Polypropylene resin foamed particles and method for manufacturing same

Abstract

Polypropylene resin foamed particles (1) have a cylindrical shape and has a through-hole (11) that penetrates the interior in the axial direction. The average pore diameter (d) of the through-hole (11) in the foamed particles (1) is less than 1 mm, and the ratio d / D of the average pore diameter (d) of the through-hole (11) to the average outer diameter (D) of the foamed particles (1) is 0.4 or less. The foamed particles (1) each have: a foamed core layer (2) composed of a polypropylene resin; and a coating layer (3) composed of a polyolefin resin and covering the side peripheral surface of the foamed core layer (2). The foam core layer (2) and the coating layer (2) contain a colored pigment. As measured under the conditions of a temperature of 230°C and a load of 2.16 kg in accordance with JIS K7210-1:2014, the melt mass flow rate MFRS of the polyolefin resin composing the coating layer (3) is higher than 15 g / 10 min.

Description

Polypropylene resin expanded particles and their manufacturing method

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

[0002] Polypropylene resin foamed bead molded articles are used in a variety of applications due to their light weight and excellent shock-absorbing properties and rigidity. Polypropylene resin foamed bead molded articles are produced, for example, by a method known as in-mold molding, in which expanded polypropylene resin beads are filled into a mold and then heated by supplying steam into the mold. In the in-mold molding method, when steam 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. Because the molded article is prone to expansion due to secondary expansion immediately after molding, it is cooled in the mold with water, air, or the like, and then released from the mold.

[0003] In recent years, the applications of expanded bead moldings have expanded, and expanded bead moldings containing a coloring pigment and having a colored appearance are sometimes required. For example, Patent Document 1 describes expanded polypropylene resin beads capable of forming such expanded bead moldings, which contain carbon black as a colorant and are obtained by expanding cylindrical polypropylene resin beads having through-holes in two stages. Expanded bead moldings molded using the expanded polypropylene resin beads obtained by the method of Patent Document 1 have a high degree of blackness and reduced color unevenness.

[0004] JP 2023-57790 A

[0005] However, upon closer observation of expanded bead moldings molded using expanded polypropylene resin beads obtained by the method of Patent Document 1, it has been found that streaky patterns are sometimes formed on the surface of the moldings. Although such streaky patterns do not affect the mechanical properties of the moldings, it has been desired to suppress the occurrence of streaky patterns depending on the application of the moldings in order to further improve the appearance of the moldings.

[0006] The present invention has been made in view of the above background, and aims to provide expanded polypropylene resin beads capable of forming expanded bead moldings having few streaks and good appearance, and a method for producing the same.

[0007] One aspect of the present invention resides in the expanded polypropylene resin particles according to the following items [1] to

[10] .

[0008] [1] A cylindrical expanded polypropylene resin bead having through holes passing through the interior in the axial direction, wherein the expanded beads have an average hole diameter d of less than 1 mm, and a ratio d / D of the average hole diameter d of the through holes to the average outer diameter D of the expanded beads is 0.4 or less, the expanded beads have an expanded core layer made of a polypropylene resin, and a coating layer made of a polyolefin resin and coating a lateral surface of the expanded core layer, the expanded core layer and the coating layer contain a coloring pigment, and the melt mass-flow rate MFR of the polyolefin resin constituting the coating layer measured according to JIS K7210-1:2014 under conditions of a temperature of 230°C and a load of 2.16 kg is S The expanded polypropylene resin particles have a modulus of elasticity of more than 15 g / 10 min.

[0009] [2] The expanded polypropylene resin beads according to [1], wherein the average circularity Cb of the outer periphery of the expanded beads is 0.95 or more in a cross section obtained by cutting the expanded beads along a plane perpendicular to the axial direction at the position where the cross-sectional area of ​​the expanded beads is greatest. [3] The expanded polypropylene resin beads according to [1] or [2], wherein the content of the coloring pigment in the foamed core layer is 0.1% by mass or more and 5% by mass or less, and the content of the coloring pigment in the coating layer is 0.1% by mass or more and 5% by mass or less. [4] The expanded polypropylene resin beads according to any one of [1] to [3], wherein the coloring pigment contained in the foamed core layer is carbon black, and the coloring pigment contained in the coating layer is also carbon black.

[0010] [5] The melt mass-flow rate (MFR) of the polypropylene resin constituting the foamed core layer measured under conditions of a temperature of 230°C and a load of 2.16 kg according to JIS K7210-1:2014. C[6] The expanded polypropylene resin particles according to any one of [1] to [4], wherein the melt mass-flow rate (MFR) of the polypropylene resin constituting the foamed core layer is measured at a temperature of 230°C and a load of 2.16 kg in accordance with JIS K7210-1:2014. C The melt mass flow rate MFR of the polyolefin resin constituting the coating layer S Ratio of MFR S / MFR C [6] The expanded polypropylene resin particles according to any one of [1] to [5], wherein the number of pores is 2 or more and 5 or less.

[0011] [7] The expanded polypropylene resin beads according to any one of [1] to [6], wherein the expanded beads have an average wall thickness t of 1.2 mm or more and 2 mm or less. [8] The expanded beads have an apparent density of 10 kg / m 3 More than 100kg / m 3 [9] The expanded polypropylene resin beads according to any one of [1] to [8], wherein the average circularity Cp of the through holes in a cross section obtained by cutting the expanded beads along a plane perpendicular to the axial direction at a position where the cross-sectional area of ​​the expanded beads is maximum is 0.90 or more.

[0012]

[10] Expanded polypropylene resin beads according to any one of [1] to [9], wherein the ratio Cb / Cp of the average circularity Cb of the outer periphery of the expanded bead to the average circularity Cp of the through holes of the expanded bead in a cross section obtained by cutting the expanded bead along a plane perpendicular to the axial direction at the position where the cross-sectional area is maximum is 0.96 or more and 1.05 or less.

[0013] Another aspect of the present invention is a method for producing expanded polypropylene resin beads according to the following item

[11] .

[0014]

[10] A method for producing cylindrical expanded polypropylene-based resin beads having through holes passing through the interior in the axial direction, the method comprising: a granulation step of producing multilayer resin beads having a cylindrical core layer made of a polypropylene-based resin and having through holes passing through the interior in the axial direction, and a coating layer made of a polyolefin-based resin and coating a lateral surface of the core layer; and an expansion step of expanding the multilayer resin beads after the granulation step to obtain the expanded beads, wherein the through holes in the multilayer resin particles have an average pore size dr of less than 0.25 mm, the ratio dr / Dr of the average pore size dr of the through holes to the average outer diameter Dr of the multilayer resin particles is 0.4 or less, the core layer and the coating layer contain a color pigment, and the melt mass-flow rate MFR of the polyolefin-based resin constituting the coating layer is measured according to JIS K7210-1:2014 under conditions of a temperature of 230°C and a load of 2.16 kg. S The method for producing expanded polypropylene resin beads, wherein the modulus of elasticity is higher than 15 g / 10 min.

[0015] According to the above-described embodiment, it is possible to provide expanded polypropylene resin beads capable of forming expanded bead moldings having few streaks and good appearance, and a method for producing the same.

[0016] FIG. 1 is a perspective view of an expanded bead. FIG. 2 is a cross-sectional view of the expanded bead of FIG. 1 in a cross section passing through the central axis of the expanded bead. FIG. 3 is a cross-sectional view taken along line III-III in FIG. 2. FIG. 4 is a perspective view showing an example of an expanded bead having a groove on the circumferential side surface. FIG. 5 is a cross-sectional view taken along line V-V in FIG. 4. FIG. 6 is an explanatory diagram showing a method for calculating the area of ​​the high-temperature peak. FIG. 7 is a plan view schematically showing the surface of a molded article. FIG. 8 is a photograph showing the appearance of the expanded bead molded article of Example 1. FIG. 9 is a photograph showing the appearance of the expanded bead molded article of Comparative Example 3.

[0017] (Expanded Polypropylene Resin Beads) As shown in FIG. 1 , an expanded polypropylene resin bead 1 (hereinafter referred to as "expanded bead 1" or simply "expanded bead") has a cylindrical shape with a through hole 11 passing through the interior in the axial direction. More specifically, the expanded bead 1 has a bottom surface 12, a top surface 13 disposed above the bottom surface 12 and having substantially the same shape as the bottom surface 12, and a peripheral side surface 14 connecting the edge of the bottom surface 12 with the edge of the top surface 13. As shown in FIG. 2 , the through hole 11 passes through the interior of the expanded bead 1 in the axial direction and opens to the bottom surface 12 and the top surface 13. Examples of the cylindrical shape include a cylinder and a square tube as shown in FIG. 1 . The number of through holes 11 provided in the expanded bead 1 may be one or more.

[0018] As shown in Figures 2 and 3, the expanded beads 1 have an expanded core layer 2 made of a polypropylene resin and a coating layer 3 made of a polyolefin resin that coats the lateral surface of the expanded core layer 2. The coating layer 3 is provided on the outer surface of the expanded beads. The expanded core layer 2 and the coating layer 3 contain coloring pigments. The melt mass-flow rate (MFR) of the polyolefin resin that composes the coating layer is S In this way, the melt mass flow rate MFR of the foamed core layer of the foamed beads is higher than 15 g / 10 min. S By covering the expanded polypropylene resin bead molded article (hereinafter also referred to as "molded article") with a coating layer made of a polyolefin resin having a high melt mass-flow rate MFR, it is possible to suppress the formation of streaky patterns on the surface of the expanded polypropylene resin bead molded article (hereinafter also referred to as "molded article"). S By using a polyolefin resin having the above structure, it is possible to suppress the formation of streaks on the surface of the molded article without impairing the rigidity of the foamed core layer. Therefore, by using the foamed beads, it is possible to suppress the formation of streaks while maintaining the rigidity of the molded article. The configurations of the foamed core layer 2 and the coating layer 3 will be described in more detail later.

[0019] [Average diameter d of through holes] The average diameter d of the through holes in the expanded beads is less than 1 mm. By setting the average diameter d of the through holes to less than 1 mm, the moldability of the expanded beads can be improved and molded polypropylene resin expanded beads with excellent surface properties and rigidity can be obtained. Furthermore, by setting the average diameter d of the through holes to less than 1 mm, even if the curing step is omitted, it is possible to obtain a molded product with a desired shape, excellent surface properties, and rigidity, with less noticeable color unevenness. From the viewpoint of more reliably achieving this effect, the average diameter d of the through holes is more preferably 0.95 mm or less, even more preferably 0.92 mm or less, particularly preferably 0.90 mm or less, and most preferably 0.85 mm or less. Note that the aforementioned "curing step" refers to a step in the production process of a molded product in which a molded product released from a molding die is left to stand for a predetermined period of time in a high-temperature atmosphere adjusted to a temperature of about 60°C to 80°C.

[0020] If the average pore diameter d of the through holes is too large, the drying time of the molded article may be excessively long, and the surface of the molded article may be prone to formation of irregularities due to gaps between the expanded beads and the through holes, and the rigidity of the molded article may be reduced.

