruding the mixed resin melt from the extruder, and pelletizing the mixed resin melt to a predetermined shape and mass. Alternatively, a mixture of the polypropylene-based resin and the polystyrene-based resin may be fed to the extruder to form the mixed resin melt. The mixed resin exhibits a morphology in which a dispersed phase composed of a polystyrene-based resin is dispersed in a continuous phase formed by a polypropylene-based resin, and it is preferable that the dispersed phase composed of the polystyrene-based resin is dispersed in streaks in the cross section of the resin particle along the extrusion direction of the resin particle. As a pelletizing method for producing resin particles, a strand cut method in which a mixed resin melt is extruded in the form of a strand from a small hole in a die attached downstream of an extrusion device, cooled in water, and then cut, an underwater cut method in which a mixed resin melt is extruded into water and cut, or a hot cut method in which a mixed resin melt is extruded into air and then cut immediately thereafter can be used.

[0029] (additives) The mixed resin may contain additives such as a cell regulator, a flame retardant, a flame retardant assistant, a cell nucleating agent, an antistatic agent, an antioxidant, an ultraviolet inhibitor, a light stabilizer, a conductive material, and a colorant, as needed.

[0030] As the flame retardant, a brominated flame retardant is preferably used, and the resin particles preferably contain a brominated flame retardant. Molded articles obtained by in-mold molding of expanded beads using the above-mentioned mixed resin as a base resin tend to be highly flammable, and this tendency becomes particularly pronounced when attempting to obtain a molded article with a high expansion ratio. Furthermore, adding a large amount of flame retardant to enhance the flame retardancy of the molded article may reduce the moldability of the expanded beads. On the other hand, the use of a brominated flame retardant can stably impart flame retardancy to the molded article without significantly impairing the in-mold formation of the expanded beads. Examples of brominated flame retardants that can be used include brominated bisphenol flame retardants and brominated styrene-butadiene copolymers. Examples of brominated bisphenol flame retardants include bromides having a bisphenol A skeleton, bromides having a bisphenol F skeleton, and bromides having a bisphenol S skeleton. More specifically, brominated bisphenol A flame retardants such as 2,2-bis(4-(2,3-dibromo-2-methylpropoxy)-3,5-dibromophenyl)propane and 2,2-bis(4-(2,3-dibromopropoxy)-3,5-dibromophenyl)propane, and brominated bisphenol S flame retardants such as bis[3,5-dibromo-4-(2,3-dibromopropoxy)phenyl]sulfone can be used as the brominated bisphenol flame retardant. Furthermore, an example of a brominated styrene-butadiene copolymer is a brominated styrene-butadiene block copolymer. The brominated flame retardant preferably contains a brominated styrene-butadiene copolymer and / or a brominated bisphenol flame retardant as the main component, and more preferably contains a brominated styrene-butadiene block copolymer as the main component. The term "main component" means that the proportion of the brominated flame retardant in the flame retardant is 50% by mass or more, preferably 60% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more. The resin particles preferably contain 0.1 to 3 parts by mass, and more preferably 0.5 to 2 parts by mass, of a brominated flame retardant per 100 parts by mass of the mixed resin, which allows the moldability of the expanded beads to be maintained while stably imparting flame retardancy to the resulting molded article.

[0031] Carbon black is preferably used as the colorant. Examples of carbon black include channel black, roller black, furnace black, thermal black, and acetylene black. Among these, furnace black is preferred from the viewpoint of dispersibility in the mixed resin. From the viewpoint of imparting a good black color to the expanded bead molding while maintaining the in-mold moldability of the expanded beads, the content of carbon black in the mixed resin is preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more. From the same viewpoint, the content of carbon black in the mixed resin is preferably 5% by mass or less, more preferably 4% by mass or less, and even more preferably 3% by mass or less.

[0032] (Other polymers) The mixed resin may contain polymers other than the polypropylene-based resin and the polystyrene-based resin, as long as the intended effects of the present invention are not impaired. In this case, the content of the other polymer in the mixed resin is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, and even more preferably 10 parts by mass or less, per 100 parts by weight of the total of the polypropylene-based resin and the polystyrene-based resin. Furthermore, in order to stably disperse the polystyrene resin in the polypropylene resin, the mixed resin may contain a compatibilizer as another polymer. Examples of the compatibilizer include styrene elastomers, specifically styrene-butadiene-styrene block copolymers (SBS), styrene-isoprene-styrene copolymers (SIS), and their hydrogenated products, such as styrene-ethylene-butylene-styrene copolymers (SEBS). Of these, styrene-ethylene-butylene-styrene copolymers (SEBS) are preferably used as the compatibilizer. The proportion of components derived from styrene in the styrene-based elastomer is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 65% ​​by mass or more. The proportion of components derived from styrene in the styrene-based elastomer is preferably 80% by mass or less, and more preferably 75% by mass or less. The melt flow rate of the styrene elastomer, measured under conditions of a temperature of 230°C and a load of 2.16 kg, is preferably 0.5 g / 10 min or more and 5 g / 10 min or less, and more preferably 1 g / 10 min or more and 3 g / min or less. From the viewpoint of improving the moldability of the expanded beads in the mold, the content of the compatibilizer in the mixed resin is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 8 parts by mass or less, per 100 parts by mass of the polypropylene resin and the polystyrene resin in total. Note that the expanded beads do not need to contain a compatibilizer, but when a compatibilizer is used, from the viewpoint of stably dispersing the polystyrene resin in the polypropylene resin, the content of the compatibilizer in the expanded beads is preferably 1 part by mass or more, more preferably 2 parts by mass or more, per 100 parts by mass of the polypropylene resin and the polystyrene resin in total.

[0033] (mass ratio of polypropylene resin to polystyrene resin) The mass ratio of polypropylene resin to polystyrene resin (polystyrene resin:polystyrene resin) is 60:40 to 90:10. If the proportion of polystyrene resin in the mixed resin is too low, the expanded bead molding tends to shrink excessively after in-mold molding, and the in-mold moldability of the expanded beads tends to decrease. On the other hand, if the proportion of polystyrene resin in the mixed resin is too high, the fusion strength between the expanded beads decreases, making it difficult to obtain an expanded bead molding with good fusion. From the viewpoint of being able to stably obtain expanded beads that have good fusion between the expanded beads and that have excellent in-mold moldability, in which shrinkage of the expanded bead molding after in-mold molding is suppressed, the mass ratio of the polypropylene-based resin to the polystyrene-based resin is preferably 60:40 to 85:15, and more preferably 60:40 to 80:20.

[0034] (Melt Flow Rate MFR RP ) Melt flow rate (MFR) of resin particles measured at a temperature of 230°C and a load of 2.16 kg RP The melt flow rate (MFR) of resin particles is 5g / 10min or more. RP If the melt flow rate MFR of the resin particles is too low, the fusion strength between the expanded beads tends to decrease, and the in-mold moldability of the expanded beads tends to decrease. RP is preferably 6 g / 10 min or more, more preferably 7 g / 10 min or more. In addition, the melt flow rate (MFR) of the resin particles RP The MFR of resin particles is 10g / 10min or less. RP If the melt flow rate MFR of the resin particles is too high, the expanded bead molding tends to shrink excessively after molding in the mold, and the in-mold moldability of the expanded beads tends to decrease. RP is preferably 9 g / 10 min or less, more preferably 8 g / 10 min or less. MFR of resin particles RP is measured using resin particles as a measurement sample in accordance with JIS K 7210-1:2014 at a temperature of 230°C and a load of 2.16 kg.

[0035] (Melting point Tm of polypropylene resin PPMR ) Melting point Tm of polypropylene resin in mixed resin PPMR From the viewpoint of suppressing shrinkage of the expanded bead molding after in-mold molding and obtaining expanded beads with excellent in-mold moldability, the melting point Tm of the polypropylene-based resin is preferably 140°C or higher, more preferably 145°C or higher, even more preferably 148°C or higher, and particularly preferably 150°C or higher. PPMRThe temperature is preferably 165°C or lower, more preferably 160°C or lower, and even more preferably 155°C or lower, from the viewpoint of easily improving the moldability of the expanded beads in a mold under low molding pressure conditions. Melting point Tm of polypropylene resin in mixed resin PPMR The DSC curve (DSC curve at the second heating) is obtained by heating the expanded beads from 30°C to 200°C at a heating rate of 10°C / min. After reaching 200°C, the expanded beads are cooled from 200°C to 30°C at a rate of 10°C / min. After the temperature reaches 200°C, the expanded beads are cooled from 200°C to 30°C at a rate of 10°C / min. Then, the expanded beads are heated for a second time from 30°C to 200°C at a rate of 10°C / min. The DSC curve at the second heating is obtained by measuring the melting point Tm of the polypropylene resin in the mixed resin. PPMR If multiple melting peaks appear on the DSC curve, the apex temperature of the melting peak with the largest area is taken as the melting point Tm of the polypropylene resin in the mixed resin. PPMR Adopted as.

