JP7610967B2 - Polypropylene resin expanded beads and expanded bead moldings - Google Patents

Description

The present invention relates to polypropylene-based resin expanded beads and polypropylene-based resin expanded bead molded articles that contain an ethylene-propylene-butene copolymer as a base resin.

Polypropylene resin foamed bead moldings are used in a variety of applications because they are lightweight and have excellent shock-absorbing properties and rigidity. Polypropylene resin foamed bead moldings can be obtained, for example, by filling a mold with polypropylene resin foamed beads and heating them with steam to secondary expand the foamed beads and melt their surfaces to fuse them together, forming them into the desired shape using an in-mold molding method. After molding, the foamed bead moldings are cooled in the mold with water or air, and then released from the mold.

Conventionally, in order to improve the moldability of expanded beads in a mold and the physical properties of expanded bead moldings, copolymers obtained by copolymerizing propylene with other monomers have been used as polypropylene-based resins. Among these copolymers, attempts have been made to use terpolymers obtained by copolymerizing propylene, 1-butene, and ethylene. For example, Patent Document 1 discloses polypropylene-based resin pre-expanded beads that use an ethylene-propylene-1-butene random copolymer having a specific melting point and melt index as the base resin. Patent Document 2 discloses polypropylene-based resin expanded beads that contain a polypropylene-based resin containing structural units consisting of 1-butene and a polypropylene-based resin with a high melting point.

If the expanded bead molding is stored at room temperature after in-mold molding, the steam that flowed into the bubbles of the expanded bead molding during in-mold molding condenses in the bubbles, creating negative pressure inside the bubbles and causing the expanded bead molding to shrink in volume and become significantly deformed. For this reason, after releasing the expanded bead molding, a curing process is usually required in which the expanded bead molding is left to stand for a predetermined period of 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.

In the manufacture of polypropylene resin expanded bead moldings, attempts have been made to omit the curing step. For example, Patent Document 3 discloses a technique for fusing expanded beads consisting of a foamed core layer and a coating layer while maintaining voids between the beads, and according to Patent Document 3, it is possible to omit the curing step. Furthermore, Patent Document 4 discloses a technique for in-mold molding of expanded beads using a polypropylene resin having a specific melting point, melt flow index, Z-average molecular weight, etc., and according to Patent Document 4, it is possible to shorten the curing time.

Japanese Patent Application Publication No. 10-316791 WO2016/60162 JP 2003-39565 A JP 2000-129028 A

There is a demand for polypropylene resin expanded beads to be moldable over a wide temperature range. In particular, from the viewpoint of reducing the burden on the equipment and reducing energy consumption, there is a demand for them to be moldable at lower temperatures. In addition, there is a demand for expanded bead moldings to have sufficient rigidity and excellent appearance. Furthermore, there is a demand for the development of polypropylene resin expanded beads that can produce expanded bead moldings with good rigidity and appearance even when the curing process is omitted.

In the techniques described in Patent Documents 1 and 2, the heating temperature range in which molding is possible is narrow, and if the curing step is omitted, the expanded bead molding shrinks and deforms significantly, making it difficult to produce an expanded bead molding with good appearance.
In the technique described in Patent Document 3, the curing step can be omitted, but voids are formed between the expanded beads of the molded body, which deteriorates the appearance of the molded body. In addition, when the expanded bead molding of Patent Document 3 is used as an energy absorbing material, for example, the rigidity and strength are insufficient. In the technique described in Patent Document 4, the curing step can be shortened, but the curing step is required. If the curing step is omitted, the expanded bead molding shrinks and deforms, making it difficult to obtain an expanded bead molding with good appearance.

The present invention has been made in view of the above background, and aims to provide polypropylene-based resin expanded beads that can be used to produce expanded bead moldings having good appearance and excellent mechanical properties such as compressive strength over a wide molding heating temperature range. Furthermore, the present invention aims to provide polypropylene-based resin expanded beads that can be used to produce expanded bead moldings having a desired shape and good appearance even if the curing step is omitted.

One aspect of the present invention is a cylindrical expanded polypropylene resin bead having a through hole,
the average pore diameter d of the through holes of the expanded beads is 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 polypropylene-based resin constituting the expanded beads is an ethylene-propylene-butene copolymer,
the total amount of the butene component content and the ethylene component content in the ethylene-propylene-butene copolymer is 2% by mass or more and 15% by mass or less, and the ratio of the butene component content [mass%] to the ethylene component content [mass%] is 2 or more;
The polypropylene resin foamed particles have a flexural modulus of 800 MPa or more.

Another aspect of the present invention is an expanded bead molding in which expanded polypropylene resin beads are fused to each other and have interconnected voids.

The polypropylene resin expanded beads described above can be used to produce expanded bead molded articles that have good appearance and excellent mechanical properties such as compressive strength at a wide range of molding heating temperatures. Furthermore, even if the curing step is omitted, expanded bead molded articles that have good appearance and excellent mechanical properties can be produced.

The foamed bead molding has a good appearance and excellent mechanical properties such as compressive strength.

FIG. 1 is an explanatory diagram showing a method for calculating the area of a high-temperature peak.

In this specification, when a numerical value or physical property value is enclosed before and after "~", the values before and after are included. 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, "weight %" and "mass %", "parts by weight" and "parts by mass" are essentially synonymous. Furthermore, in the following description, polypropylene resin expanded beads will be referred to as "expanded beads" and expanded bead molded bodies will be referred to as "molded bodies".

The expanded beads are cylindrical with through holes, have an average pore diameter d of less than 1 mm, and have a ratio [d/D] of the average pore diameter d to the average outer diameter D of 0.4 or less. The flexural modulus of the polypropylene resin constituting the expanded beads is 800 MPa or more, and the polypropylene resin contains an ethylene-propylene-butene copolymer. The total amount of the butene component content and the ethylene component content of the ethylene-propylene-butene copolymer is 2 to 15 mass %. The ratio of the butene component content C _bu to the ethylene component content C _et in the ethylene-propylene-butene copolymer (specifically, C _bu /C _et ) is 2 or more. By providing the expanded beads with the above configuration, a molded article having a good appearance and excellent strength during compression can be produced under a wide range of molding heating temperature conditions. Furthermore, even if the curing step is omitted, a molded article having a desired shape and a good appearance can be produced.