[0021] The average diameter d of the through holes is preferably 0.1 mm or more. In this case, 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 more preferably 0.2 mm or more, even more preferably 0.3 mm or more, particularly preferably 0.4 mm or more, and most preferably 0.5 mm or more.

[0022] In determining a preferred range of the average diameter d of the through holes, the above-described upper and lower limits of the average diameter d of the through holes can be combined arbitrarily. The preferred range of the average diameter d of the through holes may be, for example, 0.1 mm or more and less than 1 mm, 0.2 mm or more and 0.95 mm or less, 0.3 mm or more and 0.92 mm or less, 0.4 mm or more and 0.90 mm or less, or 0.5 mm or more and 0.85 mm or less.

[0023] The reason why the above-mentioned effect can be obtained by setting the average pore diameter d of the through holes within the above-mentioned specific range is thought to be, for example, as follows: When the expanded beads are molded in a mold, an open-cell structure consisting of open cells is formed in the molded body, i.e., minute spaces that communicate with the outside of the molded body. Specifically, the open-cell structure is formed by a complex connection of voids formed by the interconnection of through holes in a plurality of expanded beads, voids formed by the interconnection of through holes in the expanded beads with gaps between the expanded beads, voids formed by the interconnection of gaps between the expanded beads, and open-cell portions of the expanded beads that constitute the molded body.

[0024] Because the open-cell structure is connected to the outside of the molded body, when a molded body with a moderate open cell ratio is released from the mold, it is believed that outside air quickly flows into the bubbles inside the molded body through the open-cell structure. The flow of outside air into the bubbles inside the molded body then quickly balances the internal pressure of the molded body with the atmospheric pressure outside the molded body. As a result, the dimensions of the molded body are likely to stabilize early, and significant shrinkage and deformation of the molded body can be suppressed even without a curing step.

[0025] Furthermore, since the expanded beads have through holes, it is believed that when steam is supplied into the molding die, the steam can pass through the through holes. This makes it easier for the steam to reach the interior of the molding die, and it is believed that the entire expanded beads in the molding die can be easily heated. Therefore, even under low molding temperatures during in-mold molding, a molded body with excellent fusion properties and a good appearance can be obtained. As a result, the amount of heat received by the expanded beads by steam during in-mold molding can be kept low. Furthermore, the internal temperature of the molded body after demolding is prevented from becoming excessively high. As a result, it is believed that the dimensions of the molded body after in-mold molding can be easily stabilized early.

[0026] If the expanded beads do not have through-holes, the molding temperature tends to be high and it may be difficult to form a sufficient open-cell structure in the resulting molded article. Therefore, in this case, it is difficult to suppress significant shrinkage, deformation, etc. of the molded article when the curing step is omitted. In addition, in this case, the secondary expandability of the expanded beads may become excessively high, which may result in a long cooling time in the mold.

[0027] The average pore diameter d of the through holes of the expanded beads is determined as follows. First, the expanded beads are cut along a plane perpendicular to the axial direction at the position where the cross-sectional area is maximum, exposing the cut surface as shown in FIG. 3. Next, a photograph of the cut surface is taken, and the cross-sectional area (specifically, the opening area) of the through holes 11 at the cut surface is calculated. Then, the diameter of an imaginary perfect circle having the same area as the cross-sectional area of ​​the through holes 11 is calculated, and this value is used as the pore diameter of the through holes of each expanded bead. In this specification, the position where the cross-sectional area of ​​the expanded beads is maximum can be identified, for example, by 3D scanning the external shape of the expanded beads.

[0028] The above procedure is performed on 50 or more expanded beads 1, and the average diameter d of the through holes is determined as the arithmetic mean value of the diameters of the through holes 11 in these expanded beads 1. Even if the cross-sectional area of ​​the through holes 11 in each expanded bead 1 is not uniform in the axial direction, the average diameter d of the through holes of the expanded beads is determined using the diameter of the through holes 11 in the cut surface of the expanded bead 1 where the cross-sectional area is maximum, as described above.

[0029] 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 multilayer resin particles described below, the bulk 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.

[0030] [Average Circularity Cp of Through Holes] The average circularity Cp of the through holes in a cross section obtained by cutting the expanded beads along a plane perpendicular to the axial direction at the position where the cross-sectional area is greatest is preferably 0.90 or more, more preferably 0.92 or more, and even more preferably 0.95 or more. In this case, the aforementioned effects of improving the moldability of the expanded beads and shortening the cooling time during molding can be more easily achieved. Furthermore, by increasing the average circularity Cp of the through holes, the through holes become less likely to collapse due to external forces. Therefore, for example, even when the expanded beads are compressed in the mold during in-mold molding, the through holes are less likely to be blocked, thereby further improving the moldability during in-mold molding. Furthermore, by performing in-mold molding using the expanded beads, a molded product with excellent rigidity can be more easily obtained.

[0031] The average circularity Cp of the through holes of the expanded beads is determined as follows: First, the expanded beads 1 are cut along a plane perpendicular to the axial direction at the position where the cross-sectional area is maximum, to expose the cut surface as shown in Figure 3. Next, a photograph of the cut surface of the expanded beads 1 is taken, and the cross-sectional area S of the through holes 11 is measured. p (i.e., the opening area of ​​the through hole 11 on the cut surface) and the circumferential length L of the through hole 11 p (i.e., the length of the periphery of the through-hole 11 on the cross section) is measured. Then, the circularity of the through-hole 11 in each expanded bead 1 is calculated based on the following formula (1). Note that π in the following formula (1) is the ratio of the circumference of a circle to its circumference. Circularity of through-hole=4πS p / (L p ×L p ) ... (1)

[0032] The above operation is performed on 50 or more randomly selected expanded beads 1, and the average circularity Cp of the through holes is determined as the arithmetic mean value of the circularities of the through holes 11 in these expanded beads 1. Even if the cross-sectional shape of the through holes 11 of each expanded bead 1 is not uniform in the axial direction, the average circularity Cp of the through holes is determined based on the cross-sectional shape of the through holes at the position where the cross-sectional area of ​​the expanded bead 1 is maximum, as described above.

[0033] The average circularity Cp of the through holes can be adjusted to fall within the above range, for example, by changing the shape of the die for forming the through holes in the granulation step of the multilayer resin particles described below, or by adjusting the water temperature for cooling the strands, which is usually about 25° C., to a lower temperature (for example, 15° C. or lower). The upper limit of the average circularity Cp of the through holes is 1. A preferred range for the average circularity Cp of the through holes may be, for example, 0.90 or more and 1 or less, 0.92 or more and 1 or less, or 0.95 or more and 1 or less.

[0034] [Ratio d / D of the average pore size d of the through holes to the average outer diameter D of the expanded beads] The ratio d / D of the average pore size d of the through holes to the average outer diameter D of the expanded beads is 0.4 or less. By setting the ratio d / D to 0.4 or less, the secondary expandability of the expanded beads during in-mold molding can be appropriately improved, and molded articles with excellent surface properties and rigidity can be obtained over a wide range of molding pressures from low to high pressures. From the viewpoint of more reliably achieving this effect, the ratio d / D of the average pore size d of the through holes to the average outer diameter D of the expanded beads is preferably 0.35 or less, more preferably 0.3 or less, and even more preferably 0.25 or less. If the ratio d / D is too high, the secondary expandability of the expanded beads during in-mold molding may be reduced, resulting in a decrease in the surface properties and rigidity of the molded article.

[0035] The ratio d / D of the average pore size d of the through holes to the average outer diameter D of the expanded beads is preferably 0.1 or more. In this case, the through holes of the expanded beads are less likely to be crushed even when an external force is applied, so that the effect of the through holes can be more reliably exhibited. The preferred range of the ratio d / D of the average pore size d of the through holes to the average outer diameter D of the expanded beads may be, for example, 0.1 or more and 0.4 or less, 0.1 or more and 0.35 or less, 0.1 or more and 0.3 or less, or 0.1 or more and 0.25 or less.

[0036] From the viewpoint of increasing the wall thickness of the expanded beads and improving the secondary expandability of the expanded beads and the rigidity of the molded body, and from the viewpoint of more reliably suppressing deformation and shrinkage of the molded body when the curing step is omitted, 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 further improving the fillability of the expanded beads into the molding die, the average outer diameter D of the expanded beads is preferably 8 mm or less, more preferably 5 mm or less, and even more preferably 4.5 mm or less.

[0037] The preferred range of the average outer diameter D of the expanded beads can be determined by any combination of the above-mentioned upper and lower limits of the average outer diameter D. The preferred range of the average outer diameter D of the expanded beads may be, for example, 2 mm to 8 mm, 2.5 mm to 5 mm, or 3 mm to 4.5 mm.

[0038] The method for calculating the average outer diameter D of the expanded beads is as follows. First, the expanded beads 1 are cut along a plane perpendicular to the axial direction at the position where the cross-sectional area is maximum, exposing the cut surface of the expanded beads 1 as shown in Fig. 3. The cross-sectional area of ​​the expanded beads 1, including the through holes 11, on this cut surface (i.e., the area of ​​the region surrounded by the outer periphery of the expanded beads 1 on the cut surface) is calculated. Then, the diameter of an imaginary circle having an area equal to the cross-sectional area of ​​the expanded beads 1 is taken as the outer diameter of each expanded bead 1.

[0039] In this manner, the outer diameters of 50 or more expanded beads 1 are calculated, and the arithmetic mean value of the outer diameters of these expanded beads 1 is defined as the average outer diameter D of the expanded beads. Even if the cross-sectional area of ​​each expanded bead 1 is not uniform in the axial direction, the average outer diameter D of the expanded beads is determined based on the cross-sectional area of ​​the expanded beads 1 at the cross section where the cross-sectional area of ​​the expanded beads 1 is maximum, as described above.

[0040] [Average Circularity Cb of the Periphery of Expanded Beads] The average circularity Cb of the peripheral edge of the expanded beads in a cross section obtained by cutting the expanded beads at the position where the cross-sectional area is maximum along a plane perpendicular to the axial direction is preferably 0.95 or more, more preferably 0.96 or more. By performing in-mold molding using such expanded beads, the formation of streaks on the surface of the molded article can be more effectively suppressed, and molded articles with good appearance can be more easily obtained.

