Method for producing expanded polypropylene resin beads

Abstract

To provide a production method of a polypropylene resin foamed particle molding that is excellent in filling ability of a foamed particle in a molding die, can shorten a cooling time after molding in the die and can easily produce a light and good foamed particle molding.SOLUTION: In a production method of a polypropylene resin foamed particle molding, a foamed particle 1 compressed by a pressurized gas is filled in a molding die and then a heating medium is supplied into the molding die to mold the foamed particle 1 in the molding die. The foamed particle 1 has a cylindrical shape with an open hole 11. The open hole 11 has an average pore diameter d of 0.1 mm or more and less than 1 mm and a ratio d / D of the average pore diameter d of the open hole 11 to the average outer diameter D of the foamed particle 1 of 0.4 or less. The foamed particle 1 filled in the molding die has a compressibility P of 20% or more and 80% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a foamed polypropylene resin bead molded article. [Background technology]

[0002] Expanded polypropylene resin bead moldings are lightweight and have excellent shock-absorbing properties, rigidity, etc., and are therefore used in a variety of applications such as packaging materials, containers, cushioning materials, etc. In recent years, the applications of expanded polypropylene resin bead moldings have been expanding, and this has led to a demand for molding the expanded polypropylene resin beads into various shapes.

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

[0004] In in-mold molding, if the mold is not filled sufficiently with expanded beads, the appearance of the resulting molded article is likely to deteriorate. Therefore, a method known as cracking filling is available as a way to fill the mold as densely as possible with expanded polypropylene resin beads (see, for example, Patent Document 1). In cracking filling, the mold is not completely closed, but the movable mold is slightly retracted from the fixed mold in the mold opening direction, and the expanded beads are filled into the cavity, and then the mold is closed to mechanically compress the expanded beads in the cavity. Because cracking filling does not require any special equipment, it is a method commonly used in in-mold molding.

[0005] However, cracking filling has the problem that, because the expanded beads are compressed in one direction by clamping the mold, the difference in the amount of compression of the expanded beads can become large locally depending on the shape of the desired molded body, and the resulting molded body is likely to have high-density and low-density portions. In particular, when producing a molded body having a thick portion and a thin portion that is thinner than the thick portion, there is a problem that a difference in density is likely to occur between the thick portion and the thin portion. Another problem with cracking filling is that the expanded beads are not easily able to fill the thin portion.

[0006] On the other hand, one method for filling a mold with expanded polypropylene resin beads is called compression filling, in which the expanded beads are compressed by pressurized gas while being filled into the mold (see, for example, Patent Document 2). In compression filling, the expanded beads are filled into the mold in an isotropically compressed state. Furthermore, in compression filling, the expanded beads are filled while the mold is closed so that the cavity of the mold has a shape corresponding to the shape of the molded body to be obtained, so the mold does not need to be closed after filling. Therefore, compression filling can reduce local differences in the density of the molded body. Furthermore, compression filling can fill the expanded beads relatively uniformly throughout the entire mold, even when attempting to obtain a molded body having thick and thin portions or a molded body with a complex shape. Furthermore, compression filling is excellent at filling the expanded beads into thin portions, so compression filling can easily produce molded bodies having thick and thin portions. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 46-38359 [Patent Document 2] Japanese Patent Publication No. 151325 / 1983 Summary of the Invention [Problem to be solved by the invention]

[0008] When the expanded beads are filled into a mold by compression filling, it is desirable to increase the compression ratio of the expanded beads in order to improve the fillability of the expanded beads into the mold. However, increasing the compression ratio of the expanded beads in the in-mold molding method of Patent Document 2 poses the problem of requiring a significantly long time for the molded body to cool in the mold. Furthermore, increasing the compression ratio of the expanded beads in the in-mold molding method of Patent Document 2 poses the problem of reducing the fusion strength between the expanded beads. Furthermore, increasing the compression ratio of the expanded beads poses the problem of increasing the density of the resulting molded body.

[0009] The present invention has been made in view of the above background, and aims to provide a method for producing a polypropylene resin foamed bead molding that has excellent filling properties for foamed beads in a molding die, has a short cooling time after molding in the die even when the compression ratio is increased, and can easily produce a lightweight, high-quality foamed bead molding. [Means for solving the problem]

[0010] One aspect of the present invention is a method for producing a molded article made of expanded polypropylene-based resin beads, which comprises filling a mold with expanded polypropylene-based resin beads compressed by a pressurized gas, and then supplying a heating medium into the mold to in-mold the expanded polypropylene-based resin beads in the mold, The expanded polypropylene resin particles have a cylindrical shape with through holes, the average pore diameter d of the through holes of the expanded polypropylene resin beads is 0.1 mm or more and less than 1 mm, and the ratio d / D of the average pore diameter d of the through holes to the average outer diameter D of the expanded polypropylene resin beads is 0.4 or less; The method for producing a polypropylene-based resin expanded bead molded article is characterized in that, when the expanded polypropylene-based resin beads are filled in the molding die, the compressibility P of the expanded polypropylene-based resin beads, as represented by the following formula (1), is 20% or more and 80% or less. P=[{a / (b×c)}-1]×100 ···(1)

[0011] In the formula (1), a is the mass (unit: kg) of the expanded polypropylene resin particles filled in the mold, and b is the bulk density (unit: kg / m 3 ), and c is the internal volume of the mold (unit: m 3 ) [Effects of the Invention]

[0012] According to the above-described embodiment, a method for producing a polypropylene-based resin foamed bead molding can be provided, which has excellent filling properties for the foamed beads in the molding die, and even when the compression ratio is increased, the cooling time after molding in the die is short, and which allows for the easy production of a lightweight and good foamed bead molding. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram of the appearance of expanded beads. [Figure 2] FIG. 2 is a cross-sectional view taken along line II-II in FIG. [Figure 3] FIG. 3 is an explanatory diagram that schematically shows a molding apparatus used to prepare a molded body. DETAILED DESCRIPTION OF THE INVENTION

[0014] 1 and 2, the expanded beads 1 used in the production method have a cylindrical shape with through holes 11. The average hole diameter d of the through holes 11 is 0.1 mm or more and less than 1 mm, and the ratio d / D of the average hole diameter d of the through holes 11 to the average outer diameter D of the expanded beads 1 is 0.4 or less. The structure of the expanded beads 1 will be described in more detail later.

[0015] In the manufacturing method, the foamed beads having the specific shape are filled into a mold while being compressed by a pressurized gas, such as compressed air.

[0016] In the above-described manufacturing method, an internal pressure application step may be performed to apply internal pressure to the expanded beads before filling them into the mold, thereby increasing the pressure within the cells of the expanded beads before filling the mold. However, as described below, because the expanded beads have high secondary expandability, a good molded article can be easily obtained even without applying internal pressure beforehand. From the viewpoint of further simplifying the manufacturing process for molded articles and increasing productivity, the pressure within the cells of the expanded beads filled into the mold is preferably 0.1 MPa (G) or less in gauge pressure, more preferably 0.05 MPa (G) or less, and even more preferably 0.03 MPa (G) or less. It is preferable to set it to 0 MPa (G), i.e., to perform compression filling without increasing the pressure within the cells of the expanded beads. The pressure within the cells (internal pressure) can be measured, for example, by the method described in JP 2003-201361 A.

[0017] After the foamed beads have been completely filled into the mold, the pressure inside the mold is released. This allows the foamed beads, which had been compressed by the pressurized gas inside the mold, to return to their original size, filling the mold with the foamed beads. A heating medium is then supplied into the mold to heat the foamed beads. Steam, for example, can be used as the heating medium. The foamed beads inside the mold are heated by the heating medium and fuse together while undergoing secondary foaming. This allows the foamed beads inside the mold to be integrated, forming a molded product.

[0018] After the heating of the expanded beads is completed, the molded body in the mold is cooled to stabilize its shape. The molded body is then removed from the mold, completing the in-mold molding. In the above-described manufacturing method, after the in-mold molding is completed, a curing step may be performed in which the molded body is left standing in a high-temperature atmosphere to suppress shrinkage and deformation of the molded body. However, according to the above-described manufacturing method, shrinkage and deformation of the molded body can be suppressed even without the curing step, as described below. Specifically, the curing step refers to a step in which the molded body removed from the mold is left standing in an atmosphere of about 60 to 80°C for 12 hours or more to stabilize the shape of the molded body.

[0019] In the above manufacturing method, the compression ratio P of the expanded beads in the mold after the compression filling is completed is set to 20% or more and 80% or less. Here, the compression ratio P of the expanded beads is calculated by multiplying the mass a (unit: kg) of the expanded beads filled in the mold by the bulk density b (unit: kg / m 3 ) and the inner volume of the mold c (unit: m 3 ) and is expressed by the following formula (1). P=[{a / (b×c)}-1]×100 ···(1)

[0020] By setting the compression ratio P of the expanded beads to 20% or more, the expanded beads can be more easily packed into the mold, and the expanded beads can be uniformly packed throughout the mold, even when attempting to produce a molded product with a large difference in thickness between the thick and thin parts or a complex shape. This also reduces the local density variation of the resulting molded product. Furthermore, the formation of gaps between the expanded beads on the surface of the molded product can be suppressed, resulting in a good molded product.

