Expanded beads and their manufacturing method

Abstract

To provide foamed particles which enable production of a foamed particle molding that has a desired shape and is excellent in appearance and rigidity in a wide molding heating temperature range from low temperature to high temperature even when pretreatment pressing and curing steps are omitted, and a method for producing the same.SOLUTION: There are provided cylindrical foamed particles 1 having a through hole 11, and a method for producing the same. The foamed particles 1 have a foamed core layer 2 and a coating layer 3. A melting point Tms of a polyolefin-based resin constituting the coating layer 3 is lower than a melting point Tmc of a polypropylene-based resin constituting the foamed core layer 2. An average pore diameter d of the through hole 11 of the foamed particles 1 is less than 1 mm, and a ratio d / D of the average pore diameter d to an average outer diameter D of the foamed particles 1 is 0.4 or less. A flexural modulus of the polypropylene-based resin constituting the foamed core layer 2 is 1,200 MPa or more, and a melting point Tmc thereof is 158°C or lower.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a cylindrical expanded bead having through holes and a method for producing the same. [Background technology]

[0002] Expanded polypropylene resin beads are used in a variety of applications. Expanded polypropylene resin beads are used to produce expanded bead molded articles. Specifically, expanded bead molded articles are obtained by filling a mold with expanded beads, supplying steam to cause secondary expansion of the expanded beads, and melting their surfaces to fuse the expanded beads together, thereby molding them into a desired shape. Since expanded bead molded articles immediately after molding tend to expand due to secondary expansion, in order to obtain an expanded bead molded article of the desired shape, the expanded bead molded article is cooled in the mold with water, air, or the like, and then released from the mold.

[0003] In order to enhance the secondary expandability of expanded polypropylene resin beads, they are sometimes pressurized with compressed air or the like before being filled into a mold, so that a predetermined internal pressure is applied to the expanded beads before they are filled into the mold for molding. This method of pressurizing the expanded beads before filling into the mold is called pre-pressurization, and is often used to impart secondary expandability to the expanded beads because it does not require a special molding machine.

[0004] When an expanded bead molding is stored at room temperature after in-mold molding, steam that flowed into the cells of the expanded bead molding during in-mold molding condenses within the cells, creating negative pressure within the cells and causing volumetric shrinkage and significant deformation of the expanded bead molding. In particular, when the molding heating temperature is high, the expanded bead molding may shrink and deform significantly, making it impossible to obtain the desired shape. Therefore, after releasing the expanded bead molding, a curing step is usually required in which the expanded bead molding is left to stand for a predetermined time in a high-temperature atmosphere adjusted to a temperature of, for example, about 60°C to 80°C, in order to restore its shape.

[0005] In the in-mold molding of expanded polypropylene resin beads, the pre-pressurization and curing steps require capital investment and take a long time, so shortening or omitting these steps will lead to a significant improvement in the productivity of expanded bead moldings. For example, Patent Document 1 discloses a technique for fusing expanded beads consisting of a foamed core layer and a coating layer while maintaining voids between the beads, and Patent Document 1 claims that the curing step can be omitted. Furthermore, Patent Document 2 discloses a technique for in-mold molding of expanded beads using a polypropylene resin whose melting point, melt flow index, Z-average molecular weight, etc. are adjusted to fall within specific ranges, and Patent Document 2 claims that the pre-pressurization can be omitted and the curing time can be shortened. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-39565 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-129028 Summary of the Invention [Problem to be solved by the invention]

[0007] However, while the technique described in Patent Document 1 allows the curing step to be omitted, voids are formed between the expanded beads in the molded body, resulting in a significantly poor appearance of the expanded bead molded body and insufficient rigidity depending on the application. Furthermore, when the curing step is omitted, the expanded beads described in Patent Document 1 have a narrow molding heating temperature range at which an expanded bead molded body having a desired shape can be molded. The technique described in Patent Document 2 allows the curing step to be shortened, but still requires the curing step. Omitting the curing step results in significant shrinkage and deformation of the expanded bead molded body, making it difficult to obtain an expanded bead molded body having a desired shape.

[0008] The present invention has been made in view of the above background, and aims to provide expanded beads and a method for producing the same, which can produce expanded bead molded articles having a desired shape and excellent appearance and rigidity over a wide molding heating temperature range from low to high, even if the pretreatment pressurization and curing steps are omitted. [Means for solving the problem]

[0009] One aspect of the present invention is a cylindrical expanded bead having a through hole, The expanded beads have a foamed core layer made of a polypropylene-based resin and a coating layer made of a polyolefin-based resin that coats the foamed core layer, and the melting point Tms of the polyolefin-based resin that constitutes the coating layer is lower than the melting point Tmc of the polypropylene-based resin that constitutes the foamed core layer, The average pore diameter d of the through-pores of the expanded beads is 0.2 mm or more the average pore diameter d of the expanded beads is less than 1 mm, and the ratio d / D of the average outer diameter D of the expanded beads to the average pore diameter d is 0.4 or less; The expanded particles comprise a polypropylene resin constituting the foamed core layer, the polypropylene resin having a flexural modulus of 1200 MPa or more and a melting point Tmc of 158°C or less.

[0010] In another embodiment of the present invention, cylindrical multilayer resin particles having through-holes are expanded to produce a foam having an apparent density of 10 kg / m 3 More than 100kg / m 3 1. A method for producing expanded beads, comprising: The multilayer resin particle has a core layer made of a polypropylene-based resin and a coating layer made of a polyolefin-based resin that coats the core layer, and the melting point Tmrs of the polyolefin-based resin that constitutes the coating layer is lower than the melting point Tmrc of the polypropylene-based resin that constitutes the core layer, The average pore diameter dr of the through-pores of the multilayer resin particle is 0.1mm or more the average pore diameter dr of the multilayer resin particles is less than 0.25 mm, and the ratio dr / Dr of the average outer diameter Dr of the multilayer resin particles to the average pore diameter dr is 0.4 or less, The method for producing expanded beads is characterized in that the polypropylene resin constituting the core layer has a flexural modulus of elasticity of 1200 MPa or more and a melting point Tmrc of 158°C or less. [Effects of the Invention]

[0011] The expanded beads described above enable the production of expanded bead molded articles having a desired shape and excellent appearance and rigidity over a wide range of molding temperatures, from low to high, even without the pretreatment pressurization and curing steps. Furthermore, the expanded beads described above enable the drying time of the expanded bead molded articles after molding to be shortened. Thus, the present disclosure provides expanded beads that can significantly improve the production efficiency of expanded bead molded articles having excellent rigidity and appearance. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram of the appearance of expanded beads. [Figure 2] FIG. 2 is a schematic diagram of a cross section of an expanded bead. [Figure 3] FIG. 3 is an explanatory diagram showing a method for calculating the area of ​​the high-temperature peak. DETAILED DESCRIPTION OF THE INVENTION

[0013] In this specification, when an expression using "~" with a numerical value or physical property value before and after it, such as "X~Y," is used, it is synonymous with "X or more and Y or less" and is used to include the values ​​before and after it. Furthermore, when a numerical value or physical property value is expressed as a lower limit, it means that it is equal to or greater than that numerical value or physical property value, and when a numerical value or physical property value is expressed as an upper limit, it means that it is equal to or less than that numerical value or physical property value. Furthermore, "wt%" and "mass%," and "parts by weight" and "parts by mass" are essentially synonymous. Furthermore, in this specification, expanded polypropylene resin beads are appropriately referred to as "expanded beads," and expanded bead molded articles are appropriately referred to as "molded articles." Expanded beads having an expanded core layer composed of a polypropylene resin are generally called expanded polypropylene resin beads.

[0014] In this specification, "no pressure" means that no pre-pressurization step is performed in which the expanded beads are pressurized with compressed air or the like to apply a predetermined internal pressure to the expanded beads before filling the mold (i.e., pre-pressurization is omitted). In this case, the internal pressure of the expanded beads filled into the mold is usually 0 MPa (G). Furthermore, "no curing" means that no curing step is performed in which the expanded bead molding is left to stand in a high-temperature atmosphere after demolding to restore its shape (i.e., the curing step is omitted). Furthermore, the in-mold molding method of expanded beads in which an expanded bead molding is produced without pressure or curing is appropriately referred to as "no pressure, no curing molding."

[0015] 1 and 2 illustrate examples of expanded beads, but the present invention is not limited to these drawings. As shown in FIGS. 1 and 2, expanded beads 1 are cylindrical and have through holes 11. The average pore diameter d of the through holes 11 is less than 1 mm, and the ratio of the average pore diameter d to the average outer diameter D of the expanded beads 1 [d / D] is 0.4 or less. The expanded beads 1 also have an expanded core layer 2 made of a polypropylene-based resin and a coating layer 3 made of a polyolefin-based resin. The melting point Tms of the polyolefin-based resin constituting the coating layer 3 is lower than the melting point Tmc of the polypropylene-based resin constituting the expanded core layer 2. Furthermore, the flexural modulus of the polypropylene-based resin constituting the expanded core layer 2 is 1200 MPa or higher, and the melting point Tmc is 158°C or lower. By using the expanded beads 1 having the above-described configuration, expanded bead molded articles having a desired shape and excellent appearance and rigidity can be produced at a wide range of molding and heating temperatures, from low to high, even when formed without pressure or heating. Furthermore, expanded bead molded articles obtained from the expanded beads have a short drying time. Therefore, expanded bead moldings excellent in rigidity and appearance can be produced with good productivity. The expanded beads can also be molded by carrying out both or either of a pretreatment pressurization step and a curing step.

[0016] The foamed core layer of the expanded beads is composed of a polypropylene-based resin. In this specification, the polypropylene-based resin refers to a homopolymer of a propylene monomer and a propylene-based copolymer containing 50% by mass or more of structural units derived from propylene. The polypropylene-based resin is preferably a propylene-based copolymer in which propylene is copolymerized with other monomers. Specific preferred examples include copolymers of propylene and an α-olefin having 4 to 10 carbon atoms, such as an ethylene-propylene copolymer, a butene-propylene copolymer, a hexene-propylene copolymer, and an ethylene-propylene-butene copolymer. These copolymers are, for example, random copolymers or block copolymers, and are preferably random copolymers. The polypropylene-based resin may also contain multiple types of polypropylene-based resins.