[0041] The average circularity Cb of the outer peripheral edge of an expanded bead is determined as follows: First, the expanded bead is cut along a plane perpendicular to the axial direction at the position where the cross-sectional area is maximum, to expose the cut surface as shown in Figure 3. Next, a photograph of the cut surface of the expanded bead 1 is taken, and the cross-sectional area S of the expanded bead 1 including the through-holes 11 is measured. b (i.e., the area of ​​the region surrounded by the outer periphery of the expanded beads 1 on the cross section) and the periphery L of the outer periphery of the expanded beads 1 b (i.e., the length of the outer periphery of the expanded beads 1 on the cross section) is measured. Then, the circularity of the outer periphery of each expanded bead 1 is calculated based on the following formula (2). Note that π in the following formula (2) is the ratio of the circumference of a circle to its circumference. Circularity of the outer periphery of an expanded bead = 4πS b / (L b ×L b ) ... (2)

[0042] The above operation is performed on 50 or more randomly selected expanded beads 1, and the arithmetic mean value of the circularities of these expanded beads 1 is defined as the average circularity Cb of the outer periphery of the expanded beads. Even if the cross-sectional shape of each expanded bead 1 is not uniform in the axial direction, the average circularity Cb of the outer periphery of the expanded beads is determined based on the cross-sectional shape of the expanded beads 1 at the cut surface where the cross-sectional area of ​​the expanded beads 1 is maximum, as described above. The upper limit of the average circularity Cb of the outer periphery is 1. The average circularity Cb of the outer periphery of the expanded beads may preferably be in the range of 0.95 to 1, or 0.96 to 1.

[0043] The ratio Cb / Cp of the average circularity Cb of the peripheral edges of the expanded beads to the average circularity Cp of the through holes of the expanded beads is preferably 0.96 or more and 1.05 or less, and more preferably 0.98 or more and 1.02 or less. In this case, the moldability improving effect of the through holes can be more reliably exhibited, and a molded article with excellent rigidity can be obtained, and the formation of streaky patterns on the surface of the molded article can be more effectively suppressed. From the same viewpoint, the absolute value |Cp - Cb| of the difference between the average circularity Cb of the peripheral edges of the expanded beads and the average circularity Cp of the through holes is preferably 0.03 or less, more preferably 0.01 or less, and even more preferably 0.008 or less.

[0044] The average circularity Cb of the outer peripheral edge of the expanded beads is, for example, S By using a polyolefin resin having the above formula, it is possible to adjust the thickness within the above specific range.

[0045] [Grooves in Expanded Beads] From the viewpoint of further increasing the average circularity of the outer peripheral edge of the expanded beads and more effectively suppressing the formation of streaky patterns on the surface of a molded article, the ratio of the total cross-sectional area of ​​the grooves to the cross-sectional area of ​​the expanded beads excluding the through holes in a cross section obtained by cutting the expanded beads along a plane perpendicular to the axial direction at the position where the cross-sectional area is greatest is preferably 3% or less, more preferably 1.5% or less, even more preferably 1.2% or less, even more preferably 1% or less, particularly preferably 0.5% or less, and most preferably 0%, i.e., the expanded beads have no grooves on their side peripheral surfaces.

[0046] The "groove" mentioned above refers to a recess formed on the lateral surface of an expanded bead and extending along the axial direction of the expanded bead. For example, an expanded bead 102 shown in Fig. 4 has a cylindrical shape surrounded by a bottom surface 12, a top surface 13, and a lateral surface 14, and has a through-hole 11 passing through the interior in the axial direction. Three grooves 15 are formed on the lateral surface 14 of the expanded bead 102, positioned at approximately equal intervals in the circumferential direction of the expanded bead 102. When the expanded bead 102 is cut along a plane perpendicular to its axial direction, as shown in Fig. 5, the grooves 15 of the expanded bead 102 are observed as inwardly depressed portions of the contour of the expanded bead 102 on the cut surface.

[0047] The area ratio of the total cross-sectional area of ​​the grooves in the expanded beads is calculated as follows: First, the expanded beads 102 are cut along a plane perpendicular to the axial direction at the position where the cross-sectional area is maximum, to expose the cut surface as shown in Fig. 5. Next, a photograph of the cut surface of the expanded beads 102 is taken, and the cross-sectional area of ​​the expanded beads 102 excluding the through holes 11 (i.e., the area of ​​the region surrounded by the outer and inner edges of the expanded beads 102 on the cut surface) is measured.

[0048] Next, a tangent line L1 is drawn around each groove 15 in the photograph of the cross section of the expanded beads 102. The tangent line L1 touches the contour of the expanded beads 102 at two points P1 and P2 and does not pass through the interior of the expanded beads 102. The area of ​​the region surrounded by the contour of the expanded beads 102 and the tangent line L1 is then calculated, and this area is defined as the cross-sectional area of ​​each groove 15. That is, the cross-sectional area of ​​each groove 15 is the area of ​​the shaded region 151 in Figure 5. The above operation is performed for all grooves 15, and the total cross-sectional area of ​​the grooves 15 in each expanded bead 102 is calculated by adding them up.

[0049] The ratio of the total cross-sectional area of ​​the grooves 15 to the cross-sectional area of ​​the expanded beads 102 is calculated for each expanded bead 102 by dividing the total cross-sectional area of ​​the grooves 15 by the cross-sectional area of ​​the expanded beads 102 excluding the through holes 11. The above operation is performed for 100 or more expanded beads 102, and the arithmetic mean value of the ratios for these expanded beads 102 is taken as the area ratio of the total cross-sectional area of ​​the grooves in the expanded beads.

[0050] The coating layer has a melt mass flow rate MFR within the specified range. S By using a polyolefin resin having the above formula (I), the formation of grooves on the circumferential side surfaces of the expanded beads can be suppressed, and the area ratio of the total cross-sectional area of ​​the grooves in the expanded beads can be easily adjusted within the above range. The reason for this is not entirely clear, but it is thought that the residual stress remaining in the resin particles obtained in the granulation step described below is alleviated, making it difficult for wrinkles to form in the expanded beads, and that the coating layer is more easily formed on the circumferential side surfaces of the obtained expanded beads.

[0051] [Average Wall Thickness t of Expanded Beads] 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 secondary expandability of the expanded beads during in-mold molding can be further improved. Furthermore, expanded beads having an average wall thickness t within the specific range are more resistant to crushing by external forces, thereby further improving the rigidity of the molded article obtained by in-mold molding of the expanded beads. From the viewpoint of more reliably achieving these effects, the average wall thickness t of the expanded beads is more preferably 1.3 mm or more and 2 mm or less, and even more preferably 1.5 mm or more and 2 mm or less.

[0052] The average wall thickness t of the expanded beads is the average distance from the side circumferential surface of the expanded beads to the outer edge of the through-holes (i.e., the inner circumferential edge of the expanded beads). More specifically, the average wall thickness t of the expanded beads is calculated based on the following formula (3) using the average hole diameter d (unit: mm) of the through-holes and the average outer diameter D (unit: mm) of the expanded beads: t = (D - d) / 2 (3)

[0053] [Apparent density of expanded beads] The apparent density of the expanded beads is 10 kg / m 3 More than 100kg / m 3 It is preferable that the viscosity is 20 kg / m or less. 3 More than 80kg / m 3 More preferably, it is 25 kg / m or less. 3 More than 60kg / m 3 More preferably, it is 30 kg / m or less. 3 More than 50kg / m 3It is particularly preferable that the apparent density is within the above range. By performing in-mold molding using expanded beads having an apparent density within the above range, a lightweight molded body with excellent rigidity can be easily obtained. Furthermore, in the past, when producing a molded body with a particularly low density, the molded body was prone to significant deformation after demolding, making it difficult to omit the curing step. In contrast, with the expanded beads, the curing step can be omitted even when the apparent density is low, and therefore a lightweight molded body having a desired shape can be produced without curing.

[0054] The method for calculating the apparent density of expanded beads is as follows. First, 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, the expanded beads 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 determined from the rise in the liquid level. The apparent density (unit: kg / m) of the expanded beads 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.

[0055] [Foamed Core Layer] The foamed core layer of the foamed beads is composed of a polypropylene-based resin. In this specification, the term "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 foamed core layer may contain one type of polypropylene-based resin or two or more types of polypropylene-based resins.

[0056] The foam core layer is more preferably composed of a propylene-based copolymer in which propylene is copolymerized with another monomer. Preferred examples of the propylene-based copolymer include copolymers of propylene and an α-olefin having 4 to 10 carbon atoms, such as ethylene-propylene copolymer, butene-propylene copolymer, hexene-propylene copolymer, and ethylene-propylene-butene copolymer. These copolymers may be, for example, random copolymers or block copolymers, but are preferably random copolymers.

[0057] The melting point Tmc of the polypropylene resin constituting the foam core layer is preferably 155 ° C or less. In this case, a molded article having excellent appearance and rigidity can be molded at a lower molding temperature (i.e., low molding pressure). From the viewpoint of improving this effect, the melting point Tmc of the polypropylene resin constituting the foam core layer is preferably 152 ° C or less, and more preferably 148 ° C or less. 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 core layer is preferably 135 ° C or more, more preferably 138 ° C or more, and even more preferably 140 ° C or more. From the above viewpoint, the preferred range of the melting point Tmc of the polypropylene resin constituting the foam core layer may be, for example, 135 ° C or more and 155 ° C or less, 138 ° C or more and 152 ° C or less, or 140 ° C or more and 148 ° C or less.

[0058] The melting point Tmc of the polypropylene-based resin constituting the foamed core layer can be determined based on the DSC curve obtained by differential scanning calorimetry (i.e., DSC) according to JIS K7121-1987. Specifically, a test specimen made of a polypropylene-based resin is prepared, and the test specimen is conditioned according to "(2) Measuring the melting temperature after a certain heat treatment." The heating and cooling rates during conditioning are 10°C / min, and the temperature range is 30°C to 200°C. The conditioned test specimen is heated from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve. The apex temperature of the melting peak appearing in the DSC curve is then taken as the melting point Tmc of the polypropylene-based resin. Note that if multiple melting peaks appear in the DSC curve, the apex temperature of the melting peak with the largest area is taken as the melting point Tmc.

[0059] Melt mass flow rate (MFR) of polypropylene resin constituting the foam core layer CThe melt mass-flow rate (MFR) of the polypropylene resin constituting the foamed core layer is preferably 5 g / 10 min or more, more preferably 6 g / 10 min or more, and even more preferably 7 g / 10 min or more. In this case, the foamability and moldability can be further improved. On the other hand, from the viewpoint of further increasing the rigidity of the molded article, the melt mass-flow rate (MFR) of the polypropylene resin constituting the foamed core layer is preferably 5 g / 10 min or more, more preferably 6 g / 10 min or more, and even more preferably 7 g / 10 min or more. C is preferably 12 g / 10 min or less, more preferably 10 g / 10 min or less.

[0060] Melt mass flow rate (MFR) of polypropylene resin constituting the foam core layer C In determining the preferred range of the melt mass-flow rate MFR, C For example, the melt mass-flow rate MFR of the polypropylene resin constituting the foamed core layer can be C The preferred range of the MFR of the polypropylene resin constituting the foamed core layer may be 5 g / 10 min or more and 12 g / 10 min or less, 6 g / 10 min or more and 12 g / 10 min or less, or 7 g / 10 min or more and 10 g / 10 min or less. C is a value measured based on JIS K7210-1:2014 under conditions of a test temperature of 230°C and a load of 2.16 kg.