[0021] To ensure these effects, the compression ratio P of the expanded beads is preferably greater than 25%, more preferably greater than 30%. If the compression ratio P of the expanded beads is too low, it may be difficult to uniformly fill the mold with the expanded beads when producing a molded article with a large difference in thickness between the thick and thin portions or a complex shape. In this case, gaps between the expanded beads are likely to form on the surface of the molded article, which may lead to a deterioration in the appearance of the molded article.

[0022] By setting the compression rate of the expanded beads to 80% or less, the bulk density of the expanded beads can be utilized, making it possible to easily obtain a lightweight molded body. Furthermore, during in-mold molding, the heating medium can be easily supplied throughout the entire mold, improving the fusion between the expanded beads. Furthermore, even when the expanded beads are heated at a low molding heating temperature (i.e., a low molding pressure), a good molded body can be easily obtained. Furthermore, the cooling time of the molded body in the mold can be shortened.

[0023] To ensure these effects, the compression ratio P of the expanded beads is preferably 72% or less, and more preferably 65% ​​or less. If the compression ratio P of the expanded beads is too high, the density of the molded body tends to be high relative to the bulk density of the expanded beads, which may make it difficult to obtain a lightweight molded body. Furthermore, the restoring force of the expanded beads when released from the compressed state may be excessively high, which may lengthen the cooling time of the molded body in the mold. Furthermore, if the curing step is omitted, the molded body may shrink significantly or deform.

[0024] (Polypropylene resin foam particles) The structure of the expanded polypropylene resin particles used in the above-mentioned production method will be described in detail below.

[0025] The average outer diameter D of the expanded beads and the average diameter d of the through-holes 1 and 2, the expanded beads 1 have a cylindrical shape with through holes 11. The average diameter d of the through holes 11 is 0.1 mm or more and less than 1 mm, and the ratio d / D of the average diameter d of the through holes 11 to the average outer diameter D of the expanded beads 1 is 0.4 or less.

[0026] By performing compression filling using the expanded beads having the specific shape, it is possible to improve the packing property of the expanded beads into the molding die and shorten the cooling time after molding in the mold even when the compression ratio P is relatively high. Furthermore, by using the expanded beads having the specific shape, it is possible to easily obtain a lightweight and good molded product within the wide range of the compression ratio P described above.

[0027] The expanded beads may have an overall shape of, for example, a cylindrical shape or a prismatic shape. The through-holes preferably penetrate the expanded beads in the axial direction. The number of through-holes may be one or more.

[0028] If the expanded beads do not have through-holes, it may be difficult to mold the molded article at a low molding heating temperature depending on the compression ratio P, and the fusion property may be significantly reduced. Also, the cooling time of the molded article in the mold may be significantly extended. Furthermore, if the curing step is omitted, it may not be possible to suppress significant shrinkage or deformation of the molded article.

[0029] On the other hand, even if the expanded beads have through holes, if the average hole diameter d of the through holes is too large, setting the compression ratio P within the above-mentioned specific range may result in the formation of gaps between the expanded beads or irregularities due to the through holes on the surface of the molded article. Furthermore, if the compression ratio P of the expanded beads is increased to avoid the formation of gaps between the expanded beads or irregularities due to the through holes, the density of the molded article may increase significantly, making it difficult to obtain a lightweight molded article. Furthermore, if the compression ratio P is increased, the molding heating temperature required to obtain a good molded article increases, and if the curing step is omitted, significant shrinkage and deformation of the molded article may not be suppressed. These problems can be easily avoided by setting the average hole diameter d of the through holes to 1.0 mm or less. From the same perspective, the average hole diameter d of the through holes is preferably 0.95 mm or less, more preferably 0.92 mm or less, and even more preferably 0.90 mm or less.

[0030] Furthermore, by setting the average diameter d of the through holes to 0.1 mm or more, the through holes of the expanded beads are prevented from being crushed and blocked during molding in the mold, and the effects of the through holes can be more reliably exhibited. From the same viewpoint, the average diameter d of the through holes is preferably 0.2 mm or more, and more preferably 0.4 mm or more.

[0031] The average pore diameter d of the through holes can be adjusted to the above-mentioned specific range by adjusting the average pore diameter dr of the through holes in the resin particles, which will be described later, the apparent density of the expanded beads, etc. Furthermore, by using two-stage expanded beads produced by two-stage expansion, the average pore diameter d can be more easily adjusted to a small value.

[0032] The molded article obtained by the above-described manufacturing method is less likely to shrink or deform, even without a curing step. The following reasons are thought to be the reasons for this effect: Because the expanded beads have through-holes, it is believed that when a heating medium is supplied into the molding die, the heating medium can pass through the through-holes. This makes it easier for the heating medium to reach the interior of the molding die, and it is thought that the entire expanded beads in the molding die can be easily heated. Furthermore, as described below, the expanded beads have sufficient secondary foaming properties. Therefore, even under conditions where the molding heating temperature during in-mold molding is low, a molded article with excellent fusion properties and good appearance can be obtained.

[0033] Furthermore, as described above, the manufacturing method allows the molding heating temperature during in-mold molding to be low, thereby reducing the amount of heat received by the expanded beads 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. As a result, the dimensions of the molded body after in-mold molding tend to stabilize quickly.

[0034] Furthermore, the molded article after in-mold molding has minute voids originating from the through-holes of the expanded beads, and it is believed that these voids are continuous from the surface to the interior of the molded article. Therefore, when the molded article is removed from the mold after in-mold molding, air quickly flows into the bubbles inside the molded article through the voids, and as a result, it is believed that the internal pressure of the molded article quickly stabilizes.

[0035] Therefore, it is believed that the molded body obtained by the above-mentioned manufacturing method can suppress shrinkage and deformation of the molded body without performing a curing process, due to the synergistic effect of the above-mentioned effect of reducing the molding pressure and the effect of quickly stabilizing the internal pressure of the molded body.

[0036] The average pore diameter d of the through holes of expanded beads is determined as follows. First, as shown in Figure 2, an expanded bead is cut perpendicularly to the penetration direction of the through holes at a position where the area of ​​the cross section is maximum. Next, a photograph of the cut surface is taken, and the cross-sectional area of ​​the through holes (specifically, the opening area) is calculated. Then, the diameter of an imaginary perfect circle having the same area as the cross-sectional area of ​​the through holes is calculated, and this value is used as the pore diameter of the through holes of each expanded bead. The above procedure is performed on 50 or more expanded beads, and the arithmetic mean value of the obtained pore diameters of the through holes 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 area of ​​the cross section of the expanded bead is maximum, as described above.

[0037] 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 property of the expanded beads into the molding die, the average outer diameter D of the expanded beads is preferably 5 mm or less, more preferably 4.5 mm or less, and even more preferably 4.3 mm or less.

[0038] The ratio d / D of the average pore diameter d of the through holes to the average outer diameter D of the expanded beads is 0.4 or less. If the ratio d / D exceeds 0.4, the secondary expandability of the expanded beads during in-mold molding may decrease, which may result in a poor appearance of the molded article and a decrease in rigidity. From the viewpoint of further improving the secondary expandability of the expanded beads and improving the appearance and rigidity of the molded article, d / D is preferably 0.35 or less, more preferably 0.3 or less, and even more preferably 0.25 or less. On the other hand, from the viewpoint of further preventing the through holes of the expanded beads from being crushed and blocked during in-mold molding and more reliably exhibiting the effects of the through holes, the ratio d / D is preferably 0.1 or more.

[0039] The average outer diameter D of an expanded bead is determined as follows. First, as shown in Figure 2, an expanded bead is cut perpendicular to the penetration direction of the through holes at a position where the area of ​​the cross section is maximum. Next, a photograph of the cut surface of the expanded bead is taken, and the cross-sectional area of ​​the expanded bead, including the opening area of ​​the through holes, is calculated. The diameter of an imaginary circle having the same area as the cross-sectional area of ​​the expanded bead is calculated, and this value is used as the outer diameter of each expanded bead. The above procedure is performed on 50 or more expanded beads, and the arithmetic mean value of the obtained outer diameters of the expanded beads is used 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 area of ​​the cross-section of the expanded bead in the direction perpendicular to the penetration direction is maximum, as described above.

[0040] Furthermore, according to the above-mentioned production method, a good molded article can be easily obtained even when no internal pressure is applied to the expanded beads before filling the mold, as described above. Generally, in-mold molding of expanded polypropylene resin beads, an internal pressure application step is performed to enhance the secondary expandability of the expanded beads by impregnating the cells of the expanded beads with an inorganic gas such as air to increase the internal pressure of the cells. The expanded beads are then filled into the mold after being imparted with expandability. In contrast, in compression filling, the expanded beads are filled into the mold in a compressed state by pressurized gas. After filling, the pressure within the cells of the expanded beads is higher than atmospheric pressure, and the restoring force upon pressure release enhances the secondary expandability of the expanded beads. Furthermore, as described above, the expanded beads have an average pore size d and a ratio d / D within a predetermined range, demonstrating excellent secondary expandability. Therefore, according to the above-mentioned production method, the internal pressure application step can be omitted, further simplifying the production process of the molded article and increasing productivity.