[0036] (The glass transition temperature Tg of polystyrene resin PSMR ) Glass transition temperature Tg of polystyrene resin in mixed resins PSMR is the melting point Tm of the polypropylene resin described below. PP Difference from Tm PP -Tg PS However, from the viewpoint of suppressing shrinkage of the expanded bead molding immediately after molding and facilitating in-mold moldability of the expanded beads, it is preferably 80°C or higher, more preferably 90°C or higher, even more preferably 95°C or higher, and even more preferably 98°C or higher. PSMR From the viewpoint of improving the secondary expandability of the expanded beads and facilitating the in-mold moldability, the temperature is preferably 130°C or lower, more preferably 120°C or lower, even more preferably 110°C or lower, and still more preferably 105°C or lower. Glass transition temperature Tg of polystyrene resin in mixed resins PSMR The glass transition temperature can be determined as the midpoint glass transition temperature of the component derived from the polystyrene-based resin in the DSC curve obtained by heat flux differential scanning calorimetry using the expanded beads as a test piece according to JIS K 7121:2012. The glass transition temperature of the polystyrene-based resin in the mixed resin generally coincides with the glass transition temperature of the polystyrene-based resin used to form the mixed resin. When it is not easy to determine the glass transition temperature of the component derived from the polystyrene-based resin in the DSC curve, the glass transition temperature of the polystyrene-based resin may be determined by referring to the peak temperature in the differential curve of the DSC curve. In measuring the glass transition temperature, the condition of the test specimen is adjusted in accordance with "Measurement of the glass transition temperature after a certain heat treatment" described in "3. Conditioning of the test specimen (3)" of JIS K 7121:2012, and the glass transition temperature can be measured.

[0037] (Difference Tm PPMR -Tg PSMR ) Melting point Tm of polypropylene resin in mixed resin PPMR and the glass transition temperature Tg of polystyrene resin PSMR Difference from Tm PPMR -Tg PSMR From the viewpoint of improving the secondary expandability of the expanded beads and enhancing moldability in a mold, the difference Tm is preferably 35°C or higher, more preferably 37°C or higher, even more preferably 40°C or higher, still more preferably 42°C or higher, and particularly preferably 45°C or higher. PPMR -Tg PSMR From the viewpoint of suppressing excessive shrinkage immediately after molding of the expanded bead molding and improving the moldability of the expanded beads in the mold, the temperature is preferably 60°C or lower, more preferably 58°C or lower, and even more preferably 55°C or lower.

[0038] (Flexural modulus F MR ) Flexural modulus of mixed resin F MRis preferably 1000 MPa or more, more preferably 1100 MPa or more, even more preferably 1200 MPa or more, and even more preferably 1500 MPa or more, from the viewpoint of suppressing shrinkage of the expanded bead molding after in-mold molding and improving the in-mold moldability of the expanded beads. MR is preferably 2000 MPa or less, more preferably 1900 MPa or less, and even more preferably 1800 MPa or less. Flexural modulus of mixed resin F MR can be determined based on JIS K 7171:2016. The test piece can be prepared using resin particles or expanded particles. Specifically, the resin particles or expanded particles are heat-pressed to prepare a non-expanded sheet, which is then cut into a predetermined size (e.g., 80 mm length x 10 mm width x 4 mm thickness) to prepare a test piece. The flexural modulus of the mixed resin measured using a test piece made from resin particles generally coincides with the flexural modulus of the mixed resin measured using a test piece made from expanded particles.

[0039] (average aspect ratio L / D) As described above, when resin particles are obtained using an extruder, the shape of the resin particles is preferably approximately cylindrical. In this case, the average aspect ratio L / D of the resin particles is preferably 1.4 or more, more preferably 1.6 or more, even more preferably 1.8 or more, and even more preferably 2.0 or more, from the viewpoint of easily obtaining expanded beads that have good mold fillability. Resin particles composed of the above-mentioned mixed resins are more likely to shrink in the extrusion direction of the resin particles during foaming than resin particles composed of polypropylene-based resins, and the resulting expanded beads are more likely to be flattened. Therefore, the expandable beads tend to have poor mold fillability and poor in-mold moldability. However, by setting the average aspect ratio L / D of the resin particles to the above value or higher, the average aspect ratio L / D is close to 1, making it easier to obtain expanded beads that have good mold fillability. From the same viewpoint, the average aspect ratio L / D of the resin particles is preferably 5.0 or less, more preferably 4.0 or less, even more preferably 3.0 or less, and even more preferably 2.5 or less. The average aspect ratio L / D of resin particles is determined by measuring the maximum length (L) in the extrusion direction of the resin particle and the maximum cross-sectional diameter (D) of the cross section of the particle in a direction perpendicular to the longitudinal direction of the maximum length for 30 randomly selected resin particles, calculating the ratio (L / D), and then taking the arithmetic mean of these values.

[0040] (resin layer) From the viewpoint of improving the moldability of the expanded beads in a mold, the resin particles preferably have a resin layer on the surface, the resin layer having a polypropylene-based resin (S) as a base resin. In this case, the resin particles have a resin particle body having the mixed resin as a base resin and a resin layer having a polypropylene-based resin (S) as a base resin that covers the resin particle body. The resin layer may cover a portion of the particle body, or may cover the entire outer surface of the particle body. Specifically, the resin layer preferably covers 50% or more of the particle body, more preferably 70% or more, and even more preferably 80% or more. Methods for forming a resin layer on the surface of resin particles include, for example, a method of producing resin particles with a multilayer structure having a resin particle body and a resin layer by co-extrusion, and a method of coating a resin layer onto a resin particle body that has been prepared in advance. In the co-extrusion method, an extrusion device can be used that has an extruder for forming resin particle bodies, an extruder for forming resin layers, and a co-extrusion die such as a die for forming multilayer strands connected downstream of these extruders. The extruder for forming resin particle bodies is supplied with polypropylene-based resin and polystyrene-based resin for forming the mixed resin (resin particle bodies), along with optional additives, and melt-kneaded to form a mixed resin melt. The extruder for forming the resin layer is supplied with polypropylene-based resin for forming the resin layer, along with optional additives, and melt-kneaded to form a resin melt for forming the resin layer. The mixed resin melt and the resin melt for forming the resin layer are introduced into a co-extrusion die, merged, extruded from the extrusion device, and pelletized to a predetermined shape and mass, thereby obtaining multilayered resin particles with a resin layer on the surface. In addition, as a method for coating the resin particle body with a resin layer, for example, a method can be adopted in which the resin particle body and materials for constituting the resin layer are placed in a mixing device having mixing and heating functions, and then heated and mixed.

[0041] (additives) The resin layer may contain additives such as a cell regulator, a flame retardant, a flame retardant assistant, a cell nucleating agent, an antistatic agent, an antioxidant, an ultraviolet inhibitor, a light stabilizer, a conductive material, and a colorant, as needed. Carbon black is preferably used as the colorant. Examples of carbon black include channel black, roller black, furnace black, thermal black, and acetylene black. Among these, furnace black is preferred from the viewpoint of dispersibility in polypropylene-based resins. From the viewpoint of imparting a good black color to the expanded bead molding while maintaining the in-mold moldability of the expanded beads, the content of carbon black in the resin layer is preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more. From the same viewpoint, the content of carbon black in the mixed resin is preferably 5% by mass or less, more preferably 4% by mass or less, and even more preferably 3% by mass or less. As the flame retardant, a brominated flame retardant exemplified as a flame retardant that can be added to the resin particles described above can be preferably used, a brominated flame retardant containing as a main component at least one selected from the group consisting of a brominated styrene-butadiene copolymer and a brominated bisphenol flame retardant can be more preferably used, and a brominated flame retardant containing as a main component a brominated styrene-butadiene block copolymer can be even more preferably used. Furthermore, from the viewpoint of maintaining the moldability of the expanded beads in the mold while easily imparting stable flame retardancy to the resulting molded article, the amount of flame retardant in the resin layer is preferably 0.1% by mass or more and 3% by mass or less.

[0042] <Polypropylene resin (S) that makes up the resin layer> As the polypropylene-based resin (S), the polypropylene-based resins exemplified as the polypropylene-based resins used in the resin particles described above can be used. As the polypropylene-based resin (S), a propylene-based copolymer can be preferably used, and preferably at least one selected from the group consisting of a copolymer of propylene and ethylene and a copolymer of propylene, ethylene and butene can be used, and more preferably a copolymer of propylene, ethylene and butene can be used.