[0061] The foamed beads have a side peripheral surface of the foamed core layer with a melt mass flow rate MFR S By covering the molded article with a coating layer made of a polyolefin resin having a high melt mass-flow rate (MFR), the formation of streaky patterns on the surface of the molded article can be suppressed. S On the other hand, a polyolefin resin having a relatively low melt mass flow rate MFR, for example, in the above range, is used as the resin constituting the foamed core layer. C As a result, it is possible to more easily obtain the effect of suppressing the formation of streaks while maintaining the expandability and rigidity of the expanded beads.

[0062] The foamed core layer may contain a polymer other than a polypropylene-based resin, provided that the aforementioned effects are not impaired. Examples of such polymers include thermoplastic resins other than polypropylene-based resins, such as polyethylene-based resins and polystyrene-based resins, and thermoplastic elastomers. The content of polymers other than polypropylene-based resins in the foamed core layer is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less. It is particularly preferred that the content of polymers other than polypropylene-based resins in the foamed core layer is 0%, i.e., the foamed core layer contains substantially only polypropylene-based resins as polymers.

[0063] The foamed core layer may also contain additives such as a cell regulator, a crystal nucleating agent, a flame retardant, a flame retardant assistant, a plasticizer, an antistatic agent, an antioxidant, an ultraviolet light inhibitor, a light stabilizer, a conductive filler, an antibacterial agent, etc. The content of the additive in the foamed core layer is preferably, for example, 0.01 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the polypropylene-based resin.

[0064] [Coating layer] The lateral surface of the foamed core layer in the expanded beads is covered with a coating layer made of a polyolefin resin. The coating layer of the expanded beads may be in a foamed state or a non-foamed state, but is preferably in a substantially non-foamed state. The aforementioned "non-foamed state" includes a state in which the coating layer is not foamed and does not contain bubbles, and a state in which the bubbles have disappeared after foaming, and means that there is almost no bubble structure in the coating layer.

[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), more preferably 99:1 to 88:12, and even more preferably 97:3 to 90:10. In this case, the effect of suppressing the formation of streaks can be more easily achieved while maintaining the expandability, rigidity, and other properties of the expanded beads. In this specification, the term "A to B" representing a numerical range is synonymous with "A or more and B or less" and represents a numerical range including the endpoints A and B of the numerical range.

[0066] Examples of the polyolefin resin that constitutes the coating layer include polyethylene resins, polypropylene resins, and polybutene resins. From the viewpoint of adhesion to the foam core layer, the polyolefin resin that constitutes the coating layer is preferably a polyethylene resin or a polypropylene resin, and more preferably a polypropylene resin. Examples of the polypropylene resin that constitutes the coating layer include an ethylene-propylene copolymer, a butene-propylene copolymer, an ethylene-propylene-butene copolymer, and a propylene homopolymer. Among these, the polypropylene resin that constitutes the coating layer is preferably an ethylene-propylene copolymer or an ethylene-propylene-butene copolymer. On the other hand, examples of the polyethylene resin that constitutes the coating layer include linear low-density polyethylene (PE-LLD), low-density polyethylene (PE-LD), medium-density polyethylene (PE-MD), and high-density polyethylene (PE-HD).

[0067] Melt mass flow rate (MFR) of the polyolefin resin that constitutes the coating layer S The melt mass flow rate MFR of the polyolefin resin constituting the coating layer is higher than 15 g / 10 min. S By setting the melt mass-flow rate MFR in the coating layer within the above-mentioned specific range, it is possible to suppress the formation of streaky patterns on the surface of the molded article, and to easily obtain a molded article having a good appearance. S On the other hand, the polypropylene resin constituting the foamed core layer has a relatively low melt mass flow rate MFR. C As a result, it is possible to impart sufficient rigidity and expandability to the expanded beads.

[0068] The reason why such an effect can be obtained in the expanded beads is thought to be that the average circularity Cb of the outer peripheral edge of the expanded beads is increased by covering the side peripheral surface of the expanded core layer with the covering layer.

[0069] In order to more reliably obtain the above-mentioned effects, the melt mass-flow rate (MFR) of the polyolefin resin constituting the coating layer is S and the melt mass flow rate MFR of the polypropylene resin constituting the foam core layer C Difference MFR S -MFR C From the same viewpoint, the melt mass flow rate MFR of the polypropylene resin constituting the foamed core layer is preferably 10 g / 10 min or more, and more preferably 12 g / 10 min or more. C The melt mass flow rate MFR of the polyolefin resin constituting the coating layer S Ratio of MFR S / MFR C is preferably 2 or more, and more preferably 2.5 or more.

[0070] On the other hand, from the viewpoint of further suppressing peeling between the coating layer and the foamed core layer while improving the appearance of the molded article, the melt mass-flow rate (MFR) of the polyolefin resin constituting the coating layer is S and the melt mass flow rate MFR of the polypropylene resin constituting the foam core layer C Difference MFR S -MFR C From the same viewpoint, the melt mass flow rate MFR of the polypropylene resin constituting the foamed core layer is preferably 30 g / 10 min or less. C The melt mass flow rate MFR of the polyolefin resin constituting the coating layer S Ratio of MFR S / MFR C is preferably 5 or less, more preferably 4 or less.

[0071] Melt mass flow rate MFR of the polyolefin resin constituting the coating layer S and the melt mass flow rate MFR of the polypropylene resin constituting the foam core layer C Difference MFR S -MFR C In determining the preferred range of the above, the difference MFR S -MFR CThe upper and lower limits of the differential MFR can be arbitrarily combined. S -MFR C The melt mass-flow rate MFR of the polypropylene resin constituting the foamed core layer may be preferably in the range of 10 g / 10 min to 30 g / 10 min, or in the range of 12 g / 10 min to 30 g / 10 min. C The melt mass flow rate MFR of the polyolefin resin constituting the coating layer S Ratio of MFR S / MFR C In determining the preferred range of the above, the above-mentioned ratio MFR S / MFR C The upper and lower limits of the ratio MFR can be arbitrarily combined. S / MFR C The preferable range of may be 2 or more and 5 or less, or 2.5 or more and 4 or less.

[0072] The MFR of the polyolefin resin constituting the coating layer is S is a value measured based on JIS K7210-1:2014 under the conditions of a test temperature of 230°C and a load of 2.16 kg. Even when the coating layer is made of a polyethylene-based resin, the MFR S is measured under the conditions of a test temperature of 230° C. and a load of 2.16 kg.

[0073] The melting point Tms of the polyolefin resin constituting the coating layer is preferably lower than the melting point Tmc of the polypropylene resin constituting the foamed core layer. By coating the foamed core layer with a coating layer made of such a polyolefin resin, the foamed particles can be fused at a lower molding temperature (i.e., lower molding pressure) during in-mold molding. As a result, deformation and shrinkage of the molded body can be more reliably suppressed when the curing step is omitted. In addition, in this case, the water cooling time can be shortened, thereby improving productivity. From the viewpoint of more reliably achieving these effects, the difference Tmc - Tms between the melting point Tmc of the polypropylene resin constituting the foamed core layer and the melting point Tms of the polyolefin resin constituting the coating layer is preferably greater than 0°C and not more than 40°C, more preferably 3°C or more and 35°C or less, and even more preferably 5°C or more and 30°C or less.

[0074] The method for measuring the melting point Tms of the polyolefin-based resin constituting the coating layer is the same as the method for measuring the melting point Tmc of the polypropylene-based resin constituting the foamed core layer described above, except that a polyolefin-based resin is used instead of a polypropylene-based resin.

[0075] The coating layer may contain additives such as a nucleating agent, a flame retardant, a flame retardant assistant, a plasticizer, an antistatic agent, an antioxidant, an ultraviolet inhibitor, a light stabilizer, a conductive filler, an antibacterial agent, etc., within a range that does not impair the above-mentioned effects. The content of the additive in the coating layer is preferably, for example, 0.01 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the polyolefin resin.

[0076] [Coloring Pigment] The foam core layer and the coating layer contain a coloring pigment. The coloring pigment may be any color other than white, and various color tones such as black, red, blue, yellow, and green pigments may be used. The coloring pigment may be either an inorganic pigment or an organic pigment.

[0077] Examples of inorganic pigments include carbon particles such as carbon black; chromates such as yellow lead, zinc yellow, and barium yellow; ferrocyanides such as Prussian blue; sulfides such as cadmium yellow and cadmium red; oxides such as red iron oxide; and silicates such as ultramarine. Examples of organic pigments include azo pigments such as monoazo pigments, disazo pigments, azo lakes, condensed azo pigments, and chelate azo pigments; and polycyclic pigments such as phthalocyanines, anthraquinones, perylenes, perinones, thioindigo, quinacridones, dioxazines, isoindolinones, and quinophthalones.

[0078] The foamed core layer may contain one type of coloring pigment or two or more types of coloring pigments. Similarly, the coating layer may contain one type of coloring pigment or two or more types of coloring pigments. The color tone of the coloring pigment in the foamed core layer and the color tone of the coloring pigment in the coating layer may be the same as or different from each other. From the viewpoint of imparting a luxurious feel to the molded article, it is preferable that the coloring pigment in the foamed core layer and the coating layer is a black pigment. It is preferable to use carbon particles as the black pigment, and more specifically, it is preferable to use carbon black.

[0079] The content of the coloring pigment in the foamed core layer is preferably 0.1% by mass or more and 5% by mass or less, and the content of the coloring pigment in the coating layer is preferably 0.1% by mass or more and 5% by mass or less. In this case, the desired color tone can be imparted to the foamed beads while sufficiently ensuring the secondary expandability and fusion properties of the foamed beads during in-mold molding. This makes it easier to obtain a molded product having the desired color tone and good appearance. From the above viewpoints, it is more preferable that the content of the coloring pigment in the foamed core layer is 0.3% by mass or more and 4.5% by mass or less, and the content of the coloring pigment in the coating layer is 0.3% by mass or more and 4.5% by mass or less, and more preferably the content of the coloring pigment in the foamed core layer is 0.5% by mass or more and 4% by mass or less, and the content of the coloring pigment in the coating layer is 0.5% by mass or more and 4% by mass or less.

[0080] The reason for the formation of streaky patterns on the surface of the molded article is thought to be, for example, that the grooves formed on the lateral peripheral surface of the expanded beads are exposed on the surface of the molded article. Such streaky patterns on the surface of the molded article are particularly noticeable in molded articles obtained by molding expanded beads containing a color pigment. This is thought to be because the streaky patterns often appear lighter in color than the surrounding area and are less noticeable in white molded articles that do not contain a color pigment, and because, in the past, expanded beads containing a color pigment have tended to have a low average circularity, particularly at the outer periphery of the expanded beads. The expanded beads suppress the deterioration of the appearance caused by streaky patterns, even in colored molded articles obtained from expanded beads containing a color pigment.

[0081] [Closed Cell Ratio] The closed cell ratio of the expanded beads is preferably 90% or more, more preferably 92% or more, and even more preferably 95% or more, from the viewpoint of improving the moldability of the expanded beads and further increasing the rigidity of the molded body.