(Ethylene-propylene-butene copolymer)
The total amount of the butene component content and the ethylene component content in an ethylene-propylene-butene copolymer (hereinafter also simply referred to as "copolymer"), which is the base resin of the expanded beads, is 2% by mass or more and 15% by mass or less, and the mass ratio of the butene component content to the ethylene component content (butene component content/ethylene component content) is at least 2. The total of the butene component content, ethylene component content and propylene component content in the copolymer is taken to be 100% by mass.

If the total amount of the butene component content and the ethylene component content in the ethylene-propylene-butene copolymer is too small, it may be difficult to obtain a molded body with a good appearance at a low molding heating temperature. From this viewpoint, the total amount of the butene component content and the ethylene component content in the ethylene-propylene-butene copolymer is 2% by mass or more as described above. From the viewpoint of enabling molding at a lower molding heating temperature, the total amount of the butene component content and the ethylene component content in the ethylene-propylene-butene copolymer is preferably 5% by mass or more, more preferably more than 6% by mass, and even more preferably 8% by mass or more. On the other hand, if the total amount of the butene component content and the ethylene component content in the ethylene-propylene-butene copolymer is too high, it becomes difficult to suppress the shrinkage and deformation of the molded body after demolding when the curing process is omitted, and it may be difficult to obtain a molded body with a desired shape and a good appearance. From this viewpoint, the total amount of the butene component content and the ethylene component content in the ethylene-propylene-butene copolymer is 15% by mass or less as described above. From the viewpoint of making it easier to suppress significant shrinkage and deformation of the molded body after demolding, the total amount of the butene component content and the ethylene component content in the ethylene-propylene-butene copolymer is preferably 14% by mass or less, and more preferably 12% by mass or less.

In the ethylene-propylene-butene copolymer, the ratio C _bu /C _et of the butene component content C _bu to the ethylene component content C _et is 2 or more. If the ratio C _bu /C _et is too small, the expanded beads may not be molded in a wide range of molding heating temperature conditions. In addition, if the curing step is omitted, the molded body is likely to shrink and deform after demolding. From the viewpoint of a wider molding heating temperature range and of easier suppression of shrinkage and deformation of the molded body, the ratio C _bu /C _et is preferably 3 or more, more preferably 5 or more, even more preferably 6 or more, even more preferably 7 or more, particularly preferably 9 or more, and most preferably 13 or more. On the other hand, the upper limit of the ratio C _bu /C _et is not particularly limited from the above viewpoint, but is preferably 50, more preferably 30, and even more preferably 20.

From the viewpoint of being able to lower the melting point while maintaining the rigidity of the resin, the butene component content of the ethylene-propylene-butene copolymer is preferably 2% by mass or more, more preferably 5% by mass or more, even more preferably 6% by mass or more, even more preferably 7% by mass or more, particularly preferably more than 7% by mass, and most preferably 8% by mass or more. On the other hand, the butene component content of the ethylene-propylene-butene copolymer is less than 15% by mass, preferably 14% by mass or less, more preferably 12% by mass or less. If the butene component content of the ethylene-propylene-butene copolymer is within the above range, the expanded beads have a better balance between moldability and rigidity. The butene used in the butene component of the copolymer is preferably 1-butene, which is a linear α-olefin. In addition, the ethylene-propylene-butene copolymer is preferably a random copolymer.

From the viewpoint of improving moldability, the ethylene component content of the ethylene-propylene-butene copolymer is preferably 0.1 mass% or more, more preferably 0.3 mass% or more, and even more preferably 0.4 mass% or more. From the viewpoint of improving moldability when the curing step is omitted and from the viewpoint of shortening the water cooling time during in-mold molding, the ethylene component content of the ethylene-propylene-butene copolymer is preferably 5 mass% or less, more preferably 4 mass% or less, even more preferably 3 mass% or less, particularly preferably 2 mass% or less, and most preferably 1.2 mass% or less.
The propylene content of the ethylene-propylene-butene copolymer is preferably 85% by mass or more, more preferably 86% by mass or more, and even more preferably 88% by mass. The propylene content of the ethylene-propylene-butene copolymer is preferably 98% by mass or less, more preferably 95% by mass or less, and even more preferably 92% by mass or less.

It is particularly preferable that the total amount of the butene component content C _bu and the ethylene component content C _et of the ethylene-propylene-butene copolymer is more than 6% by mass and not more than 15% by mass, and the butene component content C _bu is 6% by mass or more and less than 15% by mass, and further the ratio C _bu /C _et is most preferably 6 or more. In this case, the balance between the moldability and the rigidity is excellent, and the moldability is improved when the curing step is omitted. Furthermore, the water cooling time during in-mold molding is shortened.

The ethylene-propylene-butene copolymer may contain monomer components other than the ethylene component, the propylene component, and the butene component, but is preferably composed essentially of these three monomer components, and more preferably composed only of these three monomer components.
The contents of these monomer components can be determined by IR spectrum measurement.

The ethylene component, propylene component, and butene component of the ethylene-propylene-butene copolymer refer to the ethylene-derived structural units, propylene-derived structural units, and butene-derived structural units, respectively, in the ethylene-propylene-butene copolymer. The monomer components other than the ethylene component, propylene component, and butene component refer to the structural units derived from monomers other than the ethylene-derived structural units, propylene-derived structural units, and butene-derived structural units.
The content of each monomer component in the copolymer means the content of the constituent units derived from each monomer in the copolymer.

The polypropylene resin constituting the expanded beads may contain other polymers other than ethylene-propylene-butene copolymers, as long as the intended effect of the present invention is not impaired. Examples of other polymers include other polypropylene resins such as ethylene-propylene random copolymers, propylene-butene random copolymers, and propylene homopolymers, as well as thermoplastic resins other than polypropylene resins such as polyethylene resins and polystyrene resins. The content of other polymers in the polypropylene resin constituting the expanded beads is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, and is particularly preferably 0, that is, the polypropylene resin constituting the expanded beads contains only ethylene-propylene-butene copolymers as polymers.

From the viewpoint of achieving both moldability in the mold and heat resistance, the melting point of the polypropylene resin is preferably 135°C or higher, more preferably 136°C or higher, even more preferably 137°C or higher, and particularly preferably 138°C or higher. From the viewpoint of improving moldability in the mold, the melting point of the polypropylene resin is preferably 145°C or lower, more preferably 142°C or lower, and even more preferably 140°C or lower.

The melting point of polypropylene resin is determined based on JIS K7121:1987. Specifically, the conditioning is performed in the manner described in "(2) Measuring the 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. 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.