[0043] -Melting point Tm S - Melting point Tm of polypropylene resin (S) that constitutes the resin layer S is the difference Tm S -Tg PS In order to easily increase the mechanical strength of the resulting molded article, the melting point is preferably 100°C or higher, more preferably 110°C or higher, even more preferably 120°C or higher, and even more preferably 130°C. In addition, the melting point Tm of the polypropylene-based resin (S) is preferably 100°C or higher, more preferably 110°C or higher, even more preferably 120°C or higher, and even more preferably 130°C. S The temperature is preferably 150°C or lower, more preferably 145°C or lower, from the viewpoint of easily increasing the fusion property between the expanded beads. Melting point Tm of polypropylene resin (S) that constitutes the resin layer S is the melting point Tm of polypropylene resin PP It can be obtained by the same method as above.

[0044] -Difference Tm S -Tg PS - Melting point Tm of polypropylene resin (S) that constitutes the resin layer S and the glass transition temperature Tg of the polystyrene resin used to form the mixed resin. PS Difference from Tm S -Tg PS is preferably 15°C or higher, more preferably 18°C, and even more preferably 20°C, from the viewpoint of easily obtaining expanded beads that are excellent in mechanical strength and can stably produce expanded bead molded articles with a small dimensional change rate. S -Tg PS From the viewpoint of improving the fusion property of the expanded beads and enhancing moldability in the mold, the temperature is preferably 45°C or lower, more preferably 40°C or lower.

[0045] -Difference Tm PP -Tm S - Melting point Tm of polypropylene resin used to form the mixed resin PP and the melting point Tm of the polypropylene resin (S) that constitutes the resin layer. S Difference from Tm PP -Tm SThe difference Tm is preferably 5° C. or more from the viewpoint of increasing the fusion property of the expanded beads and increasing the moldability of the expanded beads in a mold. In addition, from the viewpoint of easily obtaining expanded beads that are excellent in mechanical strength and can stably produce expanded bead molded articles with a small dimensional change rate, PP -Tm S is preferably 35°C or lower, more preferably 30°C or lower, and even more preferably 25°C or lower.

[0046] <Mass ratio of resin particle body to resin layer> When the resin particles have a resin layer on their surface, the mass ratio of the resin particle body to the resin layer (resin particle body:resin layer) using the mixed resin as the base resin is preferably 99.5:0.5 to 90:10, more preferably 99:1 to 92:8, and even more preferably 98:2 to 94:6, from the viewpoint of achieving a better balance between the fusion properties of the expanded beads and the physical properties of the resulting molded body.

[0047] <Step (B): Production of Expandable Resin Particles> Expandable resin particles can be obtained by impregnating resin particles with a blowing agent. For example, a dispersion medium and resin particles are placed in a sealed container such as an autoclave that can be sealed and can withstand heat and pressure, the resin particles are dispersed in the dispersion medium using a stirrer or the like, and a blowing agent is added to the sealed container, and the sealed container is heated and / or pressurized as necessary, and maintained to impregnate the resin particles with the blowing agent.

[0048] (dispersion medium) The dispersion medium is not particularly limited as long as it does not dissolve the resin particles, and examples thereof include water, ethylene glycol, glycerin, and alcohols such as methanol and ethanol, with water being preferred.

[0049] (dispersant) To prevent the resin particles from adhering to each other, it is preferable to further add a dispersant to the dispersion medium. Examples of dispersants include organic dispersants such as polyvinyl alcohol, polyvinylpyrrolidone, and methyl cellulose; and sparingly soluble inorganic salts such as aluminum oxide, zinc oxide, kaolin, mica, magnesium phosphate, and tricalcium phosphate. These can be used alone or in combination of two or more. Among these, sparingly soluble inorganic salts are preferred for ease of handling, and kaolin is more preferred. When a dispersant is added, it is preferable to add about 0.001 to 5 parts by mass of the dispersant per 100 parts by mass of the resin particles.

[0050] (surfactant) A surfactant can also be added to the dispersion medium. Examples of surfactants include sodium dodecylbenzenesulfonate, sodium alkylsulfonate, sodium oleate, sodium lauryl sulfate, polyoxyethylene alkyl ether sodium phosphate, polyoxyethylene alkyl ether sodium sulfate, and other anionic surfactants and nonionic surfactants commonly used in suspension polymerization. When a surfactant is added, it is preferable to add the surfactant in an amount of about 0.001 to 1 part by mass per 100 parts by mass of resin particles.

[0051] (foaming agent) The blowing agent is not particularly limited as long as it can expand the resin particles. Examples of blowing agents include inorganic physical blowing agents such as air, nitrogen, carbon dioxide, argon, helium, oxygen, and neon; aliphatic hydrocarbons such as propane, normal butane, isobutane, normal pentane, isopentane, and normal hexane; alicyclic hydrocarbons such as cyclohexane and cyclopentane; halogenated hydrocarbons such as ethyl chloride, 2,3,3,3-tetrafluoropropene, trans-1,3,3,3-tetrafluoropropene, and trans-1-chloro-3,3,3-trifluoropropene; and organic physical blowing agents such as dialkyl ethers such as dimethyl ether, diethyl ether, and methyl ethyl ether. Among these, inorganic physical blowing agents that are environmentally friendly and inexpensive are preferred. Nitrogen, air, and carbon dioxide are more preferred, and carbon dioxide is particularly preferred. These blowing agents can be used alone or in combination of two or more.

[0052] The amount of foaming agent added is determined taking into consideration the desired bulk density of the expanded beads, the type of polypropylene resin, the type of foaming agent, etc. When an organic physical foaming agent is used, the amount is preferably 5 to 50 parts by mass per 100 parts by mass of resin particles. When an inorganic physical foaming agent is used, the amount is preferably 0.1 to 30 parts by mass, more preferably 0.5 to 15 parts by mass per 100 parts by mass of resin particles.

[0053] <Step (C): Production of Expanded Beads> Expanded beads can be obtained by expanding expandable resin beads. For example, the expandable resin beads dispersed in a dispersion medium in a sealed container can be expanded by releasing the expandable resin beads together with the dispersion medium from the sealed container into an atmosphere with a lower pressure than the pressure inside the sealed container. More specifically, while maintaining the pressure inside the sealed container at a pressure equal to or higher than the vapor pressure of the blowing agent, one end of the sealed container below the water surface is opened, and the expandable resin beads together with the dispersion medium are released from the sealed container into an atmosphere with a lower pressure than the pressure inside the sealed container, usually atmospheric pressure, to expand the expandable resin beads, thereby producing expanded beads. Note that when the resin beads have a resin layer, by expanding at least the resin bead body, a multilayered expanded bead having an expanded bead body with a mixed resin as a base resin and a resin layer covering the expanded bead body can be obtained. The expandable resin particles obtained in step (B) can also be expanded by heating them with a heating medium such as hot air or steam to produce expanded particles.

[0054] When the expandable resin particles are released from a sealed container into an atmosphere with a lower pressure than the pressure inside the sealed container to cause expansion, the temperature during expansion is preferably 110°C to 170°C, more preferably 130°C to 160°C. The pressure inside the sealed container is preferably 0.5 MPa (G) to 5 MPa (G), more preferably 1 MPa (G) to 3 MPa (G). Note that pressures marked with (G) are gauge pressures, that is, pressure values ​​based on atmospheric pressure.

[0055] [Foam particles] The expanded beads produced by the production method of the present invention preferably have the following features. Expanded beads whose base resin is a mixed resin obtained by kneading a polypropylene resin and a polystyrene resin, and the melt flow rate MFR of the polypropylene resin measured under the conditions of a temperature of 230°C and a load of 2.16 kg PP is 4 g / 10 min or more and 10 g / 10 min or less, and the melt flow rate MFR PPand the melt flow rate MFR of the polystyrene resin measured under the conditions of a temperature of 230°C and a load of 2.16 kg. PS Difference between MFR PP -MFR PS the mass ratio of the polypropylene resin to the polystyrene resin is 60:40 to 90:10, and the melting point Tm PPMR is 140°C or higher, and the melting point Tm PPMR and the glass transition temperature Tg of the polystyrene resin in the mixed resin. PSMR Difference from Tm PPMR -Tg PSMR The melt flow rate MFR of the expanded beads measured under the conditions of a temperature of 230°C and a load of 2.16 kg is 35°C or more and 60°C or less. EP is 5 g / 10 min or more and 10 g / 10 min or less, and the bulk density of the expanded beads is 10 kg / m 3 More than 200kg / m 3 It is preferable that: The expanded beads satisfying the above-mentioned conditions have excellent moldability in a mold, and can be formed into expanded beads having good moldability in a mold even without subjecting the expanded beads to a pressurizing treatment using air or the like.