[0082] The closed cell content of expanded beads can be measured using an air comparison hydrometer in accordance with ASTM-D2856-70 Procedure C. Specifically, it is measured as follows. First, the expanded beads are allowed 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. After conditioning, the expanded beads are allowed to naturally pile up in a measuring cylinder until the value of the marking is about 20 cm. 3 This measurement sample is submerged in a measuring cylinder containing ethanol at a temperature of 23° C., and the apparent volume of the measurement sample is measured based on the amount of rise in the liquid level.

[0083] After the measurement sample whose apparent volume has been measured is thoroughly dried, the true volume of the measurement sample is measured using an Accupyc II 1340 manufactured by Shimadzu Corporation in accordance with Procedure C described in ASTM-D2856-70. These volume values ​​are then used to calculate the closed cell ratio (unit: %) of the measurement sample according to the following formula (4): Closed cell ratio = (Vx - W / ρ) x 100 / (Va - W / ρ) (4)

[0084] However, Vx (unit: cm 3 ) is the true volume of the expanded beads (i.e., the sum of the volume of the resin constituting the expanded beads and the total volume of the closed cells in the expanded beads), and Va (unit: cm 3 ) is the apparent volume of the expanded beads (i.e., the volume measured from the rise in the liquid level when the expanded beads are submerged in a measuring cylinder containing ethanol), W (unit: g) is the mass of the measurement sample, and ρ (unit: g / cm 3 ) is the density of the polypropylene resin constituting the foamed core layer.

[0085] The above procedure is carried out five times using different measurement samples, and the arithmetic mean value of the closed cell ratios obtained from these five measurements is taken as the closed cell ratio of the expanded beads.

[0086] [High-Temperature Peak] The expanded beads preferably have a crystalline structure in which, in a DSC curve obtained when heated from 23°C to 200°C at a heating rate of 10°C / min, an endothermic peak due to the melting inherent to the polypropylene-based resin constituting the foamed core layer and one or more melting peaks higher than this endothermic peak appear. Expanded beads with such a crystalline structure have excellent mechanical strength and moldability. Hereinafter, the endothermic peak due to the melting inherent to the polypropylene-based resin appearing in the DSC curve is referred to as the "resin-specific peak," and the melting peak appearing higher than the resin-specific peak is referred to as the "high-temperature peak." The resin-specific peak is generated by the endothermic heat generated when the crystals inherent to the polypropylene-based resin constituting the foamed core layer melt. On the other hand, the high-temperature peak is presumed to be generated by the melting of secondary crystals formed in the polypropylene-based resin constituting the foamed core layer during the manufacturing process of the expanded beads. In other words, when a high-temperature peak appears in the DSC curve, it is presumed that secondary crystals are formed in the polypropylene-based resin.

[0087] Whether or not the expanded beads have the above-mentioned crystalline structure can be determined based on a DSC curve obtained by performing differential scanning calorimetry (DSC) under the above-mentioned conditions in accordance with JIS K7121: 1987. When performing DSC, 1 to 3 mg of the expanded beads may be used as a sample.

[0088] Specifically, the DSC curve obtained when the foamed core layer is heated from 23°C to 200°C at a heating rate of 10°C / min (i.e., the first heating) as described above shows both a high-temperature peak and a peak characteristic of the polypropylene resin constituting the foamed core layer. In contrast, the DSC curve obtained when the foamed core layer is cooled from 200°C to 23°C at a cooling rate of 10°C / min after the first heating and then heated from 23°C to 200°C at a heating rate of 10°C / min again (i.e., the second heating) shows only a peak characteristic of the polypropylene resin constituting the foamed core layer. Therefore, by comparing the DSC curve obtained during the first heating with the DSC curve obtained during the second heating, the resin peak and the high-temperature peak can be distinguished. The apex temperature of this resin peak may differ slightly between the first and second heatings, but the difference is usually within 5°C.

[0089] From the viewpoint of further improving the moldability of the expanded beads and obtaining molded articles with superior rigidity, the heat of fusion of the high-temperature peak of the expanded beads is preferably 5 J / g or more and 40 J / g or less, more preferably 7 J / g or more and 30 J / g or less, and even more preferably 10 J / g or more and 20 J / g or less.

[0090] The heat of fusion of the high-temperature peak is a value determined as follows. First, the expanded beads are allowed to stand for 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm to condition the expanded beads. After the condition adjustment, 1 to 3 mg of the expanded beads are used as a sample and subjected to differential scanning calorimetry under conditions of heating from 23°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve. An example of a DSC curve is shown in FIG. 6. When the expanded beads have a high-temperature peak, the DSC curve will show a resin-specific peak ΔH1 and a high-temperature peak ΔH2 whose peak is higher than the peak of the resin-specific peak ΔH1, as shown in FIG. 6.

[0091] Next, a straight line L2 is drawn connecting point α corresponding to 80° C. on the DSC curve and point β corresponding to the melting end temperature T of the expanded beads. The melting end temperature T is the high-temperature end point of the high-temperature peak ΔH2, i.e., the intersection point of the high-temperature peak ΔH2 and the baseline on the higher temperature side of the high-temperature peak ΔH2 on the DSC curve.

[0092] After drawing the line L2, a line L3 is drawn that passes through the maximum point γ between the resin-specific peak ΔH1 and the high-temperature peak ΔH2 and is parallel to the vertical axis of the graph. This line L3 separates the resin-specific peak ΔH1 from the high-temperature peak ΔH2. The endothermic amount of the high-temperature peak ΔH2 can be calculated based on the area of ​​the portion of the DSC curve that constitutes the high-temperature peak ΔH2 and the area surrounded by the lines L2 and L3.

[0093] (Method for producing expanded beads) The method for producing expanded beads includes a granulation step for producing multilayer resin beads having a cylindrical core layer made of a polypropylene resin and having through holes, and a coating layer made of a polyolefin resin and covering the lateral surface of the core layer, and an expansion step for expanding the multilayer resin beads. Each step will be described in more detail below.

[0094] [Granulation Process] The method for producing multilayer resin particles in the granulation process is not particularly limited, but for example, the strand-cut method can be used to produce multilayer resin particles. In the strand-cut method, multilayer resin particles are produced using a co-extrusion device equipped with a core layer extruder, a coating layer extruder, and a co-extrusion die connected to these two extruders. Specifically, first, a polypropylene-based resin that will form the expanded core layer of the expanded beads, a coloring pigment, and optional additives are fed into the core layer extruder, and these are melt-kneaded in the core layer extruder to produce a resin melt-kneaded product for forming the core layer. Then, a polyolefin-based resin that will form the coating layer of the expanded beads, a coloring pigment, and optional additives are fed into the coating layer extruder, and these are melt-kneaded in the coating layer extruder to produce a resin melt-kneaded product for forming the coating layer.

[0095] The molten mixture is then discharged from each extruder and merged in a co-extrusion die to form a multilayer composite consisting of a non-foamed core layer and a non-foamed coating layer covering the lateral surface of the core layer. The composite is then extruded through small holes in a die equipped with a mechanism for forming through holes in the core layer to form a strand-like extrudate having through holes in the core layer. This extrudate is collected and cooled, and then cut to the desired length to obtain multilayer resin particles having through holes in the core layer. The method for producing multilayer resin particles is not limited to the above-described method, and other methods, such as hot cutting and underwater cutting, may also be used.

[0096] In preparing the multilayer resin particles, it is preferable to adopt a strand-cutting method in which the strand-shaped extrudate is cooled in water and then cut, which can further increase the precision of the shape of the multilayer resin particles and more easily make the shape of the multiple through holes in the final expanded beads desired.

[0097] When a strand-cutting method is used for cutting the extrudate, that is, a method in which the strand-shaped extrudate extruded from the die is cooled in water while being taken up, and then cut to an appropriate length, the particle diameter, length / outside diameter ratio, and mass per particle of the multilayer resin particle can be adjusted by appropriately changing the extrusion speed, take-up speed, cutter speed, etc. during extrusion of the resin molten kneaded product.

[0098] The average outer diameter Dr of the multilayer resin particles is preferably 0.1 mm or more and 3.0 mm or less, and more preferably 0.3 mm or more and 1.5 mm or less.

[0099] The mass of each multilayer 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. The mass of each multilayer resin particle is calculated by dividing the mass of 200 randomly selected multilayer resin particles by the number of multilayer resin particles. The mass of each multilayer resin particle obtained by the above-described method is sometimes referred to as the "average mass of the multilayer resin particles."

[0100] In the above-described production method, expanded beads having the specific shape can be easily obtained by setting the average pore size dr of the through holes in the core layer of the multilayer resin particles to less than 0.25 mm and setting the ratio dr / Dr of the average pore size dr of the through holes to the average outer diameter Dr of the multilayer resin particles to 0.4 or less. The average pore size dr of the through holes in the core layer of the multilayer resin particles and the ratio dr / Dr can be adjusted, for example, by the diameter of the small holes in the die used to form the through holes (i.e., the inner diameter of the die).

[0101] The methods for calculating the average outer diameter Dr of the multilayer resin particles and the average pore diameter dr of the through holes are the same as the methods for calculating the average outer diameter D of the expanded beads and the average pore diameter d of the through holes described above, except that multilayer resin particles are used instead of expanded beads.

[0102] [Expansion Step] In the expansion step, the multilayer resin particles are expanded. The method for expanding the multilayer resin particles is not particularly limited, but it is preferable to employ, for example, a method called a "direct expansion method" in which multilayer resin particles containing a blowing agent dispersed in an aqueous medium in a container are released together with the dispersion medium into an atmosphere with a pressure lower than that inside the container.

[0103] When expanding the multilayer resin particles by the direct foaming method, the multilayer resin particles are first placed in a container such as a pressure vessel and dispersed in a dispersion medium. At this time, a dispersant, a dispersion aid, a surfactant, or the like may be added as needed to disperse the multilayer resin particles in the dispersion medium in the container.

[0104] As the dispersion medium, 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.

[0105] It is preferable to add a dispersant to the dispersion medium. Adding a dispersant to the dispersion medium can suppress fusion between multilayer resin particles heated in a container during the foaming process. The amount of dispersant added is preferably 0.001 to 5 parts by mass per 100 parts by mass of multilayer 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.

[0106] 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 part by mass or more and 1 part by mass or less per 100 parts by mass of the multilayer resin particles.

[0107] After dispersing the multilayer resin particles in a dispersion medium, the multilayer resin particles are impregnated with a blowing agent in a container. The blowing agent impregnated into the multilayer resin particles is preferably a physical blowing agent. Examples of physical blowing agents include inorganic physical blowing agents such as carbon dioxide, air, nitrogen, helium, and argon; and organic physical blowing agents such as aliphatic hydrocarbons such as propane, butane, and hexane; cycloaliphatic hydrocarbons such as cyclopentane and cyclohexane; and halogenated hydrocarbons such as 1,3,3,3-tetrafluoropropene, 1-chloro-3,3,3-trifluoropropene, 1-chloro-2,3,3,3-tetrafluoropropene, 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 physical blowing agents may be used in combination. Furthermore, a mixture of an inorganic physical blowing agent and an organic physical blowing agent may also be used. From the viewpoint of environmental load and ease of handling, inorganic physical blowing agents are preferably used as the physical blowing agent, and carbon dioxide is more preferably used.