From the viewpoint of foaming property, the melt flow rate (MFR) of the polypropylene resin is preferably 2 to 10 g/10 min, and more preferably 5 to 9 g/10 min. The MFR of the polypropylene resin is a value measured based on JIS K7210-1:2014 at a test temperature of 230°C and a load of 2.16 kg.

The expanded beads preferably have a crystal structure in which a DSC curve obtained when 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. 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 melting inherent to the polypropylene resin constituting the expanded beads, and is considered to be due to endothermic heat generated when the crystals inherent to the polypropylene resin melt. On the other hand, the endothermic peak on the high temperature side of the resin-specific peak (i.e., high-temperature peak) is an endothermic peak that appears on the higher temperature side than the resin-specific peak in the DSC curve. When this high-temperature peak appears, it is presumed that secondary crystals exist in the resin. In addition, in the DSC curve obtained by heating from 23°C to 200°C at a heating rate of 10°C/min (i.e., the first heating) as described above, cooling from 200°C to 23°C at a cooling rate of 10°C/min, and then heating from 23°C to 200°C at a heating rate of 10°C/min again (i.e., the second heating), only the endothermic peak due to melting inherent to the polypropylene resin constituting the expanded beads is observed, so that the resin-specific peak and the high-temperature peak can be distinguished. The temperature at the apex of this resin-specific peak may differ slightly between the first and second heatings, but the difference is usually within 5°C.

The heat of fusion of the high temperature peak of the expanded beads is preferably 5 to 40 J/g, more preferably 7 to 30 J/g, and further 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.
It is believed that by setting the ratio of the heat of fusion of the high-temperature peak to the heat of fusion of the total melting peak in such a range, the expanded beads will have particularly excellent mechanical strength and excellent moldability in a mold due to the presence of secondary crystals appearing as a high-temperature peak.
Here, the heat of fusion of all melting peaks refers to the total heat of fusion determined from the areas of all melting peaks in a DSC curve.

If the flexural modulus of the polypropylene resin constituting the expanded beads is too low, the molded body may shrink and deform significantly when the curing step is omitted, and it may not be possible to obtain a molded body having a desired shape. From this viewpoint, the flexural modulus of the polypropylene resin is 800 MPa or more as described above. From the viewpoint of making it easier to suppress significant shrinkage and deformation of the molded body when the curing step is omitted, the flexural modulus is preferably 850 MPa or more, more preferably 880 MPa or more, and particularly preferably 900 MPa or more. In addition, from the viewpoint of improving moldability, the flexural modulus is preferably 1200 MPa or less, more preferably 1100 MPa or less, and even more preferably 1000 MPa or less.
The flexural modulus of the polypropylene resin can be determined based on JIS K7171:2008.

The expanded beads are cylindrical with through holes, with an average hole diameter d of less than 1 mm, and a ratio d/D of the average hole diameter d to the average outer diameter D of 0.4 or less. The expanded beads use a polypropylene resin containing the specific ethylene-propylene-butene copolymer as the base resin, and have the specific through holes, so that a molded article with good appearance and excellent strength during compression can be produced under a wide range of molding heating temperature conditions. Furthermore, even if the curing process is omitted, a good molded article can be produced. The reason why the moldability is improved by the expanded beads having through holes is thought to be that the heating medium such as steam supplied in the molding process passes through the through holes and reaches the inside of the expanded beads, so that the entire expanded beads filled in the mold are sufficiently heated, improving the secondary foaming and fusion properties of the expanded beads.

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 cylinder, a rectangular column, etc. It is more preferable that the expanded beads are cylindrical and have a cylindrical hole passing through the axial direction.
In order to improve the fusion property, the expanded beads may be coated with a coating layer. The coating layer is preferably made of, for example, a polyolefin resin having a melting point lower than that of the polypropylene resin constituting the expanded beads.

If the expanded beads do not have through holes, it may be difficult to mold a molded body at a low molding heating temperature. In addition, if the curing process is omitted, it may be difficult to manufacture a good molded body. On the other hand, even if the expanded beads have through holes, if the average pore diameter d is too large, the appearance of the molded body may be deteriorated and the strength during compression may be reduced. In addition, the secondary foaming property of the expanded beads during molding in a mold may be reduced. From this viewpoint, the average pore diameter d of the expanded beads is less than 1 mm as described above. From the viewpoint of further improving the appearance of the molded body, further improving the compression strength, and further improving the secondary foaming property of the expanded beads, the average pore diameter d of the expanded beads is preferably 0.9 mm or less, more preferably 0.85 mm or less, and even more preferably 0.8 mm or less. From the viewpoint of ease of manufacturing, the lower limit of the average pore diameter d of the expanded beads is approximately 0.2 mm or more.

The average pore diameter d of the through holes of the expanded beads is determined as follows. At least 50 expanded beads are randomly selected from the group of expanded beads and cut perpendicularly to the penetration direction of the through holes at the position where the area of the cut surface is maximum. The cut surface of each expanded bead is photographed, the cross-sectional area (specifically, the opening area) of the through hole portion is determined, and the diameter of a virtual perfect circle having the same area as that area is calculated. The arithmetic mean of these values is taken as the average pore diameter d of the through holes of the expanded beads.

From the viewpoint of improving the filling property of the expanded beads into the molding die and increasing the rigidity of the molded body, the average outer diameter D of the expanded beads is preferably 2 mm or more, more preferably 2.5 mm or more, even more preferably 3 mm or more, and is preferably 5 mm or less, more preferably 4.5 mm or less, even more preferably 4.3 mm or less.

If the ratio d/D of the average pore diameter d to the average outer diameter D of the expanded beads is too large, the appearance of the molded body may be deteriorated and the strength during compression may be reduced. In this case, the secondary expandability of the expanded beads during molding in the mold may be reduced. From this viewpoint, the ratio d/D is 0.4 or less as described above. From the viewpoint of improving the appearance of the molded body, improving the compressive strength, and improving the secondary expandability, d/D is preferably 0.35 or less, more preferably 0.3 or less, even more preferably 0.25 or less, and particularly preferably less than 0.2. From the viewpoint of ease of manufacture, the ratio d/D is preferably 0.1 or more.

The average outer diameter D of the expanded beads is determined as follows. At least 50 expanded beads randomly selected from the expanded bead group are cut perpendicular to the penetration direction of the through holes at the position where the area of the cut surface is maximum. The cut surface of each expanded bead is photographed, the cross-sectional area of the expanded bead (specifically, the cross-sectional area including the opening of the through hole) is determined, and the diameter of an imaginary perfect circle having the same area as that area is calculated. The arithmetic mean of these values is taken as the average outer diameter D of the expanded beads.