[0056] The expanded beads preferably have a resin layer on their surface, the resin layer having a polypropylene-based resin (S) as a base resin. Expanded beads having a resin layer on their surface, the resin layer having a polypropylene-based resin (S) as a base resin, can be obtained by preparing expandable resin beads by the method described in step (S) above using the resin beads having a resin layer on their surface, the resin layer having a polypropylene-based resin (S) as a base resin, and then expanding the expandable resin beads by the method described in step (C). Alternatively, the expanded beads can be obtained by placing the expanded bead bodies and the materials for forming the resin layer in a mixing device or the like having mixing and heating functions, and heating and mixing the materials. When the expanded beads have a resin layer on the surface, the mass ratio of the expanded bead body using the mixed resin as the base resin to the resin layer (expanded bead body:resin layer) is preferably 99.5:0.5 to 90:10, more preferably 99:1 to 92:8, and even more preferably 98:2 to 94:6, from the viewpoint of achieving a better balance between the fusion properties of the expanded beads and the physical properties of the resulting molded body.

[0057] <Physical properties of foamed particles> (bulk density) The bulk density of the polypropylene resin foam particles is 10 kg / m 3 If the bulk density of the expanded polypropylene beads is too low, the mechanical strength of the expanded bead molding obtained by molding the expanded beads in a mold tends to be poor. From the viewpoint of increasing the mechanical strength of the expanded bead molding, the bulk density of the expanded polypropylene beads is preferably 15 kg / m 3 More preferably, 20 kg / m 3 The bulk density of the expanded polypropylene resin particles is 200 kg / m 3 If the bulk density of the expanded polypropylene beads is too high, the expanded bead molding tends to be inferior in lightness. From the viewpoint of improving the lightness of the expanded bead molding, the bulk density of the expanded polypropylene beads is preferably 100 kg / m 3 Less than or equal to 70 kg / m 3 More preferably, 50 kg / m or less 3 or less, even more preferably 35 kg / m 3 The following is the result. The bulk density of expanded particles can be calculated as follows: First, a measuring cylinder is filled with expanded particles having a mass W1 [g], and the bottom of the measuring cylinder is lightly tapped on the floor several times to stabilize the filling height of the expanded particles in the measuring cylinder. Next, the volume V1 ([L]) of the expanded particles indicated on the measuring cylinder's scale is read. The mass W1 of the expanded particles is divided by the volume V1 (W1 / V1) and the unit is expressed as [kg / m 3 The bulk density of the expanded beads can be determined by converting the value of the bulk density into the value of the particle diameter.

[0058] (average aspect ratio L / D) The shape of the expanded beads is not particularly limited as long as it does not impair the intended effects of the present invention, and may be, for example, approximately spherical or approximately cylindrical. The average aspect ratio L / D of the expanded beads is preferably 0.7 or more, more preferably 0.8 or more, even more preferably 0.9 or more, and is preferably 1.3 or less, more preferably 1.2 or less, even more preferably 1.1 or less, from the viewpoint of improving the fillability into a molding die and further improving the in-mold moldability of the expanded beads. When the expanded beads have an approximately cylindrical shape, the average aspect ratio L / D of the expanded beads is determined by measuring the maximum length (long diameter of the expanded beads) (L) in the axial direction (height direction of the cylinder) of the expanded beads (30 randomly selected expanded beads) and the maximum cross-sectional diameter (D) of the cross section of the expanded beads in a direction perpendicular to the longitudinal direction of the maximum length, calculating the ratio (L / D) for each expanded bead, and then taking the arithmetic average of these values. In addition, when it is difficult to determine the axial direction of the expanded beads because the expanded beads are almost spherical, the maximum length of the expanded beads is taken as the major axis L of the expanded beads.

[0059] (high temperature peak) The expanded beads preferably exhibit a DSC curve obtained when the expanded beads are heated from 23°C to 200°C at a heating rate of 10°C / min, which shows a first melting peak and a second melting peak (high-temperature peak) that appears at a higher temperature than the first melting peak. The expanded beads having a high-temperature peak in the DSC curve can improve moldability in a mold. The DSC curve is obtained by performing differential scanning calorimetry (DSC) in accordance with JIS K 7122:2012 using 1 to 3 mg of the expanded beads as a test sample.

[0060] Expanded beads exhibiting a DSC curve obtained when heated from 23°C to 200°C at a heating rate of 10°C / min, which has a first melting peak and a second melting peak (high-temperature peak) appearing at a higher temperature than the first melting peak, can be obtained, 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 polypropylene-based resin used to form the mixed resin -15°C) and (the melting end temperature of the polypropylene-based resin used to form the mixed resin +10°C) and maintained at this temperature for a sufficient time, preferably about 10 to 60 minutes (maintenance step). Next, the resin particles that have undergone this maintenance step are expanded to obtain expanded particles that exhibit the above-mentioned melting peak. The maintenance step can be performed as part of the above step (B). From the viewpoint of increasing the productivity of expanded beads, it is preferable to obtain expanded beads that exhibit the above-mentioned melting peak by heating resin particles dispersed in a dispersion medium in a sealed container in the presence of a blowing agent to carry out the above-mentioned holding step, and then releasing the contents of the sealed container from the sealed container under low pressure to cause foaming.

[0061] (High temperature peak heat of fusion ΔH2) The heat of fusion ΔH2 of the expanded beads at the high temperature peak is preferably 5 J / g or more, more preferably 7 J / g or more, from the viewpoint of increasing the mechanical strength of the expanded beads and improving moldability in a mold. From the same viewpoint, the heat of fusion ΔH2 of the expanded beads at the high temperature peak is preferably 30 J / g or less, more preferably 20 J / g or less, and even more preferably 16 J / g or less. The heat of fusion ΔH2 of the high-temperature peak can be calculated as follows. First, the point on the DSC curve at 80°C is designated as α, and the point on the DSC curve corresponding to the melting end temperature is designated as β. A line (α-β) is drawn connecting these points. Next, a line parallel to the vertical axis of the graph is drawn from point γ on the DSC curve, which corresponds to the valley between the resin's intrinsic peak (first melting peak) and the high-temperature peak. The point where this line intersects with the line (α-β) is designated as δ. The area enclosed by the curve of the high-temperature peak on the DSC curve, the line segments (δ-β), and the line segments (γ-δ) is defined as the area of ​​the high-temperature peak. From this area, the heat of fusion ΔH2 of the high-temperature peak can be calculated.

[0062] (Total heat of fusion ΔH) The total heat of fusion ΔH of the expanded beads is preferably 25 J / g or more, more preferably 35 J / g or more, and even more preferably 40 J / g or more from the viewpoint of improving the mechanical strength of the expanded beads molded article. Also, the total heat of fusion ΔH of the expanded beads is preferably 120 J / g or less, more preferably 100 J / g or less, even more preferably 90 J / g or less, and even more preferably 80 J / g or less from the viewpoint of improving the in-mold moldability of the expanded beads. The total heat of fusion ΔH of the expanded beads can be determined from the area enclosed by the DSC curve between points α and β on the DSC curve obtained during the first heating of the expanded beads and the line segment (α-β).

[0063] (closed cell ratio) The closed cell ratio of the expanded beads is preferably 80% or more, more preferably 90% or more, and even more preferably 92% or more, from the viewpoint of improving the moldability of the expanded beads in a mold and improving the mechanical strength of the expanded bead molding. The closed cell ratio of the expanded beads can be measured using an air comparison hydrometer according to Procedure C of ASTM-D2856-70.

[0064] (Melt Flow Rate MFR EP ) The melt flow rate (MFR) of foamed particles measured at a temperature of 230°C and a load of 2.16 kg. EP The melt flow rate (MFR) of the expanded beads is preferably 5 g / 10 min or more. EP If the melt flow rate MFR is too low, the fusion strength between the expanded beads tends to decrease, and the in-mold moldability of the expanded beads tends to decrease. EP The melt flow rate MFR of the expanded beads is preferably 6 g / 10 min or more, more preferably 7 g / 10 min or more. EP The melt flow rate (MFR) of the expanded beads is preferably 10 g / 10 min or less. EPIf the melt flow rate MFR is too high, the expanded bead molding tends to shrink excessively after molding in the mold, and the in-mold moldability of the expanded bead tends to decrease. EP is preferably 9 g / 10 min or less, more preferably 8 g / 10 min or less. Melt flow rate (MFR) of foamed particles EP is measured using expanded beads as a measurement sample at a temperature of 230°C and a load of 2.16 kg in accordance with JIS K 7210-1:2014. Note that in the measurement, expanded beads may be used as a measurement sample after being degassed, as necessary, by a heat press or the like, as long as the physical properties of the mixed resin are not significantly impaired.