[0041] Apparent density and bulk density of foamed particles The bulk density of the foam particles is 10 kg / m 3 More than 100kg / m 3 It is preferable that the saturation is 15 kg / m or less. 3 More than 50kg / m 3 More preferably, it is 20 kg / m or less. 3More than 40kg / m 3 It is more preferable that the weight and rigidity of the molded body are improved in a well-balanced manner.

[0042] 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 more than 1.6 and not more than 2.0, and more preferably 1.7 or more and 1.9 or less. In this case, the cooling time after molding in the mold can be further shortened, and a lightweight and good expanded bead molded article can be more easily obtained. In addition, deformation and shrinkage of the molded article can be more reliably suppressed when the curing step is omitted.

[0043] The method for calculating the bulk density of expanded beads is as follows. First, the expanded beads are left to stand for 24 hours or more in an environment of 50% relative humidity, 23°C temperature, and 1 atm atmospheric pressure to condition the expanded beads. The expanded beads thus obtained are filled into a measuring cylinder so that they naturally accumulate, and the bulk volume (unit: L) of the expanded beads is read from the measuring cylinder's scale. Then, the mass (unit: g) of the expanded beads in the measuring cylinder is divided by the aforementioned bulk volume, and the value is converted into units to determine the bulk density of the expanded beads (unit: kg / m 3 The bulk density of the expanded beads is a value measured using uncompressed expanded beads in their natural state under an atmospheric pressure of 1 atm.

[0044] The method for calculating the apparent density of expanded beads is as follows. First, the expanded beads are left to stand for one day in an environment of 50% relative humidity, 23°C temperature, and 1 atm atmospheric pressure to condition the expanded beads. After measuring the mass (unit: g) of the expanded beads, they are submerged in a measuring cylinder containing alcohol (e.g., ethanol) at 23°C using a wire netting or the like, and the volume (unit: L) of the expanded beads is calculated from the rise in the liquid level. The apparent density of the expanded beads (unit: kg / m) is then calculated by converting the value obtained by dividing the mass of the expanded beads by the volume of the expanded beads. 3 ) can be calculated.

[0045] -Circularity of through holes The average circularity of the through-holes of the expanded beads is preferably 0.90 or more, more preferably 0.92 or more, and even more preferably 0.95 or more. Through-holes with a high circularity are less likely to collapse when compressed from all directions, and therefore are less likely to close even when the pressure in the molding die is released after compression and filling. Therefore, expanded beads having an average circularity of the through-holes within the above-mentioned specific range can more reliably achieve the effects of lowering the molding heating temperature and shortening the cooling time after molding in the mold, even when the compression ratio P is increased. The average circularity of the through-holes can be adjusted to within the above range in the resin bead production method described below, for example, by changing the shape of the die for forming the through-holes or by adjusting the water temperature during cooling of the extrudate, which is usually around 25°C, to a lower temperature (e.g., 15°C or lower), instead. The upper limit of the average circularity of the through-holes is 1. As described above, the average diameter d of the through-holes of the expanded beads is 0.1 mm or more and less than 1 mm. In expanded beads having such a small average pore diameter d, the through holes are easily crushed during production, and the circularity of the through holes is easily reduced. However, according to the above method, expanded beads having through holes with a high circularity can be easily obtained.

[0046] The average circularity of the through holes is calculated by the following method. First, as shown in Figure 2, an expanded bead is cut perpendicular to the penetration direction of the through holes at a position where the area of ​​the cross section is maximum. Next, a photograph of the cut section of the expanded bead is taken, and the cross-sectional area S of the through holes (i.e., the opening area of ​​the through holes on the cut section) and the perimeter C (i.e., the length of the outline of the through holes on the cut section) are determined. The circularity of the through holes of each expanded bead is a value calculated based on the following formula (2) using the cross-sectional area S, perimeter C, and pi of the through holes described above. Circularity = 4πS / (C × C) (2)

[0047] The above-mentioned procedure is performed on 50 or more expanded beads, and the obtained values ​​are arithmetically averaged to obtain the average circularity of the through holes. Even if the size of the through holes of each expanded bead is not uniform in the penetration direction, the circularity of each through hole of each expanded bead is determined by the cross-sectional area and perimeter of the through hole at the position where the area of ​​the cross section of the expanded bead is maximum, as described above.

[0048] ·Average wall thickness The average wall thickness t of the expanded beads is preferably 1.2 mm or more and 2 mm or less. In this case, the rigidity of the expanded beads is increased, so that the shape of the through holes is more likely to be maintained even when the pressure in the mold is released after compression filling, and the through holes are less likely to be blocked. As a result, the effects of the through holes, such as shortening the cooling time, can be more reliably achieved. In addition, in this case, the secondary expandability of the expanded beads can be further improved. From this viewpoint, the average wall thickness t of the expanded beads is preferably 1.3 mm or more, and more preferably 1.4 mm or more.

[0049] 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 (3). t=(Dd) / 2 (3) In the formula (3), d is the average pore size (unit: mm) of the through holes, and D is the average outer diameter (unit: mm) of the expanded beads.

[0050] Foam layer The expanded beads have a polypropylene-based resin as a base resin and a foamed layer with through-holes. 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 obtained by copolymerizing propylene with other monomers. Preferred examples of the propylene-based copolymer 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 and block copolymers, and a random copolymer is preferred. The polypropylene-based resin may contain multiple types of polypropylene-based resins. Expanded beads having a foamed layer composed of a polypropylene-based resin are generally called expanded polypropylene-based resin beads.

[0051] The foam layer may contain other polymers besides the polypropylene-based resin as long as the above-described effects 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 foam layer is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less. 0% by mass is particularly preferred, i.e., the foam layer contains substantially only polypropylene-based resins as polymers.

[0052] The polypropylene resin constituting the foam layer is preferably an ethylene-propylene random copolymer having an ethylene content of 0.5% by mass to 3.5% by mass. In this case, the rigidity of the foam layer is increased, making the through-holes less likely to be blocked even when the compression ratio P of the foamed beads is increased during compression filling. Therefore, even when the compression ratio P of the foamed beads is increased, effects such as shortening the cooling time of the molded body can be more reliably achieved. Furthermore, in this case, the rigidity of the molded body can be increased, so that shrinkage and deformation of the molded body can be more effectively suppressed even when the molded body is not subjected to a curing process after removal from the mold. From this perspective, the ethylene content of the ethylene-propylene random copolymer is more preferably 0.5% by mass to 3.5% by mass, even more preferably 0.5% by mass to 2.8% by mass, and particularly preferably 0.5% by mass to 2.0% by mass.

[0053] On the other hand, from the viewpoint of being able to mold a molded article with good fusion properties and appearance at an even lower molding heating temperature, the content of the ethylene component in the ethylene-propylene random copolymer is more preferably 1.0% by mass or more and 3.5% by mass or less, even more preferably 1.5% by mass or more and 3.5% by mass or less, and particularly preferably more than 2.0% by mass and 3.5% by mass or less. The ethylene-propylene random copolymer is a random copolymer composed of an ethylene component and a propylene component, and the total of the ethylene component and the propylene component is 100% by mass.

[0054] The content of the monomer components in the ethylene-propylene random copolymer can be calculated based on the infrared absorption spectrum of the copolymer. 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 refers to the content of the structural units derived from each monomer in the copolymer.

[0055] The melting point Tmc of the polypropylene resin constituting the foam layer is preferably 158°C or lower, more preferably 155°C or lower, even more preferably 150°C or lower, and particularly preferably 148°C or lower. In this case, it becomes possible to mold a good molded article having sufficient rigidity at a lower molding heating temperature (i.e., low molding pressure). On the other hand, from the viewpoint of further improving the heat resistance and mechanical strength of the molded article, the melting point Tmc of the polypropylene resin constituting the foam layer is preferably 135°C or higher, more preferably 138°C or higher, and even more preferably 140°C or higher.

[0056] The melting point of polypropylene resin is determined according to JIS K7121:1987. Specifically, a test specimen made of expanded polypropylene resin particles is first prepared, and the specimen is conditioned according to "(2) Measurement of melting temperature after a certain heat treatment" in JIS K7121:1987. The conditioned specimen is 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 that appears on the DSC curve is taken as the melting point Tmc. 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 Tmc.

[0057] The flexural modulus of the polypropylene-based resin constituting the foam layer is preferably 800 MPa or more and 1600 MPa or less. In this case, the rigidity of the foam layer is increased, so that the through-holes are less likely to be blocked even when the compression ratio P of the foamed beads is increased during compression filling. Therefore, even when the compression ratio P of the foamed beads is increased, effects such as shortening the cooling time of the molded body can be more reliably achieved. Furthermore, in this case, the rigidity of the molded body can be increased, so that shrinkage and deformation of the molded body can be more effectively suppressed even when the molded body is not subjected to a curing process after removal from the mold. From the viewpoint of further enhancing these effects, the flexural modulus of the polypropylene-based resin is more preferably 900 MPa or more and 1600 MPa or less, even more preferably 1000 MPa or more and 1600 MPa or less, and even more preferably 1200 MPa or more and 1600 MPa or less.