[0017] The polypropylene-based resin constituting the foam core layer may contain other polymers besides the polypropylene-based resin as long as the objectives and effects of the present disclosure are not impaired. Examples of other polymers include thermoplastic resins other than polypropylene-based resins, such as polyethylene-based resins and polystyrene-based resins, and elastomers. The content of other polymers in the polypropylene-based resin constituting the foam core layer is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less. It is particularly preferable that the content of other polymers in the polypropylene-based resin constituting the foam core layer is 0%, i.e., the foam core layer contains substantially only polypropylene-based resin as a polymer.

[0018] The polypropylene resin constituting the foamed core layer is an ethylene-propylene random copolymer, and the ethylene content in the copolymer is preferably 0.5% by mass or more and 2% by mass or less. The total of the ethylene and propylene components in the ethylene-propylene random copolymer is 100% by mass. In this case, the range of molding and heating temperatures over which pressure-free and unheated forming is possible is broadened. From the viewpoint of enabling pressure-free and unheated forming even at higher molding and heating temperatures, the ethylene content in the copolymer is more preferably 1.8% by mass or less, even more preferably 1.5% by mass or less, and particularly preferably 1.4% by mass or less. On the other hand, from the viewpoint of preventing an excessive increase in molding and heating temperature, the ethylene content in the copolymer is more preferably 0.8% by mass or more, even more preferably 1.0% by mass or more. The content of the monomer components in the copolymer can be determined by IR spectroscopy. The ethylene component and propylene component of the ethylene-propylene copolymer refer to the ethylene-derived structural units and propylene-derived structural units, respectively, in the ethylene-propylene copolymer. The content of each monomer component in the copolymer means the content of the constituent units derived from each monomer in the copolymer.

[0019] The melting point Tmc of the polypropylene-based resin constituting the foamed core layer is 158°C or lower. If the melting point Tmc exceeds 158°C, a higher molding temperature (i.e., a high molding pressure) is required to form a good foamed bead molding with excellent appearance and rigidity. In this case, it becomes difficult to suppress significant shrinkage and deformation of the molding during pressureless, unpercolating molding. Therefore, the molding temperature range in which pressureless, unpercolating molding is possible may be narrowed. From the viewpoint of enabling pressureless, unpercolating molding at lower pressures and broadening the molding temperature range, the melting point Tmc of the polypropylene-based resin constituting the foamed core layer is preferably 155°C or lower, more preferably 153°C or lower. On the other hand, from the viewpoint of further improving the heat resistance and mechanical strength of the resulting foamed bead molding, the melting point Tmc of the polypropylene-based resin constituting the foamed core layer is preferably 145°C or higher, more preferably 148°C or higher, and even more preferably 150°C or higher.

[0020] The melting point of a polypropylene resin is determined in accordance with JIS K7121:1987. Specifically, the conditioning is performed using "(2) Measurement of melting temperature after a certain heat treatment," and the conditioned test piece is heated from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve, with the apex temperature of the melting peak being taken as the melting point. Note that 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.

[0021] The foamed core layer may be composed of a polypropylene resin composition containing at least two polypropylene resins with different melting points, having a flexural modulus of 1200 MPa or more and a melting point of 158°C or less. Specifically, the foamed core layer is preferably composed of a polypropylene resin composition containing 70 to 97% by weight of a first polypropylene resin having a melting point of more than 145°C and less than 160°C, and 3 to 30% by weight of a second polypropylene resin having a melting point of more than 135°C and less than 145°C (provided that the total of the first polypropylene resin and the second polypropylene resin is 100% by weight). In this case, the secondary expandability of the foamed beads during in-mold molding can be improved without substantially reducing the rigidity of the resulting foamed bead molding. For example, the desired effect of the present invention can be easily achieved even in a molding having a thickness that varies, for example, having a thick portion and a thin portion. From the viewpoint of enhancing this effect, it is more preferable that the content of the first polypropylene resin is 75 to 95 wt % and the content of the second polypropylene resin is 5 to 25 wt % (provided that the total of the first polypropylene resin and the second polypropylene resin is 100 wt %), and it is even more preferable that the content of the first polypropylene resin is 80 to 90 wt % and the content of the second polypropylene resin is 10 to 20 wt % (provided that the total of the first polypropylene resin and the second polypropylene resin is 100 wt %). From the same viewpoint, it is more preferable that the melting point of the first polypropylene resin is greater than 150°C and less than 155°C, and that the melting point of the second polypropylene resin is greater than 140°C and less than 145°C. The melting point of the polypropylene-based resin composition is measured by the same method as that of the polypropylene-based resin described above. The flexural modulus of the polypropylene-based resin composition is measured by the same method as that of the polypropylene-based resin described below.

[0022] Furthermore, from the viewpoint of further enhancing the above-mentioned effects, it is preferable that the difference between the melting point Tmc1 of the first polypropylene resin and the melting point Tmc2 of the second polypropylene resin (i.e., Tmc1-Tmc2) be 5°C or more and less than 15°C, and more preferably 8°C or more and 13°C or less.

[0023] From the viewpoint of further improving foamability and moldability, the melt mass flow rate (i.e., MFR) of the polypropylene-based resin constituting the foamed core layer is preferably 5 g / 10 min or more, more preferably 6 g / 10 min or more. On the other hand, from the viewpoint of further improving the rigidity of the molded body, the MFR is preferably 10 g / 10 min or less, more preferably 8 g / 10 min or less. The MFR of the polypropylene-based resin is a value measured based on JIS K7210-1:2014 under conditions of a test temperature of 230°C and a load of 2.16 kg.

[0024] The flexural modulus of the polypropylene-based resin constituting the foamed core layer is 1200 MPa or more. If the flexural modulus of the polypropylene-based resin constituting the foamed core layer is less than 1200 MPa, it becomes difficult to suppress shrinkage of the molded body, especially under conditions of high molding heating temperature, and if the curing step is omitted, it may be impossible to obtain a molded body of the desired shape. In other words, the range of molding heating temperature required to obtain a molded body of the desired shape and good appearance by uncured molding is narrowed. From the viewpoint of more easily suppressing shrinkage and deformation of the molded body when the curing step is omitted, the flexural modulus of the polypropylene-based resin is preferably 1300 MPa or more, more preferably 1350 MPa or more, and even more preferably 1400 MPa or more. On the other hand, from the viewpoint of suppressing excessive increases in molding heating temperature, the flexural modulus of the polypropylene-based resin is preferably 1800 MPa or less, more preferably 1600 MPa or less, and even more preferably 1500 MPa or less. The flexural modulus of the polypropylene-based resin can be determined in accordance with JIS K7171:2008.

[0025] From the viewpoint of further improving moldability in pressureless, unheated molding, the crystallization temperature of the polypropylene resin constituting the foamed core layer of the expanded beads is preferably 105°C or higher and 115°C or lower, and more preferably 110°C or higher and 115°C or lower. The crystallization temperature of the polypropylene resin is measured using a heat flux differential scanning calorimeter in accordance with JIS K7121:2012. When multiple crystallization peaks appear on the DSC curve, the peak temperature of the crystallization peak with the highest peak height is taken as the crystallization temperature.

[0026] From the viewpoint of improving the moldability of the expanded beads and the rigidity of the molded body, the closed cell ratio of the expanded beads is preferably 80% or more, more preferably 85% or more, even more preferably 90% or more, and particularly preferably 95% or more. The closed cell ratio of the expanded beads can be measured using an air comparison hydrometer according to Procedure C described in ASTM-D2856-70.

[0027] The expanded beads have a coating layer made of a polyolefin resin that covers the foamed core layer. If the expanded beads do not have a coating layer, a higher molding temperature (i.e., a high molding pressure) is required to mold a good expanded bead molding with excellent appearance and rigidity. In this case, it becomes difficult to suppress significant shrinkage and deformation of the molding during pressureless, unheated molding, which may narrow the molding temperature range in which pressureless, unheated molding is possible. The coating layer is made of a polyolefin resin. Examples of polyolefin resins include polyethylene resins, polypropylene resins, and polybutene resins. From the viewpoint of adhesion to the foamed core layer, the polyolefin resin is preferably a polyethylene resin or a polypropylene resin, and more preferably a polypropylene resin. Examples of polypropylene resins include ethylene-propylene copolymers, ethylene-butene copolymers, ethylene-propylene-butene copolymers, and propylene homopolymers, among which ethylene-propylene copolymers and ethylene-propylene-butene copolymers are preferred.

[0028] The melting point Tms of the polyolefin resin constituting the coating layer is lower than the melting point Tmc of the polypropylene resin constituting the foamed core layer. That is, Tms < Tmc. When the melting point Tms of the polyolefin resin in the coating layer is equal to or higher than the melting point Tmc of the polypropylene resin constituting the foamed core layer (that is, when Tms ≧ Tmc), the fusion property of the foamed particles during molding decreases, molding at a low temperature becomes difficult, and there is a risk that the molding heating temperature range in which pressureless and oxygen-free molding is possible becomes narrow. From the viewpoint of making the molding heating temperature range in which pressureless and oxygen-free molding is possible wider, it is preferable that Tmc - Tms ≧ 5, more preferably Tmc - Tms ≧ 10, and even more preferably Tmc - Tms ≧ 15. From the viewpoint of suppressing peeling between the foamed core layer and the coating layer and mutual adhesion between the foamed particles, etc., it is preferable that Tmc - Tms ≦ 35, and more preferably Tmc - Tms ≦ 25.

[0029] From the viewpoint of further enhancing the fusion property of the foamed particles during molding, the melting point Tms of the polyolefin resin constituting the coating layer is preferably 120°C or higher and 145°C or lower, and more preferably 125°C or higher and 140°C or lower. The melting point of the polyolefin resin constituting the coating layer is determined based on JIS K7121:1987. Specifically, it is determined under the same conditions and by the same method as the polypropylene resin constituting the above-mentioned foamed core layer.