[0065] (Difference Tm S -Tg PS ) When the expanded beads have a resin layer on the surface of which the base resin is a polypropylene-based resin (S), the melting point Tm of the polypropylene-based resin (S) constituting the resin layer S Glass transition temperature Tg of polystyrene resin in mixed resin PS Difference from Tm S -Tg PS From the viewpoint of easily obtaining expanded beads that can stably produce expanded bead molded articles having excellent mechanical strength, the temperature is preferably 15° C. or higher, more preferably 18° C., and even more preferably 20° C. Furthermore, from the viewpoint of improving the fusibility of the expanded beads and enhancing the moldability in the mold, the temperature is preferably 45° C. or lower, more preferably 40° C. or lower.

[0066] Conventionally, when producing a foamed bead molding by molding expanded polypropylene resin beads in a mold, it has been necessary to use expanded beads having a low bulk density (particularly, expanded beads having a bulk density of 30 kg / m 3When molding is performed under conditions that make it difficult to obtain a good molded product, such as when molding using expanded beads (expanded beads described below) or when molding using a mold with a complex shape, it is necessary to perform a pretreatment, such as pressurizing the expanded beads with air or the like to increase the pressure within the cells of the expanded beads, otherwise the in-mold moldability of the expanded beads may be reduced, making it difficult to obtain a good molded product. On the other hand, the expanded beads obtained by the production method of the present invention and the expanded beads of the present invention have good fusion properties between the expanded beads even without pressure treatment, and can suppress excessive shrinkage of the expanded bead molded product after in-mold molding, resulting in excellent in-mold moldability. As a result, the expanded beads also have excellent productivity.

[0067] [Foamed bead molding] The expanded bead molding is obtained by in-mold molding of the expanded beads produced by the production method of the present invention. In-mold molding can be performed by filling the expanded beads into a molding die and heat-molding them using a heating medium such as steam. Specifically, after filling the molding die with the expanded beads, a heating medium such as steam is introduced into the molding die to heat and expand the expanded beads (secondary expansion), and the expanded beads are fused together to obtain an expanded bead molding having the shape of the molding space. The expanded beads of the present invention have excellent moldability in a mold, allowing for the production of favorable molded articles without pressure treatment. On the other hand, to further enhance the moldability of the expanded beads in a mold, for example, the expanded beads may be 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 0.01 to 0.3 MPa higher than atmospheric pressure. The expanded beads are then filled into a mold at atmospheric or reduced pressure, and a heating medium such as steam is supplied into the mold to heat-seal the expanded beads (e.g., Japanese Patent Publication No. 51-22951). Alternatively, the expanded beads may be molded by a compression-fill molding method (Japanese Patent Publication No. 4-46217), in which expanded beads pressurized to atmospheric pressure or higher are filled into a mold cavity pressurized to above atmospheric pressure with a compressed gas, and then a heating medium such as steam is supplied into the cavity to heat and seal the expanded beads. Alternatively, foamed particles with high secondary expansion power obtained under special conditions may be filled into the cavity of a mold under atmospheric pressure or reduced pressure, and then a heating medium such as steam is supplied to heat and fuse the foamed particles, as in the atmospheric pressure filling molding method (JP-B-6-49795), or a combination of the above methods (JP-B-6-22919).

[0068] <density> The density of the expanded bead molding is preferably 10 kg / m from the viewpoint of increasing the mechanical strength. 3 More preferably, 15 kg / m 3 More preferably, 20 kg / m 3 The density of the expanded bead molding is preferably 200 kg / m from the viewpoint of increasing the lightness. 3 Less than or equal to 100 kg / m 3 More preferably, 50 kg / m or less 3 or less, even more preferably 35 kg / m 3 The following is the result. The density of the expanded bead molding is calculated by dividing the mass [g] of the expanded bead molding by the volume [L] determined from the outer dimensions of the molding, and converting the result into units.

[0069] <50% compressive stress> From the viewpoint of enhancing mechanical strength, the 50% compressive stress of the expanded polypropylene resin bead molding is preferably 50 kPa or more, more preferably 100 kPa or more, and even more preferably 150 kPa or more. On the other hand, the upper limit of the 50% compressive stress of the expanded polypropylene resin bead molding is not particularly limited, but is 1 MPa or less, preferably 500 kPa or less. The 50% compressive stress of the expanded bead molding is measured in accordance with JIS K 6767:1999.

[0070] <Flame retardancy> When subjected to a flammability test specified in FMVSS (Federal Motor Vehicle Safety Standard) No. 302, the expanded bead molded article preferably conforms to the FMVSS No. 302 standard. Expanded bead molded articles that satisfy this standard can be suitably used in applications that require high flame retardancy, such as automotive components and building components. From the viewpoint of further enhancing flame retardancy, it is preferable that the burning rate of the expanded bead molding when subjected to the above-mentioned flammability test is 80 mm / min or less, more preferably 40 mm / min or less, and even more preferably 0 mm / min or less, i.e., the expanded bead molding exhibits self-extinguishing properties when subjected to the above-mentioned flammability test. [Example]

[0071] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0072] The resins, expanded beads, and expanded bead molded articles used in the examples and comparative examples were measured or evaluated as follows. The physical properties of the expanded beads were measured using expanded beads that had been conditioned by standing for 24 hours at 50% RH, 23°C, and 1 atm. The physical properties of the expanded bead molded articles were measured and evaluated using expanded bead molded articles that had been conditioned by standing for 12 hours at 50% RH, 80°C, and 1 atm after demolding.

[0073] [Measurement method] <Polypropylene resin, polystyrene resin, and mixed resin> (Melting Point) Melting point Tm of polypropylene resin PP The melting temperature was measured according to JIS K 7121:2012. The specimen conditioning method employed was "(2) Measuring the melting temperature after a certain heat treatment." Specifically, approximately 5 mg of resin conditioned for 24 hours or more at 23°C and 50% RH was used as a test specimen. The test specimen was heated from 30°C to 200°C at a heating rate of 10°C / min using a heat flux differential scanning calorimeter (Shimadzu Corporation, model number: DSC-60A). The test specimen was then cooled from 200°C to 30°C at a cooling rate of 10°C / min, and then heated again from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve. The apex temperature of the melting peak associated with the melting of the resin on the DSC curve was determined. For polypropylene-based resins, when multiple melting peaks appear on the DSC curve, the apex temperature of the melting peak with the largest area was used as the melting point of the polypropylene-based resin. In addition, the melting point Tm of the polypropylene resin in the mixed resin PPMR was determined by the above-mentioned measurement using the expanded beads as a test piece. When the expanded beads have a resin layer, a portion excluding the surface layer of the expanded beads (a portion not including the resin layer) was cut out from the expanded beads as a test piece, and the above-mentioned measurement was carried out using this test piece.

[0074] (Total heat of fusion of polypropylene resin) The total heat of fusion of the polypropylene-based resin was determined from a DSC curve obtained by subjecting the polypropylene-based resin to differential scanning calorimetry (DSC) in accordance with JIS K 7122:2012. Specifically, a DSC curve of the polypropylene-based resin at the second heating was first obtained in the same manner as in the melting point measurement described above. The point at 80°C on the obtained DSC curve at the second heating was designated as α, and the point on the DSC curve corresponding to the melting end temperature was designated as β. The total heat of fusion of the polypropylene-based resin was calculated from the area enclosed by the DSC curve in the section between points α and β and the line segment (α-β).

[0075] (Melt Flow Rate) The melt flow rates of polypropylene resin, polystyrene resin, resin particles, and expanded particles were measured in accordance with JIS K 7210-1:2014 at a temperature of 230°C and a load of 2.16 kg. In the examples and comparative examples, when the resin particles and expanded beads had a resin layer, the resin particles and expanded beads were obtained under the same conditions as in the corresponding examples and comparative examples, except that only the extruder for forming the resin particle body described below was used to obtain the resin particles without forming a resin layer on the surface, and these resin particles were used. The above measurements were carried out using these resin particles and expanded beads without a resin layer. In addition, the expanded beads that had been defoamed were used as measurement samples to measure the melt flow rate of the expanded beads.