[0058] On the other hand, from the viewpoint of being able to mold a molded article having excellent fusion properties and surface properties at an even lower molding heating temperature, the flexural modulus of the polypropylene-based resin is more preferably 800 MPa or more and 1500 MPa or less, even more preferably 800 MPa or more and 1300 MPa or less, and particularly preferably 800 MPa or more and less than 1200 MPa. The flexural modulus of the polypropylene-based resin can be determined in accordance with JIS K7171:2008.

[0059] ·Coating layer The expanded beads may have the foam layer and a coating layer that coats the foam layer. When the expanded beads have a coating layer, the foam layer coated with the coating layer is also called an "expanded core layer." The coating layer may cover the entire outer surface of the foam core layer, or may cover only a portion of the outer surface. More specifically, the expanded beads may have a multilayer structure called a core-sheath structure, for example, including a cylindrical foam core layer having through holes and a coating layer that covers the lateral surface of the foam core layer. The coating layer may be in a foamed state, but 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 50 μm.

[0060] For example, a polyolefin resin can be used as the base resin of the coating layer. 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 constituting the coating layer 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. Examples of polyethylene resins include linear low-density polyethylene, low-density polyethylene, and high-density polyethylene, among which linear low-density polyethylene is preferred.

[0061] The coating layer may contain other polymers besides polyolefin resins as long as the above-described effects are not impaired. Examples of other polymers include thermoplastic resins other than polyolefin resins, such as polystyrene resins, and elastomers. The content of other polymers in the coating layer is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less. 0% by mass is particularly preferred, i.e., the coating layer contains substantially only polyolefin resins as polymers.

[0062] The coating layer is a layer that can improve the fusion property of the expanded beads, for example. The melting point Tms of the polyolefin resin that constitutes the coating layer is preferably lower than the melting point Tmc of the polypropylene resin that constitutes the foamed core layer. In this case, the fusion property between the expanded beads during in-mold molding is further improved, so that a good molded product can be easily obtained even when the molding heating temperature during in-mold molding is lowered (i.e., the molding pressure is lowered). Furthermore, by lowering the molding pressure during in-mold molding, shrinkage and deformation of the molded product can be more easily suppressed even when a curing step is not performed. From the viewpoint of further enhancing these effects, the difference Tmc - Tms between the melting point Tmc of the polypropylene resin and the melting point Tms of the polyolefin resin is preferably 5°C or more, more preferably 6°C or more, and even more preferably 8°C or more.

[0063] Furthermore, from the viewpoint of preventing peeling between the foamed core layer and the coating layer and adhesion between the foamed particles, the difference Tmc-Tms between the melting point Tmc of the polypropylene-based resin and the melting point Tms of the polyolefin-based resin is preferably 35°C or less, more preferably 25°C or less, and even more preferably 20°C or less.

[0064] From the viewpoint of further enhancing the fusion properties of the expanded beads 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 method for measuring the melting point of the polyolefin resin constituting the coating layer is the same as the method for measuring the melting point of the polypropylene resin constituting the foamed layer described above, except that a test piece made of polyolefin resin is used instead of a test piece made of polypropylene resin. However, if multiple melting peaks appear in the DSC curve, the apex temperature of the lowest melting peak is taken as the melting point Tms.

[0065] The mass ratio of the expanded core layer to the coating layer in the expanded beads is preferably 99.5:0.5 to 85:15 (expanded core layer:coating layer). In other words, the mass ratio of the coating layer to the total mass of the expanded core layer and the coating layer (i.e., the total mass of the expanded beads) is preferably 0.5% or more and 15% or less. In this case, the effects of the coating layer described above can be reliably obtained. If the mass ratio of the coating layer is excessively low, the effects of the coating layer may be insufficient. From the viewpoint of more reliably avoiding the above-mentioned problems, the mass ratio of the coating layer is more preferably 1% or more, and even more preferably 3% or more.

[0066] On the other hand, if the mass ratio of the coating layer is excessively high, a curing step may be required to suppress shrinkage or deformation of the molded body, particularly when attempting to produce a molded body with a low apparent density. From the viewpoint of more reliably avoiding such problems, the mass ratio of the coating layer is more preferably 12% or less, and even more preferably 10% or less. The mass ratio of the expanded core layer to the coating layer in the expanded beads corresponds to the mass ratio of the core layer to the coating layer in the multilayer resin beads described below.

[0067] If necessary, additives such as cell regulators, 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 expanded particles. Examples of cell regulators that can be used 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, phenolic nucleating agents, amine-based nucleating agents, and polyethylene fluoride resin powder. When a cell regulator is added, the content of the cell regulator is preferably 0.01 to 1 part by mass per 100 parts by mass of the polypropylene resin. Carbon black is preferably used as a colorant. The content of carbon black is preferably 0.5 to 5 parts by mass per 100 parts by mass of the polypropylene resin.

[0068] (Method of manufacturing expanded polyolefin resin beads) Next, a method for producing expanded beads will be described. Expanded beads are produced by carrying out a dispersion step, a blowing agent impregnation step, and a foaming step. In the dispersion step, cylindrical resin particles having through holes are dispersed in a dispersion medium in a sealed container. In the foaming agent impregnation step, the resin particles dispersed in the dispersion medium are impregnated with a foaming agent. In the foaming step, the resin particles containing the foaming agent are expanded. The foaming step is carried out, for example, by a method in which the resin particles containing the foaming agent are released together with the dispersion medium under low pressure (i.e., a dispersion medium release foaming method). The method for producing expanded beads will be described in more detail below.

[0069] Resin particles The resin particles used to prepare the expanded beads have a polypropylene-based resin as the base resin and a cylindrical core layer with through holes. The resin particles may further have a coating layer made of a polyolefin-based resin that coats the core layer. The core layer of the resin particles corresponds to the foam layer of the expanded beads. The coating layer of the resin particles corresponds to the coating layer of the expanded beads. Therefore, when attempting to obtain single-layer expanded beads consisting only of a foam layer, it is sufficient to prepare resin particles consisting only of a core layer. When attempting to obtain multi-layer expanded beads having an expanded core layer and a coating layer, it is sufficient to prepare resin particles having a core layer and a coating layer. In the following, resin particles having a core layer and a coating layer may be referred to as "multi-layer resin particles."

[0070] Single-layer resin particles are produced, for example, as follows. First, a polypropylene resin for forming the core layer and additives, etc., added as needed, are melt-kneaded using an extruder to produce a resin melt for forming the core layer. This resin melt for forming the core layer is extruded through a small hole in the die of the extruder to produce a cylindrical extrudate with a through-hole, and the extrudate is then cut to a desired length to obtain resin particles. The method for cutting the extrudate is not particularly limited, and can be appropriately selected from a strand cut method, a hot cut method, an underwater cut method, etc.

[0071] In producing resin beads, it is preferable to employ a strand-cutting method in which a cylindrical extrudate is cooled in water at a temperature of 15° C. or less and then cut. In this case, the circularity of the through holes of the resin beads can be increased, and the average circularity of the through holes in the finally obtained expanded beads can be increased.

[0072] To produce multilayer resin particles, a coextrusion device equipped with a core layer extruder, a coating layer extruder, and a coextrusion die connected to these two extruders may be used. In the core layer extruder, a polypropylene resin for forming the core layer and optional additives are melt-kneaded to produce a melt-kneaded mixture for forming the core layer. In the coating layer extruder, a polyolefin resin for forming the coating layer and optional additives are melt-kneaded to produce a melt-kneaded mixture for forming the coating layer. These melt-kneaded mixtures are coextruded and merged in the die to form a multilayer composite consisting of a non-foamed cylindrical core layer and a non-foamed coating layer covering the outer surface of the cylindrical core layer. This composite is extruded through the small holes in the extruder die in the same manner as single-layer resin particles to produce a cylindrical extrudate with a through-hole, which is then cut to the desired length to obtain multilayer resin particles.

[0073] The particle diameter of the resin particles is preferably 0.1 mm to 3.0 mm, more preferably 0.3 mm to 1.5 mm, and the ratio of the length to the outer diameter of the resin particles is preferably 0.5 to 5.0, more preferably 1.0 to 3.0.

[0074] The average mass per resin particle is preferably 0.1 mg to 20 mg, more preferably 0.2 mg to 10 mg, even more preferably 0.3 mg to 5 mg, and particularly preferably 0.4 mg to 2 mg, and is calculated by dividing the mass of 200 randomly selected resin particles by the number of resin particles.

[0075] When the resin particles have a core layer and a coating layer, the mass ratio of the core layer to the coating layer (core layer:coating layer) is preferably 99.5:0.5 to 85:15, more preferably 99:1 to 92:8, and even more preferably 97:3 to 90:10.

[0076] By adjusting the average pore diameter dr of the through holes in the core layer of the resin beads, the average pore diameter d of the through holes in the expanded core layer of the expanded beads can be adjusted to fall within the above-mentioned specific range. More specifically, by adjusting the average pore diameter dr of the through holes in the resin beads to 0.10 mm or more and less than 0.25 mm, preferably 0.12 mm or more and less than 0.24 mm, and more preferably 0.15 mm or more and less than 0.22 mm, expanded beads having an average pore diameter d of 0.1 mm or more and less than 1 mm can be easily produced. The average pore diameter dr of the through holes in the core layer of the resin beads 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).