The average thickness t of the expanded beads is preferably 1.2 mm or more and 2 mm or less. If the average thickness t is within this range, the expanded beads will be sufficiently thick, and the secondary foaming properties during molding in the mold will be improved. In addition, from the viewpoint of making the expanded beads less susceptible to crushing by external forces and improving the compressive stress of the molded body, the average thickness t of the expanded beads is more preferably 1.3 mm or more, and even more preferably 1.5 mm or more.

The wall thickness t of an expanded bead is the distance from the surface of the expanded bead (i.e., the outer surface) to the outer edge of the through hole (i.e., the inner surface of the expanded bead), and is calculated by the following formula (1).
t=(D-d)/2...(1)
d: Average hole diameter of through holes (mm)
D: average outer diameter of foamed particles (mm)

The ratio t/D of the average wall thickness t to the average outer diameter D of the expanded beads is preferably 0.35 or more and 0.5 or less. If t/D is within the above range, the expanded beads can be well filled during in-mold molding, and the secondary foaming can be further improved. Therefore, a molded body with excellent rigidity can be produced at a lower molding heating temperature.

From the viewpoint of the balance between the light weight and rigidity of the molded body, the apparent density of the expanded beads is preferably 10 kg/m ³ or more and 150 kg/m ³ or less, more preferably 15 kg/m ³ or more and 100 kg/m ³ or less, and even more preferably 20 kg/m ³ or more and 80 kg/m ³ or less. Conventionally, when a molded body having a particularly low apparent density is produced, the molded body is easily deformed significantly after demolding, and it has been difficult to omit the curing step. In contrast, the expanded beads of the present disclosure can omit the curing step even when the apparent density is low, and a good molded body can be produced by molding without curing. Molding without curing refers to a molding method in which curing is not performed after molding in a mold.

The apparent density of the expanded beads can be determined by submerging expanded beads (weight W (g) of expanded beads) that have been left for one day in a measuring cylinder containing alcohol (e.g., ethanol) at 23°C under conditions of 50% relative humidity, 23°C, and 1 atm using a wire mesh or the like, determining the volume V (L) of the expanded beads from the rise in the water level, dividing the weight of the expanded beads by the volume of the expanded beads (W/V), and converting the unit to [kg/ ^m3 ].

From the viewpoint of 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.

The bulk density of the expanded beads is determined as follows: Expanded beads are randomly taken from the expanded bead group and placed in a measuring cylinder of 1 L volume, and a large number of expanded beads are placed up to the 1 L mark so as to form a natural pile-up state, and the mass W2 [g] of the expanded beads placed is divided by the placed volume V2 (1 L) (W2/V2) to convert the unit to [kg/ ^m3 ], thereby determining the bulk density of the expanded beads.

Expanded beads have excellent moldability in a mold, and good molded articles can be produced over a wide range of molding heating temperatures. Furthermore, good molded articles can be produced without significant shrinkage or deformation even if the curing step is omitted. The reason why the above expanded beads can produce good molded articles even if the curing step is omitted is not clear, but it is thought to be for the following reasons.

The expanded beads have a polypropylene resin having a specific or higher flexural modulus as the base resin. Therefore, it is believed that the molded body is easily prevented from shrinking after demolding, and dimensional changes are suppressed. In addition, the expanded beads are composed of an ethylene-propylene-butene copolymer having a specific copolymer composition and have through holes of a specific shape, so that molding can be performed at a lower molding pressure. Therefore, the amount of heat that the expanded beads receive from a heating medium such as steam during molding in the mold can be kept low, and dimensional changes due to thermal shrinkage of the molded body are suppressed. Furthermore, if the molded body has minute interconnected voids derived from the through holes of the expanded beads, air will flow into the bubbles inside the molded body quickly after demolding, and the internal pressure of the entire molded body will be increased, which is believed to make it easier for the dimensions of the molded body to stabilize early.
For the above reasons, it is believed that the expanded beads enable the production of good molded articles with stable dimensions even when the curing step is omitted.

[Method of manufacturing expanded beads]
The expanded beads can be produced, for example, by dispersing polypropylene-based resin particles having the above-mentioned ethylene-propylene-butene copolymer as a base resin in a dispersion medium (e.g., liquid), impregnating the resin particles with a blowing agent, and releasing the resin particles containing the blowing agent under low pressure (i.e., dispersion medium release foaming method). Specifically, it is preferable to disperse polypropylene-based resin particles having an ethylene-propylene-butene copolymer as a base resin in a dispersion medium in a sealed container, heat the mixture, and then pressurize the blowing agent to impregnate the resin particles with the blowing agent. Thereafter, it is preferable to carry out a holding step in which secondary crystals are grown at a constant temperature, and then release the contents in the sealed container under low pressure to expand the resin particles containing the blowing agent to obtain expanded beads.

(Production of polypropylene-based resin particles)
The resin particles used in the production of expanded beads are produced, for example, as follows. First, a base resin is fed into an extruder, heated and kneaded to form a resin melt. The base resin is an ethylene-propylene-butene copolymer, and additives such as a bubble nucleating agent are mixed in the extruder as necessary. The resin melt is then extruded from a small hole in a die attached to the tip of the extruder into a cylindrical strand having a through hole, and cooled and cut to obtain resin particles. The cutting method can be selected from a strand cut method, a hot cut method, an underwater cut method, and the like. In this way, cylindrical, non-foamed polypropylene-based resin particles having through holes can be obtained.

The particle diameter of the resin particles is preferably 0.1 to 3.0 mm, more preferably 0.3 to 1.5 mm. The length/diameter (specifically, 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 (calculated 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.

In addition, in the strand cut method, the particle size, length/diameter ratio, and average mass of the resin particles can be adjusted by appropriately changing the extrusion speed, take-up speed, cutter speed, etc. when extruding the molten resin.

(Production of expanded beads)
An aqueous dispersion medium is used as a dispersion medium (specifically, liquid) for dispersing the resin particles obtained as described above in a closed container. The aqueous dispersion medium is a dispersion medium mainly composed of 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.

Additives such as bubble adjusters, crystal nucleating agents, colorants, flame retardants, flame retardant assistants, plasticizers, antistatic agents, antioxidants, UV inhibitors, light stabilizers, conductive fillers, and antibacterial agents can be added to the resin particles as needed. Examples of bubble adjusters 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 bubble adjuster is added, the content of the bubble adjuster in the resin particles is preferably 0.01 to 1 part by mass per 100 parts by mass of the polypropylene-based resin.