[0030] From the viewpoint of surely suppressing peeling between the foamed core layer and the coating layer, the MFR of the polyolefin resin constituting the coating layer is preferably about the same as the MFR of the polypropylene resin constituting the foamed core layer. Specifically, it is preferably 2 to 15 g / 10 min, more preferably 3 to 12 g / 10 min, and even more preferably 4 to 10 g / 10 min. When the polyolefin resin is a polypropylene resin, its MFR is a value measured under the conditions of a test temperature of 230°C and a load of 2.16 kg based on JIS K7210-1:2014. When the polyolefin resin is a polyethylene resin, its MFR is a value measured under the conditions of a test temperature of 190°C and a load of 2.16 kg based on JIS K7210-1:2014.

[0031] The expanded beads are multilayered beads having a foamed core layer and a coating layer that coats the foamed core layer. The foamed core layer is made of a foamed polypropylene resin, and the coating layer is made of a foamed or non-foamed polyolefin resin. The coating layer is preferably in a substantially non-foamed state. "Substantially non-foamed" means that there is almost no bubble structure. The thickness of the coating layer is, for example, 0.5 to 100 μm. An intermediate layer may be further provided between the foamed core layer and the coating layer. The foamed core layer may be completely covered with the covering layer, or part of the foamed core layer may be exposed. An example of a structure in which the foamed core layer is exposed is a structure in which the side surfaces of a cylindrical foamed core layer are covered with the covering layer and the foamed core layer is exposed on the top and bottom surfaces of the cylinder.

[0032] The mass ratio (ratio by mass%) of the resin constituting the foamed core layer to the resin constituting the coating layer is preferably 99.5:0.5 to 80:20, more preferably 99:1 to 85:15, and even more preferably 97:3 to 90:10, from the viewpoint of improving moldability while maintaining the rigidity of the molded article. The mass ratio is expressed as the resin constituting the foamed core layer:the resin constituting the coating layer.

[0033] The expanded beads preferably have a crystalline structure in which a DSC curve obtained when the expanded beads are heated from 23°C to 200°C at a heating rate of 10°C / min shows an endothermic peak due to melting specific to the polypropylene resin (i.e., a peak specific to the resin) and one or more melting peaks (i.e., high-temperature peaks) on the higher temperature side of that. The DSC curve is obtained by differential scanning calorimetry (DSC) in accordance with JIS K7121:1987 using 1 to 3 mg of the expanded beads as a test sample. The resin-specific peak is an endothermic peak due to the melting inherent to the polypropylene resin that constitutes the expanded beads. It is believed to be due to the endothermic heat generated during melting of the crystals inherent to the polypropylene resin. On the other hand, the endothermic peak on the higher temperature side of the resin-specific peak (i.e., the high-temperature peak) is an endothermic peak that appears higher than the resin-specific peak on the DSC curve. The appearance of this high-temperature peak is inferred to indicate the presence of secondary crystals in the resin. Note that, as described above, a DSC curve obtained by heating from 23°C to 200°C at a heating rate of 10°C / min (i.e., the first heating), cooling from 200°C to 23°C at a cooling rate of 10°C / min, and then heating again from 23°C to 200°C at a heating rate of 10°C / min (i.e., the second heating) shows only the endothermic peak due to the melting inherent to the polypropylene resin that constitutes the expanded beads, making it possible to distinguish between the resin-specific peak and the high-temperature peak. The peak temperature of this resin-specific peak may differ slightly between the first and second heatings, but the difference is typically within 5°C.

[0034] The heat of fusion of the expanded beads at the high temperature peak is preferably 5 to 40 J / g, more preferably 7 to 30 J / g, and even more preferably 10 to 20 J / g. The ratio of the heat of fusion of the high-temperature peak to the heat of fusion of the total melting peak in the DSC curve (heat of fusion of the high-temperature peak / heat of fusion of the total melting peak) is preferably 0.05 to 0.3, more preferably 0.1 to 0.25, and even more preferably 0.15 to 0.2. By setting the ratio of the heat of fusion of the high-temperature peak to the heat of fusion of the total melting peak within this range, it is believed that the presence of secondary crystals appearing as a high-temperature peak will result in the expanded beads having particularly excellent mechanical strength and excellent in-mold moldability. Here, the heat of fusion of all melting peaks refers to the total heat of fusion determined from the area of ​​all melting peaks on a DSC curve.

[0035] As described above, expanded beads have through holes. This allows them to be molded at low pressures. This is thought to be because a heating medium such as steam supplied during the molding process passes through the through holes and reaches the inside of the expanded beads, thereby sufficiently heating the entire expanded beads filled in the mold, improving the secondary expandability and fusion properties of the expanded beads. Furthermore, because the expanded beads have a coating layer made of a resin with a lower melting point than the foam core layer, the expanded beads are easily fused together during molding. From this perspective, expanded beads can be molded at low pressures, making it possible to produce expanded bead molded articles with excellent appearance and rigidity over a wide range of molding pressures, from low to high pressures.

[0036] The cylindrical expanded beads having a through-hole preferably have at least one cylindrical hole passing through the axial direction of the columnar expanded beads such as a cylindrical or rectangular column. It is more preferable that the expanded beads are cylindrical and have a cylindrical hole passing through the axial direction.

[0037] If the expanded beads do not have through pores, molding without pressure or curing at a low molding temperature (i.e., low pressure) may result in insufficient secondary expansion and fusion, making it difficult to mold a good expanded bead molded body. Furthermore, at a high molding temperature (i.e., high pressure), it may be difficult to suppress shrinkage and deformation of the molded body. On the other hand, even if the expanded beads have through pores, if the average pore diameter d is too large, the appearance of the molded body may be poor and its rigidity may be reduced. Furthermore, at a low molding temperature (i.e., low pressure), the secondary expansion of the expanded beads during molding in a mold may be reduced, narrowing the range of molding temperatures at which molding without pressure or curing is possible. Furthermore, the drying time after molding may be long. From these perspectives, the average pore diameter d of the expanded beads is less than 1 mm, as described above. From the viewpoints of improving the appearance and rigidity of the molded article and of broadening the range of molding heating temperatures in which pressure-free and unheated molding is possible, the average pore diameter d of the expanded beads is preferably 0.95 mm or less, more preferably 0.92 mm or less, and even more preferably 0.90 mm or less. From the viewpoints of preventing the through-holes from collapsing and closing when the expanded beads are filled into a mold and of ease of production, the lower limit of the average pore diameter d of the expanded beads is preferably 0.2 mm, more preferably 0.4 mm.

[0038] The average pore diameter d of the through holes of the expanded beads is determined as follows: 50 or more expanded beads randomly selected from a group of expanded beads are cut perpendicular to the penetration direction of the through holes at the position where the cross-sectional area of ​​the cut surface is maximum. A photograph of the cut surface of each expanded bead is taken, and the cross-sectional area of ​​the through hole portion (specifically, the opening area) is determined. The diameter of an imaginary perfect circle having the same area as the cut area is calculated, and the arithmetic mean of these is used as the average pore diameter d of the through holes of the expanded beads. Note that even if the size of the through holes of each expanded bead is not uniform in the penetration direction, the through hole diameter of each expanded bead is determined by the pore diameter at the position where the cross-sectional area of ​​the expanded bead is maximum, as described above.

[0039] From the viewpoint of increasing the wall thickness of the cylindrical expanded beads and improving the secondary expandability of the expanded beads and the rigidity of the molded article, the average outer diameter D of the expanded beads is preferably 2 mm or more, more preferably 2.5 mm or more, and even more preferably 3 mm or more. On the other hand, from the viewpoint of improving the filling ability into the molding die during molding, it is preferably 5 mm or less, more preferably 4.5 mm or less, and even more preferably 4.3 mm or less. For the same reason, the aspect ratio (L / D) of the expanded beads is preferably 2 or less, more preferably 1.5 or less, and even more preferably 1.2 or less, and also preferably 0.5 or more, and more preferably 0.8 or more.

[0040] The ratio d / D of the average pore diameter d to the average outer diameter D of the expanded beads is 0.4 or less. If the ratio d / D exceeds 0.4, the appearance of the molded article may be deteriorated and the rigidity may be reduced. In this case, the secondary expandability of the expanded beads during in-mold molding may be reduced. From the viewpoints of improving the appearance of the molded article, improving the rigidity, and improving the secondary expandability, d / D is preferably 0.35 or less, more preferably 0.3 or less, and even more preferably 0.25 or less. Note that the ratio d / D is preferably 0.1 or more from the viewpoints of more stably achieving the effect of expanding the range of molding heating temperatures allowing pressureless and unheated molding, and from the viewpoint of ease of production.

[0041] The average outer diameter D of the expanded beads is determined as follows: 50 or more expanded beads randomly selected from a group of expanded beads are cut perpendicular to the penetration direction of the through holes at the position where the cross-sectional area is maximum. A photograph of the cut surface of each expanded bead is taken, and the cross-sectional area of ​​the expanded bead (specifically, the cross-sectional area including the openings of the through holes) is determined. The diameters of imaginary circles having the same area as the expanded beads are calculated, and the arithmetic mean of these is taken as the average outer diameter D of the expanded beads. Note that even if the outer diameter of each expanded bead is not uniform in the penetration direction, the outer diameter of each expanded bead is determined as the outer diameter at the position where the cross-sectional area of ​​the expanded bead in the direction perpendicular to the penetration direction is maximum, as described above.

[0042] The average wall thickness t of the cylindrical expanded beads is preferably 1.2 mm or more and 2 mm or less. If the average wall thickness t is within this range, the expanded beads will be sufficiently thick, which will improve the secondary foaming properties during in-mold molding, making it possible to perform molding without pressure or heating at a lower molding heating temperature. In addition, the expanded beads will be less likely to be crushed by external forces, which will improve the rigidity of the molded product. From these perspectives, the average wall thickness t of the expanded beads is more preferably 1.3 mm or more, and even more preferably 1.5 mm or more.

[0043] The average thickness t of the expanded beads is the distance from the surface (ie, outer surface) of the expanded beads to the outer edge of the through-hole (ie, inner surface of the expanded beads), and is calculated by the following formula (1). t=(Dd) / 2 (1) d: Average hole diameter of through hole (mm) D: Average outer diameter of foam particles (mm)

[0044] Furthermore, the ratio t / D of the average wall thickness t of the expanded beads to the average outer diameter D is preferably 0.35 or more and 0.5 or less. When t / D is within the above range, the expanded beads have good filling properties during in-mold molding, and secondary foaming properties are further improved. Therefore, molded articles with excellent rigidity can be produced at lower molding heating temperatures.