[0077] Furthermore, by adjusting the particle size and average mass of the resin particles, the average outer diameter and average wall thickness of the expanded beads can be adjusted to fall within the aforementioned ranges. More specifically, by setting the ratio dr / Dr of the average pore size dr of the through holes to the average outer diameter Dr of the resin particles to 0.4 or less, preferably 0.3 or less, more preferably 0.25 or less, and even more preferably 0.2 or less, expanded beads having a ratio d / D of the average pore size d of the through holes to the average outer diameter D of the expanded beads of 0.4 or less can be easily produced. From the viewpoint of production stability of resin beads, the average pore size dr of the through holes of the resin particles is preferably 0.1 mm or more, and the ratio dr / Dr of the average pore size dr of the through holes to the average outer diameter Dr of the resin beads is preferably 0.1 or more.

[0078] The method for calculating the average pore diameter dr of the through holes of the resin particles and the average outer diameter Dr of the resin particles is the same as the method for calculating the average pore diameter d of the through holes of the expanded beads and the average outer diameter D of the expanded beads described above, except that resin particles are used instead of expanded beads.

[0079] When the strand cutting method is used to cut the extrudate, that is, when the tubular extrudate extruded from the die is cooled in water while being withdrawn and then cut to an appropriate length, the particle diameter, length / outside diameter ratio, and average mass of the resin particles can be adjusted by appropriately changing the extrusion speed, withdrawal speed, cutter speed, etc. during extrusion of the resin melt.

[0080] ·Dispersion process In the dispersion process, the resin particles are dispersed in a dispersion medium in a sealed container. An aqueous dispersion medium containing water as a main component is used. In addition to water, the aqueous dispersion medium may contain a hydrophilic organic solvent such as ethylene glycol, glycerin, methanol, or ethanol. 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.

[0081] It is preferable to add a dispersant to the dispersion medium. Adding a dispersant to the dispersion medium can suppress fusion between resin particles heated in a container during the foaming process. The amount of dispersant added is preferably about 0.001 to 5 parts by mass per 100 parts by mass of resin particles. Organic or inorganic dispersants can be used as the dispersant, but it is preferable to use fine inorganic particles as the dispersant because of ease of handling. More specifically, examples of dispersants that can be used include clay minerals such as amsnite, kaolin, mica, and clay, as well as aluminum oxide, titanium oxide, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, and iron oxide. These dispersants may be used alone, or two or more types of dispersants may be used in combination. Among these, it is preferable to use a clay mineral as the dispersant. The clay mineral may be natural or synthetic.

[0082] 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 resin particles.

[0083] Foaming agent impregnation process In the blowing agent impregnation step, a blowing agent is supplied into a sealed container, and the resin particles in the container are impregnated with the blowing agent. To promote the impregnation of the resin particles with the blowing agent, it is preferable to supply the blowing agent into the sealed container while heating the dispersion medium and resin particles in the sealed container. A physical blowing agent is preferably used as the blowing agent. The physical blowing agent may be either an inorganic or organic physical blowing agent. Examples of inorganic physical blowing agents include carbon dioxide, air, nitrogen, helium, and argon. Examples of organic physical blowing agents include aliphatic hydrocarbons such as propane, butane, and hexane; cyclic aliphatic 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. These physical blowing agents may be used alone, or two or more of them may be used in combination. From the viewpoint of environmental load and ease of handling, it is preferable to use an inorganic physical foaming agent as the physical foaming agent, and it is more preferable to use carbon dioxide or air.

[0084] The amount of the foaming agent added is preferably 0.1 to 30 parts by mass, and more preferably 0.5 to 15 parts by mass, per 100 parts by mass of the resin particles.

[0085] Foaming process In the expansion step, for example, the contents of the sealed container are released under a pressure lower than the pressure inside the sealed container, thereby expanding at least the core layer of the resin particles, thereby obtaining expanded particles.

[0086] The internal pressure of the sealed container during expansion in the expansion step is preferably 0.5 MPa (G) or more in gauge pressure. On the other hand, the internal pressure of the sealed container is preferably 4.0 MPa (G) or less. Within the above range, expanded beads can be safely produced without risk of damage or explosion of the sealed container. Furthermore, by heating the aqueous dispersion medium at a temperature increase rate of 1 to 5°C / min in the expansion step, the temperature during expansion can be kept within an appropriate range.

[0087] Two-stage foaming process The expanded beads obtained after the expansion step may be used directly to produce a molded article, or, if necessary, a second-stage expansion step may be performed to further reduce the apparent density. In the second-stage expansion step, the expanded beads are first placed in a pressurizable sealed container, and the pressure inside the sealed container is increased using an inorganic gas such as air, thereby impregnating the expanded beads with the inorganic gas and increasing the pressure inside the cells of the expanded beads. The expanded beads are then heated in the container using a heating medium such as steam for a predetermined time, thereby further expanding the expanded beads. This allows the apparent density of the expanded beads to be further reduced.

[0088] The manufacturing method includes a filling step in which the polypropylene-based resin foamed beads are compressed with pressurized gas and filled into a pressurized molding space formed in a molding die; a pressure release step in which, after the filling step, the pressure in the molding die is released to restore the foamed beads to their compressed state; and a heating step in which, after the pressure release step, a heating medium is supplied into the molding die to heat the foamed beads, thereby fusing the foamed beads to each other.

[0089] ·Filling process In the production method, for example, as shown in FIG. 3 , a molding apparatus 2 can be used, which includes a filling hopper 21 that accommodates expanded polypropylene resin beads 1 and a transfer path 22 that connects the filling hopper 21 and a molding die 23. When using such a molding apparatus 2, first, the filling hopper 21, the transfer path 22, and the molding die 23 are pressurized with a pressurized gas to compress the expanded beads 1 accommodated in the filling hopper 21. The pressure (A) in the filling hopper 21 is preferably 0.15 MPa (G) or more and 0.30 MPa (G) or less in gauge pressure. In this case, the expanded beads 1 accommodated in the filling hopper 21 can be sufficiently compressed, and the ability to pack the expanded beads 1 into the molding die 23 can be improved. The pressurized gas that pressurizes the filling hopper 21, the transfer path 22, and the molding die 23 is preferably an inorganic gas from the viewpoint of reducing environmental impact, and air, nitrogen, or carbon dioxide is more preferred.

[0090] Next, the expanded beads 1 compressed in the filling hopper 21 are filled into the mold 23 through the transfer path 22 while maintaining the compressed state. At this time, the pressure (B) in the mold is preferably 0.12 MPa (G) or more and 0.28 MPa (G) or less. Furthermore, the difference between the pressure (A) in the filling hopper and the pressure (B) in the mold [(A) - (B)] is preferably greater than 0 MPa and less than 0.10 MPa. By setting the pressure (B) in the mold within the specific range, the compressed state of the expanded beads 1 is easily maintained from the filling hopper 21 until it reaches the mold 23. Furthermore, by setting the difference between the pressure (A) in the filling hopper and the pressure (B) in the mold [(A) - (B)] within the specific range, the transfer of the expanded beads 1 from the filling hopper 21 to the mold 23 can be more smoothly performed. As a result, the filling efficiency of the expanded beads 1 into the mold 23 can be further improved.

[0091] Pressure release process After the foamed beads 1 have been filled into the mold 23, the transfer path 22 is closed. Then, the pressure inside the mold 23 is released, causing the foamed beads 1 inside the mold 23 to return to their original shape. Even after this return, there are still gaps between the foamed beads that allow the heating medium supplied in the next heating step to pass through between the foamed beads.

[0092] ·Heating process After the pressure in the molding die 23 is released, a heating medium is supplied into the molding die to fuse the expanded beads 1 together, thereby performing in-mold molding. Steam is preferred as the heating medium.

[0093] After the heating step is completed, the molded body is cooled in the mold with water, air, etc. The molded body is then released from the mold, completing the in-mold molding. The molded body after release may be subjected to a curing step, but as mentioned above, the curing step may be omitted.

[0094] (Molded body) The molded article obtained by the above-mentioned production method is composed of a large number of foamed beads fused together. The molded article has minute voids that communicate with the outside of the molded article. The minute voids in the molded article are formed by a complex connection of voids formed by interconnected through-holes of a plurality of foamed beads, voids formed by interconnected through-holes of the foamed beads with voids formed between the foamed beads, voids formed by interconnected voids between the foamed beads, and open-cell portions of the foamed beads that constitute the molded article.

[0095] The density of the compact is 10 kg / m 3 More than 100kg / m 3 In this case, the light weight and rigidity of the molded body can be improved in a well-balanced manner. From the viewpoint of further improving the rigidity of the molded body, the density of the molded body is preferably 15 kg / m or less. 3 More preferably, it is 20 kg / m or more. 3 More preferably, it is 25 kg / m or more. 3 From the viewpoint of further improving the lightness of the molded body, the density of the molded body is particularly preferably 45 kg / m or more.3 More preferably, it is 40 kg / m or less. 3 More preferably, it is 38 kg / m or less. 3 It is even more preferable that:

[0096] Conventionally, when producing a molded body with a low density, it has been particularly difficult to omit the curing step because the molded body is prone to significant deformation after demolding. In contrast, with the expanded beads, it is possible to omit the curing step even when the apparent density is low, and a molded body with a desired shape and excellent appearance and rigidity can be produced without curing. From this perspective, it is preferable to set the density of the molded body within the above range. The density of the molded body is calculated by dividing the mass (unit: g) of the molded body by the volume (unit: L) determined from the outer dimensions of the molded body and converting the unit. Note that if it is not easy to determine the volume from the outer dimensions of the molded body, the volume of the molded body can be determined by a water immersion method.