[0076] (flexural modulus) The flexural modulus of polypropylene-based resin, polystyrene-based resin, and mixed resin was measured in accordance with JIS K 7171:2016. First, the resin or resin particles were heat-pressed at 230°C to produce a 4 mm thick sheet, and a standard test piece measuring 80 mm long, 10 mm wide, and 4 mm thick was cut out from the sheet. Using this test piece, a bending test was performed with an indenter radius R1 and a support table radius R2 of 5 mm, a support distance of 64 mm, and a test speed of 2 mm / min. In the examples and comparative examples, when the resin particles have a resin layer, the resin particles were obtained under the same conditions as the corresponding examples and comparative examples, except that only the extruder for forming the resin particle body described below was used to obtain the resin particles without forming a resin layer on the surface.Test pieces were then prepared using these resin particles without a resin layer, and the above measurements were performed to determine the flexural modulus of the mixed resin.

[0077] (glass transition temperature) Glass transition temperature Tg of polystyrene resin PSwas determined as the midpoint glass transition temperature of the DSC curve obtained by heat flux differential scanning calorimetry according to JIS K 7121: 2012. The glass transition temperature was measured in accordance with "Measurement of glass transition temperature after a certain heat treatment" described in "3. Conditioning of test specimen (3)" of JIS K 7121: 2012, in which approximately 5 mg of test specimen was placed in a container of a DSC apparatus, heated to 200°C at a heating rate of 10°C / min to dissolve, and immediately cooled to 0°C at a cooling rate of 10°C / min to condition, and the glass transition temperature was measured. In addition, the glass transition temperature Tg of the polystyrene resin in the mixed resin PSMR was determined by the above-mentioned measurement using the expanded beads as a test piece. When the expanded beads have a resin layer, a portion excluding the surface layer of the expanded beads (a portion not including the resin layer) was cut out from the expanded beads as a test piece, and the above-mentioned measurement was carried out using this test piece.

[0078] (Weight average molecular weight and number average molecular weight) The weight average molecular weight Mw and number average molecular weight Mn of the polystyrene resin were determined by gel permeation chromatography (GPC) using polystyrene as a standard substance. Specifically, 10 mg of polystyrene resin was dissolved in 10 mL of tetrahydrofuran (THF), and the result was measured by GPC, and the relative average molecular weight was calculated using polystyrene as the standard. The details of the GPC analysis conditions are as follows: Equipment used: GPC-spec high-performance liquid chromatograph manufactured by GL Sciences Inc. Column: Showa Denko K.K. columns, trade names Shodex GPC KF-806, KF-805, and KF-803, connected in series in this order. Column temperature: 40℃ Solvent: THF ·Flow rate: 1.0mL / min ·Concentration: 0.15w / v% ·Injection volume: 0.2mL Detector: UV-visible detector manufactured by GL Sciences, product name UV702 type (measurement wavelength 254 nm)

[0079] <Resin particles and foam particles> (Mixed resin morphology) The morphology of the resin particles (resin particle bodies) was confirmed using a field-emission scanning electron microscope (FE-SEM). Specifically, a resin particle was first excised, and a cross section along the extrusion direction of the resin particle was exposed near the center of the resin particle to prepare an observation sample. A cryomicrotome was used to smoothly cut the cross section along the extrusion direction of the resin particle in the observation sample at an ambient temperature of -120°C. The cross section was then exposed to ruthenium tetroxide vapor for 15 minutes, subjected to electron staining, and then subjected to an osmium coater to make the cross section conductive. SEM photographs of the cross section were then taken at 5000x magnification using an FE-SEM (SU8220, Hitachi High-Tech Corporation). The morphology of the polypropylene-based resin phase and the polystyrene-based resin phase in the mixed resin were visually observed from the SEM photographs.

[0080] (average aspect ratio L / D) The average aspect ratio L / D of the resin particles and expanded particles was determined by measuring the maximum length (L) and the maximum cross-sectional diameter (D) of the cross section of the particle in a direction perpendicular to the longitudinal direction of the maximum length for 30 randomly selected particles, calculating the ratio (L / D), and then taking the arithmetic mean of these values.

[0081] (bulk density) The bulk density of the expanded particles was determined as follows. First, a measuring cylinder was filled with expanded particles having a mass W1 [g], and the bottom of the measuring cylinder was lightly tapped on the floor several times to stabilize the filling height of the expanded particles in the measuring cylinder. Next, the volume V1 [L] of the expanded particles indicated on the measuring cylinder was read. The mass W1 [g] of the expanded particles was divided by the volume V1 [L] (W1 / V1), and the result was expressed in units of kg / m 3 The bulk density of the expanded beads was calculated by converting the value into the value of the particle diameter.

[0082] (High temperature peak heat of fusion ΔH2 and total heat of fusion ΔH) The heat of fusion ΔH2 of the high-temperature peak of the expanded beads was determined as follows. First, approximately 2 mg of expanded beads were sampled from the expanded beads. These expanded beads were used as test specimens, and a DSC curve was obtained by heating the test specimen from 23°C to 200°C at a heating rate of 10°C / min using a differential scanning calorimeter (specifically, Shimadzu Corporation, model number: DSC-60A). When the expanded beads had a resin layer, a portion excluding the surface layer of the expanded beads (a portion not including the resin layer) was cut out as a test specimen, and the above measurements were performed using this test specimen. The obtained DSC curve had a first melting peak and a second melting peak (high-temperature peak) appearing higher than the first melting peak. A line segment L1 was drawn connecting point α at a temperature of 80°C on the DSC curve and point β at the end-of-melting temperature T of the expanded beads. Next, a line L2 parallel to the vertical axis of the graph was drawn from point γ on the DSC curve, which corresponds to the valley between the first melting peak and the second melting peak (high-temperature peak), and the point where line L1 and line L2 intersected was designated as δ. The heat of fusion ΔH2 of the high-temperature peak was calculated from the area enclosed by the DSC curve showing the high-temperature peak, the line segment (δ-β), and the line segment (γ-δ). The above measurement was performed on five expanded beads, and the arithmetic mean of the obtained values ​​was designated as the heat of fusion ΔH2 of the high-temperature peak. The total heat of fusion of each expanded bead was calculated from the area enclosed by the DSC curve in the section between points α and β and the line segment L1. The above measurement was performed on five expanded beads, and the arithmetic mean of the obtained values ​​was taken as the total heat of fusion ΔH of the expanded beads. The heat of fusion of the melting peak of the expanded beads can be calculated by reference to the DSC curve shown in FIG.

[0083] (closed cell ratio) The closed cell ratio of the expanded particles was measured using an air comparison hydrometer according to procedure C of ASTM-D2856-70. Specifically, first, the bulk volume after conditioning was about 20 cm 3The expanded beads were used as measurement samples, and the mass W of the expanded beads and the apparent volume Va of the expanded beads, which was measured from the rise in the water level when the expanded beads were submerged in ethanol in a measuring cylinder, were measured. Next, the expanded beads whose apparent volume Va had been measured were thoroughly dried, and then the true volume Vx of the expanded beads was measured using an Accupyk II 1340 manufactured by Shimadzu Corporation in accordance with Procedure C described in ASTM-D2856-70. Then, based on these volume values ​​Va and Vx, the closed cell ratio was calculated using the following formula (I), and the average value of five samples (N=5) was taken as the closed cell ratio of the expanded beads. Closed cell ratio (%)=(Vx-W / ρ)×100 / (Va-W / ρ) (I) however, Vx: The true volume of the expanded beads measured by the above method, i.e., the sum of the volume of the resin constituting the expanded beads and the total volume of the closed cells in the expanded beads (unit: cm 3 ) Va: Apparent volume of foamed particles measured from the rise in water level when the foamed particles are submerged in ethanol in a measuring cylinder (unit: cm 3 ) W: Mass of foam particles (unit: g) ρ: Density of the resin that makes up the foamed particles (unit: g / cm 3 )

[0084] <Foamed bead molding> (Molded body density) The density of the expanded bead molding was determined as the arithmetic mean value of the densities of three test pieces calculated by dividing the mass of the expanded bead molding by the volume calculated based on the molding dimensions.

[0085] (50% compressive stress) The 50% compressive stress of the expanded bead molding was determined as follows. A rectangular parallelepiped test piece measuring 50 mm long, 50 mm wide, and 25 mm thick was cut out from a foamed bead molding measuring 400 mm long, 250 mm wide, and 60 mm thick, obtained by the method described below, after removing the skin layer. This test piece was compressed at a rate of 10 mm / min using an RTF-1350 manufactured by A&D Co., Ltd. in accordance with JIS K 6767:1999, to determine the load at 50% strain, and this was divided by the pressure-receiving area of ​​the test piece to determine the 50% compressive stress [kPa].