[0045] From the viewpoint of the balance between the lightness and rigidity of the molded body, the apparent density of the expanded particles is 10 kg / m 3 More than 100kg / m 3 It is preferably 15 kg / m or less, and more preferably 15 kg / m 3 More than 80kg / m 3 or less, more preferably 20 kg / m 3 More than 50kg / m 3 or less, and particularly preferably 25 kg / m 3 More than 45kg / m 3In the past, when producing a molded article having a low apparent density, the molded article was prone to significant deformation after demolding, making it difficult to omit the curing step. In contrast, with the expanded beads of the present disclosure, the curing step can be omitted even when the apparent density is low, and a molded article having a good appearance can be produced without curing.

[0046] The apparent density of the expanded particles is calculated by submerging a group of expanded particles (weight W (g)) that has been left in a measuring cylinder containing alcohol (e.g., ethanol) at 23°C for one day under conditions of 50% relative humidity, 23°C, and 1 atm using a wire netting or the like, and calculating the volume V (L) of the expanded particles from the rise in the water level, and dividing the weight of the expanded particles by the volume of the expanded particles (W / V) in units of [kg / m 3 ] can be calculated by converting

[0047] From the viewpoint of further expanding the range of molding and heating temperatures in which pressure-free and unheated molding is possible, further increasing the rigidity of the molded body, and improving the appearance, the ratio of the apparent density of the expanded beads to the bulk density of the expanded beads (i.e., apparent density / bulk density) is preferably 1.7 or more, and preferably 2.3 or less, more preferably 2.1 or less, and even more preferably 1.9 or less.

[0048] The bulk density of the expanded beads is calculated as follows: randomly extract expanded beads from the expanded bead group and place them in a measuring cylinder with a volume of 1 L. A large number of expanded beads are placed up to the 1 L mark so that they will naturally pile up, and the mass W2 [g] of the foamed beads is divided by the volume V2 (1 L) (W2 / V2) to obtain the density in units of 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.

[0049] Expanded beads have excellent moldability in a mold, and can produce molded articles with good appearance and high rigidity over a wide range of molding temperatures, from low to high. Furthermore, even if the pretreatment pressurization and curing steps are omitted, the molded articles do not shrink or deform significantly, and can produce molded articles with good appearance and high rigidity. The reason why expanded beads exhibit such effects is not entirely clear, but is thought to be as follows.

[0050] The foamed core layer of the expanded beads is made of a polypropylene resin with a flexural modulus equal to or greater than a specific value as the base resin, which is thought to help prevent the molded article from shrinking after demolding and to prevent dimensional changes even under high molding heating conditions. Furthermore, the expanded beads have a multilayer structure with an expanded core layer and a coating layer, the melting point of the polypropylene-based resin constituting the expanded core layer is below a predetermined value, and the expanded beads have through-holes of a predetermined shape. Therefore, the expanded beads have excellent fusibility and secondary foaming properties, allowing them to be molded at a lower molding heating temperature. Furthermore, molding at a low molding heating temperature can reduce the amount of heat the expanded beads receive from a heating medium such as steam during in-mold molding. Furthermore, the internal temperature of the molded body after demolding is prevented from becoming excessively high. These factors are thought to suppress dimensional changes in the molded body. Furthermore, the molded article has interconnected microvoids originating from the through-holes of the expanded beads, and therefore, after demolding, air quickly flows into the bubbles inside the molded article, increasing the internal pressure of the entire molded article, which is thought to facilitate early dimensional stabilization of the molded article. For the above reasons, even if the expanded beads are formed without pressure or heating, they have good expandability and the dimensions of the molded body are easily stable, so it is thought that good molded bodies can be produced over a wide range of molding heating temperatures, from low to high.

[0051] Expanded beads can be produced, for example, by a method in which multilayer resin particles having a core layer made of a polypropylene-based resin and a coating layer made of a polyolefin-based resin covering the core layer are dispersed in a dispersion medium (e.g., a liquid), the multilayer resin particles are impregnated with a blowing agent, and the blowing-agent-containing multilayer resin particles are released under low pressure (i.e., a dispersion medium release foaming method). Specifically, the multilayer resin particles are preferably dispersed in a dispersion medium in a sealed container, heated, and then a blowing agent is injected under pressure to impregnate the multilayer resin particles. After a holding step in which secondary crystals are grown at a constant temperature, the contents of the sealed container are preferably released under low pressure to expand at least the core layer of the blowing-agent-containing multilayer resin particles, thereby obtaining expanded beads. This production of expanded beads in a single expansion step using the dispersion medium release foaming method is also referred to as single-stage foaming.

[0052] The expanded beads of the present disclosure are preferably produced by the following production method. The manufacturing method is to expand cylindrical multilayer resin particles having through-holes to produce a foam with an apparent density of 10 kg / m 3 More than 100kg / m 3 1. A method for producing expanded beads, comprising: The multilayer resin particle has a core layer made of a polypropylene-based resin and a coating layer made of a polyolefin-based resin that coats the core layer, and the melting point Tmrs of the polyolefin-based resin that constitutes the coating layer is lower than the melting point Tmrc of the polypropylene-based resin that constitutes the core layer, the through holes of the multilayer resin particles have an average pore size dr of less than 0.25 mm, and the ratio dr / Dr of the average pore size dr to the average outer diameter Dr of the multilayer resin particles is 0.4 or less; The method for producing expanded beads is characterized in that the polypropylene resin constituting the core layer has a flexural modulus of elasticity of 1200 MPa or more and a melting point Tmrc of 158°C or less. According to this method, low-density expanded beads can be easily produced without the need for the second-stage expansion described below. In addition, when foaming the multilayer resin particles, it is preferable to impregnate the multilayer resin particles dispersed in an aqueous medium in a sealed container with a foaming agent under heating, and then release the foamed multilayer resin particles containing the foaming agent together with the aqueous medium from the sealed container to foam them. The aqueous medium is specifically a liquid such as water. Even if the average pore diameter dr of the multilayer resin particles is less than 0.25 mm, the average pore diameter d of the expanded particles may increase to 1 mm or more depending on the expansion ratio of the expanded particles to be produced (i.e., depending on the apparent density of the expanded particles). In the above-mentioned production method, multilayer resin particles with an average pore diameter dr of less than 0.25 mm are expanded to produce expanded particles with an apparent density of 10 kg / m 3 More than 100kg / m 3 The following expanded beads are manufactured: Therefore, the average pore diameter d of the expanded beads can be made, for example, less than 1 mm. Furthermore, according to the above-mentioned production method, it is possible to produce expanded beads with a high expansion ratio (i.e., low density) by, for example, one-stage expansion. Specifically, it is possible to produce expanded beads with an apparent density of 45 kg / m by one-stage expansion without carrying out the second-stage expansion described later. 3 It is possible to produce the following expanded beads:

[0053] Multilayer resin particles are produced, for example, by the following strand-cutting method. Two extruders, a core layer extruder and a coating layer extruder, are connected to a coextrusion die. The core layer extruder melt-kneads a polypropylene resin for forming the core layer with optional additives, while the coating layer extruder melt-kneads a polyolefin resin for forming the coating layer with optional additives. The melt-kneaded mixtures are then extruded and merged in the die to form a sheath-core composite consisting of a non-foamed cylindrical core layer and a non-foamed coating layer covering the outer surface of the cylindrical core layer. The composite is extruded in strand form through the holes in the nozzle attached to the tip of the extruder and cooled through a water bath. The cooled extrudate is then cut to the desired size using, for example, a pelletizer. In this way, multilayer resin particles consisting of a cylindrical core layer with through-holes and a coating layer covering the core layer can be obtained. In addition to the strand-cutting method, hot-cutting, underwater-cutting, and other methods can also be used to produce multilayer resin particles.

[0054] The particle diameter of the multilayer resin particles is preferably 0.1 to 3.0 mm, more preferably 0.3 to 1.5 mm. The length / outer diameter ratio of the resin particles is preferably 0.5 to 5.0, more preferably 1.0 to 3.0. The average mass per particle (determined from the masses of 200 randomly selected particles) is preferably 0.1 to 20 mg, more preferably 0.2 to 10 mg, even more preferably 0.3 to 5 mg, and particularly preferably 0.4 to 2 mg. The mass ratio of the core layer to the coating layer in the multilayer resin particles is preferably 99.5:0.5 to 80:20, more preferably 99:1 to 85:15, and even more preferably 97:3 to 90:10.

[0055] By adjusting the diameter dr of the through holes in the core layer of the multilayer resin particles, the average diameter d of the through holes in the expanded core layer of the expanded beads can be adjusted to the desired range. The diameter dr of the through holes in the core layer of the multilayer resin particles can be adjusted, for example, by adjusting the diameter of the small holes in the die used to form the through holes (i.e., the inner diameter of the die). Furthermore, by adjusting the particle diameter and average mass of the multilayer resin particles, the average outer diameter and average wall thickness of the expanded beads can be adjusted to the desired range. From the viewpoint of more reliably producing expanded beads having an average pore diameter d of the through holes of less than 1 mm and a ratio d / D of the average pore diameter d to the average outer diameter D of 0.4 or less, the average pore diameter dr of the through holes of the multilayer resin particles is more preferably less than 0.24 mm, and even more preferably 0.22 mm or less. From the viewpoint of the production stability of multilayer resin particles having through holes, the average pore diameter dr of the through holes of the multilayer resin particles is preferably 0.1 mm or more. From the same viewpoint, the ratio dr / Dr of the average pore diameter dr to the average outer diameter Dr of the multilayer resin particles is more preferably 0.3 or less, further preferably 0.25 or less, and particularly preferably 0.2 or less. From the viewpoint of the production stability of the multilayer resin particles having through holes, the ratio dr / Dr of the average pore diameter dr to the average outer diameter Dr of the multilayer resin particles is preferably 0.1 or more.