[0097] From the viewpoint of being able to more sufficiently suppress dimensional changes even if the curing step is omitted, the porosity of the molded body is preferably 4% or more, more preferably 4.5% or more, and even more preferably 5% or more. On the other hand, from the viewpoint of further improving the rigidity and appearance of the molded body, the porosity of the molded body is preferably 12% or less, more preferably 10% or less, and even more preferably 8% or less.

[0098] The method for measuring the porosity of a green body is as follows. First, a rectangular parallelepiped test piece is cut out from the center of the green body. This test piece is submerged in a measuring cylinder containing ethanol, and the true volume Vc (unit: L) of the test piece is determined from the rise in the ethanol liquid level. In addition, the apparent volume Vd (unit: L) is determined from the external dimensions of the test piece. The porosity (unit: %) of the green body is calculated using the true volume Vc and apparent volume Vd of the test piece according to the following formula (4). Porosity (%)=[(Vd-Vc) / Vd]×100 (4)

[0099] 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]

[0100] Examples of the method for producing the expanded polypropylene resin bead molded article are described below.

[0101] (Polypropylene resin) The properties of the polypropylene resin used in producing the expanded beads are shown in Table 1. The ethylene-propylene copolymer and ethylene-propylene-butene copolymer used in this example are both random copolymers.

[0102] [Table 1]

[0103] The physical properties of the polypropylene resin shown in Table 1 were measured as follows.

[0104] Flexural modulus 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 the supports was 64 mm, and the test speed was 2 mm / min.

[0105] Melting point The melting point of polypropylene resin was determined according to JIS K7121:1987. Specifically, a polypropylene resin test specimen was first conditioned according to "(2) Measurement of melting temperature after a certain heat treatment" in JIS K7121:1987. The conditioned test specimen was heated from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve. The melting point was determined as the apex temperature of the melting peak that appeared on the DSC curve. A heat flux differential scanning calorimeter (DSC7020, manufactured by SII Nanotechnology, Inc.) was used as the measurement device.

[0106] 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.

[0107] Next, the manufacturing methods of the molded articles in Examples 1 to 4 and Comparative Examples 1 to 10 will be described.

[0108] Example 1 The molded article of Example 1 was obtained by molding in a mold expanded beads having a cylindrical foamed core layer with through holes, which was made of PP1 shown in Table 1 as the base resin, and a coating layer covering the lateral surface of the foamed core layer, which was made of PP3 shown in Table 2 as the base resin. The expanded beads were produced as follows.

[0109] -How to make foam beads To produce the expanded beads, a coextrusion apparatus was used, equipped with a core layer extruder, a coating layer extruder, and a coextrusion die connected to these two extruders. The extrudate extruded from the coextrusion apparatus was cut using a strand-cutting method to produce multilayer resin beads. Specifically, PP1 shown in Table 1 and a cell control agent were fed into the core layer extruder and melt-kneaded in the extruder at a maximum set temperature of 245°C to obtain a melt-kneaded material for forming the core layer. Zinc borate was used as the cell control agent, and the amount of zinc borate added was 500 ppm by mass relative to the polypropylene resin.

[0110] Additionally, PP3 shown in Table 1 was melt-kneaded in an extruder for forming a coating layer at a maximum set temperature of 245°C to obtain a melt-kneaded material for forming a coating layer. These melt-kneaded materials were co-extruded and merged in a die to form a cylindrical 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 a die equipped with small holes for forming through-holes in the core layer, and then cooled in cold water at 10°C while being withdrawn. The extrudate was then cut to an appropriate length using a pelletizer to obtain multilayer resin particles consisting of a cylindrical core layer with through-holes and a coating layer covering the core layer. The mass ratio of the core layer to the coating layer in the multilayer resin particles was 95:5 (i.e., the mass ratio of the coating layer was 5%). The mass per multilayer resin particle was approximately 1.5 mg.

[0111] 1 kg of the multilayer resin particles thus obtained was placed in a 5 L sealed container together with 3 L of water as a dispersion medium. Next, 0.3 parts by mass of dispersant and 0.004 parts by mass of dispersion aid were added to the sealed container per 100 parts by mass of the multilayer resin particles, and the multilayer resin particles were dispersed in the dispersion medium. Kaolin was used as the dispersant. A surfactant (sodium alkylbenzene sulfonate) was also used as the dispersion aid.

[0112] Thereafter, carbon dioxide as a physical blowing agent was supplied into the sealed container while stirring the contents, and the temperature inside the container was raised to 150.1°C. The pressure inside the container (i.e., impregnation pressure, carbon dioxide pressure) at this time was 2.6 MPa (G) in gauge pressure. After the temperature inside the container reached 150.1°C, this temperature was maintained for 15 minutes, thereby impregnating the multilayer resin particles with the physical blowing agent. After impregnation with the physical blowing agent was completed, the sealed container was opened, and the contents were released under atmospheric pressure, thereby expanding the multilayer resin particles. In this manner, expanded beads having an expanded core layer and a coating layer were obtained. The properties of the expanded beads obtained in this manner were as shown in Table 2. The method for evaluating the properties of the expanded beads will be described later.

[0113] -Method for producing molded body To produce the molded body, a molding device 2 shown in Fig. 3 was used, which was equipped with a filling hopper 21 for storing expanded polypropylene resin beads 1 and a transfer path 22 connecting the filling hopper 21 and a molding die 23. As the molding die 23, a flat mold capable of molding a flat molded body having a length of 300 mm, a width of 250 mm, and a thickness of 60 mm was used.

[0114] First, the expanded beads obtained by the above-mentioned method were dried at a temperature of 23°C for 24 hours and then charged into the filling hopper 21. The pressure inside the cells of the expanded beads charged into the filling hopper 21 was set to 0 MPa (G) in terms of gauge pressure. That is, in this example, the expanded beads were charged into the filling hopper 21 without being subjected to an internal pressure application step for increasing the pressure inside the cells.

[0115] Next, compressed air was used as the pressurized gas to pressurize the filling hopper 21, the transfer path 22, and the molding die 23, thereby compressing the expanded beads 1 in the filling hopper 21. The pressure (A) in the filling hopper 21 and the pressure (B) in the molding die 23 were set as shown in Table 2. Thereafter, the expanded beads 1 compressed in the filling hopper 21 were supplied from the transfer path 22 into the molding die 23 while maintaining the compressed state, and the molding die 23 was filled with the expanded beads 1. The compression ratio P of the expanded beads at the time when filling was completed was set as shown in Table 2. The internal dimensions of the cavity of the molding die 23 were 300 mm long x 250 mm wide x 60 mm thick.

[0116] After the foamed beads 1 had been completely filled into the mold 23, the transfer path 22 was closed. Then, the pressure inside the mold 23 was released, causing the foamed beads 1 in the mold 23 to return to their original shape. Thereafter, steam was supplied as a heating medium into the mold to mold the foamed beads 1 in the mold, and a flat plate-shaped molded body measuring 300 mm long, 250 mm wide, and 60 mm thick was obtained.

[0117] More specifically, the in-mold molding was performed according to the following procedure. First, after closing the transfer path 22, steam was supplied from both sides of the mold in the thickness direction for 5 seconds to perform a preheating process. Then, steam was supplied from one side of the mold until a pressure 0.08 MPa (G) lower than the molding pressure shown in Table 2 was reached, performing one-sided heating. Next, steam was supplied from the other side of the mold until a pressure 0.04 MPa (G) lower than the molding pressure shown in Table 2 was reached, performing one-sided heating. Then, steam was supplied from both sides of the mold until the molding pressure shown in Table 2 was reached, performing main heating. After main heating was completed, the pressure inside the mold was released, and the molded body was cooled in the mold until the surface pressure due to the foaming force of the molded body reached 0.04 MPa (G). The molded body was then removed from the mold. The molded body after demolding was left to stand in an oven at 80°C for 12 hours to undergo a curing process. After the curing step, the molded body was conditioned by being left to stand for 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm. The properties of the molded body thus obtained were as shown in Table 2. The method for evaluating the properties of the molded body will be described later.

[0118] (Examples 2 and 3) In Examples 2 and 3, the pressure (A) in the filling hopper and the pressure in the transfer path when filling the foamed beads into the mold were changed to the values ​​shown in Table 2, and the compression ratio P of the foamed beads in the mold was made higher than in Example 1. Except for this, molded bodies were produced in the same manner as in Example 1.

[0119] Example 4 In this example, a molded body was produced in the same manner as in Example 1, except that expanded beads having a foamed core layer with PP2 as the base resin and a coating layer covering the foamed core layer with PP3 as the base resin were used, as shown in Table 2.