In the above dispersion medium release foaming method, it is preferable to add a dispersant to the dispersion medium so that the polypropylene resin particles heated in the container do not fuse to each other in the container. Any dispersant can be used as the dispersant as long as it prevents the polypropylene resin particles from fusing in the container, and both organic and inorganic dispersants can be used, but fine inorganic particles are preferable because of their 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 are used. Among these, it is preferable to use clay minerals as the dispersant. It is preferable to add about 0.001 to 5 parts by mass of the dispersant per 100 parts by mass of the resin particles.

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

As a foaming agent for foaming polypropylene resin particles, it is preferable to use a physical foaming agent. Examples of physical foaming agents include inorganic physical foaming agents and organic physical foaming agents. Examples of inorganic physical foaming agents include carbon dioxide, air, nitrogen, helium, argon, etc. Examples of organic physical foaming agents include aliphatic hydrocarbons such as propane, butane, and hexane, cyclic aliphatic hydrocarbons such as cyclopentane and cyclohexane, 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. The physical foaming agents may be used alone or in combination of two or more. In addition, inorganic physical foaming agents and organic physical foaming agents may be used in combination. From the viewpoint of environmental load and ease of handling, inorganic physical foaming agents are preferably used, and more preferably carbon dioxide. When using an organic physical foaming agent, it is preferable to use n-butane, i-butane, n-pentane, or i-pentane from the standpoint of solubility in polypropylene resin and foaming ability.

The amount of foaming agent added per 100 parts by mass of polypropylene resin particles is preferably 0.1 to 30 parts by mass, and more preferably 0.5 to 15 parts by mass.

In the manufacturing process of expanded beads, the preferred method for impregnating the resin particles with the blowing agent is to disperse the resin particles in an aqueous dispersion medium in a closed container, and then pressurize the blowing agent into the resin particles while heating.

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 it is within the above range, the expanded particles can be safely produced without risk of damage or explosion of the sealed container.

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

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

During heating in the expanded particle manufacturing process, a first-stage holding step is performed in which the temperature is held at a temperature of (melting point of polypropylene resin -20°C) or more and less than (end of melting temperature of polypropylene resin) for a sufficient time, preferably about 10 to 60 minutes. After that, the temperature is adjusted to (melting point of polypropylene resin -15°C) to (end of melting temperature of polypropylene resin +10°C). If necessary, a second-stage holding step is performed in which the temperature is held for a further sufficient time, preferably about 10 to 60 minutes. Next, the expandable resin particles containing the foaming agent are released from the sealed container under low pressure and expanded to obtain expanded particles having the above-mentioned crystal structure. The expansion is preferably performed in the sealed container at a temperature of (melting point of polypropylene resin -10°C) or more, more preferably at (melting point of polypropylene resin) or more and less than (melting point of polypropylene resin +20°C).

The expanded beads obtained as described above can be pressurized with air or the like to increase the internal pressure of the bubbles, and then heated with steam or the like to expand (two-stage expansion) to produce expanded beads with a lower apparent density.

(Production of Molded Body)
The molded body can be obtained by molding the expanded beads in a mold (i.e., the in-mold molding method). The in-mold molding method is performed by filling the expanded beads in 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, whereby the expanded beads are heated to cause secondary expansion and are fused to each other to obtain a molded body having the shape of the molding space. It is also preferable to mold the expanded beads by a pressure molding method (see, for example, JP-B-51-22951). In the pressure molding method, the expanded beads are first pressurized in advance with a pressurized gas such as air to increase the pressure in the bubbles of the expanded beads, and the pressure in the expanded beads is adjusted to a pressure 0.01 to 0.3 MPa higher than atmospheric pressure. Next, the expanded beads are filled into the mold under atmospheric pressure or reduced pressure, and then a heating medium such as steam is supplied into the mold to heat-fuse the expanded beads. The expanded beads can also be molded by a compression filling molding method (see JP-B-4-46217). In the compression filling molding method, first, the expanded beads pressurized to atmospheric pressure or higher by compressed gas are filled into a mold. Then, a heating medium such as steam is supplied into the cavity to heat the expanded beads, and the expanded beads are heat-fused. The expanded beads can also be molded by a normal pressure filling molding method (see JP-B-6-49795). In the normal pressure filling molding method, first, expanded beads with high secondary expansion power obtained under special conditions are filled into the cavity of a mold under atmospheric pressure or reduced pressure. Then, a heating medium such as steam is supplied into the cavity to heat the expanded beads, and the expanded beads are heat-fused. The expanded beads can also be molded by a method combining the above molding methods (see JP-B-6-22919).

(Molded body)
The molded body is formed, for example, by molding the expanded beads in a mold, and is composed of a large number of expanded beads fused to each other. The molded body has interconnected voids. The interconnected voids of the molded body are formed by a complex combination of voids formed by interconnecting through-holes of a plurality of expanded beads, voids formed by interconnecting through-holes of the expanded beads with voids formed between the expanded beads, voids formed by interconnecting voids between the expanded beads, and the like.

From the viewpoint of achieving a good balance between appearance and mechanical properties, and from the viewpoint of making it easier to suppress significant shrinkage and deformation of the molded body when the curing process is omitted, the porosity of the molded body is preferably 4% or more and 12% or less, and more preferably 5% or more and 10% or less.

The porosity of a molded body can be determined 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 molded body. Next, this test piece is submerged in a graduated cylinder containing ethanol, and the true volume Vc [L] of the test piece is determined from the rise in the ethanol liquid level. In addition, the apparent volume Vd [L] is determined from the outer dimensions of the test piece. The porosity of the molded body can be determined from the determined true volume Vc and apparent volume Vd according to the following formula (2).
Porosity (%)=[(Vd-Vc)/Vd]×100...(2)

From the viewpoint of the balance between light weight and rigidity, the density of the molded body is preferably 10 kg/m ³ or more and 100 kg/m ³ or less, more preferably 15 kg/m ³ or more and 80 kg/m ³ or less, and even more preferably 20 kg/m ³ or more and 50 kg/m ³ or less. Conventional molded bodies are prone to significant shrinkage and deformation after demolding when the apparent density is low, so it was necessary to provide a curing process to restore the dimensions of the molded body. The molded body of the present disclosure has stable dimensions without providing a curing process even when the apparent density is low.