[0108] The amount of the foaming agent added relative to 100 parts by mass of the multilayer resin particles is preferably 0.1 parts by mass or more and 30 parts by mass or less, and more preferably 0.5 parts by mass or more and 15 parts by mass or less.

[0109] The method for impregnating the multilayer resin particles with the blowing agent can be, for example, a method in which the blowing agent is supplied to a container and the multilayer resin particles in the dispersion medium are impregnated with the blowing agent. In this case, the impregnation of the multilayer resin particles with the blowing agent can be further promoted by heating the multilayer resin particles together with the dispersion medium.

[0110] The pressure in the container during expansion is preferably 0.5 MPa (G) or more in gauge pressure. On the other hand, the pressure in the container is preferably 4.0 MPa (G) or less in gauge pressure. If the pressure is within the above range, expanded beads can be produced safely without risk of damage to the container, explosion, etc.

[0111] When the dispersion medium is heated, the temperature of the dispersion medium can be raised at a rate of 1 to 5° C. / min, so that the temperature during foaming can be kept within an appropriate range.

[0112] After the impregnation of the multilayer resin particles with the blowing agent is completed, the contents of the container are released into an environment with a lower pressure than the container, which causes the core layer of the multilayer resin particles to expand and form a cellular structure, which is then cooled by the outside air and stabilized, yielding expanded beads.

[0113] When the multilayer resin particles are impregnated with a blowing agent, the heating and foaming are preferably carried out in the following manner. That is, first, a first-stage holding step is performed in which the temperature is maintained at a temperature of at least (the melting point of the polypropylene-based resin -20°C) but less than (the melting end temperature of the polypropylene-based resin) for a sufficient time, preferably about 10 to 60 minutes. Thereafter, the temperature is adjusted to between (the melting point of the polypropylene-based resin -15°C) and less than (the melting end temperature of the polypropylene-based resin +10°C). If necessary, a second-stage holding step is performed in which the temperature is maintained at that temperature for an additional sufficient time, preferably about 10 to 60 minutes. Thereafter, the contents of the container are preferably released to the outside while the temperature inside the container is maintained at at least (the melting point of the polypropylene-based resin -10°C), thereby foaming the multilayer resin particles. It is more preferable that the temperature inside the container during foaming be at least (the melting point of the polypropylene-based resin) but less than (the melting point of the polypropylene-based resin +20°C). By heating and expanding the multilayer resin particles in this manner, secondary crystals are formed in the polypropylene-based resin that constitutes the foamed core layer, and expanded beads that have excellent mechanical strength and moldability can be easily obtained.

[0114] In the expansion process, the multilayer resin particles may be expanded in one step by the above-mentioned direct expansion method, or the expanded particles obtained by the direct expansion method may be further expanded. When the expansion of the multilayer resin particles is performed in two steps, the first expansion step is called the first-stage expansion step, and the expanded particles obtained by the first-stage expansion step are called first-stage expanded particles. The second expansion step is called the second-stage expansion step. The expanded particles obtained by the second-stage expansion step are sometimes called second-stage expanded particles.

[0115] A specific method for expanding multilayer resin particles in two stages is as follows. First, in the first-stage expansion step, the multilayer resin particles are expanded, for example, by the direct expansion method described above, to obtain first-stage expanded particles. Then, internal pressure is applied to the first-stage expanded particles. More specifically, the first-stage expanded particles are placed in a pressure-resistant vessel, and the vessel is pressurized with air or an inorganic gas such as carbon dioxide to impregnate the first-stage expanded particles with the inorganic gas. This increases the pressure within the cells of the first-stage expanded particles to atmospheric pressure or higher. The first-stage expanded particles are then removed from the pressure-resistant vessel and heated using a heating medium such as steam or heated air in an environment with a pressure lower than the pressure within the cells, thereby subjecting the first-stage expanded particles to second-stage expansion.

[0116] (Expanded Bead Molded Article) The expanded beads can be molded in a mold to obtain an expanded bead molded article. The molded article has an open-cell structure. The open-cell structure is a minute space portion that communicates with the outside of the molded article. The open-cell structure is formed by a complex connection of voids formed by interconnected through-holes of a plurality of expanded beads, voids formed by interconnected through-holes of expanded beads with voids formed between the expanded beads, voids formed by interconnected gaps between the expanded beads, and open-cell portions of the expanded beads that constitute the molded article.

[0117] 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.

[0118] To produce the foamed bead molded article, for example, the foamed beads are filled into a mold, and then steam is supplied as a heating medium into the mold to perform in-mold molding. Specifically, the foamed beads are first filled into a mold having a cavity corresponding to the shape of the desired molded article. After filling with the foamed beads is completed, steam is supplied into the mold to heat the foamed beads. The foamed beads in the mold are heated by the steam and fuse to each other while undergoing secondary expansion. This allows the foamed beads in the mold to be integrated and form a molded article.

[0119] 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, if necessary, a curing step may be performed in which the molded body after release is left standing for a predetermined period of time in a high-temperature atmosphere adjusted to a temperature of, for example, about 60°C to 80°C. However, even if the molded body after release is not subjected to a curing step in a high-temperature atmosphere, shrinkage and deformation of the molded body can be suppressed. When the curing step is omitted, the shape of the molded body can be stabilized, for example, by leaving the molded body after release in an environment of 23°C for 12 hours.

[0120] Examples of the expanded beads, the expanded bead molded article, and the method for producing the same will now be described.

[0121] (Resin) Table 1 shows the properties of the resin used in producing the expanded beads.

[0122]

[0123] [Melting Point] The melting point of the resin was determined in accordance with JIS K7121:1987. Specifically, first, a test piece made of the resin was conditioned based on "(2) Measurement of melting temperature after a certain heat treatment" described in JIS K7121:1987. The heating and cooling temperatures during conditioning were 10°C / min, and the temperature range was 30°C to 200°C. A DSC curve was obtained by heating the conditioned test piece from 30°C to 200°C at a heating rate of 10°C / min. The apex temperature of the melting peak appearing in the DSC curve was then taken as the melting point. A heat flux differential scanning calorimeter (manufactured by SII Nanotechnology Inc., model number: DSC7020) was used as the measuring device.

[0124] [Melt Mass Flow Rate of Resin] The melt mass flow rate of polypropylene-based resin and polyethylene-based resin was measured in accordance with JIS K7210-1:2014 under conditions of a temperature of 230°C and a load of 2.16 kg. In Tables 2 to 4 described later, the melt mass flow rate of the resin constituting the coating layer is expressed as MFR. S The melt mass flow rate of the resin constituting the foam core layer is called MFR. C It is expressed as:

[0125] Next, the structure and manufacturing method of the expanded beads used in this example will be described.

[0126] 1 to 3, the expanded beads 1 of Example 1 have a cylindrical shape and have through holes 11 passing through the interior in the axial direction. The expanded beads 1 of Example 1 also have a multilayer structure including an expanded core layer 2 made of PP1 and an unexpanded coating layer 3 made of PP3 that coats the expanded core layer 2.

[0127] The method for producing the expanded beads of this example is as follows.

[0128] [Granulation Process] In this example, multilayer resin particles were prepared by the strand cutting method in the granulation process. Specifically, a co-extrusion apparatus equipped with a core layer extruder, a coating layer extruder, and a co-extrusion die connected to these two extruders was used. The extrudate extruded from the co-extrusion apparatus was cut to an appropriate length to prepare multilayer resin particles. Specifically, in the core layer extruder, PP1, carbon black as a coloring pigment, and zinc borate as a cell regulator were melt-kneaded to obtain a core layer-forming resin melt-kneaded mixture. The amount of zinc borate was 500 ppm by mass relative to the mass of PP1, and the amount of carbon black in the core layer-forming resin melt was 2.7% by mass. In parallel with this, in the coating layer extruder, PP3 and carbon black as a coloring pigment were melt-kneaded to obtain a coating layer-forming resin melt-kneaded mixture. The amount of carbon black blended into the molten kneaded resin for forming the coating layer was 2.7% by mass in the molten kneaded resin for forming the coating layer.

[0129] These resin melt blends were merged in a co-extrusion die to form a composite consisting of a non-foamed core layer and a non-foamed coating layer covering the lateral surface of the core layer. This composite was extruded through a small hole in the co-extrusion die, and then cooled in water adjusted to 10°C while withdrawing the extrudate. The extrudate was then cut to an appropriate length using a pelletizer to obtain multilayer resin particles consisting of a core layer and a coating layer covering the lateral surface of the core layer, with through-holes formed in 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 of each multilayer resin particle was approximately 1.5 mg.

[0130] [Expansion Process] In the expansion process of this example, expanded beads were produced by expanding the multilayer resin particles in two stages. In the first-stage expansion process, the multilayer resin particles were expanded by a direct expansion method to produce first-stage expanded beads. Specifically, 1 kg of the multilayer resin particles was first placed in a 5-L container together with 3 L of water as a dispersion medium. Next, 0.3 parts by mass of a dispersant and 0.004 parts by mass of a dispersion aid were added to the container relative to 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 (specifically, sodium alkylbenzene sulfonate) was used as the dispersion aid.

[0131] The container was then sealed, and carbon dioxide was added as a blowing agent to the container. The contents of the container were then stirred while being heated to the temperature shown in the "Expansion Temperature" column of Table 2. The pressure inside the container (also referred to as the impregnation pressure or carbon dioxide pressure) at this time was the value shown in the "Expansion Pressure" column of Table 2. After maintaining the above-mentioned expansion temperature for 15 minutes, the container was opened and the contents were released under atmospheric pressure, thereby obtaining first-stage expanded particles having an expanded core layer formed by the expansion of the core layer and an unexpanded coating layer covering the expanded core layer.

[0132] Next, the first-stage expanded beads were further expanded by a second-stage expansion step to obtain expanded beads. Specifically, the first-stage expanded beads were placed in a pressure-resistant container (specifically, a metal drum), and air was supplied into the pressure-resistant container to increase the pressure inside the container and impregnate the air into the bubbles. The internal pressure of the bubbles of the first-stage expanded beads removed from the pressure-resistant container was as shown in Table 2. The first-stage expanded beads were then placed in a metal drum, and steam was supplied to heat the first-stage expanded beads so that the drum pressure reached the value shown in Table 2, thereby obtaining expanded beads having the apparent density shown in Table 2.

[0133] (Example 2) The expanded beads of this example have a structure similar to that of the expanded beads of Example 1, except for the apparent density. The method for producing the expanded beads of this example is the same as that of Example 1, except that the drum pressure in the second-stage expansion step is changed as shown in Table 2.

[0134] Example 3 The expanded beads of this example have a configuration generally similar to that of the expanded beads of Example 1, except that the resin constituting the expanded core layer is changed from PP1 to PP2. The method for producing the expanded beads of this example is generally similar to the method for producing the expanded beads of Example 1, except that the resin fed into the core layer-forming extruder in the granulation step is changed from PP1 to PP2 and the expansion step conditions are changed as shown in Table 2.