[0120] Example 5 In this example, an internal pressure application step was carried out in advance to increase the pressure inside the cells of the expanded beads, which were then used for compression filling and in-mold molding. Specifically, the same expanded beads as in Example 1 were pressurized with compressed air to increase the internal pressure of the expanded beads to 0.08 MPa (G). After these expanded beads were introduced into the filling hopper 21, the pressure (A) inside the filling hopper and the pressure inside the transfer path when filling the foamed beads into the mold were changed to the values ​​shown in Table 2, and the compression ratio P of the expanded beads in the mold was made lower than in Example 1. The procedure and conditions for in-mold molding in this example were the same as in Example 1.

[0121] (Comparative Example 1) In this example, the pressure (A) in the filling hopper and the pressure in the transfer path when filling the foamed beads into the mold were changed to the values ​​shown in Table 3, so that the compression ratio P of the foamed beads in the mold was made sufficiently lower than in Example 1. A molded body was produced in the same manner as in Example 1.

[0122] (Comparative Example 2) In this example, the compression ratio P of the expanded beads in the mold was made higher than in Example 3. Specifically, the pressure (A) in the filling hopper and the pressure in the transfer path when filling the expanded beads into the mold were changed to the values ​​shown in Table 3. Furthermore, since the increased compression ratio P of the expanded beads made it impossible to obtain a good molded body at the same molding pressure as in Example 3, the molding pressure was changed to the value shown in Table 3. Except for these points, a molded body was produced in the same manner as in Example 3.

[0123] (Comparative Example 3 and Comparative Example 4) In Comparative Examples 3 and 4, solid spherical polypropylene resin foam particles having a foamed core layer without through holes and a coating layer covering the foamed core layer were used, as shown in Table 3, and molded bodies were produced in the same manner as in Example 1, except that the molding conditions were changed as shown in Table 3.

[0124] (Comparative Examples 5 to 7) In Comparative Examples 5 to 7, molded bodies were produced in the same manner as in Example 1, except that expanded beads with a large average pore diameter d of the through holes were used and the molding conditions were changed as shown in Tables 3 and 4.

[0125] (Comparative Examples 8 to 10) In Comparative Examples 8 to 10, as shown in Table 4, a foamed core layer having PP2 as the base resin and a coating layer having PP3 as the base resin that covers the foamed core layer were used, and foamed particles with a large average pore diameter d of the through holes in the foamed core layer were used. Molded bodies were produced in the same manner as in Example 1, except that the molding conditions were changed as shown in Table 4.

[0126] The methods for evaluating the properties of the expanded beads and molded articles in the examples and comparative examples shown in Tables 2 to 4 are as follows.

[0127] ·Average hole diameter d of through holes The average pore size of the through-holes in the expanded beads was determined as follows. First, the expanded beads were left to stand for 24 hours or more in an environment of 50% relative humidity, 23°C temperature, and 1 atm pressure to condition the expanded beads. One hundred expanded beads were randomly selected from the condition-conditioned expanded beads and cut perpendicular to the penetration direction of the through-holes at a position where the cross-sectional area was approximately maximum. Photographs of the cut surfaces of each expanded bead were taken, and the cross-sectional area (opening area) of the through-hole portion in the cross-sectional photograph 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 pore size d (unit: mm) of the through-holes in the expanded beads.

[0128] Bulk density The expanded beads were left to stand for 24 hours or more in an environment of 50% relative humidity, 23°C temperature, and 1 atm atmospheric pressure to condition the expanded beads. After conditioning, the expanded beads were filled into a 1 L measuring cylinder up to the 1 L mark so that they would naturally pile up. The mass (unit: g) of the expanded beads in the measuring cylinder was then divided by the bulk volume (i.e., 1 L), and the unit was converted to determine the bulk density (unit: kg / m) of the expanded beads. 3 ) was calculated.

[0129] Apparent density The expanded particles were left to stand for 24 hours or more in an environment of 50% relative humidity, 23°C temperature, and 1 atm pressure to condition the expanded particles. After measuring the mass of the expanded particles after conditioning, they were submerged in a measuring cylinder containing ethanol at 23°C using a wire net. Then, taking into account the volume of the wire net, the volume of the expanded particles was measured as read from the rise in the water level. The mass (unit: g) of the expanded particles thus obtained was divided by the volume (unit: L), and the apparent density (unit: kg / m) of the expanded particles was calculated by converting the units. 3 ) was calculated.

[0130] Average outer diameter of foam particles D The average outer diameter of the expanded beads was determined as follows. First, the expanded beads were left to stand for 24 hours or more in an environment of 50% relative humidity, 23°C temperature, and 1 atm pressure to condition the expanded beads. One hundred expanded beads were randomly selected from the expanded beads after the conditioning, and cut perpendicular to the penetration direction of the through holes at a position where the cross-sectional area was approximately maximum. Photographs of the cut surfaces of each expanded bead were taken, 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 (unit: mm) of the expanded beads. Tables 2 to 4 also show the ratio d / D of the average diameter d of the through holes to the average outer diameter D of the expanded beads.

[0131] -Circularity of through holes The circularity of expanded beads was determined as follows. First, a group of expanded beads was left to stand for 24 hours or more in an environment of 50% relative humidity, 23°C temperature, and 1 atm atmospheric pressure to condition the expanded beads. One hundred expanded beads were randomly selected from the group of expanded beads after the condition adjustment and cut perpendicular to the penetration direction of the through holes at a position where the cross-sectional area was approximately maximum. Next, a photograph of the cut surface of each expanded bead was taken, and the cross-sectional area S (opening area) and perimeter C (circumference) of the through-hole portion were determined. The circularity of the through holes of each expanded bead was calculated based on the following formula (2), and then the circularities of the through holes of the 100 expanded beads were arithmetically averaged to calculate the average circularity of the through holes. Circularity = 4πS / (C × C) (2)

[0132] ·Average wall thickness The average wall thickness t was calculated by the following formula (3) using the average outer diameter D (unit: mm) of the expanded beads obtained by the above-mentioned method and the average pore diameter d (unit: mm) of the through holes. t=(Dd) / 2 (3)

[0133] Cooling time When the expanded beads were molded in a mold, the time from the completion of the main heating to the time when the surface pressure due to the foaming force of the molded body reached 0.04 MPa (G) was measured, and this time was taken as the cooling time.

[0134] Density of the compact The mass of the compact (unit: g) is divided by the volume (unit: L) calculated from the external dimensions of the compact, and then the density of the compact (unit: kg / m) is calculated by converting the unit. 3 ) was calculated. Tables 2 to 4 also show the ratio of the density of the molded body to the bulk density of the expanded beads (molded body density / expanded bead bulk density). The smaller the density ratio, the more effectively the bulk density of the original expanded beads was utilized to obtain a lightweight molded body.

[0135] ·Moisture content In the in-mold molding, the weight Ww (unit: g) of the molded body was measured immediately after removal from the mold. The molded body was then left to dry in an oven at 80°C for 12 hours, and the weight Wd (unit: g) of the dried molded body was measured. Using these values, the moisture content (unit: %) of the molded body was calculated based on the following formula (5). Moisture content (%)=[(Ww-Wd) / Ww]×100 (5)

[0136] ·Porosity A rectangular parallelepiped test piece (20 mm long x 100 mm wide x 20 mm high) was cut out from the center of the compact. This test piece was submerged in a measuring cylinder containing ethanol, and the true volume Vc (unit: L) of the test piece was determined from the rise in the ethanol liquid level. In addition, the apparent volume Vd (unit: L) was determined from the external dimensions of the test piece. The porosity of the compact was calculated from the true volume Vc and apparent volume Vd of the compact based on the following equation (4). Porosity (%)=[(Vd-Vc) / Vd]×100 (4)

[0137] Appearance of the molded product The appearance of the molded articles was evaluated based on the results of visual observation of the articles. In the "Appearance" column in Tables 2 to 4, the following was recorded: "A" if there were few gaps between the expanded beads on the surface of the molded article and no noticeable irregularities due to through-holes, etc.; "B" if slight irregularities due to gaps between particles and / or through-holes, etc. were observed on the surface of the molded article; and "C" if significant irregularities due to gaps between particles and / or through-holes, etc. were observed on the surface of the molded article.

[0138] -Fusing ability The molded body was bent into approximately equal parts in the length direction and broken. The fracture surface of the test piece was then observed, and the number of foamed beads that had broken (material failure) and the number of foamed beads that had peeled off between the interfaces of the foamed beads were visually counted. The ratio of the number of foamed beads that had broken to the total number of foamed beads present on the fracture surface was calculated, and this percentage was used as the material failure rate (unit: %) of the test piece.

[0139] In the "Weldability" column in Tables 2 to 4, the symbol "A" was entered when the arithmetic mean value of the material failure rate was 90% or more, the symbol "B" was entered when it was 70% or more but less than 90%, and the symbol "C" was entered when it was less than 70%.