The above-mentioned expanded beads have excellent moldability at low molding heating temperatures, so the moldable temperature range in which good molded bodies can be obtained during the production of molded bodies is expanded. Furthermore, even if the curing process is omitted, molded bodies with good appearance and excellent mechanical strength can be obtained, so productivity is also excellent.

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples in any way.

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

<Polypropylene resin>
The properties of the polypropylene resins used are shown in Table 1. The ethylene-propylene-butene copolymers used in this example were all random copolymers.

(Monomer component content of polypropylene resin)
The monomer content of the polypropylene resin (specifically, ethylene-propylene-butene copolymer) was determined by a known method of determining by IR spectrum. Specifically, it was determined by the method described in Polymer Analysis Handbook (edited by Polymer Analysis Research Forum, Japan Society for Analytical Chemistry, published in January 1995, publisher: Kinokuniya Shoten, page numbers and item names: 615-616 "II.2.3 2.3.4 Propylene/ethylene copolymer", 618-619 "II.2.3 2.3.5 Propylene/butene copolymer"), that is, by a method of quantitatively determining the relationship between the value of the absorbance of ethylene and butene corrected by a predetermined coefficient and the thickness of a film-like test piece. More specifically, first, the polypropylene resin was hot pressed in a 180°C environment to form a film, and a plurality of test pieces with different thicknesses were prepared. Next, the IR spectrum of each test piece was measured to read the absorbance at 722 cm ^-1 and 733 cm ^-1 (A ₇₂₂ , A ₇₃₃ ) derived from ethylene, and the absorbance at 766 cm ^-1 (A ₇₆₆ ) derived from butene. Next, for each test piece, the ethylene component content in the polypropylene-based resin was calculated using the following formulas (3) to (5). The arithmetic average of the ethylene component contents obtained for each test piece 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)
In the formulas (3) to (5), _K'a is the apparent absorption coefficient at each wave number ( _K'a = A/ρt), _K'c is the corrected absorption coefficient, A is the absorbance, ρ is the density of the resin (unit: g/ ^cm3 ), and t is the 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 of each test piece was calculated using the following formula (6). The arithmetic average of the butene contents obtained for each test piece was defined as the ethylene content (%) in the polypropylene resin.
Butene component content (%) = 12.3 ( _A / L) ... (6)
In the formula (6), A is the absorbance, and L is the thickness (mm) of the film-like test piece.

(Melting point of polypropylene resin)
The melting point of the polypropylene resin was determined based on JIS K7121:1987. Specifically, "(2) Measurement of melting temperature after a certain heat treatment" was adopted as the conditioning, and the conditioned test piece 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 Co., Ltd., model number: DSC7020).

(Melt flow rate of polypropylene resin)
The melt flow rate (i.e., MFR) of the polypropylene-based 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.

(Flexural modulus of polypropylene resin)
A polypropylene resin was heat pressed at 230°C to prepare a 4 mm sheet, and a test piece measuring 80 mm in length, 10 mm in width, and 4 mm in thickness was cut out from the sheet. The flexural modulus of the 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 the supports was 64 mm, and the test speed was 2 mm/min.

<Expanded particles>
Table 2 shows the properties of the expanded beads.

(Average hole diameter d of through holes)
The average pore size of the through holes of the expanded beads was determined as follows. For 100 expanded beads randomly selected from the expanded beads after conditioning, the through holes were measured at the position where the area of the cut surface was maximum. The cut surface of each expanded bead was photographed, and the cross-sectional area (opening area) of the through-hole portion in the cross-sectional photograph was calculated. The diameters of the through-holes in the expanded beads were calculated, and the arithmetic mean of these was taken as the average pore size (d) of the through-holes in the expanded beads.

(Average outer diameter 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 perpendicularly to the penetration direction of the through holes at the position where the area of the cut surface was maximum. Photographs were taken of the cut surfaces of each expanded bead, and the cross-sectional area of the expanded beads (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.

(Average 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)

(Aspect ratio L/D)
Before measuring the average outer diameter D of the expanded beads and the average hole diameter d of the through holes, the maximum lengths of the through holes in the penetration direction of 100 expanded beads were measured with a vernier caliper, and the average length L of the expanded beads was calculated by arithmetically averaging these values. The average length L was then divided by the average outer diameter D to calculate the average aspect ratio L/D of the expanded beads.

(Apparent Density)
The apparent density of the expanded beads was determined as follows. First, a graduated cylinder containing ethanol at 23°C was prepared, and an arbitrary amount of the expanded beads after conditioning (mass W1 [g] of the expanded beads) was submerged in the ethanol in the graduated cylinder using a wire net. Then, taking into account the volume of the wire net, the volume V1 [L] of the expanded beads was measured from the rise in the water level. The mass W1 [g] of the expanded beads placed in the graduated cylinder was divided by the volume V1 [L] (W1/V1) and converted into units of [kg/ ^m3 ] to determine the apparent density of the expanded beads.

(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 of 1 L volume, and a large number of expanded beads were placed up to the 1 L mark so as to form a natural pile-up state, and the mass W2 [g] of the placed expanded beads was divided by the placed volume V2 (1 [L]) (W2/V2) to convert the unit to [kg/ ^m3 ] to determine the bulk density of the expanded beads.

(Heat of fusion of each peak in the DSC curve of the expanded beads)
One expanded bead was taken from the group of expanded beads after the conditioning. This expanded bead was used as a test piece, and a DSC curve was obtained by heating the test piece from 23°C to 200°C at a heating rate of 10°C/min using a differential scanning calorimeter (specifically, DSC.Q1000 manufactured by TA Instruments). An example of a DSC curve is shown in Figure 1. As shown in Figure 1, the DSC curve shows a resin-specific peak ΔH1 and a high-temperature peak ΔH2 having a peak at a higher temperature side 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 intrinsic peak ΔH1 and the high-temperature peak ΔH2, and the point where the lines L1 and L2 intersect was designated as δ. Note that point γ can also be considered as a maximum point existing between the resin intrinsic peak ΔH1 and the high-temperature peak ΔH2.
The area of the resin intrinsic peak ΔH1 is the area surrounded by the curve of the resin intrinsic peak ΔH1 portion of the DSC curve, the line segment α-δ, and the line segment γ-δ, and this was defined as the heat of fusion of the resin intrinsic 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 defined 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 average values are shown in Table 2.

<Molded body>
Table 3 shows the properties of the molded bodies.