[0086] (Flame retardant) The flame retardancy of the expanded bead molding was determined by a flammability test according to the method specified in FMVSS (Federal Motor Vehicle Safety Standards) No. 302. First, five flat test specimens measuring 356 mm long, 102 mm wide, and 13 mm thick were cut from the foamed bead moldings. Next, one longitudinal end of each test specimen was attached to the fixture of a flammability tester ("MVSS-2" manufactured by Suga Test Instruments Co., Ltd.) conforming to FMVSS No. 302, and the specimen was held horizontally. A burner flame was applied to the lower end of the other longitudinal end of the specimen, i.e., the end not held by the fixture, for 15 seconds, after which the burner flame was removed from the specimen. The burner flame height was 38 mm, and the distance from the tip of the burner to the underside of the specimen was 19 mm. The burning time was measured when the flame emanating from the specimen reached a position 38 mm from the open end of the specimen (i.e., the end not held by the fixture in the longitudinal direction). If the flame emanating from the test specimen reached a position 38 mm from the fixed end of the test specimen (i.e., the end of the test specimen held by a clamp or the like in the longitudinal direction), the measurement of the burning time was completed when the flame reached that position. If the burning of the test specimen ended before reaching a position 38 mm from the fixed end of the test specimen, the measurement of the burning time was completed when the burning ended. If the burning of the test specimen ended before reaching a position 38 mm from the fixed end of the test specimen, the burning time of the test specimen was treated as 0 seconds. The above tests were carried out using five test pieces, and the flame retardancy (burning rate) was evaluated based on the test results. The burning rate is the arithmetic average of the burning rates of five test specimens in a flammability test. When calculating the burning rate used for evaluation, test specimens that showed self-extinguishing properties in the flammability test were treated as having a burning rate of 0 mm / min. The burning rate of test specimens that did not show self-extinguishing properties was calculated using the following formula (II): B=60×D / T (II) In the above formula (II), the symbol B means the burning rate (unit: mm / min), the symbol D means the distance the flame has traveled (unit: mm), and the symbol T means the time (unit: seconds) required for the flame to travel D (unit: mm).

[0087] [Evaluation method] <Range of molding pressure that can be molded> The moldability of the expanded beads was evaluated as follows. In the production of expanded bead molded articles described below, the molding pressure (steam pressure) was changed in increments of 0.01 MPa (G) within the range of 0.32 to 0.40 MPa (G) to mold the expanded bead molded articles. The molding pressure range within which expanded bead molded articles that passed all of the evaluations of fusion, appearance, and recovery described below could be obtained was defined as the moldable molding pressure range. Note that, because the molding temperature is controlled by the molding pressure, the wider the range from the lower limit to the upper limit of the moldable molding pressure, the wider the range of molding heating temperatures within which molding is possible.

[0088] (Fusing ability) The fusion property of the expanded bead molding was evaluated by the following method. The plate-shaped molding was bent and broken, and the number of expanded beads present on the fracture surface (C1) and the number of broken expanded beads (C2) were determined. The ratio of broken expanded beads to the total number of expanded beads (C2 / C1 × 100) was calculated as the material failure rate, and a value of 60% or more was considered acceptable.

[0089] (Appearance (degree of porosity)) A 100mm x 100mm rectangle was drawn near the center of a 400mm x 250mm surface of the foamed bead molding, and a line was drawn diagonally from the corner of the rectangular area. The number of voids (gaps) of 1mm x 1mm or larger on that line was counted. A sample with fewer than five gaps was considered to pass.

[0090] (Recoverability) The surface of the expanded bead molding was observed to check for wrinkles in the expanded beads. The thickness of the central part and the four corner parts of the expanded bead molding was measured, and the ratio of the thickness of the central part to the thickest part of the four corner parts was calculated. The expanded bead molding was deemed to have passed if no wrinkles were found in the expanded beads located on the surface and the thickness ratio was 95% or more.

[0091] <Rate of change in foamed bead molding dimensions relative to mold dimensions> The longitudinal dimension (L B The vertical dimension of the mold (L A ) to the ratio of the difference between the longitudinal dimension of the mold and the longitudinal dimension of the foamed bead molding ([L A -L B ] / L A × 100) was calculated to determine the dimensional change rate of the expanded bead molding relative to the mold dimensions. The smaller the dimensional change rate, the less the molded body shrinks, meaning that a good molded body close to the dimensions of the mold is obtained. The dimensional change rate is preferably 3.0% or less, and more preferably 2.5% or less.

[0092] [Raw materials] The polypropylene-based resins and polystyrene-based resins used in the examples and comparative examples are shown in Tables 1 and 2, respectively. In Table 1, PP1 to PP4 are propylene-ethylene random copolymers, and PP5 and PP6 are propylene-ethylene-butene random copolymers. In Table 2, PS3 is a recycled raw material produced by melt-kneading pulverized extruded polystyrene resin foam in an extruder and then pelletizing it.

[0093] [Table 1]

[0094] [Table 2]

[0095] Examples 1, 5, 6, 9 and 10, and Comparative Examples 1 to 3, 6 and 7 <Preparation of resin particles> A manufacturing apparatus was prepared, which included a resin particle body forming extruder with an inner diameter of 50 mm, a multilayer strand forming die attached downstream of the resin particle body forming extruder, and a resin layer forming extruder with an inner diameter of 30 mm. The manufacturing apparatus was configured so that the downstream side of the resin layer forming extruder was connected to the multilayer strand forming die, allowing the resin layer forming melt to be layered on the surface of the mixed resin melt within the die, and also allowing co-extrusion. The polypropylene-based resin and polystyrene-based resin listed in Table 3, styrene-ethylene-butylene-styrene copolymer (SEBS) as a compatibilizer (Tuftec H1043 manufactured by Asahi Kasei Corporation, 5 parts by mass per 100 parts by mass of the total of polypropylene-based resin and polystyrene-based resin), zinc borate as a cell regulator (0.1 parts by mass per 100 parts by mass of the mixed resin), carbon black (specifically, furnace black, 2.7 parts by mass per 100 parts by mass of the mixed resin), and brominated styrene-butadiene copolymer as a flame retardant (Emerald 3000 manufactured by Chemtura, 0.8 parts by mass per 100 parts by mass of the mixed resin) were supplied to an extruder for forming resin particle bodies and melt-kneaded. In addition, the polypropylene-based resin (PP5) shown in Table 1 and carbon black (specifically, furnace black, 2.7 parts by mass per 100 parts by mass of the polypropylene-based resin for forming the resin layer) were supplied to an extruder for forming the resin layer and melt-kneaded. The mixed resin melt and the resin layer melt obtained by melt-kneading as described above were introduced into a multilayer strand-forming die (strand-forming hole diameter: 3 mm) with multiple strand-forming holes and merged inside the die. A multilayer strand with a resin layer on the surface and a two-layer (core-sheath) structure was extruded (throughput: 150 kg / h, strand take-up speed: 71 m / min). The extruded strand was water-cooled and cut using a pelletizer to obtain cylindrical resin particles with an average mass of 2 mg per particle (mass ratio of resin particle body to resin layer: resin layer = 95:5). The resin particle diameter D was 1.1 mm, the length L in the extrusion direction of the resin particle was 2.5 mm, and the average aspect ratio L / D was 2.3. Furthermore, when the morphology of the resin particles of the example was observed using the method described above, a morphology was formed in which a dispersed phase composed of a polystyrene-based resin was dispersed in a continuous phase formed of a polypropylene-based resin, and the dispersed phase composed of the polystyrene-based resin was dispersed in streaks in the cross section of the resin particle along the extrusion direction of the resin particle. A cross-sectional photograph of the resin particle of Example 1 along the extrusion direction is shown in Figure 2. In the photograph, the white part is the polystyrene-based resin. <Preparation of expanded particles> 1 kg of the obtained resin particles was supplied to a sealed container having a capacity of 5 L together with 3 L of water as an aqueous dispersion medium. Furthermore, 0.3 parts by mass of kaolin as an inorganic dispersant and 0.004 parts by mass (as an active ingredient) of a surfactant (trade name: NEOGEN, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) were added to the sealed container relative to 100 parts by mass of the resin particles. Next, carbon dioxide was injected into the sealed container as a blowing agent and the pressure was increased to 2.0 MPa (G) in gauge pressure. The pressures indicated with (G) are gauge pressures, i.e., pressure values ​​relative to atmospheric pressure. The contents of the sealed container were then heated at a rate of 2°C / min while stirring to the foaming temperature (158.0°C), and maintained at that temperature for 15 minutes. This adjustment allowed the endothermic curve of the resulting expanded beads to exhibit a high-temperature peak when measured by DSC. Thereafter, the contents of the sealed container (resin particles and water) were released under atmospheric pressure to a bulk density of 60 kg / m3 The same steps as those described above were repeated several times to obtain expanded beads to be subjected to the above-mentioned evaluations. The first-stage expanded beads obtained as described above were left to cure for 24 hours in an environment of 23°C temperature, 50% relative humidity, and 1 atm. The cured first-stage expanded beads were then filled into a pressurizable airtight container, and the pressure inside the airtight container was increased from normal pressure to pressurize the expanded beads. The pressurized state of the expanded beads was maintained for a predetermined time, allowing air to be impregnated into the cells of the expanded beads. The first-stage expanded beads were then removed from the airtight container, and first-stage expanded beads with an internal pressure of 0.5 MPa (G) were obtained. These first-stage expanded beads were then fed into a second-stage expansion device. Steam was supplied into the device to expand the first-stage expanded beads, resulting in a bulk density of 26 kg / m. 3 The expanded beads obtained were the following: The mass ratio of the expanded bead body to the resin layer in the expanded beads was expanded bead body:resin layer=95:5. The expanded beads obtained by two-stage expansion were used for the above-mentioned measurements and for producing expanded bead moldings. The measurement results are shown in Table 3 or Table 4.