[0056] The average pore diameter dr of the through holes of the multilayer resin particles is determined as follows: At least 50 multilayer resin particles randomly selected from a group of multilayer resin particles are cut perpendicular to the penetration direction of the through holes at the position where the cross-sectional area of ​​the cross-section is greatest. A photograph of the cut surface of each multilayer resin particle is taken, and the cross-sectional area of ​​the through-hole portion (specifically, the opening area) is determined. The diameter of an imaginary perfect circle having the same area as the cut surface area is calculated, and the arithmetic mean of these diameters is used as the average pore diameter dr of the through holes of the multilayer resin particles. Even if the size of the through holes of each multilayer resin particle is not uniform in the penetration direction, the through-hole diameter of each multilayer resin particle is determined by the pore diameter at the position where the cross-sectional area of ​​the multilayer resin particle is greatest, as described above.

[0057] The average outer diameter Dr of the multilayer resin particles is determined as follows: At least 50 multilayer resin particles randomly selected from a group of multilayer resin particles are cut perpendicular to the penetration direction of the through holes at the position where the cross-section area is greatest. A photograph of the cut surface of each multilayer resin particle is taken to determine the cross-sectional area of ​​the multilayer resin particle (specifically, the cross-sectional area including the openings of the through holes). The diameters of imaginary circles having the same area as the cross-sectional area are calculated, and the arithmetic mean of these is taken as the average outer diameter Dr of the multilayer resin particles. Even if the outer diameter of each multilayer resin particle is not uniform in the penetration direction, the outer diameter of each multilayer resin particle is determined as the outer diameter at the position where the cross-sectional area of ​​the multilayer resin particle in the direction perpendicular to the penetration direction is greatest, as described above.

[0058] In the strand cutting method, the particle diameter, length / outer diameter ratio, and average mass of the multilayer resin particles can be adjusted by appropriately changing the extrusion speed, take-up speed, cutter speed, etc. when extruding the resin melt.

[0059] An aqueous dispersion medium is used as a dispersion medium (specifically, a liquid) for dispersing the multilayer resin particles obtained as described above in a sealed container. The aqueous dispersion medium is a dispersion medium (specifically, a liquid) whose main component is water. The proportion of water in the aqueous dispersion medium is preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more. Examples of dispersion media other than water in the aqueous dispersion medium include ethylene glycol, glycerin, methanol, and ethanol.

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

[0061] In the dispersion medium release foaming method, it is preferable to add a dispersant to the dispersion medium to prevent the multilayer resin particles from fusing together when heated in the container. Any dispersant can be used as long as it prevents the multilayer resin particles from fusing together in the container, regardless of whether it is organic or inorganic. However, particulate inorganic materials are preferred for ease of handling. Examples of dispersants include clay minerals such as amsnite, kaolin, mica, and clay. Clay minerals may be natural or synthetic. Examples of dispersants include aluminum oxide, titanium oxide, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, and iron oxide. One or more dispersants may be used. Among these, it is preferable to use a clay mineral as the dispersant. It is preferable to add approximately 0.001 to 5 parts by mass of the dispersant per 100 parts by mass of the multilayer resin particles.

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

[0063] A physical foaming agent is preferably used as the foaming agent for expanding the multilayer resin particles. Examples of physical foaming agents include inorganic and organic foaming agents. Examples of inorganic physical foaming agents include carbon dioxide, air, nitrogen, helium, and argon. Examples of organic physical foaming agents include aliphatic hydrocarbons such as propane, butane, and hexane; alicyclic hydrocarbons such as cyclopentane and cyclohexane; and halogenated hydrocarbons such as chlorofluoromethane, trifluoromethane, 1,1-difluoromethane, 1-chloro-1,1-dichloroethane, 1,2,2,2-tetrafluoroethane, methyl chloride, ethyl chloride, and methylene chloride. Physical foaming agents may be used alone or in combination. A mixture of inorganic and organic physical foaming agents may also be used. From the standpoints of environmental impact and ease of handling, inorganic physical foaming agents are preferred, and carbon dioxide is more preferred. When an organic physical foaming agent is used, it is preferable to use n-butane, i-butane, n-pentane, or i-pentane from the viewpoints of solubility in polypropylene-based resins and foaming properties.

[0064] The amount of the foaming agent added relative to 100 parts by mass of the multilayer resin particles is preferably 0.1 to 30 parts by mass, and more preferably 0.5 to 15 parts by mass.

[0065] In the process for producing expanded beads, a preferred method for impregnating the multilayer resin particles with a blowing agent is to disperse the multilayer resin particles in an aqueous dispersion medium in a sealed container, and then pressurize the multilayer resin particles with the blowing agent while heating the dispersion.

[0066] The internal pressure of the sealed container during expansion is preferably 0.5 MPa (G: gauge pressure) or more. On the other hand, the internal pressure of the sealed container is preferably 4.0 MPa (G) or less. If the pressure is within the above range, the expanded beads can be produced safely without risk of damage or explosion of the sealed container.

[0067] By increasing the temperature of the aqueous dispersion medium in the expanded beads production process at a rate of 1 to 5°C / min, the temperature during expansion can be kept within an appropriate range.

[0068] Expanded beads having a crystalline structure in which a melting peak specific to the resin (resin specific peak) and one or more melting peaks (high temperature peaks) appear on the higher temperature side of the resin in a DSC curve obtained by differential scanning calorimetry (DSC) can be obtained, for example, as follows.

[0069] During heating in the expanded beads production process, a first-stage holding step is performed in which the mixture is held at a temperature between (the melting point of the polypropylene resin -20°C) and (the end temperature of melting of the polypropylene resin) for a sufficient time, preferably about 10 to 60 minutes. The temperature is then adjusted to a range from (the melting point of the polypropylene resin -15°C) to (the end temperature of melting of the polypropylene resin +10°C). If necessary, a second-stage holding step is performed in which the mixture is held at that temperature for an additional sufficient time, preferably about 10 to 60 minutes. The expandable resin particles containing a blowing agent are then released from the sealed container under low pressure and expanded to obtain expanded beads having the above-described crystalline structure. The expansion is preferably performed in the sealed container at a temperature between (the melting point of the polypropylene resin -10°C) and (the melting point of the polypropylene resin) and (the melting point of the polypropylene resin +20°C).

[0070] Furthermore, in the production of expanded beads having a particularly low apparent density, two-stage expansion can be performed in which the expanded beads are placed in a pressurizable sealed container, and pressurized gas such as air is injected into the container to increase the internal pressure of the expanded beads, and the expanded beads are heated in the container for a predetermined time using a heating medium such as steam to obtain expanded beads having a particularly low apparent density.

[0071] (Production of Molded Body) The molded article can be obtained by molding the expanded beads in a mold (i.e., an in-mold molding method). The in-mold molding method is carried out by filling the expanded beads into a mold and heat-molding them using a heating medium such as steam. Specifically, after filling the mold with the expanded beads, a heating medium such as steam is introduced into the mold, thereby heating the expanded beads to cause secondary expansion and fusing them together to obtain a molded article having the shape of the molding space.

[0072] (Molded body) The molded article is formed, for example, by molding expanded beads in a mold, and is composed of a large number of expanded beads fused together. The molded article has interconnected voids. The interconnected voids of the molded article are formed by a complex combination of voids formed by interconnected through-holes of a plurality of expanded beads, voids formed by interconnected through-holes of expanded beads with voids formed between the expanded beads, and voids formed by interconnected voids between the expanded beads.

[0073] The porosity of the molded body is preferably 15% or less, more preferably 12% or less, and even more preferably 10% or less from the viewpoint of improving the appearance and mechanical properties, and is preferably 4% or more, more preferably 5% or more from the viewpoint of more easily suppressing significant shrinkage and deformation of the molded body when the curing step is omitted.

[0074] The porosity of a green body can be calculated as follows. First, a rectangular parallelepiped test piece (20 mm long x 100 mm wide x 20 mm high) is cut out from the center of the green body. Next, this test piece is submerged in a measuring cylinder containing ethanol, and the true volume Vc [L] of the test piece is calculated from the rise in the ethanol liquid level. In addition, the apparent volume Vd [L] is calculated from the external dimensions of the test piece. The porosity of the green body can be calculated from the calculated true volume Vc and apparent volume Vd using the following formula (2): Porosity (%)=[(Vd-Vc) / Vd]×100...(2)

[0075] The density of the molded body is set at 10 kg / m3 in order to achieve a balance between lightness and rigidity. 3 More than 100kg / m 3 It is preferable that the saturation is 15 kg / m or less. 3 More than 80kg / m 3 Less than 20 kg / m is more preferable. 3 More than 50kg / m 3 More preferably, 25 kg / m 3 More than 45kg / m 3 It is particularly preferable that the following is satisfied. Conventional molded articles, when they have a low apparent density, tend to shrink and deform significantly after demolding, and therefore require a curing step to restore the dimensions of the molded article. The molded article of the present disclosure has stable dimensions without the need for a curing step, even when it has a low apparent density. The density of a compact is calculated by dividing the weight (g) of the compact by the volume (L) determined from the external dimensions of the compact, and then converting the result into units. If it is not easy to determine the volume from the external dimensions of the compact, the volume of the compact can be determined by the submersion method.

[0076] The expanded beads have excellent moldability from low molding heating temperatures (i.e., low molding pressures) to high molding heating temperatures (i.e., high molding pressures), and can produce molded articles with excellent appearance and rigidity, thereby widening the moldable temperature range (i.e., molding pressure range) for producing good molded articles. Furthermore, even when the pre-pressurization and post-molding curing steps of the expanded beads are omitted, that is, when molding is carried out without pressure or curing, molding is possible over a wide molding pressure range as described above, and molded articles with good appearance and excellent mechanical strength such as rigidity can be produced, thereby significantly improving the productivity of expanded bead molded articles.

[0077] The molded articles are also used as sound absorbing materials, shock absorbing materials, cushioning materials, etc. in various fields such as the field of automobiles and other vehicles, and the field of construction. [Example]

[0078] Next, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples in any way.

[0079] The resins, expanded beads, and molded articles used in the examples and comparative examples were subjected to the following measurements and evaluations. The expanded beads were evaluated after being left to stand for 24 hours at a relative humidity of 50%, 23°C, and 1 atm to condition them.

[0080] <Polypropylene resin> Table 1 shows the properties of the polypropylene resin used in the production of the expanded beads. The ethylene-propylene copolymer and ethylene-propylene-butene copolymer used in this example were both random copolymers. The density of the polypropylene resin used in this example was 900 kg / m 3 is.