(Evaluation of in-mold moldability)
The moldability in the mold is evaluated by examining the moldable range. Specifically, first, the molding steam pressure was changed by 0.02 MPa between 0.20 and 0.28 MPa (G) in increments of 0.02 MPa to mold the molded body by the method described later in <Production of molded body>. The molded body after demolding was left to stand in an oven at 80°C for 12 hours. The 12-hour stand in the oven is the curing process. After the curing process, the molded body was left to stand for 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm, to adjust the condition of the molded body. Next, the fusion property and recovery property (specifically, the recovery property of expansion or contraction after molding in the mold) of the molded body were evaluated. As a result, the one that met the following criteria was deemed to have passed, and the steam pressure at which all items were passed (i.e., the steam pressure at which a passing product could be obtained) was deemed to be the moldable steam pressure. The wider the range from the lower limit to the upper limit of the moldable steam pressure, the wider the moldable molding heating temperature range.

(Fusing ability)
The molded body was bent and broken, and the number C1 of expanded beads present on the fracture surface and the number C2 of broken expanded beads were determined, and the ratio of broken expanded beads to the above-mentioned expanded beads (i.e., material breakage rate) was calculated. The material breakage rate was calculated from the formula C2/C1×100. The above measurement was carried out five times using different test pieces, and the material breakage rate was calculated for each. When the arithmetic mean value of the material breakage rate was 80% or more, it was judged to be acceptable.

(Resilience)
The thickness of the molded product obtained using a flat mold 300 mm long, 250 mm wide, and 60 mm thick was measured near the four corners (specifically, 10 mm inward from the corners toward the center) and at the center (part that divides the product into two equal parts in both the vertical and horizontal directions). Next, the ratio (unit: %) of the thickness of the thinnest part to the thickness of the thickest part near the four corners was calculated, and a ratio of 95% or more was deemed to be acceptable.

(Evaluation of in-mold formability in untreated molding)
The fusion property and recovery property were evaluated in the same manner as in the evaluation of in-mold moldability described above, except that the molded body after demolding was left to stand for 12 hours at 23° C. without carrying out a curing step. Based on the evaluation results, the following criteria were used.
◯: Passing products were obtained at any molding steam pressure.
×: No passing product was obtained at any molding steam pressure.

In the <Production of Molded Body> described later, the molded body obtained at a molding pressure of 0.24 MPa (G) was left to stand for 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm to condition it, and the porosity, density, and 50% compressive stress of the molded body were measured, and the surface properties were evaluated. In Comparative Example 3, the molded body obtained at a molding pressure of 0.20 MPa (G) was conditioned and used for each measurement and evaluation.

(Porosity of Molded Body)
The porosity of the molded body was determined as follows. A rectangular parallelepiped test piece (20 mm long × 100 mm wide × 20 mm high) cut out from the center of the molded body was submerged in a graduated cylinder containing ethanol, and the true volume Vc [L] of the test piece was determined from the rise in the ethanol liquid level. In addition, the apparent volume Vd [L] was determined from the outer dimensions of the test piece. The porosity of the molded body was determined from the determined true volume Vc and apparent volume Vd according to the following formula (8).
Porosity (%) = [(Vd-Vc)/Vd] x 100... (8)

(Molded body density)
The density of the green body (kg/m ³ ) is calculated by dividing the weight (g) of the green body by the volume (L) determined from the outer dimensions of the green body, and converting the result into units.

(Compressive stress at 50% strain)
A test piece measuring 50 mm long x 50 mm wide x 25 mm thick was cut out from the center of the molded body so that the skin layer on the surface of the molded body was not included in the test piece. A compression test was carried out at a compression speed of 10 mm/min based on JIS K6767:1999 to determine the 50% compressive stress of the molded body.

(Surface property evaluation)
The evaluation was based on the following criteria.
A: The surface of the expanded bead molding has few interparticle gaps and shows good surface condition with inconspicuous unevenness caused by through-holes, etc. B: The surface of the expanded bead molding has some unevenness caused by interparticle gaps and/or through-holes, etc. C: The surface of the expanded bead molding has significant unevenness caused by interparticle gaps and/or through-holes, etc.

(Evaluation of water cooling time)
In the <Production of Molded Body> described below, after heating, 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 dropped to 0.04 MPa (G) (i.e., the water cooling time) was measured, and the water cooling time was evaluated based on the measured water cooling time according to the following criteria. The water cooling time was evaluated based on the water cooling time during molding at a molding pressure of 0.26 MPa (G). The reason for adopting the condition of a molding pressure of 0.26 MPa (G) for the evaluation of the water cooling time is that the higher the molding pressure, the longer the water cooling time tends to be.
A: Water cooling time is 70 seconds or less. B: Water cooling time is more than 70 seconds but less than 120 seconds. C: Water cooling time is more than 120 seconds.

The methods for producing the expanded beads and the molded articles in Examples 1 to 3 and Comparative Examples 1 to 4 will be described below.
Example 1
<Production of polypropylene-based expanded beads>
Polypropylene resin 1 (abbreviated as PP1) was melt-kneaded in an extruder at a maximum set temperature of 230°C to obtain a resin melt. PP1 is an ethylene-propylene-butene copolymer, with a butene content of 9.0% by mass and an ethylene content of 1.0% by mass. The properties of PP1 are shown in Table 1. Next, the resin melt was extruded from a small hole in a die attached to the tip of the extruder into a cylindrical strand having a through hole, and the strand was water-cooled while being taken out, and then cut into a mass of about 1.5 mg with a pelletizer. In this way, cylindrical polypropylene resin particles having a through hole were obtained. The aspect ratio of the polypropylene resin particles was 2.5. In addition, when producing the resin particles, zinc borate was supplied to the extruder as a bubble adjuster, and 500 ppm by mass of zinc borate was contained in the polypropylene resin.

1 kg of polypropylene-based resin particles was placed in a 5-L sealed container together with 3 L of water as a dispersion medium, and 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 relative to 100 parts by mass of resin particles. After adding 70 g of dry ice as a foaming agent to the sealed container, the sealed container was sealed and heated to a foaming temperature of 143.2°C while stirring the contents of the sealed container. The pressure inside the container (impregnation pressure) at this time was 3.1 MPa (G). After holding at the same temperature (i.e., 143.2°C) for 15 minutes, the contents of the container were released under atmospheric pressure to obtain foamed particles. The foamed particles were dried at 23°C for 24 hours.