[0135] Example 4 The expanded beads of this example have a configuration generally similar to that of the expanded beads of Example 1, except that the resin constituting the coating layer is changed from PP3 to PE1. The method for producing the expanded beads of this example is generally similar to the method for producing the expanded beads of Example 1, except that the resin fed into the extruder for forming the coating layer in the granulation step is changed from PP3 to PE1.

[0136] Example 5 The expanded beads of this example have a configuration generally similar to that of the expanded beads of Example 1, except that the amount of carbon black blended into the resin melt-kneaded mixture for forming the core layer and the resin melt-kneaded mixture for forming the coating layer was 1.0% by mass in each of the kneaded mixtures. The method for producing the expanded beads of this example is generally similar to the method for producing the expanded beads of Example 1, except that the amount of carbon black added to the core layer-forming extruder and the coating layer-forming extruder in the granulation step was changed so that each of the melt-kneaded mixtures was 1.0% by mass.

[0137] Example 6 The expanded beads of this example have a configuration generally similar to that of the expanded beads of Example 1, except that the amounts of carbon black blended into the resin melt-kneaded mixture for forming the core layer and the resin melt-kneaded mixture for forming the coating layer were changed to 4.0% by mass in each of the kneaded mixtures. The method for producing the expanded beads of this example is generally similar to the method for producing the expanded beads of Example 1, except that the amounts of carbon black added to the core layer-forming extruder and the coating layer-forming extruder in the granulation step were changed so that each melt-kneaded mixture was 4.0% by mass.

[0138] The expanded beads of this comparative example have a single-layer structure consisting of only an expanded core layer having through-holes. The method for producing the expanded beads of this example is generally similar to that of Example 1, except that no coating layer is formed on the circumferential surface of the core layer in the granulation step.

[0139] (Comparative Examples 2 to 4) The expanded beads of Comparative Examples 2 to 4 have generally the same configuration as the expanded beads 1 of Example 1, except that the resin constituting the coating layer was changed from PP3 to the resin shown in Table 3. The method for producing the expanded beads of these comparative examples was generally the same as the method for producing the expanded beads of Example 1, except that the resin fed into the extruder for forming the coating layer in the granulation step was changed from PP3 to the resin shown in Table 3.

[0140] (Reference Example 1) The expanded beads of this example have a structure generally similar to that of the expanded beads of Comparative Example 3, except that the average pore size d of the through holes was changed to the value shown in Table 4. The method for producing the expanded beads of this example was generally similar to the method for producing the expanded beads of Comparative Example 3, except that the die used in the granulation step was changed to a die having a shape capable of forming a core layer having a larger pore size of the through holes.

[0141] (Reference Example 2) The expanded beads of this example have a structure similar to that of the expanded beads of Comparative Example 3, except that they do not have through holes. The method for producing the expanded beads of this example is similar to that of Comparative Example 3, except that the co-extrusion die is changed to a die having a shape in which no through holes are formed.

[0142] (Reference Example 3) The expanded beads of this example have a structure similar to that of the expanded beads of Comparative Example 3, except that no color pigment is blended. The method for producing the expanded beads of this example is similar to that for Comparative Example 3, except that no color pigment is added in the granulation step.

[0143] The properties of the expanded beads of the Examples, Comparative Examples and Reference Examples are shown in Tables 2 to 4. The methods for evaluating the properties shown in Tables 2 to 4 are as follows.

[0144] (Evaluation of Expanded Beads) For the measurement and evaluation of the physical properties of the expanded beads, the expanded beads were used after being left to stand for 24 hours under conditions of 50% relative humidity, 23° C. and 1 atm to condition them.

[0145] [Bulk Density] The expanded beads after conditioning were filled into a measuring cylinder so as to naturally accumulate, and the bulk volume (unit: L) of the expanded beads was read from the scale of the measuring cylinder. Then, the mass (unit: g) of the expanded beads in the measuring cylinder was divided by the bulk volume, and the unit was further converted to obtain the bulk density (unit: kg / m) of the expanded beads. 3 ) was calculated.

[0146] [Apparent Density] After measuring the mass of the expanded beads after conditioning, the expanded beads were submerged in a measuring cylinder containing ethanol at 23°C using a wire netting. Taking the volume of the wire netting into consideration, the volume of the expanded beads was measured from the rise in the water level. The mass (unit: g) of the expanded beads thus obtained was divided by the volume (unit: L), and the units were converted to determine the apparent density (unit: kg / m) of the expanded beads. 3 The "apparent density / bulk density" column in Tables 2 to 4 lists the apparent density of the expanded beads divided by the bulk density.

[0147] [Closed Cell Ratio] The method for measuring the closed cell ratio of the expanded beads is as described above.

[0148] [Average diameter d of through holes] 100 expanded beads were randomly selected from the group of expanded beads after conditioning, and these expanded beads were cut along a plane perpendicular to the axial direction at the position where their cross-sectional area was maximum to expose the cut surface of the expanded beads. Next, a photograph of the cut surface of the expanded beads was taken, and the cross-sectional area of ​​the through holes (i.e., the opening area) on the cut surface was measured by image analysis. The diameter of a virtual perfect circle having the same area as the cross-sectional area of ​​the through holes was calculated, and this value was used as the diameter of the through holes in each expanded bead. The above operation was performed on 100 expanded beads, and the arithmetic average value of the obtained diameters of the through holes was used as the average diameter d of the through holes in the expanded beads.

[0149] [Average Outer Diameter D of Expanded Beads] One hundred expanded beads were randomly selected from the group of expanded beads after conditioning, and these expanded beads were cut along a plane perpendicular to the axial direction at the position where their cross-sectional area was largest to expose the cut surface of the expanded beads. Next, a photograph of the cut surface of the expanded beads was taken, and image analysis was performed to measure the cross-sectional area of ​​the expanded beads, including the through-holes (i.e., the area of ​​the region surrounded by the outer edge of the expanded bead on the cut surface). The diameter of a virtual circle having the same area as the cross-sectional area of ​​the expanded bead was calculated, and this value was used as the outer diameter of each expanded bead. The above procedure was performed for 100 expanded beads, and the arithmetic mean value of the obtained outer diameters of the expanded beads was used as the average outer diameter D of the expanded beads. The value obtained by dividing the average diameter d of the through-holes by the average outer diameter D of the expanded beads is shown in the "d / D" column of Tables 2 to 4.

[0150] [Average Wall Thickness t of Expanded Beads] Using the average pore diameter d of the through holes obtained by the above method and the average outer diameter D of the expanded beads, the average wall thickness t of the expanded beads was calculated according to the following formula (3): t = (D - d) / 2 (3)

[0151] [Aspect Ratio L / D of Expanded Beads] The axial lengths of 100 randomly selected expanded beads were measured with a vernier caliper, and the arithmetic mean was calculated to determine the average axial length L of the expanded beads. The aspect ratio L / D of the expanded beads was calculated by dividing the average axial length L of the expanded beads by the average outer diameter D of the expanded beads.

[0152] [Presence or Absence of Grooves and Area Ratio] First, the presence or absence of grooves extending in the axial direction on the circumferential side surface of the expanded beads was determined by the following method. Specifically, the expanded beads were cut along a plane perpendicular to the axial direction at the position where their cross-sectional area was greatest to expose the cut surface of the expanded beads. Next, a photograph of the cut surface of the expanded beads was taken, and the resulting cross-sectional photograph was observed. If the contour of the expanded beads in the cross-sectional photograph contained a portion that was depressed inward from the surrounding area, it was determined that a groove was present. If no depressed portion was present, it was determined that a groove was not present. Furthermore, for expanded beads that had grooves on their circumferential side surface, the area ratio of the grooves was calculated by the following method.

[0153] First, 100 expanded beads were randomly selected from the group of expanded beads after conditioning, and these expanded beads were cut along a plane perpendicular to the axial direction at the position where their cross-sectional area was largest to expose the cut surface of the expanded beads. Next, a photograph of the cut surface of the expanded beads was taken, and image analysis was performed to measure the cross-sectional area of ​​the expanded beads excluding the through-holes and the total cross-sectional area of ​​the grooves. These values ​​were then used to calculate the ratio (unit: %) of the total cross-sectional area of ​​the grooves to the cross-sectional area of ​​the expanded beads excluding the through-holes for each expanded bead. The method for calculating the total cross-sectional area of ​​the grooves was as described above.

[0154] The above procedure was carried out for 100 expanded beads, and the arithmetic mean value of the above ratios for these expanded beads was taken as the groove area ratio.

[0155] [Average circularity Cp of through holes] 100 expanded beads were randomly selected from the group of expanded beads after conditioning, and these expanded beads were cut along a plane perpendicular to the axial direction at the position where the cross-sectional area was maximum to expose the cut surface of the expanded beads. Next, a photograph of the cut surface of the expanded beads was taken, and the cross-sectional area S of the through holes of each expanded bead was measured by image analysis. pand the perimeter L of the through hole p These values ​​were used to calculate the circularity of the through-holes in each expanded bead based on the following formula (1): Circularity of through-hole = 4πS p / (L p ×L p ) ... (1)

[0156] The arithmetic mean value of the circularity of the through-holes in the 100 expanded beads thus obtained was taken as the average circularity Cp of the through-holes.

[0157] [Average circularity Cb of the outer periphery of the expanded beads] 100 expanded beads were randomly selected from the group of expanded beads after conditioning, and these expanded beads were cut along a plane perpendicular to the axial direction at the position where the cross-sectional area of ​​the expanded beads was maximum to expose the cut surface of the expanded beads. Next, a photograph of the cut surface of the expanded beads was taken, and the image was analyzed to determine the cross-sectional area S of the expanded beads including the through-holes. b and the peripheral length L of the outer edge of the expanded beads b These values ​​were used to calculate the circularity of the outer periphery of each expanded bead based on the following formula (2): Circularity of outer periphery of expanded bead = 4πS b / (L b ×L b ) ... (2)

[0158] The arithmetic mean value of the circularity of the peripheral edges of 100 expanded beads thus obtained was designated as the average circularity Cb of the peripheral edges of the expanded beads. In Tables 2 to 4, the "Circularity ratio Cb / Cp" column lists the value obtained by dividing the average circularity Cb of the peripheral edges of the expanded beads by the average circularity Cp of the through-holes, and the "Absolute value of the difference in circularity |Cp-Cb|" column lists the absolute value of the difference between the average circularity Cb of the peripheral edges of the expanded beads and the average circularity Cp of the through-holes.

[0159] (Evaluation of Molded Articles) Molded articles were produced using the expanded beads of Examples, Comparative Examples, and Reference Examples by the following method. First, the expanded beads were dried at 23°C for 24 hours and then impregnated with air to apply the internal pressures shown in Tables 2 to 4 to the expanded beads. Next, the expanded beads were filled into a flat mold measuring 300 mm long x 250 mm wide x 60 mm thick by a cracking filling method. The amount of cracking during filling (specifically, the ratio of the mold opening amount to the internal dimensions in the thickness direction) was 10% (i.e., 6 mm). After filling was complete, the mold was clamped in the thickness direction to mechanically compress the expanded beads.