[0096] <Manufacturing foamed bead moldings (pressureless molding)> The resulting expanded beads were filled into a mold having a molding cavity capable of molding a plate-like molded product measuring 400 mm long, 250 mm wide, and 60 mm thick, and heated using the following heating method. The heating method consisted of preheating (exhaust process) by supplying steam to the mold with drain valves on both sides of the mold open. Steam was then supplied from one side of the mold to heat it, and then steam was supplied from the other side to heat it. Subsequently, steam was supplied from both sides of the mold at a predetermined molding heating steam pressure (main heating). After the main heating was completed, the pressure was released, and the mold was water-cooled until the pressure generated on the molding surface of the mold was reduced to 0.04 MPa (G). The mold was then opened, and the expanded bead molded product was removed. The resulting expanded bead molded product was cured in an oven at 80°C for 12 hours and then slowly cooled to room temperature to obtain the plate-like expanded bead molded product. The expanded bead molded product was then evaluated as described above. The results are shown in Table 3 or Table 4. In the comparative example, since it was not possible to mold a molded article that passed all the evaluations of fusion, appearance, and recovery, measurements of the molded article density, 50% compressive stress, and flammability were not performed. In Example 1, the fusion rate of the molded body obtained when molded at the minimum molding pressure (0.38 MPa) was 80%.

[0097] Example 2 Expanded beads and expanded bead moldings were obtained in the same manner as in Example 1, except that resin beads were obtained using only the extruder for forming resin bead bodies without forming a resin layer on the surface. In Example 2, the fusion rate of the molded body obtained when molded at the minimum molding pressure (0.38 MPa) was 60%.

[0098] Example 3 Expanded beads and an expanded bead molding were obtained in the same manner as in Example 1, except that the resin forming the resin layer was changed to PP6.

[0099] Example 4 Expanded beads and expanded bead moldings were obtained in the same manner as in Example 1, except that the flame retardant was changed to a hindered amine flame retardant (trade name: NOR116, manufactured by BASF).

[0100] Examples 7 and 8 Expanded beads and expanded bead moldings were produced in the same manner as in Example 1, except that the polypropylene-based resin and polystyrene-based resin shown in Table 2 were used and the expansion temperature was set to 149.5°C.

[0101] Comparative Example 4 Expanded beads and expanded bead moldings were produced in the same manner as in Example 1, except that the polypropylene-based resin and polystyrene-based resin shown in Table 2 were used and the expansion temperature was set to 143.0°C.

[0102] Comparative Example 5 Expanded beads and expanded bead moldings were obtained in the same manner as in Comparative Example 4, except that resin beads were obtained using only the extruder for forming resin bead bodies without forming a resin layer on the surface.

[0103] Example 11 Bulk density is 18 kg / m 3 The expanded beads and the expanded bead molding were produced in the same manner as in Example 1, except that the expanded beads of the above formula were produced.

[0104] Example 12 Bulk density is 18 kg / m 3 The expanded beads and the expanded bead molding were produced in the same manner as in Example 7, except that the expanded beads of the above formula were produced.

[0105] [Table 3]

[0106] [Table 4]

[0107] As can be seen from Table 3, the expanded beads of the present invention can be molded under a wide range of molding pressures without subjecting the expanded beads to pressure treatment. Therefore, the expanded beads of the present invention can improve the production efficiency of expanded bead moldings. As can be seen from Table 4, the difference MFR PP-MFR PS In Comparative Example 1, where the difference Tm was excessively large, the closed cell ratio of the expanded beads was low. In addition, the secondary foaming property tended to be reduced, and there was no molding pressure range in which a good molded article could be molded. PP -Tg PS and difference MFR PP -MFR PS In Comparative Example 2, where the MFR was excessively large, when the molding pressure was increased until the appearance of the molded product was good, the molded product immediately after molding in the mold contracted excessively, causing a large sink mark in the center of the molded product, and there was no molding pressure range that could produce a good molded product. PP Low and differential MFR PP -MFR PS In Comparative Example 3, where the melting point Tm of the polypropylene resin was large, the secondary foaming property was reduced, and as a result, the appearance of the expanded beads was poor, and there was no range of molding pressure that could mold a good molded article. PP is low and the difference Tm PP -Tg PS In Comparative Examples 4 and 5, which had a small content of polystyrene resin, when the molding pressure was increased until the appearance of the molded product was good, a large sink mark occurred in the center of the molded product immediately after molding in the mold, and there was no molding pressure range in which a good molded product could be molded. In Comparative Example 6, which had a low content of polystyrene resin, when the molding pressure was increased until the appearance of the molded product was good, a sink mark occurred in the center of the molded product immediately after molding in the mold, and there was no molding pressure range in which a good molded product could be molded. In Comparative Example 7, which had a high content of polystyrene resin, the fusion properties were poor, and there was no molding pressure range in which a good molded product could be molded.

Claims

1. Resin particles having a base resin of a mixed resin obtained by kneading a polypropylene-based resin and a polystyrene-based resin are expanded to a bulk density of 10 kg / m ３ More than 200kg / m ３ A method for producing expanded beads, which comprises: The melting point Tm of the polypropylene resin ＰＰ is 140°C or higher, The melting point Tm of the polypropylene resin ＰＰ and the glass transition temperature Tg of the polystyrene resin. ＰＳ The difference Tm ＰＰ -Tg ＰＳ is 35°C or higher and 60°C or lower, The melt flow rate (MFR) of the polypropylene resin measured under the conditions of a temperature of 230°C and a load of 2.16 kg ＰＰ is 4 g / 10 min or more and 10 g / 10 min or less, Melt flow rate MFR of the polypropylene resin ＰＰ and the melt flow rate MFR of the polystyrene resin measured under the conditions of a temperature of 230°C and a load of 2.16 kg. ＰＳ Difference MFR ＰＰ -MFR ＰＳ is -3 g / 10 min or more and 3 g / 10 min or less, the mass ratio of the polypropylene-based resin to the polystyrene-based resin is 60:40 to 90:10; Melt flow rate (MFR) of the resin particles measured under the conditions of a temperature of 230°C and a load of 2.16 kg ＲＰ is 5 g / 10 min or more and 10 g / 10 min or less, Method for manufacturing expanded beads.

2. The melting point Tm of the polypropylene resin ＰＰ and the glass transition temperature Tg of the polystyrene resin. ＰＳ The difference Tm ＰＰ -Tg ＰＳ The method for producing expanded beads according to claim 1, wherein the temperature is 45°C or higher and 60°C or lower.

3. the resin particles have a resin layer on the surface thereof, the resin layer containing a polypropylene-based resin (S) as a base resin; The melting point Tm of the polypropylene resin (S) Ｓ and the glass transition temperature Tg of the polystyrene resin. ＰＳ The difference Tm Ｓ -Tg ＰＳ The method for producing expanded beads according to claim 1 or 2, wherein the temperature is 15°C or higher and 45°C or lower.

4. The method for producing expanded beads according to any one of claims 1 to 3, wherein the resin beads have an average aspect ratio L / D of 1.4 or more and 5.0 or less.

5. The method for producing expanded beads according to any one of claims 1 to 4, wherein the resin beads contain a brominated flame retardant.

6. The bulk density of the expanded particles is 15 kg / m ３ More than 35kg / m ３ The method for producing expanded beads according to any one of claims 1 to 5, wherein:

7. A method for producing a foamed bead molding, comprising molding the foamed beads produced by the method for producing foamed beads according to any one of claims 1 to 6 in a mold.

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