[0081] [Table 1]

[0082] (Monomer content of polypropylene resin) The monomer content of polypropylene-based resins (specifically, ethylene-propylene copolymers and ethylene-propylene-butene copolymers) was determined by a known method based on IR spectroscopy. Specifically, the method described in the Polymer Analysis Handbook (edited by the Polymer Analysis Research Forum of the Japan Society for Analytical Chemistry, published January 1995, published by Kinokuniya Shoten, pages 615-616 "II.2.3 2.3.4 Propylene / Ethylene Copolymer" and 618-619 "II.2.3 2.3.5 Propylene / Butene Copolymer") was used. This was determined by a quantitative determination method based on the relationship between the absorbance of ethylene and butene corrected by a predetermined coefficient and the thickness of a film-like test piece. More specifically, the polypropylene-based resin was first hot-pressed at 180°C to form a film, and multiple test pieces with different thicknesses were prepared. Next, the IR spectrum of each test piece was measured to determine the 722 cm spectrum derived from ethylene. -1 and 733 cm -1 Absorbance (A 722 , A 733) and butene-derived 766 cm -1 Absorbance (A 766 ) was read. Next, for each test piece, the ethylene component content in the polypropylene-based resin was calculated using the following formulas (3) to (5). The ethylene component contents obtained for each test piece were calculated as an arithmetic average, which was taken as the ethylene component content (unit: wt%) in the polypropylene-based resin. (K´ 733 ) c =1 / 0.96{(K´ 733 ) a -0.268(K´ 722 ) a}···(3) (K´ 722 ) c =1 / 0.96{(K´ 722 ) a -0.268(K´ 722 ) a}···(4) Ethylene content (%) = 0.575 {(K' 722 ) c +(K´ 733 ) c}···(5) However, in equations (3) to (5), K' a : Apparent absorption coefficient (K´) at each wave number a =A / ρt), K´ c : corrected absorption coefficient, A: absorbance, ρ: resin density (unit: g / cm 3 ), and t: thickness of the film-like test piece (unit: cm). The above formulas (3) to (5) can be applied to random copolymers. The butene content in the polypropylene resin was calculated for each test piece using the following formula (6): The arithmetic mean of the butene contents obtained for each test piece was taken as the butene content (%) in the polypropylene resin. Butene content (%) = 12.3 (A 766 / L)···(6) In the formula (6), A represents the absorbance, and L represents the thickness (mm) of the film-like test piece.

[0083] (Melting point of polypropylene resin) The melting point of polypropylene resin was determined according to JIS K7121:1987. Specifically, the conditioning method employed was "(2) Measurement of melting temperature after a certain heat treatment." The conditioned specimen was heated from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve, and the apex temperature of the melting peak was taken as the melting point. The measurement device used was a heat flux differential scanning calorimeter (manufactured by SII Nanotechnology, Inc., model number: DSC7020).

[0084] (Melt flow rate of polypropylene resin) The melt flow rate (i.e., MFR) of the polypropylene resin was measured in accordance with JIS K7210-1:2014 under conditions of a temperature of 230°C and a load of 2.16 kg.

[0085] (Flexural modulus of polypropylene resin) A 4 mm sheet of polypropylene resin was prepared by heat pressing at 230°C, and a test piece measuring 80 mm in length, 10 mm in width, and 4 mm in thickness was cut from this sheet. The flexural modulus of this test piece was determined in accordance with JIS K7171:2008. The radius R1 of the indenter and the radius R2 of the support table were both 5 mm, the distance between supports was 64 mm, and the test speed was 2 mm / min.

[0086] (Crystallization temperature of polypropylene resin) According to JIS K7121:2012, a DSC curve was obtained using a heat flux differential scanning calorimeter (SII Nanotechnology, Inc., Model No. DSC7020) by heating a polypropylene resin from 23°C to 200°C at a heating rate of 10°C / min, then cooling it from 200°C to 30°C at a cooling rate of 10°C / min. The peak temperature of the crystallization peak in this DSC curve was taken as the crystallization temperature. When multiple crystallization peaks appeared in the DSC curve, the peak temperature of the crystallization peak with the highest peak height was taken as the crystallization temperature.

[0087] <Multi-layer resin particles, foam particles> Tables 2 to 5 show the properties of the multilayer resin particles and the expanded particles.

[0088] [Table 2]

[0089] [Table 3]

[0090] [Table 4]

[0091] [Table 5]

[0092] (length of multi-layer resin particle) The length of the multilayer resin particles was determined as follows: 100 multilayer resin particles were randomly selected from the group of multilayer resin particles, and their maximum lengths were measured with a vernier caliper, and the arithmetic mean of these measurements was taken as the length of the multilayer resin particles.

[0093] (Average pore diameter dr of the through-holes of the multilayer resin particles) The average pore size of the through holes of the multilayer resin particles was determined as follows. 100 multilayer resin particles were randomly selected from the group of multilayer resin particles and cut perpendicularly to the penetration direction of the through holes at the position where the cross-section area was maximum. Photographs of the cut surfaces of each multilayer resin particle were taken, and the cross-sectional areas (opening areas) of the through holes in the cross-sectional photographs were determined. The diameters of imaginary perfect circles having the same area as the cross-sectional areas were calculated, and the arithmetic mean of these was used as the average pore size (dr) of the through holes of the multilayer resin particles.

[0094] (Average outer diameter of multilayer resin particles Dr) The average outer diameter of the multilayer resin particles was determined as follows. 100 multilayer resin particles were randomly selected from the group of multilayer resin particles and cut perpendicularly to the penetration direction of the through holes at the position where the cross-section area was greatest. Photographs of the cut surfaces of each multilayer resin particle were taken, and the cross-sectional area of ​​the multilayer resin particle (including the openings of the through holes) was determined. The diameters of imaginary circles having the same area as the cross-sectional area were calculated, and the arithmetic mean of these was used as the average outer diameter (Dr) of the multilayer resin particles.

[0095] (Average diameter of through-holes in foamed beads d) The average pore size of the through-holes in the expanded beads was determined as follows. 100 expanded beads were randomly selected from the group of expanded beads after conditioning, and cut perpendicular to the penetration direction of the through-holes at the position where the cross-sectional area was maximum. Photographs of the cut surfaces of each expanded bead were taken, and the cross-sectional areas (opening areas) of the through-holes in the cross-sectional photographs were determined. The diameters of imaginary circles having the same area as the cross-sectional areas were calculated, and the arithmetic mean of these was taken as the average pore size (d) of the through-holes in the expanded beads.

[0096] (Average outer diameter of foam particles D) The average outer diameter of the expanded beads was determined as follows. 100 expanded beads were randomly selected from the group of expanded beads after conditioning, and cut perpendicular to the penetration direction of the through-holes at the position where the cross-section area was maximum. Photographs of the cut surfaces of each expanded bead were taken, and the cross-sectional area of ​​the expanded bead (including the openings of the through-holes) was determined. The diameters of imaginary circles having the same area as the cross-sectional area were calculated, and the arithmetic mean of these was taken as the average outer diameter (D) of the expanded beads.

[0097] (Average wall thickness t) The average wall thickness of the expanded beads was calculated by the following formula (7). Average wall thickness t = (average outer diameter D - average pore diameter d) / 2 (7)

[0098] (aspect ratio L / D) Before measuring the average outer diameter D of the expanded beads and the average pore diameter d of the through holes, the maximum lengths of the through holes in the penetration direction of 100 expanded beads were measured with vernier calipers, and these were arithmetically averaged to determine the average length L of the expanded beads.The average length L was then divided by the average outer diameter D to determine the average aspect ratio L / D of the expanded beads.

[0099] (Apparent density) The apparent density of the expanded particles was determined as follows. First, a measuring cylinder containing ethanol at a temperature of 23°C was prepared, and an arbitrary amount of expanded particles after conditioning (mass W1 [g] of expanded particles) was submerged in the ethanol in the measuring cylinder using a wire mesh. Then, taking into account the volume of the wire mesh, the volume V1 [L] of the expanded particles was measured, which was read from the rise in the water level. The mass W1 [g] of the expanded particles placed in the measuring cylinder was divided by the volume V1 [L] (W1 / V1), and the unit was expressed as [kg / m 3 The apparent density of the expanded beads was calculated by converting the density into the

[0100] (bulk density) The bulk density of the expanded beads was determined as follows: Expanded beads were randomly taken from the group of expanded beads after conditioning and placed in a measuring cylinder with a volume of 1 L. A large number of expanded beads were placed up to the 1 L mark so as to form a natural pile. The mass W2 [g] of the placed expanded beads was divided by the placed volume V2 (1 [L]) (W2 / V2) to obtain the density 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.

[0101] (bulk magnification) The bulk ratio [times] of the expanded beads was calculated as follows: The density [kg / m 3 ] to the bulk density [kg / m 3 ] was calculated by dividing by [

[0102] (Heat of fusion of each peak in the DSC curve of expanded beads) After conditioning, one expanded bead was sampled from the group of expanded beads. This expanded bead was used as a test specimen, and a DSC curve was obtained by heating the specimen from 23°C to 200°C at a heating rate of 10°C / min using a differential scanning calorimeter (specifically, a DSC.Q1000 manufactured by TA Instruments). Figure 3 shows an example of a DSC curve. As shown in Figure 3, the DSC curve shows a resin-specific peak ΔH1 and a high-temperature peak ΔH2, which peaks at a higher temperature than the peak of the resin-specific peak ΔH1. Next, a line L1 was obtained by connecting point α at 80°C on the DSC curve with point β at the melting end 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 resin-specific peak ΔH1 and the high-temperature peak ΔH2, and the point where lines L1 and L2 intersect was designated δ. Point γ can also be considered the maximum point between the resin-specific peak ΔH1 and the high-temperature peak ΔH2. The area of ​​the resin-specific peak ΔH1 is the area surrounded by the curve of the resin-specific peak ΔH1 portion of the DSC curve, the line segment α-δ, and the line segment γ-δ, and this was taken as the heat of fusion of the resin-specific peak. The area of ​​the high-temperature peak ΔH2 is the area surrounded by the curve of the high-temperature peak ΔH2 portion of the DSC curve, the line segment δ-β, and the line segment γ-δ, and this was taken as the heat of fusion of the high-temperature peak. The area of ​​the total melting peak is the area surrounded by the curve of the resin-specific peak ΔH1 part of the DSC curve, the curve of the high-temperature peak ΔH2 part, and the line segment α-β (i.e., straight line L1), and this was defined as the heat of fusion of the total melting peak. The above measurements were carried out for five expanded beads, and the arithmetic mean values ​​are shown in Tables 3 and 5.