[0160] Next, steam was supplied into the mold to perform in-mold molding. In the in-mold molding, steam was first supplied into the mold for 5 seconds with the drain valve of the mold open to perform preheating. Next, the drain valve was closed, and steam was supplied from one side of the mold until a pressure 0.08 MPa (G) lower than the molding pressure during main heating was reached, performing a first one-way heating. Next, steam was supplied from the other side of the mold until a pressure 0.04 MPa (G) lower than the molding pressure during main heating was reached, performing a second one-way heating. Thereafter, steam was supplied from both sides of the mold until the molding pressure during main heating shown in Tables 2 to 4 was reached, performing main heating. After main heating was completed, the pressure inside the mold was released, and the molded body was cooled with water in the mold until the surface pressure due to the foaming force of the molded body reached 0.04 MPa (G). The time required for cooling in the mold was as shown in the "Cooling Time" column of Tables 2 to 4. The molding pressure during the main heating was set at a value at which the density of the molded body was approximately 30 kg / m except for Example 2. ３ It was set to be.

[0161] Thereafter, the expanded bead molding was removed from the mold and allowed to stand for 12 hours in an oven at 80° C. for a curing step. After the curing step, the expanded bead molding was allowed to stand for 24 hours under conditions of a relative humidity of 50%, 23° C., and 1 atm, and then its yield was evaluated.

[0162] [Density of Molded Product] The mass (unit: g) of the molded product was divided by the volume (unit: L) determined from the outer dimensions of the molded product, and then the density (unit: kg / m) of the molded product was calculated by converting the unit. 3 ) was calculated.

[0163] [50% compressive stress σ 50 A test piece having a rectangular prism shape of 50 mm long x 50 mm wide x 25 mm thick, which did not include the skin surface, i.e., the surface that had been in contact with the inner surface of the mold during molding in the mold, was cut out from the center of the molded body. A compression test was carried out at a compression rate of 10 mm / min in accordance with JIS K6767:1999, and the 50% compressive stress σ of the molded body was measured. 50 (unit: kPa) was calculated.

[0164] [Appearance] (Surface properties) The surface of the molded article was observed and the surface properties were evaluated based on the following criteria: A: The surface of the molded article has few interparticle gaps and shows a good surface condition with inconspicuous irregularities caused by through-holes, etc. B: The surface of the molded article has some irregularities caused by interparticle gaps and / or through-holes, etc. C: The surface of the molded article has significant irregularities caused by interparticle gaps and / or through-holes, etc.

[0165] (Number of Streak Patterns and Their Evaluation) A square observation area with sides of 5 cm was set in the center of a surface measuring 300 mm long and 250 mm wide on the surface of the molded article. The surface of the expanded beads present within this observation area was visually observed, and the number of streak patterns formed on the expanded beads was counted. FIG. 7 schematically shows an example of the observation area. FIG. 8 shows a photograph of the surface of molded article E1 obtained using the expanded beads of Example 1, and FIG. 9 shows a photograph of the surface of molded article C3 obtained using the expanded beads of Comparative Example 3. As shown in FIGS. 7 to 9, the surface of expanded beads 1a without streak patterns 16 is generally smooth. On the other hand, as shown in FIGS. 7 and 9, streak patterns 16 are observed as linear patterns that pass through the surface of expanded beads 1b and are lighter in color than the surrounding area. Note that through-holes 11 may be present on the surface of expanded beads 1 within the observation area.

[0166] The number of streaks that appeared in the observation area (unit: pieces / 25 cm 2 ) are as shown in the "Number of streaks" column in Tables 2 to 4. The number of streaks measured was evaluated according to the following criteria: A: Number of streaks was 10 / 25cm 2 Less than B: Number of streaks is 10 / 25cm 2 More than 60 pieces / 25cm 2Less than C: Number of streaks: 60 / 25cm 2 End

[0167]

[0168]

[0169]

[0170] As shown in Table 2, the expanded beads of Examples 1 to 6 have a cylindrical shape with through holes, and the average diameter d of the through holes and the ratio d / D of the average diameter d of the through holes to the average outer diameter D of the expanded beads are each within the above-mentioned specific ranges. Furthermore, the expanded beads of these Examples have an expanded core layer and a coating layer that coats the foamed core layer, and the melt mass-flow rate MFR of the polyolefin resin that constitutes the coating layer is S is within the specified range. Therefore, by performing in-mold molding using the expanded beads of these Examples, the formation of streaky patterns on the surface of the molded article was suppressed, and molded articles with good appearance were obtained. Furthermore, by using the expanded beads of these Examples, the water cooling time was shortened, and the productivity of molded articles was improved.

[0171] On the other hand, as shown in Table 3, the expanded beads of Comparative Example 1 did not have a coating layer, and therefore striped patterns were likely to form on the surface of the molded article. In addition, when the expanded beads of Comparative Example 1 were used, the water cooling time was long.

[0172] The coating layer of the expanded beads of Comparative Examples 2 to 4 had a melt mass-flow rate MFR S Therefore, streaky patterns tend to form on the surface of the molded article.

[0173] As shown in Table 4, the average pore diameter d of the through holes in the expanded beads of Reference Example 1 was larger than the specified range. Furthermore, the expanded beads of Reference Example 2 had no through holes. As shown in Table 4, although the melt flow rates of the polyolefin resins constituting the coating layers of these expanded beads were lower than the specified range, almost no streaky patterns were formed on the surface of the molded articles made from the expanded beads. Therefore, by comparing Examples 1 to 6 and Comparative Examples 1 to 4 with Reference Examples 1 and 2, it can be seen that the phenomenon of streaky patterns being formed on the surface of molded articles is a phenomenon specific to molded articles made from expanded beads that have through holes and whose average pore diameter d of the through holes is within the specified range. The expanded beads of Reference Example 1 had an excessively large average pore diameter of the through holes, resulting in poor surface properties. Furthermore, the expanded beads of Reference Example 2 had no through holes and required a high molding pressure.

[0174] Furthermore, the expanded beads of Reference Example 3 did not contain a coloring pigment. As shown in Table 4, although the expanded beads of these Reference Examples had a melt flow rate of the polyolefin resin constituting the coating layer lower than the specific range, no streaky patterns were observed on the surface of the molded articles made from the expanded beads. This is thought to be due to the relatively small area ratio of the grooves in the expanded beads and the white color, which prevented the streaky patterns from being observed. Therefore, a comparison of Examples 1 to 6 and Comparative Examples 1 to 4 with Reference Example 3 shows that the phenomenon of streaky patterns being formed on the surface of molded articles is more likely to occur when expanded beads containing carbon black as a colorant are molded.

[0175] The specific embodiments of the expanded polypropylene resin beads and the method for producing the same according to the present invention have been described above based on the examples. However, the specific embodiments of the expanded polypropylene resin beads and the method for producing the same according to the present invention are not limited to the embodiments, and the configurations can be changed as appropriate within the scope that does not detract from the spirit of the present invention.

Claims

1. A cylindrical polypropylene-based resin expanded bead having through holes passing through the inside in the axial direction, wherein the average hole diameter d of the through holes in the expanded bead is less than 1 mm, and the ratio d / D of the average hole diameter d of the through holes to the average outer diameter D of the expanded bead is 0.4 or less, the expanded bead has a foamed core layer made of a polypropylene-based resin, and a coating layer made of a polyolefin-based resin and coating a side peripheral surface of the foamed core layer, the foamed core layer and the coating layer contain a coloring pigment, and the melt mass-flow rate MFR of the polyolefin-based resin constituting the coating layer measured according to JIS K7210-1:2014 under conditions of a temperature of 230°C and a load of 2.16 kg is S The expanded polypropylene resin particles have a strength of more than 15 g / 10 min.

2. The polypropylene resin expanded beads according to claim 1, wherein the average circularity Cb of the outer peripheral edge of the expanded bead in a cross section obtained by cutting the expanded bead at a position where the cross-sectional area is maximum along a plane perpendicular to the axial direction is 0.95 or more.

3. A polypropylene-based resin expanded particle according to claim 1 or 2, wherein the content of the coloring pigment in the expanded core layer is 0.1% by mass or more and 5% by mass or less, and the content of the coloring pigment in the coating layer is 0.1% by mass or more and 5% by mass or less.

4. The expanded polypropylene resin particles according to any one of claims 1 to 3, wherein the coloring pigment contained in the expanded core layer is carbon black, and the coloring pigment contained in the coating layer is carbon black.

5. The melt mass-flow rate (MFR) of the polypropylene resin constituting the foamed core layer measured under conditions of a temperature of 230° C. and a load of 2.16 kg according to JIS K7210-1:2014 C The expanded polypropylene resin particles according to any one of claims 1 to 4, wherein the flow rate is 5 g / min or more and 12 g / min or less.

6. The melt mass flow rate (MFR) of the polypropylene resin constituting the foamed core layer measured under conditions of a temperature of 230° C. and a load of 2.16 kg according to JIS K7210-1:2014 C The melt mass flow rate MFR of the polyolefin resin constituting the coating layer S Ratio of MFR S / M.F.R. C The expanded polypropylene resin particles according to any one of claims 1 to 5, wherein the number of carbon atoms is 2 or more and 5 or less.

7. The expanded polypropylene resin beads according to any one of claims 1 to 6, wherein the average wall thickness t of the expanded beads is 1.2 mm or more and 2 mm or less.

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

9. The expanded polypropylene resin beads according to any one of claims 1 to 8, wherein the average circularity Cp of the through holes in a cross section obtained by cutting the expanded bead along a plane perpendicular to the axial direction at the position where the cross-sectional area is maximum is 0.90 or more.

10. A polypropylene resin expanded bead according to any one of claims 1 to 9, wherein the ratio Cb / Cp of the average circularity Cb of the outer peripheral edge of the expanded bead to the average circularity Cp of the through holes of the expanded bead in a cross section obtained by cutting the expanded bead along a plane perpendicular to the axial direction at the position where the cross-sectional area is maximum is 0.96 or more and 1.05 or less.

11. A method for producing cylindrical polypropylene-based resin expanded particles having a through hole passing through the inside in the axial direction, comprising: a granulation step of producing multilayer resin particles having a cylindrical core layer made of a polypropylene-based resin and having a through hole passing through the inside in the axial direction, and a coating layer made of a polyolefin-based resin and coating a lateral surface of the core layer; and an expansion step of expanding the multilayer resin particles after the granulation step to obtain the expanded particles, wherein the average pore size dr of the through holes in the multilayer resin particles is less than 0.25 mm, the ratio dr / Dr of the average pore size dr of the through holes to the average outer diameter Dr of the multilayer resin particles is 0.4 or less, the core layer and the coating layer contain a coloring pigment, and the melt mass-flow rate MFR of the polyolefin-based resin constituting the coating layer measured according to JIS K7210-1:2014 under conditions of a temperature of 230°C and a load of 2.16 kg is S The method for producing expanded polypropylene resin particles,

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