[0140] ·Recovery The shape recovery of the molded body was evaluated based on the shrinkage and deformation of the molded body after the curing process. Specifically, in a plan view of the molded body seen from the thickness direction, the thickness of the molded body was measured at four positions 10 mm inward from each vertex toward the center, as well as at the center. Next, the ratio (unit: %) of the thickness at the center to the thickness at the thickest position among the four measurement positions near the vertices was calculated. In the "Recoverability" column in Tables 2 to 4, a symbol "A" was entered when the thickness ratio was 95% or more, and a symbol "B" was entered when the thickness ratio was less than 95%.

[0141] In the evaluation of the molded product, a molded product that receives an "A" rating in all of the evaluations of appearance, fusion property, and shape stability can be judged to be a "good molded product."

[0142] - Shape stability when the curing process is omitted In the above-described manufacturing method, the molded body was removed from the mold and then the molded body that had not undergone the curing process was used to evaluate shape stability. Specifically, the molded body after release was conditioned by being left to stand for 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm. Then, as in the evaluation of recoverability described above, the thickness of the conditioned molded body was measured at positions near the vertices and the thickness of the central part, and the ratio (unit: %) of the thickness of the central part to the thickness of the thickest position among the four measurement positions near the vertices was calculated.

[0143] In the "shape stability" column in Tables 2 to 4, the symbol "A" was entered when the thickness ratio was 95% or more, and the symbol "B" was entered when it was less than 95%. The symbol "A" in the evaluation of shape stability when the curing step was omitted means that even when the curing step was omitted, shrinkage and deformation of the molded body were suppressed and a good molded body could be obtained. Note that if a good molded body was not obtained after the curing step, that is, if the evaluation of one or more of the above-mentioned items of appearance, fusion property, and recoverability was other than "A," the evaluation of shape stability when the curing step was omitted was not performed. For cases in which the evaluation of shape stability when the curing step was omitted was not performed, the symbol "-" was entered in the same column.

[0144] [Table 2]

[0145] [Table 3]

[0146] [Table 4]

[0147] As shown in Table 2, in Examples 1 to 5, the foamed beads having the specific shape were compressed and filled into the mold, thereby filling the mold with the foamed beads. Furthermore, the compression ratio P of the foamed beads filled into the mold was within the specific range. Therefore, the methods of Examples 1 to 5 improved the filling efficiency of the foamed beads into the mold and shortened the cooling time after in-mold molding. Furthermore, the molded articles obtained by the methods of Examples 1 to 5 had good appearances and excellent fusion properties between the foamed beads. Furthermore, the moisture content of the molded articles immediately after demolding was low, making them easy to handle. Furthermore, the manufacturing methods of Examples 1 to 5 enabled the production of good molded articles in which shrinkage and deformation were suppressed, even without a curing step.

[0148] On the other hand, as shown in Table 3, in Comparative Example 1, the compression ratio P of the expanded beads was too low, so the expanded beads were likely to be insufficiently packed into the molding die, which tended to result in gaps between the expanded beads forming on the surface of the molded article, making it difficult to obtain a molded article with a good appearance.

[0149] In Comparative Example 2, the compression ratio P of the expanded beads was too high, and the expanded beads in the molding die were prone to excessive compression. Therefore, the ratio of the density of the molded body to the bulk density of the expanded beads exceeded 2.0, making it difficult to obtain a lightweight molded body that retains the original bulk density of the expanded beads. Furthermore, the time required for cooling the molded body in the molding die was likely to be long. Furthermore, since the molding pressure needed to be increased to sufficiently fuse the expanded beads together, a curing process was required to suppress shrinkage and deformation of the molded body.

[0150] In Comparative Example 3, solid spherical expanded beads without through holes were used, which tended to lengthen the time required for cooling the molded body in the mold. Furthermore, when solid spherical expanded beads are used, a curing step is required to suppress shrinkage and deformation of the molded body.

[0151] When the compression ratio of solid spherical expanded beads is increased as in Comparative Example 4, it becomes difficult for steam to be sufficiently supplied to the interior during molding in the mold, making it difficult to sufficiently fuse the expanded beads together. Furthermore, the time required for cooling the molded product in the mold tends to be significantly longer. Therefore, it is clear that it is difficult to increase the compression ratio when solid spherical expanded beads are used.

[0152] As shown in Tables 3 and 4, in Comparative Examples 5 to 7, expanded beads with large through-hole diameters were used, so gaps between the expanded beads and gaps resulting from the through-holes were likely to form on the surface of the molded article obtained after in-mold molding. Therefore, it was difficult to obtain a molded article with a good appearance. Furthermore, even when the compression ratio was increased, the appearance could not be sufficiently improved.

[0153] In Comparative Examples 8 and 9, the through-hole diameters of the expanded beads were smaller than those in Comparative Examples 5 to 7, but were larger than those in Examples 1 to 5. Therefore, gaps between the expanded beads and gaps resulting from the through-holes are likely to form on the surface of the molded article obtained after in-mold molding. Therefore, it is difficult to obtain a molded article with a good appearance.

[0154] In Comparative Example 10, the compression ratio P of the expanded beads was increased compared to Comparative Examples 8 and 9 in order to obtain a molded product with a good appearance. As a result, the expanded beads in the mold were prone to excessive compression. Therefore, the ratio of the density of the molded product to the bulk density of the expanded beads exceeded 2.0, making it difficult to obtain a lightweight molded product. Furthermore, the molding pressure had to be increased to sufficiently fuse the expanded beads together. As a result, a curing step was required to suppress shrinkage and deformation of the molded product.

[0155] Specific embodiments of the method for producing expanded polypropylene resin bead moldings according to the present invention have been described above based on the examples. However, the specific embodiments of the method for producing expanded polypropylene resin bead moldings according to the present invention are not limited to the embodiments, and the configurations can be modified as appropriate within the scope of the present invention. [Explanation of symbols]

[0156] 1. Foam particles 11 Through hole

Claims

1. A method for producing a polypropylene-based resin foamed bead molded article, comprising filling a molding die with expanded polypropylene-based resin beads compressed by a pressurized gas, and then supplying a heating medium into the molding die to in-mold the expanded polypropylene-based resin beads in the molding die, The expanded polypropylene resin particles have a cylindrical shape with through holes, the average pore diameter d of the through holes of the expanded polypropylene resin beads is 0.1 mm or more and less than 1 mm, and the ratio d / D of the average pore diameter d of the through holes to the average outer diameter D of the expanded polypropylene resin beads is 0.4 or less; a compressibility P of the expanded polypropylene resin beads, expressed by the following formula (1), in a state in which the expanded polypropylene resin beads are filled in the molding die, of 20% or more and 80% or less: P=[{a / (b×c)}-1]×100...(1) (wherein, a in the formula (1) is the mass (unit: kg) of the expanded polypropylene resin beads filled in the mold, and b is the bulk density (unit: kg / m 3 ), and c is the internal volume of the mold (unit: m 3 )

2. 2. The method for producing a polypropylene resin foamed bead molded article according to claim 1, wherein the ratio of the apparent density of the expanded polypropylene resin beads to the bulk density of the expanded polypropylene resin beads is greater than 1.6 and not greater than 2.

0.

3. 3. The method for producing an expanded polypropylene resin bead molded article according to claim 1, wherein the through-holes of the expanded polypropylene resin beads have a circularity of 0.90 or more.

4. The method for producing a polypropylene-based resin expanded bead molding according to any one of claims 1 to 3, wherein the polypropylene-based resin that is the base resin of the polypropylene-based resin expanded beads is an ethylene-propylene random copolymer, and the ethylene-propylene random copolymer has an ethylene component content of 0.5% by mass or more and 3.5% by mass or less.

5. The method for producing a polypropylene-based resin expanded bead molding according to any one of claims 1 to 4, wherein the polypropylene-based resin that is the base resin of the polypropylene-based resin expanded beads has a flexural modulus of 800 MPa or more and 1600 MPa or less.

6. 6. The method for producing a polypropylene-based resin expanded bead molding according to claim 1, wherein the expanded polypropylene-based resin beads have a foamed core layer having a polypropylene-based resin as a base resin, and a coating layer that coats the foamed core layer, and the base resin of the coating layer is a polyolefin-based resin having a melting point lower than that of the polypropylene-based resin.

7. The bulk density of the polypropylene resin foam particles is 10 kg / m 3 More than 50kg / m 3 The method for producing an expanded polypropylene resin bead molding according to any one of claims 1 to 6, wherein the method is as follows:

8. a molding device including a filling hopper for accommodating the expanded polypropylene resin particles and a transfer path connecting the filling hopper and the molding die; a pressure (A) in the filling hopper of 0.15 MPa (G) or more and 0.30 MPa (G) or less is increased by the pressurized gas to compress the expanded polypropylene resin particles contained in the filling hopper; 8. The method for producing a polypropylene-based resin expanded bead molded article according to claim 1, wherein the pressurized gas is used to pressurize the transfer path and the mold so that the pressure (B) inside the mold is 0.12 MPa (G) or more and 0.28 MPa (G) or less, and the difference [(A) - (B)] between the pressure (A) inside the filling hopper and the pressure (B) inside the mold is more than 0 MPa and 0.10 MPa or less, and the expanded polypropylene-based resin beads are filled into the mold from the filling hopper via the transfer path while maintaining the compressed state of the expanded polypropylene-based resin beads.

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