[0103] (Closed cell ratio of foamed particles) The closed cell content of the expanded particles was measured using an air comparison type hydrometer in accordance with ASTM-D2856-70. Specifically, it was determined as follows: Bulk volume after conditioning: about 20 cm 3The expanded beads were used as measurement samples, and their apparent volume Va was accurately measured using the ethanol immersion method as described below. After measuring the apparent volume Va, the measurement sample was thoroughly dried, and the true volume Vx of the measurement sample was measured using an air comparison hydrometer 930 manufactured by Toshiba Beckman Corporation in accordance with procedure C described in ASTM-D2856-70. Based on these volume values ​​Va and Vx, the closed cell ratio was calculated using the following formula (8), 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) (8) 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 (cm 3 ) Va: The apparent volume (cm) of the foamed particles measured from the rise in the water level when the foamed particles are submerged in a measuring cylinder containing ethanol. 3 ) W: Weight of the sample for measuring foam particles (g) ρ: Density of the resin that makes up the foamed particles (g / cm 3 )

[0104] (No pressure, no fertilizer required) The pressure-free, uncured molding range was evaluated by determining the molding pressure range within which good foamed bead moldings could be produced without pretreatment pressure and curing. Specifically, moldings were first produced using the molding steam pressure range from 0.20 to 0.36 MPa (G) in increments of 0.02 MPa, as described below in "Molding Production." After demolding, the molded bodies were left to stand for 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm. The fusion properties, recovery properties, and surface properties (i.e., appearance) were evaluated as described below, and the molding steam pressure range within which moldings with a rating of "A" for both fusion properties and recovery properties were determined. The wider the range between the lower and upper limits of the molding steam pressure, the wider the molding temperature range.

[0105] <Molded body> Tables 6 and 7 show the molding conditions and properties of the compacts. Tables 8 and 9 show the evaluation results of the fusion, recovery, and surface properties (i.e., appearance) of the compacts molded at each molding steam pressure in the above evaluation (the range of possible molding without pressure or heating).

[0106] [Table 6]

[0107] [Table 7]

[0108] [Table 8]

[0109] [Table 9]

[0110] (Fusing ability) The molded body was bent and broken, and the total number of expanded beads present on the fracture surface C1 and the number of expanded beads that had broken (material failure) C2 were determined. The ratio of the number of expanded beads that had broken (C2) to the total number of expanded beads C1 (i.e., the material failure rate) was calculated. The material failure rate was calculated using the formula C2 / C1 x 100. The above measurement was performed five times using different test pieces, and the material failure rate was calculated for each. An arithmetic mean value of the material failure rate of 90% or more was evaluated as "A," a value of 70% or more but less than 90% was evaluated as "B," and a value of less than 70% was evaluated as "C."

[0111] (Recoverability) The thickness of the molded body obtained using a flat mold measuring 300 mm in length, 250 mm in width, and 60 mm in thickness was measured near the four corners (specifically, 10 mm inward from the corners toward the center) and at the center (the part dividing the body into two equal parts in both the length and width directions). Next, the ratio (unit: %) of the thickness at the center to the thickness at the thickest point near the four corners was calculated, and a ratio of 95% or more was evaluated as "A," a ratio of 90% or more but less than 95% was evaluated as "B," and a ratio of less than 90% was evaluated as "C."

[0112] (Superficiality (i.e. appearance)) The appearance was evaluated by evaluating the surface properties of the molded article, specifically, based on the following criteria. A: The surface of the expanded bead molding has almost no gaps between particles, and the surface is in a good condition with no noticeable irregularities caused by through holes or the like. B: Some irregularities due to interparticle gaps and / or through holes are observed on the surface of the expanded bead molding. It can be seen. C: The surface of the expanded bead molding is significantly uneven due to gaps between particles and / or through holes. It is recognized.

[0113] The porosity, density, and 50% compressive stress of the compacts obtained at the lowest molding pressure (MPa (G)) within the above-mentioned range of possible molding without pressure or heating were measured. Specifically, the compacts were released from the mold and allowed to stand for 12 hours at a relative humidity of 50%, 23°C, and 1 atm, and then used for each measurement. In Example 2 and Comparative Example 2, the compacts obtained at a molding pressure of 0.34 MPa (G) were allowed to stand for 12 hours at a relative humidity of 50%, 23°C, and 1 atm, and then used for each measurement.

[0114] (Porosity of molded body) The porosity of the molded body was determined as follows. A rectangular parallelepiped test piece (20 mm long x 100 mm wide x 20 mm high) was cut out from the center of the molded body and submerged in a measuring cylinder containing ethanol. The true volume Vc [L] of the test piece was determined from the rise in the ethanol liquid level. The apparent volume Vd [L] was also determined from the external dimensions of the test piece. The porosity of the molded body was calculated from the determined true volume Vc and apparent volume Vd using the following formula (2): Porosity (%)=[(Vd-Vc) / Vd]×100...(2)

[0115] (Molded object density) Molded body density (kg / m 3 ) is calculated by dividing the weight (g) of the molded body by the volume (L) determined from the outer dimensions of the molded body and converting the result into units.

[0116] (rigidity) The rigidity was evaluated by measuring the 50% compressive stress. Specifically, a test piece measuring 50 mm long x 50 mm wide x 25 mm thick was cut out from the center of the compact so that the skin layer on the surface of the compact was not included in the test piece. Based on JIS K6767:1999, a compression test was performed at a compression rate of 10 mm / min to determine the 50% compressive stress of the compact. The density of the test piece used to measure the 50% compressive stress was determined using the same method as in the measurement of the compact density above, and is shown in Tables 6 and 7 as the "density of the cut compact."

[0117] (Water cooling time) In the <Production of Molded Body> described below, after heating was completed, the pressure was released, and the time required for water cooling until the value of the surface pressure gauge attached to the inner surface of the mold fell to 0.04 MPa (G) (i.e., the water cooling time) was measured.

[0118] (drying cycle) The drying cycle was evaluated as follows. In the "Production of Green Body" section described below, green bodies were obtained using the lowest green body molding pressure (MPa (G)) within the range of possible green body molding without pressure or fermentation. These green bodies were then dried for 2 hours under conditions of 50% relative humidity, 80°C, and 1 atm, and their moisture content was measured and evaluated according to the following criteria. The moisture content (%) of the green body was calculated using the mass of the green body before drying, Ww (g), and the mass of the green body after drying, Wd (g), using the following formula (9): Good: Moisture content is 3% or less Poor: Moisture content is over 3% Moisture content (%)=[(Ww-Wd) / Ww]×100...(9)

[0119] The methods for producing the expanded beads and the molded articles in Examples 1 to 5 and Comparative Examples 1 to 6 will be described below.

[0120] Example 1 <Production of polypropylene-based expanded beads> Multilayer resin particles were produced by the strand-cutting method. First, polypropylene resin 1 (abbreviated as PP1) was melt-kneaded in an extruder for forming a core layer at a maximum set temperature of 245°C to obtain a resin melt. The properties of PP1 are shown in Table 1. Furthermore, polypropylene resin 6 (abbreviated as PP6) was melt-kneaded in an extruder for forming a coating layer at a maximum set temperature of 245°C to obtain a resin melt. Next, the resin melts from the extruder for forming the core layer and the extruder for forming the coating layer were extruded through small holes in a co-extrusion die with a hole diameter large enough to form through-holes. The resin melts merged in the die to form a sheath-core composite consisting of a non-foamed cylindrical core layer and a non-foamed coating layer covering the outer surface of the cylindrical core layer. The composite was extruded through the small holes in a die attached to the tip of the extruder into a cylindrical strand with through-holes. The strand was collected and cooled with water, and then cut into pellets with a mass of approximately 1.5 mg using a pelletizer. In this way, multilayer resin particles were obtained, each consisting of a cylindrical core layer having a through-hole and a coating layer covering the core layer. During the production of the multilayer resin particles, zinc borate was supplied to the core layer forming extruder as a cell control agent, and 500 ppm by mass of zinc borate was incorporated into the polypropylene resin.

[0121] 1 kg of multilayer resin particles and 3 L of water as a dispersion medium were placed in a 5 L sealed container. Furthermore, 0.3 parts by mass of kaolin as a dispersant and 0.004 parts by mass of a surfactant (sodium alkylbenzene sulfonate) were added to the sealed container per 100 parts by mass of the multilayer resin particles. 70 g of dry ice was added to the sealed container as a blowing agent, and the sealed container was then sealed and heated to a foaming temperature of 157.2°C while stirring. The pressure inside the container (impregnation pressure) was 3.7 MPa (G). After holding at this temperature (i.e., 157.2°C) for 15 minutes, the contents of the container were released to atmospheric pressure to obtain expanded beads. The expanded beads were then dried at 23°C for 24 hours. In this way, expanded beads with a bulk ratio of 40.0 times were obtained. These are designated as expanded beads A.

[0122] <Production of Molded Product> To produce the molded article, the expanded beads were dried at 23°C for 24 hours. The expanded beads were then filled into a flat mold measuring 300mm long x 250mm wide x 60mm thick, with the cracking amount adjusted to 20% (i.e., 12mm). The mold was then clamped and subjected to an exhaust process in which steam was supplied from both sides of the mold for 5 seconds to preheat the mold. One-sided heating was then performed by supplying steam from one side of the mold until a pressure 0.08MPa (G) lower than the desired molding pressure was reached. Next, one-sided heating was performed by supplying steam from the other side of the mold until a pressure 0.04MPa (G) lower than the desired molding pressure was reached, followed by heating until the desired molding pressure was reached (i.e., main heating). After heating, the pressure was released, and the molded article was water-cooled until the surface pressure due to the foaming force of the molded article reached 0.04MPa (G), after which it was released from the mold to obtain the molded article.

[0123] Example 2 Using the expanded beads A, molding conditions were changed as shown in Table 6 to produce a molded article.