Next, the expanded particles were placed in a pressure vessel, and air was forced into the vessel to increase the pressure inside the vessel, thereby impregnating the air bubbles and increasing the internal pressure inside the bubbles of the expanded particles. Next, steam was supplied to the expanded particles (first-stage expanded particles) removed from the pressure vessel so that the pressure inside the pressure vessel reached the pressure shown in Table 2, and the particles were heated under atmospheric pressure. The pressure inside the bubbles (gauge pressure) of the first-stage expanded particles removed from the pressure vessel was the value shown in Table 2. As a result, the apparent density of the first-stage expanded particles was reduced, and expanded particles (second-stage expanded particles) were obtained.

<Production of Molded Product>
For the manufacture of the molded body, the expanded beads obtained by the above-mentioned two-stage expansion were dried at 23°C for 24 hours and used. The dried expanded beads were pressurized with compressed air to set the internal pressure in the expanded beads to 0.10 MPa (G). Next, the expanded beads were filled into a flat mold measuring 300 mm long x 250 mm wide x 60 mm thick, with the cracking amount adjusted to 6 mm, and the mold was clamped and steam was supplied from both sides of the mold for 5 seconds to perform an exhaust process of preheating. Thereafter, steam was supplied from one side of the mold until a pressure 0.08 MPa (G) lower than the predetermined molding pressure was reached, and one-sided heating was performed. Next, steam was supplied from the other side of the mold until a pressure 0.04 MPa (G) lower than the molding pressure was reached, and then heating was performed until a predetermined molding pressure was reached (i.e., main heating). After the heating was completed, the pressure was released, and the molded body was cooled with water until the surface pressure due to the foaming force of the molded body was 0.04 MPa (G), after which the mold was opened and the molded body was taken out. Next, a curing step was carried out by leaving the molded body in an oven at 80° C. for 12 hours. In this manner, a molded body was produced.

Example 2
Expanded beads were obtained in the same manner as in Example 1, except that polypropylene resin 1 was changed to polypropylene resin 2 (abbreviated as PP2) and the expansion temperature was changed to the value shown in Table 2. In addition, a molded article was obtained in the same manner as in Example 1. PP2 is an ethylene-propylene-butene copolymer, and its monomer component contents and the like are shown in Table 1.

Example 3
Expanded beads were obtained in the same manner as in Example 1, except that polypropylene resin 1 was changed to polypropylene resin 3 (abbreviated as PP3) and the impregnation pressure and expansion temperature were changed to values shown in Table 2. A molded article was also obtained in the same manner as in Example 1. PP3 is an ethylene-propylene-butene copolymer, and its monomer component contents and the like are shown in Table 1.

(Comparative Example 1)
The expanded beads were obtained in the same manner as in Example 1, except that resin beads without through holes were produced during the production of polypropylene-based resin beads. The expanded beads in this example were approximately spherical expanded beads without through holes. A molded body was obtained in the same manner as in Example 1.

(Comparative Example 2)
In producing polypropylene-based resin particles, expanded particles were obtained in the same manner as in Example 1, except that the hole diameter of the small holes in the die for forming the through holes was changed and the carbon dioxide pressure and the expansion temperature were changed to the values shown in Table 2. Furthermore, a molded body was obtained in the same manner as in Example 1.

(Comparative Example 3)
Expanded beads were obtained in the same manner as in Example 1, except that polypropylene resin 1 was changed to polypropylene resin 4 (abbreviated as PP4) and the impregnation pressure and expansion temperature were changed to values shown in Table 2. Also, a molded article was obtained in the same manner as in Example 1. PP4 is an ethylene-propylene-butene copolymer, and its monomer component contents and the like are shown in Table 1.

(Comparative Example 4)
Expanded beads were obtained in the same manner as in Example 1, except that polypropylene resin 1 was changed to polypropylene resin 5 (abbreviated as PP5) and the impregnation pressure and expansion temperature were changed to values shown in Table 2. Also, a molded article was obtained in the same manner as in Example 1. PP5 is an ethylene-propylene copolymer, and its monomer component contents and the like are shown in Table 1.

As can be seen from Tables 2 and 3, the expanded beads of Examples 1 to 3 can be used to produce molded articles with good appearance and excellent strength when compressed over a wide range of molding pressures. Furthermore, even if the curing process is omitted, molded articles with good appearance and excellent mechanical strength can be produced.

Claims (9)

A cylindrical polypropylene-based resin foamed particle having a through hole,
the average pore diameter d of the through holes of the expanded beads is 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 polypropylene-based resin constituting the expanded beads is an ethylene-propylene-butene copolymer,
the total amount of the butene component content and the ethylene component content in the ethylene-propylene-butene copolymer is 2% by mass or more and 15% by mass or less, and the ratio of the butene component content [mass%] to the ethylene component content [mass%] is 2 or more;
The melting point of the polypropylene resin is 135° C. or higher and 145° C. or lower,
The expanded polypropylene resin particles have a flexural modulus of 800 MPa or more. A cylindrical polypropylene-based resin foamed particle having a through hole,
the average pore diameter d of the through holes of the expanded beads is 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 polypropylene-based resin constituting the expanded beads is an ethylene-propylene-butene copolymer,
the total amount of the butene component content and the ethylene component content in the ethylene-propylene-butene copolymer is 2% by mass or more and 15% by mass or less, and the ratio of the butene component content [% by mass] to the ethylene component content [% by mass] is 6 or more;
The expanded polypropylene resin particles have a flexural modulus of 800 MPa or more. The polypropylene-based resin expanded particles according to claim 1 or 2, wherein the total amount of the butene component content and the ethylene component content in the ethylene-propylene-butene copolymer is more than 6% by mass and not more than 15% by mass, and the butene component content is 6% by mass or more and less than 15% by mass. The polypropylene resin expanded particles according to any one of claims 1 to 3, wherein the ethylene component content in the ethylene-propylene-butene copolymer is 0.05% by mass or more and 3% by mass or less. The expanded polypropylene resin particles according to any one of claims 1 to 4 , wherein the average outer diameter D is 2 mm or more and 5 mm or less. The expanded polypropylene resin beads according to any one of claims 1 to 5 , wherein the average wall thickness t of the expanded polypropylene resin beads is 1.2 mm or more and 2 mm or less. The expanded polypropylene resin particles according to any one of claims 1 to 6 , wherein the ratio d/D is 0.25 or less. The expanded polypropylene resin beads according to any one of claims 1 to 7 , wherein the expanded beads have an apparent density of 10 kg/m3 or ^more and 100 kg/ ^m3 or less. 9. An expanded bead molding comprising the expanded polypropylene resin beads according to claim 1 , fused together and having interconnected voids.

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