[0124] Example 3 The expanded beads A were placed in a sealed container and pressurized with compressed air to the internal pressure shown in Table 6, after which the expanded beads were filled into a mold and molded under the molding conditions shown in Table 6 to produce a molded product.

[0125] Example 4 Expanded beads having a bulk ratio of 37.5 times were obtained in the same manner as in Example 1, except that the expansion temperature and the blowing agent impregnation pressure were changed to the values ​​shown in Table 2. The expanded beads in this example are designated "expanded beads B." Further, using expanded beads B, a molded body was produced in the same manner as in Example 1, except that the molding conditions were changed as shown in Table 6.

[0126] Example 5 Expanded beads having a bulk ratio of 40.0 times were obtained in the same manner as in Example 1, as shown in Tables 2 and 3. The expanded beads in this example are designated "expanded beads D." The blending ratio (mass ratio) of PP1 to PP2 was 80:20 (PP1:PP2). The properties of PP4 are shown in Table 1. Furthermore, expanded beads D were used, and a molded body was produced in the same manner as in Example 1, except that the molding conditions were changed as shown in Table 6.

[0127] Example 6 Expanded beads having a bulk ratio of 39.1 times were obtained in the same manner as in Example 1, as shown in Tables 2 and 3. The expanded beads in this example are designated "expanded beads J." The blending ratio (mass ratio) of PP1 to PP2 was 95:5 (PP1:PP2). The properties of PP5 are shown in Table 1. Furthermore, expanded beads J were used, and a molded body was produced in the same manner as in Example 1, except that the molding conditions were changed as shown in Table 6.

[0128] Example 7 Expanded beads having a bulk ratio of 17.7 times were obtained in the same manner as in Example 1, except that the expansion temperature and the blowing agent impregnation pressure were changed to the values ​​shown in Table 2. The expanded beads in this example are designated "expanded beads K." Further, using expanded beads K, a molded body was produced in the same manner as in Example 1, except that the molding conditions were changed as shown in Table 6.

[0129] Example 8 In the production of multilayer resin particles, expanded beads having a bulk ratio of 35.7 times were obtained in the same manner as in Example 1, except that the carbon black content in each of the core layer and coating layer was 2.7% by mass, and the expansion temperature and blowing agent impregnation pressure were changed to the values ​​shown in Table 2. The expanded beads of this example are designated "expanded beads L." Furthermore, a molded body was produced in the same manner as in Example 1, except that the expanded beads L were used and the molding conditions were changed as shown in Table 6.

[0130] (Comparative Example 1, Comparative Example 2) This example is an example of expanded beads having large through holes. In this example, expanded beads having a bulk ratio of 36.0 times as shown in Tables 4 and 5 were obtained in the same manner as in Example 1, except that the diameter of the small holes in the die used to form the through holes (i.e., the die inner diameter) was changed to enlarge the through holes, and the expansion temperature and blowing agent impregnation pressure were changed to the values ​​shown in Table 4. The expanded beads of this example are designated "expanded beads E." Furthermore, a molded body was produced in the same manner as in Example 1, except that the molding conditions were changed as shown in Table 7 using expanded beads E. Expanded beads E had a somewhat narrow range of molding pressures that allowed molding without pressure or heating. Furthermore, the rigidity and appearance of the resulting molded body were significantly inferior. Furthermore, the drying cycle was also insufficient.

[0131] (Comparative Example 3) This example is an example of expanded beads without through holes. In this example, resin beads without through holes were produced during the production of multilayer resin beads. The expansion temperature and blowing agent impregnation pressure were changed to the values ​​shown in Table 4. Otherwise, expanded beads (specifically, first-stage expanded beads) with a bulk ratio of 17.8 were obtained in the same manner as in Example 1. Next, the first-stage expanded beads were placed in a pressure-resistant vessel, and air was forced into the pressure-resistant vessel to increase the pressure inside the vessel, thereby impregnating the air into the cells and increasing the internal pressure inside the cells of the first-stage expanded beads. The pressure inside the cells of the first-stage expanded beads at this time (gauge pressure) was the value shown in Table 2. Next, the first-stage expanded beads with increased internal pressure were removed from the pressure-resistant vessel and placed in a metal drum. Steam was supplied so that the pressure inside the drum reached the steam pressure during heating shown in Table 2, and the beads were heated under atmospheric pressure. As a result, the apparent density of the first-stage expanded beads was reduced, and expanded beads (second-stage expanded beads) with a bulk ratio of 37.5 were obtained. The expanded beads thus obtained are designated "expanded beads F." Expanded beads F were approximately spherical and had no through holes. Furthermore, expanded beads F were used to produce a molded article in the same manner as in Example 1, except that the molding conditions were changed as shown in Table 7. Expanded beads F could be molded without pressure or heat within a narrow range of molding pressures.

[0132] Comparative Example 4 This example is an example of expanded beads having an expanded core layer composed of a polypropylene-based resin with a low flexural modulus. In this example, polypropylene-based resin 1 was changed to polypropylene-based resin 2 (abbreviated as PP2), and the expansion temperature and blowing agent impregnation pressure were changed to the values ​​shown in Table 4. Furthermore, as in Comparative Example 3, first-stage expanded beads were obtained, and then the apparent density of the first-stage expanded beads was reduced to produce second-stage expanded beads with a bulk ratio of 37.5. The properties of PP2 are shown in Table 1. The expanded beads of this example (i.e., second-stage expanded beads) are referred to as "expanded beads G." Furthermore, expanded beads G were used, and a molded body was produced in the same manner as in Example 1, except that the molding conditions were changed as shown in Table 7. Expanded beads G could be molded without pressure or heat over a narrow molding pressure range.

[0133] (Comparative Example 5) This example is an example of expanded beads having an expanded core layer composed of a polypropylene-based resin with a high melting point. In this example, expanded beads with a bulk ratio of 34.6 were produced in the same manner as in Example 1, except that polypropylene-based resin 1 was changed to polypropylene-based resin 3 (abbreviated as PP3) and the expansion temperature and blowing agent impregnation pressure were changed to the values ​​shown in Table 4. The properties of PP3 are shown in Table 1. The expanded beads in this example are designated "expanded beads H." Furthermore, expanded beads H were used, and a molded body was produced in the same manner as in Example 1, except that the molding conditions were changed as shown in Table 7. Expanded beads H had poor moldability. In addition, the range of molding pressures that allowed molding without pressure or heat was narrow.

[0134] (Comparative Example 6) This example is an example of expanded beads having no coating layer. In this example, expanded beads having a bulk ratio of 41.5 times were produced in the same manner as in Example 1, except that polypropylene resin 1 was extruded from a single extruder without using an extruder for forming a coating layer. The expanded beads of this example are designated "expanded beads I." Further, expanded beads I were used to produce a molded body in the same manner as in Example 1, except that the molding conditions were changed as shown in Table 7. Expanded beads I had poor moldability. In addition, the range of molding pressures that allowed molding without pressure or heating was narrow.

[0135] As can be seen from Tables 1 to 3, 6, and 8, the expanded beads of Examples 1 to 8 can be molded without pressure or curing, and even when molded without pressure or curing, expanded bead molded articles with excellent appearance and rigidity can be obtained over a wide range of molding pressures, from low to high. Furthermore, the expanded bead molded articles obtained from the expanded beads of Examples 1 to 5 require a short drying time. Therefore, expanded bead molded articles with excellent rigidity and appearance can be produced with good productivity.

Claims

1. A cylindrical expanded particle having a through hole, The expanded beads have a foamed core layer made of a polypropylene-based resin and a coating layer made of a polyolefin-based resin that coats the foamed core layer, and the melting point (Tms) of the polyolefin-based resin that constitutes the coating layer is lower than the melting point (Tmc) of the polypropylene-based resin that constitutes the foamed core layer, the average pore diameter d of the through holes of the expanded beads is 0.2 mm or more and less than 1 mm, and the ratio d / D of the average pore diameter d to the average outer diameter D of the expanded beads is 0.4 or less; The expanded beads, wherein the polypropylene-based resin constituting the foamed core layer has a flexural modulus of 1200 MPa or more and a melting point Tmc of 158°C or less.

2. The expanded beads according to claim 1, wherein the expanded beads have an average outer diameter D of 2 mm or more and 5 mm or less.

3. 3. The expanded beads according to claim 1, wherein the average wall thickness t of the expanded beads is 1.2 mm or more and 2 mm or less.

4. The expanded beads according to any one of claims 1 to 3, wherein the ratio d / D of the average pore diameter d to the average outer diameter D of the expanded beads is 0.25 or less.

5. The expanded beads according to any one of claims 1 to 4, wherein the polypropylene-based resin constituting the foamed core layer is an ethylene-propylene random copolymer, and the content of the ethylene component in the copolymer is 0.5% by mass or more and 2% by mass or less.

6. The expanded beads according to any one of claims 1 to 5, wherein the polypropylene-based resin constituting the foamed core layer has a melting point of 145°C or higher and 155°C or lower.

7. The expanded beads according to any one of claims 1 to 6, wherein the polypropylene-based resin constituting the foamed core layer has a melt mass flow rate of 6 g / 10 min or more and 10 g / 10 min or less.

8. The apparent density of the expanded beads is 10 kg / m ３ More than 100kg / m ３ The expanded beads according to any one of claims 1 to 7, wherein:

9. Cylindrical multilayer resin particles having through holes are expanded to produce a foam having an apparent density of 10 kg / m 3 More than 100kg / m 3 1. A method for producing expanded beads, comprising: the multilayer resin particle has a core layer made of a polypropylene-based resin and a coating layer made of a polyolefin-based resin that coats the core layer, and the polyolefin-based resin that constitutes the coating layer has a melting point (Tmrs) lower than the melting point (Tmrc) of the polypropylene-based resin that constitutes the core layer; the through-holes of the multilayer resin particles have an average pore diameter dr of 0.1 mm or more and less than 0.25 mm, and the ratio dr / Dr of the average pore diameter dr to the average outer diameter Dr of the multilayer resin particles is 0.4 or less, The method for producing expanded beads, wherein the polypropylene resin constituting the core layer has a flexural modulus of 1200 MPa or more and a melting point Tmrc of 158°C or less.

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